Transmitters and receptors

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8 Transmitters and receptors

1 This chapter dwells on chemical neurotransmission. The information is intended to set the stage for learning about therapeutic applications, notably in relation to the autonomic nervous system and to emotional or behavioral disorders including the psychoses.

2 The numbered flow diagrams are meant to convey a sense of motion. It may be a good idea to ‘go with the flow’ by following the possible paths taken by a given transmitter molecule (e.g. five for catecholamines).

3 Learn to distinguish between the nature of fast (ionotropic) and slow (metabotropic) receptors.

4 The transmitter or receptor agonists and antagonists mentioned here will come up in later chapters.

Electrical Synapses

Electrical synapses are scarce in the mammalian nervous system. As seen in Figure 8.1, they consist of gap junctions (nexuses) between dendrites or somas of contiguous neurons, where there is cytoplasmic continuity through 1.5-nm channels. No transmitter is involved, and there is no synaptic delay.

Figure 8.1 Structure of an electrical synapse. (A) Synaptic contact between two dendrites. (B) Enlargement from (A). (C) The gap junction between the cell membranes is bridged by close-packed ion channels. (D) Each ion channel comprises a mirror image pair of connexons. (E) Each connexon comprises six connexins, each having a wedge-shaped subunit bordering the ion channel. (F) The subunits open the ion channel by synchronous rotation.

The gap junctions are bridged by tightly packed ion channels, each comprising mirror image pairs of connexons, which are transmembrane protein groups (connexins) disposed in hexagonal format around an ion pore. Wedge-shaped connexin subunits bordering each ion pore are closely apposed when the neurons are inactive. Action potentials passing along the cell membrane cause the subunits to rotate individually, creating a pore large enough to permit free diffusion of ions and small molecules down their concentration gradients.

The overall function of these gap junctions is to ensure synchronous activity of neurons having a common action. An example is the inspiratory center in the medulla oblongata, where all the cells exhibit synchronous discharge during inspiration. A second example is among neuronal circuits controlling saccades, where the gaze darts from one object of interest to another.

Chemical Synapses

Transmitter liberation (Table 8.1 and Figure 8.2)

At resting nerve terminals, synaptic vesicles accumulate in the active zones, where they are tethered to the presynaptic densities by strands of docking proteins including actin. With the arrival of action potentials, myriads of voltage-gated calcium channels located in the presynaptic membrane are opened, leading to instant flooding of the active zone with Ca²⁺ ions. These ions cause contraction of actin filaments to bring the nearest vesicles into direct contact with the presynaptic membrane.

Table 8.1 Some named proteins involved in transmitter transport and vesicle recycling

Named protein	Function
Actin	Brings vesicle into contact with presynaptic membrane
Synaptophysin	Creates the membrane fusion pore
Calmodulin	Expels vesicle content into synaptic cleft
Clathrin	Withdraws vesicle membrane from synaptic cleft
Dynamin	Pinches the neck of the developing vesicle to complete its separation
Ligand	Receptor protein that binds with the transmitter molecule

Figure 8.2 Sequence of events following depolarization of the presynaptic membrane. (1) Opening of calcium channels (arrows) causes synaptic vesicles to be pulled into contact with the presynaptic membrane by actin filaments. Matching pairs of fusion protein macromolecules (FPMs) in the vesicle and presynaptic membrane are aligned. (2) The FPMs separate (outward arrows), permitting transmitter molecules to enter the synaptic cleft. (3) Vesicle membrane is incorporated into the presynaptic membrane while transmitter is activating the specific receptors. (4) Clathrin molecules assist inward movement of vesicle membrane. Dynamin molecules (green) assist approximation of FPM pairs (inward arrows) and pinch the neck of the emerging vesicle. (5) The vesicle is now free for recycling.

When the vesicle protein synaptophysin contacts the vesicle membrane, it becomes embedded in it and hollows out to create a fusion pore through which the vesicle discharges its contents into the synaptic cleft. The act of expulsion appears to be a function of another vesicle protein, calmodulin, activated by Ca²⁺ ions. These and other local protein responses to calcium entry are extremely rapid, the time elapsing between calcium entry and transmitter expulsion being less than 1 ms. In the case of small synaptic vesicles such as those containing glutamate or γ-aminobutyric acid (GABA), single spikes are sufficient to yield some transmitter release. In the case of peptidergic neurons, impulse frequencies of 10 Hz or more are required to induce typically slow (delay of 50 ms or more) transmitter release from the large, dense-cored vesicles.

Target cell receptor binding

Transmitter molecules bind with receptor protein molecules in the postsynaptic membrane. The two great categories of receptor are known as ionotropic and metabotropic. Each category contains some receptors whose activation leads to opening of ion pores and others whose activation leads to closure.

Ionotropic receptors

Ionotropic receptors are characterized by the presence of an ion channel within each receptor macromolecule (Figure 8.2). The transmitter binds with its specific receptor facing the synaptic cleft, causing it to change its conformation so as to open or close its ion pore. Ionotropic receptor channels are said to be transmitter-gated, or ligand-gated (from the Latin ligandum, ‘binding’), signifying their capacity to bind a transmitter molecule or a drug substitute.

