Morphine-6-Glucuronide-Induced Hyperphagia: Characterization of Opioid Action By Selective Antagonists and Antisense Mapping in Rats LIZA LEVENTHAL, ROBERT M. SILVA, GRACE C. ROSSI, GAVRIL W. PASTERNAK and RICHARD J. BODNAR Neuropsychology Doctoral Subprogram and Psychology Department, City University of New York, Flushing (L.L., R.M.S., R.J.B.) and The Cotzias Laboratory of Neuro-Oncology, Memorial Sloan-Kettering Cancer Center, New York, New York (G.C.R., G.W.P.) Accepted for publication June 22, 1998 This paper is available online at http://www.jpet.org ABSTRACT Opiate drugs such as morphine stimulate food intake in rats. The morphine metabolite, morphine-6-glucuronide (M6G), is more active than morphine in analgesic assays, and appears to act through distinct receptors. Thus, although morphine anal- gesia is decreased by antisense oligodeoxynucleotides (AS ODNs) targeting exons 1 and 4 of the MOR-1 clone, M6G analgesia is reduced by probes targeting exons 2 and 3 of the MOR-1 clone. Our study examined whether central administra- tion of M6G increased food intake in rats, and characterized this response using either selective mu, kappa 1 , delta 1 and delta 2 antagonists, or antisense directed against the various cloned opioid receptors. Central M6G (10 –1000 ng) signifi- cantly and dose-dependently increased intake after 4 hr. Whereas mu antagonism with FNA significantly and dose- dependently reduced M6G-induced hyperphagia, equimolar doses of delta 1 , delta 2 , and kappa 1 antagonists were ineffec- tive. AS ODNs directed against either exons 2 or 3 of the MOR-1 clone blocked M6G-induced hyperphagia, whereas ei- ther AS ODNs directed against exons 1 or 4, or a MS ODN directed against exon 2 were ineffective. In contrast, an AS ODN probe directed against exon 1, but not exon 2, of the MOR-1 clone reduced morphine-induced hyperphagia, an ef- fect identical to DAMGO-induced hyperphagia. Whereas M6G- induced hyperphagia was insensitive to antisense probes di- rected against the DOR-1, KOR-1 and KOR-3/ORL1 clones, these probes respectively reduced hyperphagia induced by deltorphin II, U50488H and nociceptin. Although pharmacolog- ical data indicate that M6G-induced hyperphagia acts through mu receptors, antisense data imply that the hyperphagic ac- tions of M6G are mediated by a receptor distinct from tradi- tional mu agonists, either as an alternative splice variant of the MOR-1 clone or a distinct gene. In addition to the ability of endogenous opioid peptides and peptide analogues to stimulate food intake (see reviews by Gosnell and Levine, 1996; Morley et al., 1983), morphine and other opiates such as heroin, butorphanol, codeine and levor- phanol also produce a robust feeding response (e.g., Levine and Morley, 1983; Levine et al., 1994; Sanger and McCarthy, 1980; Thornhill et al., 1976). Morphine is rapidly metabolized and glucuronidated at both the three and six positions (Jaffe and Martin, 1985). Although M6G labels mu receptors with an affinity slightly less than morphine in binding assays (Paul et al., 1989), it is 100-fold more potent (Paul et al., 1989) centrally on both thermal (Abbott and Palmour, 1988; Pas- ternak et al., 1987; Shimomura et al., 1971; Sullivan et al., 1989) and visceral (Frances et al., 1992) nociceptive tests than morphine. To investigate whether M6G, as with its parent compound, morphine, produces long-acting (4 hr) in- gestive effects, the first goal of our study was to determine whether central administration of M6G dose-dependently increased spontaneous food intake in rats. Selective opioid antagonists directed against mu (FNA), delta 1 (DALCE), delta 2 (NTII) and kappa 1 (NBNI) receptor subtypes have been used to assess opioid receptor subtype involvement of opioid agonist-induced feeding, including hy- perphagia induced by selective mu opioid agonists (see re- view by Gosnell and Levine, 1996). Using this approach, it appears that feeding stimulated by selective opioid agonists may involve more than one opioid receptor subtype. For instance, hyperphagia elicited by the selective mu opioid agonist DAMGO is blocked by pretreatment with either FNA or NBNI (Levine et al., 1990, 1991). Therefore, our second goal was to determine whether M6G-induced hy- Received for publication August 21, 1998. This work was supported in part by NIDA Grants DA05746 (L.L.), DA04194 (R.J.B.), DA07274 (G.W.P.), DA00220 (G.W.P.) and DA00310 (G.C.R.). ABBREVIATIONS: AS ODN, antisense oligodeoxynucleotides; -FNA, -funaltrexamine; CON, control; DALCE, [D-Ala 2 , Leu 5 , Cys 6 ]-enkephalin; DAMGO, [D-Ala 2 , MePhe 4 , Gly-ol 5 ]-enkephalin; Delt II, deltorphin II; icv, intracerebroventricular; KOR-1, kappa opioid receptor clone; KOR-3/ ORL1, kappa -3 -like opioid receptor clone; M6G, morphine-6-glucuronide; MOR-1, mu opioid receptor clone; MS ODN, missense oligode- oxynucleotide; NTII, naltrindole isothiocyanate; NBNI, nor-binaltorphamine. 0022-3565/98/2872-0538$03.00/0 THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS Vol. 287, No. 2 Copyright © 1998 by The American Society for Pharmacology and Experimental Therapeutics Printed in U.S.A. JPET 287:538 –544, 1998 538 at ASPET Journals on October 13, 2016 jpet.aspetjournals.org Downloaded from