一、Electrical synapses

The many kinds of synapses within the human brain fall into two general classes: electrical synapses and chemical synapses.

Although they are a distinct minority, electrical synapses are found in all nervous systems and permit direct, passive flow of electrical current from one neuron to another.

The membranes of the two communicating neurons (presynaptic and postsynaptic) come extremely close at the synapse and are actually linked together by an intercellular specialization called a gap junction.

Structure of electrical synapses

Gap junctions contain precisely aligned, paired channels called connexons（接合质）.

Connexons are present in the membranes of both the pre- and postsynaptic neurons.

Six presynaptic connexins align with six postsynaptic connexins to form a pore that connects the two cells.

The pore of a connexon channel is much larger than the pores of the voltage-gated, which permits ATP and other important intracellular metabolites, such as second messengers, to be transferred between neurons.

Connexons are composed of a special family of ion channel proteins, the connexins.

There are several different types of connexins, found in different cell types and yielding gap junctions with diverse physiological properties.

Function of electrical synapses

Electrical synapses work by allowing ionic current to flow passively through the gap junction pores from one neuron to another.

The usual source of this current is the potential difference generated locally by the presynaptic action potential.

This arrangement has a number of interesting consequences:

• transmission can be bidirectional; that is, current can flow in either direction across the gap junction, depending on which member of the coupled pair is invaded by an action.

• transmission is extraordinarily fast: because passive current flow across the gap junction is virtually instantaneous, communication can occur without the delay that is characteristic of chemical synapses.

  • These features are apparent in the operation of the first electrical synapse to be discovered, which resides in the crayfish nervous system.
  • A postsynaptic electrical signal is observed at this synapse within a fraction of a millisecond after the generation of a presynaptic action potential.
  • Such synapses interconnect many of the neurons within the circuit that allows the crayfish to escape from its predators, thus minimizing the time between the presence of a threatening stimulus and a potentially lifesaving motor response.

A more general purpose of electrical synapses is to synchronize electrical activity among populations of neurons.

• the brainstem neurons that generate rhythmic electrical activity underlying breathing are synchronized by electrical synapses.
• Electrical synapses allow synchronization of electrical activity in hippocampal interneurons.
• Electrical transmission between certain hormone-secreting neurons within the hypothalamus ensures that all cells fire action potentials at about the same time, thus facilitating a burst of hormone secretion into the circulation.

二、Chemical synapses

In a chemical synapse, the space between the pre- and postsynaptic neurons is substantially greater than at electrical synapses and is called the synaptic cleft.

The key feature of all chemical synapses is the presence of small, membranebounded organelles called synaptic vesicles within the presynaptic terminal.

The synaptic vesicles are filled with one or more neurotransmitters, which are secreted from the presynaptic neuron, and act as messengers between the communicating neurons.

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Structure of chemical synapses

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Signal transmission at chemical synapses

1. Transmitter is synthesized and then stored in vesicles
2. An action potential invades the presynaptic terminal
3. Depolarization of presynaptic terminal causes opening of voltage-gated Ca2+ channels
4. Influx of Ca2+ through channels
5. Ca2+ causes vesicles to fuse with presynaptic membrane
6. Transmitter is released into synaptic cleft via exocytosis
7. Transmitter binds to receptor molecules in postsynaptic membrane
8. Opening or closing of postsynaptic channels
9. Postsynaptic current causes excitatory or inhibitory postsynaptic potential that changes die excitability of the postsynaptic cell
10. Removal of neurotransmitter by glial uptake or enzymatic degradation
11. Retrieval of vesicular membrane from plasma membrane

Chemical neurotransmission

Can electrical information be transferred from one neuron to the next by means of chemical signaling？

In 1926, the German physiologist Otto Loewi performed a key experiment that supported this idea, which earned for him the Nobel Prize in Physiology or Medicine in 1936.

According to Loewi, the idea for his key experiment came to him in his sleep.

Loewi proved that electrical stimulation of the vagus (迷走神经) nerve slows the heartbeat by releasing a chemical signal.

• He isolated and perfused the hearts of two frogs, monitoring their beating.
• When the vagus nerve innervating the first heart was stimulated, its beating slowed.
• Remarkably, even though the vagus nerve of the second heart had not been stimulated, its beat also slowed when exposed to the perfusate from the first heart.

The vagus nerve regulates the heart rate by releasing a chemical that accumulates in the perfusate: “vagus substance,” later shown to be acetylcholine (ACh)–a neurotransmitter.

Criteria that define a neurotransmitter

1. The substance must be present within the presynaptic neuron.

2. The substance must be released in response to presynaptic depolarization, and the release must be Ca2+-dependent.

3. Specific receptors for the substance must be present on the postsynaptic cell.

Neurotransmitter

These have led to the identification of more than 100 different neurotransmitters, which can be classified into two broad categories: small-molecule neurotransmitters and neuropeptides.

In general, small-molecule neurotransmitters mediate rapid synaptic actions, whereas neuropeptides tend to modulate slower, ongoing synaptic functions.

Until relatively recently, it was believed that a given neuron produced only a single type of neurotransmitter.

It is now clear, however, that many types of neurons synthesize and release two or more different neurotransmitters–co-transmitters.

1. Metabolism of small-molecule transmitters

1. The enzymes necessary for neurotransmitter synthesis are made in the cell body of the presynaptic cell.
2. The enzymes are transported down the axon by slow axonal transport (0.5-5.0 mm/day)
3. Precursors are taken up into the terminals by specific transporters, and neurotransmitter synthesis and packaging take place within the nerve endings.
4. After vesicle fusion and release, the neurotransmitter may be enzymatically degraded.
5. The reuptake of the neurotransmitter (or its metabolites) begins another cycle of synthesis, packaging, release, and removal.

• Most small-molecule neurotransmitters are packaged in vesicles 40–60 nm in diameter, the centers of which appear clear in electron micrographs; accordingly these vesicles are referred to as small clear-core vesicles.

2. Metabolism of peptide transmitters

1. Peptide neurotransmitters, as well as the enzymes that modify their precursors, are synthesized in the cell body. Enzymes and propeptides are packaged into vesicles in the Golgi apparatus

2. Vesicles are transported to the nerve terminals via fast axonal transport (400 mm/day).

   • Peptide-containing vesicles move along these microtubule “tracks” by ATP requiring “motor” proteins such as kinesin.

