Effects of medial prefrontal cortex stimulation on the spontaneous activity of the ventral pallidal neurons in the rat

The neural connections from the medial prefrontal cortex to the ventral pallidum were investigated in urethane-anesthetized rats. Extracellular recordings were made from 124 spontaneously firing neurons in the ventral pallidum while the medial prefrontal cortex was electrically stimulated. The most frequent response to prefrontal cortex stimulation was inhibition of the firing rate of 72.3% of the neurons with orthodromic response (mean latency: 14.4 ± 1.6 ms). Excitatory responses were found in 27.7% of the neurons with orthodromic response (mean latency: 8.5 ± 1.4 ms). Frequency histograms of latencies were unimodal in both types of responses. Fifty-nine neurons (47.6% of the total tested) showed no change in spontaneous firing after medial prefrontal cortex stimulation. The electrophysiological results support previous anatomical findings of connections between the medial prefrontal cortex and the ventral pallidum. These projections play a predominantly inhibitory role in the spontaneous activity of ventropallidal neurons, and show topographical organization. This inhibition may modulate the motor performance of motivated behaviors.Key words: ventral pallidum, medial prefrontal cortex, electrophysiology.