Effect of haloperidol and its metabolites on dopamine and noradrenaline uptake in rat brain slices

Human complex exploration strategies are enriched by noradrenaline-modulated heuristics

An exploration-exploitation trade-off, the arbitration between sampling a lesser-known against a known rich option, is thought to be solved using computationally demanding exploration algorithms. Given known limitations in human cognitive resources, we hypothesised the presence of additional cheaper strategies. We examined for such heuristics in choice behaviour where we show this involves a value-free random exploration, that ignores all prior knowledge, and a novelty exploration that targets novel options alone. In a double-blind, placebo-controlled drug study, assessing contributions of dopamine (400 mg amisulpride) and noradrenaline (40 mg propranolol), we show that value-free random exploration is attenuated under the influence of propranolol, but not under amisulpride. Our findings demonstrate that humans deploy distinct computationally cheap exploration strategies and that value-free random exploration is under noradrenergic control.

Altered levels of dopamine transporter in the frontal pole and dorsal striatum in schizophrenia

The dopamine hypothesis proposes that there is a hypodopaminergic state in the prefrontal cortex and a hyperdopaminergic state in the striatum of patients with schizophrenia. Evidence suggests the hyperdopaminergic state in the striatum is due to synaptic dopamine elevation, particularly in the dorsal striatum. However, the molecular mechanisms causing disrupted dopaminergic function in schizophrenia remains unclear. We postulated that the dopamine transporter (DAT), which regulates intra-synaptic dopamine concentrations by transporting dopamine from the synaptic cleft into the pre-synaptic neuron, could be involved in dopaminergic dysfunction in schizophrenia. Therefore, we measured levels of DAT in the cortex and striatum from patients with schizophrenia and controls using postmortem human brain tissue. Levels of desmethylimipramine-insensitive mazindol-sensitive [³H]mazindol binding to DAT were measured using in situ radioligand binding and autoradiography in gray matter from Brodmann’s area (BA) 10, BA 17, the dorsal striatum, and nucleus accumbens from 15 patients with schizophrenia and 15 controls. Levels of desmethylimipramine-insensitive mazindol-sensitive [³H]mazindol binding were significantly higher in BA 10 from patients with schizophrenia (p = 0.004) and significantly lower in the dorsal striatum (dorsal putamen p = 0.005; dorsal caudate p = 0.007) from those with the disorder. There were no differences in levels of desmethylimipramine-insensitive [³H]mazindol binding in BA 17 or nucleus accumbens. These data raise the possibility that high levels of DAT in BA 10 could be contributing to lower synaptic cortical dopamine, whereas lower levels of DAT could be contributing to a hyperdopaminergic state in the dorsal striatum.

How Administration of the Beta-Blocker Propranolol Before Extinction can Prevent the Return of Fear

Combining beta-blockers with exposure therapy has been advocated to reduce fear, yet experimental studies combining beta-blockers with memory reactivation have had contradictory results. We explored how beta-blockade might affect the course of safety learning and the subsequent return of fear in a double-blind placebo-controlled functional magnetic resonance imaging study in humans (N=46). A single dose of propranolol before extinction learning caused a loss of conditioned fear responses, and prevented the subsequent return of fear and decreased explicit memory for the fearful events in the absence of drug. Fear-related neural responses were persistently attenuated in the dorsal medial prefrontal cortex (dmPFC), increased in the hippocampus 24 h later, and correlated with individual behavioral indices of fear. Prediction error-related responses in the ventral striatum persisted during beta-blockade. We suggest that this pattern of results is most consistent with a model where beta-blockade can prevent the return of fear by (i) reducing retrieval of fear memory, via the dmPFC and (ii) increasing contextual safety learning, via the hippocampus. Our findings suggest that retrieval of fear memory and contextual safety learning form potential mnemonic target mechanisms to optimize exposure-based therapy with beta-blockers.

Influence of ABCB1 genetic polymorphisms on the pharmacokinetics of risperidone in healthy subjects with CYP2D6*10/*10

BACKGROUND AND PURPOSE

The objective of this study was to investigate the combined influence of genetic polymorphisms in ABCB1 and CYP2D6 genes on risperidone pharmacokinetics.

EXPERIMENTAL APPROACH

Seventy-two healthy Korean volunteers receiving a single oral dose of 2 mg risperidone were included in this study.

