Effect of neonatal treatment with monosodium glutamate on dopaminergic andL-DOPA-ergic neurons of the medial basal hypothalamus and on prolactin andMSH secretion of rats

The effect of neonatal treatment with monosodium L-glutamate (MSG) on the d opaminergic systems of the medial basal hypothalamus has been investigated using tyrosine hydroxylase (TH) and aromatic L-amino acid decarboxylase (AA DC) immunocytochemistry. Changes in plasma levels of prolactin (PRL) and al pha -melanocyte-stimulating hormone (MSH) have also been determined in inta ct and in MSG-treated rats after inhibition of TH by alpha -methyl-p-tyrosi ne (alpha -MpT) or without inhibition of enzyme activity. Monosodium glutam ate resulted in a 40% reduction in the number of TH immunopositive tuberoin fundibular neurons, but no change in the number of AADC-positive tuberoinfu ndibular nerve cells, indicating that this reduction has occurred mainly in TH-positive but AADC-negative elements, i.e., in L-DOPA-ergic neurons. In contrast, MSG did not cause changes in the number of TH and AADC immunoreac tive neurons of the periventriculohypophysial and tuberohypophysial dopamin ergic systems, and it did not influence basal plasma PRL levels. alpha -met hyl-p-tyrosine has increased plasma PRL concentrations in both control and MSG-treated rats of both sexes, but significantly higher responses were det ected in females. None of the treatments had any effect on plasma MSH level . These findings suggest that MSG affects primarily L-DOPA-ergic neurons lo cated in the ventrolateral part of the arcuate nucleus, but not dopaminergi c neurons situated in the dorsomedial part of the arcuate nucleus; neither PRL nor MSH secretion is altered by MSG; a significant sex difference exist s in the pituitary PRL response to inhibition of TH, and this response is n ot affected by MSG. (C) 2001 Elsevier Science Inc.