Neurons involved in pain processing have been the subject of much research in the past decades, but neurons responsible for a more pesky problem -- itchy skin -- remain elusive.

In fact, many neurons seem to be involved in itch response, a process closely linked to the processing of pain in the brain, researchers reported today at the annual meeting of the Society for Neuroscience, in Washington, D.C.

To find which neurons were responsible for the itching response Glenn Giesler from the University of Minnesota and his colleagues tracked the projection of spinalthalamic neurons activated by itching stimuli to the posterior thalamus -- a brain region to which pain-sensing neurons are thought to project.

To trigger a response in itch neurons, the researchers exposed monkeys to two types of itch stimuli on their hands: a histamine injection, and a plant called cowhage, which causes a six-minute itchy reaction if smeared on the skin. Histamines and cowhage have been shown to use two different pathways to signal the itch response in the brain.

The researchers inserted electrodes into the monkey's brains to measure the neuronal firing in the thalamus, and identified a small region of the posterior thalamus in which a third of the neurons responded to both itching stimuli. However, they found no strong evidence for a universal itch-sensing neuron; some neurons responded to both itch stimuli through different signaling pathways, while others only fired in response to one or the other stimulus. In addition, two thirds of the neurons responded to an injection of capsaicin --the active chemical compound in chili peppers which triggers a painful burning response -- suggesting that the pathways of itch and pain are closely linked.

Other findings also showed the intimate relationship between itch and pain. For example, scratching an itch at the peak of its "itchiness" actually inhibits itch neuron firing in the brain by up to 40% in some cases, the researchers found. Giesler theorizes that the itch stimuli and the pain stimuli can activate the same neurons, so that when you scratch your itch, the scratching, which is a form of pain, counteracts the itch response by deactivating the sensory neuron.