Tyrosine hydroxylase expression in differentiating neurons of the rat arcuate nucleus: stimulatory influence of serotonin afferents.

Viktoria I Melnikova,André Calas,Evgenia V Proshlyakova,Jean Thibault,Michael V Ugrumov

doi:10.1155/np.2001.271

Abstract

The influence of serotonin afferents on tyrosine hydroxylase expression in differentiating neurons of the rat arcuate nucleus was studied in vivo and in vitro. In the in vivo study, pchlorophenylalanine inhibited serotonin synthesis in fetal brain from the 11th to the 20th embryonic day. We then used semiquantitative immunocytochemistry to evaluate tyrosine hydroxylase levels in neurons of the arcuate nucleus in fetuses at the 21st embryonic day or in offspring at the 35th postnatal day. Serotonin depltion significantly decreased the tyrosine hydroxylase content in neurons of males and females at the 21st embryonic day and in males at the 35st postnatal day. For the in vitro study, embryonic neurons of the arcuate nucleus were cocultured with embryonic neurons of the raphe nucleus, the main source of serotonin innervation of the brain, including the arcuate nucleus. Co-culture of the neurons resulted in a genderspecific increase of the tyrosine hydroxylase level in the neurons of the arcuate nucleus. In turn, the neurons of the raphe nucleus showed increased levels .of serotonin in both males and females, with no sexual dimorphism. Thus, our results suggest a stimulatory, long-lasting effect of serotonin afferents on tyrosine hydroxylase expression in the differentiating neurons of the rat arcuate nucleus during prenatal ontogenesis.

Highlights

Since the initial detection of dopamine in neurons of the arcuate nucleus and their axon terminals in the median eminence with the histofluorescence technique (Dahlstr6m & Fuxe, 1964; Fuxe, 1964; Bj6rklund & Nobin, 1973), the socalled tuberoinfundibular dopaminergic system has attracted the particular attention of neurobiologists and neuroendocrinologists
Fetuses: at E21, most tyrosine hydroxylase (TH)-immunoreactive neurons were located in the ventrolateral region of the arcuate nucleus, a few were found in the dorsomedial region (Fig. 1)
The THimmunoreactive neurons, mainly unipolar or bipolar, were small, pCPA treatment of pregnant rats caused a decrease in the optical density (OD) of TH-immunoreactive neurons of the arcuate nucleus in male and female fetuses, with no gender difference (Fig. 2)

Summary

Introduction

Since the initial detection of dopamine in neurons of the arcuate nucleus and their axon terminals in the median eminence with the histofluorescence technique (Dahlstr6m & Fuxe, 1964; Fuxe, 1964; Bj6rklund & Nobin, 1973), the socalled tuberoinfundibular dopaminergic system has attracted the particular attention of neurobiologists and neuroendocrinologists. This interest is explained by the great contribution of the tuberoinfundibular dopaminergic system to the inhibitory control of the adenohypophysial prolactin secretion and, thereby, to the regulation of reproduction (Neill, 1988). Serotonin afferents appear to be among the potential regulators of the differentiation of dopaminergic neurons as

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