Ethanol inhibition of baroreflex bradycardia: role of brainstem GABA receptors

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.

Central α7 and α4β2 nicotinic acetylcholine receptors offset arterial baroreceptor dysfunction in endotoxic rats

Cardiac autonomic neuropathy is a prominent feature of endotoxemia. Given the defensive role of the cholinergic pathway in inflammation, we assessed the roles of central homomeric α7 and heteromeric α4β2 nAChRs in arterial baroreceptor dysfunction caused by endotoxemia in rats. Endotoxemia was induced by i.v. administration of lipopolysaccharides (LPS, 10 mg/kg), and baroreflex activity was measured by the vasoactive method, which assesses reflex chronotropic responses to increments (phenylephrine, PE) or decrements (sodium nitroprusside, SNP) in blood pressure. Shifts caused by LPS in PE/SNP baroreflex curves and associated decreases in baroreflex sensitivity (BRS) were dose-dependently reversed by nicotine (25-100 μg/kg, i.v.). The nicotine effect disappeared after intracisternal administration of methyllycaconitine (MLA) or dihydro-β-erythroidine (DHβE), selective blockers of α7 and α4β2 receptors, respectively. The advantageous effect of nicotine on BRSPE was replicated in rats treated with PHA-543613 (α7-nAChR agonist) or 5-iodo-A-85380 (5IA, α4β2-nAChRs agonist) in dose-dependent fashions. Conversely, the depressed BRSSNP of endotoxic rats was improved after combined, but not individual, treatments with PHA and 5IA. Central α7 and α4β2 nAChR activation underlies the nicotine counteraction of arterial baroreflex dysfunction induced by endotoxemia. Moreover, the contribution of these receptors depends on the nature of the reflex chronotropic response (bradycardia vs. tachycardia).

A total closed chest sheep model of cardiogenic shock by percutaneous intracoronary ethanol injection

To develop a reproducible and stable closed chest model of ischemic cardiogenic shock in sheep, with high survival rate and potential insight into human pathology. We established a protocol for multi-step myocardial alcoholisation of the left anterior descending coronary artery by percutaneous ethanol injection. A thorough hemodynamic assessment was obtained by invasive and non-invasive monitoring devices. Repeated blood samples were obtained to determine haemoglobin and alcohol concentration, electrolytes, blood gas parameters and cardiac troponin I. After sacrifice, tissue was excised for quantification of infarction and histology. Cardiogenic shock was characterized by a significant decrease in mean arterial pressure (− 33%), cardiac output (− 29%), dP/dt_max (− 28%), carotid blood flow (− 22%), left ventricular fractional shortening (− 28%), and left ventricle end-systolic pressure–volume relationship (− 51%). Lactate and cardiac troponin I levels increased from 1.4 ± 0.2 to 4.9 ± 0.7 mmol/L (p = 0.001) and from 0.05 ± 0.02 to 14.74 ± 2.59 µg/L (p = 0.001), respectively. All haemodynamic changes were stable over a three-hour period with a 71% survival rate. The necrotic volume (n = 5) represented 24.0 ± 1.9% of total ventricular mass. No sham exhibited any variation under general anaesthesia. We described and characterized, for the first time, a stable, reproducible sheep model of cardiogenic shock obtained by percutaneous intracoronary ethanol administration.

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.

Role of neuronal nitric oxide synthase (nNOS) at medulla in tachycardia induced by repeated administration of ethanol in conscious rats

Background

Intake of ethanol (alcohol) has been shown to influence cardiovascular function; the underlying brain mechanism remains unclear. Noting that nitric oxide (NO) system in the CNS is involved in the regulation of cardiovascular function, the present study examined the role of NO in medulla in ethanol-induced cardiovascular changes.

Methods

Ethanol was administered by oral gavage at dose of 3.2 g/kg once every day for 8 consecutive days. Changes in blood pressure (BP) and heart rate (HR) in response to ethanol were measured by radiotelemetry method in freely moving female Sprague-Dawley rats. NO modulators were applied by intracerebroventricular (ICV) injection. The protein levels of nitric oxide synthase (NOS) and NO content in rostroventral medulla were measured by Western blot and nitrate/nitrite colorimetric assay kit, respectively.

Results

Ethanol intake had little effects on basal BP and HR following 8 consecutive day treatments. A significant increase in HR but not BP following ethanol intake was observed at 6th and 8th, but not at 1st and 4th day treatments as compared with saline group. A decrease in the protein expression of neuronal NOS (nNOS) but not inducible NOS or endothelial NOS and a decline in the level of NO in the medulla 30 min after ethanol administration was observed at 8th day treatment. ICV treatment with NO donors attenuated ethanol-induced tachycardia effects at 8th day treatment. Ethanol produced significantly tachycardia responses when ICV nNOS inhibitors were given at 1st day treatment.

