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*In some instances (e.g., acetylcholine, purines), agents that inhibit metabolism of the transmitter(s) have effects that are analogous to those of inhibitors of transport of other transmitters.

Receptor-effector coupling consists of ion channel mechanisms for ionotropic receptors (IR) or coupling to G proteins for GPCRs. All GPCRs modulate neuronal activity by affecting second messenger production, protein phosphorylation, and ion channel function by mechanisms described in Chapter 1. In general, Gs couples to adenylyl cyclase to activate cyclic AMP production, while coupling to Gi inhibits adenylyl cyclase; coupling to Gq activates the PLC-IP3-Ca2+ pathway; fig subunits of G proteins may modulate ion channels directly.

abbreviations: 7-OH-DPAT, 7-hydroxy-2 (di-n-propylamino) tetralin; 5-HT, 5-hydroxytryptamine (serotonin); L-AP4, L-amino-4-phosphonobutyrate; APDC, 1S, 4R-4-aminopyrrolidine2-4-dicarboxylate; AVP, arginine vasopressin; CCK, cholecystokinin; CTAP, DPhe-Cys-Tyr-DTrp-Arg-Thr-Pen-Thr-NH2; CTOP, DPhe-Cys-Tyr-DTrp-Orn-Thr-Pen-Thr-NH2; DALCE, [DAla2, Leu5, Cys6]enkephalin; DAMGO, [D-Ala2,N-Me-Phe4,Gly5-ol]- enkephalin; DDAVP, 1-desamino-8-D-arginine vasopressin; DHPG, dihydroxyphenylglycine; DPDPE, [d-Pen2, d-Pen5] enkephalin; DSBULET, Tyr-d-Ser-o-ibutyl-Gly-Phe-Leu-Thr; EPI, epinephrine; NE, norepinephrine; NK, neurokinin; NPY, neuropeptide Y; OT, oxytocin; PCP, phencyclidine; SP, substance P; SRIF, somatotropin release-inhibiting factor; THIP, 4,5,6,7-tetrahydroisoxazolo [5,4-c]-pyridone; VP, vasopressin. All other abbreviations represent experimental drugs coded by their manufacturers.

AMINO ACIDS The CNS contains high concentrations of certain amino acids, notably glutamate and gamma-aminobutyric acid (GABA). The dicarboxylic amino acids (e.g., glutamate and aspartate) produce near universal excitation, while the monocarboxylic «-amino acids (e.g., GABA, glycine, b-alanine, and taurine) produce qualitatively similar, consistent inhibitions. The availability of selective antagonists has permitted identification of selective amino acid receptors and receptor subtypes. These data, together with the development of methods for mapping the lig-ands and their receptors, demonstrate that the amino acids GABA, glycine, and glutamate are central transmitters. The structures of glycine, glutamate, GABA, and some related compounds are shown in Figure 12-4.

GABA (y-amino butyric acid)

MUSCIMOL (GABAa agonist)

BACLOFEN (GABAb agonist)

BACLOFEN (GABAb agonist)

2 - OH - SACLOFEN (GABAb antagonist)

2 - OH - SACLOFEN (GABAb antagonist)

CH3 P

CH3 P

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