Presence and effects of melatonin in Trypanosoma cruzi

The unicellular organism Trypanosoma cruzi is an eukaryote whose cell cycle mainly occurs under darkness in the insect gut. The unique external phase corresponds to the metacyclic forms, the forms that are able to infect huma ns, which appear within the insect deyections. Thus, light may be a powerfu l stressor in this unicell. Epimastigote forms (the parasite forms that gro w and transform to metacyclic forms in the insect gut) of Trypanosoma cruzi grow normally when cultured in a LD cycle of 0:24 hr, reaching exponential growth by the 7th day. A pulse of 2 hr of light (LD 2:22) was enough to bl ock the growth of the epimastigotes, an effect that was correlated with the expression of heat-shock proteins during the first 120 min of light exposu re. Thereafter, protein synthesis decreased. Light exposure of metacyclic, forms also inhibits the parasitization ability. It is known that light regu lates the production of melatonin in most animal species studied, including other unicells such as dinoflagellates. T. cruzi contains and synthesizes melatonin and, thus, light-mediated events on the parasite biological cycle could be mediated by light-induced changes in melatonin produced by this u nicell. Epimastigotes cultured under continuous darkness produce melatonin over the 24 hr period in a biphasic manner. Coinciding with the melatonin p eaks, there was high melatonin efflux from the parasite into the medium. Ep imastigotes cultured for 7 days under a LD cycle of 2:22 hr showed a 55% re duction in melatonin content, although this reduction seems not to be relat ed with the growth delay: In fact, incubation of epimastigotes with exogeno us melatonin (1 pM) did not affect parasite growth, but significantly reduc ed their transformation into metacyclic forms by the 7-8th day of treatment . Thus, the light-dependent decrease in melatonin production by the unicell may be responsible, at least partially, for the light-induce parasitizatio n inhibition. Moreover, melatonin production is highest in the metacyclic f orms. These data support a link between light, melatonin production and par asitization ability of T. cruzi and suggest the participation of the indole amine in its biological cycle.