The anticonvulsant effects of propranolol and beta-adrenergic blockade

Epileptic patients who survived sudden cardiac death have increased risk of recurrent arrhythmias and death

BACKGROUND

Cardiac arrhythmogenesis and cryptogenic epilepsy can be due to ion channel dysfunction and may coexist in the same patient. Sudden unexplained death in epilepsy (SUDEP) is a known entity with unknown cause, with the possibility of ventricular tachyarrhythmias being one of the causes. However, no prior study has investigated epileptic survivors of sudden cardiac death (SCD), recurrent life-threatening ventricular tachyarrhythmia (LTVA) and other outcomes in this patient population.

METHODS

The Antiarrhythmics Versus Implantable Cardioverter Defibrillators (AVID) Trial (n = 1016) was a multicenter trial comparing a cardioverter-defibrillator device (ICD) (n = 507) and anti-arrhythmic drugs (AADs) (n = 499) for secondary prevention of LTVAs. Mean follow-up duration was 916 ± 471 days per patient. Patients with a history of epilepsy (n = 6) in the ICD arm were included in this analysis. End points were recurrence of LTVA, cardiac death and all-cause mortality.

RESULTS

History of epilepsy (n = 6) was a significant predictor of recurrent LTVA [hazard ratio 3.53, 95% confidence interval (CI) 1.30-9.56], cardiac death (hazard ratio 4.14, 95% CI 1.30-13.14) and all-cause mortality (hazard ratio 3.82, 95% CI 1.40-10.48) in the ICD arm (n = 498). This relationship remained unchanged on multivariate analysis after controlling for baseline clinical differences.

CONCLUSION

This is the first study to investigate the effect of epilepsy on secondary prevention of LTVA. Epileptic survivors of SCD are at significantly greater risk of recurrent arrhythmias and death as compared to other survivors of recurrent LTVA. Role of coexisting channelopathies in both epilepsy and arrhythmogenesis may explain SUDEP and requires further investigation.

Propranolol blocks cardiac and neuronal voltage-gated sodium channels

Propranolol is a widely used, non-selective β-adrenergic receptor antagonist with proven efficacy in treating cardiovascular disorders and in the prevention of migraine headaches. At plasma concentrations exceeding those required for β-adrenergic receptor inhibition, propranolol also exhibits anti-arrhythmic ("membrane stabilizing") effects that are not fully explained by β-blockade. Previous in vitro studies suggested that propranolol may have local anesthetic effects. We directly tested the effects of propranolol on heterologously expressed recombinant human cardiac (NaV1.5) and brain (NaV1.1, NaV1.2, NaV1.3) sodium channels using whole-cell patch-clamp recording. We found that block was not stereospecific as we observed approximately equal IC50 values for tonic and use-dependent block by R-(+) and S-(-) propranolol (tonic block: R: 21.4 μM vs S: 23.6 μM; use-dependent block: R: 2.7 μM vs S: 2.6 μM). Metoprolol and nadolol did not block NaV1.5 indicating that sodium channel block is not a class effect of β-blockers. The biophysical effects of R-(+)-propranolol on NaV1.5 and NaV1.1 resembled that of the prototypical local anesthetic lidocaine including the requirement for a critical phenylalanine residue (F1760 in NaV1.5) in the domain 4 S6 segment. Finally, we observed that brain sodium channels exhibited less sensitivity to R-(+)-propranolol than NaV1.5 channels. Our findings establish sodium channels as targets for propranolol and may help explain some beneficial effects of the drug in treating cardiac arrhythmias, and may explain certain adverse central nervous system effects.

The mystery of sudden death: mechanisms for risks

This review addresses the possible overlapping mechanisms that may apply to the risk of sudden unexpected death occurring in epilepsy and in cardiac disease. It explores the interaction between the central and peripheral autonomic nervous systems and the cardiopulmonary systems. Included is a discussion of the potential interactive role of genetically determined subtle cardiac risk factors for arrhythmias with a predisposition for seizure-related cardiac arrhythmias. We address the possible mechanisms that are operant in producing both epileptogenic and cardiogenic arrhythmias. Finally, we speculate about potential preventive measures to minimize the risk of both sudden unexpected death in epilepsy and sudden cardiac death.

