Verapamil and flunarizine inhibit phencyclidine-induced effects: an EEG and behavioural study in rats

Reversal of ATP-binding cassette drug transporter activity to modulate chemoresistance: why has it failed to provide clinical benefit?

Enhanced drug extrusion from cells due to the overexpression of the ATP-binding cassette (ABC) drug transporters inhibits the cytotoxic effects of structurally diverse and mechanistically unrelated anticancer agents and is a major cause of multidrug resistance (MDR) of human malignancies. Multiple compounds can suppress the activity of these efflux transporters and sensitize resistant tumor cells, but despite promising preclinical and early clinical data, they have yet to find a role in oncologic practice. Based on the knowledge of the structure, function, and distribution of MDR-related ABC transporters and the results of their preclinical and clinical evaluation, we discuss probable reasons why these inhibitors have not improved the outcome of therapy for cancer patients. We also outline new MDR-reversing strategies that directly target ABC transporters or circumvent relevant signaling pathways.

Atypical antipsychotic profile of flunarizine in animal models

RATIONALE

Flunarizine is known as a calcium channel blocker commonly used in many countries to treat migraine and vertigo. Parkinsonism has been described as one of its side-effects in the elderly, which is in agreement with its recently characterized moderate D2 receptor antagonism.

OBJECTIVES

To perform a pre-clinical evaluation of flunarizine as a potential antipsychotic.

METHODS

We evaluated the action of orally administered flunarizine in mice against hyperlocomotion induced by amphetamine and dizocilpine (MK-801) as pharmacological models of schizophrenia, induction of catalepsy as a measure for extrapyramidal symptoms and impairment induced by dizocilpine on the delayed alternation task for working memory.

RESULTS

Flunarizine robustly inhibited hyperlocomotion induced by both amphetamine and dizocilpine at doses that do not reduce spontaneous locomotion (3-30 mg/kg). Mild catalepsy was observed at 30 mg/kg, being more pronounced at 50 mg/kg and 100 mg/kg. Flunarizine (30 mg/kg) improved dizocilpine-induced impairment on the delayed alternation test.

CONCLUSIONS

These results suggest a profile comparable to atypical antipsychotics. The low cost, good tolerability and long half-life (over 2 weeks) of flunarizine are possible advantages for its use as an atypical antipsychotic. These results warrant clinical trials with flunarizine for the treatment of schizophrenia.

Effects of calcium channel blockers on behaviors induced by the N-methyl-D-aspartate receptor antagonist, dizocilpine, in rats

The present study assessed the ability of voltage-sensitive calcium channel (VSCC) blockers to affect the behavioral effects of the noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist, dizocilpine, in male Wistar rats. Dizocilpine produced dose-dependent increases in locomotor activity. Nimodipine, verapamil, and flunarizine suppressed dizocilpine-facilitated vertical activity, while horizontal activity was attenuated by verapamil and nimodipine but not flunarizine. Repeated dizocilpine injections resulted in the development of sensitization to its locomotor stimulating properties. Development of sensitization was not context specific, and was observed following repeated exposures to 0.1 but not 0.056 or 0.3 mg/kg of dizocilpine. Nimodipine retarded the development of sensitization to dizocilpine's stimulating effects on horizontal activity, while verapamil suppressed sensitization to the vertical stimulating effects of dizocilpine. Flunarizine had no significant effects on sensitization to dizocilpine's locomotor stimulating properties. In rats trained to discriminate between injections of 0.056 mg/kg of dizocilpine and vehicle, none of the tested VSCC blockers was able to completely antagonize the discriminative stimulus properties of dizocilpine. Nimodipine, when administered in combination with the training dose of dizocilpine, modestly decreased the dizocilpine-lever selection. Dizocilpine dose dependently decreased the self-determined stimulation threshold implanted in rats with electrodes into the ventral tegmental area. Nimodipine exhibited some tendency to block the facilitating effects of dizocilpine, while verapamil and flunarizine had no effects. In summary, in the present experiments VSCC blockers exerted only modest interactions with the behavioral effects of dizocilpine, and it is unlikely that VSCC blockers have remarkable potential as adjunct treatment aimed at correcting the negative side effects of NMDA receptor antagonists (e.g., dizocilpine).

