Importance of central AT1 receptors for sodium intake induced by GABAergic activation of the lateral parabrachial nucleus

Control of fluid intake in dehydrated rats and evolution of sodium appetite

The objective of the present work is to examine from a new perspective the existence of causal factors not predicted by the classical theory that thirst and sodium appetite are two distinct motivations. For example, we ask why water deprivation induces sodium appetite, thirst is not "water appetite", and intracellular dehydration potentially causes sodium appetite. Contrary to the classical theory, we suggest that thirst first, and sodium appetite second, designate a temporal sequence underlying the same motivation. The single motivation becomes an "intervenient variable" a concept borrowed from the literature, fully explained in the text, between causes of dehydration (extracellular, intracellular, or both together), and respective behavioral responses subserved by hindbrain-dependent inhibition (e.g., lateral parabrachial nucleus) and forebrain facilitation (e.g., angiotensin II). A corollary is homology between rat sodium appetite and marine teleost thirst-like motivation that we name "protodipsia". The homology argument rests on similarities between behavior (salty water intake) and respective neuroanatomical as well as functional mechanisms. Tetrapod origin in a marine environment provides additional support for the homology. The single motivation hypothesis is also consistent with ingestive behaviors in nature given similarities (e.g., thirst producing brackish water intake) between the behavior of the laboratory rat and wild animals, rodents included. The hypotheses of single motivation and homology might explain why hyperosmotic rats, or eventually any other hyperosmotic tetrapod, shows paradoxical signs of sodium appetite. They might also explain how ingestive behaviors determined by dehydration and subserved by hindbrain inhibitory mechanisms contributed to tetrapod transition from sea to land.

Interference with the renin-angiotensin system reduces the palatability of 0.3 M NaCl in sodium-deplete rats

The renin-angiotensin system (RAS) controls hypertonic NaCl intake driven by sodium appetite. Here we investigated whether the antagonism of RAS interferes with hedonic and aversive orofacial motor responses, or palatability, to intraoral infusion of 0.3 M NaCl (hNaCl). Adult rats were depleted of sodium by combined sc injection of furosemide and 24 h removal of ambient sodium. In experiment 1, losartan (AT1 angiotensin II receptor antagonist, intracerebroventricular, 200 μg/μl), produced a three-fold increase in aversive orofacial motor responses to hNaCl. Losartan also suppressed hNaCl intake recorded immediately thereafter. In experiment 2, each animal had repeated recordings of hNaCl intake and orofacial responses to hNaCl distributed for 180 min. Paired recordings of intake and orofacial responses occurred within five successive blocks after the recordings of only orofacial responses when the animals were still sodium deplete (block zero). Captopril (angiotensin converting enzyme blocker, intraperitoneal, 30 mg/kg) inhibited by 75% the hedonic orofacial responses to hNaCl in blocks zero and 1. The hedonic responses to captopril remained the same throughout blocks, but became similar to vehicle from blocks 2 to 5. There was no difference in aversive responses to 0.3 M NaCl between captopril and vehicle. Captopril produced a 70-100% inhibition of hNaCl intake in blocks 1 to 5. The results suggest that angiotensin II acts in the brain increasing the palatability of hypertonic sodium during the consummatory phase of sodium appetite.

Central muscarinic and LPBN mechanisms on sodium intake

Central cholinergic activation stimulates water intake, but also NaCl intake when the inhibitory mechanisms are blocked with injections of moxonidine (α₂ adrenergic/imidazoline agonist) into the lateral parabrachial nucleus (LPBN). In the present study, we investigated the involvement of central M₁ and M₂ muscarinic receptors on NaCl intake induced by pilocarpine (non-selective muscarinic agonist) intraperitoneally combined with moxonidine into the LPBN or by muscimol (GABA_A agonist) into the LPBN. Male Holtzman rats with stainless steel cannulas implanted bilaterally in the LPBN and in the lateral ventricle were used. Pirenzepine (M₁ muscarinic antagonist, 1 nmol/1 μl) or methoctramine (M₂ muscarinic antagonist, 50 nmol/1 μL) injected intracerebroventricularly (i.c.v.) reduced 0.3 M NaCl and water intake in rats treated with pilocarpine (0.1 mg/100 g of body weight) injected intraperitoneally combined with moxonidine (0.5 nmol/0.2 μL) into the LPBN. In rats treated with muscimol (0.5 nmol/0.2 μL) into the LPBN, methoctramine i.c.v. also reduced 0.3 M NaCl and water intake, however, pirenzepine produced no effect. The results suggest that M₁ and M₂ muscarinic receptors activate central pathways involved in the control of water and sodium intake that are under the influence of the LPBN inhibitory mechanisms.

