ELSEVIER Regulatory Peptides 62 (1996) 113-118 REGULATORY PEPTIDES Chronic intracerebroventricular administration of/3-endorphin augments natural killer cell cytotoxicity in rats IngibjSrg H. Jonsdottir a,*, Catarina Johansson a, Alexzander Asea b, Kristoffer Hellstrand b, Peter Thor6n a Pavel Hoffmann a Institute qf Physiology and Pharmacology, Department of Physiology, Giheborg University, Medicinaregatan 11, S 413 90, Giiteborg, Sweden b Department of Clinical Virology, GOteborg UniL, ersity, Giiteborg, Sweden Received 26 July 1995; revised 19 December 1995; accepted 19 December t995 Abstract We have studied the effect of chronic intracerebroventricular (i.c.v.) infusion of different opioid peptides on natural killer (NK) cell mediated cytotoxicity in vivo in the spontaneously hypertensive rat (SHR). The in vivo NK cell activity was measured as the clearance of 5~ Cr-labelled YAC-1 lymphoma cells from the lung tissues. Further, the pbenotype of lymphocytes in spleen and peripheral blood was analysed by flow cytometry (FACS). All opioid drugs were administered i.c.v, for 6 days with osmotic minipumps releasing 1.0 /xl/h. /3-Endorphin (10 or 20 /zg/rat per day) significantly increased NK cell cytotoxicity in vivo. The opioid receptor antagonist naloxone (10 mg/kg, i.p.) given immediately before the injection of YAC-lymphoma cells, completely abolished the effects of i.c.v, administered /3-endorphin. Corresponding doses of /3-endorphin administered subcutaneously (s.c.) with minipumps for 6 days did not significantly affect NK cell cytotoxicity. Neither Leu- or Met-enkephalin (20 /zg/rat per day) nor dynorphin (20 /xg/rat per day) administered i.c.v. had any significant effects on NK cell activity. In /3-endorphin treated SHR, the percentage of cells with NK cell phenotype (OX52+/CD5 -) in peripheral blood was not significantly different from that of controls, while the percentage of cells with T cell phenotype (CD5+/OX52) was significantly decreased. The percentage of splenic NK cells (OX52+/CD5) and T cells (CD5 +/OX52-) was also unchanged by/3-endorphin treatment i.c.v. These results suggest that of the opioid peptides administered i.c.v., only /3-endorphin augments in vivo NK cell mediated cytotoxicity. We thus conclude that these effects most probably are centrally and opioid receptor mediated effects, since /3-endorphin in the same dose administered peripherally does not influence in vivo NK cell cytotoxicity. Keywords: SHR: Naloxone; Enkephalin; Dynorphin 1. Introduction Natural killer (NK) cells are a subpopulation of lym- phocytes which are spontaneously cytotoxic to a variety of turnout cells and virus-infected cells [1,2]. Numerous stud- ies show that endogenous opioids can augment NK cell cytolytic activity in vitro [3-6], but it has also been shown that both /3-endorphin and met-enkephalin can inhibit NK cell activity in vitro [7]. A connection between the central nervous system and the immune system has been sug- gested by many authors and opioid peptides seem to play a role in this linkage [8-12]. Effects of acute central mor- phine injections on NK cell cytotoxicity have been re- * Corresponding author. Tel.: +46 31 7733527; fax: +46 31 7733512. 0167-0115/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved Pll $0167-0115(96)00007-9 ported to suppress NK cell activity, which could be blocked with the opioid receptor antagonist naloxone [13]. Simon et al. reported that an acute injection of/3-endorphin into the nucleus raphe magnus facilitated metastatic turnout growth [14]. We have recently shown that NK cell cytotoxicity is enhanced in the spontaneously hypertensive rat (SHR) after voluntary chronic exercise for 5 weeks and in the same study we used anti-asialo GM1 antibody against NK cells to validate the involvement of NK cells in the clear- ance of the NK cell sensitive YAC-1 lymphoma cells from the lungs. (Jonsdottir et al., submitted). In this exercise model, it has been shown that both /3-endorphin levels and dynorphin-converting enzyme activity are increased in cerebrospinal fluid after voluntary chronic exercise [ 15,16]. Endogenous opioids are released in response to exercise both in animals and in humans [17,18] and it has been suggested that endogenous opioids might play a major role