Synaptic Transmission Between Neurons

Neurotransmitters Are Synthesized in Presynaptic Endings and in Neuronal Cell Bodies

Nearly all known or suspected neurotransmitters fall into one of two general categories: some are small molecules such as amines or amino acids, and the others are larger peptides (neuropeptides) or modified lipids. Small-molecule transmitters are synthesized in presynaptic cytoplasm, using locally available substrates (e.g., acetate and choline) and soluble enzymes that arrive by slow axonal transport ( Fig. 8.4A ). Making peptide transmitters, in contrast, requires the protein synthesis machinery that resides in the neuronal cell body. Neuropeptides start out there as larger precursor proteins that are packed into vesicles and dispatched by fast axonal transport to synaptic endings. Along the way, the precursors are cleaved and processed into the final, biologically active neuropeptides (see Fig. 8.4B ).

Neurotransmitters Are Packaged Into Synaptic Vesicles Before Release

Synaptic vesicles are the units of currency at chemical synapses, the sites where neurotransmitters are packaged, concentrated, and protected from catabolism while awaiting release. All presynaptic endings contain small vesicles (about 40 nm), and many also contain less numerous but larger vesicles (≥100 nm). Depending on the preparation conditions used for electron microscopy, some of the small vesicles may appear dark, and others may look clear and either round or flattened; each large vesicle contains a dark core ( Fig. 8.5 ). These differences in appearance correspond to differences in neurotransmitter content. For example, endings with clear, flattened vesicles usually contain neurotransmitters that more likely will result in inhibitory effects on the postsynaptic cell.

Small synaptic vesicles contain small-molecule transmitters, concentrated to levels far beyond that found in the cytoplasm by specific transporters in the vesicle walls. Large dense-core vesicles contain neuropeptides at lower concentrations and may contain one or more small-molecule transmitters as well. The two kinds of vesicle, like the different neurotransmitters, are manufactured and recycled differently (see Fig. 8.4 ). Small-molecule transmitters are synthesized by cytoplasmic enzymes and can therefore be manufactured and packaged for release in individual synaptic endings. Small synaptic vesicles can therefore be recycled entirely within a presynaptic ending, whereas large dense-core vesicles must be created anew in the cell body. Modified lipid neurotransmitters do not use vesicles but are stored and released directly from the lipid membrane upon activation of enzymes.

Presynaptic Endings Release Neurotransmitters Into the Synaptic Cleft

Release of vesicle containing neurotransmitter is a Ca ²⁺ -mediated secretory process. Each presynaptic density, or active zone, contains an abundance of voltage-gated Ca ²⁺ channels, together with anchoring sites for a cluster of small vesicles that are held there (“docked”) by a Ca ²⁺ -sensitive system of proteins. Depolarization of the presynaptic terminal, such as by propagation of an action potential down the axon and subsequent electrotonic spread into the terminal, causes opening of the voltage-gated Ca ²⁺ channels. Because the free intracellular Ca ²⁺ concentration is only about 10 ⁻⁷ M, Ca ²⁺ ions flow in through these channels and momentarily elevate the Ca ²⁺ concentration near the active zone by as much as 1000-fold. During the brief period before the excess Ca ²⁺ diffuses away or is sequestered, a vesicle docked nearby may fuse with the presynaptic membrane and discharge its contents into the synaptic cleft in a process called exocytosis ( Fig. 8.6A ). The whole process, from the arrival of an action potential to the release of a small vesicle's contents, can take less than 100 µsec. Because the large, dense-cored vesicles are not located adjacent to active zones, repetitive action potentials, additional Ca ²⁺ entry, and more time (tens of milliseconds) are typically required for their exocytosis (see Fig. 8.6B ). This exocytotic addition of membrane clearly cannot continue for very long, or else presynaptic endings would expand continuously. In fact, vesicle membranes are taken back up (by endocytosis ), and those used for small-molecule transmitters can be recycled in less than a minute (see Fig. 8.4A ).

Because neurotransmitter release is dependent on the entry of Ca ²⁺ , the modulation of the voltage-gated calcium channels can have a large physiological response. Several toxins as well as several drugs have been known to modulate these channels to inhibit or increase neuronal activity. In addition, there are known toxins, such as botulinum toxin, that interfere with the ability of the acetylcholine-containing vesicles, inhibiting their release, and resulting in the relaxation of skeletal muscles (see Box 8.1 ).

Neurotransmitters Diffuse Across the Synaptic Cleft and Bind to Postsynaptic Receptors

Neurotransmitters released from small vesicles find postsynaptic receptor molecules waiting for them directly across the synaptic cleft ( Fig. 8.7 ). For this reason, it takes the contents of such vesicles very little time to exert their effects. The total synaptic delay from presynaptic action potential to postsynaptic effect can be less than 200 µsec. The contents of large vesicles, in contrast, take longer to be released and often diffuse to receptors relatively far away (see Fig. 8.9 ), so their effects develop more slowly. As a self-regulation of transmitter release, at times the presynaptic neuron will also contain receptors (autoreceptors) that often result in the slowing of neurotransmitter release.

