Neuroprotective effects of carvedilol, a new antihypertensive, as a Na+ channel modulator and glutamate transport inhibitor

Effects of carvedilol on delayed rectifier and transient inactivating potassium currents in rat hippocampal CA1 neurons

1. The aims of the present study were to investigate the mechanism(s) underlying the protective effect of carvedilol against neural damage. 2. The transient inactivating potassium current (I(A) ) and the delayed rectifier potassium current (I(K) ) in rat hippocampal CA1 pyramidal neurons were recorded using whole-cell patch-clamp techniques. 3. Carvedilol (0.1-3 μmol/L) significantly inhibited I(K) with an IC(50) of 1.3 μmol/L and the inhibition was voltage independent. Over the same concentration range, carvedilol had no effect on the amplitude of I(A). At 1 μmol/L, carvedilol did not significantly change the steady state activation curves of I(A) and I(K), but did negatively shift their steady state inactivation curves. Recovery from inactivation was slowed for both I(A) and I(K). The inhibitory effect of carvedilol on I(K) was not affected by the adrenoceptor agonists phenylephrine and prazosin or the adrenoceptor antagonist isoproterenol, but propranolol was able to shift the dose-response curve of carvedilol for I(K) to the right. 4. Because I(K) is the main pathway for loss of intracellular potassium from depolarized neurons, selective obstruction of I(K) by carvedilol could be useful for neuroprotection.

Oxidative stress induced by the Fe/ascorbic acid system or model ischemia in vitro: effect of carvedilol and pyridoindole antioxidant SMe1EC2 in young and adult rat brain tissue

New effective strategies and new highly effective neuroprotective agents are being searched for the therapy of human stroke and cerebral ischemia. The compound SMe1EC2 is a new derivative of stobadine, with enhanced antioxidant properties compared to the maternal drug. Carvedilol, a non-selective beta-blocker, possesses besides its cardioprotective and vasculoprotective properties also an antioxidant effect. We compared the effect of carvedilol and SMe1EC2, antioxidants with a similar chemical structure, in two experimental models of oxidative stress in young and adult rat brain tissue. SMe1EC2 was found to improve the resistance of hippocampal neurons to ischemia in vitro in young and even in 18-month-old rats and inhibited formation of protein carbonyl groups induced by the Fe²⁺/ascorbic acid pro-oxidative system in brain cortex homogenates of young rats. Carvedilol exerted a protective effect only in the hippocampus of 2-month-old rats and that at the concentration 10-times higher than did SMe1EC2. The inhibitory effect of carvedilol on protein carbonyl formation induced by the pro-oxidative system was not proved in the cortex of either young or adult rats. An increased baseline level of the content of protein carbonyl groups in the adult versus young rat brain cortex confirmed age-related changes in neuronal tissue and may be due to increased production of reactive oxygen species and low antioxidant defense mechanisms in the adult rat brain. The results revealed the new pyridoindole SMe1EC2 to be more effective than carvedilol in neuroprotection of rat brain tissue in both experimental models involving oxidative stress.

Real-world algorithms for the optimal use of drugs and devices in the patient post myocardial infarction and the future of post myocardial infarction management

Patients with left ventricular dysfunction (LVD) are at increased risk for dying suddenly of cardiac causes. The most common causes of LVD are coronary artery disease (CAD) and myocardial infarction (MI). Aggressive intervention following MI is essential for minimizing the myocardial damage that leads to LVD and the subsequent risk for heart failure and sudden cardiac death. This article describes practical algorithms for managing the patient post MI to minimize such risks. The degree of LVD is a key factor for determining clinical management strategies in the patient post MI. Risk factor reduction and selective neurohormonal blockade, especially with angiotensin-converting enzyme inhibitors, are usually recommended in the presence or absence of LVD, along with early use of a beta blocker. In patients with LVD, more aggressive intervention includes extended use of a beta blocker. In cases of LVD progressed to heart failure, the mixed beta and alpha blocker carvedilol has improved outcomes significantly. In clinical trials, carvedilol has been demonstrated to have antiarrhythmic activity, a property that offers protection against sudden arrhythmic death in high-risk patients with LVD. Addition of an aldosterone antagonist is also advised in patients with heart failure. In selected patients with reduced ejection fractions, use of surgical/catheter treatment and device therapy offers further benefits.

