Characterization of inhibition by haloperidol and chlorpromazine of a voltage-activated K+ current in rat phaeochromocytoma cells

Is post-hypertonic lysis of human red blood cells caused by excessive cell volume regulation?

Human red blood cells (RBC) exposed to hypertonic media are subject to post-hypertonic lysis - an injury that only develops during resuspension to an isotonic medium. The nature of post-hypertonic lysis was previously hypothesized to be osmotic when cation leaks were observed, and salt loading was suggested as a cause of the cell swelling upon resuspension in an isotonic medium. However, it was problematic to account for the salt loading since the plasma membrane of human RBCs was considered impermeable to cations. In this study, the hypertonicity-related behavior of human RBCs is revisited within the framework of modern cell physiology, considering current knowledge on membrane ion transport mechanisms - an account still missing. It is recognized here that the hypertonic behavior of human RBCs is consistent with the acute regulatory volume increase (RVI) response - a healthy physiological reaction initiated by cells to regulate their volume by salt accumulation. It is shown by reviewing the published studies that human RBCs can increase cation conductance considerably by activating cell volume-regulated ion transport pathways inactive under normal isotonic conditions and thus facilitate salt loading. A simplified physiological model accounting for transmembrane ion fluxes and membrane voltage predicts the isotonic cell swelling associated with increased cation conductance, eventually reaching hemolytic volume. The proposed involvement of cell volume regulation mechanisms shows the potential to explain the complex nature of the osmotic response of human RBCs and other cells. Cryobiological implications, including mechanisms of cryoprotection, are discussed.

Effects of antipsychotic drugs and potassium channel modulators on spectral properties of local field potentials in mouse hippocampus and pre-frontal cortex

Local field potentials (LFPs) recorded intracranially display a range of location-specific oscillatory spectra which have been related to cognitive processes. Although the mechanisms producing LFPs are not completely understood, it is likely that voltage-gated ion channels which produce action potentials and patterned discharges play a significant role. It is also known that antipsychotic drugs (APDs) affect LFP spectra and a direct inhibitory effect on voltage-gated potassium channels has been reported. Additionally, voltage-gated potassium channels have been implicated in the pathophysiology of schizophrenia, a disorder for which APDs are primary therapies. In this study we sought to: i) better characterise the effects of two APDs on LFPs spectra and connectivity measures and ii) examine the effects of potassium channel modulators on LFPs and potential overlap of effects with APDs. Intracranial electrodes were implanted in hippocampus (HIP) and pre-frontal cortex (PFC) of C57BL/6J mice; power spectra, coherence and phase-amplitude cross-frequency coupling were measured. Drugs tested were APDs haloperidol and clozapine as well as voltage-gated potassium channel modulators (KVMs) 4-aminopyridine (4-AP), tetraethylammonium, retigabine and E-4031. Both APDs and KVMs significantly reduced gamma power except 4-AP, which conversely increased gamma power. Clozapine and retigabine additionally reduced gamma coherence between HIP and PFC, while 4-AP demonstrated the opposite effect. Phase-amplitude coupling between theta and gamma oscillations in HIP was significantly reduced by the administration of haloperidol and retigabine. These results provide previously undescribed effects of APDs on LFP properties and demonstrate novel modulation of LFP characteristics by KVMs that intriguingly overlap with the APD effects.

Allosteric binding site in a Cys-loop receptor ligand-binding domain unveiled in the crystal structure of ELIC in complex with chlorpromazine

Pentameric ligand-gated ion channels or Cys-loop receptors are responsible for fast inhibitory or excitatory synaptic transmission. The antipsychotic compound chlorpromazine is a widely used tool to probe the ion channel pore of the nicotinic acetylcholine receptor, which is a prototypical Cys-loop receptor. In this study, we determine the molecular determinants of chlorpromazine binding in the Erwinia ligand-gated ion channel (ELIC). We report the X-ray crystal structures of ELIC in complex with chlorpromazine or its brominated derivative bromopromazine. Unexpectedly, we do not find a chlorpromazine molecule in the channel pore of ELIC, but behind the β8-β9 loop in the extracellular ligand-binding domain. The β8-β9 loop is localized downstream from the neurotransmitter binding site and plays an important role in coupling of ligand binding to channel opening. In combination with electrophysiological recordings from ELIC cysteine mutants and a thiol-reactive derivative of chlorpromazine, we demonstrate that chlorpromazine binding at the β8-β9 loop is responsible for receptor inhibition. We further use molecular-dynamics simulations to support the X-ray data and mutagenesis experiments. Together, these data unveil an allosteric binding site in the extracellular ligand-binding domain of ELIC. Our results extend on previous observations and further substantiate our understanding of a multisite model for allosteric modulation of Cys-loop receptors.

Chlorpromazine confers neuroprotection against brain ischemia by activating BKCa channel

Chlorpromazine (CPZ) is a well-known antipsychotic drug, still widely being used to treat symptoms of schizophrenia, psychotic depression and organic psychoses. We have previously reported that CPZ activates the BKCa (KCa1.1) channel at whole cell level. In the present study, we demonstrated that CPZ increased the single channel open probability of the BKCa channels without changing its single channel amplitude. As BKCa channel is one of the molecular targets of brain ischemia, we explored a possible new use of this old drug on ischemic brain injury. In middle cerebral artery occlusion (MCAO) focal cerebral ischemia, a single intraperitoneal injection of CPZ at several dosages (5mg/kg, 10mg/kg and 20mg/kg) could exert a significant neuroprotective effect on the brain damage in a dose- and time-dependent manner. Furthermore, blockade of BKCa channels abolished the neuroprotective effect of CPZ on MCAO, suggesting that the effect of CPZ is mediated by activation of the BKCa channel. These results demonstrate that CPZ could reduce focal cerebral ischemic damage through activating BKCa channels and merits exploration as a potential therapeutic agent for treating ischemic stroke.

