Blockade of central nervous system GABAergic tone causes sympathetic-mediated increases in coronary vascular resistance in cats

A novel sympathetic neuronal GABAergic signalling system regulates NE release to prevent ventricular arrhythmias after acute myocardial infarction

AIM

Overactivation of the sympathetic nerve may lead to severe ventricular arrhythmias (VAs) after myocardial infarction (MI). Thus, targeting sympathetic nerve activity is an effective strategy to prevent VAs clinically. The superior cervical ganglion (SCG), the extracardiac sympathetic ganglion innervating cardiac muscles, has been found to have a GABAergic signalling system, the physiological significance of which is obscure. We aimed to explore the functional significance of SCG post MI and whether the GABAergic signal system is involved in the process.

METHODS

Adult male Sprague-Dawley rats were divided into seven different groups. Rats in the MI groups underwent ligation of the left anterior descending coronary artery. All animals were used for electrophysiological testing, renal sympathetic nerve activity (RSNA) testing, and ELISA. Primary SCG sympathetic neurons were used for the in vitro study.

RESULTS

The GABAA receptor agonist muscimol significantly decreased the ATP-induced increase in intracellular Ca2+ (P < 0.05). GABA treatment in MI rats significantly attenuated the level of serum and cardiac norepinephrine (NE; P < 0.05). Sympathetic activity and inducible VAs were also lower in MI + GABA rats than in MI rats (P < 0.05). Knockdown of the GABAA Rs β2 subunit (GABAA Rβ2 ) in the SCG of MI rats increased the NE levels in serum and cardiac tissue, RSNA and inducible VAs compared with vehicle shRNA (P < 0.05).

CONCLUSION

The GABAergic signalling system is functionally expressed in SCG sympathetic neurons, and activation of this system suppresses sympathetic activity, thereby facilitating cardiac protection and making it a potential target to alleviate VAs.

Acute Stress Decreases but Chronic Stress Increases Myocardial Sensitivity to Ischemic Injury in Rodents

Cardiovascular disease (CVD) is the largest cause of mortality worldwide, and stress is a significant contributor to the development of CVD. The relationship between acute and chronic stress and CVD is well evidenced. Acute stress can lead to arrhythmias and ischemic injury. However, recent evidence in rodent models suggests that acute stress can decrease sensitivity to myocardial ischemia-reperfusion injury (IRI). Conversely, chronic stress is arrhythmogenic and increases sensitivity to myocardial IRI. Few studies have examined the impact of validated animal models of stress-related psychological disorders on the ischemic heart. This review examines the work that has been completed using rat models to study the effects of stress on myocardial sensitivity to ischemic injury. Utilization of animal models of stress-related psychological disorders is critical in the prevention and treatment of cardiovascular disorders in patients experiencing stress-related psychiatric conditions.

Diazepam administered prior to coronary artery occlusion increases latency to ventricular fibrillation

Few studies have addressed the antiarrhythmic potential of pretreatment with diazepam in acute myocardial infarction. Thus, the effect of diazepam pretreatment prior to coronary artery occlusion was examined in conscious pigs. Animals were instrumented with aortic catheters to measure arterial pressure, a pulmonary artery catheter for drug administration, and a snare around the left anterior descending coronary artery for permanent occlusion one week later. Diazepam (1 mg/kg iv bolus) or vehicle was administered 10 minutes prior to occlusion. Eight of 14 animals receiving diazepam (57%) and 13 of 22 receiving vehicle animals (59%) developed ventricular fibrillation following coronary occlusion. However, the latency to ventricular fibrillation was significantly shorter (7 +/- 1 min) in animals receiving vehicle compared to animals receiving diazepam (11 +/- 1 min). Significant increases in heart rate were seen up to 5 hours after coronary occlusion only in animals receiving vehicle. The results indicate that diazepam pretreatment can increase ventricular fibrillation latency and prevent heart rate increases following acute myocardial infarction.

Increases in coronary vascular resistance initiated by blockade of CNS GABAergic tone occurs in the hindbrain

The purpose of this study was to determine the site of action in the CNS responsible for producing picrotoxin-induced sympathetic mediated increase in coronary vascular resistance. To do this, picrotoxin, was administered either into the lateral cerebral ventricle, with the perfusion restricted to the forebrain, or into the fourth ventricle, to perfuse only the hindbrain and spinal cord, in chloralose-anesthetized cats, while monitoring coronary blood flow from the anterior descending branch of the left coronary artery, arterial pressure, heart rate and electrocardiogram (ECG). There was no difference between administration into the forebrain and hind-brain in terms of changes in coronary vascular resistance, ECG, arterial pressure and sinus rate when 600 micrograms of picrotoxin was used. Administration into either area elicited significant increases in coronary vascular resistance, arterial pressure and sinus rate, as well as changes in the ST segment and occasional ventricular tachyarrhythmias. However, a separation of effects was noted between the forebrain and the hindbrain when 200 micrograms of picrotoxin was administered. Administration of this dose into the forebrain did not significantly alter coronary vascular resistance or the ST segment, although significant increases in arterial pressure and sinus rate occurred in these animals. In contrast, administration of this dose into the hindbrain elicited significant increases in coronary vascular resistance, ST segment, arterial pressure and sinus rate. These results indicate that the most sensitive site for eliciting picrotoxin-induced increase in coronary vascular lies in the hindbrain.

