Differential permeability of pineal capillaries to lanthanum ion in the rat (Rattus norvegicus), gerbil (Meriones unguiculatus) and golden hamster (Mesocricetus auratus)

Photoneuroendocrine, circadian and seasonal systems: from photoneuroendocrinology to circadian biology and medicine

This contribution highlights the scientific development of two intertwined disciplines, photoneuroendocrinology and circadian biology. Photoneuroendocrinology has focused on nonvisual photoreceptors that translate light stimuli into neuroendocrine signals and serve rhythm entrainment. Nonvisual photoreceptors first described in the pineal complex and brain of nonmammalian species are luminance detectors. In the pineal, they control the formation of melatonin, the highly conserved hormone of darkness which is synthesized night by night. Pinealocytes endowed with both photoreceptive and neuroendocrine capacities function as "photoneuroendocrine cells." In adult mammals, nonvisual photoreceptors controlling pineal melatonin biosynthesis and pupillary reflexes are absent from the pineal and brain and occur only in the inner layer of the retina. Encephalic photoreceptors regulate seasonal rhythms, such as the reproductive cycle. They are concentrated in circumventricular organs, the lateral septal organ and the paraventricular organ, and represent cerebrospinal fluid contacting neurons. Nonvisual photoreceptors employ different photopigments such as melanopsin, pinopsin, parapinopsin, neuropsin, and vertebrate ancient opsin. After identification of clock genes and molecular clockwork, circadian biology became cutting-edge research with a focus on rhythm generation. Molecular clockworks tick in every nucleated cell and, as shown in mammals, they drive the expression of more than 3000 genes and are of overall importance for regulation of cell proliferation and metabolism. The mammalian circadian system is hierarchically organized; the central rhythm generator is located in the suprachiasmatic nuclei which entrain peripheral circadian oscillators via multiple neuronal and neuroendocrine pathways. Disrupted molecular clockworks may cause various diseases, and investigations of this interplay will establish a new discipline: circadian medicine.

Reactive changes of interstitial glia and pinealocytes in the rat pineal gland challenged with cell wall components from gram-positive and -negative bacteria

Lipopolysaccharide (LPS), the major proinflammatory component of gram-negative bacteria, is well known to induce sepsis and microglial activation in the CNS. On the contrary, the effect of products from gram-positive bacteria especially in areas devoid of blood-brain barrier remains to be explored. In the present study, a panel of antibodies, namely, OX-6, OX-42 and ED-1 was used to study the response of microglia/macrophages in the pineal gland of rats given an intravenous LPS or lipoteichoic acid (LTA). These antibodies recognize MHC class II antigens, complement type 3 receptors and unknown lysosomal proteins in macrophages, respectively. In rats given LPS (50 microg/kg) injection and killed 48 h later, the cell density and immunoexpression of OX-6, OX-42 and ED-1 in pineal microglia/macrophages were markedly increased. In rats receiving a high dose (20 mg/kg) of LTA, OX-42 and OX-6, immunoreactivities in pineal microglia/macrophages were also enhanced, but that of ED-1 was not. In addition, both bacterial toxins induced an increase in astrocytic profiles labelled by glial fibrillary acid protein. An interesting feature following LPS or LTA treatment was the lowering effect on serum melatonin, enhanced serotonin immunolabelling and cellular vacuolation as studied by electron microscopy in pinealocytes. The LPS- or LTA-induced vacuoles appeared to originate from the granular endoplasmic reticulum as well as the Golgi saccules. The present results suggest that LPS and LTA could induce immune responses of microglia/macrophages and astroglial activation in the pineal gland. Furthermore, the metabolic and secretory activity of pinealocytes was modified by products from both gram-positive and -negative bacteria.

Immunohistochemical localization of D-aspartate in the rat pineal gland

Specific polyclonal antibody was raised against D-aspartate (D-Asp) which had been conjugated to glutaraldehyde and was purified by affinity chromatography. Immunohistochemical staining of rat pineal gland with the antibody demonstrated the presence of D-Asp in the cytoplasm of pinealocytes, the predominant cell type in this gland. D-Asp immunoreactivity was more evident in the distal region than in the proximal region of the gland. Pinealocytes in the distal region are presumably involved in the synthesis and secretion of the pineal hormone, melatonin, and the results of staining may indicate some yet unknown role of D-Asp in the regulation of melatonin secretion.

Regional differences in the pineal gland of the cotton rat, Sigmodon hispidus: light microscopic, electron microscopic, and immunohistochemical observations

