Influences of feeding and drinking on circadian rhythms of opioid peptides in plasma, hypothalamus and pituitary gland in rats

Changes of plasma, hypothalamic and pituitary immunoreactive beta-endorphin (IR-beta-end), methionine-enkephalin (IR-Met-enk) and ACTH (IR-ACTH) were studied under various conditions of feeding and watering in rats. When rats were fed from 17:00 to 09:00 hr and water was given ad lib, plasma IR-beta-end and IR-ACTH had parallel circadian rhythms with a peak before feeding and drinking. In the hypothalamus, IR-beta-end and IR-Met-enk showed parallel circadian rhythms with a decrease before these behaviors. When rats were fed from 09:00 to 17:00 hr, the peaks of plasma IR-beta-end and IR-ACTH shifted to one hour before the onset of feeding and drinking. When feeding and watering were restricted to 17:00-09:00 hr and 09:00-12:00 hr respectively, plasma IR-beta-end and IR-ACTH exhibited parallel circadian rhythms with two separate peaks at one hour before drinking and feeding, respectively. In the hypothalamus, IR-beta-end, IR-Met-enk and IR-ACTH showed parallel circadian rhythms with a decrease before feeding but not before drinking. When rats were fed from 17:00 to 20:00 hr, plasma IR-beta-end increased and neurohypophysial IR-beta-end and IR-Met-enk decreased at 16:00 hr, one hour before feeding. It was observed that locomotor activities increased at the time of transition from light to dark and at one hour before the onset of feeding and drinking. The present results suggest that endogenous opioid peptides may have some physiological roles in feeding and drinking behaviors.