Ultrastructural characterization of the central component of the medial preoptic nucleus

Is There Evidence for Myelin Modeling by Astrocytes in the Normal Adult Brain?

A set of astrocytic process associated with altered myelinated axons is described in the forebrain of normal adult rodents with confocal, electron microscopy, and 3D reconstructions. Each process consists of a protuberance that contains secretory organelles including numerous lysosomes which polarize and open next to disrupted myelinated axons. Because of the distinctive asymmetric organelle distribution and ubiquity throughout the forebrain neuropil, this enlargement is named paraxial process (PAP). The myelin envelope contiguous to the PAP displays focal disruption or disintegration. In routine electron microscopy clusters of large, confluent, lysosomes proved to be an effective landmark for PAP identification. In 3D assemblies lysosomes organize a series of interconnected saccules that open up to the plasmalemma next to the disrupted myelin envelope(s). Activity for acid hydrolases was visualized in lysosomes, and extracellularly at the PAP-myelin interface and/or between the glial and neuronal outer aspects. Organelles in astrocytic processes involved in digesting pyknotic cells and debris resemble those encountered in PAPs supporting a likewise lytic function of the later. Conversely, processes entangling tripartite synapses and glomeruli were devoid of lysosomes. Both oligodendrocytic and microglial processes were not associated with altered myelin envelopes. The possible roles of the PAP in myelin remodeling in the context of the oligodendrocyte-astrocyte interactions and in the astrocyte's secretory pathways are discussed.

Stigmoid bodies contain type I receptor proteins SorLA/LR11 and sortilin: new perspectives on their function

Stigmoid bodies (SBs) are structures in the cytoplasm of neurons. SBs are mostly found in the hypothalamic region of the rat and contain a protein called huntingtin-associated protein 1 (HAP1). In a recent publication, large cytoplasmic structures were shown to be immunoreactive for a type I receptor called SorLA/LR11. By light microscopic analysis, these structures appeared similar to SBs in size and in brain regional and subcellular localization. To determine whether these large puncta correspond to HAP1-containing SBs, we used antibodies specific to various domains of the apolipoprotein receptor LR11 to perform immunocytochemistry in rat and mouse brain tissue. Transfection studies using HeLa cells were conducted to demonstrate the specificity of the antibodies. We found that, in both species, antibodies to the domain II (or VSP10 for vacuolar sorting protein 10 domain) of LR11 immunoreact with large cytoplasmic structures. Co-localization immunolabeling experiments in rat brain tissue sections and in neuron cultures showed that these LR11-immunoreactive structures correspond to HAP1-positive SBs. Electron microscopy was performed in rat hypothalamus and further demonstrated the presence of LR11 in SBs and its co-localization with HAP1. LR11-containing SBs were most abundant in the hypothalamus but were also found in many brainstem nuclei, thalamus, and hippocampus. Our data also show that sortilin, another transmembrane protein containing a VPS10 domain, localizes to large cytoplasmic puncta and is found in LR11-positive and Hap1-positive SBs in hypothalamic neuron cultures.

A protocol for isolation and biochemical characterization of stigmoid bodies from rat brain

Stigmoid bodies (SBs) are structures present in the cytoplasm of neurons. Many brain regions including hypothalamus, thalamus, amygdala, septum, hippocampus, colliculi, and brainstem contain neurons with at least one SB. Despite this widespread distribution their function remains unknown. SBs contain a brain protein called huntingtin-associated protein 1 (HAP1) and have more recently been found to contain the apolipoprotein E receptor LR11 (Lipoprotein Receptor containing 11 LDL binding domains, also called SorLA for sorting protein-related receptor containing LDLR class A repeats) and sortilin. To provide a first step towards further identification of their components and perhaps shed some light on their neurobiological role, we have developed a method for isolating SBs from rat brain. The protocol relies on a combination of centrifugational forces, sucrose gradient, and immunoisolation. Samples enriched in SBs were incubated with antibodies to HAP1B or to LR11 followed by incubation with FITC conjugated secondary antibodies. Anti-FITC coated beads were incubated with samples and SB-bead complexes formed were separated by magnetic sorting without pelleting the complexes during the isolation procedure. Immunopurified SBs, visualized by light and electron microscopy, show similar ultrastructure to those present in neurons.

