TEMPORAL RELATIONSHIPS BETWEEN DIURNAL RHYTHM OF HYPOTHALAMIC CORTICOTROPIN RELEASING FACTOR, PITUITARY CORTICOTROPIN AND PLASMA CORTICOSTERONE IN RAT

Plasma corticosterone (fluorometric assay), pituitary ACTH (bioassay using isolated adrenal cells) and hypothalamic corticotropin releasing factor (CRF) (bioassay using isolated pituitary cells) were measured singly in groups of 6 female rats which were killed at 11.00, 15.00, 19.00, 21.00, 23.00, 01.00, 03.00, 05.00, 07.00 and 11.00 h, after 5 wk of adaptation to a photoperiod of 12 h light:12 h darkness. Locomotor activity was recorded continuously, using actographic cages, and the waking/sleep pattern was recorded by electroencephalography from chronically implanted control rats during the first hours of the light span. The 3 hormones measured fluctuated with a 24 h rhythmicity, with extreme values ranging between 4.12 .+-. 1.42-31.78 .+-. 194 (SE of the mean) .mu.g/100 ml for corticosterone, 4486 .+-. 269-16,629 .+-. 882 .mu.u./mg pituitary for ACTH, and 439 .+-. 20-1270 .+-. 39 .mu.u. ACTH production/hypothalamus per 105 pituitary cells. The onset of the ascending phase of the rhythm started during the first 2 h of light for CRF, 2 h later for ACTH and again 2 h later for corticosterone. The estimated acrophase of the rhythms occurred, respectively, 9.4 (CRF), 10.3 (ACTH) and 14.4 h (corticosterone) after onset of light. These phase relationships point to a central origin of the adrenal rhythm. The diurnal activation of CRF at the very beginning of the light phase was concomitant with an almost immediate reduction of the locomotor activity and onset of sleep. These correlations favor the hypothesis of a common temporal control of both the adrenal and the sleep/waking rhythms.