Rapid decreases in adrenal and plasma corticosterone concentrations after drinking are not mediated by changes in plasma adrenocorticotropin concentration.

Rats given water to drink only during a brief daily period respond to water presentation with a rapid decline in plasma corticosterone concentration. To determine whether this response is consequent to a decrease in plasma ACTH concentration or whether it reflects a sudden reduction in adrenal responsiveness to ACTH, we allowed rats access to water for 2 h/day at lights on and measured plasma ACTH and adrenal and plasma corticosterone concentrations at 3- or 5-min intervals after the onset of drinking. Adrenal and plasma corticosterone concentrations decreased significantly within 2-3 min after water presentation in the absence of concomitant changes in plasma ACTH concentration. The effect was apparent by 5 days after initiating the restricted drinking schedule and became stronger with time up to 21 days. Further characterization of the response showed that the in vitro corticosterone secretion of adrenals removed at intervals after water presentation followed the same pattern as the in vivo concentration. When empty water bottles were presented, plasma ACTH and corticosterone concentrations increased. Neither adrenal medullary function nor plasma renin concentration was found to be associated with the decline in adrenal responsiveness to ACTH after drinking. Hemisection of the spinal cord of unilaterally adrenalectomized rats attenuated the corticosterone response regardless of whether the hemisection was contralateral or ipsilateral to the remaining adrenal. These data suggest that the decreases in plasma and adrenal corticosterone concentration occurring after drinking in water-restricted rats are not dependent on changes in plasma ACTH concentrations, but may be related to changes in adrenal blood flow, steroid metabolism and distribution, or neural input to the adrenal cortex.