G-PROTEIN-COUPLED CORTICOTROPIN-RELEASING HORMONE RECEPTORS IN RAT RETINA

The presence of corticotropin-releasing hormone (CRH) receptors in rat retinal membranes was investigated by using [I-125-Tyr(0)]-ovine CRH ([I-125]oCRH) as radioligand. The receptor binding was rapid, reversib le, saturable and specific. The [I-125]oCRH binding was completely dis placed by different CRH-related peptides with a rank order of potency similar to that displayed in stimulating rat retinal adenylyl cyclase activity. Two populations of binding sites were detected: one with hig h affinity (K-d = 1.7 nM) and the other with low-affinity (K-d = 130 n M). The GTP analogue guanosine 5'-0-(3'-thiotriphosphate) reduced the high-affinity binding and increased the relative proportion of sites w ith low-affinity. Incubation of rat retinal membranes with the RM/1 an tibody, which recognizes the carboxyl-terminus of the a subunit of the G protein Gs, prevented the CRH stimulation of adenylyl cyclase. In i mmunoblots, the RM/1 antibody recognized an immunoreactive protein ban d of 45 kDa and a protein with a similar electrophoretic mobility was ADP-ribosylated by cholera toxin. These data provide evidence for the presence of specific CRH receptors in rat retina and contribute to def ine the CRH signalling system in this tissue.