Effect of Melatonin Administration and Long Day‐Length on Endocrine Cycles in the Hedgehog Erinaceus europaeus

Abstract

The effects of exogenous melatonin (subcutaneous implants containing 0.031 +/- 0.006 mg/gm body mass melatonin) or long photoperiod (18L:6D) on wild-caught adult male hedgehogs were studied. Hedgehogs were implanted with melatonin-filled or empty capsules in May, August, or September, or maintained under long photoperiod from August. Blood samples collected at monthly intervals were assayed for testosterone, melatonin, and thyroxin. Melatonin-filled capsules elevated plasma melatonin concentrations for 4-6 months. Although melatonin administration in May depressed plasma testosterone levels, testicular reactivation was advanced by 1 month the following year, and the characteristic prehibernal gain in body mass was abolished. Melatonin administration in August had no effect on plasma testosterone concentrations but reduced body mass fluctuations before and during hibernation. Hedgehogs receiving melatonin in September recovered early from hibernal body mass loss and showed a 2 month advance in testicular reactivation the following year. Maintaining hedgehogs at 18L:6D photoperiod, however, elevated plasma melatonin concentrations. Testicular reactivation the following spring was delayed by 1 month, ended 3 months early, and testosterone concentrations were depressed. All treatments depressed plasma thyroxin levels. These results suggest that elevated melatonin levels during winter are important in the regulation of endogenous endocrine cycles in the hedgehog. Hedgehogs do not respond positively to melatonin at the end of the breeding season, but are again responsive to melatonin as early as September.

Inhibitory effects of melatonin on testosterone but not on androstenedione production during winter in the vespertilionid bat, Scotophilus heathi

“…The results of the present study also supports an earlier finding in the Syrian hamster that suggests it is not the cessation of melatonin secretion by the pineal gland but rather a refractoriness of melatonin that results in the initiation of gonadal recrudescence in the spring time [Brainard et al, 19831. Further, other evidence suggest that the sensitivity to the antigonadal activity of melatonin is greatly influenced by season and photothermal conditions [Pevet et al, 1989;Fowler and Racey, 1990;Forsberg and Madej, 19901. Thus, the decline in testicular activity during winter dormancy in S. heathi could be because of increased testicular sensitivity to the melatonin signal as compared with other stages of the reproductive cycle.…”

Melatonin affects steroidogenesis and delayed ovulation during winter in vespertilionid bat, Scotophilus heathi

“…A significantly elevated progesterone level was observed during active fetal development accompanied by placental growth reaching a peak prior to parturition [24]. Fowler and Racey [26] demonstrated that elevated melatonin level is important in the regulation of endogenous hormonal cycle in hedgehog during winter. The studies on rodents showed inhibitory effect of melatonin on ovarian function [27,28].…”