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The actions of substance P (SP) on the motoneurons were investigated using the isolated newborn rat spinal cord. Potential changes were recorded intracellularly from the motoneurons. The bath application of SP produced a depolarization of the motoneuron membrane, accompanied by an increase in the membrane potential fluctuation. When the Ca concentration in the perfusing fluid was decreased to 1/ 10, the synaptic transmission was blocked, and the depolarization produced by SP was suppressed, together with a decrease in the membrane potential fluctuation. In a solution containing tetrodotoxin (TTX), the synaptic potentials were abolished and the depolarization induced by SP was decreased. Membrane fluctuations during rest and the action of SP disappeared in the TTX solution. In a solution containing DL-2-amino-5-phosphonovaleric acid (APV), the SP-induced depolarization was suppressed, but a potential fluctuation remained. The results suggest that the bath application of SP induced the release of the endogenous transmitter, possibly glutamate, and produced a depolarization of the motoneuron membrane, partly through the activation of N-methyl-D-aspartate (NMDA) receptor. At higher concentrations, in addition to the presynaptic action, SP acted on the postsynaptic membrane and depolarized the motoneuron. Substance P (SP) produces a depolarization of spinal motoneurons, and experimental evidence suggests that this neuropeptide may function as a neurotransmitter in synapses in the spinal cord (for reviews, see 18, 23). However, considerable controversy exists concerning the mechanism underlying this depolarization. The bath application of SP to the rat or frog spinal cord causes a depolarization associated with an increase in the membrane conductance of the motoneuron (16, 17, 22). The iontophoretic application of SP to cat motoneurons produces either no change (33) or a decrease (13) in the membrane conductance. The time course of SP action is generally slow compared to that of the excitatory synaptic potential or to the glutamate action (4, 7, 14, 25, 33; however, see 24). In the present study, attempts were made to characterize the action of SP on the spinal motoneurons under various conditions, Le. in media containing low Ca, tetrodotoxin (TTX) and antagonists for amino acid receptors. The results suggest that the depolarization produced by the bath application of SP at lower concentrations (less than about 200 nM) is largely attributable to the evoked release of the transmitter which depolarizes the motoneuron through the activation of amino acid receptors. At higher concentrations, the postsynaptic action of SP was also involved. MATERIALS AND METHODS Wistar rats aged from 6 to 9 days were killed under ether anesthesia, and the spinal cord with attached dorsal and ventral roots (L4-L6) were dissected in an oxygenated artificial solu-

biomedical research

&lt;b&gt;DUAL ACTIONS OF SUBSTANCE P ON THE MOTONEURONS OF THE NEWBORN RAT SPINAL CORD&lt;i&gt; IN &lt;/i&gt;&lt;/b&gt;&lt;b&gt;&lt;i&gt;VITRO &lt;/i&gt;&lt;/b&gt;