Direct Evidence that the Arcuate Nucleus-Median Eminence Tuberoinfundibular System Is Not of Primary Importance in the Feedback Regulation of Luteinizing Hormone and Follicle-Stimulating Hormone Secretion in the Castrated Rat*

Abstract

Adult rats which were received monosodium-L-glutamate (MSG, 4 mg/g BW [body weight]) on alternate days for the 1st 10 days of life manifest CNS lesions largely restricted to the retina and the arcuate nucleus of the hypothalamus in which nearly 90% of the perikarya are destroyed, leaving axons in passage intact. In animals so treated, concentrations of dopamine within the arcuate nucleus and median eminence of the hypothalamus are reduced 52% and 57%, respectively, in males and 45% and 61% in females, whereas concentrations of norepinephrine [NE] in these same 2 areas are normal. Concentrations of both NE and dopamine in 5 other hypothalamic nuclei (dorsal septal, medial preoptic, suprachiasmatic, periventricular, and dorsomedial nuclei) are unchanged. Nevertheless, despite the destruction of the arcuate nucleus cell bodies of MSG-treated rats, postcastration levels of serum FSH [follicle stimulating hormone] and LH [luteinizing hormone] in males, and FSH in females were not significantly different from FSH and LH values in castrated controls. Serum LH in castrated, MSG-treated females was slightly but significantly lower than in castrated controls. The arcuate nucleus-median eminence tuberoinfundibular neurons are not of primary importance in the tonic, negative feedback regulation of gonadotropin secretion.