WIKIMEDIA, ORRLING AND TOMER SResearchers have identified a particular neurotransmitter that is responsible for passing itchy sensations from the skin to the brain, and found a new subset of neurons in the spinal cord that transmits those signals, according to a study published this week (May 24) in Science. The findings suggest that itchiness has a neural pathway distinct from the one that mediates pain sensation.
Itchiness is triggered by the activation of sensory neurons called TRPV1 cells but these neurons also respond to heat and pain, so researchers were unsure if the sensation of itchiness might be a low level form of pain.
Analyzing proteins expressed by TPRV1 cells in mice, researchers from the National Institute of Dental and Craniofacial Research in Bethesda, Maryland, discovered that a protein called natriuretic polypeptide b (Nppb) was expressed only in a subset of the cells. Nppb-knockout mice did not respond to itch-inducing compounds, suggesting that the protein is required to produce the itch sensation.
The team found Nppb receptors on spinal cord neurons that release a molecule called gastrin-releasing peptide (GRP), a neurotransmitter suspected to relay itch signals from nerve fibers in the skin to spinal cord neurons. GRP could not be found outside the spinal cord, however, suggesting this theory was wrong. Instead, the researchers propose that GRP is released downstream of Nppb-releasing TRPV1 neurons, which are the first neurons to transmit itch signals.
The discovery will not lead to new therapies for people suffering from chronic itchiness any time soon, because neural pathways may not work the same way in humans. But “at least they have a new target,” University of Minnesota neuroscientist Glenn Giesler told ScienceNOW.