The role of innocuous touch in driving inhibition in a spinal cord pain pathway

The dorsal horn of the spinal cord is the first relay area where incoming pain, temperature and touch sensory information is processed before being sent onward to higher brain areas. When intrinsic inhibition is disrupted, innocuous inputs are allowed to activate nociceptive pathways, leading to pathological touch evoked pain. Thus it has been proposed that inhibitory neurons receive innocuous inputs, and prevent touch from activating nociceptive output neurons in the dorsal horn. Despite the importance of inhibition in the dorsal horn, studies have only begun to distinguish specific subtypes of inhibitory interneurons, the types of sensory information they receive, and how they influence nociceptive output neurons. To address whether a subset of inhibitory neurons in the superficial dorsal horn receives innocuous input, patch clamp recordings were made from EGFP-labeled inhibitory neurons. We demonstrate that a significant number of these inhibitory neurons receive innocuous input. Recordings were also made from fluorescently-labeled projection neurons that were shown to be nociceptive specific, and necessary for the development of neuropathic pain hypersensitivity. These data demonstrate that innocuous inputs drive feedforward inhibition onto the majority of projection neurons tested.;Finally, there is evidence for changes in circuits that process innocuous inputs in the dorsal horn during postnatal maturation. We demonstrate a trend of decreasing LT excitatory drive of inhibitory neurons during postnatal maturation, contrasted by an increasing prevalence of excitatory and inhibitory LT drive onto projection neurons. Taken together these results point to ongoing postnatal rearrangements to the networks activated by sensory inputs in the dorsal horn.