Depression has been linked to a reduction in serotonin and/or noradrenaline levels. In the brain these key chemicals are involved in setting people’s mood, but they have also been found in taste buds and are released from taste cells when a taste such as sugar activates the cell. Donaldson and Melichar reasoned that if serotonin or noradrenaline were reduced in someone with depression, the reduction of these chemicals throughout the body might also affect the taste buds and how they respond to different tastes, and that this would effect a depressed person’s ability to taste. They therefore decided to test whether changes in these chemicals could directly affect taste by using antidepressant drugs to enhance serotonin or noradrenaline levels in healthy volunteers – effectively creating the opposite situation to that found in depression.

Tom Heath, a Masters student on the project, first tested the volunteers for their ability to taste four different compounds – sweet, salt, bitter and sour. He then gave them antidepressant drugs that rapidly increased their levels of the neurotransmitters serotonin and noradrenaline. Three different drugs were given: an SSRI (serotonin specific reuptake inhibitor) to raise serotonin levels; an NARI (noradrenaline reuptake inhibitor) to raise noradrenaline levels; and an inactive placebo. Two hours later the volunteers were asked to take the taste tests again.

What Heath found was that when serotonin levels were increased, the volunteers could recognise sweet and bitter tastes at much lower concentrations than when their serotonin levels were normal. The effect on bitter was the most dramatic – the volunteers were able to taste it at concentration levels half of what they had been before being given the drug. The effect on sweet was also profound, with the concentrations of sugar people could taste being lowered by a third.

Increasing noradrenaline levels had different effects – instead of sweet and bitter taste recognition being enhanced, the same people could recognise bitter and sour tastes at lower concentrations. Salt taste did not seem to be affected at all by altering either of the neurotransmitters. Neither did the placebo have any effect, showing that the identified effects were real  – ie, the changes in peoples’ taste were not because they got better at performing the test or were better at guessing after taking the drugs, but that the drugs directly affected their ability to taste things at different concentrations.

These tests may help us choose the best drugs to treat depression

At each testing session the volunteers were also assessed for their anxiety levels and the researchers then determined whether their overall level of anxiety was related to their ability to taste. Some of the volunteers were quite anxious, and surprisingly, considering that these were normal healthy volunteers with no history of depression or anxiety, their level of anxiety was shown to be related to their ability to taste. The more anxious a person was, the less sensitive to bitter and salt taste they were. Taste is often thought to be determined genetically and, until now, people assumed it was fairly fixed throughout life, but these studies show that the ability to recognise different tastes can be altered by both the neurotransmitters serotonin and noradrenaline, and also by people’s mood.

These results, which were published at the end of last year in the Journal of Neuroscience, are exciting for taste research as they link previous work where serotonin and noradrenaline were shown to be important transmitters in the taste cells on the tongue with the effects of these transmitters on taste thresholds in normal people. This not only shows us that serotonin and noradrenaline are both important in changing peoples’ ability to recognise certain tastes, but  it also gives us information on which transmitters signal which tastes. These findings may also explain why anxious and depressed individuals have diminished appetite. In addition, the ability to recognise which tastes are most affected in someone with depression might give us some idea of the neurochemical basis for their disease – do they have reduced serotonin, noradrenaline, or both?

But perhaps the most exciting outcome of this research is the possibility that by identifying which neurotransmitter has been reduced, using these taste tests, we may then be able to chose the best drug with which to treat someone with depression. Until now there has been no easy way of deciding which is the best medication for an individual who is depressed.

As a result, we only get it right about 60-80 per cent of the time. It then takes up to four weeks to see if the drug is working or if we need to change it, during which time the person who is depressed may not be getting better. With a taste test we may be able to get it right first time. This research will be extended to look at taste in people with depression and anxiety in order to further explore these possibilities.

 

Dr Lucy Donaldson &  Dr Jan Melichar / Physiology