3. During the transport, the enzymes modify the propeptides to produce one or more neurotransmitter peptides.

4. After vesicle fusion and exocytosis, the peptides diffuse away and are degraded by proteolytic enzymes.

• Neuropeptides are packaged into synaptic vesicles that range from 90–250 nm in diameter. Because the center of these vesicles appear electron-dense in electron micrographs, they are referred to as large dense-core vesicles.

Molecular mechanisms of synaptic vesicle cycling

Precisely how an increase in presynaptic Ca2+ concentration goes on to trigger vesicle fusion and neurotransmitter release is not understood.

However, many important insights have come from molecular studies that have identified and characterized the proteins found on synaptic vesicles.

• Model of the molecular organization of a synaptic vesicle.
• The cytoplasmic surface of the vesicle membrane is densely covered by proteins, only 70% of which are shown here.

the protein synapsin, which reversibly binds to synaptic vesicles, may keep these vesicles tethered within the reserve pool by crosslink vesicles to each other and to **actin **filaments in the cytoskeleton.

Mobilization of these reserve pool vesicles is caused by phosphorylation of synapsin by proteins kinases, most notably the Ca2+ calmodulin-dependent protein kinase, type II (CaMKII), which allows synapsin to dissociate from the vesicles.

Once vesicles are free from their reserve pool tethers, they make their way to the plasma membrane and are then attached to this membrane by poorly understood docking reactions.

A series of priming (a process of cleaning and preparing equipment) reactions then prepares the vesicular and plasma membranes for fusion. A large number of proteins are involved in priming.

• the ATPase NSF (NEM-sensitive fusion protein) and SNAPs(soluble NSF attachment proteins).
• These two proteins work by regulating the assembly of other proteins that are called SNAREs (SNAP receptors).
• Many of the other proteins involved in priming—such as munc-13, nSec-1, complexin, snapin, syntaphilin, and tomosyn—also interact with the SNAREs

One of the main purposes of priming seems to organize SNARE proteins into the correct conformation for membrane fusion.

One of the SNARE proteins, synaptobrevin, is in the membrane of synaptic vesicles, while two other SNARE proteins called syntaxin and SNAP-25 are found primarily on theplasma membrane.

These SNARE proteins can form a macromolecular complex that spans the two membranes, thus bringing them into close apposition.

Ca2+ regulation of neurotransmitter release is conferred by synaptotagmin, a protein found in the membrane of synaptic vesicles.

Synaptotagmin acts as a Ca2+ sensor, signaling the elevation of Ca2+ within the terminal and thus triggering vesicle fusion.

How Ca2+ binding to synaptotagmin leads to exocytosis is not yet clear.

A model for Ca2+-triggered vesicle fusion. SNARE proteins on the synaptic vesicle and plasma membranes form a complex that brings together the two membranes. Ca2+ then binds to synaptotagmin, causing the cytoplasmic region of this protein to catalyze membrane fusion by binding to SNAREs and inserting into the plasma membrane.

The most important protein involved in endocytoticbudding of vesicles from the plasma membrane is clathrin.

• Clathrin has a unique structure that is called a triskelion because of its three-legged appearance

During endocytosis, clathrin triskelia attach to the vesicular membrane that is to beretrieved.

A number of adaptor proteins, such as AP-2 and AP180, connect clathrin to the proteinsand lipids of this membrane.

Such dome-like structures form coated pits that initiate membrane budding, increasing the curvature of the budding membrane until it forms a coated vesicle-like structure.

Another protein, called dynamin, causes the final pinching-off of membrane that completes the production of coated vesicles.

The clathrin coats then are removed by an ATPase, Hsc70, with another protein, auxilin, serving as a co-factor that recruits Hsc70 to the coated vesicle. Other proteins, such as synaptojanin, are also important for vesicle uncoating.

Uncoated vesicles can then continue their journey through the recycling process, eventually becoming refilled with neurotransmitter due to the actions of neurotransmitter transporters in the vesicle membrane.

Neurotransmitter receptors

Neurotransmitter receptors are proteins that are embedded in the plasma membrane of postsynaptic cells and have an extracellular neurotransmitter binding site that detects the presence of neurotransmitters in the synaptic cleft.

There are two broad families of receptor proteins that differ in their mechanism of transducing transmitter binding into postsynaptic responses.

1. The receptors containing a membrane-spanning domain that forms an ion channel.

• These receptors combine transmitter-binding and channel functions into a single molecular entity and thus are called ionotropic receptors (the Greek tropos means to move in response to a stimulus) or ligand-gated ion channels.

2. Metabotropic receptors: the eventual movement of ions through a channel depends on intervening metabolic steps.

These receptors do not have ion channels as part of their structure; instead, they have an intracellular domain that indirectly affects channels though the activation of intermediate molecules called G-proteins.

Neurotransmitter binding to these receptors activates G-proteins, which then dissociate from the receptor and interact directly with ion channels or bind to other effector proteins, such as enzymes, that make intracellular messengers that open or close ion channels.

G-proteins can be thought of as transducers that couple neurotransmitter binding to the regulation of postsynaptic ion channels. For this reason, metabotropic receptors are also called G-protein-coupled receptors.

Ionotropic receptors generally mediate rapid postsynaptic effects.

The activation of metabotropic receptors typically produces much slower responses, ranging from hundreds of milliseconds to minutes or even longer.

A given transmitter may activate both ionotropic and metabotropic receptors to produce both fast and slow postsynaptic potential (PSP) at the same synapse.

三、End plate current

Neuromuscular synapses are valuable for understanding the mechanisms that allow neurotransmitter receptors to generate postsynaptic signals..

The binding of ACh to postsynaptic receptors opens ion channels in the muscle fiber membrane, which generate the minute postsynaptic currents.

Exposure of the extracellular surface of a patch of postsynaptic membrane to ACh causes single-channel currents to flow for a few milliseconds.

The electrical actions of ACh are greatly multiplied when an action potential in a presynaptic motor neuron causes the release of millions of molecules of ACh into the synaptic cleft.

Although individual ACh receptors only open briefly, the opening of a large number of channels is synchronized by the brief duration during which ACh is secreted from presynaptic terminals.

End plate potential

The macroscopic current resulting from the summed opening of many ion channels is called the end plate current, or EPC.

