Neuropharmacology of the nucleus accumbens: systemic morphine effects on single-unit responses evoked by ventral pallidum stimulation.

Extracellular recordings of neurons in the nucleus accumbens septi of anesthetized rats have previously shown systemic opiates to have mixed effects on rates of unit activity. However, these effects become more predictable as nucleus accumbens septi neurons are functionally categorized by their responses to afferent stimulation. In the present study, the effects of systemic opiates on individual nucleus accumbens septi neurons categorized by their response patterns to ventral pallidum stimulation were examined. Across all nucleus accumbens septi units tested, these experiments showed that morphine either inhibited (42%, n = 91), excited (15%, n = 32) or had no effect (43%, n = 93) on these unit responses. Further experiments were conducted in which nucleus accumbens septi neurons were categorized on the basis of their response patterns to concurrent fimbria and ventral pallidum stimulation. In these studies, if the neuron was orthodromically evoked to respond by both fimbria stimulation and ventral pallidum stimulation, the neurons' responses to ventral pallidum stimulation (but not their fimbria-evoked responses) were consistently inhibited by morphine. In contrast, nucleus accumbens septi unit responses that were orthodromically activated by ventral pallidum stimulation but unaffected by fimbria stimulation were consistently unaffected by morphine. The major observation of this work is that the effect of systemically administered morphine on individual nucleus accumbens septi neurons can be predicted by that neuron's evoked responses to stimulation of different efferent and afferent circuits. Since these studies involve systemic administration of morphine, the results do not elucidate site of action.(ABSTRACT TRUNCATED AT 250 WORDS)