The aim of this study was to investigate changes of calretinin immunoreactivity in neurons and neuropil of the dorsal raphe nucleus (DRN) after subcutaneous administration of monosodium glutamate (MSG) to adult rats. Studies were conducted on 60-day-old male rats. The animals were divided into a control group (C) and two other groups receiving MSG at a dose of 2g/kg b.w. (I) and 4g/kg b.w. (II) subcutaneously for 3 consecutive days. Immunohistochemical peroxydese-antiperoxydase reaction was conducted with the use of a specific anti-calretinin (CR) antibody on brain slides containing DRN of 63-day-old rats. The cells and neuropil were morphologically and morphometrically analysed under the light microscope Olympus BX51. Statistically significant differences were studied with ANOVA and nonparametric Kruskal-Wallis test. In 63-day-old rats, in DRN: dorsal (DRNd), ventral (DRNv) and interfascicular (DRNif) parts, in animals receiving MSG (I and II), there was a decrease in CR- immunoreactivity in neurons and neuropil in comparison to control rats. Only in the ventrolateral part (DRNvl) a few intensively stained CR-immunoreactive cells were demonstrated. Light microscope observations were confirmed by morphometric analyses. In the DRNd and DRNv of rats receiving MSG (I and II) a decrease in average CR-immunoreactive neuron density was shown in comparison to the C group. In the DRNvl part, a statistically significant decrease in the analysed parameter was present only in I group of animals. Conversely, in DRNif no statistically significant differences were shown between studied groups of rats. In the DRN of animals receiving MSG (I and II) a decrease in average digital immunostaining intensity for CR occurred in neurons and neuropil. The obtained results demonstrated a decrease in CR immunostaining intensity level in neurons and neuropil and a decrease in density of studied protein immunoreactive cells under the influence of subcutaneous administration of MSG to adult rats. These results suggest that MSG may cause neuronal death as a result of oxidative stress or it can alter a calretinin conformation in cells after binding to calcium ions.
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