Stimulation of the mesencephalic locomotor region elicits controlled swimming in semi-intact lampreys

The role of the mesencephalic locomotor region (MLR) in initiating and cont rolling the power of swimming was studied in semi-intact preparations of la rval and adult sea lampreys. The brain and the rostral portion of the spina l cord were exposed in vitro, while the intact caudal two-thirds of the bod y swam freely in the Ringer's-containing chamber. Electrical microstimulati on (2-10 Hz; 0.1-5.0 muA) within a small periventricular region in the caud al mesencephalon elicited well-coordinated and controlled swimming that beg an within a few seconds after the onset of stimulation and lasted throughou t the stimulation period. Swimming stopped several seconds after the end of stimulation. The power of swimming, expressed by the strength of the muscl e contractions and the frequency and the amplitude of the lateral displacem ent of the body or tail, increased as the intensity or frequency of the sti mulating current were increased. Micro-injection of AMPA, an excitatory ami no acid agonist, into the MLR also elicited active swimming. Electrical sti mulation of the MLR elicited large EPSPs in reticulospinal neurons (RS) of the middle rhombencephalic reticular nucleus (MRRN), which also displayed r hythmic activity during swimming. The retrograde tracer cobalt-lysine was i njected into the MRRN and neurons (dia. 10-20 mum) were labelled in the MLR , indicating that this region projects to the rhombencephalic reticular for mation. Taken together, the present results indicate that, as higher verteb rates, lampreys possess a specific mesencephalic region that controls locom otion, and the effects onto the spinal cord are relayed by brainstem RS neu rons.