Functional hypothalamic angiotensin II and catecholamine receptor systems inside and outside the blood-brain barrier

Norepinephrine Inhibits Lipopolysaccharide-Stimulated TNF-α but Not Oxylipin Induction in n-3/n-6 PUFA-Enriched Cultures of Circumventricular Organs

Sensory circumventricular organs (sCVOs) are pivotal brain structures involved in immune-to-brain communication with a leaky blood-brain barrier that detect circulating mediators such as lipopolysaccharide (LPS). Here, we aimed to investigate the potential of sCVOs to produce n-3 and n-6 oxylipins after LPS-stimulation. Moreover, we investigated if norepinephrine (NE) co-treatment can alter cytokine- and oxylipin-release. Thus, we stimulated rat primary neuroglial sCVO cultures under n-3- or n-6-enriched conditions with LPS or saline combined with NE or vehicle. Supernatants were assessed for cytokines by bioassays and oxylipins by HPLC-MS/MS. Expression of signaling pathways and enzymes were analyzed by RT-PCR. Tumor necrosis factor (TNF)α bioactivity and signaling, IL-10 expression, and cyclooxygenase (COX)2 were increased, epoxide hydroxylase (Ephx)2 was reduced, and lipoxygenase 15-(LOX) was not changed by LPS stimulation. Moreover, LPS induced increased levels of several n-6-derived oxylipins, including the COX-2 metabolite 15d-prostaglandin-J2 or the Ephx2 metabolite 14,15-DHET. For n-3-derived oxylipins, some were down- and some were upregulated, including 15-LOX-derived neuroprotectin D1 and 18-HEPE, known for their anti-inflammatory potential. While the LPS-induced increase in TNFα levels was significantly reduced by NE, oxylipins were not significantly altered by NE or changes in TNFα levels. In conclusion, LPS-induced oxylipins may play an important functional role in sCVOs for immune-to-brain communication.

Differential responses of the vasotocin 1a receptor (V1aR) and osmoreceptors to immobilization and osmotic stress in sensory circumventricular organs of the chicken (Gallus gallus) brain

Past studies have shown that the avian vasotocin 1a receptor (V1aR) is involved in immobilization stress. It is not known whether the receptor functions in osmotic stress, and if sensory circumventricular organs may be involved. An experiment was designed with four treatment groups including a 1h immobilization acute stress (AS) group, an unstressed acute control (AC), a third given an intraperitoneal (ip) hypertonic saline injection (HS) and isotonic saline controls (IC) administered ip. One set of chick brains was perfused for immunohistochemistry while a second was sampled for quantitative RT-PCR. Plasma corticosterone (CORT) and arginine vasotocin (AVT) concentrations were significantly increased in the immobilized and hypertonic saline groups (p<0.01) compared to controls. Intense staining of the V1aR occurred throughout the organum vasculosum of the lamina terminalis (OVLT) and subseptal organ (SSO)/subfornical organ (SFO). The immunostaining allowed the boundaries of the two circumventricular organs (CVOs) to be described for the first time in avian species. Both treatment groups showed marked morphological changes in glia within the OVLT and SSO/SFO. The avian V1aR, angiotensin II type 1 receptor (AT1R), and transient receptor potential vanilloid receptor 1 (TRPV1) mRNA levels were increased in the SSO/SFO in hypertonic saline treated birds compared to isotonic controls. In contrast, the latter two genes (AT1R and TRPV1) were significantly decreased in the OVLT of birds subjected to hyperosmotic stress, while all three genes were significantly up-regulated after immobilization. Taken together, results show a possible differential function for the same receptors in two anatomically adjacent CVOs.

The central mechanism underlying hypertension: a review of the roles of sodium ions, epithelial sodium channels, the renin-angiotensin-aldosterone system, oxidative stress and endogenous digitalis in the brain

The central nervous system has a key role in regulating the circulatory system by modulating the sympathetic and parasympathetic nervous systems, pituitary hormone release, and the baroreceptor reflex. Digoxin- and ouabain-like immunoreactive materials were found >20 years ago in the hypothalamic nuclei. These factors appeared to localize to the paraventricular and supraoptic nuclei and the nerve fibers at the circumventricular organs and supposed to affect electrolyte balance and blood pressure. The turnover rate of these materials increases with increasing sodium intake. As intracerebroventricular injection of ouabain increases blood pressure via sympathetic activation, an endogenous digitalis-like factor (EDLF) was thought to regulate cardiovascular system-related functions in the brain, particularly after sodium loading. Experiments conducted mainly in rats revealed that the mechanism of action of ouabain in the brain involves sodium ions, epithelial sodium channels (ENaCs) and the renin-angiotensin-aldosterone system (RAAS), all of which are affected by sodium loading. Rats fed a high-sodium diet develop elevated sodium levels in their cerebrospinal fluid, which activates ENaCs. Activated ENaCs and/or increased intracellular sodium in neurons activate the RAAS; this releases EDLF in the brain, activating the sympathetic nervous system. The RAAS promotes oxidative stress in the brain, further activating the RAAS and augmenting sympathetic outflow. Angiotensin II and aldosterone of peripheral origin act in the brain to activate this cascade, increasing sympathetic outflow and leading to hypertension. Thus, the brain Na(+)-ENaC-RAAS-EDLF axis activates sympathetic outflow and has a crucial role in essential and secondary hypertension. This report provides an overview of the central mechanism underlying hypertension and discusses the use of antihypertensive agents.

