The role of adrenocorticotropic hormone in the inhibition of pain reactions in conscious rats.

Experiments on conscious male Sprague-Dawley rats were performed to study the effects of adrenocorticotropic hormone (ACTH) on pain reactions. Pain sensitivity was assessed in terms of the latent period of tail withdrawal in response to heat. Systemic administration of ACTH and glucocorticoids to animals with normal levels of hormone production led to increases in the latent period of the tailflick reaction. The roles of glucocorticoids and opioid peptides in ACTH-induced analgesia were addressed in experiment on animals with deficient glucocorticoid production and animals in which opiate receptors were blocked with naltrexone. Deficiency in glucocorticoid production had no effect on ACTH-induced increases in the latent period of the tailflick reaction, while blockade of opiate receptors completely eliminated this effect of ACTH. ACTH-induced analgesia in conscious rats is mediated by opiate receptors and not by glucocorticoids.