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Seetal Chavda, Philip J Luthert, Thomas E Salt; Light-Evoked Retinal Ganglion Cell Synaptic Responses and Microglial Morphology are Modulated by P2X7 Receptor Activation and Bacterial Lipopolysaccharide (LPS). Invest. Ophthalmol. Vis. Sci. 2014;55(13):2384.
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© ARVO (1962-2015); The Authors (2016-present)
ATP-gated P2X7 receptors (P2X7Rs) have shown to act as conduits for retinal ganglion cell (RGC) damage, a consequence of various neurodegenerative conditions in the visual pathway. P2X7Rs are also associated with the induction of early microglial activation, and the release of neuroinflammatory mediators, following neuronal injury. Using a dark-adapted, ex vivo mouse retinal wholemount preparation, the present study aimed to characterise the effect of P2X7R activation, and lipopolysaccharide (LPS; a microglial activator), on light-evoked NMDA receptor (NMDAR)-mediated RGC synaptic responses. The effect of LPS on microglial morphology was also investigated, under similar conditions.
ON and OFF field excitatory post-synaptic potentials (fEPSPs) were recorded from the ganglion cell layer of acutely isolated retinal wholemounts (adult, male C57BL/6 mice) in response to a light stimulus (1s-duration peak wavelength 562nm flash, repeated every 3s). The NMDAR-mediated component of the responses was pharmacologically isolated with a Mg2+-free Krebs medium containing NBQX, picrotoxin, strychnine and tetrodotoxin. Additional compounds were applied to the bathing medium. Retinae were stained for microglia (α-IBA-1/IgG-Alexa488) and visualised with confocal microscopy.
BzATP (300µM), a P2X receptor agonist, depressed the ON (78±2% of control; P<0.0001, n=21) but not the OFF RGC fEPSP peak amplitude. The effect of BzATP on the ON RGC fEPSP was reduced by the selective P2X7R antagonist, A438079 (10µM; 31±2% reduction; P<0.05, n=6). Furthermore, bath-applied LPS (10 μg/ml) depressed the ON fEPSP peak amplitude (86±3% of control; P<0.05, n=7), an effect which was reduced in the presence of A438079. Under similar conditions, microglial process area was significantly reduced with LPS treatment (74±3% of control, P<0.0001, n=71 cells, 4 retinae), suggesting a transition to an early stage of microglial activation.
In summary, LPS-mediated early microglial activation induces a depression of ON-retinal ganglion cell responses, possibly through a mechanism involving P2X7R activation. Since changes in neurotransmission and microglial function are early indicators of neuropathology, these results contribute to the understanding of neural-immune interactions in retinal disease.
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