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It is well known that mechanical or electrical stimulation of the uterine cervix activates the corpora lutea of the rat and induces pseudopregnancy as effectively as does mating with a vasectomized male (Everett, 1964). Evidence from recent studies on the effects of electrical stimulation of the hypothalamus (Everett & Quinn, 1966) and ofhypothalamic lesions (Flerk\l=o'\,1960; McCann & Friedman, 1960) suggests that an important neurogenic component for the activation of the corpora lutea exists in the hypothalamus. Little is yet known, however, of the pathways whereby information is conveyed from the genitalia to the hypothalamus controlling the secretion of pituitary luteotrophic hormone (lh). In the present study, the surgical transection of neural connections to the hypothalamus (Hal\l=a'\sz& Pupp, 1965; Hal\l=a'\sz& Gorski, 1967) was employed to investigate the pathways by which the afferent nervous impulses from the vaginal-cervical region cause the secretion of lh influencing pseudopregnancy. Surgery was performed under thiopental anaesthesia on Sprague-Dawley rats, weighing 250 to 270 g, according to the technique described by Halász & Gorski (1967). Since interruption of the anterior connection to the medial basal hypothalamus (MBH) causes anovulatory conditions (under which pseudo¬ pregnancy cannot be induced by vaginal-cervical stimulation without pre¬ treatment with gonadotrophins), only the dorsal, lateral and posterior con¬ nections to the MBH were interrupted, the MBH being left in neural contact with the anterior hypothalamus in all rats. This type of surgical transection (postero-bilateral de-afferentation, Text-fig. 1 A, B) has been reported not to interfere with ovulatory mechanisms (Halász & Gorski, 1967). From 30 days after the operation, vaginal smears were taken daily. Following completion of four normal cycles, the cervix was mechanically stimulated on the day of oestrus (4-day cycle) by tapping it rapidly (60 to 70 times) with a glass rod 2 to 3 mm in diameter. Four days later, a silk thread was placed in one of the uterine horns to test for a deciduoma reaction. Animals were killed 5 days after the traumatization. Each brain was removed and the precise localization of each de-afferentation was determined histologically. Only those animals in which the de-afferentation was verified histologically were incorporated in this report. In nine rats, perfect postero-bilateral de-afferentation of the MBH was

EFFECT OF HYPOTHALAMIC DE-AFFERENTATION ON INDUCTION OF PSEUDOPREGNANCY BY VAGINAL-CERVICAL STIMULATION IN THE RAT