Abstract
It has been suggested that the Ca 2+-dependent enzyme transglutaminase (TGase) may suppress vesicular neurotransmitter release and mediate the inhibitory effect of tetanus toxin (TetTx) on exocytosis. The aim of the present study was to test this in a model of [ 3H]5-HT release from superfused rat cortical synaptosomes. Monodansylcadaverine, a synthetic amine that acts as an alternative substrate for TGase, showed dose-dependent releasing activity which, however, was Ca 2+-independent, being maintained in a Ca 2+-free buffer (containing EGTA) or using synaptosomes preloaded with the intracellular Ca 2+ chelator BAPTA. Preincubation of synaptosomes with RS-48373, an irreversible TGase inactivator, resulted in marked (64%) and persistent inhibition of endogenous TGase but did not alter basal and K +-induced [ 3H]5-HT release. Preincubation of synaptosomes with 10 nM TetTx resulted in 52% inhibition of K +-induced [ 3H]5-HT release, and this effect was not antagonized in RS-48373-treated synaptosomes. The inhibitory effect of TetTx was significantly antagonized by 20 mM captopril, a metalloendoprotease inhibitor, confirming in rat brain synaptosomes that TetTx inhibits exocytosis by acting as a metalloendoprotease. These results suggest that TGase is not involved in controlling [ 3H]5-HT release from resting and depolarized synaptosomes, or in the inhibitory effect of TetTx. Copyright © 1996 Elsevier Science Ltd.
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