Central projections of melanopsin‐expressing retinal ganglion cells in the mouse

Abstract

A rare type of ganglion cell in mammalian retina is directly photosensitive. These novel retinal photoreceptors express the photopigment melanopsin. They send axons directly to the suprachiasmatic nucleus (SCN), intergeniculate leaflet (IGL), and olivary pretectal nucleus (OPN), thereby contributing to photic synchronization of circadian rhythms and the pupillary light reflex. Here, we sought to characterize more fully the projections of these cells to the brain. By targeting tau-lacZ to the melanopsin gene locus in mice, ganglion cells that would normally express melanopsin were induced to express, instead, the marker enzyme beta-galactosidase. Their axons were visualized by X-gal histochemistry or anti-beta-galactosidase immunofluorescence. Established targets were confirmed, including the SCN, IGL, OPN, ventral division of the lateral geniculate nucleus (LGv), and preoptic area, but the overall projections were more widespread than previously recognized. Targets included the lateral nucleus, peri-supraoptic nucleus, and subparaventricular zone of the hypothalamus, medial amygdala, margin of the lateral habenula, posterior limitans nucleus, superior colliculus, and periaqueductal gray. There were also weak projections to the margins of the dorsal lateral geniculate nucleus. Co-staining with the cholera toxin B subunit to label all retinal afferents showed that melanopsin ganglion cells provide most of the retinal input to the SCN, IGL, and lateral habenula and much of that to the OPN, but that other ganglion cells do contribute at least some retinal input to these targets. Staining patterns after monocular enucleation revealed that the projections of these cells are overwhelmingly crossed except for the projection to the SCN, which is bilaterally symmetrical.

Signalling by melanopsin (OPN4) expressing photosensitive retinal ganglion cells

“…For example, it is unclear to what extent the properties of light responses vary between the distinct subclasses of M1-type pRGCs that project to the SCN and the OPN, and how these responses may be tailored to meet the different physiological roles performed by these cells. Indeed it should be noted that in total, pRGC innervations are detected for as many as 32 distinct brain regions, 32,33,80 and in the majority of these cases the subtypes of pRGCs innervating these areas, and the specific properties of light responses transmitted to these regions has received little attention. There are also significant gaps in our understanding of the melanopsin phototransduction signalling pathway.…”

“…To date at least five distinct subtypes of pRGC have been identified, termed M1-M5-type pRGCs. 1,2,[32][33][34][35] These cells show differences in morphology and retinal connections, and exhibit light responses with markedly different kinetics and sensitivities ( Figure 1b) [32][33][34][37][38][39] (Figure 1c), and would therefore seem to be tasked with performing different physiological roles (for review see Schmidt et al 36,40 ). However, the innervations of each class of pRGC are only partially resolved, and the specific roles performed by each class of pRGC are yet to be fully determined.…”

“…1,2,32,41 M2-type pRGCs express significantly lower levels of melanopsin compared with M1-type pRGCs, and have more complex dendritic fields that stratify exclusively in the ON sublamina of the IPL. 32,34,35,42 M3-type pRGCs are rarer than either M1 or M2-type cells, and show a bistratified morphology with dendrites present in both the ON and OFF layers of the IPL, 42,43 and show an intermediary level of melanopsin expression compared to M1 -and M2-type cells. 42 The anatomy of M4 -and M5 cells is grossly similar to M2-type cells, having dendrites confined to the ON layer of the IPL, but these cell types are distinguished based on the size and complexity of their dendritic fields, and in the case of M4 cells their large soma size.…”

“…The projections of M1-type pRGCs have been well documented, using Opn4.tau.LacZ mice that selectively label M1-type pRGCs, 2,32 showing that these cells innervate a number of key brain areas associated with NIF functions, including (but not limited to) the suprachiasmatic nucleus (SCN), intergeniculate leaflet, olivary pretectal nucleus (OPN), ventrolateral preoptic area, ventral lateral geniculate nucleus, medial amygdala, lateral habenula, and the superior colliculus (SC). 32 Subsequent studies using Opn4.Cre mice have allowed the combined innervations of M1-M5-type pRGCs to be studied, 33,37 and identified a number of additional brain regions that receive input exclusively from non-M1 pRGCs, including the dorsal lateral geniculate nucleus (dLGN), the core region of the OPN, and additional innervations of the SC.…”

“…32 Subsequent studies using Opn4.Cre mice have allowed the combined innervations of M1-M5-type pRGCs to be studied, 33,37 and identified a number of additional brain regions that receive input exclusively from non-M1 pRGCs, including the dorsal lateral geniculate nucleus (dLGN), the core region of the OPN, and additional innervations of the SC. 33 However, these Opn4.Cre-based models do not allow the separation of projections from the different pRGC subtypes, as such the regions of the brain innervated specifically by M2, M3, M4, and M5 types pRGCs remain largely undetermined.…”