Melatonin inhibits metabolic activity in the rat suprachiasmatic nuclei

The pineal hormone melatonin has been implicated in the regulation of circadian rhythms and in photoperiodic control of reproduction. The effects of melatonin require the hypothalamic suprachiasmatic nucleus (SCN), a principal pacemaker controlling circadian rhythms. To determine whether SCN activity was directly affected by exogenous melatonin, rats received either melatonin or saline injections 15 min before administration of 2-deoxy-[1-¹⁴C]glucose (2-DG) at two times of day, circadian time (CT) 10 and CT14, and the brains of these rats were processed for autoradiographic determination of 2-DG uptake within the SCN. We report that SCN 2-DG uptake was inhibited by melatonin at CT10, when 2-DG uptake is normally high, and unaffected at CT14, when 2-DG uptake is normally low. This indicates that the SCN may be neural substrates through which melatonin exerts at least some of its effects on mammalian physiology.