A study by researchers in Singapore has identified optogenetic inhibitors of brain activity and successfully demonstrated their efficacy in a Drosophila melanogaster fruit fly model, which should improve our understanding of the relationship between neural circuits and behavior.

"There are many useful optogenetic tools to stimulate neural activity, but not as many effective inhibitors," lead researcher Adam Claridge-Chang, an assistant professor at Duke-National University of Singapore Medical School's (Duke-NUS) Neuroscience and Behavioural Disorders Programme and A*STAR's Institute of Molecular and Cell Biology, told BioWorld Today. This is of fundamental importance, he added, since "by inhibiting a circuit, we can find out what the role of that circuit is in normal brain function."

An improved understanding of neuronal control mechanisms, in particular how neurons control behavior by signaling to other neurons, hormone-releasing cells and muscles, might accelerate the development of new therapies for neurological and psychiatric disorders, the researchers reported Jan. 23, 2017, in Nature Methods.

"These tools are particularly useful for understanding psychiatric disorders like depression, anxiety and addiction, but have applications across all of neuroscience," said Claridge-Chang.

Optogenetics is a technique whereby light-sensitive proteins are used to control neuronal activity in living tissue, providing a useful way to improve our understanding of the neuronal control of behavior.

In optogenetics, neurons are genetically modified to express light-sensitive ion channels, which are proteins that conduct electrical signals across membranes, such that light exposure can be used to activate or inhibit electrical activity. (See BioWorld Insight, Sept. 30, 2013.)

The ability to inhibit neural circuits enables researchers to determine the importance of a particular circuit in defining behavior. Therefore Claridge-Chang and colleagues explored the use of anion channel rhodopsins (ACRs) from the alga species Guillardia theta to inhibit neural activity.

Rhodopsins are light-sensitive receptor proteins involved in visual phototransduction in many species, for example, the rhodopsins in human eyes that function as visual pigments. For algae, which are photosynthetic organisms, "it is important that they are responsive to sunlight; the ACRs would form part of a light detection system," explained Claridge-Chang.

"When the ACRs were first discovered, it was clear that these new channels were a very promising new [optogenetic] tool. However, there have been six generations of inhibitory tools that have looked promising in cultured cells, but have not been so useful in Drosophila, an important biomedical animal. So it was important to prove they were effective in these flies."

The ACRs conduct more current than these earlier generations of inhibitors. "They are rapidly responsive, require low light intensities for actuation, so they seemed ideal for inhibiting brain activity in fly behavior experiments," said Farhan Mohammad, a research fellow in Claridge-Chang's laboratory at Duke-NUS.

The group genetically modified D. melanogaster to express ACRs, and exposed the genetically modified flies to light of different colours and intensities.

In one such experiment, ACR actuation paralyzed climbing flies, making them fall. In another, illumination of ACRs in the animals' sweet-sensing cells resulted in flies that avoided green light, as if they were avoiding the silencing of a sweet taste.

At the cellular level, light actuation of ACRs produced dramatic reductions in electrical activity. "We made electrophysiological recordings from a fly nerve," said Claridge-Chang. "Illumination of flies carrying an ACR silenced electrical activity in the nerve. This was significant, as it shows that an absence of electrical activity is the best way to prove that cells have been silenced."

Taken together, these findings indicate that ACRs represent highly effective optogenetic tools for the inhibition of behavioral neuronal circuits.

"Since they are as powerful as existing non-optogenetic methods, but much faster and easier to use, there has been huge interest from the Drosophila research community in adopting [ACRs]. They make testing which circuits are necessary for a particular behavior as convenient as testing for sufficiency," said Claridge-Chang.

"Behavioral sufficiency can be proven for a [neural] circuit by activating that circuit and showing that this activation produces a behavior. For example, if activating a circuit makes an animal jump, then it is reasonable to say that activity in this circuit is 'sufficient' for the jumping behavior," the neuroscientist explained.

"Understanding any system is greatly aided by being able to remove components from that system and examine the resulting behavior," he said, noting "the ACRs make testing the necessity of circuits much more routine."

Indeed, "the ACRs are the . . . first [optogenetic inhibitors] that robustly inhibit Drosophila neuronal activity. Although our study is just newly published, this new technique is already on its way to becoming a key tool for behavior analysis." Claridge-Chang noted.

"Since this is such a fundamental, important type of experiment, [the use of the ACRs as effective optogenetic inhibitors] will accelerate neuroscience research in many different areas of preclinical and basic brain research."