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J. Duebel, S. Siegert, B. Gross, B. Roska; Functional Analysis of a Novel Amacrine Cell Type in the Mouse Retina Expressing the Vesicular Glutamate Transporter 3 (vGluT3). Invest. Ophthalmol. Vis. Sci. 2008;49(13):1418.
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Recently, a new amacrine cell type in the mammalian retina was identified (Fremeau, R.T. et al., 2002 PNAS, 99: 14488-93; Haverkamp, S., Wässle, H., 2004, J Comp Neurol., 468: 251-263; Johnson, J. et al., J Comp Neurol. 2004, 477: 386-98). While the majority of amacrine cells are inhibitory, this new type expresses a marker of excitatory neurons (vGluT3) as well as a marker of inhibitory neurons (glycine), suggesting that this is a dual-transmitter neuron. To gain insight into the function of this cell type, a transgenic mouse line has been generated which expresses a green fluorescent protein specifically in these amacrine cells through the use of the vGlut3-promoter. This transgenic mouse line allows us to repeatedly and reliably record from vGluT3-labeled amacrine cells in an identified circuit in order to analyze their properties and local interactions.
VGluT3-positive amacrine cells in the wholemount-retina were directly targeted with a patch-electrode by using a two-photon microscope. Light-evoked excitatory currents (input from bipolar cells) and inhibitory currents (input from amacrine cells) were recorded in the whole-cell configuration. The excitatory and inhibitory inputs could be separately measured by voltage clamping the cells at appropriate voltage potentials.
Preliminary results indicate that this specific cell type receives both inhibition and excitation at light ON and at light OFF (ON-OFF-inhibition and ON-OFF-excitation, respectively).
Our results concerning the excitatory synaptic input are consistent with the finding of a recent anatomical study. Haverkamp & Wässle (2004) showed that vGluT3 processes are in close apposition with axon terminals of an outer stratifying (OFF) and an inner stratifying (ON) bipolar cell type. With further experiments (e.g. double-patch recording) we will analyze the synaptic output of the vGlut3-amacrine cell in order to address whether this cell type functions as a dual-transmitter neuron.
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