Modulation of nicotinic acetylcholine receptors by strychnine

VirtuousPocketome: a computational tool for screening protein-ligand complexes to identify similar binding sites

Protein residues within binding pockets play a critical role in determining the range of ligands that can interact with a protein, influencing its structure and function. Identifying structural similarities in proteins offers valuable insights into their function and activation mechanisms, aiding in predicting protein-ligand interactions, anticipating off-target effects, and facilitating the development of therapeutic agents. Numerous computational methods assessing global or local similarity in protein cavities have emerged, but their utilization is impeded by complexity, impractical automation for amino acid pattern searches, and an inability to evaluate the dynamics of scrutinized protein-ligand systems. Here, we present a general, automatic and unbiased computational pipeline, named VirtuousPocketome, aimed at screening huge databases of proteins for similar binding pockets starting from an interested protein-ligand complex. We demonstrate the pipeline's potential by exploring a recently-solved human bitter taste receptor, i.e. the TAS2R46, complexed with strychnine. We pinpointed 145 proteins sharing similar binding sites compared to the analysed bitter taste receptor and the enrichment analysis highlighted the related biological processes, molecular functions and cellular components. This work represents the foundation for future studies aimed at understanding the effective role of tastants outside the gustatory system: this could pave the way towards the rationalization of the diet as a supplement to standard pharmacological treatments and the design of novel tastants-inspired compounds to target other proteins involved in specific diseases or disorders. The proposed pipeline is publicly accessible, can be applied to any protein-ligand complex, and could be expanded to screen any database of protein structures.

Exploring neurotransmitters and their receptors for breast cancer prevention and treatment

While psychological factors have long been linked to breast cancer pathogenesis and outcomes, accumulating evidence is revealing how the nervous system contributes to breast cancer development, progression, and treatment resistance. Central to the psychological-neurological nexus are interactions between neurotransmitters and their receptors expressed on breast cancer cells and other types of cells in the tumor microenvironment, which activate various intracellular signaling pathways. Importantly, the manipulation of these interactions is emerging as a potential avenue for breast cancer prevention and treatment. However, an important caveat is that the same neurotransmitter can exert multiple and sometimes opposing effects. In addition, certain neurotransmitters can be produced and secreted by non-neuronal cells including breast cancer cells that similarly activate intracellular signaling upon binding to their receptors. In this review we dissect the evidence for the emerging paradigm linking neurotransmitters and their receptors with breast cancer. Foremost, we explore the intricacies of such neurotransmitter-receptor interactions, including those that impinge on other cellular components of the tumor microenvironment, such as endothelial cells and immune cells. Moreover, we discuss findings where clinical agents used to treat neurological and/or psychological disorders have exhibited preventive/therapeutic effects against breast cancer in either associative or pre-clinical studies. Further, we elaborate on the current progress to identify druggable components of the psychological-neurological nexus that can be exploited for the prevention and treatment of breast cancer as well as other tumor types. We also provide our perspectives regarding future challenges in this field where multidisciplinary cooperation is a paramount requirement.

Strychnine and its mono- and dimeric analogues: a pharmaco-chemical perspective

Covering: up to November 2021Since its isolation in 1818, strychnine has attracted the attention of a plethora of chemists and pharmacologists who have established its structure, developed total syntheses, and examined its complex pharmacology. While numerous reviews on structure elucidation and total synthesis of strychnine are available, reports on structure-activity relationships (SARs) of this fascinating alkaloid are rare. In this review, we present and discuss structures, synthetic approaches, metabolic transformations, and the diverse pharmacological actions of strychnine and its mono- and dimeric analogues. Particular attention is given to its SARs at glycine receptors (GlyRs) in light of recently published high-resolution structures of strychnine-GlyR complexes. Other pharmacological actions of strychnine and its derivatives, such as their antagonistic properties at nicotinic acetylcholine receptors (nAChRs), allosteric modulation of muscarinic acetylcholine receptors as well as anti-cancer and anti-plasmodial effects are also critically reviewed, and possible future developments in the field are discussed.

