Impact of Dendritic Size and Dendritic Topology on Burst Firing in Pyramidal Cells

Neurons display a wide range of intrinsic firing patterns. A particularly relevant pattern for neuronal signaling and synaptic plasticity is burst firing, the generation of clusters of action potentials with short interspike intervals. Besides ion-channel composition, dendritic morphology appears to be an important factor modulating firing pattern. However, the underlying mechanisms are poorly understood, and the impact of morphology on burst firing remains insufficiently known. Dendritic morphology is not fixed but can undergo significant changes in many pathological conditions. Using computational models of neocortical pyramidal cells, we here show that not only the total length of the apical dendrite but also the topological structure of its branching pattern markedly influences inter- and intraburst spike intervals and even determines whether or not a cell exhibits burst firing. We found that there is only a range of dendritic sizes that supports burst firing, and that this range is modulated by dendritic topology. Either reducing or enlarging the dendritic tree, or merely modifying its topological structure without changing total dendritic length, can transform a cell's firing pattern from bursting to tonic firing. Interestingly, the results are largely independent of whether the cells are stimulated by current injection at the soma or by synapses distributed over the dendritic tree. By means of a novel measure called mean electrotonic path length, we show that the influence of dendritic morphology on burst firing is attributable to the effect both dendritic size and dendritic topology have, not on somatic input conductance, but on the average spatial extent of the dendritic tree and the spatiotemporal dynamics of the dendritic membrane potential. Our results suggest that alterations in size or topology of pyramidal cell morphology, such as observed in Alzheimer's disease, mental retardation, epilepsy, and chronic stress, could change neuronal burst firing and thus ultimately affect information processing and cognition

Dendritic integration in hippocampal dentate gyrus granule cells

Hippocampal granule cells are critical relay stations to transfer spatial information from the entorhinal cortex into the hippocampus proper. Therefore, the integrative properties of the small-caliber granule cell dendrites were examined in this thesis, using a combination of dual somato-dendritic patch-clamp recordings and two-photon glutamate uncaging. These experiments revealed unusual integrative properties that differ substantially from other principal neurons. Due to a strong dendritic voltage attenuation, the impact of individual synapses on granule cell output is low. At the same time, integration is linear, only weakly affected by input synchrony, and is independent of the spatial location of input sites. These integrative properties can enhance contrast in the generation of place-specific firing maps from entorhinal inputs and contribute to the sparse representation of space in the dentate gyrus

Generation of the complex spike in cerebellar Purkinje cells.

Each neuron of the nervous system is a machine specialised to appropriately transform its synaptic inputs into a pattern of spiking output. This is achieved through the combination of specialisations in synaptic properties and location, passive cell geometry and placement of particular active ion channels. The challenge presented to the neuroscientist is to, within each cell type, identify such specialisations in input distribution and resulting active events, and assess their relative importance in the generation of action potential output patterns. The Purkinje cell, in particular its response to climbing fibre (CF) input, is an excellent setting in which to attempt to meet this challenge. The Purkinje cell receives a single, easily isolated CF axon, which makes hundreds of synapses across the cell's highly branched, active dendritic tree, resulting in the generation of prominent dendritic calcium spikes and a distinctive, reproducible burst of fast action potentials (the complex spike) at the soma. In this thesis I have separated out the importance of the size of this input, its location and the active dendritic spikes it triggers in the generation of the complex spike. I have found that, to a large extent, the complex spike pattern is determined by the size of the CF input alone. I have characterised the complex spike (its number of spikes, their timing, height and reliability) at both constant physiological frequency and across a range of paired- pulse depression causing intervals. By alternating between whole cell current and voltage clamp in the same cell, I have recorded both the complex spikes and EPSCs generated at certain paired pulse intervals. In this way I have been able to construct the EPSC - complex spike 'input - output' relationship. This demonstrated that there is a straightforward linear transformation between the EPSC input amplitude and the number and timing of spikes in the complex spike. This applies across cells, explaining a large amount of the inter-cell variability in complex spike pattern. Input location and dendritic spikes have surprisingly little influence over the Purkinje cell complex spike. I found that complex spikes generated by dendritically distributed CF input can be reproduced by using conductance clamp to inject CF-like synaptic conductance at the soma. Both CF input and somatic EPSG injection produced complex spike waveforms that can only be easily explained by a model in which spikelets are initiated at a distant site and variably propagated to the soma. By using simultaneous somatic and dendritic recording I have demonstrated that this distant site initiation site is not in the dendrites. Somatic EPSG injection reproduced complex spikes independently of dendritic spikes, and extra dendritic spikes triggered by CF stimulation were associated with only 0.24 0.09 extra somatic spikelets in the complex spike. Rather, I have found that dendritic spikes, generated reliably by the dendritic location of CF inputs, have a role in regulating the post-complex spike pause. An extra dendritic spike generates a 3.4 0.7 mV deeper AHP and a 52 11 % longer pause before spontaneous spiking resumed. In this way, I have identified specialisations that encode the size, and thus timing, of CF inputs in the complex spike burst, whilst allowing the dendritic excitation of Purkinje cells (which is strongly associated synaptic and intrinsic plasticity) to be simultaneously encoded in the post-complex spike pause. This may reflect the complex spike's proposed dual role in both controlling ongoing movement and correcting for motor errors

