The proliferative response of human or rat T lymphocytes to phytohemag
glutinin (PHA) or concanavalin A (ConA) was measured after acute (30 m
in) or chronic (8 days) treatment with the opiate receptor antagonists
naloxone or naltrexone. Both in the rat and in the human, proliferati
on was significantly enhanced by acute treatment with the opiate recep
tor antagonists. In contrast, after chronic treatment proliferation al
ways decreased. The sudden removal of an opioid inhibitory tone might
be the basis for the increased proliferative responses observed after
acute treatment. The decrease after chronic treatment could be ascribe
d to the amplification of the inhibitory effect of endogenous opioids
due to the up-regulation of opiate receptors that follows chronic anta
gonist administration. Receptor binding studies of beta-endorphin rece
ptors on splenocytes of chronically naloxone treated rats confirmed th
is hypothesis: a higher number of beta-endorphin receptors were expres
sed on splenocytes of naloxone-treated rats compared to controls (B(ma
x) = 9.8 x 10(-12) VS. 1.1 6 x 10(-12), respectively).