Application of confocal laser scanning microscopy to the deep pineal gland and other neural tissues

Microglia in the pineal gland of the neonatal rat: characterization and effects on pinealocyte neurite length and serotonin content

Microglia in the pineal gland of 1-day-old Sprague-Dawley rats were examined by OX-42 immunocytochemistry and DiI-acetylated-LDL uptake in pineal cell suspension and were found to comprise 3-5% of the total cells in the pineal gland of the neonates. In order to investigate the effects of microglia on pinealocyte structure and function, microglia-depleted and microglia-enriched pineal cell cultures were generated from 1-day-old neonate by fluorescence activated cell sorting (FACS). After 7 days of culture, tissues were processed for either immunocytochemistry for pinealocyte S-antigen and serotonin or high performance liquid chromatography to measure serotonin. Morphometric analysis of immunoreacted cells revealed that pinealocyte neurite length was enhanced in microglia-depleted cultures and was inhibited in a microglia-enriched environment (ANOVA, P < 0.001). Serotonin content of pineal cultures decreased in microglia-depleted cultures and was elevated in microglia-enriched cultures (ANOVA, P < 0.001) without any significant change in pinealocyte numbers. These findings are consistent with a working hypothesis that microglia function to mediate neuroendocrine-immune interactions of the gland.

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.

Central and peripheral nervous structures as seen at the confocal scanning laser microscope

Central neurons and peripheral nervous structures, e.g. cutaneous free endings, perifollicular nets, Meissners corpuscles and intramuscular fibres, were studied using various impregnation methods. The confocal scanning laser microscopes (CSLMs) used were equipped with different laser sources, in order to evaluate their limitations and advantages with these techniques and to contribute to a better understanding of the general morphology of the nervous system. When staining with silver sections with clouds of tiny silver granules which are beyond the resolution power of the conventional light microscope but which show a high reflectivity with the CSLM are obtained. Golgi-Cox mercuric impregnation, however, provides specimens which are precipitate-free, thus ensuring the reliability of information obtained. It does, however, have the disadvantage of being applicable only to the central nervous system. In all cases it is an advantage for the instrument to be fitted with different lasers (e.g. Ar and He-Ne), so as to optimize the images of samples impregnated with different methods. Notwithstanding the possibility that artefacts may distort the geometry of the sample and reduce the resolution, the images presented in this paper show that with careful selection of optical sectioning distances, the use of a suitable stack of sections and, if necessary, the aid of false electronic colours and of partial or complete rotation, it is possible to achieve a more precise interpretation of the morphology and organization of complex structures, such as those of the nervous system.

Current methodologies for the study of pineal morphophysiology

Light and electron microscopes, with or without the use of immunohistochemical techniques, have been the instruments of choice for study of the pineal complex even up to recent times. Other morphological technologies have become available during the past decade that, if applied to current questions concerning pineal morphophysiology, could add considerably to our understanding of this complex system. Those technologies discussed include confocal scanning laser microscopy (in conjunction with other techniques including immunohistochemistry and three-dimensional reconstruction), tissue culture methodologies, carbocyanine dyes (i.e., DiI), in situ hybridization, and application of microinjection methodologies. It is suggested that these technologies will be necessary for morphophysiologists to not only collaborate with molecular biologists and biochemists who study the pineal complex, but to corroborate the molecular and biochemical results of our colleagues.

Immunocytochemical demonstration of nerve growth factor (NGF) receptor in the pineal gland: effect of NGF on pinealocyte neurite formation

Nerve growth factor receptor immunoreactivity (NGFRI) in the pineal gland was examined both light and electron microscopically using the monoclonal antibody 192IgG. NGFRI was located on sympathetic fibers and on perivascular cells resembling macrophage/microglia. A pineal gland dispersed cell culture model confirmed the presence of NGFRI in cells that exhibited processes of varying lengths and were distributed among pinealocytes and other flat cells. Pinealocytes in dispersed cell culture were identified immunocytochemically by their expression of S-antigen, their round shape and small size and their tendency to extend neurites in the direction of the flat cells in culture. The length of pinealocyte neurites showed a significant increase when cultured in the presence of NGF (25 ng/ml), suggesting that trophic factors, mediated by these macrophage/microglial cells, are important to the morphogenesis of these neuroendocrine cells. Neurotrophic activation of these neuroendocrine macrophage/microglia may have neuro-immunomodulatory implications leading to expression of proteins encoded by the major histocompatibility complex.

Protein gene product (PGP) 9.5 immunoreactivity in nerve fibres and pinealocytes of guinea-pig pineal gland: interrelationship with tyrosine- hydroxylase- and neuropeptide-Y-immunoreactive nerve fibres

This light-microscopic (LM) immunohistochemical study has evaluated the presence and distribution of the pan-neural and neuroendocrine marker protein gene product (PGP) 9.5 in pinealocytes and nerve fibres of guinea-pig pineal gland. The pattern of PGP 9.5-immunoreactive (ir) nerve fibres has been compared with that of fibres staining for tyrosine hydroxylase (TH) or neuropeptide Y (NPY). The vast majority of pinealocytes stained for PGP 9.5, although with variable intensity. PGP 9.5 immunoreactivity was localized in pinealocytic cell bodies and processes. Double-immunofluorescence revealed that PGP 9.5 immunoreactivity was absent from glial cells identified with a monoclonal antibody against glial fibrillary acidic protein (GFAP). PGP 9.5 immunoreactivity was also present in a large number of nerve fibres and varicosities distributed throughout the pineal gland. The number of TH-ir and NPY-ir nerve fibres was lower compared with those containing PGP 9.5 immunoreactivity. All fibres staining for NPY also stained for TH. NPY-ir nerve fibres were found to be much more numerous than previously reported for this species. The double-immunofluorescence analysis indicated that almost all TH-ir nerve fibres of the pineal gland contained PGP 9.5 immunoreactivity. However, few PGP 9.5-ir nerve fibres, located in the periphery and the central part of the gland, were TH-negative. A large number of PGP 9.5-ir fibres was concentrated in the pineal stalk. In contrast, TH-ir and NPY-ir nerve fibres were rare in this part of the pineal gland. Our data provide evidence that immunohistochemistry for PGP 9.5 may be a useful tool further to differentiate central and peripheral origins of pineal innervation.(ABSTRACT TRUNCATED AT 250 WORDS)

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

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.