Comparison of hippocampal G protein activation by 5-HT 1A receptor agonists and the atypical antipsychotics clozapine and S16924

Abstract

This study employed [(35)S]guanosine 5'- O-(3-thiotriphosphate) ([(35)S]GTPgammaS) binding to compare the actions of antipsychotic agents known to stimulate cloned, human 5-HT(1A) receptors with those of reference agonists at postsynaptic 5-HT(1A) receptors. In rat hippocampal membranes, the following order of efficacy was observed (maximum efficacy, E(max), values relative to 5-HT=100): (+)8-OH-DPAT (85), flesinoxan (62), eltoprazine (60), S14506 (59), S16924 (48), buspirone (41), S15535 (22), clozapine (22), ziprasidone (21), pindolol (7), p-MPPI (0), WAY100,635 (0), spiperone (0). Despite differences in species and tissue source, the efficacy and potency (pEC(50)) of agonists (with the exception of clozapine) correlated well with those determined previously at human 5-HT(1A) receptors expressed in Chinese hamster ovary (CHO) cells. In contrast, clozapine was more potent at hippocampal membranes. The selective antagonists p-MPPI and WAY100,635 abolished stimulation of binding by (+)8-OH-DPAT, clozapine and S16924 (p-MPPI), indicating that these actions were mediated specifically by 5-HT(1A) receptors. Clozapine and S16924 also attenuated 5-HT- and (+)8-OH-DPAT-stimulated [(35)S]GTPgammaS binding, consistent with partial agonist properties. In [(35)S]GTPgammaS autoradiographic studies, 5-HT-induced stimulation, mediated through 5-HT(1A) receptors, was more potent in the septum (pEC(50) approximately 6.5) than in the dentate gyrus of the hippocampus (pEC(50) approximately 5) suggesting potential differences in coupling efficiency or G protein expression. Though clozapine (30 and 100 microM) did not enhance [(35)S]GTPgammaS labelling in any structure, S16924 (10 micro M) modestly increased [(35)S]GTPgammaS labelling in the dentate gyrus. On the other hand, both these antipsychotic agents attenuated 5-HT (10 microM)-stimulated [(35)S]GTPgammaS binding in the dentate gyrus and septum. In conclusion, clozapine, S16924 and ziprasidone act as partial agonists for G protein activation at postsynaptic 5-HT(1A) receptors in the hippocampus. These data support a role of postsynaptic 5-HT(1A) receptors in the functional profiles of certain antipsychotic agents.

Detailed Pharmacological Characterization of 5-HT1A-Receptor-Mediated [35S]GTPγS Binding in Rat Hippocampal Membranes

“…In particular, receptor / G protein ratio (36) and NaCl concentrations (15, but see also 34) were critical to facilitate the efficacy of an inverse agonist to be captured. In previous reports using native membranes, we are unaware of any example of inverse agonism at 5-HT 1A receptors (19,20,22). In the present study, no certain evidence was obtained indicative of inverse agonism, either.…”

Native Rat Hippocampal 5-HT1AReceptors Show Constitutive Activity

“…35 S]GTP␥S binding assay failed to detect inverse agonist properties of spiperone and methiothepin in rat brain tissue (Alper and Nelson, 1998;Newman-Tancredi et al, 2003b;Odagaki and Toyoshima, 2005a,b). Alper and Nelson (1998) did report that methiothepin, but not spiperone, diminished G-protein activation at high concentrations (10 M), but this effect was not reversed by the selective neutral antagonist WAY100,635, indicating that it was not related to 5-HT 1A receptors.…”

“…Constitutive activity of 5-HT 1A receptors could also be demonstrated when the receptor was coupled to G␣ i1 in human embryonic kidney cells using a fusion protein paradigm (Milligan et al, 2001). In light of these observations, several groups have evaluated constitutive activity of 5-HT 1A receptors in brain tissue environment (Odagaki and Fuxe, 1995;Alper and Nelson, 1998;Newman-Tancredi et al, 2003b;Odagaki and Toyoshima, 2005a,b). However, constitutive activity of native 5-HT 1A receptors has not been described to date.…”

NLX-112, a novel 5-HT 1A receptor agonist for the treatment of l -DOPA-induced dyskinesia: Behavioral and neurochemical profile in rat

“…Part of the compounds' limitations may be due to poor receptor selectivity, such as blockade of dopamine D 2 receptors or other targets associated with a worsening of motor control (Bruins Slot et al, 2006;Newman-Tancredi et al, 2005;Pauwels et al, 2001;Peroutka, 1985). Alternatively, the limited efficacy of these drugs for LID reduction may be attributed to less-than-optimal agonist efficacy: buspirone, tandospirone and eltoprazine possess only modest partial agonist efficacy at 5-HT 1A receptors (Hamik et al, 1990;Newman-Tancredi et al, 2003).…”