Pyramidal neurons in layer 5 of the rat visual cortex. II. Development of electrophysiological properties

Electrophysiology and Morphology of Human Cortical Supragranular Pyramidal Cells in a Wide Age Range

The basic excitatory neurons of the cerebral cortex, the pyramidal cells, are the most important signal integrators for the local circuit. They have quite characteristic morphological and electrophysiological properties that are known to be largely constant with age in the young and adult cortex. However, the brain undergoes several dynamic changes throughout life, such as in the phases of early development and cognitive decline in the aging brain. We set out to search for intrinsic cellular changes in supragranular pyramidal cells across a broad age range: from birth to 85 years of age and we found differences in several biophysical properties between defined age groups. During the first year of life, subthreshold and suprathreshold electrophysiological properties changed in a way that shows that pyramidal cells become less excitable with maturation, but also become temporarily more precise. According to our findings, the morphological features of the three-dimensional reconstructions from different life stages showed consistent morphological properties and systematic dendritic spine analysis of an infantile and an old pyramidal cell showed clear significant differences in the distribution of spine shapes. Overall, the changes that occur during development and aging may have lasting effects on the properties of pyramidal cells in the cerebral cortex. Understanding these changes is important to unravel the complex mechanisms underlying brain development, cognition and age-related neurodegenerative diseases.

Data-driven multiscale computational models of cortical and subcortical regions

Data-driven computational models of neurons, synapses, microcircuits, and mesocircuits have become essential tools in modern brain research. The goal of these multiscale models is to integrate and synthesize information from different levels of brain organization, from cellular properties, dendritic excitability, and synaptic dynamics to microcircuits, mesocircuits, and ultimately behavior. This article surveys recent advances in the genesis of data-driven computational models of mammalian neural networks in cortical and subcortical areas. I discuss the challenges and opportunities in developing data-driven multiscale models, including the need for interdisciplinary collaborations, the importance of model validation and comparison, and the potential impact on basic and translational neuroscience research. Finally, I highlight future directions and emerging technologies that will enable more comprehensive and predictive data-driven models of brain function and dysfunction.

Electrophysiological properties of layer 2/3 pyramidal neurons in the primary visual cortex of a retinitis pigmentosa mouse model (rd10)

Retinal degeneration is one of the main causes of visual impairment and blindness. One group of retinal degenerative diseases, leading to the loss of photoreceptors, is collectively termed retinitis pigmentosa. In this group of diseases, the remaining retina is largely spared from initial cell death making retinal ganglion cells an interesting target for vision restoration methods. However, it is unknown how downstream brain areas, in particular the visual cortex, are affected by the progression of blindness. Visual deprivation studies have shown dramatic changes in the electrophysiological properties of visual cortex neurons, but changes on a cellular level in retinitis pigmentosa have not been investigated yet. Therefore, we used the rd10 mouse model to perform patch-clamp recordings of pyramidal neurons in layer 2/3 of the primary visual cortex to screen for potential changes in electrophysiological properties resulting from retinal degeneration. Compared to wild-type C57BL/6 mice, we only found an increase in intrinsic excitability around the time point of maximal retinal degeneration. In addition, we saw an increase in the current amplitude of spontaneous putative inhibitory events after a longer progression of retinal degeneration. However, we did not observe a long-lasting shift in excitability after prolonged retinal degeneration. Together, our results provide evidence of an intact visual cortex with promising potential for future therapeutic strategies to restore vision.

A simple method for poly-D-lysine coating to enhance adhesion and maturation of primary cortical neuron cultures in vitro

Introduction

Glass coverslips are used as a substrate since Harrison's initial nerve cell culture experiments in 1910. In 1974, the first study of brain cells seeded onto polylysine (PL) coated substrate was published. Usually, neurons adhere quickly to PL coating. However, maintaining cortical neurons in culture on PL coating for a prolonged time is challenging.

Methods

A collaborative study between chemical engineers and neurobiologists was conducted to find a simple method to enhance neuronal maturation on poly-D-lysine (PDL). In this work, a simple protocol to coat PDL efficiently on coverslips is presented, characterized, and compared to a conventional adsorption method. We studied the adhesion and maturation of primary cortical neurons with various morphological and functional approaches, including phase contrast microscopy, immunocytochemistry, scanning electron microscopy, patch clamp recordings, and calcium imaging.

Results

We observed that several parameters of neuronal maturation are influenced by the substrate: neurons develop more dense and extended networks and synaptic activity is enhanced, when seeded on covalently bound PDL compared to adsorbed PDL.

Discussion

Hence, we established reproducible and optimal conditions enhancing maturation of primary cortical neurons in vitro. Our method allows higher reliability and yield of results and could also be profitable for laboratories using PL with other cell types.

Dissection of insular cortex layer 5 reveals two sublayers with opposing modulatory roles in appetitive drinking behavior

The insular cortex (insula) is known to play a modulatory role in feeding and drinking. Previous studies have revealed anterior-posterior differences of subcortical projections and roles for the insula, yet the anatomical and functional heterogeneity among the cortical layers remains poorly understood. Here, we show that layer 5 of the mouse dysgranular insula has two distinct neuronal subpopulations along the entire anterior-posterior axis: The L5a population, expressing NECAB1, projects bilaterally to the lateral and capsular divisions of the central amygdala, and the L5b population, expressing CTIP2, projects ipsilaterally to the parasubthalamic nucleus and the medial division of the central amygdala. Optogenetically activating L5a and L5b neuronal populations in thirsty male mice led to suppressed and facilitated water spout licking, respectively, without avoidance against or preference for the spout paired with the opto-stimulation. Our results suggest sublayer-specific bidirectional modulatory roles of insula layer 5 in the motivational aspect of appetitive behavior.

Suprapontine Structures Modulate Brainstem and Spinal Networks

Several spinal motor output and essential rhythmic behaviors are controlled by supraspinal structures, although their contribution to neuronal networks for respiration and locomotion at birth still requires better characterization. As preparations of isolated brainstem and spinal networks only focus on local circuitry, we introduced the in vitro central nervous system (CNS) from neonatal rodents to simultaneously record a stable respiratory rhythm from both cervical and lumbar ventral roots (VRs).Electrical pulses supplied to multiple sites of brainstem evoked distinct VR responses with staggered onset in the rostro-caudal direction. Stimulation of ventrolateral medulla (VLM) resulted in higher events from homolateral VRs. Stimulating a lumbar dorsal root (DR) elicited responses even from cervical VRs, albeit small and delayed, confirming functional ascending pathways. Oximetric assessments detected optimal oxygen levels on brainstem and cortical surfaces, and histological analysis of internal brain structures indicated preserved neuron viability without astrogliosis. Serial ablations showed precollicular decerebration reducing respiratory burst duration and frequency and diminishing the area of lumbar DR and VR potentials elicited by DR stimulation, while pontobulbar transection increased the frequency and duration of respiratory bursts. Keeping legs attached allows for expressing a respiratory rhythm during hindlimb stimulation. Trains of pulses evoked episodes of fictive locomotion (FL) when delivered to VLM or to a DR, the latter with a slightly better FL than in isolated cords.In summary, suprapontine centers regulate spontaneous respiratory rhythms, as well as electrically evoked reflexes and spinal network activity. The current approach contributes to clarifying modulatory brain influences on the brainstem and spinal microcircuits during development. Novel preparation of the entire isolated CNS from newborn rats unveils suprapontine modulation on brainstem and spinal networks. Preparation views (A) with and without legs attached (B). Successful fictive respiration occurs with fast dissection from P0-P2 rats (C). Decerebration speeds up respiratory rhythm (D) and reduces spinal reflexes derived from both ventral and dorsal lumbar roots (E).

