Anesthetic blockade of the dorsolateral funiculus enhances evoked activity of spinal cord dorsal horn neurons

A previous study of cat lumbar dorsal horn neurons found reduced responsiveness to A-fiber stimulation 1.5-12 h after thoracic dorsolateral funiculus (DLF) lesions. The present study was undertaken to determine whether this was due to the loss of descending activity or to factors specifically associated with injury by examining the response properties of dorsal horn cells before and during lidocaine blockade of the ipsilateral DLF. Electric shocks applied to the dorsal columns were used to search for dorsal horn cells. Noxious and nonnoxious cutaneous mechanical stimuli and graded electrical stimuli applied to the tibial nerve were used to activate peripheral afferent fibers. Cells were classed as low threshold (LT), high threshold (HT), or multireceptive (MR), according to their responses to natural stimuli. Baseline data were collected from a total of 58 cells. Twelve of these were further studied after lidocaine injection of the DLF. All cells examined with lidocaine were in dorsal horn laminae III-V. All cells responded to activation of tibial nerve A fibers. However, the median threshold for the HT and MR cells (200 μA) was significantly higher than that of the LT cells (75 μA). Some cells in each class were also activated by C fibers (10, 70, and 64% of the LT, HT, and MR cells, respectively). For the cells that were further characterized by lidocaine blockade of the DLF, all LT cells (n = 3) responded only to A-fiber stimulation, and all HT (n = 3) and MR cells (n = 6) responded to both A- and C-fiber stimulation. For LT cells, responses evoked by mechanical and electrical stimuli were unaltered by lidocaine blockade. HT and MR cells showed enhanced responses to electrical stimulation of C fibers during DLF blockade. There was no consistent effect of the blockade on A-fiber-evoked responses. Two of three HT and four of six MR cells studied with lidocaine had spontaneous activity, which exhibited a small but significant increase during DLF blockade. Receptive fields for noxious stimulation expanded in two of six MR cells during DLF blockade. Two of three HT cells developed responses to tactile stimuli during the blockade. In two additional cells (1 HT and 1 MR), spontaneous activity and responses to C-fiber input increased after the DLF was cut. These results indicate 1) that loss of descending activity is not responsible for the decreased responsiveness seen within 90 min after DLF lesions (and suggest that other effects of spinal cord injury, such as blood flow changes or release of inhibitory transmitters from degenerating terminals, may underlie this change), 2) that descending projections in the ipsilateral DLF mediate tonic inhibition of dorsal horn neuron activity, 3) that the classification of dorsal horn cell type can be affected by release of descending inhibition.