Somatic Recordings Research Articles

Capacitance Measurements of Exocytosis From AII Amacrine Cells in Retinal Slices.

During neuronal synaptic transmission, theexocytotic release of neurotransmitters from synaptic vesicles in the presynaptic neuron evokes a change in conductance for one or more types of ligand-gated ion channels in the postsynaptic neuron. The standard method of investigation uses electrophysiological recordings of the postsynaptic response. However, electrophysiological recordings can directly quantify the presynaptic release of neurotransmitters with high temporal resolution by measuring the membrane capacitance before and after exocytosis, as fusion of the membrane of presynaptic vesicles with the plasma membrane increases the total capacitance. While the standard technique for capacitance measurement assumes that the presynaptic cell is unbranched and can be represented as a simple resistance-capacitance (RC) circuit, neuronal exocytosis typically occurs at a distance from the soma. Even in such cases, however, it can be possible to detect a depolarization-evoked increase in capacitance. Here, we provide a detailed, step-by-step protocol that describes how "Sine + DC" (direct current) capacitance measurements can quantify the exocytotic release of neurotransmitters from AII amacrine cells in rat retinal slices. The AII is an important inhibitory interneuron of the mammalian retina that plays an important role in integrating rod and cone pathway signals. AII amacrines release glycine from their presynaptic dendrites, and capacitance measurements have been important for understanding the release properties of these dendrites. When the goal is to directly quantify the presynaptic release, there is currently no other competing method available. This protocol includes procedures for measuring depolarization-evoked exocytosis, using both standard square-wave pulses, arbitrary stimulus waveforms, and synaptic input. Key features • Quantification of exocytosis with the Sine + DC technique for visually targeted AII amacrines in retinal slices, using voltage-clamp and whole-cell patch-clamp recording. • Because exocytosis occurs away from the somatic recording electrode, the sine wave frequency must be lower than for the standard Sine + DC technique. • Because AII amacrines are electrically coupled, the sine wave frequency must be sufficiently high to avoid interference from other cells in the electrically coupled network. • The protocol includes procedures for measuring depolarization-evoked exocytosis using standard square-wave pulses, stimulation with arbitrary and prerecorded stimulus waveforms, and activation of synaptic inputs.

Non-apical plateau potentials and persistent firing induced by metabotropic cholinergic modulation in layer 2/3 pyramidal cells in the rat prefrontal cortex.

Here we describe a type of depolarising plateau potentials (PPs; sustained depolarisations outlasting the stimuli) in layer 2/3 pyramidal cells (L2/3PC) in rat prefrontal cortex (PFC) slices, using whole-cell somatic recordings. To our knowledge, this PP type has not been described before. In particular, unlike previously described plateau potentials that originate in the large apical dendrite of L5 cortical pyramidal neurons, these L2/3PC PPs are generated independently of the apical dendrite. Thus, surprisingly, these PPs persisted when the apical dendrite was cut off (~50 μm from the soma), and were sustained by local calcium application only to the somatic and basal dendritic compartments. The prefrontal L2/3PCs have been postulated to have a key role in consciousness, according to the Global Neuronal Workspace Theory: their long-range cortico-cortical connections provide the architecture required for the "global work-space", "ignition", amplification, and sustained, reverberant activity, considered essential for conscious access. The PPs in L2/3PCs caused sustained spiking that profoundly altered the input-output relationships of these neurons, resembling the sustained activity suggested to underlie working memory and the mechanism underlying "behavioural time scale synaptic plasticity" in hippocampal pyramidal cells. The non-apical L2/3 PPs depended on metabotropic cholinergic (mAChR) or glutamatergic (mGluR) modulation, which is probably essential also for conscious brain states and experience, in both wakefulness and dreaming. Pharmacological tests indicated that the non-apical L2/3 PPs depend on transient receptor potential (TRP) cation channels, both TRPC4 and TRPC5, and require external calcium (Ca2+) and internal Ca2+ stores, but not voltage-gated Ca2+ channels, unlike Ca2+-dependent PPs in other cortical pyramidal neurons. These L2/3 non-apical plateau potentials may be involved in prefrontal functions, such as access consciousness, working memory, and executive functions such as planning, decision-making, and outcome prediction.

