Microelectrophoretic administration of naloxone near motoneurones fails to reproduce the effects of systemic naloxone in anaesthetized cats.

Using parallel micropipette assemblies intracellular recordings were obtained from motoneurones of barbiturate-anaesthetized cats while administering amino acids, opioid peptides and naloxone extracellularly. On motoneurones depolarized by DL-homocysteate or hyperpolarized by glycine, the opioid peptides had no effect on membrane potential. Naloxone did not increase evoked EPSP amplitudes and thus failed to reproduce the increases produced by systemic administration of this opioid antagonist. It is probable therefore that the tonic inhibition of motoneurones present in anaesthetized cats which is revealed by naloxone, does not involve release of opioid peptides near motoneurones but rather these compounds control the activity of spinal interneurones.