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Effects of the Phosphatase Inhibitors Calyculin A and Okadaic Acid on Acetylcholine Synthesis and Content of Rat Hippocampal Formation

Authors

  • Amalia M. Issa,

    Corresponding author
    1. Department of Pharmacology and Therapeutics, Center for Studies in Aging, and Department of Neurology and Neurosurgery, McGill University, Montréal, Québec, Canada
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  • Serge Gauthier,

    1. Department of Pharmacology and Therapeutics, Center for Studies in Aging, and Department of Neurology and Neurosurgery, McGill University, Montréal, Québec, Canada
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  • Brian Collier

    1. Department of Pharmacology and Therapeutics, Center for Studies in Aging, and Department of Neurology and Neurosurgery, McGill University, Montréal, Québec, Canada
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Address correspondence and reprint requests to A. M. Issa at Department of Pharmacology and Therapeutics, McGill University, McIntyre Medical Sciences Bldg., 3655 Drummond St., Montréal, Québec, Canada H3G 1Y6.

Abstract

Abstract: The biochemical mechanisms involved in the regulation of acetylcholine (ACh) turnover are poorly understood. In the experiments reported here, we examined whether inhibition of the serine/threonine phosphatases 1 and 2A by calyculin A or okadaic acid alters ACh synthesis by rat hippocampal preparations. With hippocampal slices, calyculin A (50 nM) and okadaic acid (50 nM) reduced significantly (p < 0.01) the synthesis of [3H]ACh from [3H]choline. Both calyculin A and okadaic acid produced significant depletion of endogenous tissue ACh in a concentration-dependent manner (p < 0.01). This depletion was not the result of a drug-induced increase of spontaneous ACh release, which was not changed significantly (p > 0.7) by either drug. Choline acetyltransferase (ChAT) activity from tissue exposed to calyculin A or okadaic acid was reduced in a concentration-dependent manner (p < 0.05), but these phosphatase inhibitors did not act directly on ChAT in vitro; i.e., enzymatic activity was not altered significantly (p > 0.4) in the presence of calyculin A or okadaic acid. Both high-affinity and low-affinity [3H]choline uptake by hippocampal synaptosomes were reduced significantly in a concentration-dependent manner in the presence of calyculin A or okadaic acid; these agents reduced Vmax values for high- and low-affinity choline uptake (p < 0.01) with no significant change in Km values (p > 0.1), indicating a noncompetitive inhibition. Taken together, these data suggest that phosphatase activity plays a role in presynaptic central cholinergic nerve terminal function, in particular in the modulation of ACh synthesis.

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