Serotonin release mechanisms in bovine pineal gland: stimulation by norepinephrine and dopamine

An assessment of serotonin (5HT) release was made in bovine pineal gland. Bovine pineal fragments took up [3H]5HT by a Na+-dependent process exhibiting two apparent Km, i.e. a high affinity uptake system (Km = 220 nM) and a low affinity uptake system (Km = 197 microM). A significant release of [3H]5HT was elicited by increasing K+ concentrations in the medium (20-80 mM). Exposure of bovine pineal fragments to varying doses of catecholaminergic agonists indicated that a significant [3H]5HT release was elicited at the following threshold concentrations: 10(-6) M norepinephrine (NE), 10(-7) M dopamine (DA), 10(-6) phenylephrine and 10(-6) M isoproterenol. By employing specific receptor agonists and antagonists, the 5HT release activity of adrenergic agonists was found to be mediated by alpha 1-adrenoceptors, while that of DA by D2-dopaminergic receptors. 5HT release elicited by NE or DA, as well as that by 30 mM K+, was Ca2+-dependent. Both NE and DA increase 45Ca2+ uptake in a dispersed cell preparation of bovine pineal glands. As in the case of 5HT release, the effect of NE and DA on calcium uptake was mediated by alpha 1-adrenoceptors and D2-dopaminergic receptors, respectively. These results indicate that both NE and DA control 5HT release in bovine pineal gland.