Drugs acting on calcium channels: potential treatment for ischaemic stroke

Efonidipine Exerts Cerebroprotective Effect by Down-regulation of TGF-β/SMAD-2-Dependent Signaling Pathway in Diabetic Rats

Calcium overload and hyperglycemia are risks of stroke onset in diabetics. Our study was designed to elucidate the beneficial role of calcium channel blockers by targeting voltage-gated calcium channels in diabetes-associated cerebrovascular complications. Diabetes was induced using the neonatal streptozotocin rat model. After confirmation of diabetes, middle cerebral artery occlusion (MCAO) was carried out. The pre-treatment with 1 mg/kg/day efonidipine was administered for the period of 4 weeks. After 24 h of ischemic induction surgery, the neurological score was determined, and blood was collected for determination of biochemical parameters. Treatment with efonidipine showed a significant reduction in post-ischemic brain infract volume, brain hemisphere weight difference, neurological score, Na⁺-K⁺ ATPase activity, serum CK-MB, and LDH levels in normoglycemic and hyperglycemic MCAO-induced animals. While no significant changes in glucose and lipid levels were observed by treatment, efonidipine significantly decreased the levels of malondialdehyde, acetylcholine esterase, and nitrite levels and increased the levels of antioxidant markers in both normoglycemic and hyperglycemic MCAO animals. TGF-β and VEGF were found to be down-regulated after treatment with efonidipine in gene expression study. In conclusion, the study data supports the cerebroprotective role of efonidipine in diabetic animals possibly through TGF-β/SMAD-2 signaling pathway.

Discovery and Development of Calcium Channel Blockers

In the mid 1960s, experimental work on molecules under screening as coronary dilators allowed the discovery of the mechanism of calcium entry blockade by drugs later named calcium channel blockers. This paper summarizes scientific research on these small molecules interacting directly with L-type voltage-operated calcium channels. It also reports on experimental approaches translated into understanding of their therapeutic actions. The importance of calcium in muscle contraction was discovered by Sidney Ringer who reported this fact in 1883. Interest in the intracellular role of calcium arose 60 years later out of Kamada (Japan) and Heibrunn (USA) experiments in the early 1940s. Studies on pharmacology of calcium function were initiated in the mid 1960s and their therapeutic applications globally occurred in the the 1980s. The first part of this report deals with basic pharmacology in the cardiovascular system particularly in isolated arteries. In the section entitled from calcium antagonists to calcium channel blockers, it is recalled that drugs of a series of diphenylpiperazines screened in vivo on coronary bed precontracted by angiotensin were initially named calcium antagonists on the basis of their effect in depolarized arteries contracted by calcium. Studies on arteries contracted by catecholamines showed that the vasorelaxation resulted from blockade of calcium entry. Radiochemical and electrophysiological studies performed with dihydropyridines allowed their cellular targets to be identified with L-type voltage-operated calcium channels. The modulated receptor theory helped the understanding of their variation in affinity dependent on arterial cell membrane potential and promoted the terminology calcium channel blocker (CCB) of which the various chemical families are introduced in the paper. In the section entitled tissue selectivity of CCBs, it is shown that characteristics of the drug, properties of the tissue, and of the stimuli are important factors of their action. The high sensitivity of hypertensive animals is explained by the partial depolarization of their arteries. It is noted that they are arteriolar dilators and that they cannot be simply considered as vasodilators. The second part of this report provides key information about clinical usefulness of CCBs. A section is devoted to the controversy on their safety closed by the Allhat trial (2002). Sections are dedicated to their effect in cardiac ischemia, in cardiac arrhythmias, in atherosclerosis, in hypertension, and its complications. CCBs appear as the most commonly used for the treatment of cardiovascular diseases. As far as hypertension is concerned, globally the prevalence in adults aged 25 years and over was around 40% in 2008. Usefulness of CCBs is discussed on the basis of large clinical trials. At therapeutic dosage, they reduce the elevated blood pressure of hypertensive patients but don't change blood pressure of normotensive subjects, as was observed in animals. Those active on both L- and T-type channels are efficient in nephropathy. Alteration of cognitive function is a complication of hypertension recognized nowadays as eventually leading to dementia. This question is discussed together with the efficacy of CCBs in cognitive pathology. In the section entitled beyond the cardiovascular system, CCBs actions in migraine, neuropathic pain, and subarachnoid hemorrhage are reported. The final conclusions refer to long-term effects discovered in experimental animals that have not yet been clearly reported as being important in human pharmacotherapy.

The role of glutamate in neuronal ischemic injury: the role of spark in fire

Although being a physiologically important excitatory neurotransmitter, glutamate plays a pivotal role in various neurological disorders including ischemic neurological diseases. Its level is increased during cerebral ischemia with excessive neurological stimulation causing the glutamate-induced neuronal toxicity, excitotoxicity, and this is considered the triggering spark in the ischemic neuronal damage. The glutamatergic stimulation will lead to rise in the intracellular sodium and calcium, and the elevated intracellular calcium will lead to mitochondrial dysfunction, activation of proteases, accumulation of reactive oxygen species and release of nitric oxide. Interruption of the cascades of glutamate-induced cell death during ischemia may provide a way to prevent, or at least reduce, the ischemic damage. Various therapeutic options are suggested interrupting the glutamatergic pathways, e.g., inhibiting the glutamate synthesis or release, increasing its clearance, blocking of its receptors or preventing the rise in intracellular calcium. Development of these strategies may provide future treatment options in the management of ischemic stroke.

