Differential responsiveness of the reproductive system of blind or melatonin-treated male hamsters to injections of gonadotrophin-releasing hormone (GnRH) and/or prolactin.

The combination of pituitary grafts and daily injections of gonadotrophin-releasing hormone (GnRH) completely prevents gonadal atrophy in blind (BL) and melatonin (Mel)-treated male hamsters. In order to avoid potential problems associated with the use of pituitary grafts and further define the interactions between prolactin (PRL) and GnRH in preventing reproductive regression, we injected various doses of each hormone either alone or in combination into BL or Mel-treated male hamsters. In another experiment, BL hamsters received either weekly beeswax implants of estradiol benzoate (EB) (1 mg) alone or EB implants in combination with daily injections of GnRH (2 micrograms). In each experiment the hamsters were BL and/or treated with hormones for 10 wk. Either GnRH (8 micrograms) or PRL (5 micrograms) per d partially prevented gonadal atrophy in BL hamsters. However, increasing doses of GnRH plus PRL were more effective than either hormone alone in preventing gonadal atrophy. The combined effect of these hormones was greater than the algebraic sum of their individual effects. Injections of either GnRH or PRL alone resulted in a significant maintenance of gonadal and accessory organ size in Mel-treated animals. The combination of GnRH and PRL resulted in virtually complete maintenance of testicular weight while the accessory sex organs remained atrophic. The combined effect of these hormones was equivalent to the algebraic sum of their individual effects. The treatment of BL animals with EB implants with or without GnRH did not prevent reproductive regression in spite of elevated serum PRL levels. In conclusion, the effects of GnRH and PRL were synergistic in BL hamsters and additive in Mel-treated animals. The data suggest that there is a differential responsiveness of BL v. Mel-treated hamsters to the individual as well as the combined actions of GnRH and PRL.