Chemical Neurotransmission from Retinal Ganglion Cells to Superior Colliculus Neurons under Conditions of Long-Lasting Neighboring Co-Culturing

Axons of retinal ganglion cells (RGCs) form the optic tract (OT), whose fibers transmit impulsation from the retina to subcortical visual centers. We studied the properties of chemical neurotransmission at the level of retinal projections to the above centers using our original in vitro model of neighboring coculturing of dissociated retinal cells and neurons of the superior colliculus (SC). The proposed model, as compared with recently developed similar methods, allowed us to stimulate clearly identified separate RGC axons and to record postsynaptic currents (PSCs) from neurons of the SC synaptically connected with these axons. Our experiments showed that most of these contacts are excitatory; in 12 of 15 cases, we recorded evoked excitatory PSCs (eEPSCs) in SC neurons, while inhibitory synaptic contacts between RGCs and neurons of the SC were significantly more rare. Excitatory neurotransmission was mediated by glutamate release and activation of postsynaptic NMDA and non-NMDA receptors. We found that non-NMDA receptors play the main functional role in transmission of visual signals through well-formed synaptic connections. Inhibitory synaptic transmission is mediated by activation of GABA A receptors on the postsynaptic membrane of SC neurons.