Increased expression of the N-methyl-D-aspartate receptor subunit, NR1, in immunohistochemically identified magnocellular hypothalamic neurons during dehydration

N-Methyl-D-aspartate receptors are thought to be involved in synaptic signaling within the hypothalamo-neurohypophysial system, but the extent and nature of their involvement has not been determined. In this study, in the rat, we evaluated the effect of hyperosmotic stimulation on the N-methyl-D-aspartate receptor subunit, NR1, which confers function to N-methyl-D-aspartate receptor heteromers. Co-localization of immunoreactivity for NR1 and vasopressin- or oxytocin-associated neurophysin in magnocellular neurons of the supraoptic and paraventricular hypothalamic nuclei was accomplished using double-label immunohistochemistry. Our results show that vasopressin- and oxytocin-neurophysin-positive populations contained detectable levels of NR1 labeling. Using NR1 labeling as a measure of N-methyl-D-aspartate receptor density, we examined the effect of dehydration in these nuclei. Using computer-assisted densitometry, we found significantly greater NR1 labeling densities in the magnocellular regions of both the supraoptic and paraventricular nuclei of saline-treated rats than of control rats. This increase was not due to methodological factors, since no changes in NR1 labeling density were found in a nearby nucleus, the nucleus reuniens. Western blot analysis showed similar selective increases in NR1 labeling in homogenates from the supraoptic nucleus, paraventricular nucleus and in some cases from the anterior hypothalamic area. In both immunohistochemical and western blotting experiments we did not observe a dehydration-induced increase in NR1 in other brain areas examined. Our results showing an up-regulation of NR1-containing N-methyl-D-aspartate receptors during dehydration suggest that these receptors are involved in the regulation of body water and may represent an adaptive physiological response following activation of the hypothalamo-neurohypophysial axis. In addition, these results suggest that the functional expression of N-methyl-D-aspartate receptors is dynamic and may be modified according to the physiological state of the animal.

Calbindin-D28k: role in determining intrinsically generated firing patterns in rat supraoptic neurones

1. Physiological activation of rat supraoptic nucleus (SON) neurones leads to phasic firing in vasopressin neurones and fast, continuous firing in oxytocin neurones. Using whole-cell patch clamp methods in brain slices, we investigated the role of endogenous calbindin-D28k (calbindin) in determining these intrinsically generated patterns of firing. 2. Direct introduction of calbindin (0.1-0.2 mM) into twelve of twelve phasically firing neurones suppressed Ca(2+)-dependent depolarizing after-potentials (DAPs) and changed activity from phasic to continuous firing. Bovine calcium binding protein (0.3 mM), an analogue of calbindin, had similar effects on both DAPs and firing patterns in five of five cells tested. 3. Introduction of anti-calbindin antiserum (1:2000-5000) into thirteen of thirteen continuously firing neurones unmasked DAPs and converted continuous into phasic firing. Such effects could not be mimicked either by diffusion of normal rabbit serum or antibodies directed against glial fibrillary acidic protein or against neurophysin. 4. Immunocytochemical staining with antisera directed against calbindin revealed more intense staining in the dorsal, oxytocin-rich and less intense staining in the ventral, vasopressin-rich areas of the SON. 5. Elevated intracellular Ca2+ concentration ([Ca2+]i; 0.1 mM) induced DAPs and phasic firing in all twenty-nine SON cells recorded. During chelation of intracellular Ca2+ with (1.1-11 mM) BAPTA, fifty-eight of fifty-eight neurones recorded displayed regular continuous activity and had no DAPs. 6. These data suggest that firing activities in SON cells are dependent on [Ca2+]i and that calbindin, acting as an endogenous Ca2+ buffer, is involved in regulation of intrinsic firing patterns. It is likely that calcium binding proteins have a similar influence on the firing patterns of many neuronal types throughout the nervous system.

