Central Regulation Of Cardiovascular Function Research Articles

Role of the Melanocortin System in the Central Regulation of Cardiovascular Functions.

Increasing evidence indicates that the melanocortin system is not only a central player in energy homeostasis, food intake and glucose level regulation, but also in the modulation of cardiovascular functions, such as blood pressure and heart rate. The melanocortins, and in particular α- and γ-MSH, have been shown to exert their cardiovascular activity both at the central nervous system level and in the periphery (e.g., in the adrenal gland), binding their receptors MC3R and MC4R and influencing the activity of the sympathetic nervous system. In addition, some studies have shown that the activation of MC3R and MC4R by their endogenous ligands is able to improve the outcome of cardiovascular diseases, such as myocardial and cerebral ischemia. In this brief review, we will discuss the current knowledge of how the melanocortin system influences essential cardiovascular functions, such as blood pressure and heart rate, and its protective role in ischemic events, with a particular focus on the central regulation of such mechanisms.

Effect of suvorexant on nighttime blood pressure in hypertensive patients with insomnia: The SUPER-1 study.

Orexins are neuropeptides that play a role in maintaining wakefulness and contribute to central regulation of cardiovascular function. This first multicenter, randomized, double-blind study investigated the effects of suvorexant, a reversible dual orexin receptor antagonist, on nighttime blood pressure (BP) in patients with insomnia and hypertension. After a 4-week run-in period, adult outpatients (n=82) with treated hypertension (clinic SBP <160mmHg) and insomnia were treated with suvorexant 20mg/d or placebo before bedtime for 2weeks. Twenty-four-hour ambulatory BP monitoring was performed at baseline and the end of treatment, and home BP measurements (morning and evening) were taken daily. Nighttime systolic BP (SBP), the primary endpoint, decreased slightly from baseline to week 2 in both the suvorexant and placebo groups (-4.4 vs -1.8mmHg; P=0.494). Clinic, 24-hour, daytime and morning SBP (ambulatory blood pressure monitoring) also decreased slightly and similarly from baseline in both groups. In this study, suvorexant had no overall effect on BP in patients with insomnia and treated hypertension.

Ablation of Astrocytes in the Paraventricular Nucleus Disrupts the Blood‐Brain Barrier and Increases Blood Pressure in Rats

BackgroundPerivascular astrocytes make direct connections to the cerebral vasculature. This close apposition has been shown to be critical for the regulation of blood‐brain barrier (BBB) integrity and function. Recent studies have shown that the BBB becomes disrupted in hypertensive rats, which allows circulating peptides such as angiotensin II, which is normally excluded from the central nervous system, to access brain areas critical for blood pressure control. To date, the association between astrocytes, the BBB and hypertension is still poorly understood.ObjectiveTo determine the importance of astrocytes in maintaining BBB integrity and central control of cardiovascular homeostasis.MethodsTwo groups of male Sprague‐Dawley rats (250–400g), anesthetized (pentobarbital, 60 mg/kg i.p.) and conscious freely‐moving, were bilaterally microinjected with sodium fluoroacetate (50 nL; 10 mM), a specific glial toxin, into the paraventricular nucleus of the hypothalamus (PVN) to ablate perivascular astrocytes. Blood pressure and heart rate were continuously recorded throughout. Evans blue dye (4 mL/kg) was then infused intravenously following microinjections to assess BBB integrity. Changes in baroreceptor reflex function were also studied following fluoroacetate administration.ResultsAblation of PVN astrocytes caused a significant increase in Evans Blue dye extravasation (+16.67 ± 4.9 mean density in arbitrary units, P&lt;0.05) when compared with saline injected controls, indicating a compromised BBB. Furthermore, we found a significant increase in mean arterial pressure in anesthetized (+41.02 ± 5.5 mmHg from baseline, P&lt;0.0001) and conscious rats (+23.82.96 ± 7.8 mmHg from baseline, P&lt;0.05) at 100‐minute and 2‐days following fluoroacetate injection, respectively. However, no changes were seen in the baroreceptor reflex control of heart rate.ConclusionOur results show a functional link between astrocytes, BBB disruption and central regulation of cardiovascular function. These findings suggest that a disrupted BBB, via astrocyte dysfunction, may contribute to the pathophysiology of centrally‐induced cardiovascular diseases such as hypertension.

