Ca++ and Na+ channels involved in neuronal cell death. Protection by flunarizine

Flunarizine--its effect on pentylenetetrazol-kindled seizures and on related cognitive disturbances

Epileptics are often faced with impaired intellectual processes. The basis of these impairments is still poorly understood. Kindling is an accepted model for the study of the convulsive component of epilepsy. Furthermore, it was demonstrated that pentylenetetrazol-kindled rats show diminished shuttle-box learning. Therefore, we used this model to study the influence of flunarizine, a calcium antagonist, on kindled seizures as well as related learning impairments. It was found, that acutely administered flunarizine significantly suppressed the expression of kindled seizures, but there was no effect on the developmental character of kindling. Moreover, the substance had an anticonvulsant action when administered after completion of kindling. The learning ability of kindled rats was significantly augmented when flunarizine was injected prior to each convulsive stimulation or when administered after completion of kindling. The results were explained in terms of interactions of a depressive effect on abnormal neuronal excitation, a protection against calcium-induced neurotoxicity and, finally, the vascular effect of flunarizine.

Lack of efficacy of 5-HT2A receptor antagonists to reduce brain damage after 3 minutes of transient global cerebral ischaemia in gerbils

Stimulation of the 5-HT2A receptors by serotonin has been reported to exert an excitatory effect on neocortical neurons in rats and mice, to facilitate ischaemia-induced release of excitatory amino acids and to mediate the vasomotor constrictor component of the response of blood vessels to 5-HT. 5-HT2A receptor antagonists have, therefore, been proposed as potential protectants against the effects of cerebral ischaemia. The aim of this study was to evaluate the effects of two relatively selective 5-HT2A receptor antagonists, ketanserin and ritanserin, on delayed hyperactivity and the ensuing neuronal degeneration induced by 3 minutes of bilateral carotid artery ligation in Mongolian gerbils. Effects were compared to that of flunarizine, which blocks calcium overload and served as a positive control in this paradigm. Temporal and/or rectal temperatures were measured and strictly controlled during the ischaemia and the early reperfusion phase. Locomotor activity was measured one day after the ischaemia and neuronal degeneration quantified 7 days later using an image analysis system (Quantimet 570, Leica). Global ischaemia in gerbils elicits hyperactivity associated with a delayed neuronal degeneration predominantly in the CA1 zone of the hippocampus. Ketanserin and ritanserin (3 and 10 mg/kg ip, twice daily for 3 days, pre- and postischaemia) did not protect the CA1 neurons against ischaemic damage. The postischaemic hyperactivity was inhibited only with the higher dose of ketanserin. As previously reported, flunarizine (30 mg/kg po) markedly reduced neuronal degeneration (-44.2%, p < 0.01) and totally abolished the ischaemia-induced hyperactivity. These data demonstrate that ketanserin and ritanserin are not effective protectants of the gerbil hippocampus against ischaemic damage when the body temperature of the animals is controlled, thus suggesting that 5-HT2A receptors are not directly implicated in the pathogenesis of global cerebral ischaemia in this model.

Flunarizine of limited value in children with intractable epilepsy

Fourteen ambulatory children and adolescents with intractable epilepsy were studied in an open phase II study to investigate the pharmacokinetics and pharmacodynamics of flunarizine as an add-on treatment. Flunarizine was given in increasing doses starting with 0.1-0.3 mg/kg/day until effect was observed or a steady-state plasma concentration of 50-60 ng/ml was reached. Treatment was continued for 3 months at steady state. Pharmacokinetics were determined during the immediate posttreatment period. Positive antiepileptic effect (> or = 50% reduction in seizure frequency) was observed in 4 of 14 patients (29%; 95% CI: 52-5). Independently of antiepileptic effect, 10 of 14 parents (71.4%; 95% CI: 95-48) observed positive cognitive effects. In all patients treatment was withdrawn due to either lack of effect or weight gain. Flunarizine was rapidly absorbed; mean time of peak concentration (Tmax) was 2.7 hours (range: 1-8). The mean terminal half-life was 23.2 days (range: 7-48), the total plasma clearance of flunarizine per fraction of the dose absorbed (CLp/F) was 0.28 ml/min/kg (range: 0.07-042), and the volume of distribution of flunarizine per fraction of the dose absorbed (Vd/F) was 187 L/kg (range: 99-348). We conclude that flunarizine (0.1-0.3 mg/kg/day) seems to be of limited antiepileptic value in children with intractable epilepsy. The pharmacokinetic profile of flunarizine complicates its clinical use.

