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Diversity of cell-types that collectively shape the cortical microcircuit ensures the necessary computational richness to orchestrate a wide variety of behaviors. The information content embedded in spiking activity of identified cell-types remain unclear to a large extent. Here, we recorded spike responses upon whisker touch of anatomically identified excitatory cell-types in primary somatosensory cortex in naive, untrained rats. We find major differences across layers and cell-types. The temporal structure of spontaneous spiking contains high-frequency bursts (≥100 Hz) in all morphological cell-types but a significant increase upon whisker touch is restricted to layer L5 thick-tufted pyramids (L5tts) and thus provides a distinct neurophysiological signature. We find that whisker touch can also be decoded from L5tt bursting, but not from other cell-types. We observed high-frequency bursts in L5tts projecting to different subcortical regions, including thalamus, midbrain and brainstem. We conclude that bursts in L5tts allow accurate coding and decoding of exploratory whisker touch., In order to investigate the information encoded by spiking activity in different neuronal cell types in the primary somatosensory cortex, de Kock et al performed electrophysiological recordings in untrained rats. They demonstrated that an increase in high-frequency burst spiking in thick tufted pyramids in layer 5 of the cortex allow accurate encoding of exploratory whisker touch.

Neuron morphology is frequently used to classify cell-types in the mammalian cortex. Apart from the shape of the soma and the axonal projections, morphological classification is largely defined by the dendrites of a neuron and their subcellular compartments, referred to as dendritic spines. The dimensions of a neuron’s dendritic compartment, including its spines, is also a major determinant of the passive and active electrical excitability of dendrites. Furthermore, the dimensions of dendritic branches and spines change during postnatal development and, possibly, following some types of neuronal activity patterns, changes depending on the activity of a neuron. Due to their small size, accurate quantitation of spine number and structure is difficult to achieve (Larkman, J Comp Neurol 306:332, 1991). Here we follow an analysis approach using high-resolution EM techniques. Serial block-face scanning electron microscopy (SBFSEM) enables automated imaging of large specimen volumes at high resolution. The large data sets generated by this technique make manual reconstruction of neuronal structure laborious. Here we present NeuroStruct, a reconstruction environment developed for fast and automated analysis of large SBFSEM data sets containing individual stained neurons using optimized algorithms for CPU and GPU hardware. NeuroStruct is based on 3D operators and integrates image information from image stacks of individual neurons filled with biocytin and stained with osmium tetroxide. The focus of the presented work is the reconstruction of dendritic branches with detailed representation of spines. NeuroStruct delivers both a 3D surface model of the reconstructed structures and a 1D geometrical model corresponding to the skeleton of the reconstructed structures. Both representations are a prerequisite for analysis of morphological characteristics and simulation signalling within a neuron that capture the influence of spines. Electronic supplementary material The online version of this article (doi:10.1007/s10827-011-0316-1) contains supplementary material, which is available to authorized users.

Transmitter release is triggered by highly localized, transient increases in the presynaptic Ca2+ concentration ([Ca2+]). Rapidly decaying [Ca2+] elevations were generated using Ca2+ uncaging techniques, and [Ca2+] was measured with a low-affinity Ca2+ indicator in a giant presynaptic terminal, the calyx of Held, in rat brain slices. The rise time and amplitude of evoked excitatory postsynaptic currents (EPSCs) depended on the half-width of the fluorescence transient, which was predicted by a five-binding site model of a Ca2+ sensor having relatively high affinity (Kd approx13 muM). Very fast [Ca2+] transients (half-width 2+] transients of variable amplitudes demonstrated the supralinear transfer function of the sensor. The sensitivity of release to the time course of the [Ca2+] transient may contribute to mechanisms by which the presynaptic AP waveform controls synaptic strength.

A blind patch-clamp technique for in vivo whole-cell recordings in the intact brain is described. Recordings were obtained from various neuronal cell types located 100-5,000 microm from the cortical surface. Access resistance of recordings was as low as 10 M Omega but increased with recording depth and animal age. Recordings were remarkably stable and it was therefore possible to obtain whole-cell recordings in awake, head-fixed animals. The whole-cell configuration permitted rapid dialysis of cells with a calcium buffer. In most neurons very little ongoing action potential (AP) activity was observed and the spontaneous firing rates were up to 50-fold less than what has been reported by extracellular unit recordings. AP firing in the brain might therefore be far sparser than previously thought.

