Triggering of neuroendocrine refractoriness to short-day patterns of melatonin in Siberian hamsters.

Short day lengths induce gonadal regression within 6 weeks in Siberian hamsters, but this inhibitory effect eventually wanes and reproductive competence is regained after 4-6 months in short days. These experiments were undertaken to determine whether continuous exposure to short days for several months is necessary to induce neuroendocrine refractoriness, or if a few weeks of short days are sufficient to trigger processes that culminate in refractoriness several months later. Adult male Siberian hamsters transferred from their natal long-day photoperiod of 15L (15 h light/day) to a short-day photoperiod of 10L were pinealectomized (PINx) after 0, 6, 12, 21 or 40 weeks of exposure to 10L. Intact hamsters kept in 10L manifested gonadal regression by week 6 and completed gonadal recrudescence by week 40, at which time they too were PINx. Beginning at week 40 all hamsters were infused s.c. with melatonin for 10 h/day for 6 consecutive weeks. This treatment induces gonadal regression in photosensitive hamsters. At the end of melatonin treatment, maximal gonadal regression was recorded for hamsters PINx at week 0 and those PINx after 40 weeks in long days. Hamsters PINx after 40 week of short day treatment were completely unresponsive to melatonin whereas those PINx after 6 and 12 weeks had intermediate responses. The percentage of hamsters whose reproductive apparatus was refractory to melatonin at week 40 increased with increasing duration of exposure to short days prior to PINx. Refractoriness was induced by relatively few weeks of short days in some hamsters, whereas others required much more extensive exposure. Induction of refractoriness is triggered by a fraction of the short days that hamsters experience in nature but may not be manifested until many weeks later when it coincides with gonadal recrudescence. In nature all hamsters are exposed to short days for at least 25 weeks, ensuring refractoriness in most individuals.