Blockade of L-glutamate receptors in the rostral ventrolateral medulla contributes to ethanol-evoked impairment of baroreflexes in conscious rats

Ovariectomy Exacerbates Acute Ethanol-Induced Tachycardia: Role of Nitric Oxide and NMDA Receptors in the Rostral Ventrolateral Medulla

Ethanol consumption influences cardiovascular functions. In humans, acute consumption of ethanol causes dose-dependent tachycardia. Our previous study showed that ethanol-induced tachycardia might involve decreased nitric oxide (NO) signaling in the brain's medulla. NMDA receptors, another important target of ethanol, are one of the upstream signals of nitric oxide. Reports showed the modulation of NMDA receptor function by estrogen or estrogen receptors. The present study aims to examine the hypothesis that depletion of estrogen by ovariectomy (OVX) might modulate ethanol-induced tachycardia by regulating NMDA receptor function and NO signaling in the cardiovascular regulatory nucleus of the brain. Ethanol (3.2 g/kg, 40% v/v, 10 mL/kg) or saline (10 mL/kg) was administered by oral gavage in sham or OVX female Sprague-Dawley (SD) rats. The blood pressure (BP) and heart rate (HR) were measured using the tail-cuff method. The levels of phosphoserine 896 of the GluN1 subunit (pGluN1-serine 896) and NMDA GluN1 subunits (GluN1) were determined by immunohistochemistry. The expressions of nitric oxide synthase (NOS) and estrogen receptors in the tissue were measured by Western blotting. Nitric oxide contents were measured as total nitrate-nitrite by colorimetric assay kit. In a 2-h observation, there was no significant change in BP between the saline and ethanol groups. However, compared with saline, ethanol caused an increase in HR (tachycardia) in sham control or OVX rats. Interestingly, ethanol produced more significant tachycardia in the OVX group than in the sham control group. Nitric oxide levels were lower in the area of the rostral ventrolateral medulla (RVLM) 60 min following ethanol administration in OVX compared with sham control, without significant changes in the expression of NOS and estrogen receptors (ERα and ERβ). In addition, a decrease in the immunoreactivity of pGluN1-serine 896, without significant changes in GluN1, was found in neurons of RVLM 40 min following ethanol administration in OVX compared with sham control. Our results suggest that depletion of estradiol (E2) by OVX might exacerbate the tachycardia following ethanol administration, the underlying mechanism of which might be associated with decreased NMDA receptor function and NO level in the RVLM.

Role of Alcohol Oxidative Metabolism in Its Cardiovascular and Autonomic Effects

Several review articles have been published on the neurobehavioral actions of acetaldehyde and other ethanol metabolites as well as in major alcohol-related disorders such as cancer and liver and lung disease. However, very few reviews dealt with the role of alcohol metabolism in the adverse cardiac and autonomic effects of alcohol and their potential underlying mechanisms, particularly in vulnerable populations. In this chapter, following a brief overview of the dose-related favorable and adverse cardiovascular effects of alcohol, we discuss the role of ethanol metabolism in its adverse effects in the brainstem and heart. Notably, current knowledge dismisses a major role for acetaldehyde in the adverse autonomic and cardiac effects of alcohol because of its low tissue level in vivo. Contrary to these findings in men and male rodents, women and hypertensive individuals are more sensitive to the adverse cardiac effects of similar amounts of alcohol. To understand this discrepancy, we discuss the autonomic and cardiac effects of alcohol and its metabolite acetaldehyde in a model of hypertension, the spontaneously hypertensive rat (SHR) and female rats. We present evidence that enhanced catalase activity, which contributes to cardioprotection in hypertension (compensatory) and in the presence of estrogen (inherent), becomes detrimental due to catalase catalysis of alcohol metabolism to acetaldehyde. Noteworthy, studies in SHRs and in estrogen deprived or replete normotensive rats implicate acetaldehyde in triggering oxidative stress in autonomic nuclei and the heart via (i) the Akt/extracellular signal-regulated kinases (ERK)/nitric oxide synthase (NOS) cascade and (ii) estrogen receptor-alpha (ERα) mediation of the higher catalase activity, which generates higher ethanol-derived acetaldehyde in female heart. The latter is supported by the ability of ERα blockade or catalase inhibition to attenuate alcohol-evoked myocardial oxidative stress and dysfunction. More mechanistic studies are needed to further understand the mechanisms of this public health problem.

