Korean J Physiol Pharmacol.  2020 Sep;24(5):433-440. 10.4196/kjpp.2020.24.5.433.

Inhibitory actions of borneol on the substantia gelatinosa neurons of the trigeminal subnucleus caudalis in mice

• Nguyen PTT
• Jang SH
• Rijal S
• Park SJ
• Han SK

• KMID: 2505778
• DOI: http://doi.org/10.4196/kjpp.2020.24.5.433

Abstract

The substantia gelatinosa (SG) of the trigeminal subnucleus caudalis(Vc) is the first relay site for the orofacial nociceptive inputs via the thin myelinatedAδ and unmyelinated C primary afferent fibers. Borneol, one of the valuable timehonoredherbal ingredients in traditional Chinese medicine, is a popular treatmentfor anxiety, anesthesia, and antinociception. However, to date, little is known asto how borneol acts on the SG neurons of the Vc. To close this gap, the whole-cellpatch-clamp technique was applied to elucidate the antinociceptive mechanismresponding for the actions of borneol on the SG neurons of the Vc in mice. In thevoltage-clamp mode, holding at –60 mV, the borneol-induced non-desensitizinginward currents were not affected by tetrodotoxin, a voltage-gated Na⁺ channelblocker, 6-cyano-7-nitro-quinoxaline-2,3-dione, a non-N-methyl-D-aspartate (NMDA)glutamate receptor antagonist and DL-2-amino-5-phosphonopentanoic acid, anNMDA receptor antagonist. However, borneol-induced inward currents were partiallydecreased in the presence of picrotoxin, a -aminobutyric acid (GABA)_A receptorantagonist, or strychnine, a glycine receptor antagonist, and was almost suppressedin the presence of picrotoxin and strychnine. Though borneol did not show any effecton the glycine-induced inward currents, borneol enhanced GABA-mediatedresponses. Beside, borneol enhanced the GABA-induced hyperpolarization under thecurrent-clamp mode. Altogether, we suggest that borneol contributes in part towardmediating the inhibitory GABA and glycine transmission on the SG neurons of the Vcand may serve as an herbal therapeutic for orofacial pain ailments.

Keyword

Borneol; GABA receptors; Glycine receptors; Patch-clamp techniques; Substantia gelatinosa

Figure

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  Fig. 1 Chemical structure of borneol.

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  Fig. 2 Borneol-induced inward currents are repeatable on the SG neurons of the Vc. (A) A representative trace showing the repeatable inward currents by borneol (0.3 mM) under high chloride pipette solution. (B) The before and after plot shows no significant difference in the mean inward currents between the 1st and the 2nd applications of 0.3 mM borneol (n = 17, paired t-test, p > 0.05). NS implicates not significant.

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  Fig. 3 Direct action of borneol on the SG neurons of the Vc. (A) A representative current trace showing no effect on the borneol-induced response by tetrodotoxin (TTX, 0.5 µM), a voltage-sensitive Na+ channel blocker under high chloride pipette solution. (B) The before and after plot shows no significant difference in the mean borneol-induced inward currents between the absence and the presence of tetrodotoxin (n = 13, paired t-test, p > 0.05). NS implicates not significant.

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  Fig. 4 Borneol-induced action is not mediated by ionotropic glutamate receptor activation. (A) A representative current trace showing no effect on the borneol-induced response by 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX) (10 µM) and DL-2-amino-5-phosphonopentanoic acid (AP5) (20 µM), ionotropic glutamate receptor antagonists. (B) The before and after plot shows no significant difference in the borneol-induced inward currents between the absence and the presence of CNQX and AP5 (n = 8, paired t-test, p > 0.05). NS implicates not significant.

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  Fig. 5 Borneol activates GABAA receptor on the SG neurons of the Vc. (A) A representative current trace showing the inhibition of borneol-induced inward current by picrotoxin (50 µM), a GABAA receptor antagonist. (B) The before and after plot shows significant inhibition of mean inward currents by borneol in the presence of picrotoxin (n = 10, paired t-test, ***p < 0.001).

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  Fig. 6 Borneol activates glycine receptor on the SG neurons of the Vc. A representative current trace showing the inhibition of the borneol-induced inward currents by strychnine (2 µM), a glycine receptor antagonist. (B) The before and after plot shows significant inhibition of mean inward currents by borneol in the presence of strychnine (n = 7, paired t-test, **p < 0.01).

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  Fig. 7 GABA- and/or glycine-mimetic actions of borneol. (A) A representative current trace showing inhibition of the borneol-induced inward currents by both picrotoxin (50 µM) and strychnine (2 µM). (B) The before and after plot shows significant inhibition of mean inward currents by borneol in the presence of picrotoxin and strychnine (n = 8, paired t-test, ***p < 0.001).

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  Fig. 8 Borneol enhances on GABA-induced responses but not glycine-induced responses. (A) A representative current trace showing the increase of the GABA-induced response by borneol. (B) The before and after plot shows significant increase of mean inward current by GABA (30 µM) in the presence of 0.2 mM borneol (n = 5, paired t-test, *p < 0.05). (C) A representative current trace showing no change of the glycine-induced response by borneol. (D) The before and after plot shows no significant change in the mean inward currents by glycine (30 µM) in the presence of 0.2 mM borneol (n = 8, paired t-test, p > 0.05). NS implicates not significant.

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  Fig. 9 Borneol enhances GABA-induced hyperpolarization. (A) A representative trace showing the membrane hyperpolarization induced by GABA (30 µM) alone and GABA in the presence of borneol (0.3 mM) in the current-clamp mode. (B) The before and after plot shows significant enhancement of mean membrane potential change by GABA in the absence and the presence of borneol (n = 6, paired t-test, *p < 0.05).

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