Central locus of emetic action of digitalis substances in cats

Suncus murinus: a new experimental model in emesis research

Effects of various emetic and antiemetic drugs were studied using Suncus murinus for its potential use as an experimental model in emetic research. Subcutaneous injection of nicotine bitartrate (10-15 mg/kg), veratrine sulfate (0.5-1.0 mg/kg), emetine dihydrochloride (40-80 mg/kg) and oral administration of copper sulfate (20-100 mg/kg) caused dose-dependent emesis in suncus. The ED50 of nicotine, veratrine, emetine and copper sulfate were 7.9, 0.4, 47.6 and 21.4 mg/kg, respectively. However, subcutaneously injected apomorphine hydrochloride (0.1-100 mg/kg), digitoxin (0.5-1.0 mg/kg) and orally administered emetine dihydrochloride (10-80 mg/kg) did not induce the vomiting. Chlorpromazine and promethazine decreased the emetic effect of nicotine, veratrine and copper sulfate, but scopolamine hydrobromide was not effective. These results indicate that the Suncus murinus is sensitive to various emetic and antiemetic drugs and can be used as a new experimental animal model for the emesis. Emetic behavior of suncus was discussed in comparison with other animals.

Further studies on nicotine-induced emesis: nicotinic mediation in area postrema

In unanesthetized cats the emetic action of an injection of nicotine into the cerebral ventricle through chronically implanted cannulae was investigated. Nicotine injected in doses of 0.02-1.0 mg produced dose-dependent vomiting, which was abolished after ablation of the area postrema. However, copper sulfate given intragastrically evoked vomiting in cats with an ablated area postrema. The emetic response to intracerebroventricular (ICV) nicotine as well as the vomiting produced by intragastric copper sulfate was depressed or abolished in cats pretreated with ICV reserpine. On the other hand, the emetic response to ICV nicotine and to intragastric copper sulfate was virtually unchanged in cats pretreated with ICV 6-hydroxydopamine and 5,6-dihydroxytryptamine. The duration of vomiting produced by intragastric copper sulfate, but not that of ICV nicotine, was potentiated in cats pretreated with hemicholinium-3. Ganglionic blocking agents, mecamylamine and hexamethonium, injected ICV prevented the vomiting elicited by ICV nicotine. On the other hand, selected anti-muscarinic drugs, alpha and beta adrenergic receptor antagonists, dopamine antagonists, antihistamines and a 5-hydroxytryptamine antagonist all injected into the cerebral ventricle had virtually no effect on the vomiting induced by ICV nicotine. It is postulated that nicotine evokes vomiting by its action on nicotinic receptors within the area postrema but not on catecholaminergic or serotonergic neurones. Finally, acetylcholine could also be involved in the inhibition of the complex mechanisms underlying the central regulation of vomiting.

Dimethylphenylpiperazinium-induced vomiting: nicotinic mediation in area postrema

In unanesthetized cats the emetic action of dimethylphenylpiperazinium (DMPP) was investigated, after it was injected into the cerebral ventricles through chronically implanted cannulae. DMPP injected in 0.2-2.0 mg doses into the cerebral ventricle produced dose-dependent vomiting, which was abolished after ablation of area postrema. However, copper sulfate given intragastrically evoked vomiting in cats with an ablated area postrema. Further, the emetic response to ICV DMPP and to intragastric copper sulfate was depressed or abolished in cats pretreated with ICV reserpine. The emetic response to ICV DMPP, but not that caused by intragastric copper sulfate, was potentiated in cats pretreated with ICV 5,6-dihydroxytryptamine. Ganglionic blocking agents, mecamylamine and hexamethonium, injected ICV prevented the vomiting elicited by ICV DMPP. On the other hand, selected anti-muscarinic drugs, alpha and beta adrenergic antagonists, dopamine antagonists, antihistamines and a 5-hydroxytryptamine (5-HT) antagonist all injected into the cerebral ventricles had virtually no effect on the vomiting induced by DMPP. It is postulated that DMPP evokes vomiting by its action on nicotinic receptors of nerve cells within the area postrema but not on catecholaminergic, serotonergic, or cholinergic receptors. Finally, 5-HT and acetylcholine could also be involved in the inhibition of the complex mechanisms underlying the central regulation of vomiting.

