SITES AT WHICH FORMALIN AND CAPSAICIN ACT TO STIMULATE CORTICOTROPIN SECRETION

Abstract

The sites at which injected Formalin and capsaicin act to stimulate ACTH secretion, as concluded from changes in the plasma corticosterone level, were studied in male Wistar rats. Formalin injected into an area deafferentated by denervation or spinal cord transection failed to raise the plasma corticosterone level, showing that it acted locally and that the nervous impulses it produced travelled to the hypothalamus through ascending pathways in the spinal cord. Capsaicin proved to be a potent stressor agent, and it produced a rise in the corticosterone level even when injected into a denervated area. This shows that chemonociceptor excitation at the site of injection is only one of several factors responsible for the stimulation of ACTH secretion by capsaicin. Pretreatment with capsaicin "desensitizing" the chemonociceptors in the skin and the mucous membranes merely inhibited, but did not suppress, the rise in the corticosterone level produced by a subsequent injection of capsaicin; it had no effect on the "nociceptive response" and the rise in the corticosterone level elicited by Formalin.