Conclusion

Our results suggest that medulla nNOS/NO pathways play an important role in ethanol regulation of HR.

Ser/thr phosphatases tonically attenuate the ERK-dependent pressor effect of ethanol in the rostral ventrolateral medulla in normotensive rats

We recently reported that microinjection of ethanol into the rostral ventrolateral medulla (RVLM) elicits modest increases in local extracellular signal-regulated kinase (ERK) and blood pressure (BP) in conscious normotensive rats. In this study, we tested the hypothesis that RVLM ser/thr phosphatases dampen the ERK-dependent pressor effect of ethanol in normotensive rats. We show that the pressor response elicited by intra-RVLM ethanol (10 μg) was (i) abolished following local ERK inhibition with PD98059 (1 μg) and (ii) associated with significant reduction in local phosphatase activity. Inhibition of the RVLM ser/thr phosphatase activity by okadaic acid (OKA, 0.4 μg) or fostriecin (15 pg) caused significant increases in blood pressure (BP) and potentiated the magnitude and duration of the pressor response as well as the phosphatase inhibition elicited by subsequent intra-RVLM administration of ethanol. Intra-RVLM acetaldehyde (2 μg), the main metabolic product of ethanol, caused no changes in BP or RVLM phosphatase activity but it produced significant increases in BP and inhibition of local phosphatase activity in rats treated with OKA or fostriecin. Together, the RVLM phosphatase activity acts tonically to attenuate the ERK-dependent pressor effect of ethanol or acetaldehyde in normotensive rats.

Role of rostral ventrolateral medullary ERK/JNK/p38 MAPK signaling in the pressor effects of ethanol and its oxidative product acetaldehyde

BACKGROUND

We tested the hypothesis that alterations of the phosphorylation/dephosphorylation profile of mitogen-activated protein kinases (MAPKs) in the rostral ventrolateral medulla (RVLM) underlie the pressor response elicited by ethanol (EtOH) microinjection into the RVLM of spontaneously hypertensive rats (SHRs). The studies were extended to determine whether acetaldehyde (ACA), the primary oxidative product of EtOH, replicates the molecular effects of EtOH within the RVLM and the consequent pressor response.

METHODS

Effects of EtOH or ACA on blood pressure (BP) were evaluated in the absence or presence of selective JNK (SP600125), ERK (PD98059), p38 (SB203580), or ser/thr phosphatases (okadaic acid [OKA]) inhibitor.

RESULTS

Intra-RVLM EtOH (10 μg/rat) or ACA (2 μg/rat) caused a similar ERK2-dependent pressor response because EtOH or ACA-evoked increases in BP and in RVLM p-ERK2 level were abolished after pharmacologic inhibition of ERK phosphorylation. SP600125 abrogated the pressor action of EtOH, but not ACA, thus implicating JNK in EtOH action on BP. Despite EtOH enhancement of p38 phosphorylation, pharmacological studies argued against a causal role for this kinase in EtOH-evoked pressor response. RVLM phosphatase catalytic activity was not influenced by EtOH or ACA. Interestingly, pharmacologic phosphatase inhibition (OKA), which increased RVLM p-ERK2 and BP, abrogated the pressor effect of subsequently administered EtOH or ACA.

CONCLUSIONS

Enhancement of RVLM ERK2 phosphorylation constitutes a major molecular mechanism for the pressor response elicited by intra-RVLM EtOH or its metabolite, ACA, in conscious SHRs. Further, RVLM kinases dephosphorylation does not contribute to intra-RVLM EtOH- or ACA-evoked pressor response.

Bradykinin B2 receptor-dependent enhancement of enalapril-evoked hypotension in ethanol-fed female rats

Our previous studies showed that chronic ethanol feeding attenuates centrally (clonidine) evoked and potentiates peripherally (hydralazine) evoked hypotension in female rats. In this study, we investigated whether chronic ethanol (8 weeks, 5% wt/vol) alters hemodynamic responses elicited by angiotensin-converting enzyme (ACE) inhibition (enalapril) in telemetered female rats. Given the intimate interaction between ACE and bradykinin, studies were extended to investigate the role of bradykinin receptor (B2R) in ethanol-enalapril interaction. Compared with pair-fed controls, ethanol-fed female rats exhibited (1) higher renal expressions of ACE and B2R proteins and angiotensin II levels and (2) lower blood pressure. Pharmacological inhibition of ACE and B2R supports functional role for the higher levels of these 2 proteins in ethanol-fed rats because enalapril (10 mg/kg, intraperitoneally) caused significantly greater hypotensive response in ethanol-fed rats than in control rats. Further, blockade of B2R with bradyzide (2 mg/kg, intraperitoneally) abrogated the enhanced hypotensive effect of enalapril in ethanol-fed rats but had no effect on enalapril-evoked hypotension in control rats. Finally, enalapril enhancement of spontaneous baroreflex sensitivity (BRS) in control was absent in ethanol-fed rats. These findings demonstrate that chronic ethanol produces B2R-dependent enhancement of the hypotensive response elicited by enalapril and abrogates enalapril-evoked enhancement of spontaneous baroreflex response in female rats.

