The mosquito and black fly season has started. Itching? Then try to refrain from scratching! The latest discovery published in the Science magazine brings hope to all those who will spend the summer scratching raw bites. It is a comforting thought, even though we will probably have to wait many more itchy seasons before a real relief.
Itching is one of the primary defence mechanisms of the body – although scientists are not exactly sure how and what for it evolved, they suppose that it serves chasing off or crushing the insect biting us. It can also help to remove the irritant substance from the skin. The reason seems to be trivial, but thanks to this, it is easier for us to avoid malaria, yellow fever, dengue fever, river blindness and sleeping sickness. All of these diseases occur in Africa, in the region which the Homo sapiens originated from. It is not surprising that evolution rewarded any methods for protection against them.
Since the insects, which we want to protect ourselves from, attack with a vengeance at night, the brain has learned to react with scratching even during deep sleep. Normally, we do not realize it, but every night we unconsciously reach with our hand several times and make a few quick moves that saved the life of our ancestors.
The strangest thing is that despite the universality of itching, we still do not know exactly what structures are responsible for the feeling. Once it was believed that itching is just a very weak stimulation of the nerves conducting the sensation of pain, but in the late 80s, the German researchers disproved this theory. They administered hypodermic injections with increasing doses of histamine, which is known to cause itching, to a group of volunteers. Although the examined people described increasing of the sensation, from mild to unbearable, they were always talking about itching, and never about pain.
It was, therefore, decided to try and track down the nerves responsible for conducting this sensation. Very thin electrodes, which tracked the activity of individual nerves, were placed in the skin of the studied persons. Once a single fibre was “tapped”, the volunteer was tormented by stabbing and scalding. If a nerve was conducting any of these sensations, another one was looked for. Finally, it was possible to extract a small group of fibres which were considered to be conductive of the itchy sensation.
After following them, it was found that the brain does not respond to itching with a simple reflex, as in the case of scalding. In response to itching, a number of areas responsible for, among other things, the location of sensation, but also for the emotional response and stimulus-reward reaction – the same one which is connected with drug addiction – were activated.
The researchers concluded that the itching-scratching cycle is a complex reaction of the whole brain. The need to scratch is not as violent as the primitive reflex to withdraw hand to avoid burns, but it is growing and spreads across such a large area of the brain that it is simply impossible to control it through conscious decision.
In fact, itching proved to be not the skin reaction, but the brain’s. Now, additionally, we know that our body has special neuronal “itchy paths”, which are managed by a protein called Nppb. Mice which did not produce this protein were bred. In whatever way they were teased and tickled, they did not feel itching (they felt pain, which suggests that separate mechanisms are responsible for these sensations). Administration of the protein to the spinal cord restored this ability.
It thus appears beyond all doubt that we have specifically dedicated “itchy path” in our nervous system. Will the manipulations of the described protein lead us to the production of new medications releasing us from the effects of mosquito bites? Is this a light in the tunnel for allergy sufferers, who react with itching to the very thought of the spring and summer season?
There is hope.