The mesencephalic locomotor region. I. Activation of a medullary projection site.

Previous anatomical studies from our laboratories have demonstrated descending projections of the physiologically identified mesencephalic locomotor region (MLR) to the medioventral medulla. In the present study, this area was activated electrically and chemically to determine the nature of locomotor events elicited in the precollicular-postmammillary transected cat. Low-amplitude (less than 70 microA), high-frequency (5-60 Hz) stimulation of a specific region in the medioventral medulla elicited controlled episodes of locomotion on a treadmill. Injections of cholinergic agonists into this area also were found to elicit short episodes of locomotion. Injections of cholinergic antagonists were found to block locomotion elicited by: electrical stimulation of the same area; injections of cholinergic agonists; or electrical stimulation of the MLR. Injections of substance P into the medioventral medulla were found to induce locomotion for longer periods of time than injections of cholinergic agonists. These findings demonstrate physiologically that a primary caudal termination site of the MLR is located in the medioventral medulla, and suggest that at least a portion of descending MLR projections employ acetylcholine and substance P as neurotransmitters.