Modeling of chronic selective inhibition of noradrenaline synthesis in the brain of neonatal rats

Abstract

Ontogenetic development and functioning of an organism are regulated by intracellular chemical sig naling; the signal molecules are secreted by the neu roendocrine system and function as morphogenetic or transcription factors affecting target cells and organs via specific receptors. Among the great variety of chemical signals controlling the development of target organs and tissues, noradrenaline (NA), which is syn thesized mainly by brain neurons, peripheral sym pathoadrenal system, and extra adrenal chromaffine tissue, is one of the most efficient factors exhibiting a broad range of physiological effects [1,2].We have proposed a hypothesis that, before the for mation of the blood-brain barrier (BBB), the brain functions as an endocrine organ secreting physiologi cally active substances into the general circulation and thus has a direct endocrine effect on the develop ment and functioning of peripheral target organs and tissues [3].Evidence efficiently supporting this hypothesis was obtained in our previous study using serotonin, dopamine (DA), and gonadotropin releasing hor mone as markers of endocrine functions of the devel oping brain [4].The purpose of the present work was to obtain direct evidence in support of our hypothesis that the developing brain functions as an endocrine source secreting NA into the general circulation. In particu lar, our aim was to develop a model of chronic (irre versible) suppression of NA synthesis by brain neurons in ontogeny before the BBB formation and to evaluate the changes in NA levels in the brain, peripheral blood, and peripheral organs using this model.The study was performed in male Wistar rats ana lyzed on postnatal days 2, 3, and 6 (P2, P3, and P6). The day when animals were born was considered post natal day 1. Animals were kept in a vivarium under standard conditions with unlimited food and water supply.Long term (chronic) pharmacological inhibition of NA synthesis in rat brain was ensured by adminis tration of 6 hydroxydopamine (6 OHDA), a specific neurotoxin that causes degeneration of catecholamin ergic neurons. Via DA and NA membrane transport ers, 6 OHDA is transported into the target cells and induces uncoupling of the oxidative phosphorylation chain and oxidative stress in mitochondria, which results in cell death [5]. GBR 12909, which inhibits 6 OHDA reuptake by DA neurons and thus acts as a specific neuroprotector, was administered systemi cally 60 min prior to 6 OHDA injections to prevent the death of dopaminergic neurons (DA neurons) [6].The first series of experiments was performed with 60 animals. Rats were subcutaneously injected first with 40 mg/kg GBR 12909 dihydrochloride (Tokris Bioscience, Great Britain), and then, 60 min later, with 250 or 150 μg 6 OHDA in 25 μL of 0.9% NaCl solution containing 0.1% ascorbic acid. The control animals received 0.9% NaCl instead of 6 OHDA.The second series of experiments was performed with 20 animals. Rats from the experimental group (ten animals) were administered with 150 μg 6 OHDA in 2 μL of 0.1% ascorbic acid s...