In Figure 8.3A, the excitatory channel has been opened by the transmitter, causing a major influx of sodium and a minor efflux of potassium; the excitatory postsynaptic potential (EPSP) has depolarized the cell membrane almost to firing point. In Figure 8.3B, the membrane can be hyperpolarized only to −70 mV, the chloride equilibrium potential. A greater level of depolarization requires a second messenger system (see below).

Figure 8.3 (A) A transmitter-gated excitatory ionotropic receptor. Binding of the transmitter (red, in this example representing glutamate) has opened the pore of a ‘mixed’ Na⁺−K⁺ cation channel. A large influx of sodium ions has depolarized the membrane, as shown by the excitatory postsynaptic potential (EPSP). (B) A transmitter-gated inhibitory ionotropic receptor. Binding of the transmitter (blue, representing GABA_A) has opened the pore of a chloride channel. Inward chloride conductance has been increased, and the inhibitory postsynaptic potential (IPSP) restores the membrane potential to its resting state.

Ionotropic receptors are called fast receptors on account of their immediate and brief effects on ion channels.

Metabotropic receptors

Metabotropic receptors are so called because many are capable of generating multiple metabolic effects within the cytoplasm of the neuron. The receptor macromolecule is an intrinsic membrane protein devoid of an ion channel. Its function is initiated by one or other of two subunits (α subunit, β subunit) that detaches in response to transmitter activation and moves along the inner surface of the membrane. The subunits are called G proteins, because most bind with guanine nucleotides such as guanine triphosphate (GTP) or guanine diphosphate. Their action on ion channels is usually indirect, via a second messenger system. However, some G proteins do not bind with nucleotides; instead, they activate ion channels directly (see later).

A G protein having a stimulatory effect is known as a G_s protein; a G_i protein is one with an inhibitory effect.

Three second messenger systems are well recognized.

1 The cyclic AMP system, responsible for phosphorylation of proteins.

2 The phosphoinositol system, responsible for liberating calcium from cytoplasmic stores.

3 The arachidonic acid system, responsible for production of arachidonic acid metabolites.

Cyclic AMP system

(The abbreviation cyclic AMP represents cyclic adenosine monophosphate.) In the examples shown in Figure 8.4, transmitter–receptor binding releases the alpha subunit of a G_s protein, leaving it free to link with GTP, which in turn facilitates adenosine cyclase to convert ATP to cyclic AMP. Protein kinase A in the membrane is stimulated by cyclic AMP to transfer phosphate ions from ATP to an ion channel, causing its pore to admit sodium ions, initiating depolarization of the target neuron. When the G_s protein is switched off, the membrane-attached enzyme protein phosphatase catalyzes extraction of the phosphate ions, resulting in pore closure.

Figure 8.4 The cyclic AMP (cAMP) system. The diagram shows the basic steps along the path from a G_s protein-linked receptor via cyclic AMP to an ion channel. (1) Transmitter is activating the receptor macromolecule. (2) G_s protein α subunit is freed to bind with guanosine triphosphate (GTP). (3) GTP links the unit to adenylate cyclase. (4) Adenylate cyclase catalyzes synthesis of cyclic AMP from ATP. (5) Cyclic AMP activates protein kinase A (PKA). (6) PKA transfers phosphate groups from ATP to a sodium ion channel, causing its pore to open and Na⁺ ions to rush into the cytosol, with a depolarizing effect. (7) Following inactivation of the G_s protein, dephosphorylation of the channel by a phosphatase enzyme allows the channel pore to close.

Phosphoinositol system

In the example shown in Figure 8.5, activation of another kind of G_s protein alpha subunit causes the effector enzyme phospholipase C to split a membrane phospholipid (PIP₂) into a pair of second messengers: diacylglycerol (DAG) and inositol phosphate (IP₃). DAG activates protein kinase C, which initiates protein phosphorylation. IP₃ diffuses into the cytosol, where it opens calcium-gated channels, mainly in nearby membranes of smooth endoplasmic reticulum. The Ca²⁺ ions activate certain calcium-dependent enzymes downstream, and may open calcium-gated K⁺ (outward) and Cl⁻ (inward) channels with excitatory effect.

Figure 8.5 The phosphoinositol system. The steps indicate the dual function of this system. (1) The transmitter activates the receptor macromolecule. (2) The G_s protein α subunit is freed to bind with guanosine triphosphate (GTP), which links it to phospholipase C (PLC). (3) PLC moves along the membrane and splits the enzyme phospholipase C (PLC) into diacylglycerol (DAG) and inositol triphosphate (IP₃). (4) DAG attracts enzyme protein kinase C (PKC) into the membrane, where (5) DAG is triggered to phosphorylate several proteins, potentially including ion channels. (6) IP₃ activates calcium channels in sacs of smooth endoplasmic reticulum. (7) Stored calcium spills out of the channel pore into the cytosol. (8) Ca²⁺-dependent enzymes are activated.

Arachidonic acid system

This is described later, in connection with histamine.

Metabotropic receptors are in general slow receptors. Membrane channel effects may continue for hundreds of milliseconds after a single stimulus.

