A Glutamatergic Hypothalamomedullary Circuit Mediates Thermogenesis, but Not Heat Conservation, during Stress-Induced Hyperthermia.

Stress elicits a variety of autonomic responses, including hyperthermia (stress fever) in humans and animals. In this present study, we investigated the circuit basis for thermogenesis and heat conservation during this response. We first demonstrated the glutamatergic identity of the dorsal hypothalamic area (DHAVglut2) neurons that innervate the raphe pallidus nucleus (RPa) to regulate core temperature (Tc) and mediate stress-induced hyperthermia. Then, using chemogenetic and optogenetic methods to manipulate this hypothalamomedullary circuit, we found that activation of DHAVglut2 neurons potently drove an increase in Tc, but surprisingly, stress-induced hyperthermia was only reduced by about one-third when they were inhibited. Further investigation showed that DHAVglut2 neurons activate brown adipose tissue (BAT) but do not cause vasoconstriction, instead allowing reflex tail artery vasodilation as a response to BAT-induced hyperthermia. Retrograde rabies virus tracing revealed projections from DHAVglut2 neurons to RPaVglut3, but not to RPaGABA neurons, and identified a set of inputs to DHAVglut2 → RPa neurons that are likely to mediate BAT activation. The dissociation of the DHAVglut2 thermogenic pathway from the thermoregulatory vasoconstriction (heat-conserving) pathway may explain stress flushing (skin vasodilation but a feeling of being too hot) during stressful times.

Involvement of GABAergic and Adrenergic Neurotransmissions on Paraventricular Nucleus of Hypothalamus in the Control of Cardiac Function.

Sympathetic premotor neurons of the paraventricular hypothalamus (PVN) play a role in hemodynamics adjustments during changes in body fluid homeostasis. However, PVN contribution to the tonic control of cardiac function remains to be systematically studied. In this study, we assessed whether GABAergic and adrenergic synapses, known for being active in the PVN, are involved in the control of cardiac function. Adult male Wistar rats (250-350 g; n = 27) were anesthetized with urethane (1.2-1.4 g/kg i.p.) and underwent catheterization of femoral artery to record blood pressure and heart rate. The femoral vein was used to inject the vasoactive agents phenylephrine (10 µg/kg) and sodium nitroprusside (10 µg/kg) and to supplement anesthesia. The cardiac left ventricle was catheterized to record left ventricular pressure and its derivative. Craniotomy allowed for injections (100 nL) into the PVN of: muscimol (20 mM), bicuculline methiodide (0.4 mM), propranolol (10 mM), isoproterenol (100 µM), phentolamine (13 mM), phenylephrine (30 nM). We found that: (i) inhibition of PVN by muscimol, reduced arterial pressure, cardiac chronotropy and inotropy; (ii) disinhibition of PVN neurons by bicuculline evoked positive chronotropy and inotropy, and increase blood pressure; (iii) PVN alpha adrenergic receptors control cardiac chronotropy and inotropy; (iv) beta adrenergic receptors of the PVN do not influence cardiac function; (v) afterload does not contribute to the PVN-evoked inotropy. Our results indicate that the modulation of the activity of PVN neurons exerted by GABAergic and adrenergic mechanisms contribute to the control of cardiac function.

Reduced anxiety-like behavior in transgenic rats with chronically overproduction of angiotensin-(1-7): Role of the Mas receptor.

Angiotensin-(1-7) [Ang-(1-7)], a counterregulatory peptide of the renin-angiotensin system (RAS), exerts its cardiovascular and renal functions through the G-protein-coupled receptor Mas. More recently, Ang-(1-7) has also been implicated in the control of emotional states related to fear and anxiety. Here, we tested the hypothesis that transgenic rats overexpressesing Ang-(1-7) (TGR) show reduced anxiety-like behavior in two distinct animals models, the Elevated Plus Maze (EPM) and Vogel Conflict Test (VCT). Sprague-Dawley rats (SDs) were used as controls. In addition, we also verified whether this phenotype depend on activation of the Mas receptor. In line with our hypothesis, TGR rats showed an increase in the percentage of time and entries in the open arms of the EPM. There was also an increase in the number of punished licks in VCT. These phenotypes were reversed by ICV injection of the Mas receptor antagonist, A779, but not by the AT2 and MrgD receptor antagonist, PD123319. These results suggest that chronic elevation of Ang-(1-7) levels results in a phenotype characterized by reduced anxiety-like behavior, possibly due to higher activation of the Mas receptor. Therefore, facilitation of the Ang-(1-7)/Mas receptor signaling may be further investigated as an additional strategy for the treatment of anxiety-related disorders.

