Cerebrovascular, biochemical, and cytoprotective effects of isradipine in laboratory animals

Ca2+ channel blockade reduces cocaine's vasoconstriction and neurotoxicity in the prefrontal cortex

Cocaine profoundly affects both cerebral blood vessels and neuronal activity in the brain. The vasoconstrictive effects of cocaine, concurrently with its effects on neuronal [Ca²⁺]_i accumulation are likely to jeopardize neuronal tissue that in the prefrontal cortex (PFC) could contribute to impaired self-regulation and compulsive cocaine consumption. Here we used optical imaging to study the cerebrovascular and neuronal effects of acute cocaine (1 mg/kg i.v.) and to examine whether selective blockade of L-type Ca²⁺ channels by Nifedipine (NIF) (0.5 mg/kg i.v.) would alleviate cocaine's effects on hemodynamics (measured with cerebral blood volume, HbT), oxygenation (measured with oxygenated hemoglobin, HbO₂) and neuronal [Ca²⁺]_i, which were concomitantly measured in the PFC of naive rats. Our results show that in the PFC acute cocaine significantly reduced flow delivery (HbT), increased neuronal [Ca²⁺]_i accumulation and profoundly reduced tissue oxygenation (HbO₂) and these effects were significantly attenuated by NIF pretreatment. They also show that cocaine-induced vasoconstriction is distinct from its increase of neuronal [Ca²⁺]_i accumulation though both of them contribute to hypoxemia and both effects were attenuated by NIF. These results provide evidence that blockade of voltage-gated L-type Ca²⁺ channels might be beneficial in preventing vasoconstriction and neurotoxic effects of cocaine and give support for further clinical investigations to determine their value in reducing cocaine's neurotoxicity in cocaine use disorders.

L-type calcium channels and psychiatric disorders: A brief review

Emerging evidence from genome-wide association studies (GWAS) support the association of polymorphisms in the alpha 1C subunit of the L-type voltage-gated calcium channel gene (CACNA1C) with bipolar disorder. These studies extend a rich prior literature implicating dysfunction of L-type calcium channels (LTCCs) in the pathophysiology of neuropsychiatric disorders. Moreover, calcium channel blockers reduce Ca(2+) flux by binding to the α1 subunit of the LTCC and are used extensively for treating hypertension, preventing angina, cardiac arrhythmias and stroke. Calcium channel blockers have also been studied clinically in psychiatric conditions such as mood disorders and substance abuse/dependence, yielding conflicting results. In this review, we begin with a summary of LTCC pharmacology. For each category of disorder, this article then provides a review of animal and human data. In particular, we extensively focus on animal models of depression and clinical trials in mood disorders and substance abuse/dependence. Through examining rationale and study design of published clinical trials, we provide some of the possible reasons why we still do not have definitive evidence of efficacy of calcium-channel antagonists for mood disorders. Refinement of genetic results and target phenotypes, enrollment of adequate sample sizes in clinical trials and progress in physiologic and pharmacologic studies to synthesize tissue and isoform specific calcium channel antagonists, are all future challenges of research in this promising field. © 2010 Wiley-Liss, Inc.

MR microscopy and high resolution small animal MRI: applications in neuroscience research

The application of magnetic resonance (MR) imaging in the study of human disease using small animals has steadily evolved over the past two decades and strongly established the fields of "small animal MR imaging" and "MR microscopy." An increasing number of neuroscience related investigations now implement MR microscopy in their experiments. Research areas of growth pertaining to MR microscopy studies are focused on (1). phenotyping of genetically engineered mice models of human neurological diseases and (2). rodent brain atlases. MR microscopy can be performed in vitro on tissue specimens, ex vivo on brain slice preparations and in vivo (typically on rodents). Like most new imaging technologies, MR microscopy is technologically demanding and requires broad expertise. Uniform guidelines or "standards" of a given MR microscopy experiment are non-existent. The main focus therefore of this review will be on biological applications of MR microscopy and the experimental requirements. We also take a critical look at the biological information that small animal (rodent) MR imaging has provided in neuroscience research.