Compartmentalized pooling generates orientation selectivity in wide-field amacrine cells

Orientation is one of the most salient features in visual scenes. Neurons at multiple levels of the visual system detect orientation, but in many cases, the underlying biophysical mechanisms remain unresolved. Here, we studied mechanisms for orientation detection at the earliest stage in the visual system, in B/K wide-field amacrine cells (B/K WACs), a group of giant, nonspiking interneurons in the mouse retina that coexpress Bhlhe22 (B) and Kappa Opioid Receptor (K). B/K WACs exhibit orientation-tuned calcium signals along their long, straight, unbranching dendrites, which contain both synaptic inputs and outputs. Simultaneous dendritic calcium imaging and somatic voltage recordings reveal that individual B/K dendrites are electrotonically isolated, exhibiting a spatially confined yet extended receptive field along the dendrite, which we term "compartmentalized pooling." Further, the receptive field of a B/K WAC dendrite exhibits center-surround antagonism. Phenomenological receptive field models demonstrate that compartmentalized pooling generates orientation selectivity, and center-surround antagonism shapes band-pass spatial frequency tuning. At the microcircuit level, B/K WACs receive excitation driven by one contrast polarity (e.g., "ON") and glycinergic inhibition driven by the opposite polarity (e.g., "OFF"). However, this "crossover" inhibition is not essential for generating orientation selectivity. A minimal biophysical model reproduced compartmentalized pooling from feedforward excitatory inputs combined with a substantial increase in the specific membrane resistance between somatic and dendritic compartments. Collectively, our results reveal the biophysical mechanism for generating orientation selectivity in dendrites of B/K WACs, enriching our understanding of the diverse strategies employed throughout the visual system to detect orientation.

A Multimodal Fitting Approach to Construct Single-Neuron Models with Patch Clamp and High-Density Microelectrode Arrays.

In computational neuroscience, multicompartment models are among the most biophysically realistic representations of single neurons. Constructing such models usually involves the use of the patch-clamp technique to record somatic voltage signals under different experimental conditions. The experimental data are then used to fit the many parameters of the model. While patching of the soma is currently the gold-standard approach to build multicompartment models, several studies have also evidenced a richness of dynamics in dendritic and axonal sections. Recording from the soma alone makes it hard to observe and correctly parameterize the activity of nonsomatic compartments. In order to provide a richer set of data as input to multicompartment models, we here investigate the combination of somatic patch-clamp recordings with recordings of high-density microelectrode arrays (HD-MEAs). HD-MEAs enable the observation of extracellular potentials and neural activity of neuronal compartments at subcellular resolution. In this work, we introduce a novel framework to combine patch-clamp and HD-MEA data to construct multicompartment models. We first validate our method on a ground-truth model with known parameters and show that the use of features extracted from extracellular signals, in addition to intracellular ones, yields models enabling better fits than using intracellular features alone. We also demonstrate our procedure using experimental data by constructing cell models from in vitro cell cultures. The proposed multimodal fitting procedure has the potential to augment the modeling efforts of the computational neuroscience community and provide the field with neuronal models that are more realistic and can be better validated.

Impaired dendritic spike generation in the Fragile X prefrontal cortex is due to loss of dendritic sodium channels.

Patients with Fragile X syndrome, the leading monogenetic cause of autism, suffer from impairments related to the prefrontal cortex, including working memory and attention. Synaptic inputs to the distal dendrites of layer 5 pyramidal neurons in the prefrontal cortex have a weak influence on the somatic membrane potential. To overcome this filtering, distal inputs are transformed into local dendritic Na+ spikes, which propagate to the soma and trigger action potential output. Layer 5 extratelencephalic (ET) prefrontal cortex (PFC) neurons project to the brainstem and various thalamic nuclei and are therefore well positioned to integrate task-relevant sensory signals and guide motor actions. We used current clamp and outside-out patch clamp recording to investigate dendritic spike generation in ET neurons from male wild-type and Fmr1 knockout (FX) mice. The threshold for dendritic spikes was more depolarized in FX neurons compared to wild-type. Analysis of voltage responses to simulated in vivo 'noisy' current injections showed that a larger dendritic input stimulus was required to elicit dendritic spikes in FX ET dendrites compared to wild-type. Patch clamp recordings revealed that the dendritic Na+ conductance was significantly smaller in FX ET dendrites. Taken together, our results suggest that the generation of Na+ -dependent dendritic spikes is impaired in ET neurons of the PFC in FX mice. Considering our prior findings that somatic D-type K+ and dendritic hyperpolarization-activated cyclic nucleotide-gated-channel function is reduced in ET neurons, we suggest that dendritic integration by PFC circuits is fundamentally altered in Fragile X syndrome. KEY POINTS: Dendritic spike threshold is depolarized in layer 5 prefrontal cortex neurons in Fmr1 knockout (FX) mice. Simultaneous somatic and dendritic recording with white noise current injections revealed that larger dendritic stimuli were required to elicit dendritic spikes in FX extratelencephalic (ET) neurons. Outside-out patch clamp recording revealed that dendritic sodium conductance density was lower in FX ET neurons.