Response of i(kr) and HERG currents to the antipsychotics tiapride and sulpiride

The human ether-a-go-go-related gene (hERG) channel is important for repolarization in human myocardium and is a common target for drugs that prolong the QT interval. We studied the effects of two antipsychotics, tiapride and sulpiride, on hERG channels expressed in Xenopus oocytes and also on delayed rectifier K(+) currents in guinea pig cardiomyocytes. Neither the amplitude of the hERG outward currents measured at the end of the voltage pulse, nor the amplitude of hERG tail currents, showed any concentration-dependent changes with either tiapride or sulpiride (3~300 µM). However, our findings did show that tiapride increased the potential for half-maximal activation (V(1/2)) of HERG at 10~300 µM, whereas sulpiride increased the maximum conductance (G(max)) at 3, 10 and 100 µM. In guinea pig ventricular myocytes, bath applications of 100 and 500 µM tiapride at 36℃ blocked rapidly activating delayed rectifier K(+) current (I(Kr)) by 40.3% and 70.0%, respectively. Also, sulpiride at 100 and 500 µM blocked I(Kr) by 38.9% and 76.5%, respectively. However, neither tiapride nor sulpiride significantly affected the slowly activating delayed rectifier K(+) current (I(Ks)) at the same concentrations. Our findings suggest that the concentrations of the antipsychotics required to evoke a 50% inhibition of I(Kr) are well above the reported therapeutic plasma concentrations of free and total compound.

Evaluation of five antischizophrenic agents against Toxoplasma gondii in human cell cultures

An increasing interest in the association of the presence of antibodies to Toxoplasma gondii and the development of schizophrenia in patients has been generated over the last several years. Some antischizophrenia agents have been shown to have activity against T. gondii in cell culture assays and to ameliorate behavioral changes associated with chronic T. gondii infection in rats. In the present study, we examined the effects of commonly used antipsychotic and mood stabilizing agents (haloperidol, clozapine, fluphenazine, trifluoperazine, and thioridazine) for activity against developing tachyzoites of the RH strain of T. gondii in human fibroblast cell cultures. Neither haloperidol nor clozapine had a measurable effect. Fluphenazine had an IC(50) of 1.7 µM, thioridazine had an IC(50) of 1.2 µM, and trifluoperazine had an IC(50) of 3.8 µM. Our study demonstrates that some agents used to treat schizophrenia have the ability to inhibit T. gondii proliferation in cell culture.

Chlorpromazine reduces the intercellular communication via gap junctions in mammalian cells

In the work presented herein, we evaluated the effect of chlorpromazine (CPZ) on gap junctions expressed by two mammalian cell types; Gn-11 cells (cell line derived from mouse LHRH neurons) and rat cortical astrocytes maintained in culture. We also attempted to elucidate possible mechanisms of action of CPZ effects on gap junctions. CPZ, in concentrations comparable with doses used to treat human diseases, was found to reduce the intercellular communication via gap junctions as evaluated with measurements of dye coupling (Lucifer yellow). In both cell types, maximal inhibition of functional gap junctions was reached within about 1 h of treatment with CPZ, an recovery was almost complete at about 5 h after CPZ wash out. In both cell types, CPZ treatment increased the phosphorylation state of connexin43 (Cx43), a gap junction protein subunit. Moreover, CPZ reduced the reactivity of Cx43 (immunofluorescence) at cell interfaces and concomitantly increased its reactivity in intracellular vesicles, suggesting an increased retrieval from and/or reduced insertion into the plasma membrane. CPZ also caused cellular retraction reducing cell-cell contacts in a reversible manner. The reduction in contact area might destabilize existing gap junctions and abrogate formation of new ones. Moreover, the CPZ-induced reduction in gap junctional communication may depend on the connexins (Cxs) forming the junctions. If Cx43 were the only connexin expressed, MAPK-dependent phosphorylation of this connexin would induce closure of gap junction channels.

Actions of antipsychotic drugs on pancreatic beta-cell function: contrasting effects of clozapine and haloperidol

The use of antipsychotic drugs is known to be associated with a number of adverse metabolic side effects, including diabetes mellitus. These side effects could be, at least in part, the result of impaired islet cell function, although the underlying mechanisms are unknown. We have studied the effects of the atypical antipsychotic clozapine and of the conventional drug haloperidol on electrical and secretory activity in rat pancreatic beta-cells. At a low glucose concentration (4mM), clozapine (5microM) had little or no effect on membrane potential. However, at a stimulatory glucose concentration (16mM), clozapine was found to hyperpolarize the cell membrane potential, resulting in a complete inhibition of electrical activity. In contrast, haloperidol (5microM) was found to cause a marked depolarization of the membrane potential in the presence of both low and high concentrations of glucose. Clozapine and haloperidol were found, respectively, to increase and decrease-cell input conductance, an index of K(+) permeability. Single channel recordings indicated that changes in K(ATP) channel activity contributed towards these effects. Neither clozapine nor haloperidol affected basal insulin release, although clozapine inhibited glucose-induced insulin release. It is concluded that clozapine and haloperidol exert contrasting actions on electrical activity in rat pancreatic -cells as a result of opposing effects on K(+) permeability. These findings may relate to the increased incidence of diabetes associated with clozapine treatment.