N-Methyl-D-Aspartate Receptor Signaling and Function in Cardiovascular Tissues

Excellent reviews on central N-methyl-D-aspartate receptor (NMDAR) signaling and function in cardiovascular regulating neuronal pools have been reported. However, much less attention has been given to NMDAR function in peripheral tissues, particularly the heart and vasculature, although a very recent review discusses such function in the kidney. In this short review, we discuss the NMDAR expression and complexity of its function in cardiovascular tissues. In conscious (contrary to anesthetized) rats, activation of the peripheral NMDAR triggers cardiovascular oxidative stress through the PI3K-ERK1/2-NO signaling pathway, which ultimately leads to elevation in blood pressure. Evidence also implicates Ca release, in the peripheral NMDAR-mediated pressor response. Despite evidence of circulating potent ligands (eg, D-aspartate and L-aspartate, L-homocysteic acid, and quinolinic acid) and also their coagonist (eg, glycine or D-serine), the physiological role of peripheral cardiovascular NMDAR remains elusive. Nonetheless, the cardiovascular relevance of the peripheral NMDAR might become apparent when its signaling is altered by drugs, such as alcohol, which interact with the NMDAR or its downstream signaling mechanisms.

Ethanol attenuates peripheral NMDAR-mediated vascular oxidative stress and pressor response

There are no studies on the acute effect of ethanol on peripheral N-methyl-d-aspartate receptor (NMDAR)-mediated increases in reactive oxygen species (ROS) and blood pressure (BP). We tested the hypothesis that ethanol antagonism of peripheral NMDAR dampens systemic NMDA-evoked increases in vascular ROS and BP. We investigated the effect of ethanol (1 g/kg) on BP and heart rate (HR) responses elicited by systemic bolus (125-1000 μg/kg, intra-venous [i.v.]) or infused (180 μg/kg/min) NMDA in conscious male Sprague-Dawley rats. We also hypothesized that peripheral NMDAR blockade with DL-2-Amino-5-phosphonopentanoic acid (AP-5; 5 mg/kg, i.v.) uncovers an ethanol- (1 or 1.5 g/kg) evoked hypotensive response. Ethanol attenuated the peripheral NMDAR-mediated pressor and bradycardic responses caused by NMDA infusion, and ex vivo studies revealed parallel ethanol attenuation of peripheral NMDAR-mediated increases in vascular ROS. While ethanol (1 or 1.5 g/kg) alone had no effect on BP, the higher dose caused a hypotensive response in the presence of NMDAR blockade (AP-5). Blood ethanol concentrations were not statistically different in the groups that received ethanol alone or along with NMDA or AP-5. These findings are the first to demonstrate ethanol attenuation of peripheral NMDAR-mediated pressor response, and the uncovering of ethanol-evoked hypotension in the presence of peripheral NMDAR blockade.

The novel endocannabinoid receptor GPR18 is expressed in the rostral ventrolateral medulla and exerts tonic restraining influence on blood pressure

Systemic administration of the G-protein-coupled receptor 18 (GPR18) agonist abnormal cannabidiol (Abn CBD) lowers blood pressure (BP). Whether GPR18 is expressed in the central nervous system (CNS) and plays a role in BP control is not known despite the abundance of the GPR18 ligand N-arachidonoyl glycine (NAGly) in the CNS. Therefore, we first determined whether GPR18 is expressed in the presympathetic tyrosine hydroxylase (TH) immunoreactive (ir) neurons of the brainstem cardiovascular regulatory nuclei. Second, we investigated the impact of GPR18 activation and blockade on BP and heart rate (HR) and neurochemical modulators of sympathetic activity and BP. Immunofluorescence findings revealed GPR18 expression in TH-ir neurons in the rostral ventrolateral medulla (RVLM). Intra-RVLM GPR18 activation (Abn CBD) and blockade (O-1918, 1,3-dimethoxy-5-methyl-2-[(1R,6R)-3-methyl-6-(1-methylethenyl)-2-,cyclohexen-1-yl]benzene) elicited dose-dependent reductions and elevations in BP, respectively, along with respective increases and decreases in HR in conscious male Sprague-Dawley rats. RVLM GPR18 activation increased neuronal adiponectin (ADN) and NO and reduced reactive oxygen species (ROS) levels, and GPR18 blockade reduced neuronal ADN and increased oxidative stress (i.e., ROS) in the RVLM. Finally, we hypothesized that the negligible hypotensive effect caused by the endogenous GPR18 ligand NAGly could be due to concurrent activation of CB(1)R in the RVLM. Our findings support this hypothesis because NAGly-evoked hypotension was doubled after RVLM CB(1)R blockade (SR141716, rimonabant). These findings are the first to demonstrate GPR18 expression in the RVLM and to suggest a sympathoinhibitory role for this receptor. The findings yield new insight into the role of a novel cannabinoid receptor (GPR18) in central BP control.

