The Calcium Channel Single Channel Conductance Heirarchy is N>L>T at Physiological External Calcium: Implications for Presynaptic Transmitter Release Site Gating

Abstract

A number of studies support the conclusion that single CaV channels Ca2+ nanodomains gate molecular signaling pathways. Thus, at presynaptic terminals single CaV2.2 channels trigger fusion of synaptic vesicle (SVs) by saturating a nearby calcium sensor.2 It is generally accepted that CaV1, CaV2, and CaV3 families (L, N and T, respectively) exhibit a decreasing order of single channel conductance.1 Since nanodomain dimensions are proportional to single channel current amplitude (i), high-conductance L type channels would be expected to be favored over the intermediate conductance N-type. Since the L>N>T hierarchy was determined with high Ba2+EXT, we tested the idea that this sequence may differ at physiological Ca2+EXT.We recorded i values for all three CaV families across a broad range of Ca2+EXT, spanning the physiological range. We focused on i-65mV to avoid non-linear current-to-voltage relationship complications and for direct relevance to the gating of synaptic transmission.3 A CaV2.2>CaV1>CaV3.2 hierarchy was determined for i-65mV at 1-2 mM Ca2+EXT. Mathematical modeling predicts that the CaV2.2 Ca2+ nanodomain is ∼25% more extensive than that generated by CaV1. We also calculated single channel ‘SV fusion’ domains, defined as the radii where the channel would saturate ≥50% of 5-binding site calcium sensors. With a sensor binding affinity of 10μM a single CaV2.2 can activate a calcium-fusion sensor located on the proximal face of the synaptic vesicle.4 These findings may explain why CaV2 family channels are preferred for transmitter release site gating.1. Fox AP, Nowycky MC & Tsien RW JP 394:173(1987).2. Stanley EF Neuron 11:1007(1993).3. Llinas RR, Sugimori M & Simon SM PNAS 79:2415(1982).4. Weber AM,∗ Wong FK∗, Tufford AR, Schlichter LC, Matveev V & Stanley EF Nature Neurosci in press (2010).