Cooperation in signal transduction of extracellular guanosine 5' triphosphate and nerve growth factor in neuronal differentiation of PC12 cells

Guanosine 5' triphosphate (GTP), acting synergistically with the nerve growth factor (NGF), enhances the proportion of neurite-bearing cells in cultures of PC12 rat pheochromocytoma cells. We studied the transduction mechanisms activated by GTP in PC12 cells and found that addition of GTP (100 microM) increased intracellular calcium concentration ([Ca(2+)](i)) in cells that were between 60 and 70% confluent. Addition of GTP also enhanced activation of NGF-induced extracellular regulated kinases (ERKs) and induced Ca(2+) mobilization. This mobilization, due to the activation of voltage-sensitive and ryanodine-sensitive calcium channels, as well as pertussis toxin-sensitive purinoceptors, modulates Ca(2+)-activated K(+) channels not involved in activation of ERKs. The results presented here indicate that GTP-triggered [Ca(2+)](i) increase may be a key event in GTP signal transduction, which can modulate activity of ERKs. The physiological importance of the GTP effect lies in its capacity to interact with the NGF-activated pathway to enhance neurite outgrowth from PC12 cells.

HERG binding specificity and binding site structure: evidence from a fragment-based evolutionary computing SAR study

We describe the application of genetic programming, an evolutionary computing method, to predicting whether small molecules will block the HERG cardiac potassium channel. Models based on a molecular fragment-based descriptor set achieve an accuracy of 85-90% in predicting whether the IC(50) of a 'blind' set of compounds is <1 microM. Analysis of the models provides a 'meta-SAR', which predicts a pharmacophore of two hydrophobic features, one preferably aromatic and one preferably nitrogen-containing, with a protonatable nitrogen asymmetrically situated between them. Our experience of the approach suggests that it is robust, and requires limited scientist input to generate valuable predictive results and structural understanding of the target.

Influence of i.v. haloperidol on ventricular repolarization and monophasic action potential duration in anesthetized dogs

INTRODUCTION

i.v. haloperidol is used commonly for sedation in critically ill patients. However, i.v. haloperidol has been shown to cause the life-threatening ventricular tachyarrhythmia torsades de pointes. Mechanisms by which haloperidol causes torsades de pointes have not been widely investigated in controlled studies.

STUDY OBJECTIVES

To determine the effects of i.v. haloperidol on electrophysiologic parameters known to promote torsades de pointes.

INTERVENTIONS

Monophasic action potential catheters were guided under fluoroscopy into the right and left ventricles of 14 chloralose-anesthetized dogs (haloperidol, nine dogs; placebo, five dogs). Effective refractory period (ERP), action potential duration at 90% repolarization (APD90), and QTc interval measurements were performed at baseline and after each of four doses of haloperidol (0.15, 0.5, 2.0, and 3.0 mg/kg) or placebo at three different pacing cycle lengths (450, 300, and 250 ms).

MEASUREMENTS AND RESULTS

i.v. haloperidol significantly prolonged left and right ventricular ERP by a magnitude of 12 to 20% at all pacing cycle lengths. ERP values in the placebo group did not change significantly from pretreatment values in either ventricle. Haloperidol significantly prolonged left ventricular APD90 at a pacing cycle length of 300 ms. The effects of haloperidol on right ventricular APD90 approached significance at a cycle length of 450 ms. Overall, haloperidol prolonged APD90 by 7 to 11%, with less consistent and more variable effects than those for the ERP. APD90 was not significantly altered in the placebo groups. Haloperidol produced significant prolongation in QTc intervals. The electrophysiologic effects of haloperidol were related to dose, with a plateau reached at the 0.5 mg/kg dose for ERP measurements and at the 2 mg/kg dose for the APD90 and QTc interval measurements.

CONCLUSIONS

i.v. haloperidol prolongs ventricular ERP and APD90 in intact canine hearts. These electrophysiologic effects are likely associated with the clinical torsades de pointes-inducing actions of i.v. haloperidol in critically ill patients.

Inhibition of human alpha1E subunit-mediated ca2+ channels by the antipsychotic agent chlorpromazine

Chlorpromazine is a neuroleptic antipsychotic agent with a long history of clinical use. Its primary mode of action is thought to be through modulation of monoaminergic inter-neuronal communication; however, its side-effect profile indicates substantial activities in other systems. Recent work has begun to uncover actions of this compound on ion channels. In this light we have investigated the actions of chlorpromazine on the recombinant alpha1E subunit-encoded voltage-sensitive Ca2+ channel (VSCC) that is believed to encode drug-resistant R-type currents found in neurones and other cells. Chlorpromazine produced a dose-dependent antagonism of these channels that was reversed on drug removal. The mean IC50 was close to 10 microM. At this concentration, the level of antagonism observed was dependent on the membrane potential, with greater inhibition being observed at more negative test potentials. Furthermore, chlorpromazine induced substantial changes in the steady-state inactivation properties of alpha1Ebeta3-mediated currents, although it was not seen to elicit a corresponding change in inactivation kinetics. These results are discussed with regard to the possible clinical mechanisms of chlorpromazine actions.

Inhibition of nicotinic acetylcholine receptors and calcium channels by clozapine in bovine adrenal chromaffin cells

The effects of clozapine on the activities of nicotinic acetylcholine receptors (nAChRs) and voltage-sensitive calcium channels (VSCCs) were investigated and compared with those of chlorpromazine (CPZ) in bovine adrenal chromaffin cells. [(3)H]Norepinephrine ([(3)H]NE) secretion induced by activation of nAChRs was inhibited by clozapine and CPZ with half-maximal inhibitory concentrations (IC(50)) of 10.4 +/- 1.1 and 3.9 +/- 0.2 microM, respectively. Both cytosolic calcium increase and inward current in the absence of extracellular calcium induced by nicotinic stimulation were also inhibited by clozapine and CPZ, but the greater inhibition was achieved by CPZ. In addition, [(3)H]nicotine binding to chromaffin cells was inhibited by clozapine and CPZ with IC(50) values of approximately 19 and 2 microM, respectively. On the other hand, [(3)H]NE secretion induced by high K(+) was inhibited by clozapine and CPZ with similar IC(50) values of 15.5 +/- 3.8 and 17.1 +/- 3.9 microM, respectively. Our results suggest that clozapine, as well as CPZ, inhibits nAChRs and VSCCs, thereby causing inhibition of catecholamine secretion, and that clozapine is much less potent than CPZ in inhibiting nAChRs.

