Dendrites Of Cortical Pyramidal Neurons Research Articles

Secreted semaphorins control spine distribution and morphogenesis in the postnatal CNS

The majority of excitatory synapses in the mammalian CNS are formed on dendritic spines1, and spine morphology and distribution are critical for synaptic transmission2–6, synaptic integration and plasticity7. Here, we show that a secreted semaphorin, Sema3F, is a negative regulator of spine development and synaptic structure. Mice with null mutations in genes encoding Sema3F, and its holoreceptor components neuropilin-2 (Npn-2) and plexinA3 (PlexA3), exhibit increased dentate gyrus (DG) granule cell (GC) and cortical layer V pyramidal neuron spine number and size, and also aberrant spine distribution. Moreover, Sema3F promotes loss of spines and excitatory synapses in dissociated neurons in vitro, and in Npn-2−/− brain slices cortical layer V and DG GCs exhibit increased mEPSC frequency. In contrast, a distinct Sema3A–Npn-1/PlexA4 signaling cascade controls basal dendritic arborization in layer V cortical neurons but does not influence spine morphogenesis or distribution. These disparate effects of secreted semaphorins are reflected in the restricted dendritic localization of Npn-2 to apical dendrites and of Npn-1 to all dendrites of cortical pyramidal neurons. Therefore, Sema3F signaling controls spine distribution along select dendritic processes, and distinct secreted semaphorin signaling events orchestrate CNS connectivity through the differential control of spine morphogenesis, synapse formation, and the elaboration of dendritic morphology.

Spatiotemporally Graded NMDA Spike/Plateau Potentials in Basal Dendrites of Neocortical Pyramidal Neurons

Glutamatergic inputs clustered over approximately 20-40 microm can elicit local N-methyl-D-aspartate (NMDA) spike/plateau potentials in terminal dendrites of cortical pyramidal neurons, inspiring the notion that a single terminal dendrite can function as a decision-making computational subunit. A typical terminal basal dendrite is approximately 100-200 microm long: could it function as multiple decision-making subunits? We test this by sequential focal stimulation of multiple sites along terminal basal dendrites of layer 5 pyramidal neurons in rat somatosensory cortical brain slices, using iontophoresis or uncaging of brief glutamate pulses. There was an approximately sevenfold spatial gradient in average spike/plateau amplitude measured at the soma, from approximately 3 mV for distal inputs to approximately 23 mV for proximal inputs. Spike/plateaus were NMDA receptor (NMDAR) conductance-dominated at all locations. Large Ca(2+) transients accompanied spike/plateaus over a approximately 10- to 40-microm zone around the input site; smaller Ca(2+) transients extended approximately uniformly to the dendritic tip. Spike/plateau duration grew with increasing glutamate and depolarization; high Ca(2+) zone size grew with spike/plateau duration. The minimum high Ca(2+) zone half-width (just above NMDA spike threshold) increased from distal (approximately 10 microm) to proximal locations (approximately 25 microm), as did the NMDA spike glutamate threshold. Depolarization reduced glutamate thresholds. Simulations exploring multi-site interactions based on this demonstrate that if appropriately timed and localized inputs occur in vivo, a single basal dendrite could correspond to a cascade of multiple co-operating dynamic decision-making subunits able to retain information for hundreds of milliseconds, with increasing influence on neural output from distal to proximal. Dendritic NMDA spike/plateaus are thus well-suited to support graded persistent firing.

Mapping function onto neuronal morphology.

Neurons have a wide range of dendritic morphologies the functions of which are largely unknown. We used an optimization procedure to find neuronal morphological structures for two computational tasks: first, neuronal morphologies were selected for linearly summing excitatory synaptic potentials (EPSPs); second, structures were selected that distinguished the temporal order of EPSPs. The solutions resembled the morphology of real neurons. In particular the neurons optimized for linear summation electrotonically separated their synapses, as found in avian nucleus laminaris neurons, and neurons optimized for spike-order detection had primary dendrites of significantly different diameter, as found in the basal and apical dendrites of cortical pyramidal neurons. This similarity makes an experimentally testable prediction of our theoretical approach, which is that pyramidal neurons can act as spike-order detectors for basal and apical inputs. The automated mapping between neuronal function and structure introduced here could allow a large catalog of computational functions to be built indexed by morphological structure.

