Radioimmunoassay

Plasma adrenocorticotropic hormone (ACTH) concentration was determined from samples after FST by RIA with a specific antiserum (diluted 1:120,000; graciously provided by Dr. William Engeland – University of Minnesota, Minneapolis, MN) with ¹²⁵I ACTH as a tracer label (Amersham Biosciences, Piscataway, NJ) (Jasper & Engeland, 1991). Plasma samples were diluted in steroid diluent and run in duplicate for determination of plasma corticosterone concentrations with an ¹²⁵I kit (MP Biomedicals., Orangeburg, NY). The intra-assay coefficient of variation was <10% for both the ACTH and corticosterone assays, and sensitivities (minimum detectable values) were 3.13 pg/ml and 25 ng/ml, respectively.