Gene transcription effects

It is also well established that the reflex responses to repetitive stimuli may be either progressively increased (a state of sensitization, usually induced by noxious stimuli) or diminished (a state of habituation induced by harmless stimuli). Animal experiments involving reflex arcs involving sets of sensory, internuncial, and motor neurons have shown that a characteristic of sensitization is the development of additional synaptic contacts between the internuncials and the motor neurons, together with additional transmitter synthesis and release. Characteristic of habituation is a reduction of transmitter synthesis and release. All these changes result from alterations of gene transcription. Repetitive noxious stimuli cause cyclic AMP to increase its normal rate of activation of protein kinases involved in phosphorylation of proteins that regulate gene transcription. The outcome is increased production of proteins (including enzymes) required for transmitter synthesis, and of other proteins for construction of additional channels and synaptic cytoskeletons. Repetitive harmless stimuli merely reduce the rate of transmitter synthesis and release.

Gene transcription effects are especially important in the context of forming long-term memories (Ch. 34).

Transmitters and Modulators

Several criteria should be fulfilled for a substance to be accepted as a neurotransmitter.

• The substance must be present within neurons, together with the molecules, including enzymes, required to synthesize it.

• The substance must be released following depolarization of the nerve endings that contain it, and this release must be induced by entry of calcium.

• The postsynaptic membrane must contain specific receptors that will modify the membrane potential of the target neuron.

• The isolated substance must exert the same effect when applied to a target neuron via a micropipette (microiontophoresis).

• Specific antagonist molecules, whether delivered through the circulation or by iontophoresis, must block the effect of the putative (‘thought to be’) transmitter.

• The physiologic mode of termination of the transmitter effect must be identified, whether it be by enzymatic degradation or by active transport into the parent neuron or adjacent neuroglial cells.

Many transmitters limit their own rate of release by negative feedback activation of autoreceptors in the presynaptic membrane, having the effect of inhibiting further release. Ideally, the existence of specific inhibitory autoreceptors should be established.

The term neuromodulator (L. modulare, to regulate) has been subject to several interpretations. The most satisfactory appears to derive from the terms amplitude modulation and frequency modulation in electrical engineering, signifying superimposition of one wave or signal onto another. Figure 8.6 represents a sympathetic and a parasympathetic nerve ending, close to a pacemaker cell (modified cardiac myocyte). This neighborly arrangement of nerve endings is common in the heart, and allows the respective transmitters to modulate each other’s activity. The sympathetic nerve ending liberates norepinephrine (noradrenaline), which has a stimulatory effect. The three modulators shown exert their effects via second messenger systems.

Figure 8.6 Neuromodulation occurs at nerve endings in the sinuatrial node of the heart, where sympathetic and parasympathetic nerve endings often occur in pairs. In this representation, the sympathetic system is the more active, releasing the transmitter norepinephrine, which depolarizes cardiac pacemaker cells via β₁ pacemaker membrane receptors. Circulating epinephrine exerts positive modulation on the nerve ending by increasing transmitter release via β₂ presynaptic membrane heteroreceptors. Inhibitory modulation of excess norepinephrine release is expressed via α₂ presynaptic membrane autoreceptors. At the same time, release of the inhibitory transmitter acetylcholine (ACh) from the parasympathetic bouton is inhibited via α₂ heteroreceptors.

The figure caption also refers to autoreceptors and heteroreceptors. Receptors for a particular transmitter often occur in the presynaptic as well as in the postsynaptic membrane. They are called autoreceptors. These are activated by high transmitter concentration in the synaptic cleft, and they have a negative feedback effect, inhibiting further transmitter release from the synaptic bouton. Heteroreceptors occupy the plasma membrane of neurons that do not liberate the specific transmitter. In the example shown, activity at sympathetic nerve endings is accompanied by inhibition of parasympathetic activity through the medium of heteroreceptors located on parasympathetic nerve endings.

Fate of neurotransmitters

The ultimate fate of transmitters released into synaptic clefts is highly variable. Some transmitters are inactivated within the cleft, some diffuse away into the cerebrospinal fluid via the extracellular fluid, and some are recycled either by direct uptake or indirectly via glial cells.

The principal transmitters and modulators are shown in Table 8.2. Respective receptor types are in Table 8.3.

Table 8.2 Main types of transmitters and modulators, with examples of each ^a The neuropeptides are modulators

Type	Example(s)
Amino acids	Glutamate
	γ-Aminobutyric acid (GABA)
	Glycine
Biogenic amines	Acetylcholine
	Monoamines
	Catecholamines (dopamine, norepinephrine [noradrenaline], epinephrine [adrenaline])
	Serotonin
	Histamine
Neuropeptides	Vasoactive intestinal polypeptide
	Substance P
	Enkephalin
	Endorphins
	Many others
Adenosine	—
Gaseous	Nitric oxide

^a The five monoamines contain a single amine group. Catecholamines also contain a catechol nucleus.

