Characterization of the 5-HT IB recognition site in rat brain: Binding studies with (−)[ 125I]Iodocyanopindolol

Abstract

(−)[ 125I]Iodocyanopindol ([ 125I]CYP) labels rat brain membrane sites which display high affinity for several serotonergic and β-adrenergic compounds. The binding of [ 125I]CYP to these serotonergic recognition sites was evaluated in the presence of 30 μM (−)isoprenaline in order to suppress binding to β-adrenoceptors. [ 125I]CYP binds in rat cortex membranes rapidly, reversibly and stereoselectivity to a finite number of recognition sites: B max = 180 fmol/mg, K D = 230 pM. Similar affinity values of [ 125I]CYP were obtained in membranes from rat hippocampus and striatum. Kinetic, saturation and competition experiments suggest that under these conditions [ 125I]CYP binds to a single serotonergic recognition site named 5-HT 1B. The pharmacological profile of 5-HT 1B sites is characteristic of a 5-HT 1 binding site and shows the following rank order of affinity for agonists: RU 24969, (5-methoxy-3-[1,2,3,6-tetra-hydropyridin-4-yl]1H-indole) > 5-CT, (5-carboxamidotryptamine) > 5-HT, (5-hydroxytryptamine, serotonin) > 5-OCH 3-T, (5-methoxytryptamine) ⪢ 2-CH 3-5-HT, (2-methylserotonin) > 8-OH-DPAT, (8-hydroxy-2-(di-n-propylamino)-tetralin). The rank order of affinity for antagonists is: (±)ICYP, ((±)-3-I-cyano-pindolol) > (−)21-009, (4-[3-ter-butyl-amino-2-hydroxy-propoxy]-indol-2-carbonic acid isopropyl ester) > (+)21-009 > (−)propranolol > metitepin > (−)pindolol ⪢ ketanserin > spiroperidol > mesulergine. 5-HT 1B recognition sites display low affinity for selective β 1- and β 2-adrenoceptor antagonists, e.g. atenolol, betaxolol, ICI 89-406 and ICI 118-551. The low affinity of 5-HT 1B recognition sites for some 5-HT 1A, 5-HT 1C and 5-HT 2 selective compounds (e.g. 8-OH-DPAT, mesulergine, ketanserin) suggests that 5-HT 1B recognition sites are pharmacologically different from 5-HT 1A, 5-HT 1C and 5-HT 2 recognition sites.