Neurogenesis

Neurogenesis refers to the generation of new neurons and their ability to integrate into the existing network of the brain. Neurogenesis is associated with the acquisition of new memories, which may be due to there being new neurons available to build new synapses and neural pathways that form the cellular basis of learning and memory. Birds that hoard food in autumn to see them over winter when food is scarce exhibit high rates of neurogenesis at times when they are engaging in food storing behaviours. As well as being highest in autumn, the rate of neurogenesis varies according to the harshness of the birds’ environment. A harsh environment makes them more reliant on their food stores for survival and so it makes adaptive (survival positive) sense that neurogenesis, a form of neuroplasticity, would increase if the birds are living in an environment where food is scarce.

Another example of adaptive neurogenesis is seen in rat pups that have been separated from their mother for controlled periods. In a study by Suri et al. 2013, the rats were separated from their mother for 3 hours a day from Day 2 to Day 14 of their lives. This maternal separation stress had a couple of interesting consequences. Firstly, when the rats became young adults, they were significantly quicker to learn the way out of the Morris water maze (a test of spatial learning for rodents), but when they were subsequently tested on a spatial learning task at rat “middle age” they were significantly slower than a control group of rats that had not been separated from their mothers. Secondly, the rat pups that experienced separation stress had a higher rate of neurogenesis in young adulthood than the control group. At middle age, however, it was the rats that had not experienced separation stress that had a higher rate of neurogenesis. There were epigenetic effects too. They occurred on the BDNF gene (brain-derived neurotrophic factor), a gene that is associated with learning and memory. The effect of being separated from their mother “turned up” the BDNF gene in early adulthood, but “turned it down” in middle age. So, to summarise, the experience of separation from mother enhances spatial learning, increases neurogenesis, and increases BDNF production in young adulthood, but these effects are all reversed in middle age.

How, then, does the experience of maternal separation lead to adaptive biological changes?

Separation from mother is a threat to survival, and undergoing such an experience leads our biology to compensate as best it can. Since evolution is only interested in us reproducing and only needs us to survive until that is achieved, by enhancing neurogenesis and increasing the amount of BDNF the rats have in early adolescence, they gain a learning advantage that helps them adapt and survive in their environment long enough to breed. But there is a cost to their fast-tracked learning capabilities, namely, that their capacity to adapt to their environment drops off in middle age, by which time (evolution assumes) they will have continued their genetic line. The authors state, “Early stress may transiently endow animals with a potential adaptive advantage in stressful environments but across a life span is associated with long-term deleterious effects”.

Rates of neurogenesis are affected by experience in ways that seem to promote survival which makes adaptive sense. Making new neurons must be quite biologically expensive, so you probably don’t want to be doing that unless you’re sure you need them for learning new things!

Next time, synaptic plasticity, neurons that fire together wire together!

Epigenetics

Mechanisms of neuroplasticity

Neuroplasticity refers to the capacity that neural systems have to adapt to experience. Lived experience shapes our biology in meaningful ways that helps us adapt to that experience so that we are better prepared next time it, or something similar, happens. Neuroplasticity serves evolution by helping us adapt to our environment, even picking the best strategy for reproduction within that environment so that our genetic line will be continued. Mechanisms of neuroplasticity have been selected by evolution because they are survival positive. In my next 4 blogs I introduce four mechanisms of neuroplasticity: epigenetics, neurogenesis, synaptic plasticity and white matter plasticity.

Epigenetics

Epigenetics literally means ‘above the gene’ and epigenetics is a field of study that examines experiences that result in changes to gene expression. One way this occurs is by small chemical groups becoming attached either to the DNA itself or to the core proteins that the DNA coils around. These small chemical groups make the gene itself more or less accessible. Lived experience cannot add or take away genes but the addition of these small chemical groups acts as a volume control over the gene. The gene may literally be silenced (inaccessible) so it is no longer transcribed at all or it may be turned up, or turned down. Turning a ‘the volume’ of a gene up or down really means regulating the rate of transcription of the gene, i.e. how many copies are made of the gene; copies that can then be made into protein. The epigenetic marks on genes may persist from anything from moments to a lifetime and as the pattern of epigenetic marks is copied alongside the DNA it may persist into the next generation. The experiences that leave marks on your genes to help you adapt and survive in your environment may be inherited by your children, because nature assumes that your children going to face similar challenges in a similar environment to the ones you have faced and gives them a head start.

