Brain is an important source of GnRH in general circulation in the rat during prenatal and early postnatal ontogenesis

Mechanisms of Reciprocal Regulation of Gonadotropin-Releasing Hormone (GnRH)-Producing and Immune Systems: The Role of GnRH, Cytokines and Their Receptors in Early Ontogenesis in Normal and Pathological Conditions

Different aspects of the reciprocal regulatory influence on the development of gonadotropin-releasing hormone (GnRH)-producing- and immune systems in the perinatal ontogenesis and their functioning in adults in normal and pathological conditions are discussed. The influence of GnRH on the development of the immune system, on the one hand, and the influence of proinflammatory cytokines on the development of the hypothalamic-pituitary-gonadal system, on the other hand, and their functioning in adult offspring are analyzed. We have focused on the effects of GnRH on the formation and functional activity of the thymus, as the central organ of the immune system, in the perinatal period. The main mechanisms of reciprocal regulation of these systems are discussed. The reproductive health of an individual is programmed by the establishment and development of physiological systems during critical periods. Regulatory epigenetic mechanisms of development are not strictly genetically controlled. These processes are characterized by a high sensitivity to various regulatory factors, which provides possible corrections for disorders.

Developing brain as a source of circulating norepinephrine in rats during the critical period of morphogenesis

The development of individual organs and the whole organism is under the control by morphogenetic factors over the critical period of morphogenesis. This study was aimed to test our hypothesis that the developing brain operates as an endocrine organ during morphogenesis, in rats during the perinatal period (Ugrumov in Neuro Chem 35:837-850, 2010). Norepinephrine, which is a morphogenetic factor, was used as a marker of the endocrine activity of the developing brain, although it is also secreted by peripheral organs. In this study, it was first shown that the concentration of norepinephrine in the peripheral blood of neonatal rats is sufficient to ensure the morphogenetic effect on the peripheral organs and the brain itself. Using pharmacological suppression of norepinephrine production in the brain, but not in peripheral organs, it was shown that norepinephrine is delivered from the brain to the general circulation in neonatal rats, that is, during morphogenesis. In fact, even partial suppression of norepinephrine production in the brain of neonatal rats led to a significant decrease of norepinephrine concentration in plasma, suggesting that at this time the brain is an important source of circulating norepinephrine. Conversely, the suppression of the production of norepinephrine in the brain of prepubertal rats did not cause a change in its concentration in plasma, showing no secretion of brain-derived norepinephrine to the bloodstream after morphogenesis. The above data support our hypothesis that morphogenetic factors, including norepinephrine, are delivered from the developing brain to the bloodstream, which occurs only during the critical period of morphogenesis.

Gonadotropin-Releasing Hormone in Regulation of Thymic Development in Rats: Profile of Thymic Cytokines

An increasing body of recent experimental data confirms the impact of neurohormones on fetal development and function of different body systems. The synthesis of many neurohormones starts in fetal tissues before the hypothalamic–pituitary–adrenal and hypothalamic–pituitary–gonadal systems are formed, and their high levels are detected in the bloodstream. Here, we studied the role of gonadotropin-releasing hormone (GnRH) in rat thymus development and tried to reveal possible mechanisms underlying the GnRH effects in early development. Western blotting and reverse transcription-polymerase chain reaction allowed us to identify receptor for GnRH in the fetal thymus with peak expression on embryonic days 17–18 (ED17–18). Blocking the receptors in utero on ED17 by a GnRH antagonist suppressed the concanavalin A-induced proliferative response of T cells in adults. GnRH (10⁻⁷ M) increased mRNA expression of interleukin (IL)-4, IL-10, IL-1β, interferon γ (IFNγ), and tumor necrosis factor α (TNFα) in the thymus of 18-day fetuses after an ex vivo culture for 24 h. The increased mRNA levels of the cytokines in the thymus were accompanied by increased numbers of CD4+ T helpers. Overall, the data obtained confirm the regulatory or morphogenetic effect of GnRH on fetal thymus development mediated by synthesis of thymic cytokines.

Role of Adenohypophysotropic Neurohormones in Endocrine Paraadenohypophysial Regulation of Peripheral Target Organs in Rat Ontogeny

We tested the hypothesis that brain-derived chemical stimuli contribute to direct endocrine regulation of peripheral organs during ontogeny before blood-brain barrier closure. Dopamine and gonadotropin-releasing hormone present in high concentration in peripheral blood only before blood-brain barrier closure were chosen as the chemical stimuli. It was shown than dopamine in concentrations equal to its level in the peripheral blood inhibits prolactin secretion in organotypic culture of the pituitary gland from newborn rats via specific receptors. Experiments on organotypic culture of neonatal rat testicles showed that gonadotropin-releasing hormone stimulates testosterone secretion via specific receptors. We proved that chemical stimuli entering common circulation from the brain before blood-brain barrier closure could exert direct endocrine effect on peripheral organs.

Brain as an endocrine source of circulating 5-hydroxytryptamine in ontogenesis in rats

This study was aimed to test the authors' hypothesis stating that the developing brain before the closure of the blood brain barrier (BBB) operates as an endocrine organ that secretes classical neurotransmitters and neuropeptides into the general circulation. 5-Hydroxytryptamine (5-HT) was selected as a marker of brain endocrine activity though it is also secreted by peripheral organs. 5-HT was detected in blood of rats in a biologically active concentration at any studied age, from the 21st embryonic day till the 30th postnatal day. The brain was proven to be a source of circulating 5-HT before the BBB closure by showing that the 5-HT concentration in blood decreased significantly after the inhibition of 5-HT synthesis in the brain of neonates. The 5-HT concentration in blood was not diminished after the BBB closure, apparently due to compensatory increase of 5-HT secretion by peripheral sources. Thus, brain-derived 5-HT is delivered to the general circulation before the BBB closure being potentially capable of providing endocrine regulation of target organs.

Developing brain as an endocrine organ: secretion of dopamine

This study was aimed to test our hypothesis that the developing brain operates as an endocrine organ before the establishment of the blood-brain barrier (BBB), in rats up to the first postnatal week. Dopamine (DA) was selected as a marker of the brain endocrine activity. The hypothesis was supported by the observations in rats of: (i) the physiological concentration of DA in peripheral blood of fetuses and neonates, before the BBB establishment, and its drop by prepubertal period, after the BBB development; (ii) a drop of the DA concentration in the brain for 54% and in blood for 74% on the 3rd postnatal day after the intraventricular administration of 50 μg of α-methyl-p-tyrosine, an inhibitor of DA synthesis, with no changes in the DA metabolism in peripheral DA-producing organs. Thus, the developing brain is a principal source of circulating DA which is capable of providing an endocrine regulation of peripheral organs and the brain.