Nerve injury in very young animals does not result in pain as it does later in life, probably because of an anti-inflammatory response in the spinal cord.

Maria Fitzgerald of University College London and her colleagues damaged hind-limb nerves in rat and mouse pups. They tested the sensitivity of the paws to painful stimuli, such as mechanical pressure, then recorded the excitability of spinal neurons at different ages and analysed immune profiles. They found that both pain sensitivity and neural excitability developed in the pups at an age equivalent to human adolescence, when anti-inflammatory molecules were replaced by those that promote inflammation.

The team reversed this pain suppression in injured young rats by blocking production of IL-10, an anti-inflammatory cytokine. The results could explain why such 'neuropathic' pain sometimes emerges mysteriously in human adolescents, the authors say.

J. Neurosci. 35, 457–466 (2015)