A seminal paper in the epigenetics field published in 2004 showed how the stress axis of rat pups is epigenetically programmed by the type of care they get from their mothers. Rat pups that had high levels of licking and grooming from their mother had lower levels of DNA methylation at the glucocorticoid receptor gene promoter (Weaver et al. 2004). What this means is that the rat pups that had high levels of licking and grooming, which in the rat world counts as good mothering, also had their glucocorticoid receptor gene’s volume turned up. This in turn led to the rats growing up to have a lot of glucocorticoid receptors, which both mediate the effects of the stress axis via corticosterone (in humans it is cortisol) and importantly, means that stress response can be efficiently turned off once it has done its job. The converse was also true. Rat pups that had low levels of licking and grooming, equating to less sensitive and attentive mothering, had a more methylated glucocorticoid receptor gene promoter, so had fewer glucocorticoid receptors, meaning that their stress response would persist for longer because they have fewer glucocorticoid receptors available to turn it off.

The experience of lots of licking and grooming is adaptive for the rat pups because they have lots of glucocorticoid receptors so their stress response is more efficiently terminated and this is okay because they have a sensitive, attentive and available mother to help them both to survive and to recover from any threat. The experience of low levels of licking and grooming is also adaptive because the stress response itself helps the rat pups survive. The stress response is not so readily turned off so the survival advantage conferred by the activated stress system persists and this perhaps can compensate for these rat pups not having a sensitive attentive mother available to help them survive and recover from any threat.

Rachel Yehuda studies the intergenerational effects of trauma, including how a mother’s experience of trauma leading to a diagnosis of PTSD may transfer to her children as an increased risk for PTSD (Yehuda et al. 2008). This doesn’t sound adaptive at all until you consider that the symptoms of PTSD, including anxiety, hypervigilance and insomnia are all adaptive strategies to try and remain safe in case something else traumatic occurs. Nature perhaps assumes that the mother’s children will be born into an environment that is dangerous and so prepares these children for survival in advance, by enhancing the expression of genes that contribute to these feelings of anxiety and hyper-vigilant behaviours. It is, after all the paranoid rabbit that survives. The intergenerational transmission of stress is fully discussed in a recent review by Yehuda and Bowers in Neuropsychopharmacology which is freely available (Bowers and Yehuda 2016).

Next time…neurogenesis and why chickadees, tits and squirrels have higher rates of neurogenesis in the autumn.

 

Experience: the invisible sculptor of brains

With around 86 billion neurons, that dynamically connect forming neural pathways that  become highways or byways of the mind depending on their use; the brain is the most complex structure in the universe. Given that it is an insipid fawn colour, has the texture of soft blancmange, and only weighs about 1.4 kg it’s hard to imagine how it earns such an accolade. Yet years of evolution have honed our brains not to perfection, but to an extraordinary degree of flexibility, and in the decade of the brain we are making some breathtaking discoveries about the elegant and intricate biological mechanisms that serve our survival by allowing us to adapt to our very personal and subjectively experienced environment.

