Abstract
The retinal ganglion cell layer of the eye comprises a subtype of cells characterized by their intrinsic photosensitivity and expression of melanopsin (ipRGCs). These cells regulate a variety of non-image-forming (NIF) functions such as light entrainment of circadian rhythms, acute suppression of locomotor activity (masking), and pupillary light reflex. Two neurotransmitters have been identified in ipRGCs, glutamate and pituitary adenylate cyclase-activating polypeptide (PACAP). To date, little is known about their release and interplay. Here, we describe the presence and co-localization of vesicular glutamate transporter 2 (VGLUT2; a marker of glutamate signaling) and PACAP in ipRGCs and their projections in the brain. Nine adult male Wistar rats were assigned to one of three groups; anterograde tracing (n = 3), eye enucleation (n = 3), and untreated (n = 3). Under anaesthesia, rats were transcardially perfusion-fixated, after which the brains and eyes were removed for double immunohistochemical staining using a polyclonal anti-VGLUT2 antibody and a mouse monoclonal anti-PACAP antibody. Results revealed that VGLUT2- and PACAP-immunoreactivity (-ir) were present in ipRGCs and co-localized in their projections in the suprachiasmatic nucleus, the intergeniculate leaflet, and the olivary pretectal nucleus. We conclude that there is evidence to support the use of glutamate and PACAP as neurotransmitters in NIF photoperception by rat ipRGCs, and that these neurotransmitters are co-stored and probably released from the same nerve terminals. Furthermore, we conclude that VGLUT2 is the preferred subtype of vesicular transporter used by these cells.