Differential effects of short and prolonged exposure to carvedilol on voltage-dependent Na(+) channels in cultured bovine adrenal medullary cells

We examined the effects of short and prolonged exposure to carvedilol, an antihypertensive and beta-adrenoceptor blocking drug, on voltage-dependent Na(+) channels in cultured bovine adrenal medullary cells. Carvedilol (1-100 microM) reduced (22)Na(+) influx induced by veratridine, an activator of voltage-dependent Na(+) channels. Carvedilol also suppressed veratridine-induced (45)Ca(2+) influx and catecholamine secretion in a concentration-dependent manner similar to that of (22)Na(+) influx. Prolonged exposure of the cells to 10 microM carvedilol increased [(3)H]saxitoxin ([(3)H]STX) binding, which reached a plateau at 12 h and was still observed at 48 to 72 h. Scatchard analysis of [(3)H]STX binding revealed that carvedilol increased the B(max) value (control, 14.9 +/- 0.9 fmol/10(6) cells; carvedilol, 23.8 +/- 1.2 fmol/10(6) cells) (n = 3, P < 0.05) without altering the K(d) value, suggesting a rise in the number of cell surface Na(+) channels. The increase in [(3)H]STX binding by carvedilol was prevented by cycloheximide, an inhibitor of protein synthesis, whereas carvedilol changed neither alpha- nor beta(1)-subunit mRNA levels of Na(+) channels. The carvedilol-induced increase of [(3)H]STX binding was abolished by brefeldin A and H-89, inhibitors of intracellular vesicular trafficking of proteins from the trans-Golgi network and of cyclic AMP-dependent protein kinase (protein kinase A), respectively. The present findings suggest that short-term treatment with carvedilol reduces the activity of Na(+) channels, whereas prolonged exposure to carvedilol up-regulates cell surface Na(+) channels. This may add new pharmacological effects of carvedilol to our understanding in the treatment of heart failure and hypertension.

The novel beta-blocker, carvedilol, provides neuroprotection in transient focal stroke

Increasing evidence supports a role for oxidative stress, proinflammatory cytokines, and apoptosis in the pathophysiology of focal ischemic stroke. Previous studies have found that the multi-action drug, carvedilol, is a mixed adrenergic antagonist, and that it behaves as an antioxidant and inhibits apoptosis. In the current study, the authors investigated whether carvedilol provides protection in focal cerebral ischemia and whether this protection is associated with reduced apoptosis and the downregulation of the inflammatory cytokines, tumor necrosis factor-alpha (TNF-alpha) and interleukin- 1beta (IL-1beta). Male Sprague-Dawley rats were subjected to transient middle cerebral artery occlusion (MCAO) by an intraluminal filament technique. Carvedilol (1, 3, and 10 mg/kg) was injected daily subcutaneously 2 or 4 days before the induction of ischemia. Neurologic scores, infarct volumes, TUNEL staining, and mRNA levels of TNF-alpha and IL-1beta were assessed at 24 hours reperfusion. The effect of carvedilol on microvascular cortical perfusion was studied with continuous laser-Doppler flowmetry. Twenty-four hours after MCAO, carvedilol at all three doses reduced infarct volumes by at least 40% and reduced neurologic deficits on average by 40% compared with vehicle-treated controls when given 2 or 4 days before the induction of ischemia. This protection was not mediated by changes in temperature or blood flow. Treatment with all three dose regimens resulted in fewer TUNEL positive cells compared with controls. At 24 hours reperfusion, carvedilol decreased TNF-alpha and IL-1beta expression by 40% to 50% in the ipsilateral ischemic cortex compared with the contralateral controls. The results of the current study indicate that carvedilol is neuroprotective in focal cerebral ischemia and may protect the ischemic brain by inhibiting apoptosis and attenuating the expression of TNF-alpha and IL-1beta.

