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J. M. Renna, F. R. Amthor, K. T. Keyser; On Responses in Off Rabbit Retinal Ganglion Cells Mediated in Part by nAChRs. Invest. Ophthalmol. Vis. Sci. 2008;49(13):5380.
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Strychnine is considered a selective competitive antagonist of glycine gated Cl- channels at low nanomolor concentrations (Curtis et al., 1969; Young & Snyder, 1973). At low micromolar concentrations, strychnine has been shown to block nicotinic acetylcholine receptors (nAChRs; Matsubayashi et al., 1998) and we have previously shown that strychnine can attenuate nAChR mediated retinal ganglion cell responses at low micromolar concentrations (Renna et al., 2007). Other experiments with applications of micromolar concentrations of strychnine have unmasked an ON response in OFF ganglion cells (Jensen, 1991) and this unmasked ON response has been attributed to the blockade of glycinergic suppression. We postulate the unmasked ON response is the result of the blockade of nAChRs rather than glycine receptors.
Extracellular recordings of rabbit retinal ganglion cells were made from a perfused eye cup preparation and antagonists were bath applied.
Preliminary data suggests the unmasking of the ON response is due to the blockade of nAChRs. Application of glycine receptor specific concentrations of strychnine (20 nM) did not unmask an ON response (n=4) whereas application of alpha 7/alpha 6 specific concentrations of MLA (30 nM) did unmask the ON response (n=2).
Because the ON response is not unmasked with glycine receptor specific concentrations of strychnine, it would suggest that the suppression of the ON response might be GABAergic, and preliminary MLA data indicates this suppression might be mediated through nAChRs. Further research will be required to reveal the source of inhibition as well disentangle the mechanism by which the ON response is generated in the OFF ganglion cell.
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