Neuroprotective effects by nimodipine treatment in the experimental global ischemic rat model : real time estimation of glutamate

OBJECTIVE

Glutamate is a key excitatory neurotransmitter in the brain, and its excessive release plays a key role in the development of neuronal injury. In order to define the effect of nimodipine on glutamate release, we monitored extracellular glutamate release in real-time in a global ischemia rat model with eleven vessel occlusion.

METHODS

TWELVE RATS WERE RANDOMLY DIVIDED INTO TWO GROUPS: the ischemia group and the nimodipine treatment group. The changes of extracellular glutamate level were measured using microdialysis amperometric biosensor, in coincident with cerebral blood flow (CBF) and electroencephalogram. Nimodipine (0.025 µg/100 gm/min) was infused into lateral to the CBF probe, during the ischemic period. Also, we performed Nissl staining method to assess the neuroprotective effect of nimodipine.

RESULTS

During the ischemic period, the mean maximum change in glutamate concentration was 133.22±2.57 µM in the ischemia group and 75.42±4.22 µM (p<0.001) in the group treated with nimodipine. The total amount of glutamate released was significantly different (p<0.001) between groups during the ischemic period. The %cell viability in hippocampus was 47.50±5.64 (p<0.005) in ischemia group, compared with sham group. But, the %cell viability in nimodipine treatment group was 95.46±6.60 in hippocampus (p<0.005).

CONCLUSION

From the real-time monitoring and Nissl staining results, we suggest that the nimodipine treatment is responsible for the protection of the neuronal cell death through the suppression of extracellular glutamate release in the 11-VO global ischemia model of rat.

Molecular pharmacology of high voltage-activated calcium channels

Voltage-gated calcium channels are key sources of calcium entry into the cytosol of many excitable tissues. A number of different types of calcium channels have been identified and shown to mediate specialized cellular functions. Because of their fundamental nature, they are important targets for therapeutic intervention in disorders such as hypertension, pain, stroke, and epilepsy. Calcium channel antagonists fall into one of the following three groups: small inorganic ions, large peptide blockers, and small organic molecules. Inorganic ions nonselectively inhibit calcium entry by physical pore occlusion and are of little therapeutic value. Calcium-channel-blocking peptides isolated from various predatory animals such as spiders and cone snails are often highly selective blockers of individual types of calcium channels, either by preventing calcium flux through the pore or by antagonizing channel activation. There are many structure-activity-relation classes of small organic molecules that interact with various sites on the calcium channel protein, with actions ranging from selective high affinity block to relatively nondiscriminatory action on multiple calcium channel isoforms. Detailed interactions with the calcium channel protein are well understood for the dihydropyridine and phenylalkylamine drug classes, whereas we are only beginning to understand the molecular actions of some of the more recently discovered calcium channel blockers. Here, we provide a comprehensive review of pharmacology of high voltage-activated calcium channels.

Effects of glutamate receptor agonist on extracellular glutamate dynamics during moderate cerebral ischemia

We performed real-time monitoring of the extracellular glutamate dynamics in the rat striatum in vivo using the microdialysis electrode technique, during an experimental penumbral condition of moderate global cerebral ischemia and activated glutamate receptors. The local cerebral blood flow (CBF) was measured with a laser-Doppler probe. One minute after bilateral common carotid artery occlusion (BCAO), CBF was reduced to approximately 60% of the pre-ischemic value and it remained at this level during the period of occlusion. After BCAO, a transient depolarization and a transient increase in extracellular glutamate concentration ([Glu]e) were seen. In other rats, 500 microM N-methyl-D-aspartate (NMDA) was locally micro-transfused for 30 min prior to BCAO. Upon induction of BCAO, an anoxic depolarization-like depolarization and a gradual increase in [Glu]e that continued over the duration of BCAO were seen. After BCAO was terminated, the direct current (DC) rapidly recovered to the basal level, while [Glu]e gradually decreased to the basal level. In rats that were locally micro-transfused with 500 microM Kainate prior to BCAO, DC and [Glu]e did not differ significantly from control. Pretreatment with MK-801 prior to NMDA treatment completely inhibited the NMDA-induced changes in DC and [Glu]e. Pretreatment with NBQX prior to NMDA treatment did not inhibit the NMDA-induced changes in DC and [Glu]e. Consequently, we found that activation of NMDA receptors by elevated [Glu]e exerts an important effect on [Glu]e dynamics in the spreading stroke region very early in the acute stage of cerebral ischemia in vivo.