Chlorpromazine inhibits store-operated calcium entry and subsequent noradrenaline secretion in PC12 cells

1. The effect of chlorpromazine on the store-operated Ca2+ entry activated via the phospholipase C signalling pathway was investigated in PC12 cells. 2. Chlorpromazine inhibited the sustained increase after the initial peak in the intracellular Ca2+ concentration produced by bradykinin while having no effect on the initial transient response. The inhibition was lowered by the removal of extracellular free Ca2+. However, chlorpromazine did not inhibit bradykinin-induced inositol 1,4,5-trisphosphate production. 3. Chlorpromazine inhibited the bradykinin-induced noradrenaline secretion in a concentration-dependent manner (IC(50): 24+/-5 microM, n=3). 4. To test for a direct effect of chlorpromazine on store-operated Ca2+ entry, thapsigargin, an inhibitor of microsomal Ca(2+)-ATPase, was used to induce store-operated Ca2+ entry in PC12 cells. Chlorpromazine reduced the thapsigargin-induced sustained Ca2+ level (IC(50): 24+/-2 microM, n=3), and the inhibition also occluded the inhibitory action of 1-[-[3-(4-methoxyphenyl) propoxy]-4-methoxyphenyl]-1H-imidazole hydrochloride (SK&F96365). 5. The results suggest that chlorpromazine negatively modulates the store-operated Ca2+ entry activated subsequent to PLC activation.

Ca(2+) mobilization induced by W-7 in MG63 human osteosarcoma cells

The effect of N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W-7), a widely used calmodulin inhibitor, on intracellular free Ca(2+)levels ([Ca(2+)](i)) in MG63 human osteosarcoma cells was explored using fura-2 as a Ca(2+)probe. W-7 (20-1000 micro m) induced an increase in [Ca(2+)](i)in a dose-dependent manner, with an EC(50)of 100 microm. The [Ca(2+)](i)signal comprised an initial rise and a sustained plateau without significant decay within 5 min. External Ca(2+)removal decreased the Ca(2+)signals by reducing the peak and sustained phase, indicating W-7-activated intracellular Ca(2+)release and extracellular Ca(2+)influx. W-7 (500 microm) failed to induce a [Ca(2+)](i)increase in a Ca(2+)-free medium after pre-treatment with thapsigargin (1 microm), an endoplasmic reticulum Ca(2+)pump inhibitor. Conversely, W-7 pre-treatment abolished the Ca(2+)release induced by thapsigargin. This suggests that W-7 (500 microm ) released internal Ca(2+)mainly from the endoplasmic reticulum. The addition of 3 mm Ca(2+)increased [Ca(2+)](i)dose-dependently after preincubation with 20-1000 microm W-7 in a Ca(2+)-free medium, implying that W-7 induced capacitative Ca(2+)entry. W-7-induced Ca(2+)release was not altered by inhibiting phospholipase C with 2 microm 1-(6-((17 beta - 3-methoxyestra-1,3, 5(10)-trien-17-yl)amino)hexyl)-1H-pyrrole-2,5-dione) (U73122). Tryphan blue assay demonstrated that W-7 (200 microm) caused gradual cell death within 30 min of the initial drug exposure. Together, it was found that W-7 induced [Ca(2+)](i)increases in human osteosarcoma cells by releasing internal Ca(2+)from the endoplasmic reticulum, and also by triggering Ca(2+)influx. W-7 may be cytotoxic to osteosarcoma cells.

A functional genomic study of the effects of antipsychotic agent chlorpromazine in PC12 cells

Expression profiling using methods of functional genomics can be used to investigate changes in gene transcription induced by drug treatment, which may lead to discovery of new potential drug targets. Antipsychotic agents alleviate symptoms of schizophrenia but the mechanism behind their clinical efficacy is unclear. We have used the PC12 cell line as a model to characterize effects of the antipsychotic drug chlorpromazine on gene expression using high-density complementary DNA array filters prepared from a rat brain entorhinal cortex complementary DNA library. Chlorpromazine treatment positively regulated the expression of several clones, five of which were selected for further characterization. Northern blotting experiments confirmed the increased expression of these genes after chlorpromazine treatment. Sequencing revealed that two clones were cytochrome c oxidase and three were novel genes. Characterization of the function of these genes could increase our understanding of the mechanisms of action of antipsychotic drugs, and might be beneficial for the development of more effective agents.

