Periodic Reporting for period 5 - DeCode (Dendrites and memory: role of dendritic spikes in information coding by hippocampal CA3 pyramidal neurons)
Reporting period: 2024-06-01 to 2024-11-30
Understanding how experiences establish memories is a fundamental goal of neuroscience. The hippocampus has a crucial role in episodic memory and spatial representations both in animals and humans. During such memory tasks, the hippocampus creates network codes in the form of ensembles of neurons that are activated by the same environmental feature or context; however, little is known about the mechanisms promoting the recruitment and consolidation of individual neurons into these information-coding ensembles.
The interconnected (recurrent) synaptic network of pyramidal cells (PCs) in the hippocampal CA3 area, receiving external inputs from the entorhinal cortex and the dentate gyrus, is thought to be essential for associative memory. Current models of the associative functions of CA3 are mainly based on long-lasting plasticity of synaptic connections among CA3 pyramidal cells, but recent work by us and others suggests that active, voltage-dependent properties of CA3PC dendrites may also promote ensemble functions. Dendritic voltage-dependent ion channels allow nonlinear amplification of spatiotemporally correlated synaptic inputs (such as those produced by ensemble activity) and can even generate local dendritic spikes, which may elicit specific action potential patterns (e.g. bursts) and induce synaptic plasticity. Furthermore, dendritic processing may be modulated by activity-dependent regulation of dendritic ion channels, resulting in changes in the intrinsic excitability of the cells. However, the active properties of CA3PC dendrites and their functions during spatial coding or memory tasks are poorly understood.
In this project we aimed to test the hypothesis that active integration of inputs by dendrites of individual CA3PCs plays an important role in their recruitment into specific context-coding ensembles. Evidence for active dendritic integration has been studied combining in vitro (patch-clamp electrophysiology and two-photon (2P) microscopy in slices) and in vivo (2P imaging and activity-dependent labelling in behaving rodents) approaches. Over the course of the project, we have discovered novel forms, functions and regulation of dendritic Ca2+ spikes underlying complex spike bursting and regular spiking firing patterns in CA3PCs in acute rodent brain slices, and we have investigated the functional relevance of active dendrites in CA3PCs during spatial navigation in vivo. Our work contributes to a better understanding of the cellular mechanisms underlying memory processes by the mammalian hippocampus.
The interconnected (recurrent) synaptic network of pyramidal cells (PCs) in the hippocampal CA3 area, receiving external inputs from the entorhinal cortex and the dentate gyrus, is thought to be essential for associative memory. Current models of the associative functions of CA3 are mainly based on long-lasting plasticity of synaptic connections among CA3 pyramidal cells, but recent work by us and others suggests that active, voltage-dependent properties of CA3PC dendrites may also promote ensemble functions. Dendritic voltage-dependent ion channels allow nonlinear amplification of spatiotemporally correlated synaptic inputs (such as those produced by ensemble activity) and can even generate local dendritic spikes, which may elicit specific action potential patterns (e.g. bursts) and induce synaptic plasticity. Furthermore, dendritic processing may be modulated by activity-dependent regulation of dendritic ion channels, resulting in changes in the intrinsic excitability of the cells. However, the active properties of CA3PC dendrites and their functions during spatial coding or memory tasks are poorly understood.
In this project we aimed to test the hypothesis that active integration of inputs by dendrites of individual CA3PCs plays an important role in their recruitment into specific context-coding ensembles. Evidence for active dendritic integration has been studied combining in vitro (patch-clamp electrophysiology and two-photon (2P) microscopy in slices) and in vivo (2P imaging and activity-dependent labelling in behaving rodents) approaches. Over the course of the project, we have discovered novel forms, functions and regulation of dendritic Ca2+ spikes underlying complex spike bursting and regular spiking firing patterns in CA3PCs in acute rodent brain slices, and we have investigated the functional relevance of active dendrites in CA3PCs during spatial navigation in vivo. Our work contributes to a better understanding of the cellular mechanisms underlying memory processes by the mammalian hippocampus.
In accordance with the proposed goals, we investigated the mechanisms and relevance of dendritic spikes and complex spike bursts (CSBs) in CA3PC functions.
We discovered that the propensity of CA3PCs to fire CSBs is highly heterogeneous and is regulated differently depending on the topographic position of cells within the area. We pinpointed two specific dendritic ion channel types whose region-specific activity partly explains the topographic modulation of CSB propensity. We uncovered that the synaptic input-output transformation rules triggering CSBs are non-uniform among CA3PCs: CSBs serve as an associative input pathway signal in regular spiking (RS) cells but can be triggered by any input type clustered in any individual dendritic branch in CSB-firing cells.
Using direct dendritic recordings we discovered that, unlike in other PC types, individual distal apical dendrites of CA3PCs can express two distinct spatiotemporal forms of dendritic Ca2+ spikes (a slow global and a novel fast compartmentalized type) and revealed that these distinct Ca2+ spike forms have opposing impacts on somatic firing, promoting either CSBs or strictly single action potentials, respectively.
We showed that the compound Ca2+ spike form, as measured at the soma, is also highly heterogeneous, and that a subpopulation of CA3PCs expresses dominantly short-duration Ca2+ spikes and cannot sustain prolonged CSBs under baseline conditions. Using pharmacological tools we characterized the ion channels underlying the variability of the dendritic Ca2+ spike forms. We discovered prominent regulation of the Ca2+ form by cholinergic neuromodulation, which prolongs Ca2+ spikes and promotes CSB firing, primarily in a subpopulation of CA3PCs. This mechanism may allow state-dependent, subtype-specific contribution of active dendrites to CA3PC computations.
