Molecular pharmacology of 5-HT_1D recognition sites: Radioligand binding studies in human, pig and calf brain membranes

1) The binding characteristics of [³H]5-HT (5-hydroxytryptamine, serotonin) were investigated in membrane preparations of several regions from calf, pig and human brain in the presence of 100 nmol/l 8-OH-DPAT (8-hydroxy-2[di-n-dipropylamino]tetralin) and 100 nmol/l mesulergine in order to mask 5-HT_1A and 5-HT_1C sites. 2) [³H]5-HT bound rapidly, reversibly and stereo-selectively to a population of high affinity recognition sites in membranes from pig caudate, calf caudate and human cortex, caudate and substantia nigra. 3) Saturation experiments carried out with [³H]5-HT in the presence of 100 nmol/l 8-OH-DPAT and 100 nmol/l mesulergine revealed that non-5-HT_1A non-5-HT_1C sites represented from 50 to more than 90% of the total 5-HT₁ sites (determined with [³H]5-HT in the absence of 8-OHDPAT and mesulergine), depending on the tissue source. 4) The pharmacological profile of these sites was characterized in competition experiments performed with a variety of ligands in membranes of calf, pig and human caudate membranes. Under these conditions, [³H]5-HT labelled a population of "5-HT₁-like" sites which display nanomolar affinity for tryptamines (5-carboxamidotryptamine > 5-HT >- 5-methoxytryptamine > tryptamine) and some ergolines (metergoline > methysergide). In contrast, these sites showed low affinity for drugs with high affinity and/or selectivity for 5-HT_1A (8-OH-DPAT, buspirone), 5-HT_1B (21-009, RU 24969), 5-HT_1C (mesulergine, mianserin) and 5-HT₂ sites (ketanserin, cinanserin). 5) Non-5-HT_1A non-5-HT_1C sites from calf, pig and human caudate membranes displayed a closely similar affinity profile as illustrated by the statistically very significant correlation parameters obtained when they were compared to one another. The pharmacological profile of these sites is different from that of 5-HT_1A, 5-HT_1B, 5-HT_1C, 5-HT₂ and 5-HT₃ sites but is consistent with the pharmacology of a "5-HT₁-like" receptor. It is very similar to that of the 5-HT_1D site recently described in bovine brain by Heuring and Peroutka (1987). 6) The present findings demonstrate the presence and the pharmacological similarity of 5-HT_1D sites in pig, calf and human brain.