Introduction: What Are Dendritic Spines?

Dendritic spines are cellular specializations that greatly increase the connectivity of neurons and modulate the "weight" of most postsynaptic excitatory potentials. Spines are found in very diverse animal species providing neural networks with a high integrative and computational possibility and plasticity, enabling the perception of sensorial stimuli and the elaboration of a myriad of behavioral displays, including emotional processing, memory, and learning. Humans have trillions of spines in the cerebral cortex, and these spines in a continuum of shapes and sizes can integrate the features that differ our brain from other species. In this chapter, we describe (1) the discovery of these small neuronal protrusions and the search for the biological meaning of dendritic spines; (2) the heterogeneity of shapes and sizes of spines, whose structure and composition are associated with the fine-tuning of synaptic processing in each nervous area, as well as the findings that support the role of dendritic spines in increasing the wiring of neural circuits and their functions; and (3) within the intraspine microenvironment, the integration and activation of signaling biochemical pathways, the compartmentalization of molecules or their spreading outside the spine, and the biophysical properties that can affect parent dendrites. We also provide (4) examples of plasticity involving dendritic spines and neural circuits relevant to species survival and comment on (5) current research advancements and challenges in this exciting research field.

Repetitively burst-spiking neurons in reeler mice show conserved but also highly variable morphological features of layer Vb-fated “thick-tufted” pyramidal cells

Reelin is a large extracellular glycoprotein that is secreted by Cajal-Retzius cells during embryonic development to regulate neuronal migration and cell proliferation but it also seems to regulate ion channel distribution and synaptic vesicle release properties of excitatory neurons well into adulthood. Mouse mutants with a compromised reelin signaling cascade show a highly disorganized neocortex but the basic connectional features of the displaced excitatory principal cells seem to be relatively intact. Very little is known, however, about the intrinsic electrophysiological and morphological properties of individual cells in the reeler cortex. Repetitive burst-spiking (RB) is a unique property of large, thick-tufted pyramidal cells of wild-type layer Vb exclusively, which project to several subcortical targets. In addition, they are known to possess sparse but far-reaching intracortical recurrent collaterals. Here, we compared the electrophysiological properties and morphological features of neurons in the reeler primary somatosensory cortex with those of wild-type controls. Whereas in wild-type mice, RB pyramidal cells were only detected in layer Vb, and the vast majority of reeler RB pyramidal cells were found in the superficial third of the cortical depth. There were no obvious differences in the intrinsic electrophysiological properties and basic morphological features (such as soma size or the number of dendrites) were also well preserved. However, the spatial orientation of the entire dendritic tree was highly variable in the reeler neocortex, whereas it was completely stereotyped in wild-type mice. It seems that basic quantitative features of layer Vb-fated RB pyramidal cells are well conserved in the highly disorganized mutant neocortex, whereas qualitative morphological features vary, possibly to properly orient toward the appropriate input pathways, which are known to show an atypical oblique path through the reeler cortex. The oblique dendritic orientation thus presumably reflects a re-orientation of dendritic input domains toward spatially highly disorganized afferent projections.

Quantifying neuronal structural changes over time using dynamic morphometrics

Brain circuit development involves tremendous structural formation and rearrangement of dendrites, axons, and the synaptic connections between them. Direct studies of neuronal morphogenesis are now possible through recent developments in multiple technologies, including single-neuron labeling, time-lapse imaging in intact tissues, and 4D rendering software capable of tracking neural growth over periods spanning minutes to days. These methods allow detailed quantification of structural changes of neurons over time, called dynamic morphometrics, providing new insights into fundamental growth patterns, underlying molecular mechanisms, and the intertwined influences of external factors, including neural activity, and intrinsic genetic programs. Here, we review the methods of dynamic morphometrics sampling and analyses, focusing on their applications to studies of activity-driven dendritogenesis in vertebrate systems.

Measuring Absolute Membrane Potential Across Space and Time

Membrane potential (Vmem) is a fundamental biophysical signal present in all cells. Vmem signals range in time from milliseconds to days, and they span lengths from microns to centimeters. Vmem affects many cellular processes, ranging from neurotransmitter release to cell cycle control to tissue patterning. However, existing tools are not suitable for Vmem quantification in many of these areas. In this review, we outline the diverse biology of Vmem, drafting a wish list of features for a Vmem sensing platform. We then use these guidelines to discuss electrode-based and optical platforms for interrogating Vmem. On the one hand, electrode-based strategies exhibit excellent quantification but are most effective in short-term, cellular recordings. On the other hand, optical strategies provide easier access to diverse samples but generally only detect relative changes in Vmem. By combining the respective strengths of these technologies, recent advances in optical quantification of absolute Vmem enable new inquiries into Vmem biology.

Chronic ethanol exposure differentially alters neuronal function in the medial prefrontal cortex and dentate gyrus

Alterations in the function of prefrontal cortex (PFC) and hippocampus have been implicated in underlying the relapse to alcohol seeking behaviors in humans and animal models of moderate to severe alcohol use disorders (AUD). Here we used chronic intermittent ethanol vapor exposure (CIE), 21d protracted abstinence following CIE (21d AB), and re-exposure to one vapor session during protracted abstinence (re-exposure) to evaluate the effects of chronic ethanol exposure on basal synaptic function, neuronal excitability and expression of key synaptic proteins that play a role in neuronal excitability in the medial PFC (mPFC) and dentate gyrus (DG). CIE consistently enhanced excitability of layer 2/3 pyramidal neurons in the mPFC and granule cell neurons in the DG. In the DG, this effect persisted during 21d AB. Re-exposure did not enhance excitability, suggesting resistance to vapor-induced effects. Analysis of action potential kinetics revealed that altered afterhyperpolarization, rise time and decay time constants are associated with the altered excitability during CIE, 21d AB and re-exposure. Molecular adaptations that may underlie increases in neuronal excitability under these different conditions were identified. Quantitative polymerase chain reaction of large-conductance potassium (BK) channel subunit mRNA in PFC and DG tissue homogenates did not show altered expression patterns of BK subunits. Western blotting demonstrates enhanced phosphorylation of Ca²⁺/calmodulin-dependent protein kinase II (CaMKII), and reduced phosphorylation of glutamate receptor GluN2A/2B subunits. These results suggest a novel relationship between activity of CaMKII and GluN receptors in the mPFC and DG, and neuronal excitability in these brain regions in the context of moderate to severe AUD.

Synaptic and intrinsic mechanisms underlying development of cortical direction selectivity

Modifications of synaptic inputs and cell-intrinsic properties both contribute to neuronal plasticity and development. To better understand these mechanisms, we undertook an intracellular analysis of the development of direction selectivity in the ferret visual cortex, which occurs rapidly over a few days after eye opening. We found strong evidence of developmental changes in linear spatiotemporal receptive fields of simple cells, implying alterations in circuit inputs. Further, this receptive field plasticity was accompanied by increases in near-spike-threshold excitability and input-output gain that resulted in dramatically increased spiking responses in the experienced state. Increases in subthreshold membrane responses induced by the receptive field plasticity and the increased input-output spiking gain were both necessary to explain the elevated firing rates in experienced ferrets. These results demonstrate that cortical direction selectivity develops through a combination of plasticity in inputs and in cell-intrinsic properties.

