A putative new melatonin binding site in sheep brain, MTx: preliminary observations and characteristics

In mammals, MT₁ and MT₂ melatonin receptors are high affinity G protein-coupled receptors and are thought to be involved in the integration of the melatonin signaling throughout the brain and periphery. In the present study, we describe a new melatonin binding site, named MTx, with a peculiar pharmacological profile. This site had a low affinity for 2-[¹²⁵I]-melatonin in saturation assays in hypothalamus and retina (pK_D = 9.13 {plus minus} 0.05, Bmax = 1.12 {plus minus} 0.11 fmol/mg protein and pK_D = 8.81 {plus minus} 0.50, Bmax = 7.65 {plus minus} 2.64 fmol/mg protein, respectively) and a very high affinity, in competition assays, for melatonin (pKi = 13.08 {plus minus} 0.18), and other endogenous compounds. Using autoradiography, we showed a preferential localization of the MTx in periventricular areas of the sheep brain, with a density 3 to 8 times higher than those observed for ovine MT₁ In addition, using a set of well-characterized ligands, we showed that this site did not correspond to any of the following receptors: MT₁, MT₂, MT₃ , D₁, D₂, noradrenergic, nor 5-HT₂ Based on its affinity for melatonin, MTx did not seem to be implicated in the integration of cerebral melatonin concentration variations since they were saturating for MTx. Nevertheless, it remained of prime importance because of its periventricular distribution, in close contact with the CSF, and its peculiar pharmacological profile responding to both melatoninergic and serotoninergic compounds. Significance Statement Herein a putative new melatonin binding site is described in sheep brain parts in close contact with the 3^rd ventricle. The characteristics of the pharmacological profile of this site is different from anything previously reported in the literature. The present work forms the basis of future full pharmacological characterization.