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Amacrine cells are a highly diverse group of inhibitory retinal interneurons that sculpt the responses of bipolar cells, ganglion cells, and other amacrine cells. They integrate excitatory inputs from bipolar cells and inhibitory inputs from other amacrine cells, but for most amacrine cells, little is known about the specificity and functional properties of their inhibitory inputs. Here, we have investigated GABAA receptors of the AII amacrine, a critical neuron in the rod pathway microcircuit, using patch-clamp recording in rat retinal slices. Puffer application of GABA evoked robust responses, but, surprisingly, spontaneous GABAA receptor-mediated postsynaptic currents were not observed, neither under control conditions nor following application of high-K+ solution to facilitate release. To investigate the biophysical and pharmacological properties of GABAA receptors in AIIs, we therefore used nucleated patches and a fast application system. Both brief and long pulses of GABA (3 mM) evoked GABAA receptor-mediated currents with slow, multi-exponential decay kinetics. The average weighted time constant (τw) of deactivation was ~163 ms. Desensitization was even slower, with τw ~330 ms. Non-stationary noise analysis of patch responses and directly observed channel gating yielded a single-channel conductance of ~23 pS. Pharmacological investigation suggested the presence of α2 and/or α3 subunits, as well as the γ2 subunit. Such subunit combinations are typical of GABAA receptors with slow kinetics. If synaptic GABAA receptors of AII amacrines have similar functional properties, the slow deactivation and desensitization kinetics will facilitate temporal summation of GABAergic inputs, allowing effective summation and synaptic integration to occur even for relatively low frequencies of inhibitory inputs.

Multiphoton excitation (MPE) microscopy allows subcellular structural and functional imaging of neurons and can be combined with techniques for activating postsynaptic receptors at spatial and temporal scales that mimic normal synaptic transmission. Here, we describe procedures for combining MPE imaging of dye-filled neurons with fast microiontophoresis, by which neurotransmitter agonists can be applied from high-resistance micropipettes with subcellular resolution. With adequate compensation of the pipette capacitance, the effective time constant of the pipette is reduced, and this permits application of very brief pulses of receptor agonist (≤1 ms). The consequent high temporal and spatial resolution leads to the high specificity required for single-synapse investigations. This chapter includes detailed procedures for electrophysiological whole-cell recording, structural and functional (Ca2+) MPE imaging of dye-filled neurons, targeting a microiontophoresis pipette to a specific subcellular compartment of a dye-filled neuron under visual control, and capacitance compensation of the microiontophoresis pipette, as well as examples of experimental results that can be obtained. acceptedVersion

Background: Accurate reconstruction of the morphology of single neurons is important for morphometric studies and for developing compartmental models. However, manual morphological reconstruction can be extremely time-consuming and error-prone and algorithms for automatic reconstruction can be challenged when applied to neurons with a high density of extensively branching processes. New method: We present a procedure for semi-automatic reconstruction specifically adapted for densely branching neurons such as the All amacrine cell found in mammalian retinas. We used whole-cell recording to fill All amacrine cells in rat retinal slices with fluorescent dyes and acquired digital image stacks with multi-photon excitation microscopy. Our reconstruction algorithm combines elements of existing procedures, with segmentation based on adaptive thresholding and reconstruction based on a minimal spanning tree. We improved this workflow with an algorithm that reconnects neuron segments that are disconnected after adaptive thresholding, using paths extracted from the image stacks with the Fast Marching method. Results: By reducing the likelihood that disconnected segments were incorrectly connected to neighboring segments, our procedure generated excellent morphological reconstructions of All amacrine cells. Comparison with existing methods: Reconstructing an All amacrine cell required about 2 h computing time, compared to 2-4 days for manual reconstruction. To evaluate the performance of our method relative to manual reconstruction, we performed detailed analysis using a measure of tree structure similarity (DIADEM score), the degree of projection area overlap (Dice coefficient), and branch statistics. Conclusions: We expect our procedure to be generally useful for morphological reconstruction of neurons filled with fluorescent dyes. (C) 2017 Elsevier B.V. All rights reserved.