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First published online August 9, 2007
Journal of Experimental Biology 210, 2859-2865 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.005348

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Corticosterone modulation of reproductive and immune systems trade-offs in female tree lizards: long-term corticosterone manipulations via injectable gelling material

Susannah S. French^1,*, Ryan McLemore², Brent Vernon², Gwynne I. H. Johnston¹ and Michael C. Moore¹

¹ School of Life Sciences, Arizona State University, Tempe, AZ 85287-4601, USA
² Center for Interventional Biomaterials, Harrington Department of Bioengineering, Arizona State University, Tempe, AZ 85287-9709, USA

^* Author for correspondence (e-mail: ssfrench{at}asu.edu)

Accepted 5 June 2007

Physiological trade-offs arise because multiple processes compete for the same limiting resources. While competition for resources has been demonstrated between reproduction and immune function, the regulation of this competition remains unclear. Corticosterone (CORT) is a likely mediator due to its dual role in mobilizing energy stores throughout the body and regulating physiological responses to stressors. We manipulated CORT concentrations and resources in pre-reproductive and reproductive female tree lizards (Urosaurus ornatus) to test the hypothesis that CORT regulates the distribution of limiting resources between the reproductive and immune systems. To manipulate circulating concentrations of CORT we utilized a novel method of hormone implantation, in which a polymeric compound is mixed with hormone and injected in liquid form into the animal. After injection, the liquid quickly gels in situ forming a slow release hormone implant. This method of hormone delivery eliminated the need for substantial wounds to the animal or repeated handling required by other methods. In this study, the hormone-treated animals had plasma CORT concentrations comparable to high physiological concentrations. We found that CORT treatment suppressed immune function, but only when animals were energetically compromised. We assessed immune function by measuring the healing rate of a cutaneous biopsy. Healing was suppressed in all CORT-treated reproductive animals and in all CORT-treated animals (pre-reproductive and reproductive) undergoing food restriction, but CORT had no effect in ad libitum non-reproductive females. The context-dependent action of CORT renders its response adjustable to changing environmental conditions and may allow for the suppression of specific functions depending on resource availability.

Key words: polymer, in situ gelation, wound healing, context-dependent, resources, sex steroids, drug delivery

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