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Radioimmunoassay and high performance liquid chromatography were used to determine if the gonadal atrophy induced by late afternoon injections of 5-methoxytryptamine (5-MT) in golden hamsters kept under long photoperiod could be due to the acetylation of this compound into melatonin. An increase in plasma concentrations of melatonin (10–13 nmol/l) was detected 15 min after injection of 130 nmol 5-MT. An injection of 4·3 nmol melatonin generated a similar plasma concentration of melatonin. 5-MT (130 nmol) and melatonin (4·3 nmol) were then injected daily in the late afternoon to golden hamsters kept under long photoperiod. After 8 weeks, 5-MT induced total testicular regression, while melatonin induced partial atrophy only. Thus under these experimental conditions, 5-MT had a physiological activity independent of that of melatonin.Journal of Endocrinology(1989) 121,507–512

The amount of immunocytochemically detectable vasopressin in the brain of the European hamster (Cricetus cricetus) shows a seasonal variation; i.e., dense vasopressin immunoreactivity is present in the lateral septum during summer but is absent in autumn and winter [Buijs, R. M., Pévet, P., Masson-Pévet, M., Pool, C. W., De Vries, G. J., Canguilhem, B. & Vivien-Roels, B. (1986) Brain Res. 371, 193-196]. In the winter period the European hamster hibernates. Since vasopressin in the lateral septum is known to be involved in the control of body temperature, we investigated whether infusion of vasopressin in the lateral septum during autumn-winter could influence hypothermic patterns normally seen in hibernating animals. Hamsters whose lateral septum was infused with vasopressin showed almost no periods of hypothermia, whereas hamsters treated with control infusions displayed a normal hibernation pattern. The results indicate that persistence of vasopressin release in the lateral septum of the European hamster during winter can prevent hibernation.

Daily injections of 10 μg melatonin in the late afternoon into male golden hamsters kept under a long photoperiod (14 h light: 10 h darkness) and at low ambient temperature 6 ± 1 °C) induced a complete gonadal atrophy after 4 weeks. When administered under the same conditions at doses of 25 μg, neither N-(3,5-dinitrophenyl)-5-methoxytryptamine or N-(2,4-dinitrophenyl)-5-methoxytryptamine, a putative melatonin antagonist termed ML-23 in the literature, showed any effect on testicular activity. Moreover, these two drugs were also unable to prevent melatonin-induced gonadal atrophy when injected 30 min before melatonin.The results demonstrate that in the golden hamster and in the present experimental conditions, these drugs do not have the melatonin-antagonistic properties as described in the rat.Journal of Endocrinology(1989) 123,243–247