Activation of TRPV4 channels (hVRL-2/mTRP12) by phorbol derivatives

We have studied activation by phorbol derivatives of TRPV4 channels, the human VRL-2, and murine TRP12 channels, which are highly homologous to the human VR-OAC, and the human and murine OTRPC4 channel. 4alpha-Phorbol 12,13-didecanoate (4alpha-PDD) induced an increase in intracellular Ca(2+) concentration, [Ca(2+)](i), in 1321N1 cells stably transfected with human VRL-2 (hVRL-2.1321N1) or HEK-293 cells transiently transfected with murine TRP12, but not in nontransfected or mock-transfected cells. Concomitantly with the increase in [Ca(2+)](i), 4alpha-PDD activated an outwardly rectifying cation channel with an Eisenman IV permeation sequence for monovalent cations that is Ca(2+)-permeable with P(Ca)/P(Na) = 5.8. Phorbol 12-myristate 13-acetate also induced an increase in [Ca(2+)](i) but was approximately 50 times less effective than 4alpha-PDD. EC(50) for Ca(2+) increase and current activation was nearly identical (pEC(50) approximately 6.7). Similar effects were observed in freshly isolated mouse aorta endothelial cells which express TRP12 endogenously. By using 4alpha-PDD as a tool to stimulate TRP12, we showed that activation of this channel is modulated by [Ca(2+)](i); an increase in [Ca(2+)](i) inhibits the channel with an IC(50) of 406 nm. Ruthenium Red at a concentration of 1 microm completely blocks inward currents at -80 mV but has a smaller effect on outward currents likely indicating a voltage dependent channel block. We concluded that the phorbol derivatives activate TRPV4 (VR-OAC, VRL-2, OTRPC4, TRP12) independently from protein kinase C, in a manner consistent with direct agonist gating of the channel.

Cloning and functional expression of human short TRP7, a candidate protein for store-operated Ca2+ influx

The regulation and control of plasma membrane Ca(2+) fluxes is critical for the initiation and maintenance of a variety of signal transduction cascades. Recently, the study of transient receptor potential channels (TRPs) has suggested that these proteins have an important role to play in mediating capacitative calcium entry. In this study, we have isolated a cDNA from human brain that encodes a novel transient receptor potential channel termed human TRP7 (hTRP7). hTRP7 is a member of the short TRP channel family and is 98% homologous to mouse TRP7 (mTRP7). At the mRNA level hTRP7 was widely expressed in tissues of the central nervous system, as well as some peripheral tissues such as pituitary gland and kidney. However, in contrast to mTRP7, which is highly expressed in heart and lung, hTRP7 was undetectable in these tissues. For functional analysis, we heterologously expressed hTRP7 cDNA in an human embryonic kidney cell line. In comparison with untransfected cells depletion of intracellular calcium stores in hTRP7-expressing cells, using either carbachol or thapsigargin, produced a marked increase in the subsequent level of Ca(2+) influx. This increased Ca(2+) entry was blocked by inhibitors of capacitative calcium entry such as La(3+) and Gd(3+). Furthermore, transient transfection of an hTRP7 antisense expression construct into cells expressing hTRP7 eliminated the augmented store-operated Ca(2+) entry. Our findings suggest that hTRP7 is a store-operated calcium channel, a finding in stark contrast to the mouse orthologue, mTRP7, which is reported to enhance Ca(2+) influx independently of store depletion, and suggests that human and mouse TRP7 channels may fulfil different physiological roles.

The diversity in the vanilloid (TRPV) receptor family of ion channels

Following cloning of the vanilloid receptor 1 (VR1) at least four other related proteins have been identified. Together, these form a distinct subgroup of the transient receptor potential (TRP) family of ion channels. Members of the vanilloid receptor family (TRPV) are activated by a diverse range of stimuli, including heat, protons, lipids, phorbols, phosphorylation, changes in extracellular osmolarity and/or pressure, and depletion of intracellular Ca2+ stores. However, VR1 remains the only channel activated by vanilloids such as capsaicin. These channels are excellent molecular candidates to fulfil a range of sensory and/or cellular roles that are well characterized physiologically. Furthermore, as novel pharmacological targets, the vanilloid receptors have potential for the development of many future disease treatments.

