Neurons containing a calcium-binding protein parvalbumin in the external plexiform layer of the rat olfactory bulb were identified light microscopically with the pre-embedding immunocytochemistry and were subsequently analysed with the electron microscopic serial-sectioning and three-dimensional reconstructions. In the present study we chose several different types of parvalbumin-immunoreactive neurons identified light microscopically as Van Gehuchten cell type, superficial short-axon cell type and multipolar cell type. Parvalbumin-immunoreactive somata were similar to one another in their ultrastructural characteristics, showing nuclear indentations, moderately developed Golgi apparatus and abundant mitochondria; these structural features appeared to resemble those of the short axon cells around the glomeruli and in the granule cell layer reported in previous electron microscopic studies. All neurons analysed in the present study made symmetrical synapses on to dendrites and somata of presumed mitral/tufted cells and received asymmetrical synapses from them, and occasionally formed reciprocal synapses with them. On the parvalbumin-immunoreactive processes, the asymmetrical synapses nearly equalled the symmetrical ones in number and about 30-50% of them were identified as reciprocal pairs. In contrast, no presynaptic sites were observed on parvalbumin-immunoreactive somata, and thick portions (more than approximately 2 microns in diameter) of the proximal dendrites, where they were occasionally postsynaptic in some asymmetrical and symmetrical synapses from parvalbumin-immunonegative profiles. Characteristically, parvalbumin-immunoreactive process frequently make direct contacts with one another; processes regarded light microscopically as arising from a soma or a dendrite or parvalbumin-immunoreactive neurons were sometimes revealed to be separate but directly contacting processes with electron microscopic examinations. Although puncta adherentia were occasionally observed between these contact sites, so far neither gap junctions nor chemical synapses were observed. Until now, it has been believed that in the external plexiform layer only granule cells form reciprocal synapses with mitral/tufted cells. However, the present study clearly demonstrates that interneurons different from granule cells, namely GABAergic neurons containing a calcium-binding protein parvalbumin, also make reciprocal synapses with mitral/tufted cells in the external plexiform layer. Therefore, neuronal processes making reciprocal synapses with mitral/tufted cells in the external plexiform layer cannot be determined a priori as granule cell processes.
"In the superficial part of the EPL, distal parts of mitral cell apical dendrites were outlined by inhomogeneous labeling for both the Kv4.2 and Kv4.3 subunits (Fig. 4D, E). Previous studies demonstrated that the calcium binding protein, parvalbumin is expressed in a certain subpopulation of GABAergic interneurons of the EPL (Toida et al., 1996). Double immunofluorescent experiments revealed that the Kv4.3 subunit immunopositive cells are parvalbumin immunopositive (Fig. 4G), whereas the Kv4.2 subunit immunopositive cells belong to a neurochemically different population (Fig. 4F). "
[Show abstract][Hide abstract] ABSTRACT: Theoretical and functional studies predicted a highly non-uniform distribution of voltage-gated ion channels on the neuronal surface. This was confirmed by recent immunolocalization experiments for Na+, Ca2+, hyperpolarization activated mixed cation and K+ channels. These experiments also indicated that some K+ channels were clustered in synaptic or non-synaptic membrane specializations. Here we analysed the subcellular distribution of Kv4.2 and Kv4.3 subunits in the rat main olfactory bulb at high resolution to address whether clustering characterizes their distribution, and whether they are concentrated in synaptic or non-synaptic junctions. The cell surface distribution of the Kv4.2 and Kv4.3 subunits is highly non-uniform. Strong Kv4.2 subunit-immunopositive clusters were detected in intercellular junctions made by mitral, external tufted and granule cells (GCs). We also found Kv4.3 subunit-immunopositive clusters in periglomerular (PGC), deep short-axon and GCs. In the juxtaglomerular region some calretinin-immunopositive glial cells enwrap neighboring PGC somata in a cap-like manner. Kv4.3 subunit clusters are present in the cap membrane that directly contacts the PGC, but not the one that faces the neuropil. In membrane specializations established by members of the same cell type, K+ channels are enriched in both membranes, whereas specializations between different cell types contain a high density of channels asymmetrically. None of the K+ channel-rich junctions showed any of the ultrastructural features of known chemical synapses. Our study provides evidence for highly non-uniform subcellular distributions of A-type K+ channels and predicts their involvements in novel forms of intercellular communication in the olfactory pathway.
