The ρ subunits seem to be preferentially expressed in the retina (Enz et al., 1996;Koulen et al., 1998). mRNA encoding ρ subunits, however, is present also in the superior colliculus, dorsal lateral geniculate nucleus and cerebellar Purkinje cells (Boue-Grabot et al., 1998; Wegelius et al., 1998). In addition, pharmacological effects characteristic for ρ subunit containing receptors have been reported in various brain regions (for review see Sieghart and Sperk, 2002). The absence of antibodies or a pharmacology that could distinguish between receptors containing different ρ subunits, did not allow more thorough studies on their possible existence. Since the mere existence of a subunit and the fact that it can form receptors in recombinant systems does not necessarily imply that it actually forms receptors in vivo and since such subunits could also have other functions in the brain, at the time being it only seems safe to assume the existence of one ρ-containing native receptor in the brain.
