Abstract
Purpose: :
At the second synapse of the mammalian rod pathway, AII amacrine cells receive glutamatergic input from rod bipolar cells. The synaptic conductance is mediated by Ca2+–permeable AMPA receptors with fast kinetics, and excitatory postsynaptic currents recorded in AIIs following stimulation of a single rod bipolar are characterized by a large transient component. The present study was undertaken to understand the behavior of intrinsic membrane conductances in AIIs that can interact with this synaptic conductance and shape postsynaptic integration of rod bipolar input.
Methods: :
Whole–cell voltage–clamp recordings were made at near–physiological temperatures (33–35 °C) from AII amacrine cells in of rat retina slices (P21–25; 200 µm thick). Immunohistochemistry combined with confocal microscopy was used to examine the distribution of potassium channel subunits in transverse sections (70 µm) of paraformaldehyde–fixed (4% in phosphate–buffered saline) retina.
Results: :
Depolarizing voltage steps (200 ms) from a holding potential of –80 mV elicited large, transient (A–type) potassium currents in AII amacrine cells. These currents activated near –30 mV and inactivated rapidly (τ ≈ 10 ms). At 0 mV, the average current was 2.1 ± 0.5 nA (mean ± SEM; n=6). Recovery from inactivation, as assayed by paired 50 ms steps to 0 mV delivered at intervals from 5 to 9000 ms, was very rapid (80% recovery in under 20 ms) and followed a biexponential time course: τfast = 7 ms (90%) and τslow = 500 ms (10%) (n = 3). The transient currents were blocked by 1 mM TEA (reduced to 30 ± 10% of control; n=3). Together, these data (rapid inactivation kinetics and TEA sensitivity) suggest that potassium channels containing Kv3.4 subunits mediate the A–type current in AIIs. Therefore, we used fluorescence immunohistochemistry to examine the distribution of Kv3.4 potassium channel subunits in the rat retina. Kv3.4 expression was punctate and limited to the inner and outer plexiform layers. When rod bipolar cells were labeled simultaneously with an anti–PKC antibody, Kv3.4 puncta were found to encircle their terminals.
Conclusions: :
AIIs exhibit A–type currents mediated by potassium channels with rapid kinetics indicative of Kv3.4–containing channels. These potassium channels appear to be located very close to the glutamatergic synapses made onto AIIs by rod bipolar cells. Consequently, it would seem that the A–type conductance is well–positioned to shunt strong excitatory inputs onto the distal dendrites of AIIs. Weak synaptic inputs, however, are expected to be summed linearly by the AII, as such inputs will not depolarize the cell sufficiently to activate the A–type conductance.
Keywords: retina: proximal (bipolar, amacrine, and ganglion cells) • ion channels • synapse