Table 8.3 Receptor types activated by different neurotransmitters

Ionotropic receptors	Metabotropic receptors
Glutamate (AMPA/K)	Glutamate (mGluR)
GABA_A	GABA_A
Acetylcholine (nicotinic)	Acetylcholine (muscarinic)
Glycine	Dopamine (D₁, D₂)
Serotonin (5-HT₃)	Serotonin (5-HT₁, 5-HT₂)
	Norepinephrine (noradrenaline) (α₁, α₂), epinephrine (adrenaline)
	Histamine (H₁, H₂, H₃)
	All neuropeptides
	Adenosine

Amino acid transmitters

The most prevalent excitatory transmitter in the brain and spinal cord is the amino acid l-glutamate (Figure 8.7). As an important example, all neurons projecting into the white matter from the cerebral cortex, regardless of their destinations in other areas of cortex or brainstem or spinal cord, are excitatory and use glutamate as transmitter. Glutamate is derived from α-ketoglutarate; it also provides the substrate for formation of the most common inhibitory transmitter, GABA.

Figure 8.7 The three amino acid transmitters. Glutamate is derived from α-ketoglutarate by the enzyme GABA transaminase (GABA-T); γ-aminobutyric acid (GABA) is derived from glutamate by glutamic acid decarboxylase (GAD). Glycine is the simplest amino acid.

GABA is widely distributed in the brain and spinal cord, being the transmitter in approximately one-third of all synapses. Millions of GABAergic neurons form the bulk of the caudate and lentiform nuclei, and they are also concentrated in the hypothalamus, periaqueductal gray matter, and hippocampus. Moreover, GABA is the transmitter for the large Purkinje cells, which are the only output cells of the cerebellar cortex, projecting to the dentate and other cerebellar nuclei.

GABA is synthesized from glutamate by the enzyme glutamic acid decarboxylase.

A third amino acid transmitter, glycine, is the same molecule that is used in the synthesis of proteins in all tissues. It is the simplest of the amino acids, being synthesized from glucose via serine. It is an inhibitory transmitter largely confined to internuncial neurons of the brainstem and spinal cord.

Glutamate

Glutamate acts on specific receptors of both ionotropic and metabotropic kinds. The three ionotropic ones, named after synthetic agonists that activate them, are known as AMPA, kainate, and NMDA (referring to amino-methylisoxazole propionic acid, kainate, and _N-methyl-d-aspartate, respectively). Kainate receptors are scarce; they occur in company with AMPA, hence the common use of either AMPA–K or non-NMDA to include both.

Ionotropic glutamate receptors

Activation of AMPA–K receptor channels in the postsynaptic membrane allows an immediate inrush of Na⁺ together with a small outward movement of K⁺ ions (Figure 8.8), generating the early component of the EPSP in the target neuron. Should this component depolarize the target cell membrane from −65 mV to −50 mV, this will suffice to generate electrostatic repulsion of Mg²⁺ cations that plug the NMDA receptor ion pore at rest. Na⁺ ions enter via the pore and generate action potentials. Significantly, Ca²⁺ ions also enter, and the extended period of depolarization (up to 500 ms from a single action potential) allows activation of calcium-dependent enzymes with knock-on capacity to modify the structure and even the number of synaptic contacts in the target cell. The phenomenon of activity-dependent synaptic modification is especially detectable in experimental studies of cultured slices of rat hippocampus, and is likely to be important in the generation of short-term memory traces. (For example, the anesthetic drug ketamine, which blocks the NMDA channel, also blocks memory formation.) A characteristic effect of repetitive activation of the NMDA receptor is long-term potentiation, represented by above-normal EPSP responses even some days after ‘training’. (See long-term depression, later.)

Figure 8.8 Ionotropic glutamate receptors. (1) On arrival of action potentials at the nerve terminal, (2) calcium channels open, and (3) Ca²⁺ ions cause synaptic vesicles to be pulled against the cell membrane. (4) Glutamate molecules undergo exocytosis into the synaptic cleft. (5) Transmitter binding to the AMPA–K receptor opens the ion channel, permitting large influx of sodium and a small efflux of potassium. (6) The resulting excitatory postsynaptic potential (EPSP) produces some 20 mV of depolarization, which (7) permits the glutamate ligand to activate the NMDA receptor by expulsion of its Mg²⁺ ‘plug’. Both sodium and calcium ions are admitted. (8) The NMDA-induced EPSP is sufficient to (9) trigger action potentials having an extended repolarization period.

The role of NMDA receptors in the phenomenon called glutamate excitotoxicity has been demonstrated in vascular strokes produced in experimental animals. The mass death of neurons in this kind of experiment is thought to be the result of degradation caused by excess calcium influx in accordance with the following sequence: ischemia > excess calcium influx > activation of calcium-dependent proteases and lipases > degradation of proteins and lipids > cell death. Ischemic damage may be less severe if an NMDA antagonist drug is administered soon after the initial insult.

Metabotropic glutamate receptors

More than 100 different metabotropic glutamate receptors have been identified, all of them being intrinsic membrane proteins, most occupying postsynaptic membranes and having an excitatory function, others occupying presynaptic membranes where they act as inhibitory autoreceptors.

GABA

Two major classes of GABA receptor are recognized, one being ionotropic and the other metabotropic.

