The neuroanatomical search for sexual dimorphism

In the rat preoptic area, there is a region of neuropil which shows anatomical differences between males and females. Destruction of this area does not prevent ovulation. Destruction of the suprachiasmatic nuclei prevents spontaneous cyclic ovulation in female rats, but ovulation can be induced by mating in such animals. The rats with suprachiasmatic lesions also showed major disturbances in the normal diurnal timing of many bodily functions. The ovulatory disturbance caused by suprachiasmatic lesions may be due to a defect in the normal timing mechanism for the facilitatory action of progesterone on LH secretion.