Refractoriness to melatonin occurs independently at multiple brain sites in Siberian hamsters

The mid-winter development of refractoriness to melatonin (Mel) triggers re crudescence of the atrophied reproductive apparatus of rodents. As a conseq uence, over-wintering animals with the independent regulation by day length of the several behavioral and physiological traits that vary seasonally in mammals, become reproductively competent just before the onset of spring c onditions favorable for breeding. The neural target tissues that cease to r espond to winter Mel signals have not been identified. We now report that t he suprachiasmatic nucleus of the hypothalamus, which contains the principa l circadian clock, and the reuniens and paraventricular nuclei of the thala mus, each independently becomes refractory to melatonin, Small implants of Mel that were left in place for 40 wk and that act locally on these brain n uclei, induced testicular regression within 6 wk in male Siberian hamsters; 12 wk later Mel implants no longer suppressed reproduction and gonadal rec rudescence ensued, Hamsters that were then given a systemic Mel infusion s. c. immediately initiated a second gonadal regression, implying that neurons at each site become refractory to Mel without compromising responsiveness of other Mel target tissues. Refractoriness occurs locally and independentl y at each neural target tissue, rather than in a separate "refractoriness" substrate. Restricted, target-specific actions of Mel are consistent with t he independent regulation by day length of the several behavioral and physi ological traits that vary seasonally in mammals.