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Biomedical subjects

A Beaudet

Publications and source records attributed to A Beaudet.

At least 91 records · Page 5Linked to original sources
PubMed1993

Estradiol is selectively neurotoxic to hypothalamic beta-endorphin neurons.

The neurotoxic effects of estradiol on hypothalamic arcuate neurons were examined in a model of chronic estrogenization induced by means of a single injection of estradiol valerate (EV). Eight weeks after EV treatment, a 60% decrease in the total number of beta-endorphin-immunoreactive neurons was detected in the arcuate nucleus. In contrast, the numbers of neurotensin-, somatostatin-, and tyrosine hydroxylase-immunoreactive neurons were unchanged, suggesting that the effects of estradiol were selective for beta-endorphin neurons. Further evidence for the selectivity of estradiol's actions was provided by RIAs indicating decreases in hypothalamic beta-endorphin concentrations, but not in Metenkephalin or neuropeptide-Y concentrations. Cell counts performed in Nissl-stained material using unbiased stereological methods revealed a reduction in the total number of neurons in the EV-treated group compared to that in the controls. The estimated number of neurons lost (approximately 3500) corresponded precisely with the total number of beta-endorphin neurons lost (approximately 3600), as estimated using quantitative immunocytochemistry. These results confirm the selectivity of estradiol's effect on the beta-endorphin cell population and demonstrate that the observed decrease in beta-endorphin immunoreactivity reflects actual cell loss. The evidence indicates that the selective neurotoxic effect of estradiol on hypothalamic beta-endorphin neurons contributes to reproductive senescence, suggesting that steroids may participate in disruption of the biological functions that they normally facilitate.

Animals↗
PubMed1993

Ultradian oscillation in somatostatin binding in the arcuate nucleus of adult male rats.

The ultradian rhythm of GH secretion in the male rat is generated by the reciprocal cyclic release of somatostatin (SRIF) and GH-releasing factor (GRF) from the hypothalamus. We recently demonstrated the presence of high affinity [125I]SRIF-binding sites on a subpopulation of GRF-containing arcuate neurons in rat hypothalamus. In the present study, we tested the hypothesis that these binding sites undergo rhythmic fluctuations in parallel with those of GH. Adult male rats were killed at times associated with either a peak (1100 h) or trough (1300 h) period of the GH rhythm. The hypothalamus was serially sectioned from the retrochiasmatic nucleus, rostrally, to the mammillary recess of the third ventricle, caudally, and [125I]SRIF-binding sites were labeled in vitro using high resolution autoradiography. [125I]SRIF-labeled neuronal perikarya were counted at eight cross-sectional levels across the arcuate nucleus, and binding densities were quantitated over each of these cross-sectional surfaces using computer-assisted microdensitometry. Both the number of labeled cells and the density of [125I]SRIF binding varied as a function of the time of death. The average number of [125I]SRIF-labeled cells per 10-microns thick section was 2- to 3-fold higher in rats killed at 1100 h than in those killed at 1300 h. In addition, overall binding density levels were 65% higher in animals killed at the onset of a GH peak than in those killed at the time of a GH trough. Both of these effects were apparent throughout the rostrocaudal extent of the arcuate nucleus. In contrast, [125I]SRIF binding density in the cerebral cortex did not vary between 1100 and 1300 h. These results demonstrate an ultradian variation in SRIF binding to arcuate neurons in relation to the peaks and troughs of the GH rhythm. Such rhythmicity in SRIF receptors might underlie a temporal responsiveness of arcuate GRF neurons to endogenous SRIF and may be an important mechanism by which SRIF modulates the rhythmic release of hypothalamic GRF in generation of the ultradian rhythm of GH secretion.

Activity Cycles↗
PubMed1992

Electron microscopic localization of photoaffinity-labelled delta opioid receptors in the neostriatum of the rat.

