Synaptotagmin I, Synaptobrevin II, and Syntaxin I Are Coexpressed in Rat and Gerbil Pinealocytes

Recent studies have shown that mammalian pinealocytes contain a compartment of synaptic-like microvesicles that may serve secretory functions; however, knowledge of the molecular composition of these microvesicles is still incomplete. Therefore, we have analyzed rat and gerbil pineal glands for the presence of synaptotagmin I, synaptobrevin I and II, syntaxin I, and synaptoporin (synaptophysin II) by immunoblot analyses and immunostaining of serial semithin sections. These proteins, which are components of the synaptic vesicle membrane or presynaptic plasmalemma, are thought to be essential for synaptic vesicle trafficking and exocytosis. Antibodies against synaptotagmin I, synaptobrevin II, and syntaxin I label pinealocytes (identified with an antiserum directed against synaptophysin I) in pineal glands of both species, the coexpression of the latter proteins being demonstrable at the single cell level. In contrast, pinealocytes are not or only weakly stained by the synaptoporin antibody. Immunoreactivity for synaptobrevin I is restricted to intrapineal nerve terminals, thus indicating a differential expression of synaptic vesicle protein isoforms within endocrine tissues. Immunogold staining has been performed in the gerbil pineal and reveals that synaptobrevin II and synaptotagmin I can be localized to the synaptic-like microvesicles that are concentrated in pinealocyte process terminals. Syntaxin immunoreactivity is associated with clear microvesicles and with the plasma membrane. Our findings corroborate the hypothesis that the synaptic-like microvesicles of pinealocytes can be considered as the endocrine equivalent of neuronal synaptic vesicles. Since pinealocytes of several mammalian species contain abundant clear microvesicles, the pinealocyte may serve as a paradigm for studies aimed at elucidating the biogenesis and functions of synaptic-like microvesicles in neuroendocrine cells.