| In developing neurons, calcium (Ca2+) influx through NMDA-selective ionotropic glutamate receptors is central to neuronal maturation. However, not all cells express NMDA receptors (NMDARs), suggesting that other Ca 2+-permeable glutamate receptors may be important to consider. In particular, Ca2+-permeable AMPA receptors (CP-AMPARs) are expressed throughout the adult central nervous system, but their role in development remains poorly understood. Here, we employed a modified cobalt staining technique to assay the presence of CP-AMPARs in living neurons, in combination with electrophysiology. We identified two inhibitory retinal cells, horizontal cells (HC) and AII amacrine cells, which though devoid of NMDARs express CP-AMPARs. We found that CP-AMPARs were present at all stages of the development. Yet, right before eye-opening, they exhibited a pharmacological resistance to philanthotoxin (PhTX). This finding was unexpected since external synthetic polyamines such as PhTX are routinely used to selectively block CP-AMPARs. Interestingly, polyamine-insensitive receptors failed to express in animals reared in the dark, demonstrating that light entering the eye triggers their appearance. This occurred through a BDNF (brainderived neurotrophic factor)-dependent pathway since exogenous application of the neurotrophin rescued polyamine-resistance in dark-reared rats. Polyamine-insensitivity of CP-AMPARs is assumed to originate from the presence of the AMPAR subunit GluA2. To investigate this possibility, we performed experiments on GluA2 knock-out animals. GluA2 was found to be needed to render CP-AMPARs insensitive to PhTX in retinal HC and AII amacrine cells. We further established that adult cerebellar basket, stellate and Purkinje cells also possessed CP-AMPARs that were polyamine-resistant, because of GluA2 surface expression. Finally, we provide evidence indicating that these novel receptors are implicated in disease states. Together, we demonstrate for the first time that a population of AMPARs that contain GluA2 are also Ca2+ -permeable. Their expression is driven by sensory experience centered around the GluA2 subunit. Given the absence of synaptic NMDARs, our work suggests that new mechanisms of synapse regulation may be in place during development of these synapses. |