KEY RESULTS

Significant differences were observed between the ABCB1 3435C>T genotypes for the pharmacokinetic parameters (peak serum concentration) of risperidone and the active moiety (risperidone and its main metabolite, 9-hydroxyrisperidone). There were no significant differences in the area under the serum concentration-time curves of risperidone and the active moiety among the ABCB1 2677G>T/A and 3435C>T genotypes. However, the peak serum concentration and area under the serum concentration-time curves were significantly different among the ABCB1 3435C>T genotypes in CYP2D6*10/*10.

CONCLUSIONS AND IMPLICATIONS

These findings indicate that polymorphisms of ABCB1 3435C>T in individuals with CYP2D6*10/*10, which has low metabolic activity, could play an important role in the potential adverse effects or toxicity of risperidone.

Effects of haloperidol and its pyridinium metabolite on plasma membrane permeability and fluidity in the rat brain

The use of antipsychotic drugs is limited by their tendency to produce extrapyramidal movement disorders such as tardive dyskinesia and parkinsonism. In previous reports it was speculated that extrapyramidal side effects associated with the butyrophenone neuroleptic agent haloperidol (HP) could be caused in part by the neurotoxic effect of its pyridinium metabolite (HPP(+)). Although both HPP(+) and HP have been shown to induce neurotoxic effects such as loss of cell membrane integrity, no information exists about the difference in the neurotoxic potency, especially in the potency to induce plasma membrane damage, between these two agents. In the present study, we compared the potency of the interaction of HPP(+) and HP with the plasma membrane integrity in the rat brain. Membrane permeabilization (assessed as [(18)F]2-fluoro-2-deoxy-d-glucose-6-phosphate release from brain slices) and fluidization (assessed as the reduction in the plasma membrane anisotropy of 1,6-diphenyl 1,3,5-hexatriene) were induced by HPP(+) loading (at >or=100 microM and >or=10 microM, respectively), while comparable changes were induced only at a higher concentration of HP (=1 mM). These results suggest that HPP(+) has a higher potency to induce plasma membrane damage than HP, and these actions of HPP(+) may partly underlie the pathogenesis of HP-induced extrapyramidal side effects.

Sensitive Determination of 4-(4-Bromophenyl)-4-hydroxypiperidine, a Metabolite of Bromperidol, in Rat Plasma by HPLC with Fluorescence Detection after Pre-column Derivatization Using 4-Fluoro-7-nitro-2,1,3-benzoxadiazole

The purpose of this study was to determine the level of 4-(4-bromophenyl)-4-hydroxypiperidine (BPHP), a bromperidol (BRO) metabolite, in rat plasma by HPLC with fluorescence detection after pre-column derivatization using 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F). After basic extraction of the samples with benzene, derivatization with NBD-F was conducted in borate buffer (pH 8.0) at 60 degrees C for 3 min. Mexiletine was utilized through the procedure as an internal standard (IS). Retention times of the BPHP and IS derivatives were 7.7 and 11.5 min, respectively. The regression equation for BPHP showed good linearity in the range of 0.01-1 mg/ml with the detection limit of 0.003 microg/ml. The coefficient of variation was less than 12.0%. The recovery was satisfactory. This method was applied for a pharmacokinetic study of BPHP in comparison with 4-(4-chlorophenyl)-4-hydroxypiperidine (CPHP), the corresponding haloperidol (HAL) metabolite, in rats. The ratio of the area under the plasma concentration curve (AUC) after p.o. administration of BPHP to the AUC after i.p. administration of BPHP (46%) was lower than that of CPHP (56%), indicating that intestinal absorption of BPHP is lower than that of CPHP. The ratio of BRO metabolism to BPHP (48%) was 1.8-fold higher than that of HAL metabolism to CPHP (27%); the ratio was estimated as (AUCp.o.,A-->B/AUCp.o.,B)x100, where AUCp.o.,A-->B is the AUC value of BPHP or CPHP after p.o. administration of BRO or HAL, and AUCp.o.,B is the AUC of BPHP or CPHP after administration of BPHP or CPHP, respectively. Our method provides a sensitive procedure for determination of BPHP in rat plasma and is suitable for pharmacokinetic studies of BPHP after BRO administration.

