CHRONIC ICV INFUSION OF MORPHINE AND NPFF DOWN-REGULATE MU-OPIOID RECEPTORS IN RAT-BRAIN - A QUANTITATIVE AUTORADIOGRAPHIC STUDY

Regulation of the opioid mu receptor by pharmacological manipulation h as been extensively investigated using various radioreceptor binding t echniques. However, unlike most receptor systems, where chronic agonis t administration induces a characteristic down-regulation in receptor density, prolonged morphine treatment has shown conflicting results. B oth up- and down-regulation of mu receptors by morphine have been repo rted (for review; see (1)). Our lab recently demonstrated that morphin e modulating peptide, NPFF (Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-NH2), down -regulates opioid it receptors (2). The purpose of this study was to c ompare the effects of chronic ICV infusion of morphine and NPFF on mu opioid binding sites using the in vitro techniques of whole brain homo genate receptor binding and autoradiography.