The Sound of Modern Polish Poetry: Performance and Recording after World War II by Aleksandra Kremer

Reviewed by: The Sound of Modern Polish Poetry: Performance and Recording after World War II by Aleksandra Kremer E. M. Stańczyk Kremer, Aleksandra. The Sound of Modern Polish Poetry: Performance and Recording after World War II. Harvard University Press, Cambridge, MA London, 2021. 364 pp. Notes. Index. $45.00: £39.95: €40.95. In this brilliant new monograph Aleksandra Kremer studies poetry readings by major Polish poets that have been preserved through official, semi-official and private sound recordings. Her primary focus is on seven male and three female poets — Julian Tuwim, Czesław Miłosz, Miron Białoszewski, Aleksander Wat, Wisława Szymborska, Zbigniew Herbert, Anna Kamieńska, Anna Świrszczyńska, Tadeusz Różewicz and Julian Przyboś — in addition to several others. Kremer sets herself an ambitious task: to explore how the poets’ approach to reading reflected their attitude to the Polish literary tradition, poetry performance and, importantly, poetry creation. Using the computer programme Praat, which analyses pitch, duration, loudness and timbre of sound waves, Kremer studies ‘each poet individually, trying to grasp their individuality at the crossroads of acoustics and linguistics’ (p. 27). To Kremer, a poem is not a static piece of work. Rather, the ‘the sounds of the poets’ speech […] supplement or contradict what we learn from print’ (p. 28). The book consists of five excellent chapters. Chapter one discusses what could be potentially the oldest recorded reading by a Polish poet, produced as a sound postcard in the United States in 1941. Julian Tuwim, the poet behind the recording, emulated an older tradition of reading poetry to friends and family. His sound postcard was sent as a Christmas gift to his sister in London, two decades before authors in Poland had access to comparable technology. In a similar vein, the young poet Halina Poświatowska recorded two of her works in 1959 during a stay with relatives in America. The recording was meant as a family keepsake which became all the more cherished after the poet’s untimely death eight years later. Kremer’s analysis of these early recordings [End Page 549] illustrates what she does so well throughout the monograph as a whole. She demonstrates that studying voice recordings such as these can open up space for thought-provoking comparisons; in this case — comparisons that run across generational, gender and stylistic lines. Chapter two discusses Czesław Miłosz as a reader and translator of his own poems. Kremer shows how the two roles overlapped in the poet’s biography. Miłosz’s own translations into English followed the Polish syntax and prosody which, even if unnatural in the target language, was meant to facilitate his participation — through poetry readings — in the cultural life of the United States where he lived for more than three decades. While his rhythmical and beautifully enunciated speech in Polish bore all the hallmarks of the ‘phonetic patterns from another time’ (p. 117), distinguishing his readings from both everyday speech and existing recitation styles, the poet’s recorded performances in English were visibly less skilled. Like Miłosz, Miron Białoszewski, who is the focus of chapter three, was an avid performer of his own poems. Tape-recording became central to his work in the 1960s and ’70s, as seen, for example, in the private recordings produced for close friend Jadwiga Stańczakowa. In contrast to Białoszewski’s theatrical style, readings by the Nobel Prize-winning Wisława Szymborska were understated and conversational, conjuring an atmosphere of home, even in the setting of a recording studio. The penultimate and final chapters deal with sound recordings that speak to the fragility of the poet’s body and their unwillingness or inability to deliver a ‘good’ reading, respectively. Chapter four discusses ‘taped farewells’ — recitations of poems interspersed with informal commentary — by sick and aging poets, such as Herbert, Wat, Kamieńska and Świrszczyńska. According to Kremer, these ‘highly somatic final recordings were courageous in revealing weakness, but they also aimed to overcome the body one last time in order to create variously imagined recorded afterlives’ (p. 216). Chapter five looks at the ‘unbeautiful readings’ by poets such as Różewicz who disliked performing and whose reading style...

