Modulation of GABAA receptors and of GABAergic synapses by the natural alkaloid gelsemine

Abstract

The Gelsemium elegans plant preparations have shown beneficial activity against common diseases, including chronic pain and anxiety. Nevertheless, their clinical uses are limited by their toxicity. Gelsemine, one of the most abundant alkaloids in the Gelsemium plants, have replicated these therapeutic and toxic actions in experimental behavioral models. However, the molecular targets underlying these biological effects remain unclear. The behavioral activity profile of gelsemine suggests the involvement of GABAA receptors (GABAARs), which are the main biological targets of benzodiazepines (BDZs), a group of drugs with anxiolytic, hypnotic, and analgesic properties. Here, we aim to define the modulation of GABAARs by gelsemine, with a special focus on the subtypes involved in the BDZ actions. The gelsemine actions were determined by electrophysiological recordings of recombinant GABAARs expressed in HEK293 cells, and of native receptors in cortical neurons. Gelsemine inhibited the agonist-evoked currents of recombinant and native receptors. The functional inhibition was not associated with the BDZ binding site. We determined in addition that gelsemine diminished the frequency of GABAergic synaptic events, likely through a presynaptic modulation. Our findings establish gelsemine as a negative modulator of GABAARs and of GABAergic synaptic function. These pharmacological features discard direct anxiolytic or analgesic actions of gelsemine through GABAARs but support a role of GABAARs on the alkaloid induced toxicity. On the other hand, the presynaptic effects of the alkaloid provide an additional mechanism to explain their beneficial effects. Collectively, our results contribute novel information to improve understanding of gelsemine actions in the mammalian nervous system.

Molecular Pharmacology of Gelsemium Alkaloids on Inhibitory Receptors

“…Then, we examined whether these alkaloids’ modulatory profile on GlyRs was preserved on GABA A Rs. As previously shown, gelsemine can also inhibit GABA A Rs’ function but with a significantly lower potency and efficacy than GlyRs [ 14 , 15 ]. Koumine sensitivity of recombinant α1β2γ2 GABA A Rs, the most widely expressed GABA A R subtype in the mammalian brain [ 16 ], revealed -34.0 ± 5.3% of inhibition (n = 4), which was significantly lower than the koumine-induced inhibition of GlyRs ( Figure 1 E,F).…”

“…Electrophysiological studies have determined that gelsemine is a functional modulator of glycine receptors (GlyRs) and type A GABA receptors (GABA A Rs) [ 14 , 15 ], which are the main ligand-gated ion channels controlling CNS synaptic inhibition [ 16 ]. Gelsemine exerts subunit-specific actions on GlyRs composed of α subunits.…”

“…Spinal GlyRs were also inhibited by the alkaloid and showed an IC 50 of about 42 μM [ 14 ]. On the other hand, gelsemine inhibited recombinant and native GABA A Rs and showed IC 50 values of about 55–75 μM [ 15 ]. Experimental evidence from radioligand assays and electrophysiological analyses suggest that gelsemine actions on these ion channels occurs in a competitive manner.…”

The antianxiety effects of koumine and gelsemine, two main active components in the traditional Chinese herbal medicine Gelsemium: A comprehensive review

“…Gelsemine has also been shown to be a negative modulator, rather than an agonist, of benzodiazepine‐sensitive GABA A R and GABAergic synaptic functions and to inhibit native GABA A R expressed in cultured cortical neurons. Gelsemine significantly decreased the frequency of spontaneous currents at both GABAnergic and glutamatergic synapses and inhibited GABA‐activated chloride currents through the GABA A R configuration [52] . The negative effect of gelsemine on GABA A R only requires the α and β subunits without involving the γ2 subunit, implying that gelsemine may bind to the orthosite.…”

“…Gelsemine significantly decreased the frequency of spontaneous currents at both GABAnergic and glutamatergic synapses and inhibited GABA-activated chloride currents through the GABA A R configuration. [52] The negative effect of gelsemine on GABA A R only requires the α and β subunits without involving the γ2 subunit, implying that gelsemine may bind to the orthosite. Nevertheless, GABA A R, which is composed of α and β subunits, still has regulatory sites for other ligands, such as neurosteroids, through which gelsemine can exert its effects.…”