Profile of the ecdysteroid hormone and its receptor in the salivary gland of the adult female tick, Amblyomma hebraeum

We examined the hemolymph ecdysteroid titer (by radioimmunoassay) and profile of the ecdysteroid receptor (EcR/USP; by 3H]ponasterone A binding, gel mobility shift assay, Western blot) in the salivary gland of the ixodid tick, Amblyomma hebraeum Koch (Acari: Ixodidae) throughout the tick feeding period and first 6 days post-engorgement. Throughout the slow phase of feeding, the hemolymph ecdysteroid titer was approximately 18 pg/microliter. The titer peaked at approximately 52 pg/microliter during the rapid phase of feeding, falling back to approximately 22 pg/microliter on the day of engorgement. Ecdysteroid titer rose again to approximately 750 pg/microliter by day 6 post-engorgement. EcR was undetectable by any of the three assays in unfed ticks. Following the onset of feeding, there appeared both specific ponasterone A binding and two major EcR bands detected by Western blot analysis. Both measurements were sustained throughout the feeding period, but declined after detachment when the salivary glands were degenerating. After ticks reached about 100 mg (by which time most females are mated), a discrete DNA-binding band was shown by gel mobility shift assay using Drosophila hsp27 EcRE as a probe. Moreover, the band intensified when hemolymph ecdysteroid titer reached its peak during the rapid phase of feeding; it declined along with decreasing EcR/USP levels, and with specific ligand binding activity following engorgement. This study suggests a role for the small hemolymph ecdysteroid peak during the rapid phase of feeding in initiating salivary gland degeneration.