LY393615, a novel neuronal Ca(2+) and Na(+) channel blocker with neuroprotective effects in models of in vitro and in vivo cerebral ischemia

In the present studies we have examined the effects of a new calcium channel blocker, LY393615 ((N-Butyl-[5,5-bis-(4-fluorophenyl)tetrahydrofuran-2-yl]methylamine hydrochloride, NCC1048) in a model of hypoxia-hypoglycaemia in vitro and in a gerbil model of global and in two rat models of focal cerebral ischaemia in vivo. Results indicated that LY393615 protected against hypoxia-hypoglycaemic insults in brain slices and also provided significant protection against ischaemia-induced hippocampal damage in gerbil global cerebral ischaemia when dosed at 10, 12.5 (P<0.05) or 15 mg/kg i.p. (P<0.01) 30 min before and 2 h 30 min after occlusion. The compound penetrated the brain well after a 15 mg/kg i.p. dose and had a half-life of 2.5 h. In further studies LY393615 was protective 1 h post-occlusion when administered at 15 mg/kg i.p. followed by 2 doses of 5 mg/kg i.p. 2 and 3 h later. LY393615 dosed at 15 mg/kg i.p. followed by 2 further doses of 5 mg/kg i.p. (2 and 3 h later) also produced a significant reduction in the infarct volume following Endothelin-1 (Et-1) middle cerebral artery occlusion in the rat when administration was initiated immediately (P<0.01) or 1 h (P<0.05) after occlusion. The compound was also evaluated in the intraluminal monofilament model of focal ischaemia. The animals had the middle cerebral artery occluded for 2 h, and 15 min after reperfusion LY393615 was administered at 15 mg/kg i.p. followed by 2 mg/kg/h i.v. infusion for 6 h. There was no reduction in infarct volume using this dosing protocol. In conclusion, in the present studies we have reported that a novel calcium channel blocker, LY393615, with good bioavailability protects against neuronal damage caused by hypoxia-hypoglycaemia in vitro and both global and focal cerebral ischaemia in vivo. The compound is neuroprotective when administered post-occlusion and may therefore be a useful anti-ischaemic agent.

The effects of LY393613, nimodipine and verapamil, in focal cerebral ischaemia

This study evaluates the effects of N-(2-[bis (4-fluorophenyl)methoxy]ethyl)-1-butanamine hydrochloride (LY393613), a novel neuronal (N/P/Q-type) Ca(2+) channel blocker, in ischaemia. For comparison, two commonly used L-type Ca(2+) channel blockers; nimodipine and verapamil were also evaluated. Ischaemia was induced in freely moving rats by micro-injection of endothelin-1 near the middle cerebral artery. In vivo microdialysis, laser Doppler flowmetry and histology were used to monitor ischaemia. Administration of LY393613, before and after the insult, attenuated the ischaemia-induced glutamate release, but not the dopamine release. Both nimodipine and verapamil failed to affect transmitter releases significantly, when administered post-occlusion. None of the compounds tested, produced any significant change in striatal blood flow. Histology showed that ischaemic damage was significantly less in LY393613 pre-treated rats. In conclusion, LY393613, a neuronal N/P/Q-Ca(2+) channel blocker, can attenuate ischaemic brain damage. The protective mechanism appears to be mainly the attenuation of the ischaemia-induced glutamate release, rather than its effect on cerebral hemodynamics.

Nicergoline enhances glutamate re-uptake and protects against brain damage in rat global brain ischemia

Whereas a 2-3 degrees C decrease in intraischemic brain temperature can be neuroprotective, mild brain hyperthermia significantly worsens outcome. Our previous study suggested that an ischemic injury mechanism which is sensitive to temperature may not actually increase the extracellular glutamate concentration ([Glu](e)) during the intraischemic period, but rather impairs the Glu re-uptake system, which has been suggested to be involved in the reversed uptake of Glu. We speculated that enhancing Glu re-uptake, pharmacologically or hypothermically, may shorten exposure to high [Glu](e) in the postischemic period and thereby decrease its deleterious excitotoxic effect on neuronal cells. In the present study, rats treated with nicergoline (32 mg/kg, i.p.), an ergot alkaloid derivative, showed minimal inhibition of the [Glu](e) elevation which characteristically occurs during the 10-min intraischemic period, while Glu re-uptake was dramatically improved in the postischemic period, when severe transient global ischemia was caused by mild hyperthermia. Moreover, the nicergoline (32 mg/kg, i.p.) treated rats showed reduced cell death morphologically and clearly had a far lower mortality. The present study suggests that the development of therapeutic strategies aimed at inhibition or prevention of the reversed uptake of glutamate release during ischemia, i.e., activation of the glutamate uptake mechanism, is a promising approach to reduce neural damage occurring in response to brain ischemia.

