OPIOID AND GABA MODULATION OF ACCUMBENS-EVOKED VENTRAL PALLIDAL ACTIVITY

The principle output of the nucleus accumbens innervates the ventral p allidum and rostral substantia innominata. GABA and opioid peptides ar e among the neurotransmitter candidates for this projection. The goal of the present experiments was to delineate further the physiology and pharmacology of the accumbens projection to the ventral pallidum. The trans-synaptic responsiveness of ventral pallidal and rostral substan tia innominata neurons to electrical stimulation of the nucleus accumb ens was examined concurrently with the ability of microiontophoretical ly applied morphine (an opioid agonist), naloxone (an opioid antagonis t) and bicuculline (a GABA antagonist) to modulate evoked responses. A ccumbens stimulation altered the firing rate in 60% of the 132 neurons tested. Fifty-two percent of responding neurons exhibited simple exci tations or inhibitions in response to accumbens stimulation, while 48% exhibited complex response sequences with two or more evoked componen ts. Predominant responses consisted of a short latency (< 10 ms) and s hort duration (10 ms) excitation (51% of responding neurons) and an in hibition with a variable, onset latency and, duration (52% of respondi ng neurons). Evoked responses often occurred within limited areas with in the ventral pallidum suggesting that activation of descending affer ents can influence discrete targets within the region. A large majorit y (> 80%) of neurons evoked by accumbens stimulation also exhibited a current-dependent and naloxone-sensitive increase in spontaneous firin g to microiontophoretically applied morphine. Morphine shortened the d uration of the accumbens-evoked, short latency excitation and attenuat ed the magnitude of the long-latency inhibition. Evoked responses in t he presence of morphine were opposite to those observed with naloxone, but similar to bicuculline. Thus, opioid receptor activation may be f unctionally antagonistic to GABAergic neurotransmission in the ventral pallidum. The prominence of accumbens-evoked and morphine-sensitive n eurons within the ventral pallidum corroborates the density of accumbe ns and opioid input to this brain region, and demonstrates that opioid s serve as an important influence on neuronal activity and information processing in the ventral-striatopallidal pathway.