Brain ischemia as a target for Ca2+ entry blockers

Physiologically relevant online electrochemical method for continuous and simultaneous monitoring of striatum glucose and lactate following global cerebral ischemia/reperfusion

This study demonstrates a new electroanalytical method with a high physiological relevance for simultaneous online monitoring of glucose and lactate in the striatum of the rat brain following global cerebral ischemia/reperfusion. The online analytical method is based on the efficient integration of in vivo microdialysis sampling with an online selective electrochemical detection with the electrochemical biosensors with dehydrogenases, i.e., glucose and lactate dehydrogenases, as recognition elements. The dehydrogenase-based electrochemical biosensors are developed onto the dual split-disk plastic carbon film (SPCF) electrodes with methylene green (MG) adsorbed onto single-walled carbon nanotubes (SWNTs) as the electrocatalyst for the oxidation of dihydronicotiamide adenine dinucleotide (NADH) at a low potential of 0.0 V (vs Ag/AgCl). Artificial cerebrospinal fluid (aCSF) containing NAD(+) is externally perfused from a second pump and online mixed with the brain microdialysates to minimize the variation of pH that occurred following the cerebral ischemia/reperfusion and to supply NAD(+) cofactor and O(2) for the enzymatic reactions of dehydrogenases and ascorbate oxidase, respectively. As a result, the developed online electroanalytical method exhibits a high selectivity against the electrochemically active species endogenously existing in the cerebral systems and a high tolerance against the variation of pH and O(2) following cerebral ischemia/reperfusion. This property, along with the good linearity and a high stability toward glucose and lactate as well as little cross-talk between two biosensors, substantially makes this method possible for the continuous, simultaneous, and online monitoring of glucose and lactate in the rat brain following global cerebral ischemia/reperfusion. This study establishes a new and effective platform for the investigation of the energy metabolism in physiological and pathological processes.

Neuroprotective effect on brain injury by neurotoxins from the spider Phoneutria nigriventer

The role of calcium channels blockers in ischemic condition has been well documented. The PhTx3 neurotoxic fraction of the spider Phoneutria nigriventer venom is a broad-spectrum calcium channel blocker that inhibits glutamate release, calcium uptake and also glutamate uptake in synaptosomes. In the present study we describe the effect of PhTx3 (1.0 microg/mL), omega-conotoxin GVIA (1.0 micromol/L) and omega-conotoxin MVIIC (100 nmol/L) on neuroprotection of hippocampal slices and SN56 cells subjected to ischemia by oxygen deprivation and low glucose insult (ODLG). After the insult, cell viability in the slices and SN56 cells was assessed by confocal microscopy and epifluorescence, using live/dead kit containing calcein-AM and ethidium homodimer. Confocal images of CA1 region of the rat hippocampal slices subjected to ischemia insult and treated with omega-conotoxin GVIA, omega-conotoxin MVIIC and PhTx3 showed a percentage of dead cells of 68%, 54% and 18%, respectively. The SN56 cells subjected to ischemia were almost completely protected from damage by PhTx3 while with omega-conotoxin GVIA or omega-conotoxin MVIIC the cell protection was only partial. Thus, PhTx3 provided robust ischemic neuroprotection showing potential as a novel class of agents that targets multiple components and exerts neuroprotection in in vitro model of brain ischemia.

5-aminocoumarans: dual inhibitors of lipid peroxidation and dopamine release with protective effects against central nervous system trauma and ischemia

