Gonadotrophic Hormone Secretion in Female Rats After Partial or Total Interruption of Neural Afferents to the Medial Basal Hypothalamus

Abstract

Neural connections to the medial basal hypothalamus (MBH) were partially or totally transected with a small knife. Ovulation, ovarian compensa-tory hypertrophy (OCH) and hypophysial response to castration were studied in the same animals. After complete deafferentation of the MBH, ovulation did not occur. Transection of the posterior and posterolateral connections to the MBH did not interfere with ovulation. If in addition, lateral and superior afferents were also interrupted, 8 animals out of 12 still ovulated. Ovulation did not occur in 18 rats when only the anterior connections to the MBH were interrupted by an extended frontal cut just posterior to the optic chiasm, reaching 1. 5 mm lateral from midline and extending from the base of the brain to the paraventricular nuclei above. A small frontal cut at the same level but extending only 0. 6 mm lateral from midline was ineffective in blocking ovulation. When placed 1. 5 mm behind the level of the optic chiasm this small frontal cut interfered with ovulation in 5 of 10 rats. OCH was blocked only in the animals with complete deafferentation of the MBH. In all rats with hypothalamic deafferenta-tions, gonadectomy increased pituitary and serum LH [luteinizing hormone] and castration cells developed in the pituitaries. The pituitary response to castration was diminished in the animals with complete deafferentation of the MBH. It is concluded that neural af-ferents which reach the MBH via the anterior hypothalamus are required for ovulation; above the optic chiasm these fibers are diffusely organized and converge toward the median eminence; at least 2 levels exist in the system for the negative feedback of gonadal steroids-one is represented by the MBH-pituitary complex, the other might be located in the anterior hypothalamus; the latter level can be independent of the neural afferents indispensable for ovulation; the MBH, which acts directly on the pituitary by its GTH releasing factors, is by itself capable of releasing these substances only in a tonic fashion; the nervous structures responsible for the cyclic release of the releasing factors (particularly LH-releasing factor) lie outside the MBH but influence the pituitary through it.