Microvesicle-mediated exocytosis of glutamate is a novel paracrine-like chemical transduction mechanism and inhibits melatonin secretion in rat pinealocytes

Abstract: Mammalian pinealocytes are neuroendocrine cells that synthesize and secrete melatonin, these processes being positively controlled by norepinephrine derived from innervating sympathetic neurons. Previously, we showed that pinealocytes contain a large number of microvesicles (MVs) that specifically accumulate L -glutamate through a vesicular glutamate transporter and contain proteins for exocytosis such as synaptobrevin 2 (VAMP2). These findings suggested that the MVs are counterparts of synaptic vesicles and are involved in paracrine-like chemical transduction in the pineal gland. Here, we show that pinealocytes actually secrete glutamate upon stimulation by KC1 in the presence of Ca²⁺ at 37°C. The ability of glutamate secretion disappeared when the cells were incubated at below 20°C. Loss of the activity was also observed on successive stimulation, but it was recovered after 12 hr incubation. A low concentration of cadmium chloride or ω-conotoxin GVIA inhibited the secretion. Botulinum neurotoxin E cleaved synaptic vesicle-associated protein 25 (SNAP-25) and thus inhibited the secretion. The released L -glutamate stimulated pinealocytes themselves via glutamate receptor(s) and inhibited norepinephrine-stimulated melatonin secretion. These results strongly suggest that pinealocytes are glutaminergic paraneurons, and that the glutaminergic system regulates negatively the synthesis and secretion of melatonin. The MV-mediated paracrine-like chemical transduction seems to be a novel mechanism that regulates hormonal secretion by neuroendocrine cells.