REGULATION OF PREPULSE INHIBITION BY VENTRAL PALLIDAL PROJECTIONS

The acoustic startle reflex is inhibited by the presentation of a weak auditory prestimulus 30-500 ms prior to the startling stimulus. Previ ous studies have demonstrated that prepulse inhibition (PPI) of acoust ic startle is regulated by GABAergic activity in the ventral pallidum. Ventral pallidal efferents include major projections to the pedunculo pontine tegmental nucleus (PPTg), subthalamic nucleus (STN), and medio dorsal thalamus (MD). We used lesion and intracerebral infusion techni ques to determine the relevance of these projections to the ventral pa llidal regulation of PPI. Consistent with previous results, PPTg lesio ns significantly reduced PPI in all startle sessions, while MD lesions significantly reduced PPI only under certain experimental conditions. STN lesions failed to alter PPI, but they did significantly disrupt a mphetamine-induced locomotion, verifying the behavioral effectiveness of these lesions. Infusion of the GABA-A agonist muscimol into either the PPTg or the MD significantly reduced PPI. Ventral pallidal project ions to the PPTg and to the MD thus appear to regulate PPI, possibly v ia a GABAergic mechanism. Pallidal projections to the STN may regulate other behavioral processes such as locomotor activity, but they do no t appear to regulate sensorimotor gating of the acoustic startle refle x. (C) 1997 Elsevier Science Inc.