Evidence that remodeling of insular cortex neurovascular unit contributes to hypertension-related sympathoexcitation.

The intermediate region of the posterior insular cortex (intermediate IC) mediates sympathoexcitatory responses to the heart and kidneys. Previous studies support hypertension-evoked changes to the structure and function of neurons, blood vessels, astrocytes and microglia, disrupting the organization of the neurovascular unit (NVU). In this study, we evaluated the functional and anatomical integrity of the NVU at the intermediate IC in the spontaneously hypertensive rat (SHR) and its control the Wistar-Kyoto (WKY). Under urethane anesthesia, NMDA microinjection (0.2 mmol/L/100 nL) was performed at the intermediate IC with simultaneous recording of renal sympathetic nerve activity (RSNA), heart rate (HR) and mean arterial pressure (MAP). Alterations in NVU structure were investigated by immunofluorescence for NMDA receptors (NR1), blood vessels (70 kDa FITC-dextran), astrocytes (GFAP), and microglia (Iba1). Injections of NMDA into intermediate IC of SHR evoked higher amplitude responses of RSNA, MAP, and HR On the other hand, NMDA receptor blockade decreased baseline RSNA, MAP and HR in SHR, with no changes in WKY Immunofluorescence data from SHR intermediate IC showed increased NMDA receptor density, contributing to the SHR enhanced sympathetic responses, and increased in vascular density (increased number of branches and endpoints, reduced average branch length), suggesting angiogenesis. Additionally, IC from SHR presented increased GFAP immunoreactivity and contact between astrocyte processes and blood vessels. In SHR, IC microglia skeleton analysis supports their activation (reduced number of branches, junctions, endpoints and process length), suggesting an inflammatory process in this region. These findings indicate that neurogenic hypertension in SHR is accompanied by marked alterations to the NVU within the IC and enhanced NMDA-mediated sympathoexcitatory responses likely contributors of the maintenance of hypertension.

Liposome-entrapped GABA modulates the expression of nNOS in NG108-15 cells.

BACKGROUND: Liposomes are concentric lipid vesicles that allow a sustained release of entrapped substances. GABA (γ-aminobutyric acid) is the most prevalent inhibitory neurotransmitter in the central nervous system. NEW METHOD: Using GABA-containing liposomes (GL) prepared by the freeze-thawing method, we determined the effect of sustained release of GABA on expression of neuronal nitric oxide synthase (nNOS) and GABAA receptor (GABAAR) in an in vitro neuronal model. RESULTS: Neuronal cell line NG108-15 treated with different doses of GL during 24h showed an increase in expression of GABAAR (54 and 50% with 10 and 20ng doses, respectively) and nNOS (138, 157 and 165% with 20, 50 and 100ng doses, respectively) compared with cells treated with empty liposomes (EL). Additionally, cells treated with 50ng of GL showed an increase in GABAAR (23%) after 1h followed by an increase in nNOS (55, 46 and 55%) at 8, 12 and 24h time points, respectively. Immunofluorescence experiments confirmed an increase in nNOS (134%) and basal intracellular levels of nitric oxide (84%) after GL treatment. Further, treatment of cells with GL showed a decrease in expression of a protein inhibitor of nNOS (PIN) (26, 66 and 57% with 20, 50 and 100ng doses respectively) compared with control. COMPARISON WITH EXISTING METHODS: This is first demonstration for the development of GL that allows sustained slow release of this neurotransmitter. CONCLUSION: These results suggest that a slow release of GABA can change the expression of nNOS possibly via alteration in PIN levels in neuronal cells.

