Part 2 of 3: Stress and the Adolescent Brain

                 Research is indicating that the adolescent brain is particularly sensitive to the effects of high glucocorticoid levels and therefore to stress as well.  During the teenage years, there appear to be high levels of glucocorticoid mRNA (a chemical that comes from DNA and encodes a type of blueprint for the creation of a specific type of protein) in the prefrontal cortex, which is an area that also undergoes development during this time.  This suggests that the functions that this part of the brain is responsible for, mainly cognition (reasoning, thinking, planning, sequencing, judgement, etc.) and emotion, are heavily affected by glucocorticoids and therefore by stress.  Many forms of psychopathology like depression and anxiety show up in adolescence, often following a period of particularly severe stress.  So all of this information is basically another way of saying that teenagers' emotions and decision-making abilities are prone to fluctuate according to any tiny little outside event…which we probably didn't need a neuroscientist to tell us, but now we know why!

                Interestingly, although the prefrontal cortex is greatly affected by stress, it seems that the hippocampus is spared in this period.  This is likely because the hippocampus finishes developing around 2 years of age.  Children who suffered abuse through late childhood and early adolescence did not show a decrease in hippocampal volume in adolescence, although interestingly they do show this decrease later on in adulthood.  More to come in my next post about what happens to the stressed brain as we age!

Adapted from "Effects of Stress Throughout the Lifespan on the Brain, Behaviour and Cognition" by Sonia Lupien et al. and inspired by Wayne Brake's Neuropharmacology course at Concordia University.  

Cristina McHenry 

Concordia University

Stress and the Brain: Part 1 of 3: The Stress Response

When the brain detects any kind of threat or stress, it sets a coordinated system into motion to counter the stress.  This is the stress response, and it involves autonomic, neuroendocrine, metabolic and immune components.  One of the most studied stress systems is the HPA (hypothalamic-pituitary-adrenal) axis.  To start off this component of the stress response, neurons in the hypothalamus release CRH (corticotrophin releasing hormone).  This then causes the pituitary gland to release ACTH (adrenocorticotropic hormone) into the circulatory system.  ACTH travels in the blood to the adrenal glands, which sit just above the kidneys, causing them to release glucocorticoids such as cortisol into the circulatory system. 

                Back in the brain, in an area called the hippocampus, there are receptors for these glucocorticoids, unsurprisingly called glucocorticoid receptors (GRs).  Once the stress has disappeared, glucocorticoids are supposed to bind to these receptors, which then function to shut down the hypothalamus, pituitary gland and ultimately the adrenal glands, putting an end to the stress response and returning the body to homeostasis.  Unfortunately, this system can go awry with chronic or particularly severe stress. 

Prenatal/Infant Stress and the Brain (check out my next 2 posts for Adolescent Stress and the Brain, as well as Stress and the Aging Brain)

            When pregnant moms-to-be are stressed, some of their glucocorticoids pass through the placental barrier and reach the fetus.  A certain level of glucocorticoids is required for proper nervous system development since they remodel the axons and dendrites of neurons and affect cell survival.  However, high levels of glucocorticoids have negative effects on brain development and later functioning.  Rats exposed to prenatal glucocorticoids have fewer receptors in the hippocampus later in life.  As I said before, these GRs function to shut down the stress response.  So if there are fewer GRs present in the hippocampus, the stress response is not shut down as effectively, leading to higher than normal levels of glucocorticoid activity later on (more on why this is important in the section on Stress and the Aging Brain). 

Higher prenatal glucocorticoid levels have three main effects on adult behaviour: learning impairments, greater sensitivity to drugs of abuse and increased anxiety and depression.  The learning impairments in particular are thought to be caused by the changes in the hippocampus, while drug sensitivity and anxiety/depression are thought to be caused by changes in the amygdala. 

In infancy and early childhood, the brain is surprisingly hyposensitive to stress.  Certain things can still affect brain development, however.  Good parenting actually results in a small decrease in the stress response to everyday occurrences (which is a good thing).  However, in cases of extreme deprivation, the HPA axis becomes seriously underactive, possibly due to a downregulation in the pituitary.  Basically, the hypothalamus releases so much CRH that the pituitary can't handle it anymore and gives up trying.  Remember that a certain level of HPA activity is needed in order for neurons to develop properly.  Luckily, this severe reduction in HPA activity can be fixed after a mere 10 weeks of proper care.   

