Effect of melatonin on 24-hour rhythms of ornithine decarboxylase activity and norepinephrine and acetylcholine synthesis in submaxillary lymph nodes and spleen of young and aged rats.

Young (50 days old) and old (18 months old) Sprague-Dawley rats were injected with mycobacterial Freund's adjuvant to produce an inflammatory disease of the joints and were studied the day before, and on days 6, 12 and 18 after injection. At every postinjection interval examined, old rats had significantly lower circadian amplitudes of pineal melatonin content. On day 18 of arthritis development, decreased levels of pineal melatonin were also seen in young rats. A second study, carried out 18 days after the injection of Freund's complete adjuvant and after 17 daily injections of 10 or 100 microg of melatonin in the evening, indicated that melatonin treatment restored the inflammatory response in old rats (assessed plethysmographically in hind paws) to the level found in young animals. In young rats, an inflammation-promoting effect of 100 microg melatonin could be demonstrated. As a consequence of the immune reaction, submaxillary lymph node and splenic ornithine decarboxylase activity (an index of lymph cell proliferation) augmented significantly, with acrophases of 24-hour rhythms in the afternoon for lymph nodes or in the morning for spleen. Mesor and amplitude of ornithine decarboxylase rhythm were lowest in old rats, while melatonin injection generally augmented its amplitude. Lymph node and splenic tyrosine hydroxylase activity (a presynaptic adrenergic marker) reached maximal values during early night hours while maximal values of [3H]acetylcholine synthesis (a presynaptic cholinergic marker) occurred during the afternoon in lymph nodes. Amplitude and mesor of these rhythms were lowest in old rats, an effect generally counteracted by melatonin treatment. The results suggest that inflammation is accompanied by an age-dependent, significant depression of pineal melatonin synthesis during adjuvant-induced arthritis and a decreased amplitude of the circadian rhythm of immune cell proliferation and autonomic activity in lymph nodes and spleen. These effects are counteracted by injection of melatonin, mainly in old rats.