Effects of chronic lithium on enkephalin systems and pain responsiveness

We have recently undertaken an extensive examination of the effects of chronic dietary lithium treatment on levels of brain leucine-enkephalin immunoreactivity (1-enk-IR), release of endogenous 1-enk-IR from globus pallidus prisms in vitro and behavioral responsiveness to pain. Two LiCl containing diets were used. Rats on the lower strength diet attained brain Li levels of 0.40-0.55 mEq/L, while those on the higher strength diet attained levels of 0.70-1.0 mEq/L. In one series of experiments, we sought to relate alterations of K+-stimulated, Ca++-dependent release of 1-enk-IR to alterations of the content of the peptide. After 1 week on the lower strength diet, neither measure was affected in any of the brain regions examined. Following 2 or 3 weeks of feeding with the lower strength diet, 1-enk-IR levels in the globus pallidus and nucleus accumbens were elevated. However, 3 weeks of the high strength Li diet did not lead to alterations of 1-enk-IR content. In contrast, the release of 1-enk-IR was potentiated but only in subjects in which the brain lithium exceeded a threshold level. In another series of studies, we observed that hot-plate escape latency was significantly elevated in rats fed the high strength Li diet for 3 weeks. Also, the Li-treated animals had a greater morphine-induced elevation of escape latency than controls; this effect was less effectively blocked by naltrexone. These findings suggest that chronic exposure to Li leads to transiently elevated levels of 1-enk, and, when brain Li levels are greater than 0.5 mEq/L, to a potentiation of endogenous enkephalin release. The analgesia in Li-treated subjects may eventually be related to these influences of the anti-manic drug on enkephalinergic neurotransmission.