Expression of synaptic membrane proteins in gerbil pinealocytes in primary culture

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.

Putative neurohemal release zones in the stomatogastric nervous system of decapod crustaceans

The stomatogastric nervous system (STNS) of decapod crustaceans has long been used to study the modulation of small neural circuits. Profiles in the sheath of the nerves and ganglia of the STNS, which contain only dense-core vesicles, have been described in electron microscopical studies (Friend [1976] Cell Tissue Res. 175:369-380; Kilman and Marder [1997] Soc Neurosci Abstr. 23:477; Skiebe and Ganeshina [2000] J Comp Neurol 420:373-397). These profiles resemble those found in neurohemal organs and suggest the presence of neurohemal release zones in the STNS. To map these putative neurohemal release zones, a combination of two antibodies was used in the present study. A synapsin antibody recognizing vesicle proteins of clear vesicles was combined with a synaptotagmin antibody recognizing vesicle proteins of clear and dense-core vesicles. Exclusive synaptotagmin-like staining, therefore, indicated the regions with only dense-core vesicles. Such a staining was found in a mesh in the perineural sheath of nerves in the STNS of all three species investigated. In the crayfish Cherax destructor and the lobster Homarus americanus, the stained mesh was located in the sheath of nerves connecting all four ganglia of the STNS, whereas in the crab Cancer pagurus it was found on different nerves, which are more directly exposed to the hemolymph in this species. Exclusive synaptotagmin-like staining was also found in a putative neurohemal release zone in the sheath of the circumoesophageal connectives and the postoesophageal commissure in C. destructor. These data suggest that an important source of modulation of the networks and the muscles of the stomach is a compartmentalized release of neurohormones from zones in the STNS.

The internal pH of synaptic-like microvesicles in rat pinealocytes in culture

Synaptic-like microvesicles (SLMVs) are morphological and functional equivalents of neuronal synaptic vesicles, and are responsible for the storage and secretion of classical neurotransmitters in various endocrine cells. Vacuolar H+-ATPase acidifies the internal space of these organelles and provides a driving force for the uptake of neurotransmitters. Thus, the luminal pH is an important determinant of the function of SLMVs, although its value in living cells is unknown. Here, we determined the luminal pH of SLMVs in living rat pinealocytes by means of an immunoelectronmicroscopic procedure basedon the distribution of an amphipathic amine, 3-(2,4-dinitroanilino)-3'-amino-N-methyldipropylamine (DAMP). Use of double-labeling techniques with antibodies against 2,4-dinitrophenol for DAMP and synaptophysin for SLMVs, and of frozen ultrathin sections enabled us to determine the number of immunogold particles for DAMP per microm2 of SLMVs. Using the density of gold particles, the luminal pH of SLMVs was calculated to be 5.11 +/- 0.01. Treatment with either 1 microm bafilomycin A1, a specific inhibitor of vacuolar H+-ATPase, or 50 mm ammonium chloride, a dissipater of the transmembrane pH gradient, increased the luminal pH to 6.04 +/- 0.07 and 6.05 +/- 0.11, respectively. Simultaneously, the lysosomal pH was found to be 5.14 +/- 0.07, which increased to 5.77 +/- 0.09 and 5.93 +/- 0.13 with bafilomycin A1 and ammonium chloride, respectively. It is concluded that the luminal pH of SLMVs is comparable to that of lysosomes in vivo.

Developmental potency of cultured pineal cells: an approach to pineal developmental biology

The pineal organ is still an enigma in regard to its developmental and phylogenetic origin. Little is known of the mechanism involved in determination and differentiation of pineal cells and virtually no studies have been done on the induction and tissue interactions during pinealogenesis. Interest is also centered on the evolutional transformation in structure and function, which may be related to the developmental alterations in pineal morphogenesis between the lower and higher vertebrate species. For developmental studies, avian embryos have great advantages for various experimental manipulations, such as cell and organ culture, surgical operation, and in situ transfection of developmental genes. The present review describes our cell culture studies, which have been done on developing rat and quail pineal organs, in order to elucidate the developmental potency of pineal cells and the regulatory mechanism involved in the phenotypic expression of cell properties. A number of phenotypes including numerous neuron-specific substances are shown immunohistochemically to be expressed only under culture conditions, and not observed in the mature pineal organ. As development proceeds, some of the potencies for cell differentiation are lost; hence, in the mature pineal organs most neuronal phenotypes are not expressed. Numerous factors were discovered which affect phenotypic expression of cultured pineal cells in a cell-type-specific manner. These findings, together with immunohistochemical observations on developing pineal organs, reveal that the developing pineal organ is a unique and useful model system for developmental neurobiology and that cell culture techniques offer a powerful tool for the understanding of development and cell differentiation of this particular organ.

