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M. Weick, M. B. Manookin, J. B. Demb; Intrinsic Mechanisms for Contrast Adaptation in Alpha (y-type) Retinal Ganglion Cells. Invest. Ophthalmol. Vis. Sci. 2009;50(13):1416.
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© ARVO (1962-2015); The Authors (2016-present)
Retinal ganglion cells adapt at high contrast by reducing their response gain. Adaptation in spiking responses depends on both synaptic mechanisms in retinal circuitry and intrinsic mechanisms in ganglion cell spike generation. Here, we used current injection protocols to characterize intrinsic mechanisms for adaptation in ON and OFF Alpha ganglion cells in guinea pig retina.
We made whole-cell current-clamp recordings of ganglion cells in vitro (potassium-based intracellular solution). A paired-pulse current-injection paradigm was used to characterize mechanisms for adaptation. The response to a variable test pulse of current was measured alone (baseline condition) or following a depolarizing or hyperpolarizing pre-pulse.
To the test pulse, the relationship between current injection and firing rate (I-F curve) was roughly linear. A depolarizing pre-pulse reduced the I-F slope and increased the firing threshold (i.e., increased the current required to evoke a spike). The suppression was more prominent in OFF cells. Following a depolarizing pre-pulse, the action potentials evoked by the test pulse showed a reduction in the peak derivative. A hyperpolarizing pre-pulse also reduced the I-F slope but this effect was only present in OFF cells. Neither effect was blocked by applying the Ih channel blocker ZD7288 (25 uM) or the calcium channel blocker cobalt (1 mM).
The suppressive effect of a depolarizing pre-pulse could arise from sodium channel inactivation. This mechanism would explain the reduced peak derivative of action potentials evoked by the test pulse. The suppressive effect of a hyperpolarizing pre-pulse must arise from some other mechanism, which does not involve Ih or calcium channels.
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