The Effect of an Acute Light Cycle Change on Adrenal Rhythmicity in Prepubertal Rats

Abstract

24 and 28 day old rats, raised in a 14 h light: 10 h dark (14:10) cycle, showed a clear serum corticosterone (B) peak at the beginning of the dark period (18.00 h) with an amplitude of 25–30 µg%. Other B values during the 24 h period were lower (9–12 µg%). Rats raised in a 8.5:15.5 cycle had a B peak (after onset of the dark period, at 22.00 h). Rats raised in constant light (LL) showed no variation in B throughout the 24 h. Weanling rats raised in a 8.5 :15.5 cycle appeared to synchronize more rapidly to a 14:10 cycle than did rats raised in LL. When placed in LL at weaning, rats raised either in 14:10 or 8.5:15.5 had lost their periodicity by day 24. When rats raised in 8.5:15.5 were shifted to LL or 14:10 after weaning age (day 24), the pattern of response was different. One day after the shift, rats placed in a 14:10 cycle displayed a pattern of B synchronized to the normal 14:10 pattern for rats born and raised in that light cycle. This synchrony persisted for at least 3 days. Rats placed in 14:10 at 21 days of age did not show a fully synchronized pattern until day 28. It can be concluded that weanling rats exposed to a light-dark cycle adapt quickly to a new light cycle and that older prepubertal rats shift even more quickly.

Effects of Castration on Entrainment of the Peripheral Corticosterone Rhythm in Rats before and after Puberty

“…From preliminary evidence, the prediction was made that after ovariectomy the corticosterone rhythm would regress to a prepubertal configuration, both in amplitude and in phase angle. In rats ovariectomized before puberty and exposed to a new photoperiod (a shift from 8:16 to 14:10) on that day, there was a delay in the development of the adult amplitude of serum corticosterone, as reported before [Ramaley, 1973], In a previous study [Ramaley, 1975b], prepubertal rats reared in 8:16 and placed in 14:10 at weaning age had a corticosterone peak at 22.00 h initially which shifted 1st to a blunted peak at 18.00 and 22.00 h and finally sharpened to a peak at 18.00 h, the time of the usual serum corticosterone elevation in our colony maintained on 14:10. The same pattern was noted in rats exposed to 14:10 for the 1st time at 30 days of age.…”

“…We did not examine the importance of ovarian secretions in conditioning the ability of young rats to adapt to a new light cycle. In a previous report, we noted that rats reared in a short day cycle (8:16) adapted rapidly to a challenge long day cycle (14:10), as evidenced by the timing of the serum corticosterone peak [Ramaley, 1975b]. Since rats closer to puberty adapted more rapidly than weanling rats, it seemed possible that emerging ovarian secretion might condition the shift.…”

The Development of the Serum Corticosterone Rhythm in Rats

“…It is also at around 25-26 days that rats appear to begin to synchronize their hormone secretion to photoperiod. This is indicated by the development of an adult phase relationship between the CS rhythm and photoperiod at that age [Ramaley, 1975] and the onset of the capacity for rats to generate a daily surge of prolactin initiated by cervical stimulation [Smith and R amaley, in press].…”