The stimulatory and inhibitory role of the hypothalamus in the regulation of ovulation in grass frog, Rana temporaria L.

In our previous experiments it was found that lesions placed in the infundibular hypothalamus induced an advanced ovulation in hibernating frogs, Rana temporaria. It was suggested that this premature ovulation was the effect of gonadotropin-releasing hormone (GnRH) due to the injury of some hypothalamic area inhibiting its release or its action on the pituitary gonadotrophs. To investigate this hypothesis, the following experiments were undertaken: (1) an attempt to induce ovulation with exogenous GnRH in hibernating frogs; (2) an attempt to inhibit ovulation with anti-GnRH serum in preovulatory hibernating animals nonlesioned and with lesions of the infundibular hypothalamus; and (3) administration of bromocriptine in hibernating animals to assess whether this substance, as an agonist of dopamine, possesses an inhibitory action on the ovulation. The following results were obtained: (1) lesions of the infundibular hypothalamus in hibernating preovulatory females caused an advanced ovulation during hibernation; (2) the exogenous GnRH administered to preovulatory females induced ovulation during hibernation; (3) the anti-GnRH serum injected into hibernating preovulatory lesioned females inhibited preterm ovulation during, but not after, hibernation; (4) the immunoneutralization of endogenous GnRH in nonlesioned females resulted in an inhibition of the posthibernatory ovulation; (5) bromocriptine inhibited posthibernatory ovulation in nonlesioned hibernating animals. Thus, the results of these experiments support the suggestion that induction of accelerated ovulation in lesioned hibernating animals involved the releasing action of GnRH. This action of GnRH seemed to be facilitated by the ablation of inhibitory dopaminergic function of hypothalamus in lesioned animals.