Ionotropic GABA receptors

Termed GABA_A, these are especially abundant in the limbic lobe of the brain. Each is directly linked to a chloride ion channel (Figure 8.9). Following activation of GABA_A receptors, channel pores are opened and Cl⁻ ions diffuse down their concentration gradient from synaptic cleft to cytosol. Hyperpolarization up to −80 mV or more is brought about by summation of successive inhibitory postsynaptic potentials (IPSPs) (Figure 8.10).

Figure 8.9 Drugs and the ionotropic GABA_A receptor, Green signifies agonist effect, red signifies antagonist effect. Barbiturates, benzodiazepines, and ethanol exert their hyperpolarizing effect via the natural receptor. Bicuculline antagonizes the receptor, whereas picrotoxin closes the ion pore by direct action locally.

Figure 8.10 Glutamatergic and GABAergic synapses on a multipolar neuron with spiny dendrites. Spatial summation of postsynaptic potentials is illustrated for each pair of synapses.

The sedative, hypnotic barbiturates and benzodiazepines, for example diazepam (Valium), exert their effects by activating the natural receptor. So, too, does ethanol. (Loss of social control under the influence of ethanol may follow the release of target excitatory neurons normally held in check by tonic GABAergic activity.) Some volatile anesthetics also bind with the natural receptor, prolonging the open state of the ion channel.

The chief antagonist at the receptor site is the convulsant drug bicuculline. Another convulsant is picrotoxin, which binds with protein subunits that choke the ion pore when activated.

Metabotropic GABA receptors

Termed GABA_B, these are relatively uniformly distributed throughout the brain. They are also found within peripheral autonomic nerve plexuses. Although most of their G proteins operate via second messengers, a significant number act directly, on a special class of postsynaptic K⁺ channels known as GIRK channels (G protein inwardly rectifying K⁺ channels). As shown in Figure 8.11, transmitter-binding releases the βγ subunit, which expels K⁺ ions through the GIRK channel, thereby producing an IPSP.

Figure 8.11 G-protein direct opening of a GIRK channel in a postsynaptic membrane. (A) Inactive state. (B) GABA activation of the receptor causes the G protein βγ subunit to be transferred to the GIRK channel. (C) The βγ subunit causes K⁺ ions to be expelled, thereby leading to hyperpolarization of the cell membrane.

The response properties of the target neuronal receptors are slower and weaker than those of GABA_A ionophores, requiring higher-frequency stimulation to be activated. This has led to the belief that they may be extrasynaptic in position rather than facing the synaptic cleft. This is indicated in Figure 8.12, where the belief is supported by the existence of another type of G-direct channel in extrasynaptic locations. This is a calcium channel that is also voltage-gated, and therefore participates in provision of the calcium ions needed to draw synaptic vesicles against the presynaptic membrane. Activation of a G–Ca²⁺ ligand site closes Ca²⁺ channels, thereby reducing the effectiveness of action potentials, with inhibitory effect on the parent neuron and on any nearby glutamatergic neurons.

Figure 8.12 Following transmitter release from a GABAergic neuron. (1) Transmitter binding with GABA_A receptors has a hyperpolarizing effect on the target membrane by opening chloride channels. (2) Binding with GIRK GABA_A receptors works in the same direction by opening inwardly rectifying potassium channels (GIRKs). (3) Binding with GABA_A autoreceptors dampens transmitter release from the parent neuron by closing ligand–G protein-operated calcium channels (Ca⁻). (4) Binding with GABA_A heteroreceptors on neighboring glutamatergic boutons has the same Ca⁻ effect.

In clinical disorders that involve excessive tonic muscle reflex responses (the state of spasticity, Ch. 16), the muscle relaxant baclofen is sometimes injected into the subarachnoid space surrounding the spinal cord. The drug seeps into the cord and inhibits release of glutamate from the terminals of muscle afferents, mainly by diminishing the massive calcium entry associated with excessively frequent action potentials.

Recycling of glutamate and GABA

The two routes for recycling are indicated for glutamate in Figure 8.13 and for GABA in Figure 8.14. On the left of each diagram, some transmitter molecules are retrieved from the synaptic cleft by a membrane transporter protein and reincorporated into a synaptic vesicle. On the right, transmitter molecules are being recycled through an adjacent astrocyte. Glutamate is converted to glutamine by glutamine synthetase during transit through astrocytes. Following intercellular transport into the bouton, glutamate is reassembled by glutaminase and then repacked into a synaptic vesicle. GABA is converted to glutamate by GABA transaminase during transit. Following return to the bouton, glutamate is converted (by glutamate decarboxylase) to GABA prior to storage in vesicles.

Figure 8.13 Glutamate reuptake and resynthesis. (1) Glutamate is taken up by an astrocyte, and (2) is converted to glutamine by glutamine synthetase. (3) The glutamine is transported back into the nerve terminal, (4) where it is converted to glutamate by glutaminase and (5) returned to a synaptic vesicle.

