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 mglg 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 epithelia1 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