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Circuits of Fear

Thematic project unveils functions of archaic brain structures, brought into action by a real or imagined danger signal

LUCIEN FREUDThe study of the neurochemical route of emotions in the brain of higher level mammals – and of the human being himself – is gathering more and more clues that fear, in its raw state is a sentiment that is rooted in circuits as old as the first reptiles to roam the earth. Based on a series of articles that have been published over the last few years in international magazines such as Brain Research, Behavioral Brain Research, Neuroscience and Biobehavioral Reviews, among others, researchers at the Psychobiology Laboratory of the University of São Paulo (USP) in Ribeirão Preto have come up with evidence that three extremely primitive structures on the evolutionary scale of the brain, present in animal species since the dinosaurs era, perform fundamental tasks in risk situations, potential or real, even before the cerebral amygdale is brought into action – a later structure, coming with the first mammals and directly implicated in the defense mechanism replies of the organism when faced with an aversive stimulus, something such as an environment, sound, an image or light that provokes fear.

With the help of experiments that bring on various types of stress and panic in mice – whose brain, although less complex, is somewhat similar to that of man -, scientists believe that they have found new functions for the median raphe nucleus, the inferior colliculus and the dorsal part of the periaqueductal gray matter. These are the three primitive pieces of the intricate neuronal puzzle involved in the path of fear. “Each one of these brain structures participates in a distinct manner in the generation and elaboration of different types of fear”, says the medical doctor Marcus Lira Brandão, the study’s coordinator, conducted under the umbrella of a FAPESP thematic project. “Since fear and anxiety are important components of psychiatric illnesses, to understand the circuits involved in their working is fundamental for us in finding new treatments for theses disturbances.”

Traumatic environment
According to this study, the median raphe nucleus recognizes the time and the space of an environment associated with a trauma – for example, an robbery situation – and decodes it as an aversive stimulus capable of provoking contextual conditioned fear, the type of dread associated with a traumatic environment. As to the inferior colliculus – there are two, one on either side of the brain – they enter into action in a more particular type of fear. Like a filter, a region of these auditory structures, named the central nucleus, specifically distinguishes a normal sound from another considered to be threatening. Once the filtering is done, the auditory stimulus considered normal goes to the temporal lobe, a region of the neocortex, the most rational part – and newer, from the evolutionary point of view – of the brain.

The sound taken as dangerous goes, via a scale on the auditory thalamus, to the cerebral amygdala that unlocks the typical reactions of fear: movement freezing, pupils dilation, increase in heart beat, shiver, and so on. Until then, all of the evidence had shown that the amygdala always received a crude sonorous signal, without filtering, and it itself had to carry out all of the process of separation of what could or could not be a threat to the organism.

Finally, the USP researchers have produced indications that the dorsal part of the periaqueductal gray matter seems to be linked to one of most primitive responses of defense mechanism of the organism when faced with aversive stimuli: the reaction of freezing (tense immobility). “We can see that electrical stimulation of this part of the gray matter provokes a paralysis similar to that which occurs in patients with panic syndrome disturbances”, comments Lira Brandão. Panic characterizes itself by recurring episodes of exacerbated anxiety that can last hours or even days. During these crises the patients feel that they are about to die, are afraid of going mad and frequently refuse to walk.

In spite of being involved in distinct types of fear, the three structures, over which the USP study has shed some light, have something in common: they are situated in a major structure, the mesencephalon, which makes up part of the encephalic trunk, a link between the spinal medulla and a region given the name diencephalons. So what? Someone unfamiliar with cerebral architecture would ask. The point is that in the classical evolutionary division of the human brain and of higher level mammals into three large units, the encephalon trunk is one of the areas of the most primitive part of this organ, the so called reptile brain. Present since the appearance of the dinosaurs, dozens if not hundreds of millions of years ago, this primordial portion of the brain is not, theoretically, the territory of emotions, but only of instincts of self preservation and aggression.

Emotions are the preferential domain of the second part of the brain, which was formed in the first mammals and houses the limbic system, composed of a series of structures responsible for the neuronal substrate of feelings. The third part of the brain, only present in higher order animals such as primates and man, is the neo-cortex, responsible for reasoning. In the emotive system, the cerebral amygdala has been considered a key piece, the structure that has received the greatest attention of researchers into the circuits of fear. However, according to the data from Lira Brandão’s team, the limbic system has expanded – and, in some types of fear, its primary circuits show their roots in the region of the mesencephelon.

