MICROVESICLE-MEDIATED EXOCYTOSIS OF GLUTAMATE IS A NOVEL PARACRINE-LIKE CHEMICAL TRANSDUCTION MECHANISM AND INHIBITS MELATONIN SECRETION INRAT PINEALOCYTES

Mammalian pinealocytes are neuroendocrine cells that synthesize and se crete melatonin, these processes being positively controlled by norepi nephrine 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 glutamat e transporter and contain proteins for exocytosis such as synaptobrevi n 2 (VAMP2), These findings suggested that the MVs are counterparts of synaptic vesicles and are involved in paracrine-like chemical transdu ction in the pineal gland. Here, we show that pinealocytes actually se crete glutamate upon stimulation by KCl in the presence of Ca2+ at 37 degrees C, The ability of glutamate secretion disappeared when the cel ls were incubated at below 20 degrees 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 omega-conotoxin GVIA inhibited the secretion. Botulinum neurotoxin E cleaved synaptic vesicle-associated protein 25 (SNAP-25) and thus inhibited the secret ion, The released L-glutamate stimulated pinealocytes themselves via g lutamate receptor(s) and inhibited norepinephrine-stimulated melatonin secretion. These results strongly suggest that pinealocytes are gluta minergic paraneurons, and that the glutaminergic system regulates nega tively the synthesis and secretion of melatonin. The MV-mediated parac rine-like chemical transduction seems to be a novel mechanism that reg ulates hormonal secretion by neuroendocrine cells.