Stimulation of Beta-adrenergic Receptors in the Pineal Gland Increases the Noradrenaline Stores of Its Sympathetic Nerves

The administration of isoproterenol decreases the level of serotonin in the rat pineal gland and at the same time it increases pineal noradrenaline. These effects depend on the stimulation of a beta-adrenergic receptor because they are blocked by pretreatment of the animals with propranolol; this drug by itself does not modify either serotonin or noradrenaline levels in the pineal. The elevation of noradrenaline produced by isoproterenol is selective for the pineal because it is not observed in the salivary gland innervated by postganglionic adrenergic fibers from the same origin as pineal nerves. Pineal serotonin is stored in equilibrium in two compartments, i.e., the parenchymal cells and the adrenergic nerves and thus is most probably reduced in both sites. Since noradrenaline and serotonin are detected in pineal nerve vesicles and may coexist in them, the diminution of intravesicular serotonin, by making more storage sites available, probably determines the selective increase of pineal noradrenaline. A similar modification in the ratio of intravesicular amines as a result of the physiological stimulation of pineal beta-adrenergic receptors by the adrenergic neurotransmitter may explain some of the changes observed in the content of pineal amines.