Impairment of central mediation of the arterial baroreflex by acute ethanol administration

Involvement of NMDA Receptors, Nitric Oxide, and GABA in Rostral Ventrolateral Medulla in Acute Ethanol-Induced Cardiovascular Responses in Rats

BACKGROUND

Consumption of ethanol (EtOH) (alcohol) has many effects on physiological functions, particularly those in the central nervous system (CNS) and cardiovascular system. Acute excessive intake of EtOH (alcohol intoxication) may cause hypotension and tachycardia. In this study, we examined the mechanistic involvement of glutamatergic N-methyl-d-aspartate (NMDA) receptors, nitric oxide (NO), and γ-aminobutyric acid (GABA) pathways in the CNS in acute EtOH-induced cardiovascular effects.

METHODS

EtOH was administered by intraperitoneal (IP) injection in Sprague-Dawley rats. The blood pressure (BP) and heart rate (HR) were measured in conscious and in urethane-anesthetized rats. Inhibitors were applied by intracerebroventricular (ICV) injection or by microinjection into rostral ventrolateral medulla (RVLM). Microdialysis was used to determine the level of glutamate, NO, and GABA in the RVLM.

RESULTS

IP injection of EtOH (3.2 g/kg) caused a significant decrease in BP in conscious and anesthetized rats and a late increase in HR in conscious rats. The cardiovascular effects of EtOH were significantly attenuated by ICV or by RVLM post treatment with ketamine (an NMDA receptor antagonist), N5-(nitroamidino)-L-2,5-diaminopentanoic acid (L-NNA; a NO synthase inhibitor), or bicuculline (a GABA receptor antagonist). EtOH caused an increase in the level of glutamate, NO, and GABA in the RVLM during the hypotensive responses. RVLM posttreatment with ketamine blocked the increase in NO and GABA levels; post treatment with L-NNA blocked the increase in GABA level.

CONCLUSIONS

Our results indicate that EtOH augmentation of glutamatergic NMDA receptors/NO/GABA pathways in the RVLM may participate in the hypotensive effects induced by acute administration of EtOH.

Heart rate variability biofeedback: Theoretical basis, delivery, and its potential for the treatment of substance use disorders

Heart rate variability biofeedback (HRV BFB) is a biobehavioural clinical intervention that is gaining growing empirical support for the treatment of a number of psychological disorders, several of which are highly comorbid with substance use disorders (SUDs). The present article reviews the autonomic nervous system bases of two key processes implicated in the formation and maintenance of addictive pathology-affect dysregulation and craving-and asks if HRV BFB may be an effective intervention to ameliorate autonomic nervous system dysregulation in these processes, and as such, prove to be an effective intervention for SUDs. A detailed description of HRV BFB and its delivery is provided. Preliminary evidence suggests HRV BFB may be an effective addendum to current first-line SUD treatments, though no firm conclusions can be drawn at this time; more research is needed.

Ethanol, Trauma, and the Brain

Alcohol intoxication plays a significant and causal role in various fatal injuries. In comparison to sober individuals, intoxicated people have a greater generic risk for being involved in hazardous activities that may result in fatal injuries. However, it is not clear whether the biological effects of acute alcohol intoxication result in worse injuries than those sustained by sober individuals who are injured by identical mechanisms. Alcohol intoxication has a neuroprotective effect in experimental animal models of traumatic brain injury (TBI) but the evidence for a similar effect in humans is controversial. Earlier studies found such a protective effect, but more recent large epidemiological studies have not confirmed this finding; some studies also suggest a dose-related protective or exacerbating effect of alcohol intoxication on TBI. There are two apparent alcohol-associated syndromes in which an otherwise survivable blunt force impact to the head of an intoxicated individual is fatal at the scene. The first is a fatal cardiorespiratory arrest (the so-called alcohol concussion syndrome or “commotio medullaris”); the second is “traumatic basilar subarachnoid hemorrhage” (secondary to tears in the cerebral arteries, particularly the intracranial and extracranial vertebral arteries).

Augmented central nitric oxide production inhibits vasopressin release during hemorrhage in acute alcohol-intoxicated rodents

Acute alcohol intoxication (AAI) attenuates the AVP response to hemorrhage, contributing to impaired hemodynamic counter-regulation. This can be restored by central cholinergic stimulation, implicating disrupted signaling regulating AVP release. AVP is released in response to hemorrhage and hyperosmolality. Studies have demonstrated nitric oxide (NO) to play an inhibitory role on AVP release. AAI has been shown to increase NO content in the paraventricular nucleus. We hypothesized that the attenuated AVP response to hemorrhage during AAI is the result of increased central NO inhibition. In addition, we predicted that the increased NO tone during AAI would impair the AVP response to hyperosmolality. Conscious male Sprague-Dawley rats (300-325 g) received a 15-h intragastric infusion of alcohol (2.5 g/kg + 300 mg·kg(-1)·h(-1)) or dextrose prior to a 60-min fixed-pressure hemorrhage (∼40 mmHg) or 5% hypertonic saline infusion (0.05 ml·kg(-1)·min(-1)). AAI attenuated the AVP response to hemorrhage, which was associated with increased paraventricular NO content. In contrast, AAI did not impair the AVP response to hyperosmolality. This was accompanied by decreased paraventricular NO content. To confirm the role of NO in the alcohol-induced inhibition of AVP release during hemorrhage, the nitric oxide synthase inhibitor, nitro-l-arginine methyl ester (l-NAME; 250 μg/5 μl), was administered centrally prior to hemorrhage. l-NAME did not further increase AVP levels during hemorrhage in dextrose-treated animals; however, it restored the AVP response during AAI. These results indicate that AAI impairs the AVP response to hemorrhage, while not affecting the response to hyperosmolality. Furthermore, these data demonstrate that the attenuated AVP response to hemorrhage is the result of augmented central NO inhibition.

