A Brainstem Circuit Amplifies Aversion

Neurology Clinical Psychology Life Sciences Medicine
brainstem circuit aversive value fear learning opioid withdrawal GABA neurons

A Mechanism of Brainstem Circuit Enhancing Aversion Response

Background and Motivation

Aversion response is a natural reaction in humans and animals when faced with threats or unpleasant stimuli, allowing individuals to avoid danger and playing a critical role in adaptation during evolution. However, excessive aversion can lead to a range of emotional disorders, such as depression, anxiety, bipolar disorder, and post-traumatic stress disorder (PTSD). The dynamic control and regulation of aversive signals help individuals adapt to environmental threats and adjust behavioral responses accordingly. However, the neural circuits and mechanisms that amplify aversion responses are still poorly understood. Previous research has primarily focused on the amygdala and its associated brain regions in controlling aversion and negative emotions, but activation of the amygdala often directly induces fear and anxiety behaviors rather than merely amplifying aversive signals. Therefore, finding a neural circuit capable of precisely regulating the intensity of aversion responses is of significant importance.

In this study, a team of scientists, including Liang Jingwen, Zhou Yu, and Feng Qiru, revealed a novel aversion signal amplification circuit based on the neural connections between the Interpeduncular Nucleus (IPN) and the Nucleus Incertus (NI). Their research found that the activation of GABAergic neurons in the IPN was positively correlated with the intensity of the aversion response and regulated aversion responses in fear learning and opioid withdrawal through pathways connecting to the NI. This study provides new possibilities for future development of intervention methods for emotional disorders and relapse in drug dependency.

Research Origin

This study was jointly conducted by scientists from the Chinese Institute for Brain Research (CIBR), Peking University, Hainan University, and Huazhong University of Science and Technology. The primary authors are Liang Jingwen and Zhou Yu, and the research findings were published in the November 6, 2024, issue of the journal Neuron.

Research Process and Methods

1. Experimental Subject and Labeling Method

To reveal the role of the IPN-NI circuit in aversion responses, the research team used Pax7-CreER transgenic mice to induce the expression of green fluorescent protein (EGFP) through tamoxifen, accurately labeling the GABAergic neurons in the IPN. This allowed researchers to precisely locate and manipulate Pax7 neurons in the IPN and further mark and control neuron function using adeno-associated virus (AAV) vectors. This method ensured the specificity of the experiment, enabling the research team to eliminate cross-reactions with other neurons.

2. Fiber Photometry and Neuronal Activity Detection

Researchers used fiber photometry technology to record calcium ion (Ca2+) signals in mice during free movement, observing the reaction of IPN neurons under aversive stimuli. Through Pavlovian conditioning experiments, researchers subjected mice to different intensities of electric shocks and bitter solutions (quinine) to observe the reaction intensity of IPN neurons to these aversive stimuli. The experimental results indicated that the reaction intensity of IPN neurons was positively correlated with the degree of aversion (such as shock intensity and quinine concentration), demonstrating that IPN neurons not only encode the occurrence and association of aversive events but also reflect the intensity of the stimuli.

3. Causal Analysis of Aversion Response Amplification

To verify the role of IPN neurons in aversion responses, the research team employed a loss-of-function strategy, introducing Caspase-3 into the IPN through AAV vectors to selectively ablate Pax7 neurons. The results showed that mice with these ablated neurons exhibited weaker aversion responses under aversive stimuli. Additionally, the research team utilized DREADDs technology for chemogenetic activation of IPN neurons, finding that activation of Pax7 neurons led to stronger aversion responses to mild electric shocks in mice, but this activation did not directly induce anxiety behaviors. The above results confirmed the importance of IPN neurons in aversion responses.

