Circadian disorganization in experimental arthritis.

This review discusses the experimental evidence indicating that arthritis disrupts circadian organization, which was mainly derived from animal studies employing Freund's complete mycobacterial adjuvant (FCA). The defense response to antigenic challenge, mediated in part by cytokines, includes changes in chronobiological central nervous system function, like depressed daily activity, superficial sleep or anorexia. Interferon (IFN)-gamma receptors are detectable in the central circadian pacemaker, the hypothalamic suprachiasmatic nuclei, at a time when the capacity for photic entrainment of the pacemaker became established. The disruptive effects of the systemic injection of IFN on the circadian rhythms of locomotor activity, body temperature and clock-gene mRNA expression have been documented. In the last few years we have examined a number of immune and neuroendocrine circadian rhythms in FCA-injected rats, both in the preclinical phase of arthritis (2-3 days after FCA injection) as well as in the acute phase of the disease (18 days after FCA injection). In arthritic rats, the 24-hour organization of immune and neuroendocrine responses becomes altered. A hormonal pathway involving the circadian secretion of melatonin and a purely neural pathway including, as a motor leg, the autonomic nervous system innervating the lymph nodes were identified. The significant effects of the immune-mediated inflammatory response on the diurnal rhythmicity of adenohypophysial and hypophysiotropic hormones occurred in arthritic rats. Melatonin treatment prevented the alteration in 24-hour rhythms of serum ACTH, prolactin and luteinizing hormone in rats injected with FCA. In addition, melatonin pretreatment prevented the alteration in the 24-hour variation in hypothalamic serotonin and dopamine turnover during the preclinical phase of Freund's adjuvant arthritis in rats. Some pinealectomy-induced immune changes in arthritic rats were also prevented by physiological concentrations of melatonin. Melatonin may play the role of an 'internal synchronizer' for the immune system.