Working memory

The ability to remember things for a short time is called working memory. Some people can remember many things, while other just a few. Think about an actor playing Julius Cesar this evening and Macbeth tomorrow. He must be able to remember quite a bit, but just for the performance. Recent research indicates that working-memory capacity depends on the striatal function, in particular on dopamine. It is known that as we lose our D2 receptors, we also start to lose our mind.
Why some people has larger working memory capacity than others? The first answer comes from just looking at the striatum in healthy volunteers (Roshan et al., 2008). These authors asked eleven ladies to listen some sentences to test their memory abilities later. At the same time, they injected them with a modified tyrosine (6-[18F]fluoro-L-m-tyrosine) that can give a PET signal when converted to dopamine. As they checked the ladies in the PET scan, they found that those with larger working memory capacity have a striatum with a superb ability to make dopamine.
These observations mean that the faster you can delivery dopamine, the better your working memory would be. But dopamine is not a magic thing that produce an action by itself. It needs to bind to a receptor to produce a neuronal response. So, one would expect that a person with more dopamine receptors would produce better neuronal response at the striatum. In fact, there are two types (or classes) of dopamine receptors in the striatum. The D2 receptors produce a decrease in cAMP. i.e. an inhibitory response.
Now, it gets more interesting. There are two forms of D2 receptor; a long and a short form (due to alternative spicing). The short form is probably located presynaptically, and modulates glutamate release. It turns out that there is a polymorphism of D2 receptors. This means that my gene that encodes for D2 receptor may be different from your gene. Zhang and co-workers (Zhang et al., 2007), actually found that there are 23 places where the gene differs (single-nucleotide polymorphysm of SNP). Some of these forms favour the long D2 receptor, some the short. For example the SNP19 can have GG or GT at a particular position. The GT would favour the D2 receptor long form. A fMRI of those carrying the GT form revealed a more active caudate when the subjects have to remember things. Unfortunately these authors did not determine the memory capacity of their subjects.
The take home message seems to be the more active the dopaminergic transmission at the striatum is, the larger the working memory capacity. The bad news is that we are not all equal.