SB-334867-A antagonises orexin mediated excitation in the locus coeruleus

Electrophysiological recordings from identified noradrenergic locus coeruleus (LC) neurones in rat brain slices have revealed that the orexins can cause direct and reversible depolarisation of the postsynaptic membrane. Whilst it is known that the membrane depolarisation produced by orexin-A can triple the firing rate of spontaneously active LC neurones, quantitative pharmacological analysis that determines the receptor subtype(s) mediating the orexinergic response has not yet been performed. Here we demonstrate that the effects of orexin-A are five-fold more potent than orexin-B on LC neuronal excitability. We show further that the orexin receptor antagonist SB-334867-A inhibits the effects of both agonists with pK(B) values similar to those calculated for human OX1 receptors expressed in CHO cells. Finally, we found no evidence for tonic activation of OX1 receptors in LC noradrenergic neurones despite electron microscopic evidence that orexin terminals directly contact these neurones. These data demonstrate that SB-334867-A is a useful tool compound with which to study the physiology of OX1 receptors.

Neuroprotective role of monocarboxylate transport during glucose deprivation in slice cultures of rat hippocampus

The effects of energy substrate removal and metabolic pathway block have been examined on neuronal and glial survival in organotypic slice cultures of rat hippocampus. Slice cultures resisted 24 h of exogenous energy substrate deprivation. Application of 0.5 mM alpha-cyano-4-hydroxycinnamate (4-CIN) for 24 h resulted in specific damage to neuronal cell layers, which could be reversed by co-application of 5 mM lactate. Addition of 10 mM 2-deoxyglucose in the absence of exogenous energy supply produced widespread cell death throughout the slice. This was partly reversed by co-application of 5 mM lactate. These effects of metabolic blockade on cell survival were qualitatively similar to the effects on population spikes recorded in the CA1 cell layer following 60 min application of these agents. The data suggest that monocarboxylate trafficking from glia to neurons is an essential route for supply of energy substrates to neurons particularly when exogenous energy supply is restricted.

Cloning and functional expression of a human orthologue of rat vanilloid receptor-1

Capsaicin, resiniferatoxin, protons or heat have been shown to activate an ion channel, termed the rat vanilloid receptor-1 (rVR1), originally isolated by expression cloning for a capsaicin sensitive phenotype. Here we describe the cloning of a human vanilloid receptor-1 (hVR1) cDNA containing a 2517 bp open reading frame that encodes a protein with 92% homology to the rat vanilloid receptor-1. Oocytes or mammalian cells expressing this cDNA respond to capsaicin, pH and temperature by generating inward membrane currents. Mammalian cells transfected with human VR1 respond to capsaicin with an increase in intracellular calcium. The human VR1 has a chromosomal location of 17p13 and is expressed in human dorsal root ganglia and also at low levels throughout a wide range of CNS and peripheral tissues. Together the sequence homology, similar expression profile and functional properties confirm that the cloned cDNA represents the human orthologue of rat VR1.

Characterisation of SB-221420-A - a neuronal Ca(2+) and Na(+) channel antagonist in experimental models of stroke

