Stimulation of Melatonin Synthesis in Ovine Pineals In Vitro

Static and superfused pineal slices (750 μm) have been used to study the control of melatonin synthesis by ovine pineals. Static incubates show a time‐dependent accumulation of melatonin in the medium; this is significantly increased by stimulation with norepinephrine (NE) (10 −5 M ), reaching 300% above control levels after 4 h. Perifused pineal slices show a rapid rise in melatonin release within 12–18 min in response to NE stimulation. This reaches a 3.5–4.5‐fold increase in melatonin released within 30 min. Withdrawal of NE is associated with a rapid return to prestimulated levels within 12–18 min. These time‐course characteristics compare favorably to those changes seen in vivo. The formation of [ 14 C]melatonin from [ 14 C]‐tryptophan shows a linear increase with time. In the presence of NE (10 −5 M ), the rate of synthesis is increased, albeit after an initial time lag of at least 30 min. The latter may reflect an N ‐acetyltransferase‐independent mechanism of synthesis and release. In static incubations, propranolol (10 −5 M ) inhibited NE‐induced melatonin production by about 60%. but prazosin (10 −5 M ) had no effect. As dibutyryl cyclic AMP (10 −3 M ) stimulated melatonin production, it is concluded that β‐receptors are of primary importance to the control of melatonin production, as in the rat. The role of α 1 ‐receptors is less clear, but the stimulatory action of phorbol 12‐myristate 13‐acetate on melatonin release implicates a receptor linked to phosphatidylinositol turnover.