A series of 2,3-dihydro-5-benzofuranamines (5-aminocoumarans) were developed for the treatment of traumatic and ischemic central nervous system (CNS) injury. Compounds within this class were extremely effective inhibitors of lipid peroxidation in vitro and antagonized excitatory behavior coupled with peroxidative injury induced by spinal intrathecal injection of FeCl2 (mouse-FeCl2-it assay) in vivo. Selected compounds were tested for antagonistic activity on methamphetamine (MAP)-induced hypermotility resulting from dopamine release in the mouse brain. Among the compounds synthesized, compound 26n (2,3-dihydro-2,4,6,7-tetramethyl-2-[(4-phenyl-1-piperidinyl) methyl]-5-benzofuranamine) exhibited potent effects in these assays (inhibition of lipid peroxidation, IC50 = 0.07 microM; mouse-FeCl2-it assay, ID50 = 10.4 mg/ kg, po; MAP-induced hypermotility, 98% inhibition, 10 mg/kg, ip). The S-(+)-form of compound 26n dihydrochloride (TAK-218), which has 30 times more potent antagonistic activity on MAP-induced hypermotility than the R-(-)-form, improved more significantly the survival rate in the cerebral ischemia model (rat, 1-3 mg/kg, ip) during the period of 1-14 days after ischemia and decreased functional disorders in the traumatic brain injury model (rat, 0.1-1 mg/kg, ip) 3-14 days after injury. These results imply a role for dopamine in deterioration of CNS function after ischemic and traumatic injury. TAK-218 is a promising compound for the treatment of stroke and CNS trauma and is now under clinical investigation.

Selective blockade of N-type voltage-sensitive calcium channels protects against brain injury after transient focal cerebral ischemia in rats

The neuroprotective efficacy of the selective N-type voltage-sensitive calcium channel blocker, SNX-111, was evaluated in spontaneously hypertensive rats subjected to 60 min of focal cerebral ischemia by permanent ligation of the right common carotid artery and temporary occlusion of the right middle cerebral artery. Intravenous infusion of 167 microg/kg per min SNX-111 for 30 min (5 mg/kg), initiated immediately after reperfusion, significantly reduced cortical infarct volumes measured 24 h after the ischemic insult.

Neuroprotective properties of lifarizine compared with those of other agents in a mouse model of focal cerebral ischaemia

1. Changes in the peripheral type benzodiazepine binding site density following middle cerebral artery occlusion in the mouse, have been used as a marker of neuronal damage. These sites can be identified using the selective ligand [3H]-PK 11195 located on non neuronal cells, macrophages and astroglia, within the CNS. Glial cell proliferation and macrophage invasion is an unvoidable sequelae to cerebral ischaemic injury, secondary to neuronal loss. Following occlusion of the left middle cerebral artery (left MCA) a reproducible lesion was found in the parietal cortex within 7 days which gave rise to a significant increase in [3H]-PK 11195 binding. 2. Treatment of animals with the sodium channel blocker, lifarizine, significantly reduced the ischaemia-induced increase in [3H]-PK 11195 binding when given either 30 min pre-ischaemia and three times daily for 7 days at 0.5 mg kg-1, i.p. (P < 0.01) or delayed until 15 min post-ischaemia and three times daily for 7 days at 0.5 mg kg-1, i.p. (P < 0.001). Lifarizine was an effective neuroprotective agent in this model of focal ischaemia in the mouse. 3. Lifarizine also showed a dose-related protection against the ischaemia-induced increase in [3H]-PK 11195 binding with significant protection at doses of 0.1 mg kg-1, i.p. (P < 0.05), 0.25 mg kg-1, i.p. (P < 0.01) or 0.5 mg kg-1, i.p. (P < 0.01) 15 min post-ischaemia and b.i.d. for 7 days. No significant change is seen in the Kd for [3H]-PK 11195. The first dose could be delayed for up to 4 h after cerebralartery cauterization and protection was maintained.4. Phenytoin (28 mg kg-1, i.v. 15 min and 24 h post-ischaemia) was also neuroprotective in this model(P<0.01). This agent is thought to interact with voltage-dependent sodium channels to effect its anticonvulsantactions and this mechanism may also underlie its neuroprotective actions in focal cerebralischaemia.5. Agents with other mechanisms of action were also shown to have significant neuroprotection in this model. The non-competitive NMDA antagonist, MK 801, showed significant neuroprotection in the model when given at 0.5 mg kg-1, i.p. 30 min pre-ischaemia with t.i.d. dosing for 7 days (P< 0.001). The dihydropyridine calcium antagonist, nimodipine was not protective when given using the same dosing protocol as MK 801, 0.5 mg kg-1 30 min pre-occlusion and three times daily for 7 days but showed significant protection when given at 0.05 mg kg-1 15 min post-ischaemia and three times daily for 7days. The lipid peroxidation inhibitor, tirilazad (single dose 1 mg kg-1, i.v.) showed significant neuroprotection when given 5 min post-ischaemia but not when the first dose was delayed for 4 h.