Immunoreactivity for the NMDA NR1 subunit in bulbospinal catecholamine and serotonin neurons of rat ventral medulla

Bulbospinal neurons in the ventral medulla play important roles in the regulation of sympathetic outflow. Physiological evidence suggests that these neurons are activated by N-methyl-D-aspartate (NMDA) and non-NMDA subtypes of glutamate receptors. In this study, we examined bulbospinal neurons in the ventral medulla for the presence of immunoreactivity for the NMDA NR1 subunit, which is essential for NMDA receptor function. Rats received bilateral injections of cholera toxin B into the tenth thoracic spinal segment to label bulbospinal neurons. Triple immunofluorescent labeling was used to detect cholera toxin B with a blue fluorophore, NR1 with a red fluorophore, and either tyrosine hydroxylase or tryptophan hydroxylase with a green fluorophore. In the rostral ventrolateral medulla, NR1 occurred in all bulbospinal tyrosine hydroxylase-positive neurons and 96% of bulbospinal tyrosine hydroxylase-negative neurons, which were more common in sections containing the facial nucleus. In the raphe pallidus, the parapyramidal region, and the marginal layer, 98% of bulbospinal tryptophan hydroxylase-positive neurons contained NR1 immunoreactivity. NR1 was also present in all of the bulbospinal tryptophan hydroxylase-negative neurons, which comprised 20% of bulbospinal neurons in raphe pallidus and the parapyramidal region. These results show that virtually all bulbospinal tyrosine hydroxylase and non-tyrosine hydroxylase neurons in the rostral ventrolateral medulla and virtually all bulbospinal serotonin and non-serotonin neurons in raphe pallidus and the parapyramidal region express NR1, the obligatory subunit of the NMDA receptor. NMDA receptors on bulbospinal neurons in the rostral ventral medulla likely influence sympathoexcitation in normal and pathological conditions.

Brainstem norepinephrine neurons mediate ethanol-evoked pressor response but not baroreflex dysfunction

BACKGROUND

Ethanol elicits strain-dependent blood pressure and baroreflex sensitivity responses in spontaneously hypertensive rats (SHRs) and Wistar-Kyoto (WKY) rats; the mechanisms underlying these divergent effects are not clear. The authors tested the hypothesis that differential neuronal actions of ethanol may account for these strain-dependent responses. To this end, the authors investigated the direct effects of ethanol on norepinephrine (NE)-containing neurons in the rostral ventrolateral medulla (RVLM), which modulate sympathetic neuronal activity, and on c-Jun-expressing neurons in the nucleus tractus solitarius (NTS), whose activity is inversely correlated with baroreflex sensitivity.

METHODS

In a newly developed model system in conscious, freely moving rats, the effect of intra-RVLM or intra-NTS ethanol was investigated on neuronal NE at the microinjection site (in vivo electrochemistry), blood pressure, heart rate, spontaneous baroreflex sensitivity, and c-Jun expression in the NTS.

RESULTS

Ethanol (1, 5, or 10 microg) microinjection into the RVLM elicited dose-dependent increases in RVLM NE and blood pressure in SHRs but not in WKY rats. Ethanol had no effect on the activity of the NE-containing neurons in the NTS of either strain. However, baroreflex dysfunction elicited by intra-NTS ethanol in conscious WKY rats was associated with enhanced expression of c-Jun in the NTS.

CONCLUSIONS

(1) Ethanol activation of the NE-containing neurons in the RVLM of SHRs contributes to the centrally mediated pressor response, (2) the NE-containing neurons in the NTS are not involved in ethanol-induced baroreflex dysfunction, and (3) direct activation of the c-Jun-containing neurons in the NTS is implicated in baroreflex dysfunction elicited by ethanol in normotensive rats.

