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Table
of Neurotransmitters Synaptic Transmission Neuromuscular Transmission Neurotransmitter Receptors Acetylcholine Cholinergic Agonists and Antagonists Catecholamines Serotonin GABA |
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Table of Neurotransmitters
Transmitter Molecule |
Derived From |
Site of Synthesis |
| Acetylcholine | Choline | CNS, parasympathetic nerves |
|
Serotonin 5-Hydroxytryptamine (5-HT) |
Tryptophan | CNS, chromaffin cells of the gut, enteric cells |
| GABA | Glutamate | CNS |
| Glutamate | CNS | |
| Aspartate | CNS | |
| Glycine | spinal cord | |
| Histamine | Histidine | hypothalamus |
|
Epinephrine synthesis pathway |
Tyrosine | adrenal medulla, some CNS cells |
|
Norpinephrine synthesis pathway |
Tyrosine | CNS, sympathetic nerves |
|
Dopamine synthesis pathway |
Tyrosine | CNS |
| Adenosine | ATP | CNS, peripheral nerves |
| ATP | sympathetic, sensory and enteric nerves | |
| Nitric oxide, NO | Arginine | CNS, gastrointestinal tract |
Many other neurotransmitters are derived from
precursor proteins, the so-called peptide neurotransmitters. As many as 50 different peptides
have been shown to exert their effects on neural cell function. Several of
these peptide transmitters are derived
from the larger protein pre-opiomelanocortin (POMC). Neuropeptides
are responsible for mediating sensory and emotional responses including hunger,
thirst, sex drive, pleasure and pain.
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Synaptic Transmission
Synaptic transmission refers to the propagation of nerve impulses from one nerve cell to another. This occurs at a specialized cellular structure known as the synapse, a junction at which the axon of the presynaptic neuron terminates at some location upon the postsynaptic neuron. The end of a presynaptic axon, where it is juxtaposed to the postsynaptic neuron, is enlarged and forms a structure known as the terminal button. An axon can make contact anywhere along the second neuron: on the dendrites (an axodendritic synapse), the cell body (an axosomatic synapse) or the axons (an axo-axonal synapse).
Nerve impulses are transmitted at
synapses by the release of chemicals called neurotransmitters. As a nerve impulse, or action potential, reaches the end of a presynaptic
axon, molecules of neurotransmitter are released into the synaptic space. The
neurotransmitters are a diverse group of chemical compounds ranging from simple
amines such as dopamine and
amino acids such as γ-aminobutyrate (GABA), to polypeptides such as the enkephalins. The mechanisms by which they elicit responses in
both presynaptic and postsynaptic neurons are as
diverse as the mechanisms employed by growth factor
and cytokine receptors.
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Neuromuscular Transmission
A different type of nerve transmission occurs when an
axon terminates on a skeletal muscle fiber, at a specialized structure called the neuromuscular junction. An action potential occurring at this site
is known as neuromuscular transmission.
At a neuromuscular junction, the axon subdivides into numerous terminal buttons
that reside within depressions formed in the motor end-plate. The
particular transmitter in use at the neuromuscular junction is acetylcholine.
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Neurotransmitter Receptors
Once the molecules of neurotransmitter are released from a cell as the result of the firing of an action potential, they bind to specific receptors on the surface of the postsynaptic cell. In all cases in which these receptors have been cloned and characterized in detail, it has been shown that there are numerous subtypes of receptor for any given neurotransmitter. As well as being present on the surfaces of postsynaptic neurons, neurotransmitter receptors are found on presynaptic neurons. In general, presynaptic neuron receptors act to inhibit further release of neurotransmitter.
The vast majority of neurotransmitter receptors belong
to a class of proteins known as the serpentine receptors. This class exhibits a
characteristic transmembrane structure: that is, it
spans the cell membrane, not once but seven times. The link between
neurotransmitters and intracellular signaling is carried out by association
either with G-proteins (small GTP-binding and hydrolyzing proteins) or with
protein kinases, or by the receptor itself in the
form of a ligand-gated ion channel (for example, the
acetylcholine receptor). One additional characteristic of neurotransmitter
receptors is that they are subject to ligand-induced desensitization: That is, they can become unresponsive upon prolonged exposure to their
neurotransmitter.
