β‐Adrenergic blockers prevent short photoperiod‐induced gonadal regression, but not melatonin‐induced regression in male syrian hamsters

Pineal melatonin synthesis is regulated by norepinephrine at β‐adrenergic receptors on pinealocytes. Melatonin released from the pineal is believed to be responsible for short photoperiod‐induced gonadal regression. By blocking melatonin production at the β‐adrenergic receptor with β‐adrenergic antagonists, short photoperiod‐induced gonadal regression was prevented. Propranolol or nadolol injected daily in the late afternoon prohibited short photoperiod‐produced testicular regression in male Syrian hamsters. Likewise, propranolol and nadolol pellets were able to block short photoperiod‐induced gonadal regression. Interestingly, in the presence of β‐adrenergic antagonists, gonadal regression was still produced by daily afternoon injections of melatonin. These results indicate that propranolol or nadolol can be used to “pharmacologically pinealectomize” hamsters and block the physiological action of the pineal.