Dawn of the DREADD
There are few things I find more satisfying than a good acronym, and DREADD or Designer Receptor Exclusively Activated by a Designer Drug has got to be my favorite. The name is just so evocative. Whenever I think about DREADDs, I find myself picturing some sort of Star Wars villain; maybe due to the incredibly lazy names the villains had in the prequel trilogy (General Grievous? Come on). Although it is certainly something of a mouthful, I hope to make it clear that despite this foreboding name, DREADDs are anything but dreadful, and are instead an extremely interesting and extremely powerful tool in modern neuroscience.
What is a DREADD?
To understand what DREADDs are all about, let’s tackle this frightening acronym word by word. Designer Receptors. OK, this part seems straightforward enough. A receptor is a protein that receives chemical signals via the binding of another molecule, termed the ligand. You may have heard the phrase “designer drugs” before and the “designer” in DREADD simply means that the receptor has been synthetically created via modifying an existing receptor. For instance, Bryan Roth and colleagues  created a designer receptor by expressing the human muscarinic (hM3) receptor in thousands of yeast cells. Next they mutated the hM3 receptor. These mutant hM3 receptors were then screened for their response to a drug. The scientists wanted the mutant hM3 receptor to not only be activated by this drug, but to be Exclusively Activated by it. Thus, the mutant hM3 receptor would no longer be able to bind with its native ligand, Acetylcholine, or any other known neurotransmitters. Instead, only the Designer Drug Clozapine-N-Oxide or CNO would activate the receptor, and what is more, CNO was chosen because it was known to be unable to bind to any existing receptors. Putting it all together, this new Designer Receptor would be Exclusively Activated by a Designer Drug.
The DREADD I just described, the modified hM3 receptor, when expressed in a neuron will function to increase the electrical activity of that neuron. If the DREADD is expressed in a specific brain region, or a unique cell type, CNO could be administered to effectively increase the activity of only that region or only that cell type. In addition to the excitatory hM3 DREADD, there are now many different types, including inhibitory DREADDs that will decrease the electrical activity of a neuron. Taken together, these excitatory and inhibitory DREADDs allow neuroscientists to ask a slew of fascinating questions.
Grateful for DREADDs
One of the driving goals in neuroscience is to determine how specific cells and neural circuits function to regulate behavior, physiology, and disease. Neuroscientists can use DREADDs to address these sorts of questions with exquisite specificity and control relative to what was previously available. Prior to DREADDs, if a scientist wanted to determine whether or not a region was important for memory they could either lesion it or administer a non-specific drug to increase or decrease the activity of that region. Though more temporally precise than a lesion, a pharmacological approach can lead to all sorts of off-target effects since a given drug will often interact not only with several different types or subtypes of receptors, but will also interact with those same receptors wherever they are expressed. This is particularly an issue in the brain because most neuronal receptors tend to be expressed in many different regions in both the brain and the periphery. Because DREADDs are activated only by CNO they avoid these problems. CNO, even when administered systemically, will not bind to any other receptors anywhere else in the body.
This spatial precision is an incredible boon to research, allowing scientists to investigate necessity and sufficiency of a brain region for some physiological response, behavior, or disease. If, for example, an inhibitory DREADD in the Orbital Frontal Cortex abolishes habitual behavior (like pressing a lever even when there is no reward), the scientist can conclude that the Orbital Frontal Cortex is necessary for habits . Similarly, if an excitatory DREADD is used to activate neurons that express Agouti-related protein (another great name) and this is found to cause an animal to eat, the scientist can conclude that neurons that express agouti-related protein are sufficient for this behavior .
How do DREADDs work?
Now, you may be wondering how it is that these DREADDs can be specifically expressed within a particular cell type or brain region. One method involves creating transgenic animals that will express the DREADD in a certain cell type. However, breeding transgenic animals takes a great deal of time and money and so, the most popular approach involves the use of viral vectors. Scientists can insert the gene that encodes for a DREADD (as well as a fluorescent tag so its expression and location can later be visually verified) into a virus. This virus is then surgically injected into a brain region where it will be taken up by neurons and begin expressing the DREADD (Figure 1). Cell-type specificity can be achieved by placing the DREADD gene under different promoters (see this post by Matt for a more in-depth description).
