Structure and function of the gap junctional network of photoreceptive ganglion cells

Abstract

Intrinsically photosensitive retinal ganglion cells (ipRGCs) signal not only anterogradely to drive behavioral responses, but also retrogradely to some amacrine interneurons to modulate retinal physiology. We previously found that all displaced amacrine cells with spiking, tonic excitatory photoresponses receive gap-junction input from ipRGCs, but the connectivity patterns and functional roles of ipRGC-amacrine coupling remained largely unknown. Here, we injected PoPro1 fluorescent tracer into all six types of mouse ipRGCs to identify coupled amacrine cells, and analyzed the latter’s morphological and electrophysiological properties. We also examined how genetically disrupting ipRGC-amacrine coupling affected ipRGC photoresponses. Results showed that ipRGCs couple with not just ON- and ON/OFF-stratified amacrine cells in the ganglion-cell layer as previously reported, but also OFF-stratified amacrine cells in both ganglion-cell and inner nuclear layers. M1- and M3-type ipRGCs couple mainly with ON/OFF-stratified amacrine cells, whereas the other ipRGC types couple almost exclusively with ON-stratified ones. ipRGCs transmit melanopsin-based light responses to at least 93% of the coupled amacrine cells. Some of the ON-stratifying ipRGC-coupled amacrine cells exhibit transient hyperpolarizing light responses. We detected bidirectional electrical transmission between an ipRGC and a coupled amacrine cell, although transmission was asymmetric for this particular cell pair, favoring the ipRGC-to-amacrine direction. We also observed electrical transmission between two amacrine cells coupled to the same ipRGC. In both scenarios of coupling, the coupled cells often spiked synchronously. While ipRGC-amacrine coupling somewhat reduces the peak firing rates of ipRGCs’ intrinsic melanopsin-based photoresponses, it renders these responses more sustained and longer-lasting. In summary, ipRGCs’ gap junctional network involves more amacrine cell types and plays more roles than previously appreciated.

Adeno-associated virus (AAV)-mediated Cre recombinase expression in melanopsin ganglion cells without leaky expression in rod/cone photoreceptors

“…Such leaky expression is problematic in functional assays that preserve rod/cone signaling. For instance, to study how gap-junction coupling between ipRGCs and amacrine cells might influence ipRGC photoresponses, we recently crossed Opn4 Cre mice with connexin36-flox mice to disrupt ipRGCs’ gap junctions; however, since some photoreceptors expressed Cre, rod-cone coupling was likely also affected, compelling us to block rod/cone signaling pharmacologically and measure only melanopsin-based photoresponses ( Zhao and Wong, 2021 ). We have also used Opn4 Cre/Cre mice as melanopsin knockouts to assess the behavioral roles of ipRGCs’ intrinsic photosensitivity ( Schroeder et al, 2018 ).…”