Somatic Depolarization Enhances Hippocampal CA1 Dendritic Spike Propagation and Distal Input-Driven Synaptic Plasticity

Synaptic inputs that target distal regions of neuronal dendrites can often generate local dendritic spikes that can amplify synaptic depolarization, induce synaptic plasticity, and enhance neuronal output. However, distal dendritic spikes are subject to significant attenuation by dendritic cable properties, and often produce only a weak subthreshold depolarization of the soma. Nonetheless, such spikes have been implicated in memory storage, sensory perception and place field formation. How can such a weak somatic response produce such powerful behavioral effects? Here, we use dual dendritic and somatic recordings in acute hippocampal slices of male mice to reveal that dendritic spike propagation, but not spike initiation, is strongly enhanced when the somatic resting potential is depolarized, likely as a result of increased inactivation of A-type K+ channels. Somatic depolarization also facilitates the induction of a form of dendritic spike driven heterosynaptic plasticity that enhances memory specificity. Thus, the effect of somatic membrane depolarization to enhance dendritic spike propagation and long-term synaptic plasticity is likely to play an important role in hippocampal-dependent spatial representations as well as learning and memory.SIGNIFICANCE STATEMENT Neurons receive synaptic input along their dendrites but produce action potential (AP) output at their soma. Signals arriving at the distal dendrites of pyramidal neurons (PNs) have little impact on the soma unless they combine to initiate a dendritic spike, which needs to propagate to the soma to trigger an AP. This study shows that small subthreshold depolarization of the soma powerfully enhances the propagation of dendritic spikes, through inactivation of dendritic A-type potassium channels. Enhanced dendritic spike propagation also markedly facilitates the induction of a form of plasticity driven by the distal synaptic inputs. Thus, small changes in somatic membrane potential, similar to those observed in vivo, act as a powerful gate of neuronal information transfer.

General practice during COVID-19: an FY2's perspective.

<h3>ABSTRACT</h3> Patients with Fragile X syndrome, the leading monogenetic cause of autism, suffer from impairments related to the prefrontal cortex including working memory and attention. Synaptic inputs to the distal dendrites of layer 5 pyramidal neurons in the prefrontal cortex have a weak influence on the somatic membrane potential. To overcome this filtering, distal inputs are transformed into local dendritic Na⁺ spikes, which propagate to the soma and trigger action potential output. Layer 5 extratelencephalic (ET) PFC neurons project to the brainstem and various thalamic nuclei and are therefore well positioned to integrate task-relevant sensory signals and guide motor actions. We used current clamp and outside-out patch clamp recording to investigate dendritic spike generation in ET neurons from male wild type and <i>Fmr1</i> knockout (FX) mice. The threshold for dendritic spikes was more depolarized in FX neurons compared to wild type. Analysis of voltage responses to simulated in vivo “noisy” current injections showed that a larger dendritic input stimulus was required to elicit dendritic spikes in FX ET dendrites compared wild type. Patch clamp recordings revealed that the dendritic Na⁺ conductance was significantly smaller in FX ET dendrites. Taken together, our results suggest that input-output transformation is impaired in ET neurons of the PFC in FX mice. Considering our prior findings that somatic D-type K⁺ and dendritic HCN-channel function is reduced in ET neurons, we suggest that the integration of information by PFC circuits is fundamentally altered in Fragile X syndrome. <h3>KEY POINTS</h3> Dendritic spike threshold is depolarized in Layer 5 PFC neurons in FX mice Simultaneous somatic and dendritic recording with white noise current injections revealed that larger dendritic stimuli were required to elicit dendritic spikes in FX ET neurons Outside-out patch clamp recording revealed that dendritic sodium conductance density was lower in FX ET neurons

Computationally going where experiments cannot: a dynamical assessment of dendritic ion channel currents during in vivo-like states

Background: Despite technological advances, how specific cell types are involved in brain function remains shrouded in mystery. Further, little is known about the contribution of different ion channel currents to cell excitability across different neuronal subtypes and their dendritic compartments in vivo. The picture that we do have is largely based on somatic recordings performed in vitro. Uncovering dendritic ion channel current contributions in neuron subtypes that represent a minority of the neuronal population is not currently a feasible task using purely experimental means. Methods: We employ two morphologically-detailed multi-compartment models of a specific type of inhibitory interneuron, the oriens lacunosum moleculare (OLM) cell. The OLM cell is a well-studied cell type in CA1 hippocampus that is important in gating sensory and contextual information. We create in vivo-like states for these cellular models by including levels of synaptic bombardment that would occur in vivo. Using visualization tools and analyses we assess the ion channel current contribution profile across the different somatic and dendritic compartments of the models. Results: We identify changes in dendritic excitability, ion channel current contributions and co-activation patterns between in vitro and in vivo-like states. Primarily, we find that the relative timing between ion channel currents are mostly invariant between states, but exhibit changes in magnitudes and decreased propagation across dendritic compartments. We also find enhanced dendritic hyperpolarization-activated cyclic nucleotide-gated channel (h-channel) activation during in vivo-like states, which suggests that dendritically located h-channels are functionally important in altering signal propagation in the behaving animal. Conclusions: Overall, we have demonstrated, using computational modelling, the dynamical changes that can occur to ion channel mechanisms governing neuronal spiking. Simultaneous access to dendritic compartments during simulated in vivo states shows that the magnitudes of some ion channel current contributions are differentially altered during in vivo-like states relative to in vitro.

