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Keywords:

  • inhibitory post-synaptic currents;
  • nociception;
  • pain;
  • rat;
  • spinal cord;
  • substantia gelatinosa of Rolando

Abstract

The activity of GABAergic inhibitory interneurones located in lamina II of the spinal cord is of fundamental importance for the processing of peripheral nociceptive messages. We have recently shown that 3α-hydroxy ring A-reduced pregnane neurosteroids [3α5α-neurosteroids (3α5αNS)], potent allosteric modulators of GABAA receptors (GABAARs), are synthesized in the spinal cord and limit thermal hyperalgesia during inflammatory pain. Because changes in the expression of calcium-dependent protein kinases [protein kinase C (PKC)] are observed during pathological pain in the spinal cord, we examined the possible interactions between PKC and 3α5αNS at synaptic GABAARs. Using patch-clamp recordings of lamina II interneurones in the spinal cord of 15–20-day-old rats, we showed that synaptic inhibition mediated by GABAARs and its modulation by 3α5αNS in lamina II of the spinal cord largely depend on activation of PKC. Our experimental results suggested that activation of PKC locks synaptic GABAARs in a functional state precluding further positive allosteric modulation by endogenous and exogenous 3α5αNS. This effect was fully prevented by coadministration of chelerythrin, an inhibitor of PKC. Furthermore, application of chelerythrin alone rendered synaptic GABAARs hypersensitive to endogenously produced or exogenously applied 3α5αNS. These findings confirmed that there was a significant production of endogenous 3α5αNS in lamina II of the spinal cord but also indicated that PKC-dependent phosphorylation processes were tonically activated to control GABAAR-mediated inhibition under resting conditions. We therefore can conclude that PKC activation sets an upper limit to the functional plasticity of GABAergic transmission induced by endogenous neurosteroids.