Receptors are message receivers located on the surfaces of cells, which initiate a sequence of changes in our bodies : for example, changes in the use of energy, tissue growth or the perception of pleasure and pain. These receptors receive hormones and neurotransmitters, which lock onto the receptors and trigger the event. When there is an excess of a certain hormone, first receptor resistance occurs and then the number of receptors decreases. There is some evidence that obesity is in part caused by a deficit in dopamine receptors. This leads the obese to overeat to achieve the level of satiety that normal individuals reach with less food. Bingeing raises the level of dopamine even higher, which leads to resistance and a ‘down-regulation’ of dopamine receptors, only worsening the craving problem. Genes may cause the initial problem and poor diet exacerbates it. Low levels of dopamine receptors are also found in alcoholics and drug addicts. Cocaine, or heroine, bind to the receptor, causing dopamine to accumulate since it no longer can get in.
While dopamine receptors are located largely in the brain, serotonin receptors are to be found all over the body – 80% are in other organs. Depression has been linked to low dopamine and serotonin receptor levels. Antidepressants can therefore make things worse in the long run since they temporarily raise the amount of serotonin in the body but this saturation decreases the number of serotonin receptors. In all these cases – depression, obesity and addiction – receptors react to excessive levels of hormone by becoming less sensitive to them.
The good news is we can also increase the number of receptors – for serotonin, dopamine and in the case of diabetes for insulin – since diabetes is caused by low levels of insulin receptors. Intense exercise, particularly on an empty stomach, resensitises receptors, and increases their number. In this way satiety from food and pleasure from life is more easily achieved. Fasting works in the same way. A decline in dopamine receptors is associated with ageing, but once again here is evidence a symptom of ageing can be reversed.
If we have enough dopamine receptors life becomes more than merely bearable. It is not just worth living, it becomes a source of fascination and adventure. Dopamine receptors enable us to max out on pleasure. Biology is not destiny and the decline associated with ageing is not inevitable. There is no quick fix, but a sustained exercise programme and calorie restriction can work wonders.
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