Inhibition of sunfish feeding by defensive steroids from aquatic beetles: structure-activity relationships

The vertebrate hormone deoxycorticosterone is the most commonly occurring component of defensive secretions from aquatic beetles in the family Dytiscidae. Deoxycorticosterone and the structurally related steroids pregn-4-en-20-alpha-ol-3-one and pregn-4-en-20-beta-ol-3-one were tested for their ability to inhibit feeding by bluegill sunfish, Lepomis macrochirus, in laboratory assays. Deoxycorticosterone at oral doses of 660 micrograms (2 X 10(-6) mol) per pellet caused 94% inhibition in the acceptance of artificial food pellets. At the same molar dosage, pregn-4-en-20-alpha-ol-3-one inhibited food consumption by 58%, while its epimer, pregn-4-en-20-beta-ol-3-one, did not significantly inhibit feeding. These results indicate that specific stereochemical conditions must be satisfied for the pregnenes to be noxious to L. macrochirus and suggest the existence of a receptor-ligand interaction. The potency of the three steroids in assays of feeding inhibition contradicts earlier results based on toxicity and anesthetic assays in which fish were immersed in solutions of steroids.