Harvard: Artificially activating a neural link in mice can reduce eating without chronic hunger. Anyone who has ever tried to lose weight knows it’s no fun to feel
hungry. In fact, gnawing hunger pangs can sabotage even the
best-intentioned dieter. But how exactly is it that fasting creates
these uncomfortable feelings—and consuming food takes them away? Working to unravel the complex wiring system that underlies this
intense physiological state, Harvard Medical School investigators have identified a
long-sought component of this complicated neural network.
The team reports in Nature Neuroscience
that a melanocortin 4 receptor-regulated (MC4R) circuit serves as the
neural link that inhibits and controls eating. Their discovery shows
that this brain circuit not only promotes fullness in hungry mice but
also removes the almost painful sensation of grinding hunger, findings
that could provide a promising new target for the development of
weight-loss drugs.
The Roots of Hunger
Hunger ensures survival by signaling to the body that energy reserves are low and food is needed to avoid starvation.
“One reason that dieting is so difficult is because of the unpleasant
sensation arising from a persistent hunger drive,” explained the
study’s co-senior author Bradford Lowell,
HMS professor of medicine at Beth Israel Deaconess. “Our results show
that the artificial activation of this particular brain circuit is
pleasurable and can reduce feeding in mice, essentially resulting in the
same outcome as dieting but without the chronic feeling of hunger.”
The Lowell laboratory has spent the last two decades creating a
wiring diagram of the complex neurocircuitry that underscores hunger,
feeding and appetite. Their group, along with others, made the key
discovery that agouti-peptide-expressing (AgRP) neurons, a small group
of neurons located in the brain’s hypothalamus, detect caloric
deficiency and drive intense feeding.
“When these AgRP neurons are ‘turned on,’ either by fasting or by
artificial means, laboratory animals eat voraciously,” said Lowell.
AgRP neurons sense the low energy reserves, become activated and,
through the release of inhibitory neurotransmitters, suppress the
activity of downstream neurons, which are responsible for feelings of
fullness, or satiety. This causes hunger.
But to understand exactly how the brain regulates appetite, the
researchers needed to determine which neurons were downstream of the
AgRP neurons and which were actually causing the activation and
inhibition of hunger.
“Determining the identity of these ‘satiety’ neurons is the key to
establishing the blueprint for how the brain can regulate appetite,”
said Lowell. “It was an important missing point of connection in the
wiring diagram.”
Search for Satiety Neurons
Because AgRP hunger neurons release the AgRP peptide, which is known
to block melanocortin 4 receptors, Lowell and co-corresponding authors Michael Krashes, acting section chief of the Section on Motivational Processes Underlying Appetite at NIDDK, and Alastair Garfield hypothesized that these downstream satiety neurons must have many MC4Rs.
“Our previous work had revealed that out of all the millions of
neurons in the brain, this tiny group of MC4R-expressing neurons in the
paraventricular nucleus of the hypothalamus (PVH) were important for the
regulation of feeding behavior and body weight,” said Lowell.
With this in mind, the authors set about mapping and then
manipulating the activity of these PVH MC4R neurons and then observing
the feeding behavior of the mice.
In order to gain direct access to these MC4R neurons from within the
daunting tangle of neurocircuits, the authors used a group of
genetically engineered mice that selectively express the Cre recombinase
protein in these particular brain cells. This enabled the scientists to
map their connections and remotely control their activity.
Led by Garfield, a visiting fellow in the Lowell laboratory from the
Centre for Integrative Physiology at the University of Edinburgh, the
authors employed a chemogenetic technique known as DREADDs (Designer
Receptor Exclusively Activated by Designer Drugs) to specifically and
selectively control the activity of these PVH MC4R neurons in the Cre
recombinase mice.
“Although these mice had eaten a day’s worth of calories and were
fully sated, when we used DREADDS to turn off the PVH MC4R cells, they
began to ravenously consume food for which they had no caloric need,”
said Garfield.
The opposite also held true: Artificially activating these satiety neurons prevented unfed hungry mice from eating.