Focal cerebral ischemia in the mouse: hypothermia and rapid screening of drugs

1. We have investigated the ability of several compounds to diminish both infarct area and volume induced by middle cerebral artery occlusion in the mouse. 2. Lifarizine, ipsapirone and N,N-diethyl-2-[4-(phenylmethyl)phenoxy]ethanamine HCl (DPPE) all reduced both infarct area and volume. Ifenprodil diminished the infarct area, but the effect on total infarct volume was much less pronounced. 3. In addition, we tested the protective effects of some other drugs on infarct area only. Nimodipine, verapamil, diltiazem, N-[1-[4-(4-fluorophenoxy)butyl]-4-piperidinyl]-N-methyl-2-benzothiazo lamine (R56865) and sabeluzole had no effect on infarct area. (S)-Emopamil significantly diminished infarct area. 2,3-Dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline (NBQX) also diminished infarct area significantly. 4. In some brain ischemia models hypothermia protects against ischemic damage. Mild hypothermia had no effect on infarct area in the present mouse model of focal ischemia.

Effects of Ca2+ and Na+ channel inhibitors in vitro and in global cerebral ischaemia in vivo

In the present study we have examined the effects of the small organic molecules: NNC 09-0026 ((-)-trans-1-butyl-4-(4-dimethylaminophenyl)-3-[(4-trifluoromethyl-ph eno xy) methyl] piperidine dihydrochloride); SB 201823-A (4-[2-(3,4-dichlorophenoxy)ethyl]-1-pentyl piperidine hydrochloride); NS 649 (2-amino-1-(2,5-dimethoxyphenyl)-5-trifluoromethyl benzimidazole); CNS 1237 (N-acenaphthyl-N'-4-methoxynaphth-1-yl guanidine) and riluzole on human omega-conotoxin sensitive N-type voltage-dependent Ca2+ channel currents (ICa) expressed in HEK293 cells, on Na+ channel currents (INa) in acutely isolated cerebellar Purkinje neurones in vitro and in the gerbil model of global cerebral ischaemia in vivo. Estimated IC50 values for steady-state inhibition of ICa were as follows; NNC 09-0026, 1.1 microM; CNS 1237, 4.2 microM; SB 201823-A, 11.2 microM; NS 649, 45.7 microM and riluzole, 233 microM. Estimated IC50 values for steady-state inhibition of Na+ channel currents were as follows: NNC 09-0026, 9.8 microM; CNS 1237, 2.5 microM; SB 201823-A, 4.6 microM; NS 649, 36.7 microM and riluzole, 9.4 microM. In the gerbil model of global cerebral ischaemia the number of viable cells (mean +/- S.E.M.) per 1 mm of the CA1 was 215 +/- 7 (sham operated), 10 +/- 2 (ischaemic control), 44 +/- 15 (NNC 09-0026 30 mg/kg i.p.), 49 +/- 19 (CNS 1237 30 mg/kg i.p.), 11 +/- 2 (SB 201823-A 10 mg/kg i.p.), 17 +/- 4 (NS 649 50 mg/kg i.p.) and 48 +/- 18 (riluzole 10 mg/kg i.p.). Thus NNC 09-0026, CNS 1237 and riluzole provided significant neuroprotection when administered prior to occlusion while SB 201823-A and NS 649 failed to protect. These results indicate that the Ca2+ channel antagonists studied not only inhibited human N-type voltage-dependent Ca2+ channels but were also effective blockers of rat Na+ channels. Both NNC 09-0026 and CNS 1237 showed good activity at both Ca2+ and Na+ channels and this may contribute to the observed neuroprotection.

Nicardipine and MK-801 attenuate platelet-activating factor increases following cerebral ischemia-reperfusion in gerbils

The effects of pretreatment with nicardipine (dihydropyridine Ca2+ channel antagonist), Bay K8644 (dihydropyridine Ca2+ channel agonist), and MK-801 (N-methyl-D-aspartate-receptor antagonist) on changes of platelet-activating factor (PAF) concentrations in transient ischemic brain are reported. The tissue concentration of PAF increases significantly in hippocampus, cortex and thalamus by 210%, 169% and 168% of controls without ischemia-reperfusion, respectively after 1 h of reperfusion. Nicardipine (5 mg/kg) reduces the accumulation of PAF, the remaining increases in hippocampus, cortex and thalamus being 151%, 138% and 145% of the controls, respectively. In contrast, Bay K8644 (2.5 mg/kg) enhances the accumulation of PAF, its concentrations in hippocampus, cortex and thalamus being 376%, 233% and 204% of the controls, respectively. The Bay K8644 enhancement in hippocampus is completely inhibited by pretreatment of nicardipine (5 mg/kg). MK-801 (10 mg/kg) reduces the accumulation of PAF, the remaining increases in hippocampus, cortex and thalamus being 152%, 147% and 144% of the controls, respectively. Moreover, brain tissue from animals subjected to the combined pretreatment with nicardipine (5 mg/kg) and MK-301 (10 mg/kg) indicates there is greater inhibition of ischemia-induced PAF increases than with either drug alone. These results indicate that PAF production in the ischemic brain may be regulated by Ca2+ influx through voltage-sensitive Ca2+ channels which are antagonized and agonized by nicardipine and Bay K8644, respectively and receptor-operated Ca2+ channels which are antagonized by MK-801. Because it is known that increases of intracellular Ca2+ in the brain accompany ischemia and early periods of reperfusion and that PAF exhibits neurotoxicity, the present findings support the role of PAF as a mediator in ischemia-induced brain damage at early stages of reperfusion.

