Cholinergic signal transduction cascades in rat pinealocytes: functional and ontogenetic aspects

In adult rat pinealocytes, acetylcholine activates nicotinic receptors whose stimulation causes a depolarization of the cells, opening of voltage-gated cation channels of the L-type and subsequent increase in the intracellular calcium ion concentration. These events trigger a release of glutamate that, by its action on metabotropic glutamate type 3 receptors, activates an inhibitory cyclic AMP cascade and suppresses norepinephrine-induced melatonin biosynthesis. The nicotinic response is fully developed in the third postnatal week. Prior to this timepoint, rat pinealocytes possess functional muscarinic receptors whose activation causes a rise in the intracellular calcium ion concentration through a calcium release from thapsigargin-sensitive intracellular calcium stores and an opening of store-operated calcium channels. This cascade may influence tissue differentiation and maturation of the melatonin pathway. The demonstration of functional cholinoreceptors and the ontogenetic switch from muscarinic to nicotinic signalling in rat pinealocytes supports the concept that pineal functions in mammals are influenced by neuronal inputs other than the sympathetic innervation which serves as the major regulatory system.