“Together these experiments suggested that PVH MC4R neurons function
like a brake on feeding and are essential to prevent overeating,” said
Garfield.
Much like an electronic circuit board, the brain is made up of
discrete nodes connected by intricate wiring. After the authors
determined that the PVH MC4 cells controlled satiety, Garfield said, the
next logical step was to identify the downstream node in the circuit
that was mediating this effect.
By injecting a tracer molecule into the PVH MC4R neurons, they were
able to visualize the cells in other areas of the brain that were
communicating with these cells. Their experiment revealed dense
innervation of an area in the back of the brain called the lateral
parabrachial nucleus (LPBN).
To probe the importance of this circuit in regulating appetite the
scientists used optogenetics, a technology enabling them to selectively
activate the subset of PVH MC4R neurons that target the LPBN by
delivering blue laser light via a glass optical fiber implanted in the
brains of the mice. As predicted, in the hungry mice, laser activation
of this explicit PVH MC4R → LPBN circuit engaged a “brake” that
significantly reduced the animals’ food consumption.
Confirming the Drive Reduction Theory
“There is a major hypothesis called ‘drive reduction’ that proposes
that you eat to get rid of the unpleasant feeling of hunger,” Lowell
said. “This is in contrast to other views that propose that you eat
because the taste of good food is rewarding. We, therefore, needed to
conduct a behavioral experiment that would tell us whether this PVH MC4R
→ LPBN circuit, which mice normally activate by eating, would be
pleasant if activated artificially, in the absence of any food being
consumed.”
To answer this question, co-senior author Michael Krashes and his
colleagues at the NIDDK developed a sophisticated behavioral experiment
in which laser stimulation served as a surrogate for eating food. The
experiment also enabled the mice to control the activation of their PVH
MC4R → LPBN circuit based on their spatial location.
“We built a two-chamber apparatus separated by a doorway through
which the hungry animals were free to move back and forth, as we tracked
their activity,” Krashes said. “If the animal moved into one chamber,
computer software triggered the delivery of the blue laser light, which
stimulated the animal’s PVH MC4R → LPBN brain circuit. But when the
animal returned to the other chamber, the laser switched off and the
circuit was no longer stimulated.”
The scientists followed the mice for 25 minutes to determine which
chamber the animals preferred. “Normal mice displayed no preference for
either chamber,” said Krashes, “but the genetically engineered mice,
which were able to stimulate the brain’s PVH MC4R → LPBN circuit,
greatly preferred the blue light-associated chamber, highlighting the
gratifying sensation that took away their bad feelings of hunger.”
Remarkably, Krashes said, when this experiment was repeated in mice
that had recently eaten a meal, and therefore were not experiencing the
negative feelings of hunger, turning on the satiety-promoting PVH MC4R →
LPBN circuit lost its positive value, and the animals no longer showed a
preference for the light-paired chamber.
“If the animals had found food in one particular chamber, then we
expect that they would have stayed in that chamber and would have eaten,
but since there was no food, they found another way of achieving the
same result by self-stimulating the satiety-promoting PVH MC4R → LPBN
circuit,” Krashes explained.
“Turning on the PVH-MC4R satiety neurons had the same effect as
dieting, but because it directly reduced the hunger drive it did not
cause the gnawing feelings of discomfort that often come with dieting,”
said Lowell. “Our findings suggest that the therapeutic targeting of
these cells may reduce both food consumption and the aversive sensations
of hunger—and therefore may be an effective treatment for obesity.”
This work was supported in part by the Boston Area Diabetes
Endocrinology Research Center; the University of Edinburgh Chancellor’s
Fellowship; US National Institutes of Health grants R01DK096010,
R01DK089044, R01DK071051, R01DK075632; R37DK053477, BNORC Transgenic
Core P30DK046200, BADERC Transgenic Core P30DK057521, F32DK089710;
K08DK0671561; R01DK088423 and R37DK0053301 and an American Diabetes
Association Mentor-Based Fellowship. This research was also supported,
in part, by the Intramural Research Program of the NIDDK, National
Institutes of Health (1ZIADK075087, 1ZIADK075088).