Analgesic effect of intracerebroventricular injection of GABA receptor agents and the role of opioid system

In the present study, the effect of GABA (γ-aminobutyric acid) receptor agonists and antagonists on morphine-induced antinociception was investigated in formalin test 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 g/rat) or baclofen (0.25, 0.5 and 1 g/rat) induced a dose-dependent antinociception in both phases of formalin test. The responses induced by muscimol or baclofen in both phases reduced by bicuculline or CGP35348 respectively. Morphine in combination with different doses of muscimol or baclofen tends to elicit higher response. The opioid receptor antagonist reduced the response induced by both GABA receptor agonists. It can be concluded that stimulation of GABAA and GABAB receptors are responsible for antinociception in formalin test. However, at least part of the antinociception is due to the opioid receptor mechanism.