3 NOVEL TYPES OF VOLTAGE-DEPENDENT CALCIUM CHANNELS IN RAT CEREBELLARNEURONS

With the aim of characterizing the functional and pharmacological prop erties of the different voltage-dependent Ca2+ channels expressed in a given type of CNS neuron, we obtained single Ca2+ channel recordings from rat cerebellar granule cells in primary culture. Our data show th at three novel classes of voltage-dependent Ca2+ channels are coexpres sed in cerebellar granule cells. They are pharmacologically distinct f rom dihydropyridine-sensitive L-type and omega-conotoxin-sensitive N-t ype channels, and their functional properties are different from those of P- and T-type channels. The three novel 21 pS G1-, 15 pS G2-, and 20 pS G3-type Ca2+ channels have similar inactivation properties. They show complete steady-state inactivation at -40 mV and their single-ch annel average currents have both sustained and decaying components. Th ey differ in activation threshold (-40 mV for G2, -30 mV for G3, and - 10 mV for G1, with 90 mM Ba2+ as charge carrier), mean open time (1.2 msec for G2, 1 msec for G3, 0.8 msec for G1), and single-channel curre nts (at 0 mV: 0.5 pA for G2, 0.8 pA for G3, and 1.4 pA for G1). Togeth er with the previously characterized multiple L-type Ca2+ channels (Fo rti and Pietrobon, 1993), G1-, G2-, and G3-type channels constitute th e large majority of Ca2+ channels of cerebellar granule cells in cultu re. The low activation threshold of G2-type channels and their inactiv ation properties suggest that they might be native counterparts of the recently expressed rat brain clone rbE-II (Soong et al., 1993).