For progression to clinical trials in stroke, putative neuroprotective compounds should show robust efficacy post-ischaemia in several experimental models of stroke. This paper describes the characterisation of (+)(1S, 2R)-cis-1-[4-(1-methyl-1-phenylethyl)phenoxy]-2-methylamino indane hydrochloride (SB-221420-A), a Ca(2+) and Na(+) channel antagonist. SB-221420-A inhibited (IC(50)=2.2 microM) N-type voltage-operated Ca(2+) channel currents in cultured superior cervical ganglion neurons, which were pretreated with 10 microM nimodipine to block L-type voltage-operated Ca(2+) channel currents. In dorsal root ganglion neurons pretreated with 1 microM omega-conotoxin GVIA to block N-type voltage-operated Ca(2+) channel currents, SB-221420-A inhibited the residual Ca(2+) current with an IC(50) of 7 microM. SB-221420-A also inhibited Na(+) currents in dorsal root ganglion neurons with an IC(50) of 8 microM. In rats, the pharmacokinetic profile of SB-221420-A shows that it has a half-life of 6.4 h, a high volume of distribution, is highly brain penetrating, and has no persistent metabolites. Following bilateral carotid artery occlusion in gerbils, SB-221420-A significantly reduced the level of ischaemia-induced hyperlocomotor activity and the extent of hippocampal CA1 cell loss compared to the ischaemic vehicle-treated group. SB-221420-A was also effective in focal models of ischaemia. In the mouse permanent middle cerebral artery occlusion model, SB-221420-A (10 mg/kg) administered intravenously, post-ischaemia significantly (P<0.05) reduced lesion volume compared to the ischaemic vehicle-treated group. In the normotensive rat permanent middle cerebral artery occlusion model, SB-221420-A (10 mg/kg) administered intravenously over 1 h, beginning 30 min postmiddle cerebral artery occlusion, significantly (P<0.05) reduced lesion volume from 291+/-16 to 153+/-30 mm(3), compared to ischaemic vehicle-treated controls when measured 24 h postmiddle cerebral artery occlusion. Efficacy was maintained when the same total dose of SB-221420-A was infused over a 6-h period, beginning 30 min postmiddle cerebral artery occlusion. SB-221420-A also significantly (P<0.05) reduced lesion volume following transient middle cerebral artery occlusion in normotensive rats and permanent middle cerebral artery occlusion in spontaneously hypertensive rats (SHR). Investigation of the side effect profile using the Irwin screen in mice revealed that, at neuroprotective doses, there were no overt behavioural or cardiovascular changes. These data demonstrate that robust neuroprotection can be seen post-ischaemia with SB-221420-A in both global and focal ischaemia with no adverse effects at neuroprotective doses, and indicate the potential utility of a mixed cation blocker to improve outcome in cerebral ischaemia.

Caspase inhibitors are functionally neuroprotective against oxygen glucose deprivation induced CA1 death in rat organotypic hippocampal slices

We have explored the neuroprotective efficacy of the cell penetrant caspase inhibitor, Ac-YVAD-cmk, in a hippocampal slice model of neuronal cell death induced by oxygen and glucose deprivation. Organotypic hippocampal slice cultures were prepared from 8 to 10-day-old rats and maintained for 10 to 12 days in vitro. Pre-treatment with Ac-YVAD-cmk prior to 45 min oxygen and glucose deprivation was neuroprotective as measured by propidium iodide uptake, with an EC(50) between 1 and 10 micromol/l. Ac-YVAD-cmk was also able to preserve synaptic function in the organotypic hippocampal slice cultures 24 h after oxygen and glucose deprivation. Ac-YVAD-cmk prevented the increase in histone-associated DNA fragmentation induced by oxygen and glucose deprivation. Interleukin-1beta did not reverse the protective effect of Ac-YVAD-cmk, and interleukin-1 receptor antagonist alone was not protective. These results show that caspase inhibitors are neuroprotective in a hippocampal slice culture system, using structural, biochemical and electrophysiological endpoints, and that this effect is not a result of inhibition of interleukin-1beta production.

Cloning, localisation and functional expression of the human orthologue of the TREK-1 potassium channel

We have cloned human TREK-1, one of the newly emerging mammalian family of 2-P domain potassium channels. The channel has 411 amino acids with a 41-amino-acid extension at the C-terminus when compared with the cloned mouse TREK-1 channel. Expression of hTREK-1 produced a substantial hyperpolarising shift in resting membrane potential accompanied by the induction of large, outwardly rectifying, non-inactivating currents which were potassium selective. Pharmacologically, hTREK-1-mediated currents were only blocked to a limited extent by classic potassium channel blockers or open channel pore blockers known to potently inhibit other channels. The channel was reversibly potentiated by arachidonic acid. CNS distribution of hTREK-1 is widespread with higher levels being observed in caudate, putamen, amygdala, thalamus and spinal cord. Only low levels of expression were seen in the majority of peripheral regions. Thus, hTREK-1, although functionally and pharmacologically similar to mouse TREK-1, appears to have a more CNS-specific distribution.