Dendritic spikes control synaptic plasticity and somatic output in cerebellar Purkinje cells.

Neurons receive the vast majority of their input onto their dendrites. Dendrites express a plethora of voltage-gated channels. Regenerative, local events in dendrites and their role in the information transformation in single neurons are, however, poorly understood. This thesis investigates the basic properties and functional roles of dendritic spikes in cerebellar Purkinje cells using whole-cell patch clamp recordings from the dendrites and soma of rat Purkinje cells in brain slices. I show that parallel fibre (PF) evoked dendritic spikes are mediated by calcium channels, depend on membrane potential and stimulus intensity and are highly localized to the spiny branches receiving the synaptic input. A determining factor in the localization and spread of dendritic calcium spikes is the activation of large-conductance, calcium dependent potassium (BK) channels. I provide a strong link between dendritic spikes and the endocannabinoid dependent short-term synaptic plasticity, depolarization-induced suppression of excitation (DSE). Gating the dendritic spikes using stimulus intensity or membrane potential, I show that the threshold of DSE is identical to that of the dendritic spikes and the extent of DSE depends on the number of dendritic spikes. Blocking BK channels increases the spatial spread of dendritic spikes and enables current injection or climbing fibre (CF) evoked dendritic spikes to suppress PF inputs via DSE. By monitoring dendritic spikes during strong PF stimulation-induced long-term depression (LTD), I also provide a link between long-term synaptic plasticity and dendritic excitability. By showing that blocking CB1 cannabinoid receptors reduces the intensity requirement for LTD, I provide a connection between the short- and long-term changes in PF strength triggered by dendritic spikes I also investigate the effect dendritic spikes have on somatic action potential output. Contrary to pyramidal cells, where dendritic spikes boost the output of the neuron, the average Purkinje cell output becomes independent from the output strength for inputs triggering dendritic spikes. However, the temporal pattern of the output is strongly affected by dendritic spikes. I show that this phenomenon depends on BK channel activation resulting in a pause in somatic firing following dendritic spikes. In summary, I present a description of PF evoked local dendritic spikes and demonstrate their functional role in controlling the synaptic input and action potential output of cerebellar Purkinje cells

Influence of the electrotonic architecture on single neurons dynamics : a computational approach

