Variability in leptin and adrenal response in juvenile Steller sea lions (Eumetopias jubatus) to adrenocorticotropic hormone (ACTH) in different seasons

Eight free-ranging juvenile Steller sea lions (SSL; 6 males, 2 females; 14–20 months) temporarily held under ambient conditions at the Alaska SeaLife Center were physiologically challenged through exogenous administration of adrenocorticotropic hormone (ACTH). Four individuals (3 males, 1 female) underwent ACTH challenge in each of two seasons, summer and winter. Following ACTH injection serial blood and fecal samples were collected for up to 3 and 96 h, respectively. A radioimmunoassay (RIA) was validated for leptin, and using a previously validated RIA for cortisol, collected sera were analyzed for both cortisol and leptin. ACTH injection resulted in a 2.9-fold increase (P = 0.164) in leptin which preceded a 3.2-fold increase (P = 0.0290) in cortisol by 105 min in summer. In winter, a 1.7-fold increase in leptin (P = 0.020) preceded a 2.1-fold increase (P = 0.001) in serum cortisol by 45 min. Mean fecal corticosteroid maxima were 10.4 and 16.7-fold above baseline 28 and 12 h post-injection and returned to baseline 52 and 32 h post-injection, in summer and winter, respectively. Data indicate acute activity in juvenile adrenal glands is detectable in feces approximately 12–24 h post-stimulus in either season, with a duration of approximately 40 h in summer and 20 h in winter. Changes in serum cortisol proved statistically significant both seasons and elevated concentrations were detected by 30 min post-stimulus (baseline 64.8 ± 4.2; peak 209.5 ± 18.3 ng/ml: summer; baseline 87.0 ± 15.7; peak 237.6 ± 10.0 ng/ml: winter), whereas the changes that occurred in serum leptin proved to be significant only in winter (baseline 6.4 ± 0.6; peak 18.7 ± 7.0 ng/ml: summer; baseline 4.2 ± 0.5; peak 7.5 ± 0.6 ng/ml: winter). Changes in fecal corticosteroids proved significant only in summer (baseline 117.8 ± 36.7; peak 1219.3 ± 298.4 ng/g, P = 0.038: summer; baseline 71.8 ± 13.7; peak 1198.6 ± 369.9 ng/g, P = 0.053: winter) due to a high degree of individual variability in winter months. The data indicate that ACTH stimulates leptin production earlier than cortisol in both summer and winter, and that while the leptin response appears most variable in summer, fecal corticosteroids are most variable in winter.