Because the current flowing during the EPC is normally inward, it causes the postsynaptic membrane potential to depolarize. This depolarizing change in potential is the EPP, which typically triggers a postsynaptic action potential by opening voltage-gated Na+ and K+channels.

Reversal potentials

When the potential of the postsynaptic muscle cell is controlled by the voltage clamp method, the magnitude of the membrane potential clearly affects the amplitude and polarity of EPCs

• When the postsynaptic membrane potential is made more negative than the resting potential, the amplitude of the EPC becomes larger, whereas this current is reduced when the membrane potential is made more positive.
• At approximately 0 mV, no EPC is detected, and at even more positive potentials, the current reverses its polarity, becoming outward rather than inward.
• The potential where the EPC reverses, about 0 mV in the case of the neuromuscular junction, is called the reversal potential.

Postsynaptic current and potential

Although this discussion has focused on the neuromuscular junction, similar mechanisms generate postsynaptic responses at all chemical synapses.

The general principle is that transmitter binding to postsynaptic receptors produces a postsynaptic conductance change as ion channels are opened (or sometimes closed).

The postsynaptic conductance is increased if—as at the neuromuscular junction— channels are opened, and decreased if channels are closed.

This conductance change typically generates an electrical current, the postsynaptic current (PSC), which in turn changes the postsynaptic membrane potential to produce a postsynaptic potential (PSP).

The conductance changes and the PSPs that typically accompany them are the ultimate outcome of most chemical synaptic transmission, concluding a sequence of electrical and chemical events that begins with the invasion of an action potential into the terminals of a presynaptic neuron.

Excitatory and inhibitory postsynaptic potentials

In many ways, the events that produce PSPs at synapses are similar to those that generate action potentials in axons; in both cases, conductance changes produced by ion channels lead to ionic current flow that changes the membrane potential.

PSPs are called excitatory (or EPSPs) if they increase the likelihood of a postsynaptic action potential occurring, and inhibitory (or IPSPs) if they decrease this likelihood.

In both cases, neurotransmitters binding to receptors open or close ion channels in the postsynaptic cell.

Whether a postsynaptic response is an EPSP or an IPSP depends on the type of channel that is coupled to the receptor, and on the concentration of permeant ions inside and outside the cell.

In fact, the only distinction between postsynaptic excitation and inhibition is the reversal potential of the PSP in relation to the threshold voltage for generating action potentials in the postsynaptic cell.

1. Excitatory postsynaptic potentials

As an example of excitatory postsynaptic action, consider a neuronal synapse that uses glutamate as the transmitter.

Many such synapses have receptors that, like the ACh receptors at neuromuscular synapses, open ion channels that are nonselectively permeable to cations.

When these glutamate receptors are activated, both Na+ and K+ flow across the postsynaptic membrane, yielding an Erev of approximately 0 mV for the resulting postsynaptic current.

If the resting potential of the postsynaptic neuron is –40 mV, the resulting EPSP will depolarize by bringing the postsynaptic membrane potential toward 0 mV.

Thus, a glutamate-induced EPSP will increase the probability that this neuron produces an action potential, defining the synapse as excitatory.

2. Inhibitory postsynaptic potentials

As an example of inhibitory postsynaptic action, consider a neuronal synapse that uses GABA as its transmitter.

At such synapses, the GABA receptors typically open channels that are selectively permeable to Cl–, and the action of GABA causes Cl– to flow across the postsynaptic membrane into the cell and produce a hyperpolarizing IPSP.

This hyperpolarizing IPSP will take the postsynaptic membrane away from the action potential threshold of –40 mV, clearly inhibiting the postsynaptic cell.

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A simple rule distinguishes postsynaptic excitation from inhibition: An EPSP has a reversal potential more positive than the action potential threshold, whereas an IPSP has a reversal potential more negative than threshold.

Summation of Synaptic Potentials

The PSPs produced at most synapses in the brain are much smaller than those at the neuromuscular junction; indeed, EPSPs produced by individual excitatory synapses may be only a fraction of a millivolt and are usually well below the threshold for generating postsynaptic action potentials.

How then, can such synapses transmit information if their PSPs are subthreshold?

The answer is that neurons in the central nervous system are typically innervated by thousands of synapses, and the PSPs produced by each active synapse can sum together—in space and in time—to determine the behavior of the postsynaptic neuron.

A microelectrode records the postsynaptic potentials produced by the activity of two excitatory synapses (E1 and E2) and an inhibitory synapse (I).

activation of either one of the excitatory synapses alone (E1 or E2) produces a subthreshold EPSP.

activation of both excitatory synapses at about the same time causes the two EPSPs to sum together.

If the sum of the two EPSPs (E1 + E2) depolarizes the postsynaptic neuron sufficiently to reach the threshold potential, a postsynaptic action potential results.

Summation thus allows subthreshold EPSPs to influence action potential production.

Likewise, an IPSP generated by an inhibitory synapse (I) can sum (algebraically speaking) with a subthreshold EPSP to reduce its amplitude (E1 + I) or can sum with suprathreshold EPSPs to prevent the postsynaptic neuron from reaching threshold (E1 + I + E2).

In short, the summation of EPSPs and IPSPs by a postsynaptic neuron permits a neuron to integrate the electrical information provided by all the inhibitory and excitatory synapses acting on it at any moment.

Whether the sum of active synaptic inputs results in the production of an action potential depends on the balance between excitation and inhibition.

Normally, the balance between EPSPs and IPSPs changes continually over time, depending on the number of excitatory and inhibitory synapses active at a given moment and the magnitude of the current at each active synapse.

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Summary:

四、Neurotransmitters and Their Receptors

Categories of Neurotransmitters

Small-molecule transmitters

• acetylcholine
• amino acids
• purines
• biogenic amines

Neuropeptides: more than 100 peptides, usu. 3~36 a.a. long

Small-molecule Neurotransmitters

1. Acetylcholine

Acetylcholine (ACh) was the first substance identified as a neurotransmitter

• How?

ACh serves as a transmitter at skeletal neuromuscular junctions, neuromuscular synapse between the vagus nerve and cardiac muscle fibers, synapses in the ganglia of the visceral motor system, and in the central nervous system (actions not well understood).

Acetylcholine is synthesized in nerve terminals from the precursors acetyl coenzyme A (acetyl CoA, which is synthesized from glucose) and choline, in a reaction catalyzed by choline acetyltransferase (CAT)

After synthesis in the cytoplasm of the neuron, a vesicular ACh transporter (VAChT) loads approximately 10,000 molecules of ACh into each cholinergic vesicle.

