Opn5, also known as neuropsin, is an opsin that functions as a photoreceptive protein. It is involved in various physiological functions such as vision, photoentrainment of circadian and circannual rhythms, photoperiodism, and background adaptation [2]. It forms an independent group among vertebrate opsins, with mammals typically having one Opn5 gene (Opn5m), while non-mammalian vertebrates have additional subtypes like Opn5L1 and Opn5L2 [2].

In KO mouse models, Opn5-null mice show overactive brown adipose tissue (BAT), increased body temperature, and exaggerated thermogenesis when cold-challenged, indicating that Opn5 in hypothalamic preoptic area (POA) neurons normally suppresses BAT thermogenesis [3]. In terms of myopia, violet light prevents lens defocus-induced myopia in mice, and this effect depends on the retinal expression of Opn5, suggesting its role in emmetropization [1]. Also, Opn5-null mice display impaired photoentrainment and phase-shifting to UVA light, although Opn5 alone is not sufficient for photoentrainment of the suprachiasmatic nuclei (SCN) clock [4].

In conclusion, Opn5 plays crucial roles in thermoregulation, myopia prevention, and circadian photoentrainment as revealed through KO mouse models. These findings contribute to understanding the mechanisms of related diseases such as myopia and disorders related to thermoregulation and circadian rhythm disruption.