Thujone-rich essential oil from Aloysia deserticola (Rica-rica) decreases hippocampal inhibitory synaptic transmission in mice

Background

Aloysia deserticola Phil. is an endemic aromatic plant from the Chilean Andean highlands traditionally consumed as an herbal infusion. Although its essential oil has been chemically characterised, direct electrophysiological evidence for central nervous system actions remains limited. Because thujone-type monoterpenes have been described as non-competitive inhibitors of gamma-aminobutyric acid type A receptor (GABA_AR)-mediated signalling, we hypothesised that Aloysia deserticola essential oil (AD-EO) could modulate inhibitory neurotransmission in hippocampal circuits.

Aims

This study aimed to define the chemical profile of AD-EO and determine its acute effects on perisomatic inhibitory synaptic currents in dorsal hippocampal CA1 pyramidal neurons.

Methods

Essential oil was obtained by steam-water distillation and analysed by gas chromatography–mass spectrometry. Whole-cell voltage-clamp recordings were performed in acute dorsal hippocampal slices from postnatal day 30–45 C57BL/6 mice. Evoked inhibitory postsynaptic currents (eIPSCs) were recorded from CA1 pyramidal neurons at 0 mV in the presence of the NMDA and AMPA receptor antagonists MK-801 and CNQX, respectively, using perisomatic stimulation in the stratum pyramidale. AD-EO was bath-applied under serial dilution conditions of 0.2, 0.02, 0.01 µL/mL and 0.7 nL/mL, from the least diluted to the most diluted preparation, with estimated thujone-equivalent concentrations of approximately 1mM, 103, 51.7 and 3.8µM, respectively. Synaptic amplitude, paired-pulse ratio, rise and decay slopes, and coefficient of variation-based analyses were quantified. Normality was assessed using the Shapiro–Wilk test. Paired comparisons were analysed using paired Student’s t-test or Wilcoxon matched-pairs signed-rank test, group comparisons using one-way analysis of variance followed by Tukey’s post hoc test, and coefficient of variation relationships using linear regression.

Results

Gas chromatography–mass spectrometry identified 13 compounds, showing a strongly thujone-enriched chemotype dominated by α-thujone (89.06%) and β-thujone (3.38%), which together represented 92.44% of the total chromatographic area. Bath application produced a concentration-dependent depression of inhibitory synaptic transmission. At the highest concentration (~1mM), eIPSCs amplitude decreased from 101.8±4.3% at baseline to 45.6±3.4% 10 minutes after application, p<0.0001. At the lowest concentration (~3.8µM), amplitude was also reduced from 98.0±4.0% to 81.8±3.7%, p=0.003, although this effect was smaller than that observed at the highest concentration, p=0.003. Intermediate concentrations (~103 and ~51.7µM) reduced responses to 57.2±11.6% and 62.2±7.3% of baseline, both p<0.0001. The highest concentration decreased rise slope and accelerated decay kinetics, indicating changes in the inhibitory current waveform. Paired-pulse ratio increased at the highest and intermediate concentrations, while coefficient-of-variation (CV) analysis showed significant 1/CV²–amplitude relationships at these same concentrations but not at the lowest concentration. This pattern supports a predominantly postsynaptic effect, with an additional presynaptic contribution becoming evident under intermediate-to-less diluted AD-EO conditions, corresponding to higher estimated thujone-equivalent bath concentrations.

Conclusions

Together, these findings indicate that thujone-rich AD-EO rapidly decreases inhibitory synaptic efficacy in CA1 pyramidal neurons, with evidence consistent with a major postsynaptic contribution and an additional presynaptic component at intermediate-to-high concentrations. These results provide direct electrophysiological evidence that AD-EO modulates hippocampal inhibitory signalling and support its future evaluation as a candidate pharmacological tool to probe conditions characterised by altered inhibitory synaptic function.

References

No references