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Hamster refractoriness: the role of insensitivity of pineal target tissues.

Hamsters exposed to short days undergo gonadal collapse followed by recrudescence and insensitivity to the regressive effects of such photoperiods. This refractoriness may be due to exhaustion of the pineal gland or desensitization of its target. Hamsters whose gonads had spontaneously recrudesced were injected with melatonin (25 micrograms per injection) once daily (known to induce regression in intact hamsters) or twice daily (reported to arrest reproduction in pinealectomized hamsters) for 7 weeks. In neither case did refractory hamsters respond to melatonin treatment. The gonads of intact hamsters treated with melatonin for 21 weeks regressed and spontaneously recrudesced along a normal time course. These results indicate that gonadal refractoriness is due to insensitivity of the target tissues of the pineal gland and imply that melantonin participates in photoperiodic regulation of reproduction in the golden hamster.