Morphine-Induced Suppression of Cytotoxic T Lymphocyte Activity in Alloimmunized Mice Is Not Mediated through a Naltrindole-Sensitive Delta Opioid Receptor

Abstract

The effect of chronic morphine exposure on natural killer (NK) activity in vivo and the generation of cytotoxic T lymphocytes (CTLs) in vitro and in vivo was investigated. Chronic exposure to morphine (10^–5–10^–11 M) in vitro had no effect on the generation of antigen-driven effector cells. However, the daily administration of morphine (50.0 mg/kg, s.c.) into alloimmunized mice (C57BL/6 into C3H/HeN) for 11 days resulted in a decrease in peritoneal and splenic CTL activity but not splenic NK activity. In addition, there was a 60% decrease in the number of thymocytes recovered from chronic morphine-treated mice compared to vehicle-treated controls. However, the overall percentage of CD4+CD8-, CD4-CD8+ and CD4+CD8+ thymocytes did not change between the two groups of treated animals. Pretreatment of the mice with the δ₁-selective antagonist, (E)-7-benzylidine-7-dihydronaltrexone (BNTX, 0.6 mg/kg, s.c.) did not block morphine-mediated suppression of splenic CTL activity but did block morphine-induced suppression of peritoneal lymphocyte CTL activity. In addition, BNTX pretreatment alone augmented splenic NK activity and such augmentation was blocked following chronic morphine exposure. In contrast, the 5-selective antagonist, naltrindole (20.0 mg/kg, s.c), had no effect alone nor antagonized the action of morphine on CTL activity. Splenic CTL effector cells from either treated group of animals lysed their target (EL-4 lymphoma) through a Ca²⁺-dependent mechanism. Collectively, the results indicate morphine suppresses CTL activity through an indirect pathway, insensitive to naltrindole rather than through direct lymphocyte opioid receptors.