Neuron-specific responses to acetylcholine within the spinal dorsal horn circuits of rodent and primate

Excitatory and inhibitory neurotransmission within the spinal dorsal horn is tightly controlled to regulate transmission of nociceptive signals to the brain. One aspect of this control is modulation of neuronal activity through cholinergic signaling. Nociceptive neurons in the dorsal horn express both nicotinic and muscarinic cholinergic receptors and activation of these receptors reduces pain in humans, while inhibition leads to nociceptive hypersensitivity. At a cellular level, acetylcholine (ACh) has diverse effects on excitability which is dependent on the receptor and neuronal subtypes involved. In the present study we sought to characterize the electrophysiological responses of specific subsets of lamina II interneurons from rat and marmoset spinal cord. Neurons were grouped by morphology and by action potential firing properties. Whole-cell voltage-clamp recordings from lamina II dorsal horn neurons of adult rats showed that bath applied acetylcholine increased, decreased or had no effect on spontaneous synaptic current activity in a cell-type specific manner. ACh modulated inhibitory synaptic activity in 80% of neurons, whereas excitatory synaptic activity was affected in less than 50% of neurons. In whole-cell current clamp recordings, brief somatic application of ACh induced cell-type specific responses in 79% of rat lamina II neurons, which included: depolarization and action potential firing, subthreshold membrane depolarization, biphasic responses characterized by transient depolarization followed by hyperpolarization and membrane hyperpolarization alone. Similar responses were seen in marmoset lamina II neurons and the properties of each neuron group were consistent across species. ACh-induced hyperpolarization was blocked by the muscarinic antagonist atropine and all forms of acetylcholine-induced depolarization were blocked by the nicotinic antagonist mecamylamine. The cholinergic system plays an important role in regulating nociception and this study contributes to our understanding of how circuit activity is controlled by ACh at a cellular level in primate and rodent spinal cord.

Chronotoxicity of Semen Strychni is associated with circadian metabolism and transport in mice

OBJECTIVES

We aimed to determine the circadian responses of mice to Semen Strychni and to investigate the role of pharmacokinetics in generating chronotoxicity.

METHODS

Total extract of Semen Strychni was administered by oral gavage to wild-type (WT) and Bmal1-/- (a circadian clock-deficient model) mice at different circadian time points for toxicity (including survival) and pharmacokinetic characterization. Nephrotoxicity and neurotoxicity were evaluated by measuring plasma creatinine and creatine kinase BB (CK-BB), respectively. Drug metabolism and transport assays were performed using liver/intestine microsomes and everted gut sacs, respectively.

KEY FINDINGS

Semen Strychni nephrotoxicity and neurotoxicity as well as animal survival displayed significant circadian rhythms (the highest level of toxicity was observed at ZT18 and the lowest level at ZT2 to ZT6). According to pharmacokinetic experiments, herb dosing at ZT18 generated higher plasma concentrations (and systemic exposure) of strychnine and brucine (two toxic constituents) compared with ZT6 dosing. This was accompanied by reduced formation of both dihydroxystrychnine and strychnine glucuronide (two strychnine metabolites) at ZT18. Bmal1 ablation sensitized mice to Semen Strychni-induced toxicity (with increased levels of plasma creatinine and CK-BB) and abolished the time dependency of toxicity. Metabolism of Semen Strychni (strychnine and brucine) in the liver and intestine microsomes of WT mice was more extensive at ZT6 than at ZT18. These time differences in hepatic and intestinal metabolism were lost in Bmal1-/- mice. Additionally, the intestinal efflux transport of Semen Strychni (strychnine and brucine) was more extensive at ZT6 than ZT18 in WT mice. However, the time-varying transport difference was abolished in Bmal1-/- mice.

CONCLUSIONS

Circadian responses of mice to Semen Strychni are associated with time-varying efflux transport and metabolism regulated by the circadian clock (Bmal1). Our findings may have implications for optimizing phytotherapy with Semen Strychni via timed delivery.

