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L.-L. Zhang, M. A. Freed; Two-Photon Photolysis of Glutamate Reveals Hot Spots on the Dendrites of Retinal Ganglion Cells. Invest. Ophthalmol. Vis. Sci. 2008;49(13):5802.
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Retinal ganglion cell dendrites are the major site for integration of visual information before the signals are relayed to the rest of the brain. Although dendritic integration of synaptic inputs has been modeled based on reasonable assumptions about membrane resistance, capacitance and voltage sensitive channel density, a direct view of synaptic integration would be invaluable.
We recorded in the whole cell mode from the ON-center ganglion cell soma in an intact in vitro preparation of the guinea pig retina. L-AP4 (50 uM) was added in the bath solution to block light responses. The pipette contained 2 mM Lucifer yellow or 20 uM Alexa-488 to reveal dendritic morphology. The cell was voltage clamped at the reversal potential for inhibitory currents, isolating glutamatergic currents. Femto-second laser pulses of 720 nm wavelength and 1 ms duration were delivered by a fast scanning system along the dendrites.
Focal photolysis of MNI-caged-L-glutamate (200 uM) produced inward currents responses equivalent to spontaneous miniature EPSCs in amplitude and time course (~10 pA, 5 ms). Functional mapping of the inward currents revealed hot spots of glutamate sensitivity.
This is a method of mapping the integration of transmitter sensitivity across the dendritic arbor. If applied to cells of different type, it promises to show how the dendritic tree’s size, branching structure, and regenerative currents determine the integration of synaptic input.
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