Published: November 20, 2015
The typical itch sensation that provokes a need to scratch is most often elicited by light tactile stimuli such as the brush of a feather against the skin or an insect crawling up one’s hand.
Vibrating fine vellous hair on the skin initiates this mechanical itch, which can serve as a type of defense mechanism for the body against some external irritant. Itch signals, in fact, are received by the same part of the brain that receives pain signals, but itching is an entirely unique sensation. When a sensory nerve ending such as that in the skin is stimulated, a nerve impulse travels along a sensory neuron to the spinal cord, from which a signal will be sent down a motor neuron to contract a body part, in this case movement of muscle to scratch. A study published in the journal “Science” explores the neural pathway involved in the promotion and inhibition of the mechanical itching sensation.
The study focused mainly on the inhibitory neurons found in the dorsal horn of the spinal cord that express a certain neuropeptide, Y. To understand the contribution of these inhibitory neurons to the sensation of mechanical itch and scratching, the authors administered a diphtheria toxin, used to reduce the number of inhibitory neurons in the spinal cord, to mice of precise genetic lineage known to express the neuropeptide Y neurons.
Two weeks post-injection, the mice displayed spontaneous, frequent scratching resulting in skin lesions. To further test the effects of the injection, the researchers delivered a mechanical itch to both the experimental and control mice, and the researchers observed increased scratching in those mice with a reduced number of inhibitory neurons and relatively low amounts of scratching among the control mice.
They also tested sensitivity to pinches versus brush strokes and found that there was an increase in hairy-skin neuronal activity when the mice were brush-stroked and no change in neuronal activity when the were pinched. The study found that inhibitory neurons expressing the neuropeptide Y contribute to tactile inhibition of itch by receiving inputs from low-threshold mechanoreceptors of hairy skin. These inhibitory neurons selectively gate low-threshold mechanical itch yet do not affect mechanical pain.
This study could help researchers better comprehend chronic itch conditions of humans, including eczema and diabetic neuropathy as well as any of those insensitive to antihistamine drugs. The mice with reduced neuropeptide Y inhibitory neurons display quite similar phenotypes to those humans with chronic itch conditions. Quifu Ma, a neurobiologist at Harvard University and one of the researchers in this study, said, “In the future, we may have some way to manipulate neuron activity to inhibit itching.”
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