COUP-TFI/Nr2f1 Orchestrates Intrinsic Neuronal Activity during Development of the Somatosensory Cortex

The formation of functional cortical maps in the cerebral cortex results from a timely regulated interaction between intrinsic genetic mechanisms and electrical activity. To understand how transcriptional regulation influences network activity and neuronal excitability within the neocortex, we used mice deficient for Nr2f1 (also known as COUP-TFI), a key determinant of primary somatosensory (S1) area specification during development. We found that the cortical loss of Nr2f1 impacts on spontaneous network activity and synchronization of S1 cortex at perinatal stages. In addition, we observed alterations in the intrinsic excitability and morphological features of layer V pyramidal neurons. Accordingly, we identified distinct voltage-gated ion channels regulated by Nr2f1 that might directly influence intrinsic bioelectrical properties during critical time windows of S1 cortex specification. Altogether, our data suggest a tight link between Nr2f1 and neuronal excitability in the developmental sequence that ultimately sculpts the emergence of cortical network activity within the immature neocortex.

Maturation of pyramidal cells in anterior piriform cortex may be sufficient to explain the end of early olfactory learning in rats

Studies have shown that neonate rodents exhibit high ability to learn a preference for novel odors associated with thermo-tactile stimuli that mimics maternal care. Artificial odors paired with vigorous strokes in rat pups younger than 10 postnatal days (P), but not older, rapidly induce an orientation-approximation behavior toward the conditioned odor in a two-choice preference test. The olfactory bulb (OB) and the anterior olfactory cortex (aPC), both modulated by norepinephrine (NE), have been identified as part of a neural circuit supporting this transitory olfactory learning. One possible explanation at the neuronal level for why the odor-stroke pairing induces consistent orientation-approximation behavior in <P10 pups, but not in >P10, is the coincident activation of prior existent neurons in the aPC mediating this behavior. Specifically, odor-stroke conditioning in <P10 pups may activate more mother/nest odor's responsive aPC neurons than in >P10 pups, promoting orientation-approximation behavior in the former but not in the latter. In order to test this hypothesis, we performed in vitro patch-clamp recordings of the aPC pyramidal neurons from rat pups from two age groups (P5–P8 and P14–P17) and built computational models for the OB-aPC neural circuit based on this physiological data. We conditioned the P5–P8 OB-aPC artificial circuit to an odor associated with NE activation (representing the process of maternal odor learning during mother–infant interactions inside the nest) and then evaluated the response of the OB-aPC circuit to the presentation of the conditioned odor. The results show that the number of responsive aPC neurons to the presentation of the conditioned odor in the P14–P17 OB-aPC circuit was lower than in the P5–P8 circuit, suggesting that at P14–P17, the reduced number of responsive neurons to the conditioned (maternal) odor might not be coincident with the responsive neurons for a second conditioned odor.

Corticostriatal plasticity in the nucleus accumbens core

Small fluctuations in striatal glutamate and dopamine are required to establish goal-directed behaviors and motor learning, while large changes appear to underlie many neuropsychological disorders, including drug dependence and Parkinson's disease. A better understanding of how variations in neurotransmitter availability can modify striatal circuitry will lead to new therapeutic targets for these disorders. Here, we examined dopamine-induced plasticity in prefrontal cortical projections to the nucleus accumbens (NAc) core. We combined behavioral measures of male mice, presynaptic optical studies of glutamate release kinetics from prefrontal cortical projections, and postsynaptic electrophysiological recordings of spiny projection neurons within the NAc core. Our data show that repeated amphetamine promotes long-lasting but reversible changes along the corticoaccumbal pathway. In saline-treated mice, coincident cortical stimulation and dopamine release promoted presynaptic filtering by depressing exocytosis from glutamatergic boutons with a low-probability of release. The repeated use of amphetamine caused a frequency-dependent, progressive, and long-lasting depression in corticoaccumbal activity during withdrawal. This chronic presynaptic depression was relieved by a drug challenge which potentiated glutamate release from synapses with a low-probability of release. D1 receptors generated this synaptic potentiation, which corresponded with the degree of locomotor sensitization in individual mice. By reversing the synaptic depression, drug reinstatement may promote allostasis by returning corticoaccumbal activity to a more stable and normalized state. Therefore, dopamine-induced synaptic filtering of excitatory signals entering the NAc core in novice mice and paradoxical excitation of the corticoaccumbal pathway during drug reinstatement may encode motor learning, habit formation, and dependence.

Hold Your Methods! How Multineuronal Firing Ensembles Can Be Studied Using Classical Spike-Train Analysis Techniques

Responses of neuronal populations play an important role in the encoding of stimulus related information. However, the inherent multidimensionality required to describe population activity has imposed significant challenges and has limited the applicability of classical spike train analysis techniques. Here, we show that these limitations can be overcome. We first quantify the collective activity of neurons as multidimensional vectors (patterns). Then we characterize the behavior of these patterns by applying classical spike train analysis techniques: peri-stimulus time histograms, tuning curves and auto- and cross-correlation histograms. We find that patterns can exhibit a broad spectrum of properties, some resembling and others substantially differing from those of their component neurons. We show that in some cases pattern behavior cannot be intuitively inferred from the activity of component neurons. Importantly, silent neurons play a critical role in shaping pattern expression. By correlating pattern timing with local-field potentials, we show that the method can reveal fine temporal coordination of cortical circuits at the mesoscale. Because of its simplicity and reliance on well understood classical analysis methods the proposed approach is valuable for the study of neuronal population dynamics.

Biophysically realistic neuron models for simulation of cortical stimulation

OBJECTIVE

We implemented computational models of human and rat cortical neurons for simulating the neural response to cortical stimulation with electromagnetic fields.

APPROACH

We adapted model neurons from the library of Blue Brain models to reflect biophysical and geometric properties of both adult rat and human cortical neurons and coupled the model neurons to exogenous electric fields (E-fields). The models included 3D reconstructed axonal and dendritic arbors, experimentally-validated electrophysiological behaviors, and multiple, morphological variants within cell types. Using these models, we characterized the single-cell responses to intracortical microstimulation (ICMS) and uniform E-field with dc as well as pulsed currents.

MAIN RESULTS

The strength-duration and current-distance characteristics of the model neurons to ICMS agreed with published experimental results, as did the subthreshold polarization of cell bodies and axon terminals by uniform dc E-fields. For all forms of stimulation, the lowest threshold elements were terminals of the axon collaterals, and the dependence of threshold and polarization on spatial and temporal stimulation parameters was strongly affected by morphological features of the axonal arbor, including myelination, diameter, and branching.

SIGNIFICANCE

These results provide key insights into the mechanisms of cortical stimulation. The presented models can be used to study various cortical stimulation modalities while incorporating detailed spatial and temporal features of the applied E-field.

