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 pallidum, through its projections to the mediodorsal nucleus of the thalamus, has been considered as the main output structure of the prefrontal-basal ganglia circuits. However, as shown recently, the ventral pallidum 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 position of the ventral pallidum in the prefrontal-basal ganglia circuit originating from the prelimbic and medial orbital areas. Following injections of biocytin (an anterograde tracer) into the region of the core of the nucleus accumbens receiving excitatory inputs from the prelimbic and medial orbital areas, axonal terminal fields were observed in a delineated dorsal region of the ventral pallidum. When the biocytin injections were made into this ventral pallidal region, anterogradely labelled fibres were observed in both the dorsomedial substantia nigra pars reticulata and the medial subthalamic nucleus, but not in the mediodorsal nucleus of the thalamus. Confirming these anatomical observations, electrical stimulation of the core of the nucleus accumbens induced 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 responding cells, 43% were antidromically driven from the subthalamic nucleus, 30% from the substantia nigra pars reticulata and only 6% from the mediodorsal nucleus of the thalamus. These data demonstrate that the region of the ventral pallidum involved in the prefrontal cortex-basal ganglia circuit originating from the prelimbic and medial orbital areas represents essentially a ventral subcommissural extension of the external segment of the globus pallidus since it exhibits similar extrinsic connections and functional characteristics. In conclusion, in this 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.