The functional role of the nucleus accumbens in the control of the substantia nigra: electrophysiological investigations in intact and striatum-globus pallidus lesioned rats.

The effects of electrical stimulation of the nucleus accumbens on the activity of identified substantia nigra neurons were studied in intact and lesioned rats. The latter had both the caudate-putamen complex and globus pallidus destroyed by electrolytic lesions. In intact rats a total of 42 of 107 neurons (39.2%) responded to stimulation of the nucleus accumbens. Of the 107 neurons 32 (29.8%) were inhibited and 10 (9.4%) were excited. Pure short inhibitions, long latency inhibitions and excitations followed by inhibition were found in both parts of the substantia nigra. Pure long lasting inhibitions were determined on pars compacta cells only. In lesioned animals, in which the coactivation of striatal and/or cortical fibers traversing the accumbens region was avoided, the percentage of responsive neurons decreased to 20% (23/115). The predominant responses recorded in this situation were pure inhibitions of pars compacta cells (14/46) and long latency inhibitions of pars reticulata neurons (7/69). No pure excitation or excitation-inhibition sequence was recorded. In the two sets of experiments 5 cells were activated antidromically from the nucleus accumbens. The results provide electrophysiological evidence for an inhibitory pathway from the nucleus accumbens to the substantia nigra. The low percentage of responsive neurons, the lack of excitatory responses, the paucity of reciprocal connections and the different inhibitory effects on the two populations of nigral neurons demonstrate that the functional role of the nucleus accumbens in controlling the substantia nigra differs from that exerted by the striatum.