α2 Receptors in the lateral parabrachial nucleus generates the pressor response of the cardiovascular chemoreflex, effects of GABAA receptor

The lateral parabrachial nucleus (LPBN) is a pontine area involved in cardiovascular chemoreflex. This study was performed to find the effects of reversible synaptic blockade of the LPBN on the chemoreflex responses, and to find the roles of GABA_A receptor and α₂-adenoreceptor (α₂-AR) in chemoreflex. It also aimed to seek possible interaction between GABA and noradrenergic systems of the LPBN in urethane-anesthetized male rats. Cardiovascular chemoreflex was activated by intravenous injection of potassium cyanide (KCN, 80 μg/kg). The cardiovascular responses of chemoreflex were evaluated before (control), 5 and 15 min after microinjection of each drug (100 nl) into the LPBN. Microinjections of cobalt chloride (5 mM), a reversible synaptic blocker, into the LPBN greatly attenuated the chemoreflex pressor and bradycardic responses indicating that the LPBN plays a main role in chemoreflex. Local injection of yohimbine (10 nmol), an α₂-AR antagonist, attenuated the pressor response with no effect on bradycardic response, suggesting that α₂-adrenoreceptors are involved in producing the pressor response of the chemoreflex. Microinjection of bicuculline methiodide (BMI, 100 pmol), a GABA_A antagonist, into the LPBN augmented the pressor response and attenuated the bradycardic response, indicating that GABA inhibits the sympathetic output to the heart and vasculature. Sequential injection of yohimbine and BMI had no significant effect on the pressor response but attenuated the bradycardia. In conclusion, the LPBN is essential for the chemoreflex responses. The pressor response of the chemoreflex, at least partly, is produced by α₂- adenoreceptors. GABA in the LPBN inhibits the cardiovascular system. Finally, there is no interaction between GABAergic and adrenergic neurons of the LPBN in producing the cardiovascular chemoreflex.

Role of the lateral parabrachial nucleus in the control of sodium appetite

In states of sodium deficiency many animals seek and consume salty solutions to restore body fluid homeostasis. These behaviors reflect the presence of sodium appetite that is a manifestation of a pattern of central nervous system (CNS) activity with facilitatory and inhibitory components that are affected by several neurohumoral factors. The primary focus of this review is on one structure in this central system, the lateral parabrachial nucleus (LPBN). However, before turning to a more detailed discussion of the LPBN, a brief overview of body fluid balance-related body-to-brain signaling and the identification of the primary CNS structures and humoral factors involved in the control of sodium appetite is necessary. Angiotensin II, mineralocorticoids, and extracellular osmotic changes act on forebrain areas to facilitate sodium appetite and thirst. In the hindbrain, the LPBN functions as a key integrative node with an ascending output that exerts inhibitory influences on forebrain regions. A nonspecific or general deactivation of LPBN-associated inhibition by GABA or opioid agonists produces NaCl intake in euhydrated rats without any other treatment. Selective LPBN manipulation of other neurotransmitter systems [e.g., serotonin, cholecystokinin (CCK), corticotrophin-releasing factor (CRF), glutamate, ATP, or norepinephrine] greatly enhances NaCl intake when accompanied by additional treatments that induce either thirst or sodium appetite. The LPBN interacts with key forebrain areas that include the subfornical organ and central amygdala to determine sodium intake. To summarize, a model of LPBN inhibitory actions on forebrain facilitatory components for the control of sodium appetite is presented in this review.

Natriorexigenic effect of DAMGO is decreased by blocking AT1 receptors in the central nucleus of the amygdala

