Three novel types of voltage-dependent calcium channels in rat cerebellar neurons

With the aim of characterizing the functional and pharmacological properties of the different voltage-dependent Ca2+ channels expressed in a given type of CNS neuron, we obtained single Ca*+ channel recordings from rat cerebellar granule cells in primary culture. Our data show that three novel classes of voltage-dependent Ca*+ channels are coexpressed in cerebellar granule cells. They are pharmacologically distinct from dihydropyridine-sensitive L-type and a-conotoxin-sensitive N-type channels, and their functional properties are different from those of P- and T-type channels.
The three novel 2 1 pS G I-, 15 pS G2-, and 20 pS G3-type Ca*+ channels have similar inactivation properties. They show complete steady-state inactivation at -40 mV and their single-channel average currents have both sustained and decaying components. They differ in activation threshold (-40 mV for G2, -30 mV for G3, and - 10 mV for Gl, with 90 my Baz+ as charge carrier), mean open time (1.2 msec for G2, 1 msec for G3,O.E msec for Gl), and single-channel currents (at 0 mV: 0.5 pA for G2, 0.8 pA for G3, and 1.4 pA for Gl ).
Together with the previously characterized multiple L-type Ca*+ channels (Forti and Pietrobon, 1993), Gl-, G2-, and G3- type channels constitute the large majority of Ca2+ channels of cerebellar granule cells in culture. The low activation threshold of GP-type channels and their inactivation properties suggest that they might be native counterparts of the recently expressed rat brain clone rbE-II (Soong et al., 1993).