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Circadian rhythms of plasma melatonin in the Adelie penguin ( Pygoscelis adeliae ) in constant dim light and artificial photoperiods
Author(s) -
Cockrem J.F.
Publication year - 1991
Publication title -
journal of pineal research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.881
H-Index - 131
eISSN - 1600-079X
pISSN - 0742-3098
DOI - 10.1111/j.1600-079x.1991.tb00457.x
Subject(s) - melatonin , circadian rhythm , photoperiodism , biology , pygoscelis , medicine , endocrinology , pineal gland , ecology , botany , foraging
The response of plasma melatonin in Adelie penguins ( Pygoscelis adeliae ) to constant dim light and to light/dark cycles was measured to determine the capacity of the pineal gland to secrete melatonin after exposure to continuous daylight for 2 months. Penguins were moved in mid‐summer from the natural photoperiod to either constant dim light (n = 10), to a 12L:12D light/dark cycle (n = 5), or to a 12L:12D light/dark cycle with a 30 min light pulse (50–155 lux) on the third (n = 4) or sixth (n = 5) “night.” Blood samples were collected regularly through cannulae for up to 33 h. The birds in dim light were sampled after 2 days, with samples obtained over at least 24 h from 7 birds. Three of these birds had melatonin rhythms (peak levels 66.7–130.2 pg/ml) whereas the other 4 birds had constant low levels (<44 pg/ml). The phase of the rhythm was similar for all 3 birds. This is consistent with the pacemaker that regulates the circadian rhythm of melatonin secretion being entrained to a period of 24 h when the penguins were exposed to the natural photoperiod. Mean melatonin levels (42.7 ± 2.5 pg/ml) were elevated compared to those previously reported in penguins under natural daylight. All penguins held under a 12L:12D light/dark cycle had melatonin rhythms. The phase and form of these rhythms were similar to those reported for other birds, and they appeared to be circadian rhythms entrained by the light/dark cycle. Melatonin levels after a 30 min light pulse increased to peak levels at 0700 h that were 131.9 ± 10.1% (third night) and 130.2 ± 21.5% (sixth night) of those at 0030 h, as compared to 93.5 ± 15.9% for birds without a pulse. This difference was not statistically significant, but it is consistent with data from other species where a light pulse at a similar time during the “night” reduces melatonin levels after the pulse and delays the time of nocturnal peak levels. These results indicate that the capacity of the Adelie penguin pineal gland to secrete melatonin rhythmically is not lost after exposure to 2 months of continuous daylight. Also, melatonin secretion in the Adelie penguin may respond to relative changes in light intensity, even when the lowest light intensities are much greater than those considered experimentally to be bright light.