DORSAL HORN CELLS CONNECTED TO THE LISSAUER TRACT AND THEIR RELATION TO THE DORSAL-ROOT POTENTIAL IN THE RAT

We have examined the role of dorsal horn cells that respond ts Lissaue r tract stimulation in regulating primary afferent depolarization (PAD ). PAD was monitored by recording the dorsal root potential (DRP) in t he roots of the lumbar cord. Recordings were made of the discharges of Lissauer tract-responsive cells, and their discharges were correlated with the DRPs occurring spontaneously and those evoked by stimulation . Electrical microstimulation of the Lissauer tract(<10 mu A; 200 mu s ) was used to activate the tract selectively and evoke a characteristi c long-latency DRP. Cells that were excited by Lissauer tract stimulat ion were found in the superficial laminae of the dorsal horn. They exh ibited low rates of ongoing discharge and responded to Lissauer tract stimulation typically with a burst of impulses with a latency to onset of 5.6 +/- 2.7 ms (mean +/- SD) and to termination of 13.6 +/- 4.1 ms (n = 109). Lissauer tract-responsive cells in L-5 were shown to recei ve convergent inputs from cutaneous and muscle afferents as they respo nded to stimulation of the sural nerve (100%, n = 19) and the nerve to gastrocnemius (95%, n = 19). The latency of the response to sural ner ve stimulation was 3.7 +/- 1.5 ms and to gastrocnemius nerve stimulati on, 8.3 +/- 3.6 ms. Stimulation through a microelectrode at a depth of 1.5 mm in the sensorimotor cortex (100 mu A, 200 mu s) evoked a respo nse in 17 of 31 Lissauer tract-responsive cells (55%) with a latency t o onset of 21.9 +/- 2.8 ms (n = 17). Stimulation of the sural nerve, n erve to gastrocnemius or sensorimotor cortex was shown to depress the response of Lissauer tract-responsive cells to a subsequent Lissauer t ract stimulus. The ongoing discharges of Lissauer tract-responsive cel ls were correlated to the spontaneous DRP using spike-triggered averag ing. Of 123 cells analyzed in this way, 117 (95%) were shown to be cor related to the DRP. in addition, the peaks of spontaneous negative DRP s in spinally transected animals were detected in software. Perievent time histograms triggered from these peaks showed the discharge of Lis sauer tract-responsive cells to be correlated to the spontaneous DRPs in 57 of 62 cells (92%) recorded. We conclude that these data provide compelling evidence that the Lissauer tract, and the dorsal horn cells that it excites, mediate the PAD evoked from multiple neural pathways .