Cytoprotection does not preserve brain functionality in rats during the acute post-stroke phase despite evidence of non-infarction provided by MRI

In animal models of stroke the promise of a therapy is commonly judged from infarct size measurements, assuming that a reduction in infarct size results in reduction of the functional deficits. We have evaluated the validity of the concept that structural integrity translates into functional integrity during the acute post-stroke period (24 h). Unilateral permanent middle cerebral artery occlusion (pMCAO) in Fischer F344 rats leads to infarcts comprising the ipsilateral striatum and cortical structures, including the somatosensory cortex. Infarct volumes were assessed using magnetic resonance imaging (MRI) methods (T(2), diffusion, perfusion MRI). The functional integrity of the somatosensory cortex was assessed by functional MRI (fMRI) measuring changes in local cerebral blood volume, and by assessing the forelimb grip strength and the beam-walking performance of the animals. Treatment with the calcium antagonist isradipine (2.5 mg/kg injected s.c. immediately after pMCAO) reduced the total infarct size by more than 40% compared to vehicle-injected controls. In particular, the ipsilateral somatosensory cortex appeared normal in diffusion- and T(2)-weighted MRI images. In sham-operated rats simultaneous electrical stimulation of both forepaws led to similar activation of both somatosensory cortices, while in pMCAO animals given vehicle only the contralateral cortex showed an fMRI response. Similarly, in pMCAO rats treated with isradipine, functional activation following bilateral electrical stimulation was only detected in the contralateral somatosensory cortex despite the normal appearance of the ipsilateral cortex in MRI images. Furthermore, fMRI responses to pharmacological stimulation with bicuculline were virtually absent in the ipsilateral somatosensory cortices both in vehicle- and isradipine-treated rats. Finally there was no significant difference between vehicle- and isradipine-treated animals upon the performance of beam-walking test or in forelimb grip strength. It is concluded that during the acute (24 h) post-occlusion period, structural integrity in the somatosensory cortex revealed by MRI does not translate into preservation of function.

Calcium antagonist isradipine reduces metabolic alterations in acute cerebral ischemia in spontaneously hypertensive rats

The present study was designed to examine the effect of a calcium antagonist isradipine (PN200-110: PN) on local cerebral blood flow and brain tissue metabolism after 1-hour supratentorial ischemia induced by bilateral carotid artery ligation (BCL) in spontaneously hypertensive rats (SHR). PN, dissolved in ethanol plus polyethylene glycol 400, diluted with saline to make the final concentration of 0.25mg/ml and 2.5mg/ml, was administered subcutaneously either 30 min prior to BCL or just after the induction of incomplete cerebral ischemia (n = 7 in each group). Vehicle injection was served as a control group (n = 7). Cerebral blood flow in the parietal cortex (CBF) and the cerebellar cortex (CeBF) was measured by hydrogen clearance technique, and the supra- and infratentorial metabolites of the brain frozen in situ were determined by the enzymatic method. Blood pressure was lowered, but CBF was increased by PN administration in pre-BCL treatment study. After 1 hour of BCL, CBF decreased to around 10% or less of the resting value, being insignificant among the groups. Brain adenosine triphosphate was better preserved in PN-administered groups. The increase in lactate level tended to reduce dose dependently by PN treatment. PN also reduced the metabolic alterations in brain tissue with significance, even when administered just after the induction of forebrain ischemia. It is considered that pre- as well as post-BCL administration of PN is beneficial to attenuate the metabolic alterations in incomplete forebrain ischemia in SHR.

