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In optogenetics, specific neurons are modified to produce light sensitive proteins that activate the cell when they are hit with light. The main drawback is that it is hard to get the light deep into the brain. Ultrasound, on the other hand, can reach everywhere — and if it is strong enough — directly activate anything. If there was a way to make ultrasound selective for some neurons, but not others, somebody might get rich.

Enter sonogenetics, 2015’s brand new neuroscience technique of the year. To use it, you simply apply the same bit of genetic engineering you would use to introduce light-sensitive proteins, but instead pop in vibration-sensitive proteins. If you are able do that, a wonderful thing can happen: provided you dial down the ultrasonic power to a nice safe level, a level well below the threshold that would normally get any neuron jumping, you can target just those neurons that you introduced to fancy mechanoreceptors.

The first researchers to completely grasp this promethean power whole in their minds, and actually pull off the technique in the lab, hail from California’s Salk institute and the UCSD. Writing yesterday in the journal Science, they took sonogenetics out for a test drive in the worm. To fully appreciate what the worm can now do for us, try to might imagine what it would be like to be on a first-name basis with all 100 billion or so of the neurons in your nervous system. If you tasked each neuron only with the job of being conscious of itself, you might just about be able to do it. Unfortunately, you probably couldn’t concentrate on much else.

The beauty of using the roundworm c. Elegans, is that the researchers who make a living studying it actually have an endearing name for every one of its 302 neurons. That kind of familiarity should never be underestimated. Not only that, researchers know exactly (or at least have a good idea about) the kinds of behaviors that each of those neurons control or detect. Typically these behaviors might take the form of combinations of a certain kinds of flexes or turns in response to a specific class of stimuli. To top things off, they also have at their disposal a variety of techniques to transfer almost any gene they desire into virtually any neuron, or subset of neurons, causing them to express the corresponding protein as if it were their own.

For the experiments here, several crafty genetic manipulations were required. First, in order to figure out how the worms were sensing the ultrasound, the researchers knocked out some of the usual suspects. These worms are known to express a certain class of mechanosensitive ion channels, namely TRP-4, in just six neurons: Four sensory neurons, and in two so-called interneurons that deal exclusively in dopamine. Dopamine isn’t really critical here; we just mention it cause some folks get excited about it. These TRP-4 channels, like many typically mechano-sensitive ion channels, are normally embedded in the outer plasma membrane of the cell. When they get stretched, they open and can ultimately ‘fire’ the cell.

When the researchers then hit the worms who lacked the TRP-4 with ultrasound, the worm’s response significantly decreased. This response was still there at a low level, but it dropped enough to lead them to believe that TRP-4 is a major player. To prove it, they expressed the protein in key sensory neurons that normally lack the protein.

Lest anyone doubt that these kinds of manipulations are little short of magic, the researchers took things up a notch. They could already determine that the worms detected the ultrasound because those newly minted mechanosensitive neurons ended up generating the same kinds of behaviors that they normally do for other stimuli. To then quantify these effects at the subcellular level, the researchers beefed up these same neurons with special calcium-sensitive proteins that visibly fluoresce (at least when seen in a fast and sensitive scope) when activated.

There is one wiggle in all this, which we left for the end. That’s because the main bug in everything above is also its main feature. In order to efficiently couple the ultrasonic power into the worm body — and by couple we also mean any amplification or focusing therein — the worm was embedded within a sea of perfluorohexane microbubbles. These bubbles, just a few microns in diameter, are standard ultrasonic accessories that can do a lot of things in addition to simply enhancing contrast. When you hit them with the right frequency, power, and peak negative pressure (refraction), they can flex and resonate in sync. Optimal bubble expansions were found with peak pressures of around a MPa for 10 ms pulses at a frequency of 2.25 MHz.

Above pressures of 2.5 MPa it was found that inertial cavitation, and the subsequent shockwaves that followed, compromised cell membrane integrity. However, peak pressure isn’t the only thing that can bring a cell down. From a power point of view, the researchers were cautious not to exceed tissue temperature limits. Using thermocouple sensors, they found temperature increases of less than 0.1 °C, which they suggest the worms are unlikely to feel. That’s good for several reasons: it’s nice to know that neurons won’t be cooked to death, and its also critical to know that the worms were responding to the mechanical effects rather than thermal effects.

Now, the feature we mentioned, is that if you flip the microbubble geometry — in other words put the microbubbles into the animal instead of the animal into the microbubbles — you get a reversible way to turn the whole thing on. By injecting the bubbles into the bloodstream, perhaps even human bloodstreams, you get a window of around 60 minutes to play. The researchers probably are not in any position to make claims about what happens to all the microbubbles. However, if it is presumed that they are confined to capillary beds, one might estimate their range for activating neurons.

In fact, the authors say that since neurons roughly 25 μm beneath the cuticle were activated through another 0.5 μm of worm skin, mechanical deformation should have ample penetrance. With average distances of a mere 20 μm for most of our own neurons from a capillary, we might be optimistic the technique could be extended to higher creatures like us.