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J Pharmacol Exp Ther. 1992 Feb;260(2):614-26.

Characterization of 5-hydroxytryptamine1B receptors in rat spinal cord via [125I]iodocyanopindolol binding and inhibition of [3H]-5-hydroxytryptamine release.

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Department of Anesthesiology, University of Texas Health Science Center, San Antonio.


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 30 microM isoproterenol. The Kd 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-HT1A, 5-HT1C, 5-HT2, 5-HT3 or alpha-2 receptor sites. Similar to the pharmacological characteristics of the 5HT1B 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 greater than 5-methoxytryptamine greater than 5-HT. Compounds with activity at 5-HT1A, 5-HT1C, 5-HT2 or 5-HT3 sites were inactive. A correlation analysis of selective 5-HT1B compounds comparing the pKD 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-HT1B receptor, and affinity of compounds for this site predicts action at the 5-HT1B autoreceptor.(ABSTRACT TRUNCATED AT 400 WORDS).

[Indexed for MEDLINE]

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