Effects of β-adrenoceptor antagonists in the neural nitric oxide release induced by electrical field stimulation and sodium channel activators in the rabbit corpus cavernosum

Beta-Adrenoceptor antagonists may present receptor-independent mechanisms, such as blockade of voltage-gated sodium channels. This study aimed to investigate the effects of non-selective (propranolol), and selective beta1- (atenolol, metoprolol and betaxolol) and beta2-adrenoceptor (ICI 118,551) antagonists in the nitric oxide (NO)-mediated rabbit corpus cavernosum relaxations induced by either electrical field stimulation (EFS) or activators of voltage-gated sodium channels. The sodium channel blockers tetrodotoxin and saxitoxin abolished the relaxations induced by EFS or sodium channel activators of binding site-2 (aconitine and veratridine), site-3 (Ts3 toxin), site-4 (Ts1 toxin) and site-5 (brevetoxin-3). The beta-adrenoceptor antagonists failed to affect the relaxations induced by EFS, aconitine and veratridine. Relaxations induced by Ts3 and Ts1 toxins, as well as brevetoxin-3, were markedly reduced by prior addition of propranolol, betaxolol and ICI 118,551. During the established relaxation induced by Ts3 toxin, propranolol failed to restore the basal tone. In conclusion, beta-adrenoceptor antagonists may cause an allosteric inhibition at the binding site-3, -4 and -5 of voltage-gated sodium channels, leading to blockade of neural NO release.

Anticonvulsant profile and mechanism of action of propranolol and its two enantiomers

The anticonvulsant properties of the ss-adrenoceptor antagonist propranolol and its two enantiomers were examined in various screening tests in order to characterize the anticonvulsant profile as well as the possible molecular mechanism of action. These compounds dose-dependently raised the threshold for tonic electroshock seizures in mice and were effective in the traditional maximal electroshock test (ED (50)s 15- 20 mg kg (-1)i.p.). In combination with clinically used antiepileptics, the anticonvulsant effectiveness of the latter was significantly increased. In the pentylenetetrazol (85 mg kg (-1)s.c.) seizure threshold test, ( +/-)- and ( +)-propranolol were not effective in preventing clonic seizures. In unrestrained rats with chronically implanted electrodes in the dorsal hippocampus, propranolol and its ( +)-enantiomer equieffectively reduced the duration of electrically-evoked hippocampal afterdischarges (10 and 20 mg kg (-1)i.p.) and raised the focal stimulation threshold (20 mg kg (-1)i.p.). In amygdala-kindled rats, both drugs ( >or= 10 mg kg (-1)i.p.) reduced the seizure severity from stage 5 (generalized clonic-tonic) to stage 3 (unilateral forelimb) seizures. Furthermore, whole-cell patch-clamp experiments showed that ( +)- as well as ( -)-propranolol ( 10(-6)to 10(-4)M) depressed the fast inward sodium current in a concentration- and use-dependent manner in cultured rat cardiomyocytes and inhibited picrotoxin-induced burst firing activity of mouse spinal cord neurones in culture. In conclusion, propranolol and its two enantiomers have anticonvulsant effects in models for generalized tonic-clonic and complex partial seizures which may be accounted for by the sodium channel blocking and not by the ss-adrenoceptor blocking activity.