In vivo magnetic resonance methods in pharmaceutical research: current status and perspectives

In the last decade, in vivo MR methods have become established tools in the drug discovery and development process. In this review, several successful and potential applications of MRI and MRS in stroke, rheumatoid and osteo-arthritis, oncology and cardiovascular disorders are dealt with in detail. The versatility of the MR approach, allowing the study of various pathophysiological aspects in these disorders, is emphasized. New indication areas, for the characterization of which MR methods have hardly been used up to now, such as respiratory, gastro-intestinal and skin diseases, are outlined in a subsequent section. A strength of MRI, being a non-invasive imaging modality, is the ability to provide functional, i.e. physiological, readouts. Functional MRI examples discussed are the analysis of heart wall motion, perfusion MRI, tracer uptake and clearance studies, and neuronal activation studies. Functional information may also be derived from experiments using target-specific contrast agents, which will become important tools in future MRI applications. Finally the role of MRI and MRS for characterization of transgenic and knock-out animals, which have become a key technology in modern pharmaceutical research, is discussed. The advantages of MRI and MRS are versatility, allowing a comprehensive characterization of a diseased state and of the drug intervention, and non-invasiveness, which is of relevance from a statistical, economical and animal welfare point of view. Successful applications in drug discovery exploit one or several of these aspects. In addition, the link between preclinical and clinical studies makes in vivo MR methods highly attractive methods for pharmaceutical research.

Ca2+ channel antagonists and neuroprotection from cerebral ischemia

Stroke is the third leading cause of death and the main disabling neurologic disease. The finding in experimental studies that neuronal death does not occur immediately after ischemic injury has encouraged the development of neuroprotective agents. Various Ca2+ channel antagonists, that is, L-type-selective or non-selective derivatives from classical Ca2+ channel antagonists, have been examined for their ability of neuroprotection through improvement of cerebral blood circulation or inhibition of Ca2+ overload induced by excessive glutamate release. Although some of the antagonists showed efficient neuroprotection in animal models, systemic hypotension limited the utility of these drugs, and none of the compounds showed beneficial effects in treatments for acute ischemic stroke in clinical trials. Drugs other than Ca2+ channel antagonists developed on the basis of the glutamate-Ca2+ overload hypothesis were shown also to lack clinical benefit. Recently, some mechanisms have been proposed to interpret neuronal death in relation to hyperexcitability or apoptosis after ischemic insult. In these hypotheses, activation of the Ca2+ channel types selectively expressed in neuronal tissues is proposed as a critical step of the pathways toward neurodegeneration. Thus, it is increasingly recognized that developing highly selective compounds for neuronal Ca2+ channels is not only important for treatment of stroke but also for elucidation of mechanisms that underlie neurodegeneration.

Effects of a calcium antagonist, lacidipine, on experimental focal cerebral ischemia in rats

We investigated the effects of lacidipine on focal cerebral ischemia in rats, and these effects were compared with those of nicardipine. Drugs were administered orally 5 min after middle cerebral artery occlusion (MCAO). Neurological scores as described by Bederson et al. (Stroke 17, 472-476, 1986) and cerebral infarct size (CIS) determined by the 2,3,5-triphenyltetrazolium chloride staining method were measured 24 hr after MCAO. Cerebral blood flow (CBF) and energy metabolites were determined by the hydrogen clearance method and an enzymatic method, respectively. In the drug-untreated group, we observed low-CBF of approximate 13 ml/100 g/min during 0.5-6 hr of occlusion and extensive cerebral infarction associated with severe neurologic deficits (ND). Lacidipine at 1 and 3 mg/kg, although it lowered blood pressure, improved low-CBF to approximate 20 ml/100 g/min during 1.5-6 hr of occlusion and increased tissue levels of ATP 6 hr after MCAO in a dose-dependent manner. Nicardipine at 30 mg/kg also improved low-CBF and increased tissue levels of ATP significantly. However, the improvement of low-CBF by nicardipine was transient. Lacidipine at 3 mg/kg reduced CIS and ameliorated ND significantly. In contrast, nicardipine at 30 mg/kg could not ameliorate ND in spite of a significant reduction of CIS similar to that of lacidipine (3 mg/kg). These results suggest that the improvement of focal cerebral ischemia by lacidipine may be partly due to long-lasting improvement of collateral blood supply to the ischemic area.

