Neuropharmacology of the nucleus accumbens: iontophoretic applications of morphine and nicotine have contrasting effects on single-unit responses evoked by ventral pallidal and fimbria stimulation.

Extracellular recordings within the nucleus accumbens (NAS) of halothane anesthetized rats have revealed that iontophoretically applied morphine and nicotine have contrasting effects on neuronal responses evoked by fimbria or VP stimulation. Iontophoretically applied morphine inhibited NAS single-unit responses evoked by VP stimulation but did not affect unit responses evoked by fimbria stimulation. In contrast, iontophoretically applied nicotine had no effect on NAS single-unit responses evoked by VP stimulation but inhibited single-unit responses evoked by fimbria stimulation. Spontaneously active NAS units were inhibited by iontophoretically applied morphine but were unaffected by nicotine. In addition, experiments were conducted to determine whether NAS unit responses to electrical stimulation of the VP were likely to involve cell body as opposed to axonal activations. Selective cell body stimulation by glutamate micro-infusions into the VP region excited spontaneously active VP single-units. Concurrently recorded NAS unit responses to electrical stimulation of the VP were also excited. These results are consistent with the idea that NAS evoked responses to VP electrical stimulation involve somal activation. Generally, these results suggest a specific neuropharmacological organization of the NAS. Analysis of the effects of morphine and nicotine on other NAS circuits will establish a systems level understanding of NAS responses to reinforcers.