Fear and forget

Amygdala intercalated neurons are required for expression of fear extinction.
http://www.ncbi.nlm.nih.gov/pubmed/18615014

Disclaimer: I appreciate all the positive feedback on the blog that I've received over the last month. I'm glad you guys are enjoying it! That having been said, I do get the occasional request for a shorter blog entry. So I suppose once a month, you guys deserve something you can read and digest in 5 minutes or less. So here goes nothing. I hope you appreciate the fanservice.

Today's Article
There are many fearful things in this world. Not the least of them is returning home at 10 PM only to realize you have a blog entry due by the next day. In all seriousness, fear is an adaptive mechanism geared towards survival. It allows us to mount a superior response in situations where we require additional attention. For example, fear is that involuntary emotion that causes us to run very, very fast the other way when we see an angry editor (or hungry lion) bearing down on us. Nevertheless, it is critical that we do not become consumed by the long-term effects of fear, which can become crippling and debilitating. Post-traumatic stress disorder (PTSD) affects sufferers with extreme anxiety. The relationship between long-term fear (anxiety) and and acute fear is still poorly understood, but this study attempts to shed further light upon the normal pathways by which fear is handled in healthy individuals.

Today's researchers asked whether the process of reducing fear after a harrowing situation, and removing the emotion of fear from memories, can be traced to a single region of the brain. This process is called 'fear extinction'. In fact, they found it could be traced to a single cell type, named the ITC neuron. These neurons reside in discrete clusters within the central fear processing center of the brain, the amygdala. Using a nifty biochemical trick, the researchers were able to piggyback a toxic molecule onto a chemical signal that these cells normally respond to in the brain, but neighboring cells don't recognize. This technique is also seeing use in the treatment of cancer. They applied this concoction in the vicinity of the ITC cell clusters in a rat's brain. Once inside the cell, this toxic molecule specifically killed the ITC cells.

The researchers then tested the rats for their responses to fear. They found that rats with the ITC neurons missing were able to respond normally to acutely fearful situations, but they continued to show elevated fear much longer than normal rats (up to a week). This result was strongly suggestive that ITC neurons pay a significant role in fear extinction. Although the length of the study was relatively short, it provides hope that specific neurons whose activity might be targeted by drugs or other therapies, are involved in the processes that underly excessive fear. The study does not describe a link between this region and any of the (rather poor) animal models of PTSD. Nevertheless, it is intriguing that one neuron type could be responsible for such a complex behavior as fear extinction. Substantial further work remains to validate these cells as a therapeutic target for anxiety or PTSD, but the discovery of ITC neurons it is a significant milestone. An interesting next step for this research would be to use functional imaging to study these clusters in the amygdala of patients suffering from PTSD. This technique would allow us to determine whether this brain region is functioning abnormally - it could be that a PTSD event causes such a sustained, high level of fear that these circuits are 'overloaded' so to speak. If so we might be able to study these cells more carefully for drug targets. Treatment of deep brain regions is still very difficult, but some intervention and possibly prophylaxis (for soldiers) might be possible.