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.
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.