Figure 8.14 GABA reuptake and resynthesis. On the left, GABA molecules are being recycled intact. On the right, GABA is taken up by an astrocyte, then (1) GABA is converted to glutamate by GABA transaminase. (2) Glutamate is converted to glutamine by glutamine synthetase. (3) Glutamine is taken up by the axon and converted to glutamate by glutaminase. (4) Glutamate is converted to GABA by glutamate decarboxylase and (5) returned to a synaptic vesicle.

The remarkable autoimmune disorder known as stiff person syndrome, caused by blockade of glutamate decarboxylase, is described in Chapter 29.

Glycine

Glycine is synthesized from glucose via serine. Its main function as a transmitter is to provide tonic negative feedback on to motor neurons in the brainstem and spinal cord. Inactivation of glycine, for example by strychnine poisoning, results in agonizing convulsions (Clinical Panel 8.1).

Clinical Panel 8.1 Strychnine poisoning

Strychnine is a glycine receptor blocker. The victim of strychnine poisoning suffers agonizing convulsions because of liberation of α motor neurons from the tonic inhibitory control of Renshaw cells (Figure CP 8.1.1). The convulsions resemble those induced by the tetanus toxin, described in Chapter 6. This is no surprise, because tetanus toxin prevents the release of glycine from Renshaw cells. Postmortem studies of normal human brain, using radiolabeled strychnine, have shown glycine receptors to be especially abundant on internuncial neurons in the nucleus of the trigeminal nerve supplying the jaw muscles, and in the nucleus of the facial nerve supplying the muscles of facial expression. These two muscle groups are especially affected in both types of convulsive attack.

Figure CP 8.1.1 The negative feedback loop whereby Renshaw cells inhibit excessive firing by motor neurons. ACh, acetylcholine. (1) Fiber from a descending motor pathway is exciting a spinal motor neuron. (2) The motor neuron is stimulating muscle contraction. (3) A recurrent branch stimulates a Renshaw cell. (4) The Renshaw cell provides sufficient inhibition to prevent overactivity of the motor neuron.

Recycling

Glycine is rapidly taken up by an axonal transporter and re-stored in synaptic vesicles.

Biogenic amine transmitters

Acetylcholine

Acetylcholine (ACh) is a highly significant transmitter. In the central nervous system (CNS), activity of cholinergic neurons projecting from the basal region of the forebrain to the hippocampus is essential for learning and memory; degeneration of these neurons is consistently associated with the onset of Alzheimer disease. In the peripheral nervous system (PNS), all motor neurons to skeletal muscle are cholinergic; all preganglionic neurons supplying the ganglia of the sympathetic and parasympathetic systems are cholinergic, as is the postganglionic nerve supply of the parasympathetic system to cardiac muscle, the smooth muscle of the intestine and bladder, and the smooth muscles of the eye involved in accommodation for close-up vision.

Acetylcholine is formed when an acetyl group is transferred to choline from acetyl coenzyme A (acetyl CoA) by the enzyme choline acetyltransferase (Figure 8.15), which is unique to cholinergic neurons. The choline is actively transported into the neuron from the extracellular space. Acetyl CoA is synthesized in mitochondria that are concentrated in the nerve terminal and also provide the enzyme. Following release, ACh is degraded in the synaptic cleft by acetylcholinesterase (AChE), yielding choline and acetic acid. These molecules are largely recaptured and recycled to form fresh transmitter.

Figure 8.15 (A) Synthesis of acetylcholine (ACh) from acetyl coenzyme A (acetyl CoA) and choline catalyzed by choline acetyltransferase (ChAT). (B) Degradation of ACh catalyzed by acetylcholinesterase (AChE). Dashed arrows indicate recycling of acetic acid and choline.

Some steps in synthesis, degradation, and recycling of ACh are also shown in Figure 8.16.

Figure 8.16 (A) History of acetylcholine molecules in the central nervous system. The postsynaptic membrane shown contains nicotinic receptors (nAChR). (1) Choline taken up from the extracellular fluid is sent to the nerve ending. (2) The choline is acetylated by acetyl coenzyme A released by mitochondria, the reaction being catalyzed by choline acetyltransferase (ChAT). (3) Completed ACh molecules are taken up by synaptic vesicles. (4) Liberated ACh bonds briefly with its receptor. (5) Acetylcholinesterase (AChE) hydrolyzes the molecule. (6) The choline moiety is transported back into the cytosol. (7) Formation of a fresh molecule of ACh is mediated by the transferase, en route to a synaptic vesicle. (8) The acetate moiety is transported into the cytosol. (9) Mitochondria use the acetic acid to produce fresh acetyl CoA. (B) The ligand-gated nicotinic receptor, indicating the inrush of sodium and potassium ions associated with ACh–receptor binding.

Both ligand-gated and G protein-coupled ACh receptors are recognized. The former are called nicotinic, because they were first discovered to be activated by nicotine extracted from the tobacco plant. The latter are called muscarinic because activated by muscarine extracted from the poisonous mushroom Amanita muscaria.

Monoamines

Catecholamines

As indicated in Table 8.2, the catecholamines comprise dopamine, norepinephrine, and epinephrine (adrenaline). As shown in Figure 8.17, all three are derived from the amino acid tyrosine.

Figure 8.17 Synthetic pathway of the catecholamine transmitters.