Increase in coronary vascular resistance produced by stimulating neurons in the region of the area postrema of the cat

Recently, Somberg, in a preliminary report (1983), noted that electrical stimulation of the area postrema causes an increase in coronary vascular resistance. The increase in resistance was mediate by an increase in sympathetic activity as it was counteracted by alpha-adrenergic receptor blockade. The purpose of our study was to confirm the findings of Somberg by using the method of chemical-induced activation of cell bodies in the area postrema. This was done by bilateral topical application of kainic acid to the area postrema of cats while monitoring coronary blood flow, heart rate, arterial blood pressure and the ECG. Topical application of kainic acid produced an increase in coronary vascular resistance (43 +/- 13%, P less than 0.05), S-T segment changes, increases in heart rate (60 +/- 10, P less than 0.05) and arterial blood pressure (88 +/- 20, P less than 0.05) and ventricular tachyarrhythmias. All of these effects were prevented by pretreating animals with the alpha-adrenoceptor blocking agent, phentolamine. These results suggest that chemical excitation of area postrema neurons produces coronary constriction that is mediated by the sympathetic nervous system and alpha-adrenoceptors on coronary vessels.

Coronary vasospasm: a neuro-behavioral event?

Evidence exists for the presence of a neuro-anatomy that can produce cardiac vasospasm. Recent experimental and clinical data suggest that this neuro-anatomic mechanism can be activated by behavioral events in the psycho-social surround of the patient with coronary artery disease. I hypothesize that this mechanism can produce myocardial ischemia and can explain the variability in onset of clinical angina.

Adrenergic coronary tone during submaximal exercise in the dog is produced by circulating catecholamines. Evidence for adrenergic denervation supersensitivity in the myocardium but not in coronary vessels

The goal of this study was to test the hypothesis that circulating catecholamines are primarily responsible for alpha-adrenergic coronary vasoconstriction during submaximal exercise. The experimental series consisted of chronic studies in which a regional left ventricular sympathectomy was performed with phenol. Myocardial perfusion to the innervated and sympathectomized left ventricular regions was measured in these animals during (1) a control period, (2) treadmill exercise, (3) exercise during beta-adrenergic blockade, and (4) exercise during combined alpha- + beta-adrenergic blockade. We found no differences in myocardial perfusion between the innervated and sympathectomized regions or the transmural distribution of perfusion during any of these interventions. Thus, there is no evidence for neurogenic alpha-adrenergic coronary vasoconstriction. However, during exercise in the presence of alpha- and beta-blockade, coronary resistance (mmHg X min X 100 g/ml) was significantly less in both the innervated (0.65 +/- 0.07) and sympathectomized (0.68 +/- 0.07) regions than during beta-blockade, 0.90 +/- 0.17 and 0.89 +/- 0.16, respectively. This suggests that coronary alpha-adrenergic constriction was produced by circulating catecholamines. This concept of humorally mediated, alpha-adrenergic coronary vasoconstriction was strengthened by in vivo and in vitro studies that demonstrated that alpha-adrenergic supersensitivity of the coronary vasculature was not present. Myocardial beta-adrenergic supersensitivity was observed in the phenol regional sympathectomy model; however, this effect was blocked by propranolol (1 mg/kg). This indicates that alpha-adrenergic vasoconstriction in both myocardial regions during submaximal exercise is produced by circulating catecholamines. The major conclusion of this study is that, during submaximal exercise in the canine, alpha-adrenergic coronary vasoconstrictor tone is predominantly due to circulating catecholamines rather than direct neural effects.

Digitalis: neurally mediated arrhythmogenic and coronary vasoconstrictor properties

Studies of digitalis have revealed an appreciable role of the autonomic nervous system in the modulation of the cardiovascular toxicity of digitalis. Neuraxis transection experiments and studies using digitalis compounds that fail to enter the central nervous system suggest that centers located in the floor of the fourth cerebral ventricle play a substantial part in the neural arrhythmogenic effects of the digitalis glycosides. Electrical stimulation of the area postrema in partially digitalized animals, not otherwise manifesting ventricular arrhythmias, elicits ventricular tachycardia. Neuraxis transection experiments and studies with a charged digitalis derivative also suggest localization of the coronary vasoconstrictive properties of digitalis to the area postrema. The results suggest that this region of the brainstem is sensitive to a neurally active substance, digitalis, causing peripheral sympathetic effects of considerable consequence.