Glutamate uptake activity in retina Müller cells: Circadian modulation.

Signal transmission in the visual pathway is carried by glutamate, the major excitatory neurotransmitter in the vertebrate brain. Ionotropic and metabotropic receptors mediate the actions of this amino acid. A tight regulation of extra-synaptic glutamate concentrations is needed for the selective activation of its receptors as a function of the day/night cycle. Moreover, proper extra synaptic glutamate levels regulation is also important for the avoidance of an excitotoxic insult. A family of plasma membrane glutamate transporters expressed in glia cells carries out the clearance of this amino acid from the synaptic cleft. Taking into consideration the well-established light-dependent differential glutamate release, we decided to evaluate the expression and uptake activity of Müller glia glutamate transporters, since these cells are responsible for more than 90 % of the total retinal glutamate uptake activity. To this end, we used the Moorfields/Institute of Ophthalmology-Müller 1 cell line and a rat retina Müller glia primary culture synchronized through a 50 % serum strategy. The glutamate transport activity was evaluated with a [3H] D-Aspartate uptake assay. A time-dependent differential glutamate transporter regulation was present demonstrating that the night/dark cycle regulates the transporter function. This data supports the notion that Müller glia has intrinsic photoreceptor properties and further strengthens the role of radial glia cells in retina physiology.