Activation of µ‐opioid receptors and block of K_IR3 potassium channels and NMDA receptor conductance by l‐ and d‐methadone in rat locus coeruleus

Abstract

BACKGROUND AND PURPOSEMethadone activates opioid receptors to increase a potassium conductance mediated by G-protein-coupled, inwardly rectifying, potassium (KIR3) channels. Methadone also blocks KIR3 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. EXPERIMENTAL APPROACHIntracellular and whole-cell recording were made from locus coeruleus neurons in brain slices and the activation of m-opioid receptors and blockade of KIR3 and NMDA channels with l-and d-methadone was examined. KEY RESULTSThe 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 KIR3 conductance induced by activation of a2-adrenoceptors was decreased with high concentrations of l-and d-methadone (10-30 mM). In addition, methadone blocked the resting inward rectifying conductance (KIR). 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 AND IMPLICATIONSMethadone activated m-opioid receptors at low concentrations in a stereospecific manner. KIR3 and NMDA receptor channel block was not stereospecific and required substantially higher concentrations. The separation in the concentration range suggests that the activation of m-opioid receptors rather than the channel blocking properties mediate both the therapeutic and toxic actions of methadone. AbbreviationsACSF, artificial cerebrospinal fluid; AMPA, a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid; b-CNA, b-chlornaltrexamine; KIR, inward rectifying channel; KIR3, G-protein-coupled, inwardly rectifying, potassium channel; LC, locus coeruleus; ME, [Met]

Characterization of the antinociceptive effects of the individual isomers of methadone after acute and chronic administrations

“…The greater than predicted potency difference based on binding affinity at the higher stimulus intensity could reflect the d -isomer acting as a lower efficacy agonist relative to the l -isomer. This is consistent with whole cell current- and voltage-clamp recordings which showed a greater intrinsic activity for l -methadone relative to d -methadone (Matsui and Williams, 2010). However, further testing with combinations of the two isomers will be required to demonstrate definitively that, consistent with these in vitro assays, d -methadone functions as a partial efficacy μ agonist in vivo.…”

Identification of Functional Circuitry between Retrosplenial and Postrhinal Cortices during Fear Conditioning

“…CTb-488 was chosen as the retrograde tracer because of the high specificity of transport and the bright label provided by this compound (Conte et al, 2009). In addition, previous studies have shown that detection of fluorescently tagged CTb labeling is compatible with immunohistochemical techniques (Matsui and Williams, 2010). CTb-488 was infused into POR using the following coordinates with respect to lambda (in mm): +0.5, −0.1 anteroposterior, ±0.4 lateral and −4.4, −3.8 ventral from the skull surface, based on previous studies (Bucci et al, 2000; Burwell and Hafeman, 2003) and on the rat brain atlas of Paxinos and Watson (2007).…”

Regulation ofµ-Opioid Receptors: Desensitization, Phosphorylation, Internalization, and Tolerance

“…Estimates of agonist efficacy to determine RAVE values will be dependent on the effector under study. In addition, secondary actions of agonists such as the block of potassium channels by high concentrations of methadone will affect accurate determinations of efficacy Matsui and Williams, 2010). Likewise, the development of tolerance varies considerably with the assay.…”