Involvement of transforming growth factor α in the photoperiodic regulation of reproduction in birds

The molecular mechanism underlying photoperiodism is not well understood in any organism. Long-day-induced conversion of prohormone T-4 to bioactive T-3 within the mediobasal hypothalamus (MBH) is critical for the photoperiodic regulation of reproduction. However, because thyroidectomy does not completely block the photoperiodic response in some species, the existence of a thyroid hormone-independent regulatory mechanism appears certain. To identify this novel mechanism, differential subtractive hybridization analysis was performed using MBH of quail kept under short-day and longday conditions. This analysis identified a gene encoding TGF alpha. Expression of TGF alpha mRNA was induced in the median eminence by the stimulus of long days, and this induction was observed at dusk on the first long day. This rapid induction of TGF alpha mRNA was similar to induction of the thyroid hormone-activating enzyme gene [Dio2 (type 2 iodothyronine deiodinase)], which is the earliest event yet determined in the photoinduction process. Expression analysis of epidermal growth factor receptors revealed strong expression of erbB4 and weak expression of erbB1 and erbB2 in the median eminence. Intracerebroventricular infusion of physiological dose of TGF alpha induced LH secretion and testicular growth under short-day conditions. Finally, we demonstrate that T-3 implantation and TGF alpha infusion into the MBH, either of which causes testicular growth, do not affect the expression of TGF alpha and Dio2, respectively. Thus, long-day-induced activation of the TGF alpha signaling pathway appears to mediate a thyroid hormone-independent pathway for the photoperiodic regulation of reproduction.