Activation of µ-opioid receptors and block of Kir3 potassium channels and NMDA receptor conductance by L- and D-methadone in rat locus coeruleus.
OBJECTIVE
Methadone activates opioid receptors to increase a potassium conductance mediated by G-protein-coupled, inwardly rectifying, potassium (K(IR) 3) channels. Methadone also blocks K(IR) 3 channels and N-methyl-D-aspartic acid (NMDA) receptors. However, the concentration dependence and stereospecificity of receptor activation and channel blockade by methadone on single neurons has not been characterized.


METHODS
Intracellular and whole-cell recording were made from locus coeruleus neurons in brain slices and the activation of µ-opioid receptors and blockade of K(IR) 3 and NMDA channels with L- and D-methadone was examined.


RESULTS
The potency of L-methadone, measured by the amplitude of hyperpolarization was 16.5-fold higher than with D-methadone. A maximum hyperpolarization was caused by both enantiomers (∼30 mV); however, the maximum outward current measured with whole-cell voltage-clamp recording was smaller than the current induced by [Met](5) enkephalin. The K(IR) 3 conductance induced by activation of α(2) -adrenoceptors was decreased with high concentrations of L- and D-methadone (10-30 µM). In addition, methadone blocked the resting inward rectifying conductance (K(IR) ). Both L- and D-methadone blocked the NMDA receptor-dependent current. The block of NMDA receptor-dependent current was voltage-dependent suggesting that methadone acted as a channel blocker.


CONCLUSIONS
Methadone activated µ-opioid receptors at low concentrations in a stereospecific manner. K(IR) 3 and NMDA receptor channel block was not stereospecific and required substantially higher concentrations. The separation in the concentration range suggests that the activation of µ-opioid receptors rather than the channel blocking properties mediate both the therapeutic and toxic actions of methadone.
Matsui
Aya
A
Vollum Institute, Oregon Health and Science University, Portland, OR 97239, USA.
Williams
John T
JT
D000818:Animals; D051676:G Protein-Coupled Inwardly-Rectifying Potassium Channels / Q000037:antagonists & inhibitors* / Q000502:physiology; D008125:Locus Coeruleus / Q000187:drug effects* / Q000378:metabolism; D008297:Male; D008691:Methadone / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology*; D026902:Potassium Channel Blockers / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D016194:Receptors, N-Methyl-D-Aspartate / Q000037:antagonists & inhibitors* / Q000502:physiology; D017450:Receptors, Opioid, mu / Q000378:metabolism*; D013237:Stereoisomerism
D051676:G Protein-Coupled Inwardly-Rectifying Potassium Channels; D026902:Potassium Channel Blockers; D016194:Receptors, N-Methyl-D-Aspartate; D017450:Receptors, Opioid, mu; D008691:Methadone