Neuroprotective activities of carvedilol and a hydroxylated derivative: role of membrane biophysical interactions

Carvedilol is a vasodilating beta-blocker and antioxidant approved for treatment of mild to moderate hypertension, angina, and congestive heart failure. SB 211475 (4-[2-hydroxyl-3-[[2-(2-methoxyphenoxy)ethyl]amino]propoxyl]-9H-++ +carbazol-3-ol), a hydroxylated carvedilol analogue, is an even more potent antioxidant in several assay systems. Carvedilol also has neuroprotective capacity with modulatory actions at N-methyl-D-aspartate (NMDA) receptors and Na+ channels. In the present study, we demonstrated that in cultured rat cerebellar neurons, SB 211475 has 28-fold greater antioxidant activity than carvedilol, but is 2- to 6-fold less potent, respectively, at inhibiting neurotoxic activities at Na+ channels and at NMDA receptor channels. To determine a biophysical rationale for these differential activities, small angle x-ray scattering data were obtained from model lipid and brain membrane bilayers containing either carvedilol, SB 211475, or dihydropyridine calcium channel blockers. Electron density profiles revealed that the location of SB 211475 was restricted to the glycerol backbone/hydrocarbon interface and significantly reduced membrane width by 5%, whereas the time-averaged location for carvedilol and flunarizine also extended to the hydrated surface of the bilayer. Comparison of carvedilol with several dihydropyridines showed a correlation between high ClogP values (lipophilicity), Na+ channel inhibitory potency, and bilayer localization. The antioxidant activity of SB 211475 could be explained by restricted intercalation into the glycerol phosphate/hydrocarbon interface, creating an increase in volume associated with the phospholipid acyl chains, which would then become resistant to lipid peroxidation. Differential channel modulation may also be explained by these membrane structural results, which indicate that carvedilol and the less spatially restricted dihydropyridine molecules are more likely to inhibit transmembrane receptor channels.

The neuroprotective drug vinpocetine prevents veratridine-induced [Na+]i and [Ca2+]i rise in synaptosomes

The effect of the neuroprotective drug, vinpocetine on the veratridine-evoked [Na+]i and [Ca2+]i rise in isolated nerve terminals was studied. Vinpocetine, in a pharmacologically relevant concentration range (0.4-10 microM)i reduced the increase of [Na+]i induced by veratridine (100 microM). The effect of the drug was concentration-dependent with 10 microM vinpocetine completely preventing the increase of [Na+]i. The [Ca2+]i rise in response to veratridine was also prevented by vinpocetine. In addition, the [Ca2+]i signal induced by depolarization with 20 mM K+ was reduced by vinpocetine (1-20 microM). This effect was not influenced by preincubation with 1 microM TTX and was also observed when Na+ was replaced by N-methyl glucamine in the medium. It is concluded that vinpocetine is capable of inhibiting voltage-dependent Na+ and Ca2+ channels, respectively, and these effects might contribute to the neuroprotection exerted by the drug.

Profile of SB-204269, a mechanistically novel anticonvulsant drug, in rat models of focal and generalized epileptic seizures

