Chapter 4
The Chemical Basis for Neuronal Communication
Fundamentals of Chemical Neurotransmission
Fast and Slow Synaptic Transmission
Information Flow Across Chemical Synapses
Synthesis, Storage, and Release of Chemical Messengers
Composition of Vesicle Membranes
Receptors and Receptor Subtypes
Regulation of Neuronal Excitability
Maintenance of the Synaptic Environment
Drug-Induced Parkinson Disease
Neurons in the human brain communicate primarily by the release of small quantities of chemical messengers, most of which are commonly called neurotransmitters. These chemicals alter the electrical activity of neurons after they interact with receptors on cell surfaces. Therapeutic alteration of brain function requires an understanding of the processes that regulate the synthesis and release of neurotransmitters and the means by which receptors alter neuronal electrical activity and biochemical function.
OVERVIEW
The brain contains approximately 100 billion (1011) neurons, each of which can make as many as 100,000 terminal contacts. Thus it has been estimated that the human brain contains approximately 1016 connections between neurons. Communication at most of these connections is mediated by chemical messengers. The transfer of information between neurons takes place at structurally and functionally specialized locations called synapses. Most synapses use chemical messengers that are released in discrete units (quanta) from presynaptic axonic or dendritic terminals in response to depolarization of the terminal.
Rapid diffusion of a chemical messenger across the synaptic cleft is followed by binding of this substance to receptors spanning the postsynaptic membrane. There is a resultant alteration in the electrical, biochemical, or genetic properties of that neuron. Less frequently, chemical messengers may also be released at sites without synaptic specializations. These messengers diffuse more widely than do neurotransmitters released at synaptic sites, and they influence receptors located at distant sites and on more than one neuron. Whether synaptic or nonsynaptic, chemical communication in the nervous system depends on (1) the nature of the presynaptically released chemical messenger, (2) the type of postsynaptic receptor to which it binds, and (3) the mechanism that couples receptors to effector systems in the target cell.
Western medicine is based fundamentally on modification of biologic function by drugs. Much of this alteration is focused on processes of chemical neurotransmission, whether in the central nervous system (CNS) or the periphery. The focus in this chapter is on basic elements of chemical neurotransmission. A model synapse, the noradrenergic synapse, is introduced, and a series of therapeutically important drugs is highlighted to illustrate potential medical targets in such a synapse.
FUNDAMENTALS OF CHEMICAL NEUROTRANSMISSION
Neurotransmitters
Specific criteria that define whether a chemical messenger can be identified as a neurotransmitter are listed in Table 4-1. Although a wide variety of putative neurotransmitters have been identified, these criteria have been met for only a few chemical substances. In general, these transmitters can be categorized as small-molecule messengers (having fewer than 10 carbon atoms) or larger neuropeptides (containing 10 or more carbon atoms).
Table 4-1 Criteria Necessary to Define a Substance as a Neurotransmitter
Small-molecule chemical messengers are classed as amino acids, biogenic amines, and nucleotides or nucleosides (Table 4-2). Amino acid neurotransmitters include γ-aminobutyric acid (GABA), glycine, aspartate, and glutamate. The vast majority of signaling within the nervous system is carried by amino acid neurotransmitters, specifically GABA and glutamate. For example, it has been estimated that roughly every fifth nerve cell and one of every six synaptic contacts uses GABA as a neurotransmitter. The biogenic amines include the familiar neurotransmitters acetylcholine, dopamine, norepinephrine, epinephrine, serotonin, and histamine. The nucleotide-nucleoside class includes adenosine and adenosine triphosphate (ATP). Nitric oxide, which functions as an endogenous nitrovasodilator in the cardiovascular system, has also been identified as a putative neurotransmitter.
Table 4-2 Substances Believed to Act as Chemical Messengers in the Central Nervous System
ATP, adenosine triphosphate; GABA, γ-aminobutyric acid.
More than 40 neuropeptides have been identified in brain tissue. These include methionine enkephalin (met-enkephalin) and leucine enkephalin (leu-enkephalin) as well as larger peptides, such as endorphins, calcitonin gene–related peptide (CGRP), arginine vasopressin, cholecystokinin, and many others (Table 4-2).
With a few exceptions, one being nitric oxide, the chemical messengers used by neurons are stored in secretory vesicles and released from them by exocytosis. In the case of neurotransmitters, these vesicles are found mainly in the presynaptic nerve terminals.
Fast and Slow Synaptic Transmission
The diffusion of a chemical message across the synaptic cleft can be quite rapid. At the neuromuscular junction, for example, it takes only about 50 microseconds for acetylcholine to reach the postsynaptic membrane. Total synaptic delay, the time from presynaptic release of neurotransmitter to the activation or inhibition of the postsynaptic neuron, is variable. This variability is influenced by the transduction mechanisms in the postsynaptic neuron.