Dissertação de mestrado, Ciência Cognitiva, Universidade de Lisboa, Faculdade de Ciências, Faculdade de Letras, Faculdade de Medicina, Faculdade de Psicologia, 2014Na presente dissertação, investigamos de forma sistemática a forma como a morfologia dendrítica subjaz as diferenças na atividade elétrica neuronal que estão na base da geração de potenciais de ação. De forma a atingir este objetivo desenvolvemos uma medida que quantifica as duas maiores fontes de variabilidade morfológica: métrica e topologia, e ainda outros componentes estruturais como canais iónicos. Baseado na nova medida, propomos um novo mecanismo de sincronização que relaciona a estrutura dendritica à modulação de currente axial que flui da árvore dendrítica até ao soma. Esta hipótese afirma que quanto mais simétrica a estrutura electrotónica da célula é, mais currente irá chegar ao soma das dendrites devido à sincronização obtida em virtude da simetria estrutural. De forma a testar a hipótese de sincronização foram simuladas duas experiências usando modelos multi-compartimentais computacionais de células de Purkinje, Piramidais e células do córtex Visual. Na primeira abordagem, as estruturas das células foram quantificadas utilizando a nova medida e depois comparadas com a quantidade de currente axial proviniente das dendrites que atingia o soma. Na segunda abordagem, os potenciais de voltagem são medidos ao nível do compartimento axo-somático de forma a se poder analisar se diferenças encontradas na condição axial induzem diferenças na atividade de spiking da célula. Os resultados apoiam a hipótese de sincronização, pois neurónios com estruturas electrotónicas com níveis de simetria mais elevados, exibem os níveis mais elevados de currente axial a chegar ao soma para o mesmo estímulo. As diferenças encontradas na condição axial correlacionaram-se com o tempo que os neurónios levaram a atingir um potencial de ação, com os neurónios mais simétricos a requerer menos tempo para o fazer. No entanto, diferenças significativas não emergiram nos padrões de potenciais de ação, mas estes resultados podem ser explicados por algumas limitações no protocolo de estimulação. Em suma, os nossos resultados mostram que a medida desenvolvida é uma alternativa promissora às abordagens morfométricas tradicionais, pois pode ser utilizada com confiança para quantificar diferenças estruturais, podendo ser aplicada a vários tipos de neurónios, providenciando uma ligação entre estrutura e função.In this dissertation, we systematically investigate how dendritic morphology underlies the differences in the electrical dynamics of the cell that lead to spiking behaviour. To accomplish this goal we develop a new measure that provides a quantitative account of the two most relevant sources of morphological variability: metrics and topology, as well as of other structural components such as ion channels. Supported by the new measure, we propose a new synchronization mechanism that relates dendritic structure to the modulation of axial current that flows from the dendrites to the soma. This hypothesis states that the more symmetric the electrotonic structure of a cell is, the more current will reach the soma from the dendrites due to the synchronism obtained by virtue of structural symmetry. To test the synchronization hypothesis two simulation-based experiments using detailed multi-compartmental computational models of Purkinje, Pyramidal and Visual cortical cells were conducted. In the first approach, by means of the novel measure, the structure of the cells are quantified, and compared with the amount of axial current reaching the soma from the dendritic tree. In the second approach, voltage traces are measured at the axo-somatic compartment to analyse whether differences found in the axial current condition induce differences in the output spiking patterns. Our results support the synchronization hypothesis, as neurons with electrotonic structures with higher levels of symmetry exhibited the highest amount of current reaching the soma for the same stimulus. These differences correlated with the time that neurons required to spike, with more symmetrical neurons requiring less time to do so. Nevertheless, significant differences fail to emerge in the output spike trains, but these results can be explained by some limitations in the stimulation protocol. Overall, the results show that the proposed measure is a promising alternative to traditional morphometrics measures as it can be used with confidence to quantify structural differences, and can be applied across different types of neurons while providing a bridge between structure and function

Computational modeling of prefrontal cortex circuits

Dissertation presented to obtain the Ph.D degree in BiologyThe most outstanding feature of the human brain is its ability to perform highly complex cognitive tasks and one key region of the brain involved in these elaborated tasks is the prefrontal cortex. However, little is known about the basic neuronal processes that sustain these capacities. This dissertation describes the computational study of the biophysical properties of neurons in the prefrontal cortex that underlie complex cognitive processes with special emphasis in working memory, the ability to keep information online in the brain for a short period of time while processing incoming external stimuli.(...

