A barrel-related interneuron in layer 4 of rat somatosensory cortex with a high intrabarrel connectivity.
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.
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
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Mentions: All experimental procedures were carried out according to the animal welfare guidelines of the Max Planck Society. Wistar rats aged between 18 and 22 days were anesthetized with isoflurane, decapitated, and slices through the somatosensory cortex were cut in cold extracellular solution using a vibrating microslicer (DTK-1000, Dosaka, Kyoto, Japan). The method described by Agmon and Connors (1991) was used with minor modifications required for rat brain. The tissue was sliced at slow speed and high vibration frequency into 300-µm-thick “semi-coronal” slices (Land and Kandler 2002). Slices were collected with a pipette, and subsequently incubated (30–60 min) in an artificial saline solution bubbled with 95% O2 and 5% CO2 containing 6 mM MgCl2 to reduce synaptic activity at room temperature (22–24 °C) before recording. After the incubation, slices were transferred to the experimental setup and selected for best visibility of barrels in cortical layer 4. During the experiment, slices were continuously superfused with an extracellular solution containing (in mM): 125 NaCl, 2.5 KCl, 25 glucose, 25 NaHCO3, 1.25 NaH2PO4, 2 CaCl2, and 1 MgCl2 and bubbled with 95% O2 and 5% CO2. The pipette (intracellular) solution used for both pre- and postsynaptic cell had the following composition (mM): 135 potassium gluconate, 4 KCl, 10 HEPES, 10 phosphocreatine-Na, 4 ATP-Mg (pH 7.2, osmolarity 291 mOsm). Biocytin (Sigma, Munich, Germany) at a concentration of 3 mg/mL was routinely added to the internal solution and neurons were filled during 1–2 h of recording. Slices were placed in the recording chamber under an upright microscope (Axioskop FS1, Carl Zeiss, Göttingen, Germany; fitted with w2.5 plan/0.075 NA and w40 W/0.80 objectives) with the pial surface pointing to the front and the hippocampus to the right. The barrel field was visualized at low magnification under bright-field illumination and can be identified in layer 4 as narrow dark stripes with evenly spaced, light “hollows” (Agmon and Connors 1991; Feldmeyer et al. 1999). Barrel structures are present in 6 or 7 slices but a continuous band of barrels is visible only in 2 or 3 slices just above the fimbria–fornix and the lateral ventricle (Fig. 1A; Agmon and Connors 1991). Individual L4 neurons were identified at ×40 magnification using infrared differential interference contrast (IR-DIC) microscopy (Fig. 1B).Figure 1.
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.