DREADDs and Optogenetics
Because DREADDs take advantage of genetic tools, the technology is sometimes referred to as chemogenetics. You may be familiar with the term optogenetics (see previous Neuwrite articles here and here), which is something like chemogenetics’ more popular cousin.
Optogenetics utilizes similar genetic techniques as DREADDs, but uses pulses of light to increase or decrease the activity of neurons rather than CNO. While both chemo- and optogenetics are extremely powerful tools that address similar questions, they differ in a few meaningful ways. Optogenetics has the advantage of much greater temporal control, since the technique relies upon pulses of light that move…at the speed of light. In contrast, DREADDs are a more blunt approach since CNO will remain in the brain for several hours. However, DREADDs have the advantage of being much easier to use and much less invasive relative to optogenetics. Optogenetics requires the surgical implantation of optical fibers for stimulation and often some sort of recording device which can cause unintended lesions. Additionally, the head mount for the optical fibers is quite bulky and can interfere with some sensitive behaviors. DREADDs require a much less invasive surgery or, if a transgenic animal is used, no surgery at all. Due to their relative ease of use, DREADDs are more suited for initial and exploratory studies attempting to figure out whether or not some region or cell type is involved in a behavior, while optogenetics is more suited for further studies that require more precision.
DREADDing the future?
DREADDs are more than just a cool name. They have enormous potential to help investigate and manipulate neural circuits. Indeed, DREADDs have already contributed to our understanding in vastly different fields, from how some neurons influence visual discrimination  to the circuits involved in the rewarding effects of addictive drugs .
Clearly, DREADDs are an extremely powerful tool for basic neuroscience research, but more than that, they have great potential as translational treatments in the future. For instance, a recent study found that chemogenetic inhibition of neural activity reduced plaque formation in a rodent model of Alzheimer’s Disease . Though this was a rodent study and human treatments are undoubtedly many, many years away, DREADDs are quite promising as possible therapeutics, especially considering that DREADDs have recently been successfully used in non-human primates . The specificity offered by DREADDs is a vast improvement over current drug treatments, which are about as specific as a shotgun. How important this is really cannot be overstated, as it would allow much more precise targeting of treatments and the elimination of nearly any side-effects. Significant caution is of course warranted since messing around with the genome can have horrific unintended consequences and, though it is a cliche, much more testing is needed to determine if DREADD technology can be safely and successfully used as a treatment. Although their therapeutic potential is tantalizing, even if DREADD technology never makes the jump to humans, it will remain an incredibly potent technology for unraveling the incredible intricacy of the brain.
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 Krashes MJ, Koda S, ChianPing Y, Rogan SC, Adams AC, Cusher DS., Maratos-Flier E., Roth BL., Lowell BB. 2011. Rapid, reversible activation of AgRP neurons drives feeding behavior in mice. J. Clin. Investig. 121(4):1424–28
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Yuan P., Grutzendler J. (2016) Attenuation of β-Amyloid Deposition and Neurotoxicity by Chemogenetic Modulation of Neural Activity. The Journal of Neuroscience. 36(2): 632-641. doi: 10.1523/JNEUROSCI.2531-15.2016.
Eldridge MA., Lerchner W., Saunders RC., Kaneko H., Krausz KW., Gonzales FJ., Ji B., Higuchi M., Minamimoto T., Richmond BJ. 2016. Chemogenetic disconnection of monkey orbitofrontal and rhinal cortex reversibly disrupts reward value. Nature Neuroscience. 19(1): 37-39. doi: 10.1038/nn.4192
Title Image: http://faculty.sites.uci.edu/mahlerlab/research/
DREADD Receptor: http://www.cell.com/cms/attachment/2007958605/2030599709/gr3.jpg
Figure 1: http://www.loviclab.com/Methods
DREADD publication graph: http://amaprod.silverchaircdn.com/data/Journals/NEUR/934619/s_nbs150004f1.png