Chronic Treatment with Ivabradine Does Not Affect Cardiovascular Autonomic Control in Rats.

A low resting heart rate (HR) would be of great benefit in cardiovascular diseases. Ivabradine-a novel selective inhibitor of hyperpolarization-activated cyclic nucleotide gated (HCN) channels- has emerged as a promising HR lowering drug. Its effects on the autonomic HR control are little known. This study assessed the effects of chronic treatment with ivabradine on the modulatory, reflex and tonic cardiovascular autonomic control and on the renal sympathetic nerve activity (RSNA). Male Wistar rats were divided in 2 groups, receiving intraperitoneal injections of vehicle (VEH) or ivabradine (IVA) during 7 or 8 consecutive days. Rats were submitted to vessels cannulation to perform arterial blood pressure (AP) and HR recordings in freely moving rats. Time series of resting pulse interval and systolic AP were used to measure cardiovascular variability parameters. We also assessed the baroreflex, chemoreflex and the Bezold-Jarish reflex sensitivities. To better evaluate the effects of ivabradine on the autonomic control of the heart, we performed sympathetic and vagal autonomic blockade. As expected, ivabradine-treated rats showed a lower resting (VEH: 362 ± 16 bpm vs. IVA: 260 ± 14 bpm, p = 0.0005) and intrinsic HR (VEH: 369 ± 9 bpm vs. IVA: 326 ± 11 bpm, p = 0.0146). However, the chronic treatment with ivabradine did not change normalized HR spectral parameters LF (nu) (VEH: 24.2 ± 4.6 vs. IVA: 29.8 ± 6.4; p > 0.05); HF (nu) (VEH: 75.1 ± 3.7 vs. IVA: 69.2 ± 5.8; p > 0.05), any cardiovascular reflexes, neither the tonic autonomic control of the HR (tonic sympathovagal index; VEH: 0.91± 0.02 vs. IVA: 0.88 ± 0.03, p = 0.3494). We performed the AP, HR and RSNA recordings in urethane-anesthetized rats. The chronic treatment with ivabradine reduced the resting HR (VEH: 364 ± 12 bpm vs. IVA: 207 ± 11 bpm, p < 0.0001), without affecting RSNA (VEH: 117 ± 16 vs. IVA: 120 ± 9 spikes/s, p = 0.9100) and mean arterial pressure (VEH: 70 ± 4 vs. IVA: 77 ± 6 mmHg, p = 0.3293). Our results suggest that, in health rats, the long-term treatment with ivabradine directly reduces the HR without changing the RSNA modulation and the reflex and tonic autonomic control of the heart.

Coordinated autonomic and respiratory responses evoked by alerting stimuli: Role of the midbrain colliculi.

Threatening stimuli trigger rapid and coordinated behavioral responses supported by cardiorespiratory changes. The midbrain colliculi can generate coordinated orienting or defensive behavioral responses, and it has been proposed that collicular neurons also generate appropriate cardiovascular and respiratory responses to support such behaviors. We have shown previously that under conditions where collicular neurons are disinhibited, coordinated cardiovascular, somatomotor and respiratory responses can be evoked independently of the cortex by auditory, visual and somatosensory stimuli. Here we report that these natural stimuli effectively increase inspiratory time most likely though phase switching. As a result the pattern of phrenic and sympathetic coupling is an inspiratory-related sympathoexcitation. We propose that blockade of tonic GABAergic input in the midbrain colliculi permits alerting stimuli to drive command neurons that generate coordinated cardiovascular, respiratory and motor outputs. The outputs of these command neurons likely interact with the central respiratory pattern generator, however the precise output pathways mediating the coordinated autonomic and respiratory responses remain to be determined.

Disinhibition of the midbrain colliculi unmasks coordinated autonomic, respiratory, and somatomotor responses to auditory and visual stimuli.

