1. Neuroscience
Download icon

Optogenetics: Exciting inhibition in primates

  1. Wim Vanduffel  Is a corresponding author
  2. Xiaolian Li
  1. Department of Neurosciences, KU Leuven Medical School, Belgium
  2. Leuven Brain Institute, KU Leuven Medical School, Belgium
  3. Department of Radiology, Harvard Medical School, United States
  4. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, United States
Insight
  • Cited 0
  • Views 1,035
  • Annotations
Cite this article as: eLife 2020;9:e59381 doi: 10.7554/eLife.59381

Abstract

A new genetic marker enables precise control over a group of inhibitory neurons in monkeys.

Main text

At the age of 30, Louis Leborgne lost his ability to speak, and was only able to utter one syllable – ‘tan’. After his death 21 years later in 1861, a brain biopsy by his physician Paul Broca revealed that his inability to speak was caused by a lesion in the left frontal cortex, in a region of the brain now known as Broca’s area (Mohammed et al., 2018). More instructive though less known are Ernest Auburtin’s experiments on the exposed brain of a patient who had attempted suicide. When Auburtin softly pressed on specific parts of the frontal cortex, the patient lost his ability to speak, whereas his other cognitive functions remained unaffected (Stookey, 1963).

Whilst both studies showed the power of causally linking specific regions of the brain to specific behaviors, Auburtin’s experiments were more telling because of the reversible nature of his actions on an otherwise intact brain. However, these experiments – and many ‘causal’ techniques developed since then – lack spatio-temporal precision: that is, they cannot identify precisely where and when signals are produced or interrupted. Optogenetics overcomes these problems by using genetic manipulation or viral vectors to express light-sensitive proteins (such as opsins) in neurons: illumination with specific wavelengths of light can then induce neuronal responses that are precisely synchronized with the frequency of stimulation (Boyden et al., 2005). Moreover, specific cell types within a pool of diverse neurons can be precisely targeted, thereby achieving astonishing spatial resolution (Shemesh et al., 2017).

To reveal the causal relationship between a brain region and specific behavior, one can either increase local brain activity (Gerits et al., 2012), or decrease it (Afraz et al., 2015). Moreover, a region of the brain can be silenced by either inhibiting excitatory neurons or activating inhibitory neurons. A major drawback with the former approach, however, is that a period of silence induced by light is typically followed by a period of unwanted 'rebound' activity. Activating a local network of inhibitory neurons is more elegant because it takes advantage of natural inhibition in the brain and avoids the problem of rebound effects. This approach has proved successful in rodents, but efforts to translate it to primates have been limited. Recently, however, a new viral vector (Dlx5/6) that only targets inhibitory interneurons was demonstrated in multiple species, including primates (Dimidschstein et al., 2016).

Now, in eLife, Abhishek De (University of Washington), Yasmine El-Shamayleh (Columbia University) and Gregory Horwitz (University of Washington) report on the behavioral effectiveness of this approach in macaques (De et al., 2020). The researchers injected a Dlx5/6 vector carrying channelrhodopsin (an opsin that activates neurons) into the primary visual cortex of the primates and used immunohistochemical techniques to confirm that it primarily targeted inhibitory neurons that used the neurotransmitter GABA. When this region was illuminated via an optic fiber connected to an electrode, about two-thirds of the neurons increased their activity, whereas one third suppressed their activity. The latencies for the former group of neurons (that is, the interval between light stimulation and neuronal response) were short, which suggests that they were GABAergic interneurons that expressed channelrhodopsin. The latencies for the inactivated group of neurons were longer, indicating they were downstream from directly activated interneurons.

Therefore, using the new optogenetic technique to induce inhibition in the primary visual cortex (meaning this part of the cortex can no longer process visual information) should result in a temporary blind spot in the monkey’s visual field. To test this behaviorally, De, El-Shamayleh and Horwitz used a saccade test, during which they presented the monkey a point on a screen. When the position of the point overlapped with the receptive fields of the stimulated inhibitory neurons (blind spot), the monkeys were less likely to detect the stimulus (Figure 1). Also, performance on a contrast detection task was severely diminished when inhibitory neurons were activated. In this test, monkeys had to indicate (with eye movements) whether a contrast-adjusted faint stimulus was positioned either on the left or the right side of the screen. Optogenetic stimulation reduced their sensitivity to detect this cue on a grey background with the same mean luminance when the stimulus was placed inside the activated receptive fields. Thus, this experiment confirmed that optogenetics only affected the monkeys’ sensitivity to the visual stimulus, but not the ability to move their eyes.

Local visual sensitivity is reduced in macaques by activating inhibitory neurons.

