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Mechanism of generation of spontaneous miniature outward currents (SMOCs) in retinal amacrine cells.

Mitra P, Slaughter MM - J. Gen. Physiol. (2002)

Bottom Line: Ca(2+) permeation through the kainic acid receptor had no effect.Internal Ca(2+) chelation with 10 mM BAPTA eliminated SMOCs, whereas 10 mM EGTA had no effect.This amplified Ca(2+) signal in turn activates BK channels in a discontinuous fashion, resulting in randomly occurring SMOCs.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology and Biophysics, School of Medicine, State University of New York at Buffalo, Buffalo, NY 14214, USA.

ABSTRACT
A subtype of retinal amacrine cells displayed a distinctive array of K(+) currents. Spontaneous miniature outward currents (SMOCs) were observed in the narrow voltage range of -60 to -40 mV. Depolarizations above approximately -40 mV were associated with the disappearance of SMOCs and the appearance of transient (I(to)) and sustained (I(so)) outward K(+) currents. I(to) appeared at about -40 mV and its apparent magnitude was biphasic with voltage, whereas I(so) appeared near -30 mV and increased linearly. SMOCs, I(to), and a component of I(so) were Ca(2+) dependent. SMOCs were spike shaped, occurred randomly, and had decay times appreciably longer than the time to peak. In the presence of cadmium or cobalt, SMOCs with pharmacologic properties identical to those seen in normal Ringer's could be generated at voltages of -20 mV and above. Their mean amplitude was Nernstian with respect to [K(+)](ext) and they were blocked by tetraethylammonium. SMOCs were inhibited by iberiotoxin, were insensitive to apamin, and eliminated by nominally Ca(2+)-free solutions, indicative of BK-type Ca(2+)-activated K(+) currents. Dihydropyridine Ca(2+) channel antagonists and agonists decreased and increased SMOC frequencies, respectively. Ca(2+) permeation through the kainic acid receptor had no effect. Blockade of organelle Ca(2+) channels by ryanodine, or intracellular Ca(2+) store depletion with caffeine, eradicated SMOCs. Internal Ca(2+) chelation with 10 mM BAPTA eliminated SMOCs, whereas 10 mM EGTA had no effect. These results suggest a mechanism whereby Ca(2+) influx through L-type Ca(2+) channels and its subsequent amplification by Ca(2+)-induced Ca(2+) release via the ryanodine receptor leads to a localized elevation of internal Ca(2+). This amplified Ca(2+) signal in turn activates BK channels in a discontinuous fashion, resulting in randomly occurring SMOCs.

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A subtype of retinal third order neurons shows a voltage-graded spectrum of outward currents. (A) Cell was held at −80 mV and currents were elicited in normal Ringer's by 500 ms depolarizing voltage steps from −90 to 70 mV in 10 mV increments. The sodium current is shown by arrow marked as INa. Note the appearance of the Ito at −40 mV (arrow). The magnification of the figure does not allow outward currents to be discerned between −90 to −50 mV (overlapping traces at these voltages shown by curved arrow). (B) At higher gain, SMOCs seen in normal Ringer's at −50 mV. (C) Whole cell outward currents elicited at −20 and 40 mV, comparing the amplitude of the sustained Iso and the apparent amplitude of the transient Ito. (D) Normalized I-V plot of the whole cell outward currents. Region enclosed within arrows indicates the voltage range of SMOC appearance. The magnitude of Iso (closed circles; measured at the end of the 500-ms pulse) and the apparent magnitude of Ito (open squares) have been normalized to their peak values. Data is an average of eight cells.
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fig1: A subtype of retinal third order neurons shows a voltage-graded spectrum of outward currents. (A) Cell was held at −80 mV and currents were elicited in normal Ringer's by 500 ms depolarizing voltage steps from −90 to 70 mV in 10 mV increments. The sodium current is shown by arrow marked as INa. Note the appearance of the Ito at −40 mV (arrow). The magnification of the figure does not allow outward currents to be discerned between −90 to −50 mV (overlapping traces at these voltages shown by curved arrow). (B) At higher gain, SMOCs seen in normal Ringer's at −50 mV. (C) Whole cell outward currents elicited at −20 and 40 mV, comparing the amplitude of the sustained Iso and the apparent amplitude of the transient Ito. (D) Normalized I-V plot of the whole cell outward currents. Region enclosed within arrows indicates the voltage range of SMOC appearance. The magnitude of Iso (closed circles; measured at the end of the 500-ms pulse) and the apparent magnitude of Ito (open squares) have been normalized to their peak values. Data is an average of eight cells.

