Synaptic physiology II

Conventional neurotransmitters comprise three groups

All conventional neurotransmitters are stored in synaptic vesicles (SVs) and are released in quantal fashion by Ca ²⁺ -dependent exocytosis. These conventional transmitters can be divided into three groups. One group includes ACh, γ-aminobutyric acid (GABA) , glutamate, glycine ( Fig. 13.1 ), and certain purines. They are stored in approximately 40-nm diameter, electron-lucent SVs. These transmitters are released and act at well-defined synapses with “classical” structures: the presynaptic cells, which have active zone densities in their boutons, are separated by narrow (20- to 40-nm) synaptic clefts from the postsynaptic cells, which have specialized postsynaptic densities ( Fig. 12.3 ). This type of synaptic transmission is sometimes referred to as wiring transmission because direct connection of presynaptic and postsynaptic cells can be viewed as “hard wiring.” Transmission at electrical synapses ( Chapter 12 ) also is a type of wiring transmission.

A second group of conventional neurotransmitters comprises biogenic monoamines : serotonin (5-hydroxytryptamine or 5-HT), histamine , and the catecholamines ^a

a Catecholamines all contain a catechol (1,2-dihydroxybenzene) moiety.

(dopamine, norepinephrine [NE], and epinephrine [Epi], which are sequentially derived from tyrosine) ( Fig. 13.1 ). These transmitters are usually stored in small dense-core (electron-opaque) SVs, which are approximately 40 to 70 nm in diameter. Biogenic amines can be released at classical synapses where the closely apposed postsynaptic cells have specialized postsynaptic densities. Biogenic amine transmitters are also released at en passant synapses where the presynaptic “terminals” are simply SV-containing varicosities (enlargements) that occur along axons as they pass postsynaptic cells. At en passant synapses, the postsynaptic cells often exhibit typical postsynaptic density regions to which the transmitter receptors are confined. In many instances, however, the transmitter receptors are more diffusely distributed on the postsynaptic cell surface. A good example of the latter is the synapse between a sympathetic neuron and a vascular smooth muscle cell.

At en passant synapses, the released transmitter may activate several postsynaptic cells. This type of neurotransmission is sometimes referred to as volume transmission , because the transmitter activates all postsynaptic cells that express the appropriate receptor within the volume of tissue in which the transmitter concentration is sufficiently high. In contrast to the situation at classical synapses, neurotransmitters must diffuse across greater distances to reach postsynaptic cells; hence the synaptic delay is correspondingly longer for volume transmission.

The third group of conventional transmitters, the neuropeptides, range in length from 3 to approximately 100 amino acid residues, although most have fewer than 30 residues. These peptide neurotransmitters are often stored in large dense-core vesicles (∼100–150 nm diameter) in which monoamines are frequently cosequestered. These SVs are often concentrated in, and release their contents from, boutons at classical synapses. Neuropeptide transmitter release is distinctive in requiring bursts of presynaptic APs, whereas most other conventional transmitters are usually released by a single AP. Ca ²⁺ -evoked release from large dense-core vesicles may also occur at various places along the presynaptic axon where no apparent synaptic structures are present. This type of transmitter release therefore more closely resembles the secretion of most hormones by endocrine cells, which do not have specialized active zones where the secretory vesicles cluster before release. Indeed, the storage and secretion of peptide hormones in the magnocellular neurons (large cell neurons) of the hypothalamus ^b

Roger Guillemin and Andrew Schally shared (with Rosalyn Yalow) the 1977 Nobel Prize in Medicine or Physiology for their discoveries of peptide “releasing” hormones, which are stored in and released by hypothalamic neurons. In 1931, Ulf von Euler first discovered a peptide neurotransmitter, substance P. von Euler and Julius Axelrod did pioneering research on norepinephrine as a neurotransmitter, for which they shared the 1970 Nobel Prize in Medicine or Physiology with Bernard Katz ( Chapter 12 ).

were originally believed to be a peculiarity of these neuroendocrine cells. We now recognize that this phenomenon is much more widespread in the nervous system.

Unconventional neurotransmitters are not stored in synaptic vesicles

In contrast to the aforementioned hydrophilic neurotransmitters, a few neurotransmitters are hydrophobic gases or lipids. These lipid-soluble transmitters cannot be stored in SVs, which have lipid bilayer membranes. The examples discussed here are the gaseous neurotransmitters , nitric oxide (NO) and carbon monoxide (CO), and several lipids known as endocannabinoids , such as 2-arachidonoylglycerol (2-AG) ( Fig. 13.1 ). The endogenous cannabinoids are so named because many of their effects are mimicked by the major psychoactive agent in marijuana, Δ ⁹ -tetrahydrocannabinol, an exogenous cannabinoid that is derived from the hemp plant, Cannabis sativa. Even though the unconventional transmitters are not stored in SVs, they fit the definition of synaptic neurotransmitters given earlier. They are generated in and released by a neuron, and they act on an adjacent neuron or effector cell or back on the neuron that released the molecule.

