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Mean Open Time (mean + open_time)
Selected AbstractsCharacterization of the single-channel properties of NMDA receptors in laminae I and II of the dorsal horn of neonatal rat spinal cordEUROPEAN JOURNAL OF NEUROSCIENCE, Issue 10 2001G. Mark Green Abstract The single-channel properties of native NMDA receptors in laminae I and II of the dorsal horn of the neonatal rat spinal cord were studied using outside-out patch-clamp techniques. These receptors were found to have several features that distinguish them from native NMDA receptors elsewhere in the CNS. Single-channel currents activated by NMDA (100 nm) and glycine (10 µm) exhibited five distinct amplitude components with slope-conductance values of 19.9 ± 0.8, 32.9 ± 0.6, 42.2 ± 1.1, 53.0 ± 1.0 and 68.7 ± 1.5 pS. Direct transitions were observed between all conductance levels but transitions between 69-pS openings and 20-, 33- and 42-pS openings were rare. There was no significant difference in the frequency of direct transitions from 42- to 20-pS compared to 20- to 42-pS transitions. The Kb (0 mV) for Mg2+ was 89 µm. The Mg2+ unblocking rate constant was similar to other reported values. However, the Mg2+ blocking rate constant was larger than other reported values, suggesting an unusually high sensitivity to Mg2+. The NR2B subunit-selective antagonist, ifenprodil, had no significant effect on overall channel activity but significantly decreased the mean open time of 53-pS openings. These results suggest neonatal laminae I and II NMDA receptors are not simply composed of NR1 and NR2B subunits or NR1 and NR2D subunits. It is possible that these properties are due to an as yet uninvestigated combination of two NR2 subunits with the NR1 subunit or a combination of NR3A, NR2 and NR1 subunits. [source] BDNF, NT-3 and NGF induce distinct new Ca2+ channel synthesis in developing hippocampal neuronsEUROPEAN JOURNAL OF NEUROSCIENCE, Issue 11 2000Pietro Baldelli Abstract Neurotrophins exert short- and long-term effects on synaptic transmission. The mechanism underlying these forms of synaptic plasticity is unknown although it is likely that intracellular Ca2+ and presynaptic Ca2+ channels play a critical role. Here we show that BDNF, NGF and NT-3 (10,100 ng/mL) exhibit a selective long-term up-regulation of voltage-gated Ca2+ current densities in developing hippocampal neurons of 6,20 days in culture. NGF and NT-3 appear more effective in up-regulating L-currents, while BDNF predominantly acts on non-L-currents (N, P/Q and R). The effects of the three neurotrophins were time- and dose-dependent. The EC50 was comparable for BDNF, NGF and NT-3 (10,16 ng/mL) while the time of half-maximal activation was significantly longer for NGF compared to BDNF (58 vs. 25 h). Despite the increased Ca2+ current density, the neurotrophins did not alter the voltage-dependence of channel activation, the kinetics parameters or the elementary properties of Ca2+ channels (single-channel conductance, probability of opening and mean open time). Neurotrophin effects were completely abolished by coincubation with the nonspecific Trk-receptor inhibitor K252a, the protein synthesis blocker anisomycin and the MAP-kinase inhibitor PD98059, while cotreatment with the PLC-, blocker, U73122, was without effect. Immunocytochemistry and Western blotting revealed that neurotrophins induced an increased MAP-kinase phosphorylation and its translocation to the nucleus. The present findings suggest that on a long time scale different neurotrophins can selectively up-regulate different Ca2+ channels. The action is mediated by Trk-receptors/MAP-kinase pathways and induces an increased density of newly available Ca2+ channels with unaltered gating activity. [source] A Novel Background Potassium Channel in Rat Atrial CellsEXPERIMENTAL PHYSIOLOGY, Issue 4 2000Z. Shui A K+ channel activated by intracellular ATP has been observed in inside-out patches from rat atrial cells. The channel has a slope conductance of 130 ± 5 pS in symmetrical 140 mM K+ solution, and is almost independent of voltage over the range from -80 to +80 mV. There is no detectable inactivation during application of ATP over a few minutes. In the presence of 3 mM intracellular ATP, channel openings occur as bursts with a mean open time of 1.7 ms, a mean closed time of 0.4 ms, a mean burst duration of 18 ms and a mean burst interval of 41 ms. Kinetic analysis suggests that ATP mainly affects the burst duration and the burst interval of the channel. Based on the properties above, the channel differs from other known K+ channels in cardiac cells and may contribute to background K+ current. [source] Ca2+ -dependent components of inactivation of unitary cardiac L-type Ca2+ channelsTHE JOURNAL OF PHYSIOLOGY, Issue 1 2010Ira R. Josephson A Ca2+ ion-dependent inactivation (CDI) of L-type Ca2+ channels (LCC) is vital in limiting and shaping local Ca2+ ion signalling in a variety of excitable cell types. However, under physiological conditions the unitary LCC properties that underlie macroscopic inactivation are unclear. Towards this end, we have probed the gating kinetics of individual cardiac LCCs recorded with a physiological Ca2+ ion concentration (2 mm) permeating the channel, and in the absence of channel agonists. Upon depolarization the ensemble-averaged LCC current decayed with a fast and a slow exponential component. We analysed the unitary behaviour responsible for this biphasic decay by means of a novel kinetic dissection of LCC gating parameters. We found that inactivation was caused by a rapid decrease in the frequency of LCC reopening, and a slower decline in mean open time of the LCC. In contrast, with barium ions permeating the channel ensemble-averaged currents displayed only a single, slow exponential decay and little time dependence of the LCC open time. Our results demonstrate that the fast and slow phases of macroscopic inactivation reflect the distinct time courses for the decline in the frequency of LCC reopening and the open dwell time, both of which are modulated by Ca2+ influx. Analysis of the evolution of CDI in individual LCC episodes was employed to examine the stochastic nature of the underlying molecular switch, and revealed that influx on the order of a thousand Ca2+ ions may be sufficient to trigger CDI. This is the first study to characterize both the unitary kinetics and the stoichiometry of CDI of LCCs with a physiological Ca2+ concentration. These novel findings may provide a basis for understanding the mechanisms regulating unitary LCC gating, which is a pivotal element in the local control of Ca2+ -dependent signalling processes. [source] | ||||||||||