Transduction mechanisms can be divided into fast and slow types. Fast chemical neurotransmission operates with a total synaptic delay of only a few milliseconds, whereas slow chemical neurotransmission usually requires hundreds of milliseconds. In both cases, the receptors on the postsynaptic membranes are glycoproteins that span the lipid bilayer membrane and transduce an extracellular chemical signal into a functional change in the target neuron. The difference relates to the complexity of the transduction mechanism.
In fast chemical neurotransmission, the postsynaptic receptor is itself an ion channel. This type of transmission is associated exclusively with small-molecule neurotransmitters. The binding of transmitter stimulates the channel to open, permitting a flux of ions across the membrane that alters the membrane potential. The process is fast because it is direct. Ion channels in this type of neurotransmission are called ligand-gated or receptor-gated ion channels; the ions normally involved are sodium, potassium, calcium, and chloride. Movement of these ions causes a change in the transmembrane electrical potential, which, if it exceeds threshold, may lead to generation of an action potential.
In slow chemical neurotransmission, the signal is transduced by a mechanism involving G protein–coupled receptors. These proteins and their action are discussed later in the chapter. Briefly, the binding of the transmitter (frequently a neuropeptide) causes the receptor to activate a G protein, which in turn binds to and influences an effector protein, which elicits the cellular effect. In some cases, the effector protein is an ion channel, which is induced to open or close. Transduction in these cases can be almost as rapid as in fast neurotransmission. More often, the effector is an enzyme that produces an intracellular second messenger, such as cyclic adenosine monophosphate (cAMP), whose cytoplasmic concentration is altered in response to the reception of a signal (binding of the transmitter) at the cell surface and that elicits intracellular responses to the signal. Second messengers can produce a plethora of cellular responses, ranging from the opening or closing of membrane ion channels to alterations in gene expression. These effects are mediated by complex sequences of chemical events, which is why they are relatively slow.
Information Flow Across Chemical Synapses
Transmission of information at a chemical synapse involves the following general sequence of events (Fig. 4-1): (1) secretory vesicle synthesis and transport to the synaptic terminal; (2) for small-molecule neurotransmitters, loading of the transmitter into the vesicle (for neuropeptides, this step accompanies vesicle synthesis); (3) depolarization of the presynaptic terminal; (4) vesicle docking with the presynaptic membrane, exocytosis of its contents, and trans-synaptic diffusion of the transmitter; (5) binding of transmitter to, and activation of, the postsynaptic receptor; (6) transduction of the signal, resulting in a postsynaptic response and one or two terminal steps; and (7) active reuptake of the transmitter by the presynaptic terminals or by glia or (8) enzymatic degradation of the transmitter in the synaptic cleft. These final events eliminate transmitter from the synaptic cleft and thereby terminate its action.
In many synapses, the amount of transmitter that a presynaptic terminal releases in response to an action potential can be regulated from outside the cell. Two regulatory mechanisms are (1) presynaptic receptor–mediated autoregulation and (2) retrograde transmission. In presynaptic receptor–mediated autoregulation, the neuron self-regulates the subsequent quantal release of its own chemical messenger. As a neurotransmitter enters the synaptic cleft, it stimulates not only postsynaptic receptors but also receptors located on the membranes of the terminal from which it was released. This constantly updates the presynaptic neuron concerning neurotransmitter synthesis and release and the efficiency of information transfer. In most cases, autoregulation is inhibitory. Loss or reduction of this input is interpreted as a reduction in signaling ability, and the presynaptic neuron increases the subsequent synthesis and release of stored neurotransmitter.
In retrograde transmission, the postsynaptic neuron responds to synaptic activation by releasing a second chemical messenger. This messenger diffuses back across the synapse and alters the function of the presynaptic terminal. Nitric oxide is currently the best example of a mediator of retrograde transmission.
SYNTHESIS, STORAGE, AND RELEASE OF CHEMICAL MESSENGERS
Neuronal chemical messengers are stored in two types of vesicles: small vesicles (also called synaptic vesicles) and large dense-cored vesicles. Synaptic small vesicles (~50 nm in diameter) appear clear and empty in electron micrographs and contain small-molecule chemical messengers such as GABA, glutamate, and acetylcholine. A subset of these small vesicles, with electron-dense cores, are found in both central and peripheral neurons. These vesicles contain the catecholamine family of biogenic amines (dopamine, norepinephrine, and epinephrine). Synaptic vesicles cluster near the exocytotic surface of a presynaptic nerve terminal in regions called active zones (Fig. 4-1).
Large dense-cored vesicles (~75 to 150 nm in diameter) are less numerous and appear in other intraneuronal locations as well as in the axon terminal. The electron-opaque, dense core is composed of soluble proteins that are mainly one or more neuropeptides. This core may also contain a small chemical messenger—often a biogenic amine, co-stored with a neuropeptide.
Neurons in certain hypothalamic nuclei contain a third type of vesicle called the neurosecretory vesicle. These vesicles are large (~150 to 200 nm in diameter), contain neurohormones, and are especially concentrated in axon terminals in the neurohypophysis (the posterior pituitary).