Cystathionine-β-Synthase Gene Transfer Into Rostral Ventrolateral Medulla Exacerbates Hypertension via Nitric Oxide in Spontaneously Hypertensive Rats.

Rostral ventrolateral medulla (RVLM) plays a crucial role in the central regulation of cardiovascular functions. Cystathionine-β-synthase (CBS) is a major hydrogen sulfide (H2S)-generating enzyme that has been identified mainly in the brain. The present study was designed to examine CBS expression and determine its roles and mechanisms of regulating sympathetic outflow and blood pressure (BP) in the RVLM in spontaneously hypertensive rats (SHR). CBS expression was decreased in the RVLM in SHR compared to Wistar-Kyoto (WKY) rats. Accumulating evidences suggest that H2S interacts with nitric oxide (NO) to regulate cardiovascular function. Therefore, we hypothesize that the decrease in CBS expression in the RVLM may be involved in the disorder of l-arginine/NO pathway, which subsequently affects BP in SHR. Overexpression of CBS in the RVLM caused significant increases in BP, heart rate, and urinary norepinephrine excretion in SHR but not in WKY. Acute experiments were carried out at day 7 after gene transfer. NO metabolite levels, neuronal NO synthase, and γ-amino butyric acid were decreased in SHR after CBS gene transfer. Furthermore, pressor responses to microinjection of NG-monomethyl-l-arginine into RVLM were blunt in SHR transfected with AdCBS compared to SHR transfected with AdEGFP. Overexpression of CBS in the RVLM elicits enhanced pressor responses in SHR, but not in WKY, and the NO system is involved in these effects. The results suggest that alterations of H2S signaling in the brain may be associated with the development of hypertension.

Developmental changes in GABAergic neurotransmission to presympathetic and cardiac parasympathetic neurons in the brainstem

Cardiovascular function is regulated by a dynamic balance composed of sympathetic and parasympathetic activity. Sympathoexcitatory presympathetic neurons (PSNs) in the rostral ventrolateral medulla project directly to cardiac and vasomotor sympathetic preganglionic neurons in the spinal cord. In proximity to the PSNs in the medulla, there are preganglionic cardiac vagal neurons (CVNs) within the nucleus ambiguus, which are critical for parasympathetic control of heart rate. Both CVNs and PSNs receive GABAergic synaptic inputs that change with challenges such as hypoxia and hypercapnia (H/H). Autonomic control of cardiovascular function undergoes significant changes during early postnatal development; however, little is known regarding postnatal maturation of GABAergic neurotransmission to these neurons. In this study, we compared changes in GABAergic inhibitory postsynaptic currents (IPSCs) in CVNs and PSNs under control conditions and during H/H in postnatal day 2-5 (P5), 16-20 (P20), and 27-30 (P30) rats using an in vitro brainstem slice preparation. There was a significant enhancement in GABAergic neurotransmission to both CVNs and PSNs at age P20 compared with P5 and P30, with a more pronounced increase in PSNs. H/H did not significantly alter this enhanced GABAergic neurotransmission to PSNs in P20 animals. However, the frequency of GABAergic IPSCs in PSNs was reduced by H/H in P5 and P30 animals. In CVNs, H/H elicited an inhibition of GABAergic neurotransmission in all ages studied, with the most pronounced inhibition occurring at P20. In conclusion, there are critical development periods at which significant rearrangement occurs in the central regulation of cardiovascular function.

Angiotensin-converting enzyme 2: central regulator for cardiovascular function.