Hyponatremia and the syndrome of inappropriate secretion of antidiuretic hormone (SIADH)

The syndrome of inappropriate ADH secretion (SIADH), also recently referred to as the "syndrome of inappropriate antidiuresis", is an often underdiagnosed cause of hypotonic hyponatremia, resulting for instance from ectopic release of ADH in lung cancer or as a side-effect of various drugs. In SIADH, hyponatremia results from a pure disorder of water handling by the kidney, whereas external Na+ balance is usually well regulated. Despite increased total body water, only minor changes of urine output and modest edema are usually seen. Renal function and acid-base balance are often preserved, while neurological impairment may range from subclinical to life-threatening. Hypouricemia is a distinguishing feature. The major causes and clinical variants of SIADH are reviewed, with particular emphasis on iatrogenic complications and hospital-acquired hyponatremia. Effective treatment of SIADH with water restriction, aquaretics, or hypertonic saline + loop diuretics, as opposed to worsening of hyponatremia during parenteral isotonic fluid administration, underscores the importance of an early accurate diagnosis and careful follow-up of these patients.

Angiotensin II induces inward currents in subfornical organ neurones of rats

The action of angiotensin II on subfornical organ (SFO) neurones was studied using whole-cell current and voltage-clamp recordings in rat slice preparations. In the current-clamp mode, membrane depolarization in response to angiotensin II was accompanied by an increased frequency of action potentials and an increased membrane conductance. In the voltage-clamp mode, angiotensin II elicited inward currents in a dose-dependent manner. The net angiotensin II-induced inward currents were voltage-independent, with a mean reversal potential of -29.8 +/- 6.2 mV. Amplitudes of the angiotensin II-induced inward currents were decreased during perfusion with a low sodium medium. The angiotensin II-induced inward currents were blocked by the AT1 antagonist losartan, and were partially blocked by the AT2 antagonist PD-123319. Neurones which were sensitive to angiotensin II were found in the peripheral region of the SFO, whereas neurones in the central region were less sensitive to angiotensin II. These results suggest that angiotensin II induces inward currents, with opening of nonselective cation channels through mainly AT1 receptors in a subpopulation of SFO neurones of rats.

Serotonin and stress-induced increases in renin secretion are not blocked by sympathectomy/adrenal medullectomy but are blocked by beta antagonists

The present study examined the role of the sympathetic nervous system as a mediator of the message from the CNS to the kidneys to stimulate the secretion of renin. Two procedures that increase the secretion of renin were tested: administration of the serotonin releaser fenfluramine, which increases renin release without altering blood pressure [53], and subjecting the rats to the 'psychological' stressor of conditioned emotional response (CER) stress. Pretreatment of rats with either the beta antagonist sotalol or the beta 1-selective antagonist atenolol completely prevented the increase in plasma renin activity and concentration caused by fenfluramine (5 mg/kg i.p.) injection. However, chemical sympathectomy with 6-hydroxydopamine (6-OHDA) combined with surgical adrenal medullectomy did not prevent the increase in plasma renin activity and concentration following fenfluramine injection. Since beta-antagonists have been previously shown to prevent the renin response to CER stress, we also tested whether the sympathetic nervous system mediates the renin response to CER stress. Chemical sympathectomy combined with adrenal medullectomy did not prevent the effect of CER stress on renin release. The completeness of the sympathectomy/adrenal medullectomy was verified biochemically by measuring plasma epinephrine and both plasma and renal norepinephrine concentrations. Plasma epinephrine and renal norepinephrine levels were reduced to below 1% of control while plasma norepinephrine was reduced to below 8% of control values. In conclusion, our data support previous reports suggesting that activation of CNS pathways increases the secretion of renin. However, the message from the brain to release renin from the kidneys does not exclusively involve either the sympathetic innervation of the kidneys or adrenal epinephrine. Although beta 1 receptors are involved in mediating this phenomenon, their location or mechanism remains unknown and will be discussed.

Nitric oxide synthase in the subfornical organ of Mongolian gerbil (Meriones unguiculatus), mouse and rat

The present paper aims to contribute to the knowledge of the histophysiologic role of the subfornical organ. Using nicotinamide adenine dinucleotide phosphate diaphorase histochemistry (NADPH-diaporase) and nitric oxide synthase immunocytochemistry, we detected a high concentration of the neuronal isoform of nitric oxide synthase in neurons of the subfornical organ of the adult Mongolian gerbil (Meriones unguiculatus), mouse and rat. Our results suggest that neurons of the subfornical organ produce a considerable amount of nitric oxide which acts, not only as a neurotransmitter, but could also diffuse into cerebral blood vessels and cerebrospinal fluid.

