Effects of melatonin on 24-h rhythms of neuroendocrine and immune changes in Freund's adjuvant-induced arthritis

Old rats had significantly lower values of the amplitude of pineal melatonin content at any time point studied. Complete Freund's adjuvant administration reduced the amplitude of melatonin rhythm. On day 18 of arthritis development, decreased pineal melatonin contents were also seen in young rats. On day 18, a global depressive effect of arthritis development on pineal melatonin content was found. However, prolactin levels increased in old as compared to young rats. Freund's adjuvant administration brought about changes in the secretory pattern of prolactin in young and old animals that are opposite to those described for melatonin. Cell proliferation as assessed by ornithine decarboxylase activity exhibited significant 24-h variations with maximal activity of the enzyme during daily hours, acrophases varying from 13:15 to 16:52 h (lymph nodes). The mesor and amplitude of ornithine decarboxylase activity were lowest in old rats. Melatonin (100 &mgr;g) augmented amplitude of daily rhythm in submaxillary lymph node ornithine decarboxylase activity significantly. These data indicate that melatonin and prolactin change differentially during arthritis development in young and old animals, perhaps indicating opposite effects in the induction and maintenance of adjuvant-arthritis. The changes are compatible with the values of ornithine descarboxylase activity.