Vitamin D and serotonin in winter.

Calcitriol accumulates in the nuclei of adrenal medullary cells, stimulating the tyrosine hydroxylase gene expression. The transcription of the CREM gene is induced by adrenergic input to the pineal gland at night. Depending on the photoperiod of the prior night length, the CREM gene is either subsensitive or supersensitive to induction. Guided by the memory of past photoperiods, the induced changes in melatonin synthesis may mediate either the inhibitory or stimulatory effects on bodily functions. Calcitriol might inhibit the binding of melatonin to the nuclear retinoid Z receptors, which would result in the increased serotonin levels. Serotonin synthesis is hypothesized to be dependent on the duration of light exposure the previous summer.