Extraretinal mediation of responses to temperature and light in hatchling alligators

Extraocular phototransduction and circadian timing systems in vertebrates

It is widely accepted that, for organisms with eyes, the daily regulation of circadian rhythms is made possible by light transduction through those organs. Yet, it has been demonstrated repeatedly in recent years that ocular light receptors that mediate vision, at least in mammals, are not the same photoreceptors involved in circadian regulation. Moreover, it has been recognized for many years that circadian regulation can occur in organisms without eyes. In fact, extraocular circadian phototransduction (EOCP) appears to be a phylogenetic rule for the vast majority of species. EOCP has been reported in every nonmammalian species studied to date. In mammals, however, the story is very different. This paper presents findings from studies that have examined specifically the capacity for EOCP in vertebrate species. In addition, the literature addressing noncircadian aspects of extraocular phototransduction is briefly discussed. Finally, possible mechanisms underlying EOCP are discussed, as are some of the implications of the presence, or absence, of EOCP across phylogeny.

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.