Abnormal neocortical development in mice lacking cGMP-dependent protein kinase I

Cyclic GMP-dependent protein kinase type I (cGKI) is a key signaling intermediate important for synaptic potentiation in the hippocampus and cerebellum, but its expression and function in cortical development have not been elucidated. The expression of cGKI in the developing mouse neocortex was evaluated by immunofluorescence labeling, and effect of cGKI deletion on cortical development was studied in adult cGKI knockout mice. cGKI was expressed at highest levels at embryonic stages in young neurons and radial glial fibers, corresponding to the major period of radial migration and laminar development of pyramidal neurons (embryonic day E13.5–E14.5), declining upon maturation (E17.5–postnatal day P28). The cerebral cortex of homozygous null mutant mice lacking cGKI exhibited heterotopic collections of neurons in the upper cortical layers and abnormal invaginations of layer I, in accord with a neuronal migration or positioning defect. Some cGKI mutant mice displayed defects in midline development resulting in partial fusion of cerebral hemispheres with adjacent neuronal heterotopias. Apical dendrites of cortical pyramidal neurons were misoriented in the cerebral cortex of cGKI null mutants, as shown in reporter mice expressing yellow fluorescent protein in layer V pyramidal neurons and by Golgi impregnation. These results demonstrate a role for cGKI signaling in cortical development related to neuronal migration/positioning that is important for dendritic orientation and connectivity.

Regulation of HCN channel surface expression by a novel C-terminal protein-protein interaction.

Hyperpolarization-activated cation currents (I(h)) are carried by channels encoded by a family of four genes (HCN1-4) that are differentially expressed within the brain in specific cellular and subcellular compartments. HCN1 shows a high level of expression in apical dendrites of cortical pyramidal neurons and in presynaptic terminals of cerebellar basket cells, structures with a high density of I(h). Expression of I(h) is also regulated by neuronal activity. To isolate proteins that may control HCN channel expression or function, we performed yeast two-hybrid screens using the C-terminal cytoplasmic tails of the HCN proteins as bait. We identified a brain-specific protein, which has been previously termed TRIP8b (for TPR-containing Rab8b interacting protein) and PEX5Rp (for Pex5p-related protein), that specifically interacts with all four HCN channels through a conserved sequence in their C-terminal tails. In situ hybridization and immunohistochemistry show that TRIP8b and HCN1 are colocalized, particularly within dendritic arbors of hippocampal CA1 and neocortical layer V pyramidal neurons. The dendritic expression of TRIP8b in layer V pyramidal neurons is disrupted after deletion of HCN1 through homologous recombination, demonstrating a key in vivo interaction between HCN1 and TRIP8b. TRIP8b dramatically alters the trafficking of HCN channels heterologously expressed in Xenopus oocytes and human embryonic kidney 293 cells, causing a specific decrease in surface expression of HCN protein and I(h) density, with a pronounced intracellular accumulation of HCN protein that is colocalized in discrete cytoplasmic clusters with TRIP8b. Finally, TRIP8b expression in cultured pyramidal neurons markedly decreases native I(h) density. These data suggest a possible role for TRIP8b in regulating HCN channel density in the plasma membrane.

Computational subunits in thin dendrites of pyramidal cells.

The thin basal and oblique dendrites of cortical pyramidal neurons receive most of the synaptic inputs from other cells, but their integrative properties remain uncertain. Previous studies have most often reported global linear or sublinear summation. An alternative view, supported by biophysical modeling studies, holds that thin dendrites provide a layer of independent computational 'subunits' that sigmoidally modulate their inputs prior to global summation. To distinguish these possibilities, we combined confocal imaging and dual-site focal synaptic stimulation of identified thin dendrites in rat neocortical pyramidal neurons. We found that nearby inputs on the same branch summed sigmoidally, whereas widely separated inputs or inputs to different branches summed linearly. This strong spatial compartmentalization effect is incompatible with a global summation rule and provides the first experimental support for a two-layer 'neural network' model of pyramidal neuron thin-branch integration. Our findings could have important implications for the computing and memory-related functions of cortical tissue.

The PDZ-binding domain is essential for the dendritic targeting of 5-HT 2A serotonin receptors in cortical pyramidal neurons in vitro

The 5-HT 2A serotonin receptor represents an important molecular target for atypical antipsychotic drugs and for most hallucinogens. In the mammalian cerebral cortex, 5-HT 2A receptors are enriched in pyramidal neurons, within which 5-HT 2A receptors are preferentially sorted to the apical dendrites. In primary cortical cultures, 5-HT 2A receptors are sorted to dendrites and not found in the axons of pyramidal neurons. We identified a sorting motif that mediates the preferential targeting of 5-HT 2A receptors to the dendrites of cortical pyramidal neurons in vitro. We constructed green fluorescent protein-tagged 5-HT 2A receptors wherein potential sorting motifs were disrupted, and subsequently employed either the Semliki Forest virus or calcium phosphate for the transient expression of recombinant 5-HT 2A receptors in cultured cortical pyramidal neurons. Using dual-labeling immunofluorescent confocal microscopy, we quantified the axonal and dendritic sorting patterns of endogenous and recombinant 5-HT 2A receptors. We discovered that disruption of the PDZ-binding domain of the 5-HT 2A receptor greatly attenuates the dendritic targeting of 5-HT 2A receptors without inappropriately sorting 5-HT 2A receptors to axons. The PDZ-binding domain is therefore a necessary signal for the preferential targeting of the 5-HT 2A receptor to the dendritic compartment of cultured cortical pyramidal neurons, the first such role ascribed to this protein–protein interaction motif of any G protein-coupled receptor.

