Seasonal change in the phase relations of circadian neurotransmitter rhythms in the anterior hypothalamus

Serotonin and norepinephrine concentrations were determined by high-performance liquid chromatography with electrochemical detection in the right and left anterior hypothalamic nuclei (AHN) of scotosensitive (reproductively inhibited by short daylengths) and scotorefractory male Syrian hamsters maintained on short daylengths (LD 10:14). Marked circadian rhythms of serotonin and nonrepinephrine concentrations were observed in the right and left AHN of both scotosensitive and scotorefractory hamsters. In the right and left AHN of scotorefractory hamsters there was a 12-16-hour phase relationship between the daily peaks of serotonin and nonrepinephrine concentrations. However, in the left AHN of scotosensitive hamsters there was a 0-4 hour phase relation between the daily peaks of the two neurotransmitters. Seasonal conditions are thought to be the result of interactions between circadian and neuroendocrine rhythms. The AHN may represent a neural pacemaker area where circadian neural rhythms can interact temporally to produce seasonal complexes of physiology conditions.<>