A Study on the Effect of Intracuneiformis Nucleus Microinjection of GABAA Receptor Agonist and Antagonist on Antinociceptive Effects of Morphine by Formalin Test in Rat

In the present study, the effects of intracuneiformis nucleus microinjection of gamma-aminobutyric acidA (GABAA) receptor agonist and antagonist on antinociception inducced by morphine were investigated with formalin test in rat. Intracuneiformis nucleus microinjection of morphine (۱۰µgr/rat) and Bicuculline (۵۰, ۱۰۰ ng/rat) induced antinociception in the both first and second phases of formalin test. Muscimol (۲۵,۵۰,۱۰۰ ngr/rat ) produced antinociception only in the second phase of formalin test. Morphine in combination with muscimol but not with bicuculline elicited potentiation. The responses induced by muscimol were not affected by bicuculline. The opioid receptor antagonist (naloxone) did not reduce the response induced by muscimol in the both first and second phases of formalin test. It may be concluded that central GABAA receptor stimulation induced antinociception in formalin test. However, the antinociception induced by GABAA receptor agonist may be partly mediated through nucleus cuneiformis opioid receptor mechanisms. The apparent antinociceptive effects of muscimol may be due to its leakage to rostal portion of Cnf and reduction in motion activity, so have no relation to its real antinociceptive effects.