The effects of a purified toxic extract of Prymnesium patelliferum on transport of ions through the plasma membrane of synaptosomes

Extract of the ichthyotoxic marine alga Prymnesium patelliferum has been shown to have several different effects on the transport of neurotransmitters across nerve membranes. It inhibits the sodium dependent uptake of L-glutamate and GABA and enhances the calcium-dependent release of acetylcholine. We have therefore investigated the effects of a purified toxic extract of P. patelliferum on some membrane properties using rat brain synaptosomes. We found that under conditions where the algal extract inhibited the uptake of L-glutamate, it increased the intracellular concentrations of Na+ and Ca2+, stimulated efflux of K+ determined as 86Rb efflux, and depolarized the synaptosomal membrane. There was no effect on Na+/K(+)-ATPase or ouabain-insensitive ATPase activities. Further, there was no leakage of the cytosolic marker LDH, indicating that the various effects of the algal extract were not due to nonspecific leakage or lysis of the synaptosomes. The rise in the cytosolic concentration of free Ca2+ induced by the algal extract was dependent on extracellular Ca2+, and was inhibited by flunarizine (1-100 microM) but not by the Ca2+ channel blockers omega-conotoxin GVIA (1 microM), diltiazem (100 microM), nifedipine (100 microM) or verapamil (100-500 microM). The increase in Na+ influx induced by the algal extract was insensitive to tetrodotoxin (3 microM) and procaine (100 microM), whereas both the Na+ influx and the membrane depolarization were inhibited by flunarizine (1-100 microM). The increase in K+ efflux was insensitive to flunarizine (5-100 microM). From these results it appears that the toxic extract of P. patelliferum increases the permeability of synaptosomes to Ca2+, Na+ and K+ and that these effects may be responsible for the plasma membrane depolarization and the disturbance of the neurotransmitter transport processes.

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.

Ion channel involvement in anoxic depolarization induced by cardiac arrest in rat brain

Anoxic depolarization (AD) and failure of ion homeostasis play an important role in ischemia-induced neuronal injury. In the present study, different drugs with known ion-channel-modulating properties were examined for their ability to interfere with cardiac-arrest-elicited AD and with the changes in the extracellular ion activity in rat brain. Our results indicate that only drugs primarily blocking membrane Na+ permeability (NBQX, R56865, and flunarizine) delayed the occurrence of AD, while compounds affecting cellular Ca2+ load (MK-801 and nimodipine) did not influence the latency time. The ischemia-induced [Na+]e reduction was attenuated by R56865. Blockade of the ATP-sensitive K+ channels with glibenclamide reduced the [K+]e increase upon ischemia, indicating an involvement of the KATP channels in ischemia-induced K+ efflux. The KATP channel opener cromakalim did not affect the AD or the [K+]e concentration. The ischemia-induced rapid decline of extracellular calcium was attenuated by receptor-operated Ca2+ channel blockers MK-801 and NBQX, but not by the voltage-operated Ca2+ channel blocker nimodipine, R56865, and flunarizine.

Endothelium-dependent rhythmic contractions induced by cyclopiazonic acid, a sarcoplasmic reticulum Ca(2+)-pump inhibitor, in the rabbit femoral artery

The vascular responses to cyclopiazonic acid (CPA), an inhibitor of the Ca(2+)-ATPase in the sarcoplasmic reticulum, were investigated in the rabbit femoral artery, suspended in an organ chamber for isometric tension recordings. CPA produced rhythmic contractions in the femoral artery which had been contracted with phenylephrine. CPA, however, did not induce the rhythmic responses in endothelium-denuded arteries. NG-nitro-L-arginine methyl ester and methylene blue, inhibitors of the formation and the action of nitric oxide, respectively, failed to antagonize the CPA-induced rhythmic contractions in the phenylephrine-contracted artery. In contrast, the CPA-induced rhythmic contractions were abolished by charybdotoxin, a Ca(2+)-activated K+ channel antagonist, but not by glibenclamide, a blocker of the ATP-sensitive K+ channel. Nifedipine also inhibited the CPA-induced rhythmic contractions in the endothelium-intact artery and relaxed the endothelium-denuded artery treated with CPA. These results indicate that the CPA-induced rhythmic contractions in the phenylephrine-contracted rabbit femoral artery may be attributed to the periodic inactivation of the voltage-dependent Ca2+ channel, presumably regulated by the Ca(2+)-activated K+ channel. The activation of the K+ channel by CPA might occur only when the endothelium is present.

