Presynaptic internal Ca2+ stores contribute to inhibitory neurotransmitter release onto mouse cerebellar Purkinje cells

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Abstract
1. Miniature inhibitory postsynaptic currents (mIPSCs) were recorded in mouse Purkinje cells in the presence of 1 micro M tetrodotoxin (TTX). Under these conditions, which eliminated Ca(2+) influx through voltage-dependent Ca(2+) channels (VDCCs), the contribution of Ca(2+) stores to spontaneous GABA release was examined. 2. The plant alkaloid ryanodine acts as an inhibitor of endoplasmic reticulum ryanodine-sensitive Ca(2+) release channels (ryanodine receptors) at low micromolar concentrations. Ryanodine effects were confined to a subpopulation of cells tested. At 10 micro M ryanodine, 4/12 cells showed a significant increase in mean mIPSC frequency of +19.6+/-4.0% (n=4). 3. The sarco/endoplasmic reticulum Ca(2+)-ATPase (SERCA) pump inhibitor cyclopiazonic acid (CPA) produced a more robust effect. In 8/10 cells, 25 micro M CPA caused a significant increase in mean mIPSC frequency; the mean increase being +26.0+/-3.0% (n=8). Similar results were seen with thapsigargin (1-2 micro M), another SERCA pump inhibitor. 4. Ruthenium red (RuR) has been proposed to either act directly on the release machinery or block Ca(2+) pumps on internal stores. At 10 micro M RuR, all cells showed a rapid, large increase in mean mIPSC frequency of +90.4+/-16.4% (n=9). This increase was greater than that seen by agents known to modulate Ca(2+) stores and was more consistent with a direct action. At this concentration, RuR also occluded the effects of CPA. 5. For all reagents, there were no obvious effects on mean mIPSC amplitude. However, the effects on mIPSC frequency were consistent with a presynaptic action and indicate that Ca(2+) stores may contribute to spontaneous GABA release onto mouse Purkinje cells.