How does the brain deal with fear? A study in mice tells us more

In the functionalist paradigm of the neurosciences – widely used today – we must be able to account for the different states of consciousness starting from the neuron as an explanatory cause because the states physical and the mental states are one and the same entity. Among these states of consciousness, fear has been extensively studied for more than half a century. The pioneering experiments are those of John Downer or Joseph LeDoux, two American researchers who demonstrate the central place occupied by the amygdala in the occurrence of stereotyped behavior in the face of dangerous stimuli and its role in the formation of aversive memories.

Valerie Doyereresearch director of the Memory, Emotion and Time team at the NeuroPSI Institute of the University of Paris-Saclay and CNRS, discusses the formation of this literature, now abundant: IThere has been a long debate in the scientific community over whether the formation of aversive acquired memory is carried out by the amygdala or by structures at the thalamic level. Today, the literature leans in favor of the amygdala even if a vision too centered on the structure is probably too simplistic. Like the senses, memory is multimodal, it needs several elements to form. The increasingly significant idea is that it is probably interactions between several structures that are at the origin of the phenomenon”.

Recently, a new study published in the journal Cell Reports brings new elements to this literature by demonstrating the existence of a central neural circuit in the occurrence of stereotyped behaviors to aversive stimuli and in the formation of aversive memory. But, before explaining the ins and outs of this research, it should be noted that, despite the obsession of the scientific literature with the sensation of fear, the amygdala is also associated with the transmission of neutral and positive stimuli: “ In structure-focused dynamics, the amygdala has tended to be thought of as solely associated with processing aversive stimuli while other structures, such as the nucleus accumbens of the striatum, have been more associated with pleasure and craving. . However, current research tends to show that this segmentation is simplistic and does not take into account the particularities highlighted by the experiments. We find the aversive in the structures usually associated with pleasure and vice versa. explains Valérie Doyère.

What we already knew about the amygdala and fear

Like all scientific work, this experiment is part of a corpus of already known data. So far, research has characterized neurons well — these are CGRP neurons for Calcitonin Gene related Peptide — which project into the different anatomical regions of the amygdala (in particular its central, striatal and lateral part). Moreover, the brain circuits between posterior thalamic areas such as the parvocellular nucleus and the aforementioned anatomical regions of the amygdala were already known.

On the strength of these elements, the authors of the study will want to add a stone to this edifice by characterizing histologically and anatomically these links: “ This experiment is performed with the objective of showing that two circuits from the thalamus and the brainstem that project to the central and lateral amygdala form two parallel networks depending on the sensory stimuli involved in the occurrence of stereotyped aversive behaviors”, says Valérie Doyère.

The contribution of this research: The amygdala as a processing center for aversive signals

Firstly, this experiment demonstrates — using calcium imaging techniques which testify to the fact that a neuron has just discharged and lines of mice genetically modified to express a fluorescent protein — that the famous CGRP neurons (in two distinct regions, the subfascicular part of the parvocellular nucleus of the thalamus and the external lateral parabrachial nucleus of the brainstem) are well activated by different aversive sensory stimuli (electric shock, strong lightloud sound, foul odor and unpleasant taste): “ The advantage of this imaging technique compared to the old methods is that we obtain a spatial individualization of the structures within which the neurons in question are activated. Thanks to this, we can go and see specifically whether or not specific clusters of cells react to this or that stimulus. One interesting thing about their discovery is that the majority of neurons are found to respond to several different stimuli. This is interesting in that it suggests that associations between aversive stimuli can potentially occur at the cellular level,” explains Valérie Doyère.

In a second step, the scientists highlight the existing connections between these populations of CGRP neurons and the sensory relays thanks to a clever method although widely used in this field of research: “ They inject a virus with a fluorescent marker at a specific location which will be picked up by the pre-synaptic side of the neuron and which has the ability to go up the neuronal circuit in the opposite direction. Using immunological-histo-chemical methods, we can then visualize, in the dead animal, the neurons of fluorescent origin that have been marked by the virus by carrying out its journey in reverse of the usual direction. develops Valérie Doyère.

Next, the researchers inhibit the expression of CGRP neurons and show that, depending on the regions in which the populations of neurons are deactivated, the rodents no longer manifest certain stereotyped behaviors in the face of certain aversive stimuli: The experiment shows that the localization of the CGRP neurons is of importance in the aversive character of the stimuli. Unfortunately, it is not known if the stimulus is no longer transmitted altogether due to the deactivation of the neurons or if the transmission of the stimulus is still effective but no longer includes the aversive component”, point Valérie Doyère.

This reinforces the idea of ​​the parallel circuit with dominant stimuli for certain clusters of neurons. To ascertain this point, the investigators also inhibited afferent neurons from both regions in the central and lateral amygdala. The results obtained are concordant and show that a parallel circuit exists except for one nuance: “It is known that the lateral amygdala also projects into the central amygdala and the central amygdala projects back to the parabrachial nucleus of the brainstem. So this parallel circuit eventually comes together and takes the form of a loop rather than two networks that would be radically distinct,” notes Valérie Doyere.

Finally, the researchers wanted to study more closely the role of the CGRP neurons of these two parallel circuits in the learning between a neutral signal and an unconditioned stimulus, i.e. innately aversive). This is called a Pavlovian task after the psychology researcher Ivan Pavlov who set up this type of protocol. The researchers then observe that, if the CGRP cells do not participate in the formation of aversive memory, the two circuits do participate: “ During the Pavlovian task with the electric shock, the CGRP cells do not activate during the final test, that is, when the animal is supposed to have associated the signal with the aversion. On the other hand, when the experimenters come to stimulate the neural circuits identified using an optogenetic method thus replacing the electric shock, they indeed observe the same behavioral response of immobilization in the animal. This means that it is probably at the level of the cells of the amygdala that the memory association takes place, even if the researchers do not measure the activation of the latter. concludes Valérie Doyère.

Is it really the feeling of fear that we measure?

Before leaving, we must raise an important nuance. In the title and in our introduction, we talked about fear by worry of simplification. In reality, it would have been more accurate to speak of a defensive response. Indeed, there is a chasm quite significant between defensive and physiological responses associated with aversive stimuli and feelings of fear or anxiety. The debate concerning these states of consciousness is mixed and the answers provided do not yet achieve consensus. However, there are strong arguments that suggest that the neural circuits we have discussed so far are not responsible for feelings of fear and anxiety. In a 2016 article in theAmerican Journal of PsychiatryJoseph LeDoux and Daniel Pine question this problem and propose a dual model of fear.

Indeed, they point to the fact that the unique neural circuitry of fear is arguably a chimera. To support their thesis, they argue that physiological and behavioral markers of fear are weakly correlated in humans with conscious feelings of fear and anxiety, that patients with amygdala damage are still able to feel fear (even if they are no longer able to recognize it) or that subliminal stimuli can trigger defensive and physiological responses without causing feelings of fear.

These different points lead them to theorize two circuits, the first being similar to the classic single circuit determining the stereotyped responses to an aversive stimulus, the other having a strong cognitive component determining the sensation of fear. For their opponents, the subjective sensation of fear is only the reflection of a deficiency in our understanding of the physiology of the latter.

These two visions are therefore radically incompatible and refer to different philosophical positions: reductionism on the one hand and emergentism on the other. For the former, sensations flow logically and necessarily from neuronal physiology, while for the others, this sensation is a property that emerges from the same components but within a more complex system that they form when they interact.