A new optogenetics method can selectively stimulate inhibitory neurons and so reduce activity in a specific brain region. To test this method in macaques, De, El-Shamayleh and Horwitz injected the viral vector Dlx5/6 carrying channelrhodopsin – which activates inhibitory neurons (pink) – into the primary visual cortex of primates. Upon stimulation with light (torch), these inhibitory neurons were activated (top graph, blue line), subsequently leading to a deactivation of the excitatory neurons (grey: bottom graph, blue line). In a visual task, the monkeys were presented with a point that could appear randomly in different locations, and the subjects had to detect this point using an eye movement (green arrows). Activation of inhibitory neurons induced a blind spot in the visual field of the monkey; hence, monkeys were less likely to detect the point when it appeared in the blind spot (as indicated by the red cross). IPS: impulses per second (which corresponds to the firing rate of the neuron).

The work of De, El-Shamayleh and Horwitz constitutes another important milestone bridging the gap between research into the rodent brain and research into the monkey brain (Han et al., 2009; Gerits and Vanduffel, 2013), and it confirms that it is possible to successfully change behavior using selective activation of inhibitory neurons in primates (Dimidschstein et al., 2016). In the future, methods to reversibly control specific neurons in large brains with millisecond precision may become possible in monkeys (Marshel et al., 2019) and would allow to unravel the precise neuronal mechanisms underlying higher-order cognitive abilities unique to primates. Ultimately, it may be possible to recover lost brain functions in patients like Louis Leborgne, or patients with psychiatric disorders linked to a malfunctioning inhibitory circuitry of the brain.

References

Article and author information

Author details

  1. Wim Vanduffel

    Wim Vanduffel is in the Department of Neurosciences, KU Leuven Medical School, Leuven, Belgium, the Leuven Brain Institute, Leuven, Belgium, the Department of Radiology, Harvard Medical School, Boston, United States and the Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, United States

    For correspondence
    wim@nmr.mgh.harvard.edu
    Competing interests
    No competing interests declared
    ORCID icon "This ORCID iD identifies the author of this article:" 0000-0002-9399-343X
  2. Xiaolian Li

    Xiaolian Li is in the Department of Neurosciences, KU Leuven Medical School, Leuven, Belgium, and the Leuven Brain Institute, Leuven, Belgium

    Competing interests
    No competing interests declared
    ORCID icon "This ORCID iD identifies the author of this article:" 0000-0003-3648-7554

Publication history

  1. Version of Record published: July 1, 2020 (version 1)

Copyright

© 2020, Vanduffel and Li

This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited.

Metrics

  • 1,035
    Page views
  • 71
    Downloads
  • 0
    Citations

Article citation count generated by polling the highest count across the following sources: Crossref, PubMed Central, Scopus.

Download links

A two-part list of links to download the article, or parts of the article, in various formats.

Downloads (link to download the article as PDF)

Download citations (links to download the citations from this article in formats compatible with various reference manager tools)

Open citations (links to open the citations from this article in various online reference manager services)

Further reading

    1. Neuroscience
    Filip Sobczak et al.
    Research Article Updated

    Pupil dynamics serve as a physiological indicator of cognitive processes and arousal states of the brain across a diverse range of behavioral experiments. Pupil diameter changes reflect brain state fluctuations driven by neuromodulatory systems. Resting-state fMRI (rs-fMRI) has been used to identify global patterns of neuronal correlation with pupil diameter changes; however, the linkage between distinct brain state-dependent activation patterns of neuromodulatory nuclei with pupil dynamics remains to be explored. Here, we identified four clusters of trials with unique activity patterns related to pupil diameter changes in anesthetized rat brains. Going beyond the typical rs-fMRI correlation analysis with pupil dynamics, we decomposed spatiotemporal patterns of rs-fMRI with principal component analysis (PCA) and characterized the cluster-specific pupil–fMRI relationships by optimizing the PCA component weighting via decoding methods. This work shows that pupil dynamics are tightly coupled with different neuromodulatory centers in different trials, presenting a novel PCA-based decoding method to study the brain state-dependent pupil–fMRI relationship.

    1. Neuroscience
    Debora Fusca, Peter Kloppenburg
    Research Article

    Local interneurons (LNs) mediate complex interactions within the antennal lobe, the primary olfactory system of insects, and the functional analog of the vertebrate olfactory bulb. In the cockroach Periplaneta americana, as in other insects, several types of LNs with distinctive physiological and morphological properties can be defined. Here, we combined whole-cell patch-clamp recordings and Ca2+ imaging of individual LNs to analyze the role of spiking and nonspiking LNs in inter- and intraglomerular signaling during olfactory information processing. Spiking GABAergic LNs reacted to odorant stimulation with a uniform rise in [Ca2+]i in the ramifications of all innervated glomeruli. In contrast, in nonspiking LNs, glomerular Ca2+ signals were odorant specific and varied between glomeruli, resulting in distinct, glomerulus-specific tuning curves. The cell type-specific differences in Ca2+ dynamics support the idea that spiking LNs play a primary role in interglomerular signaling, while they assign nonspiking LNs an essential role in intraglomerular signaling.