Inhibition by lifarizine of intracellular Ca2+ rises and glutamate exocytosis in depolarized rat cerebrocortical synaptosomes and cultured neurones

1. The effects of lifarizine (RS-87476) on intracellular Ca2+ rises and the release of glutamate from rat cerebrocortical synaptosomes depolarized with 30 mM KCl were investigated by use of entrapped fura 2 and exogenous glutamate dehydrogenase. 2. Prior (1 min) addition of lifarizine decreased 30 mM KCl-induced total glutamate release, with 3 microM and 10 microM causing 39% and 72% averaged decreases from controls. The calcium-dependent component of glutamate release (approx. 40% of total) was similarly decreased by 47% and 74%, whereas the calcium-independent component was decreased by only 32% and 43% respectively. 3. In parallel experiments with fura-2-loaded synaptosomes, lifarizine reduced the depolarization-induced increases in intracellular [Ca2+], suggesting that this is the means by which the decreases in glutamate release are brought about. Lifarizine inhibited both the plateau and the spike phases of the Ca2+ increases suggesting that, in addition to its known sodium channel blocking properties, it may also inhibit more than one class of calcium channel in the synaptosomes. 4. Lifarizine at 1 microM and 3 microM also inhibited the rises in intracellular [Ca2+] in rat cultured cortical neurons depolarized with 60 mM KCl. 5. These effects of lifarizine on intracellular Ca2+ and glutamate exocytosis may contribute to its neuroprotective action.

Block of P-type Ca2+ channels by the NMDA receptor antagonist eliprodil in acutely dissociated rat Purkinje cells

The effect of eliprodil on P-type Ca2+ channels was investigated in acutely dissociated rat Purkinje neurons, by using the whole-cell patch-clamp technique. Eliprodil inhibited in a reversible manner the omega-agatoxin-IVA-sensitive Ba2+ current elicited by step depolarizations from a -80 mV holding voltage (IC50 = 1.9 microM). The Ba2+ current showed steady-state inactivation (V1/2 = -61 mV) which was shifted toward more positive values when the intracellular Ca2+ buffering was increased. In these conditions, the potency of eliprodil was decreased (IC50 = 8.2 microM), suggesting a modulation by intracellular Ca2+ of the eliprodil blockade. The potency of eliprodil was not modified at more depolarized holding potentials and was not dependent on the frequency at which the step-depolarizations were applied (0-0.2 Hz) indicating a lack of voltage and use dependence of the eliprodil blockade. When eliprodil was applied in the patch-pipette at a concentration which causes full block when applied externally, the Ba2+ current amplitude was not affected and external application of eliprodil was still efficacious, indicating an extracellular location of the binding site. Analysis of the time course of recovery from Ca2+ channel blockade obtained by concomitant application of eliprodil with Cd2+, omega-agatoxin-IVA or fluspirilene, indicated that these later compounds did not interact with eliprodil, suggesting that eliprodil acts at a different site. These results demonstrate that eliprodil blocks P-type Ca2+ channels in cerebellar Purkinje neurons and suggest that this property may contribute to its neuroprotective activity.

Neuroprotective efficacy of lifarizine (RS-87476) in a simplified rat survival model of 2 vessel occlusion

1. A new, modified rat two vessel occlusion model (with hypotension) was established and the neuroprotective efficacy of the novel agent lifarizine (RS-87476) was examined. 2. The two vessel occlusion model used in the study was a modification of the model described in the literature, whereby we have obviated the need to use a muscle relaxant and intubate the trachea to provide ventilatory support by providing a tight fitting face mask attached to the ventilator. Furthermore, the need to combine exsanguination and additional pharmacological means of inducing the mandatory hypotension (50 mmHg), required to decrease brain blood perfusion pressure, has been removed by simply manipulating the concentration of the already present halothane anaesthetic. 3. The appropriate level of hypotension having been reached, microvascular clips were applied to bilaterally occlude the common carotid arteries for 12 min. This resulted in a loss of the cortical EEG activity. Local cerebral blood flow was measured 6 min into the occlusion period, using the fully quantitative [14C]-iodoantipyrine autoradiographic technique, in a separate group of rats (n = 5). This illustrated the lack of any blood flow, in the areas under study, during the period when there was an isoelectric cortical EEG pattern. 4. The high grade global ischaemic lesion which occurred gave quantifiable neuronal damage in several vulnerable regions of the brain, namely, the hippocampal CA1 sub-field, cortex, thalamus, striatum, and cerebellar brain stem (Purkinje cells). 5. Following the global ischaemic insult the rats were allowed to recover for 72 h before assessment of the damage, during which time one group of rats (n = 11) received 100 micrograms kg-1 lifarizine i.a. 5 min post-occlusion, 500 micrograms kg-1 lifarizine i.p. 15 min post-occlusion, and 500 micrograms kg-1 lifarizine i.p. twice daily for 72 h. A second group of rats (n = 12) was treated with appropriate volumes of vehicle (0.4 ml kg-1 i.a. and 2 ml kg-1 i.p.) at identical time points. 6. Histopathological damage was assessed, from cresyl violet and haematoxyline/eosin stained sections, using a scoring system of 0-6 (no damage-complete neuronal death). The dosing regimen of lifarizine gave reduced damage in the hippocampal CA1 sub-field (4.1 +/- 0.3 to 2.8 +/- 0.6) and striatum (1.7 +/- 0.3 to 1.2 +/- 0.3) and significant neuroprotection in the anterior cortex (2.0 +/- 0.2 to 1.2 +/- 0.2; p < 0.05), thalamus (1.5 +/- 0.2 to 0.8 +/- 0.2; p < 0.01), posterior cortex (1.5 +/- 0.2 to 1.0 +/- 0.2; p < 0.05) and cerebellar brain stem (0.9 +/- 0.2 to 0.4 +/- 0.1; p < 0.01). The overall mean brain score was significantly reduced (from 1.5 +/- 0.1 to 0.9 +/- 0.2). 7. These data show that the newly modified 2 vessel occlusion model produced a quantifiable level of ischaemic damage and that the novel agent lifarizine is neuroprotective in the model.