The distribution of delta opioid receptors, selectively labelled in vitro with the photoaffinity probe monoiodo azido-DTLET ([D-Thr2,pN3Phe4, Leu5]enkephaly-Thr6), was analyzed by light and electron microscopic radioautography in sections from rat neostriatum. Preliminary experiments indicated that up to 65% of specific 125I-azido-DTLET binding to rat striatal sections was still detectable following prefixation of the brain with 0.5% glutaraldehyde. These experiments also showed that up to 20-30% of the specifically bound radioactivity was covalently linked following ultraviolet irradiation and was thereby retained in tissue during subsequent postfixation and dehydration steps. Accordingly, the topographic distribution of the covalently attached azido-DTLET molecules was similar to that seen in fresh frozen sections and characteristic of that previously described for delta sites. Light and electron microscopic examination of the label in prefixed, striatal sections irradiated with ultraviolet light revealed that a significant proportion of specifically bound 125I-azido-DTLET molecules was intraneuronal. Specifically, 16% of the labelled binding sites were found in dendrites, 12% in perikarya and 4% in axon terminals. These results suggest that an important proportion of delta opioid binding sites labelled in the neostriatum correspond to receptors that are undergoing synthesis, transport and/or recycling. They also imply that a major fraction of delta sites are associated with intrastriatal neurons, as opposed to afferent axons. Approximately 44% of the labelled binding sites were associated with neuronal plasma membranes. Although most of these were found at the level of axodendritic (20%) and dendrodendritic (7%) appositions, comparison of the labelling incidence of these two compartments with their frequency of occurrence in tissue suggested that delta sites are fairly widely dispersed along neuronal plasma membranes. Only a small proportion (smaller than that of mu or kappa sites labelled in the same region) was associated with synaptic specializations. These results support the concept that delta receptors correspond to molecular entities that are distinct from mu and kappa sites and suggest that delta ligands act primarily nonjunctionally on the plasma membrane of striatal neurons.

Affinity Labels↗
PubMed1992

Neurotensin terminals form synapses primarily with neurons lacking detectable tyrosine hydroxylase immunoreactivity in the rat substantia nigra and ventral tegmental area.

A light and electron microscopic double antigen localization technique was employed to examine the fine structural relationship between neurotensin-containing axon terminals and dopaminergic neurons in the substantia nigra and ventral tegmental area of the rat. At the light microscopic level, neurotensin-immunoreactive terminals were densely distributed throughout the substantia nigra pars compacta and ventral tegmental area in close proximity to tyrosine hydroxylase-immunoreactive somata and dendrites. On electron microscopic examination, direct synaptic connections were identified between neurotensin-immunoreactive axon terminals and tyrosine hydroxylase-immunopositive perikarya and dendrites. However, only 8.2% and 8.8% of the neurotensin-immunoreactive axonal profiles detected in the substantia nigra and ventral tegmental area, respectively, were found in direct apposition with tyrosine hydroxylase-immunostained elements. In turn, only 9.3% and 10.0% of tyrosine hydroxylase immunoreactive dendrites sampled from the substantia nigra and ventral tegmental area, respectively, were seen in contact with neurotensin immunopositive axon terminals. However, neurotensin-immunoreactive and tyrosine hydroxylase-immunolabelled elements were frequently identified in close anatomical proximity (less than 5 microns) to one another. These results are interpreted in light of the selective association of neurotensin receptors with dopaminergic neurons in the substantia nigra and ventral tegmental area to suggest a predominantly parasynaptic mechanism of action for neurotensin in the ventral midbrain.

Animals↗
PubMed1992

Synaptic localization of kappa opioid receptors in guinea pig neostriatum.

Distribution of kappa opioid receptors was examined by EM radioautography in sections of guinea pig neostriatum with the selective 125I-labeled dynorphin analog [D-Pro10]dynorphin-(1-11). Most specifically labeled binding sites were found by probability circle analysis to be associated with neuronal membrane appositions. Because of limitations in resolution of the method, the radioactive sources could not be ascribed directly to either one of the apposed plasma membranes. Nevertheless, three lines of evidence favored a predominant association of ligand with dendrites of intrinsic striatal neurons: (i) the high frequency with which labeled interfaces implicated a dendrite, (ii) the enrichment of dendro-dendritic interfaces, and (iii) the occurrence of dendritic profiles labeled at several contact points along their plasma membranes. A small proportion of labeled sites was associated with axo-axonic interfaces, which may subserve the kappa opioid-induced regulation of presynaptic dopamine and acetylcholine release documented in guinea pig neostriatum. Although most membrane-associated kappa sites were found at extrasynaptic locations, approximately 23% were associated with synaptic specializations. This proportion is markedly higher than that previously reported for either mu or delta sites in rat neostriatum. Whether labeled synapses represent preferential sites of action for kappa ligands remains to be established. In any event, these results support the hypothesis that in mammalian brain kappa opioid receptors are conformationally and functionally distinct from mu and delta types.

Amino Acid Sequence↗
PubMed1992

Association of neurotensin receptors with VIP-containing neurons and serotonin-containing axons in the suprachiasmatic nucleus of the rat.

