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Diurnal rhythm of response to chronic intrahypothalamic melatonin injections in the white-footed mouse, peromyscus leucopus.

A pronounced diurnal change in responsiveness to intrahypothalamic melatonin injections was observed in the white-footed mouse, Peromyscus leucopus. 7 weeks of chronic daily afternoon (12 h after lights-on) injections of 500 ng melatonin into the vicinity of the suprachiasmatic nuclei (SCN) caused a 66% reduction in female reproductive tract weight relative to saline-injected controls (p less than 0.01), with 83% of these animals having an imperforate vagina, and no animals having mature follicles. Subcutaneous injections of 500 ng melatonin had little effect. Chronic morning (2 h after lights-on) injections of melatonin into the vicinity of the SCN had little effect on the maintenance of normal reproductive tract weight, with 18% of these animals having an imperforate vagina, and 71% having mature follicles. These results indicate that the daily rhythm of melatonin antigonadal action is due to change in responsiveness to melatonin of target neurons in the region of the SCN.