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| 2.87 | The title is: Pharmacological Targeting of GABAA Receptors (Including α1β2γ2L Subtype) by Borneol Enantiomers [(+)-Borneol, (-)-Borneol] and Isoborneol: A Focus on GABAA Channels and Receptor Subunits |
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Title: Pharmacological Targeting of GABAA Receptors (Including α1β2γ2L Subtype) by Borneol Enantiomers [(+)-Borneol, (-)-Borneol] and Isoborneol: A Focus on GABAA Channels and Receptor Subunits
Positive Allosteric Modulation of GABAA Receptors by Borneol and Its Enantiomers
Borneol enantiomers [(+)-borneol, (-)-borneol] function as positive allosteric modulators (PAMs) of GABAA receptors, including the α1β2γ2L subtype [1][2][3][4]. (+)-borneol, a bicyclic monoterpene, positively modulates human recombinant α1β2γ2L GABAA receptors at low GABA concentrations [3]. Borneol’s stimulatory action at GABAA receptors aligns with its traditional medicinal uses, as GABAA receptor activation mediates effects like anxiolysis, sedation, and anesthesia [4].
GABAA Receptor Modulation and Related Effects of Borneol and Other Monoterpenes
| Parameter | Compound(s) | Details | Citation |
|---|---|---|---|
| Positive modulation potency rank (wild-type α1β2γ2s receptors, 100 μM (-)-borneol enhancing EC20 GABA responses) | (+)-menthol > (-)-menthol > (-)-borneol > (-)-menthone = (+)-camphor = (-)-camphor > (-)-carvone = (+)-carvone | Rank order of potency for positive modulation | [5] |
| Effect of picrotoxin on borneol-induced inward currents | Borneol | Currents reduced from 37.6 ± 6.49 pA to 18.0 ± 3.27 pA (n = 10, p < 0.001) by 50 μM picrotoxin (GABAA antagonist) | [6] |
| Potentiation of GABA-induced inward currents | 0.2 mM borneol | Potentiated 30 μM GABA-induced currents from 164 ± 40.8 pA to 300 ± 64.9 pA (n = 5, p < 0.05) | [6] |
| Enhancement of GABA-induced hyperpolarization | 0.3 mM borneol | Hyperpolarization increased from 6.58 ± 1.19 mV to 9.87 ± 1.02 mV (n = 6, p < 0.05) | [6] |
| Anticonvulsant activity | (-)-borneol | Protected against pentylenetetrazol (PTZ)- and maximal electroshock (MES)-induced convulsions in mice; activity linked to GABAergic neurotransmission via flumazenil experiments | [3] |
| Anesthetic potency (Xenopus tadpoles) | (+)-borneol | Loss of righting reflex EC50: 32.1 ± 9.1 μM; moderate correlation (r = 0.68) between anesthetic potency and estimated 100% GABAA receptor current potentiation for monoterpenes | [7] |
Electrophysiological studies confirm borneol’s interaction with GABAA receptors, as picrotoxin—a non-competitive GABAA antagonist—attenuates borneol-induced inward currents [6]. Additionally, (-)-borneol demonstrates anticonvulsant effects in mice, while (+)-borneol exhibits anesthetic potency in Xenopus tadpoles, with links to GABAA receptor modulation observed across these assays [3][7].
Subtype-Specific Modulation of α1β2γ2L GABAA Receptors by Borneol Enantiomers and Isoborneol
Human recombinant α1β2γ2L GABAA receptors are pharmacologically targeted by borneol enantiomers [(+)-borneol, (-)-borneol] and isoborneol, with distinct subtype-specific modulation patterns observed across varying GABA concentrations [3][8][9]. The magnitude of potentiation by these monoterpenes depends strongly on GABA concentration, as summarized below:
Modulation of Cl⁻ Conductance by Borneol Enantiomers and Isoborneol at α1β2γ2L GABAA Receptors Across GABA Concentration Ranges
| GABA Concentration Range | (+)-borneol Enhancement | (-)-borneol Enhancement | Isoborneol Enhancement | Citation |
|---|---|---|---|---|
| Extremely low (EC1–4) | 1095% | 884% | Not specified | [8] |
| Low (EC5–14) | 1251% | 1106% | 986% | [8] |
| Moderately low (EC15–24) | 848% | 903% | 419% | [8] |
| Intermediate (EC25–39) | Up to 300% | Up to 300% | Up to 300% | [8] |
| Intermediate (EC40–59) | 200% (discernible) | Not specified | Not specified | [8] |
| Maximal (EC100) | 19% potentiation | 21% reduction | 44% reduction | [8][9] |
At maximal (EC100) GABA concentrations, the enantiomers exhibit divergent effects, with (+)-borneol enhancing the response while (-)-borneol and isoborneol reduce it [8][9]. Additionally, (+)-borneol at 500 μM shifts the GABA dose-response curve leftward at α1β2γ2L receptors, decreasing the mean EC50 by 34 ± 8% without significantly reducing the maximal GABA response [8].
