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J. M. Ackert, R. Farajian, S. Chheda, B. Volgyi, S. A. Bloomfield; ON Direction Selective Ganglion Cells Display a Directional OFF Response Received via Gap Junctions With Polyaxonal Amacine Cells. Invest. Ophthalmol. Vis. Sci. 2007;48(13):1162.
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© ARVO (1962-2015); The Authors (2016-present)
To study the response properties of ON direction selective (DS) ganglion cells in the rabbit retina.
Dual, simultaneous extracellular recordings were obtained from pairs of neighboring ON DS ganglion cells visualized with transcleral infrared illumination in a flattened retinal preparation of the albino rabbit. Responses were examined before and after application of 100 µm picrotoxin (PTX), 100 µM L-AP4, 25 µM 18-beta-glycyrrhetinic acid (18-GA), 5 µM neostigmine (NEO), and/or 100 µM hexamethonium (HEX).
Somata of ON DS ganglion cells were identified in the living retina by their characteristic labeling with the vital dye Azure blue. Pairs of neighboring ON DS cells were then targeted for simultaneous, dual recordings. We reported at ARVO last year that application of PTX reveals an OFF response in ON DS cells. This OFF response occurred only at light offset and was insensitive to the application of L-AP4, indicating that it is generated by the OFF pathway. The OFF response revealed by PTX application was direction selective, but the preferred direction was always opposite or orthogonal to that of the ON response. Since the directional OFF response was generated in the presence of PTX, its selectivity cannot be mediated by GABA. However, application of either the cholinesterase inhibitor, NEO, or the nicotinic acetylcholine receptor blocker, HEX, reversibly abolished the direction selectivity of the OFF response. ON DS ganglion cells are coupled to a subtype of polyaxonal amacrine cell. Confocal microscopy showed that the dendrites of the coupled polyaxonal amacrine meandered for long distances in sublamina-a of the IPL before descending into sublamina-b. These dendrites co-stratified with both bands of starburst cell (ChAT) processes. We found that application of the gap junction blocker, 18-GA, completely and reversibly abolished the OFF response in ON DS cells.
Our results suggest that ON DS ganglion cells receive a direction selective OFF response via the polyaxonal amacrine cells to which they are coupled. This OFF response appears to be masked by a tonic GABAergic inhibition. The direction selectivity of the OFF response does not appear to be mediated by GABA, but by cholinergic circuitry. These data suggest that amacrine cells carry direction selective signals. They thus indicate an unexpected complexity in the circuitry subserving direction selectivity.
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