Fast Inhibitory Decay Facilitates Adult-like Temporal Processing in Layer 5 of Developing Primary Auditory Cortex

The protracted maturational process of temporal processing in layer 4 (L4) of primary auditory cortex (A1) has been extensively studied. Accumulating evidences show that layer 5 (L5) receives direct thalamic inputs as well. How the temporal responses in L5 may developmentally emerge remains unclear. Using in vivo loose-patch recordings in rat A1, we found that putative pyramidal (Pyr) neurons in developing L5 exhibited adult-like stimulus-following ability but less bursting shortly after hearing onset. L5 Pyr neurons in adult A1 exhibited phase-locking similar to L4 neurons, while L5 fast-spiking (FS) neurons showed greater phase-locking at 7 and 12.5 pps. In developing L5, whole-cell recordings revealed inhibition with decay constant comparable to that in adult L5, thereby avoiding the summation of inhibition that contributed to the strong adaptation in L4. Given the targets of L5 outputs, the relatively precocious temporal processing in L5 might contribute to temporal response maturation in connected cortical and subcortical areas. Our findings were in agreement with the idea that L5 may be a "hub" for processing cortical inputs and outputs that can operate independently of L4.

Norepinephrine Modulates Pyramidal Cell Synaptic Properties in the Anterior Piriform Cortex of Mice: Age-Dependent Effects of β-adrenoceptors

Early odor preference learning in rodents occurs within a sensitive period [≤postnatal day (P)10–12], during which pups show a heightened ability to form an odor preference when a novel odor is paired with a tactile stimulation (e.g., stroking). Norepinephrine (NE) release from the locus coeruleus during stroking mediates this learning. However, in older pups, stroking loses its ability to induce learning. The cellular and circuitry mechanisms underpinning the sensitive period for odor preference learning is not well understood. We first established the sensitive period learning model in mice – odor paired with stroking induced odor preference in P8 but not P14 mice. This learning was dependent on NE-β-adrenoceptors as it was prevented by propranolol injection prior to training. We then tested whether there are developmental changes in pyramidal cell excitability and NE responsiveness in the anterior piriform cortex (aPC) in mouse pups. Although significant differences of pyramidal cell intrinsic properties were found in two age groups (P8–11 and P14+), NE at two concentrations (0.1 and 10 μM) did not alter intrinsic properties in either group. In contrast, in P8–11 pups, NE at 0.1 μM presynaptically decreased miniature IPSC and increased miniature EPSC frequencies. These effects were reversed with a higher dose of NE (10 μM), suggesting involvement of different adrenoceptor subtypes. In P14+ pups, NE at higher doses (1 and 10 μM) acted both pre- and postsynaptically to promote inhibition. These results suggest that enhanced synaptic excitation and reduced inhibition by NE in the aPC network may underlie the sensitive period.

Fezf2 expression in layer 5 projection neurons of mature mouse motor cortex

The mature cerebral cortex contains a wide diversity of neuron phenotypes. This diversity is specified during development by neuron-specific expression of key transcription factors, some of which are retained for the life of the animal. One of these key developmental transcription factors that is also retained in the adult is Fezf2, but the neuron types expressing it in the mature cortex are unknown. With a validated Fezf2-Gfp reporter mouse, whole-cell electrophysiology with morphology reconstruction, cluster analysis, in vivo retrograde labeling, and immunohistochemistry, we identify a heterogeneous population of Fezf2(+) neurons in both layer 5A and layer 5B of the mature motor cortex. Functional electrophysiology identified two distinct subtypes of Fezf2(+) neurons that resembled pyramidal tract projection neurons (PT-PNs) and intratelencephalic projection neurons (IT-PNs). Retrograde labeling confirmed the former type to include corticospinal projection neurons (CSpPNs) and corticothalamic projection neurons (CThPNs), whereas the latter type included crossed corticostriatal projection neurons (cCStrPNs) and crossed-corticocortical projection neurons (cCCPNs). The two Fezf2(+) subtypes expressed either CTIP2 or SATB2 to distinguish their physiological identity and confirmed that specific expression combinations of key transcription factors persist in the mature motor cortex. Our findings indicate a wider role for Fezf2 within gene expression networks that underpin the diversity of layer 5 cortical projection neurons.

Anatomy and physiology of the thick-tufted layer 5 pyramidal neuron

The thick-tufted layer 5 (TTL5) pyramidal neuron is one of the most extensively studied neuron types in the mammalian neocortex and has become a benchmark for understanding information processing in excitatory neurons. By virtue of having the widest local axonal and dendritic arborization, the TTL5 neuron encompasses various local neocortical neurons and thereby defines the dimensions of neocortical microcircuitry. The TTL5 neuron integrates input across all neocortical layers and is the principal output pathway funneling information flow to subcortical structures. Several studies over the past decades have investigated the anatomy, physiology, synaptology, and pathophysiology of the TTL5 neuron. This review summarizes key discoveries and identifies potential avenues of research to facilitate an integrated and unifying understanding on the role of a central neuron in the neocortex.

Motor cortex maturation is associated with reductions in recurrent connectivity among functional subpopulations and increases in intrinsic excitability

Behavior is derived from the configuration of synaptic connectivity among functionally diverse neurons. Fine motor behavior is absent at birth in most mammals but gradually emerges during subsequent postnatal corticospinal system maturation; the nature of circuit development and reorganization during this period has been largely unexplored. We investigated connectivity and synaptic signaling among functionally distinct corticospinal populations in Fischer 344 rats from postnatal day 18 through 75 using retrograde tracer injections into specific spinal cord segments associated with distinct aspects of forelimb function. Primary motor cortex slices were prepared enabling simultaneous patch-clamp recordings of up to four labeled corticospinal neurons and testing of 3489 potential synaptic connections. We find that, in immature animals, local connectivity is biased toward corticospinal neurons projecting to the same spinal cord segment; this within-population connectivity significantly decreases through maturation until connection frequency is similar between neurons projecting to the same (within-population) or different (across-population) spinal segments. Concomitantly, postnatal maturation is associated with a significant reduction in synaptic efficacy over time and an increase in intrinsic neuronal excitability, altering how excitation is effectively transmitted across recurrent corticospinal networks. Collectively, the postnatal emergence of fine motor control is associated with a relative broadening of connectivity between functionally diverse cortical motor neurons and changes in synaptic properties that could enable the emergence of smaller independent networks, enabling fine motor movement. These changes in synaptic patterning and physiological function provide a basis for the increased capabilities of the mature versus developing brain.

Pyramidal cell development: postnatal spinogenesis, dendritic growth, axon growth, and electrophysiology

Here we review recent findings related to postnatal spinogenesis, dendritic and axon growth, pruning and electrophysiology of neocortical pyramidal cells in the developing primate brain. Pyramidal cells in sensory, association and executive cortex grow dendrites, spines and axons at different rates, and vary in the degree of pruning. Of particular note is the fact that pyramidal cells in primary visual area (V1) prune more spines than they grow during postnatal development, whereas those in inferotemporal (TEO and TE) and granular prefrontal cortex (gPFC; Brodmann's area 12) grow more than they prune. Moreover, pyramidal cells in TEO, TE and the gPFC continue to grow larger dendritic territories from birth into adulthood, replete with spines, whereas those in V1 become smaller during this time. The developmental profile of intrinsic axons also varies between cortical areas: those in V1, for example, undergo an early proliferation followed by pruning and local consolidation into adulthood, whereas those in area TE tend to establish their territory and consolidate it into adulthood with little pruning. We correlate the anatomical findings with the electrophysiological properties of cells in the different cortical areas, including membrane time constant, depolarizing sag, duration of individual action potentials, and spike-frequency adaptation. All of the electrophysiological variables ramped up before 7 months of age in V1, but continued to ramp up over a protracted period of time in area TE. These data suggest that the anatomical and electrophysiological profiles of pyramidal cells vary among cortical areas at birth, and continue to diverge into adulthood. Moreover, the data reveal that the “use it or lose it” notion of synaptic reinforcement may speak to only part of the story, “use it but you still might lose it” may be just as prevalent in the cerebral cortex.

