Touch stimulates neurogenesis
Mice exposed to novel tactile stimuli produced new immature neurons in the spinal cord, suggesting that neurogenesis may play a role in touch and vice versa
Novel touch stimuli can stimulate neurogenesis in the spinal cord of mice, according to a study published online today (16 November) in Molecular Psychiatry
, suggesting that neurogenesis may be an important component of touch sensation.
|Image: Wikimedia commons, Aaron Logan|
"This is a very interesting and unexpected result," said neuroscientist linkurl:Pierre-Marie Lledo;http://www.pasteur.fr/ip/easysite/go/03b-00002q-01k/perception-and-memory of the Institut Pasteur in France, who was not involved in the research. Neurogenesis in the spinal cord has predominately only been documented in vitro, he said. "To see indeed you have neurogenesis in vivo in the dorsal horn of the spinal cord is rather puzzling and very interesting," and suggests a new mechanism by which organisms may be able to process complex tactile environments, Lledo noted.
Recently, however, neuroimmunologist linkurl:Michal Schwartz;http://www.weizmann.ac.il/neurobiology/labs/schwartz/ of The Weizmann Institute of Science and her colleagues discovered proliferating neural progenitor cells in the dorsal horn of the mouse spinal cord. Because this part of the spinal cord is known to be composed of predominately sensory neurons, "it gave us an idea that [these new neurons] are participating in pain and/or touch sensation," Schwartz said.
To test this idea, the team placed mice in enriched cages containing sandpaper, gravel, or sponge substrates, or a combination, for 2 hours and measured new cell production in the spinal cord. Just 2 hours after exposure to the enriched environments, the mice showed a dramatic increase in the number of new cells in the dorsal horn. The amount of neurogenesis was greater in mice exposed to environments with a combination of substrate types, suggesting that cell proliferation may be a response not only to the novelty of an environment, but to its diversity as well.
"We had not expected to have such an amazing effect," said Ravid Shechter, a graduate student in Schwartz's lab who helped execute the experiments. "It was a very fast response to the environment."
"It was a huge surprise," agreed neuroimmunologist and coauthor linkurl:Asya Rolls,;http://med.stanford.edu/profiles/postdocs/researcher/Asya_Rolls a prior member of the Schwartz lab and a current postdoc at Stanford University. "[Neurogenesis is] an additional component that was never even suggested in this field [of touch sensation]."
To test the role of neurogenesis in habituation to stimuli, the team exposed the mice to the environments repeatedly over a 7-day period, or permanently housed them in the enriched cages. In contrast to the single exposure experiments, repeated exposures did not result in increased neurogenesis, and continuous exposure even seemed to inhibit the process. Further analysis of the cells revealed that instead of proliferating, the newly formed cells had begun to differentiate, mostly into GABAergic immature neurons. As inhibitory neurons, these GABAergic cells may play a role in habituation.
The immature neurons tended to die within just four weeks, however, Lledo noted. What drives neuronal death is unclear, but even at a young age, these neurons can be active. "The same category of neurons the same age in the olfactory bulb, we have been able to demonstrate about one week after birth these newborn neurons indeed release GABA." Thus, even though the neurons fail to mature, they may still have functional consequences, he said.
Indeed, the findings show a striking similarity to the process of adult neurogenesis in the olfactory bulb, said Lledo, where exposure to different odors has been found to stimulate cell proliferation. "If you look at the brain of a mouse living in a very clean and boring cage, the number of neurons is quite reduced," he said. "But as soon as you change the odorants every day, you stimulate the neurons two or three [fold]." Thus, the process of neurogenesis and differentiation may be "a more general phenomenon of plasticity in the sensory organs," he said.
The finding that neurogenesis may be an integral component of touch sensation may have implications for pain management and the treatment of pain diseases, Lledo added. Because these new GABAergic neurons, which are known to be inhibitory, are generated where pain fibers terminate in the spinal cord, "more newborn neurons located here will provide more inhibition to these nocioceptive fibers, and therefore will change the threshold of pain."
Furthermore, the results may provide some answers with regard to touch treatments used in alternative medicine, Schwartz said, for which the mechanisms are currently a bit of a mystery. "In this regard, [our results] may give a scientific basis to unexplained effects of touch treatment."
R. Shechter, et al., "Touch gives new life: mechanosensation modulates spinal cord adult neurogenesis," Molecular Psychiatry, 1-11, 2010.
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