Facilitation of central imidazoline I(1)-site/extracellular signal-regulated kinase/p38 mitogen-activated protein kinase signalling mediates the hypotensive effect of ethanol in rats with acute renal failure

BACKGROUND AND PURPOSE

This study investigated the role of central sympathetic activity and related mitogen-activated protein kinase (MAPK) signalling in the cardiovascular effects of ethanol in a model of acute renal failure (ARF).

EXPERIMENTAL APPROACH

The effects of pharmacological interventions that inhibit peripheral or central sympathetic activity or MAPK on the cardiovascular actions of ethanol in rats with ARF induced by glycerol were evaluated.

KEY RESULTS

Glycerol (50%, 10 mL.kg(-1), i.m.) caused progressive increases and decreases in blood pressure (BP) and heart rate (HR) respectively. Subsequent i.v. ethanol (0.25 or 1 g.kg(-1)) elicited dose-related changes in BP (decreases) and HR (increases). These effects were replicated after intracisternal (i.c.) administration of ethanol. Blockade of nicotinic cholinoceptors (nAChR, hexamethonium, 20 mg.kg(-1)) or alpha(1)-adrenoceptors (prazosin, 1 mg.kg(-1)) attenuated cardiovascular effects of ethanol. Ethanol hypotension was also attenuated after the centrally acting sympatholytic drug moxonidine (selective I(1)-site agonist, 100 microg.kg(-1) i.v.), but not guanabenz (selective alpha(2)-receptor agonist, 30 microg.kg(-1), i.v.), suggesting involvement of central circuits of I(1) sites in ethanol-evoked hypotension. Selective blockade I(1) sites (efaroxan) but not alpha(2) (yohimbine) adrenoceptors abolished the hypotensive response to ethanol. Intracisternal administration of PD98059 or SB203580, inhibitors of extracellular signal-regulated kinase (ERK 1/2) and p38 MAPK, respectively, reduced the hypotensive action of moxonidine or ethanol. When used simultaneously, the two MAPK inhibitors produced additive attenuation of ethanol hypotension.

CONCLUSIONS AND IMPLICATIONS

Sympathoinhibitory pathways of central I(1)-sites and downstream ERK/p38 MAPK signalling were involved in the hypotensive action of ethanol in ARF.

Direct evidence for imidazoline (I1) receptor modulation of ethanol action on norepinephrine-containing neurons in the rostral ventrolateral medulla in conscious spontaneously hypertensive rats

BACKGROUND

Enhancement of the rostral ventrolateral medulla (RVLM) presympathetic (norepinephrine, NE) neuronal activity represents a neurochemical mechanism for the pressor effect of ethanol. In this study, we tested the hypothesis that ethanol action on RVLM presympathetic neurons is selectively influenced by the signaling of the local imidazoline (I1) receptor. To support a neuroanatomical and an I1-signaling selectivity of ethanol, and to circumvent the confounding effects of anesthesia, the dose-related neurochemical and blood pressure effects of ethanol were investigated in the presence of selective pharmacological interventions that cause reduction in the activity of RVLM or nucleus tractus solitarius (NTS) NE neurons via local activation of the I1 or the alpha2-adrenergic receptor in conscious spontaneously hypertensive rats.

RESULTS

Local activation of the I1 receptor by rilmenidine (40 nmol) or by the I1/alpha2 receptor mixed agonist clonidine (1 nmol), and local activation of the alpha2-adrenergic receptor (alpha2AR) by the pure alpha2AR agonist alpha-methylnorepinephrine (alpha-MNE, 10 nmol) caused reductions in RVLM NE, and blood pressure. Intra-RVLM ethanol (1, 5, or 10 microg), microinjected at the nadir of the neurochemical and hypotensive responses, elicited dose-dependent increments in RVLM NE and blood pressure in the presence of local I1--but not alpha2-receptor activation. Only intra-NTS alpha-MNE, but not rilmenidine or clonidine, elicited reductions in local NE and blood pressure; ethanol failed to elicit any neurochemical or blood pressure responses in the presence of local activation of the alpha2AR within the NTS.