Stay tuned for Parts 2 & 3!

Cristina McHenry

Concordia University

Adapted from "Effects of Stress Throughout the Lifespan on the Brain, Behaviour and Cognition" by Sonia Lupien et al. and inspired by Wayne Brake's Neuropharmacology course at Concordia University.  

Electricity and the brain

Part 3 of 3 : Electric currents that create the action potential

The negative voltage of the neuron attracts the positive charges on the outside, but they can not enter the neuron because the neuronal membrane prevents them. That is when another group of proteins steps in. These proteins are sensitive to the voltage of the neuron (voltage-dependent ion channels). They are specific for a single atom, have a structure that reminds us of a tube with a door that opens when a certain potential is reached. Thus, when a neuron receives a stimulus (from the 5 senses or another neuron), there is a fluctuation of the neuron’s resting potential (Figure 4). If a specific potential is reached, the voltage-dependent proteins door opens and lets in the positive charges. This movement of charges is an electrical current that can be measured in amperes, usually on the order of picoampere (10^-12 ampere).

When we think of brain electricity, there are two very important voltage-dependent channels: sodium channels (Na) and potassium channels (K). When the neuron is depolarized so as to achieve a certain potential (threshold potential), these channels open and let their specific atoms (ions) go through (Figure 4). Note that sodium channels have faster kinetics (open faster) than potassium channels ; they will open first, sodium will quickly enter the neuron and there is then an increase of positive charges inside the neuron, up until the sodium channels close. At that time, the voltage in the neuron rises to about 40millivolts (mV). Then the potassium channels come into play. They allow potassium ions to leave the neuron to eliminate the positive charges inside the neuron and allow the neuron to come back to a negative voltage. This very fast sequence of opening and closing is called the action potential. This action potential will develop in the neuron cell body and spread along the axon like a wave in water until it reaches the end where it can communicate with another neuron, allow muscle contraction, allow the release of hormones, etc..

Eric Trudel 

McGill University

Electricity and the brain

Part 2 of 3 : The neuronal membrane and the establishment of neuronal voltage

The "walls" of neurons (neuronal membrane) are composed of special fats that totally separate the interior of the neuron from the outside. If we take these special fats and place them in the tank mentioned above, they will spontaneously form a sphere with a uniform distribution of minerals (those previously dissolved in the aquarium in part 1) inside and outside the sphere (electrically neutral). We then have a draft of a neuron.

The neuronal membrane contains several proteins that distinguish neurons from all other cells. One group in particular, called pumps or exchangers, transfers one or more atoms to the other side of the membrane (Figure 3). Therefore, if we introduce this group of proteins in the membrane of the example of the aquarium above, one group of proteins will be responsible for removing the sodium atoms inside the neuron, another group will be responsible to concentrate potassium atoms inside the neuron, etc. (Table 1). The net result will be a decrease of positive charges (Na +, Ca2 +) inside the neuron and the inside will be negatively charged, thus forming a potential difference (about -60millivolt) ; this is called the resting potential.

Eric Trudel 

McGill University

Sight Recovery and it's Discontents

Many of us take for granted that we have the ability to see the world around us. Vision is an invaluable sense that leads us to make dozens of behavioural decisions every second. Historically, the fate of individuals blinded since childhood due to accident or congenital diseases were destined to never see again.

Although these procedures are rare, there are methods which can restore their sight. For example, corneal graft transplantation involves removing the cornea from a deceased donor and surgically placing it onto the eye of a blind patient after removing the unusable cornea. Another technique is corneal stem cell transplantation. Limbal stem cells (immature epithelial cells) from healthy corneas are removed and cultured into a new cornea in ex vivo. And now, new research in the field of bioenegineering is investigating the use of synthetic corneas made from complex polymers.
The ability to give someone their sight back is extraordinary. However, regaining vision after decades of blindness may not always be a happy or easy experience. There are two sight recovery cases which demonstrate the powerful effects of vision restoration.

