AFTERNOON INJECTIONS OF MELATONIN IN THE DJUNGARIAN HAMSTER PHODOPUS-SUNGORUS - LONG-LASTING SEX-SPECIFIC EFFECTS AND INFLUENCE OF ACUTE TREATMENT ON THE ENDOGENOUS PINEAL MELATONIN RHYTHM

The effects of exogenous melatonin on gonadal function and on the endo genous melatonin profiles of male and female Djungarian hamsters were investigated. Daily afternoon subcutaneous injections of melatonin (50 mu g) in hamsters kept in long photoperiods (LD 16:8) led to gonadal inhibition in all the treated females (n=13) but in only 2 of 13 males within the treatment period of 8 weeks (uteri: 244+/-11 mg in control s vs. 79+/-4 mg in treated hamsters, P<0.001; ovaries: 13.6+/-0.6 mg i n controls vs. 7.9+/-0.7 mg in treated hamsters, P<0.001; testes: 1,02 1+/-54 mg in controls vs. 732+/-100 mg in treated hamsters, P<0.05; an d accessory glands: 641+/-38 mg in controls vs. 548+/-70 mg in treated hamsters, P>0.05). These results indicate that there are some circums tances under which the gonadal responses of the reproductive organs of male and female Djungarian hamsters differ. Interestingly, pineal con centrations of melatonin were found to be significantly higher in male s (P<0.01), possibly indicating a more robust endogenous supply with t he hormone, whereas serum melatonin levels were not significantly diff erent between males and females. However, the subcutaneous injection o f melatonin (5, 25, and 125 mu g) exerted no acute effect on the endog enous, circadian melatonin-profile, independent of dosage and sex.