Computationally going where experiments cannot: a dynamical assessment of dendritic ion channel currents during in vivo-like states

Background: Despite technological advances, how specific cell types are involved in brain function remains shrouded in mystery. Further, little is known about the contribution of different ion channel currents to cell excitability across different neuronal subtypes and their dendritic compartments in vivo. The picture that we do have is largely based on somatic recordings performed in vitro. Uncovering dendritic ion channel current contributions in neuron subtypes that represent a minority of the neuronal population is not currently a feasible task using purely experimental means. Methods: We employ two morphologically-detailed multi-compartment models of a specific type of inhibitory interneuron, the oriens lacunosum moleculare (OLM) cell. The OLM cell is a well-studied cell type in CA1 hippocampus that is important in gating sensory and contextual information. We create in vivo-like states for these cellular models by including levels of synaptic bombardment that would occur in vivo. Using visualization tools and analyses we assess the ion channel current contribution profile across the different somatic and dendritic compartments of the models. Results: We identify changes in dendritic excitability, ion channel current contributions and co-activation patterns between in vitro and in vivo-like states. Primarily, we find that the relative timing between ion channel currents are mostly invariant between states, but exhibit changes in magnitudes and decreased propagation across dendritic compartments. We also find enhanced dendritic hyperpolarization-activated cyclic nucleotide-gated channel (h-channel) activation during in vivo-like states, which suggests that dendritically located h-channels are functionally important in altering signal propagation in the behaving animal. Conclusions: Overall, we have demonstrated, using computational modelling, the dynamical changes that can occur to ion channel mechanisms governing neuronal spiking. Simultaneous access to dendritic compartments during simulated in vivo states shows that the magnitudes of some ion channel current contributions are differentially altered during in vivo-like states relative to in vitro.

Computationally going where experiments cannot: a dynamical assessment of dendritic ion channel currents during in vivo-like states.

Background: Despite technological advances, how specific cell types are involved in brain function remains shrouded in mystery. Further, little is known about the contribution of different ion channel currents to cell excitability across different neuronal subtypes and their dendritic compartments in vivo. The picture that we do have is largely based on somatic recordings performed in vitro. Uncovering dendritic ion channel current contributions in neuron subtypes that represent a minority of the neuronal population is not currently a feasible task using purely experimental means. Methods: We employ two morphologically-detailed multi-compartment models of a specific type of inhibitory interneuron, the oriens lacunosum moleculare (OLM) cell. The OLM cell is a well-studied cell type in CA1 hippocampus that is important in gating sensory and contextual information. We create in vivo-like states for these cellular models by including levels of synaptic bombardment that would occur in vivo. Using visualization tools and analyses we assess the ion channel current contribution profile across the different somatic and dendritic compartments of the models. Results: We identify changes in dendritic excitability, ion channel current contributions and co-activation patterns between in vitro and in vivo-like states. Primarily, we find that the relative timing between ion channel currents are mostly invariant between states, but exhibit changes in magnitudes and decreased propagation across dendritic compartments. We also find enhanced dendritic hyperpolarization-activated cyclic nucleotide-gated channel (h-channel) activation during in vivo-like states, which suggests that dendritically located h-channels are functionally important in altering signal propagation in the behaving animal. Conclusions: Overall, we have demonstrated, using computational modelling, the dynamical changes that can occur to ion channel mechanisms governing neuronal spiking in vitro and in vivo. In particular, we have shown that the magnitudes of some ion channel current contributions are differentially altered during in vivo-like states relative to in vitro.

Heterogeneities in intrinsic excitability and frequency-dependent response properties of granule cells across the blades of the rat dentate gyrus.

