CHRONIC INTRACEREBROVENTRICULAR INFUSION OF THE ANTIOPIOID PEPTIDE, PHE-LEU-PHE-GLN-PRO-GLN-ARG-PHE-NH2 (NPFF), DOWN-REGULATES MU-OPIOID BINDING-SITES IN RAT-BRAIN

Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-NH2 (NPFF), an endogenous mammalian an tiopioid peptide, has been shown by other laboratories to attenuate th e acute antinociceptive effects of morphine, the development of morphi ne tolerance, and naloxone-induced withdrawal in morphine-dependent ra ts. The present study determined the effect of chronic NPFF on mu opio id receptors and mRNA for the endogenous opioids dynorphin and enkepha lin. Rats received ICV infusions of either saline or NPFF (5 mug/h) fo r 13 days via Alzet 2002 osmotic minipumps. Homogenate binding studies , which used whole brain membranes, demonstrated that NPFF decreased t he B(max) of mu binding sites (labeled by [H-3][D-Ala2-MePhe4 Gly-ol5] enkephalin) from 262 +/- 12 to 192 +/- 12 fmolmg protein, and increase d the K(d) from 1. 1 to 2.3 nM. Quantitative receptor autoradiography and in situ hybridization experiments were conducted with sections col lected at the level of the striatum. The density of mu opioid binding sites labeled by [H-3][D-Ala2-MePhe4,Gly-ol5]enkephalin was decreased in all brain areas measured except the corpus callosum, and there was no change in dynorphin mRNA or enkephalin mRNA in the caudate, the nuc leus accumbens, or the ventral pallidum. Rats chronically administered ICV morphine sulfate (20 mug/h) for 14 days developed tolerance to mo rphine and a low degree of dependence, as measured by naloxone-precipi tated withdrawal. Chronic administration of NPFF concurrently with mor phine sulfate did not significantly alter naloxone-induced withdrawal signs or the development of morphine tolerance. Viewed collectively wi th previous findings that chronic ICV infusion of anti-NPFF IgG upregu lates mu receptors, these data provide additional evidence that the de nsity of CNS mu receptors is tonically regulated by NPFF in the extrac ellular fluid. The action of NPFF to decrease mu receptors is consiste nt with an antiopioid role for this peptide; however, the fact that NP FF (administered into the lateral ventricle) did not appreciably alter expression of morphine tolerance and dependence contrasts with previo us findings and reinforces the view that this effect is most reliably seen after third ventricle administration.