Effect of melatonin on 24H changes in plasma protein levels during the preclinical phase of Freund's adjuvant arthritis in rats

The 24h rhythms in plasma protein concentration were examined in rats on th e third day after injection of Freund's complete adjuvant or adjuvant's veh icle, performed 3h after light on. In rats treated with adjuvant's vehicle, peak values of albumin and gamma,globulin occurred during the nocturnal ac tivity span (P < .02 and P < .0001, respectively), while those of alpha-1, alpha-2, and beta globulins were found late during the rest span (P < .002, P < .0001, and P < .0004, respectively). Freund's adjuvant administration abolished temporal changes in plasma albumin and beta globulin levels. It a lso decreased the amplitude of daily changes in alpha-1 and alpha-2 globuli n (P < .05) and diminished mean values of alpha-2 globulin (P < .01). Pretr eatment of rats with melatonin (30 mug daily) for 11 days, 11h after light on, counteracted mycobacterial adjuvant-induced suppression of the 24h rhyt hms in albumin and alpha-1, alpha-2, and beta globulins. The results furthe r support the existence of preventive properties of a pharmacological dose of melatonin in situations in which a lost of circadian rhythmicity is expe cted.