The role of the dorsal column somatosensory pathway in ulnar nerve inhibition of cocaine-induced locomotion. (A) Dorsal column lesions (DC X) were introduced bilaterally at the C3 level of the spinal cord (A1), as shown in a representative image of toluidine blue staining (A2). Mechanical stimulation (MS) of the ulnar site (MS) reduced cocaine-induced locomotion (Sham + MS; n = 10), compared to the control group (DC X; n = 6), which was blocked by surgical lesion of DC prior to MS (DC X + MS; n = 7; A3). *p < 0.05 vs. Sham + MS. (B) When CN lesions (CN X) were performed bilaterally 7–10 days prior to the experiment (B1 and B2), MS of the ulnar site failed to reduce cocaine locomotion (CN X + MS; n = 10), compared to the control groups of CN X (CN X; n = 5), Sham (n = 5) and Sham + MS (n = 5; B3). *p < 0.05 vs. CN X + MS. (C) Representative waveforms and peri-stimulus time histograms of in vivo extracellular recordings in CN neurons (C1–2). Ulnar MS markedly increased CN WDR (n = 6) and LT (n = 7) spiking activity (C3). *p < 0.05 vs. Baseline. (D) Increased expression of c-fos immunopositive cells following ulnar stimulation (MS, n = 6; D1 & D3) in the dorsomedial portion of the CN (D2), compared to the control group (Con, n = 6; D1). Bar = 20 µm. *p < 0.05 vs. Con (D3).