Modulation of Synaptic Function in Retinal Amacrine Cells

Amacrine cells are interneurons that have diverse functions in retinal signal processing. In order to study signaling and modulation in retinal amacrine cells, we employ a simplified culture system containing identifiable GABAergic amacrine cells. Immunocytochemistry experiments indicate that GABAergic amacrine cells express metabotropic glutamate receptor 5 (mGluR5), a group I mGluR usually linked to the IP3 signaling pathway. Ca(2+) imaging experiments using an mGluR5-specific agonist indicate that these receptors are functional and when activated, can stimulate temporally diverse Ca(2+) elevations. To begin to establish the role of these receptors in modulating amacrine cell function, we have used electrophysiological methods to ask whether ion channels are the targets of mGluR5-dependent modulation. Here we discuss our results indicating that activation of mGluR5 leads to enhancement of currents through GABA(A) receptors. This enhancement is dependent upon elevations in cytosolic Ca(2+) and activation of protein kinase C (PKC). To explore the consequences of Ca(2+) elevations in another context, we have used nitric oxide (NO) donors to mimic the effects of activating the Ca(2+)-dependent synthetic enzyme for NO, neuronal nitric oxide synthase. We find that exposure to NO donors also enhances the amplitude of currents through GABA(A) receptors. Together, these results indicate that glutamate from presynaptic bipolar cells has the potential to work through multiple mechanisms to regulate the function of amacrine-to-amacrine cell GABAergic synapses.