Bud extracts from Tilia tomentosa Moench inhibit hippocampal neuronal firing through GABAA and benzodiazepine receptors activation

Ethnopharmacological relevance: Tilia tomentosa Moench bud extracts (TTBEs) is used in traditional medicine for centuries as sedative compound. Different plants belonging to the Tilia genus have shown their efficacy in the treatment of anxiety but still little is known about the mechanism of action of their bud extracts. Aim of the study: To evaluate the action of TTBEs as anxiolytic and sedative compound on in vitro hippocampal neurons. Material and methods: The anxiolytic effect of TTBEs was assayed by testing the effects of these compounds on GABAA receptor-activated chloride current of hippocampal neurons by means of the patch-clamp technique and microelectrode-arrays (MEAs). Results: TTBEs acutely administered on mouse hippocampal neurons, activated a chloride current comparable to that measured in the presence of GABA (100 mM). Bicuculline (100 mM) and picrotoxin (100 mM) blocked about 90% of this current, while the remaining 10% was blocked by adding the benzodiazepine (BDZ) antagonist flumazenil (30 mM). Flumazenil alone blocked nearly 60% of the TTBEs activated current, suggesting that TTBEs binds to both GABAA and BDZ receptor sites. Application of high doses of TTBEs on spontaneous active hippocampal neurons grown for 3 weeks on MEAs blocked the synchronous activity of these neurons. The effects were mimicked by GABA and prevented by picrotoxin (100 mM) and flumazenil (30 mM). At minimal doses, TTBEs reduced the frequency of synchronized bursts and increased the cross-correlation index of synchronized neuronal firing. Conclusions: Our data suggest that TTBEs mimics GABA and BDZ agonists by targeting hippocampal GABAergic synapses and inhibiting network excitability by increasing the strength of inhibitory synaptic outputs. Our results contribute toward the validation of TTBEs as effective sedative and anxiolytic compound.