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Background Physical injury, including surgery, can result in chronic pain; yet chronic pain following childbirth, including cesarean delivery in women, is rare. The mechanisms involved in this protection by pregnancy or delivery have not been explored. Methods We examined the effect of pregnancy and delivery on hypersensitivity to mechanical stimuli of the rat hindpaw induced by peripheral nerve injury (spinal nerve ligation) and after intrathecal oxytocin, atosiban and naloxone. Additionally, oxytocin concentration in lumbar spinal cerebrospinal fluid was determined. Results Spinal nerve ligation performed at mid-pregnancy resulted in similar hypersensitivity to nonpregnant controls, but hypersensitivity partially resolved beginning after delivery. Removal of pups after delivery prevented this partial resolution. Cerebrospinal fluid concentrations of oxytocin were greater in normal postpartum rats prior to weaning. To examine the effect of injury at the time of delivery rather than during pregnancy, spinal nerve ligation was performed within 24 h of delivery. This resulted in acute hypersensitivity that partially resolved over the next 2–3 weeks. Weaning of pups resulted only in a temporary return of hypersensitivity. Intrathecal oxytocin effectively reversed the hypersensitivity following separation of the pups. Postpartum resolution of hypersensitivity was transiently abolished by intrathecal injection of the oxytocin receptor antagonist, atosiban. Conclusions These results suggest that the postpartum period rather than pregnancy protects against chronic hypersensitivity from peripheral nerve injury and that this protection may reflect sustained oxytocin signaling in the central nervous system during this period.

Proinflammatory cytokines may sensitize primary sensory neurons and facilitate development of neuropathic pain processes after peripheral nerve injury. The goal of this study was to determine whether responses of dorsal root ganglion (DRG) neurons to exogenous tumor necrosis factor alpha (TNF-alpha) are altered in a chronically compressed DRG (CCD) injury model. Extracellular recordings from teased dorsal root microfilaments demonstrated that acute topical application of TNF-alpha to the DRG for 15 min evoked C- and Abeta-fiber responses in both normal and CCD rats. However, the response latency was significantly shorter, and the peak discharge rate was higher, in CCD fibers than in normal fibers. Intracellular recordings from small- and large-sized neurons showed that TNF-alpha induced greater depolarization and greater decrease in rheobase in CCD neurons than in normal neurons. The proportion of both small- and large-sized neurons that were responsive to TNF-alpha increased significantly after CCD injury. Furthermore, TNF-alpha altered the discharge patterns of large, spontaneously active neurons in addition to enhancing their discharge rates. However, the depolarization caused by TNF-alpha in such neurons was minor (<2 mV). Inflammatory cytokines such as TNF-alpha increased the sensitivity of sensory neurons in normal and CCD rats. The CCD injury itself, on the other hand, increased neuronal responses to inflammatory cytokines.

Local perfusion of the dorsal root ganglion (DRG) with tumor necrosis factor alpha (TNF-alpha) in rats induces cutaneous hypersensitivity to mechanical stimuli. Thus we investigated the cellular mechanisms of TNF-alpha-induced mechanical hyperalgesia. The L(4) and L(5) DRGs with the sciatic nerves attached were excised from rats for in vitro dorsal root microfilament recording. After baseline recording for 15 min, TNF-alpha (0.001, 0.01, 0.1, or 1 ng/ml) was applied to the DRG for 15 min, followed by washout for at least 30 min. Alternatively, H-89 or Rp-cAMPS, two specific cAMP-dependent protein kinase (PKA) inhibitors, was added to the perfusion solution for 15 min prior to TNF-alpha application. TNF-alpha (1 ng/ml) induced neuronal discharges in 67% (14/21) of C fibers and 27% (4/15) of Abeta fibers when applied topically to the DRG. Acute TNF-alpha application not only evoked discharges in silent fibers, but also enhanced ongoing activity of spontaneously active fibers and increased neuronal sensitivity to electrical stimulation of the peripheral nerves. H-89 (10 microM) and Rp-cAMPS (100 microM) each completely blocked the TNF-alpha-evoked response in most C and Abeta fibers tested but did not affect fiber conductivity. Our results demonstrates that exogenous inflammatory cytokines such as TNF-alpha can elicit a PKA-dependent response in sensory neurons and thus strongly suggest that endogenous TNF-alpha may contribute to the development of certain pathological pain states.