Vol. 292, Issue 2, 684-691, February 2000

5-Hydroxytryptamine_1A Receptor-Stimulated [³⁵S]GTPS Binding in Rat Brain: Absence of Regional Differences in Coupling Efficiency¹

Emanuel Meller, Hua Li, Kenneth D. Carr and Jacob M. Hiller

Millhauser Laboratories, Department of Psychiatry, New York University Medical Center, New York, New York.

In hippocampal membranes, the selective 5-hydroxytryptamine (5-HT_1A) receptor agonists 8-hydroxy-dipropylaminotetralin (8-OH-DPAT) and N,N-dipropyl-5-carboxamidotryptamine (N,N-DP-5-CT) stimulated guanosine-5'-O-(3-thio)triphosphate ([³⁵S]GTPS) binding by 130 to 140%; binding stimulated by nonselective agonists (5-HT and 5-CT) was ~30% greater. However, the selective 5-HT_1A receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl-cyclohexanecarboxamide (WAY100,635) completely abolished the increases produced by 8-OH-DPAT and N,N-DP-5-CT but only eliminated 70% of that elicited by 5-CT. The rank potency order of the tested agonists was identical with their rank order of affinity for 5-HT_1A receptors [5-CT  N,N-DP-5-CT > R-(+)-8-OH-DPAT > 5-HT > ipsapirone]. Racemic 8-OH-DPAT and the partial agonist ipsapirone exhibited lower intrinsic activity than R-(+)-8-OH-DPAT. R-(+)-8-OH-DPAT also stimulated [³⁵S]GTPS binding in cortex, but not in striatum, which lacks 5-HT_1A receptors. Partial irreversible inactivation of 5-HT_1A receptors, in vitro with phenoxybenzamine (0.3 or 1 µM) or in vivo with N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (1 mg/kg), reduced the maximal response produced by R-(+)-8-OH-DPAT but did not alter its EC₅₀. In autoradiographic sections, R-(+)-8-OH-DPAT stimulated [³⁵S]GTPS binding in 5-HT_1A receptor-rich regions (dorsal hippocampus, 123%; lateral septum, 111%; midhippocampus, 110%; dorsal raphe nucleus, 83%; medial prefrontal cortex, ~60%). The EC₅₀ of R-(+)-8-OH-DPAT did not vary significantly among brain regions (46-96 nM). Partial irreversible blockade of 5-HT_1A receptors in brain sections (phenoxybenzamine, 10 µM) reduced the maximal response without altering the EC₅₀ in both the hippocampus and dorsal raphe. Despite prior evidence that dorsal raphe somatodendritic 5-HT_1A autoreceptors exhibit high receptor/effector coupling efficiency (receptor reserve) compared with postsynaptic receptors in hippocampus, there was no evidence of a difference at the level of receptor/G protein coupling.