Repeated touch and needle prick stimulation in the neonatal period increases the baseline mechanical sensitivity and post-injury hypersensitivity of adult spinal sensory neurons

Noxious stimulation at critical stages of development has long-term consequences on somatosensory processing in later life, but it is not known whether this developmental plasticity is restricted to nociceptive pathways. Here we investigate the effect of repeated neonatal noxious or innocuous hind-paw stimulation upon adult spinal dorsal horn cutaneous mechanical sensitivity. Neonatal Sprague-Dawley rats of both sexes received four unilateral left hind-paw needle pricks (NP, n=13) or four tactile (cotton swab touch) stimuli, per day (TC, n=11) for the first 7 days of life. Control pups were left undisturbed (n=17). When adult (6-8 weeks), lumbar wide dynamic range (WDR) neuron activity in lamina III-V was recorded using in vivo extracellular single unit electrophysiology. Spike activity evoked by cutaneous dynamic tactile (brush), pinch and punctate (von Frey hair) stimulation and plantar receptive field areas were recorded, at baseline and 2 and 5 days following left plantar hind-paw incision. Baseline brush receptive fields, von Frey hair and pinch sensitivity were significantly enhanced in adult NP and TC animals compared to undisturbed controls, although effects were greatest in NP rats. Post-incision injury sensitivity of adult WDR neurons to both noxious and dynamic tactile hypersensitivity was significantly greater in NP animals compared to TC and undisturbed controls. We conclude that both repeated touch and needle prick stimulation in the neonatal period can alter adult spinal sensory neuron sensitivity to both innocuous and noxious mechanical stimulation. Thus, spinal sensory circuits underlying touch and pain processing are shaped by a range of early life somatosensory experiences.This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (CC BY), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited