Comparison between the influence of the intravenous and intracerebroventricular injection of a nitric oxide donor on adrenocorticotropic hormone release and hypothalamic neuronal activity

Central Administration of Angiotensin-(1-7) Improves Vasopressin Impairment and Hypotensive Response in Experimental Endotoxemia

Angiotensin-(1-7) [Ang-(1-7)]/Mas receptor is a counter-regulatory axis that counteracts detrimental renin-angiotensin system (RAS) effects, especially regarding systemic inflammation, vasopressin (AVP) release, and hypothalamic-pituitary-adrenal (HPA) activation. However, it is not completely understood whether this system may control centrally or systemically the late phase of systemic inflammation. Thus, the aim of this study was to determine whether intracerebroventricular (i.c.v.) administration of Ang-(1-7) can modulate systemic inflammation through the activation of humoral pathways in late phase of endotoxemia. Endotoxemia was induced by systemic injection of lipopolysaccharide (LPS) (1.5 mg/kg, i.v.) in Wistar rats. Ang-(1-7) (0.3 nmol in 2 µL) promoted the release of AVP and attenuated interleukin-6 (IL-6) and nitric oxide (NO) levels but increased interleukin-10 (IL-10) in the serum of the endotoxemic rats. The central administration of Mas receptor antagonist A779 (3 nmol in 2 µL, i.c.v.) abolished these anti-inflammatory effects in endotoxemic rats. Furthermore, Ang-(1-7) applied centrally restored mean arterial blood pressure (MABP) without affecting heart rate (HR) and prevented vascular hyporesponsiveness to norepinephrine (NE) and AVP in animals that received LPS. Together, our results indicate that Ang-(1-7) applied centrally promotes a systemic anti-inflammatory effect through the central Mas receptor and activation of the humoral pathway mediated by AVP.

Role of hypothalamic corticotropin-releasing factor in mediating alcohol-induced activation of the rat hypothalamic-pituitary-adrenal axis

Alcohol stimulates the hypothalamic-pituitary-adrenal (HPA) axis through brain-based mechanisms in which endogenous corticotropin-releasing factor (CRF) plays a major role. This review first discusses the evidence for this role, as well as the possible importance of intermediates such as vasopressin, nitric oxide and catecholamines. We then illustrate the long-term influence exerted by alcohol on the HPA axis, such as the ability of a first exposure to this drug during adolescence, to permanently blunt neuroendocrine responses to subsequent exposure of the drug. In view of the role played by CRF in addiction, it is likely that a better understanding of the mechanisms through which this drug stimulates the HPA axis may lead to the development of new therapies used in the treatment of alcohol abuse, including clinically relevant CRF antagonists.

The hypothalamo-pituitary axis responses to lipopolysaccharide-induced endotoxemia in mice lacking inducible nitric oxide synthase

Nitric oxide (NO) generated by inducible NO synthase (iNOS) may be implicated in the biological responses of the central nervous system to immune stimuli. To elucidate the role of iNOS in the hypothalamo-pituitary axis in responses to endotoxemia, using iNOS knockout (KO) mice, we examined the levels of c-fos, a neural activational marker, and corticotropin-releasing hormone (CRH) gene transcription in the paraventricular nucleus (PVN) and central amygdala (CeAMY) during lipopolysaccharide (LPS)-induced endotoxemia. In addition, the serum adrenocorticotropic hormone (ACTH) levels were also examined during endotoxemia. Following the intraperitoneal administration of LPS (1 mg/kg), the levels of the c-fos gene expression significantly increased in the PVN and the CeAMY regardless of the genotype. However, the disruption of the iNOS gene resulted in a significant decrease in the c-fos gene induction in the PVN in comparison to that observed in control mice. LPS administration caused a significant increase in CRH mRNA levels in the PVN and CeAMY regardless of genotype. However, the LPS-induced upregulation of CRH mRNA was significantly attenuated in the PVN of iNOS KO mice in comparison to that in the control mice. In contrast, no such genotype differences in the neural activity or CRH gene transcription were observed in the CeAMY. The serum ACTH responses to LPS were also significantly blunted in the iNOS KO mice in comparison to the control mice. These results suggest that iNOS-derived NO may therefore play a stimulatory role in the activity of the hypothalamo-pituitary axis during endotoxemia.

Comparison between the influence of shocks and endotoxemia on the activation of brain cells that contain nitric oxide

We sought to identify the brain circuitry that underlies the stimulatory role of nitric oxide (NO) role on the hypothalamic-pituitary-adrenal (HPA) axis. Specifically, we determined whether electrofootshocks (60 min) or the intravenous administration of lipopolysaccharide (LPS, 100 microg/kg)-activated neurocircuitries that express either neuronal NO synthase (nNOS), a constitutive enzyme responsible for NO formation, or L-citrulline, an amino acid that is produced in equimolar amounts with NO. Shocks significantly increased the number of cells showing Fos immunoreactivity (ir) in the paraventricular nucleus (PVN) of the hypothalamus, the lateral hypothalamus (LH), amygdaloid complex (AD) and thalamus (TH), and to a lesser extent, in the hippocampus (HP), caudate putamen (CP) and frontal cortex (FC). However, shocks did not alter the number of nNOS-positive cells nor increased citrulline signals in these brain regions. LPS significantly upregulated the number of cells with fos-like ir in the PVN, LH, AD, TH, HP, CP and FC, but only increased the number of cells positive for citrulline in the PVN, 87% of which co-expressed Fos. Thus, while shocks did not alter nNOS gene expression or citrulline levels in the brain regions studied, LPS significantly increased the number of PVN cells expressing citrulline without concomitant changes in other brain areas. Endotoxemia also upregulated significantly more PVN cells that co-expressed Fos and nNOS, compared to shocks. As NO stimulates the PVN circuitries that participate in shocks- and LPS-induced ACTH release, the lack of changes in nNOS or citrulline levels due to shocks suggests that, in this model, constitutively formed NO may modulate HPA axis activity in the absence of changes in its synthesis.

Role of specific adrenergic receptors in mediating the adrenocorticotropic hormone response to increased nitric oxide levels

We investigated the role played by catecholamine-dependent pathways in modulating the ability of the nitric oxide (NO) donor 3-morpholino-sydnonimine (SIN-1) to release adrenocorticotropic hormone (ACTH) following its intracerebroventricular (i.c.v.) or intravenous (i.v.) injection. We first showed that the nonspecific adrenergic agonist noradrenaline, the alpha- or beta-adrenergic agonists phenylephrine or dobutamine, or the noradrenergic uptake inhibitor desipramine, all significantly stimulated ACTH secretion by freely moving, nonanaesthetized rats. We then observed that destruction of noradrenergic nerve endings with the neurotoxin 6-hydroxydopamine, respectively abolished and significantly decreased the ACTH response to the i.c.v. or i.v. administration of SIN-1. Finally, we sought to identify the type of adrenergic receptor(s) mediating the influence of catecholamines. beta-Adrenergic receptors did not appear to be involved in the stimulatory effect of SIN-1 regardless of its route of injection. By contrast, alpha 2-adrenergic receptors played an important role in the ACTH response to i.v. or i.c.v. administered SIN-1. Collectively, these results indicate that while hypothalamic alpha 1- and beta-adrenergic receptors are important for hypothalamic-pituitary-adrenal (HPA) axis activity, only alpha 2-adrenergic receptors are involved in modulating the ability of NO to release ACTH. Our laboratory and others have previously reported that NO increased hypothalamic noradrenaline levels, while conversely noradrenaline up-regulated levels of NO synthase, the enzyme responsible for NO formation; and that injection of corticotropin-releasing factor into the brain ventricles releases catecholamines and stimulates NO formation. Taken together with these observations, our results point to complex functional relationships between NO, catecholamines and the HPA axis.