Acute tolerance to ethanol inhibition of NMDA-induced responses in rat rostral ventrolateral medulla neurons

The present study was performed to examine the effects of acute ethanol exposure on N-methyl-D-aspartate (NMDA)-induced responses and the development of acute tolerance in rat rostral ventrolateral medulla (RVLM) in vivo and in vitro. Repeated microinjections of NMDA (0.14 nmol) into the RVLM every 30 min caused reproducible increases in mean arterial pressure in urethane-anesthetized rats weighing 325-350 g. Intravenous injections of ethanol (0.16 or 0.32 g, 1 ml) inhibited NMDA-induced pressor effects in a blood-concentration-dependent and reversible manner. The inhibitory effect of ethanol was reduced over time during continuous infusion of ethanol or on the second injection 3.5 h after prior injection of a higher dose of ethanol (0.32 g). A high dose of ethanol (0.32 g) had no significant effects on alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, gamma-aminobutyric acid and glycine-induced changes in blood pressure. In vitro studies showed that ethanol (10- 100 mM) dose-dependently inhibited inward currents elicited by pressure ejection of NMDA (10 mM) in RVLM neurons of neonatal brainstem slice preparations. When the superfusion time of ethanol (100 mM) was increased to 50 min, its inhibitory effect decreased gradually after 30-40 min in 60% of RVLM neurons examined. These data suggested that ethanol inhibition and subsequent tolerance development is associated with changed sensitivity to NMDA in the RVLM, which may play important roles in the ethanol regulation of cardiovascular function.

An association between ethanol-evoked enhancement of c-jun gene expression in the nucleus tractus solitarius and the attenuation of baroreflexes

BACKGROUND

An increased expression of Fos, the protein product of the immediate early gene c-fos, in the nucleus tractus solitarius (NTS) is associated with dysfunction in baroreceptor reflex control of heart rate. Our previous studies demonstrated that ethanol attenuates baroreflex sensitivity (BRS) in rats and humans. In this study, we tested the hypothesis that enhanced expression of the immediate early gene c-jun (an index of neuronal activity) in the NTS contributes to the baroreflex dysfunction caused by ethanol.

METHODS

Conscious male spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats were used to measure blood pressure, heart rate, and baroreflex sensitivity (Oxford method). The c-jun messenger RNA (mRNA) expression in NTS was measured by in situ hybridization.

RESULTS

Ethanol elicited dose-dependent attenuation in BRS in WKY rats, which was associated with significant increases in c-jun mRNA in the NTS. In contrast, ethanol had no effect on BRS or c-jun mRNA in the NTS of the SHRs; the latter exhibited significantly lower BRS and higher c-jun mRNA in the NTS compared with WKY rats.

CONCLUSIONS

An increased basal level of c-jun mRNA in the NTS may contribute to the reduced BRS in the SHR, and ethanol enhancement of neuronal activity of the NTS, expressed as increased c-jun mRNA expression, may contribute to its attenuation of BRS, which highlights the NTS as a neuroanatomical target for ethanol action on baroreflexes.

Effect of ethanol on reductions in norepinephrine electrochemical signal in the rostral ventrolateral medulla and hypotension elicited by I1-receptor activation in spontaneously hypertensive rats

BACKGROUND

The mechanism of the antagonistic hemodynamic interaction between ethanol and centrally acting sympatholytics is not known. In this study, we tested the hypothesis that the imidazoline (I1)-receptor modulation of norepinephrine (NE) release within the rostral ventrolateral medulla (RVLM) plays a pivotal role in this clinically relevant hemodynamic interaction. METHOD In anesthetized spontaneously hypertensive rats, the effects of centrally acting sympatholytics on RVLM NE electrochemical signal were investigated by in vivo electrochemistry along with cardiovascular responses in the absence and presence of ethanol. In vivo microdialysis in conscious spontaneously hypertensive rats was used to confirm the electrochemical findings. RESULTS Clonidine (30 microg/kg, intravenously) or rilmenidine (400, 600, or 800 microg/kg) significantly reduced RVLM NE electrochemical signal (index of neuronal activity) and mean arterial pressure; rilmenidine effects were dose-related, and ethanol (1 g/kg) counteracted these responses. Ethanol (1 g/kg) pretreatment increased both RVLM NE electrochemical signal and blood pressure but did not influence the reductions in both variables elicited by subsequently administered clonidine. The alpha2-adrenergic antagonist 2-methoxyidazoxan (30 microg/kg) counteracted rilmenidine (800 microg/kg)-evoked responses. In vivo microdialysis in conscious spontaneously hypertensive rats confirmed the electrochemical findings since clonidine- (30 microg/kg, intravenously) evoked reductions in RVLM NE and the associated hypotension were counteracted by ethanol (1 g/kg). CONCLUSIONS (1) Ethanol counteracts centrally mediated hypotension, at least in part, by increasing RVLM NE; (2) the interaction involves the I1 receptor modulation of RVLM neuronal activity; (3) the alpha2-adrenergic receptor contributes to the electrochemical and cardiovascular effects of high doses of rilmenidine, and (4) the RVLM is a neuroanatomical target for systemically administered ethanol.

