Multiple melatonin target tissues mediate termination of photorefractoriness by long day lengths in Siberian hamsters

The development of refractoriness to the short-day melatonin rhythm in mid-winter triggers recrudescence of the photoinhibited reproductive system of many rodents. As a result, over-wintering animals attain reproductive competence prior to the onset of spring conditions that favor successful reproduction. While in the photorefractory state, hamsters are insensitive to short day lengths and the associated long-duration melatonin rhythm. Prior to regaining sensitivity to short day length inhibition of reproduction, hamsters must first be exposed to 10 to 12 weeks of long, summer-like day lengths and the associated short-duration melatonin rhythm. The neural melatonin target tissues that mediate the breaking of photorefractoriness by long day lengths have not been identified. Long day length information is thought to be communicated to the reproductive axis through the actions of melatonin at the reuniens nucleus of the thalamus (NRe) and the SCN of the hypothalamus. The authors report that the SCN and the NRe also participate in the breaking of reproductive photorefractoriness by long day lengths. Micro-implants of melatonin that were left in place for 12 weeks during exposure to long day lengths and that act locally on these brain nuclei to obscure the endogenous melatonin rhythm, and thus ambient day length information, blocked the breaking of refractoriness. Identical melatonin implants located in another melatonin target tissue, the paraventricular nucleus of the thalamus, did not interfere with the breaking of reproductive refractoriness. By contrast, breaking of refractoriness of the seasonal body mass response did not follow the pattern exhibited by the reproductive response. The results suggest that these melatonin target tissues serve distinct but overlapping roles in the photoperiodic mechanism.