Corticosteroid modulation and stress-induced analgesia in rats

Early evidence has indicated the presence and involvement of specific neural systems which can inhibit the responses to painful stimuli. More recently, further advances suggest that the opiate system may interact with other systems to modulate the analgesia produced by the opiates or various stressors. Since corticosteroids were found to be elevated under the conditions of different stress-induced analgesia (SIA), there may be interactions between the pain-inhibiting systems and the corticosteroids. Recently it was reported that acute stress or long-term adrenalectomy can result in release of beta-endorphin (beta E) and ACTH from the pituitary gland, which can be blocked by dexamethasone. In our early studies we have shown partial antagonism of the SIA by dexamethasone and complete antagonism after naloxone. In this report it was found that chronic treatment of the rats with 0.02% metyrapone in drinking water for 8 weeks resulted in minor hyperalgesia. The chronic pretreatment with metyrapone resulted in a significant potentiation of the analgesia induced via the cold swim stress model, which was reversed by 1 mg/kg (IP) naloxone. Also, hyperalgesia was noted 18 days after the bilateral adrenalectomy of the rats as measured in our laboratory by the hot plate method and as reported by Heybach and Vernikos-Danellis in 1978. These results suggest that the corticosteroid modulation (pituitary-adrenal axis) may have a role in regulating the SIA, and this may implicate the interactions of the corticosteroids with pain-inhibiting systems.