Effects of ethanol consumption on vasopressin and neuropeptide Y immunoreactivity and mRNA expression in peripheral and central areas related to cardiovascular regulation

Results from previous studies have demonstrated that ethanol influences central neural mechanisms involved in the control of blood pressure. We studied the effects of ethanol consumption on vasopressin and neuropeptide Y immunoreactivity and mRNA expression in the nucleus tractus solitarius and paraventricular hypothalamic nucleus, as well as in the petrosal and nodose ganglia of rats. The ethanol-fed rats received liquid diet ad libitum containing 37.5% ethanol-derived calories (6.7% volume/volume), and the pair-fed rats received the same volume of diet containing isocaloric amounts of maltose-dextrin substituted for ethanol for 3 or 28 days. Arterial blood pressure was evaluated in a separate group of rats, which was unchanged by 3 days, but elevated by 21% after 28 days of ethanol consumption. Vasopressin immunoreactivity and mRNA signal were not detected in the ganglia, nor were they changed in the nucleus tractus solitarius and paraventricular hypothalamic nucleus, by 3 days of ethanol consumption. However, after 28 days of ethanol liquid diet consumption, vasopressin-positive terminals were decreased in the nucleus tractus solitarius and vasopressin immunoreactivity cell bodies and mRNA signal were decreased in the paraventricular hypothalamic nucleus. Neuropeptide Y-immunoreactive terminals were increased in the nucleus tractus solitarius only after 28 days of ethanol liquid diet consumption, but they were decreased in the paraventricular hypothalamic nucleus in rats treated with ethanol for 3 or 28 days. We concluded that the levels of both vasopressin and neuropeptide Y neurotransmitters are changed by long-term ethanol consumption in the neuronal pathways related to control of blood pressure.

Inhibition by ethanol of NMDA-induced responses and acute tolerance to the inhibition in rat sympathetic preganglionic neurons in vitro and in vivo

N-methyl-d-aspartate (NMDA) receptors have been demonstrated to be a pivotal target for ethanol action. The present study examined the actions of acute ethanol exposure on NMDA-induced responses and the acute tolerance to ethanol actions in rat sympathetic preganglionic neurons (SPNs) in vitro and in vivo. NMDA (50 microM) applied every 5 min induced reproducible membrane depolarizations of SPNs in neonatal spinal cord slice preparations. Ethanol (50 - 100 mM) applied by superfusion for 15 min caused a sustained decrease in NMDA-induced depolarizations in a dose-dependent and reversible manner. When the superfusion time of ethanol (100 mm) was increased to 50 min, NMDA-induced depolarizations were attenuated initially but a gradual recovery was seen in approximately 40% of SPNs tested. Repeated injections of NMDA (2 nM) intrathecally at 30 min interval caused reproducible increases in mean arterial pressure (MAP) in urethane-anesthetized rats. Intravenous injections of ethanol (0.16 or 0.32 g, 1 ml) inhibited NMDA-induced pressor effects in a blood concentration-dependent manner. The inhibition by ethanol of NMDA-induced pressor effects was reduced over time during continuous infusion of ethanol or on the second injection 3.5 h after prior injection of a higher dose of ethanol. Ethanol, at concentrations significantly inhibited NMDA-induced responses, had no significant effects on alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-induced responses. The study demonstrated the selective inhibition by ethanol of NMDA-induced responses and the development of acute tolerance to the inhibitory effects in SPNs both in vitro and in vivo. These effects may play important roles in the ethanol regulation of cardiovascular function.