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.

Haemodynamics and tissue specificity with isradipine

The significant effects of isradipine in producing arterial vasodilation and mild negative chronotropic effects without significant negative inotropic effects suggests that this agent should provide excellent antihypertensive efficacy in the treatment of perioperative hypertension. Isradipine may prove to be a safe antihypertensive treatment in patients with impaired ventricular function (cardiac failure), impaired myocardial perfusion (ischaemia), and in cases of selected conduction abnormalities or arrhythmias. The demonstration that its effects are limited to vascular resistance rather than vascular capacitance is an important distinguishing feature of isradipine compared with other antihypertensive agents. Finally, the potential application of this dihydropyridine calcium antagonist for cytoprotection and its effects on atherosclerosis remain exciting therapeutic prospects.

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

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

Diffusion- and T2-weighted imaging: evaluation of oedema reduction in focal cerebral ischaemia by the calcium and serotonin antagonist levemopamil

Magnetic resonance imaging (MRI) was used to evaluate beneficial drug effects in a rat model of focal cerebral ischaemia. Extent of cerebral oedema was measured on T2-weighted images 24 hr after permanent middle cerebral artery occlusion (MCAO) in spontaneously hypertensive rats. Areas of increased signal intensity strongly correlate with histochemically determined areas of ischaemia in corresponding brain planes (r = 0.84; p < .001). In a separate cohort of animals, spatial progression of oedema formation was studied at 3, 24, 48, and 72 hr after MCAO showing a maximum extent at 48 hr. Early events in cerebral ischaemia were monitored using diffusion-weighted imaging. Effects of levemopamil [formerly (S)-emopamil], a calcium and serotonin antagonist, and the reference compound isradipine were quantified on high resolution T2-weighted spin-echo images 24 hr after MCAO. Combined pre- and posttreatment with isradipine showed a 21% inhibition of oedema progression. Application of a single dose (10 mg/kg) of levemopamil either 30 min before or 2 hr after MCAO revealed a diminution of oedematous areas by 19% and 25%, respectively. Levemopamil reduces the extent of ischaemic brain oedema in an established stroke model.

Pharmacologic management of neonatal cerebral ischemia and hemorrhage: old and new directions

New developments in pharmacologic management of cerebral ischemia and hemorrhage are reviewed. A number of agents with diverse modes of action have now been shown to be neuroprotective in adult and neonatal animal models when administered either before or after a hypoxic-ischemic insult. As experience improves with these agents in hypoxic-ischemic injury and periventricular-intraventricular hemorrhage in human neonates, there is reason to be optimistic that effective neuroprotective strategies will soon be clinically available.

Alteration of voltage-dependent calcium channels in canine brain during global ischemia and reperfusion

Elevated intracellular calcium (iCa2+) plays an important role in the pathophysiology of ischemic brain damage. The mechanisms by which iCa2+ increases are uncertain. Recent evidence implicates the voltage-dependent calcium channel (VDCC) as a likely site for the alteration in Ca2+ homeostasis during ischemia. The purpose of this study was to determine whether VDCCs are altered by global ischemia and reperfusion in a canine cardiac arrest, resuscitation model. We employed the radioligand, [3H]PN200-110, to quantitate the equilibrium binding characteristics of the VDCCs in the cerebral cortex. Twenty-five adult beagles were separated into four experimental groups: (a) nonischemic controls, (b) those undergoing 10-min ventricular fibrillation and apnea, (c) those undergoing 10-min ventricular fibrillation and apnea followed by spontaneous circulation and controlled respiration for 2 and (d) 24 h. Brain cortex samples were taken prior to killing of the animal, frozen immediately in liquid nitrogen, and crude synaptosomal membranes isolated by differential centrifugation/filtration. After 10 min of ischemia the maximal binding (Bmax) of [3H]PN200-110 increased to greater than 250% of control values (control Bmax 11.16 +/- 0.98; ischemic 28.35 +/- 2.78 fmol/mg protein; p less than 0.05). Bmax returned to near control values after 2 h of reperfusion but remained significantly greater than the control at 24 h. Although the affinity constant (Kd) (control = 0.12 +/- 0.03 nM) appeared to increase with ischemia and normalize with reperfusion, the changes were not statistically significant. We conclude that the binding of [3H]PN200-110 to L-type VDCCs is increased after 10 min of global ischemia/anoxia produced by ventricular fibrillation and apnea in the dog.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurocytotoxicity: pharmacological implications

