Hypnotic effect
Melatonin has a soporific effect which is greatest during the day. This may be through inhibiting the wake promoting effect of the SCN or by a direct action on other sleep mechanisms, perhaps in the thalamus or hypothalamus. Its effect is probably mediated by activation of inhibitory GABA-ergic neurones, possibly through the action of cytokines. It does not appear to alter the proportions of NREM and REM sleep, but if given during the day usually leads to stages 1 and 2 NREM sleep.

Inhibition of reproductive function
Melatonin reduces prolactin and luteinizing hormone levels and delays puberty. The fall in its serum concentration at puberty may enable sexual development to take place. A high peak melatonin level at night is associated with infertility in both males and females.

The steadily changing seasonal duration of exposure to light alters the duration for which melatonin is secreted each night. This may influence reproductive activity, although humans are less sensitive to this photoperiod signal than many animals which have well-defined seasonal reproductive cycles.

Effects on immune function
Resection of the pineal gland is followed by immuno-suppression and changes in the thymus, probably due to loss of melatonin secretion. Melatonin’s effects on immune function are:

1 augmentation of natural killer (NK) cell activity in killing tumour and virus infected cells;
2 binding to T-lymphocytes, especially CD4+, and augmenting T-helper cell activity increasing their production of interferon gamma and interleukin-1 (IL-1), and release of IL-2;
3 increasing antibody responses, probably by augmenting T-helper cell function;
4 preventing apoptosis of T-lymphocytes;
5 antagonizing corticosteroid-induced immunosuppression, probably through its effects on cytokine availability.

Vasodilation. The heat loss through the skin due to peripheral vasodilation when melatonin is secreted in the evening lowers the body temperature and facilitates sleep onset.

Mild anticonvulsant action

References