Non-sympathetic synaptic innervation of the pinealocyte of the Mongolian gerbil (Meriones unguiculatus): an electron microscopic study.

Direct synaptic innervation of pinealocytes was observed in the superficial pineal gland of the Mongolian gerbil (Meriones unguiculatus) by electron microscopy. This innervation consisted of nerve fibres terminating in boutons with clear transmitter vesicles with a diameter of 40-60 nm. The boutons made synaptic junctions with the cell membrane of the pinealocyte displaying thickenings of both the pre- and postsynaptic membranes. Such boutons persisted in the gland 1 week after removal of both superior cervical ganglia. In contrast, all the sympathetic boutons containing transmitter vesicles with a small dense core disappeared after ganglionectomy. This direct synaptic innervation reveals a neuronal character of the pinealocyte and might underlie reports of action potentials in electrophysiological recordings from the gland.