Taurine immunoreactivity in the rat supraoptic nucleus: prominent localization in glial cells

Taurine is an inhibitory amino acid that hyperpolarizes magnocellular neurosecretory neurons. To determine which cell types in the rat supraoptic nucleus contain taurine, we used a monoclonal antibody raised against a taurine conjugate. Preembedding immunocytochemistry was carried out at the light and electron microscopic levels using diaminobenzidine and gold-substituted silver-intensified peroxidase as markers. We report the presence of taurine in all cellular compartments of the supraoptic nucleus, except axons, with variable labeling intensities among the different compartments. Few cell bodies of magnocellular neurons were immunoreactive, but many distal dendrites and some proximal ones showed weak-to-moderate levels of immunoreactivity. Strong immunoreactivity was found over glial cell bodies and their processes, in particular in the ventral glial lamina of the supraoptic nucleus. Large astrocytic processes labeled with the taurine antibody included the endfeet participating in the glial limitans around capillaries and at the ventral surface of the hypothalamus. Other types of immunoreactive astrocytic profiles were found scattered within the neuropil where these processes participated in different interactions with the neuronal elements of the supraoptic nucleus. Immunoreactive glial expansions, sometimes even the main process of the glial cell, engulfed axonal boutons. Other labeled glial processes were found between two magnocellular perikarya or closely apposed to the membrane of axonal boutons contacting the neuronal cell bodies. The frequent finding of closely apposed glial and dendritic elements bearing different levels of taurine-like immunoreactivity suggests that exchange of taurine between those two compartments may occur. We propose that taurine could be released from supraoptic glia by a small decrease in osmolarity or by receptor-mediated mechanisms during conditions of low hormonal (vasopressin and/or oxytocin) needs. Such released taurine could then act on presynaptic or postsynaptic sites, or both, to exert its neuromodulatory actions.

VGF expression in the brain

VGF is a neuronal polypeptide first identified as a cDNA clone in a gene library from nerve growth factor-stimulated PC12 cells. In the present paper, the expression of VGF is examined for the first time throughout the adult rat central nervous system with immunocytochemistry and Northern blot analysis. VGF RNA was found in all brain regions studied, including hypothalamus, hippocampus, cerebellum, olfactory bulb, and cortex. In contrast to the relatively strong immunostaining of hypothalamic neurons, the level of VGF RNA expression in the hypothalamus was relatively low in comparison with other brain regions. With the aid of antisera raised against bacterially produced recombinant proteins containing parts of the VGF sequence, immunoreactive neurons were detected throughout the brain, including regions of the olfactory tubercle, caudate-putamen, thalamus, cortex, amygdala, hypothalamus, midbrain, and hippocampus. VGF-immunoreactive neurons did not contain detectable amounts of nerve growth factor receptor; other neurons that showed nerve growth factor receptor immunoreactivity expressed no VGF immunoreactivity. The lack of colocalization of VGF and nerve growth factor receptor suggests that, unlike expression in PC12 cells, VGF expression in neurons from the central nervous system does not require nerve growth factor stimulation. Within the hippocampus, the location of VGF-immunoreactive cells was suggestive of inhibitory interneurons. VGF-immunoreactive axons and terminals were found throughout the brain. These observations extend our earlier work on VGF expression in the hypothalamus to other regions of the brain and support the conclusion that although VGF expression is only detected in subsets of neurons in each brain region, these subsets are widely distributed throughout the central nervous system.

Converging GABA- and glutamate-immunoreactive axons make synaptic contact with identified hypothalamic neurosecretory neurons

To study the neurochemical identity of axons in synaptic contact with identified hypothalamic neurosecretory neurons in rats, we combined retrograde axonal transport of a marker molecule with postembedding immunogold staining for amino acid neurotransmitters. After intravenous injections of horseradish peroxidase, neurosecretory neurons with axons in the median eminence or neurohypophysis transported the peroxidase retrogradely back to the cell body of origin. Serial ultrathin sections from the paraventricular and arcuate nuclei were immunostained with glutamate or GABA antisera. Peroxidase-labeled neurons and their dendrites received synaptic contact from colloidal gold-labeled axons immunoreactive for GABA or for glutamate. Axons which were highly immunoreactive for GABA and other axons immunoreactive for glutamate but not for GABA consistently made converging synaptic contact with the same peroxidase-labeled cell. Some of the peroxidase-labeled neurons from the arcuate nucleus which were postsynaptic to both GABA and glutamate axons were themselves identified as being GABA immunoreactive. Serial ultrathin sections revealed that multiple presynaptic axons immunoreactive for glutamate or GABA made repeated contacts with single neurons. These results suggest a widespread convergence of the major inhibitory and excitatory amino acid transmitter on the neurons which control both the anterior and posterior pituitary hormones.

