Temporal Changes in Gene Expression in the Arcuate Nucleus Precede Seasonal Responses in Adiposity and Reproduction

In anticipation of seasonal climate changes, Siberian hamsters display a strategy for survival that entails profound physiological adaptations driven by photoperiod. These include weight loss, reproductive quiescence, and pelage growth with shortening photoperiod and vice versa with lengthening photoperiod. This study reports gene expression changes in the hypothalamus of Siberian hamsters switched from short days (SD) to long days (LD), and also in photorefractory hamsters. Siberian hamsters were maintained in either LD or SD for 14 wk, conditions that generate physiological states of obesity under LD and leanness under SD. After 14 wk, SD lighting was switched to LD and gene expression investigated after 0, 2, 4, and 6 wk by in situ hybridization. Genes encoding nuclear receptors (RXR/RAR), retinoid binding proteins (CRBP1 and CRABP2), and histamine H3 receptor were photoperiodically regulated with significantly lower expression in SD, whereas VGF mRNA expression was significantly higher in SD, in the dorsomedial posterior arcuate nucleus. After a SD-to-LD switch, gene expression changes of CRABP2, RAR, H3R, and VGF occurred relatively rapidly toward LD control levels, ahead of body weight recovery and testicular recrudescence, whereas CRBP1 responded less robustly and rxrgamma did not respond at the mRNA level. In this brain nucleus in photorefractory animals, the CRABP2, RAR, H3R, and VGF mRNA returned toward LD levels, whereas CRBP1 and rxrgamma remained at the reduced SD level. Thus, genes described here are related to photoperiodic programming of the neuroendocrine hypothalamus through expression responses within a subdivision of the arcuate nucleus.