| The regulatory mechanisms that govern planarian regeneration have generated considerable research interest; yet the exact nature of the mechanism is not well understood. Much circumstantial evidence indicates that regeneration occurs under the influence of the trophic substance mediated by nerves. Although chemical in nature, the trophic agent (or agents) is not known. Some insight into this problem has been gained by the study of the action of selected cholinergic, adrenergic and serotonergic agonists and antagonists on regeneration. Of these only the serotonergic antagonist, methysergide maleate (MSM), had any significant effect. Placed in the ambient water (22(DEGREES)C), 2.1 x 10('-5) M MSM retarded Dugesia dorotocephala regeneration by three days as judged by the appearance of eyespots. Using formaldehyde-induced fluorescence, the presence of serotonin was detected on the second day post transection during the regeneration process. However, this early appearance was retarded by two days in regenerating worms treated with MSM. This delay along with the three-day overall delay in the appearance of eyespots may be explained by the competitive blocking action of MSM. The blocking of serotonin at the time of transection appears to delay the activation of the adenylate cyclase and the subsequent increase in cyclic AMP levels necessary to initiate cell differentiation. A radio-immunoassay study clearly demonstrated that cyclic AMP levels in MSM-treated blastemas were less than those in untreated controls. A comparison of MSM-treated and untreated blastemal tissue prepared by the zinc iodide-osmium technique revealed a well-developed network of nerve structures in the untreated tissue. MSM-treated tissue at comparable stages exhibited a paucity of nerve structures and those present which did stain, did so less intensely. The reduction of osmium during fixation is accompanied by the oxidation of amine groups. Staining occurs because of the formation of an osmium complex at the sites of cellular biogenic amines in the nervous tissue. The lower intensity stain may indicate that there are lower concentrations of the biogenic amines, including serotonin, in the developing nerve structures of the blastemas retarded by MSM treatment. The data strongly suggest that serotonin is a necessary agent in the regenerative scheme of events. |
