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A barrel-related interneuron in layer 4 of rat somatosensory cortex with a high intrabarrel connectivity.

Koelbl C, Helmstaedter M, Lübke J, Feldmeyer D - Cereb. Cortex (2013)

Bottom Line: Three distinct clusters of FS L4 interneurons were identified based on their axonal morphology relative to the barrel column suggesting that these neurons do not constitute a homogeneous interneuron population.We found on average 3.7 ± 1.3 putative inhibitory synaptic contacts that were not restricted to perisomatic areas.In conclusion, we characterized a novel type of barrel cortex interneuron in the major thalamo-recipient layer 4 forming dense synaptic networks with L4 spiny neurons.

View Article: PubMed Central - PubMed

Affiliation: Department of Cell Physiology, Max Planck Institute of Medical Research, Jahnstr. 20, D-69120 Heidelberg, Germany Current address: Section of Cardiovascular Medicine, Boston University Medical Center, 88 East Newton Street, Boston, MA 02118, USA.

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Average axonal domains and firing patterns of the 3 different types of fast-spiking L4 interneurons. (A1–3) Representative Neurolucida reconstructions of L4 interneurons that were classified as cluster 1 (A1), cluster 2 (A2), and cluster 3 (A3) cells, respectively (axons are given in blue, the somatodendritic domain in red). (B1–3) Average axonal length density maps (blue) of the 3 different clusters, respectively. White, thin contour lines enclosing 70%, 80%, and 90% of the integrated axonal density are shown superimposed. The red triangles depict the location of the L4 interneuron somata with respect to the center of the barrel. Note that cluster 1 interneurons have axonal domains that project both throughout cortical layer 2/3, 4, and 5 with the highest density in layer 2/3. In contrast, cluster 3 interneurons have an axonal domain that resides almost exclusively within the home barrel. Cluster 2 interneurons constitute an intermediate class between cluster 1 and 3 neurons with an axon largely confined to the home barrel with only short vertically ascending axonal collaterals in lower layer 2/3. (C1–3) Examples of AP firing patterns elicited by rectangular current pulses injected at the soma of the 3 L4 interneuron types. The maximum firing frequency in all of these neurons was ∼300 Hz, that is, all 3 types can be classified as interneurons with FS characteristics. There were no significant differences in the firing characteristics and, thus, the L4 interneurons cannot be discriminated by these criteria. (D) Immunohistochemistry of a typical cluster 3 neuron. AMCA streptavidin was used to determine the location and morphology of the neuron, which was positive for PV but negative for CB.
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BHT263F3: Average axonal domains and firing patterns of the 3 different types of fast-spiking L4 interneurons. (A1–3) Representative Neurolucida reconstructions of L4 interneurons that were classified as cluster 1 (A1), cluster 2 (A2), and cluster 3 (A3) cells, respectively (axons are given in blue, the somatodendritic domain in red). (B1–3) Average axonal length density maps (blue) of the 3 different clusters, respectively. White, thin contour lines enclosing 70%, 80%, and 90% of the integrated axonal density are shown superimposed. The red triangles depict the location of the L4 interneuron somata with respect to the center of the barrel. Note that cluster 1 interneurons have axonal domains that project both throughout cortical layer 2/3, 4, and 5 with the highest density in layer 2/3. In contrast, cluster 3 interneurons have an axonal domain that resides almost exclusively within the home barrel. Cluster 2 interneurons constitute an intermediate class between cluster 1 and 3 neurons with an axon largely confined to the home barrel with only short vertically ascending axonal collaterals in lower layer 2/3. (C1–3) Examples of AP firing patterns elicited by rectangular current pulses injected at the soma of the 3 L4 interneuron types. The maximum firing frequency in all of these neurons was ∼300 Hz, that is, all 3 types can be classified as interneurons with FS characteristics. There were no significant differences in the firing characteristics and, thus, the L4 interneurons cannot be discriminated by these criteria. (D) Immunohistochemistry of a typical cluster 3 neuron. AMCA streptavidin was used to determine the location and morphology of the neuron, which was positive for PV but negative for CB.

Mentions: For the quantitative morphological analysis of L4 interneurons, we used the same method as described for L2/3 interneurons in rat barrel cortex (Helmstaedter et al. 2009a). In brief, the whole reconstruction was aligned with the bright-field photograph of the slice taken during the experiment. We generated 2D maps (density maps) of 3D axonal and dendritic domains in relation to the anatomically defined cortical layers, by using custom-made software (“RembrandtII”; Helmstaedter et al. 2009a) written in Igor Pro (Wavemetrics, Lake Oswego, OR, USA). “Contour lines” were computed as “iso-density lines” that enclosed a 70%, 80%, and 90%, respectively, of the total axonal length of the neuron. So-called “innervation domains” were calculated for the inhibitory connection between L4 interneurons and spiny neurons. Reconstructions of the dendritic trees of 19 postsynaptic L4 spiny neurons were processed as described above to obtain average interpolated maps of dendritic length density aligned to the center of the home barrel. For each of the axonal projection types represented by the different clusters (see below), the barrel-aligned axonal density maps of presynaptic interneurons were pointwise multiplied with the barrel-aligned dendritic density maps of the postsynaptic L4 spiny neurons. Maps were not normalized, so the product density was comparable among interneuron types. Three contour lines at the same absolute product density are shown for all innervation domains for comparison (Fig. 3B).


