Gap Junctions Propagate Melanopsin-like Responses among Ganglion Cells in Efemp1R345W Mice

Abstract

Purpose: In mice with an R345W mutation in Efemp1, there is a remarkable increase in the number of retinal ganglion cells having melanopsin-like responses to light (wildtype control 12%; Efemp1^R345W 23%). We define melanopsin-like responses electrophysiologically as those having relatively long latency and duration and persisting when input from the rods and cones has been blocked pharmacologically. The purpose of this study was to determine the cause of this physiological phenotype, and how it affects vision.

Methods: IHC was used to count the number of melanopsin positive cells in retinas of wild type (wt) and Efemp1^R345W mice. Retinal ganglion cell activity and responses to light were recorded using multielectrode array methods. A pharmacologic cocktail was used to block all synaptic transmission, revealing cells with rod- and cone-independent responses to light. Gap junction blockers 18β-glycyrrhetinic acid and/or meclofenamic acid were then added to show whether electrotonic synapses propagated responses from melanopsin ganglion cells to other ganglion cell types.

Results: There was no difference in the number of melanopsin-positive ganglion cells between wildtype and Efemp1^R345W mice. Efemp1^R345W retinas had twice as many cells with melanopsin-like responses under synaptic blockade, but many cells had physiological types (ON, ON-OFF, OFF, etc) not consistent with previously identified melanopsin ganglion cells. Gap junction blockers decreased the number of cells with melanopsin-like responses to typical control numbers.

Conclusions: We have found that Efemp1^R345W causes an increase in the number of ganglion cells with melanopsin-generated responses to light, but no increase in the number of cells expressing melanopsin. Rather, our findings point to abnormal signal propagation from melanopsin ganglion cells to other classes of ganglion cells, via gap junctions.

Keywords: 531 ganglion cells • 532 gap junctions/coupling • 715 signal transduction: pharmacology/physiology