Hippocampus is well-known as the center of spatial navigation and learning. Prof. Häusser and his team have recently published a very exciting article about direct modification of spatial navigation memories via holographical optogenetic stimulation. They applied virtual reality to train the animals for certain tasks and used two-photon deep imaging to visualize the place cell ensembles. They claimed stimulation of only a dozen unique place cell could be enough changing the behavior of the animal. This is the first direct evidence for a casual role of in place cell in navigation. This is a huge step to understand the memory encoding of the hippocampus. Congratulations for the authors!
Graphical abstract of the experimental design and results:
Understanding of the dynamics of neuronal ensembles in the brain is as much important as the examination of the contribution of a single neuron during neuronal processes. State of the art rigs support the idea to examine large neuronal populations. Moreover, these populations tend to be cooperative with each other, but it is still unknown how stable these ensembles and their connections. Dr. Rafael Yuste and his lab’s goal is to understand the long-term stability of these neuronal ensembles in mouse visual cortex. They applied two-photon volumetric calcium microcopy to perform chronic calcium imaging for several weeks. They found visually-evoked ensembles were quite stable. Their results suggest that these neuronal populations implement long-term memories.
For more information: https://www.biorxiv.org/content/10.1101/2020.10.28.359117v1
Optogenetics is a technique that grants the manipulation of neuronal activity. This method allows for targeted excitation and/or inhibition of specific neuronal populations.
A particular protein of interest is expressed in the targeted cells. This protein – called the optogenetic actuator - has the unique characteristic to be a light-sensitive ion channel. When illuminated with the corresponding wavelength, the channel allows the flow of ions through the cell membrane. This method offers spatiotemporal control of neuronal excitability in living tissue.
For use in vivo, optogenetics requires the implant of an optical fiber (stereotaxic surgery) - to provide the illumination necessary to control the neuronal activity. With most available solutions, the optical fiber is connected to a stimulation unit that provides the light. This technique has the disadvantage of requiring invasive implantation of equipment such as the optical fiber in the animal’s brain.
However, a new channelrhodopsin developed by Deisseroth’s lab allows for the activation of specific neural populations at unprecedented depths of up to 7 mm with millisecond precision. This tool would make possible implant-free deep brain optogenetics.
More information following this link: https://www.nature.com/articles/s41587-020-0679-9.
Recent paper from Professor Bernd Kuhn’s lab shows interesting results about the activity of Layer 6 corticothalamic neurons. They used two-photon long-term deep imaging (up to 8-900 µm) to visualize calcium responses during various behavioral tasks from locomotion to sleep. They found that Layer 6 corticothalamic neurons are either visual stimulus activated, suppressed, or quiet. Moreover, these ensembles complement each other and cause constant heterogeneous activity during any behavior state. That was the very first time to examine Layer 6 neurons with this method.
2P reconstruction of examined area (scale in µm).
In our News post this week, we recommend reading this detailed review article on the current techniques for investigating the brain extracellular space. In this paper, Tønnesen’s lab provides an introduction to in vitro and in vivo current neuroscience lab methods such as point-source diffusion measurement, electron microscopy, magnetic resonance imaging, widefield fluorescence microscopy, scanning fluorescence microscopy, and super-resolution fluorescence microscopy. These techniques are mainly based on optical imaging and electrical recordings.
This multiplication of methods has offered scientists the opportunity to diversify their approach to a specific scientific question. This article highlights the strengths and weaknesses of each technology. In order to diversify the observations of a given sample, the authors recommend combining several levels of study and methodology to broaden the project perspectives.
Overview of the Current Techniques for Investigating the Brain Extracellular Space - Soria et al.
Presenting our latest addition: the 3D accelerometer!
Connecting behavioral observations to nervous system activity has become increasingly important. Diverse technologies are available for measuring the neuronal activity in freely moving rodents, such as in vivo imaging – e.g. fiber photometry – and electrophysiology recordings. Additionally, it is equally crucial to precisely detect, quantify, and classify animal behaviors.
The 3D accelerometer is used to detect movement onsets and discern specific behaviors, such as rearing. This tool allows head movement measurements at high temporal resolution and along 3 axes independently of each other. Lightweight and compact, the sensor board is easily mounted and can be carried by both mice and rats.
3D accelerometer device, including the sensor board and the sensor cable with connectors
An example of accelerometer measurements of head pitch and roll can be found in this article:
Retrograde tracing, optogenetic manipulation, fiber photometry, cell-specific electrophysiology… Neuroscientists benefit from an increasingly wide range of tools to image and study the activity of neuronal populations. All these experiments rely on the same crucial step: intracerebral surgery. These procedures require specific equipment, from the stereotaxic instruments to the anesthesia methods.
Traditionally carried out through injection, the anesthesia is nowadays mostly performed with the inhalation of anesthetic gas, thanks to its rapid induction and limited recovery time. However, surgical procedures can sometimes be challenging when they are conducted on neonatal mice. A team from Cambridge (Hinze Ho et al.) has decided to tackle this issue by developing its own protocol to control the anesthesia of P0-2 mice. Combined with the design of an anesthetization mould, this method ensures an improved recovery for the rodents.
Access to the research article following this link: https://doi.org/10.1016/j.jneumeth.2020.108824.
Description of the surgery procedure - Hinze Ho et al.
Two-photon microscopy has revolutionized the ability to image neural activity in the brain of a living animal. Compared to single-photon microscopy, this technique allows for targeted excitation of small volumes in deep tissue. Moreover, the two-photon technique provides tissue penetration up to 800-1000 μm.
As powerful as it is, the imaging depth of a stand-alone two-photon microscope does not allow for deep brain region imaging. One technique used to image deeper is to couple the two-photon microscope with a gradient index (GRIN) lens. Implanted in the tissue, the GRIN lens allows the user to record from deep areas through long working distance objectives.
Even though two-photon GRIN lens-coupled imaging allows for optical sectioning and 3D imaging, this technique has limitations regarding axial scanning speed and contrast. By adding a tunable acoustic GRIN (TAG) lens to their setup, Chien et al. designed a new system allowing in vivo imaging of neurons in deep mouse brain areas with high-contrast and high-temporal resolution. The publication is available on bioRxiv following this link https://www.biorxiv.org/content/10.1101/2020.09.19.304675v1.
In vivo functional imaging of neuronal activity from SCN of a head-fixed anesthetized mouse - Chien et al. 2020
Dopamine is a neurotransmitter involved in key functions of the brain, e.g. motor control, reward, arousal, or motivation. To better understand the modulation of dopamine, it is fundamental to benefit from tools allowing both the manipulation of neuronal circuits and the recording of fluorescent signals in the brain, particularly in freely moving animals.
To perform such experiments, it is recommended that optogenetic actuators and imaging sensors spectra do not overlap. Therefore, expanding the fluorescent indicators palette is the key to successful multi-color imaging experiments.
In this new Nature Method article, Patriarchi et al. designed yellow-shifted and red-shifted dopamine sensors. Following their development of dLight1, a green fluorescent protein-based DA sensor, the team created YdLight1 – 525nm emission peak - and RdLight1 – 588nm emission peak. Exciting work!
More information can be found following this link: https://doi.org/10.1038/s41592-020-0936-3.
There was a long time ago when the first brain signal was recorded. Since then science has shown multiple ways to record brain activity in realtime. Somehow the public was shocked when Neuralink has demonstrated this last week. Probably because of Elon Musk the co-founder of Neuralink. Media follows Elon Musk's every step and big announcements. Proving that the implants work was a big step for Neuralink, although the technique was a long time ago described.