Acute effects of a single afternoon melatonin injection on LH, prolactin, and pineal melatonin titers in the Syrian hamster.

Young adult male Syrian hamsters maintained in an LD 14:10 photo-period (lights out at 8 p.m.) were given a single injection of 25 micrograms melatonin or ethanolic-saline at 4 p.m. Animals from each group were sacrificed at 5 p.m., 4 a.m., and 10 a.m. following the injections. Pituitary and plasma LH and prolactin as well as pineal melatonin levels were determined in all animals by RIA. Pineal melatonin levels were significantly (p greater than 0.025) reduced at 10 a.m. in the melatonin treated group. Melatonin administration also suppressed pituitary LH levels at 4 a.m. and plasma LH levels at 10 a.m. Plasma prolactin concentrations at 4 a.m. and pituitary prolactin levels at both 5 p.m. and 10 a.m. were significantly elevated in melatonin treated hamsters compared with those given ethanolic-saline. These results indicate that exogenous melatonin has immediate effects on several hormones important in the regulation of reproductive events in this species. This acute injection model may therefore aid in the elucidation of melatonin's mechanism of action in the Syrian hamster.