Nature: Why does infection make people have no love? The relevant neurons have finally been found!

*For medical professionals only

Chills and fever, lack of appetite, and general weakness do not want to move, I believe that everyone has experienced these symptoms when they are infected and sick, and these reactions are called disease behaviors [1].

Although these symptoms are "sinful", they play a vital role in fighting infections and restoring physical function [2].

Obviously, disease behavior is driven by the brain, but how does the brain know that there is an infection in the body?

Recently, Jeffrey M.
Friedman's team from the Howard Hughes Medical Institute at Rockefeller University published a blockbuster study in the journal Nature[3] that revealed the neural mechanisms
by which the brain produces diseased behavior after infection.

They injected the animals with lipopolysaccharides (LPS, the main component of bacterial endotoxins) to mimic bacterial infections and found that some neurons in the solitary bundle nucleus (NTS) and posterior region (AP) of the brainstem were visibly activated
.

Further studies found that such neurons are characterized by the expression of ADCYAP1 (a neuropeptide), inhibition of ADCYAP1 neurons can alleviate the disease behavior of mice after LPS injection, while activation of ADCYAP1 neurons triggers disease behavior in healthy mice without infection or injection of LPS, indicating that ADCYAP1 neurons play an important role
in the production of disease behavior.

Screenshot of the first page of the paper

Let's take a look at how this study unfolded
.

First, the researchers simulated the infection
by injecting LPS into mice.
Although injecting LPS does not fully mimic all aspects of bacterial infection, Shengcheng can control the condition of animals well and minimize
differences between animals.

Injection of three doses of LPS (0.
1 mg/kg, 0.
5 mg/kg, 2.
5 mg/kg, lethal dose of 10 mg/kg) can cause mice to develop diseased behavior and a dose-response dependence
.
After 15 min of LPS injection, the mouse eating, drinking and activity were significantly reduced, while eating, drinking and activity
were almost completely stopped after 1 day of injection.

Injection of LPS caused mice to develop diseased behaviors and reduced eating, drinking and activity

At the same time, injecting LPS can reduce the core body temperature of mice in the medium to high dose group, but the effect is not obvious
in the low dose group.
After taking it all together, the researchers used the medium dose (0.
5 mg/kg) in subsequent experiments to conduct the study
.

Next, the researchers looked for activated neurons by staining FOS of the brain-complete tissue of mice after injecting LPS
.
The results showed that compared with the control group, many brain regions were activated after LPS injection, and the brainstem vagus dorsal complex neuron activation composed of AP (incomplete blood-brain barrier, can be exposed to related substances in circulation), NTS (vagus nerve input nucleus) and dorsal motor nucleus of the vagus nerve was very significant, and the strength and number of FOS-expressing cells in these three parts were very significantly increased (NTS was 8 times, AP was 32 times, and the dorsal motor nucleus of the vagus nerve was 10 times).

。

After LPS injection, the activation of neurons in the dorsal complex of the brainstem vagus nerve in mice was remarkable

To clarify the role of NTS and AP in post-infection disease behavior, the researchers injected AAV-DIO-Gq-mCherry into the NTS-AP region
of TRAP2 mice (expressing triphenoxylamine-induced Cre recombinase that knocks into the Fos gene locus).

3 weeks after the injection of AAV, simultaneous injection of 4-hydroxytriphenylamine and LPS enables Cre to be expressed transiently (about 6 h) in FOS-expressing neurons, thereby activating DREADD
in those cells activated by LPS.

After a 3-week recovery period, clonzapine (CNO, DREADD ligand) or intra-physiological saline intraperitoneal injection was given to measure changes in rats in
feeding, drinking, exercise capacity, body temperature and body weight.

Neurons expressing FOS in the NTS-AP region are activated by chemical genetics

The results of the study showed that within 30 minutes after the injection of the CNO, the neurons expressing FOS in the NTS-AP region were reactivated, significantly reducing the intake, water intake, and exercise of
the mice.

12 h after CNO injection, mice barely eat, drink or exercise
.
At the same time, the body temperature and body weight of the mice also decreased
after the CNO injection.