Influence of some beta-adrenoceptor antagonists on the anticonvulsant potency of antiepileptic drugs against audiogenic seizures in DBA/2 mice

The two enantiomers of propranolol antagonize generalized tonic-clonic seizures in DBA/2 mice with the (-)-enantiomer being about 1.5 times more potent than the (+)-enantiomer. Metoprolol was less active and atenolol was unable to affect audiogenic seizures. In combination with conventional antiepileptic drugs, both propranolol enantiomers tested in doses not affecting the occurrence of audiogenic seizures increased the anticonvulsant activity of diazepam, phenobarbital, valproate and lamotrigine and tended to increase that of carbamazepine and phenytoin. The effect was more pronounced with the (-)-enantiomer. This increase was associated with an enhancement of motor impairment, however, the therapeutic index of combined treatment of the antiepileptic drugs with both propranolol enantiomers was more favourable than the combination with saline alone. Metoprolol was also able to decrease the ED(50) values of the antiepileptic drugs, whereas atenolol showed no effects. Since neither enantiomer of propranolol significantly influenced the total and free plasma levels of the antiepileptics, pharmacokinetic interactions are not likely. In addition, (+)- and (-)-propranolol did not significantly affect the hypothermic effects of the antiepileptics tested. In conclusion, both enantiomers of propranolol and metoprolol showed an additive anticonvulsant effect when co-administered with some conventional antiepileptic drugs, most notably diazepam, phenobarbital, lamotrigine and valproate, implicating a possible therapeutic relevance of such drug combinations.

Suppression of amygdala kindling with massed stimulation: effect of noradrenaline antagonists

Afterdischarge (AD) triggered by brief, daily stimulation of the amygdala progressively increases in complexity and duration and, over days, develops into generalized convulsions. This progression, called kindling, is delayed by noradrenaline (NA). When brief stimulation of the amygdala occurs too frequently (massed), there is a suppression of AD growth and little evidence of kindling. Previously we showed that depletion of NA before massed amygdala stimulation prevented the suppression of AD growth described above, and readily precipitated generalized seizures. In the present report, we examined the role of NA in maintaining this suppression of AD growth, after it was well established. We showed that suppression of AD development during the first 15 massed stimulations (interstimulus interval of 5 min) was reduced by subsequent injection of the NA alpha 2 antagonist, yohimbine, with most rats exhibiting occasional generalized convulsions. Conversely, rats exposed to the beta antagonist, propranolol, like controls, not only showed suppressed AD growth, but also elevated AD thresholds. Three weeks later, only a small positive transfer to daily kindling was observed in all groups. We conclude that alpha 2 NA receptors help maintain suppression of AD growth induced by massed stimulation of the amygdala, while beta receptors provide only a small proepileptic influence. These results and those from the 'rapid' kindling model (Lothman et al., Brain Research, 360 (1985) 83-91) are compared, and related to NA receptor subtype variations in the amygdala and hippocampus.

Effect of catecholaminergic drugs on quinolinate- and kynurenine-induced seizures in mice

Administration of reserpine, trifluperidol, chlorpromazine, haloperidol, spiroperidol, and thioproperazine to adult mice shortened the latency and increased the number of animals with clonic seizures induced by 1-kynurenine sulfate or its metabolite quinolinic acid. Haloperidol dose-dependently intensified kynurenine-induced seizures and did not alter pentylenetetrazole seizures. Dopamine abolished the effect of haloperidol while serotonin was ineffective. Pretreatment with 6-hydroxydopamine potentiated kynurenine-induced seizures, but not quinolinic acid-induced seizures. The seizure thresholds of kynurenine and quinolinic acid were not affected by pretreatments with yohimbine, clonidine, piperoxan, phentolamine and tricyclic antidepressants. Apomorphine and amphetamine (i.p.), noradrenaline and adrenaline (i.c.v.) possess anticonvulsant action against kynurenine and not against quinolinic acid. The data obtained suggest a similarity of kynurenine and known convulsants in the involvement of the catecholaminergic processes in their convulsant action. Quinolinic acid markedly differs from kynurenine in its mechanism of action as indicated by their interactions with numerous endogenous substances.

