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Y. Xia, R. Carroll, S. Nawy; Activity Regulated AMPA Receptor Trafficking in Mammalian Retina . Invest. Ophthalmol. Vis. Sci. 2005;46(13):1124.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: The rapid cycling of AMPA receptors (AMPARs) into and out of the plasma membrane is known to occur in many brain regions, and is thought to play a role in important forms of synaptic plasticity such as LTP and LTD. However, the question of whether rapid cycling of AMPARs occurs in retinal neurons has not yet been examined. Methods: We focused our studies on cells in the ganglion cell layer using two independent approaches to test the idea that AMPARs undergo rapid turnover in these cells. One approach was to use whole cell recording to measure the amplitude of AMPA currents, as a read–out of the number of receptors at the surface. Inclusion of GDP–ßS or a dynamin–blocking peptide is known to block endocytosis of AMPARs, without affecting exocytosis. Under conditions where cycling occurs, receptors reach the surface but cannot leave, causing a run–up, or potentiation of the AMPA response. A second approach using cultured cells was to directly visualize receptor turnover using an antibody for the AMPAR subunit GluR2. Results: In both cultured cells and in intact retina, we observed a high rate of receptor cycling, but only when ongoing activity was low. In cultures enriched for amacrine, but not ganglion cells (i.e., low level of glutamatergic synaptic transmission), AMPA mediated responses were markedly potentiated over a period of 30 minutes when endocytosis was prevented by dialysis with GDP–ßS (approximately 2.5 fold increase) or a dynamin blocking peptide (1.8 fold), but remained stable with a standard internal solution. In support of this finding, we found that only 20% of receptors labeled with a GluR2 antibody remained at the surface after 45 minutes. To look at conditions of high activity, we added ganglion cells, or hippocampus pyramidal neurons to the cultures. Now spontaneous synaptic activity was readily observed, and in these cells there was little or no potentiation of the AMPA response by GDP–ßS, implying a low rate of cycling. Similar results were obtained from cells in the intact retina: when ongoing synaptic activity was blocked by incubation in 100 µM CNQX overnight or with visual deprivation for 48 hrs, GDP–ßS induced a 1.8 fold increase in the amplitude of AMPA–elicited responses obtained by focal application of AMPA, suggesting that both manipulations up–regulated receptor cycling rate in intact retina. Conclusions: Our results suggest that there is a strong link between synaptic activity and AMPAR trafficking in retina neurons such that ongoing synaptic activity reduces turnover rates and stabilizes the postsynaptic receptor population.
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