The Role of the Pineal in Mediating Photoperiodic Control of Growth and Division Synchrony and Capillary Sequestration of Plasmodium berghei in Mice

Abstract

Control over growth and division synchrony as well as vascular capture and release of late growth forms is shown to be mediated by the pineal body. Vascular trapping of these late growth forms seems to be most intense in the liver, spleen, and bone marrow. Since we observed these older parasites adhering to the vascular wall, vascular trapping is probably by a mechanism other than phagocytosis. Certain types of photoperiodic rhythm have been shown to influence the growth of Plasmodium berghei, a malarial parasite of rodents. Two effects of the photoperiodic rhythm have so far been observed. The first is the regulation of growth and division synchrony, and the second appears to be the tendency of segmenting parasites to sequester or to be sequestered in certain parts of the vascular system (Arnold, Lalli, and Martin, 1969). Growth and division of malarial parasites of many species have been entrained to 1-, 2-, and 3-day patterns, primarily by light rhythms. Each species of host and parasite has special characteristics. Infection of mice by P. berghei is a particularly interesting example of synchrony because the generation time of the parasite is 24 hr, and thus fits a diurnal pattern (Arnold et al., loc. cit.). One of the more interesting aspects of the problem of synchrony in P. berghei is that light rhythms require specific periods of light and dark exposure for effective augmentation of growth and division synchrony. As we have previously shown only certain intensities and spectra of light produce this synchronizing effect on parasite growth. In addition to the process of growth and division synchrony, the photoperiodic rhythm Received for publication 19 July 1968. * Supported by U. S. Army Contract No. DA-49193-MD-2545 from the U. S. Army Research and Development Command Office of the Surgeon General. This paper is contribution No. 405 from the Army Research Program on Malaria. appears to modify the sequestration of segmenting parasites in the vascular system. This phenomenon is frequently observed with several species of malarial parasites in addition to P. berghei in mice. A considerable amount of morbidity and mortality of the host, for instance, is produced by capillary blockade caused by the sequestration of mature schizonts of P. falciparum in man (Rigdon and Fletcher, 1945; Edington, 1967). For the most part, vascular sequestration of the parasite involves its older dividing asexual forms (Alger, 1963). This process of sequestration has been called margination by some authors because these older parasites appear to stick to the walls of the capillaries. It seems likely that once an erythrocytic form of the parasite is phagocytized, it is destroyed. Therefore, the capture and release of living parasites by the vascular endothelium would require a mechanism other than that of phagocytosis since these parasites are not destroyed. The capture and release of parasites by the vascular system is accentuated by the photoperiodic mechanisms. For example, we showed in a previous study that the segmenting forms in a synchronous infection were largely absent from the circulating blood at 3 AM when the light intensity was under 100 foot candles (ft-c). Normally, at 3 AM we would expect a maximum number of mature schizonts. When the light intensity was raised to 1,000 ft-c, sequestration seemed to be complete again. In this present study, we hope to