Testosterone effects on the neuronal ultrastructure in the medial preoptic nucleus of male Japanese quail

Dorsolateral neurons of the medial preoptic nucleus (POM) of male Japanese quail are sensitive to the plasma levels of testosterone: their volume and optical density in Nissl-stained sections increase in castrated birds treated with testosterone. The present study was performed on castrated male quail treated or not with Silastic implants filled with testosterone to describe the ultrastructural variations induced by testosterone in these neurons. Gonadally intact male birds were included as controls. The ultrastructure of neurons, taken from the dorsolateral portion of the POM, was dramatically affected by the endocrine manipulations. Quantitative evaluations demonstrated a significant decrease in castrated birds of the rough endoplasmic reticulum (RER), of free polyribosomes, of Golgi complexes, and of dense bodies; these changes paralleled the decrease in cell size. The cell size and the percentage of volume occupied by the intracellular organelles in castrated birds treated with testosterone were comparable to values observed in controls. These ultrastructural changes are similar to those observed in neuronal targets for other gonadal hormones, supporting the idea that testosterone stimulates the development of cytoplasmic structures involved in protein synthesis and secretion. In addition, exposure to testosterone affects the synaptic inputs to POM. These ultrastructural changes are presumably related to the physiological effects (e.g., activation of male sexual behavior) exerted by testosterone on this preoptic region.

Ultrastructural characterization of the sexually dimorphic medial preoptic nucleus of male Japanese quail

The medial preoptic nucleus is a sexually dimorphic structure whose cytoarchitecture, afferent and efferent connections, and functions have been previously described. No detailed ultrastructural study has, however, been performed to date. Here we describe the ultrastructural organization of this important preoptic structure of the male quail. Neuronal cell bodies of the medial preoptic nucleus generally show extensive development of protein-synthesis-related organelles (rough endoplasmic reticulum, polysomes), and of secretory structures (Golgi complexes, secretory vesicles, dense bodies). Previous morphometrical studies at the light-microscopical level have demonstrated the presence of a medial and a lateral neuronal population distinguished by the size of their cell bodies (the medial neurons are smaller than the lateral neurons). The present ultrastructural investigation confirms the difference in size, but no difference has been observed in the ultrastructural organization of the neurons. In both the medial and the lateral part, the nucleus is characterized by a large variety of cell bodies, including some that, on the basis of their ultrastructure, can be considered as putative peptidergic neurons. Close contacts are frequently observed between adjacent cell bodies that are normally arranged in clusters. Various types of synaptic endings are also present, suggesting a rich supply of nerve fibers. A few glial cells are scattered within the nucleus. In view of the crucial role of this region in regulating quail sexual behavior, the large heterogeneity of neurons and of afferent nervous fibers suggest that this region might have an important role in the integration of information arriving from different brain regions.

Evidence of a direct projection of the vomeronasal organ to the medial preoptic nucleus and hypothalamus

In rodents the vomeronasal system plays an important role in modulating the hypothalamic control of gonadotropin secretion. It is accepted that the anatomical pathways by which the vomeronasal organ influences the neuroendocrine aspects of the reproductive behavior involves a polysynaptic system, including the accessory olfactory bulb, the 'vomeronasal amygdala', the bed nucleus of the stria terminalis and medial preoptic and ventromedial hypothalamic nuclei. We found that 72 h after unilateral transection of the vomeronasal nerves of an ipsilateral orthograde degeneration is seen in the neuropil of the medial preoptic, ventromedial, and arcuate hypothalamic nuclei. This represents an anatomical evidence of a monosynaptic link between the vomeronasal neuroepithelium and the medial preoptic nucleus, and hypothalamus.

Immunocytochemical localization and quantitation of estrogen-binding cells in the male and female (virgin, pregnant, lactating) mouse brain

Estrogen-binding cells in the brains of male, virgin, pregnant, and lactating female mice were localized and quantified using an immunocytochemical technique. Nuclear estrogen-receptors were detected in cells of the midbrain periventricular gray, the cortical and medial amygdaloid nucleus, the arcuate nucleus and ventromedial nucleus of the hypothalamus, the anterior hypothalamic area and the preoptic area in animals of all 4 experimental groups. In virgin females, immunostaining was also seen in the hippocampus, the entorhinal and piriform cortex and in the septal nuclei. In pregnant females the anterior amygdaloid area and the bed nucleus of the stria terminalis were labelled additionally. After the first 5 days of lactation, estrogen-binding cells could no longer be detected in the hippocampus, entorhinal and piriform cortex and anterior amygdaloid area. These apparent qualitative differences in the pattern of estrogen-receptor immunostaining among the experimental groups are further accentuated by significant quantitative differences in the absolute numbers and relative densities of labelled cells in a given nucleus. Pregnant females have the highest numbers of estrogen-binding cells in many nuclei, whereas lactating females have the highest densities of labelled cells. These differences suggest a dynamic regulation of the number and distribution of estrogen-target cells in the respective brain areas initiated by, or as a result of hormonal states of the animals. Functional implications are discussed.