THYROIDECTOMY OF HOUSE SPARROWS (PASSER-DOMESTICUS) PREVENTS PHOTOINDUCED TESTICULAR GROWTH BUT NOT THE INCREASED HYPOTHALAMIC GONADOTROPIN-RELEASING-HORMONE

Thyroidectomy of starlings (Sturnus vulgaris) prevents the decrease in hypothalamic gonadotrophin-releasing hormone (GnRH) which normally oc curs at the onset of photorefractoriness. To extend this observation t o another species, changes in testicular mass, bill colour, moult, and hypothalamic GnRH content were monitored in photostimulated and nonph otostimulated intact and thyroidectomized house sparrows (Passer domes ticus). Photostimulated intact birds rapidly increased testicular mass and GnRH. Later, testicular mass and GnRH decreased, and birds moulte d, as they became photorefractory. Nonphotostimulated intact birds sho wed an increase in testicular mass and GnRH. Neither photostimulated n or nonphotostimulated thyroidectomized birds showed a marked increase in testicular mass, but both showed an increase in GnRH. Photostimulat ed thyroidectomized birds showed no subsequent decrease in GnRH and th ey did not moult. It is suggested that in this species, thyroidectomy inhibits the release of GnRH as well as preventing the downregulation of GnRH synthesis normally associated with the development of photoref ractoriness. (C) 1998 Academic Press.