Mutation of histidine 286 of the human P2X4 purinoceptor removes extracellular pH sensitivity

1. Effects of external pH on the human P2X4 purinoceptor, an ATP-activated ion channel, were studied using the Xenopus oocyte expression system. 2. Changing the external pH from 7.4 to 6.5 significantly reduced, whilst an increase to pH 8 enhanced, maximum ATP-activated current amplitude, without changing the current- voltage relationship of the ATP-activated current. 3. Diethyl pyrocarbonate (DEPC; 10 mM) treatment of P2X4-injected oocytes had no effect on the pH sensitivity of the ATP-activated current. 4. Site-directed mutagenesis of histidine 286 (H286) to alanine completely abolished the pH sensitivity of the P2X4 receptor at all agonist concentrations. ATP potency showed a small (fourfold) leftward shift. Mutagenesis of the other three histidines present in the P2X4 sequence had no effect on pH sensitivity. 5. The results show that pH modulation of P2X4 in the pathophysiological range is mediated by protonation of H286. This provides direct confirmation that pH sensitivity resides in the P2X4 channel protein rather than the agonist species.

Rank-order inhibition by omega-conotoxins in human and animal autonomic nerve preparations

The inhibitory effects of the omega-conotoxins GVIA, MVIIA and MVIIC on electrically-evoked, tetrodotoxin (10(-7) M)-sensitive, autonomic nerve activity were studied using human, rat or guinea-pig vas deferens and intestinal tissues. In each preparation from each species, nM concentrations of omega-conotoxins GVIA and MVIIA prevented the neuronally-mediated contractions, whereas omega-conotoxin MVIIC was either markedly less potent (IC(50)'s 1.4 or 2.9 log units more than for omega-conotoxin GVIA in guinea-pig ileum and rat vas deferens, respectively) or was without significant activity (human vas deferens, human Taenia coli) when tested at similar concentrations. In contrast the differences in potency between omega-conotoxins GVIA and MVIIC were considerably less when assayed directly on Ca(2+) channel currents evoked from rat superior cervical ganglion neurons in culture (approximately 0.1 log unit difference) and from a stable cell line expressing rat alpha(1B), alpha(2)delta, beta(1b) Ca(2+) channel subunits (approximately 0.9 log unit). These different rank-orders of inhibitory activity of the conotoxins support the suggestion that there are pharmacologically distinct N-type Ca(2+) channels in the peripheral nervous system, and that this tissue-dependent difference is seen in man.

Orexin A activates locus coeruleus cell firing and increases arousal in the rat

The localization of orexin neuropeptides in the lateral hypothalamus has focused interest on their role in ingestion. The orexigenic neurones in the lateral hypothalamus, however, project widely in the brain, and thus the physiological role of orexins is likely to be complex. Here we describe an investigation of the action of orexin A in modulating the arousal state of rats by using a combination of tissue localization and electrophysiological and behavioral techniques. We show that the brain region receiving the densest innervation from orexinergic nerves is the locus coeruleus, a key modulator of attentional state, where application of orexin A increases cell firing of intrinsic noradrenergic neurones. Orexin A increases arousal and locomotor activity and modulates neuroendocrine function. The data suggest that orexin A plays an important role in orchestrating the sleep-wake cycle.

Lack of effect of the novel anticonvulsant SB-204269 on voltage-dependent currents in neurones cultured from rat hippocampus

The novel anticonvulsant SB-204269 inhibits epileptiform afterdischarges induced by high K+ in rat hippocampal slices. Its effects on voltage-gated Na+ currents, measured from cultured hippocampal neurones using whole cell patch clamp, were compared to the effects of existing anticonvulsants. SB-204269 produced no significant tonic block of Na+ currents nor any voltage-dependent and frequency-dependent block at doses 50 to 500 fold higher than its anticonvulsant EC50 of 0.2 microM. In contrast, lamotrigine, phenytoin and carbamazepine at 50 microM, blocked Na+ currents in a voltage-dependent manner. SB-204269 also had no effect on action potential discharges evoked by elevating external K+. These data suggest that direct blockade of voltage-gated channels does not contribute to the anticonvulsant properties of SB-204269 and further support the hypothesis that this compound has a novel mechanism of action.