The aim of the present study was to identify cellular elements bearing high-affinity neurotensin (NT) binding sites in the suprachiasmatic nucleus (SCN) of the rat hypothalamus. Because the distribution of these binding sites had previously been reported to conform to that of both vasoactive intestinal peptide (VIP)-containing nerve cell bodies and serotonin (5-HT)-containing axons, the following experimental approaches were used: (1) the overlap between autoradiographically labeled NT binding sites and immunocytochemically labeled VIP neurons was examined in adjacent 5-microns-thick sections taken across the entire rostrocaudal extent of the SCN; and (2) the density of NT binding sites was examined by quantitative autoradiography following cytotoxic lesioning of 5-HT afferents. Double-labeling studies demonstrated precise overlap between 125I-NT binding and VIP immunostaining throughout the SCN. Moreover, at high magnification intensely VIP-immunoreactive neurons were found in direct register with 125I-NT-labeled cells visualized in adjacent sections. Densitometric autoradiographic studies demonstrated a significant reduction in specific 125I-NT binding within the SCN following intracerebroventricular injection of the neurotoxin, 5,7-dihydroxytryptamine. Taken together, these results indicate that within the SCN, NT receptors are present both presynaptically on serotonin axons and postsynaptically on the perikarya and dendrites of VIP-containing neurons.

5,7-Dihydroxytryptamine↗
PubMed1992

Light and electron microscopic localization of retrogradely transported neurotensin in rat nigrostriatal dopaminergic neurons.

We previously demonstrated the existence of a retrograde axonal transport of radioactivity to the substantia nigra pars compacta following injection of mono-iodinated neurotensin in rat neostriatum. In the present study, the topographical and cellular distribution of this retrogradely transported material was examined by light and electron microscopic autoradiography. Four and a half hours after unilateral injection of [125I]neurotensin in the caudoputamen, retrogradely labelled neuronal cell bodies were detected by light microscopic autoradiography throughout the ipsilateral substantia nigra pars compacta as well as within the ventral tegmental area and retrorubral field. In semithin sections, silver grains were prevalent over the perinuclear cytoplasm of neuronal cell bodies but were also detected over neuronal nuclei. Analysis of electron microscopic autoradiographs revealed that the vast majority (greater than 85%) were associated with neuronal perikarya, unmyelinated and myelinated axons, dendrites and terminals. Within the soma, a significant proportion of silver grains (16% of somatic grains) was detected over the nucleus. However, the majority were identified over the cytoplasm where they often encompassed cytoplasmic organelles, including rough endoplasmic reticulum, mitochondria, Golgi apparatus, lysosomes, and multi-vesicular bodies. In dendrites and axons, a substantial percentage of silver grains (63-89%) was localized over the plasma membrane. A minor proportion (13% of total) of the autoradiographic labelling was detected over myelin sheaths, astrocytes, and oligodendrocytes. The present results are consistent with previous light-microscopic evidence for internalization and retrograde transport of intrastriatal neurotensin within nigrostriatal dopaminergic neurons. They further suggest that retrogradely transported neurotensin may be processed along a variety of intracellular pathways including those mediating degradation in lysosomes and recycling in rough endoplasmic reticulum. The detection of specific autoradiographic labelling in the nucleus supports the concept that neurotensin alone, or complexed to its receptor, might be involved in the regulation of gene expression through direct or indirect interactions with nuclear DNA. Consequently, the retrograde transport of neurotensin in nigrostriatal dopaminergic neurons might provide a vehicle through which events occurring at the level of the axon terminal may initiate long-term biological responses.

Animals↗
PubMed1992

Colocalization of somatostatin receptors and growth hormone-releasing factor immunoreactivity in neurons of the rat arcuate nucleus.

Recent studies from our group have demonstrated an association of [125I]-labeled somatostatin (SRIF)-binding sites with a subpopulation of arcuate (ARC) neurons. The distribution of these cells was similar to that of growth hormone-releasing factor (GRF)-immunoreactive neurons, which led us to propose that at least some SRIF receptors may be directly localized to GRF-containing cells. To test this hypothesis, we have visualized radiolabeled SRIF-binding sites and GRF immunoreactivity (ir) in adjacent sections of the hypothalamus, by combined radioautography and immunohistochemistry. Adult male rats were sacrificed by decapitation and the brains were rapidly frozen and serially sectioned on a cryostat. Fifteen pairs of adjacent 6-microns-thick sections, taken at 100-microns intervals through the rostrocaudal extent of the ARC nucleus, were alternately processed for [125I]-SRIF radioautography and GRF immunohistochemistry. GRF-ir and [125I]-SRIF-labeled cells were mapped at each level and quantified with the aid of a camera lucida. The maps were subsequently superimposed to determine the extent of [125I]-SRIF/GRF-ir colocalization. GRF-ir perikarya [13.2 +/- 4.4 (mean +/- SE) cells per section] were mainly localized in the ventrolateral portion of the ARC nucleus and predominated within the caudal-most tier. [125I]-SRIF-labeled cells (35.6 +/- 6.5 cells per section) were more numerous, more evenly distributed, and extended further rostrally and caudally than GRF-ir cells. Superimposition of the camera lucida maps indicated that, overall, 33.5 +/- 10.8% of the GRF-ir cells were labeled with [125I]-SRIF in adjacent sections.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1992

Monosodium glutamate-induced reductions in hypothalamic beta-endorphin content result in mu-opioid receptor upregulation in the medial preoptic area.