Membrane-delimited coupling between sigma receptors and K+ channels in rat neurohypophysial terminals requires neither G-protein nor ATP

Receptor-mediated modulation of ion channels generally involves G-proteins, phosphorylation, or both in combination. The sigma receptor, which modulates voltage-gated K+ channels, is a novel protein with no homology to other receptors known to modulate ion channels. In the present study patch clamp and photolabelling techniques were used to investigate the mechanism by which sigma receptors modulate K+ channels in peptidergic nerve terminals. The sigma receptor photoprobe iodoazidococaine labelled a protein with the same molecular mass (26 kDa) as the sigma receptor protein identified by cloning. The sigma receptor ligands pentazocine and SKF10047 modulated K+ channels, despite intra-terminal perfusion with GTP-free solutions, a G-protein inhibitor (GDPbetaS), a G-protein activator (GTPgammaS) or a non-hydrolysable ATP analogue (AMPPcP). Channels in excised outside-out patches were modulated by ligand, indicating that soluble cytoplasmic factors are not required. In contrast, channels within cell-attached patches were not modulated by ligand outside a patch, indicating that receptors and channels must be in close proximity for functional interactions. Channels expressed in oocytes without receptors were unresponsive to sigma receptor agonists, ruling out inhibition through a direct drug interaction with channels. These experiments indicate that sigma receptor-mediated signal transduction is membrane delimited, and requires neither G-protein activation nor protein phosphorylation. This novel transduction mechanism is mediated by membrane proteins in close proximity, possibly through direct interactions between the receptor and channel. This would allow for more rapid signal transduction than other ion channel modulation mechanisms, which in the present case of neurohypophysial nerve terminals would lead to the enhancement of neuropeptide release.

Block of rat brain recombinant SK channels by tricyclic antidepressants and related compounds

SK channels are small conductance, Ca(2+)-activated K(+) channels that underlie neuronal slow afterhyperpolarization and mediate spike frequency adaptation. Using the patch clamp technique, we tested the effects of eight clinically relevant psychoactive compounds structurally related to the tricyclic antidepressants, on SK2 subtype channels cloned from rat brain and functionally expressed in the human embryonic kidney cell line, HEK293. Amitriptyline, carbamazepine, chlorpromazine, cyproheptadine, imipramine, tacrine and trifluperazine blocked SK2 channel currents with micromolar affinity. The block was reversible and concentration-dependent. The potency differed according to chemical structure. In contrast, the cognitive enhancer linopirdine was ineffective at blocking these channels. Our results point to a distinct pharmacological profile for SK channels.

Haloperidol prolongs diastolic phase of Ca(2+) transient in cardiac myocytes

Haloperidol (HPL), a widely used antipsychotic drug, is known to induce serious ventricular arrhythmias. However, the mechanism underlying their induction is not clear. We therefore examined the effects of HPL on the intracellular Ca(2+) ([Ca(2+)](i)) transient and on cell motion in cultured cardiac myocytes, as well as the pathways involving the HPL-induced abnormality of Ca(2+) homeostasis. HPL prolonged the diastolic phase of the Ca(2+) transient, with a mid-diastolic re-elevation of [Ca(2+)](i). The re-elevation of [Ca(2+)](i) was shown to be provoked by Ca(2+) release from sarcoplasmic reticulum (SR), which can trigger delayed afterdepolarization, the major arrhythmogenic factor. The re-elevation of [Ca(2+)](i) coincided with cell re-contraction during diastole. The induction of this abnormality by HPL appears to be independent of the mechanisms of the antipsychotic action.

Dopamine receptor antagonists modulate glucose uptake in rat pheochromocytoma (PC12) cells

A variety of dopaminergic ligands were evaluated for their ability to alter glucose transport in PC12 cells. Certain antipsychotic drugs which targeted D2 dopamine receptors, such as pimozide, fluphenazine and chlorpromazine, inhibited glucose uptake (with IC50's in the range of 2-40 microM). By contrast, haloperidol and sulpiride (also D2 antagonists) showed marginal activity. The atypical antipsychotic drug, clozapine (a D4 antagonist), also effectively inhibited glucose transport by the cells. Ligands specific for D1 receptors did not interfere with glucose uptake. Time course studies revealed that a short incubation with the drugs (1-5 min) was sufficient to block glucose transport. These findings may have implications for the adverse effects of these drugs and for the interpretation of imaging studies of brain glucose metabolism in patients on antipsychotic medications.

Chlorpromazine-induced inhibition of catecholamine secretion by a differential blockade of nicotinic receptors and L-type Ca2+ channels in rat pheochromocytoma cells

We investigated the effect of chlorpromazine (CPZ), a phenothiazine neuroleptic, on catecholamine secretion in rat pheochromocytoma (PC12) cells. CPZ inhibited [3H]norepinephrine ([3H]NE) secretion induced by 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP), an agonist of nicotinic acetylcholine receptors (nAChRs) with an IC50 value of 1.0 +/- 0.2 microM. The DMPP-induced rise in cytosolic free Ca2+ concentration [Ca2+]i was inhibited by CPZ with an IC50 of 1.9 +/- 0.1 microM. The DMPP-induced increase in cytosolic free Na+ concentration [Na+]i was also inhibited by CPZ with a similar potency. Furthermore, the binding of [3H]nicotine to PC12 cells was inhibited by CPZ with an IC50 value of 2.7 +/- 0.6 microM, suggesting that the nAChRs themselves are inhibited by CPZ. In addition, both 70 mM K+-induced [3H]NE secretion and [Ca2+]i increase were inhibited by CPZ with IC50 of 7.9 +/- 1.1 and 6.2 +/- 0.3 microM, respectively. Experiments with Ca2+ channel antagonists suggest that L-type Ca2+ channels are mainly responsible for the inhibition. We conclude that CPZ inhibits catecholamine secretion by blocking nAChRs and L-type Ca2+ channels, with the former being more sensitive to CPZ.

