b-Endorphin fragments (DTgE and DEgE) reduce opiate withdrawal in guinea pig ileum Anna Capasso a, *, Alberto Loizzo b a Dipartimento di Scienze Farmaceutiche, Universita ` di Salerno, Via Ponte Don Melillo, 84084 Fisciano, Salerno, Italy b Istituto Superiore di Sanita `, Rome, Italy Abstract The effect exerted by two b-endorphin fragments (DTgE and DEgE) was investigated on the acute opioid dependence induced by m , k and d receptor agonists in vitro. After a 4-min in vitro exposure to morphine (less selective m agonist), DAGO (highly selective m agonist), U50-488H (highly selective k agonist) and b-endorphin (selective m – d agonist), a strong contracture of guinea pig isolated ileum was observed after the addition of naloxone. This effect was also observed when rabbit isolated jejunum was pretreated with deltorphin (highly selective d agonist). DTgE or DEgE injection treatment before or after morphine, DAGO, U50-488H, b-endorphin or deltorphin were able to both prevent and reverse the naloxone-induced contracture after exposure to the opioid agonists in a concentration-dependent fashion. Our results indicate that both DTgE or DEgE are able to reduce significantly opioid dependence in vitro, suggesting an important functional interaction between b-endorphin fragments and opioid dependence induced by m , k and d receptors. D 2001 Elsevier Science Inc. All rights reserved. Keywords: b-Endorphin fragments; Guinea pig ileum; Opioid dependence 1. Introduction A model of dependence can be induced and measured in vitro by using guinea pig ileum (Collier, 1980; Collier et al., 1981; Lujan and Rodriguez, 1981; Chal, 1983, 1986). Tissues from untreated animals, after a brief exposure to opioids, show a strong naloxone-induced contracture (Eisenberg, 1982; Valeri et al., 1990a,b, 1992; Morrone et al., 1990, 1993) indicating that cellular mechanisms of dependence may occur very rapidly fol- lowing occupation of receptors and that these mechanisms are operative within the myenteric plexus (North and Karras, 1978). Until a few years ago, it was generally agreed that m opioid receptors played a central role in the development of opiate dependence (De Launder et al., 1984; Gmerek and Woods, 1985), whereas the involvement of d and k opioid receptors was overlooked largely because of the lack of specific agonists and antagonists. Recent evidence, however, indicates that d and k as well as m receptors are involved in the development of opiate dependence both in vivo and in vitro (Gmerek et al., 1985; Valeri et al., 1990b, 1992; Abelhamid et al., 1991). Recently, several studies have focused on the possible interference exerted by some b-endorphin derivatives (i.e., DTgE and DEgE) on the opioid system (Van Ree et al., 1978; Pedigo et al., 1980; Schoemaker and Nickolson, 1980). Available data regarding this subject in the literature are few and controversial. It has been reported that the removal of the N-terminal amino acid residue tyrosine from g-endorphin resulting in DTgE leads to a loss of opioid activity, with a loss of capacity to interfere with opioid receptors (De Wied et al., 1978; Neil et al., 1981; Pieretti et al., 1993). On the other hand, Neil et al. (1981) reported that chronic systemic DTgE treatment in rats increased mor- phine analgesia, resembling the effect observed after chronic naloxone exposure (Tang and Collins, 1978; Bardo and Miller, 1984). 0278-5846/02/$ – see front matter D 2001 Elsevier Science Inc. All rights reserved. PII:S0278-5846(01)00274-3 Abbreviations: DAGO; D-Ala 2 -N-methyl-Phe-Gly 5 -ol)enkephalin; DEgE; Des-enkephalin-g-endorphin; DTgE; Des-tyrosine-g-endorphin; U50-488H; trans(±)-3,4-Dichloro-N-methyl-N-(2(1pyrrolidynyl) cyclo- hexyl)-benzeneacetamid * Corresponding author. Tel./fax: +39-89-964357. E-mail address: annacap@unisa.it (A. Capasso). Progress in Neuro-Psychopharmacology & Biological Psychiatry 26 (2002) 313 – 319