Basal activity of the hypothalamo-pituitary-interrenal (HPI) axis changes over development in larval amphibians, but development of the responsiveness of this axis to an external stressor has not been studied. We compared developmental changes in whole-body corticosterone content of two anuran amphibian species, Rana pipiens (family Ranidae) and Xenopus laevis (family Pipidae). We also examined developmental changes in the responsiveness of the HPI axis by subjecting tadpoles of different developmental stages to a laboratory shaking/confinement stress and to ACTH injection. We measured whole-body corticosterone content as an indicator of the activity of the HPI axis. Whole-body corticosterone content of R. pipiens remained low during premetamorphosis and prometamorphosis but increased dramatically at metamorphic climax and remained elevated in juvenile frogs. By contrast, whole-body corticosterone content of X. laevis was highest during premetamorphosis, declined at the onset of prometamorphosis, increased at metamorphic climax and remained at climax levels in juvenile frogs. Premetamorphic and prometamorphic tadpoles of both species showed strong corticosterone responses to both shaking stress and ACTH injection. The magnitude and pattern of response differed among developmental stages, with premetamorphic tadpoles of both species showing greater responsiveness to stress and ACTH. Our results show that interrenal responsiveness is developed in premetamorphic tadpoles, suggesting that at these stages tadpoles are capable of mounting an increase in stress hormone production in response to changes in the external environment. Our results also highlight the importance of comparative studies in understanding the development of the stress axis.
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