Differentiation of the inhibitory effects of calcium antagonists on abnormal behaviors induced by methamphetamine or phencyclidine

The abnormal behaviors induced by methamphetamine (MAP: 1 mg/kg) or phencyclidine (PCP: 10 mg/kg) were measured using behavioral analysis following the microinjection of one of three calcium (Ca) antagonists (nifedipine: Nif, nicardipine: Nic and flunarizine: Flu) into the rat caudate putamen or amygdala. The intraperitoneal administration of MAP or PCP induced abnormal behaviors in a time-dependent manner; the maximum locomotor activity was measured 30-45 min after the administration of MAP or PCP. This hyperactivity was sustained for more than 2 h. Following microinjection of these Ca antagonists into the caudate putamen, each showed a different pattern of inhibition on MAP- or PCP-induced abnormal behaviors. Nic and Flu were effective at reducing these abnormal behaviors, in contrast, Nif was ineffective. In particular, the inhibitory effect of Nic was stronger than that of Flu. Microinjection of these Ca antagonists into the amygdala did not show any reductive effect on the hyperactivity induced by MAP or PCP. These results demonstrate that these Ca antagonists have different pharmacological properties and that both L- and T-type Ca2+ channels modulate the dopamine release in the rat caudate putamen. These results further lead us to suggest the presence of a subtype of L-type Ca2+ channels in the caudate putamen.

Calcium channels mediate angiotensin II-induced drinking behaviour and c-fos expression in the brain

It is widely accepted that calcium ions are critically important in both short- and/or long-lasting responses of neurons to a stimulus. We have shown previously that NMDA receptors play a role in dipsogenic responses and c-fos expression induced by intracerebroventricular (i.c.v.) infusion of angiotensin II (Ang II). Since NMDA receptors are known to be linked to receptor-operated calcium channels, this study determined whether voltage dependent calcium channels are also involved in Ang II-induced behavioural (drinking) and endocrine responses as well as c-fos expression. The antidipsogenic actions of three L-type calcium channel antagonists, nifedipine, diltiazem and verapamil on Ang II-induced drinking behaviour were studied. These bind to the dihydropyridine, phenylalkylamine and benzothiazepine sites respectively. Rats (Lister-hooded) pre-treated i.c.v. with either 25 or 100 microg nifedipine, followed by 25 pmol Ang II, drank significantly less water than controls during the first 15 min after infusion. However, rats pre-treated with i.c.v. 100 microg diltiazem or verapamil showed no change in Ang II-induced drinking behaviour. The antidipsogenic actions of N- and P-type calcium channel antagonists omega-conotoxin GVIA and omega-conotoxin MVIIC were also evaluated. Rats pre-treated with 5 pmol or 20 pmol omega-conotoxin GVIA did show a slight but not significant suppression of water intake, particularly after the higher dose. Rats pre-treated with omega-conotoxin MVIIC drank almost the same amount of water as those pre-treated with saline. Nifedipine was found to suppress both Ang II-induced corticosterone release and c-fos expression in the following areas: organum vasculosum of the lamina terminalis (OVLT), median preoptic nucleus (MNPO), hypothalamic paraventricular nucleus (PVN) and supraoptic nucleus (SON). The results described in this paper provide evidence that calcium channels play important roles in the Ang II-induced behavioural and endocrine responses, and in the expression of the immediate-early gene c-fos. This suggests that an L-type calcium channel may participate both short- and longer-term neuronal actions of Ang II.