Angiotensin-converting enzyme 2 (ACE2) is a new component of the renin-angiotensin system (RAS). Accumulating evidence shows that ACE2 provides protective effects in peripheral tissues and has great potential for the treatment of RAS-related diseases. The role of ACE2 in the central nervous system is not well established. However, in recent years, much more progress has been made on the studies of this carboxypeptidase in the central regulation of blood pressure and cardiovascular function in general. It has been shown that brain ACE2 interacts with the other components of the RAS (ACE, angiotensin II, and angiotensin II type 1 receptor), protects baroreflex and autonomic function, stimulates nitric oxide release, reduces oxidative stress, and prevents the development of or attenuates hypertension. These data support the critical role of ACE2 in the central regulation of cardiovascular function. This review summarizes recently published data on the central effects of ACE2 in the regulation of cardiovascular function.

Blood pressure regulation and cardiac autonomic control in mice overexpressing α- and γ-melanocyte stimulating hormone

Melanocyte stimulating hormones (MSH) derived from pro-opiomelanocortin have been demonstrated to participate in the central regulation of cardiovascular functions. The aim of the present study was to elucidate the chronic effects of increased melanocortin activation on blood pressure regulation and autonomic nervous system function. We adapted telemetry to transgenic mice overexpressing α- and γ-MSH and measured blood pressure, heart rate and locomotor activity, and analyzed heart rate variability (HRV) in the frequency-domain as well as baroreflex function by the sequence technique. Transgenic (MSH-OE) mice had increased systolic blood pressure but their heart rate was similar to wild-type (WT) controls. The 24-h mean of systolic blood pressure was 132 ± 7 mmHg in MSH-OE and 113 ± 4 mmHg in WT mice. Locomotor activity was decreased in the MSH-OE mice. Furthermore, MSH-OE mice showed slower adaptation to mild environmental stress in terms of blood pressure changes. The low frequency (LF) power of HRV tended to be higher in MSH-OE mice compared to WT mice, without a difference in overall variability. The assessment of baroreflex function indicated enhanced baroreflex effectiveness and more frequent baroreflex operations in MSH-OE mice. Baseline heart rate, increased LF power of HRV and increased baroreflex activity may all reflect maintenance of baroreflex integrity and an increase in cardiac vagal activity to counteract the increased blood pressure. These results provide new evidence that long-term activation of the melanocortin system elevates blood pressure without increasing heart rate.

Angiotensin-Converting Enzyme 2 Overexpression in the Subfornical Organ Prevents the Angiotensin II–Mediated Pressor and Drinking Responses and Is Associated With Angiotensin II Type 1 Receptor Downregulation

We recently reported the presence of angiotensin-converting enzyme (ACE)2 in brain regions controlling cardiovascular function; however, the role of ACE2 in blood pressure regulation remains unclear because of the lack of specific tools to investigate its function. We hypothesized that ACE2 could play a pivotal role in the central regulation of cardiovascular function by regulating other renin-angiotensin system components. To test this hypothesis, we generated an adenovirus expressing the human ACE2 cDNA upstream of an enhanced green fluorescent protein (eGFP) reporter gene (Ad-hACE2-eGFP). In vitro characterization shows that neuronal cells infected with Ad-hACE2-eGFP (10 to 100 multiplicities of infection), but not Ad-eGFP (100 multiplicities of infection), exhibit dose-dependent ACE2 expression and activity. In addition, an active secreted form was detected in the conditioned medium. In vivo, Ad-hACE2-eGFP infection (2x10(6) plaque-forming units intracerebroventricularly) produced time-dependent expression and activity (with a peak at 7 days) in the mouse subfornical organ. More importantly, 7 days after virus infection, the pressor response to angiotensin (Ang) II (200 pmol intracerebroventricularly) was significantly reduced in Ad-hACE2-eGFP-treated mice compared with controls. Furthermore, subfornical organ-targeted ACE2 overexpression dramatically reduced the Ang II-mediated drinking response. Interestingly, ACE2 overexpression was associated with downregulation of the Ang II type 1 receptor expression both in vitro and in vivo. These data suggest that ACE2 overexpression in the subfornical organ impairs Ang II-mediated pressor and drinking responses at least by inhibiting the Ang II type 1 receptor expression. Taken together, our results show that ACE2 plays a pivotal role in the central regulation of blood pressure and volume homeostasis, offering a new target for the treatment of hypertension and other cardiovascular diseases.

