Neurophysiological responses to melatonin in the SCN of short-day sensitive and refractory hamsters

The pineal hormone melatonin plays a central role in the regulation of seasonal reproductive cycles in mammals and several studies have implicated the suprachiasmatic nucleus (SCN) as a target on which melatonin acts. The Syrian hamster is a long-day breeder which exhibits gonadal regression when housed in short (< 12.5 h) daily photoperiods or injected daily with melatonin in long photoperiods. In the present paper we address the question whether melatonin affects firing rates of SCN neurones and whether these effects change as the animals become refractory to short photoperiods. In long-day (LD14:10) hamsters SCN neurones were suppressed (31%), activated (15%) or unaffected (54%) by melatonin. In contrast, there was an increased proportion of melatonin insensitive cells (88%) in short-day (refractory) hamsters. Melatonin-responsive cells were found primarily during the late projected day and early projected night in both long-day and short-day animals. This reduced responsiveness of SCN neurones to melatonin in hamsters refractory to short-day exposure may represent part of the mechanism underlying the development of gonadal refractoriness and the onset of gonadal growth in anticipation of long spring photoperiods.