Morphological and physiological diversity of M1 intrinsically photosensitive retinal ganglion cells

Abstract

Purpose : Melanopsin-expressing, intrinsically photosensitive retinal ganglion cells (ipRGCs), comprise five subtypes, M1 to M5, according to their different morphological and physiological properties. Recent work demonstrated that the M1 subtype can be subdivided into two different molecular subpopulations based on whether they express the transcription factor Brn3b (Brn3b-positive or Brn3b-negative cells). These two populations show divergent brain targeting and functions in non-image-forming behaviors. However, whether these cells have distinct physiological and morphological properties is unknown.

Methods : To compare distributions of Brn3b-positive and Brn3b-negative M1 cells, whole mount retinas of Opn4^tau-LacZ mice (N=4), in which M1 ipRGCs are exclusively labeled with β-galactosidase (β-gal), were immunostained for β-gal and Brn3b. To compare physiological properties, in vitro single cell recording was performed in a whole cell configuration. M1 cells were fluorescently identified by a Cre inducible reporter gene in the Opn4^Cre mouse line (N=6) and morphologically identified using fluorescence-conjugated hydrazide included in the internal solution. The recorded cells were filled with neurobiotin during recording and then incubated with streptavidin to more clearly analyze morphological properties, and were immunostained for Brn3b. Statistical analysis was performed using one-way ANOVA with Fisher's LSD post hoc test.

Results : We found that Brn3b-positive and Brn3b-negative M1 cells were differentially distributed across the retina, with a higher proportion of Brn3b-positive M1 cells in the ventral retina. Surprisingly, we found that Brn3b-positive and Brn3b-negative M1 cells exhibited differences in their resting membrane potential, intrinsic membrane properties, and light response characteristics. These M1 subtypes also differed in their morphological properties. Brn3b-positive M1 cells have a longer total dendritic length and larger dendritic field than Brn3b-negative M1 cells.

Conclusions : Brn3b-positive and Brn3b-negative M1 cells differ across multiple parameters including distribution, physiological properties, and morphological characteristics. These findings support the idea that different molecular M1 populations differ in their connectivity in the brain and influence on non-image forming behaviors.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.