How nerve cells communicate in the fear network
It is the day of my PhD defense. Everyone came: Family, friends, colleagues, my doctoral supervisor and the other reviewers. They are sitting in the auditorium, curiously waiting to hear about my three-year research project. Facing them, I go weak at the knees. My heart is beating fast, I am breathing rapidly and my hands are clammy and a bit shaky. What I am experiencing here, with the adrenaline pumping through my body, is known as the autonomic stress response or – to put it simply – fear.
A network in the brain regulates fear and anxiety
A whole network of structures is active in my brain right now: the so called fear network. One important and particularly well investigated part of it is the amygdala. The amygdala is located laterally in our temporal lobe and receives information about all sensory stimuli, such as facial expressions or the voices of the reviewers. The amygdala’s job is now to emotionally evaluate these sensory inputs. Do I perceive the facial expressions as friendly, neutral or critical? It also stores negative experiences and associations; I have had controversies with one of the reviewers in the past. If all goes to plan, the amygdala triggers the stress response described above.
Another important component of the fear network is the midline thalamus, which is located right in the middle of our hemispheres. It receives information about the emotional state of our body or our “state of mind”. That could, for instance, mean information about our stress level, which the midline thalamus then forwards to the amygdala. However, compared to the amygdala, the exact role of the midline thalamus in fear – or emotional behavior in general – is less clear.
What is certain, though, is that once the fear network is thrown off balance, fear and anxiety become a problem. They can manifest in panic attacks or as post-traumatic stress disorder. It is, therefore, important to understand how the fear network in the brain works and how it is regulated. In my research, I specifically investigated how information is conveyed between the midline thalamus and amygdala and whether this process is influenced by specific anxiolytic substances, namely opioids.
Exploring how neurons communicate in a network: optogenetics and patch-clamp
In the brain, communication does not occur through WhatsApp voicemail, but through electric impulses and chemical messengers (neurotransmitters). In simplified terms, imagine two neurons, where one is a sender and the other a recipient. When a neuron is active, meaning when it generates electrical impulses, it can send messages to downstream neurons via a neuronal wire, the axon, which relays the electrical impulse to the contact site near the recipient neuron. When this happens, chemical transmitters are released from the end of the sender neuron to the start of the recipient neuron, which receives the forwarded message. The message can read “Quiet!” or “Pass it on!” depending on the transmitter that is released. Neurons sending “Pass it on!” messages use the activating transmitter glutamate, while neurons sending “Quiet!” messages use the transmitter GABA, which can inhibit the recipient neuron. Now, we expand our view – communication in the brain involves more than one sender and one recipient neuron. Rather, the brain is a complex network of about 86 billion neurons. In the mouse brains that I explore to answer my research questions, there are 71 billion neurons. How can I listen only to the conversation between the thalamus and amygdala neurons?
This is were modern optogenetic techniques come into play. They allow me to modify the sending neurons in the mouse’s midline thalamus such that I can activate them externally through light flashes. In this way, I can experimentally make neurons send massages. As I assumed that the neurons in the midline thalamus and amygdala are interconnected, I tried to detect messages from the thalamus in the amygdala neurons. Therefore, I used a slice preparation from the mouse brain and identified single neurons in the amygdala, into which I inserted a super thin recording electrode. Using this so-called patch-clamp technique, I was actually able to record incoming electrical activity. In this case, I detected “Pass it on!” messages from the thalamic neurons that I was activating with light flashes as they released the transmitter glutamate. Thus, I was able to show that midline thalamic neurons are directly wired with neurons in the amygdala. This line of communication was not known in detail before.
Opioids alter the communication between midline thalamic and amygdala neurons
A characteristic feature of midline thalamic neurons is that they possess a high number of binding sites for opioids. Well-known opioids include the pain reliever morphine and the drug heroin but also the endogenous hormone endorphin. Not only do all these compounds get you high, but they also act to relieve anxiety and fear.
So, in a next step I investigated whether opioids influence the communication between midline thalamus and amygdala neurons. As before, I activated neurons in the midline thalamus through light flashes and recorded the electrical activity generated by incoming messages in amygdala neurons. However, once the neurons came into contact with opioids, they showed significantly decreased electrical activity. Messages from the midline thalamus to the amygdala were not transferred with the same efficiency as before. From this I concluded that opioids do not stop the conversation between midline thalamic neurons and the amygdala, but they tune down the volume. This probably occurs because less glutamate transmitter is released at the contact site between thalamus and amygdala neurons.
Considering that opioids cause neurons in the amygdala to not quite understand “Pass on!” messages from the midline thalamus anymore, these amygdala neurons may not be sufficiently activated to, in turn, forward information to the next contact site with a downstream neuron. In fact, in further experiments I was able to show that opioids downregulate information transfer to neurons that mediate fear responses, such as those that increase heart rate. Put simply, opioids disturb the communication between the midline thalamus and amygdala, and may thereby ameliorate anxiety.
When do opioids come into play in midline thalamic-amygdala networks? A scenario
We know endogenous opioids help us cope with stress and make us react less negatively or anxiously. This may be because they impede the midline thalamus from sending information to the amygdala. In stressful situations, midline thalamic neurons are particularly active, and if they forward information to the amygdala, this could trigger anxious responses. Since this line of communication is inhibited by opioids, as I have shown in my research, subjects with higher endogenous opioid levels may cope better with stressful situations and react with less anxiety or panic. One important matter for future studies will be to determine whether the pathway I investigated is substantially involved in mediating these processes.
In my stressful situation of presenting my thesis, I was able to calm my fears by slowing my breath and thinking positively. And eventually, my reviewers were satisfied.
Background information "Science in a way that everyone can understand"
This article is the result of a communication training for junior scientists. The participants learned techniques for writing an interesting, easily readable text. Subsequently, they wrote an article about their research that also non-experts should understand and translated it into English. The communication team of the Cells-in-Motion Cluster of Excellence initiated the project and supported the participants in individual coaching sessions. The English support office of the University of Münster helped to optimise the translations. The aims: Reflecting terms of content and language when editing their own research topic should benefit the participants in their communication with the public but also within the scientific community. They also gain experience in working with communication departments and photographers.
Further articles resulting from this project: