Brief and prolonged effects of Lissauer tract stimulation on dorsal horn cells

Increased excitability of dorsal horn neurones may play a critical role in producing some pain states and there is evidence that the excitability of neurones lying throughout the dorsal horn is subject to regulation by cells in its most superficial laminae. This paper examines the effect on dorsal horn cell receptive fields and excitability of the specific activation of Lissauer's tract, a tract containing propriospinal axons which arise from cells in the substantia gelatinosa and which project to the substantia of neighbouring spinal segments. Experiments were carried out on anaesthetised spinal rats in the L3-4 spinal segments with microelectrode stimulation on the surface of the Lissauer tract (LT) and microelectrode recording of single cells or small groups of cells that responded to gentle brushing on the skin. Single shocks or brief trains of low-level stimuli to the LT produced a characteristic long-latency dorsal root potential (DRP) on the L3 dorsal root and a brief burst of firing in superficial cells with no stimulation of primary afferents. Generally, this was accompanied by no excitation of deeper dorsal horn cells but commonly by a period of inhibition, often followed by facilitation. We then turned to the effect of long periods (30-90min) of continual LT stimulation because we had seen hints of prolonged facilitation of the deeper cells after periods of such stimulation. Trains of 5 stimuli separated by 2ms and repeated every 200ms were used with individual pulses of 200 micros duration and less than 10 microA amplitude. This resulted in a shift of the effect on deep cells from primarily inhibition to mainly facilitation. We then examined in detail the effect of these long periods of LT stimulation on the size of receptive fields (RFs) of dorsal horn cells first on single units and then by repeated mapping of the RFs of small groups of cells. Control periods of 60min with no LT stimulation produced no significant RF changes but 30, 60 or 90min of LT stimulation produced successively greater expansions of RFs. When the LT stimulus was turned off after 1h, the RFs remained expanded for at least 2h. The spike height of these cells remained unchanged. The effect was not influenced by the NMDA antagonist MK801 but was imitated by the GABA(A) antagonist picrotoxin. It is concluded that the prolonged expansion of RFs could not be produced by modulation of descending control since the animals had spinal transections. Neither was it dependent on an NMDA-sensitive mechanism. With these data it is not possible to conclude whether the mechanism is pre-synaptic, post-synaptic or both. It is proposed that the most likely explanation is a decrease in the normal tonic inhibition operated in part by a GABA dependent mechanism. This phenomenon may play a role in prolonged pathological states of increased spinal cord excitability.