Neuroscience
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Respiration-related membrane potential fluctuations were recorded in hypoglossal (XII) motoneurons and pre-Bötzinger complex (pre-BötC) interneurons in medullary slices from perinatal rats. Bath application of serotonin (5-HT) evoked a ketanserine-sensitive depolarization (approximately 11 mV) and tonic spike discharge in XII motoneurons, whereas pre-BötC neurons responded with a <6 mV depolarization and no tonic discharge. The membrane effects were accompanied by an increase in respiratory frequency by up to 260% in 64% of preparations. ⋯ In conclusion, 5-HT-evoked tonic excitation of respiratory XII motoneurons is mediated by postsynaptic 5-HT2 receptors. The excitatory effects on respiratory rhythm are also primarily attributable to postsynaptic 5-HT2 receptors of pre-BötC neurons. Additional modulatory effects on the interneurons appear to be mediated by postsynaptic 5-HT1A receptors.
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Intracerebral microdialysis was employed in awake freely moving rats to investigate the effects of nociceptin/orphanin FQ receptor ligands on glutamate extracellular levels in the substantia nigra pars reticulata. Nociceptin/orphanin FQ, ineffective at 0.1 microM, induced a prolonged stimulation of nigral glutamate levels at 1 and 10 microM (mean effect of 137+/-9 and 167+/-13%, respectively, of basal values). These effects were prevented by the novel nociceptin/orphanin FQ receptor antagonist [Nphe(1)]nociceptin/orphanin FQ(1-13)NH(2) (100 and 300 microM, respectively) but not by the non-selective opioid receptor antagonist naloxone (10 microM). [Nphe(1)]nociceptin/orphanin FQ(1-13)NH(2) (100 microM) inhibited by itself glutamate outflow (maximal reduction to 71+/-4%) while naloxone was ineffective. ⋯ Intranigral perfusion with tetrodotoxin (1 microM) or with the dopamine D(2) receptor antagonist raclopride (1 microM), failed to affect basal glutamate output and prevented the facilitatory effect of nociceptin/orphanin FQ (10 microM). However, perfusion with the GABA(A) receptor antagonist bicuculline (10 microM) increased local glutamate extracellular levels by itself and attenuated the effect of the peptide. Our data suggest that nociceptin/orphanin FQ increases glutamate extracellular levels in the substantia nigra pars reticulata via activation of nociceptin/orphanin FQ receptors located on non-glutamatergic, possibly dopaminergic and GABAergic, neuronal elements.
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Cortical malformations resulting from aberrant brain development can be associated with mental retardation, dyslexia, and intractable forms of epilepsy. Despite emerging interest in the pathology and etiology of cortical malformations, little is known about the phenotype of cells within these lesions. In utero exposure to the DNA methylating agent methylazoxymethanol acetate (MAM) during a critical stage in neurodevelopment results in animals with distinct clusters of displaced neurons in hippocampus, i.e. nodular heterotopia. ⋯ Normotopic CA1 pyramidal neurons (e.g. pyramidal cells with normal lamination adjacent to a heterotopia) in the MAM brain exhibited molecular and electrophysiological properties that were nearly identical to those of age-matched CA1 pyramidal neurons from control rats. We conclude that neuronal heterotopiae in the hippocampus of MAM-exposed rats are comprised of neurons with a Layer II-III supragranular cortex phenotype. The MAM model, therefore, may serve as a useful tool in examination of the factors influencing aberrant brain development and epilepsy.
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Glutamate-gated ion channels are widely expressed in neurons where they serve a host of cellular functions. An appealing, but yet unexplored, way to delineate the functions of particular glutamate receptor subtypes is to direct the expression of dominant-negative and gain-of-function mutant subunits. We tested the ability of two dominant-negative subunits, an alpha-amino-3-hydroxy-5-methyl-isoxazolproprionic acid receptor subunit and a kainate receptor subunit, to silence recombinant and neuronal glutamate receptors. ⋯ When expressed in cerebellar granule cells, the dominant-negative subunits silenced native channels in a subtype-specific fashion. Immunocytochemical staining of control and transfected neurons, as well as studies with a gain-of-function glutamate receptor-1 mutant, indicated that the mutant subunits were expressed at levels roughly equal to the total abundance of related native subunits, and both dominant-negatives suppressed native channel expression 60-65% when tested 24 h post-transfection. If co-assembly of the mutant subunits with related native subunits is combinatorial, this level of suppression gives receptor half-lives of approximately 20 h.
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Delta-catenin (or neural plakophilin-related arm-repeat protein/neurojungin) is primarily a brain specific member of the p120(ctn) subfamily of armadillo/beta-catenin proteins that play important roles in neuronal development. Our previous studies have shown that the ectopic expression of delta-catenin induces the formation of dendrite-like extensions and that the overexpression of delta-catenin promotes dendritic branching and increases spine density. Here we demonstrate that delta-catenin displays a dendritic distribution pattern in the adult mouse brain and is co-enriched with postsynaptic density-95 (PSD-95) in the detergent insoluble postsynaptic scaffolds. ⋯ In dissociated hippocampal neurons overexpressing delta-catenin, glutamate stimulation leads to a rapid redistribution of delta-catenin that can be attenuated by 6-cyano-7-nitroquinoxaline-2,3-dione and dizocilpine, selective inhibitors of ionotropic glutamate receptors. Upon glutamate receptor activation, delta-catenin becomes down-regulated and its association with NR2A and mGluR1alpha in cultured neurons is diminished. These findings support a possible functional connection between delta-catenin and the glutamatergic excitatory synaptic signaling pathway during neuronal development.