Evidence for Neural Control of the Release of Pituitary Ovulating Hormone in the Golden Syrian Hamster

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Abstract
Golden Syrian hamsters (Mesocricetus auratus) exposed daily since birth to fluorescent light from 6:00 AM to 6:00 PM and displaying regular 4-day vaginal estrous cycles ovulated between 1:00 and 3:00 AM on the day characterized by a thick vaginal discharge consisting mainly of nucleated epithelial cells and few, if any, leukocytes. A critical period was found on the preceding day between 2:00 and 4:00 PM before which hypophysectomy or injection of the centrally active drug phenylisopropylhydrazine blocked ovulation and after which both procedures were ineffective. Ovulation in hamsters was also inhibited by tranylcypromine, iproniazid and pargyline. All these drugs produced marked behavioral excitement. Pentobarbital, chlorpromazine and reserpine, which are known to inhibit ovulation in rats, were ineffective in hamsters in doses which evoked marked behavioral depression.