Suppression of pentylenetetrazol-elicited seizure activity by intraosseous propranolol in pigs

The intraosseous (IO) route provides a rapid and effective alternative venous access in the pediatric population when the conventional intravenous (IV) route cannot be easily obtained. DL-propranolol, a beta-adrenoceptor antagonist, exhibits antiepileptic activity in various animal seizure models. This study assessed the efficacy of IO propranolol in suppressing pentylenetetrazol (PTZ)-induced seizure activity in pigs. Domestic swine (13-20 kg) were prepared for recordings of arterial blood pressure, ECG and electrocortical activity. Seizure activity was induced by pentylenetetrazol (PTZ; 100 mg/kg; IV). Sixty seconds after the onset of seizure activity, the animals either received no drug (control) or propranolol (IV or IO via an 18-gauge spinal needle placed in the right proximal tibia). A transient increase (16.3-50.0%) in the mean arterial blood pressure (MAP) was observed following PTZ administration. Both IO and IV propranolol significantly suppressed the seizure duration (SD) (sec/min interval) at 1 min following drug administration; SD control, 36.3 +/- 4.8; IV propranolol, 12.3 +/- 5.1; IO propranolol, 18.3 +/- 6.0. In addition, both IV and IO propranolol produced a maximal decrease of 32-38% in the basal heart rate; and reduced the transient increase in MAP elicited by PTZ, with no significant effect on the basal MAP. The data demonstrate that 1) propranolol possesses anticonvulsant activity against PTZ-induced seizure in the pig, and 2) the intraosseous route is a rapid and effective alternative venous access for propranolol administration in swine.

Involvement of brain transmitters in the modulation of shock-induced aggression in rats by propranolol and related drugs

(+/-)Propranolol (1, 3, 10 and 30 mg/kg) exhibited a differential effect on footshock aggression (FSA) in rats. Lower doses (1 and 3 mg/kg) of the drug facilitated FSA, whereas an inhibitory effect was observed with higher doses (10 and 30 mg/kg) of the same. (+)Propranolol (30 mg/kg) and UM-272 (1 and 10 mg/kg) as well as physostigmine (0.1 and 0.5 mg/kg) all produced inhibition of FSA. Similar FSA inhibitory effects were also observed with salbutamol (1 and 5 mg/kg). Pretreatment with atropine and not methylatropine attenuated the anti-aggressive effect of (+/-)propranolol (10 mg/kg) without appreciably altering the facilitatory effect (1 mg/kg) of the drug on FSA. In addition, at the anti-aggressive doses, (+/-)propranolol (10 mg/kg) and UM-272 (10 mg/kg), significantly inhibited brain cholinesterase enzyme activity when compared to saline controls. (+/-)Propranolol (10 mg/kg) also inhibited significantly the aggression induced by reserpine-apomorphine treatment. It is inferred that a central cholinergic and dopaminergic mechanism is involved in the anti-aggressive effect of (+/-)propranolol, whereas the low dose induced facilitation of affective aggression could be attributed to central beta-adrenoceptor blockade.

A possible role for beta-adrenergic receptors in the expression of audiogenic seizures

DBA/2 mice are genetically prone to audiogenic seizures and, when compared with seizure resistant C57BL/6 mice, were found to have an increased density of beta-adrenergic receptors in their midbrain at the age of peak seizure susceptibility. Propranolol, a beta-receptor blocking agent, attenuated all stages of the seizure syndrome. However, a comparison of the effects of its d- and l-isomers suggested that propranolol's anticonvulsant activity was due to its local anesthetic-like action. Pindolol, a more potent beta blocker that is at least 100 times less potent than propranolol with respect to local anesthetic-like activity, produced anticonvulsant effects in approximately the same systemic dose range as propranolol. This indicates that pindolol's anticonvulsant activity could be due to beta blockade and, taken together, these data suggest that beta-adrenergic receptors may play a role in the expression of audiogenic seizures in these animals.