During quiet wakefulness and sleep, and under anesthesia, the membrane potentials of neocortical pyramidal neurons show synchronous, slow oscillations, so-called up-down states (UDS), that can be detected in the local field potential (LFP). The influence of this synchronized, spontaneous neocortical activity on the hippocampus is largely unknown. We performed the first in vivo whole-cell recordings from hippocampal dorsal CA1 interneurons and found that their membrane potentials were phase-locked to neocortical up-down states with a small delay. These results provide strong evidence for cortico-hippocampal interaction and suggest that neocortical activity drives hippocampal interneurons during UDS.

AN important step towards understanding the molecular basis of the functional diversity of voltage-gated K+ channels in the mammalian brain has been the discovery of a family of genes encoding rat brain K+ channel-forming (RCK) proteins1–6. All species of these RCK proteins form homomultimeric voltage-gated K+ channels with distinct functional characteristics in Xenopus laevis oocytes following injection of the respective cRNAs2. RCK-specific mRNAs are coexpressed in several regions of the brain7, suggesting that RCK proteins also assemble into heteromultimeric K+ channels. In addition expression experiments with fractionated poly(A)+ mRNA have suggested that heteromultimeric K+ channels may occur in mammalian brain8. We report here that heteromultimeric K+ channels composed of two different RCK proteins (RCK1 and RCK4) assemble after cotransfection of HeLa cells with the corresponding cDNAs and after coinjection of the corresponding cRNAs into Xenopus oocytes. The heteromultimeric RCK1, 4 channel mediates a transient potassium outward current, similar to the RCK4 channel but inactivates more slowly, has a larger conductance and is more sensitive to block by dendrotoxin and tetraethylammonium chloride.

The Torpedo and calf acetylcholine receptors and hybrids composed of subunits from the two species have been produced in Xenopus oocytes by the use of the cloned complementary DNAs. Single-channel current measurements indicate that these receptors form channels of similar conductance but with different gating behaviour.

Sodium currents, INa, were recorded from Xenopus laevis oocytes which had been injected with mRNA synthesized by in vitro transcription of the rat brain sodium channel II cDNA (Noda et al. 1986 a, b). Patch pipettes were used to apply depolarizing voltage steps and to record macroscopic sodium currents of between 50 and 750 pA from cell-attached patches of the oocyte membrane. With a combination of whole-cell and patch clamp recording, the properties of the implanted sodium channels could be studied in detail. They were analyzed according to the model of Hodgkin and Huxley (1952 a) assuming three activation gates. The activation of the sodium currents is characterized by an equilibrium potential of -29 mV and an apparent gating charge of 8.7 e0. At -64 mV half of the sodium currents were inactivated. From single-channel current recordings, an elementary sodium channel conductance of 19 pS and an average open time of 0.43 ms were obtained at -32 mV membrane potential and 16 degrees C. The single-channel and activation properties of rat brain sodium channel II are therefore comparable to those found in peripheral nerve and skeletal muscle, but inactivation occurs at less negative potentials. This could be a specific property of the brain sodium channels and may underlie the maintained inward sodium currents reported in brain neurones (French and Gage 1985).

The structure-function relationship of the nicotinic acetylcholine receptor (AChR) has been effectively studied by the combination of complementary DNA manipulation and single-channel current analysis. Previous work with chimaeras between the Torpedo californica and bovine AChR delta-subunits has shown that the region comprising the hydrophobic segment M2 and its vicinity contains an important determinant of the rate of ion transport through the AChR channel. It has also been suggested that this region is responsible for the reduction in channel conductance caused by divalent cations and that segment M2 contributes to the binding site of noncompetitive antagonists. To identify those amino acid residues that interact with permeating ions, we have introduced various point mutations into the Torpedo AChR subunit cDNAs to alter the net charge of the charged or glutamine residues around the proposed transmembrane segments. The single-channel conductance properties of these AChR mutants expressed in Xenopus laevis oocytes indicate that three clusters of negatively charged and glutamine residues neighbouring segment M2 of the alpha-, beta-, gamma- and delta-subunits, probably forming three anionic rings, are major determinants of the rate of ion transport.