INVOLVEMENT OF PHOSPHATASE-ACTIVITIES IN THE RUN-DOWN OF GABA(A) RECEPTOR FUNCTION IN RAT CEREBELLAR GRANULE CELLS IN CULTURE

Run-down of GABA activated Cl- currents was found when rat cerebellar granule cells in culture were studied by the whole-cell patch-clamp te chnique in the absence of ATP in the pipette medium. This event could be prevented, even in the absence of ATP, by using the perforated-patc h technique or by adding to the pipette medium either a blocker of pro tein tyrosine phosphatase, sodium vanadate, or deltamethrin, a blocker of the protein serine/threonine phosphatase calcineurin. Conversely, run-down could be partially induced, even in the presence of ATP, by b lockers of tyrosine kinases. A reduction of GABA(A) receptor activity was also found in outside-our membrane patches when ATP was not on the membrane inside. The run-down phenomenon involved all three conductan ce levels found in these patches: 11, 20 and 30 pS. In all three cases it was due to a reduction of channels' open probability. The single-c hannel experiments showed that also in this case run-down was prevente d by either sodium vanadate or deltamethrin on the membrane cytoplasmi c side. Overall, through relatively unphysiological conditions (cells in culture and patch-clamp techniques), the study of the run-down phen omenon shows that the tyrosine phosphorylation state of GABA(A) recept ors is of importance in maintaining it in a proper functional state. T he data also show that tyrosine phosphorylation state is controlled by a protein tyrosine phosphatase, whose activity in turn is blocked via serine/threonine phosphorylation. (C) 1998 IBRO. Published by Elsevie r Science Ltd.