Opioid and GABA modulation of accumbens-evoked ventral pallidal activity

The principle output of the nucleus accumbens innervates the ventral pallidum and rostral substantia innominata. GABA and opioid peptides are 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 substantia innominata neurons to electrical stimulation of the nucleus accumbens was examined concurrently with the ability of microiontophoretically applied morphine (an opioid agonist), naloxone (an opioid antagonist) and bicuculline (a GABA antagonist) to modulate evoked responses. Accumbens stimulation altered the firing rate in 60% of the 132 neurons tested. Fifty-two percent of responding neurons exhibited simple excitations or inhibitions in response to accumbens stimulation, while 48% exhibited complex response sequences with two or more evoked components. Predominant responses consisted of a short latency (< 10 ms) and short duration (10 ms) excitation (51% of responding neurons) and an inhibition with a variable, onset latency and, duration (52% of responding neurons). Evoked responses often occurred within limited areas within the ventral pallidum suggesting that activation of descending afferents can influence discrete targets within the region. A large majority (> 80%) of neurons evoked by accumbens stimulation also exhibited a current-dependent and naloxone-sensitive increase in spontaneous firing to microiontophoretically applied morphine. Morphine shortened the duration of the accumbens-evoked, short latency excitation and attenuated the magnitude of the long-latency inhibition. Evoked responses in the presence of morphine were opposite to those observed with naloxone, but similar to bicuculline. Thus, opioid receptor activation may be functionally antagonistic to GABAergic neurotransmission in the ventral pallidum. The prominence of accumbens-evoked and morphine-sensitive neurons within the ventral pallidum corroborates the density of accumbens and opioid input to this brain region, and demonstrates that opioids serve as an important influence on neuronal activity and information processing in the ventral-striatopallidal pathway.