In the case of the median raphe nucleus, the scientists also managed to specify what neurotransmitter – a substance released by an excited neuron in charge of passing forward the received stimulus to another neuron – does the job of taking the fear signals of this structure to the other areas of the brain. It is the serotonin, one of the most important neurotransmitters whose action has known effects on the patterns of sleeping, moods, sexual behavior and blood vessel constriction, just to cite an few examples. The simple inhibition of the transmission routes of serotonin in mice, an experiment carried out in the USP laboratories, prevented the message of contextual fear from going any further and reaching the other structures responsible for the carrying out defensive replies, including the hippocampus and the amygdala.

The neurotransmitter responsible for carrying the auditory stimuli of the lower colliculus to other structures has not been determined yet. However, there is a good lead. This circuit seems to be modulated by dopamine, a neurotransmitter usually associated to schizophrenia – a serious mental problem that upsets reasoning, leads to a confusion of emotions and provokes the loss of contact with reality, causing delirium, above all voiced. Or that is to say, the raising of the level of dopamine increases the efficiency of the transmission of aversive auditory stimulus. Nevertheless, it has not as yet been possible to determine if the blocking of the transmission channels interrupts the flow of this information to the brain. “We need more studies to see if there are not other neurotransmitters involved in the process”, testified Lira Brandão. With respect to the action of the periaqueductal gray matter in freezing reply, the experiments with neurotransmitters are still in the initial phase and do not allow forany greater comment.

For survival
It has to be made clear what types of fear the USP researchers have been dealing with. In a general manner, the study’s goal is fear in its most primitive form. It is the instant tremor that we feel, both we and animals, when faced by any situation interpreted by our brain as being of life or death. As a type of conditioned reflex, non-rational, this unconscious and primordial panic works in favor of our survival instinct. It is fear that makes us (re)act, without thinking, when faced with something perceived as a threat, real or imaginary. For example, without it animals would not manage to escape from their predators. To be efficient in the task of guaranteeing the perpetuation of the species, this fear presents itself, very often in an exacerbated and unfounded manner.

In evolution, the strategy of better safe than sorry seems to have been more efficient. Or as said the North American neurologist Joseph Le Doux from New York University, author of the excellent book about emotions and the brain (The Emotional Brain ), “it is better to confuse a piece of stick with a snake than a snake with a piece of stick”. This primitive fear is therefore a form of dread very different from rational fear that which a student feels on taking a test for which he has not studied. The student is hesitant when confronted by the test because he consciously knows that he is not prepared for the examination.

Some forms of primitive fear inhabit the human mind and that of animal’s right from birth. They are inherent suspicions, genetically passed on by our ancestors – and don’t originate from traumatic experiences lived through by the people who now demonstrate these fears. In mice, the aversion to open places fits intothis definition. In man, the fear of high altitude is one of these cases. Nobody needs to fall from a ten meter wall in order to be apprehensive about the fall. We enter the world “programmed” with this fear. This type of fear is studied under the name of unconditioned fear. This form of fear is being studied by the USP researchers but it is not their main focus of interest. Their attention is more directed to understanding the neuronal circuits used in the elaboration of some types of conditioned fear, which emerges as a function of a traumatic experience, technically called aversive stimulus.

In this line of work, the study of conditioned contextual fear in mice has produced interesting results. How is this type of emotion created in animals? It is worth describing a classical experiment to understand the process of induction of fear associated with an environment. Mice are placed in different locations of the breeding unit in which they live: a closed compartment, lit by a red light (neutral for rodents, it works only for the animals to visualize the discriminative characteristics of the environment-context), where their reactions can be filmed by a small video camera. Within this strange place, the feet of the mice systematically receive shocks of moderate intensity for the animal (0.6 milliamps). Every twenty seconds they receive an electrical stimulus that lasts for one second. In a single session the procedure is repeated ten times, which makes the animals associate the compartment – the strange atmosphere, the context – with electrical shock.

The consequences of the trauma are visible the following day: it is enough to place the mice in the same compartment, or in a similar environment for the fear to instantaneously take hold of the animals. The animals freeze their movements, raise their hair, dilate their pupils, urinate and defecate uncontrollably and their heart beat takes off. That is, they exhibit all of the typical responses of one who faces a situation codified by the brain as dangerous. The contextual fear exhibited by the rats is similar to the one experienced by a person who fears walking through narrow and dark streets for having been robbed in a poorly lit alley late at night. With some variations of procedures, the researchers can induce in the laboratory mice, as well as contextual fear, the fear of sonorous and luminous stimuli.

New functions
It was while unchaining this range of dreads in rats that Lira Brandão’s team reached the results of their research, which pointed to new functions for three brain structures. In the case of the rats with contextually conditioned fear, the scientists verified that rodents with an inactive median aphe nucleus, chemically or surgically blocked, did not present the typical replies of those who were in a situation of danger. “But those who had the nucleus preserved exhibited the expected replies”, says the biologist Viviane Avanzi, who participates in the studies conducted by the Psychobiology Laboratory. In 1978, researchers of USP itself in Ribeirão Preto, led by Frederico Graeff (today retired), had already put together evidence that the structure could perform this function, but they ended up not going on with their studies, which were only recently taken up again.