We have established an in vivo (two-photon microscope combined with virtual reality) system for imaging hippocampal CA1 and CA3 PCs, developed software tools to program data acquisition protocols and monitor behavioral performance, and worked out spatial context discrimination training paradigms. In experiments in head-fixed mice navigating in various virtual spatial contexts, we have imaged the activity of populations of PCs or subcellular compartments of individual PCs and investigated the forms of dendritic activity and their roles in neuronal activity and tuning.
Using novel computational and experimental approaches we demonstrated how long-term synaptic plasticity depends on the spatial pattern of inputs as well as local and global dendritic integrative mechanisms in hippocampal neurons.
The results arising from the project have been presented at several prominent scientific conferences and in part have been published (or are in the process of publication) in prestigious neuroscience journals.
We discovered that the propensity of CA3PCs to fire CSBs is highly heterogeneous and is regulated differently depending on the topographic position of cells within the area. We pinpointed two specific dendritic ion channel types whose region-specific activity partly explains the topographic modulation of CSB propensity. We uncovered that the synaptic input-output transformation rules triggering CSBs are non-uniform among CA3PCs: CSBs serve as an associative input pathway signal in regular spiking (RS) cells but can be triggered by any input type clustered in any individual dendritic branch in CSB-firing cells.
Using direct dendritic recordings we discovered that, unlike in other PC types, individual distal apical dendrites of CA3PCs can express two distinct spatiotemporal forms of dendritic Ca2+ spikes (a slow global and a novel fast compartmentalized type) and revealed that these distinct Ca2+ spike forms have opposing impacts on somatic firing, promoting either CSBs or strictly single action potentials, respectively.
We showed that the compound Ca2+ spike form, as measured at the soma, is also highly heterogeneous, and that a subpopulation of CA3PCs expresses dominantly short-duration Ca2+ spikes and cannot sustain prolonged CSBs under baseline conditions. Using pharmacological tools we characterized the ion channels underlying the variability of the dendritic Ca2+ spike forms. We discovered prominent regulation of the Ca2+ form by cholinergic neuromodulation, which prolongs Ca2+ spikes and promotes CSB firing, primarily in a subpopulation of CA3PCs. This mechanism may allow state-dependent, subtype-specific contribution of active dendrites to CA3PC computations.
We have established an in vivo (two-photon microscope combined with virtual reality) system for imaging hippocampal CA1 and CA3 PCs, developed software tools to program data acquisition protocols and monitor behavioral performance, and worked out spatial context discrimination training paradigms. In experiments in head-fixed mice navigating in various virtual spatial contexts, we have imaged the activity of populations of PCs or subcellular compartments of individual PCs and investigated the forms of dendritic activity and their roles in neuronal activity and tuning.
Using novel computational and experimental approaches we demonstrated how long-term synaptic plasticity depends on the spatial pattern of inputs as well as local and global dendritic integrative mechanisms in hippocampal neurons.
The results arising from the project have been presented at several prominent scientific conferences and in part have been published (or are in the process of publication) in prestigious neuroscience journals.
The project has uncovered novel mechanisms and unexpected fine-tuned regulation of active dendritic properties, as well as complex dendritic input-output functions in CA3PCs.
The discovery of distinct types of dendritic Ca2+ spikes with drastically different spatiotemporal properties provided a conceptual advance, as it revealed that (contrary to the widely held view) Ca2+ spikes can be highly heterogeneous among neuron types and even subtypes, and can have opposing functional and computational impacts on neuronal output.
The demonstration that cholinergic neuromodulation critically regulates the Ca2+ spikes and thereby bursting properties of a subpopulation of CA3PCs can advance our understanding of the state-dependent computations performed by the CA3 region.
Our work, along with other studies in the field, supports the new notion that CA3PCs do not form a homogenous circuit but are composed of morpho-functionally heterogeneous subpopulations with potentially different computational roles.
We expect that the results of our in vivo imaging experiments will provide important new insights into the unique roles of dendritic Ca2+ spikes in shaping the activity of CA3PCs during spatial navigation and memory.
These results altogether may significantly alter our concept of how the hippocampal CA3 region processes and stores information.
The discovery of distinct types of dendritic Ca2+ spikes with drastically different spatiotemporal properties provided a conceptual advance, as it revealed that (contrary to the widely held view) Ca2+ spikes can be highly heterogeneous among neuron types and even subtypes, and can have opposing functional and computational impacts on neuronal output.
The demonstration that cholinergic neuromodulation critically regulates the Ca2+ spikes and thereby bursting properties of a subpopulation of CA3PCs can advance our understanding of the state-dependent computations performed by the CA3 region.
Our work, along with other studies in the field, supports the new notion that CA3PCs do not form a homogenous circuit but are composed of morpho-functionally heterogeneous subpopulations with potentially different computational roles.
We expect that the results of our in vivo imaging experiments will provide important new insights into the unique roles of dendritic Ca2+ spikes in shaping the activity of CA3PCs during spatial navigation and memory.
These results altogether may significantly alter our concept of how the hippocampal CA3 region processes and stores information.