The transmitters are synthesized in the nerve terminals, the requisite tyrosine and enzymes having been sent there by rapid transport. Newly synthesized transmitter must be packaged immediately into a synaptic vesicle by a monoamine transporter protein lodged in the vesicular membrane, because the catabolic enzyme monoamine oxidase (MAO) permeates the cytosol. On release, most of the transmitter binds with one or more specific receptors in the postsynaptic membrane and (where present) with an autoreceptor in the presynaptic membrane. Of the remainder, some is inactivated by catechol-O-methyl transferase (COMT), an enzyme liberated from the postsynaptic membrane into the synaptic cleft (Figure 8.18). The rest is taken up by a specific uptake transporter, and is either collected by a vesicular protein transporter or is inactivated by MAO.

Figure 8.18 History of the main catecholamine transmitter molecules. (1) Transmitter dopamine (DA) or norepinephrine (NE) molecules are transported into synaptic vesicles. (2) Following depolarization-induced calcium entry to the cytosol, synaptic vesicles are pulled into contact with the presynaptic membrane of the terminal bouton. (3) Liberated transmitter molecules have three possible fates. Most bind with G protein-coupled receptors in the postsynaptic membrane, initiating second messenger events. (4) Specific transmitter reuptake transporters (DAT, NET) return transmitter molecules to the cytosol. (5) Some of these molecules are repackaged for further use. (6) Surplus molecules in the cytosol are degraded by mitochondria-derived monoamine oxidase (MAO) enzyme. The metabolites float away in extracellular fluid destined to weep through ventricular walls into the cerebrospinal fluid, where some metabolites may be detected. (7) A second group of transmitter molecules within the synaptic cleft are broken down by catechol-O-methyl transferase (COMT). (8) A third group function as D₂ or α₂ autoreceptors, inhibiting further transmitter release. (9) A second set of metabolites drifting away in the extracellular fluid. (10) Some intact transmitter molecules drifting away to activate receptors elsewhere.

Dopamine

Dopamine is of particular interest in the clinical contexts of Parkinson disease, drug addiction, and schizophrenia. It is synthesized from tyrosine in two steps (Figure 8.17), being converted to DOPA (dihydroxyphenylalanine) by amino acid hydroxylase, and from DOPA to dopamine by dopa decarboxylase, an enzyme restricted to catecholaminergic neurons. The two main sets of dopaminergic neurons are located in the midbrain. They are the substantia nigra and the ventral part of the tegmentum called the ventral tegmental area (VTA).

The substantia nigra belongs functionally to the basal ganglia (Ch. 28). A dopaminergic nigrostrial pathway projects from substantia nigra to striatum (caudate nucleus and putamen). This pathway controls a motor loop of neurons feeding forward to the motor cortex. Degeneration of neurons in the substantia nigra is a classic feature of Parkinson disease, in which normal movements are disrupted by rigidity of the musculature and/or tremor.

The VTA projects groups of dopaminergic neurons into the forebrain. One group, called mesocortical, projects to the prefrontal cortex; overactivity of this system has been invoked to explain some clinical features of schizophrenia (Ch. 29). The other, called mesolimbic, projects to several limbic nuclei including the nucleus accumbens (bedded in the ventral striatum); dopamine liberation within the nucleus accumbens appears to be the basis of the dopamine rush, or dopamine high, associated with several kinds of drug addiction (Ch. 34).

Receptors

Dopamine receptors are all G protein-coupled. D₁ and D₂ receptors are recognized, each having more than one subtype. D₁ receptors activate G_s proteins and are excitatory, activating adenylate cyclase with consequent receptor phosphorylation. D₂ receptors activate G_i proteins and are inhibitory; they may inactivate adenylate cyclase, and may also promote hyperpolarization by opening GIRK ion channels and/or inhibiting voltage-gated Ca²⁺ channels. Both kinds are numerous in the striatum, where they are required for the proper execution of learned motor programs including locomotion (Ch. 28).

Norepinephrine

In the CNS, noradrenergic neurons are concentrated in the cerulean nucleus (locus ceruleus) in the floor of the fourth ventricle. From here, they project to all parts of the gray matter of the brain and spinal cord (see Figure 24.4). These neurons are important for regulation of the sleep–wake cycle and the control of mood.

In the PNS, norepinephrine is liberated by sympathetic nerve endings, notably throughout the cardiovascular system, where it maintains the blood pressure. It is an integral component of the ‘fight or flight’ response to danger. Formation of norepinephrine takes place in neurons containing dopamine β-hydroxylase. This enzyme is remarkable in being restricted to the inner surface of the membrane of synaptic vesicles.

Receptors

All are G protein-gated. They are broadly grouped into alpha and beta sets, with two principal subtypes within each. Some details are provided in Table 8.4.

Table 8.4 Main features of noradrenergic receptors

Catecholamine recycling

Recycling of dopamine and norepinephrine occurs via specific reuptake transporters, as indicated in Figure 8.18. The figure indicates that not all the molecules are recycled into synaptic vesicles within the parent neuron. Any of three other fates are possible: some are metabolized in or near the synaptic cleft by the enzyme COMT; others are carried for up to 100 µm by the extracellular fluid, perhaps bonding to isolated specific membrane heteroreceptors on other neurons as depicted in Figure 6.6 (‘volume transmission’); and others achieve reuptake only to be metabolized by the enzyme MAO liberated by nearby mitochondria.

