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ctional adaptation of the spinal pain reflex networks during development. The basic concept is that spinal interneurons, grouped in modules, spontaneously start up muscle twitches that, in turn, cause a sensory feedback informing about the consequence of the resulting movement pattern. This sensory feedback is then "imprinted" on the circuitry of the module.

In a sense, the spinal module asks the question "What function do I have?" and then receives a sensory echo answering the question. The strength of the connections in the circuitry can then be set appropriately. That tactile (i.e., innocuous), rather than noxious, (i.e., painful) input is used to set the gain in the pain pathways solves the puzzle of how pain systems can be functionally adapted during early life despite the rare occurrence of noxious stimuli.

In one study, the authors examined the development of a withdrawal reflex in 161 rats after a brief noxious heat stimulus to the tail. Tail withdrawal movements were erroneous in more than 50 percent of the cases up to approximately postnatal day 10. Thereafter, tail movements gradually become appropriate and reached adult levels (i.e., less than 10% error) at postnatal day 21. Interestingly, this adaptation occurred even when the tail was deprived of any noxious stimuli by either encasement in a tube or application of topical anesthetics.

However, removal of tail hairs to prevent tactile sensation blocked pain responses and adaptation, indicating that low-threshold mechanoreceptors are necessary for adaptation to the noxious stimulus. Anesthetizing the tail in adult rats did not alter the reflex. These findings indicate that this cross-modality adaptive response during early development is driven by normally activated mechanosensors that influence the strength of synapses involved in the withdrawal reflex. It seems in this case, "no pain, no gain" is not the operating principle. In another animal study, it was demonstrated that to
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Contact: Dawn McCoy
dawn@sfn.org
202-462-6688
Society for Neuroscience
10-Nov-2003

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