1. Earlier optimization of structure-activity relationships in a novel series of 4-(benzoylamino)-benzopyrans, led to the discovery of SB-204269 (trans-(+)-6-acetyl-4S-(4-fluorobenzoylamino)-3,4-dihydro-2, 2-dimethyl-2H-benzo[b]pyran-3R-ol, hemihydrate), a potent orally-active anticonvulsant in the mouse maximal electroshock seizure threshold (MEST) test. 2. Studies have now been undertaken to determine the effects of SB-204269 in a range of seizure models and tests of neurological deficits in rats. In addition, the compound has been evaluated in a series of in vitro mechanistic assays. 3. SB-204269 proved to be an orally-effective anticonvulsant agent, at doses (0.1-30 mg Kg-1) devoid of overt behavioural depressant properties, in models of both electrically (MEST and maximal electroshock (MEST)) and chemically (i.v. pentylenetetrazol (PTZ) infusion)-evoked tonic extension seizures. However, the compound did not inhibit PTZ-induced myoclonic seizures at doses up to 30 mg kg-1, p.o. 4. SB-204269 also selectively reduced focal electrographic seizure activity in an in vitro elevated K+ rat hippocampal slice model at concentrations (0.1-10 microM) that had no effect on normal synaptic activity and neuronal excitability. 5. In all of these seizure models, SB-204269 was equivalent or better than the clinically established antiepileptic drugs carbamazepine and lamotrigine, in terms of anticonvulsant potency and efficacy. 6. Unlike SB-204269, the corresponding trans 3S,4R enantiomer, SB-204268, did not produce marked anticonvulsant effects, an observation in accord with previous findings for other related pairs of trans enantiomers in the benzopyran series. 7. In the rat accelerating rotarod test, a sensitive paradigm for the detection of neurological deficits such as sedation and motor incoordination, SB-204269 was inactive even at doses as high as 200 mg kg-1, p.o. This was reflected in the excellent therapeutic index (minimum significantly effective dose in the rotarod test/ED50 in the MES test) for SB-204269 of > 31, as compared to equivalent values of only 7 and 13 for carbamazepine and lamotrigine, respectively. 8. At concentrations (> or = 10 microM) well above those required to produce anticonvulsant activity in vivo (i.e. 0.1 microM in brain), SB-204269 did not interact with many of the well known mechanistic targets for established antiepileptic drugs (e.g. Na+ channels or GABAergic neurotransmission). Subsequent studies have shown that the anticonvulsant properties of SB-204269 are likely to be mediated by a novel stereospecific binding site present in the CNS. 9. The overall efficacy profile in rodent seizure models, together with a minimal liability for inducing neurological impairment and an apparently unique mechanism of action, highlight the therapeutic potential of SB-204269 for the treatment of refractory partial and generalized tonic-clonic seizures.

Tumor necrosis factor-alpha. A mediator of focal ischemic brain injury

BACKGROUND AND PURPOSE

Tumor necrosis factor-alpha (TNF-alpha) is a pleiotropic cytokine that rapidly upregulates in the brain after injury. The present study was designed to explore the pathophysiological significance of brain TNF-alpha in the ischemic brain by systematically evaluating the effects of lateral cerebroventricular administration of exogenous TNF-alpha and agents that block the effects of TNF-alpha on focal stroke and by examining the potential direct toxic effects of TNF-alpha on cultured neurons to better understand how TNF-alpha might mediate stroke injury.

METHODS

TNF-alpha (2.5 or 25 pmol) was administered intracerebroventricularly to spontaneously hypertensive rats 24 hours before permanent or transient (80 minutes and 160 minutes) middle cerebral artery occlusion (MCAO). Animals were examined 24 hours later for neurological deficits and ischemic hemisphere necrosis and swelling. In some of these studies, neutralizing anti-TNF-alpha monoclonal antibody (mAb) (60 pmol) was injected intracerebroventricularly 30 minutes before exogenous TNF-alpha (25 pmol). In addition, the effects of blocking endogenous TNF-alpha on permanent focal ischemic injury were determined with the use of either mAb (60 pmol) or soluble TNF receptor I (sTNF-RI) (0.3 or 0.7 nmol) administered intracerebroventricularly 30 minutes before and 3 and 6 hours after MCAO. Finally, the direct neurotoxic effects of TNF-alpha were studied in cultured rat cerebellar granule cells exposed to TNF-alpha (10 to 2000 U/mL for 6 to 24 hours), and neurotransmitter release, glutamate toxicity, and oxygen radical toxicity were studied.

