In Vitro Synthesis of Dopamine and Noradrenaline in the Isolated Rat Pineal Gland: Day‐Night Variations and Effects of Electrical Stimulation

Isolated rat pineal glands were incubated in vitro in a medium containing [ I4 C]dopamine or [ 14 C]tyrosine, and the tissue contents of l4 C‐labelled and total dopamine and noradrenaline were determined by HPLC followed by electrochemical detection and scintillation spectrometry. During incubation with [ l4 C]dopamine, the labelled amine accumulated in pineal glands and was partially converted into [ l4 C]noradrenaline. Nomifensine, a neuronal amine uptake blocker, largely inhibited the accumulation of [ l4 C]dopamine and the formation of [ 14 C]noradrenaline. These experiments demonstrated dopamine β‐hydroxylase activity in the sympathetic nerves of the pineal gland. During incubation with [ 14 C]tyrosine, formation of [ l4 C]dopamine and [ 14 C]noradrenaline was observed in the pineal tissue, indicating that noradrenaline can also be synthesized from dopamine, endogenously formed in the gland. Electrical stimulation of the stalk region of the pineal gland during incubation with [ l4 C]dopamine enhanced the accumulation of [ l4 C]dopamine and synthesis of [ 14 C]noradrenaline. Electrical stimulation also enhanced the formation of [ l4 C]dopamine during incubation with [ l4 C]tyrosine. Compared to that at midday, the tissue content of endogenous noradrenaline at midnight was enhanced by 50% and that of dopamine by 450%. The in vitro accumulation of [ 14 C]dopamine, as well as the synthesis of [ 14 C]dopamine and [ 14 C]noradrenaline, was also increased at midnight. In conclusion, sympathetic nerves in the rat pineal gland contain tyrosine hydroxylase and dopamine β‐hydroxylase, the two enzymes required for the synthesis of noradrenaline. The capacity of the pineal gland to synthesize dopamine and noradrenaline is enhanced during the night.