Effect of intracerebroventricular injection of GABA receptor agents on morphine-induced antinociception in the formalin test.
ABSTRACT In the present study, the effects of gamma-aminobutyric acid (GABA) receptor agonists and antagonists on antinociception induced by morphine in the formalin test were investigated in rats. Intraperitoneal (i.p.) injection of different doses of morphine (1, 3, 6 and 9 mg/kg) and intracerebroventricular (i.c.v.) injection of different doses of muscimol (0.5, 1 and 2 microg per rat) or baclofen (0.25, 0.5 and 1 microg per rat) induced a dose-related antinociception in the both first and second phases of the formalin test. The responses induced by muscimol or baclofen in both phases were reduced by bicuculline or CGP35348 [p-(3-aminopropyl)-p-diethoxymethyl-phosphinic acid], respectively. Bicuculline alone has produced antinociception in the second phase and CGP35348 alone has had antinociception in both phases of the formalin test. Morphine in combination with different doses of muscimol or baclofen did not elicit potentiation. The opioid receptor antagonist naloxone reduced the response induced by muscimol in the second phase and baclofen in both phases of the formalin test. It may be concluded that central GABA(A) and GABA(B) receptor stimulation induces antinociception in the formalin test. However, the antinociception induced by GABA receptor agonists may be mediated partly through supraspinal opioid receptor mechanisms and, for the GABA(B) receptor agonist, through spinal and supraspinal opioid receptor mechanisms.
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ABSTRACT: This paper is the twenty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2002 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).Peptides 09/2003; 24(8):1241-302. · 2.43 Impact Factor