mGluR1 Regulates the Interspike Interval Threshold for Dendritic Ca(2+) Transients in the Cerebellar Purkinje Cells

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