NMDA induces persistent inward and 1 outward currents that cause rhythmic 2 bursting in adult rodent motoneurons

21 N-Methyl-D-aspartate (NMDA) receptors are of critical importance for 22 locomotion in the developing neonatal spinal cord in rats and mice. However, due to 23 profound changes in the expression of NMDA receptors in development between the 24 neonatal stages and adulthood, it is unclear whether NMDA receptors are still an 25 important component of locomotion in the adult rodent spinal cord. To shed light on 26 this issue, we have taken advantage of recently developed preparations allowing the 27 intracellular recording of adult motoneurons that control the tail in the sacrocaudal 28 spinal cord of adult mice and rats. We show that in the adult sacrocaudal spinal cord 29 NMDA induces rhythmic activity recorded on the ventral roots, often coordinated 30 from left to right, as in swimming motions with the tail (fictive locomotion). The 31 adult motoneurons themselves are intrinsically sensitive to NMDA application. That 32 is, when motoneurons are synaptically isolated with tetrodotoxin (TTX), NMDA still 33 causes spontaneous bursts of rhythmic activity, depending on the membrane 34 potential. We show that these bursts in motoneurons depend on an NMDA-mediated 35 persistent inward current and are terminated by the progressive activation of a 36 persistent outward current. These results indicate that motoneurons, along with the 37 central pattern generator, can actively participate in the production of swimming38 like locomotor activity in adult rodents. 39