Administration of Monosodium Glutamate to Neonatal Male Rats: Alterations in the Gonadotrophs and in Gonadotrophin Secretion

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Abstract
We have studied the effect of administration of L-monosodium glutamate (MSG) to neonatal rats on gonadotroph morphology and gonadotrophin secretion in the prepubertal male rat. Rats were injected with MSG (4 mg/g body weight) or with 0.1 ml saline/10 g body weight on days 1, 3, 5, 7 and 9 of life (day of birth = day 0) and were used for experiment on day 40. Trunk blood was collected from 8 saline- and 9 MSG-injected rats for assay of serum luteinizing hormone (LH) and follicle-stimulating hormone (FSH) concentrations. One-half of the anterior pituitary gland was assayed for LH and FSH concentrations and the other half was placed in culture medium for a 30-min preincubation and then placed in fresh medium for a 2-hour incubation (basal LH and FSH release). An additional 4 rats in each group were killed and the pituitary glands were prepared for histological examination and immunocytochemical staining of LH and FSH cells and morphometric examination of these cells at the light microscopic level. The morphometric analyses were compared with those performed previously by us on adult male rats. The results suggest that in 40-day-old male rats treated neonatally with MSG (1) serum LH concentration is maintained at normal levels despite a small anterior pituitary gland; (2) gland LH concentration is elevated resulting in normal gland LH stores; (3) the total gland basal LH release rate is normal and this may play the major role in the maintenance of normal serum LH levels; (4) gland FSH concentration is normal but gland FSH stores are reduced which may contribute to the trend towards a lower total gland basal FSH release rate and subsequently reduced serum FSH concentrations; (5) individual gonadotrophs are smaller but store increased amounts of gonadotrophin, and (6) FSH cells are fewer in number relative to the number of LH cells and like the saline-injected controls ‘all’ FSH cells contain LH. The results also suggest that the percentage of gonadotrophs that are LH or LH/FSH cells are similar in prepubertal and adult male rats and that there are more gonadotrophs per unit volume of anterior pituitary gland in the prepubertal males.