Recent data suggest that brain damage in ischemia, hypoglycemia, and several other brain diseases is caused by excitotoxic mechanisms which are triggered by presynaptic release of glutamate and related excitatory amino acids, and which involve an abnormal postsynaptic influx of calcium into cells containing a high density of glutamate receptors. This contention is supported by results demonstrating reduction of infarct size in focal ischemia due to middle cerebral artery (MCA) occlusion, and amelioration of neuronal necrosis in hypoglycemic coma, by antagonist which block the NMDA type of glutamate receptor. These results underscore the pathogenetic role of calcium influx into energy-compromised cells since the NMDA receptor-linked ion channel has a high conductance to calcium. The issue has been clouded by the inability of NMDA antagonists to ameliorate brain damage due to cardiac arrest, or to forebrain ischemia in rats and gerbils. In these conditions, however, an AMPA receptor blocker (NBQX) has been found efficacious. These results demonstrate that the pathophysiology of ischemic lesions is different in the cardiac arrest type of ischemia and in lesions due to MCA occlusion, and demand an explanation of the differences in therapeutic response. Tentatively, the cardiac arrest type of ischemia is so dense that multiple calcium conductances are activated in the energy-deprived tissue, explaining why any drug which acts on only one of them (such as an NMDA antagonist) cannot prevent cellular calcium overload. Furthermore the ultimate brain damage, which is often conspicuously delayed, may be secondary to upregulation of synaptic efficacy, causing increased calcium cycling and calcium-related damage. In this situation, an AMPA receptor blocker may be efficacious because it blocks "fast" excitation and Na+ influx, an "upstream" event which causes "downstream" calcium influx via multiple pathways. In the perifocal ("penumbra") zone of a stroke lesion, the situation is different since depolarisation is initially moderate and/or intermittent. Furthermore, since ATP is still produced (albeit at a reduced rate) the problem is one of a disturbed pump/leak relationship. Then, blockade of a major calcium-carrying channel by NMDA receptor blockers, or of the trigger to depolarisation by an AMPA receptor antagonist, may improve the pump/leak relationship and carry cells in the penumbra over a critical period.

Isradipine

Isradipine is a potent dihydropyridine calcium channel blocker. It is highly selective for vascular smooth muscle, with very few negative inotropic or chronotropic effects. It may have minor depressant effects on the sinoatrial node, hence reducing the incidence of reflex tachycardia. The drug is extensively metabolized in the liver, with several pharmacologically inactive metabolites. As the elimination half-life is about 9 h the drug is usually given twice daily, but a once-daily modified release form is under investigation. Isradipine is effective monotherapy in essential hypertension, and has been successfully combined with pindolol and captopril. On the basis of more limited evidence it also appears to be beneficial in stable angina and in congestive cardiac failure. A trial is also under way to assess its antiatherogenic properties. Adverse effects are those predicted for a vasodilator calcium antagonist, and may be less frequent than for equivalent doses of nifedipine. This needs to be confirmed by more extensive clinical experience. Overall, isradipine can be considered favourably in essential hypertensives where a calcium channel blocker is indicated, particularly if it is desirable to avoid myocardial depression or to minimize reflex tachycardia. Its role in other cardiovascular disease awaits further evaluation.