The hypothesis that the inferior colliculus, a structure known to be involved in the capture of sounds, works as an auditory stimuli filter, separating the dangerous signals from the harmless, gained ground after Lira Brandão and his laboratory colleagues, a team of fifteen young researchers, made some surprising discoveries. Firstly they discovered that electrical stimuli produced in the colliculus, whose impact is similar to that of known aversive sounds to the animals, causes replies of fear. Afterwards, the most important of all, they realized that these reactions could be neutralized by the administration of tranquilizers directly into the colliculus. Consequently, we concluded that the fear of sonorous stimuli is measured by the colliculus”, reports another researcher in the group, the psychologist Jorge Manuel Nobre.

The link between the dorsal portion of the periaqueductal gray matter and the reaction of freezing was determined through the electrical stimulation of this region of this structure. There was a suspicion that the gray matter on its own, or perhaps only the ventral part, was involved with the mesencephalon neural mechanisms that lead to paralysis (freezing or immobility) associated with fear. To demonstrate that the ventral portion was not the determining factor in this reaction, the researchers damaged it, making it useless, and then gave electrical stimuli of low intensity to the dorsal part. It was enough to provoke the typical reaction of muscular freezing.

“With our research we don’t want to minimize the importance of the amygdala in the circuit of fear. The aversive stimulus needs to arrive at it so that the process of the defensive reaction of the organism can be set off”, warns d Lira Brandão. “The objective of our studies is to better understand all of the circuit of fear, highlighting the importance of various other structures, which receive and integrate this information even before it arrives at the amygdala.”

Why it is difficult to control fear and emotions

As everybody knows through their own experience, it is more difficult to control one’s emotion than reason. Love, hate, happiness, sadness, fear and empathy are feelings apparently spontaneous to man, which take control of people independently from their desire. For example, nobody stops hating their worst enemy simply because it has been put into his head that hating them does no good. Almost by definition, reason is controllable – not emotions. Analyzing the nerve connections that link the parts of the brain, where reason and emotion originate from the most diverse types of stimuli, neurologists have formulated a theory to explain why the human suffers to dominate emotions – fear being among them -, a step that is considered to be the father of reason.

According to this approach, which may not be pleasant to all psychologists and psychiatrists, the key to understanding this mystery is in the fact of there being more nerve connections linked to the amygdala (the brain structure that is the key in the determination of physical responses and behavior provoked by fear and other emotions) to the neo-cortex (part of the brain responsible for cognitive and rational thinking) than those connecting the neo-cortex to the amygdala.

In other words, the extension of the neural network, capable of carrying information from the amygdala to the neo-cortex is significantly larger than the number of qualified routes that can be made in the opposite journey. “The two structures communicate, one with the other, but this communication is asymmetric”, explains Marcus Lira Brandão, of the Psychobiology Laboratory of São Paulo University in Ribeirão Preto. “During aversive situations, our emotional replies predominate over those of reason.”

The North American neurologist Joseph Le Doux from New York University, a renowned student of conditioned fear, is the greatest supporter of this vision. For him, this asymmetric communication helps to explain why psychiatric therapy doesn’t always give good results with anxiety victims and other mental problems. A more efficient treatment for these disturbances, according to the adepts of this prognosis, could be achieved if drugs were to be developed that would facilitate the interaction of the neo-cortex with the amygdale.

Though emotions have already been the target of studies with a biological focus since the time of Charles Darwin, in the second half of the 19th century, neurology has begun to explore in more depth this unstable and irrationally challenging territory of the brain circuits involved with these feelings (and behaviors) for only the last two decades. In so called neurobiology of emotions, fear becomes the preferred target of studies since it is at the biological root of various mental disturbances and for being a condition of easy identification and induction in the laboratory.

In a short period of time, this research line, dealt with in centers throughout the world, placed in evidence various structures of the encephalic chord that participate in the generation and elaboration of fear and that hooks up to a small gray structure situated in the middle part of the brain, whose format reminds one of an almond – the amygdala, which, as a matter of fact, are two, one in each brain hemisphere, and clearly have nothing to do with the hormonal glands of the throat.

The project
Neurobiology of Fear and Anxiety (98/11187-2); Modality: Thematic Project; Coordinator: Marcus Lira Brandão – Psychobiology Laboratoryof USP in Ribeirão Preto; Investment: R$ 641,059.54