Microstimulation mapping of the basal forebrain in the anesthetized rat: the "preoptic locomotor region"

Previous studies have indicated that the basal forebrain at the level of the preoptic area contains neurons which participate in the initiation of locomotion. This study attempted to localize those neurons by mapping sites at which 25- and 50-microA stimulation (50 Hz, 0.5 ms cathodal pulses, 10-s trains) initiated hindlimb stepping. 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 was applied through 50-70-microns diameter pipettes filled with 2 M NaCl at approximately 1600 sites in the basal forebrain, adjacent thalamus, and striatum. A circumscribed grouping of 25-microA locomotor sites, centered in the lateral preoptic area, defined the preoptic locomotor region. It extended into the ventral bed nucleus of the stria terminalis, the lateral part of the medial preoptic area, the anterior hypothalamic area, the medial and rostral parts of the ventral pallidum, medial substantia innominata, and the horizontal limb of the diagonal band. This general region is known to project to the midbrain locomotor region and the ventral tegmental area; it is proposed to initiate locomotion in service of primary motivational systems. Among the structures generally negative for locomotor sites were the dorsal and ventral striata, septal complex, bed nucleus of stria terminalis, and lateral ventral pallidum and substantia innominata. These findings indicate that low current stimulation applied to a circumscribed area centered in the lateral preoptic area produces locomotor stepping in the anesthetized rat. Whether the activated elements in this preoptic locomotor region are cells or fibers is not yet known. The degree of localization afforded by these findings indicates that the areas that are most likely to contain the mediating elements are quite limited in extent.