The brain is one of the most important sources of dopamine in the systemic circulation in the perinatal period of ontogenesis in rats.

This study was designed to test the authors' hypothesis that dopamine passes from dopamine-synthesizing cells in the brain to the systemic circulation prior to the formation of the blood-brain barrier during ontogenesis. High-performance liquid chromatography studies demonstrated that peripheral blood dopamine levels before formation of the blood-brain barrier-in rat fetuses and neonates-are significantly higher than after formation of the barrier in adult rats, providing indirect evidence in support of the hypothesis. Furthermore, formation of the blood-brain barrier is accompanied by a significant increase in dopamine levels in the rat brain. Direct evidence for the hypothesis was obtained in the form of a sharp decrease in blood dopamine levels in fetuses after lesioning of dopamine-synthesizing neurons in the brain by encephalectomy.

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

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 depletion significantly decreased the tyrosine hydroxylase content in neurons of males and females at the 21st embryonic day and in males at the 35th 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 gender-specific 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.

Hypothalamo-pituitary control of the cell-mediated immunity in rat embryos: role of LHRH in regulation of lymphocyte proliferation.

The role of the neuroendocrine system in the development of cell-mediated immunity has been studied in fetal rats. The spontaneous and mitogen-induced proliferation of liver lymphocytes and thymocytes was evaluated in vitro in rats at the 22nd prenatal day following surgical ablation of the forebrain (encephalectomy) or of the entire brain and pituitary (decapitation) in rat fetuses in utero at the 18th day. Non-operated and sham-operated fetuses served as controls. The ablation of the entire brain and pituitary in rat fetuses resulted in an increase (40-60%) of spontaneous proliferation of liver and thymic cells in comparison with sham-operated fetuses. The ablation of the forebrain including the hypothalamus caused a decrease in the mitogenic proliferative response of thymocytes and liver lymphocytes for 40 and 20%, respectively. The ablation of the entire brain including the hypothalamus and pituitary resulted in a 80% decrease of the proliferative response of thymocytes and in the full suppression of proliferation of liver lymphocytes. The immune proliferative response was restored by the LHRH administration either systemically to operated fetuses (0.2 microg/fetus) or to the cell culture (10(-9) and 10(-7) M). It was concluded that the central nervous system was important for maturation of the immune system in rats during the prenatal period. In particular, neuroendocrine system are likely to play a major role as LHRH treatment in vitro and in vivo appeared to contribute to this regulation.

Prolactin secretion and its dopamine inhibitory control in rat fetuses.

This study has determined in rats the ontogenetic schedule of the onset of pituitary prolactin (PRL) synthesis and release as well as of the establishment of the dopamine (DA) inhibitory control of PRL secretion. RIA recognized PRL traces in the pituitary at the 18th embryonic day (E18), although a clearly detectable amount of this hormone was first measured at E20, suggesting the onset of PRL synthesis. The PRL level in the pituitary increased significantly by E22, in females to a higher extent than in males. Decapitation of fetuses did not cause any change in the PRL plasma level in males showing no PRL release from the pituitary until term. Conversely, there was a slight but significant fall of plasma PRL in decapitated females, suggesting PRL release from the pituitary. An inhibition of DA receptors on lactotropes of fetuses resulted in an increased level of plasma PRL at E20, but not at E18, while the pituitary content of PRL remained unchanged. The same treatment at E22 caused a significant increase of the PRL concentration in plasma and a concomitant fall in the pituitary that could be prevented by preliminary encephalectomy. These data show that the tuberoinfundibular DA system begins to inhibit PRL release from lactotropes between E20 and E22, completely arresting PRL release from the pituitary in males but not in females.

Androgen-dependent sex differences in the hypothalamic serotoninergic system.

This study has attempted to fulfill our knowledge on the sex differences in the hypothalamic serotoninergic (5-HT) system in adult rats, and also to evaluate the role of neonatal androgens in the appearance of this sexual dimorphism. Such integrative characteristics of the 5-HT system as 5-HT content and specific uptake were estimated and compared in the anterior and middle hypothalami in intact adult females and males, as well as in neonatally castrated adult males. According to our data, the 5-HT content and [3H]5-HT specific uptake both in the anterior and middle hypothalami of intact females exceeded significantly those in intact males. Neonatal castration of males resulted in an increase of the 5-HT content and [3H]5-HT uptake in both hypothalamic regions up to their levels in intact females. Thus, the present study provides new information on the sex differences in the hypothalamic 5-HT system, which are apparently related to the neonatal masculinization of the hypothalamus.

Ontogenesis of the hypothalamic catecholaminergic system in rats: synthesis, uptake and release of catecholamines.

The development of the hypothalamic catecholaminergic system during ontogenesis in rats has been studied with glyoxylic acid histofluorescent method in vivo and with isotopic biochemical technique in vitro. It has been demonstrated that at the 15th fetal day the catecholaminergic system was functionally inactive at least in its ability for the uptake and K(+)-stimulated release of catecholamines. Since the 16th fetal day, hypothalamic neuronal elements gained an ability for synthesis of catecholamines, their specific uptake and K(+)-evoked release. Over the subsequent two days, the intensity of the fluorescent intraneuronal product rose considerably showing the increase of either synthesis or accumulation of catecholamines. Simultaneously, the values of the uptake and K(+)-stimulated release of the exogenous radioactively-labelled dopamine increased significantly. The intensity of the fluorescence of the hypothalamic neuronal elements dropped from 20th fetal until the ninth postnatal day, whereas the specific uptake doubled over the same period reaching its adult level. By the 21st postnatal day the reaccumulation of the fluorescent product occurred.

Development of the hypothalamic 5-hydroxytryptamine system during ontogenesis in rats: uptake and release of 5-hydroxytryptamine in vitro.

The development of the hypothalamic 5-hydroxytryptamine system has been evaluated in vitro according to [3H]5-hydroxytryptamine uptake and release in fetuses (16-20th fetal day), neonates (9th postnatal day) and adults (45th day of life). At the 16th fetal day the hypothalamic neural elements were characterized by specific uptake of 5-hydroxytryptamine and its spontaneous release; the next day K+-stimulated Ca2+-dependent release appeared. By the 18th fetal day, the 5-hydroxytryptamine uptake doubled and was retained at this level both in older fetuses and in postnatal rats. The K+-stimulated release of [3H]5-hydroxytryptamine increased considerably during the perinatal period, reaching an adult level by the 9th postnatal day. These data indicate the sprouting of 5-hydroxytryptamine fibers to the hypothalamus and the maturation of their membrane mechanisms for the bidirectional transport of 5-hydroxytryptamine early during ontogenesis-to a significant extent before the 18th fetal day.

Studies on the hypothalamic control of thyroid function in the rabbit fetuses.

In order to reveal the onset in ontogenesis of hypothalamic control of thyroid function 22-23-day-old fetuses of rabbits were encephalectomized. The concentrations of T3 and T4 in thyroid glands and blood serum were measured radioimmunologically on the 29th and 30th day of prenatal life. Significant differences of T3 and T4 concentrations are revealed only in thyroid glands of 30-day-old encephalectomized fetuses. Treatment of encephalectomized fetuses by TRH 30 min prior to fixation removed the effect of encephalectomy on hormone concentration and on thyroid structure. The represented data are considered as a proof of the establishment of hypothalamic cotrol of hormone synthesis in thyroid gland beginning from the 30th day of prenatal life. The regulation of hormone release from the gland is probably established at a later stage of maturation.