Korean J Physiol Pharmacol.  2018 Jul;22(4):419-425. 10.4196/kjpp.2018.22.4.419.

The effect of µ-opioid receptor activation on GABAergic neurons in the spinal dorsal horn

• Kim YR
• Shim HG
• Kim CE
• Kim SJ

• KMID: 2414266
• DOI: http://doi.org/10.4196/kjpp.2018.22.4.419

Abstract

The superficial dorsal horn of the spinal cord plays an important role in pain transmission and opioid activity. Several studies have demonstrated that opioids modulate pain transmission, and the activation of µ-opioid receptors (MORs) by opioids contributes to analgesic effects in the spinal cord. However, the effect of the activation of MORs on GABAergic interneurons and the contribution to the analgesic effect are much less clear. In this study, using transgenic mice, which allow the identification of GABAergic interneurons, we investigated how the activation of MORs affects the excitability of GABAergic interneurons and synaptic transmission between primary nociceptive afferent and GABAergic interneurons. We found that a selective µ-opioid agonist, [D-Ala², NMe-Phe⁴, Gly-ol]-enkephanlin (DAMGO), induced an outward current mediated by K⁺ channels in GABAergic interneurons. In addition, DAMGO reduced the amplitude of evoked excitatory postsynaptic currents (EPSCs) of GABAergic interneurons which receive monosynaptic inputs from primary nociceptive C fibers. Taken together, we found that DAMGO reduced the excitability of GABAergic interneurons and synaptic transmission between primary nociceptive C fibers and GABAergic interneurons. These results suggest one possibility that suppression of GABAergic interneurons by DMAGO may reduce the inhibition on secondary GABAergic interneurons, which increase the inhibition of the secondary GABAergic interneurons to excitatory neurons in the spinal dorsal horn. In this circumstance, the sum of excitation of the entire spinal network will control the pain transmission.

Keyword

DAMGO; GABAergic interneurons; Opioid; Pain; Substantia gelatinosa

MeSH Terms

Analgesics, Opioid
Animals
Enkephalin, Ala(2)-MePhe(4)-Gly(5)-
Excitatory Postsynaptic Potentials
GABAergic Neurons*
Interneurons
Mice
Mice, Transgenic
Nerve Fibers, Unmyelinated
Neurons
Spinal Cord
Spinal Cord Dorsal Horn*
Substantia Gelatinosa
Synaptic Transmission
Analgesics, Opioid
Enkephalin, Ala(2)-MePhe(4)-Gly(5)-

Figure

• 

  Fig. 1 DAMGO, a selective µ-opioid receptor agonist, produced outward currents in GABAergic interneurons of the SG.(A) Identification of monosynaptic inputs from primary nociceptive C fibers to GABAergic interneurons. Superimposition traces of evoked EPSCs induced by monosynaptic C fiber inputs to GABAergic interneurons (conduction velocity (CV)=0.3 m/s). (B) In voltage clamp mode, DAMGO produced reproducible outward currents of about 39.55±2.16 pA in GABAergic interneurons. Holding potential (VH)=−70 mV.

• 

  Fig. 2 DAMGO-induced current is mediated by K+ channels.(A) Representative traces of current responses in the absence and presence of DAMGO in GABAergic interneurons. Voltage steps of 500 ms durations were commanded from −120 mV to −30 mV with 10 mV steps before and during superfusion with DAMGO. Holding potential (VH)=−70 mV. (B) The current-voltage (I-V) relationship of the DAMGO-induced current (n=6). (C) Schematic diagram of experimental protocol in (Ca) and representative traces of current responses of GABAergic interneurons (Cb) in the aCSF, DAMGO, and DAMGO added Ba2+. Voltage steps were commanded from −120 mV to −30 mV with 10 mV steps each lasting 500 ms. Holding potential (VH)=−70 mV. The I-V relationship shows that DAMGO-induced current is completely blocked by extracellular Ba2+ (Cc). (n=6, p<0.05, Two-way RM ANOVA).

• 

  Fig. 3 DAMGO-induced current is dependent on external K+ concentrations.(A) Representative traces of current responses at normal (2.5 mM) and high (8.5 mM) concentrations of the external K+ during DAMGO treatment. Voltage steps of 500 ms durations were commanded from −120 mV to −30 mV with 10 mV steps. (B) The DAMGO-induced current significantly increased at high K+ concentrations (n=5, p<0.05, Two-way RM ANOVA). Holding potential (VH)=−70 mV.

• 

  Fig. 4 DAMGO affects excitatory synaptic transmission between primary nociceptive C fibers and GABAergic interneurons in SG.(A) Representative traces before, during, and after DAMGO treatment. (B) Evoked EPSCs were significantly decreased by DAMGO in GABAergic interneurons which receive monosynaptic nociceptive input from C fibers. The evoked EPSCs were decreased to 42.24±10.41% of baseline and recovered by 9.92±7.02 of baseline (n=12; p<0.01, paired t-test). Data are presented as mean±SEM.

• 

  Fig. 5 A diagram to illustrate a possible mechanism by which DAMGO works in the SG.(A) In spinal dorsal horn, excitatory neurons normally receive an inhibitory control of GABAergic interneurons, and as a result, pain transmission is properly modulated. In addition, these GABAergic interneurons inhibit each other. (B) Under DAMGO, the excitability of GABAergic interneurons which receive direct excitatory inputs from primary nociceptive C fibers is decreased. If so, the inhibition of another GABAergic interneurons (*) toward excitatory neurons can be enhanced. As a results, the sum of excitation of the entire spinal circuit will reduce the pain transmission.

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