The hypotensive action of rilmenidine is dependent on functional N-methyl-D-aspartate receptor in the rostral ventrolateral medulla of conscious spontaneously hypertensive rats

Rilmenidine is a second-generation centrally acting antihypertensive drug that acts mainly through the activation of the imidazoline (I(1)) receptor in the rostral ventrolateral medulla (RVLM). To investigate the contribution of the N-methyl-D-aspartate receptor (NMDAR) to the hypotensive action of rilmenidine, experiments were undertaken in conscious male spontaneously hypertensive rats (SHRs). Microinjection of cumulative doses of rilmenidine (10, 20, and 40 nmol) at 10- to 15-min intervals, into the RVLM elicited dose-dependent hypotensive and bradycardic response. Pretreatment with intra-RVLM 2-amino-5-phosphonopentanoic acid (AP5) (2 nmol), a selective NMDAR antagonist, not only abolished the hypotensive response elicited by intra-RVLM rilmenidine (40 nmol) but also converted it to a pressor response (-24 +/- 1 versus 17 +/- 7 mm Hg; P < 0.05) and significantly attenuated the bradycardic response (-72 +/- 18 versus -24 +/- 20 bpm; P < 0.05). The blood pressure response to intra-RVLM N-methyl-D-aspartate (NMDA) depended on the dose applied. Whereas intra-RVLM NMDA (>20 pmol) produced the expected pressor response, a lower dose (10 pmol) reduced mean arterial pressure (MAP) (-14 +/- 3 mm Hg) and heart rate (-21 +/- 12 bpm). The divergent MAP responses were attenuated by intra-RVLM AP5 (2 nmol), which implicates the NMDAR in the pressor as well as the depressor response. The present findings suggest that the NMDAR in the RVLM of the SHR 1) exerts dual effects on blood pressure, with the response type depending on the level of NMDAR activation, and 2) plays a pivotal role in the hypotension mediated by I(1) receptor activation in the RVLM.

Effect of long-term ethanol feeding on brainstem alpha(2)-receptor binding in Wistar-Kyoto and spontaneously hypertensive rats

Our previous studies have shown that ethanol attenuates baroreflex function in Wistar-Kyoto (WKY) but not in spontaneously hypertensive rats (SHRs). The present study determined the effects of chronic ethanol administration on alpha(2)-binding sites in brainstem areas that modulate baroreflexes. In vitro autoradiography was utilized to evaluate the effect of a 3-month ethanol feeding on the density (B(max)) and affinity (K(D)) of alpha(2)-adrenoceptors in the middle (mNTS) and rostral (rNTS) portions of the nucleus tractus solitarius of SHRs and WKY rats. Autoradiographic examination of brainstem sections preincubated with [125I]p-iodoclonidine revealed no inter-strain differences in alpha(2)-binding in control rats. Ethanol feeding caused strain-dependent changes in alpha(2)-binding activity, which comprised significant (P<0.05) decreases in the density of alpha(2)-binding sites in both areas of the NTS in SHRs versus no effect in WKY rats. These findings do not favor a role for brainstem alpha(2)-adrenoceptors in ethanol-induced attenuation of baroreflexes. Interestingly, the ethanol-evoked reduction in the NTS alpha(2)-receptor density in SHRs may explain reported findings that ethanol abolishes the hypotensive effect of the alpha(2)-adrenoceptor agonist clonidine in this rat model.

Pharmacological activation of nociceptin receptors in the nucleus tractus solitarius inhibits baroreceptor reflex in pentobarbital-anesthetized rats

Nociceptin receptors are densely distributed in the nucleus tractus solitarius pre- and postsynaptically. This study tested whether nociceptin receptors in this brain area are involved in the modulation of baroreceptor reflex. In pentobarbital-anesthetized rats, pharmacological activation of nociceptin receptors with bilateral microinjection of a synthetic peptide agonist, nociceptin, into the nucleus tractus solitarius attenuated baroreflex sensitivity as demonstrated by a marked reduction in baroreflex bradycardia induced by a single dose of intravenous phenylephrine. The inhibitory effect of nociceptin was dose dependent (0.04, 0.2 and 1nmol) and was blocked by pretreatment with microinjection of 1nmol nocistatin, a peptide that can functionally reverse the action of nociceptin. In contrast, injection of an opioid receptor antagonist, naloxone (5nmol), did not modify the inhibition of baroreflex sensitivity induced by nociceptin. Neither nocistatin nor naloxone injected into the nucleus alone had any detectable effect on baseline blood pressure and heart rate and baroreflex bradycardia. These data indicate that the newly discovered nociceptin receptors in the central nervous system possess an inhibitory influence on baroreflex transmission at the level of the nucleus tractus solitarius.

