Ionotropic Receptors Ligand-Gated Ion Channels

Several receptors that recognize the excitatory amino acid glutamate (e.g., the NMDA, kainate, and quisqualate classes) represent another group of ligand-gated ion channels. The NMDA receptor has been the focus of intense interest in recent years. This receptor appears to be activated primarily by membrane depolarizations that occur during periods of heightened neuronal activity. The activated NMDA receptor channel allows both Na+ and Ca2+ to flow into the cell. The detrimental 

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Figure 19.2. Schematic representation of some receptor superfamilies. This figure represents the common mechanisms of the four major receptor families discussed in Section 19.2. Example 1 is of the G protein-coupled receptor. Example 2 represents the ionotropic receptor (ligand-gated ion channel) superfamily, on which multiple binding sites are known to exist (see Figure 19.1). Example 3 represents the steroid receptor superfamily. Example 4 depicts the enzyme-linked receptor superfamily.

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