New and mature neurons in adult circuits: telling memories apart

Project Information

NEWRON

Grant agreement ID: 678790

DOI

10.3030/678790

Project closed

EC signature date 7 July 2016

Start date 1 October 2016

End date 30 September 2022

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 498 955,00

EU contribution

€ 1 498 955,00

Coordinated by

INSTITUT PASTEUR
France

Periodic Reporting for period 4 - NEWRON (New and mature neurons in adult circuits: telling memories apart)

Reporting period: 2021-04-01 to 2022-09-30

Summary of the context and overall objectives of the project

How does the brain store and retrieve distinct memories of episodes that closely resemble each other? This task is critically important for our everyday lives, as it allows us to distinguish between similar places, routes, events, or people. A brain region called the hippocampus has been suggested to serve this purpose: when similar information enters the hippocampus, its input gate, the “dentate gyrus”, is thought to produce non-overlapping memory representations in a process termed “pattern separation”.
Intriguingly, during adult life, the dentate gyrus is also one of the few mammalian brain regions that is constantly supplied with new neurons, providing new circuit elements that can incorporate into the neuronal network. How the activity of new adult-born neurons and mature granule cells combines to drive the production and storage of distinct memories represents a new frontier in understanding brain function.
However, to determine how these neurons transform similar synaptic input patterns into decorrelated spike output patterns representing distinct memories, we need to monitor and manipulate their activity during behaviour. To address these challenges, we combine molecular, physiological and optical approaches in the mouse hippocampus during navigation in a virtual reality environment. We use intracellular recordings to assess how hippocampal neurons convert synaptic inputs into spike output, single- and 2-photon Ca2+ imaging to monitor population activity in the hippocampal circuit, and optogenetic tools to causally test the involvement of specific cell types. These experiments allow us to address the following key questions:
Which behaviours trigger activity in identified newborn and mature dentate gyrus neurons?
How do synaptic inputs drive spiking in newborn and mature neurons during behaviour?
How does neuronal activity in different hippocampal areas discriminate between small changes in the environment, and how does this discrimination relate to behaviour?
How are small changes in an environment encoded by synaptic inputs to newborn and mature neurons?
Can we differentially manipulate the activity of new and mature cells to selectively interfere with animal behaviour during pattern separation and completion tasks?
Understanding the role of new and mature neurons in hippocampal function will give us critical insights into the cellular mechanisms of memory formation. It will also provide fundamental knowledge that is required to unravel the mechanisms underlying debilitating diseases that affect the hippocampus, such as Alzheimer’s dementia.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

We have accomplished the following research achievements since the beginning of the project:

We have addressed the question how and where in the hippocampal circuit different memory representations of similar spatial environments are produced. To tackle this challenge, we have established methods that allow us to measure the activity of identified populations of hippocampal neurons in navigating animals: we perform in vivo 2-photon Ca2+ imaging from hippocampal subregions of head-fixed mice navigating in a virtual-reality environment while performing a visual memory task. To validate our findings, we also use an alternative strategy, where we perform single-photon widefield microendoscope calcium imaging from dentate granule cells, in mice freely navigating along a linear track. Our results have pinpointed the input region of the hippocampus, the dentate gyrus, as the region that can tell subtle changes between familiar and novel environments apart; in contrast, activity in the output region of the hippocampus (CA1) determines the spatial behavior of the animal.
Together, our findings address the fundamental question how and where in the hippocampal circuit distinct memory representations of similar experiences are produced, and how these representations affect memory-guided behaviour.

We have studied how neurons in the hippocampus integrate synaptic inputs during pattern separation tasks. To study these mechanisms, we perform in vivo whole-cell patch-clamp recordings from hippocampal neurons of mice navigating in a virtual environment. This technique allows us to measure both the synaptic inputs that a neuron receives, and the action potential output that it produces. We have developed a behavioural paradigm that allows us to measure correlations between synaptic input patterns in familiar and novel environments and to probe synaptic responses to elementary sensory stimuli. Using this technique, we have tackled the fundamental question how the brain knows when to store a new memory rather than recalling an old one. We have revealed that the entry stage to the hippocampus, the dentate gyrus, shows synaptic subthreshold responses to novel environments, which can drive a shift in the downstream hippocampal circuit from reactivating existing neuronal assemblies to creating new ones.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

We have developed new virtual-reality tools for rodents that allow us to instantaneously apply subtle changes to the visual environment, while monitoring whether the animal can behaviourally distinguish between these different scenes. We simultaneously record the activity of populations of identified hippocampal neurons, or synaptic inputs to individual hippocampal neurons. This paradigm allows us to relate the animal’s spatial perception of different environments with their neuronal representation in the hippocampus. Our strategy goes beyond the state of the art as we can apply rigorously quantifiable small changes to the environment while monitoring the animal’s behavioural response along with the neuronal response to these changes, which is critical for determining the role of hippocampal subfields in memory-guided behavior.
We have established methods to record synaptic inputs to identified granule cells while an animal is exploring novel and familiar virtual environments, using whole-cell patch-clamp recordings. This technology enables us to measure synaptic responses to subtle changes in the environment.
We have set up a spatial light modulator that allows us to stimulate functionally identified populations of specific neurons in behaving animals. This strategy allows us to causally test pattern separation theories.
We have established tools for viral delivery of genetically encoded calcium indicators to adult-born neurons in the hippocampus in vitro, and are in the process of applying this technology in vivo. Our method is novel in that it allows us to precisely birth date (+-2 days) the adult-born neuronal population, in contrast to previous work where only a broad population of adult-born neurons at various maturational stages (1 up to 8 weeks post mitosis) was labelled for functional imaging.
We are using a novel widefield single-photon microendoscopy system that allows us to measure the activity of hippocampal neurons in freely moving animals. We have devised a task where we can test the effects of subtle parameter changes such as ambient odours, or running direction, on neuronal representations. At the same time, this strategy also allows us to distinguish between the effects of spatial versus contextual variables on neuronal activity.

Dentate gyrus neurons expressing GCaMP6f (green), with adult-born neurons expressing tdTomato (red)

Virtual reality setup for rodents, based on a spherical dome screen projection.