Serotonin and melatonin inhibit phytohemagglutinin- (PHA) induced interferon-gamma (IFN-gamma) production by lymphocytes. In this paper, it is shown that IFN-gamma-increased tryptophan uptake by lymphocytes and macrophages led to an enhanced production of serotonin. When IFN-gamma and serotonin were added together to a lymphocyte culture, N-acetyl serotonin and melatonin production was increased, whereas the path to 5-hydroxy-indoleacetic acid remained unchanged. Therefore, the stimulated IFN-gamma production of serotonin and melatonin by lymphocytes and macrophages and the inhibition of IFN-gamma synthesis by these indoleamines suggest a hypothesis for an immunoregulatory circuit.

The cellular bases of the powerful cytolytic activity of the human protozoan parasite Entamoeba histolytica were explored by studying the effect of the virulent strain HM1:IMSS on epithelial monolayers of MDCK cells using a combination of time-lapse microcinematography and transmission and scanning electron microscopy. Early alterations of the epithelial cell membranes were detected by measuring changes in the transepithelial electrical resistance of MDCK monolayers mounted in Ussing chambers. The aggressive mechanism of E. histolytica trophozoites was found to be a complex, multifactorial phenomenon that included hit-and-run damage to the plasma membrane of effector cells mediated through contact, phagocytosis of lysed or apparently intact, but detached, MDCK cells, and intracellular degradation of ingested cells. Following contact with amebas, the epithelial monolayers showed a pronounced lowering of transepithelial resistance, opening of tight junctions, distortion of microvilli, surface blebbing, and the presence of minute focal discontinuities in the plasma membrane. There was no evidence of amebic exocytosis, membrane fusion, or junction formation between the parasite and host plasma membranes. Although modifications in the epithelial cell membranes usually preceded lysis, the cytolytic activity of the parasite did not exclusively involve damage to the plasma membrane of the cultured host cells but also was mediated by avid phagocytosis, the displacement and separation of neighboring cells by means of pseudopodial activity, and the "pinching-off" of the peripheral cytoplasm of epithelial cells.

The formation of blisters is a transient phenomenon that led Dr. Leighton to describe its observation with time-lapse photography in the MDCK monolayer to a "gently boiling oatmeal." One may ask why is it transient and why most areas are momentarily not blistering. The observations discussed in this article indicate that (a) at the blister, junctions are really tight; (b) when transported fluid is allowed to escape through a permeable support, junctions are also tight, but (c) when the support is impermeable the junctions allow ouabain and peroxidase through. This suggests that, if the attachment of the monolayer is strong enough, the accumulation of fluid (Figure 7) bursts the junctions. If, on the contrary, junctions withstand the pressure, factor 3 of Figure 7 prevails over the others, and a blister is formed. In all the rest of the monolayer junctions seem to be open. This is in keeping with the observation by Rabito et al.,27 that if the monolayers are prepared on weakened supports, blisters are much bigger than under control conditions. It also allows the measurement of ionic fluxes and labeling of pumping sites at the basolateral membrane. As a corollary, one may say that some of the factors known to affect blistering 26 may very well act through modifications in the occluding junctions.

Different concentrations of indoleamines, serotonin and melatonin, inhibited phytohemagglutinin stimulated DNA synthesis. Thus, 10(-3) to 10(-4) M of either indoleamine acted at the optimal phytohemagglutinin concentration, while 10(-3) to 10(-7) M acted at suboptimal phytohemagglutinin levels. The serotonin effect was reversed by the serotonergic S1-S2 receptor antagonist methysergide but not by the S2 antagonist ketanserin. This indicates that only the S1 receptor is involved in the inhibitory effect. Inhibition of lymphoproliferation by indoleamines was also exerted on pokeweed mitogen and protein A from Staphylococcus aureus stimulations. Serotonin and melatonin also inhibited phytohemagglutinin and protein A from Staphylococcus aureus induction of interferon-gamma synthesis. The initial uptake of Ca2+ was not affected by indoleamines, suggesting that it is not the mechanism of their inhibitory effects. As interferon-gamma induced tryptophan uptake by T lymphocyte- and macrophage-depleted populations, and tryptophan is the metabolic precursor of serotonin and melatonin, a new immunoregulatory circuit is postulated.

No abstract