Differential sensitisation to central cardiovascular effects of angiotensin II in rats with a myocardial infarct: Relevance to stress and interaction with vasopressin

The purpose of the present study was to elucidate if rats with myocardial infarction manifest altered responsiveness to central cardiovascular effects of low doses of angiotensin II (ANG II), and if ANG II and vasopressin (VP) cooperate in the central regulation of cardiovascular functions at rest and during stress. Conscious Sprague–Dawley rats with myocardial infarction induced by left coronary artery ligation, or sham-ligated (SL) controls were infused intracerebroventricularly with artificial cerebrospinal fluid (aCSF), ANG II, ANG II + VP or ANG II + V1a receptor antagonist (V1ANT) 4 weeks after cardiac surgery. In the infarcted but not in the SL rats, the resting mean arterial blood pressure (MABP) was significantly elevated by infusions of ANG II and ANG II + VP, while infusion of ANG II + V1ANT was not effective. During administration of aCSF, the pressor, and tachycardic responses to an air-jet stressor were significantly greater in the infarcted than in the SL rats. In the SL rats, the pressor responses to the stressor were significantly greater during infusions of ANG II, ANG II + VP and ANG II + V1ANT than during infusion of aCSF. The tachycardic response in the SL rats was enhanced only by the combined infusion of ANG II + VP. In the infarcted rats, the pressor and the tachycardic responses to the stressor were similar in all groups. It is concluded that: (1) under resting conditions the infarcted rats manifest sensitisation to the central pressor effect of ANG II and that this effect depends on concomitant stimulation of V1a VP receptors, (2) central ANG II may enhance the pressor response to an alarming stressor in the SL rats through an action which does not depend on the concomitant stimulation of V1a receptors, (3) the cooperative action of ANG II and VP is required for intensification of the tachycardic response to the alarming stressor in the SL rats and (4) exaggeration of the cardiovascular responses to the alarming stressor in the infarcted rats cannot be further augmented by an additional stimulation of central ANG II receptors or combined stimulation of ANG II and VP receptors.

Microinjections of melanin concentrating hormone into the nucleus tractus solitarius of the rat elicit depressor and bradycardic responses

The presence of melanin-concentrating hormone (MCH) containing processes, projecting from the lateral hypothalamus to the medial nucleus tractus solitarius (mNTS), has been reported in the rat. It was hypothesized that MCH acting within the mNTS may modulate the central regulation of cardiovascular function. This hypothesis was tested in urethane-anesthetized, artificially ventilated, adult male Wistar rats. Microinjections (100 nl) of MCH (0.25, 0.5, 0.75, and 1 mM) into the mNTS of anesthetized rats elicited decreases in mean arterial pressure (20.4±1.6, 50.7±3.3, 35.7±2.8 and 30.0±2.6 mm Hg, respectively). The decreases in heart rate in response to these concentrations of MCH were 40.0±8.7, 90.0±13.0, 48.0±7.3 and 48.0±8.0 beats/min, respectively. Maximum cardiovascular responses were elicited by a 0.5 mM concentration of MCH. Cardiovascular responses to MCH were similar in unanesthetized mid-collicular decerebrate rats. Control microinjections of normal saline (100 nl) did not elicit any cardiovascular response. Ipsilateral or bilateral vagotomy significantly attenuated MCH-induced bradycardia. Prior microinjections of PMC-3881-PI (2 mM; MCH-1 receptor antagonist) into the mNTS blocked the cardiovascular responses to microinjections of MCH. Microinjection of MCH (0.5 mM) into the mNTS decreased efferent greater splanchnic nerve activity. Direct application of MCH (0.5 mM; 4 nl) to barosensitive nucleus tractus solitarius (NTS) neurons increased their firing rate. These results indicate that: 1) MCH microinjections into the mNTS activate MCH-1 receptors and excite barosensitive NTS neurons, causing a decrease in efferent sympathetic activity and blood pressure, and 2) MCH-induced bradycardia is mediated via the activation of the vagus nerves.

