Vesicular glutamate transporters define two sets of glutamatergic afferents to the somatosensory thalamus and two thalamocortical projections in the mouse
ABSTRACT The ventral posterior nucleus of the thalamus (VP) receives two major sets of excitatory inputs, one from the ascending somatosensory pathways originating in the dorsal horn, dorsal column nuclei, and trigeminal nuclei, and the other originating from the cerebral cortex. Both systems use glutamate as neurotransmitter, as do the thalamocortical axons relaying somatosensory information from the VP to the primary somatosensory cortex (SI). The synapses formed by these projection systems differ anatomically, physiologically, and in their capacity for short-term synaptic plasticity. Glutamate uptake into synaptic vesicles and its release at central synapses depend on two isoforms of vesicular glutamate transporters, VGluT1 and VGluT2. Despite ample evidence of their complementary distribution, some instances exist of co-localization in the same brain areas or at the same synapses. In the thalamus, the two transcripts coexist in cells of the VP and other nuclei but not in the posterior or intralaminar nuclei. We show that the two isoforms are completely segregated at VP synapses, despite their widespread expression throughout the dorsal and ventral thalamus. We present immunocytochemical, ultrastructural, gene expression, and connectional evidence that VGluT1 in the VP is only found at corticothalamic synapses, whereas VGluT2 is only found at terminals made by axons originating in the spinal cord and brainstem. By contrast, the two VGluT isoforms are co-localized in thalamocortical axon terminals targeting layer IV, but not in those targeting layer I, suggesting the presence of two distinct projection systems related to the core/matrix pattern of organization of thalamocortical connectivity described in other mammals.
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ABSTRACT: Most glutamatergic inputs in the neocortex originate from the thalamus or neocortical pyramidal cells. To test whether thalamocortical afferents selectively innervate specific cortical cell subtypes and surface domains, we investigated the distribution patterns of thalamocortical and corticocortical excitatory synaptic inputs in identified postsynaptic cortical cell subtypes using intracellular and immunohistochemical staining combined with confocal laser scanning and electron microscopic observations in 2 thalamorecipient sublayers, lower layer 2/3 (L2/3b) and lower layer 5 (L5b) of rat frontal cortex. The dendrites of GABAergic parvalbumin (PV) cells preferentially received corticocortical inputs in both sublayers. The somata of L2/3b PV cells received thalamic inputs in similar proportions to the basal dendritic spines of L2/3b pyramidal cells, whereas L5b PV somata were mostly innervated by cortical inputs. The basal dendrites of L2/3b pyramidal and L5b corticopontine pyramidal cells received cortical and thalamic glutamatergic inputs in proportion to their local abundance, whereas crossed-corticostriatal pyramidal cells in L5b exhibited a preference for thalamic inputs, particularly in their distal dendrites. Our data demonstrate an exquisite selectivity among thalamocortical afferents in which synaptic connectivity is dependent on the postsynaptic neuron subtype, cortical sublayer, and cell surface domain. © The Author 2015. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: firstname.lastname@example.org.Cerebral Cortex 06/2015; DOI:10.1093/cercor/bhv124 · 8.31 Impact Factor
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ABSTRACT: We studied how nicotinic acetylcholine receptors (nAChRs) regulate glutamate release in the secondary motor area (Fr2) of the dorsomedial murine prefrontal cortex, in the presence of steady agonist levels. Fr2 mediates response to behavioral situations that require immediate attention and is a candidate for generating seizures in the frontal epilepsies caused by mutant nAChRs. Morphological analysis showed a peculiar chemoarchitecture and laminar distribution of pyramidal cells and interneurons. Tonic application of 5 μM nicotine on layer V pyramidal neurons strongly increased the frequency of spontaneous glutamatergic excitatory postsynaptic currents (EPSCs). The effect was inhibited by 1 μM dihydro-β-erythroidine (DHβE, which blocks α4-containing nAChRs), but not by 10 nM methyllicaconitine (MLA, which blocks α7-containing receptors). EPSCs were also stimulated by 5-iodo-3-[2(S)-azetidinylmethoxy]pyridine (5IA85380), selective for β2-containing receptors, in a DHβE-sensitive way. We next studied the association of α4 with different populations of glutamatergic terminals, by using as markers the vesicular glutamate transporter type 1 (VGLUT1) for cortico-cortical synapses, and type 2 (VGLUT2) for thalamo-cortical projecting fibers. Immunoblots showed higher expression of α4 in Fr2, as compared to the somatosensory (SS) cortex. Immunofluorescence showed intense VGLUT1 staining throughout the cortical layers, whereas VGLUT2 immunoreactivity displayed a more distinct laminar distribution. In layer V, co-localization of α4 nAChR subunit with both VGLUT1 and VGLUT2 was considerably stronger in Fr2 than in SS cortex. Thus, in Fr2, α4β2 nAChRs are expressed in both intrinsic and extrinsic glutamatergic terminals and give a major contribution to control glutamate release in layer V, in the presence of tonic agonist levels. Synapse, 2013. © 2013 Wiley Periodicals, Inc.Synapse 06/2013; 67(6). DOI:10.1002/syn.21655 · 2.43 Impact Factor
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ABSTRACT: The vesicular glutamate transporters, VGLUT1 and VGLUT2, reportedly display complementary distribution in the rat brain. However, co-expression of them in single neurons has been reported in some brain areas. We previously found co-expression of VGLUT1 and VGLUT2 mRNAs in a number of single neurons in the principal sensory trigeminal nucleus (Vp) of the adult rat; the majority of these neurons sent their axons to the thalamic regions around the posteromedial ventral nucleus (VPM) and the posterior nuclei (Po). It is well known that trigeminothalamic (T-T) projection fibers arise not only from the Vp but also from the spinal trigeminal nucleus (Vsp), and that trigeminocerebellar (T-C) projection fibers take their origins from both of the Vp and Vsp. Thus, in the present study, we examined the expression of VGLUT1 and VGLUT2 in Vp and Vsp neurons that sent their axons to the VPM/Po regions or the cortical regions of the cerebellum. For this purpose, we combined fluorescence in situ hybridization (FISH) histochemistry with retrograde tract-tracing; immunofluorescence histochemistry was also combined with anterograde tract-tracing. The results indicate that glutamatergic Vsp neurons sending their axons to the cerebellar cortical regions mainly express VGLUT1, whereas glutamatergic Vsp neurons sending their axons to the thalamic regions express VGLUT2. The present data, in combination with those of our previous study, indicate that glutamatergic Vp neurons projecting to the cerebellar cortical regions express mainly VGLUT1, whereas the majority of glutamatergic Vp neurons projecting to the thalamus co-express VGLUT1 and VGLUT2.Brain Structure and Function 02/2013; DOI:10.1007/s00429-012-0495-1 · 4.57 Impact Factor