The postsynaptic actions of ACh at many cholinergic synapses (the neuromuscular junction in particular) is not terminated by reuptake but by a powerful hydrolytic enzyme, acetylcholinesterase (AChE)

This enzyme is concentrated in the synaptic cleft, ensuring a rapid decrease in ACh concentration after its release from the presynaptic terminal.

AChE has a very high catalytic activity (about5000 molecules of ACh per AChE molecule per second) and rapidly hydrolyzes ACh into acetate and choline.

The choline produced by ACh hydrolysis is recycled by being transported back into nerve terminals, where it is used to resynthesize ACh. By a high-affinity, Na+-dependent choline co-transporter (ChT).

Drugs Interacting with Cholinergic Enzymes

Organophosphates: including some potent chemical warfare agents such as “Sarin”, which has been classified as a weapon of mass destruction.

Sarin was made notorious in 1995 when a group of terrorists released this nerve gas in Tokyo’s underground rail system.

Organophosphates can be lethal because they inhibit AChE, allowing ACh to accumulate at cholinergic synapses. This buildup of ACh depolarizes the postsynaptic cell and renders it refractory to subsequent ACh release, causing neuromuscular paralysis and other effects.

The high sensitivity of insects to AChE inhibitors has made organophosphates popular insecticides.

Nicotinic Ach Receptor (nAChR)

Many of the postsynaptic actions of ACh are mediated by the nicotinic ACh receptor (nAChR), so named because the CNS stimulant nicotine also binds to these receptors.

• Nicotine consumption produces some degree of euphoria, relaxation, and eventually addiction, effects believed to be mediated by nAChRs.
• nAChRs are nonselective cation channels that generate excitatory postsynaptic responses.
• nAChRs are the best-studied type of ionotropic neurotransmitter receptor, and unraveling their molecular organization has provided deep insights into the workings of ionotropic receptors.
• Nicotinic receptors are large protein complexes consisting of five subunits
  • Neuromuscular junction nAChR contains two α subunits, which are combined with up to three other types of subunits–β, δ, and either γ or ε–in the ratio 2α:1β :1δ:1γ/ε
    • Each of the α subunits has a binding site that binds a single molecule of ACh. Both ACh binding sites must be occupied for the receptor to be activated, so that only relatively high concentrations of ACh activate these receptors.
    • These subunits also bind other ligands, such as nicotine and α-bungarotoxin
  • Neuronal nAChRs differ from those of muscle in that they (1) lack sensitivity to α-bungarotoxin, and (2) comprise only two receptor subunit types (α and β), which are present in a ratio of 3α:2β.

Structure of the nACh receptor

Each subunit of the receptor contains a large extracellular region (which in α subunits contains the ACh binding site) as well as four membrane-spanning domains.

The transmembrane domains of the five individual subunits together form a channel with a central membrane-spanning pore

The width of this pore is substantially larger than that of the pores of voltage-gated ion channels, consistent with the relatively poor ability of nACh receptors to discriminate between different cations. Within this pore is a constriction that may represent the gate of the receptor.

Binding of ACh to the α subunits is thought to cause a conformational change that rearranges the receptor transmembrane domains, thereby opening the gate and permitting ions to diffuse through the channel pore.

Subunits forming ionotropic neurotransmitter receptors

Muscarinic ACh receptors (mAChRs)

A second class of ACh receptors is activated by muscarine, a poisonous alkaloid found in some mushrooms, and thus they are referred to as muscarinic ACh receptors (mAChRs)

mAChRs are metabotropic and mediate most of the effects of ACh in the brain.

Like other metabotropic receptors, mAChRs have seven helical membrane-spanning domains.

ACh binds to a single binding site on the extracellular surface of the mAChR; this binding site is within a deep channel that is formed by several of the transmembrane helices.

Binding of ACh to this site causes a conformational change that permits G-proteins to bind to the cytoplasmic domain of the mAChR.

Five subtypes of mAChR are known and are coupled to different types of Gproteins, thereby causing a variety of slow postsynaptic responses

Muscarinic ACh receptors are highly expressed in the striatum and various other forebrain regions, where they activate inward rectifier K+ channels or Ca2+activated K+ channels, thereby exerting an inhibitory influence on dopamine-mediated motor effects

In other parts of the brain, such as the hippocampus, mAChRs are excitatory and act by closing KCNQ-type K+ channels

These receptors are also found in the ganglia of the peripheral nervous system.

mAChRs mediate peripheral cholinergic responses of autonomic effector organs such as heart, smooth muscle, and exocrine glands and are responsible for the inhibition of heart rate by the vagus nerve.

Numerous drugs act as mAChR agonists or antagonists; mAChR blockers that are therapeutically useful include atropine (used to dilate the pupil), scopolamine (effective in preventing motion sickness), and ipratropium (useful in the treatment of asthma)

2. Glutamate

Glutamate is the most important transmitter for normal brain function

Nearly all excitatory neurons in the central nervous system are glutamatergic, and it is estimated that more than half of all brain synapses release this neurotransmitter

Glutamate is a nonessential amino acid that does not cross the blood-brain barrier and therefore must be synthesized in neurons from local precursors.

The most prevalent precursor for glutamate synthesis is glutamine, which is taken up into presynaptic terminals by the system A transporter 2 (SAT2) and is then metabolized to glutamate by the mitochondrial enzyme glutaminase

Glutamate synthesized in the presynaptic cytoplasm is packaged into synaptic vesicles by vesicular glutamate transporters (VGLUTs, at least three)

Once released, glutamate is removed from the synaptic cleft by the excitatory amino acid transporters (EAATs)

Some EAATs are present in glial cells and others in presynaptic terminals.

Glutamate transported into glial cells via EAATs is converted into glutamine by the enzyme glutamine synthetase.

Glutamine is then transported out of the glial cells by a different transporter, the system N transporter 1 (SN1), and transported into nerve terminals via SAT2

This overall sequence of events is referred to as the glutamate-glutamine cycle.

This cycle allows glial cells and presynaptic terminals to cooperate both to maintain an adequate supply of glutamate for synaptic transmission and to terminate postsynaptic glutamate action

Glutamate Receptors

There are several types of ionotropic glutamate receptors: AMPA receptors, NMDA receptors, and kainate receptors which are named after the agonists that activate them: AMPA (α-amino-3-hydroxyl-5-methyl-4isoxazole-propionate), NMDA (N-rnethyl-D-aspartate), and kainic acid.

