Hypothalamic-Pituitary Function in Adult Rats Treated Neonatally with Monosodium Glutamate*

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Abstract
Large doses of monosodium-i-glutamate (MSG) administered to rats early in life induce permanent lesions in the retina and the hypothalamic arcuate nucleus. Despite normal serum LH and FSH concentrations, the reproductive capabilities of MSG-treated rats are diminished. The present study was conducted to determine hypothalamic-pituitary responsiveness to exogenous LHRH, naloxone (NAL), or n-methyl-aspartic acid (NMA) in MSG-treated rats. In addition, the effects of castration on plasma LH pulses were studied in male rats. Sprague-Dawley rats were injected sc with MSG (4 mg/g) or NaCl at 2, 4, 6, 8, and 10 days of age. Treatment with MSG produced arcuate lesions, obesity, and hypogonadism in males and females and reduced serum testosterone concentrations (P ≤ 0.05) in males. Concentrations of serum LH and FSH and hypothalamic LHRH were not affected significantly in either sex. In both male and female MSG-treated rats, NAL, NMA, and LHRH stimulated LH secretion equal to or greater than in saline controls. Castrated male MSG-treated rats had significantly lower (P < 0.05) mean plasma LH concentrations and lower mean pulse amplitude than saline-treated controls, but LH pulse frequencies were not altered significantly. These data suggest the following. 1) Exogenous LHRH and two stimuli (NAL and NMA) thought to release LHRH from the hypothalamus stimulate LH secretion in MSG-treated rats. Pituitary gland LH responsiveness to LHRH (150 ng, sc) is significantly greater after MSG treatment, whereas the LH responsiveness to NAL and NMA is similar in MSG- and saline-treated rats. 2) The hypothalamic LHRH of MSG-treated animals is bioactive. 3) Under the conditions of our experiments, male MSG-treated castrates (105 days of age) secrete LH pulses at the same frequency as saline-treated controls but with lower amplitude, resulting in a lower mean serum LH concentration. 4) Male MSG-treated castrates (105 days of age) release less LHRH than the saline-treated controls.