Alterations in ascending dorsal horn neurons by a surgical incision in therat foot

Background: Little is known about the mechanisms of pain caused by a surgic al incision. The authors have developed a rat model of postoperative pain c haracterized by decreased withdrawal thresholds to punctate mechanical stim uli after plantar incision. The present studies examined the response chara cteristics of dorsal horn neurons receiving input from the plantar aspect o f the foot before and after a plantar incision placed adjacent to the low t hreshold area of the receptive field (RF). Methods: Individual dorsal horn neurons from the lumbar enlargement were an tidromically identified and characterized as low threshold, wide dynamic ra nge (WDR), and high threshold (HT) based on their responses to brush and pi nch. Thresholds (in millinewtons), the pinch RF, and stimulus-response func tions (SRFs) to von Frey filaments characterized the neurons. SRFs were ana lyzed using area under the curve. Changes in background activity, punctate mechanical thresholds, SRFs, and RF were recorded after an incision was mad e adjacent to the most sensitive area of the RF. Results: In all cells, an incision increased background activity; this rema ined elevated in 3 of 9 HT and 16 of 28 WDR neurons 1 h later. The SRFs wer e enhanced in 10 of 27 WDR neurons and in 2 of 8 HT cells after incision. O nly the WDR neurons were responsive to filaments that produced withdrawal r esponses after incision in behavioral experiments. Increases in the RFs out side of the injured area occurred after incision in 15 of 29 WDR and 2 of 9 Ill cells, Conclusion A plantar incision caused dorsal horn cell activation and centra l sensitization. Because the threshold of HT neurons did not decrease to th e range of the withdrawal responses in behavioral experiments, particular W DR dorsal horn neurons likely contribute to the reduced withdrawal threshol d observed in behavioral experiments, Both WDR and HT neurons are capable o f transmitting enhanced responses to strong punctate mechanical stimuli aft er incision.