Effects of N-methyl-D-aspartate (NMDA) and non-NMDA receptor antagonists on excitation of the tooth-pulp-evoked C1 spinal neurons in the rat.

To examine whether there is a difference between the effects of iontophoretically applied N-methyl-D-aspartate (NMDA) and non-NMDA receptor antagonists on the activity of C1 spinal neurons with input from the tooth pulp, extracellular single recordings were performed in pentobarbital-anesthetized rats. The activity of 16 C1 spinal neurons and the amplitude of the digastric electromyogram increased proportionally by 1.0-3.5 times the threshold for jaw-opening reflex. After iontophoretic application (10, 30 and 50 nA, 5 min) of NMDA receptor blocker (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a, d]cycloheptene-5,10-imine hydrogen maleate or non-NMDA receptor blocker (6-cyano- 7-nitroquinoxaline-2,3-dione), the mean number of spikes responding to the tooth pulp stimulation at x3.5 threshold for jaw-opening reflex was significantly decreased. Under these conditions, there were no significant differences between the amplitudes of the dEMG before and after applications of both NMDA and non-NMDA receptor antagonists. These results suggest that the release of endogenous excitatory neurotransmitters is necessary for activation of C1 spinal neurons that are associated with the transmission of nociceptive information, and that both NMDA and non-NMDA receptors contribute to the mechanism of excitation of tooth-pulp-evoked C1 spinal neurons.