The midbrain superior and inferior colliculi have critical roles in generating coordinated orienting or defensive behavioral responses to environmental stimuli, and it has been proposed that neurons within the colliculi can also generate appropriate cardiovascular and respiratory responses to support such behavioral responses. We have previously shown that activation of neurons within a circumscribed region in the deep layers of the superior colliculus and in the central and external nuclei of the inferior colliculus can evoke a response characterized by intense and highly synchronized bursts of renal sympathetic nerve activity and phrenic nerve activity. In this study, we tested the hypothesis that, under conditions in which collicular neurons are disinhibited, coordinated cardiovascular, somatomotor, and respiratory responses can be evoked by natural environmental stimuli. In response to natural auditory, visual, or somatosensory stimuli, powerful synchronized increases in sympathetic, respiratory, and somatomotor activity were generated following blockade of GABAA receptors in a specific region in the midbrain colliculi of anesthetized rats, but not under control conditions. Such responses still occurred after removal of most of the forebrain, including the amygdala and hypothalamus, indicating that the essential pathways mediating these coordinated responses were located within the brain stem. The temporal relationships between the different outputs suggest that they are driven by a common population of "command neurons" within the colliculi.

Synchronized activation of sympathetic vasomotor, cardiac, and respiratory outputs by neurons in the midbrain colliculi.

The superior and inferior colliculi are believed to generate immediate and highly coordinated defensive behavioral responses to threatening visual and auditory stimuli. Activation of neurons in the superior and inferior colliculi have been shown to evoke increases in cardiovascular and respiratory activity, which may be components of more generalized stereotyped behavioral responses. In this study, we examined the possibility that there are "command neurons" within the colliculi that can simultaneously drive sympathetic and respiratory outputs. In anesthetized rats, microinjections of bicuculline (a GABA(A) receptor antagonist) into sites within a circumscribed region in the deep layers of the superior colliculus and in the central and external nuclei of the inferior colliculus evoked a response characterized by intense and highly synchronized bursts of renal sympathetic nerve activity (RSNA) and phrenic nerve activity (PNA). Each burst of RSNA had a duration of ∼300-400 ms and occurred slightly later (peak to peak latency of 41 ± 8 ms) than the corresponding burst of PNA. The bursts of RSNA and PNA were also accompanied by transient increases in arterial pressure and, in most cases, heart rate. Synchronized bursts of RSNA and PNA were also evoked after neuromuscular blockade, artificial ventilation, and vagotomy and so were not dependent on afferent feedback from the lungs. We propose that the synchronized sympathetic-respiratory responses are driven by a common population of neurons, which may normally be activated by an acute threatening stimulus.

Chronic infusion of angiotensin-(1-7) into the lateral ventricle of the brain attenuates hypertension in DOCA-salt rats.

Angiotensin-(ANG)-(1-7) is known by its central and peripheral actions, which mainly oppose the deleterious effects induced by accumulation of ANG II during pathophysiological conditions. In the present study we evaluated whether a chronic increase in ANG-(1-7) levels in the brain would modify the progression of hypertension. After DOCA-salt hypertension was induced for seven days, Sprague-Dawley rats were subjected to 14 days of intracerebroventricular (ICV) infusion of ANG-(1-7) (200 ng/h, DOCA-A7) or 0.9% sterile saline. As expected, on the 21st day, DOCA rats presented increased mean arterial pressure (MAP) (≈40%), and impaired baroreflex control of heart rate (HR) and baroreflex renal sympathetic nerve activity (RSNA) in comparison with that in normotensive control rats (CTL). These changes were followed by an overactivity of the cardiac sympathetic tone and reduction of the cardiac parasympathetic tone, and exaggerated mRNA expression of collagen type I (≈9-fold) in the left ventricle. In contrast, DOCA rats treated with ANG-(1-7) ICV had an improvement of baroreflex control of HR, which was even higher than that in CTL, and a restoration of the baroreflex control of RSNA, the balance of cardiac autonomic tone, and normalized mRNA expression of collagen type I in the left ventricle. Furthermore, DOCA-A7 had MAP lowered significantly. These effects were not accompanied by significant circulating or cardiac changes in angiotensin levels. Taken together, our data show that chronic increase in ANG-(1-7) in the brain attenuates the development of DOCA-salt hypertension, highlighting the importance of this peptide in the brain for the treatment of cardiovascular diseases.

