Role of the locus coeruleus on blood pressure response and atrial natriuretic peptide secretion following extracellular volume expansion

Noradrenergic neuromodulation in ageing and disease

The locus coeruleus (LC) is a small brainstem structure located in the lower pons and is the main source of noradrenaline (NA) in the brain. Via its phasic and tonic firing, it modulates cognition and autonomic functions and is involved in the brain's immune response. The extent of degeneration to the LC in healthy ageing remains unclear, however, noradrenergic dysfunction may contribute to the pathogenesis of Alzheimer's (AD) and Parkinson's disease (PD). Despite their differences in progression at later disease stages, the early involvement of the LC may lead to comparable behavioural symptoms such as preclinical sleep problems and neuropsychiatric symptoms as a result of AD and PD pathology. In this review, we draw attention to the mechanisms that underlie LC degeneration in ageing, AD and PD. We aim to motivate future research to investigate how early degeneration of the noradrenergic system may play a pivotal role in the pathogenesis of AD and PD which may also be relevant to other neurodegenerative diseases.

Sodium Intake and Disease: Another Relationship to Consider

Sodium (Na+) is crucial for numerous homeostatic processes in the body and, consequentially, its levels are tightly regulated by multiple organ systems. Sodium is acquired from the diet, commonly in the form of NaCl (table salt), and substances that contain sodium taste salty and are innately palatable at concentrations that are advantageous to physiological homeostasis. The importance of sodium homeostasis is reflected by sodium appetite, an "all-hands-on-deck" response involving the brain, multiple peripheral organ systems, and endocrine factors, to increase sodium intake and replenish sodium levels in times of depletion. Visceral sensory information and endocrine signals are integrated by the brain to regulate sodium intake. Dysregulation of the systems involved can lead to sodium overconsumption, which numerous studies have considered causal for the development of diseases, such as hypertension. The purpose here is to consider the inverse-how disease impacts sodium intake, with a focus on stress-related and cardiometabolic diseases. Our proposition is that such diseases contribute to an increase in sodium intake, potentially eliciting a vicious cycle toward disease exacerbation. First, we describe the mechanism(s) that regulate each of these processes independently. Then, we highlight the points of overlap and integration of these processes. We propose that the analogous neural circuitry involved in regulating sodium intake and blood pressure, at least in part, underlies the reciprocal relationship between neural control of these functions. Finally, we conclude with a discussion on how stress-related and cardiometabolic diseases influence these circuitries to alter the consumption of sodium.

Locus coeruleus mediates cold stress-induced polycystic ovary in rats

Previous reports about the rat ovary have shown that cold stress promotes ovarian morphological alterations related to a polycystic ovary (PCO) condition through activation of the ovarian sympathetic nerves. Because the noradrenergic nucleus locus coeruleus (LC) is activated by cold stress and synaptically connected to the preganglionic cell bodies of the ovarian sympathetic pathway, this study aimed to evaluate the LC's role in cold stress-induced PCO in rats. Ovarian morphology and endocrine and sympathetic functions were evaluated after 8 wk of chronic intermittent cold stress (4 C, 3 h/d) in rats with or without LC lesion. The effect of acute and chronic cold stress upon the LC neuron activity was confirmed by Fos protein expression in tyrosine hydroxylase-immunoreactive neurons. Cold stress induced the formation of follicular cysts, type III follicles, and follicles with hyperthecosis alongside increased plasma estradiol and testosterone levels, irregular estrous cyclicity, and reduced ovulation. Considering estradiol release in vitro, cold stress potentiated the ovarian response to human chorionic gonadotropin. Ovarian norepinephrine (NE) was not altered after 8 wk of stress. However, LC lesion reduced NE activity in the ovary of cold-stressed rats, but not in controls, and prevented all the cold stress effects evaluated. Cold stress increased the number of Fos/tyrosine hydroxylase-immunoreactive neurons in the LC, but this effect was more pronounced for acute stress as compared with chronic stress. These results show that cold stress promotes PCO in rats, which apparently depends on ovarian NE activity that, under this condition, is regulated by the noradrenergic nucleus LC.

