A handful of people around the world have never known the meaning of physical pain – not because they live incredibly sheltered lives, but because their nerves lack a crucial ion channel that helps transmit signals between adjacent nerve cells. A new study reveals that our sense of smell depends on this same protein gate, establishing a previously unrecognised link between the perception of pain and scent.
Jan Weiss of the University of Saarland School of Medicine in Homburg, Germany, and his colleagues recruited three people who cannot feel pain because they have a rare condition known as congenital analgesia. Weiss wanted to know whether people with this disorder have difficulty with other senses.
The trio of participants – two of whom were siblings – could see and hear well and had never complained about a lousy sense of smell, but the researchers decided to put their noses to the test anyway. When the participants sniffed cotton wool pads soaked in balsamic vinegar, orange, mint, perfume and coffee, they failed to identify any of the odours. In contrast, nine healthy volunteers and the siblings' parents performed just fine, breathing deeply from the pleasant orange and mint scents and turning sharply away from the vinegar.
Weiss and his team already knew that people who cannot experience physical pain usually lack a sodium ion channel called Nav1.7 in the membranes of nerve cells in the dorsal root ganglion and in the ganglia that are part of the autonomic nervous system, and wondered whether this loss could also explain the smelling problems. To find out, they examined tissue samples taken from the nose and olfactory system of normal people during surgery. The examinations revealed Nav1.7 channels in the cell membranes of the neurons that stipple these tissues.
Knockout noses
Weiss bred mice that lacked Nav1.7 in their olfactory neurons and discovered that, although the neurons still produced electrical signals in response to odours, they no longer transferred the signals to other neurons as they typically would.
The behaviour of these "knockout" mice also implied that they could not smell. Mice are generally intrigued by the aromas of potential mates and of food, but when the researchers presented the knockout mice with the scents of male and female urine, peanut butter and milk, they showed no interest. The mice were similarly unfazed when researchers exposed them to a chemical that foxes secrete from their anal glands which usually sends rodents scurrying in the opposite direction. When the researchers separated mother knockout mice from their pups, the smell-impaired mums failed to corral their offspring – a behaviour that probably relies on smell.
Co-author Frank Zufall, also of the University of Saarland, says the connection between smell and pain was completely unexpected. "We don't know why these two systems use the same channel," Zufall says, "but it's possible this is a more general sodium ion channel for sensory systems. Earlier evidence has shown it's expressed in taste cells."
Joost Drenth of the Radboud University Nijmegen Medical Centre in the Netherlands says it is a very interesting paper and is likewise curious to find out how else Nav1.7 is involved in our sensory systems. Pain seems to be linked, at least anecdotally, to other senses: a blinding light, a deafening noise and spicy chilli pepper can all be painful.
Zufall adds that the discovery has important implications. Knowing that a particular ion channel is necessary for smell suggests that in the future we may be able to help people who have lost this sense. And because sodium ion channels are often targets for painkillers, it's helpful to know that they could also disrupt the sense of smell as an unintentional side effect.
No comments:
Post a Comment