Mentions: Fig. 1 A shows current recordings obtained from such a cell in normal Ringer's, under the whole cell voltage clamp. The cell was held at −80 mV and depolarized from −90 to 70 mV in 10 mV increments, in pulses of 500 ms. The family of traces obtained illustrates the current signature for this cell type. An inward TTX-sensitive (500 nM) Na+ current (marked as INa in Fig. 1 A) activates at −30 mV. Large outward currents are activated in response to depolarization. These outward currents are primarily K+ fluxes, as they reverse near 0 mV when bath Ringer's contained high K+ equivalent to that in the recording pipette (n = 11; unpublished data). Since these cells have high input resistances, very small currents are seen in the voltage range between −90 and −70 mV. The first induced outward current appears at −60 mV. These currents, termed SMOCs, are spike-shaped and TTX-insensitive and occur within the narrow voltage range of −60 to −40 mV (n = 12). Fig. 1 B shows SMOCs elicited in normal Ringer's at −50 mV. SMOCs at −50 mV have a mean amplitude of 33 ± 0.5 pA, mean time to peak of 2.8 ± 0.1 ms, and a mean 95% decay time of 5.5 ± 0.1 ms (average of four cells, ∼1,000 events). In this set of 1,000 analyzed events, SMOCs appeared at a frequency of 9.8 ± 0.5 Hz and the highest SMOC amplitude was 92 pA.


Mechanism of generation of spontaneous miniature outward currents (SMOCs) in retinal amacrine cells.

Mitra P, Slaughter MM - J. Gen. Physiol. (2002)

A subtype of retinal third order neurons shows a voltage-graded spectrum of outward currents. (A) Cell was held at −80 mV and currents were elicited in normal Ringer's by 500 ms depolarizing voltage steps from −90 to 70 mV in 10 mV increments. The sodium current is shown by arrow marked as INa. Note the appearance of the Ito at −40 mV (arrow). The magnification of the figure does not allow outward currents to be discerned between −90 to −50 mV (overlapping traces at these voltages shown by curved arrow). (B) At higher gain, SMOCs seen in normal Ringer's at −50 mV. (C) Whole cell outward currents elicited at −20 and 40 mV, comparing the amplitude of the sustained Iso and the apparent amplitude of the transient Ito. (D) Normalized I-V plot of the whole cell outward currents. Region enclosed within arrows indicates the voltage range of SMOC appearance. The magnitude of Iso (closed circles; measured at the end of the 500-ms pulse) and the apparent magnitude of Ito (open squares) have been normalized to their peak values. Data is an average of eight cells.
© Copyright Policy
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2311394&req=5