Control of Membrane Excitability by Potassium and Chloride Leak Conductances

The permeability of the neuronal membrane to different ions determines both resting membrane potential (RMP) and input conductance. These parameters determine the cells response to synaptic input. In this thesis I have examined how the molecular properties of potassium and chloride ion channels can influence neuronal excitability in ways that have not previously been considered. For example, two‐pore domain potassium (K2P) channels open at rest to generate a persistent potassium ion efflux. In addition to its accepted role in setting the RMP, I have tested the hypothesis that this conductance is sufficient to repolarise the membrane during an action potential (AP) in the absence of voltage‐dependent potassium channels (Kv). We tested this prediction using heterologous expression of TASK3 or TREK1 K2P channels combined with conductance injection to simulate the presence of a voltage‐gated sodium conductance. These experiments demonstrated that K2P channels are sufficient to support APs during short and prolonged depolarising current pulses. The membranes permeability to chloride ions can also be affected by extrasynaptic GABAA receptors containing the delta subunit (δ‐GABAARs) that produce a tonic conductance due to their high apparent affinity for GABA. The anaesthetics Propofol and THIP are both believed to alter neuronal excitability by enhancing this persistent chloride flux. We have examined how this anaesthetic action is affected by the steady‐state ambient GABA concentrations that are believed to exist in vivo. Surprisingly, the anaesthetic enhancement of δ‐GABAARs is lost at low ambient GABA concentrations. Therefore, I would suggest that the anaesthetic potency of these drugs is affected by the resting ambient GABA concentration in a manner that has not previously been appreciated. In the current Thesis I have examined the molecular and pharmacological properties of two very different ion channel families that both generate a leak conductance, and I will present models that link the behaviour of these ion channels to their ability to modulate neuronal excitability

A model for cerebral cortical neuron group electric activity and its implications for cerebral function

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2002.Includes bibliographical references (p. 245-265).The electroencephalogram, or EEG, is a recording of the field potential generated by the electric activity of neuronal populations of the brain. Its utility has long been recognized as a monitor which reflects the vigilance states of the brain, such as arousal, drowsiness, and sleep stages. Moreover, it is used to detect pathological conditions such as seizures, to calibrate drug action during anesthesia, and to understand cognitive task signatures in healthy and abnormal subjects. Being an aggregate measure of neural activity, understanding the neural origins of EEG oscillations has been limited. With the advent of recording techniques, however, and as an influx of experimental evidence on cellular and network properties of the neocortex has become available, a closer look into the neuronal mechanisms for EEG generation is warranted. Accordingly, we introduce an effective neuronal skeleton circuit at a neuronal group level which could reproduce basic EEG-observable slow ( 3mm). The effective circuit makes use of the dynamic properties of the layer 5 network to explain intra-cortically generated augmenting responses, restful alpha, slow wave (< 1Hz) oscillations, and disinhibition-induced seizures. Based on recent cellular evidence, we propose a hierarchical binding mechanism in tufted layer 5 cells which acts as a controlled gate between local cortical activity and inputs arriving from distant cortical areas. This gate is manifested by the switch in output firing patterns in tufted(cont.) layer 5 cells between burst firing and regular spiking, with specific implications on local functional connectivity. This hypothesized mechanism provides an explanation of different alpha band (10Hz) oscillations observed recently under cognitive states. In particular, evoked alpha rhythms, which occur transiently after an input stimulus, could account for initial reogranization of local neural activity based on (mis)match between driving inputs and modulatory feedback of higher order cortical structures, or internal expectations. Emitted alpha rhythms, on the other hand, is an example of extreme attention where dominance of higher order control inputs could drive reorganization of local cortical activity. Finally, the model makes predictions on the role of burst firing patterns in tufted layer 5 cells in redefining local cortical dynamics, based on internal representations, as a prelude to high frequency oscillations observed in various sensory systems during cognition.by Fadi Nabih Karameh.Ph.D