Neuropeptide Y–Induced Phase Shifts of PER2::LUC Rhythms Are Mediated by Long-Term Suppression of Neuronal Excitability in a Phase-Specific Manner

Abstract

Endogenous circadian rhythms are entrained to the 24-h light/dark cycle by both light and nonphotic stimuli. During the day, nonphotic stimuli, such as novel wheel-induced exercise, produce large phase advances. Neuropeptide Y (NPY) release from the thalamus onto suprachiasmatic nucleus (SCN) neurons at least partially mediates this nonphotic signal. The authors examined the hypothesis that NPY-induced phase advances are accompanied by suppression of PER2 and are mediated by long-term depression of neuronal excitability in a phase-specific manner. First, it was found that NPY-induced phase advances in PER2::LUC SCN cultures are largest when NPY (2.35 µM) is given in the early part of the day (circadian time [CT] 0–6). In addition, PER2::LUC levels in NPY-treated (compared to vehicle-treated) samples were suppressed beginning 6–7 h after treatment. Similar NPY application to organotypic Per1::GFP SCN cultures resulted in long-term suppression of spike rate of green fluorescent protein–positive (GFP+) cells when slices were treated with NPY during the early or middle of the day (zeitgeber time [ZT] 2 or 6), but not during the late day (ZT 10). Furthermore, 1-h bath application of NPY to acute SCN brain slices decreased general neuronal activity measured through extracellular recordings. Finally, NPY-induced phase advances of PER2::LUC rhythms were blocked by latent depolarization with 34.5 mM K⁺ 3 h after NPY application. These results suggest that NPY-induced phase advances may be mediated by long-term depression of neuronal excitability. This model is consistent with findings in other brain regions that NPY-induced persistent hyperpolarization underlies mechanisms of energy homeostasis, anxiety-related behavior, and thalamocortical synchronous firing. (Author correspondence: [email protected])

Keywords:

• Circadian
• Electrophysiology
• Entrainment
• Nonphotic
• NPY
• Suprachiasmatic nucleus

ACKNOWLEDGMENTS

This work was supported by National Institutes of Health, grant K99/R00 GM086683 (K.L.G.). We thank Karla Eubanks for technical assistance and Dr. Martin Young for helpful comments on the writing of the manuscript.

Declaration of Interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.