Effects of direct- and indirect-acting serotonin receptor agonists on the antinociceptive and discriminative stimulus effects of morphine in rhesus monkeys

Jun Xu Li, Wouter Koek, Kenner C. Rice, Charles P. France

Research output: Contribution to journalArticle

17 Scopus citations

Abstract

Serotonergic (5-HT) systems modulate pain, and drugs acting on 5-HT systems are used with opioids to treat pain. This study examined the effects of 5-HT receptor agonists on the antinociceptive and discriminative stimulus effects of morphine in monkeys. Morphine increased tail-withdrawal latency in a dose-related manner; 5-HT receptor agonists alone increased tail-withdrawal latency at 50 °C but not 55 °C water. The antinociceptive effects of morphine occurred with smaller doses when monkeys received an indirect-acting (fenfluramine) or direct acting (8-OH-DPAT, F13714, buspirone, quipazine, DOM, and 2C-T-7) agonist. The role of 5-HT receptor subtypes in these interactions was confirmed with selective 5-HT 1A (WAY100635) and 5-HT 2A (MDL100907) receptor antagonists. None of the 5-HT drugs had morphine-like discriminative stimulus effects; however, fenfluramine and 5-HT 2A receptor agonists attenuated the discriminative stimulus effects of morphine and this attenuation was prevented by MDL100907. The 5-HT 1A receptor agonists did not alter the discriminative stimulus effects of morphine. Thus, 5-HT receptor agonists increase the potency of morphine in an assay of antinociception, even under conditions where 5-HT agonists are themselves without effect (ie, 55 °C water), without increasing (and in some cases decreasing) the potency of morphine in a drug discrimination assay. Whereas 5-HT 2A receptor agonists increase the potency of morphine for antinociception at doses that have no effect on the rate of operant responding, 5-HT 1A receptor agonists increase the potency of morphine only at doses that eliminate operant responding. These data suggest that drugs acting selectively on 5-HT receptor subtypes could help to improve the use of opioids for treating pain.

Original languageEnglish (US)
Pages (from-to)940-949
Number of pages10
JournalNeuropsychopharmacology
Volume36
Issue number5
DOIs
Publication statusPublished - Apr 1 2011

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Keywords

  • antinociception
  • drug discrimination
  • drug interaction
  • morphine
  • rhesus monkey
  • serotonin

ASJC Scopus subject areas

  • Pharmacology
  • Psychiatry and Mental health

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