These findings suggest that neurons in the NTS-AP region can indeed trigger disease behavior
.

Activation of neurons in the NTS-AP region can induce disease behavior

To identify specific populations of neurons that may mediate disease behavior, the researchers sequenced neurons in NTS and APs activated after LPS injection, and compared them with neurons that were not injected with LPS, and found 8 neuron populations
activated by LPS.

8 populations of 8 neurons activated by LPS were found by snRNA sequencing

Through the selective regulation of these neuronal populations, the study found that one of these types of neurons, which are characterized
by the expression of ADCYAP1 (neuropeptide molecule), plays a key role in regulating disease behavior.

Inhibition of ADCYAP1 neurons can alleviate disease behavior in mice after LPS injection, while selective activation of the ADCYAP1 neuron population can induce disease behavior in healthy mice without infection or injected LPS, which means that ADCYAP1 neurons play an important role
in the neural mechanisms of disease behavior.

Interestingly, although inhibition of ADCYAP1 neurons can alleviate certain disease behaviors (e.
g.
, feeding, drinking, exercising), it does not affect core body temperature, which may be mediated
by other neurons.
Earlier, in addition to NTS and AP, many other brain regions are activated by LPS stimulation, including the hypothalamic preoptic region (which is involved in thermoregulation
).

Earlier this year, another team of researchers also published research in the journal Nature and found that the endplate vasculature (OVLT) and surrounding ventral medial pre-optic region (VMPO) in the hypothalamus play an important role in regulating core body temperature, warming behavior, and appetite after LPS stimulation[4].

The immune signals generated by infection can cross the blood-brain barrier, triggering the local production of corresponding molecules that promote the activation
of VMPO and OVLT neurons.

Combining the two studies, perhaps these two brain regions—AP and NTS in the brainstem, and OVLT and VMPO in the hypothalamus—have complementary or overlapping roles in controlling disease behavior, with the former contributing to decreased exercise and the latter contributing to changes in body temperature, while both controlling food and water intake
.

Interestingly, AP and OVLT are two of the few brain regions in the brain that lack a blood-brain barrier, which allows relevant signals produced by pathogens such as bacteria in the body to directly contact APs and OVLTs and interact with local neurons, inducing immune signals in the brain and mediating related immune responses [5].

It has been previously reported that both AP and OVLT regulate both food and water intake, as well as cardiovascular function, sleep, and body temperature [6
].
Therefore, it is not surprising
that disease behavior is mediated by multiple brain regions together.

Pathogen-induced disease behavioral mechanism map

Overall, the study discovered the neural mechanisms that reveal LPS-induced disease behavior and identified ADCYAP1 neurons, a subclass
of neurons that are important for inducing disease behavior.
It is possible that the substances produced by pathogens are activated through an incomplete blood-brain barrier or activation of peripheral vagus or spinal nerves, and neurons in NTS and APs, thereby inducing disease behavior
.

But the study did not reveal the regulatory mechanisms of ADCYAP1 neurons downstream after activation in NTS and APs, as well as the duration of disease behavior
.
At the same time, does this neural circuit play a similar role in regulating symptoms caused by viral infection? These are all things that need to be explored further in future research
.

References

1.
Konsman JP, Parnet P, Dantzer R: Cytokine-induced sickness behaviour: mechanisms and implications.
Trends Neurosci 2002, 25(3):154-159.

2.
Harden LM, Kent S, Pittman QJ, Roth J: Fever and sickness behavior: Friend or foe? Brain Behav Immun 2015, 50:322-333.

3.
Ilanges A, Shiao R, Shaked J, Luo J-D, Yu X, Friedman JM: Brainstem ADCYAP1+ neurons control multiple aspects of sickness behaviour.
Nature 2022.

4.
Osterhout JA, Kapoor V, Eichhorn SW, Vaughn E, Moore JD, Liu D, Lee D, DeNardo LA, Luo L, Zhuang X et al: A preoptic neuronal population controls fever and appetite during sickness.
Nature 2022, 606(7916):937-944.

5.
Daneman R, Prat A: The blood-brain barrier.
Cold Spring Harb Perspect Biol 2015, 7(1):a020412.

6.