A novel neuropeptide–endocrine interaction controlling ecdysteroid production in ixodid ticks

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Abstract
Ixodid (hard) ticks are blood–feeding arthropods that require a blood meal to complete each stage of development. However, the hormonal events coordinating aspects of feeding and development are only poorly understood. We have delineated a new neuropeptide–endocrine interaction in the adult tick, Amblyomma hebraeum, that stimulates the synthesis of the moulting hormones, the ecdysteroids. In adult female ticks, ecdysteroid synthesis could be demonstrated in integumental tissue incubated in vitro with a synganglial (central nervous system) extract, but not in its absence. Stimulation by the synganglial extract is both time– and dose–dependent, but is completely abolished by trypsin treatment, suggesting that the activity is due to a peptide/protein. Integumental tissue ecdysteroidogenesis is also stimulated by elevation of the cAMP concentration using forskolin and 3–isobutyl–1–methyl–xanthine, or by 8–bromo–cAMP. This suggests the involvement of at least a cAMP second messenger system in the neuropeptide–ecdysteroidogenesis axis, without precluding a role for other second messengers as well. Despite involving a quite different steroidogenic tissue, the foregoing system has some parallels with the known prothoracicotropic hormone (neuropeptide)–prothoracic gland endocrine axis of insects.