Received for publication August 10, 2006.
Revised October 13, 2006.
Accepted for publication December 13, 2006.

Native rat hippocampal 5-HT_1A receptors show constitutive activity

Abstract

Previous studies have shown that human 5-HT_1A receptors stably expressed in transfected cell lines show constitutive G-protein activity, as revealed by the inhibitory effect of inverse agonists, such as spiperone, on basal [³⁵S]GTPS binding. In the present study, we evaluated the constitutive activity of native rat 5-HT_1A receptors in hippocampal membranes. Using anti-Go-antibody capture coupled to scintillation proximity assay under low sodium (30 mM) conditions, we observed high basal [³⁵S]GTPS binding to G_o subunits (defined as 100%). In these conditions, 5-HT and the prototypic selective 5-HT_1A agonist (+)8-OH-DPAT, both stimulated [³⁵S]GTPS binding to G_o to a similar extent, raising binding to approximately 130% of basal with pEC₅₀ values of 7.91 and 7.87, respectively. The 5-HT_1A-selective neutral antagonist WAY100,635 could block these effects in a competitive manner with pKb values (5-HT: 9.57; (+)8-OH-DPAT: 9.52) that are consistent with pKi value at r5-HT_1A receptors (9.33). In this native receptor system, spiperone and methiothepin reduced basal [³⁵S]GTPS binding to G_o in a concentration-dependent manner to 90% of basal with pIC50 of 7.37 and 7.98, respectively. The inhibition of basal [³⁵S]GTPS binding induced by maximally effective concentrations of spiperone (10 µM) or methiothepin (1 µM) was antagonised by WAY100,635 in a concentration-dependent manner (pKb: 9.52 and 8.87, respectively), thus indicating that this inverse agonism was mediated by 5-HT_1A receptors. These data provide the first demonstration that native rat serotonin 5-HT_1A receptors can exhibit constitutive activity in vitro.

Key words: Serotonin, Gi family, Func. analysis receptor/ion channel mutants

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