Estrogen enhancement of baroreflex sensitivity is centrally mediated

We have recently shown that estrogen enhances baroreceptor control of reflex bradycardia in conscious rats. The present study replicated this finding in pentobarbital sodium-anesthetized rats, and the study was extended to investigate whether this effect of estrogen is centrally or peripherally mediated. Hemodynamic responses to electrical stimulation of the central end of the aortic depressor or the vagal efferent nerve were evaluated in pentobarbital sodium-anesthetized sham-operated (SO), ovariectomized (OVX), and OVX estradiol-treated Sprague-Dawley rats. Phenylephrine (1-16 microgram/kg iv) elicited dose-dependent pressor and bradycardic responses. Regression analysis of the baroreflex curves, relating changes in mean arterial pressure and heart rate, revealed a significantly smaller baroreflex sensitivity in OVX compared with SO anesthetized rats (-0.54 +/- 0.05 and -0.91 +/- 0.12 beats. min-1. mmHg-1, respectively; P < 0.05). Treatment of OVX rats with 17beta-estradiol (E2, 50 microgram. kg-1. day-1 for 2 days subcutaneously) significantly enhanced baroreflex sensitivity to a level similar to that of SO rats (P < 0.05). The enhancing effect of E2 on the baroreflex-mediated bradycardia, observed in conscious and anesthetized rats, seems to be selective because the baroreflex-mediated tachycardic responses measured in a separate group of conscious rats were not altered by ovariectomy or E2 administration. Electrical stimulation of the aortic nerve elicited frequency-dependent depressor and bradycardic responses that were significantly smaller in OVX compared with SO values (P < 0.05). Treatment of OVX rats with E2 restored the hemodynamic responses to aortic stimulation to near SO levels. On the other hand, hemodynamic responses to vagal stimulation were not affected by OVX or treatment with E2. These findings suggest that enhancement of reflex bradycardia by estrogen is centrally mediated and involves interaction with central projections of the aortic nerve.

Effect of oral ethanol intake on muscle sympathetic nerve activity and cardiovascular functions in humans

The aim of the present study was to clarify the acute effect of alcohol drinking on muscle sympathetic nerve activity and other cardiovascular variables in young healthy human volunteers. Seven volunteers (25.0 +/- 4.7 years in age, weighing 59.9 +/- 5.9 kg) were instructed to lay down on a bed, and muscle sympathetic nerve activity (MSNA) was microneurographically recorded from the tibial nerve, simultaneously with an electrocardiogram, blood pressure with Finapres and autosphygmomanometer, cardiac output by impedance cardiography, and skin blood flow by laser Doppler flowmetry. After a 1-h rest, the subjects drank alcohol (0.6 g/kg) and were remained in resting position for 105 min. Blood ethanol levels indicated that they were moderately intoxicated. Heart rate constantly increased until 30 min after the ingestion, and maintained a peak level. MSNA was slightly but significantly suppressed just after the ingestion, and was gradually enhanced until the end of the experiment, showing a significant difference from the control level until 40 min in burst rate and until 25 min in total MSNA after the ingestion. Blood pressure showed an abrupt and transient increase at first, then gradual decrease until 30 min after the ingestion, and maintenance of the resting level until the end of the experiment. Cardiac output showed no constant tendency and no significant differences after the ingestion with wide interindividual variation. Skin blood flow increased 15 min after the ingestion.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

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)

Neuronal and cardiovascular responses to adenosine microinjection into the nucleus tractus solitarius

