Decoding the nightly melatonin signal through circadian clockwork

Photoperiod regulates the timing of seasonal cycles in reproduction, energy metabolism, moult and other seasonal characteristics, and the effects are transduced through changes in the duration of nocturnal melatonin secretion from the pineal gland. Short daily melatonin signals (4–8 h/day) activate a summer physiology, while long signals (>10 h/day) produce a winter phenotype. Decoding signal duration occurs in specific target cells in the brain and pituitary gland, each governing a different component of the seasonal adaptation. The pars tuberalis (PT) of the pituitary regulates prolactin release and provides a tractable model system to investigate the molecular decoding mechanism. In the PT, melatonin onset at dusk activates cryptochrone (Cry1) gene expression and melatonin offset at dawn activates period (Per1) gene expression, thus the Cry/Per interval varies directly with nightlength, and inverse to daylength. It is proposed that photoperiod-induced changes in this phase-relationship dictates the level of CRY/PER protein heterodimer formation, and in turn, the level of transcriptional drive to the genes that control PT output – up-regulated under long days stimulating prolactin secretion and a summer physiology, and – down-regulated by short days in winter. The melatonin signal is thus decoded through circadian clock genes.