Calpain activation and inhibition in organotypic rat hippocampal slice cultures deprived of oxygen and glucose

It has been suggested that, after ischaemia, activation of proteases such as calpains could be involved in cytoskeletal degradation leading to neuronal cell death. In vivo, calpain inhibitors at high doses have been shown to reduce ischaemic damage and traumatic brain injury, however, the relationship between calpain activation and cell death remains unclear. We have investigated the role of calpain activation in a model of ischaemia based on organotypic hippocampal slice cultures using the appearance of spectrin breakdown products (BDPs) as a measure of calpain I activation. Calpain I activity was detected on Western blot immediately after a 1-h exposure to ischaemia. Up to 4 h post ischaemia, BDPs were found mainly in the CA1 region and appeared before uptake of the vital dye propidium iodide (PI). 24 h after the insult, BDPs were detected extensively in CA1 and CA3 pyramidal cells, all of which was PI-positive. However, there were many more PI-positive cells that did not have BDPs, indicating that the appearance of BDPs does not necessarily accompany ischaemic cell death. Inhibition of BDP formation by the broad-spectrum protease inhibitor leupeptin was not accompanied by any neuroprotective effects. The more specific and more cell-permeant calpain inhibitor MDL 28170 had a clear neuroprotective effect when added after the ischaemic insult. In contrast, when MDL 28170 was present throughout the entire pre- and post-incubation phases, PI labelling actually increased, indicating a toxic effect. These results suggest that calpain activation is not always associated with cell death and that, while inhibition of calpains can be neuroprotective under some conditions, it may not always lead to beneficial outcomes in ischaemia.

Time course of the initial [Ca2+]i response to extracellular ATP in smooth muscle depends on [Ca2+]e and ATP concentration

In response to extracellular application of 50 microM ATP, all individual porcine aortic smooth muscle cells respond with rapid rises from basal [Ca2+]i to peak [Ca2+]i within 5 s. The time from stimulus to the peak of the [Ca2+]i response increases with decreasing concentration of ATP. At ATP concentrations of 0.5 microM and below, the time to the [Ca2+]i peak varies more significantly from cell to cell than at higher concentrations, and each cell shows complicated initiation and decay kinetics. For any individual cell, the lag phase before a response decreases with increasing concentration of ATP. An increase in lag time with decreasing ATP concentration is also observed in the absence of extracellular Ca2+, but the lag phase is more pronounced, especially at concentrations of ATP below 0.5 microM. Whole-cell patch-clamp electrophysiology shows that in porcine aortic smooth muscle cells, ATP stimulates an inward current carried mainly by Cl- ion efflux with a time course similar to the [Ca2+]i changes and no detectable current from an ATP-gated cation channel. A simple signal cascade initiation kinetics model, starting with nucleotide receptor activation leading to IP3-mediated Ca2+ release from IP3-sensitive internal stores, fits the data and suggests that the kinetics of the Ca2+ response are dominated by upstream signal cascade components.

SB-205384: a GABA(A) receptor modulator with novel mechanism of action that shows subunit selectivity

1. SB-205384, and its (+) enantiomer (+)-SB-205384 were tested for their modulatory effects on human GABA(A) receptor subunit combinations expressed in Xenopus oocytes by electrophysiological methods. 2. The slowing of the decay rate induced by SB-205384 on native GABA-activated currents in rat neurones was also seen on GABA(A) currents in oocytes expressing human GABA(A) subunits. This temporal effect was observed for the alpha3beta2gamma2 subunit combination with little effect in subunit combinations containing either alpha1 or alpha2. 3. Potentiation of the peak amplitude of the GABA-activated currents by SB-205384 or (+)-SB-205384 was less specific for a particular subunit combination, although the greatest effect at 10 microM drug was seen on the alpha3beta2gamma2 subunit combination. 4. In contrast, zolpidem, a benzodiazepine site modulator, did not significantly slow decay rates of GABA(A) currents in oocytes expressing the alpha3beta2gamma2 subunit combination. Zolpidem, as expected, did selectively potentiate GABA-activated currents on oocytes expressing the gamma2 subunit compared to those containing the gamma1. 5. The results show that the novel kinetic modulatory profile of SB-205384 is selective for the alpha3beta2gamma2 subunit combination. This suggests that the compound is binding to a novel regulatory site on the subunit complex.

