A new class of synaptic response involving calcium release in dendritic spines

In the classical view, transmission of signals across synapses in the mamma lian brain involves changes in the membrane potential of the postsynaptic c ell. The use of high-resolution cellular imaging has revealed excitatory sy napses at which postsynaptic, transient alterations in calcium ion concentr ation are tightly associated with electrical responses (reviewed in ref. 1) . Here, by investigating the synapse between parallel glutamatergic fibres and Purkinje cells in the mouse cerebellum, we identify a class of postsyna ptic responses that consist of transient increases in dendritic Ca2+ concen tration but not changes in somatic membrane potential. Our results indicate that these synaptic Ca2+ transients are mediated by activation of metabotr opic glutamate-responsive mGluR1-type receptors(2-4) and require inositol-1 ,4,5-trisphosphate-mediated Ca2+ release(5,6) from to postsynaptic microdom ains, which range, depending on the frequency of stimulation, from individu al spines to small spinodendritic compartments. Thus, the synaptic Ca2+-rel ease signal may be one of the critical cues that determine the input specif icity of longterm depression, a well-established form of activity-dependent plasticity at these synapses(7-9).