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Acetylcholine
Acetylcholine (ACh) is a simple molecule synthesized from choline and acetyl-CoA through the action of choline acetyltransferase. Neurons that synthesize and release ACh are termed cholinergic neurons. When an action potential reaches the terminal button of a presynaptic neuron a voltage-gated calcium channel is opened. The influx of calcium ions, Ca2+, stimulates the exocytosis of presynaptic vesicles containing ACh, which is thereby released into the synaptic cleft. Once released, ACh must be removed rapidly in order to allow repolarization to take place; this step, hydrolysis, is carried out by the enzyme, acetylcholinesterase. The acetylcholinesterase found at nerve endings is anchored to the plasma membrane through a glycolipid.
Synthesis of Acetylcholine
ACh receptors are ligand-gated
cation channels composed of four different
polypeptide subunits arranged in the form [(α2)(β)(γ)(δ)]. Two main classes of ACh receptors have been identified on the basis of their
responsiveness to the toadstool alkaloid, muscarine,
and to nicotine, respectively: the muscarinic
receptors and the nicotinic
receptors. Both receptor classes are abundant in the human brain. Nicotinic receptors are further divided into
those found at neuromuscular junctions and those found at neuronal synapses.
The activation of ACh receptors by the binding of ACh leads to an influx of Na+ into the cell and
an efflux of K+, resulting in a depolarization of the postsynaptic
neuron and the initiation of a new action potential.
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Cholinergic Agonists and Antagonists
Numerous compounds have been identified that act as either agonists or antagonists of cholinergic neurons. The principal action of cholinergic agonists is the excitation or inhibition of autonomic effector cells that are innervated by postganglionic parasympathetic neurons and as such are referred to as parasympathomimetic agents. The cholinergic agonists include choline esters (such as ACh itself) as well as protein- or alkaloid-based compounds. Several naturally occurring compounds have been shown to affect cholinergic neurons, either positively or negatively.
The responses of cholinergic neurons can also be enhanced by administration of cholinesterase (ChE) inhibitors. ChE inhibitors have been used as components of nerve gases but also have significant medical application in the treatment of disorders such as glaucoma and myasthenia gravis as well as in terminating the effects of neuromuscular blocking agents such as atropine.
Natural Cholinergic Agonist and Antagonists
Source of Compound |
Mode of Action |
|
Agonists |
||
| Nicotine | alkaloid prevalent in the tobacco plant | activates nicotinic class of ACh receptors, locks the channel open |
| Muscarine | alkaloid produced by Amanita muscaria mushrooms | activates muscarinic class of ACh receptors |
| α-Latrotoxin | protein produced by the black widow spider | induces massive ACh release, possibly by acting as a Ca2+ ionophore |
Antagonists |
||
| atropine (and related compound Scopolamine) | alkaloid produced by the deadly nightshade, Atropa belladonna | blocks ACh actions only at muscarinic receptors |
| Botulinus toxin | eight proteins produced by Clostridium botulinum | inhibits the release of ACh |
| α-Bungarotoxin | protein produced by Bungarus genus of snakes | prevents ACh receptor channel opening | d-Tubocurarine | active ingredient of curare | prevents ACh receptor channel opening at motor end-plate |
Catecholamines
The principal catecholamines are norepinephrine, epinephrine and dopamine. These compounds are formed from phenylalanine and tyrosine. Tyrosine is produced in the liver from phenylalanine through the action of phenylalanine hydroxylase. The tyrosine is then transported to catecholamine-secreting neurons where a series of reactions convert it to dopamine, to norepinephrine and finally to epinephrine (see also Specialized Products of Amino Acids).
Synthesis of the Catecholamines from Tyrosine.
Catecholamines exhibit peripheral nervous system excitatory and inhibitory effects as well as actions in the CNS such as respiratory stimulation and an increase in psychomotor activity. The excitatory effects are exerted upon smooth muscle cells of the vessels that supply blood to the skin and mucous membranes. Cardiac function is also subject to excitatory effects, which lead to an increase in heart rate and in the force of contraction. Inhibitory effects, by contrast, are exerted upon smooth muscle cells in the wall of the gut, the bronchial tree of the lungs, and the vessels that supply blood to skeletal muscle.
In addition to their effects as neurotransmitters, norepinephrine and epinephrine can influence the rate of metabolism. This influence works both by modulating endocrine function such as insulin secretion and by increasing the rate of glycogenolysis and fatty acid mobilization.
The catecholamines bind to two different classes of receptors termed the α- and β-adrenergic receptors. The catecholamines therefore are also known as adrenergic neurotransmitters; neurons that secrete them are adrenergic neurons. Norepinephrine-secreting neurons are noradrenergic. The adrenergic receptors are classical serpentine receptors that couple to intracellular G-proteins. Some of the norepinephrine released from presynaptic noradrenergic neurons recycled in the presynaptic neuron by a reuptake mechanism.
