Melatonin accelerates the reentrainment of the circadian adrenocortical rhythm in inverted illumination cycle.

The effect of melatonin, para-chlorophenyl alanine (PCPA), gamma-aminobutyric acid (GABA) and dibutyryl-cAMP (d-cAMP) on the photoentrainment of the circadian adrenocortical rhythm was studied by the intrahypothalamic (near the SCN) implantation of a paraffin pellet mixed with each agent, in the rat. 2 weeks after implantation, the rats were moved to a room with a reversed lighting cycle (from LD to DL). Blood samples were taken from individual rats at 4-hour intervals for 24 h, and 2, 4, 6 and 10 days after movement, by the tail tip incision method. The effects of implantation of PCPA, GABA and d-cAMP on the reentrainment to the illumination cycle reversed were indistinguishable from those due to implantation of a pellet of paraffin alone. In these groups, more than half of the rats did not reentrain to the light cycle reversed in 4 (less than 18.1%) and 6 days (less than 36.3%), but did reentrain in 10 days. However, melatonin caused reentrainment of the circadian adrenocortical rhythm to reversed illumination between 4 (45.4%) and 6 days (72.7%). Pinealectomized rats receiving a melatonin pellet also exhibited an accelerated reentrainment to the light cycle reversed, when compared with the rat which received a pellet of paraffin alone. However, pinealectomized rats which received a pellet of paraffin alone did not require more time than intact rats to reentrain to the light cycle reversed. These results indicate that melatonin accelerates the reentrainment of the circadian adrenocortical rhythm to the light cycle reversed.