Pharmacological protection against the cytotoxicity induced by 6-hydroxydopamine and H2O2 in chromaffin cells

We present in this report the characteristics of the damage induced by 6-hydroxydopamine and H2O2 on bovine chromaffin cells in primary culture. Cytotoxicity was quantified using catecholamine cell contents, lactate dehydrogenase (LDH) release, trypan blue exclusion and morphological appearance. An excellent correlation between these four parameters was found. The cytotoxic effects of 6-hydroxydopamine were Ca(2+)-independent. In spite of this, the Ca2+ channel antagonists R56865 (N-[1-(4-(fluorophenoxy)butyl)]-4-piperidinyl-N-methyl-2-benzo-thiazo lamine) lidoflazine exhibited marked cytoprotective effects against both 6-hydroxydopamine and H2O2. The selective dopamine uptake blocker, bupropion, increased the viability of 6-hydroxydopamine and H2O2-treated cells from 20% to around 80%. Catalase drastically protected against the cytotoxic effects of 6-hydroxydopamine and H2O2. In contrast, desferrioxamine gave better protection against H2O2 cytotoxicity; glutathione and N-acetylcysteine only afforded substantial protection against 6-hydroxydopamine. Three main conclusions emerge from this study. (1st) 6-Hydroxydopamine causes chromaffin cell damage via a mechanism probably related to the production of free radicals, but unrelated to Ca2+ ions. Cytoprotection afforded by R56865 and lidoflazine must be unrelated to their Ca2+ antagonist properties. This suggests a novel component in the cytoprotective mechanism of action of these drugs. (2nd) The strong cytoprotective effects of bupropion seem to be unrelated to its ability to block the plasmalemmal dopamine carrier. (3rd) Bovine adrenal chromaffin cells in primary cultures are a suitable model for adult neurons to study the basic mechanism of cell damage, and to screen new drugs with putative neuroprotective properties.

Effects of calcium channel blockers on the response to endothelin-1, bradykinin and sodium nitroprusside in porcine ciliary arteries

Calcium channel blockers are increasingly used in ophthalmology, for instance in patients with visual field defects caused by vasospasm. Endothelin is a new vasoactive peptide which also has been implicated in hypoperfusion of the ophthalmic circulation. This study investigated the effects of the calcium channel blockers on the response to endothelin-1, bradykinin and sodium nitroprusside in isolated porcine ciliary arteries (diameter 200-250 microns). Isolated porcine ciliary arteries were suspended in myograph systems filled with modified Krebs-Ringer solution (37 degrees C; 95% O2/5% CO2) for isometric tension recording. Endothelin-1 (10(-12) -10(-7) M) induced potent concentration-dependent contractions of porcine ciliary arteries (PD50 = 8.3 +/- 0.1; n = 7). Lacidipine (10(-5) -10(-7) M) and nifedipine (10(-5) M) significantly reduced the contractions and decreased the sensitivity to endothelin-1 as compared to control (P < 0.03). On the other hand, endothelium-dependent relaxations to bradykinin (10(-10) -10(-6) M) and endothelium-independent relaxations to sodium nitroprusside (10(-10) -10(-4) M) remained unaffected by the calcium channel blocker. These findings demonstrate that in porcine ciliary arteries, the calcium channel blockers selectively inhibit endothelin-1-induced contractions, while leaving endothelium-dependent and endothelium-independent relaxations unaffected. This property of calcium channel blockers may contribute to the clinical efficacy of this class of drugs in patients with ocular vasospasms.