All of these receptors are glutamate-gated cation channels that allow the passage of Na+ and K+ , similar to the nAChR

Hence AMPA, kainate, and NMDA receptor activation always produces excitatory postsynaptic responses

Most central synapses possess both AMPA and NMDA receptors

Antagonist drugs that selectively block either AMPA or NMDA receptors are often used to identify synaptic responses mediated by each receptor type

Excitatory postsynaptic currents (EPSCs) produced by NMDA receptors are slower and last longer than those produced by AMPA receptors

EPSCs generated by AMPA receptors usually are much larger than those produced by other types of ionotropic glutamate receptors, so that AMPA receptors are the primary mediators of excitatory transmission in the brain.

Kainate receptors are less well defined

• In some cases, these receptors are found on presynaptic terminals and serve as a feedback mechanism to regulate glutamate release.
• When found on postsynaptic cells, kainate receptors generate EPSCs that rise quickly but decay more slowly than those mediated by AMPA receptors

(1) AMPA Receptors

Like all ionotropic receptors, AMPA receptors are composed of multiple subunits. The four different AMPA receptor subunits are designated GluA1 to GluA4.

Each subunit has several different domains, including an extracellular ligand-binding domain that is responsible for binding glutamate, and a transmembrane domain that forms part of the ion channel.

Four different AMPA receptor subunits (GluA1 to GluA4) are organized into the tetrameric structure

The extracellular structure of AMPA receptors is asymmetrical and therefore looks different when viewed from its front and side surfaces

• The AMPA receptor is Y-shaped, with the large extracellular domains of the subunits narrowing down as the receptor passes through the plasma membrane
• After rotating the receptor by 90 degrees, the extracellular ligand-binding domains have a characteristic “clamshell” shape, with glutamate and other ligands binding within the opening of the clamshell

The transmembrane domain consists of helices that form both the channel pore and a gate that occludes the pore when glutamate is not bound to the receptor

• Binding of glutamate causes the clamshell structure to “shut”
• This movement then causes the gate helices within the transmembrane domain to move and thereby open the channel pore.

(2) NMDA Receptors

NMDA receptors have physiological properties that set them apart from the other ionotropic glutamate receptors.

• The most significant is that the pore of the NMDA receptor channel allows the entry of Ca2+ in addition to Na+ and K+
• Another key property is that Mg2+ blocks the pore of this channel at hyperpolarized membrane potentials, while depolarization pushes Mg2+ out of the pore
  • NMDA receptors pass cations (most notably Ca2+) only when the postsynaptic membrane potential is depolarized, such as during activation of strong excitatory inputs and/or during action potential firing in the postsynaptic cell.—the coincident presence of both glutamate and postsynaptic depolarization to open NMDA receptors
• Another unusual feature of NMDA receptors is that their gating requires a co-agonist—the amino acid glycine, which is present in the ambient extracellular environment of the brain

3. GABA and Glycine

Glycine

(1) Introduction

About half of the inhibitory synapses in the spinal cord use glycine; most other inhibitory synapses use GABA.

Glycine is synthesized from serine by the mitochondrial isoform of serine hydroxymethyltransferase

Glycine is transported into synaptic vesicles via the same vesicular inhibitory amino acid transporter that loads GABA into vesicles, VIATT.

Once released from the presynaptic cell, glycine is rapidly removed from the synaptic cleft by glycine transporters in the plasma membrane.

• Mutations in the genes coding for some of these transporters result in hyperglycinemia, a devastating neonatal disease characterized by lethargy, seizures, and mental retardation.

(2) Glycine Receptors

Glycine receptors are ligand-gated Cl– channels whose general structure closely mirrors that of the GABAA receptors.

Glycine receptors are pentamers consisting of mixtures of four types of α subunits, along with an accessory β subunit.

• These receptors are potently blocked by strychnine, which may account for the toxic properties of this plant alkaloid.
• Binding of glycine to a ligandbinding site on the extracellular domains causes a conformational change that opens the pore, thereby enabling Cl– and other permeant anions to flow through the pore.
• This Cl– flux inhibits the postsynaptic neuron
• Blocking these receptors, by binding of strychnine to the same ligandbinding site, closes the pore.

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Most inhibitory synapses in the brain and spinal cord use either γ-aminobutyric acid (GABA) or glycine as neurotransmitters.

As many as a third of the synapses in the brain use GABA as their inhibitory neurotransmitter, and GABA is most commonly found in local circuit interneurons.

The predominant precursor for GABA synthesis is glucose, which is metabolized to glutamate by the tricarboxylic acid cycle enzymes

The enzyme glutamic acid decarboxylase (GAD), which is found almost exclusively in GABAergic neurons, catalyzes the conversion of glutamate to GABA

GABA

GAD requires a co-factor, pyridoxal phosphate, for activity

• Because pyridoxal phosphate is derived from vitamin B6, a deficiency of this vitamin can lead to diminished GABA synthesis
• The significance of this fact became clear after a disastrous series of infant deaths was linked to the omission of vitamin B6 from infant formula.
• The absence of vitamin B6 greatly reduced the GABA content of the brain, and the subsequent loss of synaptic inhibition caused seizures that in some cases were fatal.

Once GABA is synthesized, it is transported into synaptic vesicles via a vesicular inhibitory amino acid transporter (VIAAT)

The mechanism of GABA removal is similar to that for glutamate: Both neurons and glia contain high-affinity Na+-dependent co-transporters for GABA which are termed GATs

Most GABA is eventually converted to succinate, which is metabolized further in the tricarboxylic acid cycle that mediates cellular ATP synthesis.

• Two mitochondrial enzymes are required for this degradation: GABA transaminase and succinic semialdehyde dehydrogenase.
• There are also other pathways for degradation of GABA, the most noteworthy of which results in the production of γ-hydroxybutyrate, a GABA derivative that has been abused as a “date rape” drug.
  • Oral administration of γhydroxybutyrate can cause euphoria, memory deficits, and unconsciousness

GABAergic synapses employ two types of postsynaptic receptors, called GABAA and GABAB.

GABAA are ionotropic receptors, while GABAB are metabotropic receptors

The ionotropic GABAA receptors are GABA-gated anion channels, with Cl– being the main permeant ion under physiological conditions.

