Multiple Components of Ca2+Channel Facilitation in Cerebellar Granule Cells: Expression of Facilitation during Development in Culture

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Abstract
The contribution of pharmacologically distinct Ca2+ channels to prepulse-induced facilitation was studied in mouse cerebellar granule cells. Ca2+ channel facilitation was measured as the percentage increase in the whole-cell current recorded during a test pulse before and after it was paired with a positive prepulse. The amount of facilitation was small in recordings made during the first few days in tissue culture but increased substantially after 1 week. L-type channels accounted for the largest proportion of facilitation in 1-week-old cells (60–70%), whereas N-type channels contributed very little (∼3%). The toxins ω-agatoxin IVa or ω-conotoxin MVIIC (after block of N-, L-, and P-type channels) each blocked a small percentage of facilitation (∼12 and 14%, respectively). Perfusion of cells with GTP-γ-S enhanced the facilitation of N-type channels, whereas it inhibited facilitation of L-type channels. During development in vitro, the contribution of L-type channels to the whole-cell current decreased. Single-channel recordings showed the presence of 10 and 15 pS L-type Ca2+ channels in 1-d-old cells. After 1 week in culture, a ∼25 pS L-type channel dominated recordings from cell-attached patches. Positive prepulses increased the activity of the 25 pS channel but not of the smaller conductance channels. The expression of Ca2+channel facilitation during development may contribute to changes in excitability that allow frequency-dependent Ca2+influx during the period of active synaptogenesis.