YSM Issue 90.2

neuroscience
FOCUS
Researchers at the
Yale School of Medicine,
led by Ivan de
Araujo, associate professor
of Psychiatry, are looking
for answers. The team of
scientists identified a sub-region
of the amygdala, an olive-shaped
region of the brain, as the epicenter
for predatory hunting. The amygdala
has long been recognized as a contributor
to emotions like fear and aggression,
but these researchers dug deeper to understand
the molecular pathways which
drive predatory instincts. Shining flashes
of laser light into the mice’s brains, the researchers
could turn on or off the neurons
that fire when mice hunt. Accordingly,
the researchers could transform the mice
at will from passive creatures to gluttonous
predators. Through their findings, the
team of scientists uncovered clues relating
to the evolution of the brain.
Firing the Neurons
Previous studies showed that when rats
hunt insects, the central amygdala (CeA),
an almond-shaped sub-region of the amygdala,
surges in activity. This led the researchers
at Yale to question how the CeA of
predators’ brains coordinate the plethora of
muscles involved with hunting. “Predatory
behavior is a particularly complex task because
an animal needs a number of different
muscles to run, jump on things, and use its
head to kill,” De Araujo said.
First, the researchers injected the
central amygdala of the mice with a virus
that contained a gene encoding a
light-sensitive ion channel. Ion channels
allow positively charged sodium ions to
flow into the negatively charged axon of
the neuron. The change in charge distribution
along the length of the axon,
called depolarization, causes the neuron
to fire. Thus, the virus made neurons in
the CeA of the mice more prone to firing.
The signal traveled through the body until
it reached certain target muscles, such
as the jaw and neck muscles.
To control when the neurons fired, the
researchers inserted an optrode, an optic
fiber cable connected to an electrode.
Through this cable, the researchers could
send a specific wavelength of laser light
into the mice’s brains to turn on the neurons
involved when a mouse hunts. This
technique, called optogenetics, allowed
the researchers to use the energy from
the laser to manually stimulate the parts
of brains in mice which control hunting
and biting.
“Optogenetics has two great advantages.
First, you can target and study a specific
group of neurons. Second, the control
over when the neurons fire is specific. The
technique is very transient and very fast,”
said Wenfei Han, first author of the paper.
Optogenetic activation triggered only the
neurons injected with the virus to fire more
intensely according to electromyogram
(EMG) recordings, which measure electrical
activity in muscles.
Initiating the Hunt
When the CeA was stimulated using optogenetics,
the normally indifferent, docile mice
transformed into bestial predators. For example,
when a cricket was placed in the cage, the
mice captured and killed their prey in a much
shorter time than they did when the CeA was
not stimulated. The mice attacked even inanimate
objects; when a bottle cap was placed in
front of them, the mice grasped and bit the cap
as if it were prey. Such attacks were not observed
when the laser was off.
Upon laser stimulation, the mice attacked
the closest object they could find. When a food
pellet was placed in the opposite corner of the
cage and the laser was turned on, some mice
started to eat the cotton bed on which they were
resting before stimulation. In some cases, the
mice would grab onto the laser cable itself and
chew straight through it. Unsurprisingly, when
the cable split, the mice ceased hunting and returned
to their previous docility. Even when no
food was placed in the cage, the CeA-stimulated
mice positioned themselves in an eccentric
feeding position, tensing their hind legs and
holding their front legs to their mouths
as if they were eating. The researchers
called this phenomenon “fictive
feeding,” as it reflected the feeding
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