title: Functional evidence for physiological mechanisms to circumvent neurotoxicity of cardenolides in an adapted and a non-adapted hawk-moth species.
abstract: Because cardenolides specifically inhibit the Na(+)K(+)-ATPase, insects feeding on cardenolide-containing plants need to circumvent this toxic effect. Some insects such as the monarch butterfly rely on target site insensitivity, yet other cardenolide-adapted lepidopterans such as the oleander hawk-moth, Daphnis nerii, possess highly sensitive Na(+)K(+)-ATPases. Nevertheless, larvae of this species and the related Manduca sexta are insensitive to injected cardenolides. By radioactive-binding assays with nerve cords of both species, we demonstrate that the perineurium surrounding the nervous tissue functions as a diffusion barrier for a polar cardenolide (ouabain). By contrast, for non-polar cardenolides such as digoxin an active efflux carrier limits the access to the nerve cord. This barrier can be abolished by metabolic inhibitors and by verapamil, a specific inhibitor of P-glycoproteins (PGPs). This supports that a PGP-like transporter is involved in the active cardenolide-barrier of the perineurium. Tissue specific RT-PCR demonstrated expression of three PGP-like genes in hornworm nerve cords, and immunohistochemistry further corroborated PGP expression in the perineurium. Our results thus suggest that the lepidopteran perineurium serves as a diffusion barrier for polar cardenolides and provides an active barrier for non-polar cardenolides. This may explain the high in vivo resistance to cardenolides observed in some lepidopteran larvae, despite their highly sensitive Na(+)K(+)-ATPases.
authors.affiliation: Molekulare Evolutionsbiologie, Biozentrum Grindel, Martin-Luther-King-Platz 3, 20146 Hamburg, Germany. georg.petschenka@googlemail.com
authors.forename: Georg
authors.initials: G
authors.lastname: Petschenka
authors.forename: Christian
authors.initials: C
authors.lastname: Pick
authors.forename: Vera
authors.initials: V
authors.lastname: Wagschal
authors.forename: Susanne
authors.initials: S
authors.lastname: Dobler
mesh_terms: D018435:ATP Binding Cassette Transporter, Subfamily B / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D000222:Adaptation, Physiological; D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D004077:Digoxin / Q000378:metabolism*; D020387:Food Chain; D006458:Hemolymph / Q000378:metabolism; D019476:Insect Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D007814:Larva / Q000254:growth & development / Q000502:physiology; D009036:Moths / Q000254:growth & development / Q000502:physiology*; D030007:Nerium / Q000737:chemistry; D009498:Neurotoxins / Q000378:metabolism*; D010042:Ouabain / Q000378:metabolism*; D010525:Peripheral Nerves / Q000502:physiology; D010539:Permeability; D016133:Polymerase Chain Reaction; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism; D013045:Species Specificity
chemical_list: D018435:ATP Binding Cassette Transporter, Subfamily B; D019476:Insect Proteins; D009498:Neurotoxins; D010042:Ouabain; D004077:Digoxin; D000254:Sodium-Potassium-Exchanging ATPase