Central AT1 receptor blockade restores baroreflex sensitivity and lowers blood pressure in ACE2 knockout mice

We recently showed the presence of ACE2 in brain regions involved in cardiovascular (CV) regulation. Although angiotensin (Ang)(1-7) is known to increase cardiac baroreflex sensitivity (CBS), the role of ACE2 is unclear. Therefore, we investigated ACE2 role in modulating CBS in ACE2 KO (n=5), exhibiting a high blood pressure (BP) phenotype, and control littermates (n=4). Anesthetized mice were implanted with carotid and jugular catheters for BP and heart rate monitoring and drug infusion, respectively. CBS (bpm/mmHg) was determined following random infusion of phenylephrine (50 ng/min iv) and sodium nitroprusside (5 μg/min iv). In ACE2 KO mice, CBS is significantly altered (−2.8±0.3) compared to control mice (−4.4±0.3; P<0.05) suggesting higher brain Ang-II levels in this model. To confirm the participation of central Ang-II, the experiment was repeated following central blockade of AT1 receptors with Losartan (10 μg/200 nl icv). Although Losartan had no effect on baseline BP in control mice (+1±1 mmHg; P>0.05), it significantly decreased BP in ACE2 KO mice (−11±2 mmHg; P<0.05). Moreover, AT1 receptor blockade partially restored CBS in ACE2 KO mice (−3.3±0.3; P<0.05). These data show that ACE2 is tonically involved in the maintenance of baseline BP and in the central regulation of CV function, offering a new target for the treatment of hypertension and other CV diseases. Support: NIH NS052479 and P20 RR018766

Cardiovascular actions of adrenocorticotropin microinjections into the nucleus tractus solitarius of the rat

The presence of adrenocorticotropin (ACTH) containing cells and melanocortin (MC) receptors has been reported in the nucleus tractus solitarius (NTS) of the rat. The importance of the NTS in the regulation of cardiovascular function is also well established. Based on these reports, it was hypothesized that ACTH acting within the NTS may modulate the central regulation of cardiovascular function. To test this hypothesis, cardiovascular effects of ACTH in the NTS were investigated in intact urethane-anesthetized and unanesthetized decerebrate, artificially ventilated, adult male Wistar rats. Microinjections of ACTH (0, 0.5, 1, 2, and 4 mM) into the medial subnucleus of NTS (mNTS) elicited decreases in mean arterial pressure (MAP; 0±0, 24.4±3.5, 35.7±4.3, 44.5±5.8 and 53.7±5.6 mm Hg, respectively) and heart rate (HR; 0±0, 25.7±5.3, 35.5±6.4, 47.5±12.1 and 55.0±5.6 beats/min, respectively). The onset and duration of the responses to microinjections of ACTH (0.5–4 mM) were 5–10 s and 45–120 s, respectively. Control microinjections of artificial cerebrospinal fluid (aCSF) did not elicit any response. The volume of all microinjections was 100 nl. The concentrations of ACTH that elicited depressor and bradycardic responses when microinjected into the mNTS (e.g. 1 or 2 mM, 100 nl), did not elicit a response when injected i.v. ( n=5) or i.c.v. ( n=2) indicating that there was no leakage of the drug from the injection site in the mNTS. Microinjections of MC3/4 receptor antagonists (acetyl-[Nle 4, Asp 5, d-2-Nal 7, Lys 10]-cyclo-alpha-MSH amide, fragments 4–10 (SHU9119) and agouti-related protein (83-132) amide) into the mNTS blocked the responses to ACTH. Microinjections of ACTH (2 mM) into the mNTS decreased efferent greater splanchnic nerve activity. Bilateral vagotomy significantly attenuated ACTH-induced bradycardia. These results indicated that: 1) microinjections of ACTH into the mNTS elicited depressor and bradycardic responses, 2) these responses were mediated via MC3/4 receptors, 3) the depressor effects were mediated via a decrease in the activity of the sympathetic nervous system, and 4) the bradycardic responses were vagally mediated.

