Pinealocytes, endocrine cells that synthesize and secrete melatonin, possess a large number of synaptic-like microvesicles (MVs) containing synaptophysin. By monitoring cross-reactivity with anti-synaptophysin antibody, the MVs were highly purified from bovine pineal glands. The purified MVs were morphologically similar to but distinct from neuronal synaptic vesicles by their lack of synapsin I. Immunological study indicated that the MVs contained vacuolar H+-ATPase, synaptotagmin and synaptobrevin 2 (VAMP2). The MVs accumulated l-glutamate at the expense of ATP hydrolysis by vacuolar H+-ATPase. No uptakes of melatonin, serotonin, noradrenaline, γ-aminobutyrate or acetylcholine were observed. These results indicated that the MVs are organelles for storage of l-glutamate in pinealocytes and suggested a possibility that pinealocytes transmit glutamate signals by MVs-mediated exocytosis.
