New strategy for stimulating neurogenesis may lead to drugs to improve cognition and mood

Researchers at Columbia University Medical Centre have developed a new way to stimulate neurone production in the adult mouse brain, demonstrating that neurones acquired in the brain's hippocampus during adulthood improve certain cognitive functions.

In recent years, scientists have been exploring whether stimulating neurogenesis (the formation of new neurones) in the adult brain has a beneficial effect on cognition or mood. Until now, studies have relied on interventions, such as exercise and enriched environments, that affect numerous other processes in the brain in addition to increasing adult hippocampal neurogenesis.

The research, led by Rene Hen, PhD, professor of Neuroscience and Pharmacology, in the Departments of Neuroscience and Psychiatry at Columbia University and the New York State Psychiatric Institute, appears in the Advance Online Publication of the journal Nature. Amar Sahay, PhD, a postdoctoral fellow, is the lead author on the study.

After boosting the number of neurones in the hippocampus, an area of the brain involved in memory and mood, the researchers tested the mice in both learning and mood-related tasks and looked for changes in behaviour. The researchers found specific effects on learning tasks that involve a process called pattern separation, which is the ability to distinguish between similar places, events and experiences.

'This process is crucial for learning because it enables us to know whether something is familiar or novel,' said Dr Hen. 'If it is familiar, you move on to the next bit of information; if it's novel, you want to be able to recognise that it's new and give it meaning. These mice, with just more adult-born neurones, and no other changes in the brain, basically learn better in tasks where they have to discriminate between similar contexts.'

Earlier strategies for manipulating neurogenesis, according to the investigators, were broader and less specific. 'In addition to stimulating neurogenesis, these earlier methods exerted many other effects on the brain. As a result, you never knew with these older manipulations what's due to neurogenesis, or what's due to the other effects that these manipulations cause, and, indeed, what we find is that when you stimulate just adult neurogenesis, you actually get a subtle effect. Unlike broader manipulations, it does not affect all forms of learning, it's very specific to tasks that require pattern separation,' said Dr Hen.

Pattern separation is not only important for learning; it may also be important for anxiety disorders, including post traumatic stress disorder (PTSD) and panic disorder. People with PTSD, say the researchers, have a more generalised fear response, so that when they are placed in a situation that reminds them of even one aspect of their trauma, they frequently have a full fear response.

'I think a good example of this is someone who has developed PTSD as a result of 9/11. For them, the simple sight of an airplane or high tower may be enough to reawaken the initial traumatic episode and bring back the full aversive memory. Sometimes these generalisations become so pervasive that people basically don't want to leave their home anymore because everything reminds them of the original event,' said Dr Hen.

The normal adaptive response, say the authors of the study, is to separate similar events or experiences. 'Even though I may remember 9/11, when I see an airplane over NYC, I am able to recognise that it's a different situation and process it accordingly, while someone in the same situation with PTSD may re-experience the traumatic memory of 9/11 and have a panic attack. So this may be one reason why stimulating neurogenesis to improve pattern separation may contribute to treatment of some of these anxiety disorders,' said Dr Hen.

Enhancing pattern separation, by either the method the Columbia researchers used, or other strategies, may also be useful in treating some of the learning deficits seen in people with normal or pathological ageing, such as Alzheimer's disease. In fact, there is already evidence that pattern separation declines during normal ageing.

'This paper, as a consequence, may stimulate a whole area of research in humans to try to determine who in the population may have a pattern separation deficit, and whether it is restricted to the emotional domain, or is present even while performing tasks devoid of emotional salience. Once these studies are done in humans, it may be possible to treat these people with specifically targeted drugs or more personalised therapies,' said Dr Hen.

The researchers say that the genetic strategy used to stimulate neurogenesis in their experiments can be mimicked pharmacologically, potentially leading to the development of new drugs to reverse pattern separation deficits. One such class of drugs the investigators are currently testing - BAX inhibitors - works by blocking cell death.

'These drugs are basically doing the same thing that we did with our genetic manipulation - namely, increasing the survival of the young neurones which normally undergo a process of cell death that eliminates at least half of these neurones. Now instead of dying, the neurones will go on to survive,' said Dr Sahay.

Some BAX inhibitors have been developed for stroke research, where the goal has also been to prevent neurones from dying. The Columbia researchers plan to begin testing the BAX inhibitors in mice shortly. And if they produce cognitive benefits, the testing will be extended to clinical trials to determine if there's also a beneficial effect in humans.

'I think we're getting closer to harnessing neurogenesis to improve cognition and mood in humans. This research may also help explain a bit of a mystery in the field, which we still don't understand, regarding how the hippocampus can be involved with both cognition - which is its classic function - and in mood and anxiety-related functions. Perhaps the fact that pattern separation affects both the cognitive and mood domains is the beginning of an answer to that paradox,' said Dr Hen.