Patient SB has become a famous case study in vision research. He was blind from age 10 months due to complications with a smallpox vaccination. At the age of 52, he received a corneal graft operation to restore his sight. SB thought that vision would be positive and life-altering. However, when his surgical recovery was complete and the bandages came off, he was ultimately "disappointed in what he saw." SB went from being a successful blind person to an unsuccessful sighted individual. Although it is hard to understand if you have not experienced it, vision can be a shocking and confusing blur (no pun intended!).

If you are not accustomed to that level of mental stimulation, the variety of information as well as the sheer quantity of input we get from sight is very overwhelming. Your brain needs to essentially learn to see. Therefore, patients like SB need to be given the proper psychosocial support in order to function and mentally adjust to their drastically different lives. Unfortunately for SB, he did not receive adequate support to help him through this transformation. Within two years of regaining his sight, SB became clinically depressed and died after catching a brief illness. SB was simply not ready to handle his new sensory modality.

This story, greatly contrasts that of patient M.M. who lost his sight due to a chemical accident at the age of 3. When M.M was 43 years old he got a stem cell transplant to restore his sight. M.M very much enjoyed living in the visual world, adapted well and continues to master his new abilities. An important lesson to learn from these two cases is that it takes a long time for the brain to get used to processing this new kind of information. Furthermore, it takes motivation and patience to handle such drastic changes to daily life. More generally speaking, gaining back any sort of key ability (not just vision) is intense and it is not always an easy process to endure. It is important to judge the expectations of the patient as well as provide a solid network of coping strategies in order to facilitate a functional recovery. As scientific research finds new and inventive ways to cure physical disabilities, psychological resources are needed for an effective recovery. It should be seen as essential as the medical procedure itself.

Lisa Kirsch
McGill University

Out of Sight, Out of Mind, but Not Entirely Out of Order: What Cortical Blindness Reveals about Consciousness

Everybody knows that our thoughts are linked to our brains in some way. We say things like "Use your brain!" to encourage people to think things through, or "Can I pick your brain?" in the hopes of gaining thoughtful insight from a friend. But while we take it for granted that our thoughts – our very consciousness – is a product of our brains, centuries of scientific evidence was collected before this idea worked its way into our everyday language. One of the most fascinating demonstrations of the principle that our brains generate our conscious experience comes from the frontiers of neuropsychology, with a curious syndrome called “blindsight.”

Blindsight is a strange phenomenon - an oxymoron! Surely, there can be no such thing as “blind” sight. How could an individual see and be blind at the same time?

Remarkably, such a phenomenon does exist in a population of individuals who are cortically blind. This form of blindness occurs when there is tissue damage to the main visual area of the brain - the primary visual cortex, or “v1.” V1 lies at the very back of the brain and is fundamental for visual experience. While our eyes receive light signals from the world around us, v1 interprets those signals to generate what we experience as “seeing.” Individuals with cortical blindness have properly functioning eyes, but their v1 is damaged.

For at least a century, doctors knew about cortical blindness, and they assumed that it resulted in the same visual impairment as ordinary blindness – the inability to see. But roughly 35 years ago, it was discovered that cortically blind patients were able to do things that ordinarily blind individuals could not. A series of carefully designed studies in the 1970’s revealed that cortically blind patients were able to point to the location of objects and distinguish between various shapes, all the while claiming that they could not see the objects they were locating or the shapes that they were discriminating! At the time, the discovery transformed the way scientists understood vision and generated much speculation about the nature of consciousness.

Why consciousness?

Blindsight tells us something about consciousness because the syndrome revealed that cortically blind patients are not really blind; they are just unaware of what they see.

We now know that vision is the product of the unified effort of many different brain networks, each specializing in a particular aspect of our visual experience. Like a machine, the brain has many different wires traveling back and forth between different locations. These brain wires (i.e., neurons) carry information that gives rise to specific mental states and behaviors. In our visual brain, some of these networks specialize in recognizing faces; others specialize in recognizing colors, and still others are used for interpreting the lines and curves that comprise letters and familiar objects.

At a more basic level, the visual pathways of the brain can be reduced to two main networks: One pathway connects our eyes to v1, and a second pathway connects our eyes to a region of the brain that controls movement. While the first pathway is fundamental for our visual experience, allowing us to see the rich detail in our environment, the second pathway is more primal, allowing us to respond reflexively to sudden movement even before we are fully aware of what we have seen. This second pathway bypasses v1 and serves a motor, rather than a visual, function. It allows you to flinch in response to something flying in your direction, even if you are not aware of the projectile. It also helps your direct your gaze to something important, like a car moving towards you.

