Effects of iontophoretically applied substance P, calcitonin gene-related peptide on excitability of dorsal horn neurones in rats

Spontaneous pain, allodynia and hyperalgesia are well known phenomena follo wing peripheral nerve or tissue injury, and it is speculated that secondary hyperalgesia and allodynia, are generally thought to depend on a hyperexci tability (sensitization) of neurons in the dorsal hem. It is supposed that the sensitization may be due to various actions of neurotransmitters (SP, C GRP, excitatory amino acids) released from the primary afferent fibers. In this study, we examined effects of the iontophoretically applied SP and CGR P on the response to EAA receptor agonists (NMDA and non-NMDA) in the WDR d orsal horn neurones and see if the effects of SP or CGRP mimic the characte ristic response pattern known in various pain models. The main results are summarized as follows : 1) SP specifically potentiated NMDA response. 2) CG RP non-specifically potentiated both NMDA and AMPA responses. Potentiation of NMDA response, however, was significantly greater than that of AMPA resp onse. 3) 50% of SP applied cells and 15.8% of CGRP applied cells showed rec iprocal changes(potentiation of NMDA response and suppression of AMPA respo nse). These results are generally consistent with the sensitization charact eristics in diverse pain models and suggests that the modulatory effects of SP and CGRP on NMDA and non-NMDA (AMPA) response are, at least in part, co ntribute to the development of sensitization in various pain models.