Keywords: Kv3 Potassium Channels; Retinal Ganglion Cells; Fast-spiking; High frequency firing; Action Potentials; Drug effects; Whole-Cell Recordings; Patch Clamp Techniques; Reverse Transcriptase Polymerase Chain Reaction.
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Ionic conductances underlying excitability in tonically firing retinal ganglion cells of adult rat
This work was carried out in 2006-2007 at Bogomoletz institute of physiology and International center for molecular physiology (Kiev, Ukraine).
Yuriy O. Kolodin carried out electrophysiological recordings and wrote manuscript.
Oleksii O. Grygorov carried out polymerase chain reactions .
Intrinsic firing properties of retinal ganglion cells (RGCs) of mature (1 month old) rat were studied in retinal flat-mounted preparations using whole cell current clamp recordings. In response to 500-ms depolarizing current step the majority (94.1%) of the examined RGCs (n=85) displayed sustained firing that lasted for the duration of the depolarization period (tonic RGCs). In addition, 63.5% of the cells had clearly a tonic fast-spiking phenotype with the steady-state firing frequency in the range 50-124 Hz. The rest few (5.9%) RGCs always displayed transient firing accommodated within duration of the current steps (phasic RGCs). Ionic conductances underlying excitability in tonically firing neurons were studied by applications of selective pharmacological blockers. Application of TTX (1 μM) caused reversible disappearance of action potentials (APs) in response to stimulus. Suppression of Ca2+ influx through voltage-activated Ca2+ channels by 200 μM Cd2+ resulted mainly in moderate increase of steady-state firing frequency and increase of single AP repolarization rate, however, without abolishing the basic pattern of tonic firing. Physiological roles of different types of voltage-gated potassium channels were studied using applications of respective blockers. It was found that potassium conductance highly sensitive to external TEA (1 mM) or 4-aminopyridine (4-AP, 200 μM) is responsible for fast repolarization and afterhyperpolarizati on of a single AP, providing the cells with the ability for high-frequency firing. Potassium conductance sensitive to α-dendrotoxin (α-DTX, 100 nM) did not play such a role. The known specificity of these drugs strongly suggested that this 4-AP and TEA-sensitive conductance is mediated by Kv3 potassium channels. The prominent role of Kv3 conductance was also suggested by fast-spiking phenotype of the cells. Single-cell RT-PCR experiments confirmed the expression of Kv3.1 and Kv3.2 mRNA in the RGCs. Thus, in tonically firing rat RGCs, TTX-sensitive Na+ and Kv3 K+ currents generate a basic firing pattern, while Ca2+ and Ca2+-dependent conductances only moderately regulate discharge frequency.
kv3 potassium channe, retinal ganglion cel