The influence of total or partial isolations of the medial basal hypothalamus (MBH) on pineal-mediated gonadal responses was studied in adult male hamsters. Light deprivation, due to exposure of hamsters to light: dark cycles of 1:23 (in hours) or blinding, led to involution of the testes, seminal vesicles and coagulating glands and to a statistically insignificant depression in bioassayable pituitary follicle stimulating hormone. The effects of darkness and the pineal on the pituitary-gonadal axis were completely prevented by total or anterior hypothalamic disconnections; posterior disconnections negated the responses in less than one-half the animals. The following tentative explanations for the results were discussed: 1. the hypothalamic cuts interrupted pathways which pass through the hypothalamus and transmit photic information between the eyes and the pineal gland, thus rendering the pineal unresponsive to photoperiodic stimuli; 2. the surgical manipulations within the hypothalamus caused a "central sympathectomy" which interfered with sympathetic control of pineal biosynthetic activity; 3. the hypothalamic cuts interrupted axons of extra-MBH pineal responsive neurons that are inhibitory to gonadotropin-regulating cells within the MBH; 4. the hypothalamic cuts severed axons of extra-MBH neurons which determine the sensitivity of gonadotropin-regulating cells within the MBH, thus rendering the latter neurons unresponsive to the pineal influence.
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