Melatonin receptor density is regulated in rat pars tuberalis and suprachiasmatic nuclei by melatonin itself.

The pineal hormone melatonin interacts with the neuroendocrine system via high affinity binding sites. The density of these receptors is increased after 72 h constant light or pinealectomy in both rat pars tuberalis and suprachiasmatic nuclei. The present results show that, in rat PT and SCN, this increase can be inversed by a single melatonin injection (50 micrograms s.c. in 0.1 ml NaCl, 4 h before sacrifice). At the time of sacrifice, plasma melatonin concentrations of injected animals were no longer significantly different when compared to control values. Thus, this observed effect is not a result of a receptor occupation, but demonstrates a direct regulation of melatonin receptors by melatonin itself.