60 THE SCIENTIST | the-scientist.com
LAB TOOLSVSEVOLOD BELOUSOV, RUSSIAN ACADEMY OF SCIENCESinvisible infrared range. His team tested
the method by directing the infrared
beam at embryos embedded in agarose.
They are now developing ways of using
this system in actively behaving zebra-
fish larvae (Nat Comms, 8:15362, 2017).WARMING UP TO MAMMALS
Optogenetics is widely used in mammals,
but its invasiveness and the low conduc-
tance of channelrhodopsins—the pho-
toreceptive channel proteins that are
modified for use in optogenetics stud-
ies—have led researchers to explore
thermogenetics.
ThermoTRPs have a much higher con-
ductance, turning on neurons with little
external stimulation. But finding a thermo-
TRP that works at around 37°–38° C, the
physiological temperature of mammals,
has so far been challenging. Neither rat
TRPM8, nor fly TRPA1 nor rattlesnake
TRPA1 can be used in mammals because
their activation thresholds are far below
mammalian body temperatures.
TRPA1 from the rat snake, however, is
active around 38.5° C, which is fairly close
to mammalian brain temperature, says
Belousov. His lab reported that this channel
could activate cultured rodent neurons (NatComms, 8:15362, 2017), but its performance
in behaving mice remains to be shown.
Scientists have also used rat TRPV1 to
activate cultured mammalian cells. Even
though the temperature at which half the
TRPV1 channels are activated is thought
to be around 42° C, Arnd Pralle, a bio-
physicist at the University at Buffalo, New
York, successfully used TRPV1 at 39° C
to activate neurons in the motor cortex,
dorsal striatum, and in the ridge between
dorsal and ventral striatum of freely mov-
ing mice. At that temperature, only about
15 percent to 20 percent of TRPV1 chan-
nels open, but the resulting calcium cur-
rent is enough to turn on the neurons
(eLife, 6:e27069, 2017).
“You’re not trying to keep the temper-
ature at 43° C for several minutes,” says
Polina Anikeeva, a neural nanotechnol-
ogist at MIT. “Ultimately, for all of the
methods, the name of the game is trying
to get a really short spike and then imme-
diately turning off your stimulus.”Unlike in fruit flies and zebrafish, of
course, ambient warming doesn’t raise the
temperature of warm-blooded animals.
The field desperately needs new ways to
wirelessly deliver energy deep into the tis-
sue, says Belousov. The focused infrared
beam that his team developed works as
a vehicle for thermal delivery at single-
cell resolution until about 2–3 mm deep
into the brain by adjusting wavelength
and pulse duration. But stimulating areas
deeper in the brain still requires the surgi-
cal implantation of fibers.
Although limited, infrared radiation
is still absorbed by the body, curbing pen-
etration depth, says Anikeeva. “The only
field that our body truly does not couple
to is magnetic.”
Pralle, Anikeeva, and others are
therefore developing “magnetothermal”
approaches, in which magnetic nanopar-
ticles are injected into the brain and acti-
vated by a high-intensity magnetic field. The
neuro-localized magnets then convert that
localized magnetic energy into heat, which
in turn activates TRPV1 channels. Using this
technique, Pralle’s team was able to tweak
the brain regions responsible for walking,
rotational, and freezing behavior in unteth-
ered mice (eLife, 6:e27069, 2017).
Recently, Pralle’s team demonstrated
the use of a thermosensitive chloride
channel, anoctamin 1 (TMEM16A), to
silence neurons in rat hippocampal cul-
tures using magnetic nanoparticles. The
channel’s big benefit is that upon activa-
tion, around 38°–39° C, it inhibits neu-
rons (Front Neurosci, 12:560, 2018).
“The end goal is really to use all these
techniques to turn different regions of the
brain on and off and analyze the circuits
that play together,” says Pralle. gRED HEAT: These mouse primary embryonic
neurons glow red because they are expressing
the rat snake thermoreceptor TRPA1 tagged with
a red fluorescent protein.Ultimately, for all of the methods, the name of the game is
trying to get a really short spike and then immediately turning
off your stimulus.
—Polina Anikeeva, MIT