Regulatory peptide involvement in the reproductive biology of flatworm parasites

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Abstract
Summary Parasites are invariably characterized by a prodigious egg output and channel much of their metabolic activity towards reproduction. While certain aspects of reproduction, such as gametogenesis and egg formation, have received attention, surprisingly little is known about the mechanisms regulating reproductive physiology and behaviour. It is likely that these mechanisms will involve neuropeptides because, in the absence of endocrine glands and a circulatory system, it is the secretory (peptidergic) component of the nervous system that will serve as an endocrine system. The last few years have witnessed a growing awareness of the range of peptidergic molecules produced by parasites, a diversity that overshadows the number of transmitters produced by the conventional nervous system. Neuropeptides are distributed throughout the CNS; in addition, peptidergic elements occur in those components of the PNS that innervate the gonads and muscularised ducts of the male and female reproductive systems. Peptidergic cells are also associated with the eggforming chamber, or ootype, and the implications of this for the control of egg formation and as a possible target for chemotherapy are discussed.