This study investigated neuronal, blood pressure, and heart rate responses to adenosine microinjection into caudal and rostral NTS of anesthetized rats. The site of recording and microinjection was verified chemically by observing the responses to a test dose of l-glutamate (5 nmol) and histologically at the conclusion of the experiment. Neuronal firing rate increased (+29.4 +/- 5.3%) and decreased (-48 +/- 9.4%) in response to l-glutamate microinjection into the rostral and caudal NTS, respectively. These opposite neuronal responses were followed by depressor (-32.4 +/- 8.3 vs. -36 +/- 5.5 mmHg) and bradycardic (-25.2 +/- 7.7 vs. -25.8 +/- 3.4 beats/min) responses to l-glutamate microinjection into the two subareas of the NTS. Microinjection of a submaximal dose (1 nmol) of adenosine into the NTS produced site-dependent cardiovascular responses which were preceded by similar inhibition of neuronal firing (-60 +/- 4 vs. -55.9 +/- 1.7%). Whereas adenosine microinjection into the rostral NTS elicited modest pressor (+10.1 +/- 2.1 mmHg) and tachycardic (+9 +/- 3.9 beats/min) responses, its microinjection into the caudal NTS produced depressor (-29.2 +/- 5.3 mmHg) and bradycardic (-14.6 +/- 1.7 beats/min) responses. These findings suggest that compared to l-glutamate, adenosine produces opposite (rostral) and similar (caudal) neuronal and cardiovascular effects in the two subareas of the NTS. In the caudal NTS, adenosine (0.1, 1, and 10 nmol) elicited dose-related inhibitory neuronal and cardiovascular responses that were attenuated by systemic theophylline but not 8-(p-sulfophenyl) theophylline (8-SPT) administration. The neuronal and cardiovascular responses to adenosine microinjection into the caudal NTS were also attenuated by microinjection of 8-SPT into the same area. Finally, single-unit activity inhibited by adenosine or l-glutamate microinjection into the caudal NTS was also inhibited by baroreceptor loading and excited by baroreceptor unloading. These findings suggest a) l-glutamate elicits opposite neuronal responses in the rostral and caudal NTS; b) the distinct hemodynamic responses elicited by adenosine in the two subareas may be related, at least in part, to their differing responses to l-glutamate; and c) the similarity between the neuronal responses to adenosine and l-glutamate microinjection into the caudal NTS and the response of the same neurons to baroreceptor activation support the hypothesis that adenosine plays a neuromodulatory role in the processing of baroreceptor information.

Selective sensitization by L-glutamate of baroreflex-mediated bradycardia following microinjection into the rostral ventrolateral medulla

This study examined the role of L-glutamate receptors in different sites in the rostral ventrolateral medulla (RVL) on baseline mean arterial pressure (MAP), heart rate (HR), sympathetic efferent discharge (SED) as well as baroreflex control of HR and SED. Depending on the site, the hemodynamic responses varied from an increase to a decrease in MAP which was accompanied by a similar or an opposite change in HR; the change in SED correlated positively with the change in MAP. Because injection of the test dose of glutamate (5 nM) into the deepest portion of the RVL produced the most pronounced increases in MAP, which was accompanied by a brisk bradycardia, we decided to investigate the nature of these responses. Dose-related sympathoexcitatory, pressor and bradycardic effects were obtained in response to 1, 3, 5 and 10 nM glutamate. The glutamate antagonist, glutamate diethylester (GDEE) abolished these responses and decreased baseline MAP, HR and SED. That glutamate-evoked bradycardia was baroreceptor-mediated was supported by: (1) the bradycardia was not only eliminated but was also converted to a small but significant tachycardic response in sinoaortic denervated rats and following cardiac muscarinic blockade; (2) glutamate substantially sensitized the baroreceptor HR and had no effect on SED response when tested by phenylephrine; and (3) GDEE abolished the sensitizing action of glutamate on the baroreflex control of HR. We conclude that glutamatergic pathways in the deepest portion of the RVL are tonically active and subserve sympathoexcitatory, pressor and tachycardic effects. Also, glutamate differentially sensitizes the baroreceptor reflex pathways that control HR by mainly activating the vagal component of the reflex, an effect which masks its tachycardic effect.

Impairment of baroreceptor reflex control of heart rate but not sympathetic efferent discharge by central neuroadministration of ethanol

We investigated the acute hemodynamic effects of ethanol microinjection into brain areas known to influence cardiovascular function and reflexes. In chloralose-anesthetized rats, ethanol had no effect on baseline mean arterial pressure, heart rate, or sympathetic efferent discharge when microinjected into the nucleus tractus solitarius, the dorsal motor nucleus of the vagus, the rostral ventrolateral medulla, or the posterior hypothalamus. On the other hand, ethanol microinjection into the anterior hypothalamus caused a site-dependent pressor effect and an increase in sympathetic efferent discharge. Baroreceptor heart rate response but not sympathetic efferent discharge response was impaired by ethanol microinjection into the nucleus tractus solitarius, the dorsal motor nucleus of the vagus, and the rostral ventrolateral medulla, suggesting that ethanol involves one or more of these areas in its inhibitory effect on baroreceptor heart rate response and that ethanol has a selective action on baroreceptor reflex control of heart rate. The findings that 1) the effect was dose dependent and 2) injection of ethanol outside of, or an equal volume of cerebrospinal fluid into, the nucleus tractus solitarius had no effect on the response strongly suggest that the observed effect on baroreceptor heart rate response was ethanol mediated. Ethanol microinjection into the dorsal motor nucleus of the vagus impaired the heart rate response, thus raising the possibility that leakage of ethanol to that area from the nucleus tractus solitarius might have contributed to its effect. These findings show that ethanol has a pressor and sympathoexcitatory site of action within the anterior hypothalamus and that it selectively impairs baroreceptor heart rate response via a central site of action; the mechanisms by which ethanol produces these effects remain to be elucidated.