Ionotropic receptors are ligand-gated ion channels for which the ligands are neurotransmitters

Ionotropic receptors are ligand-gated channels ( Chapter 8 ) comprising four or five subunits whose arrangement defines a central, gated pore. Binding of the transmitter to the ligand-binding site on the ionotropic receptor induces a conformational change that opens the channel. Ionotropic receptors generally mediate fast synaptic responses because the neurotransmitters directly gate ion channels. These receptors are important targets for pharmacotherapy.

Metabotropic receptors are not ion channels but are proteins with seven transmembrane helices

Binding of a neurotransmitter activates a metabotropic receptor, which in turn activates a G-protein . These receptors are therefore members of the G-protein–coupled receptor (GPCR) family. ^c

Alfred G. Gilman and Martin Rodbell were awarded the 1994 Nobel Prize for Physiology or Medicine for discovering G-proteins and characterizing their role in cell signaling.

This is the largest family of membrane proteins in the human genome; its members influence almost all biological responses and are the targets of approximately 60% of clinically useful drugs. Most metabotropic receptors function as homodimers or heterodimers (or even oligomers). Binding of a neurotransmitter molecule to one monomer is usually sufficient to activate the dimer fully, and binding of a transmitter molecule to the second monomer may even reduce the activity of the dimer.

A G-protein is a heterotrimeric protein complex comprising α-, β-, and γ-subunits; the α-subunit has a binding site for guanine nucleotide . A wide range of different α-, β-, and γ-subunits is expressed, giving rise to many possible αβγ trimer combinations that can couple diverse receptors to downstream effectors ( Fig. 13.2 ). In the quiescent state, a guanosine diphosphate (GDP) is bound to the α-subunit of a G-protein (α _GDP βγ). Some α _GDP βγ complexes are bound to appropriate GPCR dimers, thus forming inactive pentameric complexes. On activation by a neurotransmitter molecule, the metabotropic receptor catalyzes the exchange of guanosine triphosphate (GTP) for GDP to generate α _GTP βγ. This enables the α _GTP βγ complexes to dissociate into α _GTP and βγ moieties, each of which can interact with downstream effectors such as phospholipase C, protein kinase C (PKC), or adenylyl cyclase, to initiate a cellular response ( Fig. 13.2 ). The signal is terminated when the α-subunit hydrolyzes the bound GTP, causing the reversion to α _GDP , which then can recombine with βγ to regenerate the inactive α _GDP βγ trimer. Because G-protein signaling depends on multiple enzymatic processes, synaptic responses mediated by metabotropic receptors are necessarily slower than those mediated by ionotropic receptors.

Metabotropic receptor–coupled G-proteins are molecular switches that modulate postsynaptic activity in a variety of ways. For example, the α-subunit of G-proteins activates enzymes that generate second messengers such as inositol trisphosphate (IP ₃ ), diacylglycerol (DAG) ( Fig. 13.2 ), and the cyclic mononucleotides , cAMP and cGMP. The IP ₃ releases Ca ²⁺ from the sarcoplasmic reticulum/endoplasmic reticulum (S/ER), and DAG and the cyclic nucleotides can gate ion channels or stimulate protein kinases to phosphorylate other proteins. The βγ subunits can directly modulate Ca ²⁺ channels and K ⁺ channels (e.g., inward rectifier, Kir, channels; Box 8.5 ). Phosphorylation at multiple sites on the cytoplasmic loop of the GPCR regulates its activity. This and the variety of transmitter receptors and their subtypes, and the variety of G-protein subunit isoforms, result in a rich tapestry of responses.

GPCR activation ceases when the neurotransmitter dissociates from the receptor or when the receptor is desensitized

When the transmitter molecule dissociates from the GPCR dimer, the receptor reverts to its inactive conformation and can no longer catalyze GTP-GDP exchange until it is reactivated by another transmitter molecule. Activated GPCRs can be phosphorylated by G protein–coupled receptor kinases (GRKs) , members of the serine/threonine kinase family. This permits the binding of β-arrestin (a scaffold protein , an organizer of signaling molecules) to the GPCR and thereby sterically hinders coupling to G-protein complexes. This process is a form of desensitization . The GPCR-β–arrestin complexes can be retrieved from the plasma membrane to enable recycling of the GPCRs. The GPCR-β–arrestin complexes also can bind and activate various kinases such as the extracellular signal–regulated kinase (ERK) , whose downstream actions are G-protein-independent.

Rather than providing a comprehensive description of all aspects of neurotransmitter action in the brain, we will concentrate on a few representative examples in which the underlying mechanisms have been elucidated. Descriptions of more complex behaviors, in which the mechanisms are less well understood, are left to specialized neuroscience texts.