Rat and gerbil pinealocytes contain the synaptosomal-associated protein 25 (SNAP-25)

Developmental and diurnal expression of the synaptosomal-associated protein 25 (Snap25) in the rat pineal gland

Snap25 (synaptosomal-associated protein) is a 25 kDa protein, belonging to the SNARE-family (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) of proteins, essential for synaptic and secretory vesicle exocytosis. Snap25 has by immunohistochemistry been demonstrated in the rat pineal gland but the biological importance of this is unknown. In this study, we demonstrate a high expression of mRNA encoding Snap25 in all parts of the rat pineal complex, the superficial-, and deep-pineal gland, as well as in the pineal stalk. Snap25 showed a low pineal expression during embryonic stages with a strong increase in expression levels just after birth. The expression showed no day/night variations. Neither removal of the sympathetic input to the pineal gland by superior cervical ganglionectomy nor bilateral decentralization of the superior cervical ganglia significantly affected the expression of Snap25 in the gland. The pineal expression levels of Snap25 were not changed following intraperitoneal injection of isoproterenol. The strong expression of Snap25 in the pineal gland suggests the presence of secretory granules and microvesicles in the rat pinealocyte supporting the concept of a vesicular release. At the transcriptional level, this Snap25-based release mechanism does not exhibit any diurnal rhythmicity and is regulated independently of the sympathetic nervous input to the gland.

The molecular architecture of ribbon presynaptic terminals

The primary receptor neurons of the auditory, vestibular, and visual systems encode a broad range of sensory information by modulating the tonic release of the neurotransmitter glutamate in response to graded changes in membrane potential. The output synapses of these neurons are marked by structures called synaptic ribbons, which tether a pool of releasable synaptic vesicles at the active zone where glutamate release occurs in response to calcium influx through L-type channels. Ribbons are composed primarily of the protein, RIBEYE, which is unique to ribbon synapses, but cytomatrix proteins that regulate the vesicle cycle in conventional terminals, such as Piccolo and Bassoon, also are found at ribbons. Conventional and ribbon terminals differ, however, in the size, molecular composition, and mobilization of their synaptic vesicle pools. Calcium-binding proteins and plasma membrane calcium pumps, together with endomembrane pumps and channels, play important roles in calcium handling at ribbon synapses. Taken together, emerging evidence suggests that several molecular and cellular specializations work in concert to support the sustained exocytosis of glutamate that is a hallmark of ribbon synapses. Consistent with its functional importance, abnormalities in a variety of functional aspects of the ribbon presynaptic terminal underlie several forms of auditory neuropathy and retinopathy.

Analysis of SNAP-25 immunoreactivity in hippocampal inhibitory neurons during development in culture and in situ

Synaptosomal associated protein of 25 kDa (SNAP-25) is a component of the soluble N-ethylmaleimide-sensitive fusion protein (NSF) attachment protein receptor (SNARE) complex which plays a central role in synaptic vesicle exocytosis. We have previously demonstrated that adult rat hippocampal GABAergic synapses, both in culture and in brain, are virtually devoid of SNAP-25 immunoreactivity and are less sensitive to the action of botulinum toxin type A, which cleaves this SNARE protein [Neuron 41 (2004) 599]. In the present study, we extend our findings to the adult mouse hippocampus and we also provide demonstration that hippocampal inhibitory synapses lacking SNAP-25 labeling belong to parvalbumin-, calretinin- and cholecystokinin-positive interneurons. A partial colocalization between SNAP-25 and glutamic acid decarboxylase is instead detectable in developing mouse hippocampus at P0 and, at a lesser extent, at P5. In rat embryonic hippocampal cultures at early developmental stages, SNAP-25 immunoreactivity is detectable in a percentage of GABAergic neurons, which progressively reduces with time in culture. Consistent with the presence of the substrate, botulinum toxin type A is partially effective in inhibiting synaptic vesicle recycling in immature GABAergic neurons. Since SNAP-25, beside its role as a SNARE protein, is involved in additional processes, such as neurite outgrowth and regulation of calcium dynamics, the presence of higher levels of the protein at specific stages of neuronal differentiation may have implications for the construction and for the functional properties of brain circuits.

Synaptic-like microvesicles in mammalian pinealocytes

The recent deciphering of the protein composition of the synaptic vesicle membrane has led to the unexpected identification of a compartment of electron-lucent microvesicles in neuroendocrine cells which resemble neuronal synaptic vesicles in terms of molecular structure and function. These vesicles are generally referred to as synaptic-like microvesicles (SLMVs) and have been most intensively studied in pancreatic beta-cells, chromaffin cells of the adrenal medulla, and pinealocytes of the pineal gland. This chapter focuses on the present knowledge of SLMVs as now well-established constituents of mammalian pinealocytes. I review the results of morphological, immunocytochemical, and biochemical studies that were important for the characterization of this novel population of secretory vesicles in the pineal organ. The emerging concept that SLMVs serve as a device for intercellular communication within the pineal gland is outlined, and unanswered questions such as those pertaining to the physiological function and regulation of pineal SLMVs are discussed.

Expression of synaptic membrane proteins in gerbil pinealocytes in primary culture

Pinealocytes of various mammalian species contain abundant synaptic-like microvesicles (SLMVs) which are considered the endocrine equivalent of neuronal synaptic vesicles. Although the pinealocytes may thus be a suitable cellular model for experimental in vitro studies of SLMVs, nothing is known about the presence of SLMVs in isolated pinealocytes maintained under tissue culture conditions. In the present investigation, we prepared dissociated primary cultures of gerbil pinealocytes to study the expression and distribution of protein components of synaptic vesicles/SLMVs and the presynaptic plasmalemma in pinealocytes kept in vitro. Using immunofluorescence microscopy, we found that cultured pinealocytes readily expressed all synaptic membrane proteins investigated, i.e., synaptophysin, synaptotagmin I, synaptobrevin II, syntaxin I and SNAP-25. Punctuate immunoreactivity for the vesicle-associated proteins could be detected throughout the cell bodies of pinealocytes and was also distributed into all of their processes which began to develop within the first days in culture. Outgrowing processes exhibited growth cone-like structures which were enriched in synaptic vesicle-associated proteins. After 1 week in vitro, pinealocytes had frequently formed an elaborate network of long interwoven processes. Accumulations of synaptic vesicle-associated proteins were observed in varicosities and terminal swellings of the processes. The vesicle-rich process swellings often established synaptic-like process swellings often established synaptic-like contacts with somata and processes of other pinealocytes. Some of the pinealocyte processes possessed additional axon-like properties as demonstrated by their lack of immunoreactivity for the somato-dendritic marker MAP2 and the transferrin receptor. The comparison of the staining patterns for synaptophysin and the endocytotic marker transferrin receptor by confocal laser scanning microscopy revealed a largely differential intracellular distribution of the two proteins. This may indicate that a substantial fraction of pinealocyte SLMVs by-passes the early endosomal-related recycling pathway of SLMVs. Herewith, we have shown that isolated gerbil pinealocytes maintained in primary culture can acquire morphological and neurochemical traits which closely mimick those observed in vivo. In particular, these cultures permit experimental studies of the compartment of pinealocyte SLMVs which seem to make up a major secretory pathway for paracrine intrapineal communication.