Liposome-encapsulated neuropeptides for site-specific microinjection.

This paper describes an experimental approach based on nanotechnology for assessing the chronic actions of short-lived neuropeptides at specific sites of the brain. This methodology combines the advantages of two different techniques: the microinjection of a suspension of peptide-containing liposomes into a specific site of the brain, and the use of liposomes as a local and sustained release nanosystem of the peptide.

Cardiovascular and thermal responses evoked from the periaqueductal grey require neuronal activity in the hypothalamus.

Stimulation of neurons in the lateral/dorsolateral periaqueductal grey (l/dlPAG) produces increases in heart rate (HR) and mean arterial pressure (MAP) that are, according to traditional views, mediated through projections to medullary autonomic centres and independent of forebrain mechanisms. Recent studies in rats suggest that neurons in the l/dlPAG are downstream effectors responsible for responses evoked from the dorsomedial hypothalamus (DMH) from which similar cardiovascular changes and increase in core body temperature (T(co)) can be elicited. We hypothesized that, instead, autonomic effects evoked from the l/dlPAG depend on neuronal activity in the DMH. Thus, we examined the effect of microinjection of the neuronal inhibitor muscimol into the DMH on increases in HR, MAP and T(co) produced by microinjection of N-methyl-D-aspartate (NMDA) into the l/dlPAG in conscious rats. Microinjection of muscimol alone modestly decreased baseline HR and MAP but failed to alter T(co). Microinjection of NMDA into the l/dlPAG caused marked increases in all three variables, and these were virtually abolished by prior injection of muscimol into the DMH. Similar microinjection of glutamate receptor antagonists into the DMH also suppressed increases in HR and abolished increases in T(co) evoked from the PAG. In contrast, microinjection of muscimol into the hypothalamic paraventricular nucleus failed to reduce changes evoked from the PAG and actually enhanced the increase in T(co). Thus, our data suggest that increases in HR, MAP and T(co) evoked from the l/dlPAG require neuronal activity in the DMH, challenging traditional views of the place of the PAG in central autonomic neural circuitry.

Microinjection of muscimol into the periaqueductal gray suppresses cardiovascular and neuroendocrine response to air jet stress in conscious rats.

Microinjection of the neuronal inhibitor muscimol into the dorsomedial hypothalamus (DMH) suppresses increases in heart rate (HR), mean arterial pressure (MAP), and circulating levels of adrenocorticotropic hormone (ACTH) evoked in air jet stress in conscious rats. Similar injection of muscimol into the caudal region of the lateral/dorsolateral periaqueductal gray (l/dlPAG) reduces autonomic responses evoked from the DMH, leading to the suggestion that neurons in the l/dlPAG may represent a descending relay for DMH-induced increases in HR and MAP. Here, we examined the role of neuronal activity in the caudal l/dlPAG on the increases in MAP, HR, and plasma ACTH seen in air jet stress in rats. Microinjection of muscimol into the caudal l/dlPAG reduced stress-induced increases in HR and MAP, while identical injections into sites just dorsal or into the rostral l/dlPAG had no effect. Microinjection of a combination of the glutamate receptor antagonists 2-amino-5-phosphonopentanoate (AP5) and 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo[f]quinoxaline-2,3-dione (NBQX) into the caudal l/dlPAG decreased stress-induced increases in HR alone only at the end of the 20-min stress period but significantly accelerated return to baseline. Surprisingly, microinjection of muscimol into the caudal l/dlPAG also reduced the stress-induced increase in plasma ACTH by 51%. Compared with unstressed control rats, rats exposed to air jet stress exhibited approximately 3 times the number of Fos-positive neurons in the l/dlPAG. These findings suggest that neurons in the l/dlPAG are activated in air jet stress and that this activity contributes to increases in HR, MAP, and plasma ACTH.