Bicuculline sensitive depressor response to ethanol infusion into the lateral hypothalamus

Decreased GABA function in the hypothalamus increases mean arterial pressure (MAP) and heart rate (HR). Since ethanol acts on GABA-A receptors, blocking GABA-A receptors can prevent a decrease of MAP and HR by ethanol in the lateral hypothalamus (LH). Ethanol at 5-30 mM, with or without 25 ng/microl bicuculline, was infused into the LH, and the activity of the site was validated with 100 nmoles of serotonin. Male rats were anesthetized with pentobarbital, and the femoral artery was catheterized to measure MAP and HR. Microinfusion was performed with a 28-gauge cannula placed into the LH. Serotonin increased MAP and HR within 15 sec. Ethanol decreased the MAP by -21.15 +/- 3.92 mmHg and HR by -53.61 +/- 14.95 BPM, at 15 min, which recovered by 15 min after the infusion was terminated. These maximum decreases were produced by 20 mM ethanol giving a U-shaped dose response. The aCSF vehicle had no effect. Bicuculline prevented ethanol-induced changes and had no effect when administered alone. Both serotonin and ethanol have direct effects on LH neurons with cardiovascular function. Ethanol produces this effect through GABA-A receptors.

Excess catecholamine syndrome. Pathophysiology and therapy

In addition to genetic factors, lifestyle has a predominant influence on primary hypertension and noninsulin-dependent diabetic mellitus (NIDDM). We initiated studies using radiotelemetry for characterizing molecular events linked with excess calorie intake and psychologic stress. An increased calorie intake was associated with raised (p < 0.05) systolic and diastolic blood pressure as well as heart rate independent of day-night cycle. Sympathetic activity was in excess when related to the unchanged motility. The hyperkinetic hypertension is expected to result in adverse remodeling of resistance vessels and to aggravate insulin resistance. To examine adverse effects of psychological stress, rats were subjected to intermittent food pellet feeding. Urinary catecholamines and cardiac norepinephrine stores were increased (p < 0.05). The depressed (p < 0.05) rate of Ca2+ uptake of sarcoplasmic reticulum is expected to contribute to cellular Ca2+ overload. These lifestyle influences strengthen the notion of an excess catecholamine syndrome which requires selective reduction of sympathetic outflow of the brain by I1-receptor agonists.

Ethanol abolishes clonidine-induced impairment of baroreflex control of heart rate in conscious rats

Our previous studies showed that the ability of ethanol or clonidine to alter the baroreflex control of heart rate (baroreflex sensitivity, BRS) depends on the functional activity of aortic baroreflexes. In this study, we investigated the interaction between the two drugs on BRS in conscious rats with intact baroreflexes (shamoperated, SO) and after aortic baroreceptor denervation (ABD). The slope of the curve relating increments in mean arterial pressure induced by phenylephrine to corresponding reflex bradycardic responses was taken as an index of BRS. Ethanol (1 g/kg i.v.) significantly (p < 0.05) attenuated BRS in SO rats (-1.7 +/- 0.13 versus -1.04 +/- 0.15 beats/min/mm Hg) but not in ABD rats. Clonidine (30 microg/kg, i.v.) elicited significantly (p < 0.05) greater hypotensive responses in conscious ABD compared with SO rats. The BRS was not affected by clonidine administration in SO rats but showed significant (p < 0.05) reductions in ABD rats. Ethanol (1 g/kg, i.v.) had no effect on the hypotensive response to subsequently administered clonidine in ABD and SO rats; however, the effect of the two drugs on BRS was variable. In ABD rats, the BRS values before and after administration of ethanol and clonidine were similar, suggesting that pretreatment with ethanol counteracted clonidine-evoked attenuation of BRS in this rat preparation. In SO rats, the ethanol-clonidine combination produced a significant (p < 0.05) decrease in BRS, similar to the effect of ethanol when administered alone. These data confirm earlier findings that the aortic baroreflex arc modulates the interaction of ethanol and clonidine with baroreflex function. Further, the ability of ethanol to abolish clonidine-induced attenuation of BRS in ABD rats may relate to the compound effects of the two drugs on neuronal pathways participating in the central processing of baroreflexes in these rats.