GABA: a dominant neurotransmitter in the hypothalamus

To study the organization and distribution of the inhibitory amino acid neurotransmitter GABA in the medial hypothalamus, we used a postembedding immunocytochemical approach with colloidal gold. Quantitative analysis showed that half (49%) of all synapsing boutons studied were immunoreactive for GABA, based on immunogold staining of the suprachiasmatic, arcuate, supraoptic, and paraventricular nuclei. This was corroborated with pre-embedding peroxidase immunostaining with antisera against glutamate decarboxylase, the GABA synthetic enzyme. These data suggest that GABA is the numerically dominant neurotransmitter in the hypothalamus, and emphasize the importance of inhibitory circuits in the hypothalamus. Serial ultrathin sections were used to reconstruct GABA immunoreactive boutons and axons in three dimensions. With this type of analysis we found less morphological heterogeneity between GABA immunoreactive boutons than with single ultrathin sections. Single sections sometimes showed boutons containing only small clear vesicles, and other with both clear vesicles and small dense core vesicles. However, with serial sections through individual boutons, dense core vesicles were consistently found at the periphery of the pre-synaptic GABA immunoreactive boutons, suggesting probable co-localization of GABA with unidentified peptides in most if not all boutons throughout the hypothalamus. A positive correlation was found between the density of small clear vesicles and the intensity of immunostaining with colloidal gold particles. GABA immunoreactive axons generally made symmetrical type synaptic specializations, although a small percentage made strongly asymmetrical synaptic specializations. Vesicles in GABA immunoreactive boutons were slightly smaller than those in non-reactive boutons. Synaptic efficacy is related to the position of the synapse on the post-synaptic neuron. While the majority of GABA immunoreactive axons made synaptic contact with dendrites, the distribution of GABA immunoreactive synapses on somata and dendrites was the same as would be expected from a random distribution of all boutons. No preferential innervation of cell bodies by GABA immunoreactive terminals was found. Serial ultrathin sections showed that a GABA immunoreactive axon would sometimes make repeated synaptic contacts with a single postsynaptic neuron, indicating a high degree of direct control by the presynaptic GABAergic cell. Other immunoreactive axons made synaptic contact with a number of adjacent dendrites and cells, suggesting a role for GABA in synchronizing the activity of hypothalamic neurons. Based on the density of immunogold particles per unit area, varying concentrations of immunoreactive GABA were found in different presynaptic boutons in the hypothalamus.

Hypothalamic expression of a novel gene product, VGF: immunocytochemical analysis

VGF is the designation for a new 712 amino acid protein, regulated by nerve growth factor (NGF) in PC12 cells, that has not been previously described in the CNS. Northern blot analysis with a nick-translated VGF cDNA probe revealed a single band of mRNA in the brain with a molecular weight identical to that found in PC12 cells. The current paper presents a series of immunocytochemical studies of VGF expression with a focus on the hypothalamus. Two different antisera were raised against nonoverlapping amino acid sequences of a bacterial-expressed protein from the VGF gene cloned from PC12 cells. VGF immunoreactivity is strongly expressed in the rat suprachiasmatic nucleus (SCN), particularly in the dorsomedial part of the nucleus. The administration of colchicine to block axonal transport facilitates detection of the VGF immunoreactivity also in the ventrolateral suprachiasmatic nucleus. This protein appears to be the first one of limited neuronal distribution which is found in both dorsomedial SCN and ventrolateral SCN. Immunostaining of serial 1 micron SCN sections reveals co-localization of VGF in cells which also contain vasopressin or vasoactive intestinal polypeptide. Weaker immunoreactivity is also found in the magnocellular paraventricular and supraoptic nuclei, where the VGF immunoreactivity co-localizes with oxytocin or vasopressin. Mutant Brattleboro rats which do not express vasopressin showed strong VGF immunoreactivity both in the dorsomedial SCN and in cells of the magnocellular neuronal systems, including cells which normally express vasopressin. When axonal transport of the protein is blocked by colchicine, VGF-immunoreactive cells in the hypothalamic arcuate, parvocellular paraventricular, and tuberomammillary nuclei can also be detected, in addition to weakly immunoreactive scattered cells in the hippocampus, amygdala, thalamus, and cortex. VGF immunoreactivity is strong in the axonal projections of SCN and weak in the axons of the paraventricular and supraoptic nuclei. With ultrastructural studies, VGF immunoreactivity is found in presynaptic boutons in the SCN and in axons in the neurohypophysis. Weak axonal staining is present in some regions of the hypothalamus and in the external and internal zones of the median eminence. Immunoreactivity is absent from the intermediate lobe of the hypophysis. In neonatal rats strong VGF immunoreactivity is found throughout the SCN at postnatal day 4 but not in the adjacent hypothalamus. VGF immunoreactivity is also seen in other areas of the brain in neonatal rats, including the lateral geniculate nucleus; while the staining in the dorsal lateral geniculate disappears in the adult, that in the intergeniculate leaflet, a visual center which projects to the SCN, remains.(ABSTRACT TRUNCATED AT 400 WORDS)

