Neuronal signalling mechanisms between retinal ganglion cells and superior colliculus neurons during visual processing

Abstract

The superior colliculus (SC) is a conserved midbrain structure present in all vertebrates, crucial for integrating sensory signals to initiate motor commands. The mammalian SC has three layers: superficial, intermediate, and deep, aligned to the visual field. In mice, the superficial layer processes visual information from 85-90% of retinal ganglion cells (RGCs) originating from the retina. The functional connectivity between RGCs and superficial SC cell types in mice is unclear, and whether it exhibits synaptic plasticity remains unexplored. Additionally, the direct RGC input's role in binocular vision processing is unknown. This thesis investigates the biophysical and synaptic properties of the retinocollicular pathway through optogenetics and electrophysiology.

To investigate monosynaptic RGC input to neurons in the superficial SC we virally expressed ChR2 in the retina and optogenetically stimulating RGC axons with light. We classified superficial SC neurons in vitro based on their electrophysiological and morphological properties, revealing four main cell types: wide-field (WF), horizontal, narrow-field (NF), and stellate. Light-evoked RGC monosynaptic responses were observed in all four cell types. We found RGC input to WF neurons evoked action potentials at more negative membrane potentials than during current injection, which was not observed in other cell types. In the presence of tetrodotoxin and 4-aminopyridine, RGC input evoked putative calcium spikes in horizontal, NF and stellate neurons, but not WF neurons. Our findings indicate that all four cell types of the superficial SC receive monosynaptic input from the retina.

We investigated neurotransmitters in the retinocollicular pathway. Most light-induced monosynaptic RGC responses were mediated by glutamate, but some synaptic responses were not affected by glutamatergic antagonists. Different receptor antagonists revealed that stellate and horizontal cells receive GABAergic and cholinergic input via GABAA and nicotinic acetylcholine receptors. Thus, neurons in the superficial SC receive glutamatergic, GABAergic, and cholinergic RGC input in a cell-specific manner.

We investigated synaptic plasticity at the RGC to WF neuron synapse using a spike-time-dependent plasticity (STDP) protocol during optogenetic RGC activation, inducing long-term potentiation (LTP) and long-term depression (LTD) in WF neurons. LTP depended on NMDA receptors and L-type Ca2+ channels, while LTD required only L-type Ca2+ channel activation. By investigating changes in the paired-pulse ratio and coefficient of variation, we determined that LTP was presynaptic and LTD postsynaptic. Similar findings in WF neurons identified via Ntsr1-tdTomato mice indicate that the RGC to WF synapse exhibits activity-dependent plasticity, suggesting its role in visual processing.

We focused on direct retinal input's role in binocular processing in the SC. Binocular neurons exist in all three SC layers, but RGCs innervate only the superficial layer, primarily from the contralateral retina. Optogenetic RGC responses from the contralateral eye appeared throughout the superficial layer, while responses from the ipsilateral eye were seen only in the anteromedial region between the superficial and intermediate layers. This shows that RGC input convergence from both eyes occurs only in this limited SC area. Consequently, binocular visual information to other SC parts must use different pathways.

Collectively, this thesis investigates visual processing in the SC with a focus on the biophysical and synaptic characteristics of the direct connection between the retina and specific cells types in the superficial SC. It provides a comprehensive examination of this critical visual pathway, which plays a key role in orientation movements as well as innate defensive behaviours.