Bidirectional actions of nociceptin/orphanin FQ on A delta-fibre-evoked responses in rat superficial spinal dorsal horn in vitro.

The present study investigated the modulatory actions of nociceptin/orphanin FQ on excitatory glutamatergic transmission in spinal dorsal horn. In transverse spinal cord slices with an attached dorsal root, mono- and polysynaptic A delta-fibre-evoked extracellular field potentials were recorded from superficial dorsal horn. Nociceptin/orphanin FQ showed bidirectional effects on monosynaptic transmission with a potentiation at lower concentrations (100-300 nM) and a dose-dependent depression at higher concentrations (1-3 microM). The polysynaptic field potential was dose-dependently depressed by nociceptin/orphanin FQ (100 nM-3 microM). None of the actions of nociceptin/orphanin FQ was reversed by the non-specific opioid receptor antagonist naloxone, the N-methyl-D-aspartate receptor antagonist D-2-amino-5-phosphonovaleric acid or the peptide nocistatin. The bidirectional actions of nociceptin/orphanin FQ on the monosynaptic field potential may provide an in vitro model for the bidirectional actions of nociceptin/orphanin FQ in behavioural studies showing hyperalgesia at low doses of intrathecal nociceptin/orphanin FQ and analgesia at higher doses.