Glucocorticoid modulation of neuronal activity and hormone secretion induced by blood volume expansion

The present study evaluated the involvement of glucocorticoid in the activation of vasopressinergic and oxytocinergic neurons of hypothalamic nuclei and plasma levels of vasopressin (AVP), oxytocin (OT), atrial natriuretic peptide (ANP) and corticosterone (CORT) in response to both isotonic and hypertonic blood volume expansion (BVE). Rats were subjected to isotonic (0.15 M NaCl, 2 ml/100 g b.w., i.v.) or hypertonic (0.30 M NaCl, 2 ml/100 g b.w., i.v.) BVE with or without pre-treatment with dexamethasone (1 mg/kg, i.p.). Results showed that isotonic BVE increased OT, ANP and CORT, and decreased AVP plasma levels. On the other hand, hypertonic BVE enhanced AVP, ANP, OT, and CORT plasma concentrations. Both hypertonic and isotonic BVE induced an increase in the number of Fos-OT double-labeled magnocellular neurons in the PVN and SON. Pre-treatment with dexamethasone reduced OT secretion, as well as Fos-OT immunoreactive neurons in response to both isotonic and hypertonic BVE. We also observed that dexamethasone pre-treatment had no effect on AVP secretion in response to hypertonic BVE, although this effect was associated with a blockade of Fos expression in the vasopressinergic magnocellular neurons in the PVN and SON. In conclusion, these data suggest that, not only the rapid OT release from storages, but also the oxytocinergic cellular activation induced by BVE are modulated by glucocorticoids. However, this pattern of response was not observed for AVP cells, suggesting that dexamethasone is not likely to influence rapid release of AVP but seems to modulate the activation of these neurons in response to hypertonic BVE.

Immunohistochemical mapping of natriuretic peptide receptor-A in the brainstem of Macaca fascicularis

Natriuretic peptide receptor-A (NPR-A) mediates the biological effects of atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP), and is involved in maintaining cardiovascular homeostasis. In this immunohistochemical study we examined the distribution of NPR-A in the brainstem of the cynomolgus monkey. NPR-A immunoreactivity was localized to neurons in specific brainstem regions. NPR-A-immunoreactive perikarya were found in the red nucleus and the oculomotor nucleus in the midbrain, the parabrachial nucleus and the locus coeruleus in the pons, and the dorsal motor nucleus of the vagus, the hypoglossal nucleus, the cuneate nucleus, the gracile nucleus, the nucleus ambiguus, the lateral reticular nucleus, the reticular formation, and the inferior olivary nucleus in the medulla oblongata. Extensive networks of immunoreactive fibers were apparent in the red nucleus, the oculomotor nucleus, the principal sensory trigeminal nucleus, and the parabrachial nucleus. Double immunostaining revealed NPR-A immunoreactivity in cholinergic neurons of the parabrachial nucleus, the dorsal motor nucleus of vagus, the hypoglossal nucleus, and the nucleus ambiguus. However, there was no colocalization of NPR-A and tyrosine hydroxylase in the locus coeruleus. The wide anatomical distribution of NPR-A-immunoreactive structures suggests that natriuretic peptides, besides having a role in the central regulation of endocrine and cardiovascular homeostasis, may also mediate diverse physiological functions.

Locus Coeruleus Lesions Decrease Oxytocin and Vasopressin Release Induced by Hemorrhage

The role of the noradrenergic nucleus Locus Coeruleus (LC) on hemorrhage-induced vasopressin (AVP) and oxytocin (OT) secretion was examined. Rats with LC lesion were submitted to three 1-min hemorrhage sessions at 5-min intervals; 15% of the total blood volume was withdrawn in each session. OT and AVP were measured in plasma, paraventricular (PVN) and supraoptic (SON) nuclei and in posterior pituitary (PP). LC Lesion did not affect basal plasma AVP or OT levels, but partly blocked the increase in plasma AVP and OT induced by hemorrhage. Hemorrhage produced decreases in content of AVP and OT in the PVN and SON and increased levels in the PP. These responses were attenuated in the lesioned group, but only in the PVN and PP. Data suggest a stimulatory role of the inputs from LC to PVN neurons on hemorrhage-induced OT and AVP secretion and that, this pathway is critical in the hypo-volemic neuroendocrine reflex.

