MULTIPLE COMPONENTS OF CA2- EXPRESSION OF FACILITATION DURING DEVELOPMENT IN CULTURE( CHANNEL FACILITATION IN CEREBELLAR GRANULE CELLS )

The contribution of pharmacologically distinct Ca2+ channels to prepul se-jnduced 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 i t was paired with a positive prepulse. The amount of facilitation was small in recordings made during the first few days in tissue culture b ut increased substantially after ? week. L-type channels accounted for the largest proportion of facilitation in I-week-old cells (60-70%), whereas N-type channels contributed very little (similar to 3%). The t oxins omega-agatoxin IVa or omega-conotoxin MVIIC (after block of N-, L-, and P-type channels) each blocked a small percentage of facilitati on (similar to 12 and 14%, respectively). Perfusion of cells with GTP- gamma-S enhanced the facilitation of N-type channels, whereas it inhib ited 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 C a2+ channels in 1-d-old cells. After 1 week in culture, a similar to 2 5 pS L-type channel dominated recordings from cell-attached patches. P ositive prepulses increased the activity of the 25 pS channel but not of the smaller conductance channels. The expression of Ca2+ channel fa cilitation during development may contribute to changes in excitabilit y that allow frequency-dependent Ca2+ influx during the period of acti ve synaptogenesis.