Excitatory action of the bird antidiuretic hormone vasotocin on neurons in the subfornical organ

The responsiveness of spontaneously active neurons in the subfornical organ (SFO) of adult ducks to angiotensin II (ANGII) and the bird specific antidiuretic hormone, arginine vasotocin (AVT), the analog of the mammalian arginine vasopressin (AVP), were investigated in brain slices with extracellular recording technique. 65% (n = 66) of the neurons increased their activity after superfusion with ANGII, the rest were unresponsive. Application of AVT activated 52% (n = 68) of the investigated neurons and like ANGII never caused an inhibition of the spontaneously active SFO neurons. A close correlation exists between the ANGII and AVT sensitivity of duck SFO neurons, because 29 out of 33 neurons were excited by AVT as well as ANGII. The relatively weak antagonistic effect of the V1-type receptor antagonist Pmp-Tyr (Me)-Arg8-vasopressin on the AVT induced excitation suggests a different pharmacology of the bird AVT receptor as compared to the mammalian AVP receptor. The excitatory response of ANGII and AVT on the very same neurons suggest a similar function of both peptides on SFO mediated effects in vivo, such as an increase in water intake. However, peripheral AVT concentrations, unlike ANGII concentrations in the blood are not high enough to activate SFO neurons from the blood side of the blood brain barrier. Therefore AVT is presumably released from synapses of neurons originating within or projecting to the SFO. The identity of the ANGII and AVT reactive neurons suggests that synaptically released AVT should facilitate SFO mediated drinking.

Ontogenesis of the angiotensin II (ANGII) receptor system in the duck brain

The ontogenetic development of the central nervous angiotensin II (ANGII) receptor system in the duck was studied at embryonic days E20 and E27 and at postnatal days P3 and P14 by computerized semiquantitative autoradiography employing the receptor antagonist 125I[1Sar,8Ile]ANGII as radioligand. For circumventricular structures involved in the sensing of brain-intrinsic (AV3V region) or blood-borne (subfornical organ, SFO) ANGII, binding sites for 125I[1Sar,8Ile]ANGII were first detectable at E27, with a steady rise in binding density up to P14. The choroid plexus of the lateral (PCVL) and third (PCVIII) cerebral ventricles responsible for cerebrospinal fluid (CSF) production were endowed with maximal ANGII receptor densities at E20 with subsequent reduction to constant medium (PCVIII) or low (PCVL) values. Besides the choroid plexus, the magnocellular paraventricular nucleus (PVN) was the only structure presenting ANGII specific binding sites at E20, although at low density. As for the SFO and AV3V region, labelling of ANGII binding sites in the PVN increased continuously during development to high values at P14. Nuclear components of the limbic system (archistriatum, amygdala and habenular complex) did not reveal specific labelling by the radioligand at E20 with constant, moderate binding densities evaluated for E27, P3 and P14. In the duck brain, functionally related structures exhibited a homogeneous ontogenetic development of their ANGII receptor system.

The alpha2-adrenergic receptor system in the hypothalamus of the Pekin duck

In the present study, we have employed the monoradioiodinated alpha 2-agonist clonidine ([125I]-CLO) to characterize duck hypothalamic alpha 2-adrenoceptors and to localize alpha 2-specific binding sites in the duck brain. To validate the alpha 2-specificity of [125I]-CLO using an enriched duck hypothalamic membrane fraction, a radioreceptor assay was established by altering the membrane protein concentration, time, temperature and ionic milieu of incubation, and in the presence or absence of protease inhibitors. Competitive displacement studies revealed the following sequence of potency to displace [125I]-CLO: yohimbine greater than (-)-epinephrine greater than clonidine greater than (-)-norepinephrine greater than phentolamine greater than (-)-phenylephrine greater than (-)-isoproterenol greater than prazosin. The non-hydrolyzable guanosine 5'-triphosphate analog guanylylimidodiphosphate markedly inhibited [125I]-CLO binding suggestive of G-protein involvement. With regard to the histological distribution, diencephalic structures, such as the habenula and the nucleus reticularis of the thalamus, were densely labeled by [125I]-CLO. In the hypothalamus, alpha 2-adrenoceptors were detected in the antidiuretic hormone-synthesizing nucleus paraventricularis, the nucleus praeopticus medialis, the nucleus anterior medialis hypothalami, the nucleus magnocellularis praeopticus, the nucleus commissurae pallii, the nucleus inferior hypothalami and the regio lateralis hypothalami. Circumventricular organs, such as the plexus choroidei, organum subfornicale, organum paraventriculare and the corpus pineale, were endowed with alpha 2-specific binding sites, as were the cell layers of the tectum opticum. In addition, telencephalic structures revealed high receptor densities.(ABSTRACT TRUNCATED AT 250 WORDS)