• Activation of these GABAA receptors causes an influx of negatively charged Cl– that inhibits postsynaptic cells.

GABAA Receptors

Like nACh receptors, GABAA receptors are pentamers. The five GABAA receptor subunits are assembled into a structure quite similar to that of the nAChR

• The transmembrane domains of the subunits form a central pore that includes a ring of positive charges that presumably serve as the binding site for Cl–.

GABAA receptors: binding sites for numerous ligands

• GABA binds in pockets found at the interface between the extracellular domains of the subunits; many other types of ligands also bind to these sites
  • Benzodiazepines such as diazepam (Valium) and chlordiazepoxide (Librium) are anxiety-reducing drugs that enhance GABAergic transmission by binding to the extracellular domains.
  • The same site binds the hypnotic zolpidem (Ambien), which is widely used to induce sleep
  • Ketamine also binds to the extracellular domain of GABA receptors.

The transmembrane domains of GABAA receptors also serve as the targets for numerous ligands.

• Inhalant anesthetics and steroid
• Ethanol: at least some aspects of drunken behavior are caused by ethanol-mediated alterations in ionotropic GABA receptors.

GABAB Receptors

Like the ionotropic GABAA receptors, the metabotropic GABAB receptors are inhibitory.

• GABAB-mediated inhibition is often due to the activation of K+ channels
• A second action of GABAB receptors is to block Ca2+ channels, which also inhibits postsynaptic cells.

The structure of GABAB receptors is similar to that of other metabotropic receptors, although GABAB receptors assemble as heterodimers of B1 and B2 subunits

4. Biogenic Amines

Biogenic amine transmitters regulate many brain functions and are also active in the peripheral nervous system.

Because biogenic amines are implicated in such a wide variety of behaviors (ranging from central homeostatic functions to cognitive phenomena such as attention), it is not surprising that defects in biogenic amine function are implicated in most psychiatric disorders

Many drugs of abuse also act on biogenic amine pathways.

There are five well-established biogenic amine neurotransmitters:

• the three catecholamines
  • dopamine
  • norepinephrine (noradrenaline)
  • epinephrine (adrenaline)
• histamine
• serotonin

Catecholamines

• All the catecholamines (so named because they share the catechol moiety) are derived from a common precursor, the amino acid tyrosine.
• The first step in catecholamine synthesis is catalyzed by tyrosine hydroxylase in a reaction requiring oxygen as a co-substrate and tetrahydrobiopterin as a co-factor to synthesize dihydroxyphenylalanine (DOPA)

Dopamine

(1) Introduction

Dopamine is present in several brain regions, although the major dopamine-containing area of the brain is the corpus striatum, which receives major input from the substantia nigra and plays an essential role in the coordination of body movements

• In Parkinson’s disease, for instance, the dopaminergic neurons of the substantia nigra degenerate, leading to a characteristic motor dysfunction
• Dopamine is also believed to be involved in motivation, reward, and reinforcement and many drugs of abuse work by affecting dopaminergic circuitry in the CNS.

(2) Drugs affecting dopamine pathway

Dopamine is produced by the action of DOPA decarboxylase on DOPA

Following its synthesis in the cytoplasm of presynaptic terminals, dopamine is loaded into synaptic vesicles via a vesicular monoamine transporter (VMAT).

Dopamine action in the synaptic cleft is terminated by reuptake of dopamine into nerve terminals or surrounding glial cells by a Na+-dependent dopamine co-transporter, termed DAT.

• Cocaine apparently produces its psychotropic effects by inhibiting DAT, thereby increasing dopamine concentrations in the synaptic cleft.
• Amphetamine, another addictive drug, also inhibits DAT as well as the transporter for norepinephrine.

The two major enzymes involved in the catabolism of dopamine are monoamine oxidase (MAO) and catechol O-methyltransferase (COMT). Both neurons and glia contain mitochondrial MAO and cytoplasmic COMT

• Inhibitors of these enzymes, such as phenelzine and tranylcypromine, are used clinically as antidepressants.

(3) Dopamine Receptors

Once released, dopamine acts exclusively by activating G-protein-coupled receptors.

The structure of this kind of receptors, such as the D3 dopamine receptor, closely parallels that of other metabotropic receptors

Activation of these receptors generally contributes to complex behaviors; for example, administration of dopamine receptor agonists causes hyperactivity and repetitive, stereotyped behavior in laboratory animals.

Activation of another type of dopamine receptor in the medulla inhibits vomiting. Thus, antagonists of these receptors are used as emetics to induce vomiting after poisoning or a drug overdose.

Norepinephrine (Noradrenaline)

Norepinephrine is used as a neurotransmitter in the locus coeruleus, a brainstem nucleus that projects diffusely to a variety of forebrain targets and influences sleep and wakefulness, attention, and feeding behavior.

Norepinephrine synthesis requires dopamine β-hydroxylase, which catalyzes the production of norepinephrine from dopamine.

Norepinephrine is then loaded into synaptic vesicles via the same VMAT involved in vesicular dopamine transport.

Norepinephrine is cleared from the synaptic cleft by the norepinephrine transporter (NET), a Na+-dependent co-transporter that also is capable of taking up dopamine

• NET is a molecular target of amphetamine, which acts as a stimulant by producing a net increase in the release of norepinephrine and dopamine
• A mutation in the NET gene is a cause of orthostatic intolerance, a disorder that produces lightheadedness while standing up

Like dopamine, norepinephrine is degraded by MAO and COMT.

Norepinephrine acts on αand β-adrenergic receptors. Both types of receptor are G-protein-coupled.

The β-adrenergic receptor was the first identified metabotropic neurotransmitter receptor

Agonists and antagonists of adrenergic receptors, such as the β-blocker propanolol, are used clinically for a variety of conditions ranging from cardiac arrhythmias to migraine headaches.

Epinephrine (adrenaline)

Epinephrine is found in the brain at lower levels and also is present in fewer brain neurons than other catecholamines.

Epinephrine-containing neurons in CNS are primarily in the lateral tegmental system and in the medulla and project to the hypothalamus and thalamus.

The enzyme that synthesizes epinephrine, phenylethanolamine-N-methyltransferase, is present only in epinephrine-secreting neurons.

The metabolism and receptors of epinephrine are very similar to those of norepinephrine

Histamine

(1) Introduction

Histamine is found in neurons in the hypothalamus that send sparse but widespread projections to almost all regions of the brain and spinal cord.

