Muscimol injections in the nucleus raphé dorsalis block the antinociceptive effect of morphine in rats: apparent lack of 5-hydroxytryptamine involvement in muscimol's effect.

The effect of morphine (3 mg kg-1 subcutaneously) on tail flick in the tail immersion test was studied in rats which had received a muscimol (100 ng) injection either in the nucleus raphé dorsalis (DR) or medianus (MR). The levels of 5-hydroxyindoleacetic acid (5-HIAA) were measured in the hippocampus and striatum of muscimol-injected animals. Muscimol injections in the DR reduced 5-HIAA concentrations in the striatum but not in hippocampus, whereas in animals which had received muscimol in the MR a selective decrease in hippocampal 5-HIAA levels was found. Muscimol injections in the DR blocked the effect of morphine while no effect was seen in animals which had received muscimol in the MR. An injection of 5,7-dihydroxytryptamine (6 micrograms in 3 microliter) in the DR did not change the effect of morphine or muscimol. These findings indicate that muscimol-sensitive neurones in the DR, which are probably not 5-hydroxytryptaminergic, are involved in the effect of morphine on tail flick in tail immersion. The muscimol-sensitive neurones involved in this effect of morphine do not seem to exist in the MR.