Pyramidal cells in V1 of African rodents are bigger, more branched and more spiny than those in primates

Pyramidal cells are characterized by markedly different sized dendritic trees, branching patterns, and spine density across the cortical mantle. Moreover, pyramidal cells have been shown to differ in structure among homologous cortical areas in different species; however, most of these studies have been conducted in primates. Whilst pyramidal cells have been quantified in a few cortical areas in some other species there are, as yet, no uniform comparative data on pyramidal cell structure in a homologous cortical area among species in different Orders. Here we studied layer III pyramidal cells in V1 of three species of rodents, the greater cane rat, highveld gerbil, and four-striped mouse, by the same methodology used to sample data from layer III pyramidal cells in primates. The data reveal markedly different trends between rodents and primates: there is an appreciable increase in the size, branching complexity, and number of spines in the dendritic trees of pyramidal cells with increasing size of V1 in the brain in rodents, whereas there is relatively little difference in primates. Moreover, pyramidal cells in rodents are larger, more branched and more spinous than those in primates. For example, the dendritic trees of pyramidal cells in V1 of the adult cane rat are nearly three times larger, and have more than 10 times the number of spines in their basal dendritic trees, than those in V1 of the adult macaque (7900 and 600, respectively), which has a V1 40 times the size that of the cane rat. It remains to be determined to what extent these differences may result from development or reflect evolutionary and/or processing specializations.

Alterations of neocortical pyramidal neurons: turning points in the genesis of mental retardation

Pyramidal neurons (PNs) represent the majority of neocortical cells and their involvement in cognitive functions is decisive. Therefore, they are the most obvious target of developmental disorders characterized by mental retardation. Genetic and non-genetic forms of intellectual disability share a few basic pathogenetic signatures that result in the anomalous function of PNs. Here, we review the key mechanisms impairing these neurons and their participation in the cortical network, with special focus on experimental models of fetal exposure to alcohol. Due to the heterogeneity of PNs, some alterations affect selectively a given cell population, which may also differ depending on the considered pathology. These specific features open new possibilities for the interpretation of cognitive defects observed in mental retardation syndromes, as well as for novel therapeutic interventions.

Acetylcholine encodes long-lasting presynaptic plasticity at glutamatergic synapses in the dorsal striatum after repeated amphetamine exposure

Locomotion and cue-dependent behaviors are modified through corticostriatal signaling whereby short-term increases in dopamine availability can provoke persistent changes in glutamate release that contribute to neuropsychiatric disorders, including Parkinson's disease and drug dependence. We found that withdrawal of mice from repeated amphetamine treatment caused a chronic presynaptic depression (CPD) in glutamate release that was most pronounced in corticostriatal terminals with a low probability of release and lasted >50 d in treated mice. An amphetamine challenge reversed CPD via a dopamine D1-receptor-dependent paradoxical presynaptic potentiation (PPP) that increased corticostriatal activity in direct pathway medium spiny neurons. This PPP was correlated with locomotor responses after a drug challenge, suggesting that it may underlie the sensitization process. Experiments in brain slices and in vivo indicated that dopamine regulation of acetylcholine release from tonically active interneurons contributes to CPD, PPP, locomotor sensitization, and cognitive ability. Therefore, a chronic decrease in corticostriatal activity during withdrawal is regulated around a new physiological range by tonically active interneurons and returns to normal upon reexposure to amphetamine, suggesting that this paradoxical return of striatal activity to a more stable, normalized state may represent an additional source of drug motivation during abstinence.

Population rate dynamics and multineuron firing patterns in sensory cortex

Cortical circuits encode sensory stimuli through the firing of neuronal ensembles, and also produce spontaneous population patterns in the absence of sensory drive. This population activity is often characterized experimentally by the distribution of multineuron "words" (binary firing vectors), and a match between spontaneous and evoked word distributions has been suggested to reflect learning of a probabilistic model of the sensory world. We analyzed multineuron word distributions in sensory cortex of anesthetized rats and cats, and found that they are dominated by fluctuations in population firing rate rather than precise interactions between individual units. Furthermore, cortical word distributions change when brain state shifts, and similar behavior is seen in simulated networks with fixed, random connectivity. Our results suggest that similarity or dissimilarity in multineuron word distributions could primarily reflect similarity or dissimilarity in population firing rate dynamics, and not necessarily the precise interactions between neurons that would indicate learning of sensory features.

A role for silent synapses in the development of the pathway from layer 2/3 to 5 pyramidal cells in the neocortex

The integration of neurons within the developing cerebral cortex is a prolonged process dependent on a combination of molecular and physiological cues. To examine the latter we used laser scanning photostimulation (LSPS) of caged glutamate in conjunction with whole-cell patch-clamp electrophysiology to probe the integration of pyramidal cells in the sensorimotor regions of the mouse neocortex. In the days immediately after postnatal day 5 (P5) the origin of the LSPS-evoked AMPA receptor (AMPAR)-mediated synaptic inputs were diffuse and poorly defined with considerable variability between cells. Over the subsequent week this coalesced and shifted, primarily influenced by an increased contribution from layers 2/3 cells, which became a prominent motif of the afferent input onto layer 5 pyramidal cells regardless of cortical region. To further investigate this particular emergent translaminar connection, we alternated our mapping protocol between two holding potentials (-70 and +40 mV) allowing us to detect exclusively NMDA receptor (NMDAR)-mediated inputs. This revealed distal MK-801-sensitive synaptic inputs that predict the formation of the mature, canonical layer 2/3 to 5 pathway. However, these were a transient feature and had been almost entirely converted to AMPAR synapses at a later age (P16). To examine the role of activity in the recruitment of early NMDAR synapses, we evoked brief periods (20 min) of rhythmic bursting. Short intense periods of activity could cause a prolonged augmentation of the total input onto pyramidal cells up until P12; a time point when the canonical circuit has been instated and synaptic integration shifts to a more consolidatory phase.

Analysis of slow (theta) oscillations as a potential temporal reference frame for information coding in sensory cortices

While sensory neurons carry behaviorally relevant information in responses that often extend over hundreds of milliseconds, the key units of neural information likely consist of much shorter and temporally precise spike patterns. The mechanisms and temporal reference frames by which sensory networks partition responses into these shorter units of information remain unknown. One hypothesis holds that slow oscillations provide a network-intrinsic reference to temporally partitioned spike trains without exploiting the millisecond-precise alignment of spikes to sensory stimuli. We tested this hypothesis on neural responses recorded in visual and auditory cortices of macaque monkeys in response to natural stimuli. Comparing different schemes for response partitioning revealed that theta band oscillations provide a temporal reference that permits extracting significantly more information than can be obtained from spike counts, and sometimes almost as much information as obtained by partitioning spike trains using precisely stimulus-locked time bins. We further tested the robustness of these partitioning schemes to temporal uncertainty in the decoding process and to noise in the sensory input. This revealed that partitioning using an oscillatory reference provides greater robustness than partitioning using precisely stimulus-locked time bins. Overall, these results provide a computational proof of concept for the hypothesis that slow rhythmic network activity may serve as internal reference frame for information coding in sensory cortices and they foster the notion that slow oscillations serve as key elements for the computations underlying perception.

