PRESENCE OF LINKS BETWEEN ZINC AND MELATONIN DURING THE CIRCADIAN CYCLE IN OLD MICE - EFFECTS ON THYMIC ENDOCRINE ACTIVITY AND ON THE SURVIVAL

Links between zinc and melatonin in old melatonin treated mice with a reconstitution of thymic functions have been recently documented. Conc omitant increments of the nocturnal peaks of zinc and melatonin, with a synchronization of their circadian patterns, are achieved in old mic e after melatonin treatment. A recovery of the nocturnal peaks of thym ulin plasma levels and of the number of thymulin-secreting cells with a synchronization of their circadian patterns are also achieved. The e xistence of significant positive correlations between melatonin and zi nc and between melatonin and thymulin or the number of thymulin-secret ing cells supports the presence of links between zinc and melatonin al so during the circadian cycle with a beneficial effect on thymic funct ions. The altered circadian pattern of corticosteron in old mice is no rmalized by melatonin. The existence of inverse correlations between c orticosteron and melatonin, between corticosteron and zinc and between corticosteron and thymulin or the number of thymulin-secreting cells during the whole circadian cycle, suggests the involvement of glucocor ticoids pathway in the melatonin thymic reconstitution, via zinc. The presence of an interplay among zinc, melatonin, glucocorticoids and th ymulin may be, therefore, supported during the circadian cycle. 'In vi tro' experiments from old thymic explants show a direct action of zinc , rather than melatonin, on thymulin production, further suggesting th at the action of melatonin on the thymic efficiency is mediated by the zinc bioavailability. The beneficial effect of the links between zinc and melatonin on thymic functions during the circadian cycle, may be extended to a prolonged survival in aging, where, however, zinc may be more involved. (C) 1998 Elsevier Science B.V. All rights reserved.