New anticonvulsant drugs. Focus on flunarizine, fosphenytoin, midazolam and stiripentol

In the past decade, several new antiepileptic drugs have been tested. Most recently, 5 new antiepileptic drugs have been launched onto European and US markets. These include vigabatrin, oxcarbazepine and lamotrigine in Europe, and felbamate and gabapentin in the US. In addition to these, 3 additional drugs are in the clinical investigational stage: flunarizine, fosphenytoin and stiripentol. A fourth agent is midazolam, which was originally introduced in 1986, but recently has shown effectiveness in the treatment of status epilepticus. Flunarizine is a selective calcium channel blocker that has shown anticonvulsant properties in both animal and human studies. It is a long-acting anticonvulsant that clinical studies have shown to have effects similar to those of phenytoin and carbamazepine in the treatment of partial, complex partial and generalised seizures. Fosphenytoin was developed to eliminate the poor aqueous solubility and irritant properties of intravenous phenytoin. It is rapidly converted to phenytoin after intravenous or intramuscular administration. In clinical studies, this prodrug showed minimal evidence of adverse events and no serious cardiovascular or respiratory adverse reactions. It may have a clear advantage over the present parenteral formulation of phenytoin. Midazolam is a benzodiazepine that is more potent than diazepam as a sedative, muscle relaxant and in its influence on electroencephalographic measures. It has been shown to be an effective treatment for refractory seizures in status epilepticus. Stiripentol has anticonvulsant properties as well as the ability to inhibit the cytochrome P450 system. There are significant metabolic drug interactions between stiripentol and phenytoin, carbamazepine and phenobarbital (phenobarbitone). Stiripentol has been studied in patients with partial seizures, refractory epilepsy and refractory absence seizures with some efficacious results.

The ability of diphenylpiperazines to prevent neuronal death in dorsal root ganglion neurons in vitro after nerve growth factor deprivation and in vivo after axotomy

The mechanism of neuroprotection by the calcium channel antagonist flunarizine against neuronal death is unknown. We investigated the ability of other calcium channel antagonists (cinnarizine, nimodipine, nicardipine, diltiazem, and verapamil), calmodulin antagonists, and calpain inhibitors to prevent neuronal death in rat dorsal root ganglion neurons in vitro after nerve growth factor (NGF) deprivation and the ability of cinnarizine and diltiazem to protect in vivo after axotomy. In vitro, only neurons treated with cinnarizine or flunarizine were protected from death after withdrawal. In vivo, cinnarizine, but not diltiazem, protected dorsal root ganglion neurons in rats after unilateral sciatic nerve crush. Intracellular calcium concentration ([Ca2+]i) was evaluated with fura 2 after NGF deprivation in vitro. Neurons "committed to die" 24 h after NGF deprivation displayed a decline in [Ca2+]i before visible morphological deterioration consistent with cell death. The influx of extracellular calcium was not necessary to produce neuronal death. Neurons deprived of NGF gradually lost the ability to respond to elevated external potassium with an increase in [Ca2+]i during the first 24 h after trophic factor deprivation. After 24 h, neurons deprived of NGF could not be rescued by readministration of NGF. Neurons protected from cell death with diphenylpiperazines maintained their response to high external potassium, suggesting continued membrane integrity. We speculate that diphenylpiperazines may protect sensory neurons via an unknown mechanism that stabilizes cell membranes.