Effects of sigma ligands on the ability of rimcazole to inhibit PCP hsp70 induction

Phencyclidine (PCP) can result in schizophrenia-like behavior. It binds at the PCP site on the NMDA-receptor calcium channel and at the sigma receptor. PCP also induces the heat shock gene hsp7O in retrosplenial cortex neurons. An antipsychotic drug, rimcazole, inhibits PCP hsp7O induction. Rimcazole binds predominantly to sigma-2 sites. It is hypothesized that sigma ligands without antipsychotic properties and with some sigma-2 affinity should partially reverse the effects of rimcazole. (+)-3-PPP, (+)-cyclazocine, and (+)-pentazocine bind predominantly to sigma-I sites. (+)-3-PPP is also a modest sigma-2 ligand. Female Sprague-Dawley rats (200-260 g) were injected intraperitoneally (IP) with (+)-3-PPP (50 mg/kg), rimcazole (60 mg/kg) and, after 5 min, with PCP (40 mg/kg). Brains were sectioned (100 mu m) and presence of the hsp7O gene protein product, HSP7O, was determined immunocytochemically. (+)-3-PPP significantly (0 <0.05) diminished the ability of rimcazole to inhibit PCP hsp7O induction in the retrosplenial cortex. (+)-Cyclazocine (15mg/kg, IP) and (+)-pentazocine (8Omg/kg, IP) given in an analogous manner did not diminish the ability of rimcazole to inhibit PCP hsp7O induction.

DNQX inhibits phencyclidine (PCP) and ketamine induction of the hsp70 heat shock gene in the rat cingulate and retrosplenial cortex

Phencyclidine (PCP) and ketamine are known to block NMDA receptor mediated excitotoxicity by non-competitively blocking the NMDA receptor calcium channel. PCP and ketamine have the paradoxical effect of also inducing the heat shock gene, hsp70, in the cingulate and retrosplenial cortex of the rat. The present study shows that DNQX, a specific AMPA receptor antagonist, given as either a 5 mg/kg or 10 mg/kg intraperitoneal dose or into the lateral cerebral ventricle (5 microliters of 0.5 mg/ml) significantly diminished PCP (40 mg/kg) and ketamine (80, 100, 120 mg/kg) hsp70 induction in the posterior cingulate and retrosplenial cortex. The most dramatic decrease of hsp70 induction was seen with the intraventricular dose of DNQX. Present findings show that the AMPA receptor has a role in PCP/ketamine induction of hsp70 in the cortex. DNQX inhibition of PCP/ketamine hsp70 induction was likely related to AMPA receptor antagonism which prevented excess calcium influx via voltage-gated calcium channels.

Effects of calcium channel inhibitors on ethanol effects and pharmacokinetics in healthy volunteers

Previous studies have shown that calcium channel antagonists alter the effects of alcohol in animals and humans. We selected a phenylalkylamine, verapamil, and a dihydropyridine, nimodipine, to determine whether these drugs would affect the subjective or psychomotor effects of ethanol in humans. Subjects ingested verapamil (80 mg, PO), nimodipine (30 and 60 mg, PO), or placebo 60 min before drinking an alcohol (0.7 g/kg) or placebo beverage. Subjects' mood, psychomotor performance, physiological status, and blood alcohol levels were assessed up to 3 h after beverage ingestion. Alcohol increased "drunk" ratings and impaired psychomotor performance (p < 0.05). Blood alcohol levels were decreased by nimodipine pretreatment, but not by verapamil pretreatment. Subjective and psychomotor effects of alcohol were not altered as a function of nimodipine or verapamil pretreatment. Nimodipine, verapamil, and alcohol, either alone or in combination, had no effect on blood pressure or heart rate.

Influence of non-L-type calcium channel antagonists on phencyclidine-induced effects in rats

Omega-conotoxin (1 and 2 micrograms/10 microliter i.c.v.), a N-type calcium channel blocker, and amiloride (7.5 and 15 micrograms/10 microliter i.c.v.), a T-type calcium antagonist, significantly prevented the EEG and behavioural effects induced by phencyclidine (PCP, 5 mg/kg i.p.) in rats. In accordance with previous studies showing the significant influence of L-type calcium blockers in the same model, these results confirm that the modulation of calcium currents plays a key role in the expression of PCP-induced effects.