Elsevier

Brain Research

Volume 788, Issues 1–2, 30 March 1998, Pages 215-222
Brain Research

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

https://doi.org/10.1016/S0006-8993(97)01551-5Get rights and content

Abstract

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.

Introduction

Tardive dyskinesia (TD) is a well known side effect of haloperidol (HP, Fig. 1) treatment that is slow to develop and often irreversible [14]. TD has been attributed to the supersensitivity of dopamine receptors [7], but this mechanism is not consistent with a number of factors documented in TD patients [14]. Rollema et al. [20]provided evidence that a key metabolite of HP, haloperidol pyridinium (HPP+, Fig. 1), shared some structural similarity and toxic actions with MPP+ and suggested that HPP+ may induce TD by causing neuronal injury via nerve terminal transport and inhibition of mitochondrial respiration. In a related study, Bloomquist et al. [3]demonstrated that HPP+ was, in fact, cytotoxic to cultured dopaminergic and serotonergic mesencephalic neurons. In addition, MPP+ and HPP+ blocked the uptake of both serotonin and dopamine into mouse brain synaptosomes, suggesting an interaction with the neuronal transporters for these biogenic amines [3]. These findings were confirmed and extended in subsequent studies 10, 11, 12. Thus, the available data suggest that HPP+ has a mode of action similar to that of MPP+, and that the MPTP/MPP+ model of neurotoxicity may be valid for assessing the role of HP metabolites in the development of TD. An important component of the action of MPP+ is its specific uptake by dopaminergic nerve terminals [16]. Thus, comparative studies were initiated on the inhibitory potency, reversibility, and kinetic nature of the interaction of HPP+ and other HP metabolites (HPTP and RHPP+, Fig. 1) with the dopamine and serotonin transporters of mouse brain synaptosomes. We also assessed the ability of HP metabolites to release [3H]dopamine and [3H]serotonin from preloaded synaptosomes in the presence and absence of the uptake blockers mazindol and fluoxetine, in order to determine the role of transporter-mediated uptake in the actions of these compounds. Additional studies tested the ability of these compounds to release preloaded GABA from cortical synaptosomes as a measure of the specificity of their effects. Preliminary versions of these results have appeared 2, 4.

Section snippets

Chemicals and animals

Fluoxetine, mazindol, HP, HPTP, HPP+, RHPP+, MPTP, MPP+ and pargyline were kindly provided by Neal Castagnoli, Jr., Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, VA. Veratridine (VTD), rotenone (ROT), bovine serum albumin (fraction V), Coomassie brilliant blue G250, and phosphoric acid were purchased from Sigma Chemical (St. Louis, MO). Samples of [3H]dopamine (20 Ci/mmol) and [3H]GABA (88 Ci/mmol) were purchased from New England Nuclear (DuPont,

Inhibition of dopamine and serotonin uptake

Synaptosomes prepared from striatal tissue were used to evaluate inhibitory effects of MPTP, MPP+, HP, HPTP, HPP+ and RHPP+ on dopamine uptake. In nearly all cases, compounds showed inhibition of about 90% at maximally effective concentrations, where the residual probably represents nonspecific binding of label to the synaptosomes. MPP+ (Fig. 2A) was the most potent and effective blocker of dopamine uptake, showing 91% maximal inhibition and an IC50 value of about 4 μM (Table 1). For

Discussion

For blocking the uptake of serotonin and dopamine, there was considerably more activity present for the metabolites of HP than for the parent compound, confirming that conversion of HP to HPTP, HPP+, and RHPP+ was bioactivating metabolism. The tetrahydropyridines, MPTP and HPTP, were the most potent blockers of serotonin uptake in their respective structural series. The greater potency of MPTP than MPP+ in this regard is consistent with previous studies 16, 17. In contrast, the

Acknowledgements

The authors would like to thank Drs. N. Castagnoli and C. Van der Schyf for their review of the manuscript. Financial support was provided by a National Institute of Health Biomedical Research Support Grant (to J.R.B.) and a Virginia Commonwealth predoctoral fellowship (to A.M.W.).

References (23)

  • J Bloomquist et al.

    In vitro neurotoxicity assessment of oxidative and reductive metabolites of haloperidol

    Fundam. Appl. Toxicol., The Toxicologist

    (1995)
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