Male rat hypothalamic extraretinal photoreceptor Opsin3 is sensitive to osmotic stimuli and light

Hypothalamic opsin 3 suppresses MC4R signaling and potentiates Kir7.1 to promote food consumption

Mammalian opsin 3 (OPN3) is a member of the opsin family of G-protein-coupled receptors with ambiguous light sensitivity. OPN3 was first identified in the brain (and named encephalopsin) and subsequently found to be expressed in other tissues. In adipocytes, OPN3 is necessary for light responses that modulate lipolysis and glucose uptake, while OPN3 in human skin melanocytes regulates pigmentation in a light-independent manner. Despite its initial discovery in the brain, OPN3 functional mechanisms in the brain remain elusive. Here, we investigated the molecular mechanism of OPN3 function in the paraventricular nucleus (PVN) of the hypothalamus. We show that Opn3 is coexpressed with the melanocortin 4 receptor (Mc4r) in a population of PVN neurons, where it negatively regulates MC4R-mediated cAMP signaling in a specific and Gαi/o-dependent manner. Under baseline conditions, OPN3 via Gαi/o potentiates the activity of the inward rectifying Kir7.1 channel, previously shown to be closed in response to agonist-mediated activation of MC4R in a Gαs-independent manner. In mice, we found that Opn3 in Mc4r-expressing neurons regulates food consumption. Our results reveal the first mechanistic insight into OPN3 function in the hypothalamus, uncovering a unique mechanism by which OPN3 functions to potentiate Kir7.1 activity and negatively regulate MC4R-mediated cAMP signaling, thereby promoting food intake.

Defining spatial nonuniformities of all ipRGC types using an improved Opn4cre recombinase mouse line

Intrinsically photosensitive retinal ganglion cells (ipRGCs) play a crucial role in several physiological light responses. In this study, we generate an improved Opn4cre knockin allele (Opn4cre(DSO)), which faithfully reproduces endogenous Opn4 expression and improves compatibility with widely used reporters. We evaluated the efficacy and sensitivity of Opn4cre(DSO) for labeling in retina and brain and provide an in-depth comparison with the extensively utilized Opn4cre(Saha) line. Through this characterization, Opn4cre(DSO) demonstrated higher specificity in labeling ipRGCs with minimal recombination escape. Leveraging a combination of electrophysiological, molecular, and morphological analyses, we confirmed its sensitivity in detecting all ipRGC types (M1-M6) and defined their unique topographical distribution across the retina. In the brain, the Opn4cre(DSO) line labels ipRGC projections with minimal labeling of cell bodies. Overall, the Opn4cre(DSO) mouse line represents an improved tool for studying ipRGC function and distribution, offering a means to selectively target these cells to study light-regulated behaviors and physiology.

A new Opn4cre recombinase mouse line to target intrinsically photosensitive retinal ganglion cells (ipRGCs)

Intrinsically photosensitive retinal ganglion cells (ipRGCs) play a crucial role in several physiological light responses. In this study we generate a new Opn4cre knock-in allele (Opn4cre(DSO)), in which cre is placed immediately downstream of the Opn4 start codon. This approach aims to faithfully reproduce endogenous Opn4 expression and improve compatibility with widely used reporters. We evaluated the efficacy and sensitivity of Opn4cre(DSO) for labeling in retina and brain, and provide an in-depth comparison with the extensively utilized Opn4cre(Saha) line. Through this characterization, Opn4cre(DSO) demonstrated higher specificity in labeling ipRGCs, with minimal recombination escape. Leveraging a combination of electrophysiological, molecular, and morphological analyses, we confirmed its sensitivity in detecting all ipRGC types (M1-M6). Using this new tool, we describe the topographical distributions of ipRGC types across the retinal landscape, uncovering distinct ventronasal biases for M5 and M6 types, setting them apart from their M1-M4 counterparts. In the brain, we find vastly different labeling patterns between lines, with Opn4cre(DSO) only labeling ipRGC axonal projections to their targets. The combination of off-target effects of Opn4cre(Saha) across the retina and brain, coupled with diminished efficiencies of both Cre lines when coupled to less sensitive reporters, underscores the need for careful consideration in experimental design and validation with any Opn4cre driver. Overall, the Opn4cre(DSO) mouse line represents an improved tool for studying ipRGC function and distribution, offering a means to selectively target these cells to study light-regulated behaviors and physiology.