Neuroscience
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GABA(A) receptors are ligand-operated chloride channels assembled from five subunits in a heteropentameric manner. Using immunocytochemistry, we investigated the distribution of GABA(A) receptor subunits deriving from 13 different genes (alpha1-alpha6, beta1-beta3, gamma1-gamma3 and delta) in the adult rat brain. Subunit alpha1-, beta1-, beta2-, beta3- and gamma2-immunoreactivities were found throughout the brain, although differences in their distribution were observed. ⋯ In most pontine and cranial nerve nuclei and in the medulla, only subunit alpha1-, beta2- and gamma2-immunoreactivities were strong, whereas the inferior olive was significantly labeled only for subunits beta1, gamma1 and gamma2. In this study, a highly heterogeneous distribution of 13 different GABA(A) receptor subunit-immunoreactivities was observed. This distribution and the apparently typical patterns of co-distribution of these GABA(A) receptor subunits support the assumption of multiple, differently assembled GABA(A) receptor subtypes and their heterogeneous distribution within the adult rat brain.
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The change in synaptic inputs through primary afferent C- and A-fibers during postnatal development was examined in substantia gelatinosa neurons of a rat spinal cord slice with an attached L5 dorsal root by use of the blind whole-cell patch-clamp technique; the synaptic responses were compared between the slices obtained from immature (postnatal days 21-23) and mature (postnatal days 56-60) male rats. The mono- and/or polysynaptic afferent inputs were monitored by recording glutamatergic excitatory postsynaptic currents and potentials evoked by stimulating C- and A-fibers, the identification of which was based on the values of threshold stimulus intensity and of the conduction velocity of the fibers, determined by intracellular recordings from dorsal root ganglion neurons. Immature substantia gelatinosa neurons received synaptic inputs through Abeta-, Adelta- and C-afferents, with proportions of 51%, 46% and 36%, respectively. ⋯ In both immature and mature rats, repetitive stimulation of C-afferents did not elicit any slow responses, which are longer in duration than the monosynaptic excitatory postsynaptic currents, although C-fibers are known to contain not only excitatory amino acids, but also neuropeptides such as substance P, which is thought to be involved in the production of slow responses. These results indicate that both C- and Adelta-afferents innervating substantia gelatinosa neurons are reorganized following maturation, accompanied by a withdrawal or elimination of Abeta-fibers from the substantia gelatinosa, probably due to a competition among the fibers during development. In spite of the developmental increase in C-fiber inputs, mature as well as immature substantia gelatinosa neurons did not display any slow synaptic responses, which appear to be mediated by transmitters other than excitatory amino acids.
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Intracellular recordings were made from thalamocortical neurons in slices of rat dorsal lateral geniculate nucleus in vitro, where ionotropic glutamate receptors and ionotropic and metabotropic GABA receptors had been blocked. The activation of specific metabotropic glutamate receptors by exogenous agonists and by the electrical stimulation of the corticothalamic pathway was then assessed using selective antagonists. The specific group I agonist (S)-3, 5-dihydoxyphenylglycine and the non-selective agonist (1S, 3R)-1-aminocyclo-pentane-1,3-dicarboxylic acid both caused a concentration-dependent depolarization of membrane potential. ⋯ However, they were not blocked by 6-methyl-2-(phenylethyl)-pyridine, a highly selective mGlu5 receptor antagonist. Thus, the membrane potential depolarization of thalamocortical neurons caused either by exogenous agonists or by the stimulation of cortical fibres resulted from the specific activation of mGlu1 but not mGlu5 receptors. This result is consistent with the location of this receptor type on the distal dendrites of thalamocortical neurons in the dorsal lateral geniculate nucleus of the thalamus.
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We have investigated with histochemical techniques the expression of peptides and other neurochemical markers in the hypothalamus and olfactory bulb of male mice, in which the genes encoding the alpha and beta thyroid hormone receptors (TRalpha1, TRbeta1 and TRbeta2) have been deleted. Thyrotropin-releasing hormone messenger RNA levels were increased in the hypothalamic paraventricular nucleus and in the medullary raphe nuclei of mutant mice lacking the thyroid hormone receptors alpha1 and beta (alpha1(-/-)beta(-/-)), as compared to wild-type mice. In contrast, galanin messenger RNA levels were lower in the hypothalamic paraventricular nucleus of mutant animals, as was galanin-like immunoreactivity in the internal layer of the median eminence. ⋯ The tyrosine hydroxylase messenger RNA levels were also slightly reduced. In contrast, the levels of galanin and neuropeptide Y messenger RNA in this region were unchanged in thyroid hormone receptor alpha1(-/-)beta(-/-) mice as compared to wild-type mice. Together these studies reveal many regional and neurochemically selective alterations in neuronal phenotype of mice devoid of all known thyroid hormone receptors.
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The serotonergic metacerebral giant cell (C1) of Helix pomatia was isolated with its bifurcate axon and plated in culture under five conditions: (i) with no target; (ii) with the appropriate target B2 near the stump of the bigger branch (CBC); (iii) with B2 near the stump of the smaller branch (CC); (iv) with a wrong target (C3) near the stump of the CBC branch and (v) with B2 and C3 positioned near the CBC and CC stump, respectively. The counting of anti-serotonin antibody-labelled varicosities of the C1 neuron showed that the presence of the appropriate target in either axonal domain both down-regulated the number of varicosities of the contralateral neuritic field, and increased their average size, whereas the wrong target induced an overall reduction of the number of C1 neuron varicosities, and inhibited the evoked transmitter release. The action potential-evoked calcium concentration increase in the neuritic terminals of the C1 neuron cultured alone, or in presence of the appropriate target, reached a value significantly higher than that reached in presence of the wrong target. These results provide evidence that the postsynaptic neuron regulates both morphological and functional development of presynaptic terminals.