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Excitatory amino acid (EAA) receptor agonists were tested for their effect on identified rat spinal neurons. Only 75% of the spinal neurons tested increased their firing rate in response to iontophoretic application of one or more of the EAA receptor agonists, N-methyl-d-aspartate (NMDA), quisqualate (Quis),(RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid HBr (AMPA), and kainate (KA). NMDA and Quis or AMPA activated primarily nociceptive neurons (60% of these neurons were projection neurons) in the rat spinal cord. KA-activated neurons were primarily classified as low threshold neurons. Both NMDA and AMPA, at subthreshold doses, significantly increased neuronal responses to peripheral noxious mechanical stimulation; NMDA also significantly increased neuronal responses to peripheral noxious thermal stimulation. Iontophoretically applied phencyclidine (PCP) decreased NMDA-induced …