Calcium, energy metabolism and the development of selective neuronal loss following short-term cerebral ischemia

Short-term cerebral ischemia results in the delayed loss of specific neuronal subpopulations. This review discusses changes in energy metabolism and Ca2+ distribution during ischemia and recirculation and considers the possible contribution of these changes to the development of selective neuronal loss. Severe ischemia results in a rapid decline of ATP content and a subsequent large movement of Ca2+ from the extracellular to the intracellular space. Similar changes are seen in tissue subregions containing neurons destined to die and those areas largely resistant to short-term ischemia, although differences have been observed in Ca2+ uptake between individual neurons. The large accumulation of intracellular Ca2+ is widely considered as a critical initiating event in the development of of neuronal loss but, as yet, definitive evidence has not been obtained. the increased intracellular Ca2+ content activates a number of additional processes including lipolysis of phospholipids and degradation or inactivation of some specific proteins, all of which could contribute to altered function on restoration of blood flow to the brain. Reperfusion results in a rapid recovery of ATP production. Cytoplasmic Ca2+ concentration is also restored during early recirculation as a result of both removal to the extracellular space and uptake into mitochondria. Within a few hours of recirculation, subtle increases in intracellular Ca2+ and a reduced capacity for mitochondrial respiration have been detected in some ischemia-susceptible regions. Both of these changes could potentially contribute to the development of neuronal loss. More pronounced alterations in Ca2+ homeostasis, resulting in a second period of increased mitochondrial Ca2+, develop with further recirculation in ischemia-susceptible regions. The available evidence suggests that these increases in Ca2+, although developing late, are likely to precede the irreversible loss of neuronal function and may be a necessary contributor to the final stages of this process.

Reduction by lifarizine of the neuronal damage induced by cerebral ischaemia in rodents

1. The objective of this study was to evaluate the broad neurocytoprotective potential of the novel sodium-calcium ion channel modulator, lifarizine (RS-87476), in two rodent 72 h survival models of forebrain ischaemia. 2. Under fluothane anaesthesia, rats were subjected to 10 min four vessel occlusion and gerbils to either (i) 5 or (ii) 10 min bilateral carotid artery occlusion. 3. Rats were dosed parenterally solely post-ischaemia (reperfusion) in a series of five studies covering a range of intra-arterial/intraperitoneal (i.a./i.p.) combination doses from 2/10, 5/20, 20/100, 50/200 and 100/500 micrograms kg-1, where the initial loading dose was injected i.a. at 5 min. An i.p. dose was given at 15 min and repeated twice daily. In a sixth study, treatment at 50/200 micrograms kg-1 was deferred for 1 h. 4. Gerbils were treated (i) 15 min pre-ischaemia with either (a) 250, (b) 500 micrograms kg-1 i.p., or (c) 5 mg kg-1 by gavage (p.o.) for 3 days then at 1 h pre-ischaemia. Animals treated as (ii) received 500 micrograms kg-1 i.p. 15 min pre-ischaemia. The above doses were repeated twice daily for 3 days post-ischaemia for the respective groups. 5. In rats, the protective effect of lifarizine was regionally and cumulatively assessed in six brain regions (anterior and posterior neocortex, hippocampal CA1 subfield, thalamus, striatum, cerebellar Purkinje cells-brain stem) at each dose level. Cumulative (total) means +/-s.e.mean neurohistopathological scores(0-4) of 1.16+/-0.09 (n=5), 1.02+/-0.10 (n=5), 0.93+/-0.06 (n=6), 0.79+/-0.09 (n=9) and 0.45+/-0.16(n = 7), respectively, were obtained for the above treatment groups compared to the control (2.01 +/- 0.17,n = 16) group (P<0.0035). The score for the 1 h deferred treatment group was also significant at 0.77 +/- 0.10, n =5 (P< 0.0035). The normal group without ischaemia showed a score of 0.52 +/- 0.09 (n = 6).6. In gerbils, (i) percentage delayed neuronal death (DND) of hippocampal pyramidal cells in the CA1subfield was prevented at 250 (a) and 500 microg kg-' i.p. (b) (27.2+/- 14.6, n=6 and 26.9+/- 10.4%, n= 10 respectively, P<0.02) compared to controls (78.3+/-8.5%, n= 12) and by 5 mg kg-1 p.o. (c) (2.9+/-0.8%,n =l1, P <0.002). Mean +/- s.e.mean total brain scores (0-4) for each of 4 different features denoting cerebral 'oedema' were lower for normal brains (1.60 +/-0.34, n =6) and reduced in animals dosed at 250(a) (3.00+/-0.79, n=6) and 500 microg kg-1 i.p. (b) (3.75 0.36, n= 10) compared to controls (6.58+/-1.00,n = 12) (P< 0.02 -0.03). There was a linear relationship (r = 0.97) between the 'oedema' scores and percentage CA1 DND. Percentage CA1 DND in response to 10 min ischaemia (ii) was reduced(53.0+/-21.0%, n=6, P<0.05) compared to controls (100.0+/-0.0%, n=7).7 The significant neuroprotection shown by lifarizine in rodents substantiates findings in other species.These observations, together with its effect on ion channels and efficacy at extremely low doses offers novelty and suggests a broad spectrum of activity in ischaemia.

