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Distinct ipRGCs
Further research has shown that ipRGCs project to different brain nuclei to control both non-image forming and image forming functions. These brain regions include the SCN, where input from ipRGCs is necessary to photoentrain circadian rhythms, and the olivary pretectal nucleus (OPN), where input from ipRGCs control the pupillary light reflex (PLR). Hattar and colleagues conducted research that demonstrated that ipRGCs project to hypothalamic, thalamic, stratal, brainstem and limbic structures. Although ipRGCs were initially viewed as uniform population, further research revealed that there are several subtypes with distinct morphology and physiology. Since 2011, Hattar's laboratory has contributed to these findings and has successfully distinguished subtypes of ipRGCs.

Diversity of ipRGCs
Hattar and colleges utilized Cre-based strategies for labeling ipRGCs to reveal that there are at least five ipRGC subtypes that project to a number of central targets. Five classes of ipRGCs, M1 through M5, have been characterized to date in rodents. These classes differ in morphology, dendritic localization, melanopsin content, electrophysiological profiles, and projections.

Diversity in M1 cells
Hattar and co-workers discovered that the same even among the subtype of ipRGC, there can be designated sets of ipRGCs that differentially control circadian versus pupillary behavior. In experiments with M1 ipRGCs, Hattar and colleagues discovered that the transcription factor Brn3b is expressed by M1 ipRGCs that target the OPN, but not by ones that target the SCN. Using this knowledge, they designed an experiment to cross Melanopsin-Cre mice with mice that conditionally expressed a toxin from the Brn3b locus. This allowed them to selectively ablate only the OPN projecting M1 ipRGCS. This resulted in a loss of pupil reflexes, however, this did not impair circadian photo entrainment. This demonstrated that the M1 ipRGC consists of molecularly distinct subpopulations that innervate different brain regions and execute specific light-induced functions. This isolation of a 'labeled line' consisting of differing molecular and functional properties in a highly specific ipRGC subtype was an important first for the field. It also underscored the extent to which molecular signatures can be used to distinguish between RGC populations that would otherwise appear the same, which in turn facilitates further investigation into their specific contributions to visual processing.