European Journal of Neuroscience 05/2008; 27(7):1686-99. DOI:10.1111/j.1460-9568.2008.06141.x · 3.18 Impact Factor
"). Mitral/tufted cells are glutamatergic , and serial section reconstruction studies have shown that they form excitatory synapses onto PV-IR EPL interneurons, which form inhibitory synapses onto mitral/tufted cells, 30–50% of which are reciprocal (Toida et al. 1996; Crespo et al. 2001). EPL interneurons, therefore, appear to receive robust, ongoing, AMPA/kainate receptor– mediated glutamatergic excitation from mitral/tufted cells, even in the absence of olfactory stimulation, and they in turn appear to inhibit mitral/tufted cells within the EPL. "
[Show abstract][Hide abstract] ABSTRACT: Altered distribution of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor subunit GluR1 has been linked to stimulation-dependent changes in synaptic efficacy, including long-term potentiation and depression. The main olfactory bulb (OB) remains plastic throughout life; how GluR1 may be involved in this plasticity is unknown. We have previously shown that neonatal naris occlusion reduces numbers of interneuron cell bodies that are immunoreactive for GluR1 in the external plexiform layer (EPL) of the adult mouse OB. Here, we show that immunoreactivity of mouse EPL interneurons for GluR1 is also dramatically reduced following olfactory deafferentation in adulthood. We further show that expression of glutamic acid decarboxylase (GAD) 65, 1 of 2 GAD isoforms expressed by adult gamma-aminobutyric acidergic interneurons, is reduced, but to a much smaller extent, and that in double-labeled cells, immunoreactivity for the Ca(2+)-binding protein parvalbumin (PV) is also reduced. In addition, GluR1 expression is reduced in presumptive tufted cells and interneurons that are negative for GAD65 and PV. Consistent with previous reports, sensory deafferentation resulted in little neuronal degeneration in the adult EPL, indicating that these differences were not likely due to death of EPL neurons. Together, these results suggest that olfactory input regulates expression of the GluR1 AMPA receptor subunit by tufted cells that may in turn regulate GluR1 expression by interneurons within the OB EPL.
Chemical Senses 03/2008; 33(2):201-10. DOI:10.1093/chemse/bjm079 · 3.16 Impact Factor
"The response latency and large synaptic jitter in responses of the interneurons to ON stimulation are consistent with this view, and with anatomical studies showing the excitatory synaptic inputs of EPL interneurons are primarily from M/T cell dendrites within the EPL (Toida et al., 1996). Because external tufted (ET) cells within the GL proper burst spontaneously at ∼3Hz (Hayar et al., 2004a), and some ET cells extend dendrites and/or axons into the EPL (Macrides and Schneider, 1982; Hayar et al., 2004a), they could impose the EPSC bursts of the interneurons. "
[Show abstract][Hide abstract] ABSTRACT: In the external plexiform layer (EPL) of the main olfactory bulb, apical dendrites of inhibitory granule cells form large numbers of synapses with mitral and tufted (M/T) cells, which regulate the spread of activity along the M/T cell dendrites. The EPL also contains intrinsic interneurons, the functions of which are unknown. In the present study, recordings were obtained from cell bodies in the EPL of mouse olfactory bulb slices. Biocytin-filling confirmed that the recorded cells included interneurons, tufted cells, and astrocytes. The interneurons had fine, varicose dendrites, and those located superficially bridged the EPL space below several adjacent glomeruli. Interneuron activity was characterized by high frequency spontaneous excitatory postsynaptic potential/currents that were blocked by the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione and largely eliminated by the voltage-sensitive Na+ channel blocker, tetrodotoxin. Interneuron activity differed markedly from that of tufted cells, which usually exhibited spontaneous action potential bursts. The interneurons produced few action potentials spontaneously, but often produced them in response to depolarization and/or olfactory nerve (ON) stimulation. The responses to depolarization resembled responses of late- and fast-spiking interneurons found in other cortical regions. The latency and variability of the ON-evoked responses were indicative of polysynaptic input. Interneurons expressing green fluorescent protein under control of the mouse glutamic acid decarboxylase 65 promoter exhibited identical properties, providing evidence that the EPL interneurons are GABAergic. Together, these results suggest that EPL interneurons are excited by M/T cells via AMPA/kainate receptors and may in turn inhibit M/T cells within spatial domains that are topographically related to several adjacent glomeruli.
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