Effects of propranolol and a number of its analogues on sodium channels

To assess the relative contributions that the sodium channel blocking activity of propranolol may play in a variety of its therapeutic applications, its effects were examined in vitro with a sodium channel specific 22Na+ uptake system, using rat brain membranes. Propranolol inhibited 22Na+ uptake in the rat brain membrane preparation by acting as a competitive inhibitor of the binding of the sodium channel opening agent veratridine, with an IC50 for this action of 6.5 microM. This is approximately one order of magnitude higher in concentration than that necessary for expression of the beta-adrenergic antagonism of propranolol. The binding of propranolol and its action to block sodium channels were demonstrably different from those of the neurotoxins tetrodotoxin and saxitoxin. Propranolol had effects on sodium channels that are similar, although not identical to those of the local anesthetics procaine and lidocaine. The concentrations of propranolol and a number of its analogues which produced 50% inhibition of 22Na+ uptake (IC50 values ranging from 4 to greater than 100 microM) were similar to the concentrations of these same analogues which were required to produce negative inotropic and antiarrythmic effects (ED40) on isolated rabbit atria [D. O. Rauls and J. K. Baker, J. med. Chem, 22, 81 (1979)]. These effects showed correlations of 0.945 and 0.936, respectively, with the 22Na+ uptake inhibition. It is concluded from this information that a substantial proportion of the negative inotropic and antiarrythmic effects of propranolol is due to its action on sodium channels.

Role of central beta-adrenoceptors in the control of pentylenetetrazol-induced convulsions in rats

1 The role of central beta-adrenoceptors in the anticonvulsant effect of beta-adrenoceptor antagonists has been examined. 2 Oral administration of (-)- and (+)-propranolol (0.05-1 mg/kg) and (+/-)-pindolol (0.025-0.5 mg/kg) produced a dose-dependent decrease in duration of convulsions produced by pentylenetetrazol (PTZ 50 mg/kg, i.p.) in rats. 3 At the EC50 level, (-)-propranolol is seven times more effective than the (+)-isomer. 4 Oral administration of (-)-, (+)- or (+/-)-practolol (1-10 mg/kg) or (-)- or (+)-timolol (1-10 mg/kg), two beta-adrenoceptor antagonists that do not penetrate the blood brain barrier, had no significant effect on the duration of PTZ-induced convulsions. 5 Intracerebroventricular administration of (-)-propranolol (0.5 microgram/kg) or (-)-timolol (0.25 microgram/kg) produced highly significant anticonvulsant effects whereas the (+)-isomers at the same dose level were ineffective. (+/-)-Pindolol (0.25 microgram/kg) was also much more effective given by this route than when given orally. The (+)- and (-)-isomers of the beta 1-adrenoceptor selective antagonist practolol (10 microgram/kg) exerted only weak anticonvulsant effects. 6 This study provides evidence that beta-adrenoceptor antagonists exert an anticonvulsant effect through central beta 2-adrenoceptors. At high dose levels, additional anticonvulsant activity is associated with membrane stabilization in those antagonists which possess this property.

Physiological evidence (hypothermia) for central effects of beta-blocker agents

In recent studies carried out during the last decade, it has been found that beta-blocker agents have many central effects. These findings raise the question how beta-blocker agents act when they influence central effects. In spite of intensive research efforts on this subject, no answer has been offered so far. Several studies indicate that the effect of beta-blocker agents on the dopaminergic receptors in the brain is very similar to their effect on beta-adrenergic receptors. In a set of experiments we checked the effect of two beta-blocker agents: oxprenolol and dl-propranolol, and the effect of d-propranolol on hypothermia induced by d-amphetamine and apomorphine at an ambient temperature of 4 degrees C. It was found that the two beta-blockers and the d-propranolol significantly potentiated the effect of hypothermia induced by d-amphetamine and apomorphine, an effect which is mediated by the central dopaminergic system. However, the activity of the beta-blockers was agonistic. Furthermore, the influence of d-propranolol on hypothermia shows that this dopaminergic agonistic activity of the beta-blockers is not related to peripheral or central beta-receptor blockage.