Estradiol valerate (EV) treatment in the rat induces a lesion of the hypothalamic arcuate nucleus, resulting in significant decreases in hypothalamic beta-endorphin. In addition, the EV treatment causes a selective increase in mu-opioid binding in the medial preoptic area (MPOA). Since beta-endorphin neurons located in the arcuate nucleus project extensively to the MPOA, we have hypothesized that the EV-induced loss of these afferents induces a compensatory upregulation of mu-opioid receptors in opioid target neurons. In order to test this hypothesis, we have utilized monosodium glutamate (MSG) treated animals as a model of beta-endorphin cell loss and hence of beta-endorphin deafferentation of the MPOA. Neonatal MSG treatment has been shown to result in the destruction of 80-90% of arcuate neurons accompanied by pronounced decreases in beta-endorphin concentrations in both arcuate nucleus and MPOA. mu-Opioid binding sites were radioautographically labeled in sections from the MPOA of sham- and MSG-injected animals using the methionine enkephalin analogue 125I-FK 33-824 and quantitated by computer-assisted densitometry. The remainder of the hypothalamus of these same animals was utilized for the determination of the beta-endorphin concentration. The hypothalami of rats treated with MSG exhibited 62% (p < 0.01) less beta-endorphin than saline-injected controls. In addition, the mean mu-opioid-binding densities in the MPOA were 24% (p < 0.05) above controls in the MSG-treated group.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1992

Vitamin E protects hypothalamic beta-endorphin neurons from estradiol neurotoxicity.

Estradiol valerate (EV) treatment has been shown to result in the destruction of 60% of beta-endorphin neurons in the hypothalamic arcuate nucleus. Evidence suggests that the mechanism of EV-induced neurotoxicity involves the conversion of estradiol to catechol estrogen and subsequent oxidation to free radicals in local peroxidase-positive astrocytes. In this study, we examined whether treatment with the antioxidant, vitamin E, protects beta-endorphin neurons from the neurotoxic action of estradiol. Our results demonstrate that chronic vitamin E treatment prevents the decrement in hypothalamic beta-endorphin concentrations resulting from arcuate beta-endorphin cell loss, suggesting that the latter is mediated by free radicals. Vitamin E treatment also prevented the onset of persistent vaginal cornification and polycystic ovarian condition which have been shown to result from the EV-induced hypothalamic pathology.

Animals↗
PubMed1991

Binding and internalization of iodinated neurotensin in neuronal cultures from embryonic mouse brain.

The binding and internalization of labeled neurotensin were studied by means of biochemical and light microscopic radioautography techniques in primary cultures of neurons from whole cerebral hemispheres of mouse embryos. Saturable, high affinity neurotensin binding was detected 5-7 days postplating in cells incubated with 0.1 nM 125I-Tyr3-neurotensin at 37 degrees C or 10 degrees C. The binding capacity at equilibrium was 3 times higher at 37 degrees C than at 10 degrees C. Moreover, whereas virtually all the radioactivity bound at 10 degrees C was membrane-bound (i.e. was readily washable by a hypertonic, high pH, NaCl solution), more than 70% of the radioactivity bound at 37 degrees C was intracellular (i.e. resisted the same treatment). Light microscopic radioautograms of whole cells revealed that approximately 16% of neurons were labeled with 125I-Tyr3-neurotensin at either 37 degrees C or 10 degrees C. The labeling was observed over cell bodies and processes, and the density of silver grains associated with perikarya, as compared to processes, was proportionally higher at 37 degrees C than at 10 degrees C. Semi-thin (1 micron thick) sections through cells incubated at 37 degrees C confirmed that a major fraction of the radioactivity was intracellular and showed that it was mainly confined to the cytoplasm. These results indicate that 125I-Tyr3-neurotensin binds to a distinct subset of primary cultured neurons and that a large proportion of the bound radioactivity undergoes rapid internalization in a temperature-dependent manner. It is proposed that this internalization is ligand-induced and that it may play a role in the modulation of central neurotensin receptor levels.

Animals↗
PubMed1991

5,7-Dihydroxytryptamine injections increase glial fibrillary acidic protein in the hypothalamus of adult rats.

The distribution and levels of glial fibrillary acidic protein (GFAP) were determined in the adult rat hypothalamus following axotomy of serotonin (5-HT) neurons. Seven days after unilateral intrahypothalamic injection of the 5-HT neurotoxin, 5,7-dihydroxytryptamine, there was a marked increase in the number of GFAP-labelled astrocytes in the ipsilateral hypothalamus of 5,7-DHT-treated as compared to sham-treated rats. In addition, levels of GFAP were significantly increased 7 days after 5,7-DHT injection.

5,7-Dihydroxytryptamine↗