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EJ Warren, CN Allen, RL Brown, DW Robinson; Intrinsic Membrane Properties and Morphological Characteristics of Rat Retinal Ganglion Cells that Project to the Suprachiasmatic Nucleus . Invest. Ophthalmol. Vis. Sci. 2002;43(13):2774.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To determine if the morphology and intrinsic membrane properties of retinal ganglion cells (RGCs) projecting to the circadian system are different from those that project to the primary visual targets. Methods: Stereotaxic injections of fluorescent microspheres into the suprachiasmatic nucleus (SCN) were utilized to unequivocally identify SCN-projecting RGCs in retinal wholemounts. Whole-cell patch-clamp recording was used to record the membrane properties in retrogradely-labeled RGCs and to fill the cells with Lucifer yellow and Biocytin. Following recordings, the wholemounts were fixed in 4% paraformaldehyde and subsequently processed with a standard ABC reaction. Each neuron was then reconstructed using Neurolucida software. Results: RGCs projecting to the SCN exhibited smooth dendritic arbors that arose from 2 to 3 thicker primary dendrites. Their dendritic fields varied somewhat with regards to several morphometric parameters, including the number of branch points (4 to 26), the dendritic extent (329 to 874 µm), the area of influence (45758 to 148698 µm2), the dendritic distribution index (8 to 9) and the circularity index (0.64 to 0.83). The average resting potential of these morphologically characterized neurons was - 44mV. While the majority of cells were relatively silent at rest, about 20% exhibited robust spiking activity. In response to membrane depolarization, some cells elicited transient spiking patterns, whilst others responded in a more sustained fashion at rates ranging from 5.3 to 11.8 Hz. In all cases the spike shape was relatively consistent with a half-width of 1.4 ms, a rise time of 0.6 ms and a decay time of 0.9 ms. Input resistances were distributed broadly with an average of 985 MΩ. Conclusion: While the morphology of SCN-projecting neurons generally resembles that described for Type III RGCs, there are subtle differences. Moreover, the SCN-projecting RGCs show significant functional and morphological heterogeneity.
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