Engineering a surrogate human heteromeric α/β glycine receptor orthosteric site exploiting the structural homology and stability of acetylcholine-binding protein

Protein-engineering methods have been exploited to produce a surrogate system for the extracellular neurotransmitter-binding site of a heteromeric human ligand-gated ion channel, the glycine receptor. This approach circumvents two major issues: the inherent experimental difficulties in working with a membrane-bound ion channel and the complication that a heteromeric assembly is necessary to create a key, physiologically relevant binding site. Residues that form the orthosteric site in a highly stable ortholog, acetylcholine-binding protein, were selected for substitution. Recombinant proteins were prepared and characterized in stepwise fashion exploiting a range of biophysical techniques, including X-ray crystallography, married to the use of selected chemical probes. The decision making and development of the surrogate, which is termed a glycine-binding protein, are described, and comparisons are provided with wild-type and homomeric systems that establish features of molecular recognition in the binding site and the confidence that the system is suited for use in early-stage drug discovery targeting a heteromeric α/β glycine receptor.

Exploring the differences in metabolic behavior of astrocyte and glioblastoma: a flux balance analysis approach

Brain cancers demonstrate a complex metabolic behavior so as to adapt the external hypoxic environment and internal stress generated by reactive oxygen species. To survive in these stringent conditions, glioblastoma cells develop an antagonistic metabolic phenotype as compared to their predecessors, the astrocytes, thereby quenching the resources expected for nourishing the neurons. The complexity and cumulative effect of the large scale metabolic functioning of glioblastoma is mostly unexplored. In this study, we reconstruct a metabolic network comprising of pathways that are known to be deregulated in glioblastoma cells as compared to the astrocytes. The network, consisted of 147 genes encoding for enzymes performing 247 reactions distributed across five distinct model compartments, was then studied using constrained-based modeling approach by recreating the scenarios for astrocytes and glioblastoma, and validated with available experimental evidences. From our analysis, we predict that glycine requirement of the astrocytes are mostly fulfilled by the internal glycine-serine metabolism, whereas glioblastoma cells demand an external uptake of glycine to utilize it for glutathione production. Also, cystine and glucose were identified to be the major contributors to glioblastoma growth. We also proposed an extensive set of single and double lethal reaction knockouts, which were further perturbed to ascertain their role as probable chemotherapeutic targets. These simulation results suggested that, apart from targeting the reactions of central carbon metabolism, knockout of reactions belonging to the glycine-serine metabolism effectively reduce glioblastoma growth. The combinatorial targeting of glycine transporter with any other reaction belonging to glycine-serine metabolism proved lethal to glioblastoma growth.

Role of glycine in nociceptive and non-nociceptive bladder reflexes and pudendal afferent inhibition of these reflexes in cats

AIM

This study examined the role of glycinergic transmission in nociceptive and non-nociceptive bladder reflexes and in inhibition of these reflexes by pudendal nerve stimulation (PNS).

METHODS

Cystometrograms (CMGs) were performed in α-chloralose anesthetized cats by intravesical infusion of saline or 0.25% acetic acid (AA) to trigger, respectively, non-nociceptive or nociceptive bladder reflexes. PNS at 2 or 4 times threshold (T) intensity for inducing anal twitch was used to inhibit the bladder reflexes. Strychnine (a glycine receptor antagonist) was administered in cumulative doses (0.001-0.3 mg/kg, i.v.) at 60-120 min intervals.

RESULTS

Strychnine at 0.001-0.3 mg/kg significantly (P < 0.05) increased bladder capacity and reduced contraction amplitude during saline CMGs but did not change these parameters during AA CMGs except at the 0.3 mg/kg dose which increased bladder capacity. Strychnine did not alter PNS inhibition during saline CMGs except at the highest dose at 2T intensity, but significantly (P < 0.05) suppressed PNS inhibition during AA CMGs after 0.001-0.003 mg/kg doses at 2T and 4T intensities. During AA CMGs strychnine (0.3 mg/kg) also unmasked a post-PNS excitatory effect that significantly reduced bladder capacity after termination of PNS.