Simultaneous determination of haloperidol and bromperidol and their reduced metabolites by liquid–liquid extraction and automated column-switching high-performance liquid chromatography

This study describes a new simultaneous determination of haloperidol and bromperidol and their reduced metabolites by modification of automated column-switching high-performance liquid chromatography. The test compounds were extracted from 1ml of plasma using chloroform-hexane (30:70 (v/v)), and the extract was injected into a hydrophilic metaacrylate polymer column for clean-up and a C(18) analytical column for separation. The mobile phases consisted of phosphate buffer (0.02M, pH 4.6), perchloric acid (60%) and acetonitrile (54:1:45 (v/v)) and was delivered at a flow-rate of 0.6ml/min. The peak was detected using a UV detector set at 215nm. The method was validated for the concentration range 1-100ng/ml, and good linearity (r >0.999) was confirmed. Intra-day coefficient variations (CVs) for haloperidol, reduced haloperidol, bromperidol and reduced bromperidol were less than 2.5, 3.1, 2.4 and 2.5%, respectively. Inter-day CVs for corresponding compounds were 3.9, 5.1, 2.6 and 4.4%, respectively. Relative errors ranged from -5 to 10% and mean recoveries were 96-100%. The limit of quantification was 1.0ng/m for each compound. This method shows good specificity with respect to commonly prescribed psychotropic drugs, and it could be successfully applied for pharmacokinetic studies and therapeutic drug monitoring, particularly in patients receiving both haloperidol and bromperidol.

Ritanserin counteracts both rat vacuous chewing movements and nigro-striatal tyrosine hydroxylase-immunostaining alterations induced by haloperidol

The effect of subchronic co-administration of ritanserin (1.5 mg/kg, i.p., twice a day) and haloperidol (1 mg/kg, i.p., twice a day) on rat vacuous chewing movements and on tyrosine hydroxylase-immunostaining was investigated. Ritanserin significantly reduced rat vacuous chewing movements observed following 2, 3 and 4 weeks of haloperidol administration and after 5 days of haloperidol withdrawal. Furthermore, ritanserin prevented the reduction of striatal tyrosine hydroxylase-immunostaining and the shrinkage of nigral dopaminergic cell bodies induced by haloperidol. The present results indicate that ritanserin may possess protective properties on both dopaminergic nigro-striatal neuron alterations and vacuous chewing movements induced by haloperidol, and provide further evidence indicating a possible association between these two haloperidol-induced effects.

Poor reliability of therapeutic drug monitoring data for haloperidol and bromperidol using enzyme immunoassay

Therapeutic drug monitoring (TDM) services for plasma concentrations of haloperidol and bromperidol using enzyme immunoassay (EIA) methods are available in Japan, whereas high-performance liquid chromatographic (HPLC) methods are preferred in other countries. To compare these methods, we took 54 plasma samples for haloperidol and 91 plasma samples for bromperidol from schizophrenic patients receiving haloperidol or bromperidol, and the samples were measured using both commercial EIA and HPLC methods. Significant linear correlations were found between the two methods in determining haloperidol (EIA = 1.351 x HPLC + 1.39; r = 0.934, P < 0.001) and bromperidol (EIA = 1.420 x HPLC + 0.712; r = 0.956, P < 0.001) concentrations, but plasma concentrations using the EIA kits were approximately 92% (95% CI; 53-131%) and 62% (54-70%) higher than those using HPLC for haloperidol and bromperidol, respectively. Mean (and range) plasma concentrations of reduced metabolites were 54% (30-92%) and 55% (29-111%) of those of haloperidol and bromperidol, respectively. The present study suggests that reduced metabolites are included to a considerable degree in TDM data using the EIA kits. Therefore, some limitation of TDM data of haloperidol and bromperidol using the EIA kits, ie, high precision but poor accuracy, should be kept in mind.

Sub-chronic treatment with classical but not atypical antipsychotics produces morphological changes in rat nigro-striatal dopaminergic neurons directly related to "early onset" vacuous chewing

In the present work, we investigated if an impairment of dopaminergic neurons after subchronic haloperidol treatment might be a possible physiopathologic substrate of the "early onset" vacuous chewing movements (VCMs) in rats. For this purpose, different antipsychotics were used to analyse a possible relationship between VCMs development and morphological alterations of tyrosine-hydroxylase-immunostained (TH-IM) neurons. Rats treated twice a day with haloperidol displayed a significant increase of VCMs that was both time- (2-4 weeks) and dose (0.1-1 mg/kg) dependent. Immunocytochemical analysis showed a shrinkage of TH-IM cell bodies in substantia nigra pars compacta and reticulata and a reduction of TH-immunostaining in the striatum of haloperidol treated rats with the arising of VCMs. No differences were observed in TH-IM neurons of ventral tegmental area and nucleus accumbens vs. control rats. The atypical antipsychotics risperidone (2 mg/kg, twice a day), amisulpride (20 mg/kg, twice a day) and clozapine (10 mg/kg, twice a day) did not produce any nigro-striatal morphological changes or VCMs. TH-IM nigro-striatal neuron morphological alterations and VCMs were still present after three days of withdrawal in rats treated for four weeks with haloperidol (1 mg/kg). Both the main morphological changes and the behavioural correlate disappeared after three weeks of withdrawal. These results suggest that haloperidol induces a morphological impairment of the dopaminergic nigro-striatal neurons which is directly associated with the arising, permanency and disappearance of VCMs in rats.