Alterations of neocortical development and maturation in autism: insight from valproic acid exposure and animal models of autism

Autism spectrum disorder (ASD) is a behaviourally defined brain disorder affecting approximately 1 in 88 children. Many pathological studies have shown that ASD is frequently associated with grey and white matter changes that can be described by their deviations from the normal trajectory of cortical maturation. For example, during the early (i.e. <2 years) postnatal period there is marked and selective tissue overgrowth in the higher-order temporal and frontal networks involved in emotional, social, and communication functions. In this focused review we first summarize some basic principles of neocortical neural organization and how they are disrupted in ASD. We will then highlight some of the potential mechanisms by which the normal developmental trajectory and organization of neocortical networks can be altered based on animal studies of valproic acid, a teratogen widely used in animal models of ASD. We argue that the trajectory of postnatal cerebral neocortex development may be influenced by several cellular and molecular mechanisms that may all converge to produce a neuropathology characterized by premature or accelerated neuronal growth.

Developmental shift of short-term synaptic plasticity in cortical organotypic slices

Although short-term synaptic plasticity (STP) is ubiquitous in neocortical synapses its functional role in neural computations is not well understood. Critical to elucidating the function of STP will be to understand how STP itself changes with development and experience. Previous studies have reported developmental changes in STP using acute slices. It is not clear, however, to what extent the changes in STP are a function of local ontogenetic programs or the result of the many different sensory and experience-dependent changes that accompany development in vivo. To address this question we examined the in vitro development of STP in organotypic slices cultured for up to 4 weeks. Paired recordings were performed in L5 pyramidal neurons at different stages of in vitro development. We observed a shift in STP in the form of a decrease in the paired-pulse ratio (PPR) (less depression) from the second to fourth week in vitro. This shift in STP was not accompanied by a change in initial excitatory postsynaptic potential (EPSP) amplitude. Fitting STP to a quantitative model indicated that the developmental shift is consistent with presynaptic changes. Importantly, despite the change in the PPR we did not observe changes in the time constant governing STP. Since these experiments were conducted in vitro our results indicate that the shift in STP does not depend on in vivo sensory experience. Although sensory experience may shape STP, we suggest that developmental shifts in STP are at least in part ontogenetically determined.

Visual deprivation alters dendritic bundle architecture in layer 4 of rat visual cortex

The effect of visual deprivation followed by light exposure on the tangential organisation of dendritic bundles passing through layer 4 of the rat visual cortex was studied quantitatively in the light microscope. Four groups of animals were investigated: (I) rats reared in an environment illuminated normally--group 52 dL; (II) rats reared in the dark until 21 days postnatum (DPN) and subsequently light exposed for 31 days-group 21/31; (III) rats dark reared until 52 DPN and then subsequently light exposed for 3 days--group 3 dL; and (IV) rats totally dark reared until 52 DPN--group 52 DPN. Each group contained five animals. Semithin 0.5-1-μm thick resin-embedded sections were collected from tangential sampling levels through the middle of layer 4 in area 17 and stained with Toluidine Blue. These sections were used to quantitatively analyse the composition and distribution of dendritic clusters in the tangential plane. The key result of this study indicates a significant reduction in the mean number of medium- and small-sized dendritic profiles (diameter less than 2 μm) contributing to clusters in layer 4 of groups 3 dL and 52 dD compared with group 21/31. No differences were detected in the mean number of large-sized dendritic profiles composing a bundle in these experimental groups. Moreover, the mean number of clusters and their tangential distribution in layer 4 did not vary significantly between all four groups. Finally, the clustering parameters were not significantly different between groups 21/31 and the normally reared group 52 dL. This study demonstrates, for the first time, that extended periods of dark rearing followed by light exposure can alter the morphological composition of dendritic bundles in thalamorecipient layer 4 of rat visual cortex. Because these changes occur in the primary region of thalamocortical input, they may underlie specific alterations in the processing of visual information both cortically and subcortically during periods of dark rearing and light exposure.

Postnatal development of intrinsic and synaptic properties transforms signaling in the layer 5 excitatory neural network of the visual cortex

Information flow in neocortical circuits is regulated by two key parameters: intrinsic neuronal properties and the short-term activity-dependent plasticity of synaptic transmission. Using multineuronal whole-cell voltage recordings, we characterized the postnatal maturation of the electrophysiological properties and short-term plasticity of excitatory synaptic transmission between pairs of layer 5 (L5) pyramidal neurons (n = 158) in acute slices of rat visual cortex over the first postnatal month. We found that the intrinsic and synaptic properties of L5 pyramidal neurons develop in parallel. Before postnatal day 15 (P15), intrinsic electrophysiological properties were tuned to low-frequency operation, characterized by high apparent input resistance, a long membrane time constant, and prolonged somatic action potentials. Unitary excitatory synaptic potentials were of large amplitude (P11-P15; median, 514 μV), but showed pronounced use-dependent depression during prolonged regular and physiologically relevant presynaptic action potential firing patterns. In contrast, in mature animals we observed a developmental decline of the peak amplitude of unitary EPSPs (P25-P29; median, 175 μV) paralleled by a decrease in apparent input resistance, membrane time constant, and somatic action potential duration. Notably, synaptic signaling of complex action potential firing patterns was also transformed, with P25-P29 connections faithfully signaling action potential trains at frequencies up to 40 Hz (1st to 50th action potential ratio, 0.91 ± 0.12). Postnatal refinement of intrinsic properties and short-term plasticity therefore transforms the capacity of the L5 excitatory neural network of the visual cortex to generate and process patterns of action potential firing and contribute to network activity.

Postnatal development of A-type and Kv1- and Kv2-mediated potassium channel currents in neocortical pyramidal neurons

Potassium channels regulate numerous aspects of neuronal excitability, and several voltage-gated K(+) channel subunits have been identified in pyramidal neurons of rat neocortex. Previous studies have either considered the development of outward current as a whole or divided currents into transient, A-type and persistent, delayed rectifier components but did not differentiate between current components defined by α-subunit type. To facilitate comparisons of studies reporting K(+) currents from animals of different ages and to understand the functional roles of specific current components, we characterized the postnatal development of identified Kv channel-mediated currents in pyramidal neurons from layers II/III from rat somatosensory cortex. Both the persistent/slowly inactivating and transient components of the total K(+) current increased in density with postnatal age. We used specific pharmacological agents to test the relative contributions of putative Kv1- and Kv2-mediated currents (100 nM α-dendrotoxin and 600 nM stromatoxin, respectively). A combination of voltage protocol, pharmacology, and curve fitting was used to isolate the rapidly inactivating A-type current. We found that the density of all identified current components increased with postnatal age, approaching a plateau at 3-5 wk. We found no significant changes in the relative proportions or kinetics of any component between postnatal weeks 1 and 5, except that the activation time constant for A-type current was longer at 1 wk. The putative Kv2-mediated component was the largest at all ages. Immunocytochemistry indicated that protein expression for Kv4.2, Kv4.3, Kv1.4, and Kv2.1 increased between 1 wk and 4-5 wk of age.

