Multiple reports indicate interactions between opioids and the immune
system. Opioids call have immunosuppressive or immunoenhancing effects
depending on the in vivo or the in vitro models used. Moreover, repor
ted in vivo effects of opioids on immune responses may be direct or in
direct. Pharmacological studies aimed at characterizing opioid recepto
rs on immune cells have brought confusing and controversial data, due
in part to the low number of receptors on these cells and also to the
presence of ''non-opioid'' receptors, i.e. receptors that cannot be bl
ocked by ''classical'' opioid ligands such as naloxone. The first mole
cular cloning of an opioid receptor was reported recently for the mous
e delta receptor (1,2). Subsequently, the cDNAs for the mu and kappa r
eceptors were cloned by several groups. We used a reverse-transcriptas
e-polymerase chain reaction (RT-PCR) strategy for detecting expression
of the three cloned opioid receptors on immune cells. The human immun
e cells tested express no or low level of message for the delta and ka
ppa receptors and undetectable mRNA for the mu receptor. The murine im
mune cells tested show variable levels of message for the delta recept
or and undetectable mRNA for the mu and kappa receptors.