Circadian variation in plasma corticosterone in the American alligator, Alligator mississippiensis, and the effects of ACTH injections.

Crocodilians are unique among the reptiles in that they lack a pineal gland. Since the pineal is believed to play a role in the regulation of photoperiodic control of circadian rhythms in activity and corticosterone secretion in vertebrates, plasma corticosterone was measured by radioimmunoassay (RIA) in samples collected over a 24-hr period from captive-reared, juvenile crocodilians (Alligator mississippiensis) held under a constant temperature but exposed to natural photoperiod. A clear biphasic rhythm in plasma corticosterone levels was observed, with a peak at 0800 hr and a second peak at 2000 hr, which corresponds to the biphasic activity cycle of this reptile. A single injection of 8 IU of mammalian ACTH into juvenile alligators resulted in a significant increase in corticosterone levels by 1 hr; hormone levels remained elevated for 24 hr, returning to baseline by 48 hr. Whereas handling and bleeding the alligators caused a significant increase in plasma corticosterone at 1, 4, and 24 hr, hormone levels in the ACTH-injected animals were significantly higher than the saline-injected controls at these times. Corticosterone levels in alligators range between 0.1 and 6 ng/ml as measured by RIA. These values are an order of magnitude lower than values previously reported for crocodilians.