Singapore: Switching off specific brain regions in a
laboratory animal is an important type of experiment used to better
understand how the brain works. A study published in Nature Methods by Singapore-based researchers identified effective inhibitors of brain activity in the important animal model Drosophila melanogaster,
the common vinegar fly. These new tools are enabling researchers to
better understand the relationship between neural circuits and
behaviour, expanding our knowledge of the brain.
Neurons (brain cells) process information and
control behaviour by sending signals to other neurons,
hormone-releasing cells and muscles. A fuller understanding of the
neuronal control of behaviour would accelerate the development of
therapies for neurological and psychiatric disorders.
One of the ways researchers have tried to
understand the neuronal control of behaviour is with optogenetics, a
technique that uses light-sensitive proteins to control neuronal
activity in living tissue. In optogenetics, neurons are genetically
modified to express light-sensitive ion channels (proteins that conduct
electricity), such that light exposure may be used to activate or
inhibit electrical activity.
“There are many useful optogenetic tools to
stimulate neural activity but not as many effective inhibitors,”
explained Assistant Professor Adam Claridge-Chang, who led the research
at Duke-NUS Medical School (Duke-NUS) and A*STAR’s Institute of
Molecular and Cell Biology (IMCB).
Being able to inhibit neural circuits
provides researchers the ability to determine the importance of a
particular circuit in defining behaviour. In view of that, Asst Prof
Claridge-Chang with Dr Farhan Mohammad and other colleagues explored the
use of anion channelrhodopsins (ACRs) from an alga species (Guillardia theta) to inhibit neural activity.
In reading the paper that first described the
ACRs, Dr Mohammad realized that ACRs conducted more current compared to
other tools. “They are rapidly responsive, require low light
intensities for actuation, so they seemed ideal for inhibiting brain
activity in fly behaviour experiments,” said Dr Mohammad, a Research
Fellow in the Claridge-Chang group.
The group genetically modified flies to
express ACRs, and exposed these animals to light of different colours
and intensities. In one of the experiments, ACR actuation paralysed
climbing flies, causing them to fall abruptly. In another, illumination
of ACRs in the animals’ sweet-sensing cells resulted in flies that
avoided green light, as though they were avoiding the silencing of a
sweet taste. At the cellular level, light actuation of ACRs produced
dramatic reductions in electrical activity.
The work done at Duke-NUS and A*STAR’s IMCB
indicated that ACRs are highly effective optogenetic tools for the
inhibition of behavioural circuits.
“Since they are as powerful as existing methods, but much faster and easier to use, there has been huge interest from the Drosophila research
community in adopting these tools,” reported Asst Prof Claridge-Chang,
from the Duke-NUS Neuroscience and Behavioural Disorders Programme.
“They make testing which circuits are necessary for a particular
behaviour as convenient as testing for sufficiency.”
“Understanding any system is greatly aided by
being able to remove components from that system and examine the
resulting behaviour,” explained Asst Prof Claridge-Chang. “The ACRs are
the seventh generation of optogenetic inhibitors, but the first that
robustly inhibit Drosophila neuronal activity. Although our
study is just newly published, this new technique is already on its way
to becoming key tool for behaviour analysis.”
This research was supported by a Singapore
Ministry of Education Tier 2 project grant (MOE-2013-T2-2-054), project
grants (1231AFG030 and 1431AFG120) from the A*STAR Joint Council Office,
a Biomedical Research Council block grant to the Institute of Molecular
and Cell Biology and the Duke-NUS Signature Research Programme, with
funding from the Singapore Ministry of Health. Additional funding came
from A*STAR Scientific Scholars Fund, the National Research Foundation
Fellowship (NRF-NRFF2015-06), a block grant from the Temasek Life
Sciences Laboratory and the Duke-NUS PhD Programme in Integrated Biology
and Medicine.