Locomotor stepping elicited by electrical stimulation of the lateral hypothalamus requires an ipsilateral descending...

The effects of unilateral midbrain lesions on stepping produced by ipsilateral medial forebrain bundle (mfb) stimulation was determined in acute experiments. Rats were anesthetized with nembutal, mounted in a stereotaxic apparatus and suspended over a wheel which rotated when the rat stepped. Stimulation consisted of 10-sec trains of 50 Hz cathodal pulses (0.5 msec) at currents up to 200 microA delivered through monopolar electrodes. The stimulation sites were in the mfb at the level of the subthalamic nucleus. One side of the midbrain was randomly selected to receive a radio-frequency lesion. Certain midbrain lesions (N = 11) abolished or severely reduced stepping elicited from ipsilateral mfb stimulation but produced no effect on contralaterally elicited stepping. Areas of common damage in these lesions included the medial half of the medial lemniscus and the ventral tegmental area (VTA). Lesions that were without effect (N = 18) generally spared the dorsal VTA but damaged extensively either the ventrolateral VTA, medial lemniscus, substantia nigra, red nucleus, or central gray. A supplemental experiment further implicated a critical descending system since neither unilateral nor bilateral mfb lesions blocked stepping elicited by VTA stimulation. Together these results suggest that a descending path through the ipsilateral dorsal VTA mediates stepping elicited by mfb stimulation in the anesthetized rat.