Background: Little is known about whether peripheral nerve injury during the early postnatal period modulates synaptic efficacy in the immature superficial dorsal horn (SDH) of the spinal cord, or whether the neonatal SDH network is sensitive to the proinflammatory cytokine TNFalpha under neuropathic conditions. Thus we examined the effects of TNFalpha on synaptic transmission and intrinsic membrane excitability in developing rat SDH neurons in the absence or presence of sciatic nerve damage.
Results: The spared nerve injury (SNI) model of peripheral neuropathy at postnatal day (P)6 failed to significantly alter miniature excitatory (mEPSCs) or inhibitory (mIPSCs) postsynaptic currents in SDH neurons at P9-11. However, SNI did alter the sensitivity of excitatory synapses in the immature SDH to TNFalpha. While TNFalpha failed to influence mEPSCs or mIPSCs in slices from sham-operated controls, it significantly increased mEPSC frequency and amplitude following SNI without modulating synaptic inhibition onto the same neurons. This was accompanied by a significant decrease in the paired-pulse ratio of evoked EPSCs, suggesting TNFalpha increases the probability of glutamate release in the SDH under neuropathic conditions. Similarly, while SNI alone did not alter action potential (AP) threshold or rheobase in SDH neurons at this age, TNFalpha significantly decreased AP threshold and rheobase in the SNI group but not in sham-operated littermates. However, unlike the adult, the expression of TNFalpha in the immature dorsal horn was not significantly elevated during the first week following the SNI.
Conclusion: Developing SDH neurons become susceptible to regulation by TNFalpha following peripheral nerve injury in the neonate. This may include both a greater efficacy of glutamatergic synapses as well as an increase in the intrinsic excitability of immature dorsal horn neurons. However, neonatal sciatic nerve damage alone did not significantly modulate synaptic transmission or neuronal excitability in the SDH, which could reflect a relatively weak expression of TNFalpha in the injured spinal cord at early ages. The above data suggest that although the sensitivity of the SDH network to proinflammatory cytokines after nerve injury is present from the first days of life, the profile of spinal cytokine expression under neuropathic conditions may be highly age-dependent.