Ionotropic glutamate receptor-mediated responses in the rat primary somatosensory cortex evoked by noxious and innocuous cutaneous stimulation in vivo.

To examine the involvement of different ionotropic glutamate receptors in the mediation of responses evoked by noxious cutaneous stimulation, single unit recordings were made from 31 neurons in the primary somatosensory (SI) cortex of rats anesthetized with urethane. To compare synaptic receptor pharmacology across somatosensory submodalities, 13 of the neurons were also tested with an innocuous, cutaneous air jet stimulus. Mechanical (HT) responses, evoked by a 5-s noxious pinch, decayed gradually upon termination of the stimulus and lasted on average for 15.1+/-1.9 s (+/-SEM; n=10). An increase in baseline activity was also observed during noxious stimulus trials of 5-min stimulus intervals. A correlation between increase in mechanical or thermal HT responses and baseline activity was found for some neurons. However, the normalized ratios of the mechanical or thermal HT response to baseline activity during iontophoretic application of (RS)-3-(2-carboxypiperazine-4-yl)-propyl-l-phosphonic acid (CPP), an N-methyl-D-aspartic acid (NMDA) receptor antagonist (0.6+/-0.1; n=11, or 6-nitro-7-sulfamoylbenz[f]quinoxaline-2,3-dione (NBQX), an (RS)-alpha-amino-3-hydroxy5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptor antagonist (0.8+/-0.1; n=11), suggest that the reductions in baseline activity did not account for the reductions of the mechanical or thermal HT responses observed, which were reduced proportionally more than the baseline activity. A 10-ms air jet evoked a biphasic increase in action potentials above an average background activity of 7+/-2 spikes/s (n=13). The early phase of this low-threshold (LT) response was within two or three 10-ms bins and had an average firing rate of 74+/-11 spikes/s evoked in the first 10-ms bin (n=13). In eight neurons, the early LT response was followed by a lower frequency excitatory component lasting an average of 415+/-92 ms. Iontophoretic application of CPP reduced responses evoked by a noxious pinch (21+/-10% of control responses; n=19) and a noxious thermal stimulus (24+/-18%; n=5). The fast component of the LT responses was only reduced to 85+/-4% (n=12). A slower component of the LT responses, when present, was also reduced by CPP (15+/-19%; n=4). Iontophoretic application of NBQX reduced responses evoked by a noxious pinch (42+/-12%; n=19) and a noxious thermal stimulus (63+/-16%; n=8). The fast component of the LT responses was reduced to 43+/-6% (n=12) and the slower component to 32+/-20% (n=6). These data show that both NMDA and AMPA/kainate receptors are involved in the mediation of SI high-threshold responses. This same combination of glutamate receptors also mediates low-threshold synaptic responses.