Regulation of the L-type calcium channel alpha-1 subunit by chronic depolarization in the neuron-like PC12 and aortic smooth muscle A7r5 cell lines

The regulation of L-type voltage-dependent Ca2+ channels by chronic depolarization was studied in the aortic smooth muscle A7r5 and neuron-type PC12 cell lines, by probing the expression and the functional state of their constitutive alpha-1 subunits. PC12 cells showed, after prolonged exposure to a high-K+ depolarizing solution, a 25% reduction of the functional Ca2+ channel density which was accompanied by a decrease of the alpha-1 subunit mRNA expression. In A7r5 cells submitted to a similar protocol of depolarization, 45Ca2+ uptake measurements revealed a fall in the functional activity of L-type Ca2+ channels which was not related to a modulation of their mRNA expression, but arose from a long-term voltage-dependent channel inactivation. Accordingly, the lag time and the mechanisms of recovery were different in the two cell types. In PC12 cells, when restoring physiological culture conditions, de novo synthesis of alpha-1 subunits allowed the recovery of the original density of L-type Ca2+ channels at the membrane surface. As for the A7r5 cells, we showed that after chronic depolarization, the complete restoration of the resting membrane potential and the related Ca2+ channel activity required a 2-day incubation in physiological medium and could probably be related to a normalization of the increased intracellular Ca2+ concentration. In contrast, it is noteworthy that, in PC12 cells, the only transient increase of intracellular Ca2+ content in the first hours of depolarization could account for the long-term down-regulation of L-type Ca2+ channels.

Actions of the novel neuroprotective agent, lifarizine (RS-87476), on voltage-dependent sodium currents in the neuroblastoma cell line, N1E-115

1. The actions of the neuroprotective agent, lifarizine (RS-87476-190), on voltage-dependent Na+ currents have been examined in the neuroblastoma cell line, N1E-115, using the whole-cell variant of the patch clamp technique. 2. At a holding potential of -80 mV, lifarizine reduced the peak Na+ current evoked by a 10 ms depolarizing step with an IC50 of 1.3 microM. At holding potentials of -100 and -60 mV the IC50 concentrations of lifarizine were 7.3 microM and 0.3 microM, respectively. 3. At a holding potential of -100 mV, most channels were in the resting state and the IC50 value for inhibition of Na+ current should correspond to the dissociation constant of lifarizine for resting channels (KR). KR was therefore estimated to be 7.3 microM. 4. In the absence of lifarizine, recovery from inactivation following a 20 s depolarization from -100 mV to 0 mV was complete within 2 s. However, in the presence of 3 microM lifarizine recovery took place in a biexponential fashion with time constants of 7 s and 79 s. 5. Lifarizine (1 microM) had no effect on steady-state inactivation curves when conditioning pre-pulses of 1 s duration were used. However, when pre-pulse durations of 1 min were used the curves were shifted to the left by lifarizine by about 10 mV. Analysis of the shifts induced by a range of lifarizine concentrations revealed that the apparent affinity of lifarizine for the inactivated state of the channel (K1) was 0.19 microM. 6. Lifarizine (1 microM) had no effect on chloramine-T-modified Na+ currents, suggesting no significant open channel interaction. 7. Taken together, these data show that lifarizine is a potent voltage-dependent inhibitor of Na+currents in NIE-115 cells and that the voltage-dependence arises from an interaction of the compound with the inactivated state of the channel. The possible contribution of Na+ current inhibition to the neuroprotective actions of lifarizine is discussed.