D2 receptor blockade by flunarizine and cinnarizine explains extrapyramidal side effects. A SPECT study

Twenty-six patients under treatment with the calcium channel blockers flunarizine (Fz) or cinnarizine (Cz) were examined-with single-photon emission computed tomography using [123I]iodobenzamide as a ligand. The striatal dopamine D2 receptor-binding potential was determined and found to be reduced by 14 to 63% (39.5 +/- 15.0%; p < 0.0001) in patients compared with age-matched control values. This reduction was larger in 12 patients with extrapyramidal symptoms and was only slowly reversible after discontinuation of treatment. Patients treated for > 6 months had significantly larger reductions than patients treated for a shorter period. Parkinsonian symptoms were only seen in patients older than 50 years. Our findings prove a neuroleptic-like action of Fz and Cz, which seems to be the major reason for their extrapyramidal side effects. Older age and long-term treatment are predisposing factors for these effects.

Neuroprotective profile of lifarizine (RS-87476) in rat cerebrocortical neurones in culture

1. The ability of the neuroprotective agent, lifarizine (RS-87476), to mitigate veratridine-, cyanide- and glutamate-induced toxicity in rat embryonic cerebrocortical neurones in primary culture has been compared with that of tetrodotoxin (TTX), nitrendipine, (+)-MK-801 and (-)-MK-801. Lactate dehydrogenase (LDH) released into the culture medium was used as the indicator of cell viability. 2. Incubation of cultures for 16 h in a medium containing veratridine (10(-4) M), sodium glutamate (10(-3) M) or sodium cyanide (10(-3) M) resulted in consistent elevations of LDH activity in the culture medium. The ability of compounds to attenuate these elevations was expressed as the concentration required to inhibit the increases in LDH release by 50% (IC50). 3. Neurotoxicity induced by veratridine was inhibited by lifarizine (IC50 = 4 x 10(-7) M), TTX (IC50 = 3 x 10(-8) M) and nitrendipine (IC50 = 3 x 10(-5) M). In contrast, (+)-MK-801 (up to 3 x 10(-5) M) was ineffective against this insult. 4. Glutamate-induced neurotoxicity was inhibited by (+)-MK-801 (IC50 = 1.4 x 10(-8) M) and to a lesser extent by (-)-MK-801 (IC50 = 1 x 10(-7) M), but was unaffected by lifarizine, TTX or nitrendipine (up to 10(-6) M). 5. (+)-MK-801 was effective against sodium cyanide-induced neurotoxicity (IC50 = 1.9 x 10(-8) M), whereas lifarizine and TTX (up to 10(-6) M) and nitrendipine (up to 3 x 10(-6) M) were without protective activity against this insult. 6. The results demonstrate that lifarizine potently protects rat cortical neurones in vitro against a neurotoxic insult that requires activation of sodium channels for its expression, and that the compound is ineffective against insults mediated by N-methyl-D-aspartate receptor activation. The weak efficacy of nitrendipine against veratridine-induced cell death argues against the involvement of L-type calcium channels in this insult. These data are consistent with the notion that the neuroprotective activity oflifarizine observed in vivo may be mediated by inhibition of neuronal sodium currents.

[Pharmacologic brain protection: specific agents]

Dysfunctional sodium influx is the first step in the ischaemic cascade. It has been recently demonstrated that reducing ionic flux through voltagegated Na channels shortens the NMDA receptor activity of cultured hippocampal slices in which oxidative phosphorylation and glycolysis have been blocked. The implication of this finding is that blocking initial events in the ischaemic cascade, events which do not directly cause neuronal damage, will reduce the damage done by downstream events. It also seems intuitively reasonable to suppose that truncating initial steps of the ischaemic cascade, as distinct from blocking glutamate receptors and scavening free radicals, will reduce the probability of interfering with endogenous mechanisms of repair. Clinically useful, substantive, prophylactic, pharmacological cerebral protection will come from drugs that work upstream. And for pharmacological protection that can only be initiated subsequent to an ischaemic event, the more we learn about endogenous repair, or genetic pharmacology, the closer we will come to maximizing the benefits and minimizing the costs of downstream intervention.