Ovariectomy abolishes ethanol-induced impairment of baroreflex control of heart rate in conscious rats

Our previous studies have shown that ethanol attenuates baroreflex control of heart rate in male rats. The present study investigated whether this effect of ethanol is gender-related, and whether it involves hormonal factors. The effect of intragastric administration of ethanol or equal volume of water on baroreflex-mediated decreases in heart rate in response to increments in blood pressure evoked by phenylephrine were evaluated in conscious age-matched male and female Sprague-Dawley rats as well as in ovariectomized rats. Baroreflex curves relating changes in blood pressure and associated heart rate responses were constructed, and the slopes of the regression lines were taken as a measure of baroreflex sensitivity. Phenylephrine (1-16 microg kg(-1), i.v.) elicited dose-dependent pressor responses that were similar in all groups of rats. However, the associated reflex bradycardic responses depended on the rat preparation and the dose of ethanol employed. In water-treated (control) animals, significantly (P < 0.05) lesser reflex bradycardic responses were observed in female compared with male rats (baroreflex sensitivity, -1.21 +/- 0.12 vs. -1.67 +/- 0.12 beats min(-1) mmHg(-1)). Ovariectomy resulted in a further reduction in baroreflex sensitivity (-0.82 +/- 0.06 beats min(-1) mmHg(-1)), suggesting a favorable role for ovarian hormones in baroreflex modulation. In male rats, ethanol (0.25, 0.5, or 1 g kg(-1), intragastric) elicited dose-related decreases in reflex bradycardic responses. The reduction in the regression coefficient obtained by the two higher doses (0.5 and 1 g kg(-1)) of ethanol was statistically significant compared with control values. The ability of ethanol to reduce baroreflex sensitivity appears to be gender-independent as it was similarly demonstrated in intact female rats. In contrast, ethanol had no effect on reflex bradycardic responses in ovariectomized rats at any of the doses tested. The data suggest that ethanol reduces baroreflex control of heart rate irrespective of the rat gender. Further, the lack of an effect of ethanol on baroreflex sensitivity in ovariectomized rats may suggest a role for ovarian hormones in ethanol-evoked baroreflex attenuation.

The role of glutamatergic neurotransmission in the pathophysiology of alcoholism

Recent evidence suggests that ethanol abuse produces its diverse effects on the brain to a substantial degree by disrupting the function of the major excitatory neurotransmitter, glutamate. Ethanol, at concentrations associated with behavioral effects in humans, inhibits the N-methyl-D-aspartate (NMDA) receptor, which mediates the post-synaptic excitatory effects of glutamate. Tolerance to ethanol results in up-regulation of the NMDA receptor so that abrupt withdrawal produces a hyperexcitable state that leads to seizures, delerium tremens, and excitotoxic neuronal death. Ethanol's inhibition of the NMDA receptor in the fetal brain likely contributes to the CNS manifestations of fetal alcohol syndrome. Therapeutic strategies aimed at correcting glutamatergic dysregulation in alcoholism need to be explored.

Ethanol microinjection into the area postrema selectively attenuates baroreflex sensitivity measured by vasopressin in conscious rats

Microinjection of ethanol (10 microg) into the area postrema (AP) of conscious rats attenuated baroreflex sensitivity (BRS) measured by arginine vasopressin (AVP) (-1.73 +/- 0.13 versus -2.47 +/- 0.16 bpm/mmHg), but not by phenylephrine (PE) (-1.94 +/- 0.26 versus -1.82 +/- 0.20 bpm/mmHg). Intra-AP injection of the V1 receptor antagonist D(CH2)5Tyr(Me)AVP replicated the differential effects of ethanol on BRS measured by AVP (-1.89 +/- 0.11 versus -2.52 +/- 0.10 bpm/mmHg) and PE (-2.10 +/- 0.12 versus -2.09 +/- 0.19 bpm/mmHg). Intra-AP artificial cerebrospinal fluid (ACSF) did not change BRS measured by AVP or PE. These data suggest that ethanol attenuates the facilitatory action of AVP on baroreflexes via its interaction with AVP neurons in the AP.