Evidence that the delta-selective alkylating agent, fit, alters the mu-noncompetitive opiate delta binding site

Recombinant mu-delta receptor as a marker of opiate abuse

The brain is particularly vulnerable to drugs of abuse changing the neuroreceptor functions. Opiates interact and overstimulate heterogeneous opioid receptors leading to their desensitization, internalization, and activation of recombinant opioid receptor. The molecular properties of rat and human brain recombinant mu-delta receptor were compared with those of purified mu- and delta-receptors. cDNA coding the unique fragment of recombinant mu-delta receptor was isolated and sequenced. We hypothesized that recombinant mu-delta receptor may be a hallmark of opiate abuse. Peptide fragments of the mu- (MOR), delta- (DOR), and recombinant mu-delta- (MDOR) receptors were used as antigens to assess the presence of autoantibodies in the blood of rats that self-administered heroin and cocaine, as well as drug abusers. Significant steady elevation of MDOR autoantibodies were measured in sera of rats that self-administered heroin compared to that for cocaine and vehicle animals. The appearance and increased level of MDOR autoantibodies in opiate abusers correlated with severity of the disorder and duration of drug exposure.

Opioid peptide receptor studies. 1. Identification of a novel delta-opioid receptor binding site in rat brain membranes

Our laboratory was among the first to propose the existence of delta receptor subtypes: a delta site thought to be associated with a mu-delta-opioid receptor complex termed the delta cx binding site and delta site not associated with the mu-delta-opioid receptor complex, termed the delta ncx site. In previous studies, we assayed the delta cx site with [3H][D-Ala2,D-Leu5]enkephalin using rat brain membranes depleted of delta ncx sites by pretreatment with the site-directed acylating agent, (+)-trans-SUPERFIT. In the present study, we investigated, using (+)-trans-SUPERFIT-pretreated membranes, the possibility of heterogeneity of the delta cx binding site. Two sites were resolved: the delta cx-1 site at which mu ligands are potent noncompetitive inhibitors and delta ligands are weak competitive inhibitors, and the delta cx-2 site where delta ligands are potent and mu ligands are weak, mixed competitive-noncompetitive inhibitors. Although the delta cx-2 site has a delta-like ligand-selectivity profile, several experiments distinguished it from the delta ncx site. Two lines of evidence suggest that the delta ncx site corresponds to the cloned delta receptor. One, the delta receptor was cloned from the NG108-15 cell line, and this receptor, like the delta ncx binding site, irreversibly binds SUPERFIT and (+)-trans-SUPERFIT. Secondly, administration of delta-antisense DNA selectively decreases delta ncx binding. Viewed collectively, the major finding of this study is the discovery of a novel SUPERFIT-insensitive and delta-antisense-insensitive delta cx-2 binding site.

Probing the opioid receptor complex with (+)-trans-SUPERFIT. II. Evidence that mu ligands are noncompetitive inhibitors of the delta cx opioid peptide binding site

Previous studies delineated two classes of delta binding sites; a delta binding site not associated with the opioid receptor complex, termed the delta ncx site, and a delta site associated with the opioid receptor complex, termed the delta cx site. The delta ncx site has high affinity for [D-Pen2,D-Pen5]enkephalin, and is synonymous with what is now identified as the delta 1 binding site. Pretreatment of membranes with the delta-selective acylating agents FIT, or (+)-trans-SUPERFIT, deplete membranes of the delta ncx binding site, which permits the selective labeling of the delta cx binding site with [3H][D-Ala2,Leu5]enkephalin. The present study compared the properties of the delta cx binding site present in brain membranes pretreated with (+)-trans-SUPERFIT with the properties of the delta cx site present in untreated membranes. The major findings are: 1) pretreatment of membranes with (+)-trans-SUPERFIT decreased the IC50 values of delta-preferring drugs, and increased the IC50 values of mu-preferring drugs, for the delta cx binding site; 2) the degree of delta selectivity was highly correlated with the magnitude of the (+)-trans-SUPERFIT-induced shift in the IC50 values; 3) the ligand-selectivity patterns of the mu and delta cx sites present in (+)-trans-SUPERFIT-pretreated membranes were poorly correlated; 4) whereas mu-preferring drugs were noncompetitive inhibitors of [3H][D-Ala2,Leu5]enkephalin binding to the delta cx site, delta-preferring drugs were competitive inhibitors.(ABSTRACT TRUNCATED AT 250 WORDS)

Physiological regulation of neurohypophyseal kappa-opiate receptors

Quantitative autoradiography was used to examine the characteristics of kappa-opiate receptor binding in neural lobe sections from dehydrated rats and water-sated homozygous Brattleboro rats. The density of kappa-sites was decreased by 40% and 44%, respectively, after 5 days of water deprivation or hypertonic saline. Both the density and affinity of kappa-receptor sites were lower in homozygous Brattleboro rats. We suggest that kappa-receptor down-regulation occurs as a result of elevated release of opioid peptides induced by chronic osmotic stimulation.

Selective activation of opioid receptors differentially affects lordosis behavior in female rats

The effects of opioid peptides that are highly selective ligands for mu receptors (morphiceptin). delta receptors (delta-receptor peptide), kappa receptors (dynorphin 1-9), and the mu/delta complex (beta-endorphin), were tested on lordosis behavior in ovariectomized rats primed with estrogen and progesterone. Intracerebroventricular infusions of beta-endorphin or morphiceptin both inhibited and facilitated lordosis in a dose-dependent fashion whereas all doses of delta-receptor peptide facilitated lordosis. Dynorphin 1-9 had no significant effect at any dose, although a trend toward increased lordosis quotients was observed 30 min after infusion. The effects of beta-endorphin, morphiceptin, and delta-receptor peptide were reversed with naloxone, although naloxone alone had no effect on lordosis behavior. These results indicate that the specific activation of opioid receptor subtypes differentially affects lordosis behavior. It appears that binding to high-affinity mu 1 receptors exerts an inhibitory influence on lordosis, whereas binding to low-affinity mu 2 receptors or delta receptors exerts a facilitatory influence. Binding to kappa receptors does not appear to affect lordosis behavior.

Morphine tolerance increases mu-noncompetitive delta binding sites

In light of more recent knowledge concerning endogenous opioid peptides and their multiple opiate receptors, we reevaluated the effects of morphine tolerance on opiate receptor binding parameters. Rats were implanted with morphine or placebo pellets, and [3H][D-Ala2,D-Leu5]enkephalin ([3H]DADL) was used to label brain membranes. Utilizing the technique of binding surface analysis, we observed a selective 47% up-regulation of lower affinity [3H]DADL binding sites (mu-noncompetitive delta binding sites) in morphine pelleted rats. To corroborate these results, we treated brain membranes with the site directed alkylating agent FIT (N-phenyl-N-[1-(2-p-isothiocyanato)phenyl-ethyl)-4-piperidinyl] propanamide), which results in membranes highly enriched with the lower affinity [3H]DADL binding site. Scatchard plots of [3H]DADL binding to FIT-treated membranes also revealed that chronic morphine treatment produced a 60-65% up-regulation of the mu-noncompetitive delta binding site. These data indicate that chronic morphine alters a selective subpopulation of opiate receptors that may play a role in the mechanisms of opiate tolerance and physical dependence.