Role of calcitonin gene-related peptide in nociception : interactions with substance P and opioids

Abstract

This thesis presents a study of calcitonin gene-related peptide (CGRP) and its antagonist CGRP8-37 and the interactions with substance P and opioids on the transmission of presumed nociceptive information at the spinal cord level in the rat. Significant bilateral increases in hindpaw withdrawal latency (HWL) to heat and mechanical stimulation were induced dose-dependently by intrathecal administration of 5 or 10 but no 1 nmol of CGRP8-37. Intrathecal injection of 10 nmol of CGRP had no effects on the HWLs intrathecal injection of 5 nmol of substance P decreased the latency to both HWLs.

Intrathecal administration of 10 nmol of CGRP8-37 not only reversed the SP-induced decrease in latency to both hindpaw withdrawal responses but also mediated significant increases in response latency compared to basal levels in the hot-plate and Radall Selitto tests. The increased effects induced by CGRP8-37 were reversed dose-dependently by intrathecal administration of 11 or 22 but not 2.75 nmol of the opioid antagonist naloxone. Furthermore, mu and delta opioid receptors, but not the kappa, were shown to be involved in the effects. These results suggest that CGRP has an important role in transmission of presumed nociceptive information in the dorsal born of the spinal cord where at CGRP interacts with endogenous opioid system and substance P.

After unilateral ligation of the left common sciatic nerve to induce mononeuropathy, bilateral decreases in HWLs were observed which were more pronounced on the lesioned than the contralateral side in the hot-plate and the Randall Selitto tests. Compared with sham operated rats the content of CGRP-like immunoreactivity in mononeuropathic rats was significantly decreased in the left dorsal horn of the spinal cord and left dorsal root ganglia. Significant bilateral increases in HWLs more pronounced in intact rats than the mononeuropathic, were induced by intrathecal injection of 5 or 10 bu not 1 nmol of CGRP8-37. CGRP8-37's effects were reversed dose-dependently by intrathecal administration of 11 or 22 but not by 2.75 nmol of naloxone. These results indicate that CGRP contribute to the spinal transmission of presumed nociceptive information in mononeuropathic rats. Subcutaneous injection of 0.1 ml of 2% carrageenan into the plantar region of rat left hindpaw caused a significant increase in its volume, leaving the right side unaffected.

However, decreases in HWLs to heat and mechanical stimulation were bilateral. The decreased HWL, to heat stimulation continued for 14 days following caraageenan injection. The decreased HWL, to mechanical stimulation lasted for 2 days, then reversed to basal value and thereafter was increased from day 3 to day 14. After unilateral injection of carrageenan significant bilateral increases in the concentration of CGRP-like immunoreactivity were found in the perfusate of both hindpaws and in the cerebrospinal fluid, but not in plasma, Intrathecal administration of 10 nmol of CGRP8-37 induced significant increases in the HWL, which were less pronounced in rats with inflammation than in normal control rats. CGRP8-37's effects were reversed by intrathecal administration of 88 nmol of naloxone. Intrathecal injection of 26.6 nmol of morphine produced increases of greater significance in HWLs than 10 nmol of CGRP8-37. The results show that experimentally induced unilateral hindpaw inflammation induces a bilateral decrease in HWLs to noxious heat and mechanical stimulation, whit CGRP having a role in the transmission of presumed nociceptive information in the spinal cord of inflammatory rats, and the possible involvement of the opioid system in the CGRP8-37 related effect.

Conclusion: CGRP plays an important role in the transmission of presumed nociceptive information in the spinal cord of intact, inflammatory and mononeuropathic rats. Interactions of CGRP with substance P and opioids were also demonstrated.