Effects of neonatal administration of monosodium glutamate on four neuropeptide concentrations in the rat brain

The present study was designed to measure concentrations of four neuropeptides in different brain regions in monosodium glutamate(MSG)-treated rats and to assess molecular forms of each peptide with gel and high performance liquid chromatography (HPLC). MSG(4mg/kg body weight) or 10% NaCl was injected subcutaneously on postnatal days 1, 3, 5, 7 and 9 to male littermates which were subsequently used on postnatal day 100. Rats were sacrificed by decapitation, and the brains were dissected into ten discrete regions. The brain extracts were subjected to measurement of four neuropeptides; somatostatin (SRIF), neuropeptide Y (NPY), atrial natriuretic polypeptide(ANP), and a novel pituitary polypeptide 7B2 by specific radioimmunoassays. Significant increase (p less than 0.01) in midbrain SRIF content was observed in MSG-treated rats, though there was no significant change in hypothalamic SRIF content. Significant reduction (p less than 0.05) in hypothalamic NPY content was also found in MSG-treated rats. Hypothalamic ANP content was similar in both MSG-treated and control rats. A significant increase of 7B2 content was found in substantia nigra/ventral tegmentum and hypothalamus (p less than 0.05 or p less than 0.01, respectively) in MSG-treated rats. These four immunoreactivities were further characterized by gel permeation or high pressure liquid chromatography (HPLC). Chromatographic analysis of SRIF immunoreactivity revealed that there were two distinctive peaks and smaller molecular weight component corresponding to SRIF. Fractionation of NPY or 7B2 immunoreactivity by gel permeation showed a single major peak which was identical to the synthetic NPY or 7B2 immunoreactivity from porcine pituitary extract. HPLC analysis for ANP immunoreactivity also showed that the major immunoreactive component corresponded to synthetic rat ANP. MSG treatment could not produce any major alterations in proportions of molecular forms studied. These results suggest that MSG treatment in neonates might produce the alterations in SRIF, NPY and 7B2 content in the discrete brain regions including the hypothalamus.