Naloxone fails to block substance P-induced excitation of spinal nociceptive units.
In chloralose-anaesthetized cats the iontophoretic application of substance P produced an excitation of dorsal horn nociceptive units, as found in earlier studies. Naloxone (0.1-0.2 mg/kg, IV) failed to abolish this response to substance P in any of the 9 cats studied. In 4 additional cats nociceptive units were tested with morphine (1.0-3.2 mg/kg, IV) and substance P in case the antinociceptive effect of descending pathways is mediated via an excitatory action, but in each case morphine and substance P produced opposite effects (depression and excitation, respectively), suggesting that this was not the case. The present results fail to support a naloxone-antagonizable inhibitory effect of substance P in the spinal cord.
D000818:Animals; D002415:Cats; D004305:Dose-Response Relationship, Drug; D005073:Evoked Potentials, Somatosensory / Q000187:drug effects; D005727:Ganglia, Spinal / Q000187:drug effects; D009020:Morphine / Q000494:pharmacology; D009270:Naloxone / Q000494:pharmacology*; D009433:Neural Inhibition / Q000187:drug effects; D009619:Nociceptors / Q000187:drug effects*; D013116:Spinal Cord / Q000187:drug effects*; D013373:Substance P / Q000494:pharmacology*; D009435:Synaptic Transmission / Q000187:drug effects*
D013373:Substance P; D009270:Naloxone; D009020:Morphine