Failure of nimodipine to prevent brain damage in a global brain ischemia model in the rat

In view of our negative results with the calcium antagonist nimodipine as a cerebroprotective agent in a cardiopulmonary resuscitation model in the rat, we examined the protective effects of nimodipine in the four-vessel (carotid and vertebral) occlusion model, a model of global brain ischemia without important cardiovascular depression. Survival and neurological status were monitored and after 72 h the hippocampus was resected and examined for histological evaluation. The animals were treated blindly and randomly with either nimodipine, its solvent or saline given subcutaneously. In two separate studies, high doses (total dose: 5 mg/kg) or low doses of nimodipine (total dose: 1.6 mg/kg) were administered. In the high dose series, the survival rates at 72 h in the nimodipine group, the saline group and the solvent group were 4% (2/44), 19% (7/37) and 20% (8/41), respectively; in the low dose series, the figures were 26% (13/50), 34% (15/44) and 39% (18/46), respectively. The differences between nimodipine, solvent and saline were not statistically significant. Likewise, no differences in neurological status or histological brain damage were found. These data suggest that nimodipine offers no cerebral protection in global brain ischemia and may even be toxic, especially when given in high doses.

Nimodipine decreases resuscitability in a cardiopulmonary arrest model in the rat

Although calcium has been implicated in ischemia-induced brain death or dysfunction, many animal studies do not show a beneficial effect of calcium-entry blockers given after resuscitation from a cardiopulmonary arrest (CPA). This may be due to the fact that treatment was started too late; we, therefore, evaluated the effect of the calcium-entry blocker nimodipine administered at the earliest feasible postischemic moment, i.e. at the start of the resuscitation attempts. In anesthetized Wistar rats, CPA was induced by an intra-cardiac injection of KCl, and maintained for 7 min by chest restriction. At the start of the resuscitation attempts, 50 rats were blindly and randomly assigned to intravenous treatment with either nimodipine (10 micrograms/kg over 2 min, followed by 1 micrograms/kg per min for 60 min; n = 25) or saline (n = 25). In the nimodipine group, significantly less rats could be resuscitated (11/25 versus 20/25) and the survival rate at the end of the 7 days evaluation period tended to be lower (5/25 versus 11/25). In the rats surviving after 7 days, there was no difference between both groups in incidence of seizures, neurological status and histological lesions in the hippocampus. It is concluded that nimodipine, in the dose tested and given during resuscitation in this rat model, has a detrimental effect on resuscitability and no beneficial effect on the neurological outcome in the surviving animals.

Extracellular ions during veratridine-induced neurotoxicity in hippocampal slices: neuroprotective effects of flunarizine and tetrodotoxin

Veratridine, by blocking Na+ channel inactivation and shifting activation to more negative membrane potentials, causes Na(+)-influx and a persistent tendency for depolarization. Veratridine is neurotoxic to cultured neurones, and this neurotoxicity can be blocked by the class IV calcium antagonist, flunarizine. We were interested to know whether similar effects could be found in a functional differentiated tissue containing adult neurones and glial cells. We examined this in hippocampal slices using extracellular potential recordings and ion-selective microelectrodes sensitive to [Na+]o, [Ca2+]o and [K+]o. Veratridine blocked synaptic transmission in CA1, and induced several episodes of spreading depression (SD). This was followed by a long-lasting increase in [K+]o and a continuous decrease in [Ca+]o. Following veratridine exposure to hypoxia only revealed a small negative DC shift and small shifts in extracellular ions; indicating that the cells had lost the ability to maintain ion homeostasis before the hypoxia, and that veratridine had been neurotoxic. In hippocampal slices obtained from guinea pigs which had been pretreated with 40 mg/kg x 2 flunarizine orally the time before the first SD induced by veratridine was doubled. Although the ion shifts during the first SD were similar to controls, flunarizine reduced the time of recovery of [Ca2+]o, [K+]o and DC potential. The increase in [K+]o baseline and the massive decrease in [Ca2+]o baseline seen following the SDs in the solvent group were smaller in the flunarizine-treated slices. During the subsequent hypoxic period the negative DC shift was 8x larger in the flunarizine group, and the shifts in [K+]o, [Na+]o and [Ca2+]o were bigger. Tetrodotoxin also delayed the first SD during veratridine and increased the size of the DC shift during the subsequent hypoxic period. Both flunarizine and tetrodotoxin therefore protected adult brain tissue containing glia from the neurotoxicity of veratridine. These findings suggest that persistent Na(+)-influx and the consequent Ca2(+)-influx produce neurotoxicity, and that the ability to attenuate this neurotoxicity may be important in the mechanism of action of cerebroprotective drugs from different pharmacological classes.

