Hohlweg found that the administration of large amount of estrogen induces ovulation in immature rats. It is suggested that this ovulation may be evoked by the steroid action in the central nervous system, especially on the hypothalamus. Along with our previous finding the acetylcholine content in the rat hypothalamus fluctuated at 6 to 10 hrs. after the estrogen injection, the similar fluctuations of the choline acetylase and the monoamine oxidase activities were also seen at that time following the administration of estradiol.
The results obtained in present studies are quite in keeping with the observation that the activation of hypothalamus for ovulation may occur several hours after the injection of estradiol, the fact indicating the participation of cholinergic and adrenergic mechanisms involved in the hypothalamus in the Hohlweg's phenomenon.