Developmental changes in interrenal responsiveness in anuran amphibians

Basal activity of the hypothalamo-pituitary-interrenal (HPI)axis changes over development in larval amphibians, but developmentof the responsiveness of this axis to an external stressor hasnot been studied. We compared developmental changes in whole-bodycorticosterone content of two anuran amphibian species, Ranapipiens (family Ranidae) and Xenopus laevis (family Pipidae).We also examined developmental changes in the responsivenessof the HPI axis by subjecting tadpoles of different developmentalstages to a laboratory shaking/confinement stress and to ACTHinjection. We measured whole-body corticosterone content asan indicator of the activity of the HPI axis. Whole-body corticosteronecontent of R. pipiens remained low during premetamorphosis andprometamorphosis but increased dramatically at metamorphic climaxand remained elevated in juvenile frogs. By contrast, whole-bodycorticosterone content of X. laevis was highest during premetamorphosis,declined at the onset of prometamorphosis, increased at metamorphicclimax and remained at climax levels in juvenile frogs. Premetamorphicand prometamorphic tadpoles of both species showed strong corticosteroneresponses to both shaking stress and ACTH injection. The magnitudeand pattern of response differed among developmental stages,with premetamorphic tadpoles of both species showing greaterresponsiveness to stress and ACTH. Our results show that interrenalresponsiveness is developed in premetamorphic tadpoles, suggestingthat at these stages tadpoles are capable of mounting an increasein stress hormone production in response to changes in the externalenvironment. Our results also highlight the importance of comparativestudies in understanding the development of the stress axis.