CONCLUSION

The findings support the neuroanatomical selectivity of ethanol, and support the hypothesis that the neurochemical (RVLM NE), and the subsequent cardiovascular, effects of ethanol are selectively modulated by I1 receptor signaling in the RVLM.

Upregulation of vascular inducible nitric oxide synthase mediates the hypotensive effect of ethanol in conscious female rats

Previous reports from our laboratory have shown that ethanol elicits hypotension in female but not in male rats and that this effect of ethanol is estrogen dependent (El-Mass MM and Abdel-Rahman AA. Alcohol Clin Exp Res 23: 624-632, 1999; El-Mass MM and Abdel-Rahman AA. Clin Exp Hypertens 21: 1429-1445, 1999). In the present study, we tested the hypothesis that ethanol lowers blood pressure in female rats via upregulation of the inducible nitric oxide synthase (iNOS) in vascular tissues. The effects of pretreatment with NG-nitro-L-arginine (NOARG; nonselective nitric oxide synthase inhibitor) or aminoguanidine (selective iNOS inhibitor) on hemodynamic responses elicited by intragastric (ig) ethanol were determined in conscious female rats. Changes in vascular (aortic) iNOS protein expression evoked by ethanol in the presence and absence of aminoguanidine were also measured by immunohistochemistry. Compared with control (water treated) female rats, ethanol (1 g/kg ig) elicited hypotension that was associated with a significant increase in the aortic iNOS activity. The hypotensive effect of ethanol was virtually abolished in rats infused with the nitric oxide synthase inhibitor NOARG, suggesting a role for nitric oxide in ethanol hypotension. The inability of ethanol to lower blood pressure in NOARG-treated rats cannot be attributed to the presence of elevated blood pressure in these rats because ethanol produced hypotension when blood pressure was raised to comparable levels with phenylephrine infusion. Selective inhibition of iNOS by aminoguanidine (45 mg/kg ip), which had no effect on baseline blood pressure, abolished both the hypotensive action of subsequently administered ethanol and the associated increases in aortic iNOS content. These findings implicate vascular iNOS, at least partly, in the acute hypotensive action of ethanol in female rats.

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.

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.

A theoretical model of the role of brain stem nuclei in alcohol-mediated arrhythmogenesis in older adults

Uncertainty about the mechanism of alcohol-mediated arrhythmogenesis and the effect of alcohol use on arrhythmic risk among older adults is an increasing concern in light of population aging and recent reports that moderate alcohol consumption may protect older adults against coronary artery disease. In this review, a theoretical model of the role of brain stem nuclei in alcohol-mediated arrhythmogenesis in older adults is developed. The model is based on the hypothesis that the effects of alcohol on central autonomic pathways of cardiac control may alter the threshold for alcohol-mediated arrhythmogenesis among older adults. Findings from multiple lines of research including cellular, electrophysiological, epidemiological, experimental, and clinical studies in human, animal, and in vitro models were synthesized in developing the model. Suggestions for future research on the topic of alcohol-mediated arrhythmogenesis in older adults are offered.

An association between the estrogen-dependent hypotensive effect of ethanol and an elevated brainstem c-jun mRNA in female rats

We have recently demonstrated that chronic ethanol administration lowers blood pressure (BP) in female rats and this effect is significantly attenuated by ovariectomy. The present study investigated whether ethanol hypotension is estrogen dependent. Further, since estrogen regulates AP-1 activity, the study was extended to determine whether estrogen/c-jun interaction is involved in the estrogen-dependent hypotensive effect of ethanol. Changes in BP and heart rate (HR) were evaluated in radiotelemetered pair-fed sham-operated (SO), ovariectomized (OVX), and OVX estradiol (E2)-treated rats receiving liquid diet with or without ethanol (5%, w/v) for 12 weeks. The in situ hybridization technique was used to measure the c-jun mRNA expression in two brainstem areas, the nucleus tractus solitarius (NTS) and the rostral ventrolateral medulla (RVLM). Ethanol feeding caused significant (P<0.05) decreases in BP in SO rats that started at week 1 and reached its maximum (approximately 10 mmHg) at week 6 and remained at that level till the end of week 12. In OVX rats, ethanol had no effect on BP during the first 5 weeks after which a decrease of 5 mmHg was demonstrated and remained thereafter. Estrogen replacement (17beta-estradiol subcutaneous pellet, 14.2 microg/day) restored the hypotensive effect of ethanol to a level similar to that of SO rats both in terms of magnitude and duration. Densitometric analysis of the in situ hybridization autoradiograms revealed that OVX and E2 replacement had no effect on c-jun mRNA expression in the NTS or RVLM. Ethanol feeding produced a significant (twofold) increase in c-jun mRNA expression in the RVLM of SO rats versus no effect in the NTS. The increased expression of c-jun mRNA observed following ethanol treatment in the RVLM of SO rats was abolished in OVX rats and restored to SO levels after E2 replacement. These findings suggest a link between the estrogen-dependent hypotensive effect of chronically administered ethanol and the increased expression of c-jun mRNA in the brainstem of female rats.