The central histamine projections mediate arousal and attention, similar to central Ach and norepinephrine projections. Histamine also controls the reactivity of the vestibular system.

Allergic reactions or tissue damage cause release of histamine from mast cells in the bloodstream

Histamine is produced from the amino acid histidine by a histidine decarboxylase and is transported into vesicles via the same VMAT as the catecholamines

(2) Histamine receptors

The four known histamine receptors are all metabotropic receptors. Histamine receptors

Antihistamines that cross the blood-brain barrier, such as diphenhydramine (Benadryl®), act as sedatives by interfering with the roles of histamine in CNS arousal.

Antagonists of the H1 receptor also are used to prevent motion sickness, perhaps because of the role of histamine in controlling vestibular function.

H2 receptors control the secretion of gastric acid in the digestive system, allowing H2 receptor antagonists to be used in the treatment of a variety of upper gastrointestinal disorders (e.g., peptic ulcers).

Serotonin

(1) Introduction

Serotonin, or 5-hydroxytryptamine (5-HT), was initially thought to increase vascular tone by virtue of its presence in blood serum (hence the name serotonin)

Serotonin is found primarily in groups of neurons in the raphe region of the pons and upper brainstem, which have widespread projections to the forebrain, and regulate sleep and wakefulness.

5-HT occupies a place of prominence in neuropharmacology because a large number of antipsychotic drugs that are valuable in the treatment of depression and anxiety act on serotonergic pathways.

5-HT is synthesized from the amino acid tryptophan, which is an essential dietary requirement.

Tryptophan is taken up into neurons by a plasma membrane transporter and hydroxylated in a reaction catalyzed by the enzyme tryptophan-5-hydroxylase, the rate-limiting step for 5-HT synthesis.

The synaptic effects of serotonin are terminated by transport back into nerve terminals via a specific serotonin transporter (SERT) that is present in the presynaptic plasma membrane and is encoded by the 5HTT gene.

Many antidepressant drugs are selective serotonin reuptake inhibitors (SSRIs) that inhibit transport of 5-HT by SERT

• Perhaps the best-known example of an SSRI is the antidepressant drug Prozac.

(2) Serotonin receptors

Most 5-HT receptors (encoded by HTR genes) are metabotropic with a monomeric structure typical of G-protein-coupled receptors

• These have been implicated in behaviors, including the emotions, circadian rhythms, motor behaviors, and state of mental arousal.
• Impairments in the function of these receptors have been implicated in numerous psychiatric disorders, such as depression, anxiety disorders, and schizophrenia, and drugs acting on serotonin receptors are effective treatments for a number of these conditions
• The psychedelic drug LSD (lysergic acid diethylamide) presumably causes hallucinations by activating multiple types of metabotropic 5-HT receptors.
• Activation of 5-HT receptors also mediates satiety and decreased food consumption, which is why serotonergic drugs are sometimes useful in treating eating disorders.

One group of serotonin receptors, the 5-HT3 receptors, are ligand-gated ion channels formed from combinations of the five 5-HT3 subunits.

ATP and other purines

All synaptic vesicles contain ATP, which is co-released with one or more “classic” neurotransmitters.

ATP acts as an excitatory neurotransmitter in motor neurons of the spinal cord, as well as in sensory and autonomic ganglia.

Postsynaptic actions of ATP have also been demonstrated in CNS, specifically for dorsal horn neurons and in a subset of hippocampal neurons.

(1) Purinergic receptors

Three classes of purinergic receptors are known

• One class consists of ionotropic receptors called P2X receptors.
  • The structure of these receptors is unique among ionotropic receptors because each subunit has a transmembrane domain that crosses the membrane only twice
  • Only three of these subunits are required to form a trimeric receptor.
  • As in all ionotropic receptors, a pore is located in the center of the P2X receptor and forms a nonselective cation channel
  • Thus, P2X receptors mediate excitatory postsynaptic responses.

The other two classes of purinergic receptors are G-protein-coupled metabotropic receptors.

• The two classes differ in their sensitivity to agonists–one type is preferentially stimulated by adenosine, whereas the other is preferentially activated by ATP
• Structure of a metabotropic A~2A ~adenosine receptor

Peptide Neurotransmitters

Many peptides known to be hormones also act as neurotransmitters

Some peptide transmitters have been implicated in modulating emotions

Others, such as substance P and the opioid peptides, are involved in the perception of pain

Still other peptides, such as melanocyte-stimulating hormones, adrenocorticotropin, and β-endorphin, regulate complex responses to stress.

The mechanisms responsible for the synthesis and packaging of peptide transmitters are fundamentally different from those used for the small-molecule neurotransmitters and are much like the synthesis of proteins that are secreted from non-neuronal cells (pancreatic enzymes, for instance).

Peptide-secreting neurons generally synthesize polypeptides that are much larger than the final, “mature” peptide.

Processing these polypeptides, which are called pre-propeptides (or pre-proproteins), takes place within the neuron’s cell body by a sequence of reactions that occur in several intracellular organelles.

Pre-propeptides are synthesized in the rough endoplasmic reticulum, where the signal is removed.

The remaining polypeptide, called a propeptide, then traverses the Golgi apparatus and is packaged into vesicles in the trans-Golgi network.

The final stages of peptide neurotransmitter processing occur after packaging into vesicles and involve proteolytic cleavage, modification of the ends of the peptide, glycosylation, phosphorylation, and disulfide bond formation.

Propeptide precursors can give rise to more than one species of neuropeptide, which means that multiple neuroactive peptides can be released from a single vesicle

In addition, neuropeptides often are co-released with small-molecule neurotransmitters.

Thus, peptidergic synapses often elicit complex postsynaptic responses.

Peptides are catabolized into inactive amino acid fragments by enzymes called peptidases, usually located on the extracellular surface of the plasma membrane.

The biological activity of the peptide neurotransmitters depends on their amino acid sequence.

Based on their sequences, neuropeptide transmitters have been loosely grouped into five categories:

• the brain/gut peptides;
• opioid peptides;
• pituitary peptides;
• hypothalamic releasing hormones;
• a catch-all category containing other, not easily classified, peptides

(1) Substance P

The study of neuropeptides began more than 60 years ago with the accidental discovery of substance P, a powerful hypotensive agent and an example of the first category of peptide.

