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Ca2+ transients can be observed in the distal dendrites of Purkinje cells (PCs) despite their lack of action potential backpropagation. These Ca2+events in distal dendrites require specific patterns of PC firing, such as complex spikes (CS) or simple spikes (SS) of burst mode. Unlike CS, whichcan act directly on voltage-gated calcium channels in the dendrites through climbing fiber inputs, the condition that can produce the Ca2+ events indistal dendrites with burst mode SS is poorly understood. Here, we propose the interspike interval threshold (ISIT) for Ca2+ transients in the distaldendrites of PC. We found that to induce the Ca2+ transients in distal dendrites the frequency of spike firing of PC should reach 250 Hz (3 ms ISI). Metabotropic glutamate receptor 1 (mGluR1) activation significantly relieved the ISIT and established cellular conditions in which spike firingwith 50 Hz (19 ms ISI) could induce Ca2+ transients in the distal dendrites. In contrast, blocking T-type Ca2+ channels or depleting the endoplasmicreticulum Ca2+ store resulted in a stricter condition in which spike firing with 333 Hz (2 ms ISI) was required. Our findings demonstrate that thePC has strict ISIT for dendritic Ca2+ transients, and this ISIT can be relieved by mGluR1 activation. This strict restriction of ISIT could contributeto the reduction of the signal-to-noise ratio in terms of collecting information by preventing excessive dendritic Ca2+ transients through the spontaneousactivity of PC.
