A comparative study of the effects of morphine in the dorsal periaqueductal gray and nucleus accumbens of rats submitted to the elevated plus-maze test

We studied the effects of morphine injected either systemically or into the dorsal periaqueductal gray (DPAG) or nucleus accumbens (NA) using conventi onal and ethological analyses of behavior of rats submitted to the elevated plus-maze test with, transparent walls. Intraperitoneal morphine (0,1 mg/k g and 0.3 mg/kg) increased both standard and ethological measures, expressi ng general exploratory activity such as total arm entries, end-exploration, scanning, head-dipping, and rearing. Morphine 10 (7.6 mu g/mu l) and 30 nm ol (23 mu g/mu l) injected into nucleus accumbens produced similar effects, which were blocked by i.p. naltrexone (2.0 mg/kg), an opioid antagonist wi th good affinity for mu-opioid receptors. Morphine injected into the DPAG p roduced either anti-aversive (10 nmol) or aversive effects (30 nmol), which respectively reduced and increased entries and time spent in the open arms and behaviors associated with risk assessment (peeping out, stretched atte nd postures, and Rat hack approach). The proaversive effects were inhibited by i.p. norbinaltorphimine (2.0 mg/kg), a selective inhibitor for K-opioid receptors. These findings support the contention that at least some of the motivational effects of morphine may be due to activation of opioid mechan isms in nucleus accumbens, and DPAG has neural substrates for antiaversive and aversive effects of morphine. Moreover, on the basis of previous and pr esent data obtained in this laboratory, it is suggested that stimulation of mu-opioid receptors inhibits and stimulation of K-receptors activates he n eural substrate of aversion in the DPAG. On the other hand, the increase in exploratory behavior due to interaction of morphine with mu-opioid recepto rs in the nucleus accumbens may be due to the stimulation of the interface between neural substrates of motivation and motor output in this structure.