The synaptic potential mediated by metabotropic glutamate receptors is not associated with a substantial elevation of cytosolic free calcium concentration in Purkinje cells

Brief tetanic stimulation of parallel fibres can evoke a slow excitatory postsynaptic potential (EPSP) in cerebellar Purkinje cells that is mediated by metabotropic glutamate receptors (mGluRs). It is likely that the receptor subtype involved is mGluR1, which couples to the production of diacylglycerol and inositol-1,4,5-trisphosphate (IP₃). We therefore examined whether the mGluR-EPSP is associated with an increase in cytosolic free calcium [Ca²⁺](i) using simultaneous Ca²⁺ imaging and electrophysiological recordings. An mGluR-EPSP could be evoked in all nine Purkinje cells tested. In all but one this potential was not associated with measurable changes in [Ca²⁺](i), whereas single calcium spikes produced large Ca²⁺ transients. In the one Purkinje cell where [Ca²⁺](i) was elevated, the rise was estimated to be roughly 20-fold smaller than that produced by a single Ca²⁺ spike.