Timescales of multineuronal activity patterns reflect temporal structure of visual stimuli

The investigation of distributed coding across multiple neurons in the cortex remains to this date a challenge. Our current understanding of collective encoding of information and the relevant timescales is still limited. Most results are restricted to disparate timescales, focused on either very fast, e.g., spike-synchrony, or slow timescales, e.g., firing rate. Here, we investigated systematically multineuronal activity patterns evolving on different timescales, spanning the whole range from spike-synchrony to mean firing rate. Using multi-electrode recordings from cat visual cortex, we show that cortical responses can be described as trajectories in a high-dimensional pattern space. Patterns evolve on a continuum of coexisting timescales that strongly relate to the temporal properties of stimuli. Timescales consistent with the time constants of neuronal membranes and fast synaptic transmission (5–20 ms) play a particularly salient role in encoding a large amount of stimulus-related information. Thus, to faithfully encode the properties of visual stimuli the brain engages multiple neurons into activity patterns evolving on multiple timescales.

Projection-specific neuromodulation of medial prefrontal cortex neurons

Mnemonic persistent activity in the prefrontal cortex (PFC) constitutes the neural basis of working memory. To understand how neuromodulators contribute to the generation of persistent activity, it is necessary to identify the intrinsic properties of the layer V pyramidal neurons that transfer this information to downstream networks. Here we show that the somatic dynamic and integrative properties of layer V pyramidal neurons in the rat medial PFC depend on whether they project subcortically to the pons [corticopontine (CPn)] or to the contralateral cortex [commissural (COM)]. CPn neurons display low temporal summation and accelerate in firing frequency when depolarized, whereas COM neurons have high temporal summation and display spike frequency accommodation. In response to dynamic stimuli, COM neurons act as low-pass filters, whereas CPn neurons act as bandpass filters, resonating in the theta frequency range (3-6 Hz). The disparate subthreshold properties of COM and CPn neurons can be accounted for by differences in the hyperpolarization-activated cyclic nucleotide gated cation h-current. Interestingly, neuromodulators hypothesized to enhance mnemonic persistent activity affect COM and CPn neurons distinctly. Adrenergic modulation shifts the dynamic properties of CPn but not COM neurons and increases the excitability of CPn neurons significantly more than COM neurons. In response to cholinergic modulation, CPn neurons were much more likely to display activity-dependent intrinsic persistent firing than COM neurons. Together, these data suggest that the two categories of projection neurons may subserve separate functions in PFC and may be engaged differently during working memory processes.

Molecular dissection of I(A) in cortical pyramidal neurons reveals three distinct components encoded by Kv4.2, Kv4.3, and Kv1.4 alpha-subunits

The rapidly activating and inactivating voltage-gated K(+) (Kv) current, I(A), is broadly expressed in neurons and is a key regulator of action potential repolarization, repetitive firing, backpropagation (into dendrites) of action potentials, and responses to synaptic inputs. Interestingly, results from previous studies on a number of neuronal cell types, including hippocampal, cortical, and spinal neurons, suggest that macroscopic I(A) is composed of multiple components and that each component is likely encoded by distinct Kv channel alpha-subunits. The goals of the experiments presented here were to test this hypothesis and to determine the molecular identities of the Kv channel alpha-subunits that generate I(A) in cortical pyramidal neurons. Combining genetic disruption of individual Kv alpha-subunit genes with pharmacological approaches to block Kv currents selectively, the experiments here revealed that Kv1.4, Kv4.2, and Kv4.3 alpha-subunits encode distinct components of I(A) that together underlie the macroscopic I(A) in mouse (male and female) cortical pyramidal neurons. Recordings from neurons lacking both Kv4.2 and Kv4.3 (Kv4.2(-/-)/Kv4.3(-/-)) revealed that, although Kv1.4 encodes a minor component of I(A), the Kv1.4-encoded current was found in all the Kv4.2(-/-)/Kv4.3(-/-) cortical pyramidal neurons examined. Of the cortical pyramidal neurons lacking both Kv4.2 and Kv1.4, 90% expressed a Kv4.3-encoded I(A) larger in amplitude than the Kv1.4-encoded component. The experimental findings also demonstrate that the targeted deletion of the individual Kv alpha-subunits encoding components of I(A) results in electrical remodeling that is Kv alpha-subunit specific.

Properties of glutamatergic synapses in immature layer Vb pyramidal neurons: coupling of pre- and postsynaptic maturational states

Following initial contact formation, glutamatergic synapses in cortical neurons undergo pronounced functional maturation. These maturational events, occurring both pre- and postsynaptically, have been well described in the developing hippocampus. In this paper, we characterized glutamatergic synapses in immature layer Vb pyramidal neurons of the mouse somatosensory cortex during early postnatal development. At postnatal day 7, a significant subpopulation of glutamatergic synapses exhibited a low release probability that was accompanied by strong paired-pulse facilitation of AMPA EPSCs (paired-pulse ratio C > or = 2). Increasing extracellular Ca(2+) concentration increased release probability and led to paired-pulse depression. During further postnatal development, these functionally immature synapses disappeared. As shown pharmacologically,these synapses expressed postsynaptic NMDA receptors containing NR2B subunits, while NMDA receptors with NR2A subunits were lacking. Taken together, a low release probability presynaptically was coupled to postsynaptic NR2B signaling. This subpopulation of neocortical synapses thus differed from the majority of synapses in the developing hippocampus, where high release probability is coupled to NR2B signaling. The novel type of functionally immature glutamatergic synapse described here might play an important role in early developmental synapse elimination and in the activity-dependent refinement of the neocortical synaptic microcircuitry.

Distributed fading memory for stimulus properties in the primary visual cortex

It is currently not known how distributed neuronal responses in early visual areas carry stimulus-related information. We made multielectrode recordings from cat primary visual cortex and applied methods from machine learning in order to analyze the temporal evolution of stimulus-related information in the spiking activity of large ensembles of around 100 neurons. We used sequences of up to three different visual stimuli (letters of the alphabet) presented for 100 ms and with intervals of 100 ms or larger. Most of the information about visual stimuli extractable by sophisticated methods of machine learning, i.e., support vector machines with nonlinear kernel functions, was also extractable by simple linear classification such as can be achieved by individual neurons. New stimuli did not erase information about previous stimuli. The responses to the most recent stimulus contained about equal amounts of information about both this and the preceding stimulus. This information was encoded both in the discharge rates (response amplitudes) of the ensemble of neurons and, when using short time constants for integration (e.g., 20 ms), in the precise timing of individual spikes (<or= approximately 20 ms), and persisted for several 100 ms beyond the offset of stimuli. The results indicate that the network from which we recorded is endowed with fading memory and is capable of performing online computations utilizing information about temporally sequential stimuli. This result challenges models assuming frame-by-frame analyses of sequential inputs.

A color-based visualization technique for multielectrode spike trains

Multi electrode recordings of neuronal activity provide an overwhelming amount of data that is often difficult to analyze and interpret. Although various methods exist for treating multielectrode datasets quantitatively, there is a particularly prominent lack of techniques that enable a quick visual exploration of such datasets. Here, by using Kohonen self-organizing maps, we propose a simple technique that allows for the representation of multiple spike trains through a sequence of color-coded population activity vectors. When multiple color sequences are grouped according to a certain criterion, e.g., by stimulation condition or recording time, one can inspect an entire dataset visually and extract quickly information about the identity, stimulus-locking and temporal distribution of multi-neuron activity patterns. Color sequences can be computed on various time scales revealing different aspects of the temporal dynamics and can emphasize high-order correlation patterns that are not detectable with pairwise techniques. Furthermore, this technique is useful for determining the stability of neuronal responses during a recording session. Due to its simplicity and reliance on perceptual grouping, the method is useful for both quick on-line visualization of incoming data and for more detailed post hoc analyses.