CONCLUSIONS

Glycinergic inhibitory neurotransmission in the central nervous system plays an unexpected role to tonically enhance the magnitude and reduce the bladder volume threshold for triggering the non-nociceptive bladder reflex. This is attributable to inhibition by glycine of another inhibitory mechanism. Glycine also has a minor role in PNS inhibition of the nociceptive bladder reflex. Neurourol. Urodynam. 35:798-804, 2016. © 2015 Wiley Periodicals, Inc.

A hybrid electrical/chemical circuit in the spinal cord generates a transient embryonic motor behavior

Spontaneous network activity is a highly stereotyped early feature of developing circuits throughout the nervous system, including in the spinal cord. Spinal locomotor circuits produce a series of behaviors during development before locomotion that reflect the continual integration of spinal neurons into a functional network, but how the circuitry is reconfigured is not understood. The first behavior of the zebrafish embryo (spontaneous coiling) is mediated by an electrical circuit that subsequently generates mature locomotion (swimming) as chemical neurotransmission develops. We describe here a new spontaneous behavior, double coiling, that consists of two alternating contractions of the tail in rapid succession. Double coiling was glutamate-dependent and required descending hindbrain excitation, similar to but preceding swimming, making it a discrete intermediary developmental behavior. At the cellular level, motoneurons had a distinctive glutamate-dependent activity pattern that correlated with double coiling. Two glutamatergic interneurons, CoPAs and CiDs, had different activity profiles during this novel behavior. CoPA neurons failed to show changes in activity patterns during the period in which double coiling appears, whereas CiD neurons developed a glutamate-dependent activity pattern that correlated with double coiling and they innervated motoneurons at that time. Additionally, double coils were modified after pharmacological reduction of glycinergic neurotransmission such that embryos produced three or more rapidly alternating coils. We propose that double coiling behavior represents an important transition of the motor network from an electrically coupled spinal cord circuit that produces simple periodic coils to a spinal network driven by descending chemical neurotransmission, which generates more complex behaviors.

The involvement of accumbal glycine receptors in the dopamine-elevating effects of addictive drugs

The ability of drugs of abuse to increase mesolimbic levels of dopamine is a characteristic associated with their rewarding effects. Exactly how these effects are produced by different substances is not as well characterised. Our previous work in rats has demonstrated that accumbal glycine receptors (GlyRs) are involved in mediating the dopamine-activating effects of ethanol, and in modulating ethanol intake. In this study the investigation of GlyR involvement was extended to include several different drugs of abuse. By using microdialysis and electrophysiology we compared effects of addictive drugs, with and without the GlyR antagonist strychnine, on dopamine levels and neurotransmission in nucleus accumbens. The dopamine-increasing effect of systemic ethanol and the drug-induced change in neurotransmission in vitro, as measured by microdialysis and field potential recordings, were dependent on GlyRs in nAc. Accumbal GlyRs were also involved in the actions of tetrahydrocannabinol and nicotine, but not in those of cocaine or morphine. These data indicate that accumbal GlyRs play a key role in ethanol-induced dopamine activation and contribute also to that of cannabinoids and nicotine.

From toxins targeting ligand gated ion channels to therapeutic molecules

Ligand-gated ion channels (LGIC) play a central role in inter-cellular communication. This key function has two consequences: (i) these receptor channels are major targets for drug discovery because of their potential involvement in numerous human brain diseases; (ii) they are often found to be the target of plant and animal toxins. Together this makes toxin/receptor interactions important to drug discovery projects. Therefore, toxins acting on LGIC are presented and their current/potential therapeutic uses highlighted.

