Adrenocortical and thyroid systems of rats during the initial period of nociceptive influences.

Two phases were identified in the initial period of formation of the adrenocortical reaction to acute nociception. The first phase (at 10-15 sec) consisted of an "urgent" mobilization of the " basal" reserves of hormonally active substances and was characterized by suppression of corticoliberin activity in the hypothalamus and extrahypothalamic brain structures in rats, with a parallel increase in the plasma concentration of adrenocorticotropic hormone, draining of corticosteroid reserves in the adrenals, and decreases in blood corticosterone levels on the background of increases in corticosterone levels in target organs. The second phase, the hypercompensation phase, involved progressive increases in the titers of study substances. Significant changes in adrenal and blood aldosterone levels were seen only 2.5 min after nociception. Rearrangements of thyroid status were also biphasic. However, unlike the primary adrenocortical response, the first phase of the stress-related thyroid response involved a marked increase in plasma thyroxine and triiodothyronine, while the second involved a selective decrease in the thyroxine concentration to normal levels.