Fentanyl-related 4-heteroanilido piperidine OHM3295 augments splenic natural killer activity and induces analgesia through opioid receptor pathways

Recently, the fentanyl-related compound OHM3295 has been shown to induce a naltrexone-sensitive, dose-related analgesia in CD1 mice. However, unlike morphine or fentanyl, which are potent immunosuppressive drugs, OHM3295 has been found to augment splenic natural killer (NK) activity in a dose-related and naltrexone-reversible manner. The present study investigated the type (delta, kappa or mu) of opioid receptor involved in analgesia and immunomodulation after acute administration of OHM3295. CD1 mice pretreated with β-funaltrexamine (β-FNA, 40.0 mg/kg) showed an insignificant induction of analgesia (8.4 ± 3.7%) after 3.2 mg/kg OHM3295, whereas mice pretreated with vehicle, norbinaltorphimine (10.0 mg/kg) or naltrindole (20.0 mg/kg) exhibited 43.6 ± 12.6% of maximal analgesia, as determined by the tail- flick latency test. Consistent with previous results, acute administration of OHM3295 (3.2 mg/kg) augmented splenic NK activity (20.7 ± 3.4 lyric units [LU]) relative to vehicle-treated mice (8.2 ± 0.7 LU). Pretreatment with β- FNA (40.0 mg/kg) completely blocked (9.0 ± 1.9 LU) OHM3295-mediated augmentation of NK activity, whereas pretreatment with norbinaltorphimine (10.0 mg/kg) partially blocked (15.8 ± 2.2 LU) the drug-induced effect. However, pretreatment with naltrindole (20.0 mg/kg) did not antagonize OHM3295-induced increases in splenic NK activity but rather further enhanced (32.3 ± 4.2 LU) the effect. NK-enriched effector cells from OHM3295-treated mice displayed an increase in conjugation with YAC-1 target cells, an increase in the percent killing of target cells and a significant increase in the number of active killer cells compared with NK-enriched effector cells from vehicle-treated mice. In addition, the expression of the early activation antigen CD69 was elevated on NK1.1⁺ cells from OHM3295-treated mice relative to vehicle-treated mice after incubation with targets. These results implicate kappa and mu opioid receptor involvement in OHM3295- mediated augmentation of splenic NK activity and identify a potential mechanism for this enhancement.