In addition to modulating GABA-evoked responses, (+)-borneol, (-)-borneol, and isoborneol exert direct actions on α1β2γ2L GABAA receptors in the absence of GABA. (+)-borneol generates the greatest direct effect, producing up to 89% of the maximal GABA response in a concentration-dependent and reversible manner (threshold concentration of 300 μM), followed by (-)-borneol (84%) and isoborneol (51%) [8][9]. The direct action of (+)-borneol is partially inhibited by bicuculline (up to 76% inhibition at 100 μM) and picrotoxinin (up to 60% inhibition at 30 μM), suggesting interaction with their respective binding sites, but is unaffected by flumazenil (a benzodiazepine [BZD] antagonist)—indicating no binding to the high-affinity BZD site [8][9]. Similarly, flumazenil does not block the enhancement of GABA responses by (+)-borneol at these receptors [8].
Structural features influence activity: the hydroxyl group is critical for potent modulation, as (-)-bornyl acetate (acetyl group substitution) and camphor (ketone group substitution) exhibit lower activity than the borneol enantiomers [8][9]. The orientation of the hydroxyl group also matters: borneol enantiomers have hydroxyl substituents opposite to the geminal dimethyl bridge, while isoborneol’s hydroxyl group is oriented in the same direction as the bridge, resulting in intermediate efficacy [9]. In vivo, (-)-borneol pretreatment (50, 100, 200 mg/kg, i.p.) protects against pentylenetetrazole (PTZ)- and maximal electroshock (MES)-induced convulsions in mice, increasing the latency of clonic convulsions and preventing tonic convulsions—effects that suggest modulation of GABAergic neurotransmission [3][9].
Collectively, these data demonstrate that borneol enantiomers and isoborneol are subtype-specific modulators of α1β2γ2L GABAA receptors, with distinct potency and efficacy profiles dependent on GABA concentration and structural features.
Anticonvulsant and Anxiolytic Effects of Borneol Linked to GABAergic Neurotransmission
The GABA receptor functions as a ligand-gated ion channel [10]. Borneol enantiomers and their derivatives exhibit anticonvulsant, anxiolytic, and GABAergic-modulating effects, with evidence linking these activities to GABAA receptor interaction and GABAergic neurotransmission [11]. Key findings on their pharmacodynamic effects are summarized below:
Borneol Enantiomer and Derivative Effects on Seizures, Anxiety, and GABAergic Signaling
| Compound/Form | Model/Assay | Dose/Administration | Key Observations | Citation |
|---|---|---|---|---|
| (+)-Borneol | Human recombinant α1β2γ2L GABAA receptors | Low GABA concentrations | Positive modulator of receptor activity | [3] |
| (-)-Borneol | PTZ- and MES-induced convulsions (mice) | 50, 100, 200 mg/kg, i.p. | Increased time onset of clonic convulsions; prevented MES-induced tonic convulsions | [3] |
| Borneol | MES-induced seizures | 300 mg/kg, i.p. | 75%, 37.5%, and 12.5% protection at 15, 30, and 60 min post-administration | [12] |
| Borneol | Geller conflict test | 800 mg/kg | Anxiolytic anti-conflict effect blocked by bicuculline (GABAA antagonist) | [13] |
| Borneol | PTZ-induced kindling model | 5, 10, 25 mg/kg, i.p. | Significantly decreased epileptogenic process vs. diazepam (1 mg/kg) | [13] |
| Borneol-containing essential oil (ZMEO) | PTZ-induced seizure models | Not specified | Flumazenil (benzodiazepine site antagonist) reversed anticonvulsant effects (reduced latency, increased clonic frequency, abolished mortality protection) | [14] |
| Borneol (BAO) | Mg²⁺-free-treated neurons; kainic acid (KA)-exposed models | Not specified | Regulated GABA signaling via GAD/GIRK1 pathway; ameliorated abnormal discharge; corrected decreased GABAA receptor expression | [15] |
Collectively, these findings confirm that the anticonvulsant and anxiolytic effects of borneol enantiomers are tied to GABAA receptor modulation and the regulation of GABAergic neurotransmission [11].
Analgesic Mechanisms of Borneol Independent of GABAA Receptors
Topical borneol-induced analgesia in the capsaicin model was unaffected by intrathecal injection of the GABAA antagonist bicuculline; however, bicuculline effectively attenuated analgesia from the GABAA agonist muscimol in a parallel positive control experiment, suggesting that GABAA-mediated mechanisms do not play a major role in this form of analgesia [16]. Additionally, TRPM8 exhibits greater sensitivity to borneol compared to other known targets, including GABAA [16].
References
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