Nucleus pulposus application onto rat spinal dorsal nerve roots leads to a persistent increase in spinal C-fibre responses, possibly due to upregulation of IL1α, IL1β and TNF

Abstract

Sensitization of sensory neurons after noxious conditioning may be involved in many chronic pain states, including radiating low back pain and sciatica following disc herniation. Here, we examine such sensitization induced by two types of noxious conditioning: I) electrical sciatic high frequency stimulation (HFS), and II) nucleus pulposus (NP), harvested from intervertebral discs, applied onto the spinal dorsal nerve roots. In addition, we investigate the gene expression of the proinflammatory cytokines IL1α, IL1β, TNF and the protease MMP1 in NP tissue. Electrophysiological extracellular potentials were recorded from the spinal dorsal horn in anaesthetized Sprague-Dawley- or Lewis rats. A single test stimulus was applied to the sciatic nerve every 4th minute and the A- and C-fibre responses were separated according to latencies. For the gene expression analysis, total RNA was isolated from NP tissue and mRNA expression quantified by RT-qPCR. First, the spinal neuronal responses were studied by field potential recordings following HFS conditioning of the sciatic nerve. The HFS conditioning produced a clear long-term potentiation (LTP), which outlasted the experimental time period of 180 minutes. Next, the spinal neuronal responses were studied by single unit recordings following NP application onto the dorsal nerve roots. The NP conditioning produced a persistent increase in the spinal C-fibre responses, also outlasting the experimental time period of 180 minutes. In addition, the present study demonstrated a significant upregulation in the gene expression of IL1α, IL1β and TNF 180 minutes after application of NP onto the spinal dorsal nerve roots. No changes, however, were seen in the expression of MMP1. In summary, the HFS caused a robust LTP in the spinal cord. Furthermore, application of NP onto the spinal dorsal nerve roots induced an LTP-like phenomenon which was also associated with an increase in gene expression of IL1α, IL1β and TNF in NP tissue 180 minutes after application. The present data suggests that herniated NP in contact with the dorsal nerve roots may cause a persistent spinal hyperexcitability of nociceptive neurons, possibly due to biochemical mediators intrinsic to the NP tissue.