General systemic effects and electrocardiographic changes following injections of digitalis glycosides into the lateral ventricle of the brain

Comparison of electrical resistance, bubble withdrawal, and stereotaxic method for cannulation of cerebral ventricles

A comparison was made of three methods of lateral intraventricular (LV) cannulation in the rat brain. Rats in a control group (n = 8) were cannulated using traditional stereotaxic coordinates. In a second group (n = 8), electrical resistance was used to localize the ventricle and differentiate it from surrounding brain areas. In a third group (n = 8), a bubble withdrawal technique was employed where movement of a bubble in polyethylene tubing was monitored as the cannula was lowered in brain. All three methods produced accurate LV cannulation, but the electrical resistance technique was associated with optimal placement within the LV. Using the resistance procedure, greater access was obtained to posterior portions of the ventricular system. Also described is a novel stereotaxic cannula holder that may be used in the bubble withdrawal procedure or in obtaining acute samples of cerebrospinal fluid.

Vertebral artery injection of ouabain does not hasten arrhythmogenesis

Cats were infused with ouabain into either their vertebral artery or their femoral vein, and latencies to early and late ventricular arrhythmia and death were determined. No significant differences were found between the 2 groups. These data question the likelihood that the area postrema is, as has been recently proposed, the primary site of action for digitalis-induced cardiac arrhythmias.

Interactions between the autonomic nervous system and the cardiovascular effects of ouabain in guinea-pigs

Anaesthetized guinea-pigs were intoxicated with an intravenous infusion of ouabain. This infusion induced a marked pressor response which was reduced in bilaterally adrenalectomized or pithed animals. Ouabain produced initial bradyarrhythmias in 60% of guinea-pigs. Bilateral vagotomy or pretreatment with atropine abolished the bradyarrhythmias and sensitized the animals to the arrhythmic effects of ouabain. Pithing or beta-adrenoceptor blockade reduced the potency of ouabain for producing arrhythmias, but bilateral adrenalectomy did not give protection. Preferential alpha 2-adrenoceptor stimulation with clonidine (10-300 micrograms . kg-1 i.v.) also reduced the arrhythmogenic effects of ouabain, whereas no protection was found with St 91, a clonidine related compound which does not cross the blood-brain barrier. The effect of clonidine was antagonized by piperoxan. Preferential alpha 2-adrenoceptor blockade with piperoxan (6 mg . kg-1 i.v.) did not change the pressor response to ouabain, but sensitized the animals to the arrhythmogenic effects of ouabain. In contrast, the preferential alpha 1-antagonistic agent AR-C 239 (0.3 mg . kg-1 i.v.) abolished the pressor response to ouabain and in addition increased the dose of ouabain required to produced ventricular premature beats and ventricular fibrillation. These experiments indicate: (i) that ouabain produces in guinea-pigs a pressor response which seems to be due to catecholamine release from the adrenal medulla probably by an action on the central nervous system; (ii) that the ventricular arrhythmias induced by ouabain are due in part to stimulation of the central nervous system leading to an increase in beta-adrenoceptor activity; (iii) that central alpha-adrenoceptors appear to be involved in the arrhythmogenic effects of ouabain, as clonidine reduced these effects. On the other hand piperoxan, a preferential alpha 2-adrenoceptor antagonist did not change the pressor response to ouabain and increased the arrhythmogenic effects whereas AR-C 239 had opposite effects.

A simple method for the verification of a successful cannulation of the rat cerebral ventricles

A simple and inexpensive method for the cannulation of the cerebral ventricles of the rat is described. The required tools consist of a stereotaxic instrument, a standard steel injection cannula and a polyethylene tube.

Localization of the neurally mediated arrhythmogenic properties of digitalis

Available evidence suggests that the propensity of digitalis glycosides to produce cardiac arrhythmias is due in part to their neuroexictatory effects. We have performed experiments in cats which support the existence of a neurogenic component in the etiology of digitalis-induced ventricular arrhythmias. Our data further indicate that the locus of this neural effect lies within an area of the medulla 2 millimeters above to 2 millimeters below the obex. These findings, when considered with the effects of polar cardiac glycosides that do not cross the blood-brain barrier, suggest that the area postrema may be the site of neural activation.

