Calcitonin gene-related peptide content, basal outflow and electrically-evoked release from monoarthritic rat spinal cord in vitro

In this study, Freund's adjuvant-induced monoarthritis in the rat hind paw was used to induce chronic pain and inflammation. In order to compare the basal outflow, electrically-evoked release and total content of calcitonin gene-related peptide like immunoreactivity (CGRP-LI) with previously reported changes in substance P (SP-LI), the lumbar enlargement of monoarthitic (complete Freund's adjuvant-treated, CFA rat) and control (incomplete Freund's adjuvant-treated, IFA rat) spinal cords were used. During the 4-wk period after injection, neither the basal nor the evoked release of CGRP-LI from CFA cords differed from controls. By contrast, we have previously reported that SP-LI release from CFA rat spinal cords was significantly higher than from controls, 21 days after inoculation with Freund's adjuvant. Electrically-evoked CGRP-LI release from 21-day CFA rat spinal cord slices was not modified by superfusion with a GABA(B) antagonist, CGP 36742 (100 mu M) which could greatly increase SP-LI release. However, the release of both peptides was significantly increased to the same extent in IFA and normal tissue but to a lesser extent in CFA cords, by superfusion with the opioid antagonist naloxone (1 mu M). In conclusion, CGRP-LI, unlike SP-LI, did not appear to be susceptible to any changes in the lumbar enlargement of the rat spinal cord during inflammation of the hind paw. In addition, CGRP-LI release was increased by antagonism of opiate but not GABA(B) receptors, suggesting that during chronic inflammation of one hind paw, the GABA(B)ergic system, unlike the opioid system, might be activated to selectively inhibit the enhanced SP-LI release but not CGRP-LI release which is not changed.