Simultaneous immunogold labeling of GABAergic terminals and vasopressin-containing neurons in the rat paraventricular nucleus

The GABAergic innervation of vasopressin-containing cells in the magnocellular part of the paraventricular nucleus was studied at the electron-microscope level using antibodies against GABA and vasopressin. The detection of both GABA and vasopressin on the same ultrathin section, performed with a double-labeling immunogold method, revealed GABAergic terminals in symmetrical synaptic contact with vasopressin-containing neurons. These GABAergic terminals displayed mitochondria, clear synaptic vesicles and varying numbers of electron-dense vesicles. Vasopressin-immunoreactivity was associated with neurosecretory granules, whereas GABA-immunoreactivity was found above mitochondria, clear synaptic vesicles and some electron-dense vesicles. This study, demonstrating the extensive participation of GABA in the innervation of magnocellular vasopressin-secreting neurons, suggests that this inhibitory neurotransmitter regulates vasopressin secretion at the level of the paraventricular nucleus.

First visualization of dopaminergic neurons with a monoclonal antibody to dopamine: a light and electron microscopic study

A monoclonal antibody recently synthesized against dopamine (DA) was tested in rat and mouse brain sections after further treatment by PAP immunocytochemistry at the light and electron microscopic levels. Distribution of DA-immunoreactive cell bodies was examined in the substantia nigra (sn), the ventral tegmental area (vta), and the raphe nuclei. DA-immunoreactive fibers were investigated in two DA projection systems, the striatum and the septum. Many dopaminergic cell bodies were found in the sn and the vta. Some scattered DA neurons were encountered in the pars reticulata of the sn. The dorsal raphe and linearis raphe nuclei displayed sparse immunoreactive neurons and a dense plexus of DA fibers. Immunoreactive fibers were observed in the entire striatum, more dense in the ventral part. In the septum, immunonegative neurons were outlined by thin DA fibers in synaptic contact with their somata or dendrites. According to our observations, this DA monoclonal antibody seems to be a selective and sensitive tool for studying the dopaminergic neuronal circuitry at both histological and ultrastructural level.

Comparative study of dopamine- and noradrenaline-immunoreactive terminals in the paraventricular and supraoptic nuclei of the rat

The distribution of dopaminergic and noradrenergic terminal fields of the paraventricular (PVN) and supraoptic (SON) nuclei of the rat was investigated at the optic and electron microscopical level using antibodies directed against dopamine (DA) and noradrenaline (NA). The DA innervation was uniform among these nuclei, although more important in the PVN than in the SON. NA fibers were preferentially distributed in the parvocellular parts of the PVN and in areas of the magnocellular nuclei where vasopressinergic neurons were mainly located. Both DA and NA terminals synaptically contacted magnocellular neurons on their cell body or dendrites. This study thus provides morphological evidence for a double and independent catecholaminergic control, by DA and NA, on neuroendocrine mechanisms at the hypothalamic level.