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.

Sexually dimorphic hemodynamic effects of intragastric ethanol in conscious rats

This study investigated the gender-related differences in hemodynamic effects of small to moderate doses of intragastrically (i.g.) administered ethanol in conscious rats. Changes evoked by ethanol (0.25, 0.5 or 1 g/kg) in mean arterial pressure (MAP), heart rate, cardiac index (CI), stroke volume (SV), and total peripheral resistance (TPR) were followed for 90 min in age-matched male and female Sprague-Dawley rats. Baseline values of MAP (121+/-2 vs. 124+/-2 mm Hg) were similar whereas CI (55+/-2 vs. 43+/-2 ml/min/100 g) and TPR (2.2+/-0.1 vs. 3.0+/-0.1 mm Hg/ml/min/100 g) were significantly (P<0.05) higher and lower, respectively, in female compared with male rats. In male rats, the middle dose (0.5 g/kg) of ethanol caused a slight increase in MAP due to significant (P<0.05) increases in CO whereas the other two doses (0.25 and 1 g/kg) had no effect on MAP. In female rats, MAP was not affected by ethanol (0.25 and 0.5 g/kg) and showed a significant reduction by the higher dose (1 g/kg) that was associated with decreases in CO and SV while TPR did not change. The hypotensive effect of ethanol (1 g/kg) in female rats started after 50 min, was maximal (13+/-1.7 mm Hg) at 70 min and remained so for the remaining 20 min of the study. Blood ethanol concentrations were similar in male and female rats. These findings suggest that the hemodynamic responses to i.g. ethanol are gender-related and that ethanol-evoked hypotension in female rats appears to involve a reduction in cardiac output.

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 cardiovascular startle response: anxiety and the benzodiazepine receptor complex

Benzodiazepine receptor (BZR) agonists are prototypic anxiolytic agents, whereas BZR inverse agonists exert anxiogenic effects. The effects of these compounds offer a potentially important pharmacological model system to examine the central mechanisms of anxiety. In accord with its putative anxiogenic properties, we previously found that the BZR partial inverse agonist, FG 7142, enhances the cardiovascular defensive response to a nonsignal acoustic stimulus in rats. In contrast, we found in the present study that this agent attenuates both the somatic and cardiovascular components of the acoustic startle response. BZR agonists and inverse agonists are known to modulate the basal forebrain cortical cholinergic system, and we consider the potential involvement of this system in the disparate psychophysiological actions of FG 7142 and in anxiety states in general.

Ethanol inhibition of stress-related tachycardia involves medullary NMDA receptors

In rats, neurons in the perifornical area of the hypothalamus send descending projections to the commissural part of the nucleus tractus solitarii as demonstrated by an anterograde tracer study. In urethane-anaesthetised rats, stimulation of neurons in the perifornical area by microinjection of bicuculline or 6-OH-saclofen causes tachycardia and inhibits baroreflex bradycardia. The effects elicited from the perifornical area are similar in magnitude to those elicited from the adjacent dorsomedial nucleus, also called the hypothalamic defense area. Microinjection into the nucleus tractus solitarii of the NMDA (N-methyl-D-aspartate) receptor antagonist, AP-7 (2-amino-7-phosphonoheptanoic acid), inhibits the tachycardic response to stimulation of the perifornical area. Injection of ethanol intravenously or into the nucleus tractus solitarii also inhibits this tachycardic response, but causes no further inhibition when combined with AP-7. We conclude that the perifornical area is part of the hypothalamic defense area, and it is under strong, tonic GABAergic inhibition mediated by both GABAA and GABAB receptors. Furthermore, descending input from the perifornical area to the nucleus tractus solitarii is via an NMDA synapse, and ethanol inhibits stress-related tachycardia by inhibiting these NMDA receptors in the nucleus tractus solitarii.

Ethanol inhibits chemoreflex excitation of reticulospinal vasomotor neurons

In anesthetized and ventilated rats, activation of carotid chemoreceptors with intracarotid administration of 100 nmol sodium cyanide rapidly excited the spinal cord-projecting vasomotor neurons in the rostroventrolateral reticular nucleus (RVL) of the medulla oblongata and sympathetic nerves and increased arterial pressure. The chemoreflex sympathoexcitatory pressor responses were attenuated by an acute systemic administration of ethanol at 0.45 g/kg, but not at 45 mg/kg. The ethanol effects were observed at the level of RVL-spinal vasomotor neurons, in attenuating the neuronal responses to the chemoreflex excitation and direct iontophoresis of N-methyl-D-aspartic acid (NMDA) but without altering responses of the carotid sinus nerves to intracarotid cyanide. The effect of ethanol on the RVL neurons was further defined as blocking NMDA-evoked inward current in the corresponding spontaneously active RVL neurons in vitro. The results indicate that acute ethanol intoxication markedly influences NMDA receptor activation and arterial chemoreflexes. The relevance of the type of action to clinical hypertension in chronic and heavy drinkers is discussed.

