Neuroanatomical distribution of the orphan GPR50 receptor in adult sheep and rodent brains

GPR50, formerly known as melatonin-related receptor, is one of three subtypes of the melatonin receptor subfamily, together with the MT1 and MT2 receptors. By contrast to these two high-affinity receptor subtypes and despite its high identity with the melatonin receptor family, GPR50 does not bind melatonin or any other known ligand. Specific and reliable immunological tools are therefore needed to be able to elucidate the physiological functions of this orphan receptor that are still largely unknown. We have generated and validated a new specific GPR50 antibody against the ovine GPR50 and used it to analyse the neuroanatomical distribution of the GPR50 in sheep, rat and mouse whole brain. We demonstrated that GPR50-positive cells are widely distributed in various regions, including the hypothalamus and the pars tuberalis of the pituitary, in all the three species studied. GPR50 expressing cells are abundant in the dorsomedial nucleus of the hypothalamus, the periventricular nucleus and the median eminence. In rodents, immunohistochemical studies revealed a broader distribution pattern for the GPR50 protein. GPR50 immunoreactivity is found in the medial preoptic area (MPA), the lateral septum, the lateral hypothalamic area, the bed nucleus of the stria terminalis, the vascular organ of the laminae terminalis and several regions of the amygdala, including the medial nuclei of amygdala. Additionally, in the rat brain, GPR50 protein was localised in the CA1 pyramidal cell layer of the dorsal hippocampus. In mice, moderate to high numbers of GPR50-positive cells were also found in the subfornical organ. Taken together, these results provide an enlarged distribution of GPR50 protein, give further insight into the organisation of the melatoninergic system, and may lay the framework for future studies on the role of the GPR50 in the brain.