Baroreflex control of heart rate and renal sympathetic nerve activity in rats with low brain angiotensinogen.

The main objective of the present study was to evaluate baroreceptor control of heart rate (HR) and renal sympathetic nerve activity (RSNA) in transgenic rats (TG) with low angiotensinogen production in glial cells, TGR(ASrAogen)-680. In addition, the sympathetic and vagal autonomic tonus to the heart was investigated. As previously shown, TG rats presented a lower arterial pressure (AP) and HR. However, TG rats had decreased AP variability during the night (8.9+/-0.4 mmHg vs 9.8+/-0.3 mmHg, in SD) accompanied by an increase in HR variability (39+/-1 beats/min vs 35+/-1 beats/min, in SD) and augmented locomotor activity during the night (3.5+/-0.3 counts/min vs 2.5+/-0.2 counts/min, in SD). In addition, TG rats presented increased baroreflex sensitivity for the RSNA (slope of line that correlates decreases in RSNA and increases in AP=1.36+/-0.18 vs 0.77+/-0.1, in SD) and an increased sensitivity for both the baroreflex bradycardia (0.79+/-0.04 ms/mmHg vs 0.52+/-0.04 ms/mmHg, in SD) and tachycardia (1.46+/-0.1 ms/mmHg vs 0.93+/-0.01 ms/mmHg, in SD). Further, TG rats had increased vagal tonus (25+/-3 beats/min vs 11+/-4 beats/min in SD) without significant change in the sympathetic tonus to the heart. These results confirm and extend previous observations showing that glial angiotensinogen, the main source of brain RAS peptides, importantly modulates sympathetic tonus, at least to the renal nerve, and vagal tonus to the heart.

Microinjection of muscimol into caudal periaqueductal gray lowers body temperature and attenuates increases in temperature and activity evoked from the dorsomedial hypothalamus.

Microinjection of the neuronal inhibitor muscimol into the midbrain lateral/dorsolateral periaqueductal gray (l/dlPAG) suppresses increases in heart rate (HR) and mean arterial pressure (MAP) evoked by microinjection of the GABA(A) receptor antagonist bicuculline methiodide (BMI) into the dorsomedial hypothalamus (DMH) in rats. Injection of BMI into the DMH also increases body temperature (Tco) and motor activity. Here, our goal was to extend previous findings by examining the effect of microinjection of muscimol into the PAG on these thermogenic and behavioral responses in conscious freely moving rats. Microinjection of muscimol (300 pmol and 1 nmol) alone into the l/dlPAG reduced baseline Tco without affecting activity, HR, or MAP. Similar injection of a dose that failed to alter baseline Tco (100 pmol) suppressed the increases in Tco evoked from the DMH and significantly attenuated DMH-induced increases in locomotor activity. Whereas microinjection of 1 nmol muscimol into the ldlPAG abolished the increases in Tco evoked from the DMH and in fact lowered body temperature to a degree similar to that seen after this dose of muscimol alone, 1 nmol muscimol at adjacent sites outside the targeted region of the PAG had no significant effect on DMH-induced increases in Tco or any other parameter. These results indicate a role for neuronal activity in the l/dlPAG in (1) the temperature and behavioral responses to disinhibition of neurons in the DMH, and (2) the maintenance of basal body temperature in conscious freely moving rats.

