Pharmacological Differentiation and Characterization of 5‐HT1A, 5‐HT1B, and 5‐HT1CBinding Sites in Rat Frontal Cortex

Abstract

Abstract:Drug interactions with 5‐HT 1(5‐hydroxytryptamine type 1) binding site subtypes were analyzed in rat frontal cortex. 8‐Hydroxy‐N, N‐dipropyl‐2‐aminotetralin (8‐OH‐DPAT) displays high affinity (Ki 3.3 ± 1 n M) for 29 ± 3% of total [3 H]5‐HT binding in rat frontal cortex and low affinity (K, 9,300 ± 1,000) for 71 ± 4% of the remaining 5‐HT 1 sites. Therefore, non‐5‐HT 1A binding in rat frontal cortex was defined as specific [3 H]5‐HT binding observed in the presence of 100 n M 8‐OH‐DPAT. 5‐Methoxy 3‐(1,2,3,6‐tetrahydro‐4‐pyridinyl) 1 H indole (RU 24969), 1–(m‐trifluoromethylphenyl)piperazine (TFMPP), mianserin, and methysergide produce shallow competition curves of [3 H]5‐HT binding from non‐5‐HT 1A sites. Addition of 10−‐3M GTP does not increase the apparent Hill slopes of these competition curves. Computer‐assisted iterative curve fitting suggests that these drugs can discriminate two distinct subpopulations of non‐5‐HT 1A binding sites, each representing ∼35% of the total [3 H]5‐HT binding in the rat frontal cortex. All three 5‐HT, binding site subtypes display nanomolar affinity for 5‐HT and 5‐methoxytryptamine. A homogeneous population of 5‐HT 1A sites can be directly labeled using [3 H]8‐OH‐DPAT. These sites display nanomolar affinity for 8‐OH‐DPAT, WB 4101, RU 24969, 2‐{4‐(4‐(2‐pyrimidinyl)‐1‐piperazinyl]butyl}‐1,2‐benzisothiazol‐3‐(2 H)one‐1,1‐dioxidehydrochloride (TVX Q 7821), 5‐methoxydimethyltryptamine, and N‐lysergic acid diethylamide. The potencies of RU 24969, TFMPP, and quipazine for [3 H]5‐HT binding are increased by addition of 100 n M 8‐OH‐DPAT and 3,000 n M mianserin to the [3 H]5‐HT binding assay. Moreover, the drugs have apparent Hill slopes near 1 under these conditions. This subpopulation of total [3 H]5‐HT binding is designated 5‐HT 1B. By contrast, methysergide and mianserin become more potent inhibitors of residual [3 H]5‐HT binding to non‐5‐HT 1A sites in the presence of 100 n M 8‐OH‐DPAT and 10 n M RU 24969. The drug competition curves under these conditions have apparent Hill slopes of near unity and these sites are designated 5‐HT 1C. Drug competition studies using a series of 24 agents reveals that each 5‐HT, subtype site has a unique pharmacological profile. These results suggest that radioligand studies can be used to differentiate three distinct subpopulations of 5‐HT, binding sites labeled by [3 H]5‐HT in rat frontal cortex.