Experience shapes brains in more ways than we think and this has profound implications for the way we understand ourselves, how we understand and respond to criminal behaviour and what sense we make of mental health and illness. One obvious way that experience shapes brains is the way we acquire language. We learn to speak, think, read and write in the language and vernacular, complete with cultural and familial colloquialisms, and the accent and tone that we hear from the mouths around us. The same is is true, although less obviously, for the emotional and relational language of attachment and bonding. Broadly speaking, as infants, our safety and therefore chances of survival are increased by maintaining proximity to our caregivers, who are bigger, stronger and more competent in the environment than we are. Our caregivers have a particular attitude towards their own vulnerability and independence and ascribe a value and probably some judgement to these aspects of themselves, which will subconsciously and consciously get communicated to their infant through their interactions. The infant’s brain is a master pattern detector and is biologically driven to maintain proximity to their caregiver, and so the infant will learn to display more or less vulnerability or independence, depending on which garners more of the caregiver’s approval and is most successful at maintaining the infant in a close proximity to her/him. These adaptive strategies for maintaining proximity are the attachment patterns described by Mary Ainsworth in the 1960s.

But how are these nuanced emotional experiences held in the wet matter of the brain? Physical, neurobiological changes occur in the brain as a result of experience and this is neuroplasticity, the capacity of our brains to remember what is needed, to forget what isn’t, and to adapt to circumstance. Experience leads to changes on our DNA, to changes in the rate we produce new neurons, and the success with which they integrate into the existing network. These mechanisms that translate lived experience into biology are epigenetics and neurogenesis respectively. Experience builds new connections in the brain, ‘neurons that fire together, wire together’ and the repeated traffic down particular neural pathways generates myelin which makes the traffic down that route faster and more efficient. These mechanisms are synaptic and white matter plasticity. All of these processes underlie forms of learning, which occurs at a cellular, genetic and non-conscious level right up through to what is consciously learned, such as a new short cut, and what is rehearsed so very many times that it becomes almost automatic, such as learning to play our favourite piece of music.

Neuroplasticity is exciting, it offers optimism, supporting our hopes for growth and change, but knowing that experience profoundly shapes the brain brings a depth of understanding that reveals some unsettling implications and demands both our compassion and our full attention. Adults who have had experiences of abuse and neglect in childhood are over-represented in both the psychiatric and the prison populations. If those experiences of abuse and neglect in childhood make actual neurobiological changes to our brain that shape how we respond, then at what point are we responsible? At what point do we have choice and how much choice do we have? What could this mean for our ‘justice system’? Equally, we think of illness as something inside a person that isn’t working as it should, but if the environment has led to neurobiological changes occurring in the brain then what does it mean when we identify the pathology in the person and not the environment? The brain is the organ of adaptation. It adapts to a supportive or benign environment and it adapts to an adverse environment. A healthy brain is a brain that can adapt to the environment it encounters. Regarding one adaptation as ‘healthy’ and the other adaptation as ‘illness’ suggests an error in our thinking, we are ascribing different values to the exact same process. What makes the adaptations desirable in the healthy person and undesirable in the person we label as ill, is not the process of adaptation itself or anything necessarily intrinsic to the person but is a feature of the environment they have adapted to. We are locating the problem in the wrong place and our solutions follow suit. Neuroplasticity may demand its own paradigm, a shift away from the dichotomy of health and illness to a model where distress and difficulty are compassionately and richly understood to be adaptations to environments that could not support all of our needs. If our brains adapt to experience and this leads to changes that whilst adaptive are not desirable, can we choose new experiences that will generate new adaptations that are desirable? Do we need to choose and seek out healing experiences with intention and wisdom?

Experience leads to adaptive changes in the brain that we are only beginning to understand the biological mechanisms for and that as a society we will be confronted by. What I hope to do with this blog is explore the neuroscience, the philosophical implications and the meaning I find for myself in all aspects of neuroplasticity. By way of a rudely late introduction, I’m a philosopher, neuroscientist, mental health nurse and psychotherapist in training. I also have a story of complex trauma and self-harm, and through nimble manoeuvring earlier in my life, have avoided psychiatric diagnosis and intervention which would now be superfluous. From this rich, unusual and candid perspective I’m excited to bring you evidence that I find, and questions that occur to me so you can engage with what neuroplasticity means to you in your life and work.