O hipotálamo dorsomedial e a organização da resposta cardiovascular ao estresse emocional: uma perspectiva funcional / The dorsomedial hypothalamus and the organization of the cardiovascular response to emotional stress: a functional perpective

RESUMO: O estresse emocional resulta em ativação de vias específicas do sistema nervoso central, que produzem respostas autonômicas, comportamentais e endócrinas. Sabe-se que situações de estresse recorrentes ou prolongadas podem resultar em vários estados patológicos, como por exemplo, a hipertensão arterial.O Hipotálamo tem papel fundamental na integração das respostas fisiológicas ao estresse emocional. Particularmente, estudos têm mostrado que um núcleo especifico do hipotálamo, o hipotálamo dorsomedial (DMH), é um componente fundamental das vias centrais mediadoras das respostas cardiovasculares ao estresse emocíonal. A inibição dos neurônios dessa área reduz os aumentos de freqüência cardíaca e de pressão arterial em ratos quando submetidos à situações de estresse emocional. Ao contrário, a ativação farmacológica dos neurônios do DMH produz aumento na frequência cardíaca, pressão arterial, hormônio adrenocorticotrópico (ACTH), atividade locomotora e na atividade simpática para diversos leitos vasculares. A similaridade dessa resposta com aquela produzida durante a situação real de estresse emocíonal sugere que esta área é fundamental na integração da resposta fisiológica ao estresse.A presente revisão tem como objetivo mostrar, através de resultados de estudos recentes, as vias centrais utilizadas pelo DMH na organização da resposta cardiovascular ao estresse emocional.

Blockade of AT1 receptors in the rostral ventrolateral medulla increases sympathetic activity under hypoxic conditions.

The role of ANG type 1 (AT1) receptors in the rostral ventrolateral medulla (RVLM) in the maintenance of sympathetic vasomotor tone in normotensive animals is unclear. In this study, we tested the hypothesis that AT1 receptors make a significant contribution to the tonic activity of presympathetic neurons in the RVLM of normotensive rats under conditions where the excitatory input to these neurons is enhanced, such as during systemic hypoxia. In urethane-anesthetized rats, microinjections of the AT1 receptor antagonist candesartan in the RVLM during moderate hypoxia unexpectedly resulted in substantial increases in arterial pressure and renal sympathetic nerve activity (RSNA), whereas under normoxic conditions the same dose resulted in no significant change in arterial pressure and RSNA. Under hypoxic conditions, and after microinjection of the GABA(A) receptor antagonist bicuculline in the RVLM, subsequent microinjection of candesartan in the RVLM resulted in a significant decrease in RSNA. In control experiments, bilateral microinjections in the RVLM of the compound [Sar1,Thr8]ANG II (sarthran), which decreases sympathetic vasomotor activity via a mechanism that is independent of AT1 receptors, significantly reduced arterial pressure and RSNA under both normoxic and hypoxic conditions. The results indicate that, at least under some conditions, endogenous ANG II has a tonic sympathoinhibitory effect in the RVLM, which is dependent on GABA receptors. We suggest that the net effect of endogenous ANG II in this region depends on the balance of both tonic excitatory and inhibitory actions on presympathetic neurons and that this balance is altered in different physiological or pathophysiological conditions.

Cardiovascular responses evoked by leptin acting on neurons in the ventromedial and dorsomedial hypothalamus.

Leptin, a circulating hormone produced by adipose tissue, is believed to act on the hypothalamus to increase sympathetic vasomotor activity, in addition to its well-known effects on appetite and energy expenditure. In this study, we determined the cardiovascular effects of direct application of leptin to specific cell groups within the hypothalamus that are known to be activated by circulating leptin. In rats anesthetized with urethane, microinjections of leptin (16 ng in 20 nL solution) were made into the ventromedial hypothalamic nucleus, dorsomedial hypothalamic nucleus, and paraventricular nucleus. Compared with vehicle solution, microinjections of leptin into the ventromedial hypothalamic nucleus evoked significant increases in arterial pressure and renal sympathetic nerve activity, but not heart rate. In contrast, microinjections of leptin into the dorsomedial hypothalamic nucleus evoked significant increases in arterial pressure and heart rate but not renal sympathetic nerve activity, whereas microinjections of leptin into the paraventricular nucleus had no significant effect on any of the measured cardiovascular variables. These results indicate that the ventromedial and dorsomedial hypothalamic regions might be important sites at which leptin activation leads to increases in sympathetic vasomotor activity and heart rate, as occurs in obesity-related hypertension.