MU AND DELTA OPIATE RECEPTORS IN RAT-BRAIN ARE AFFECTED BY GTP BUT NOT BY MIF-1

The possible competition of MIF-1 [MSH release inhibiting factor-1] (Pro-Leu-Gly-NH2) with 3H-naloxone or 3H-D-Ala-D-Leu-enkephalin (DADLE) ro mu and delta opiate receptors in the rat striatum was examined under conditions of varying temperatures and concentrations of Na and GTP. MIF-1, in concentrations ranging from 10-14-10-3 M, failed to compete with either 3H-DADLE or 3H-naloxone for delta or mu receptors, GTP(2 .mu.M) facilitated binding (17.7 .+-. 1.9%) at all concentrations of MIF-1 in the 1H-DADLE assay. GTP facilitated binding in the 3H-naloxone assay (11.5 .+-. 1.3%) under conditions used to potentiate antagonist binding (addition of NaCl and incubation at 4.degree. C). GTP showed optimal facilitation at a concentration of about 2 .mu.M in both the 3H-DADLE and 3H-naloxone antagonist conditions. At higher doses of GTP, 3H-naloxone binding remained high, but 3H-DADLE binding decreased to near or below control values. MIF-1 did not affect these GTP responses, indicating further the lack of effect of this peptide on opiate binding.