Production and testing of a needle- implantable device for estimating long term tissue availability of the hormone corticosterone in rats is proposed. This work contributes to the long term goal of developing a similar device for measurement of the tissue availability of a broad range of compounds in both research and medical settings. The device operates on the diffusion- sink principle: a compartment is created in which the concentration of the compound of interest is maintained at a negligible level, by action of a binding protein with high affinity and specificity for that compound; consequently, the compound continuously diffuses into the device at a rate that depends on its availability in the immediate environment and the diffusive resistance offered by the walls of the device. By measuring the accumulation of a compound by a subcutaneously implanted device over a suitable time interval, a direct estimate of its tissue availability is obtained. Compactness and high capacity are gained by using concentrated binding proteins purified by affinity chromatography. Potential applications include measurement of time integrated cortisol availability in clinical psychiatry for differential diagnosis of endogenous depression or melancholia.
