Initiating or blocking locomotion in spinal cats by applying noradrenergicdrugs to restricted lumbar spinal segments

After an acute low thoracic spinal transection (T13), cats can be made to w alk with the hindlimbs on a treadmill with clonidine, an alpha2-noradrenerg ic agonist. Because previous studies of neonatal rat spinal cord in vitro s uggest that the most important lumbar segments for rhythmogenesis are L1-L2 , we investigated the role of various lumbar segments in the initiation of walking movements on a treadmill of adult cats spinalized (T13), 5-6 d earl ier. The locomotor activities were evaluated from electromyographic and vid eo recordings. The results show that: (1) localized topical application of clonidine in restricted baths over either the L3-L4 or the L5-L7 segments w as sufficient to induce walking movements. Yohimbine, an alpha2-noradrenerg ic antagonist, could block this locomotion when applied over L3-L4 or L5-L7 ; (2) microinjections of clonidine in one or two lumbar segments from L3 to L5 could also induce locomotion; (3) after an intravenous injection of clo nidine, locomotion was blocked by microinjections of yohimbine in segments L3, L4, or L5 but not if the injection was in L6; (4) locomotion was also b locked in all cases by additional spinal transections at L3 or L4. These re sults show that it is possible to initiate walking in the adult spinal cat with a pharmacological stimulation of a restricted number of lumbar segment s and also that the integrity of the L3-L4 segments is necessary to sustain the locomotor activity.