Role of angiotensin type 1A receptors in the sympathetic activation of angiotensin-dependent hypertension
Nikola JancovskiD.A. CarterDeli ChenY.-T. ChoongThu-Phuc NguyenЕ. В. ЛукошковаAngela A. ConnellyJaspreet K. BassiGeoffrey A. HeadClément MenuetAndrew M. Allen
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Catecholaminergic cell groups
Sympathetic nervous system
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The present study describes the distribution and cellular morphology of catecholaminergic neurons in the CNS of two species of monotreme, the platypus <i>(Ornithorhynchus anatinus)</i> and the short-beaked echidna <i>(Tachyglossus aculeatus)</i>. Tyrosine hydroxylase immunohistochemistry was used to visualize these neurons. The standard A1–A17, C1–C3 nomenclature was used for expediency, but the neuroanatomical names of the various nuclei have also been given. Monotremes exhibit catecholaminergic neurons in the diencephalon (A11, A12, A13, A14, A15), midbrain (A8, A9, A10), rostral rhombencephalon (A5, A6, A7), and medulla (A1, A2, C1, C2). The subdivisions of these neurons are in general agreement with those of other mammals, and indeed other amniotes. Apart from minor differences, those being a lack of A4, A3, and C3 groups, the catecholaminergic system of monotremes is very similar to that of other mammals. Catecholaminergic neurons outside these nuclei, such as those reported for other mammals, were not numerous with occasional cells observed in the striatum. It seems unlikely that differences in the sleep phenomenology of monotremes, as compared to other mammals, can be explained by these differences. The similarity of this system across mammalian and amniote species underlines the evolutionary conservatism of the catecholaminergic system.
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The initial part of the noradrenergic cerebral system--the locus caeruleus neurons--has been studied light and electron microscopically at various time after injection of 6- hydrodopamine (6-OHDA), a substance possessing a selective neurotoxic effect on the catecholaminergic mediatory system. Intracisternal injection of 6-OHDA (300 mcg) produces a number of reactive rearrangements in the neurons and large dendrites. Nevertheless, the death of the neural cells is not observed even by the 36th day.
Locus coeruleus
Catecholaminergic cell groups
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Catecholamines (norepinephrine, dopamine, epinephrine) act in the brain as chemical neurotransmitters and represent integrative component of many anatomical and functional interrelationships, which play important role in the maintenance of the basic physiological processes and homeostasis of living organism. In the brain, several well circumscribed conglomerations of catecholaminergic neurons and dopaminergic and noradrenergic pathways can be recognized. Although they are represented by only a few thousands of catecholaminergic neurons (in rat about 5) located only in certain brain areas, their rich arborization provides extensive innervation over the whole brain. Catecholamines are significantly involved in conveying of viscero- and somato-sensitive signals to integrative centrers located in higher brain areas and participate in the regulation of all vitally important systems under basal conditions as well as during stress. Their normal physiological activity is important for the maintenance of healthy functioning of the organism. Brainstem aggregations of catecholaminergic neurons, localized predominantly in autonomic regions, are involved in conveying the afferent peripheral stress and cardiovascular signals. The hypothalamic paraventricular nucleus, which represents an integrative center of the stress response, receives a rich catecholaminergic innervation from the caudal brain. On the other hand, catecholaminergic neurons localized in the ventrolateral rostral medulla form an important component of circuits involved in the regulation of the cardiovascular system. Central catecholamines are also involved in many other important brain circuits, however, with respect to the limited space of this review, they could not be included.
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Brainstem catecholaminergic neurons significantly participate in the regulation of neuroendocrine system activity, particularly during stressful conditions. However, so far the precise quantitative characterisation of basal and stress-induced changes in gene expression and protein levels of catecholaminergic biosynthetic enzymes in these neurons has been missing. Using a quantitative reverse transcription-polymerase chain reaction method, we investigated gene expression of catecholamine biosynthetic enzymes in brainstem noradrenergic and adrenergic cell groups in rats under resting conditions as well as in acutely and repeatedly stressed animals. For the first time, we described quantitative differences in basal levels of catecholamine biosynthetic enzyme mRNA in brainstem catecholaminergic ascending and descending projecting cell groups. Moreover, we found and defined some differences among catecholaminergic cell groups in the time-course of mRNA levels of catecholaminergic enzymes following a single and especially repeated immobilisation stress. The data obtained support the assumption that brainstem catecholaminergic cell groups represent a functionally differentiated system, which is highly (but specifically) activated in rats exposed to stress. Therefore, potential interventions for the treatment of stress-related diseases need to affect the activity of brainstem catecholaminergic neurons not uniformly but with some degree of selectivity.
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