The peculiar name derives from the fact that this molecule was an unidentified component of powder extracts from brain and intestine.

Substance P is an 11-amino-acid peptide present in high concentrations in the human hippocampus, neocortex, and also in the gastrointestinal tract; hence its classification as a brain/gut peptide

It is also released from C fibers, the small-diameter afferents in peripheral nerves that convey information about pain and temperature (as well as postganglionic autonomic signals)

Substance P is a sensory neurotransmitter in the spinal cord, where its release can be inhibited by opioid peptides released from spinal cord interneurons, resulting in the suppression of pain .

(2) Opioid Peptides

An especially important category of peptide neurotransmitters is the family of opioids, so named because they bind to the same postsynaptic receptors that are activated by opium.

The active ingredients in opium are a variety of plant alkaloids, predominantly morphine

Morphine, named for Morpheus, the Greek god of dreams, is still in use today and is one of the most effective analgesics, despite its addictive potential.

The opioid peptides were discovered in the 1970s during a search for endorphins-endogenous compounds that mimicked the actions of morphine

The endogenous ligands of the opioid receptors have now been identified as a family of more than 20 opioid peptides that fall into three classes:

Opioid peptides are widely distributed throughout brain and are often co-localized with other small-molecule neurotransmitters, such as GABA, 5-HT.

In general, the opioids tend to be depressants. When injected intracerebrally in experimental animals, they act as analgesics

Opioids are also involved in complex behaviors such as sexual attraction and aggressive/submissive behaviors.

(3) Neuropeptide receptors

Virtually all neuropeptides initiate their effects by activating G-protein-coupled receptors.

The study of these metabotropic peptide receptors in the brain has been difficult because few specific agonists and antagonists are known

Neuropeptide receptor activation is especially important in regulating the postganglionic output from sympathetic ganglia and the activity of the gut.

Peptide receptors, particularly the neuropeptide Y receptor, are also implicated in the initiation and maintenance of feeding behavior leading to satiety or obesity

5. Unconventional neurotransmitters

These chemical signals can be considered as neurotransmitters because of their roles in interneuronal signaling and because their release from neurons is regulated by Ca2+

However, they are unconventional in comparison to other neurotransmitters because they are not stored in synaptic vesicles and are not released from presynaptic terminals via exocytotic mechanisms.

In fact, these unconventional neurotransmitters need not be released from presynaptic terminals at all and are often associated with retrograde signaling (that is, from postsynaptic cells back to presynaptic terminals).

• Endocannabinoids; Nitric oxide (NO).

Endocannabinoids

Endocannabinoids are a family of related endogenous signals that interact with cannabinoid receptors

These receptors are the molecular targets of Δ^9^-tetrahydrocannabinol, the psychoactive component of the marijuana plant, Cannabis

Anandamide and 2-arachidonoylglycerol (2-AG) have been established as endocannabinoids.

These signals are unsaturated fatty acids with polar head groups and are produced by enzymatic degradation of membrane lipids

Production of endocannabinoids is stimulated by a second messenger within postsynaptic neurons, typically a rise in postsynaptic Ca2+ concentration, allowing these hydrophobic signals to diffuse through the postsynaptic membrane to reach cannabinoid receptors on other nearby cells.

Endocannabinoid action is terminated by carrier-mediated transport of these signals back into the postsynaptic neuron, where they are hydrolyzed by the enzyme fatty acid hydrolase (FAAH).

At least two types of cannabinoid receptors have been identified, with most actions of endocannabinoids in the CNS mediated by the CB1 type.

• The CB1 receptor is a G-protein-coupled receptor related to the metabotropic receptors for ACh, glutamate, and other conventional neurotransmitters.

The best-documented action of endocannabinoids is the inhibition of communication between presynaptic inputs and their postsynaptic target cells.

In both the hippocampus and the cerebellum (among other brain regions), endocannabinoids serve as retrograde signals that regulate GABA release at certain inhibitory synapses

• At such synapses, depolarization of the postsynaptic neuron causes a transient reduction in inhibitory postsynaptic responses.
• Depolarization reduces synaptic transmission by elevating the concentration of Ca2+ in the postsynaptic neuron; this rise in Ca2+ triggers synthesis and release of endocannabinoids from the postsynaptic cells.
• The endocannabinoids then bind to CB1 receptors on presynaptic terminals, inhibiting the amount of GABA released in response to presynaptic action potentials, and thereby reducing inhibitory transmission.

Nitric oxide (NO)

It is a gas produced by the action of nitric oxide synthase, an enzyme that converts the amino acid arginine into a metabolite (citrulline) and simultaneously generates NO.

Within neurons, NO synthase is regulated by Ca2+ binding to the Ca2+ sensor protein calmodulin

Once produced, NO can permeate the plasma membrane, meaning that NO generated inside one cell can travel through the extracellular medium and act inside nearby cells.

This property makes NO a potentially useful agent for coordinating the activities of multiple cells in a localized region and may mediate certain forms of synaptic plasticity that spread within small networks of neurons.

All of the known actions of NO are mediated within its cellular targets; for this reason, NO often is considered a second messenger rather than a neurotransmitter.

Some of the actions of NO are due to the activation of the enzyme guanylyl cyclase, which then produces the second messenger cGMP within target cells.

NO decays spontaneously by reacting with oxygen to produce inactive nitrogen oxides; thus, its signals last for only a short time (seconds or less)

NO signaling evidently regulates a variety of synapses that also employ conventional neurotransmitters; so far, presynaptic terminals that release glutamate are the best-studied NO targets in the CNS

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Summary

Glutamate is the major excitatory neurotransmitter in the brain, whereas GABA and glycine are the major inhibitory neurotransmitters.

The actions of these small-molecule neurotransmitters are typically faster than those of the neuropeptides.

Two broadly different families of neurotransmitter receptors have evolved to carry out the postsynaptic signaling actions of neurotransmitters

Ionotropic or ligand-gated ion channels combine the neurotransmitter receptor and ion channel in one molecular entity, and therefore give rise to rapid postsynaptic electrical responses

Metabotropic receptors regulate the activity of postsynaptic ion channels indirectly, usually via G-proteins, and induce slower and longer-lasting electrical responses.

The postsynaptic response at a given synapse is determined by the combination of receptor subtypes, G-protein subtypes, and ion channels that are expressed in the postsynaptic cell.