Evidence for microvesicular storage and release of glycine in rodent pinealocytes

Prompted by previous studies suggesting a regulatory role for the inhibitory amino acid gamma-aminobutyric acid (GABA) within the mammalian pineal gland, we carried out a study of rat and gerbil pineal organs to elucidate whether there is evidence for a vesicular storage and release of GABA and/or glycine. Immunohistochemistry revealed the presence of the vesicular inhibitory amino acid transporter in pinealocytes. Moreover, we found that, in addition to glutamate and aspartate, cultured pinealocytes also released glycine upon stimulation by depolarizing concentrations of KCl, whereas the content of GABA in the culture medium did not exceed the detection limit either under control conditions or following KCl application. Therefore, we propose that glycine is a further component of the paracrine signaling system within the pineal organ which is based on the compartment of synaptic-like microvesicles (SLMVs) inside pinealocytes.

Vesicular monoamine transporter 1 is responsible for storage of 5-hydroxytryptamine in rat pinealocytes

Vesicular monoamine transporters (VMATs) are involved in chemical transduction in monoaminergic neurons and various endocrine cells through the storage of monoamines in secretory vesicles. Mammalian pinealocytes contain more 5-hydroxytryptamine (5-HT) than any other cells and are expected to contain VMAT, although no information is available so far. Upon the addition of ATP, radiolabeled 5-HT was taken up by a particulate fraction prepared from cultured rat pinealocytes. The 5-HT uptake was inhibited significantly by bafilomycin A1 (an inhibitor of vacuolar H+-ATPase), 3,5-di-tert-butyl-4-hydroxybenzylidenemalononitrile (a proton conductor), or reserpine (an inhibitor of VMAT). RT-PCR analysis suggested that VMAT type 1 (VMAT1), but not type 2, is expressed. Antibodies against VMAT1 recognized a single polypeptide with an apparent molecular mass of approximately 55 kDa, and specifically immunostained pinealocytes. VMAT1 immunoreactivity was high in the vesicular structures in the varicosities of long branching processes and was associated with 5-HT, but not with synaptophysin, a marker protein for microvesicles. The 5-HT immunoreactivity in the long branching processes disappeared upon incubation with reserpine. These results indicate that 5-HT, at least in part, is stored in vesicles other than microvesicles in pinealocytes through a mechanism similar to that of various secretory vesicles.

Interstitial glial cells of the gerbil pineal gland display immunoreactivity for the metabotropic glutamate receptors mGluR2/3 and mGluR5

Recent studies have strengthened the hypothesis that neuroactive amino acids such as L-glutamate play an important role in the physiology of the mammalian pineal gland. In particular, there is now considerable evidence that L-glutamate is liberated from electron-lucent microvesicles of pinealocytes for a paracrine modulation of melatonin synthesis and release which may at least partially be mediated by the metabotropic glutamate receptor mGluR3. In order to expand our incomplete knowledge of possible pineal target cells and signal transduction mechanisms which are involved in glutamate-dependent intercellular communication, we have performed an immunohistochemical study of the gerbil pineal gland with antibodies directed against the metabotropic glutamate receptors mGluR2/3 and mGluR5. Using microwave irradiation of cryostat sections prior to immunostaining, strong immunoreactivity for both receptor subtypes was constantly observed in a subpopulation of pineal cells. Interestingly, these mGluR-positive cells could be identified as interstitial glial cells since they were labeled by antibodies against the intermediate filament protein vimentin in double immunofluorescence histochemistry. This indicates that interstitial glial cells in the gerbil possess the capacity to express at least two metabotropic glutamate receptors coupled to different intracellular signal transduction pathways. Therefore, it can be concluded that the glutamatergic communication system of the pineal gland may not only enable paracrine crosstalk among pinealocytes but probably also relies on interactions between pinealocytes and interstitial cells analogous to neuronal-glial signaling.

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.

Differential expression of SNAP-25 isoforms and SNAP-23 in the adrenal gland

In the rat adrenal gland, we previously observed that SNAP-25 is not restricted to the plasmalemma in noradrenergic cells as it is in adrenergic cells, and hypothesized that SNAP-25 isoform expression is different in the two phenotypes. Expression of SNAP-25 isoforms and SNAP-23 was examined by immunoblotting, immunofluorescence, and RT-PCR. Amplifications of SNAP-25 mRNAs were combined with Southern hybridization, restriction enzyme analysis, and sequencing of cloned PCR products to compare SNAP-25 isoform expression in rat and bovine adrenal glands. SNAP-25 and SNAP-23 mRNA and protein are expressed in the glands; SNAP-23 is enriched in the adrenal cortex, whereas SNAP-25 is restricted to the adrenal medulla. Furthermore, high levels of SNAP-25 and low levels of SNAP-23 are observed in the PC12 cells, whereas both SNAP-25 and SNAP-23 are expressed in adrenal medullary cultures. In all extracts, the SNAP-23 mRNA corresponded to SNAP-23a. SNAP-25a is the major form expressed in rat adrenal glands (75%), as it is in PC12 cells (80%), but both SNAP-25a and SNAP-25b (40% vs. 60%) are expressed in bovine adrenal medulla in situ and in culture. In addition, an enriched population of adrenergic cells (93%) expressed a higher level of SNAP-25b (70%), suggesting that this isoform may not be restricted to fast neurotransmission.