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!