A study on the effect of intra-cuneiformis 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 intra-cuneiformis nucleus microinjection of gamma-aminobutyric acid A (GABAA) receptor agonist and antagonist on antinociception induced by morphine were investigated with formalin test in rat. Intra-cuneiformis nucleus microinjection of morphine (10 µg/rat) and bicuculline (50, 100 ng/rat) induced antinociception in the both 1st and 2nd phases of formalin test. Muscimol (25, 50, 100 ngr/rat) produced antinociception only in the 2nd phase of formalin test. Morphine in combination with muscimol but no with bicuculline elicited potentiation. The responses included by muscimol were not affected by bicuculline. The opioid receptor antagonist (Naloxone) did not reduce the response induced by muscimol in the both 1st and 2nd 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.