Effect of neonatal treatment with MSG (monosodium glutamate) on thyroid of the adult male rats.

Abstract

Monosodium glutamate (MSG), administered to newborn rats produces extensive lesions in neurons of the arcuate nuclei of the hypothalamus. The cells represent the site of neurohormone production, including the production of both growth-hormone releasing hormone (GHRH) and somatostatin. Studies were performed on male Wistar strain rats, subcutaneously injected with MSG, at 4 mg/g body weight, on days 2, 4, 6, 8 and 10 of life. When the rats reached the age of 18 months, they were additionally stimulated with a single dose of TSH (Ambinon). When the rats reached the age of 6, 12, or 18 months, their thyroids were isolated and fixed in Bouin's solution. In HE-stained preparations, planimetric and volumetric proportions occupied by the epithelial fraction, colloid and stroma, the number of thyroid follicles per mm2 and the thickness of epithelium were determined. Serum T3 and T4 levels were quantified by RIA. Significance of differences was tested using Student's t test. The weight of experimental rat thyroids showed no significant variations as compared to the controls but were greatest in the group of 12-month-old rats. The same was noted for the volumetric fractions of epithelium, colloid and stroma. The planimetric fractions occupied by epithelium, colloid and stroma in the thyroid remained unchanged and amounted to 60%, 31% and 9%, respectively. The number of follicles per mm2 thyroid cross-section in the MSG-treated 6-, 12- or 18-month-old rats was 131.3, 116.2 and 130.4, respectively, and did not differ from control values. Thickness of follicular epithelium showed no significant variations. Serum T3 levels in the rats examined after 6, 12 or 18 months were increased by 67%, 89% and 33%, respectively, as compared to serum T4 levels. When compared to the controls, the serum T4 level was lower only in the 12-month-old MSG-treated rats and the serum T3 level was higher in 18-month-old MSG-treated rats. The ability of the thyroid to respond to Ambinon stimulation was preserved. The results of our investigations suggest that the rat hypothalamic centers involved in regulation of the pituitary-thyroid axis are slightly affected by neonatal administration of MSG.