Neuropeptide Y differentially suppresses per1 and per2 mRNA induced by light in the Suprachiasmatic nuclei of the golden hamster

Neuropeptide Y (NPY), present in an input pathway to the suprachiasmatic nuclei (SCN), can block the effects of light on circadian rhythms. The authors have studied this interaction using an in vitro brain slice technique. Effects of NPY on light-induced period1 and period2 mRNA in the SCN were examined in vitro following a light pulse during early subjective night. Golden hamsters (n = 91) were housed under a 14:10 LD cycle and then moved to constant dim red light for 3 days. Hamsters were exposed to a 5-min light pulse previously shown to induce phase shifts and prepared for in vitro application of NPY. Hypothalamic slices containing the SCN were maintained in vitro for 40 min to 4 h after the light pulse, then quick-frozen. Sections were evaluated by in situ hybridization with [S-35]-labeled cRNA probes for per mRNA. Rapid light induction of both per1 and per2 by 40 min and 1 h after the light pulse, respectively, was apparent, with NPY inhibition of this response significant by at least these same time points. However, although striking suppression of per2 mRNA by the NPY continued through the peak for per2 at 2 h, per1 mRNA levels rebounded quickly to equal the per1 induction peak at 1 h and mirrored the control light induction pattern for per1 thereafter. Delaying NPY to 30 min after slice preparation demonstrated that NPY is capable of suppressing peak per1 levels. These results confirm the feasibility of measuring light-induced gene expression in the SCN in vitro. A differential regulation of per1 and per2 transcription might be of critical importance for the modulation of circadian responses to light.