fig1: A subtype of retinal third order neurons shows a voltage-graded spectrum of outward currents. (A) Cell was held at −80 mV and currents were elicited in normal Ringer's by 500 ms depolarizing voltage steps from −90 to 70 mV in 10 mV increments. The sodium current is shown by arrow marked as INa. Note the appearance of the Ito at −40 mV (arrow). The magnification of the figure does not allow outward currents to be discerned between −90 to −50 mV (overlapping traces at these voltages shown by curved arrow). (B) At higher gain, SMOCs seen in normal Ringer's at −50 mV. (C) Whole cell outward currents elicited at −20 and 40 mV, comparing the amplitude of the sustained Iso and the apparent amplitude of the transient Ito. (D) Normalized I-V plot of the whole cell outward currents. Region enclosed within arrows indicates the voltage range of SMOC appearance. The magnitude of Iso (closed circles; measured at the end of the 500-ms pulse) and the apparent magnitude of Ito (open squares) have been normalized to their peak values. Data is an average of eight cells.
Mentions: Fig. 1 A shows current recordings obtained from such a cell in normal Ringer's, under the whole cell voltage clamp. The cell was held at −80 mV and depolarized from −90 to 70 mV in 10 mV increments, in pulses of 500 ms. The family of traces obtained illustrates the current signature for this cell type. An inward TTX-sensitive (500 nM) Na+ current (marked as INa in Fig. 1 A) activates at −30 mV. Large outward currents are activated in response to depolarization. These outward currents are primarily K+ fluxes, as they reverse near 0 mV when bath Ringer's contained high K+ equivalent to that in the recording pipette (n = 11; unpublished data). Since these cells have high input resistances, very small currents are seen in the voltage range between −90 and −70 mV. The first induced outward current appears at −60 mV. These currents, termed SMOCs, are spike-shaped and TTX-insensitive and occur within the narrow voltage range of −60 to −40 mV (n = 12). Fig. 1 B shows SMOCs elicited in normal Ringer's at −50 mV. SMOCs at −50 mV have a mean amplitude of 33 ± 0.5 pA, mean time to peak of 2.8 ± 0.1 ms, and a mean 95% decay time of 5.5 ± 0.1 ms (average of four cells, ∼1,000 events). In this set of 1,000 analyzed events, SMOCs appeared at a frequency of 9.8 ± 0.5 Hz and the highest SMOC amplitude was 92 pA.

Bottom Line: Ca(2+) permeation through the kainic acid receptor had no effect.Internal Ca(2+) chelation with 10 mM BAPTA eliminated SMOCs, whereas 10 mM EGTA had no effect.This amplified Ca(2+) signal in turn activates BK channels in a discontinuous fashion, resulting in randomly occurring SMOCs.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology and Biophysics, School of Medicine, State University of New York at Buffalo, Buffalo, NY 14214, USA.

ABSTRACT
A subtype of retinal amacrine cells displayed a distinctive array of K(+) currents. Spontaneous miniature outward currents (SMOCs) were observed in the narrow voltage range of -60 to -40 mV. Depolarizations above approximately -40 mV were associated with the disappearance of SMOCs and the appearance of transient (I(to)) and sustained (I(so)) outward K(+) currents. I(to) appeared at about -40 mV and its apparent magnitude was biphasic with voltage, whereas I(so) appeared near -30 mV and increased linearly. SMOCs, I(to), and a component of I(so) were Ca(2+) dependent. SMOCs were spike shaped, occurred randomly, and had decay times appreciably longer than the time to peak. In the presence of cadmium or cobalt, SMOCs with pharmacologic properties identical to those seen in normal Ringer's could be generated at voltages of -20 mV and above. Their mean amplitude was Nernstian with respect to [K(+)](ext) and they were blocked by tetraethylammonium. SMOCs were inhibited by iberiotoxin, were insensitive to apamin, and eliminated by nominally Ca(2+)-free solutions, indicative of BK-type Ca(2+)-activated K(+) currents. Dihydropyridine Ca(2+) channel antagonists and agonists decreased and increased SMOC frequencies, respectively. Ca(2+) permeation through the kainic acid receptor had no effect. Blockade of organelle Ca(2+) channels by ryanodine, or intracellular Ca(2+) store depletion with caffeine, eradicated SMOCs. Internal Ca(2+) chelation with 10 mM BAPTA eliminated SMOCs, whereas 10 mM EGTA had no effect. These results suggest a mechanism whereby Ca(2+) influx through L-type Ca(2+) channels and its subsequent amplification by Ca(2+)-induced Ca(2+) release via the ryanodine receptor leads to a localized elevation of internal Ca(2+). This amplified Ca(2+) signal in turn activates BK channels in a discontinuous fashion, resulting in randomly occurring SMOCs.

Show MeSH
Related in: MedlinePlus