μ-Opioid receptor (μ-OR) activation with agonist [D-Ala², N-Me-Phe⁴, Gly⁵-ol]-enkephalin (DAMGO) in the central nucleus of the amygdala (CeA) induces sodium (0.3M NaCl) intake in rats. The purpose of this study was to examine the effects of pre-injections of losartan (AT1 angiotensin receptor antagonist) into the CeA on 0.3 M NaCl and water intake induced by DAMGO injected bilaterally in the same area in rats submitted to water deprivation-partial rehydration (WD-PR) and in rats treated with the diuretic furosemide (FURO) combined with a low dose of the angiotensin-converting enzyme inhibitor captopril (CAP) injected subcutaneously (FURO/CAP). Male Sprague-Dawley rats with stainless steel cannulas implanted bilaterally into the CeA were used. In WD-PR rats, bilateral injections of DAMGO (2 nmol in 0.5 μL) into the CeA induced 0.3 M NaCl and water intake, and pre-treatment with losartan (108 nmol in 0.5 μL) injected into the CeA reduced 0.3 M NaCl and water intake induced by DAMGO. In FURO/CAP rats, pre-treatment with losartan (108 nmol in 0.5 μL) injected into the CeA attenuated the increase in 0.3M NaCl and water intake induced by DAMGO (2 nmol in 0.5 μL) injected into the same site. The results suggest that the natriorexigenic effect of DAMGO injected into the CeA is facilitated by endogenous angiotensin II acting on AT1 receptors in the CeA, which drives rats to ingest large amounts of hypertonic NaCl.

Angiotensinergic and cholinergic receptors of the subfornical organ mediate sodium intake induced by GABAergic activation of the lateral parabrachial nucleus

Bilateral injections of the GABA(A) agonist muscimol into the lateral parabrachial nucleus (LPBN) induce 0.3 M NaCl and water intake in satiated and normovolemic rats, a response reduced by intracerebroventricular (icv) administration of losartan or atropine (angiotensinergic type 1 (AT₁) and cholinergic muscarinic receptor antagonists, respectively). In the present study, we investigated the effects of the injections of losartan or atropine into the subfornical organ (SFO) on 0.3M NaCl and water intake induced by injections of muscimol into the LPBN. In addition, using intracellular calcium measurement, we also tested the sensitivity of SFO-cultured cells to angiotensin II (ANG II) and carbachol (cholinergic agonist). In male Holtzman rats with cannulas implanted bilaterally into the LPBN and into the SFO, injections of losartan (1 μg/0.1 μl) or atropine (2 nmol/0.1 μl) into the SFO almost abolished 0.3M NaCl and water intake induced by muscimol (0.5 nmol/0.2 μl) injected into the LPBN. In about 30% of the cultured cells of the SFO, carbachol and ANG II increased intracellular calcium concentration ([Ca²⁺](i)). Three distinct cell populations were found in the SFO, i.e., cells activated by either ANG II (25%) or carbachol (2.6%) or by both stimuli (2.3%). The results suggest that the activation of angiotensinergic and cholinergic mechanisms in the SFO is important for NaCl and water intake induced by the deactivation of LPBN inhibitory mechanisms with muscimol injections. They also show that there are cells in the SFO activated by both angiotensinergic and cholinergic stimuli, perhaps those involved in the responses to muscimol into the LPBN.

Manganese-enhanced magnetic resonance imaging for mapping of whole brain activity patterns associated with the intake of snack food in ad libitum fed rats

Non-homeostatic hyperphagia, which is a major contributor to obesity-related hyperalimentation, is associated with the diet’s molecular composition influencing, for example, the energy content. Thus, specific food items such as snack food may induce food intake independent from the state of satiety. To elucidate mechanisms how snack food may induce non-homeostatic food intake, it was tested if manganese-enhanced magnetic resonance imaging (MEMRI) was suitable for mapping the whole brain activity related to standard and snack food intake under normal behavioral situation. Application of the MnCl₂ solution by osmotic pumps ensured that food intake was not significantly affected by the treatment. After z-score normalization and a non-affine three-dimensional registration to a rat brain atlas, significantly different grey values of 80 predefined brain structures were recorded in ad libitum fed rats after the intake of potato chips compared to standard chow at the group level. Ten of these areas had previously been connected to food intake, in particular to hyperphagia (e.g. dorsomedial hypothalamus or the anterior paraventricular thalamic nucleus) or to the satiety system (e.g. arcuate hypothalamic nucleus or solitary tract); 27 areas were related to reward/addiction including the core and shell of the nucleus accumbens, the ventral pallidum and the ventral striatum (caudate and putamen). Eleven areas associated to sleep displayed significantly reduced Mn²⁺-accumulation and six areas related to locomotor activity showed significantly increased Mn²⁺-accumulation after the intake of potato chips. The latter changes were associated with an observed significantly higher locomotor activity. Osmotic pump-assisted MEMRI proved to be a promising technique for functional mapping of whole brain activity patterns associated to nutritional intake under normal behavior.