The dentate gyrus (DG), the input gate to the hippocampus proper, is anatomically segregated into three different sectors, namely, the suprapyramidal blade, the crest region, and the infrapyramidal blade. Although there are well-established differences between these sectors in terms of neuronal morphology, connectivity patterns, and activity levels, differences in electrophysiological properties of granule cells within these sectors have remained unexplored. Here, employing somatic whole cell patch-clamp recordings from the rat DG, we demonstrate that granule cells in these sectors manifest considerable heterogeneities in their intrinsic excitability, temporal summation, action potential characteristics, and frequency-dependent response properties. Across sectors, these neurons showed positive temporal summation of their responses to inputs mimicking excitatory postsynaptic currents and showed little to no sag in their voltage responses to pulse currents. Consistently, the impedance amplitude profile manifested low-pass characteristics and the impedance phase profile lacked positive phase values at all measured frequencies and voltages and for all sectors. Granule cells in all sectors exhibited class I excitability, with broadly linear firing rate profiles, and granule cells in the crest region fired significantly fewer action potentials compared with those in the infrapyramidal blade. Finally, we found weak pairwise correlations across the 18 different measurements obtained individually from each of the three sectors, providing evidence that these measurements are indeed reporting distinct aspects of neuronal physiology. Together, our analyses show that granule cells act as integrators of afferent information and emphasize the need to account for the considerable physiological heterogeneities in assessing their roles in information encoding and processing.NEW & NOTEWORTHY We employed whole cell patch-clamp recordings from granule cells in the three subregions of the rat dentate gyrus to demonstrate considerable heterogeneities in their intrinsic excitability, temporal summation, action potential characteristics, and frequency-dependent response properties. Across sectors, granule cells did not express membrane potential resonance, and their impedance profiles lacked inductive phase leads at all measured frequencies. Our analyses also show that granule cells manifest class I excitability characteristics, categorizing them as integrators of afferent information.

1,25-Dihydroxyvitamin D modulates L-type voltage-gated calcium channels in a subset of neurons in the developing mouse prefrontal cortex

Schizophrenia has been associated with a range of genetic and environmental risk factors. Here we explored a link between two risk factors that converge on a shared neurobiological pathway. Recent genome-wide association studies (GWAS) have identified risk variants in genes that code for L-type voltage-gated calcium channels (L-VGCCs), while epidemiological studies have found an increased risk of schizophrenia in those with neonatal vitamin D deficiency. The active form of vitamin D (1,25(OH)2D) is a secosteroid that rapidly modulates L-VGCCs via non-genomic mechanisms in a range of peripheral tissues, though its non-genomic effects within the brain remain largely unexplored. Here we used calcium imaging, electrophysiology and molecular biology to determine whether 1,25(OH)2D non-genomically modulated L-VGCCs in the developing prefrontal cortex, a region widely implicated in schizophrenia pathophysiology. Wide-field Ca2+ imaging revealed that physiological concentrations of 1,25(OH)2D rapidly enhanced activity-dependent somatic Ca2+ levels in a small subset of neurons in the developing PFC, termed vitamin D-responsive neurons (VDRNs). Somatic nucleated patch recordings revealed a rapid, 1,25(OH)2D-evoked increase in high-voltage-activated (HVA) Ca2+ currents. Enhanced activity-dependent Ca2+ levels were mediated by L-VGCC but not associated with any changes to Cacna1c (L-VGCC pore-forming subunit) mRNA expression. Since L-VGCC activity is critical to healthy neurodevelopment, these data suggest that suboptimal concentrations of 1,25(OH)2D could alter brain maturation through modulation of L-VGCC signalling and as such may provide a parsimonious link between epidemiologic and genetic risk factors for schizophrenia.

Spikelets in pyramidal neurons: generating mechanisms, distinguishing properties, and functional implications.

Spikelets are small spike-like depolarizations that are found in somatic recordings of many neuron types. Spikelets have been assigned important functions, ranging from neuronal synchronization to the regulation of synaptic plasticity, which are specific to the particular mechanism of spikelet generation. As spikelets reflect spiking activity in neuronal compartments that are electrotonically distinct from the soma, four modes of spikelet generation can be envisaged: (1) dendritic spikes or (2) axonal action potentials occurring in a single cell as well as action potentials transmitted via (3) gap junctions or (4) ephaptic coupling in pairs of neurons. In one of the best studied neuron type, cortical pyramidal neurons, the origins and functions of spikelets are still unresolved; all four potential mechanisms have been proposed, but the experimental evidence remains ambiguous. Here we attempt to reconcile the scattered experimental findings in a coherent theoretical framework. We review in detail the various mechanisms that can give rise to spikelets. For each mechanism, we present the biophysical underpinnings as well as the resulting properties of spikelets and compare these predictions to experimental data from pyramidal neurons. We also discuss the functional implications of each mechanism. On the example of pyramidal neurons, we illustrate that several independent spikelet-generating mechanisms fulfilling vastly different functions might be operating in a single cell.

Paradoxical Excitatory Impact of SK Channels on Dendritic Excitability.