A barrel-related interneuron in layer 4 of rat somatosensory cortex with a high intrabarrel connectivity.

Koelbl C, Helmstaedter M, Lübke J, Feldmeyer D - Cereb. Cortex (2013)

Average axonal domains and firing patterns of the 3 different types of fast-spiking L4 interneurons. (A1–3) Representative Neurolucida reconstructions of L4 interneurons that were classified as cluster 1 (A1), cluster 2 (A2), and cluster 3 (A3) cells, respectively (axons are given in blue, the somatodendritic domain in red). (B1–3) Average axonal length density maps (blue) of the 3 different clusters, respectively. White, thin contour lines enclosing 70%, 80%, and 90% of the integrated axonal density are shown superimposed. The red triangles depict the location of the L4 interneuron somata with respect to the center of the barrel. Note that cluster 1 interneurons have axonal domains that project both throughout cortical layer 2/3, 4, and 5 with the highest density in layer 2/3. In contrast, cluster 3 interneurons have an axonal domain that resides almost exclusively within the home barrel. Cluster 2 interneurons constitute an intermediate class between cluster 1 and 3 neurons with an axon largely confined to the home barrel with only short vertically ascending axonal collaterals in lower layer 2/3. (C1–3) Examples of AP firing patterns elicited by rectangular current pulses injected at the soma of the 3 L4 interneuron types. The maximum firing frequency in all of these neurons was ∼300 Hz, that is, all 3 types can be classified as interneurons with FS characteristics. There were no significant differences in the firing characteristics and, thus, the L4 interneurons cannot be discriminated by these criteria. (D) Immunohistochemistry of a typical cluster 3 neuron. AMCA streptavidin was used to determine the location and morphology of the neuron, which was positive for PV but negative for CB.
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BHT263F3: Average axonal domains and firing patterns of the 3 different types of fast-spiking L4 interneurons. (A1–3) Representative Neurolucida reconstructions of L4 interneurons that were classified as cluster 1 (A1), cluster 2 (A2), and cluster 3 (A3) cells, respectively (axons are given in blue, the somatodendritic domain in red). (B1–3) Average axonal length density maps (blue) of the 3 different clusters, respectively. White, thin contour lines enclosing 70%, 80%, and 90% of the integrated axonal density are shown superimposed. The red triangles depict the location of the L4 interneuron somata with respect to the center of the barrel. Note that cluster 1 interneurons have axonal domains that project both throughout cortical layer 2/3, 4, and 5 with the highest density in layer 2/3. In contrast, cluster 3 interneurons have an axonal domain that resides almost exclusively within the home barrel. Cluster 2 interneurons constitute an intermediate class between cluster 1 and 3 neurons with an axon largely confined to the home barrel with only short vertically ascending axonal collaterals in lower layer 2/3. (C1–3) Examples of AP firing patterns elicited by rectangular current pulses injected at the soma of the 3 L4 interneuron types. The maximum firing frequency in all of these neurons was ∼300 Hz, that is, all 3 types can be classified as interneurons with FS characteristics. There were no significant differences in the firing characteristics and, thus, the L4 interneurons cannot be discriminated by these criteria. (D) Immunohistochemistry of a typical cluster 3 neuron. AMCA streptavidin was used to determine the location and morphology of the neuron, which was positive for PV but negative for CB.
Mentions: For the quantitative morphological analysis of L4 interneurons, we used the same method as described for L2/3 interneurons in rat barrel cortex (Helmstaedter et al. 2009a). In brief, the whole reconstruction was aligned with the bright-field photograph of the slice taken during the experiment. We generated 2D maps (density maps) of 3D axonal and dendritic domains in relation to the anatomically defined cortical layers, by using custom-made software (“RembrandtII”; Helmstaedter et al. 2009a) written in Igor Pro (Wavemetrics, Lake Oswego, OR, USA). “Contour lines” were computed as “iso-density lines” that enclosed a 70%, 80%, and 90%, respectively, of the total axonal length of the neuron. So-called “innervation domains” were calculated for the inhibitory connection between L4 interneurons and spiny neurons. Reconstructions of the dendritic trees of 19 postsynaptic L4 spiny neurons were processed as described above to obtain average interpolated maps of dendritic length density aligned to the center of the home barrel. For each of the axonal projection types represented by the different clusters (see below), the barrel-aligned axonal density maps of presynaptic interneurons were pointwise multiplied with the barrel-aligned dendritic density maps of the postsynaptic L4 spiny neurons. Maps were not normalized, so the product density was comparable among interneuron types. Three contour lines at the same absolute product density are shown for all innervation domains for comparison (Fig. 3B).

Bottom Line: Three distinct clusters of FS L4 interneurons were identified based on their axonal morphology relative to the barrel column suggesting that these neurons do not constitute a homogeneous interneuron population.We found on average 3.7 ± 1.3 putative inhibitory synaptic contacts that were not restricted to perisomatic areas.In conclusion, we characterized a novel type of barrel cortex interneuron in the major thalamo-recipient layer 4 forming dense synaptic networks with L4 spiny neurons.

View Article: PubMed Central - PubMed

Affiliation: Department of Cell Physiology, Max Planck Institute of Medical Research, Jahnstr. 20, D-69120 Heidelberg, Germany Current address: Section of Cardiovascular Medicine, Boston University Medical Center, 88 East Newton Street, Boston, MA 02118, USA.

Show MeSH
Related in: MedlinePlus