Inwardly permeating Na ions generate the voltage dependence of resurgent Na current in cerebellar Purkinje neurons.
Voltage-gated Na channels of cerebellar Purkinje neurons express an endogenous open-channel blocking protein. This blocker binds channels at positive potentials and unbinds at negative potentials, generating a resurgent Na current and permitting rapid firing. The macroscopic voltage dependence of resurgent current raises the question of whether the blocker directly senses membrane potential or whether voltage dependence is conferred indirectly. Because we previously found that inwardly permeating Na ions facilitate dissociation of the blocker, we measured voltage-clamped currents in different Na gradients to test the role of permeating ions in generating the voltage dependence of unblock. In reverse gradients, outward resurgent currents were tiny or absent, suggesting that unblock normally requires "knockoff" by Na. Inward resurgent currents at strongly negative potentials, however, were larger in reverse than in control gradients. Moreover, occupancy of the blocked state was prolonged both in reverse gradients and in control gradients with reduced Na concentrations, indicating that block is more stable when inward currents are small. Accordingly, reverse gradients shifted the voltage dependence of block, such that resurgent currents were evoked even after conditioning at negative potentials. Additionally, in control gradients, peak resurgent currents decreased linearly with driving force during the conditioning step, suggesting that the stability of block varies directly with inward Na current amplitude. Thus, the voltage dependence of blocker unbinding results almost entirely from repulsion by Na ions occupying the external pore. The lack of voltage sensitivity of the blocking protein suggests that the blocker's binding site lies outside the membrane field, in the permeation pathway.
Aman
Teresa K
TK
Interdepartmental Neuroscience Program and Department of Neurobiology and Physiology, Northwestern University, Evanston, Illinois 60208, USA.
Raman
Indira M
IM
D000818:Animals; D002463:Cell Membrane Permeability / Q000502:physiology; D002531:Cerebellum / Q000166:cytology / Q000378:metabolism*; D015640:Ion Channel Gating / Q000502:physiology; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D008810:Mice, Inbred C57BL; D011689:Purkinje Cells / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism*; D026941:Sodium Channel Blockers / Q000378:metabolism; D015222:Sodium Channels / Q000378:metabolism*
D008565:Membrane Proteins; D026941:Sodium Channel Blockers; D015222:Sodium Channels; D012964:Sodium
Neuron. 2005 Jan 20;45(2):233-44
15664175
Neuron. 1991 Nov;7(5):743-53
1742023
Mol Cell Neurosci. 2008 Aug;38(4):607-15
18599309
Biophys J. 2001 Feb;80(2):729-37
11159440
J Neurosci. 2003 Jun 15;23(12):4899-912
12832512
J Neurosci. 2009 Feb 18;29(7):2027-42
19228957
J Gen Physiol. 1977 Nov;70(5):567-90
591912
J Neurosci. 2002 Apr 15;22(8):3100-7
11943813
Biophys J. 2001 Mar;80(3):1262-79
11222290
Neuron. 1994 Apr;12(4):819-29
8161454
J Gen Physiol. 1971 Oct;58(4):413-37
5112659
J Gen Physiol. 1995 Oct;106(4):617-40
8576700
J Gen Physiol. 1997 Jul;110(1):23-33
9234168
Neuron. 1995 Oct;15(4):951-60
7576643
J Neurophysiol. 2004 Nov;92(5):2831-43
15212420
Nature. 1983 Dec 1-7;306(5942):436-41
6316158
J Neurosci. 1997 Jun 15;17(12):4517-26
9169512
Biophys J. 2007 Mar 15;92(6):1938-51
17189307
J Gen Physiol. 1996 Aug;108(2):89-104
8854339
Biophys J. 1994 Nov;67(5):1806-15
7858119
J Gen Physiol. 1977 Jun;69(6):915-26
302321