Autoradiographic localization of 3H-digoxin binding by neural cells in the medulla

The purpose of this investigation was to localize binding sites for the cardiac glycoside digoxin in the medulla of the rat in vivo. Adult male Sprague-Dawley rats were injected (IV) with 3H-digoxin and killed 30 minutes later. Autoradiographs of medullas showed evidence of 3H-digoxin binding to small- and medium-sized neural cells in the regions of the nucleus solitarius, dorsal motor nucleus of the vagus, area postrema, and in the zone between the area postrema and the underlying neuropil. However, the parasympathetic preganglionic neurons of the dorsal motor nucleus were not labeled. The 3H-digoxin-labeled cells in the medulla were located mainly in the commissural and medial portions of nucleus solitarius at the level of the area postrema. Animals injected with unlabeled digoxin followed by 3H-digoxin showed reduced binding of radioactivity. The small- and medium-sized neurons of the caudal portions of the nucleus solitarius are internuncial in position with respect to cardiovascular afferents of the glossopharyngeal and vagus nerves and sympathetic and parasympathetic cardiovascular efferent neurons of the medulla. The results of this study suggest that these 3H-digoxin-labeled cells, presumably neurons of nucleus solitarius, may possess high affinity binding sites for digoxin. Further, the area postrema, which lacks a blood-brain barrier, may provide a portal of entry for 3H-digoxin into regions of the medulla known to contain neurons that play a role in the regulation of cardiac rhythm.

Inotropic and toxic effects of a polar cardiac glycoside derivative in the dog

It has been suggested that central nervous system (CNS) neuroexcitation plays an important role in digitalis-induced cardiac arrhythmias. To elucidate further the role of the CNS in digitalis-induced arrhythmias, the inotropic and toxic effects of a highly polar semisynthetic cardiac glycoside, 3beta-O-(4 amino-4,6 dideoxy-beta-D-galactopyranosyl)-digitoxigenin (ASI-222) were compared to those of digoxin and correlated with plasma and cerebrospinal fluid (CSF) concentrations of each drug. Thirteen dogs anesthetized with sodium pentobarbital were given repeated intravenous doses of digoxin or ASI-222. Ventricular tachycardia was elicited at a mean dose of digoxin of 0.12 mg/kg, compared with 0.09 mg/kg for ASI-222 (not significant). Terminal ventricular fibrillation occurred after 0.18 mg/kg of digoxin, a value significantly larger than the ASI-222 dose (0.14 mg/kg, P less than 0.05) required to produce lethal arrhythmias. Digoxin produced a 21% increase in LV dP/dt at a plasma digoxin concentration of 20.0 +/- 2 ng/ml (mean +/- SEM) 30 minutes after 0.05 mg/kg; the CSF digoxin concentration at this time averaged 0.7 +/- 0.1 ng/ml. At death, the plasma digoxin concentration was 88 +/- 16 ng/ml and CSF digoxin concentration was 5.7 +/- 1.6 ng/ml. ASI-222 produced a similar 25% increase in LV dP/dt 30 minutes after administration of 0.05 mg/kg, with a plasma concentration of 18 +/- 2 ng/ml as determined by a newly developed radioimmunoassay. The plasma ASI-222 concentration at death, 95 +/- 18 ng/ml, was similar to that of digoxin. However, CSF samples at 30 minutes and at death showed no detectable levels of ASI-222. Thus, despite similar inotropic and toxic responses and similar plasma drug concentrations compared to digoxin, ASI-222 was demonstrated to have limited if any access to the CNS as judged by CSF concentrations. These findings suggest that direct CNS stimulation does not play a primary part in the genesis of digitalis-induced cardiac arrhythmias in this experimental model, or that CNS effects are mediated by an area or areas lacking an effective blood-brain barrier.

CNS site of antiarrhythmic action of diphenylhydantoin (DPH) in the cat

Diphenylhydantoin (DPH) is known to be a potent anticonvulsant agent, useful in treating and preventing grand mal seizures. More recently, DPH was reported also to be a potent antiarrhythmic agent acting by means of a depressant action on the heart. The present experiments demonstrated that DPH has also a potent antiarrhythmic action when administered to the CNS. The posterolateral hypothalamus was stimulated in cats to evoke cardiac arrhythmias of varying severity both during and after stimulation. In general, it was found that the post-stimulus arrhythmias were obtained more readily than those during stimulation. The mean effective dose of DPH required to prevent the arrhythmias via the i.v. route was 11.9 mg/kg, and that via the vertebral artery route and via the fourth ventricular route was only 1.9--1.4 and 1.4 mg/kg, respectively. These results suggest that though DPH has identifiable antiarrhythmic action on the heart itself it has a strong antiarrhythmic effect via the central nervous system as well.