Spontaneous Activity of Dopaminergic Amacrine Cells in the Intact Mouse Retina

Abstract

Purpose:: To study the electrophysiological characteristics of dopaminergic amacrine (DA) cells in the intact mouse retina.

Methods:: Patch clamp recordings were obtained from GFP-labeled DA cells identified by UV and infrared illumination in a mouse superfused, flattened retina preparation. Ionic currents were recorded in voltage clamp at a holding potential of -70mV. Membrane voltage variations were recorded in current-clamp by holding the cells near their resting potential (-70mV). The patch pipette solution contained biocytin, so that the identity of the recorded cells could be confirmed by staining with fluorescent streptavidin combined with antibodies to tyrosine hydroxylase.

Results:: Mouse DA cells are wide-field, interplexiform amacrines characterized by (i) a dendritic field ~700µm diameter, consisting of a small number of thick, sparsely branching processes, and (ii) multiple, usually unbranching, varicose axons, ~800µm in length. Both dendrites and axons reside in sublamina a, stratum 1 of the IPL. DA cells had an average membrane capacitance of 42.2 +/- 1.1 pF (mean +/- SEM; n=8). When depolarized to +50mV in 10mV steps, they exhibited a very large Na⁺ current, peaking at -40mV, a fast transient and a large delayed outward current. In current clamp mode, DA cells spontaneously generated rhythmic bursts of action potentials, each riding on a 10-15mV slow depolarization. This was followed by a distinct hyperpolarization that merged onto a slowly rising, depolarizing ramp leading to the next burst episode. The spontaneous activity was insensitive to the application of the AMPA/kainate glutamate receptor blocker CNQX at a 10µM concentration. Pacemaking activity was also unaffected by application of picrotoxin. In keeping with the results in dissociated DA cells, 1mM CsCl, an inhibitor of the hyperpolarization-activated current I_h, did not significantly modify the spontaneous activity.

Conclusions:: DA cells in the intact mouse retina preparation exhibit spontaneous spike activity characterized by rhythmic burst firing. This contrasts with the rhythmic firing of individual action potentials previously reported for dissociated DA cells. The spontaneous burst discharges of DA cells in the intact mouse retina appear to be independent of both excitatory and inhibitory synaptic inputs mediated by AMPA/kainate and GABA_A receptors.

Keywords: dopamine • amacrine cells • retina: proximal (bipolar, amacrine, and ganglion cells)