Synaptophysin – a common constituent of presumptive secretory microvesicles in the mammalian pinealocyte: A study of rat and gerbil pineal glands

Abstract

Recent studies have established that pinealocytes of the mammalian pineal gland contain marker molecules of neuroendocrine cells or paraneurons like the synaptic vesicle-associated protein synaptophysin (p38). The objective of this study was to identify the subcellular synaptophysin-positive compartment and to characterize in detail the intracellular distribution of this protein in rat and gerbil pinealocytes. An analysis of serial semithin sections of plastic-embedded pineals immunostained for synaptophysin, including computer-assisted optical density measurements of synaptophysin immunoreactivities, demonstrated unequivocally that synaptophysin was highly concentrated in dilated process terminals of the pinealocytes. More than 75% of these process terminals were found to border or lie within the pericapillary space. At the ultrastructural level, they contained accumulations of small clear vesicles of variable size that turned out to be the site of synaptophysin immunoreactivity when immunogold staining was performed. In addition, microvesicles surrounding synaptic ribbons were also immunolabeled. Hence, the pinealocyte is the first neuroendocrine cell type that has now been shown to concentrate synaptophysin-positive microvesicles in perivascular process endings. This observation lends strong support to the hypothesis that small clear vesicles in neuroendocrine cells in general, and in pinealocytes in particular, serve secretory functions. The quantitative analysis of completely sectioned process endings revealed that the microvesicles outnumber by far the amount of dense core vesicles and therefore cannot arise by endocytosis of dense core vesicle membranes. Thus, small synaptic-like vesicles probably constitute an independent secretory pathway of the paraneuronal pinealocytes. In the present study, we could also establish the absence of immunoreactivity for synapsin I (belonging to a family of neuron-specific nerve terminal phosphoproteins) from pinealocytes. Synapsin I immunoreactivity was only detectable in intrapineal nerve terminals and varicosities. Taken together, the immunostaining patterns of the pineal gland obtained with antibodies directed against synaptic vesicle-associated proteins render the mammalian pinealocyte a very special type of neuroendocrine cell or paraneuron rather than a "classic" neuron.

Microvesicles isolated from bovine pineal gland specifically accumulate l‐glutamate

“…MVs were distinct from neuronal synaptic vesicles by their lack of synapsin I (Fig. 3), confirming previous histochemical result [10]. The lack of synapsin I in the MVs ruled out a possibility that synaptic vesicles derived from nerve endings projected into pineal gland are contaminated into the MVs fraction.…”

“…3), MVs may store glutamate and extrude it by exocytosis. Process terminals and synaptic ribbon-region seem to be the sites of exocytosis of glutamate, because MVs are condensed in these areas [10]. Although further studies such as detection of the glutamatedriven intercellular signal transfer and precise localization of glutamate receptor will be necessary, the putative MVs-mediated signal transduction system may be important for understanding of the mechanism of regulation of melatonin secretion.…”

“…Pinealocytes are known to contain relatively high concentration of glutamate [9] and a large number of synaptic-like microvesicles (MVs) [10]. Recent evidence indicated that MVs *Corresponding author.…”

Functional Expression of a GLT‐1 Type Na+‐Dependent Glutamate Transporter in Rat Pinealocytes

“…Besides melatonin, pinealocytes secrete L-glutamate through exocytosis (Moriyama et al, 1996;Yamada et al, l996a); pinealocytes contain a large number of microvesicles (MVs), also termed synaptic-like microvesicles (Redecker and Bargsten, 1993;Moriyama and Yamamoto, 1 995a), that accumulate L-glutamate through a specific vesicular transporter energetically coupled with vacuolar type H~-ATPase (Moriyama and Yamamoto,I995a,b). Then, glutamate is secreted from the cells through the exocytosis of MVs in Ca2-dependent and botulium neurotoxin-sensitive manners (Yamada et al, 1996a,b).…”

Complexin-I-deficient sperm are subfertile due to a defect in zona pellucida penetration

“…Animals were transcardially perfused with prewash and fixative solution (4% paraformaldehyde in PBS) as detailed (Redecker & Bargsten 1993). Testes were immersed in fixative overnight at 4 8C.…”