Characterization of 5-hydroxytryptamine(1B) receptors in rat spinal cord via [125I]iodocyanopindolol binding and inhibition of [3H]-5- hydroxytryptamine release

I. Matsumoto, M. R. Combs, D. J. Jones

Research output: Contribution to journalArticle

30 Scopus citations

Abstract

The aim of the present study in rat spinal cord synaptosomes was to compare the pharmacological characteristics of the serotonin (5-HT)(1B) receptor defined by [125I]iodocyanopindolol([125I]ICYP) binding and the 5-HT autoreceptor defined by inhibition of [3H]-5-HT release. In Percoll gradient Fractions 3 and 4 of spinal cord synaptosomes, a single saturable binding site for [125I]ICYP with a maximum binding of 70 and 134 fmol/mg, respectively, was demonstrated in the presence of 30μM isoproterenol. The K(d) of 0.16 nM did not vary between fractions. Competition for [125I]ICYP binding by various 5-HT agonists and antagonists also indicated a single site model based on a Hill coefficient of approximately 1.0. The most potent compounds at displacing [125I]ICYP binding were RU 24969 (5-methoxy-3- [1,2,3,6-tetrahydropyridin-4-yl]-1H-indole), 5-carboxyamidotryptamine HCl, 5- methoxytryptamine, 5-HT and CGS 12066B (7-trifluoromethyl-4(4 methyl-1- pyrolo[1,2-a]-quinoxaline malate). [125I]ICYP binding was not altered by compounds with activity at 5-HT(1A), 5-HT(1C), 5-HT2, 5-HT3 or alpha-2 receptor sites. Similar to the pharmacological characteristics of the 5HT(1B) site defined by [125I]ICYP, compounds most active at inhibiting 15 mM K+- stimulated release of [3H]-5-HT were RU24969 = 5-carboxyamidotryptamine HCl = CGS 12066B > 5-methoxytryptamine > 5-HT. Compounds with activity at 5- HT(1A), 5-HT(1C), 5-HT2 or 5-HT3 sites were inactive. A correlation analysis of selective 5-HT(1B) compounds comparing the pK(D) for displacement of [125I]ICYP vs. the IC50 for inhibition of [3H]-5-HT release demonstrated the pharmacological similarity of the presynaptic inhibitory 5- HT autoreceptor and the 5-HT receptor site defined by [125I]ICYP binding in spinal cord synaptosomes (r = 0.791, P = .0193). Although [125I]ICYP binding was unaltered, alpha-2 agonists such as clonidine, norepinephrine and UK 14304 {(5-bromo-6-[2-imidazolin-2-ylamino]-quinoxaline} as well as the alpha-2 antagonists rauwolscine and yohimbine also decreased the K+- stimulated release of [3H]-5-HT and phentolamine, an alpha-2 antagonist increased release. The action of these alpha-2 compounds to alter [3H]-5-HT release suggests the presence of heteroreceptors localized on 5-HT terminals in the spinal cord. These results point out that [125I]ICYP identifies the 5-HT(1B) receptor, and affinity of compounds for this site predicts action at the 5-HT(1B) autoreceptor. However, in the presence of multiple presynaptic regulatory sites, the pharmacology of presynaptic regulatory receptors, assessed through inhibition of transmitter release, does not necessarily correlate when binding of ligands which label only a single site for regulation. Thus, limitations exist in the interpretation of pharmacological studies on the characteristics of binding vs. release for systems where both autoreceptors and heteroreceptors reside on neuronal terminals and regulate release.

Original languageEnglish (US)
Pages (from-to)614-626
Number of pages13
JournalJournal of Pharmacology and Experimental Therapeutics
Volume260
Issue number2
StatePublished - Jan 1 1992

ASJC Scopus subject areas

  • Molecular Medicine
  • Pharmacology

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