Flatworm neuropeptides — present status, future directions

Abstract

Peptides represent a highly-diverse group of multipurpose chemical messengers produced by the nervous, endocrine and immune systems. In the nervous system, peptides function as neurotransmitters or neuromodulators, modifying the action of other transmitter substances. Neuronal peptides can also serve as blood-borne neurohormones. In the endocrine system, peptides function as classical blood-borne hormones or as localised paracrine mediators. In the immune system, peptides serve immune functions. The bidirectional communication between the immune and neuroendocrine systems allows immunologic peptides to play hormonal roles and neuroendocrine peptides to serve immunoregulatory functions. The fact that the three systems share the same peptide molecules gives rise to highly versatile and complex mechanisms that are involved in the maintenance of homeostasis. A more detailed discussion of regulatory peptides is beyond the scope of this brief review. For further information, the reader is referred to the many excellent reviews and texts in this field, including Blalock (1989), Holmgren (1989), Polak (1989) and Turner (1987).