Calcium dynamics at the mossy fiber/cerebellar granule cell synapse

Granule cells form the largest neuronal population in the mammalian brain. They process information entering into the cerebellar cortex through the mossy fibers. Cytosolic calcium regulates a large variety of neuronal fonctions and, interestingly, it has been shown that post synaptic calcium concentration regulates long term potentiation (LTP) at the mossy fiber/granule cell synapse. In the present study, we have used calcium imaging combined with patch clamp recording techniques in acute slice preparation to investigate calcium dynamics at the mossy fiber/cerebellar granule cell synapse. We have evaluated the contribution of the different mechanisms involved in the initiation of the calcium signal at the synaptic level: voltage dependent calcium channels, NMDA receptors, metabotropic glutamate receptors and intracellular calcium stores. During voltage clamp recordings, using the fluorescent calcium probe Oregon green BAPTA 1, we have measured the calcium transients induced by a 200 ms depolarisation at 0 mV and by high frequency mossy fiber stimulation (1s at 100 Hz) in control condition and in the presence of various antagonists (APV, AIDA,thapsigargin). We demonstrate that calcium transients are localized at the dendritic endings and that VDCC and NMDA receptors are the major systems triggering the calcium signal. In addition, calcium induced calcium release plays a prominent role in amplification of the calcium signal. In addition, using current clamp recordings, we have caracterized the calcium response elicited during LTP induction.