4. Downstream Pathways from IPN to NI

Through cell type-specific sparse labeling and micro light-sheet scanning tomography, researchers successfully reconstructed the morphology of neurons projecting from the IPN to the NI, observing that these neurons mainly projected to the NI pars compacta (NIC) and NI pars dissipata (NID). To verify the synaptic connections between the IPN and the NI, researchers used whole-cell patch-clamp recordings and found that optogenetic activation of IPN neuron axon terminals could evoke postsynaptic currents in NI neurons, further proving that IPN neurons formed functional monosynaptic connections in the NI. Additionally, experiments showed these connections were primarily GABAergic inhibitory connections, confirming that the IPN, through its GABAergic projection neurons, regulates the activity of NI neurons.

5. Regulation of Aversion Response in Opioid Withdrawal

To further explore the role of the IPN-NI pathway in opioid withdrawal, the research team conducted a naloxone-induced morphine withdrawal experiment, recording significant increases in Pax7 neuron activity in the NI. Additionally, mice with ablated IPN neurons projecting to the NI showed significantly weakened aversion responses without affecting responses to rewards, indicating that activation of IPN neurons amplified aversion memory and learning induced by withdrawal.

Research Results and Key Findings

This study revealed the amplifying role of IPN-Pax7 neuron activity on the intensity of responses to aversive stimuli. The activity of IPN neurons is closely associated with the aversive value of stimuli and regulates fear learning and withdrawal aversion responses through downstream pathways connecting to the NI. In fear conditioning experiments, the activity of IPN neurons not only increased under conditioned stimuli but also remained active during the extinction phase after the shocks stopped. The role of IPN-Pax7 neurons in aversion responses also applies to opioid withdrawal contexts.

Specifically, projections of IPN-Pax7 neurons to the downstream NI enhance aversion responses, causing mice to exhibit stronger avoidance behavior when faced with mild aversive stimuli (such as mild electric shocks or quinine solutions). By chemogenetically activating these neurons projecting to the NI, mice’s freezing responses were significantly increased. This effect was not only apparent during the conditioning stage but remained effective during subsequent memory consolidation and expression stages.

Research Significance and Application Value

This study provides new insights into the neural mechanisms of aversion response amplification, revealing the role of the IPN-NI pathway as an amplifier of aversive signals. This pathway not only regulates the amplification of aversive signals under normal circumstances but also promotes aversion memory and learning in opioid withdrawal contexts, demonstrating its critical role in withdrawal. The above research offers potential targets for future treatments of emotional disorders and relapse in drug addiction.

Moreover, the aversion amplification effect of IPN-Pax7 neurons in withdrawal contexts shows extensive clinical prospects. Particularly in the process of opioid withdrawal, the excessive enhancement of aversion signals often hinders addicts from overcoming drug dependence. Therefore, specific regulation of the IPN-NI pathway may provide a new therapeutic approach to help addicts in withdrawal.

Research Highlights

  1. Novel Aversion Signal Amplification Circuit: The IPN-NI pathway is revealed for the first time as an amplifier of aversive signals, providing a new perspective on the neural mechanisms regulating aversion responses.
  2. Specific Aversion Enhancement Function: The projection of IPN neurons through the NI selectively amplifies aversive signals without directly inducing general anxiety or changes in activity.
  3. Regulation of Withdrawal Aversion: IPN-Pax7 neurons have an amplifying role in the aversion responses of opioid withdrawal, highlighting their critical role in the withdrawal process.
  4. Potential Clinical Application Value: The IPN-NI pathway may become a potential target for future treatment of emotional disorders and addiction relapse, offering new possibilities for intervention in emotion and drug dependence.

Summary

Research by Liang Jingwen and colleagues reveals the unique role of brainstem circuits in the amplification of aversion responses. By regulating the IPN-NI pathway, the response intensity to aversive stimuli in mice is significantly enhanced. This study provides important evidence for understanding the neural mechanisms of aversion responses and their applications in drug withdrawal and emotional disorders, proposing the possibility of regulating aversion memory and learning through the IPN-NI circuit, paving new directions for future treatments of emotions and addictions.