Metabolic studies on haloperidol and its tetrahydropyridinyl dehydration product (HPTP) in C57BL/6 mouse brain preparations

The neuroleptic agent haloperidol (HP) and its tetrahydropyridinyl dehydration product HPTP are biotransformed by humans, baboons and rodents to the HP pyridinium (HPP(+)) and reduced HP pyridinium (RHPP(+)) species, potential neurotoxic metabolites that have been detected in the brain. HPP(+), however, does not pass the mouse blood-brain barrier since it is not detected in the brain following systemic administration. We report here that C57BL/6 mouse brain preparations catalyze the oxidation of HP and HPTP to HPP(+). The initial rate of HPP(+) formation from HPTP by whole brain homogenates was estimated to be approximately 20 times faster than that observed with HP as substrate. HPTP also was converted to HPP(+) by mouse brain microsomal preparations and brain slices. These results suggest that the presence of HPP(+) in the C57BL/6 mouse brain following systemic administration of HPTP may be due primarily to its in situ metabolism to HPP(+). Attempts to identify the catalyst responsible for these biotransformations, however, have not been successful.

Pharmacokinetics of haloperidol: an update

Haloperidol is commonly used in the therapy of patients with acute and chronic schizophrenia. The enzymes involved in the biotransformation of haloperidol include cytochrome P450 (CYP), carbonyl reductase and uridine diphosphoglucose glucuronosyltransferase. The greatest proportion of the intrinsic hepatic clearance of haloperidol is by glucuronidation, followed by the reduction of haloperidol to reduced haloperidol and by CYP-mediated oxidation. In studies of CYP-mediated disposition in vitro, CYP3A4 appears to be the major isoform responsible for the metabolism of haloperidol in humans. The intrinsic clearances of the back-oxidation of reduced haloperidol to the parent compound, oxidative N-dealkylation and pyridinium formation are of the same order of magnitude, suggesting that the same enzyme system is responsible for the 3 reactions. Large variation in the catalytic activity was observed in the CYP-mediated reactions, whereas there appeared to be only small variations in the glucuronidation and carbonyl reduction pathways. Haloperidol is a substrate of CYP3A4 and an inhibitor, as well as a stimulator, of CYP2D6. Reduced haloperidol is also a substrate of CYP3A4 and inhibitor of CYP2D6. Pharmacokinetic interactions occur between haloperidol and various drugs given concomitantly, for example, carbamazepine, phenytoin, phenobarbital, fluoxetine, fluvoxamine, nefazodone, venlafaxine, buspirone, alprazolam, rifampicin (rifampin), quinidine and carteolol. Overall, drug interaction studies have suggested that CYP3A4 is involved in the biotransformation of haloperidol in humans. Interactions of haloperidol with most drugs lead to only small changes in plasma haloperidol concentrations, suggesting that the interactions have little clinical significance. On the other hand, the coadministration of carbamazepine, phenytoin, phenobarbital, rifampicin or quinidine affects the pharmacokinetics of haloperidol to an extent that alterations in clinical consequences would be expected. In vivo pharmacogenetic studies have indicated that the metabolism and disposition of haloperidol may be regulated by genetically determined polymorphic CYP2D6 activity. However, these findings appear to contradict those from studies in vitro with human liver microsomes and from studies of drug interactions in vivo. Interethnic and pharmacogenetic differences in haloperidol metabolism may explain these observations.