Quercetin Inhibits α3β4 Nicotinic Acetylcholine Receptor-Mediated Ion Currents Expressed in Xenopus Oocytes

Quercetin mainly exists in the skin of colored fruits and vegetables as one of flavonoids. Recent studies show that quercetin, like other flavonoids, has diverse pharmacological actions. However, relatively little is known about quercetin effects in the regulations of ligand-gated ion channels. In the previous reports, we have shown that quercetin regulates subsets of homomeric ligand-gated ion channels such as glycine, 5-HT(3A) and α7 nicotinic acetylcholine receptors. In the present study, we examined quercetin effects on heteromeric neuronal α3β4 nicotinic acetylcholine receptor channel activity expressed in Xenopus oocytes after injection of cRNA encoding bovine neuronal α3 and β4 subunits. Treatment with acetylcholine elicited an inward peak current (I(ACh)) in oocytes expressing α3β4 nicotinic acetylcholine receptor. Co-treatment with quercetin and acetylcholine inhibited I(ACh) in oocytes expressing α3β4 nicotinic acetylcholine receptors. The inhibition of I(ACh) by quercetin was reversible and concentration-dependent. The half-inhibitory concentration (IC(50)) of quercetin was 14.9±0.8 µM in oocytes expressing α3β4 nicotinic acetylcholine receptor. The inhibition of I(ACh) by quercetin was voltage-independent and non-competitive. These results indicate that quercetin might regulate α3β4 nicotinic acetylcholine receptor and this regulation might be one of the pharmacological actions of quercetin in nervous systems.

The activation of nicotinic acetylcholine receptors enhances the inhibitory synaptic transmission in the deep dorsal horn neurons of the adult rat spinal cord

Somatosensory information can be modulated by nicotinic acetylcholine receptors (nAChRs) in the superficial dorsal horn of the spinal cord. Nonetheless, the functional significance of nAChRs in the deep dorsal horn of adult animals remains unclear. Using whole-cell patch-clamp recordings from lamina V neurons in the adult rat spinal cord, we investigated whether the activation of nAChRs could modulate the inhibitory synaptic transmission in the deep dorsal horn. In the presence of CNQX and APV to block excitatory glutamatergic synaptic transmission, bath applications of nicotine (100 microM) significantly increased the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) in almost all neurons tested. The effect of nicotine was mimicked by N-methyl-4-(3-pyridinyl)-3-butene-1-amine (RJR-2403, 100 microM), an alpha 4 beta 2-nAChR agonist, and was also mimicked by choline (10 mM), an alpha 7-nAChR agonist. The effect of nicotine was completely blocked by the nAChR antagonist mecamylamine (5 microM). In the presence of tetrodotoxin (0.5 microM), nicotine (100 microM) significantly increased the miniature IPSC frequency. On the other hand, RJR-2403 (100 microM) or choline (10 mM) did not affect miniature IPSCs. The application of nicotine (100 microM) also evoked a large inward current in all lamina V neurons tested when cells were held at -60 mV. Similarly, RJR-2403 (100 microM) induced inward currents in the majority of lamina V neurons examined. On the other hand, choline (10 mM) did not elicit any detectable whole-cell currents. These results suggest that several nAChR subtypes are expressed on the presynaptic terminals, preterminals, and neuronal cell bodies within lamina V and that these nAChRs are involved in the modulation of inhibitory synaptic activity in the deep dorsal horn of the spinal cord.

Galanthamine and non-competitive inhibitor binding to ACh-binding protein: evidence for a binding site on non-alpha-subunit interfaces of heteromeric neuronal nicotinic receptors

