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Propranolol blockade of short photoperiod-induced gonadal regression: modification by melatonin injections or
1Department of Anatomy, College of Medicine, Texas A&M University, College Station 77843-1114.
Abstract:
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.
Insights
Propranolol prevented short photoperiod-induced testicular weight loss in hamsters by inhibiting melatonin synthesis. Melatonin implants also blocked this regression, indicating the pineal gland
Area of Science:
- Endocrinology
- Chronobiology
- Neuroendocrinology
Background:
- Short photoperiods can induce gonadal regression in male Syrian hamsters.
- The pineal gland and melatonin play crucial roles in seasonal reproduction.
- Understanding the mechanisms of photoperiodic regulation is vital for reproductive biology.
Purpose of the Study:
- To investigate the effects of propranolol and melatonin on photoperiod-induced testicular changes in male Syrian hamsters.
- To elucidate the site of action for propranolol in the context of seasonal reproduction.
- To differentiate between melatonin synthesis inhibition and post-pineal action.
Main Methods:
- Male Syrian hamsters were exposed to short photoperiods (10L:14D) for ten weeks.
- Daily injections of saline, propranolol, or melatonin were administered.
- Melatonin implants were used in some groups.
- Testicular weights, serum hormone levels, and amine levels were measured.
Main Results:
- Propranolol significantly prevented testicular weight decline caused by short photoperiods.
- Daily melatonin injections caused gonadal regression, which propranolol did not prevent.
- Melatonin implants effectively blocked testicular regression from both short photoperiods and melatonin injections.
- No significant differences in hypothalamic or cortical amine levels were observed across treatment groups.
Conclusions:
- Propranolol likely acts at the pineal gland to inhibit melatonin synthesis, rather than affecting post-pineal melatonin action.
- Melatonin's role in mediating photoperiodic responses is confirmed.
- These findings support a specific mechanism for propranolol's effect on seasonal reproductive changes.