Forelimb and hindlimb stepping by the anesthetized rat elicited by electrical stimulation of the diencephalon and mesencephalon

Thirty rats anesthetized with a combination of chloralose, urethane and nembutal or nembutal alone were fixed in a stereotaxic apparatus and suspended over a moving treadmill belt. Electrical stimulation (100 microA, 10-sec trains, 0.5-msec cathodal pulses, 50-Hz pulse frequency) was applied every 200-microns through 109 movable electrodes. The patterns of stepping elicited ranged from well-coordinated stepping of all four limbs to spastic stepping of only one limb. In the hypothalamus, stepping-positive sites were found in and dorsal to the medial forebrain bundle, the paraventricular, dorsomedial and posterior nuclei, and the supramammillary areas. Dorsally, effective regions extended to the medial zona incerta and the fields of Forel. In the thalamus only the parafascicular nucleus and the anterior parts of the rhomboid and reuniens nuclei were positive. Positive sites were particularly dense in the ventral tegmental area. In the dorsal midbrain positive sites were most common at caudal levels dorsal and dorsolateral to the central gray. At sites in the area of the medial raphe, stepping was elicited only at the offset of the train. The distribution of positive sites resembled that found in unanesthetized rats and indicates that pathways at which electrical stimulation elicits stepping are separated into dorsal and ventral systems at the level of the midbrain.