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Gelsemine is one of the principal alkaloids produced by the Gelsemium genus of plants belonging to the Loganiaceae family. The extracts of these plants have been used for many years, for a variety of medicinal purposes. Coincidentally, recent studies have shown that gelsemine exerts anxiolytic and analgesic effects on behavioural models. Several lines of evidence have suggested that these beneficial actions were dependent on glycine receptors, which are inhibitory neurotransmittergated ion channels of the CNS. However, it is currently unknown whether gelsemine can directly modulate the function of glycine receptors. EXPERIMENTAL APPROACHWe examined the functional effects of gelsemine on glycine receptors expressed in transfected HEK293 cells and in cultured spinal neurons by electrophysiological techniques. KEY RESULTSGelsemine directly modulated recombinant and native glycine receptors and exerted conformation-specific and subunit-selective effects. Gelsemine modulation was voltage-independent and was associated with differential changes in the apparent affinity for glycine and in the open probability of the ion channel. In addition, the alkaloid preferentially targeted glycine receptors in spinal neurons and showed only minor effects on GABA A and AMPA receptors. Furthermore, gelsemine significantly diminished the frequency of glycinergic and glutamatergic synaptic events without altering the amplitude. CONCLUSIONS AND IMPLICATIONSOur results provide a pharmacological basis to explain, at least in part, the glycine receptor-dependent, beneficial and toxic effects of gelsemine in animals and humans. In addition, the pharmacological profile of gelsemine may open new approaches to the development of subunit-selective modulators of glycine receptors. Abbreviations

Glycine receptors (GlyRs) are members of the pentameric ligandgated ionic channel family (pLGICs) and mediate fast inhibitory neurotransmission in the brain stem and spinal cord. The function of GlyRs can be modulated by positive allosteric modulators (PAMs). So far, it is largely accepted that both the extracellular (ECD) and transmembrane (TMD) domains constitute the primary target for many of these PAMs. On the other hand, the contribution of the intracellular domain (ICD) to the PAM effects on GlyRs remains poorly understood.To gain insight about the role of the ICD in the pharmacology of GlyRs, we examined the contribution of each domain using a chimeric receptor. Two chimeras were generated, one consisting of the ECD of the prokaryotic homologue Gloeobacter violaceus ligand-gated ion channel (GLIC) fused to the TMD of the human α 1 GlyR lacking the ICD (Lily) and a second with the ICD (Lily-ICD). The sensitivity to PAMs of both chimeric receptors was studied using electrophysiological techniques. The Lily receptor showed a significant decrease in the sensitivity to four recognized PAMs. Remarkably, the incorporation of the ICD into the Lily background was sufficient to restore the wild-type α 1 GlyR sensitivity to these PAMs. Based on these data, we can suggest that the ICD is necessary to form a pLGIC having full sensitivity to positive allosteric modulators.

Glycine receptors (GlyRs) are anion-permeable pentameric ligand-gated ion channels (pLGICs). The GlyR activation is critical for the control of key neurophysiological functions, such as motor coordination, respiratory control, muscle tone and pain processing. The relevance of the GlyR function is further highlighted by the presence of abnormal glycinergic inhibition in many pathophysiological states, such as hyperekplexia, epilepsy, autism and chronic pain. In this context, previous studies have shown that the functional inhibition of GlyRs containing the α3 subunit is a pivotal mechanism of pain hypersensitivity. This pathway involves the activation of EP2 receptors and the subsequent PKAdependent phosphorylation of α3GlyRs within the intracellular domain (ICD), which decrease the GlyR-associated currents and enhance neuronal excitability. Despite the importance of this mechanism of glycinergic dis-inhibition associated with dysfunctional α3GlyRs, our current understanding of the molecular events involved is limited. Here, we report that the activation of PKA signaling pathway decreases the unitary conductance of α3GlyRs. We show in addition that the substitution of the PKAtargeted serine with a negatively charged residue within the ICD of α3GlyRs and of chimeric receptors combining bacterial GLic and α3GlyR was sufficient to generate receptors with reduced conductance. Thus, our findings reveal a potential biophysical mechanism of glycinergic dis-inhibition and suggest that post-translational modifications of the ICD, such as phosphorylation, may shape the conductance of other pLGICs. Glycine receptors (GlyRs) belong to the pentameric ligand-gated ion channel (pLGIC) family. GlyRs are anion-permeable channels, allowing the fast influx of chloride and the control of neuronal excitability. An individual GlyR subunit is composed by an extracellular domain (ECD), four transmembrane domains (TM1-4) and an intracellular domain between the TM3 and TM4 domains (ICD) 1-4. To date, a single β subunit and four α subunits (α1-4) has been described. The α subunits share a high degree of sequence identity (≈75%). Nevertheless, they exhibit important differences in their biophysical and pharmacological properties as well as in their distribution along the CNS 1,3,4. In the mammalian CNS, GlyR activity critically controls neurophysiological functions such as motor coordination, respiratory control, muscle tone, as well as pain processing 2,3,5-13. The importance of glycinergic inhibition was first recognized in studies using the GlyR antagonist strychnine 14,15. Later, genetic studies found that mutations in the GlyR α1 and β genes are linked to hyperekplexia in humans 16. More recent evidence has shown that specific GlyR subunits may play key roles in several diseases. For example, while the α1 subunit has been linked to tumorigenesis and alcohol intoxication 17,18 , mutations in the α2 subunit have been linked to autism 19. Alterations in the RNA processing of α3 subunits generates hyperactive receptors, which have been rela...

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.