Mechanism of neurotoxicity of cardiotonic glycosides

1 In cats intracerebroventricular administration of 5, 10, 20 mug of peruvoside, a cardiac glycoside obtained from the plant, Thevetia neriifolia, and 10 and 20 mug of ouabain, produced marked neurotoxicity. This was dose-related. 2 Prior administration reserpine (2 mg/kg i.m., 500 mug i.c.v.) or tetrabenazine (25 mg/kg i.v., 50 mg/kg i.v. and 2 mg/,g i.c.v.) suppressed the neurotoxicity, but lithium carbonate (100 mg/,g i.p., 2 mg 2.c.v.) and haloperidol (200 mug i.c.v.) were ineffective. 3 Prior administration of 2-bromolysergic acid diethylamide (BOL-148, 200 mug i.c.v.) or p-chlorophenylalanine (PCPA) (400 mg/kg i.p.) suppressed the neurotoxicity induced by peruvoside and ouabain. 4 Perfusion of the lateral ventricles of cats with 10, 20 and 30 mug of peruvoside or ouqbain produced a massive release of 5-hydroxytryptamine (5-HT). This was dose-related. Prior administration PCPA suppressed the release of 5-HT. 5 The results of the findings indicate the involvement of 5-HT in the genesis of neurotoxicity induced by peruvoside or ouabain.

The importance of a central adrenergic mechanism in the cardiovascular responses to ouabain

Central administration of ouabain in the ventricular system of vagotomized dogs and cats elicited increases in blood pressure, cardiac contractile force and cardiac rate followed by ventricular arrhythmia. Spinal transection (C2) or hexamethonium treatment abolished the central effects of ouabain. Bilateral stellate ganglionectomy prevented the tachycardia and reduced the pressor response to the lower dose (15 mug) of ouabain; bilateral adrenalectomy only reduced the pressor effect. Neither of the procedures alone was adequate to inhibit the cardiovascular responses following a higher dose (90 mug) of ouabain whereas a combination of both procedures blocked these responses completely. Tachyphylaxis to the cardiovascular effects of very high doses of intracerebroventricularly (i.c.v.)-administered ouabain was shown to be of central origin. Prior depletion of brain catecholamines by Ro 4-1284 or 6-hydroxy -dopamine, and central adrenergic neuron blockade by bretylium prevented the centrogenic effects of ouabain. Similarly, central alpha- or beta-adrenoceptor blockade also prevented the responses to i.c.v. orabain. The results suggest that ouabain causes the release of adrenergic mediator (noradrenaline) in the brain, probably by a depolarization of a primary neuron(s), and that the central adrenergic mechanism is responsible for the genesis of the cardiovascular effects. Both alpha- and beta-adrenoceptors seem to be important in the central control of cardiovascular function. The most sensitive site for ouabain action was found to be the posterior hypothalamus in the vicinity of the third ventricle.

Effects of microinjection of ouabain into the hypothalamus in cats

1. In cats anaesthetized with chloralose, repeated injections of 20 mug ouabain were made either into the cerebral ventricles, or into the ventromedial nucleus of the hypothalamus (VMH), or intravenously whilst the electrocardiogram, arterial blood pressure and respiration were recorded.2. The injections produced cardiac arrhythmias preceded by sinus bradycardia, variable changes in arterial blood pressure and respiratory depression. Death occurred either from ventricular fibrillation or from cardiac arrest.3. The arrhythmias which occurred after the injections into the cerebral ventricles were not peripheral effects produced after absorption of the ouabain into the blood stream, because with intravenous injections larger amounts were required to produce the arrhythmias and to cause death than with intraventricular injections. The arrhythmias which resulted in death were due to an action on the VMH. With microinjections of ouabain into this region of the brain death occurred earlier and after smaller doses than after intraventricular injections.4. While sinus bradycardia was abolished by bilateral vagotomy other cardiac arrhythmias were prevented by acute cardiac sympathectomy and cervical cord transection. Thus both the sympathetic and parasympathetic nervous systems appear to be involved in the production of these arrhythmias.5. Since some of the cardiac arrhythmias obtained with ouabain in anaesthetized cats resemble the cardiotoxic effects seen in clinical practice during treatment with digitalis glycosides it is concluded that these effects, too, are, at least in part, central in origin, caused by an action on the VMH and mediated mainly via the sympathetic nervous system.