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Online Journal of Veterinary Research


Volume 25(2): 143-153, 2021.

Effect of 23 hour light-dark cycles on pineal structure and serum melatonin

in young and adult rabbits (Oryctolagus cuniculus).


Ahmed Ali Amer, Bahaa Fakhri Hussein


College of Veterinary Medicine, University of Baghdad, Iraq.




Amer AA, Hussein BF., Effect of 23 hour light-dark cycles on pineal structure and serum melatonin in young and adult rabbits, Onl J Vet Res., 25(2): 143-153, 2021. We describe effect of light-dark cycles on pineal histology and serum melatonin at different ages in rabbits. Three groups of 5 rabbit pups, weaners and adults each were exposed to 24h light, 23h dark/1h light and 12h light/12h dark (controls) cycles for 14, 30 or 180 days. Compared with controls, light increased (P < 0.05) melatonin ~40-52% in 14 and 30 day old rabbits but did not affect melatonin in those kept dark at 14 days. In adults we found no difference between controls and 24h light exposures but did note 42% higher levels in rabbits kept dark. In 30 day weaners, however, melatonin was ~30% higher in those kept dark compared with light exposure (P < 0.05). In pups darkness was associated with ~0.19mm invagination of mesenchymal tissue from roof of diencephalon pineal gland whereas in those kept in 24h light reached 0.25 mm. We observed increased pineal mitotic activity in animals exposed to light. In 30 day old rabbits pineal length reached 3mm in darkness and 2.81mm in light exposure. In adults we measured 7.15 mm in those kept in the dark, 5.40 mm in light, and controls 6.10 mm.


KEY WORDS: Rabbit light/dark cycles, melatonin, histology. pineal gland.