European Journal of Pharmacology, 81 (1982) 459-468 459 Elsevier Biomedical Press IDENTIFICATION OF ~-ENDORPHIN-(6-17) AS THE PRINCIPAL METABOLITE OF DES-TYROSINE-v-ENDORPHIN (DTvE) IN VITRO AND ASSESSMENT OF ITS ACTIVITY IN NEUROTRANSMITrER RECEPTOR BINDING ASSAYS HANS SCHOEMAKER, THOMAS P. DAVIS, NORMAN W. PEDIGO I ANDREW CHEN, ERIC S. BERENS, PAUL RAGAN 2, NICHOLAS C. LING * and HENRY I. YAMAMURA 3 Departments of Pharmacology, Biochemistry and Psychiatry, and the Arizona Research Laboratories, University of Arizona, Health Sciences Center, Tucson, A Z 85724, and * Neuroendocrinology Laboratories, The Salk Institute, La Jolla, CA 92112, U.S.A. Received 28 January 1982, revised MS received 31 March 1982, accepted 13 April 1982 H. SCHOEMAKER, T.P. DAVIS, N.W. PEDIGO, A. CHEN, E.S. BERENS, P. RAGAN, N.C. LING and H.I. YAMAMURA, Identification of fl-endorphin-(6-17) as the principal metabolite of des-tyrosine-y-endorphin (DTyE) in vitro and assessment of its activity in neurotransmitter receptor binding assays, European J. Pharmacol. 81 (1982) 459-468. Des-tyrosine-y-endorphin (fl-endorphin-(2-17); DTyE) lacks direct in vitro activity at dopaminergic receptors, but does inhibit in vivo [3H]spiperone binding in various rat brain areas. The principal objective of these studies was to test the hypothesis that DT'yE may exert its selective, neuroleptic-like activity through an active metabolite. Accordingly, DTyE was incubated at 37°C in a whole rat brain homogenate of neutral pH after which samples were prepared for HPLC analysis. The major, heat-stable metabolite of DTyE was identified as the clinically active, fl-endorphin related fragment, fl-endorphin-(6-17). The fl-endorphin sequences 4-17, 5-17, 10-17, 12-17 and 2-16 were also present but in minor amounts. Identical results were obtained studying DTy E metabolism using rat striatal tissue slices. Neurotransmitter receptor binding experiments showed that fl-endorphin-(6-17) was inactive at central dopaminergic, serotonergic, muscarinic, benzodiazepine and opiate receptors measured in vitro. Thus, like DTyE, fl-endorphin-(6-17) differs from classical neuroleptics in that it does not inhibit in vitro [3H]spiperone binding in the corpus striatum, frontal cortex or mesolimbic areas of the rat brain. It may be that DTyE and fl-endorphin-(6-17) exert their selective neuroleptic-like activity through an indirect inhibition of central dopaminergic activity, possibly in combination with an in vivo antagonism of the postsynaptic dopamine receptor. High performance liquid chromatography (HPLC) Neuroleptics Des-tyrosine-'t-endorphin Endorphin metabolism Dopamine 1. Introduction Recent interest has been focused on the fl-en- dorphin (flE)-related fragments "t-endorphin (fiE- (I-17); yE) and des-tyrosine-y-endorphin (fiE-(2- t Present address: Department of Pharmacology, Louisiana State University Medical Center, 1100 Florida Avenua, New Orleans, LA 70119, U.S.A. 2 Present address: National Naval Medical Center, Depart- ment of Psychiatry C-93, Bethesda, MD 20014, U.S.A. 3 To whom all correspondence should be addressed: Depart- ment of Pharmacology, University of Arizona, Tucson, Arizona 85724, U.S.A. 0014-2999/82/0000-0000/$02.75 © 1982 Elsevier Biomedical Press 17); DTyE). These fragments, and others such as a-endorphin (fl-(1-16)), may be involved in vari- ous aspects of behavioral adaptation (De Wied et al., 1978a,b; Van Ree et al., 1980). Several yE related fragments, and most notably DTyE, have shown neuroleptic-like activity in a variety of be- havioral tests such as the extinction of active and passive avoidance behavior (De Wied et al., 1978b), the attenuation of ventral tegmental (Dorsa et al., 1979) and nucleus accumbens (Van Ree and Otte, 1980) self-stimulation behavior, and the inhibition of methylphenidate induced locomotor activity (Davis et al., 1981). Clinical studies also indicate