Effects of Ca2+ channel antagonists on chromaffin cell death and cytosolic Ca2+ oscillations induced by veratridine

Exposure of bovine chromaffin cells to 30 microM veratridine for 24 h led to 70-80% cell death as reflected by phase contrast microscopy, trypan blue exclusion, lactate dehydrogenase (LDH) release and cell catecholamine contents. Na+ deprivation, Ca2+ deletion or tetrodotoxin (5 microM) prevented the veratridine-induced cell damage. Nimodipine and verapamil, but not omega-conotoxin GVIA afforded 20-30% protection. Flunarizine protected the cells by 80% and R56865 by 60%. Stimulation of fura-2-loaded single bovine chromaffin cells with 30 microM of 1,1-dimethyl-4-phenylpiperazinium (DMPP) or 59 mM K+ caused fast increases in cytosolic Ca2+ concentrations, ([Ca2+]i). The [Ca2+]i rose from 0.1 to peaks of 1.9 microM, which quickly declined to near basal levels with a t1/2 of around 30 s. In spite of sustained stimulation with these two depolarizing agents, the [Ca2+]i remained low and did not undergo oscillations. In contrast, veratridine (30 microM) caused large and frequent oscillatory changes in the [Ca2+]i which were long-lasting and did not disappear even 30 min after washing out the toxin. The [Ca2+]i oscillations were reversibly suppressed by Na+ or Ca2+ removal and by 5 microM tetrodotoxin. Selective L-type Ca2+ channel blockers (10 microM nimodipine or verapamil) or N-type Ca2+ channel blockers (1 microM omega-conotoxin GVIA) did not affect the [Ca2+]i oscillations. In contrast, flunarizine or R56865 (10 microM each) suppressed the oscillations of [Ca2+]i. The results demonstrate that bovine chromaffin cells have the necessary machinery to develop prolonged and repetitive [Ca2+]i oscillations in the presence of veratridine; however, 'physiological' depolarizing stimuli did not cause oscillations.(ABSTRACT TRUNCATED AT 250 WORDS)

Postnatal development of voltage-sensitive Na+ channels in rat brain

Previous studies have shown that mammalian neuronal excitability increases with age, and this excitability may be related to development of Na+ channels. In addition, evidence suggests that Na+ channels are involved in the neuronal response to O2 deprivation. Because of this, we wished to examine the pharmacologic properties and neuroanatomical distribution of the Na+ channels in newborn brain and as a function of age. In this study, we used ligand-binding techniques and autoradiography with 3H-saxitoxin (STX) to investigate Na(+)-channel distribution in brains of rats at postnatal days 0, 3, 10, 21, 35, and 120. We found that (1) in each area examined, the Scatchard plots for STX binding were linear in both immature and mature brains in a ligand concentration range of 0.4-64 nM; the slopes, however, were different between areas or ages, with Kd values ranging between 1 and 5 nM; (2) STX-binding density was more than tenfold lower in the rostral brain and cerebellum at birth than in the adult and increased with age; (3) binding density in the newborn brainstem was higher than in other areas such as the cortex and cerebellum, which is opposite to the distribution in the adult; and (d) the brainstem had a different developing pattern with an early-peak density level (P10-21) and a lower adult level.(ABSTRACT TRUNCATED AT 250 WORDS)

Voltage-dependent ion channels in glial cells

Glial cells, although non-excitable, express a wealth of voltage-activated ion channels that are typically characteristic of excitable cells. Since these channels are also observed in acutely isolated cells and in brain slices, they have to be considered functional in the intact brain. Numerous studies over the past 10 years have yielded detailed characterizations of glial channels permitting comparison of their properties to those of their neuronal counterparts. While for the most part such comparisons have demonstrated a high degree of similarity, they also provide evidence for the expression of some uniquely glial ion channels. An increasing number of studies indicate that the expression of "glial" channels is influenced by the cells' microenvironment. For example, the presence of neurons can induce or inhibit (depending on the preparation and type of channel studied) the expression of glial ion channels. Like ion channels in excitable cells, glial channels can be functionally regulated by activation of second-messenger pathways, allowing for short-term modulation of their membrane properties. Although the extent to which most of the characterized ion channels are involved in glial function is presently unclear, a growing body of data suggests that certain channels play an active role in glial function. Thus inwardly rectifying K+ channels in concert with delayed rectifying K+ channels are thought to be involved in the removal and redistribution of excess K+ in the brain, a process referred to as "spatial buffering". Glial K+ channels may also be crucial in modulating glial proliferation. Cl- channels and stretch-activated cation channels are believed to be involved in volume regulation. Na+ channels appear to be important in fueling the glial Na+/K(+)-pump, and Ca2+ channels are likely involved in numerous cellular events in which intracellular Ca2+ is a critical second messenger.