RESULTS

TNF-alpha increased the percent hemispheric infarct induced by permanent MCAO in a dose-related manner from 13.1 +/- 1.3% (vehicle) to 18.9 +/- 1.7% at 2.5 pmol (P < .05) and 27.1 +/- 1.3% at 25 pmol (P < .0001). The high dose of TNF-alpha increased ischemia-induced forelimb deficits from 1.6 +/- 0.2 to 2.3 +/- 0.2 (P < 0.1). TNF-alpha (2.5 pmol) also increased the infarction induced by 80 or 160 minutes of transient MCAO from 1.9 +/- 0.9% to 4.3 +/- 0.4% (P < .01) and from 14.2 +/- 1.3% to 21.6 +/- 2.2% (P < .05), respectively. The exacerbation of infarct size, swelling, and neurological deficit after exogenous TNF-alpha was reversed by preinjection of 60 pmol mAb. Blocking endogenous TNF-alpha also significantly reduced focal ischemic brain injury. Treatment with 60 pmol mAb before and after permanent MCAO significantly reduced infarct size compared with control (nonimmune) antibody treatment by 20.2% (P < .05). Reduced brain infarction also was produced by brain administration of 0.3 nmol (decreased 18.2%) or 0.7 nmol (decreased 26.1%, P < .05) sTNF-RI before and after focal stroke. The intracerebroventricular administration of TNF-alpha or sTNF-RI did not alter brain or body temperature, blood gases or pH, blood pressure, blood glucose, or general blood chemistry. In cultured cerebellar granule cells, the application of TNF-alpha did not directly affect neurotransmitter release or glutamate or oxygen free radical toxicity.

CONCLUSIONS

These studies demonstrate that exogenous TNF-alpha exacerbates focal ischemic injury and that blocking endogenous TNF-alpha is neuroprotective. The specificity of the action(s) of TNF-alpha was demonstrated by antagonism of its effects with specific anti-TNF-alpha tools (ie, mAb and sTNF-RI). TNF-alpha toxicity does not appear to be due to a direct effect on neurons or modulation of neuronal sensitivity to glutamate or oxygen radicals and apparently is mediated through nonneuronal cells. These data suggest that inhibiting TNF-alpha may represent a novel pharmacological strategy to treat ischemic stroke.

Na+ channels as targets for neuroprotective drugs

Drugs that block voltage-dependent Na+ channels are well known as local anaesthetics, antiarrhythmics and anticonvulsants. Recent studies show that these compounds also provide a powerful mechanism of cytoprotection in animal models of cerebral ischaemia, hypoxia or head trauma. In this article Charles Taylor and Brian Meldrum review evidence indicating that Na+ channel modulators are neuroprotective and describe recent ideas for the molecular sites of action of voltage-dependent Na+ channel blockers. Clinical trials with several compounds are now in progress for stroke and traumatic head injury, and the therapeutic potential for this group of compounds is discussed.

Neuroprotective effects of tetrodotoxin as a Na+ channel modulator and glutamate release inhibitor in cultured rat cerebellar neurons and in gerbil global brain ischemia

BACKGROUND AND PURPOSE

Studies examining the role of tetrodotoxin-sensitive ion channels in hypoxic-ischemic neuronal damage have concluded that sodium influx is an important initiating event. We examined the neuroprotectant effect of tetrodotoxin on both cultured cerebellar neurons and on CA1 hippocampal neurons of gerbils exposed to brain ischemia.

METHODS

We studied neuroprotective mechanisms using cultured rat cerebellar granule cells exposed to veratridine, which induced cytotoxicity, neurotransmitter release, and calcium influx. Survival of gerbil CA1 neurons was examined by direct neuron counts 7 days after 6 minutes of global ischemia with reperfusion.

RESULTS

Tetrodotoxin protected cultured neurons in a dose-dependent manner from veratridine-induced toxicity (protective concentration [PC50] = 22 nmol/L). Veratridine induced [3H]aspartate efflux that was sodium dependent, only 25% calcium dependent, and was inhibited by tetrodotoxin (inhibitory concentration [IC50] = 60 nmol/L). Veratridine initiated increases in intracellular calcium that were also reversed by tetrodotoxin (IC50 = 63 nmol/L); reversal was dependent on the sodium-calcium exchanger and the sodium-potassium pump. Neuroprotection of 90% (n = 10; P = .001 versus vehicle) of gerbil CA1 hippocampal neurons was achieved by pretreatment with 2 ng of tetrodotoxin delivered three times intracerebroventricularly, without causing hypothermia.

CONCLUSIONS

Sodium channel blockers like tetrodotoxin may have utility in treatment of ischemic neuronal injury by preventing excessive neuronal depolarizations, limiting excitotoxic glutamate release through reversal of the sodium-dependent glutamate transporter, preventing intracellular calcium overload, preserving cellular energy stores, and allowing recovery of ionic homeostasis through operation of the sodium-calcium exchanger.