Propranolol blockade of short photoperiod-induced gonadal regression: modification by melatonin injections or implants.

Male Syrian hamsters were exposed to short photoperiods (10L:14D) for ten weeks and received daily ethanolic saline (1:10), propranolol (125 micrograms, sc) or melatonin (25 micrograms, sc) injections at 16:45. Some hamsters also received melatonin implants (1 mg in 24 mg beeswax, sc) every two weeks in addition to the above treatments. Testicular weights were determined at the beginning, every three weeks, and at the end of the experiment. Serum hormone levels, hypothalamic amine levels and cortical amine levels were also determined at the conclusion of the experiment. Propranolol prevented the decline in testes weight produced by short photoperiod (P less than 0.001 versus control). Propranolol was not able to prevent daily injections of melatonin from producing gonadal regression. Melatonin implants blocked testicular regression produced by both short photoperiod and melatonin injections. No differences in hypothalamic or cortical amine levels were observed between the treatment groups. These results add further support to the suggestion that propranolol is acting at the pineal gland preventing melatonin synthesis and not at a post-pineal central site which would affect melatonin binding and action.