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- Amplitude-Dependent Spike-Broadening and Enhanced Ca Signali...Amplitude-Dependent Spike-Broadening and Enhanced Ca Signaling in GnRH-Secreting Neurons
Biophysical Journal, Volume 79, Issue 3, 1 September 2000, Pages 1310-1323
Fredrick Van Goor, Andrew P. LeBeau, Lazar Z. Krsmanovic, Arthur Sherman, Kevin J. Catt and Stanko S. Stojilkovic
AbstractIn GnRH-secreting (GT1) neurons, activation of Ca-mobilizing receptors induces a sustained membrane depolarization that shifts the profile of the action potential (AP) waveform from sharp, high-amplitude to broad, low-amplitude spikes. Here we characterize this shift in the firing pattern and its impact on Ca influx experimentally by using prerecorded sharp and broad APs as the voltage-clamp command pulse. As a quantitative test of the experimental data, a mathematical model based on the membrane and ionic current properties of GT1 neurons was also used. Both experimental and modeling results indicated that inactivation of the tetrodotoxin-sensitive Na channels by sustained depolarization accounted for a reduction in the amplitude of the spike upstroke. The ensuing decrease in tetraethylammonium-sensitive K current activation slowed membrane repolarization, leading to AP broadening. This change in firing pattern increased the total L-type Ca current and facilitated AP-driven Ca entry. The leftward shift in the current-voltage relation of the L-type Ca channels expressed in GT1 cells allowed the depolarization-induced AP broadening to facilitate Ca entry despite a decrease in spike amplitude. Thus the gating properties of the L-type Ca channels expressed in GT1 neurons are suitable for promoting AP-driven Ca influx in receptor- and non-receptor-depolarized cells.
Abstract | Full Text | PDF (255 kb) - Subthreshold Sodium Current from Rapidly Inactivating Sodium...Subthreshold Sodium Current from Rapidly Inactivating Sodium Channels Drives Spontaneous Firing of Tuberomammillary Neurons
Neuron, Volume 33, Issue 4, 14 February 2002, Pages 587-600
Abraha Taddese and Bruce P Bean
SummaryA role for “persistent,” subthreshold, TTX-sensitive sodium current in driving the pacemaking of many central neurons has been proposed, but this has been impossible to test pharmacologically. Using isolated tuberomammillary neurons, we assessed the role of subthreshold sodium current in pacemaking by performing voltage-clamp experiments using a cell's own pacemaking cycle as voltage command. TTX-sensitive sodium current flows throughout the pacemaking cycle, even at voltages as negative as −70 mV, and this current is sufficient to drive spontaneous firing. When sodium channels underlying transient current were driven into slow inactivation by rapid stimulation, persistent current decreased in parallel, suggesting that persistent sodium current originates from subthreshold gating of the same sodium channels that underlie the phasic sodium current. This behavior of sodium channels may endow all neurons with an intrinsic propensity for rhythmic, spontaneous firing.
Summary | Full Text | PDF (446 kb) - Na Action Potentials in Human PhotoreceptorsNa Action Potentials in Human Photoreceptors
Neuron, Volume 30, Issue 2, 1 May 2001, Pages 451-458
Fusao Kawai, Masayuki Horiguchi, Hiromitsu Suzuki and Ei-ichi Miyachi
SummaryMammalian photoreceptors are hyperpolarized by a light stimulus and are commonly thought to be nonspiking neurons. We used the whole-cell patch-clamp technique on surgically excised human retina to examine whether human photoreceptors can elicit action potentials. We discovered that human rod photoreceptors express voltage-gated Na channels, and generate Na action potentials, in response to membrane depolarization from membrane potentials of −60 or −70 mV. Na spikes in human rods were elicited at the termination of a light response that hyperpolarized the potential well below −50 mV. This served to amplify the release of a neurotransmitter when a bright light is turned off, and thus selectively amplify the off response to the light signal.
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Copyright © 1964 The Biophysical Society. All rights reserved.
Biophysical Journal, Volume 4, Issue 2, 77-91, 1 March 1964
doi:10.1016/S0006-3495(64)86770-9
Articles
An Analysis of the Striated Muscle Fiber Action Current
Howard Jenerick
Abstract
A method is presented for determining the magnitude of the ionic current associated with the propagated spike potential. Parameters of the action current are then compared to various aspects of the response as recorded in a phase plane. The findings also include evidence for (a) Na+ ion as the major inward current carrier, (b) a fairly constant membrane conductance during the terminal phase (∼1 msec.) of the spike potential, and (c) the influence of Ca++ ion concentration on the action current.
