Intrinsic photosensitivity of a deep brain photoreceptor

Summary In addition to having a photoreceptive retina and pineal organ, non-mammalian vertebrates are known to have photoreceptors in the deeper regions of the brain that mediate seasonal changes in physiology and behavior [1]. Cerebrospinal fluid (CSF)-contacting neurons extend knob-like dendrites into the ventricular cavity, where they form ciliated terminals [2]. The dendritic structures of photoreceptor cells in the developing retina and the pineal organ resemble those of CSF-contacting neurons. Thus, CSF-contacting neurons have been suggested to function as deep brain photoreceptors. Although the localization of a novel short-wavelength-sensitive photopigment (OPN5) that detects violet and ultraviolet (UV) light has been recently demonstrated in CSF-contacting neurons [3,4], there has been no direct evidence of photosensitivity in deep brain CSF-contacting neurons. Here, we report that the OPN5-positive CSF-contacting neurons in the paraventricular organ (PVO) of the quail mediobasal hypothalamus (MBH) are intrinsically photosensitive and are involved in the regulation of seasonal reproduction.