Codeine presynaptically inhibits the glutamatergic synaptic transmission in the nucleus tractus solitarius of the guinea pig.
Although codeine is the most prominent and centrally acting antitussive agent, the precise sites and mode of its action have not been fully understood yet. In the present study, we examined the effects of codeine on synaptic transmission in second-order neurons of the nucleus tractus solitarius (NTS), which is the first central relay site receiving tussigenic afferent fibers, by using whole-cell patch-clamp recordings in guinea-pig brainstem slices. Codeine (0.3-3 mM) significantly decreased the amplitude of excitatory postsynaptic currents (EPSCs) evoked by electrical stimulation of the tractus solitarius in a naloxone-reversible and concentration-dependent manner, but it had no effect on the decay time of evoked EPSCs (eEPSCs). The inhibition of eEPSCs was accompanied by an increased paired-pulse ratio of two consecutive eEPSCs. The inward current induced by application of AMPA remained unchanged after codeine application. A voltage-sensitive K+ channel blocker, 4-aminopyridine (4-AP) attenuated the inhibitory effect of codeine on eEPSCs. These results suggest that codeine inhibits excitatory transmission from the primary afferent fibers to the second-order NTS neurons through the opioid receptors that activate the 4-AP sensitive K+ channels located at presynaptic terminals.
Ohi
Y
Y
Department of Molecular and Medical Pharmacology, Graduate School of Medicine and Pharmaceutical Sciences, University of Toyama, 2630 Sugitani, Toyama 930-0194, Japan.
Kato
F
F
Haji
A
A
D015761:4-Aminopyridine / Q000494:pharmacology; D018750:6-Cyano-7-nitroquinoxaline-2,3-dione / Q000494:pharmacology; D000701:Analgesics, Opioid / Q000494:pharmacology*; D000818:Animals; D003061:Codeine / Q000494:pharmacology*; D003627:Data Interpretation, Statistical; D004594:Electrophysiology; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D019706:Excitatory Postsynaptic Potentials / Q000187:drug effects; D018698:Glutamic Acid / Q000502:physiology*; D006168:Guinea Pigs; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D009270:Naloxone / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D026902:Potassium Channel Blockers / Q000494:pharmacology; D015221:Potassium Channels / Q000187:drug effects; D018091:Receptors, AMPA / Q000187:drug effects; D017661:Receptors, Presynaptic / Q000187:drug effects*; D017552:Solitary Nucleus / Q000187:drug effects / Q000502:physiology*; D009435:Synaptic Transmission / Q000187:drug effects*
D000701:Analgesics, Opioid; D018691:Excitatory Amino Acid Antagonists; D009292:Narcotic Antagonists; D026902:Potassium Channel Blockers; D015221:Potassium Channels; D018091:Receptors, AMPA; D017661:Receptors, Presynaptic; D009270:Naloxone; D018698:Glutamic Acid; D018750:6-Cyano-7-nitroquinoxaline-2,3-dione; D015761:4-Aminopyridine; D003061:Codeine