NMDA receptors in the spinal cord exert excitatory influences on spinal motor output in rats

The role of NMDA receptors in the regulation of spinal motor output was studied in rats. Muscle tension of the hind limbs of decerebrate animals and spinal reflex potentials in anesthetized animals were recorded. Intrathecal injection as well as systemic or intra-4th ventricular injection of (+)-5-methyl-10-11-dihydro-5H-dibenzo[a,d]cyclohepta-5-10-imine maleate (MK-801) reduced muscle tension. Systemic MK-801 did not alter monosynaptic reflexes either in intact or spinal rats, but attenuated polysynaptic reflexes in spinal rats. Thus spinal NMDA receptors participate in spinal motor output in the presence of some specific factors such as descending facilitation and preceding segmental depolarization, which remove the Mg2+ blocking.