GABAB receptors modulate a tetanus-induced sustained potentiation of monosynaptic inhibitory transmission in the rat nucleus tractus solitarii in vitro

Whole-cell recordings were made in the nucleus tractus solitarii (NTS) in transverse brainstem slices from rats. Monosynaptic GABAA-receptor-mediated inhibitory postsynaptic currents (IPSCs) or potentials (IPSPs) were evoked (0.1-0.2 Hz) by electrical stimulation within and medial to the tractus solitarius in the presence of the ionotropic glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM) or 6,7-dinitroquinoxaline-2,3-dione (DNQX; 10 microM) and D-amino-5-phosphonopentanoic acid (APV; 50 microM). A brief period of tetanic stimulation (20 Hz, 2 s) resulted in posttetanic (< 5 min, 69 of 73 recordings) and sustained potentiation (> 15 min, 31 of 73 recordings) of the IPSP/Cs. Sustained potentiation was not due to alterations in the reversal potential of IPSP/Cs. Both pre- and post-tetanus IPSP/Cs were completely blocked by the GABAA antagonist bicuculline (10 microM). Postsynaptic responses to pressure ejection of the GABAA-receptor agonist muscimol were unaltered in cells displaying sustained potentiation. Sustained potentiation of IPSP/Cs could be induced by tetanus in the presence of either metabotropic glutamate receptor antagonists or bicuculline. However, sustained potentiation could not be induced in the presence of the GABAB-receptor antagonists 2-OH-saclofen (400 microM) or CGP35348 (3-amino-propyl-(diethoxymethyl)phosphinic acid, 100 microM), although a subsequent tetanus following washout induced sustained potentiation. Posttetanic potentiation was unaffected by GABAB-receptor antagonists. These data suggest that neuronal or terminal excitability of GABAergic interneurons in the NTS is enhanced following brief periods of increased frequency of activation in vitro. This novel phenomenon within the rat medulla may be involved in the temporal modulation of autonomic reflex sensitivity observed during certain behavioral states, such as the defense reaction.

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

This study investigated the effect of ethanol microinjected into the rostral ventrolateral medulla on the cardiovascular responses to intrarostral ventrolateral medulla administration of the excitatory amino acids L-glutamate and N-methyl-D-aspartate (NMDA) and on baroreflex-mediated heart rate responses (baroreflex sensitivity) in conscious freely moving Sprague-Dawley rats. L-Glutamate (5 nmol) or NMDA (25, 50, and 100 pmol) microinjected into the rostral ventrolateral medulla elicited pressor and bradycardiac responses. The cardiovascular responses elicited by both L-glutamate and NMDA were significantly (p < 0.05) attenuated by intrarostral ventrolateral medulla ethanol (10 micrograms) or 2-amino-5-phosphonopentanoic (2 nmol), a selective NMDA receptor antagonist, but not by ACSF. Enhancement of the cardiovascular responses to L-glutamate by intrarostral ventrolateral medulla p-chloromercuriphenylsulfonic acid (0.1 nmol), a glutamate uptake inhibitor, was reversed by subsequent ethanol, but not ACSF, microinjection. None of the treatments influenced baseline blood pressure or heart rate. Ethanol or 2-amino-5-phosphonopentanoic acid microinjected into the rostral ventrolateral medulla significantly (p < 0.05) attenuated baroreflex sensitivity tested by phenylephrine. In contrast, p-chloromercuriphenylsulfonic acid significantly (p < 0.05) enhanced baroreflex sensitivity (-2.14 +/- 0.09 vs. -3.08 +/- 0.18); subsequent ethanol microinjection reversed this enhancement (-2.90 +/- 0.21 vs. -1.86 +/- 0.24). Equal volume of ACSF had no effect on baroreflex sensitivity of pretreated rats (-3.22 +/- 0.31 vs. -2.98 +/- 0.34). These results suggest that ethanol exerts a marked inhibitory action on glutamatergic pathways within the rostral ventrolateral medulla that act to enhance baroreflex sensitivity.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effects of ethanol on baroreceptor heart rate responses of conscious spontaneously hypertensive and normotensive rats