Long-term effects of postnatal hypoxia and flunarizine on the dopaminergic system

Long-term changes of learning behavior and of the striatal dopaminergic system were observed in a rat model of early postnatal hypoxia. Striatal dopamine (DA) concentration, K(+)-stimulated DA release from slices, and DA uptake into crude synaptosomal preparations (S1 fractions) were used as markers of the striatal DAergic system. Active avoidance learning was tested as behavioral criterion. Cyclodextrin and flunarizine were found to produce long-term effects on the DAergic system in control animals. While cyclodextrin normalized hypoxia-induced effects in DA release, flunarizine prevented those in DA uptake and improved avoidance learning.

A comparison of the protective effect of dexamethasone to other potential prophylactic agents in a neonatal rat model of cerebral hypoxia-ischemia

It has recently been reported that pretreatment with a single dose of dexamethasone (0.1 mg/kg) 24 hours before hypoxia in 7-day-old rat pups is protective against an hypoxic-ischemic insult (unilateral carotid artery occlusion followed by 3 hours of hypoxia in 8% O2). The authors now examine whether pretreatment 6 hours before insult is equally effective and compare other agents potentially suitable for prophylaxis in neonatal hypoxia-ischemia, including the calcium antagonists flunarizine (30 mg/kg pretreatment), nimodipine (0.5 mg/kg pretreatment), and the 21-aminosteroid U-74389F (10 mg/kg pre- and posttreatment). For each active agent, there was also a vehicle-treated control group. Comparison of the mean area of ipsilateral infarction on brain coronal sections showed that there was no statistically significant difference between the various control groups (mean area of infarction 66% +/- 4%). Pretreatment with dexamethasone 6 hours prior to hypoxia offered complete protection with no infarction. A beneficial effect was seen following pretreatment with flunarizine (mean area of infarction 33.6% +/- 7.8%), although this degree of damage was still significantly different from that seen with dexamethasone pretreatment. Pretreatment with nimodipine or U-74389F offered no protection (mean area of infarction 77.5% +/- 4% and 59% +/- 10%, respectively). Unlike findings in adult animals and clinical studies, the current studies show that dexamethasone may have a role in the treatment of neonatal hypoxia-ischemia and deserves reappraisal.

K(+)-stimulated 45Ca2+ flux into rat neocortical mini-slices is blocked by omega-Aga-IVA and the dual Na+/Ca2+ channel blockers lidoflazine and flunarizine

High-threshold neuronal voltage-sensitive Ca2+ channels (VSCCs) have been classified into at least three subtypes, including L, N, and P, based on biophysical and pharmacological criteria. We examined K(+)-induced 45Ca2+ flux into rat neocortical mini-slices to determine which of these subtype(s) might be involved in this phenomenon. Neither the L-type Ca2+ channel antagonist isradipine at 10 microM nor the N-type antagonist omega-conotoxin GVIA at 1 microM were effective antagonists of 45Ca2+ flux in this model. However, the P-type Ca2+ channel antagonist, omega-Aga-IVA, blocked 70% of flux at 200 nM, with an IC50 of 17 nM, strongly implicating P-type Ca2+ channel involvement in K(+)-stimulated Ca2+ entry into mammalian nerve terminals. About 30% of the flux response was resistant to the action of omega-Aga-IVA, suggesting that a still uncharacterized subtype of VSCC is involved in Ca2+ entry into mammalian nerve terminals. Both the omega-Aga-IVA sensitive and insensitive components of 45Ca2+ flux were blocked by the diphenylalkylpiperazines, lidoflazine and flunarizine (IC50 = 6.4 microM and 11 microM, respectively), which have dual Na+/Ca2+ channel blocking actions.

