EXPOSURE OF RAT SPINAL NEURONS TO NMDA, AMPA AND KAINATE PRODUCES ONLY SHORT-TERM ENHANCEMENTS OF RESPONSES TO NOXIOUS AND NONNOXIOUS STIMULI

The ability of excitatory amino acids (EAAs) to modulate nociceptive a nd non-nociceptive responses was tested on spinal neurones of the anae sthetized rat. NMDA (N-methyl-D-aspartate), AMPA a-amino-3-hydroxy-5-m ethyl-4-isoxazole-propionate) and kainate were applied by iontophoreti c ejection to increase the background firing rate of each cell to simi lar to 25 spikes/s. Responses to noxious heat and pinch and innocuous tap stimuli were enhanced to similar degrees by all three EAAs and ret urned to control immediately following termination of EAA ejection. Th is result shows that, whilst NMDA does enhance synaptic responses of s pinal neurones, this effect is little or no greater than for AMPA or k ainate. Furthermore, the rapid recovery of nociceptive responses indic ates that more than NMDA receptor activation alone is required to indu ce longer-term enhancement of nociceptive responses (hyperalgesia).