Neurons controlled by DREADD

16 07 2009

A big advance in non-invasive neuronal remote control was published today in Neuron. Several groups have been working on expressing non-endogenous or customized receptors into neurons so that specific genetically selected neurons can be turned on or off.  Channelrhodopsin, halorhodopsin and Opto-XRs do this via a light-gated membrane channels or receptors. Ligand-gated alternatives are the Drosophila allatostatin receptor, and RASSLs, GPCRs with customized binding sites. Each one of these has particular drawbacks.  The opsins require coupling of a light fiber into the brain, and high expression of some opsins can cause cytotoxicity.  The allatostatin ligand needs to be perfused directly into the brain. RASSLs show background activity in the absence of the applied ligand, which also can cause toxicity. Bryan Roth’s group has been pioneering RASSLs and has produced second-generation receptors which avoids these drawbacks.  In these new receptors, DREADDs, the background activity is completely abolished and the ligand has no off target effects.  

 

DREADDs have no effect on the spiking activity of the hippocampus in the absence of CNO (top). Subcutaneous injection of CNO causes bursts of action potentials in DREADD expressing hippocampus (bottom).

DREADDs have no effect on the spiking activity of the hippocampus in the absence of CNO (top). Subcutaneous injection of CNO causes bursts of action potentials in DREADD expressing hippocampus (bottom).

The DREADD, dubbed hM3Dq, in the paper, Remote Control of Neuronal Activity in Transgenic Mice Expressing Evolved G Protein-Coupled Receptors, allows selective activation of a genetically targeted population of neuron in a totally non-invasive way.  Simply inject the ligand, CNO, and the activity of the expressing neurons will rise in a dose dependent manner.  Onset is rather slow, starting around 10 minutes post-injection and peaking within 45 minutes.  Offset takes hours, so this isn’t the right technology to explore precise temporal coding of spike trains. But, when combined with the genetic targeting information from the Allen Brain Atlas, this tool will find great use in demonstrating the function of specific brain regions and even specific cell types within a brain region. 

The authors have also published an inhibiting DREADD, hM4Di, which can turn off targeted neurons.  I’ve personally tested a variety of neuronal silencing technologies in the last 6 months, including the hM4Di inactivating DREADD.  In in utereo electroporated cortical slices, the expression of these receptors had no discernible effect on the morphology, eletrophysiological parameters or cell health.  When CNO was puffed onto the slice, the amount of current injection required to elicit a spike doubled or tripled. CNO did nothing to non-expressing neurons. The cell returned to normal within seconds of washout of the drug.  I haven’t tested the hM3Dq activating DREADD, but from my experience with hM4Di, I highly recommend these tools for getting the control you want with minimal fiddling with light fibers or expression levels.


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4 responses

16 07 2009
Robert

I was just curious if there exists an ion protein that could be activated using a form of electromagnetic radiation that could penetrate the skull?

Something analogous to optogenetics except without the need to have a light source and could be activated non-invasively.

Also what do you think of the potential of ultrasound to manipulate brain chemistry?

16 07 2009
andrewhires

Great question re: electromagnetic stimulation.

Roger has been pitching this idea to new post-doc’s in the Tsien Lab for the past five years. In theory, you could engineer a very fast rectifying voltage-gated ion channel into the neurons of your choice, then apply a rapidly (~2000Hz) oscillating electric field. The channel will open and shut fast enough to drive positive ions into the cell (or negative ions out) through the channel, while the endogenous channels won’t gate fast enough for the field to effect the non-expressing cells.

In practice, this is a very, very difficult project. One post-doc, who ended up developing the ChIEF Channelrhodopsins, tried for his first year in the lab. He never saw conclusive evidence of current induction. You need to do a lot of protein engineering of the channel and a lot of electrical engineering for the electromagnetic field generation. Getting the field density high enough, oscillating that quickly requires massive power. I think it would take a serious collaborative effort between two specialist labs to make progress. Although there was a poster on this a few years ago at SFN… I’ll try to dig up the reference.

As for ultrasound, is the idea that you vibrate a brain region and it gets heated or excited, increasing spike rates? Ultrasound the raphe nucleus and have feelings of love and empathy wash over you? Maybe… Assuming you could drive activity in the first place, I’m not so sure the resolution of ultrasound would be high enough. I haven’t kept up with the ultrasound literature. I am planning rectify this by covering an interesting paper in Nature Photonics about deep tissue ”imaging” of fluorescent proteins using ultrasonic echos of the light absorbance in a coming post!

17 07 2009
Robert

I don’t know if you’ve read the paper or not about ultrasound neuromodulation. It’s open access.
http://www.plosone.org/article/info:doi/10.1371/journal.pone.0003511

I think it toggles open sodium channels due to mechanical energy. It has a better spatial targeting accuracy than deep brain stimulation and it can reach any brain region. I’m not quite sure what the resolution of stimulation is. They mentioned something about using protein specific ultrasound resonances in the paper, but that is more speculative.

16 09 2009
Light-switchable protein interactions « Brain Windows

[…] of genetic tools that control neural activity (Channelrhodopsins, Halorhodopsins, DREADDs) in functionally defined populations, such as neurons that are active during a particular task or […]

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