Comparison of the immediate effects of surgical incision on dorsal horn neuronal receptive field size and responses during postnatal development.

Background: Pain behavior in response to skin incision is developmentally regulated, but little is known about the underlying neuronal mechanisms. The authors hypothesize that the spatial activation and intensity of dorsal horn neuron responses to skin incision differ in immature and adult spinal cord.

Methods: Single wide-dynamic-range dorsal horn cell spike activity was recorded for a minimum of 2 h from anesthetized rat pups aged 7 and 28 days. Cutaneous pinch and brush receptive fields were mapped and von Frey hair thresholds were determined on the plantar hind paw before and 1 h after a skin incision was made.

Results: Baseline receptive field areas for brush and pinch were larger and von Frey thresholds lower in the younger animals. One hour after the incision, brush and pinch receptive field area, spontaneous firing, and evoked spike activity had significantly increased in the 7-day-old animals but not in the 28-day-old animals. Von Frey hair thresholds decreased at both ages.

Conclusions: Continuous recording from single dorsal horn cells both before and after injury shows that sensitization of receptive fields and of background and afferent-evoked spike activity at 1 h is greater in younger animals. This difference is not reflected in von Frey mechanical thresholds. These results highlight the importance of studying the effects of injury on sensory neuron physiology. Injury in young animals induces a marked and rapid increase in afferent-evoked activity in second-order sensory neurons, which may be important when considering long-term effects and analgesic interventions.