The role of GABA in the central regulation of AVP and ANP release and blood pressure due to angiotensin and carbachol, and central GABA release due to blood pressure changes

Hypothalamic paraventricular nucleus activation contributes to neurohumoral excitation in rats with heart failure

Heart failure (HF) is a serious cardiovascular disease and is characterized by exaggerated sympathetic activity. In this paper, we review these limited studies, with particular emphasis on examining the role of the paraventricular nucleus (PVN) in the neurohumoral excitation in HF. The PVN is an important neuroendocrine and preautonomic output nucleus, and is considered as the important central site for integration of sympathetic nerve activity. Accumulating evidences demonstrate that a number of neurohumoral processes are involved in the pathophysiology of HF, such as renin-angiotensin system (RAS), proinflammatory cytokines (PICs), neurotransmitters, and reactive oxygen species (ROS). Recent studies about neurohumoral regulation indicate that angiotensin II type1 receptor (AT₁-R) is the important product mediated by cytoplasmic nuclear factor-kappa B (NF-κB) which is up-regulated along with elevated PICs and angiotensin II (ANG II) in the PVN of HF rats. These findings suggest that the NF-κB mediates the cross-talk between RAS and PICs in the PVN in HF. The further studies indicate that the interaction between AT₁-R and NF-κB in the PVN contributes to oxidative stress and sympathoexcitation by modulating neurotransmitters in heart failure, and the superoxide activates NF-κB in the PVN and contributes to neurohumoral excitation. In conclusion, the neurohumoral excitation in HF is based on the interaction of RAS, PICs, ROS, NF-κB and neurotransmitters in the PVN; and the activated NF-κB in the PVN modulates the neurotransmitters and contributes to sympathoexcitation in rats with heart failure.

Changes in vasopressin release and autonomic function induced by manipulating forebrain GABAergic signaling under euvolemia and hypovolemia in conscious rats

The anteroventral third ventricular region (AV3V) is a pivotal area for osmotic responses and integration of autonomic functions. The purpose of this study was to investigate whether the gamma-aminobutyric acid (GABA)-ergic activity in the AV3V may be involved in the regulation of arginine vasopressin (AVP) secretion and related phenomena under the conditions with or without hypovolemia. Experiments were performed in conscious rats. We found that AV3V infusion with the GABA(A) receptor antagonist bicuculline in euvolemic rats caused prompt increases in plasma AVP, osmolality, glucose, arterial pressure and heart rate. The effects of the bicuculline infusion were abolished by prior infusion of a GABA(A) receptor agonist, muscimol. When repeated twice with a 10-min interval, removal of systemic blood (10 mL/kg body weight) lowered arterial pressure and enhanced plasma AVP, osmolality, glucose and angiotensin II. Muscimol infusion in the AV3V, but not in the cerebral ventricle, inhibited the responses of plasma AVP and glucose, despite having no effect in a sham hemorrhagic state. The inhibition of the AVP response by the muscimol infusion was also verified in rats given a combined stimulus of bleeding plus an osmotic load. In contrast, AV3V infusion with the GABA(B) receptor agonist baclofen tended to intensify the hemorrhagic responses of plasma AVP and glucose, despite its potency to prevent the hemorrhagic fall in arterial pressure. These results, taken together with our previous data, suggest that hypovolemic stimuli, like hyperosmotic stimuli, may promote AVP secretion by causing the inhibition of AV3V GABA(A)-ergic activity responsible for potentiation of glutamatergic activity.

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.

Intracerebroventricular carbachol induces FOS immunoreactivity in lamina terminalis neurons projecting to the supraoptic nucleus

Central application of the non-selective cholinergic receptor agonist, carbachol, induces water intake, vasopressin (VP) release and an acute increase in arterial blood pressure. Forebrain sites, particularly those located along the lamina terminalis (LT) (i.e. the subfornical organ (SFO), organum vasculosum (OV) and the median preoptic nucleus (MePO)) and in the hypothalamus, have been proposed as the major targets for producing the effects induced by intracerebroventricular (i.c.v.) carbachol injections. However, the functional and neuroanatomical relationship among carbachol-activated cells along the LT and hypothalamic areas such as the supraoptic nuclei (SON), is unclear. The present study investigated the i.c.v. carbachol-induced activity of the soma of LT projections which descend from the SFO, OV and MePO and terminate in the region of the SON. Cells along the LT were retrogradely labeled from SON-targeted injections of fluoro-gold, and FOS-immunoreactivity (FOS-ir) was used to assess activation. A significant number of cells in the SFO, OV and MePO were double-labeled for both FOS-ir and fluoro-gold. The FOS labeling in the cells of the LT-associated structures was significantly reduced by pretreatment with the i.c.v. muscarinic antagonist, atropine. Taken together, the results indicate that neurons located in structures located along the LT and projecting to the region of the SON are activated by i.c.v. carbachol and that these receptors are likely to be muscarinic.