Catecholamine Catabolism
Epinephrine and norepinephrine are catabolized to inactive compounds through the sequential actions of catecholamine-O-methyltransferase (COMT) and monoamine oxidase (MAO). Compounds that inhibit the action of MAO have been shown to have beneficial effects in the treatment of clinical depression, even when tricyclic antidepressants are ineffective. The utility of MAO inhibitors was discovered serendipitously when patients treated for tuberculosis with isoniazid showed signs of an improvement in mood; isoniazid was subsequently found to work by inhibiting MAO.
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Metabolism of the catecholamine neurotransmitters. Only clinically important enzymes are included in this diagram. The catabolic byproducts of the catecholamines, whose levels in the cerebrospinal fluid are indicative of defects in catabolism, are in blue underlined text. Abbreviations: TH = tyrosine hydroxylase, DHPR = dihydropteridine reductase, H2B = dihydrobiopterin, H4B = tetrahydrobiopterin, MAO = monoamine oxidase, COMT = catecholamine-O-methyltransferase, MHPG = 3-methoxy-4-hydroxyphenylglycol, DOPAC = dihydroxyphenylacetic acid. |
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Serotonin
Serotonin (5-hydroxytryptamine, 5HT) is formed by the hydroxylation and decarboxylation of tryptophan (see also Specialized Products of Amino Acids).
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Pathway for serotonin synthesis from tryptophan. Abbreviations: THP = tryptophan hydroxylase, DHPR = dihydropteridine reductase, H2B = dihydrobiopterin, H4B = tetrahyrobiopterin, 5-HT = 5-hydroxytryptophan, AADC = aromatic L-amino acid decarboxylase. |
The greatest concentration of 5HT (90%) is found in the enterochromaffin cells of the gastrointestinal tract. Most of the remainder of the body's 5HT is found in platelets and the CNS. The effects of 5HT are felt most prominently in the cardiovascular system, with additional effects in the respiratory system and the intestines. Vasoconstriction is a classic response to the administration of 5HT.
Neurons that secrete 5HT are termed serotonergic. Following the release of 5HT, a portion is taken back up by the presynaptic serotonergic neuron in a manner similar to that of the reuptake of norepinephrine.
The function of serotonin is exerted upon its interaction with specific receptors. Several serotonin receptors have been cloned and are identified as 5HT1, 5HT2, 5HT3, 5HT4, 5HT5, 5HT6, and 5HT7. Within the 5HT1 group there are subtypes 5HT1A, 5HT1B, 5HT1D, 5HT1E, and 5HT1F. There are three 5HT2 subtypes, 5HT2A, 5HT2B, and 5HT2C as well as two 5HT5 subtypes, 5HT5a and 5HT5B. Most of these receptors are coupled to G-proteins that affect the activities of either adenylate cyclase or phospholipase Cγ. The 5HT3 class of receptors are ion channels.
Some serotonin receptors are presynaptic
and others postsynaptic. The 5HT2A receptors mediate platelet
aggregation and smooth muscle contraction. The 5HT2C receptors are
suspected in control of food intake as mice lacking this gene become obese from
increased food intake and are also subject to fatal seizures. The 5HT3
receptors are present in the gastrointestinal tract and are related to
vomiting. Also present in the gastrointestinal tract are 5HT4 receptors
where they function in secretion and peristalsis. The 5HT6 and 5HT7
receptors are distributed throughout the limbic system of the brain and the 5HT6
receptors have high affinity for antidepressant drugs.
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GABA
Several amino acids have distinct excitatory or inhibitory effects upon the nervous system. The amino acid derivative, γ-aminobutyrate, also called 4-aminobutyrate, (GABA) is a well-known inhibitor of presynaptic transmission in the CNS, and also in the retina. The formation of GABA occurs by the decarboxylation of glutamate catalyzed by glutamate decarboxylase (GAD). GAD is present in many nerve endings of the brain as well as in the β-cells of the pancreas. Neurons that secrete GABA are termed GABAergic.
GABA Synthesis
GABA exerts its effects by binding to two distinct
receptors, GABA-A and GABA-B. The GABA-A receptors form a Cl-
channel. The binding of GABA to GABA-A receptors increases the Cl- conductance of presynaptic
neurons. The anxiolytic drugs of the benzodiazepine
family exert their soothing effects by potentiating
the responses of GABA-A receptors to GABA binding. The GABA-B receptors are
coupled to an intracellular G-protein and act by increasing conductance of an
associated K+ channel.
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Michael W. King, Ph.D / IU School of Medicine / miking at iupui.edu