Evidence for an opioid inhibitory effect on T cell proliferation

The proliferative response of human or rat T lymphocytes to phytohemagglutinin (PHA) or concanavalin A (ConA) was measured after acute (30 min) or chronic (8 days) treatment with the opiate receptor antagonists naloxone or naltrexone. Both in the rat and in the human, proliferation was significantly enhanced by acute treatment with the opiate receptor antagonists. In contrast, after chronic treatment proliferation always 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 ascribed to the amplification of the inhibitory effect of endogenous opioids due to the up-regulation of opiate receptors that follows chronic antagonist administration. Receptor binding studies of β-endorphin receptors on splenocytes of chronically naloxone treated rats confirmed this hypothesis: a higher number of β-endorphin receptors were expressed on splenocytes of naloxone-treated rats compared to controls (B_max = 9.8 × 10^-12 vs. 1.16 × 10^-12, respectively).