Rhythms in immunoreactive melatonin in the retina and Harderian gland of rats: persistence after pinealectomy.

Immunoreactive melatonin levels were measured in the retina and Harderian gland of adult male rats throughout a 24 hour period. The animals were maintained under a light:dark cycle of 14:10 (lights on at 0600h). In intact animals, immunoreactive melatonin values in both organs exhibited a 24h rhythm with peak levels being measured at 0800h, 2 hours after lights on. Pinealectomy significantly increased peak levels at 0800h in both the retina and the Harderian gland. Gonadectomy abolished the peak retinal melatonin levels at 0800h. Likewise, continual light exposure for 1 week depressed the melatonin peak in the retina but not in the Harderian gland.