Postnatal development of dendritic synaptic integration in rat neocortical pyramidal neurons

The dendritic tree of layer 5 (L5) pyramidal neurons spans the neocortical layers, allowing the integration of intra- and extracortical synaptic inputs. Here we investigate the postnatal development of the integrative properties of rat L5 pyramidal neurons using simultaneous whole cell recording from the soma and distal apical dendrite. In young (P9-10) neurons, apical dendritic excitatory synaptic input powerfully drove action potential output by efficiently summating at the axonal site of action potential generation. In contrast, in mature (P25-29) neurons, apical dendritic excitatory input provided little direct depolarization at the site of action potential generation but was integrated locally in the apical dendritic tree leading to the generation of dendritic spikes. Consequently, over the first postnatal month the fraction of action potentials driven by apical dendritic spikes increased dramatically. This developmental remodeling of the integrative operations of L5 pyramidal neurons was controlled by a >10-fold increase in the density of apical dendritic Hyperpolarization-activated cyclic nucleotide (HCN)-gated channels found in cell-attached patches or by immunostaining for the HCN channel isoform HCN1. Thus an age-dependent increase in apical dendritic HCN channel density ensures that L5 pyramidal neurons develop from compact temporal integrators to compartmentalized integrators of basal and apical dendritic synaptic input.

Spontaneous neuronal burst discharges as dependent and independent variables in the maturation of cerebral cortex tissue cultured in vitro: a review of activity-dependent studies in live 'model' systems for the development of intrinsically generated bioelectric slow-wave sleep patterns

A survey is presented of recent experiments which utilize spontaneous neuronal spike trains as dependent and/or independent variables in developing cerebral cortex cultures when synaptic transmission is interfered with for varying periods of time. Special attention is given to current difficulties in selecting suitable preparations for carrying out biologically relevant developmental studies, and in applying spike-train analysis methods with sufficient resolution to detect activity-dependent age and treatment effects. A hierarchy of synchronized nested burst discharges which approximate early slow-wave sleep patterns in the intact organism is established as a stable basis for isolated cortex function. The complexity of reported long- and short-term homeostatic responses to experimental interference with synaptic transmission is reviewed, and the crucial role played by intrinsically generated bioelectric activity in the maturation of cortical networks is emphasized.

Maturation of intrinsic and synaptic properties of layer 2/3 pyramidal neurons in mouse auditory cortex

We investigated the development of L2/3 pyramidal cell (PC) circuitry in juvenile mice from postnatal day 10 (P10) to P29. Using whole cell recordings in an in vitro thalamocortical slice preparation, we examined the connection architecture and intrinsic and synaptic properties of PCs. The excitatory connections between PCs were highly localized: the probability of connection between PCs declined with intersomatic distance from 0.18 to about 0.05 over 150 microm, but did not vary with age. However, the mean and variance of the intrinsic and synaptic properties of PCs changed dramatically between P10 and P29. The input resistance, membrane time constant, and resting membrane potential decreased, leading to reduced neural excitability in older animals. Likewise, there were age-dependent decreases in the amplitude and decay time of the excitatory postsynaptic potentials as well as short-term synaptic depression. Both the intrinsic and synaptic properties underwent a transitional period between P10 and P18 prior to reaching steady state at P19-P29. We show that these properties combine to produce age-related differential synaptic responses to low- and high-frequency synaptic input that may contribute to differences in auditory processing during development.

Development of Cholinergic Modulation and Graded Persistent Activity in Layer V of Medial Entorhinal Cortex

During muscarinic modulation, principal neurons from layer V of rat medial entorhinal cortex (mEC) respond to repeated applications of a brief stimulus with a graded change in persistent firing frequency. This pattern of discharge has been proposed to represent an intrinsic mechanism for short-term memory operations. To investigate the implementation of persistent activity in mEC during development, we characterized the electrophysiological properties of layer V principal neurons in the mEC over a range of postnatal stages. We observed significant differences in both passive (resistance, time constant, and resting membrane potential) and active properties (threshold, action potential, and adaptation) of principal neurons from rats aged 5–7, 10–13, 16–19, and 21–23 days. We also examined the properties of muscarinic-dependent persistent activity in EC slices from different age groups. Recordings were conducted using the perforated-patch whole cell technique because persistent activity runs down in the ruptured-patch configuration. Although no neuron in the youngest group exhibited graded persistent activity in response to muscarinic receptor activation, this activity was recorded in the 10- to 13-day-old group and its occurrence increased from 69% in the 16- to 19-day-old group to 76% in the 21- to 23-day-old group. This postnatal increase in neurons endowed with persistent firing properties in mEC was found to parallel the increase in density of ChAT-positive immunostaining of fibers and the developmental changes in M1 muscarinic receptor mRNA levels. All these data suggest that the implementation of mnemonic properties in mEC principal neurons matches the ontogenic development of afferent cholinergic circuits and their signaling components.

Dopamine-dependent long term potentiation in the dorsal striatum is reduced in the R6/2 mouse model of Huntington's disease

The striatum is critically important in motor, cognitive and emotional functions, as highlighted in neurological disorders such as Huntington's disease (HD) where these functions are compromised. The R6/2 mouse model of HD shows progressive motor and cognitive impairments and alterations in striatal dopamine and glutamate release. To determine whether or not dopamine-dependent neuronal plasticity is also altered in the dorsolateral striatum of R6/2 mice, we compared long term potentiation (LTP) and long term depression (LTD) in striatal slices from R6/2 mice with that seen in slices from wild type (WT) mice. In adult WT mice (aged 8-19 weeks), frequency-dependent bidirectional plasticity was observed. High frequency stimulation (four 0.5 s trains at 100 Hz, inter-train interval 10 s) induced LTP (134+/-5% of baseline), while low frequency stimulation (4 Hz for 15 min) induced LTD (80+/-5% of baseline). LTP and LTD were significantly blocked by the N-methyl-D-aspartic acid (NMDA) receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid (D-AP5) (to 93+/-6% and 103+/-8% of baseline respectively), indicating that they are both dependent on NMDA glutamate receptor activation. LTP was significantly blocked by the dopamine D1 receptor antagonist R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH-23390) (98+/-8% of baseline), indicating that LTP is dependent on activation of dopamine D(1)-type receptors, whereas LTD was not significantly different (90+/-7%). In adult R6/2 mice (aged 8-19 weeks), LTP was significantly reduced (to 110+/-4% of baseline), while LTD was not significantly different from that seen in WT mice (85+/-6%). These data show that R6/2 mice have impaired dopamine-dependent neuronal plasticity in the striatum. As dopamine-dependent plasticity is a proposed model of striatum-based motor and cognitive functions, this impairment could contribute to deficits seen in R6/2 mice.

Background synaptic activity is sparse in neocortex

Neurons are continually exposed to background synaptic activity in vivo. This is thought to influence neural information processing, but background levels of excitation and inhibition remain controversial. Here we show, using whole-cell recordings in anesthetized rats, that spontaneous depolarizations ("Up states") in neocortical pyramidal neurons are driven by sparse, mostly excitatory synaptic activity (less than five inputs per millisecond; approximately 10% inhibitory). The mean synaptic conductance change is small (<10 nS at the soma) and opposed by anomalous rectification, resulting in a net increase in input resistance during Up states. These conditions enhance the effectiveness of each synapse at depolarized potentials. Hence, neocortical networks are relatively quiet at rest, and the effect of synaptic background is weaker than previously thought.