POSITION OF THE VENTRAL PALLIDUM IN THE RAT PREFRONTAL CORTEX BASAL GANGLIA CIRCUIT

The ventral pallidum receives major inputs from the nucleus accumbens, a striatal region related to the prefrontal cortex. The ventral palli dum, through its projections to the mediodorsal nucleus of the thalamu s, has been considered as the main output structure of the prefrontal- basal ganglia circuits. However, as shown recently, the ventral pallid um also sends efferents to the subthalamic nucleus and the substantia nigra, suggesting that it could participate in intrinsic basal ganglia circuits. The aim of the present investigation was to determine the p osition of the ventral pallidum in the prefrontal-basal ganglia circui t originating from the prelimbic and medial orbital areas. Following i njections of biocytin (an anterograde tracer) into the region of the c ore of the nucleus accumbens receiving excitatory inputs from the prel imbic and medial orbital areas. axonal terminal fields were observed i n a delineated dorsal region of the ventral pallidum. When the biocyti n injections were made into this ventral pallidal region, anterogradel y labelled fibres were observed in both the dorsomedial substantia nig ra pars reticulata and the medial subthalamic nucleus, but not in the mediodorsal nucleus of the thalamus. Confirming these anatomical obser vations, electrical stimulation of the core of the nucleus accumbens i nduced an inhibition of the spontaneous activity (D=34.9+/-13.3 ms, L= 9.2+/-3.3 ms) in 46.5% of the ventral pallidal cells. Among these resp onding cells, 43% were antidromically driven from the subthalamic nucl eus, 30% from the substantia nigra pars reticulata and only 6% from th e mediodorsal nucleus of the thalamus. These data demonstrate that the region of the ventral pallidum involved in the prefrontal cortex-basa l ganglia circuit originating from the prelimbic and medial orbital ar eas represents essentially a ventral subcommissural extension of the e xternal segment of the globus pallidus since it exhibits similar extri nsic connections and functional characteristics. In conclusion, in thi s prelimbic and medial orbital channel, the ventral pallidum cannot be considered as a major output structure but is essentially involved in intrinsic basal ganglia circuits. (C) 1997 IBRO. Published by Elsevie r Science Ltd.