Rapid neurotransmission is mediated through a superfamily of Cys-loop receptors that includes the nicotinic acetylcholine (nAChR), gamma-aminobutyric acid (GABA(A)), serotonin (5-HT(3)) and glycine receptors. A class of ligands, including galanthamine, local anesthetics and certain toxins, interact with nAChRs non-competitively. Suggested modes of action include blockade of the ion channel, modulation from undefined extracellular sites, stabilization of desensitized states, and association with annular or boundary lipid. Alignment of mammalian Cys-loop receptors shows aromatic residues, found in the acetylcholine or ligand-binding pocket of nAChRs, are conserved in all subunit interfaces of neuronal nAChRs, including those that are not formed by alpha subunits on the principal side of the transmitter binding site. The amino-terminal domain containing the ligand recognition site is homologous to the soluble acetylcholine-binding protein (AChBP) from mollusks, an established structural and functional surrogate. We assess ligand specificity and employ X-ray crystallography with AChBP to demonstrate ligand interactions at subunit interfaces lacking vicinal cysteines (i.e. the non-alpha subunit interfaces in nAChRs). Non-competitive nicotinic ligands bind AChBP with high affinity (K(d) 0.015-6 microM). We mutated the vicinal cysteine residues in loop C of AChBP to mimic the non-alpha subunit interfaces of neuronal nAChRs and other Cys loop receptors. Classical nicotinic agonists show a 10-40-fold reduction in binding affinity, whereas binding of ligands known to be non-competitive are not affected. X-ray structures of cocaine and galanthamine bound to AChBP (1.8 A and 2.9 A resolution, respectively) reveal interactions deep within the subunit interface and the absence of a contact surface with the tip of loop C. Hence, in addition to channel blocking, non-competitive interactions with heteromeric neuronal nAChR appear to occur at the non-alpha subunit interface, a site presumed to be similar to that of modulating benzodiazepines on GABA(A) receptors.

Supraspinal input is dispensable to generate glycine-mediated locomotive behaviors in the zebrafish embryo

The anatomy of the developing zebrafish spinal cord is relatively simple but, despite this simplicity, it generates a sequence of three patterns of locomotive behaviors. The first behavior exhibited is spontaneous movement, then touch-evoked coiling, and finally swimming. Previous studies in zebrafish have suggested that spontaneous movements occur independent of supraspinal input and do not require chemical neurotransmission, while touch-evoked coiling and swimming depend on glycinergic neurotransmission as well as supraspinal input. In contrast, studies in other vertebrate preparations have shown that spontaneous movement requires glycine and other neurotransmitters and that later behaviors do not require supraspinal input. Here, we use lesion analysis combined with high-speed kinematic analysis to re-examine the role of glycine and supraspinal input in each of the three behaviors. We find that, similar to other vertebrate preparations, supraspinal input is not essential for spontaneous movement, touch-evoked coiling, or swimming behavior. Moreover, we find that blockade of glycinergic neurotransmission decreases the rate of spontaneous movement and impairs touch-evoked coiling and swimming, suggesting that glycinergic neurotransmission plays critical yet distinct roles for individual patterns of locomotive behaviors.

Pharmacological characterisation of strychnine and brucine analogues at glycine and alpha7 nicotinic acetylcholine receptors

Strychnine and brucine from the plant Strychnos nux vomica have been shown to have interesting pharmacological effects on several neurotransmitter receptors, including some members of the superfamily of ligand-gated ion channels. In this study, we have characterised the pharmacological properties of tertiary and quaternary analogues as well as bisquaternary dimers of strychnine and brucine at human alpha1 and alpha1beta glycine receptors and at a chimera consisting of the amino-terminal domain of the alpha7 nicotinic receptor (containing the orthosteric ligand binding site) and the ion channel domain of the 5-HT3A serotonin receptor. Although the majority of the analogues displayed significantly increased Ki values at the glycine receptors compared to strychnine and brucine, a few retained the high antagonist potencies of the parent compounds. However, mirroring the pharmacological profiles of strychnine and brucine, none of the analogues displayed significant selectivity between the alpha1 and alpha1beta subtypes. The structure-activity relationships for the compounds at the alpha7/5-HT3 chimera were significantly different from those at the glycine receptors. Most strikingly, quaternization of strychnine and brucine with substituents possessing different steric and electronic properties completely eliminated the activity at the glycine receptors, whereas binding affinity to the alpha7/5-HT3 chimera was retained for the majority of the quaternary analogues. This study provides an insight into the structure-activity relationships for strychnine and brucine analogues at these ligand-gated ion channels.

