Inositol-1,4,5-trisphosphate receptor-mediated Ca mobilization is not required for cerebellar long-term depression in reduced preparations

Cerebellar longterm depression (LTD) is a cellular model system of informat ion storage in which coincident parallel fiber and climbing fiber activatio n of a Purkinje neuron (PN) gives rise to a sustained attenuation of parall el fiber-PN synaptic strength. Climbing fiber and parallel fiber inputs may be replaced by direct depolarization of the PN and exogenous glutamate pul ses, respectively. The parallel fiber-PN synapse has a high-density of mGlu R1 receptors that are coupled to phosphoinositide turnover. Several lines o f evidence indicated that activation of mGluR1 by parallel fiber stimulatio n is necessary for the induction of cerebellar LTD. Because phosphoinositid e hydrolysis has two initial products, 1,2-diacylglycerol and inositol-1,4, 5-trisphosphate (IP3), we wished to determine whether IP3 signaling via IP3 receptors and consequent Ca mobilization were necessary for the induction of cerebellar LTD. First, ratiometric imaging of free cytosolic Ca was perf ormed on both acutely dissociated and cultured PNs. It was determined that the threshold for glutamate pulses to contribute to LTD induction was below the threshold for producing a Ca transient. Furthermore, the Ca transients produced by depolarization alone and glutamate plus depolarization were no t significantly different. Second, the potent and selective IP3 receptor ch annel blocker xestospongin C was not found to affect the induction of LTD i n either acutely dissociated or cultured PNs at a concentration that was su fficient to block mGluR1-evoked Ca mobilization. Third, replacement of mGlu R activation by exogenous synthetic diacylglycerol in an LTD induction prot ocol was successful. Taken together, these results suggest that activation of an IP3 signaling cascade is not required for induction of cerebellar LTD in reduced preparations.