Similarities in the refractoriness of the reproductive system of hamsters to melatonin and to darkness

In the first study, adult male hamsters were exposed to short days (light:dark cycles of 2:22) and, as expected, they experienced gonadal involution. When the animals were returned to long day conditions (14:10), gonadal regeneration ensued and could not be inhibited by daily afternoon melatonin injections. Likewise, after normal reproductive function was re-established the animals were refractory to the inhibitory influence of afternoon melatonin injections. In the second study, adult male hamsters were exposed to naturally short days beginning in October. By the following April the gonads had re-established a high level of activity. Initiation of afternoon melatonin injections at this time revealed that the animals were refractory to the indole. The refractory period to melatonin was eventually interrupted by the exposure of the hamsters to long days throughout the summer months. Refractoriness to melatonin may be a consequence of the down regulation of the melatonin receptors.