Composition of Vesicle Membranes
All vesicles are composed of a lipid bilayer membrane, spanned by a variety of proteins. Some proteins are common to both large dense-cored vesicles and synaptic vesicles, such as those that form calcium channels and the proteins synaptotagmin and SV2. Other proteins are found in high concentrations only in synaptic vesicles; these include synaptophysin and synaptobrevin. The differences in protein content reflect the different roles that large dense-cored vesicles and synaptic vesicles play in neurons.
Vesicles also contain proteins that act to accumulate small chemical messengers. These take the form of membrane pumps or transporters, most of which are coupled to the transport of protons. Synaptic vesicles contain at least four classes of proton-coupled transporters for chemical messengers, each specific for a different type of messenger. One class, the vesicular monamine transporter (VMAT), drives the accumulation of biogenic amines, including the catecholamines dopamine, norepinephrine, and epinephrine as well as the monoamine serotonin. Others are specific for acetylcholine, glutamate, and GABA or glycine. Large dense-cored vesicles can also accumulate small chemical messengers in addition to their neuropeptides. However, it is believed that the transporters involved are different from those used by synaptic vesicles.
Biosynthesis
In terms of biosynthesis, an important difference between synaptic vesicles and large dense-cored vesicles is that the former can be recycled and refilled in the axon terminal, whereas the latter are both made and filled in the neuronal soma and are not recycled. This reflects the fact that small-molecule neurotransmitters can be synthesized in axon terminals, whereas neuropeptides, because they are synthesized on ribosomes and processed through the endoplasmic reticulum and Golgi complex, can be made only in the soma (Fig. 4-2). The cis face of the Golgi complex (also called the proximal or forming face) is prototypically concave toward the nucleus of the cell, whereas the trans face (distal or maturation face) is convex (Fig. 4-2). Peptides from the endoplasmic reticulum enter the cis face of the Golgi complex and are sorted and packaged into vesicles that bud from its trans face.
Large dense-cored vesicles contain neuropeptide messengers and are filled during the process of vesicle synthesis in the Golgi complex. These vesicles are translocated, by fast axonal transport (range, 4 to 17 mm/hr), from the cell body to axonal or dendritic release sites (Fig. 4-3). Frequently, neuropeptides are synthesized in the form of large precursor peptides that may be cleaved to yield more than one secreted bioactive neuropeptide. Maturation of neuropeptides can require covalent chemical modification of amino acid side chains, often with the addition of small chemical groups. Examples of the types of chemical modifications include the addition of methyl groups (methylation), sugar moieties (glycosylation), and sulfate groups (sulfation). This process of maturation can occur within the endoplasmic reticulum, during packaging of peptides into large dense-cored vesicles within the Golgi complex, or during axonal transport.
In general, synaptic vesicles are formed initially by budding from the Golgi apparatus within the cell body (Figs. 4-2 and 4-4). After transport to and release from the presynaptic terminal, however, the lipoprotein membrane components of the synaptic vesicles are recycled in a continuous process that occurs within nerve terminals (Fig. 4-4). Synthesis of the chemical messenger in a synaptic vesicle can occur while the vesicle is in the nerve terminal rather than in the cell body.
Some small-molecule neurotransmitters are synthesized in the cytosol of the axon and axon terminal and then transported into synaptic vesicles, whereas others are synthesized in the vesicle itself. The synthesis of acetylcholine is an example of the first of these mechanisms. The soluble enzyme choline acetyltransferase (CAT) catalyzes the acetylation of choline from acetyl coenzyme A (CoA) to yield the neurotransmitter acetylcholine. A high-affinity vesicular membrane transport protein concentrates this transmitter in cholinergic synaptic vesicles. Synthesis of the catecholamine norepinephrine is an example of the second mechanism. In the case of norepinephrine, synthesis occurs within the synaptic vesicle. The immediate precursor to norepinephrine, dopamine, is concentrated within the noradrenergic synaptic vesicle by a transporter specific for biogenic amines (the VMAT). Only then is dopamine converted to norepinephrine by the action of the enzyme dopamine β-hydroxylase, which is attached to the luminal border of the vesicular membrane.
Transporters for small chemical messengers concentrate compounds inside the vesicle to levels 10 to 1000 times higher than those found in the cytosol. The energy required for this transport is derived from an ATP-driven proton pump. The exchange of protons for the chemical messenger allows accumulation of the chemical messenger inside the vesicle.
Localization
As mentioned earlier, synaptic vesicles are preferentially concentrated in active zones of the nerve terminal (Figs. 4-4 and 4-5). These zones are biochemically and anatomically specialized for neurotransmitter release. Large numbers of voltage-sensitive calcium channels are clustered in the plasma membrane of active zones. Consequently, depolarization of the axon terminal (or in special cases the dendrites) results in a high local concentration of calcium. This calcium causes synaptic vesicles to bind to the plasma membrane and stimulates exocytotic release of vesicle contents into the synaptic cleft. Active zones also contain high concentrations of the filamentous protein synapsin, which aids in the clustering of synaptic vesicles.
Figure 4-5.