Glycine-gated chloride channels depress synaptic transmission in rat hippocampus

An inhibitory role for strychnine-sensitive glycine-gated chloride channels (GlyRs) in mature hippocampus is beginning to be appreciated. We have reported previously that CA1 pyramidal cells and GABAergic interneurons recorded in 3- to 4-wk-old rat hippocampal slices express functional GlyRs, dispelling previous misconceptions that GlyR expression ceases in early development. However, the effect of GlyR activation on cell excitability and synaptic circuits in hippocampus has not been fully explored. Using whole cell current-clamp recordings, we show that activation of strychnine-sensitive GlyRs through exogenous glycine application causes a significant decrease in input resistance and prevents somatically generated action potentials in both CA1 pyramidal cells and interneurons. Furthermore, GlyR activation depresses the synaptic network by reducing suprathreshold excitatory postsynaptic potentials (EPSPs) to subthreshold events in both cell types. Blockade of postsynaptic GlyRs with the chloride channel blocker 4, 4'-diisothiocyanatostilbene-2-2'-disulfonic acid (DIDS) or altering the chloride ion driving force in recorded cells attenuates the synaptic depression, strongly indicating that a postsynaptic mechanism is responsible. Increasing the local glycine concentration by blocking reuptake causes a strychnine-sensitive synaptic depression in interneuron recordings, suggesting that alterations in extracellular glycine will impact excitability in hippocampal circuits. Finally, using immunohistochemical methods, we show that glycine and the glycine transporter GlyT2 are co-localized selectively in GABAergic interneurons, indicating that interneurons contain both inhibitory neurotransmitters. Thus we report a novel mechanism whereby activation of postsynaptic GlyRs can function to depress activity in the synaptic network in hippocampus. Moreover, the co-localization of glycine and GABA in hippocampal interneurons, similar to spinal cord, brain stem, and cerebellum, suggests that this property is likely to be a general characteristic of inhibitory interneurons throughout the CNS.

Strychnine induces embryotoxicity in rat neurulation

Administering strychnine, a potent antagonist of glycine receptors, to pregnant rats caused marked toxic effects on the ensuing embryos. The embryotoxic effects of strychnine were compared with those induced by retinal palmitate during rat neurulation; and it was found that strychnine was stronger than retinal palmitate in a number of abnormalities such as anencephaly, general aplasy and abnormal cerebral vesicles. Although the glycine receptor beta1 subunit mRNA was found to be expressed in the embryos when strychnine was administered to the mother rats, its presence may not fully account for the toxic effects and it may be that strychnine is targeting also other molecules, such as the nicotinic receptor that has been found early in development.

The rat spinal cord slice: Its use in generating pharmacological evidence for cholinergic transmission using the α7 subtype of nicotinic receptors in the central autonomic nucleus

Lamina X surrounds the central canal of the spinal cord and is an important site for the convergence of somatic and visceral afferent inputs relaying nociceptive information. Lamina X contains sympathetic preganglionic neurons (SPN) in the so-called central autonomic nucleus which may participate to viscero-autonomic reflexes. Here, we describe a transversal slice preparation of postnatal rat thoracolumbar spinal cord which allows the detailed characterization of the morphology, electrophysiological properties, synaptic activities and receptor pharmacology of neurons surrounding the central canal. By means of the patch clamp technique, in its whole cell configuration, and by the use of various pharmacological tools, we show here that lamina X neurons of the central autonomic nucleus express functional alpha7 nicotinic receptors which are located postsynaptically on SPNs where they are involved in a fast cholinergic transmission. Thus, this in vitro preparation is useful to study the mechanisms and the pharmacology of viscero-autonomic reflexes.

