Locomotor stepping elicited by electrical stimulation of the hypothalamus persists after lesion of descending fibers of passage.

Locomotion initiated by electrical stimulation of the lateral hypothalamus could be due to activation of local neurons or of fibers of passage descending from locomotor regions in the basal forebrain. This study mapped hypothalamic sites for electrically elicited locomotion in six rats with electrolytic lesions of the ipsilateral basal forebrain sources of descending fibers of passage. For mapping, anesthetized rats were held in a stereotaxic apparatus supported by a sling so that stepping movements rotated a wheel. Anesthesia was maintained by periodic injections of Nembutal (7 mg/kg) supplemented by lidocaine injections. Stimulation (25 and 50 microA, 50 Hz, 0.5 msec cathodal pulses, 10 sec trains) was applied through 50-80 microns diameter pipettes filled with 2 M saline. In all cases, locomotor stepping could be elicited by stimulation in sites ipsilateral to the lesion at currents of 50 microA or less. In the one case in which 25-microA sites were not found in the lateral hypothalamus, the lesion extended caudally to within 1 mm of the stimulation sites. These findings do not exclude a locomotor role for fibers of passage but they suggest that activation of lateral hypothalamic neurons is sufficient to initiate locomotion.