Millivolt-scale DC shifts in the human scalp EEG: evidence for a nonneuronal generator.

Slow shifts in the human scalp-recorded EEG, including those related to changes in brain CO(2) levels, have been generally assumed to result from changes in the level of tonic excitation of apical dendrites of cortical pyramidal neurons. We readdressed this issue using DC-EEG shifts elicited in healthy adult subjects by hypo- or hypercapnia. A 3-min period of hyperventilation resulted in a prompt negative shift with a rate of up to 10 microV/s at the vertex (Cz) and an extremely steep dependence (up to 100 microV/mmHg) on the end-tidal Pco(2). This shift had a maximum of up to -2 mV at Cz versus the temporal derivations (T3/T4). Hyperventilation-like breathing of 5% CO(2)-95% O(2), which does not lead to a significant hypocapnia, resulted in a near-complete block of the negative DC shift at Cz. Hypoventilation, or breathing 5% CO(2) in air at normal respiratory rate, induced a positive shift. The high amplitude of the voltage gradients on the scalp induced by hyperventilation is not consistent with a neuronal origin. Instead, the present data suggest that they are generated by extracortical volume currents driven by a Pco(2)-dependent potential difference across epithelia separating the cerebrospinal fluid and blood. Since changes in respiratory patterns and, hence, in the level of brain Pco(2), are likely to occur under a number of experimental conditions in which slow EEG responses have been reported (e.g., attention shifts, preparatory states, epileptic seizures, and hypoxic episodes), the present results call for a thorough reexamination of the mechanisms underlying scalp-recorded DC-EEG responses.

Attention, consciousness, and electrical wave activity within the cortical column

The triangular circuit theory of attention is extended to activity of pyramidal neurons in cortical columns. The expanded theory accounts for both the brief attention of locating and identifying objects and the sustained attention of preparation for and maintenance of attention to objects, attributes, and locations. It is proposed that sustained attention is expressed in a cortical column by repeated surges of current in the long apical dendrites of pyramidal neurons, which form a wave. When the wave is sufficiently intense in a cluster of these neurons, it can be measured at the scalp as EEG oscillations. The wave of attention may or may not produce output pulses from the neuron. Selective attention may be expressed as enhanced processing of pulses arriving at the basal dendrites and leaving at the soma of these pyramidal neurons. It is also proposed that wave activity in the apical dendrites of cortical pyramidal neurons generates subjective experience. When wave activity is produced by the attention circuit in a cortical area beyond the primary sensory areas, it generates our foreground experience of an object. When wave activity is produced in the primary sensory areas by signals from sensory receptors, it generates our background experience, or consciousness.

Non-invasive Studies on Human Brain Functions by using SQUID Gradiometer

A non-invasive technique, magnetoencephalography (MEG) can measure magnetic fields originating mainly from the intracellular current flow through the apical dendrites of cortical pyramidal neurons, which is caused by excitatory synaptic inputs. Our recent MEG studies on sensory and motor functions of the human brain are reviewed. MEG provides an effective non-invasive approach to the studies of human brain functions, especially when combined with appropriate animal experiments.

Premovement slow cortical potentials and required muscle force in self-paced hand movements in the monkey

Surface negative—deep positive, slowly increasing potentials prior to self-paced hand movements were recorded in the contralateral premotor, motor and somatosensory cortices, with chronically implanted electrodes. Such premovement slow potentials in the 3 cortical areas changed their magnitudes with the required muscle force in the hand movement, and the potentials in the different cortices appeared to differ slightly in the manner of change. These results may suggest that the EPSPs in the superfacial parts of apical dendrites of cortical pyramidal neurons principally via certain thalamo-cortical projections are induced prior to movements in the cortices so as to adjust the required force on anticipation, and that the premotor, motor and somatosensory cortices play some different functional roles in preparatory processes for the movement performance.

Effects of differential environments on plasticity of dendrites of cortical pyramidal neurons in adult rats

Thickening of the frontal cortex and especially the occipital cortex was observed in adult rats after exposure to the “enriched” condition. An increase in branching and in length of terminal segments was found in the dendritic tree of pyramidal cells in layers II and III of the visual cortex of the adult rat after exposure to “standard” and enriched conditions. These exposures began at day 112 and continued 30 days. The increase observed in the basal dendritic tree of pyramidal cells in the superficial layers was significantly greater in the enriched conditions than in the standard condition. It appeared, furthermore, that branching occurred predominantly on basal terminal segments of all orders at a considerable distance from the tip. This mode of growth is similar to that observed in the cortex of normal immature rats. The differential conditions did not influence the bifurcation angles. The dendritic and cortical changes and changes reported in the literature indicate that the effects of differential experience are not limited to a short, “critical” period.