This study investigated the acute effects of ethanol (0.25, 0.5, or 1 g/kg, iv) on mean arterial pressure, heart rate, and gain of the baroreceptor reflex control of heart rate (BRS) in conscious spontaneously hypertensive rats (SHRs) and age-matched Wistar rats. BRS was substantially lower in the SHRs as compared with the Wistar rats when measured by phenylephrine (-0.95 +/- 0.16 vs. -2.02 +/- 0.22 beats/min/mm Hg) or nitroprusside (-1.90 +/- 0.2 vs -3.02 +/- 0.32 beats/min/mm Hg). None of the doses of ethanol influenced BRS in the SHR. In contrast, ethanol attenuated BRS in Wistar rats, but this effect was dependent on the dose used and the type of response. The lower doses attenuated the reflex tachycardic response, but had no effect on the reflex bradycardic response. On the other hand, the 1 g/kg dose of ethanol attenuated the reflex bradycardic but not the tachycardic response. Ethanol produced a pressor effect (15-25 mm Hg; 5-min duration), which was neither dose- nor strain-dependent. However, only in the SHR mean arterial pressure remained elevated (10-15 mm Hg above control) for 20 min in response to the 0.5 g/kg dose of ethanol, and its recovery coincided with the occurrence of a slowly developing negative chronotropic response. Ethanol produced a dose-related negative chronotropic effect in both strains of rats that was of longer duration in the SHR, particularly with the 1 g/kg dose. It is possible that the bradycardic effect of ethanol influenced negatively its pressor effects.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition by ethanol of the cardiovascular effects of glutamate in the nucleus of the solitary tract

This study investigates whether ethanol affects the cardiovascular changes evoked by the excitatory amino acid glutamate in the nucleus of the solitary tract (NTS). Male Sprague-Dawley rats were anesthetized with urethane and instrumented for microinjection of drugs into the NTS. In 28 animals, an initial dose-response curve for glutamate (37, 74, and 148 pmol/60 nL) was obtained. In eight animals (control group), the dose-response curve was unchanged when it was tested after microinjection of 60 nL of saline into the NTS. In contrast, the prior intra-NTS administration of ethanol (25 or 50 mM) consistently inhibited the hypotensive and bradycardic effects of glutamate. The specificity of this inhibitory response was corroborated by the inability of intra-NTS administrations of ethanol (50 mM) to affect the hypotensive and bradycardic responses to nicotine (922 pmol. 1.84 nmol, and 3.96 nmol). These results indicate that the cardiovascular effects of glutamate can be antagonized by ethanol in the NTS. This inhibitory effect, in turn, may provide a basis for the hypertensinogenic action of ethanol in humans.

Acute alcohol administration stimulates baroreceptor discharge in the dog

It has been shown that alcohol administration causes baroreceptor reflex inhibition. The site of action of alcohol could reside anywhere within the baroreceptor reflex arc. Therefore, the goal of this study was to determine the effects of acute administration of alcohol on carotid sinus baroreceptor discharge characteristics. In pentobarbital-anesthetized dogs, the carotid sinus was isolated and perfused. Single unit baroreceptor discharge was recorded from the carotid sinus nerve along with carotid sinus diameter using sonomicrometry. Carotid sinus pressure-baroreceptor discharge and carotid sinus pressure-diameter curves were constructed. Perfusion of the carotid sinus with alcohol (100 mmol/L) significantly decreased the pressure threshold from 91.1 +/- 2.8 to 86.4 +/- 2.9 mm Hg (p < 0.05) and increased the peak discharge rate from 45.8 +/- 3.4 to 52.8 +/- 3.6 spikes per second (p < 0.01). The same phenomenon was seen during perfusion of the carotid sinus with acetaldehyde (2.5 mmol/L) but was not seen during perfusion with acetate (2.5 mmol/L). During perfusion of the carotid sinus with alcohol, the carotid sinus pressure-carotid sinus diameter relation did not change. The baroreceptor sensitization induced by alcohol is not an endothelium-dependent mechanism, because endothelial denudation did not block this alcohol-induced effect. Measurement of the duration of postexcitatory depression of carotid sinus baroreceptors, which is related to Na+,K(+)-ATPase activity, showed that perfusion of the carotid sinus with alcohol or acetaldehyde significantly reduced the duration of postexcitatory depression, indicating that the alcohol- and acetaldehyde-induced effect on baroreceptor discharge is most likely mediated by an inhibition of Na+,K(+)-ATPase.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of intravenous infusion of ethanol on regional blood flow in conscious rats