Dendritic excitability regulates how neurons integrate synaptic inputs and thereby influences neuronal output. As active dendritic events are associated with significant calcium influx they are likely to be modulated by calcium-dependent processes, such as calcium-activated potassium channels. Here we investigate the impact of small conductance calcium-activated potassium channels (SK channels) on dendritic excitability in male and female rat cortical pyramidal neurons in vitro and in vivo Using local applications of the SK channel antagonist apamin in vitro, we show that blocking somatic SK channels enhances action potential output, whereas blocking dendritic SK channels paradoxically reduces the generation of dendritic calcium spikes and associated somatic burst firing. Opposite effects were observed using the SK channel enhancer NS309. The effect of apamin on dendritic SK channels was occluded when R-type calcium channels were blocked, indicating that the inhibitory impact of apamin on dendritic calcium spikes involved R-type calcium channels. Comparable effects were observed in vivo Intracellular application of apamin via the somatic whole-cell recording pipette reduced the medium afterhyperpolarization and increased action potential output during UP states. In contrast, extracellular application of apamin to the cortical surface to block dendritic SK channels shifted the distribution of action potentials within UP states from an initial burst to a more distributed firing pattern, while having no impact on overall action potential firing frequency or UP and DOWN states. These data indicate that somatic and dendritic SK channels have opposite effects on neuronal excitability, with dendritic SK channels counter-intuitively promoting rather than suppressing neuronal output.SIGNIFICANCE STATEMENT Neurons typically receive input from other neurons onto processes called dendrites, and use electrical events such as action potentials for signaling. As electrical events in neurons are usually associated with calcium influx they can be regulated by calcium-dependent processes. One such process is through the activation of calcium-dependent potassium channels, which usually act to reduce action potential signaling. Although this is the case for calcium-dependent potassium channels found at the cell body, we show here that calcium-dependent potassium channels in dendrites of cortical pyramidal neurons counter-intuitively promote rather than suppress action potential output.

Capacitance measurement of dendritic exocytosis in an electrically coupled inhibitory retinal interneuron: an experimental and computational study.

Exocytotic release of neurotransmitter can be quantified by electrophysiological recording from postsynaptic neurons. Alternatively, fusion of synaptic vesicles with the cell membrane can be measured as increased capacitance by recording directly from a presynaptic neuron. The “Sine + DC” technique is based on recording from an unbranched cell, represented by an electrically equivalent RC‐circuit. It is challenging to extend such measurements to branching neurons where exocytosis occurs at a distance from a somatic recording electrode. The AII amacrine is an important inhibitory interneuron of the mammalian retina and there is evidence that exocytosis at presynaptic lobular dendrites increases the capacitance. Here, we combined electrophysiological recording and computer simulations with realistic compartmental models to explore capacitance measurements of rat AII amacrine cells. First, we verified the ability of the “Sine + DC” technique to detect depolarization‐evoked exocytosis in physiological recordings. Next, we used compartmental modeling to demonstrate that capacitance measurements can detect increased membrane surface area at lobular dendrites. However, the accuracy declines for lobular dendrites located further from the soma due to frequency‐dependent signal attenuation. For sine wave frequencies ≥1 kHz, the magnitude of the total releasable pool of synaptic vesicles will be significantly underestimated. Reducing the sine wave frequency increases overall accuracy, but when the frequency is sufficiently low that exocytosis can be detected with high accuracy from all lobular dendrites (~100 Hz), strong electrical coupling between AII amacrines compromises the measurements. These results need to be taken into account in studies with capacitance measurements from these and other electrically coupled neurons.

Robustness to Axon Initial Segment Variation Is Explained by Somatodendritic Excitability in Rat Substantia Nigra Dopaminergic Neurons.

In many neuronal types, axon initial segment (AIS) geometry critically influences neuronal excitability. Interestingly, the axon of rat SNc dopaminergic (DA) neurons displays a highly variable location and most often arises from an axon-bearing dendrite (ABD). We combined current-clamp somatic and dendritic recordings, outside-out recordings of dendritic sodium and potassium currents, morphological reconstructions and multicompartment modeling on male and female rat SNc DA neurons to determine cell-to-cell variations in AIS and ABD geometry, and their influence on neuronal output (spontaneous pacemaking frequency, action potential [AP] shape). Both AIS and ABD geometries were found to be highly variable from neuron to neuron. Surprisingly, we found that AP shape and pacemaking frequency were independent of AIS geometry. Modeling realistic morphological and biophysical variations helped us clarify this result: in SNc DA neurons, the complexity of the ABD combined with its excitability predominantly define pacemaking frequency and AP shape, such that large variations in AIS geometry negligibly affect neuronal output and are tolerated.SIGNIFICANCE STATEMENT In many neuronal types, axon initial segment (AIS) geometry critically influences neuronal excitability. In the current study, we describe large cell-to-cell variations in AIS length or distance from the soma in rat substantia nigra pars compacta dopaminergic neurons. Using neuronal reconstruction and electrophysiological recordings, we show that this morphological variability does not seem to affect their electrophysiological output, as neither action potential properties nor pacemaking frequency is correlated with AIS morphology. Realistic multicompartment modeling suggests that this robustness to AIS variation is mainly explained by the complexity and excitability of the somatodendritic compartment.

