GABA-gated cation channels

The 5-HT3 receptor is not coupled to G proteins. It directly activates a 5-HT-gated cation channel, which leads to depolarization of a variety of cells. As a result, there is an increase in the release of DA, NA, GABA, ACh, and 5-HT at central sites (Stanford, 2001). The 5-HT3 receptor is present in cortical and subcortical structures (Table 9.7). 

All cell membranes contain transmembrane proteins that form ion channels. These ion channels are usually selectively permeable to particular ions. Some channels, such as GABA-gated ion channels, are permeable to Cl ions and are inhibitory in nature because they make the inside of the nerve or muscle cells more negative as the Cl ions enter. Some ion channels are permeable to the cations Na and K, and an example of this type is the nicotinic acetylcholine-gated channel. Nicotinic channels have an excitatory effect when they open because Na ions enter and K ions leave through these channels. The cell becomes more positive inside and depolarizes. If the cell is a muscle cell, calcium accumulates in the cytoplasm and it contracts. We have found that all over the surface of Ascaris muscle there are GABA receptors (Martin, 1980) as well as nicotinic acetylcholine channels (Martin, 1982 Robertson and Martin, 1993). 

Ionotropic receptors (bottom left) are ligand-gated ion channels. When they open as a result of the transmitter s influence, ions flow in due to the membrane potential (see p. 126). If the inflowing ions are cations (Na"", C, Ca ""), depolarization of the membrane occurs and an action potential is triggered on the surface of the postsynaptic cell. This is the way in which stimulatory transmitters work (e.g., acetylcholine and glutamate). By contrast, if anions flow in (mainly Cl ), the result is hyperpolarization of the postsynaptic membrane, which makes the production of a postsynaptic action potential more dif cult. The action of inhibitory transmitters such as glycine and GABA is based on this effect. 

Ionotropic receptors are ligand-gated ion channels (left half of the table). The receptors for stimulatory transmitters (indicated in the table by a ) mediate the inflow of cations (mainly Na""). When these open after binding of the transmitter, local depolarization of the postsynaptic membrane occurs. By contrast, inhibitory neurotransmitters (GABA and glycine) allow cr to flow in. This increases the membrane s negative resting potential and hinders the action of stimulatory transmitters hyperpolarization, 0). 

These characteristics are shared with subunits that form other ion channels/receptors and thus define a receptor superfamily, usually referred to as the nicotinic family. All members in this superfamily function as either cation- or anion-selective channels, thereby mediating fast excitatory or inhibitory synaptic transmission. In mammalian cells, the cation-selective members include nicotinic and 5HTg receptors, whereas the anion-selective members include GABA, GAB Ac, and glycine receptors. Anion-selective channels in this family are also found in invertebrates these channels are gated by glutamate, 5-HT, histidine, and acetylcholine (1). 

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