Neurotransmitter Action Is Terminated by Uptake, Degradation, or Diffusion

Neurotransmitter molecules must be removed quickly once they have had a chance to bind to receptors so that the postsynaptic membrane will be prepared for subsequent releases of transmitter. This is accomplished by almost every means imaginable ( Fig. 8.8 ). Binding of neurotransmitter and receptor is a reversible event, so receptors and removal mechanisms compete for transmitter. Some transmitter simply diffuses away, but this is a slow process. In most synapses, transmitter is reabsorbed into the presynaptic ending or taken up by neighboring glial cells or even by the postsynaptic neuron. In some synapses, enzymes in the synaptic cleft degrade free transmitter. Reabsorbed transmitters or their metabolic products, like vesicle membranes, are often recycled for use in a subsequent synaptic event.

Different kinds of neurotransmitters have different preferred mechanisms of removal. For example, serotonin, norepinephrine, and dopamine are transported rapidly back into the presynaptic ending through large protein transporters for repackaging into synaptic vesicles. Acetylcholine, in contrast, is split into acetate and choline by acetylcholinesterase in the synaptic cleft; the choline is then transported back into the presynaptic ending and used for the resynthesis of more acetylcholine. Neuropeptides are either degraded by extracellular peptidases or swallowed up by the postsynaptic cell while still attached to their receptors ( Fig. 8.9 ).

Rapid Synaptic Transmission Involves Transmitter-Gated Ion Channels

Early studies of synaptic transmission demonstrated that a single action potential in the motor nerve ending at a neuromuscular junction causes a large but brief EPSP, ^c

c Often referred to as an end-plate potential because motor end plate is another term for neuromuscular junction.

sufficient to cause an action potential and contraction in the muscle fiber ( Fig. 8.10B ). Closer analysis revealed small, brief depolarizing events in the postjunctional muscle membrane during the periods between action potentials (see Fig. 8.10A ), each corresponding to a brief period of channel opening and depolarizing current flow. We now know that each of these small depolarizing events is the response of the postsynaptic membrane to the spontaneous release of one synaptic vesicle. Each vesicle contains about 10,000 acetylcholine molecules, which diffuse across the synaptic cleft and bind briefly to acetylcholine receptors, which are ligand-gated ion channels permeable to Na ⁺ and K ⁺ ions. Acetylcholinesterase competes with the receptors for the released acetylcholine, so the channels stay open for only a millisecond or two. During this time, they allow a current flow that tries to move the membrane potential to a value around 0 mV (between the Na ⁺ and K ⁺ equilibrium potentials; see Chapter 7 , Calculating the Membrane Potential, Eq. 7.17 ). Because 2 msec is considerably less than the time constant of the muscle membrane, the membrane potential never reaches 0 mV. Instead, it rises rapidly at the beginning of the postsynaptic current flow and then decays slowly at the end of the current flow (see Fig. 8.10A ). Entry of depolarizing current is localized to the site of the receptors, so the postsynaptic potential decays electrotonically (see Fig. 8.10C and D ). However, the presynaptic ending at each neuromuscular junction contains more than 1000 active zones ( Fig. 8.11 and see Fig. 8.7 ), so a single action potential in the motor axon causes the nearly simultaneous release of hundreds of vesicles full of acetylcholine at closely spaced sites, which in turn causes the large EPSP normally seen (see Fig. 8.9B ).

The basic elements of neuromuscular transmission have proved to be generally true of fast synaptic transmission throughout the nervous system ( Fig. 8.12A ). Depolarization-induced Ca ²⁺ entry causes the release of one or more packets of neurotransmitter, called quanta, each of which is generally assumed to be the contents of a single vesicle docked at an active zone. The neurotransmitter then diffuses across the synaptic cleft and binds transiently to a transmitter-gated (i.e., ligand-gated) ion channel. The selectivity of the channel determines the postsynaptic effect. Opening channels selective for monovalent cations, as in the example of the neuromuscular junction, causes an EPSP. In contrast, opening channels selective for Cl ⁻ moves the membrane potential toward the Cl ⁻ equilibrium potential; this is the most common basis of fast IPSPs in the central nervous system (CNS). The selective concentration of receptors on the postsynaptic membrane ensures that fast postsynaptic potentials are localized spatially. Transmitter-gated ion channels are also referred to as ionotropic receptors (from the same Greek word that gave rise to tropism, a movement toward or away from a stimulus, as in the heliotropic growth of plants in the direction of sunlight).