Recently, a group from Naples reports that D-Aspartic acid functions as a neurotransmitter in both a mammal the rats (Rattus norvegicus), and a mollusk (Loligo vulgaris). D-Aspartic acid (D-Asp) has been known to scientists for well over a century. However, its role as a neurotransmitter was only now confirmed by the work presented by D’Aniello et.al.

The brain is usually thought of in its own category when considering our organs; deservedly so, since it seems to be the seat of our personal identity, our selves.  Still, the molecules we think of as neurotransmitters (GABA, glutamate, serotonin, etc.) we most often isolated from other parts of the body, and have roles in biology unconnected with neurotransmission.

Presence in the brain is of course not the qualifying factor for describing any particular molecule as a neurotransmitter. The D’Abuello et.al demonstrate not only the presence of D-Asp acid in high concentrations in synaptic vesicles, but also show that there are specific post-synaptic receptors for D-Asp which trigger signal-transduction of cAMP upon binding of the D-Asp ligand.

What did the group find?
Kleinfeld’s group devised a technique that uses elaborately-named “cell-based neurotransmitter fluorescent engineered reporters” (CNiFERs for short) to examine how neurotransmitter receptors are activated. CNiFERs are cells that have been engineered to change color when acted upon by a specific neurotransmitter. The group created CNiFERs that responded to acetylcholine and implanted these cells into the frontal cortex of adult rats. When stimulated, the CNiFERs fluoresced to indicate the presence of acetylcholine in the frontal cortex.

Next, the group injected the rats with clozapine and olanzapine – neuroleptic drugs (antipsychotics) that are often used to treat schizophrenia. The implanted cells ceased to fluoresce, indicating that the drugs were blocking the transmission of acetylcholine. In other words, the group could see how neuroleptics were affecting the frontal cortex simply by looking at its color!

Why is this important?
This is very exciting and potentially very important. It is theoretically possible to create CNiFERs that respond to any neurotransmitter and to implant cells in any part of the brain. The mechanisms by which many psychiatric drugs work is largely a mystery, with selective serotonin reuptake inhibitors (SSRIs, used to treat depressions) being a case in point. With CNiFERs, researchers have a potentially powerful tool to understand how biochemical signals are relayed through the brain and the sites where they are active. This may lead to more effective treatments for disorders and lay bare many of the secrets of biochemistry that have been hidden for so long.

What was so important about Caspi et al.?
The original Caspi paper of 2003 looked at a sample of 847 New Zealanders, and examined whether gene-environment interactions might lead to depression. The research team were particularly interested in genes involved in serotonin transportation and reception - serotonin has long been recognized as an important biochemical in the neurobiology of depression. They identified short and long versions of 5-HTT’s regulatory region. The short allele led to decreased gene expression and therefore fewer serotonin transporters in the the cell. Results of the study found that individuals with the short variant of the serotonin transporter 5-HTT predicted the onset of depression – but only if the individual was exposed to repeated environmental stress. The finding was important because it provided a blueprint for understanding gene-environment interactions: genes came to be viewed not so much as rigid determinants of fate, but part-players in a complex narrative.

What has changed?
A lot. The new paper by Merikangas and colleagues failed to substantiate the gene-environment association for 5-HTT. Normally, this might be viewed as a minor setback, but this study was a meta-analysis of 14 papers covering approximately 12,500 individuals – a huge data-pool by anybody’s standards. The group did confirm an association between stressful circumstances and depression (hardly big news). However, the serotonin transporter, 5-HTT did not enter into the equation. For many researchers in the field this is very disappointing and may force a rethink in our whole understanding of the disorder.

Caspi and co-author Terrie Moffitt are sticking by their guns, however. Science quotes them as saying that the new resarch “ignores the complete body of scientific evidence”, including studies in mice who have an elevated stress response as a result of their 5-HTT genotype. This may be the case, but Caspi and Moffitt are now swimming against the tide. It hard to argue with a sample of 12,500. Moreover, other sources had already begun to question the validity of the 5-HTT association. Recent whole-genome association studies, which have scanned a huge amount of genomes and found several thousand biomarkers did not turn up anything on 5-HTT.
While it is not time yet to close the book entirely on the association, I think it is time for some serious re-evaluation.