K+ channel modulation in rodent neurohypophysial nerve terminals by sigma receptors and not by dopamine receptors

1. Sigma receptors bind a diverse group of chemically unrelated ligands, including pentazocine, apomorphine (a dopamine receptor agonist) and haloperidol (a dopamine receptor antagonist). Although sigma binding sites are widely distributed, their physiological roles are poorly understood. Here, the whole-terminal patch-clamp technique was used to demonstrate that sigma receptors modulate K+ channels in rodent neurohypophysis. 2. Previous work suggested that dopamine type 4 (D4) receptors modulate neurohypophysial K+ current, so this study initially tested the role of dopamine receptors. Experiments using transgenic mice lacking D2, D3 or D4 receptors indicated that the reduction of K+ current by PPHT and U101958 (ligands thought to be selective for dopamine receptors) is not mediated by dopamine receptors. The sensitivity of the response to U101958 (a drug that binds to D4 receptors) was the same in both wild-type and D4 receptor-deficient mice. 3. Experiments with other ligands revealed a pharmacological signature inconsistent with any known dopamine receptor. Furthermore, dopamine itself (at 100 microM) had no effect. Thus, despite the activity of a number of putative dopamine receptor ligands, dopamine receptors play no role in the modulation of neurohypophysial K+ channels. 4. Because of the negative results regarding dopamine receptors, and because some of the dopamine receptors ligands used here are known to bind also to sigma receptors, experiments were conducted to test for the involvement of sigma receptors. In rat neurohypophysis the sigma receptor ligands SKF10047, pentazocine, and ditolylguanidine all reversibly inhibited K+ current in a concentration-dependent fashion, as did haloperidol and apomorphine (ligands that bind to both dopamine and sigma receptors). The activity of these and other ligands tested here matches the reported binding specificity for sigma receptors. 5. Fifteen candidate endogenous sigma receptor ligands, including biogenic amines (e.g dopamine and serotonin), steroids (e.g. progesterone), and peptides (e.g. neuropeptide Y), were screened for activity at the sigma receptor. All were without effect. 6. Haloperidol reduced K+ current proportionally at all voltages without shifting the voltage dependence of activation and inactivation. Sigma receptor ligands inhibited current through two distinct K+ channels, the A-channel and the Ca2+-dependent K+ channel. In rat, all drugs reduced current through both channels proportionally, suggesting that both channels are modulated by a single population of sigma receptors. In contrast, mouse peptidergic nerve terminals either have two receptors which are sensitive to these drugs, or a single receptor that is differentially coupled to ion channel function. 7. The inhibition of voltage-activated K+ current by sigma receptors would be expected to enhance the secretion of oxytocin and vasopressin from the neurohypophysis.

Inhibition by fluoxetine of voltage-activated ion channels in rat PC12 cells

The effects of fluoxetine (Prozac) on voltage-activated K+, Ca2+ and Na+ channels were examined using the whole-cell configuration of the patch clamp technique in rat pheochromocytoma (PC12) cells. When applied to the external bath solution, fluoxetine (1, 10, 100 microM) decreased the peak amplitude of K+ currents. The K+ current inhibition by fluoxetine (10 microM) was voltage-independent and the fraction of current inhibition was 39.7-51.3% at all voltages tested (0 to +50 mV). Neither the activation and inactivation curves nor the reversal potential for K+ currents was significantly changed by fluoxetine. The inhibition by fluoxetine of K+ currents was use- and concentration-dependent with an IC50 of 16.0 microM. The inhibition was partially reversible upon washout of fluoxetine. The action of fluoxetine was independent of the protein kinases, because the protein kinase C or A inhibitors (H-7, staurosporine, Rp-cAMPS) did not prevent the inhibition by fluoxetine. Intracellular infusion with GDPbetaS or pretreatment with pertussis toxin did not block the inhibitory effects of fluoxetine. The inhibitory action of fluoxetine was not specific to K+ currents because it also inhibited both Ca2+ (IC50 = 13.4 microM) and Na+ (IC50 = 25.6 microM) currents in a concentration-dependent manner. Our data indicate that when applied to the external side of cells, fluoxetine inhibited voltage-activated K+, Ca2+ and Na+ currents in PC12 cells and its action on K+ currents does not appear to be mediated through protein kinases or G proteins.

The cell death response to gamma-radiation in MCF-7 cells is enhanced by a neuroleptic drug, pimozide

Neuroleptic drugs that bind sigma sites were tested for their ability to inhibit growth and radiosensitize MCF-7 human breast cancer cells. Inhibition of growth by approximately 50% occurred in cells exposed to pimozide (0.6 microM), haloperidol (10 microM), and the sigma ligand DTG (1,3-di(2-tolyl)guanidine, 20 microM), but no growth inhibition occurred in cells exposed to clozapine, a neuroleptic drug lacking sigma binding activity, or dextromethorphan, a selective sigma 1 binding ligand. Pimozide (2.5 microM), but not haloperidol (3.6 microM), enhanced the sensitivity of MCF-7 cells to gamma radiation in clonogenic survival assays. Pimozide significantly decreased MCF-7 clonogenic survival following a 5 or 8 Gy dose of gamma radiation, and the dose of radiation required for 1% survival (survival enhancement ratio, SER) was decreased by a factor of 2. Exposure of normal WI-38 human embryonic lung cells to pimozide did not increase their sensitivity to gamma radiation. Pimozide (2.5 microM) activated early apoptotic changes in MCF-7 cells that were detected by the uptake of Hoechst 33342 dye, and 10 microM pimozide activated a complete apoptotic pathway resulting in the death of > 90% of the cells within 24 hours. MCF-7 cells exposed to gamma radiation alone (8 Gy) showed giant cell formation, mitotic arrest, and a limited degree of apoptosis and necrosis. Within 50 hours of treatment with a combination of radiation and pimozide, cell numbers were sharply reduced compared with cultures exposed to either radiation or pimozide alone. We conclude that pimozide augmented the sensitivity of MCF-7 cells to radiation-induced cell killing through a mechanism not shared by haloperidol, but suggest that concentration of pimozide in MCF-7 cells as a result of an enrichment of sigma 2 sites might target the radiosensitization.