Furosemide inhibits regenerative cortical spreading depression in anaesthetized cats

Ionic perturbations occur during cortical spreading depression (SD), a phenomenon implicated in migraine pathophysiology. We studied the effect of 0.2, 2 and 20 mg kg-1 i.v. (n=4) furosemide on cortical direct current (d.c.) potential, cerebrovascular laser Doppler flux (rCBF[LDF]), artery diameter and NO concentration in the parietal cortex of the anaesthetized cat during repetitive SD. In vehicle-treated animals (n=4), SD activity was sustained for 50+/-1.8 min. However, duration of SD activity was significantly reduced when compared to vehicle to 39+/-6.6 (n=4), 34+/-8.5 (n=4) and 27.3+/-11.3 min (n=4), at 0.2, 2 and 20 mg kg-1 i.v. furosemide respectively. It is hypothesized that the mechanism of inhibition of SD d.c. activity by furosemide may be through alterations in cortical ion buffering capacity or inhibition of cell swelling in neurones or glia. These mechanisms may represent potential novel drug targets in future migraine therapy.

Effect of SB-205384 on the decay of GABA-activated chloride currents in granule cells cultured from rat cerebellum

1. 4-Amino-7-hydroxy-2-methyl-5,6,7,8,-tetrahydrobenzo[b]thieno[2,3-b]pyrid ine-3-carboxylic acid, but-2-ynyl ester (SB-205384) and other gamma-aminobutyric acid(A) (GABA(A)) receptor modulators were tested for their effects on GABA-activated chloride currents in rat cerebellar granule cells by use of the whole-cell patch clamp technique. 2. The major effect of SB-205384 on GABA(A)-activated current was an increase in the half-life of decay of the response once the agonist had been removed. This is in contrast to many GABA(A) receptor modulators that have previously been shown to potentiate GABA-activated currents. 3. This profile could be explained if SB-205384 stabilizes the channel in open and desensitized states so that channel closing is dramatically slowed. Such a modulatory profile may produce a novel behavioural profile in vivo.

The effects of SB 206284A, a novel neuronal calcium-channel antagonist, in models of cerebral ischemia

The effects of SB 206284A, 1-[7-(4-benzyloxyphenoxy)heptyl] piperidine hydrochloride, have been investigated in vitro on calcium and sodium currents in rat-cultured dorsal root ganglion (DRG) neurones and potassium-mediated calcium influx in rat synaptosomes. Cardiovascular hemodynamic effects in both anesthetized and conscious rats, and neuroprotective activity in in vivo cerebral ischemia models were also investigated. In the rat DRG cells, SB 206284A caused almost complete block of the sustained inward Ca2+ current (IC50 = 2.4 microM), suggesting that the compound is an effective blocker of slowly inactivating, high-voltage calcium current. SB 206284A reduced locomotor hyperactivity in the gerbil bilateral carotid artery occlusion model without affecting ischemia-induced damage in the hippocampal CA1 region. In the rat middle cerebral artery occlusion model, SB 206284A reduced lesion volume in the posterior forebrain, and in the rat photochemical cortical lesion model, lesion volume was reduced even when treatment was delayed until 4 hours after occlusion. At neuroprotective doses, SB 206284A had no cardiovascular effects. These findings show that SB 206284A is a novel calcium channel antagonist that shows neuroprotective properties.

Selective N-type calcium channel antagonist omega conotoxin MVIIA is neuroprotective against hypoxic neurodegeneration in organotypic hippocampal-slice cultures

BACKGROUND AND PURPOSE

Neuroprotection by antagonists of both L-type and N-type calcium channels occurs in in vivo models of ischemia. The site of action of calcium channel antagonists is unclear, however, and it is likely that a combination of vascular and direct neuronal actions occurs. We have investigated the effects of blocking neuronal calcium channels using an organotypic hippocampal-slice model of ischemia.

METHODS

Organotypic hippocampal-slice cultures prepared from 10-day-old rats were maintained in vitro for 14 days. Cultures were exposed to either 3 hours of oxygen deprivation (hypoxia) or 1 hour of combined oxygen and glucose deprivation (ischemia). Neuronal damage was quantified after 24 hours by propidium iodide fluorescence.