Epinephrine

Neuronal production of epinephrine in the CNS appears to be confined to a group of cells in the upper lateral part of the medulla oblongata. Only these contain the enzyme (phentolamine N-methyltransferase) that provides the final link in the catecholamine chain (Figure 8.17). Some of these neurons project upward to the hypothalamus, others to the lateral gray horn of the spinal cord. Their functions are not yet clear.

In the PNS, the chromaffin cells of the adrenal medulla liberate epinephrine as a hormone into the capillary bed. The epinephrine augments sympathetic effects on the circulatory and other systems during the alarm response to danger. As shown in Figure 13.6, the chromaffin cells are modified sympathetic ganglion cells receiving synaptic contacts from preganglionic cholinergic neurons. One function of circulating epinephrine, illustrated in Figure 13.5, is to boost norepinephrine output at sympathetic nerve terminals by activating β₂ heteroreceptors there.

Serotonin

In the medical literature, more has been written about serotonin than about any other neurotransmitter. Depletion of serotonin has a well-established connection with depression. Abnormalities of serotonin metabolism have been implicated in other behavioral disorders, including anxiety states, obsessive-compulsive disorders, and bulimia.

As indicated in Figures 21.2 and 21.5 Figure 21.2 Figure 21.5, serotonergic cell bodies occupy the midregion or raphe (seam) of the brainstem. Their axonal ramifications are quite prodigious, penetrating to every region of the gray matter of brain and spinal cord.

Serotonin, commonly referred to as 5-HT because it is 5-hydroxytryptamine, is derived from the dietary amino acid tryptophan, present in the circulation. It is actively transported across the blood–brain barrier into the brain extracellular fluid, then transported into serotonergic neurons. Formation of serotonin from tryptophan is a two-step process (Figure 8.19). Tryptophan is converted to 5-hydroxytryptophan by the enzyme tryptophan hydroxylase, and this is converted to serotonin by 5-hydroxytrytophan decarboxylase.

Figure 8.19 Synthesis of serotonin from tryptophan.

Receptors

Seven groups of receptors have been identified. Ongoing research seeks to refine drug therapy so as to target individual receptors thought to be responsible, by either over- or underactivity, for specific disorders, notably in the psychiatric domain.

Table 8.5 provides some details for a selected shortlist of serotonin receptors. The table includes a reference to receptors on the somas and dendrites of the parent cell. These are targets of recurrent axon collaterals, as indicated in Figure 8.20.

Table 8.5 Some serotonin receptors of clinical interest

Figure 8.20 Serotonergic autoreceptors. 5-HT_1A reduce both excitability and serotonin synthesis. 5-HT_1D reduce serotonin release.

(After Nestler et al. 2001, with permission of McGraw-Hill)

Recycling

Recycling follows the same general pattern as for the catecholamines. Here again, the final step of transmitter synthesis takes place within the terminal bouton, and the released molecules may activate either presynaptic autoreceptors on the parent neuron or isolated heteroreceptors on other neurons nearby. There appears to be no degradatory enzyme in or near the synaptic cleft comparable with COMT, but MAO is present within the parent bouton (as shown in Figure 8.21).

Figure 8.21 History of serotonin transmitter molecules in the central nervous system. (1) Serotonin is transported into synaptic vesicles. (2) Transmitter molecules undergo exocytosis into the synaptic cleft. (3) A postsynaptic serotonin receptor is being targeted. (4) The (therapeutically important) serotonin reuptake transporter returns transmitter to the terminal cytosol. (5) Some transmitter is repackaged into synaptic vesicles. (6) Some is degraded instead by (therapeutically important) monoamine oxidase (MAO). (7) Some activates presynaptic autoreceptors. (8) A 5-HT_1D presynaptic autoreceptor is retarding further transmitter release. (9) Some diffuses through the extracellular space, using ‘volume transmission’ to activate receptors on other neurons.

Monoamines and abnormal emotional or behavioral states

Abnormal monoamine function has been implicated in a great variety of abnormal emotional or behavioral states, including depression, insomnia, anxiety disorders, panic attacks, and specific phobias. Brain areas involved are discussed in Chapters 23 and 29.

Histamine

Histamine is synthesized from histidine by histidine decaboxylase, as shown in Figure 8.22. The somas of histaminergic neurons appear to be confined to the posterior part of the hypothalamus, where they occupy the small tuberomammillary nucleus shown in Figure 26.1. However, their axons extend widely, mainly to all parts of the cerebral cortex. The main function of histaminergic neurons is to participate with cholinergic and serotonergic neurons in maintaining the awake state. These neurons are active in the awake state and silent during sleep (see Ch. 24). Activation is a function of the peptide orexin locally produced by lateral hypothalamic neurons. The compulsive daytime sleep disorder known as narcolepsy (Ch. 30) appears to result from failure of orexin production.