Light microscopic, electron microscopic and immunohistochemical observations of the various portions of the pineal gland of the cotton rat (Sigmodon hispidus) were made. The volume of the proximal half occupied about 30% of the whole organ, and pinealocytes were slightly smaller in size in the proximal portion than elsewhere. The distal and intermediate portions contained few interstitial cells and numerous astrocytes, but the proximal portion lacked interstitial cells and had more abundant astrocytes than elsewhere. Astrocytes, which were immunoreactive for glial fibrillary acidic protein, mainly lined the pericapillary spaces in the distal and intermediate portions, but in the proximal portion these cells often surrounded isolated or groups of pinealocytes. In the distal and intermediate portions, abundant sympathetic fibers and less numerous non-sympathetic, peptidergic fibers were mainly localized in the pericapillary spaces; these fibers were sparsely distributed in the parenchyma close to interstitial cells or astrocytes. In the proximal portion, non-sympathetic fibers were scarce and sympathetic fibers were distributed abundantly and almost exclusively in the parenchyma. Most of the sympathetic fibers were adjacent to astrocytes and, occasionally, made specialized contact with them. Fenestrae in the capillary endothelium were numerous in the distal portion but absent in the proximal portion. Thus, marked differences in structure existed between the distal and proximal portions of the pineal gland of the cotton rat suggesting that both portions are functionally dissimilar. In addition, the present study indicates that the proximal portion of the cotton rat was well developed and showed morphological features similar to the deeply situated pineal glands of other mammals.

Immunocytochemistry and calcium cytochemistry of the mammalian pineal organ: A comparison with retina and submammalian pineal organs

Morphologically the mammalian pineal organ is a part of the diencephalon. It represents a neural tissue histologically ("pineal nervous tissue") and is dissimilar to endocrine glands. Submammalian pinealocytes resemble the photoreceptor cells of the retina, and some of their cytologic characteristics are preserved in the mammalian pinealocytes together with compounds demonstrable by cyto- and immunocytochemistry and participating in photochemical transduction. In our opinion, the main trend of today's literature on pineal functions--only considering the organ as a common endocrine gland--deviates from this structural and histochemical basis. In mammals, similar to the lower vertebrates, the pinealocytes have a sensory cilium developed to a different extent. The axonic processes of pinealocytes form ribbon-containing synapses on secondary pineal neurons, and/or neurohormonal terminals on the basal lamina of the surface of the pineal nervous tissue facing the perivascular spaces. Ribbon-containing axo-dendritic synapses were found in the rat, cat, guinea pig, ferret, and hedgehog. In the cat, we found GABA-immunoreactive interneurons, while the secondary nerve cells, whose axons enter the habenular commissure, were GABA-immunonegative. GABA-immunogold-labeled axons run between pinealocytes and form axo-dendritic synapses on intrapineal neurons. There is a similarity between the light and electron microscopic localization of Ca ions in the mammalian and submammalian pineal organs and retina of various vertebrates. Calcium pyroantimonate deposits--showing the presence of Ca ions--were found in the outer segments of the pineal and retinal photoreceptors of the frog. In the rat and human pineal organ, calcium accumulated on the plasmalemma of pinealocytes and intercellularly among pinealocytes. The formation of pineal concrements in mammals may be connected to the high need for Ca exchange of the pinealocytes for their supposed receptor and effector functions.

The ultrastructure of mammalian pinealocytes: a systematic investigation

Pinealocytes are not only the principal cellular components of the pineal gland, but they are also the principal synthetic machinery of this enigmatical gland with highly diverse and often questionable empyreal roles assigned to it. Ultrastructural descriptions of pinealocytes belonging to some 70 species of mammals (a mere 2% or less of the over 4,200 mammalian species) have been summarized from the available literature with new observations on 12 species of chiropterans. Space limitation precluded any treatment of the supporting glia, neural elements, and the perivascular spaces. A detailed table lists nearly all mammalian species whose pineal ultrastructure has been investigated. Blanks in this table point to the necessity of studies on those particular groups. A tabular listing of unusual structures reported within the pinealocyte cytoplasm points out the impending experimental work on these species. Such studies using the latest techniques might provide clearer insights into the functional role of the pineal gland as an important and integral component of the neuroendocrine axis. Whereas sufficient structural information now exists on cytoplasmic organelles such as synaptic ribbons and spherules, annulate lamellae, subsurface cisterns, and the several types of synaptic arrangements seen in relation to the pinealocyte soma and its processes, the functional role of these structures in pineal synthetic processes remains to be elucidated.

Permeability of the pineal gland of the viscacha (Lagostomus maximus maximus) to the lanthanum hydroxide

Lanthanum hydroxide together with a fixative solution was injected into the third ventricle of the viscacha. The tracer penetrated freely into the pineal gland demonstrating an extensive network of intercommunicating intercellular spaces. Numerous gap junctions were observed indicating that they are the predominant interparenchymal contacts. The capillaries were characterized by non-fenestrated endothelial cells lining a moderate-sized perivascular space. Precipitates of lanthanum hydroxide were found in the basal membranes of the perivascular and interendothelial spaces, but not in the vascular lumina. The tracer stopped at tight junctions; these constitute a barrier between the pinealocytes and the blood compartment. Active micropinocytosis observed in the capillary walls suggest a transendothelial transport.

Effects of lanthanum in cellular systems. A review

Lanthanum belongs to the group of elements known as "lanthanons," which also includes cerium, europium, promethium, and thulium. It is the most electropositive element of the rare earth group, is uniformly trivalent, and is similar in its chemical properties to the alkaline earth elements. The effects of this element and its compounds on cellular systems are of considerable interest because of their increasing use in industry and as a substitute or antagonist for calcium in a variety of cellular reactions. Lanthanum is also being employed extensively in studying anatomical barriers, membrane structure, and subcellular transport systems, particularly the calcium pathway.