The anticonvulsant lamotrigine enhances Ih in layer 2/3 neocortical pyramidal neurons of patients with pharmacoresistant epilepsy

Alterations of the hyperpolarization activated nonselective cation current (Ih) are associated with epileptogenesis. Accordingly, the second-generation antiepileptic drug lamotrigine (LTG) enhances Ih in rodent hippocampus. We directly evaluated here whether LTG fails to enhance Ih in neocortical slices from patients with pharmacoresistant epilepsy. With somatic current clamp recordings we observed that LTG depolarized the membrane potential, decreased the input resistance and increased the “sag” in human layer 2/3 neocortical pyramidal neurons when confounding IKir was blocked. In subsequent voltage clamp recordings we confirmed a LTG induced increase of Ih that was qualitatively similar to the one we found in rat neocortical and hippocampal pyramidal neurons. This increase is sufficient to curtail single excitatory postsynaptic potentials (EPSPs) and reduces their temporal summation in human neocortical pyramidal neurons under physiological conditions, i.e. without blocking any other currents, as estimated by sharp microelectrode recordings.Taken together LTG increases Ih and thereby alters neuronal excitability, even in neurons of pharmacoresistant patients. However, whether this increase fully countervails the deficits of Ih in epileptic patients, remains elusive.

Simultaneous dendritic voltage and calcium imaging and somatic recording from Purkinje neurons in awake mice

Spatiotemporal maps of dendritic signalling and their relationship with somatic output is fundamental to neuronal information processing, yet remain unexplored in awake animals. Here, we combine simultaneous sub-millisecond voltage and calcium two-photon imaging from distal spiny dendrites, with somatic electrical recording from spontaneously active cerebellar Purkinje neurons (PN) in awake mice. We detect discrete 1−2 ms suprathreshold voltage spikelets in the distal spiny dendrites during dendritic complex spikes. Spikelets and their calcium correlates are highly heterogeneous in number, timing and spatial distribution within and between complex spikes. Back-propagating simple spikes are highly attenuated. Highly variable 5–10 ms voltage hotspots are localized to fine dendritic processes and are reduced in size and frequency by lidocaine and CNQX. Hotspots correlated with somatic output but also, at high frequency, trigger purely dendritic calcium spikes. Summarizing, spatiotemporal signalling in PNs is far more complex, dynamic, and fine scaled than anticipated, even in resting animals.

Dendritic GIRK Channels Gate the Integration Window, Plateau Potentials, and Induction of Synaptic Plasticity in Dorsal But Not Ventral CA1 Neurons.

Studies comparing neuronal activity at the dorsal and ventral poles of the hippocampus have shown that the scale of spatial information increases and the precision with which space is represented declines from the dorsal to ventral end. These dorsoventral differences in neuronal output and spatial representation could arise due to differences in computations performed by dorsal and ventral CA1 neurons. In this study, we tested this hypothesis by quantifying the differences in dendritic integration and synaptic plasticity between dorsal and ventral CA1 pyramidal neurons of rat hippocampus. Using a combination of somatic and dendritic patch-clamp recordings, we show that the threshold for LTP induction is higher in dorsal CA1 neurons and that a G-protein-coupled inward-rectifying potassium channel mediated regulation of dendritic plateau potentials and dendritic excitability underlies this gating. By contrast, similar regulation of LTP is absent in ventral CA1 neurons. Additionally, we show that generation of plateau potentials and LTP induction in dorsal CA1 neurons depends on the coincident activation of Schaffer collateral and temporoammonic inputs at the distal apical dendrites. The ventral CA1 dendrites, however, can generate plateau potentials in response to temporally dispersed excitatory inputs. Overall, our results highlight the dorsoventral differences in dendritic computation that could account for the dorsoventral differences in spatial representation.SIGNIFICANCE STATEMENT The dorsal and ventral parts of the hippocampus encode spatial information at very different scales. Whereas the place-specific firing fields are small and precise at the dorsal end of the hippocampus, neurons at the ventral end have comparatively larger place fields. Here, we show that the dorsal CA1 neurons have a higher threshold for LTP induction and require coincident timing of excitatory synaptic inputs for the generation of dendritic plateau potentials. By contrast, ventral CA1 neurons can integrate temporally dispersed inputs and have a lower threshold for LTP. Together, these dorsoventral differences in the threshold for LTP induction could account for the differences in scale of spatial representation at the dorsal and ventral ends of the hippocampus.