When faced with a depressed patient, clinicians often have to choose a proper course of treatment based on a guess as to which neurotransmitter in the brain is being disrupted--serotonin, noradrenaline or both. According to Jan Melichar, a psychiatrist at the University of Bristol, doctors "get it right about 60 to 80 percent of the time," but they have to wait up to one month to see if they chose correctly. A report appearing in the December 6 issue of The Journal of Neuroscience could result in an easier way of putting patients on a path to mental stability.
The authors, coauthored by Melichar and Lucy Donaldson, a neuroscientist and physiologist also at the University of Bristol, found that a person's sense of taste--known to be genetic and once thought to be fixed--is in fact plastic and responds to changes in neurotransmitter levels as well as to different moods. The research group gave 20 volunteers either one of two classes of common antidepressants--serotonin specific reuptake inhibitors (SSRIs) or noradrenaline reuptake inhibitors (NARIs)--or a placebo. (Each drug increases the level of its specific neurotransmitter in the brain.) Before taking the treatments and then again two hours after, the subjects took taste tests where researchers gave them solutions of tastants in different concentrations and told them what taste to expect: sour, salt, sweet or bitter. The participants then had to indicate at what concentration they could detect taste.
The team discovered that those who took SSRIs reported an increased sensitivity to sweet and bitter tastes, detecting them at concentrations of 27 percent and 53 percent lower, respectively, than before ingestion of the drug. Among those who took NARIs, they detected sour and bitter tastes at concentrations 22 percent and 39 percent lower, respectively, than before treatment. Neither group responded differently to salty tastes. Donaldson doesn't know exactly why the salt threshold remained unaffected in the short term. "It may be because there's something to do with the way that salt is detected by the taste cells within the tongue," she speculates, noting that she and Melichar believe the effects of their manipulations are taking place primarily in the taste buds and not in the brain. The team, however, did find that a decreased sensitivity to salt correlated well with higher general anxiety levels among the 20 study participants--as did sensitivity to bitterness--although no one in the study suffered from either anxiety disorders or depression.
"For a long time we thought that taste wasn't plastic," explains Linda Kennedy, a neurobiologist at Clark University in Worcester, Mass., who also studies the taste system. These findings, she continues, provide evidence that "internal stimuli will change taste sensitivity." She also notes that the connection between mood and taste sensitivity may have some applications for fighting obesity. The Bristol team, for its part, believes the work can be expanded to create a test for prescribing the right antidepressants to patients. "What we're working towards, hopefully, is to try and use taste tests and taste reactions in people as a sort of marker for the levels of those neurotransmitters in people with depression, so that we can tell if they've got a serotonin problem or a noradrenaline problem or both," Donaldson says. She admits, however, that the group still has only worked thus far with normal subjects and needs to see how tastes may be altered for people who are depressed as well as how these effects may change over the term of the illness. In addition, they would need to do both shorter- and longer-term studies to determine how these neurotransmitter manipulations play out over time. As Donaldson explains: "That will give us an idea of how much it is going on in the brain and how much of it is actually going on in the taste bud."