Activators and inactivators of calcium channels: effects in the central nervous system

The interactions of calcium antagonists or channel activators with the different classes of calcium channel are reviewed with particular emphasis on interactions with neuronal tissue; reasons for the failure of calcium antagonists to inhibit neurotransmitter release under normal circumstances are outlined. Calcium antagonists may be protective in several pathological situations and the possibilities of protection against ischaemic damage in the central nervous system are evaluated.

Flunarizine. A reappraisal of its pharmacological properties and therapeutic use in neurological disorders

Flunarizine is a class IV calcium antagonist with a pharmacological profile which suggests its therapeutic potential in a number of neurological and cerebrovascular disorders. It is an effective prophylactic treatment for common or classic migraine in children and adults, and it appears at least as effective as a number of other agents which act by different pharmacological mechanisms, including pizotifen (pizotyline), cinnarizine, methysergide, nimodipine, metoprolol, propranolol, aspirin and cyclandelate. Flunarizine is also effective in reducing the frequency of seizures, when used as an 'add-on' treatment, in some patients with partial or generalised epilepsy resistant to maximal therapy with a combination of several conventional antiepileptic drugs. Placebo-controlled studies show that flunarizine is effective in the treatment of vertigo and associated symptoms of either peripheral or central origin, and in the treatment of cerebrovascular insufficiency where psychological symptoms, rather than vertigo, are the primary symptoms. In the treatment of vertigo, flunarizine appears at least as effective as cinnarizine and more effective than nicergoline, betahistine dichlorhydrate, pentoxifylline (oxpentifylline) and vincamine. Flunarizine therefore is useful in the prophylaxis of migraine, an effective treatment for vertigo and a worthwhile alternative as 'add-on' therapy in patients with epilepsy resistant to conventional drugs.

The effect of calcium entry blocker S-emopamil on cerebrocortical metabolism and blood flow changes evoked by graded hypotension

Our studies demonstrate that in the explanation of the protective effect of the calcium antagonist (S)-emopamil the possibility of beneficial metabolic causes (lower O2 consumption) must also be considered beside of blood flow increasing effects. It is suggested that (S)-emopamil may be a useful drug for the treatment of cerebro-ischemic disorders.

Calcium antagonism and vascular smooth muscle

Vasoconstriction results from an exaggerated increase of intracellular Ca2+ concentration which initiates the contractile process within the vascular smooth muscle. The dependency of these cells on extracellular Ca2+ to trigger the contractile process when exposed to naturally occurring vasoactive substances such as those released from aggregating blood platelets varies in different vascular areas. This is one of the factors that determine the different sensitivity to the inhibitory effect of various calcium antagonist. A blood vessel can be more reactive to some calcium antagonists than to others, depending on the vascular area. Experiments on isolated cerebral arteries suggest that inhibition of cerebral vasoconstriction is observed with substances such as flunarizine under conditions of vascular hyperresponsiveness generated by acute or chronic pathological conditions or triggered by interaction between vasoactive substances. In this regard marked differences exist between the individual calcium antagonists. Those that are selective for slow Ca2+ channels will inhibit myocardial contractile force and decrease vascular myogenic activity (e.g., at the arteriolar level). Such inhibitory activity is not observed with flunarizine, which affects Ca2+ entry rather selectively, when calcium overload is imposed upon the vasculature, in particular at cerebrovascular sites. This suggests a potential use of this compound in a number of neurological disorders related to cerebral ischemia.