Voltage-activated potassium channels in mammalian neurons and their block by novel pharmacological agents

1. Electrophysiological studies have shown that a number of different types of potassium (K) channel currents exist in mammalian neurons. Among them are the voltage-gated K channel-currents which have been classified as fast-inactivating A-type currents (KA) and slowly inactivating delayed-rectifier type currents (KDR). 2. Two major molecular superfamilies of K channel have been identified; the KIR superfamily and the Shaker-related superfamily with a number of different pore-forming alpha-subunits in each superfamily. 3. Within the Shaker-related superfamily are the KV family, comprising of at least 18 different alpha-subunits that almost certainly underlie classically defined KA and KDR currents. However, the relationship between each of these cloned alpha-subunits and native voltage-gated K currents remains, for the most part, to be established. 4. Classical pharmacological blockers of voltage-gated K channels such as tetraethylammonium ions (TEA), 4-aminopyridine (4-AP), and certain toxins lack selectivity between different native channel currents and between different cloned K channel currents. 5. A number of other agents block neuronal voltage-gated K channels. All of these compounds are used primarily for other actions they possess. They include organic calcium (Ca) channel blockers, divalent and trivalent metal ions and certain calcium signalling agents such as caffeine. 6. A number of clinically active tricyclic compounds such as imipramine, amitriptyline, and chlorpromazine are also potent inhibitors of neuronal voltage-gated K channels. These compounds are weak bases and it appears that their uncharged form is required for activity. These compounds may provide a useful starting point for the rational design of novel selective K channel blocking agents.

The effects of neuroleptic and tricyclic compounds on BKCa channel activity in rat isolated cortical neurones

1. The actions of several neuroleptic and tricyclic compounds were examined on the large conductance Ca(2+)-activated K+ (BKCa) channel present in neurones isolated from the rat motor cortex. 2. Classical neuroleptic compounds including chlorpromazine and haloperidol applied to the intracellular surface of inside-out patches produced a concentration-dependent reduction in BKCa channel activity. Similar effects were observed when these compounds were applied to the extracellular surface of outside-out patches. 3. In contrast, the atypical neuroleptic compounds clozapine and sulpiride did not affect BKCa channel activity (100 nM-1 mM) in either inside-out or outside-out patches, while 10 microM pimozide produced 73% of the inhibition produced by 10 microM chlorpromazine. 4. BKCa channel activity was also unaffected by application of structurally related tricyclic compounds including the anti-cholinesterase tacrine and the anti-epileptic carbamazepine. The tricyclic antidepressant drug amitriptyline was found to inhibit BKCa channel activity but was much less effective than the classical neuroleptic compounds. 5. It is concluded that compounds belonging to the classical neuroleptic group of drugs inhibit BKCa channel activity in the rat motor cortex in a structurally-specific manner. This observation may be of clinical significance as it may contribute to some of the side effects associated with classical neuroleptic drug therapy.

Stable expression and characterization of the human brain potassium channel Kv2.1: blockade by antipsychotic agents

We have cloned the cDNA encoding the voltage-dependent K+ channel Kv2.1 from human brain (hKv2.1). RNase protection and RT-PCR (reverse transcriptase-PCR) experiments reveal abundant Kv2.1 transcripts in human brain with virtually no expression detectable in human heart. hKv2.1 has been stably transfected into a human glioblastoma cell line, and transformed cells display large, slowly activating outward currents. The kinetics, steady-state activation and inactivation parameters, and external tetraethylammonium sensitivity were all similar to those described previously for hKv2.1 channels transiently expressed in Xenopus oocytes or other mammalian cell lines. A number of dopamine receptor antagonist/antipsychotic agents were shown to block hKv2.1. Trifluoperizine, trifluperidol and pimozide produced time-dependent blockade of hKv2.1 with IC50 values of approx. 1-2 microM. The diphenylbutylpiperidine fluspirilene was shown to be 4-5-fold more potent than the other agents tested inhibiting hKv2.1 current with an IC50 value of 297 nM. The block produced by fluspirilene was both time- and frequency-dependent. Furthermore, fluspirilene (1 microM) shifted the midpotential of the hKv2.1 steady-state inactivation curve by approx. 15 mV in the hyperpolarizing direction. These results demonstrate the usefulness of this transfection system for the pharmacological characterization of hKv2. 1. Fluspirilene proved to be a relatively potent blocker of hKv2.1 and may provide a useful starting point for the development of more potent and selective agents active against this brain K+ channel.

Radioligand binding and photoaffinity labelling studies show a direct interaction of phenothiazines at 5-HT3 receptors

The effects of a range of phenothiazines were examined on 5-hydroxytryptamine3 (5-HT3) receptors in membranes from NIE-115 neuroblastoma cells using radioligand binding. Chlorpromazine, fluphenazine, perphenazine, trifluoperazine and prochlorperazine inhibited specific binding of both the 5-HT3 receptor antagonist [3H]GR65630 and agonist [3H]meta-chlorophenylbiguanide (mCPBG), with Ki values ranging from 0.4 to 3.9 microM. The mode of action of chlorpromazine was further examined using photoaffinity labelling in the presence and absence of 5-HT. Saturation radioligand binding data with both [3H]GR65630 and [3H]mCPBG showed that photoaffinity labelling with chlorpromazine (1 microM) caused a decrease in the maximum number of binding sites observed (35% and 28% for agonist and antagonist, respectively). This decrease was not observed when the membranes were incubated in the presence of 5-HT. The results demonstrate a direct interaction of a range of phenothiazines at the 5-HT3 receptor binding site.