In blindsight, the pathway responsible for visual experience is damaged, but the pathway that coordinates movement in response to objects in our environment is preserved. Thus, although individuals with blindsight cannot see, they can respond reflexively to a sudden movement.

Blindsight provides evidence that the brain is responsible for our conscious experience because damage to the brain tissue of v1 diminishes visual awareness. Individuals with blindsight can still process what they see in important ways. Through the non-visual, reflexive pathway, they can identify the location of objects, avoid obstacles in their path, and direct their gaze to moving objects. What they lack is the awareness of the objects they are locating and the obstacles they are avoiding – it is their very visual consciousness that is impaired as a result of damage to v1, and therefore, v1 is crucial for such consciousness.

-Levi Riven

Female Mice Hot for Guys Who Cry

Attempts at mixing up love potions go back at least to Tristan and Isolde, but since the discovery of pheromones it has become increasingly clear that many animals have only to look to their armpits - and an assortment of other glands - for an irresistible concoction of their own. Pheromones are small molecules that animals release into the air, and when another animal of the same species detects them, the pheromone will cause a specific social behavior – perhaps most notably, an increase in sexual receptivity. While many people are excited by the prospect of chemicals that might improve their sex lives, pheromones make neuroscientists randy for another reason: they represent an opportunity to dissect how the brain works. As far as we can tell, the brain is a huge network of smaller circuits. This means that to do things like control movement and store memories, the brain relies on circuits of nerve cells that communicate with one another to store info or output some kind of behavior. Presumably, the behavior associated with a pheromone relies on a specific neural circuit that is activated when a pheromone is detected by an animal. So, once a pheromone is isolated, you can use it to search out the neural circuit in the brain that is responsible for the behavior; just spray the chemical into the air and watch for a change in behavior of the animal you are studying. Theoretically, experiments like this may help determine exactly how small sets of nerve cells generate behavior. This is a big deal, because although we think small circuits run the brain, we know surprisingly little about the details of how these circuits are wired up.

This summer, a Japanese group in pursuit of a circuit to call their own published a study examining the role of a potential pheromone called ESP1 in sexual behavior of female mice. ESP1 has a quirk, though: it is released in the tears of male mice. Although tears and sex may seem an unlikely pairing to us humans, previous results suggested that ESP1 might be a sex pheromone. Besides, it would be the smell of the tears, not the look of them, causing the excitement. Banking on this, the group characterized ESP1, the proteins that detect it (its receptor) and the nerve cells that use this receptor to tell whether ESP1 is present in the air. By sticking electrodes up a female mouse’s nose, they found that ESP1 sprayed in the surroundings causes electrical responses in neurons of the vomeronasal organ - long thought to be solely responsible for pheromone detection. This organ contains a number of small populations of neurons that respond very specifically to single pheromone types. The group confirmed that ESP1 was indeed responsible for the electrical activity by testing mice whose ESP1 receptors were non-functional. As expected, females without functioning receptors showed no activity in their vomeronasal organ when exposed to ESP1.

The electrical activity in the ESP1-detecting neurons is likely relayed further into the brain to control sexual behavior by activating a neural circuit. To check this, the group exposed normal females and females with non-functional ESP1 receptors to ESP1, introduced them to male mice and watched the magic unfold. Normally, female mice play hard-to-get, and rightly so: although mouse courtship begins with the suitor chasing his crush around like we all used to do in the school yard, he quickly jumps straight to 3rd base, attempting “anogenital investigation.” He then immediately tries to mount – romance has apparently been dead in the mouse world for quite some time. When the female is finally convinced by his wares she initiates lordosis, a perhaps not unfamiliar backward curvature of the spine, facilitating the entry of the penis into the vagina. Females exposed to ESP1 before males were introduced were almost 5 times more likely to initiate lordosis and allow mating than those not exposed to ESP1, while the mutant females, who had no ESP1-evoked activity in their vomeronasal organs, were no more likely than unexposed females. Although most of these experiments were done by exposing the females to purified ESP1, the researchers also compared female responses to males who had ESP1 naturally in their tears and those who didn’t. The results weren’t quite as strong as with purified ESP1, but males who had ESP1 were more successful in their quest.