Selective neurotoxins, chemical tools to probe the mind: the first thirty years and beyond

For centuries, starting with the advent of the microscope, cytotoxins have been known to non-selectively destroy nerves and other tissue cells. However, neurotoxins restricted in effect to one kind of neuron are an invention of the 20th century. One might reasonably trace the origins of this field to 1960 when the Nobel Laureates, R. Levi- Montalcini and S Cohen, showed that an antibody to nerve growth factor effectively prevented development of sympathetic nerves in the absence of overt changes in dorsal root ganglia and other neural and non-neural tissues. The year 1967 marks discovery of 6-hydroxydopamine, the first of dozens of chemically-selective neurotoxins. As stated by the physiologist W.B. Cannon, neural function can be deduced by denoting absence-deficits. A wealth of knowledge in neuroscience has been realized through use of neurotoxins. In the 21st century we foresee neurotoxins for virtually all neurochemically-identifiable or receptor-specific neurons, acting at/via functional proteins or characteristic DNA sites. These tools will provide us with a better means to probe the mind and thereby lead to a fuller understanding of the intricate roles of identifiable neuronal systems in integrative neuroscience.

Metabolism, pharmacogenetics, and metabolic drug-drug interactions of antipsychotic drugs

1. Antipsychotic drugs are extensively metabolised by cytochrome P450 (CYP) enzymes. 2. Dispositions of a number of antipsychotic drugs have been shown to cosegregate with polymorphism of CYP2D6. 3. Metabolic drug-drug interactions have frequently been observed when antipsychotics are coadministered with other drugs. 4. Many antipsychotic drugs are converted to active metabolites which can contribute to the therapeutic or side effects of the parent drug. 5. Information concerning the individual CYP isoenzymes involved in the metabolism of antipsychotic drugs is important for the safe clinical use of this group of drugs.

Effects of haloperidol metabolites on neurotransmitter uptake and release: possible role in neurotoxicity and tardive dyskinesia

This research explored the effects of haloperidol (HP) metabolites on biogenic amine uptake and release, and compared them to those of MPTP and its toxic metabolite, MPP+. In synaptosome preparations from mouse striatum and cortex, the HP metabolites haloperidol pyridinium (HPP+), reduced haloperidol pyridinium (RHPP+), and haloperidol tetrahydropyridine (HPTP) inhibited the presynaptic uptake of dopamine and serotonin, with greater affinity for the serotonin transporter. HPP+ was the most potent inhibitor of dopamine uptake, and HPTP of serotonin uptake, both with IC50 values in the low micromolar range. RHPP+ was less active than the other metabolites, but was more active than the parent compound, HP. Inhibition of uptake was reversed when free drug was removed by centrifugation and then resuspension of the synaptosomes in fresh buffer, suggesting that inhibition of uptake was due to interaction with the transporters and was not due to irreversible cytotoxicity. HPP+ showed noncompetitive inhibition of both serotonin and dopamine uptake, suggesting that it has a relatively slow dissociation rate for its interaction with the transporter proteins. In experiments on amine release, HPP+ and HPTP were four-fold less potent than MPP+ for releasing preloaded dopamine from striatal synaptosomes, and only MPP+-dependent release was antagonized by the uptake blocker, mazindol. In contrast, RHPP+ displayed little ability to release either amine neurotransmitter. HPTP was about two-fold more potent than MPP+ for releasing serotonin from cortical synaptosomes, whereas HPP+ was less active than MPP+. The specific serotonin transport blocker fluoxetine was only able to antagonize release induced by MPP+. These results suggest that HP metabolites bind to the transporters for dopamine and serotonin, but are not transporter substrates. In contrast to their potent effects on amine release, HPP+ and HPTP were unable to release preloaded GABA from cortical synaptosomes. The implications of these results concerning a possible role of HP metabolites in the development of tardive dyskinesia are discussed.

Determination of 4-(4-chlorophenyl)-4-hydroxypiperidine, a metabolite of haloperidol, by gas chromatography with electron-capture detection

An electron-capture gas chromatographic procedure was developed for the analysis of 4-(4-chlorophenyl)-4-hydroxypiperidine (CPHP), a metabolite of haloperidol. The assay involved basic extraction of this metabolite from the biological samples, followed by back-extraction with HCl. After basification of the acid phase, extractive derivatization with pentafluorobenzoyl chloride in toluene was conducted. The pentafluorobenzoyl derivative was quantified on a gas chromatograph equipped with a fused-silica capillary column, an electron-capture detector and a printer-integrator. N-(3-Trifluoromethylphenyl)piperazine was carried through the procedure as an internal standard and calibration curves were determined for each assay run. The procedure was demonstrated to be linear and reproducible and was utilized to detect and quantify CPHP in urine, plasma, brain and liver samples from rats treated with haloperidol. The structure of the derivatized metabolite was confirmed by gas chromatography-mass spectrometry.