The effects of intravenous infusions of ethanol and saline (0.9% NaCl) on mean arterial pressure (MAP), heart rate (HR), total peripheral resistance (TPR), cardiac contractility (dP/dtmax) and systemic haemodynamics were studied in conscious, unrestrained rats by the radioactive microsphere technique. Saline (0.03 and 0.06 mL min-1 kg-1 for 12 min each dose) in the time-control group did not affect MAP, HR, TPR, dP/dtmax or vascular conductances in any organs or beds. While the low dose ethanol (2.4 mg min-1 kg-1) did not alter MAP, HR, TPR, systemic haemodynamics or dP/dtmax, the high dose (4.8 mg min-1 kg-1) slightly reduced MAP and TPR but did not affect HR, cardiac output or dP/dtmax. Both doses of ethanol vasodilated the intestine and spleen, but vasoconstricted the skin. The high dose caused additional vasodilatation in the heart and testes and the low dose also constricted the skeletal muscle bed. Our results show that ethanol, at non-hypotensive or slightly hypotensive doses, has marked vasodilator effects in the heart, intestine, spleen and testes.

Ethanol modulation of GABA receptor-activated Cl- currents in neurons of the chick, rat and mouse central nervous system

Modulation of gamma-aminobutyric acidA (GABAA) receptor function by drugs such as ethanol may depend on the genetic heterogeneity of GABAA receptor subunits, which vary across species and cell types. For this reason, the effects of ethanol on gamma-aminobutyric acid receptor-activated chloride currents (IGABA) were examined using whole-cell voltage-clamp recordings in primary cultures of neurons obtained from different species (chick, mouse and rat) and from different brain regions (cerebral cortex, hippocampus, cerebellum and spinal cord), and in acutely dissociated neurons from rat neocortical slices. Low concentrations (1-50 mM) of ethanol produced an enhancement of IGABA in some cells from each brain region examined. In cells obtained from the rat and chick cerebral cortex, 40-58% of cells exhibited an ethanol-sensitive IGABA. Moreover, a statistically significant variation in the response to ethanol was found in rat cortical neurons obtained from different litters. In mouse hippocampal neurons, potentiation of IGABA was obtained with ethanol concentrations (1-10 mM) well below those needed to inhibit neuronal responses to N-methyl-D-aspartic acid (30-50 mM), suggesting a differential sensitivity of these two receptor mechanisms to ethanol. Potentiation of IGABA by ethanol was reversed by the benzodiazepine receptor partial inverse agonist RO15-4513 (ethyl 8-azido-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a][1,4]benzodiazepine - 3-carboxylate), but was not affected by chelation of intracellular calcium. Furthermore, high concentrations of GABA attenuated the ability of ethanol to enhance IGABA. These results are consistent with the hypothesis that ethanol facilitates coupling between receptor binding and chloride channel activation.

Inhibition of baroreflex bradycardia by ethanol involves both GABAA and GABAB receptors in the brainstem of the rat

The effects of ethanol on baroreceptor reflex bradycardia and its interactions with cardiovascular effects mediated by GABA receptors in the dorsal vagal complex were studied in urethane-anaesthetised rats. Ethanol, 1 g kg-1 administered i.v. or 25-200 nmol microinjected bilaterally into the dorsal vagal complex, inhibited the reflex bradycardic response to bolus i.v. doses of phenylephrine both in spontaneously breathing and in paralysed, artificially ventilated animals, and this effect could be prevented by pretreatment with the GABA-depleting agent, 3-mercaptopropionate in both groups of rats. Ethanol, 1 g kg-1 i.v., did not influence the bradycardic response to electrical stimulation of the cervical vagus. Microinjection of muscimol into the dorsal vagal complex caused a dose-dependent pressor response and inhibited baroreflex bradycardia. The pressor response was potentiated and a tachycardic response to muscimol emerged following microinjection of ethanol into the dorsal vagal complex. Similar administration of baclofen caused dose-dependent increases in blood pressure and heart rate and inhibited baroreflex bradycardia. Injection of ethanol into the dorsal vagal complex potentiated the pressor response to a low dose of baclofen but did not affect the tachycardic response. Bicuculline, 10 pmol/side into the dorsal vagal complex, blocked the effects of muscimol but not those of baclofen and reduced the baroreflex inhibitory action of ethanol. 2-Hydroxysaclofen, microinjected at 400 pmol to 1 nmol/side, blocked the effects of baclofen but not those of muscimol. 2-Hydroxysaclofen or phaclofen, 2 mg kg-1 s.c., prevented the baroreflex inhibitory action of ethanol and also prevented ethanol potentiation of the pressor and, less effectively, the tachycardic effects of muscimol. It is concluded that ethanol inhibits baroreflex bradycardia through potentiation of the actions of endogenous GABA in the dorsal vagal complex. Both GABAA and GABAB receptors appear to be involved in this action of ethanol.