Eicosanoid is an umbrella term for all of the biologically active metabolites of arachidonic and two other C20 polyunsaturated fatty acids (PUFAs). Insect scientists are most familiar with the prostaglandins (PGs; a glossary of abbreviations is presented in Table 1), which make up one of several groups of eicosanoids. PGs are responsible for release of egg-laying behavior in a few insect species (Stanley-Samuelson andLoher, 1986), and PGs and certain other eicosanoids appear to mediate hemocytic immune responses to bacterial infections in insects (Stanley-Samuelson et al., 1991). PGs were recently implicated in regulation of Malpighian tubule physiology (Petzel andStanley-Samuelson, 1992; Petzel, 1993). The significance of our increasing knowledge of eicosanoids is that they represent a tier of regulatory physiology in invertebrates that has largely gone unrecognized hitherto. On that basis, we gain from our studies of eicosanoids a more complete understanding of the complexity of regulation of invertebrate physiology. Because most of our background knowledge of eicosanoids arises from work on mammalian physiology and pathophysiology, we also gain a broader appreciation of comparative physiology.
As an index of the growing awareness of the importance of eicosanoids in insects and other invertebrates, this and the following two chapters in this volume are devoted to the subject. In this chapter, I shall present an overview of the nomenclature, structures and biology of eicosanoids in insects and selected other invertebrates. I pay only passing attention to two major areas, the roles of PGs in Malpighian tubules and arachidonate metabolism in ticks and tick salivary glands, which are covered in detail in the following two chapters. After a general discussion of the roles of eicosanoids in insects, I consider the current state of our understanding, compared to the mammalian model of eicosanoid action.
Brady (1983) reviewed the very early findings on PGs in insects; Bundy (1985) discussed non-mammalian sources of PGs; Stanley- Samuelson andLoher (1986) reviewed the details of PGs in insect reproduction, which remains contemporary. In the last several years, I articulated the idea of a comparative eicosanoid physiology in invertebrates (Stanley-Samuelson, 1987; 1991). Two reviews present a broader focus with respect to fatty acid metabolism and eicosanoids in insects (Stanley-Samuelson et al., 1988; Blomquist et al., 1991). A comprehensive review of eicosanoids in insects is forthcoming, which will cover fatty acid nutritional requirements and their relationship to eicosanoids, a detailed history of the field, and a discussion of the ecological significance of eicosanoids (Stanley-Samuelson, 1993). These topics are not covered in this chapter.