R56865 and flunarizine as Na(+)-channel blockers in isolated Purkinje neurons of rat cerebellum

Dose-related blocking effects of R56865, flunarizine and nimodipine on voltage-activated Na+ currents recorded in the whole-cell voltage clamp mode were studied in acutely isolated Purkinje neurons of rat cerebellum. The dose-dependences of blocking action were obtained for all drugs at a holding potential of -110 mV and rare stimulation. At stimulation frequencies 5 and 15 Hz the block produced by R56865 was increased showing a shift of dose-dependence to lower concentrations of antagonist. This shift was less pronounced for flunarizine, practically absent for nimodipine, and increased for all drugs with an increase in the amplitude of stimulating voltage pulse. With the change in holding potential to -80 mV the block produced by R56865 and flunarizine increased showing a dose-dependence shift to lower concentrations of antagonists. All the drugs tested induced parallel shifts of the steady-state voltage-dependence of inactivation of Na+ channels to more negative membrane potentials. R56865, and to a lesser extent flunarizine, slowed down the recovery of Na+ channels from steady-state inactivation increasing the relative number of channels which showed slow recovery. In the absence of Na+ current inactivation (treatment by intracellular pronase) R56865 at a concentration of 1 microM blocked modified channels preferentially in the open state, while the block produced by flunarizine showed no dependence on voltage pulse protocol. R56865 was shown to decrease the cell leakage while other drugs produced little or no effect. It is concluded that R56865 and flunarizine block Na+ currents predominantly by interacting with inactivated Na+ channels. The higher ability of R56865 to block open channels and to increase slow inactivation underlies its higher frequency-dependence. These characteristics suggest the use of R56865 and flunarizine in the treatment of cerebral ischemia.

R56865 as Ca(2+)-channel blocker in Purkinje neurons of rat: comparison with flunarizine and nimodipine

The blocking action of recently synthesized benzothiazolamine derivative R56865 was compared with that of dihydropyridine (nimodipine) and diphenylalkylamine (flunarizine) on low-voltage-activated and non-inactivating high-voltage-activated Ca2+ currents. The experiments were carried out on freshly isolated Purkinje neurons of rat cerebellum using patch-clamp technique in the whole-cell configuration. Among the substances tested R56865 was found to be the most effective blocker of the Ca2+ current. In the sequence R56865, flunarizine and nimodipine, apparent Kd values for low-voltage-activated current are 0.1, 0.9 and 3.5 microM, and for high-voltage-activated current 3.1, 9.5 and 38 microM, respectively. The current-voltage relationships for both types of currents displayed little or no shift under either flunarizine or R56865 but showed a 10-mV shift in the positive direction under the action of nimodipine. The steady-state inactivation curves for low-voltage-activated calcium currents were shifted under the action of R56865, flunarizine and nimodipine (in concentrations which blocked 50-60% of the current) to more negative membrane potentials for 20, 10 and 6 mV, respectively. In contrast to R56865, flunarizine blocked both types of Ca2+ channel in a use-dependent manner. It is concluded that the order of potency of Ca2+ antagonist for both types of channels studied is R56865 > flunarizine > nimodipine. Strong shift of steady-state inactivation relationship by R56865 can further facilitate its blocking action in in vivo conditions.

Neuroanatomical distribution and binding properties of saxitoxin sites in the rat and turtle CNS

Since saxitoxin (STX) binds to voltage-sensitive sodium channels and blocks their function, it has been widely used in the study of these channels. There is, however, limited information on STX binding properties and the neuroanatomical distribution of the Na+ channel as a function of brain region in the rat and in lower vertebrates such as the turtle. In the present study, we used a broad range of 3H-STX concentration (up to 64 nM) to examine saturation profiles and density distribution in both adult rat and turtle brains. We found that (1) STX sites do not vary greatly in affinity (most Kds = 2 to 5 nM) in various regions of the adult rat brain; (2) STX binding distribution was very heterogeneous in the rat with much higher density in the cortex, hippocampus, amygdala, and cerebellum than in the brainstem and spinal cord; (3) STX sites are mostly localized in layers made mostly of neurons with low density in white matter; and (4) turtle brain STX sites had similar binding properties, but its brain had much fewer STX sites than the rat, especially in the cerebellum and rostral areas such as the cortex. We conclude that (a) adult brain sodium channels have similar STX binding affinity in spite of the existence of multiple sodium channel subtypes; (b) the brainstem is very different from rostral brain areas in channel density; and (c) the turtle brain has a much lower sodium channel density than the rat brain.