Carotid body dopamine content and release by short-term hypoxia: effect of haloperidol and alpha methyl paratyrosine

Dopamine (DA) is thought to modulate the transduction of the hypoxic stimulus by the glomus cell in the carotid body (CB). The hypothesis tested here is that presynaptic DA D2 receptors (D2's) located on the type 1 cell function as autoreceptors to control DA release and/or synthesis. The aim of the study was to compare the effects of blocking D2's with haloperidol and DA synthesis with alpha methyl paratyrosine (AMPT) on the in vitro carotid body DA response to hypoxia. 54 CB's sampled from adult rabbits were incubated for one hour in a surviving medium bubbled with either 100% O2 or 8% O2 Sixteen CB's served as control (100% O2: n = 8, 8% O2: n = 8), 18 (100% O2: n = 8, 8% O2: n = 10) were sampled from rabbits pretreated with AMPT and 20 (100% O2: n = 12, 8% O2: n = 8) were incubated with micromolar concentrations of haloperidol. At the end of exposure. DA contained in the carotid body (DACB) and released in the surviving medium (DAr) were measured by HPLC. In 100% O2 DACB was not different between either AMPT or haloperidol and control, but DAr was significantly higher in the haloperidol group compared with control (mean +/- SE: 26.6 +/- 7.4 versus 7.6 +/- 2.0 pmol/h, P < 0.02). In 8% O2, control DACB (576 +/- 133 pmol/CB) was significantly higher than AMPT or haloperidol (respectively 228 +/- 29.6 and 246 +/- 49.9 pmol/CB, P < 0.01) and control DAr (234 +/- 72.3 pmol/h) was also significantly higher than AMPT or haloperidol (respectively 28.8 +/- 5.2 and 40.6 +/- 11.4 pmol/h, P < 0.01). Finally, DAr was significantly larger in 8% O2 than in 100% O2 in control and AMPT groups (P < 0.01), but not in the haloperidol group. The increase in DAr by haloperidol in the resting CB is consistent with the blockade of D2's regulating DA release. The decreased DAr in 8% O2 after AMPT suggests that increased DA synthesis contributes to maintain DA secretion by the type I cell exposed to short term hypoxia. The lack of difference in DAr between 8% O2 and 100% O2 after haloperidol probably reflects non specific--i.e., D2 independent--effect of micromolar concentration of haloperidol on DA synthesis and/or sodium-calcium exchangers during hypoxia.

Implication of ATP receptors in brain functions

The possible implication of P2-purinoceptors in brain functions is reviewed. Involvement of P2-purinoceptors in memory and learning (Section 2) is suggested by ATP release from hippocampal slices [Wieraszko, A., Goldsmith, G. and Seyfried, T. N. (1989) Brain Res. 485, 244-250], induction of fast synaptic currents in cultured hippocampal neurons [Inoue, K., Nakazawa, K., Fujimori, W. and Takanaka, A. (1992a) Neurosci. Lett. 134, 294-299] and long-lasting enhancement of the population spikes [Wieraszko, A. and Seyfried, T. N. (1989) Brain Res. 491, 356-359; Nishimura, S., Mohri, M., Okada, Y. and Mori, M. (1990) Brain Res. 525, 165-169; Fujii, S., Kato, H., Furuse, H., Ito, K., Osada, H., Hamaguchi, T. and Kuroda, Y. (1995) Neurosci, Lett. 187, 130-132], as well as ATP release on glutamate stimulation to evoke an increase in intracellular Ca2+ in hippocampal cells [Inoue, K., Koizum, S. and Nakazawa, K. (1995) NeuroReport 6, 437-440]. Moreover, mRNAs for certain types of P2x-purinoceptors are present in the hippocampus [Collo, G., North, R. A., Kawashima, E., Merlo-Pich, E., Neidhart, S., Surprenant, A. and Buell, G. (1996) J. Neurosci. 16, 2495-2507]. It is likely, therefore, that ATP may be involved in modulation of synaptic efficiency in the hippocampus. The implication of ATP in schizophrenia is suggested by the fact that antipsychotic drugs inhibit ATP-evoked responses in PC12 cells [Koizumi, S., Ikeda, M., Nakazawa, K., Inoue, K., Ito, K. and Inoue, K. (1995b) Biochem. Biophys. Res. Commun. 210, 624-630] without blocking the action of dopamine D2 receptors. Involvement of P2-purinoceptors in Sections 4 ("Pain and cognition") and 5 ("Central regulation of the autonomic system") are also discussed.

Inhibition by antipsychotic drugs of L-type Ca2+ channel current in PC12 cells

Inhibition by antipsychotic drugs of voltage-gated L-type Ca2+ channels was characterized in rat neuronal cell line pheochromocytoma PC12 cells. Under whole-cell voltage-clamp, haloperidol and chlorpromazine (1-100 microM) inhibited Ba2+ current permeating through Ca2+ channels. Fluspirilene and pimozide the Ba2+ current at lower concentrations (fluspirilene, 0.1 pM to 1 nM; pimozide 10 pM to 1 microM). Effects of dopamine receptor antagonists and calmodulin antagonists were tested because antipsychotic drugs are known to exhibit these pharmacological activities. Sulpiride (1 and 10 microM), an antagonist to dopamine D2 receptors, and SCH-23390 (R(+)-7-chloro-8-hydroxy-3-methyl-l-phenyl-2,3,4,5-tetrahydro-1H-3- benzazepine; 1 and 10 microM), an antagonist to dopamine D1 receptors, also inhibited the Ba2+ current. As for calmodulin antagonists, W-7 (N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide; 10 and 100 microM) as well as calmidazolium (10 nM to 1 microM) reduced the Ba2+ current. The inhibition by haloperidol or fluspirilene of the Ba2+ current was not affected when GTP in intracellular solution was replaced with GDP beta S. These properties of the Ca2+ channel inhibition are discussed by comparing with those of the K+ channel inhibition and in relation to therapeutic relevance.