Leptin phase-advances the suprachiasmatic circadian pacemaker in vitro

The primary circadian dock in mammals is located in the suprachiasmatic nucleus (SON). Two rhythms controlled by the SON clock are feeding and metabolic rate. The hormone leptin is a potent regulator of food intake and energy metabolism. Immunocytochemical and in situ hybridization studies have indicated the presence of leptin receptors in several hypothalamic nuclei. Also, rodents with mutations in the leptin receptor or leptin protein exhibit abnormal circadian rhythms. These data raised the possibility that leptin may modulate SON clock activity. Therefore, we investigated the effects of leptin on SCN neuronal activity rhythms in vitro. Leptin was applied at various times to rat brain slices containing the SCN. Extracellular recordings of SCN neurons were used to calculate the time-of-peak in SCN activity. Phase shifts of the SCN-clock were determined by comparing the time-of-peak in treated slices to that of control slices. Results showed that leptin phase-advarices the SCN clock by about 2h when applied during the subjective day and into the early subjective night. To investigate whether these phase shifts might be occurring through stimulation of leptin receptors, we performed leptin receptor immunocytochemistry using two different antibodies to the leptin receptor. The K-20 antiserum is immunoreactive With the amino-terminus of all leptin receptor isoforms and B-3 is specific for the transmembrane forms. We found positive leptin receptor immunoreactivity in the SCN using both antibodies, supporting the hypothesis that the observed phase shifts are the result of a direct action of leptin in the SCN.