A comparative autoradiographic study of the density of [3H]SR95531, [3H]MK-801 and [3H]cGMP binding in the locus coeruleus and central pontine grey of spontaneously hypertensive and Wistar-Kyoto rats

The Spontaneously Hypertensive rat (SHR) has been previously shown to have a host of neurochemical differences compared with their normotensive counterpart, the Wistar-Kyoto (WKY) rat. Using quantitative receptor autoradiography, the density of GABA(A) and NMDA receptors and [3H]cGMP binding within the locus coeruleus (LC) and central pontine grey (CGPn) were compared in the SHR and WKY rat using the radioligands [3H]SR95531, [3H]MK-801 and [3H]cGMP respectively. It was found that [3H]SR95531 binding was significantly greater in both the LC and CGPn of the SHR compared with the WKY rat (unpaired t test; P < 0.05). Greater binding densities of [3H]MK-801 and [3H]cGMP were also observed in the LC of the SHR compared with the WKY rat; however, no differences in the binding density of these two ligands were observed in the CGPn. It is suggested that these neurochemical differences within the LC of the SHR may relate to phenotypic differences between SHR and WKY rats that have previously been reported.

Neurochemical brain groups activated after an isotonic blood volume expansion in rats

In order to establish the involvement of particular neurochemical brain groups in the response to blood volume expansion, we analyzed Fos-labeling in combination with immunolabeling for serotonin, tyrosine hydroxylase, vasopressin and oxytocin, 90 min after a sham or i.v. isotonic blood volume expansion (BVE) in unanesthetized, unrestrained rats. We also examined the changes in concentration of oxytocin, atrial natriuretic peptide and vasopressin plasma, induced by blood volume load, to confirm our previous studies. The results demonstrate the participation of specific paraventricular and supraoptic nucleus groups of cells (oxytocinergic-vasopressinergic), serotoninergic dorsal raphe nucleus cells and catecholaminergic A1/A2/A6 groups (in the caudal ventrolateral medulla, nucleus of the solitary tract and locus coeruleus respectively), in the regulatory response to BVE. They provide detailed neuroanatomical evidence to support previous observations showing the contribution of these neurochemical systems in the neural, behavioral and endocrine response to isotonic BVE.

Neuroendocrine control of body fluid metabolism

Mammals control the volume and osmolality of their body fluids from stimuli that arise from both the intracellular and extracellular fluid compartments. These stimuli are sensed by two kinds of receptors: osmoreceptor-Na+ receptors and volume or pressure receptors. This information is conveyed to specific areas of the central nervous system responsible for an integrated response, which depends on the integrity of the anteroventral region of the third ventricle, e.g., organum vasculosum of the lamina terminalis, median preoptic nucleus, and subfornical organ. The hypothalamo-neurohypophysial system plays a fundamental role in the maintenance of body fluid homeostasis by secreting vasopressin and oxytocin in response to osmotic and nonosmotic stimuli. Since the discovery of the atrial natriuretic peptide (ANP), a large number of publications have demonstrated that this peptide provides a potent defense mechanism against volume overload in mammals, including humans. ANP is mostly localized in the heart, but ANP and its receptor are also found in hypothalamic and brain stem areas involved in body fluid volume and blood pressure regulation. Blood volume expansion acts not only directly on the heart, by stretch of atrial myocytes to increase the release of ANP, but also on the brain ANPergic neurons through afferent inputs from baroreceptors. Angiotensin II also plays an important role in the regulation of body fluids, being a potent inducer of thirst and, in general, antagonizes the actions of ANP. This review emphasizes the role played by brain ANP and its interaction with neurohypophysial hormones in the control of body fluid homeostasis.