[3H]8‐Hydroxy‐2‐(Di‐n‐Propylamino)Tetralin Binding to Pre‐ and Postsynaptic 5‐Hydroxytryptamine Sites in Various Regions of the Rat Brain

Abstract

The specific binding of [3 H]8‐hydroxy‐2‐(di‐n‐propylamino)tetralin ([3 H]8‐OH‐DPAT) to 5‐hydroxytryptamine (5‐HT)‐related sites was investigated in several regions of the rat brain. Marked differences were observed in the characteristics of binding to membranes from hippocampus, striatum, and cerebral cortex. Hippocampal sites exhibited the highest affinity (KD > 2 n M) followed by the cerebral cortex (KD > 6 n M) and the striatum (KD > 10 n M). Ascorbic acid inhibited specific [3 H]8‐OH‐DPAT binding in all three regions but millimolar concentrations of Ca 2+, Mg 2+, and Mn 2+ enhanced specific binding to hippocampal membranes, whereas only Mn 2+ increased it in the cerebral cortex and all three cations inhibited specific binding to striatal membranes. Guanine nucleotides (0.1 m M GDP, GTP) inhibited binding to hippocampal and cortical membranes only. As intracerebral 5,7‐dihydroxytryptamine markedly decreased [3 H]8‐OH‐DPAT binding sites in the striatum, but not in the hippocampus, the striatal sites appear to be on serotoninergic afferent fibers. In contrast, in the hippocampus the sites appear to be on postsynaptic 5‐HT target cells, as local injection of kainic acid decreased their density. Both types of sites appear to be present in the cerebral cortex. The postsynaptic hippocampal [3 H]8‐OH‐DPAT binding sites are probably identical to the 5‐HT 1A, subsites, but the relationship between the presynaptic binding sites and the presynaptic autoreceptors controlling 5‐HT release deserves further investigation.