Inhibitory effects of calcium channel antagonists on motor dysfunction induced by intracerebroventricular administration of paraquat

This study reports the effects of Ca2+ channel blockers (Ca antagonists) on intraneuronal Ca2+ ([Ca2+]i) movements and on the disturbance of rotarod performance produced in rats by intracerebroventricular administration of paraquat. Paraquat (50 nmol) produced a decrement in rotarod performance which was present at 30 min. and maximal at 60 min. and was not associated with overt behavioural changes; larger doses of paraquat (100-400 nmol intracerebroventricularly) produced paresis and convulsions which severely disrupted rotarod behaviour. The disruption of rotarod performance after paraquat (50 nmol intracerebroventricularly) was significantly reduced by giving Ca antagonists (flunarizine, verapamil and nicardipine) not only intraperitoneally 15 min. after paraquat but also intracerebroventricularly immediately before paraquat. The order of pharmacological potency was flunarizine > or = verapamil > nicardipine. In contrast, intracerebroventricular administration of Bay K 8644, a Ca agonist, enhanced the disruption of rotarod performance caused by paraquat (50 nmol). In in vitro studies, paraquat markedly potentiated the rapid increase in [Ca2+]i levels evoked by 50 mM KCl in rat brain synaptosomal fraction, although paraquat alone produced a small prolonged rise in [Ca2+]i levels which had a slow onset. The above results suggest that paraquat induced neurotoxicity is associated with increased [Ca2+]i levels in brain neuronal cells, and that paraquat might effect on membrane activity instability.

Arachidonic acid-induced Ca2+ release from isolated sarcoplasmic reticulum

Arachidonic acid has been shown to release Ca2+ from isolated skeletal and cardiac sarcoplasmic reticulum (SR) vesicles. The release took place nearly equally well from all fractions of the SR and was only partially inhibited by ruthenium red, suggesting that some other pathway is involved in addition to the SR Ca2+ release channel. Arachidonic acid increased SR Ca2+ efflux even in the presence of several different SR Ca2+ pump inhibitors. It also had considerably less effect on uptake measured in the presence of oxalate and did not appear to inhibit Ca(2+)-dependent ATPase activity. Thus, the SR Ca2+ pump also appears to be minimally perturbed by arachidonic acid. Arachidonyl CoA was more effective at releasing Ca2+ than the parent compound. Arachidonic acid effects were not inhibited by lipoxygenase or cyclooxygenase inhibitors, suggesting that no eicosanoids are involved in the effects under study here. Flunarizine, cinnarizine and propyl-methylenedioxyindene inhibited the Ca2+ release induced by arachidonic acid. The effects of arachidonic acid appear to depend on the ratio of arachidonic acid to membrane vesicles.

Effects of L-type calcium channel antagonists on the serotonin-depleting actions of MDMA in rats

The calcium channel antagonists verapamil nifedipine and flunarizine all increased the threshold for convulsions induced by N-methyl-D-aspartate in rats. By contrast, only flunarizine blocked the long-term serotonin-depleting effects of 3,4-methylenedioxymethamphetamine. Flunarizine was also the only drug that antagonized methamphetamine-induced stereotypy. These findings suggest that calcium influx through L-type channels does not participate in the neurotoxic mechanism of MDMA, and that the neuroprotective actions of flunarizine are probably related to its anti-dopaminergic activity.

Effects of Ca2+ antagonists and antiepileptics on tetrodotoxin-sensitive Ca(2+)-conducting channels in isolated rat hippocampal CA1 neurons

All Ca2+ antagonists blocked tetrodotoxin-sensitive Ca2+ current (TTX-ICa) more potently than Na+ current (INa). Phenytoin and MK-801, at concentrations which had no effect on INa, could block TTX-ICa concentration-dependently. Valproic acid and phenobarbital had no effect on both TTX-ICa and INa. In particular, flunarizine and phenytoin have more potent inhibitory effects on TTX-ICa than other test drugs. These results suggest that the abnormal excess-excitation of TTX-sensitive Ca(2+)-conducting channels may be one of the trigger factors generating epilepsy.