Although the exact circuitry involved in the sexual behavior has yet to be studied in depth, one thing is clear: female mice are turned on by the smell of tears in their man’s eyes. I wouldn’t count on crying as a go-to pick up line though; humans do respond physiologically to pheromones, but where in the brain and how we detect them is still a mystery, and anything similar to ESP1 has yet to be found in human tears. For now we should either get the good stuff by mail or keep counting on our armpits. (Two for one: the latter is apparently good for the environment.)

What rodents are telling us about human behavior and autism

A tales of two voles

Prairie voles are extremely social creatures, preferring to spend the majority of their time with other prairie voles. In contrast, the closely related montane voles are extremely asocial creatures, choosing a solitary lifestyle over one where they’d have to be tied down. Eerily, prairie voles and humans share many other social behaviors: after a male and female prairie vole decide to spend the night together, they fall madly in love (called pair-bonding), move into a shared nest and raise their children together (‘Till death do us part: 75% of prairie voles stay together until one partner dies). Like with humans, affairs are fairly common, jealousy is rampant and some males (>40%) just never settle down (known in the field as “wanderers”). Prairie voles are even known to enjoy an alcoholic drink every once in a while. With these similarities, it’s no wonder the prairie vole has been the model system of choice to study the physiological basis of social behavior, and why the montane vole has served as the perfect “asocial” control.

Researchers have focused much of their attention on the role of two neuropeptide hormones, oxytocin and vasopressin, in mediating these social behaviors. Oxytocin and vasopressin are well-known regulators of peripheral tissues involved in birth, lactation and water homeostasis. However, their receptors (the cell-surface molecules which bind to these hormones and transmit their signals to the inside of cells) are also found scattered throughout the brain. Turns out, these hormones are also released by prairie voles when they “spend the night”, suggesting that these hormones may induce the post-mating behaviors of prairie voles: partner preference, aggression towards intruders and parental care (characteristics of social monogamy… referred to as monogamy for the rest of the article). Indeed, researchers can induce partner preference and aggression in virgin male prairie voles by injecting them with vasopressin and can block these behaviors by blocking the vasopressin receptor during mating. The same can be done in female prairie voles, except with oxytocin and an oxytocin receptor blocker, reflecting gender-specific differences in how these hormones affect social behaviors. Only thing is, montane voles also release oxytocin and vasopressin following mating and giving them additional vasopressin fails to induce monogamy—what then results in the difference in post-mating behaviors seen between prairie and montane voles? Surprisingly, when researchers looked at the brains of montane and prairie voles, they found striking differences in the location of vasopressin and oxytocin receptors. Prairie voles had more receptors in the ventral pallidum and nucleus accumbens, areas associated with reward and reinforcement, whereas montane voles lacked receptors in these areas. Importantly, other monogamous species, such as marmosets and California mice, show a similar distribution of receptors to the prairie voles. These results suggest that monogamous species may perceive social attachments as pleasurable and rewarding, therefore reinforcing these behaviors, whereas these behaviors are not reinforced in non-monogamous species.

Of course, the goal of all these studies is to understand and possibly treat some human behaviors. Autism can be a particularly devastating neurodevelopmental disorder characterized by severe social deficits such as lack of eye contact, empathy and social attachment. It is therefore not surprising that researchers are looking to see whether oxytocin and vasopressin are dysregulated in autistic patients. Indeed, some cases of autism are associated with reduced levels of circulating oxytocin or complete deletion or mutations in the oxytocin or vasopressin receptor genes. Can administration of oxytocin or vasopressin help with the symptoms of autism? Surprisingly, administration of oxytocin to humans increases social behaviors such as eye contact, trust and empathy and reduces social anxiety. Initial trials with oxytocin and high-functioning autistics showed improvements in their ability to make eye contact and other social behaviors. Therefore, although a link between autism and the oxytocin or vasopressin systems is tenuous and much research remains to be done, research on voles may surprisingly hold the key to understanding human social and asocial behaviors.

For more information:
Oxytocin, vasopressin and autism (free full-text): http://rstb.royalsocietypublishing.org/content/361/1476/2187.long

Those binge-drinking voles:
http://www.oregonlive.com/health/index.ssf/2010/07/voles_a_party_animal_sheds_lig.html