Angiotensin‐II receptors regulate ACE2 expression in the brain

We previously reported the presence of ACE2 in brain nuclei involved in the central regulation of blood pressure (BP). Using real-time PCR, we tested the hypothesis that angiotensin-II receptors blockade would modulate ACE2 expression in these brain regions. Non-transgenic (NT) and transgenic mice (n=6 per group) with brain-selective overexpression of AT1a receptors (NSE-AT1) were instrumented with radiotelemeters for continuous BP monitoring. Baseline BP (mmHg) was recorded for 1 wk before subcutaneous implantation of osmotic pumps containing saline, an AT1 blocker (losartan, 10 mg/Kg/day) or an AT2 antagonist (PD123319, 10 mg/Kg/day). Mice were infused and BP recorded for 2 wks. In losartan-treated mice, mean BP decreased significantly (NT:−10±3; NSE-AT1:−12±3; P<0.05) by 2 wks but remained unchanged in the saline groups (NT:−1±2; NSE-AT1:-2±3). On the other hand, AT2 blockade increased BP only in NSE-AT1 (12±5; P<0.05) mice. Basal ACE2 mRNA expression was increased in the nucleus of tractus solitarius (NT:1±0.1; NSE-AT1:1.3±0.2; P<0.05) and decreased in the ventrolateral medulla (NT:1±0.1; NSE-AT1:0.7±0.1; P<0.05) of NSE-AT1 mice but was similarly decreased by losartan (NT:0.5±0.1; NSE-AT1:0.7±0.1 and NT:0.6±0.1; NSE-AT1:0.4±0.1, respectively, P<0.05) in both genotypes. On the contrary, AT2 blockade increased ACE2 expression in these regions (NT:1.9±0.4; NSE-AT1:1.5±0.2 and NT:1.9±0.3, respectively, P<0.05). These data suggest that both AT1 and AT2 receptors regulate ACE2 expression in brain regions controlling BP, confirming the involvement of brain ACE2 in the central regulation of cardiovascular function. (NS52479)

The cardiovascular effects of nitric oxide and carbon monoxide in the nucleus tractus solitarii of rats.

Nitric oxide (NO) and carbon monoxide (CO) are endogenously synthesized gaseous molecules that act as neurotransmitters in both central and peripheral nervous systems. Previously, we have shown the involvement of NO and CO in central cardiovascular regulation and baroreflex modulation. In this study we investigated the possible interaction of NO and CO in the nucleus tractus solitarii (NTS) on cardiovascular effects in rats. Male Sprague-Dawley rats were anesthetized with urethane, and mean blood pressure (MBP) and heart rate (HR) were monitored intra-arterially. l-Arginine (3.3 nmol), the precursor of NO, or hematin (1 nmol), a heme molecule cleaved by heme oxygenase (HO) to yield CO, were microinjected unilaterally into the NTS. Cardiovascular effects were evaluated before and after microinjection of the HO inhibitor zinc deuteroporphyrin 2,4-bis glycol (ZnDPBG: 1 nmol) or the NO synthase (NOS) inhibitors N -monomethyl-l-arginine (l-NMMA: 10, 33 and 100 nmol) and N-nitro-l-arginine methyl ester (l-NAME: 10, 33 and 100 nmol). Unilateral microinjection of l-arginine or hematin into the NTS produced decreases in blood pressure and heart rate. These cardiovascular effects of both l-arginine and hematin were attenuated by prior administration of the NOS inhibitors l-NMMA or l-NAME in a dose-dependent manner. However, prior administration of ZnDPBG attenuated only the cardiovascular effects of hematin but not l-arginine. These results demonstrated that the HO/CO pathway might couple to the activation of NOS via the liberation of NO, to participate in central regulation of cardiovascular function. They also suggested a possible interaction between the NO/NOS and CO/HO systems in the NTS of rats.

