Protein Coupled Receptors

Most receptors in the CNS do not have intrinsic ionic conductance channels within their structure but instead regulate cellular activity by the generation of various "second messengers." Receptors of this class do not generally interact directly with the various second-messenger-generating enzymes but instead transmit information to the appropriate "effector" by the activation of interposed coupling proteins. These are the G protein-coupled receptor families. The G protein-coupled receptors (GPCRs, which constitute more than 80% of all known receptors in the body, and number about 300) all span the plasma membrane seven times (see Figure 1-1). GPCRs have been the focus of extensive research in psychiatry in recent years (Catapano and Manji 2007). The amino terminus is on the outside of the cell and plays a critical role in recognition of the ligand; the carboxy terminus and third intracellular loop are inside the cell and regulate not only coupling to different G proteins but also "cross talk" between receptors and desensitization (see Figure 1-1).

G proteins are so named because of their ability to bind the guanine nucleotides guanosine triphosphate (GTP) and guanosine diphosphate (GDP). Receptors coupled to G proteins include those for catecholamines, serotonin, ACh, various peptides, and even sensory signals such as light and odorants (Table 1-1). As we discuss later in the chapter, multiple subtypes of G proteins are known to exist, and they play critical roles in amplifying and integrating signals.

TABLE 1-1. Key features of G protein subunits

G protein class

Members

Effector(s)/Functions

Examples of receptors

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