Molecular characterization of circannual interval timing in the preoptic area and anteroventral periventricular nucleus in the Djungarian hamster (Phodopus sungorus).

Most temperate animals exhibit seasonal rhythms in reproductive physiology and behaviour. Gonadotropin-releasing hormone (GnRH) cells in the preoptic area (POA) and kisspeptin (Kiss) cells in the anteroventral periventricular (AvPv) nucleus are critical for timing photoperiod-induced changes in seasonal reproduction. In response to prolonged exposure to photoperiod cues, many rodents exhibit endogenously generated programmed changes in physiology that reflect a circannual timing mechanism. Here we used transcriptome sequencing coupled with measurements of surface body temperature and testes mass across a simulated circannual interval timer to characterize the molecular changes involved in the seasonal control of reproduction and surface body temperature. Adult male Djungarian hamsters were collected after exposure to short photoperiod on 4-week intervals from 4 to 32 weeks. The POA and AvPv transcriptomes from short-photoperiod hamsters were compared to a long photoperiod reference group. Our analyses confirmed robust photoperiodic regulation of Kiss1 mRNA expression in the AvPv. Weighted gene co-expression network analyses (WGCNAs) identified hundreds (POA: 211, AvPv: 415) of transcripts that were associated with testes mass. Gene set enrichment analysis (GSEA) identified 'gonadotropin secretion' and 'glial cell proliferation' as the primary pathways associated with circannual interval timing. Vimentin had a strong negative association with testes mass, indicating AvPv glial morphology may increase during gonadal involution. In the POA, pathways associated with 'regulation at synapse' and 'regulation of synaptic plasticity' were highly enriched and transcripts positively associated with gonadal involution indicated that ribosomal plasticity and intracellular calcium signalling are key mechanisms involved in seasonal reproduction. WGCNAs identified 567 transcripts in the POA that were significantly associated with surface body temperature. GSEA discovered thyrotrophin-releasing hormone was significantly negatively associated with surface body temperature. Overall, the POA and AvPv transcriptome datasets provide the foundation to expand our understanding of the molecular representation of seasonal time in the mammalian hypothalamus.