Involvement of nuclei in the hypothalamus in cardiac sympathoexcitatory reflexes in cats

The hypothalamus is considered to be an important area in the central regulation of cardiovascular function. However, its role in processing excitatory cardiovascular reflexes induced by stimulation of cardiac afferents has not been established. In the present study, using c-Fos immunoreactivity, we located neurons in the hypothalamus activated by inputs from cardiac sympathetic afferents. Following bilateral barodenervation and cervical vagotomy in anesthetized cats, bradykinin (BK, 1–10 μg, in 0.1 ml; n=7) was applied repetitively (6×, every 20 min) to the anterior epicardial surface of the left ventricle. This chemical stimulation caused consistent excitatory cardiovascular reflexes characterized by increases in blood pressure (BP) and heart rate (HR), while the vehicle for BK (0.9% saline, n=6) produced no such responses. Compared to control cats, c-Fos immunoreactive cells were significantly increased ( P<0.05) in the arcuate nucleus (ARC), dorsal hypothalamic area (HDA), dorsomedial nucleus, paraventricular hypothalamic nucleus (PVN) and periventricular nucleus in the BK-treated animals. More neurons double-labeled with c-Fos and nitric oxide synthase (NOS) were observed in the PVN following epicardial application of BK ( P<0.05). There was no significant increase in co-localization of these two labelings in the other nuclei. These results suggest that several nuclei in the hypothalamus respond to activation of cardiac sympathetic afferents, leading to sympathoexcitatory reflexes. Nitric oxide (NO) may function as a neurotransmitter or as a neuromodulator in the PVN during these cardiac–cardiovascular responses.

Activation of Serotonergic Pathways from the Midbrain Raphe System to the Subfornical Organ by Hemorrhage in the Rat

The role of serotonergic neural pathways from the midbrain raphe nuclei to the subfornical organ (SFO) in the central regulation of cardiovascular function and body fluid balance was investigated in adult male rats under urethane anesthesia. Eleven neurons in the dorsal raphe nucleus (DR) were antidromically activated by electrical stimulation of the SFO. Of these neurons, 6 displayed an excitatory response following hemorrhage (10 ml/kg bwt) while the remaining 5 neurons were unresponsive. Ninety-four neurons in the SFO were tested for a response to electrical stimulation of the DR or hemorrhage. Electrical stimulation of the DR caused orthodromic excitation (19%) or inhibition (5%) of the activity of SFO neurons. In 14 of 18 SFO neurons that displayed the excitation to the stimulation of the DR, hemorrhage (30 to 50 mm Hg suppression in mean arterial pressure) produced an increase of their discharge, while the stimulus was without effect in the remaining neurons responsive to the stimulation of the DR. The effects of hemorrhage on serotonin (5-HT) release in the region of the SFO were examined using intracerebral microdialysis techniques. Hemorrhage significantly increased 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) concentrations in the region of the SFO. The present data suggest that the serotonergic pathways from the DR to the SFO may relay activation of the peripheral baroreceptors to SFO neurons which result in enhanced excitability, indicating the involvement of the pathways in the regulation of cardiovascular function.

Brain natriuretic peptide-mediated changes in the extracellular neurotransmitter turnover in the rostral ventrolateral medulla

Changes in the rostral ventrolateral medullary neurotransmitter levels and associated cardiovascular functions in response to local administration of brain natriuretic peptide were investigated in urethane-anesthetized Sprague–Dawley rats. Unilateral injections of various doses of brain natriuretic peptide into the rostral ventrolateral medulla region led to significant reductions in both blood pressure and heart rate. To identify the changes occurring in the extracellular neurochemical profile, brain natriuretic peptide was perfused at the rate of 1.5 μl/min for a period of 1 h through a microdialysis probe implanted stereotaxically into the rostral ventrolateral medulla area and the dialysate was assayed every 15 min for both catechols and indoleamine. Both norepinephrine and epinephrine concentrations were significantly reduced. Dihydroxyphenylacetic acid concentration showed no significant change in response to brain natriuretic peptide perfusion. On the other hand, serotonin turnover estimated by the measurement of its metabolite (5-hydroxyindoleacetic acid) concentration increased during the perfusion of brain natriuretic peptide. Blood pressure and heart rate also showed significant reduction during the perfusion of brain natriuretic peptide. These results suggest that brain natriuretic peptide may be relevant in the central regulation of cardiovascular functions by modulating monoamine neurotransmitters.