RESULTS

Three hours of anoxia produced damage exclusively in CAT pyramidal cells. This damage was prevented by preincubation with omega conotoxin MVIIA, a selective N-type calcium channel blocker, and omega conotoxin MVIIC, which blocks N-type and other presynaptic neuronal calcium channels. The dihydropyridine nifedipine and the mixed calcium channel blocker SB201823-A were not protective. Furthermore, if addition of conotoxin MVIIA was delayed until after the hypoxic episode, a dose-dependent neuroprotective effect was observed, with an IC50 of 50 nmol/L. In contrast to hypoxia, none of the compounds was neuroprotective in the model of oxygen-glucose deprivation, although it was determined that extracellular calcium was essential for the generation of ischemic damage.

CONCLUSIONS

These studies present clear evidence that neuroprotection by selective N-type calcium channel antagonists is mediated directly through neuronal calcium channels. In contrast, the neuroprotective effects of dihydropyridines may be mediated through vascular calcium channels or indirectly through actions in other brain regions.

SB 201823-A antagonizes calcium currents in central neurons and reduces the effects of focal ischemia in rats and mice

BACKGROUND AND PURPOSE

Excessive calcium entry into depolarized neurons contributes significantly to cerebral tissue damage after ischemia. We evaluated the ability of a novel neuronal calcium channel blocker, SB 201823-A, to block central neuronal calcium influx in vitro and to reduce ischemic injury in two rodent models of focal stroke.

METHODS

Patch-clamp electrophysiology and intracellular Ca2+ imaging in rat hippocampal and cerebellar neurons were used to determine effects on neuronal calcium channel activity. Middle cerebral artery occlusion was performed in Fisher 344 rats and CD-1 mice to determine the effects on rodent focal ischemic injury and neurological deficits. Cardiovascular monitoring in conscious rats was conducted to determine cardiovascular liabilities of the compound.

RESULTS

In cultured rat hippocampal cells, calcium current measured at plateau was reduced by 36 +/- 8% and 89 +/- 4% after 5 and 20 mumol/L SB 201823-A, respectively. In cerebellar granule cells in culture, pretreatment with 2.5 mumol/L SB 201823-A totally prevented initial calcium influx and reduced later calcium influx by 50 +/- 2.5% after N-methyl-D-aspartate/glycine stimulation (P < .01). KCl depolarization-induced calcium influx also was reduced by more than 95%. In rats, a single treatment with 10 mg/kg IV SB 201823-A beginning 30 minutes after focal ischemia decreased (P < .05) hemispheric infarct by 30.4% and infarct volume by 29.3% and reduced (P < .05) forelimb deficits by 47.8% and hindlimb deficits by 36.3%. In mice, treatments with 10 mg/kg IP SB 201823-A beginning 30 minutes after focal ischemia significantly reduced infarct volume by 41.5% (P < .01). No blood pressure effects were observed with the therapeutic dose of the compound.

CONCLUSIONS

These results indicate that the new neuronal calcium channel blocker SB 201823-A can block stimulated calcium influx into central neurons and can provide neuroprotection in two models of focal cerebral ischemia without affecting blood pressure. Data from several different studies now indicate that the neuronal calcium channel antagonists are a promising therapy for the postischemic treatment of stroke.

SB 201823-A, a neuronal Ca2+ antagonist is neuroprotective in two models of cerebral ischaemia

We have characterised the Ca2+ channel blocking properties of a new non-peptide Ca2+ channel antagonist, SB 201823-A, in cultures of rat sensory neurones. The IC50 for SB 201823-A against total Ca2+ current in sensory neurones was 4.9 microM. SB 201823-A showed little selectivity for sub-types of neuronal Ca2+ channel but was selective for Ca2+ channels over Na+ and K+ channels. Efficacy against other types of cation channel such as agonist gated channels was not assessed. SB 201823-A was neuroprotective in vivo when administered post-ischaemia in one focal and one global model of neuronal ischaemia. In the rat photothrombotic focal lesion model, SB 201823-A administered i.p. 10 min post-ischaemia resulted in a dramatic reduction in lesion volume. In the gerbil bilateral carotid artery occlusion global model, SB 201823-A dosed i.p. 30 min post-occlusion resulted in both histological and functional improvements when compared to vehicle treated animals. These data suggest that such novel neuronal Ca2+ channel antagonists may have potential in ameliorating both the pathological and functional consequences of stroke in man.