The metabotropic glutamate receptor subtype 1 (mGluR1) is widely expressed in the CNS. Its activation at parallel fiber-Purkinje cell synapses is crucial for cerebellar function. Synaptic stimulation of mGluR1 at parallel fiber synapses is followed by two G-protein-dependent processes: production of IP₃ leading to release of Ca²⁺ ions from intracellular stores (Takechi, Eilers & Konnerth, Nature, 1998) and initiation of a slowly activating EPSC (sEPSC).

We studied mGluR1-dependent signaling in mice deficient for different subunits of transient receptor potential channels (TRPCs) by using whole-cell recordings and Ca²⁺ imaging in acute cerebellar slices. In contrast to earlier suggestions, the mGluR-mediated sEPSC persists in the absence of TRPC1 and in the combined absence of TRPC1, TRPC4 and TRPC6. We found that mGluR1-mediated synaptic transmission at the parallel fiber-Purkinje cell synapse requires TRPC3. Both the synaptically evoked slow EPSC and inward currents evoked by local application of the mGluR-specific agonist DHPG are completely absent in TRPC3-deficient mice. Ca²⁺ release from internal stores was not affected by the absence of TRPC3. Single cell quantitative RT-PCR analysis showed that TRPC3 is the by far dominating TRPC subunit of Purkinje cells. Importantly, TRPC3-deficient mice show a distinct defect in their walking behavior while the other TRPC-mutants examined (TRPC1-/-, TRPC1/4 (-/-)2 and TRPC1/4/6 (-/-)3) are behaviorally inconspicuous. Thus, our results establish TRPC3 as a new type of postsynaptic channel that mediates mGluR-mediated transmission in cerebellar Purkinje cells and is important for motor coordination (Hartmann et al., Neuron, 2008). In peripheral organs TRPC channel signaling is linked to the STIM and Orai proteins. Using PCR analyses we detected mRNA for STIM1 and 2 and Orai1-3 in the cerebellum. The analysis of their expression in individual Purkinje cells is in progress.

Using two-photon Ca²⁺ imaging we measured synaptically evoked TRPC3-dependent Ca²⁺ influx signal in spines and dendrites of Purkinje cells. Our results show that the mGluR-dependent Ca²⁺ signal has two distinct components: a large Ca²⁺ release signal from stores and an about 10-fold smaller Ca²⁺ influx signal. The Ca²⁺ release component is prominent in spines and has a fast rise time, while the Ca²⁺ influx signal has a slow onset and is more evenly distributed in spines and dendrites. By analyzing TRPC3-deficient mice we determined the kinetic properties of the pure, mGluR-mediated synaptic Ca²⁺ release signal in spines and dendrites.

Our results provide strong evidence that Ca²⁺ release from stores, but not TRPC3-mediated Ca²⁺ influx underlies activity-dependent synaptic plasticity, like long-term depression, in cerebellar Purkinje cells.