Membrane properties of area postrema neurons

Intrinsic membrane properties, voltage-dependent sodium and voltage-dependent potassium currents of area postrema neurons in culture have been characterized with respect to their voltage dependence, time dependence and sensitivity to specific blocking agents. The area postrema is a hindbrain circumventricular organ which is known to have an important role in the central regulation of cardiovascular function. This study is the first to describe the biophysical properties of ion channels present in rat area postrema neurons. Recordings in current-clamp mode revealed a mean resting membrane potential of -55.0 ± 1.6 ( n = 24) mV and an input resistance of 213.6 ± 23 M Ω. For the 24 neurons tested, the evoked action potential had a mean threshold of 38.8 ± 2 mV and a mean amplitude of 107.3 ± 15 mV. Our results show that the area postrema possesses only one principle sodium current which is completely abolished by 5 μM tetrodotoxin (TTX) ( n = 28). This current activated near −50 mV and reached peak amplitude at −30 mV. The area postrema does not possess a TTX insensitive sodium current. The area postrema has at least two types of potassium currents. All area postrema neurons studied with tetraethylamonium (TEA) ( n = 40) showed the presence of a slowly activating outward current which was present at voltages greater than −40 mV and was blocked by 10 mM TEA. In addition, 75% of the neurons studied ( n = 30/40) also showed a rapidly inactivating, 4-AP sensitive IA type current which activated near −30 mV. Angiotensin II attenuated both the peak and the steady-state potassium currents, suggesting that angiotensin 11 may modulate area postrema activity by inhibiting voltage-gated potassium channels.

A method for restraining awake rats using head immobilization

A method for restraining awake rats using head implant immobilization is described. In order to reduce stress to the individual animal, rats were restrained side by side in pairs during the adaptation and experimental periods. This technique was used in studies of central regulation of cardiovascular function. In particular, microdialysis probes were placed stereotaxically in deep nuclei of the cerebellum while mean arterial pressure and heart rate were monitored in the awake rat. After initial habituation, normal levels of heart rate and blood pressure obtained during restraint indicated that this is essentially a nonstressful procedure. This technique could also be used for a variety of other experimental conditions where head movement is not desirable, such as during oculomotor or vestibular experiments.

Vasopressin receptors of the vasopressor (V1) type in the nucleus of the solitary tract of the rat mediate direct neuronal excitation

The existence of vasopressin-sensitive neurons in the nucleus of the solitary tract of the rat and the presence in this brain area of vasopressin binding sites were investigated using extracellular single-unit recordings from brain-stem slices and light microscopic autoradiography. About 45% of the recorded neurons responded to vasopressin at 5-2000 nM by a reversible, concentration-dependent increase in firing rate. The action of vasopressin was direct, was suppressed by a vasopressor antagonist, and was mimicked by a vasopressor agonist. Oxytocin was 10-100 times less efficient than vasopressin and a specific antidiuretic agonist was without effect. Using light microscopic autoradiography and 3H-arginine vasopressin as a ligand, high-affinity vasopressin binding sites were found distributed over the whole rostrocaudal extent of the nucleus of the solitary tract. Binding was displaced by unlabeled vasopressor agonist but not by unlabeled antidiuretic agonist. Thus, the nucleus of the solitary tract contains V1-type vasopressin receptors which are, at least in part, located on neuronal membranes and whose activation generates bioelectrical signals. Solitary tract vasopressin-sensitive neurons may be the target of a vasopressinergic innervation originating in the hypothalamic paraventricular nucleus and could be involved in the central regulation of cardiovascular functions.