EVIDENCE FOR MELATONIN SYNTHESIS IN RODENT HARDERIAN-GLAND - A DYNAMIC IN-VITRO STUDY

Melatonin content and release from Harderian glands (HGs) has been mea sured by an in vitro perifusion technique in three rodent species: Wis tar rat, Syrian hamster, and Siberian hamster. Melatonin immunoreactiv e concentrations in HGs of animals killed at 10.00 hr were 0.31 +/- 0. 031 pg/mg gland in male Wistar rat, 0.54 +/- 0.026 pg/mg gland in male Siberian hamster, 0.17 +/- 0.070 and 0.20 +/- 0.059 pg/mg gland in ma le and female Syrian hamster, respectively. In all species examined, i solated HGs perifused for 9-15 hr released melatonin but did not stabi lize their melatonin release rate. No sex-related difference could be noted in the HG melatonin release rate. The total amount of melatonin released over a 15 hr long perifusion was about 0.075 +/- 0.004 ng/15 h/mg gland and 0.063 +/- 0.010 ng/15 hr/mg gland in male and female Wi star rat, respectively; 0.155 +/- 0.019 ng/15 hr/mg gland and 0.141 +/ - 0.006 ng/15 hr/mg gland in male and female Siberian hamster, respect ively; 0.035 +/- 0.003 ng/15 hr/mg gland and 0.045 +/- 0.004 ng/15 hr/ mg gland in male and female Syrian hamster, respectively. This amount, which is higher than the tissue levels, demonstrates the de novo mela tonin synthesis. This is confirmed by the fact that infusion of the in doleamine precursor, tryptophan (TRP), stimulated melatonin secretion from HGs. The melatonin release is increased by 2.5-fold in male and f emale Wistar rat, 1.5-fold in male and female Siberian hamster, and 2. 0- and 3.0-fold in male and female Syrian hamster, respectively. Treat ment with a TRP hydroxylase inhibitor, para-chlorophenylalanine, reduc ed basal melatonin release and inhibited the TRP-induced melatonin sti mulation. Kinetics and amounts of melatonin released were not affected by pinealectomy, ruling out a possible plasmatic origin of the HG mel atonin. Isoproterenol, a beta-adrenergic agonist, and dibutyryl cyclic AMP, a cyclic AMP analogue, failed to stimulate HG melatonin secretio n. In conclusion, these results confirm the presence of melatonin in t he HGs and demonstrate that melatonin is synthesized in and released f rom isolated rodent HGs.