Modulation of ischemic signal by antagonists of N-methyl-D-aspartate, nitric oxide synthase, and platelet-activating factor in gerbil hippocampus

Cerebral ischemia in the gerbil results in early hippocampal changes, which include transient activation and/or translocation of protein kinase C (PKC), increased enzymatic activity of ornithine decarboxylase (ODC), and elevated DNA binding ability of activator protein-1 (AP1). The time-course of all three of these postischemic responses was found to be almost parallel, peaking at 3 hr after the ischemic insult. The effectiveness of known modulators of postischemic morphological outcome (MK-801, L-NAME, and gingkolides BN 52020 and BN 52021) in counteracting the induction of PKC, ODC, and AP1 formation was tested. These drugs were administrated as followed: MK-801 (a noncompetitive inhibitor of NMDA channel), 0.8 mg/kg i.p., 30 min before ischemia, and 5 min after the insult; L-NAME (competitive inhibitor of NO synthase), 10 mg/kg i.p., 30 min before ischemia, and 5 mg/kg, 5 min after ischemia; BN52020 and BN52021 (inhibitors of platelet-activating factor: PAF receptors) were administered as a suspension in 5% ethanol in water by oral route, 10 mg/kg for 3 days before ischemia. Three of these drugs, MK-801, L-NAME, and BN52021, significantly reduced ischemia-elevated activity of PKC and ODC, whereas AP1 formation was only partially attenuated. Our observations implicate the existence of different mechanism(s) for postischemic PKC and ODC activation, which in turn is engaged in AP1 induction.

Block of human voltage-sensitive Na+ currents in differentiated SH-SY5Y cells by lifarizine

1. The ability of lifarizine (RS-87476) to block human voltage-sensitive Na+ channel currents was studied by use of whole cell patch clamp recording from differentiated neuroblastoma cells (SH-SY5Y). 2. The Na+ conductance in differentiated SH-SY5Y cells (24.0 +/- 2.4 nS, n = 11) was half-maximally activated by 10 ms depolarizations to -37 +/- 2 mV and was half-maximally inactivated by predepolarizing pulses of 200 ms duration to -86 +/- 3 mV (n = 11). 3. At low stimulus frequencies (0.1 to 0.33 Hz) voltage-dependent sodium currents were completely blocked, in a concentration-dependent manner, by extracellular application of either tetrodotoxin (EC50 = 4 +/- 1 nM, n = 12) or by lifarizine (EC50 = 783 +/- 67 nM, n = 9). The onset of block by lifarizine (tau = 91 +/- 14 s at 10 microM) was considerably slower than that of tetrodotoxin (tau = 16 +/- 3 s at 100 nM). 4. Lifarizine (1 microM) reduced the peak sodium conductance in each cell (from 26.4 +/- 2.0 nS to 15.1 +/- 2.7 nS, n = 4) without changing the macroscopic kinetics of sodium current activation or inactivation (V1/2 = -35 1 mV and -87 +/- 4 mV respectively, n = 4). Similarly, lifarizine (1 microM) did not affect the reversal potential of the macroscopic sodium current (+14 +/- 5 mV in control and +16 +/- 2 mV in 1 microM lifarizine; n = 4) or reactivation time-constant (tau = 14.0 +/- 4.4 ms). 5. Block of the sodium channel open state by tetrodotoxin (30 nM) did not prevent the inhibition caused by a subsequent application of lifarizine (3 micro M). In contrast the depression caused by lifarizinewas readily reversible after pretreatment of cells with the local anaesthetic, lignocaine (1O mM).6. These data demonstrate that lifarizine is a use- and voltage-dependent antagonist of human voltage sensitive sodium currents. The slow kinetics and pharmacology of the block by lifarizine indicate that access of this drug to the channel is more restricted than that of tetrodotoxin and may involve an allosteric site or state of the channel that is also regulated by local anaesthetics.

Na+ currents that fail to inactivate

Textbook accounts give the impression that Na+ channels are short-acting binary switches: depolarization opens them, but only for about one millisecond. In contrast to this simplified view, a small but significant fraction of the total Na+ current in neurons occurs because channels open after long delays or in long-duration bursts of openings. Such non-inactivating Na+ current acts physiologically in neurons to amplify synaptic potentials and enhance endogenous rhythmicity, and also to aid repetitive firing of action potentials. In glial cells it also may regulate Na(+)-K+ ATPase activity. The evidence for non-inactivating Na+ current in a variety of neurons and glia is reviewed, along with a brief discussion of its ion channel substrate and its relevance for neurological diseases and drug therapy.

New synthetic ligands for L-type voltage-gated calcium channels

The pharmacology of the L-type Ca2+ channel has been the subject of considerable basic and clinical investigation over the past two decades primarily because of the clinical activities of nifedipine, verapamil and diltiazem. However, it is quite clear that this Ca2+ channel is, in common with other pharmacologic receptors, a multiple drug receptor. There are probably as many as six or more discrete drug binding sites associated with this Ca2+ channel. Continued investigation of these sites may yield both new therapeutic agents, structural clues to ligands active at other classes of Ca2+ channel and structures active at other classes of ion channel.