Seasonal alterations in the daily rhythms in hypothalamic expression of genes involved in the photoperiodic transduction and neurosteroid-dependent processes in migratory blackheaded buntings

Abstract

This study investigated seasonal alterations in the daily rhythms of hypothalamic expression of genes involved in the photoperiodic regulation of annual cycles in birds. We measured 4-hourly mRNA expression of genes involved in the photoperiodic transduction (OPN5, EYA3, CGA, TSHβ, DIO2, DIO3) and neurosteroid-dependent processes (AR, CYP19, ERα, ERβ) in the hypothalamus of migratory blackheaded buntings photoinduced with photosensitive, photostimulated (early and late stimulated) and photorefractory seasonal states. There were significant differences in daily mRNA profiles between the photoperiodic states. Particularly, increased CGA, TSHβ and DIO2, and decreased DIO3 mRNA levels in the early photostimulated state, as compared to photosensitive state, suggests that there is a role of thyroid hormones in photostimulation in buntings. Similar differences in the expression of genes coding for the aromatase enzyme (CYP19) and receptors for estrogen (ERα, ERβ), but not androgen (AR), indicates that there is seasonal alteration in the neuroestrogen-mediated functions. Further, peak expression times of CGA, TSHβ and DIO2 genes at hour 14-15 of the day in early stimulated state evidenced molecular regulation of the daily rhythm of photoinducibility in buntings. Most significantly, however, we found an attenuated daily rhythm in thyroid hormone modulatory genes and switch of peak expression time from day to night in CYP19 mRNA rhythm in the subsequent late photostimulated state, although testicular maturation still persisted. Perhaps, these alterations in daily rhythms signalled the initiation of processes underlying other seasonal phenologies in parallel with gonadal response, such as the manifestation of the nighttime flight in buntings. These results show alterations in daily rhythms underlying the transcriptional regulation of the photoperiod-induced seasonal states in migratory blackheaded buntings.

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