Inhibition of human alpha 7 nicotinic acetylcholine receptors by open channel blockers of N-methyl-D-aspartate receptors

1. Human alpha 7 nicotinic acetylcholine (ACh) receptors were expressed in Xenopus oocytes and the effects of the N-methyl-D-aspartate (NMDA) receptor open channel blockers memantine and cerestat on this receptor were examined using two-electrode voltage-clamp recordings and 125I-alpha-bungarotoxin (125I-alpha-bgtx) binding. 2. Memantine and cerestat produced complete inhibition of ACh-induced inward currents with affinities similar to that reported for native NMDA receptors. Cerestat, IC50 1.7 (-1; +2) microm, was more potent than memantine, IC50 5 (-3;+8) microM, and the effects of both drugs were fully and rapidly reversible. 3. Inhibition of alpha 7 receptor function was voltage-independent, and it occurred at concentrations far lower than those needed to inhibit (never completely) binding of 125I-alpha-bgtx to alpha 7 receptors, suggesting that the effects of memantine or cerestat are noncompetitive. 4. These results provide evidence that human alpha 7 receptors are inhibited by memantine and cerestat and suggest that caution should be applied when using these compounds to study systems in which NMDA and nACh receptors co-exist.

A pharmacologically distinct nicotinic ACh receptor is found in a subset of frog semicircular canal hair cells

Frog vestibular organs are endowed with a prominent cholinergic efferent innervation whose stimulation results in several different effects, thereby suggesting diversity in the expression of postsynaptic acetylcholine (ACh) receptors. The application of ACh can mimic efferent stimulation in producing both an inhibition and a facilitation of afferent discharge which are thought to be mediated by at least two distinct ACh receptors present on vestibular hair cells, i.e., alpha9-containing nicotinic receptors (alpha9nAChR) and muscarinic receptors (mAChR), respectively. Using patch-clamp and multiunit vestibular afferent recordings, we demonstrate the presence of an additional excitatory hair cell nicotinic ACh receptor pharmacologically distinct from both alpha9nAChR and mAChR. In order of increasing potency, this distinct receptor was activated by ACh, carbachol, and particularly by the selective nicotinic agonist 1,1-dimethyl-4-phenyl-piperazinium (DMPP). This DMPP-sensitive nicotinic receptor (RDMPP) was antagonized by the classic nicotinic antagonist d-tubocurarine, but refractory to strychnine, atropine, and propylbenzilylcholine mustard, at concentrations that completely block alpha9nAChR and/or mAChR. Activation of RDMPP on application of ACh or DMPP to a subpopulation of isolated posterior semicircular canal (SCC) hair cells resulted in a large depolarization (18.0 +/- 1.2 mV). The current underlying this depolarization was typically small (80.1 +/- 21.6 pA) and showed an inward rectification starting around -45 mV. Given their respective EC50s (47 nM vs. 20 microM), RDMPP was nearly 400 times more sensitive to ACh than alpha9nAChR and thus responded to concentrations of ACh considered too low to be effective at stimulating alpha9nAChR. Despite this remarkable sensitivity, exogenous ACh readily stimulated the mAChR in the intact posterior SCC preparation but failed to activate RDMPP unless the acetylcholinesterase inhibitor physostigmine was present, or high concentrations of ACh were used (>3 mM). In frog, RDMPP most likely underlies the rapid excitatory response seen during efferent stimulation.

Fast synaptic transmission mediated by alpha-bungarotoxin-sensitive nicotinic acetylcholine receptors in lamina X neurones of neonatal rat spinal cord

Using patch clamp recordings on neonatal rat spinal cord slices, we have looked for the presence of alpha-bungarotoxin-sensitive nicotinic ACh receptors (nAChRs) on sympathetic preganglionic neurones (SPNs) surrounding the central canal of the spinal cord (lamina X) and examined whether they were implicated in a fast cholinergic synaptic transmission. SPNs were identified either by their morphology using biocytin in the recording electrode and/or by antidromic stimulation of the ventral rootlets. The selective alpha7-containing nAChR (alpha7*nAChR) agonist choline (10 mM) induced a fast, rapidly desensitizing inward current, which was fully blocked by alpha-bungarotoxin (alpha-BgT; 50 nM) and strychnine (1 microM), two antagonists of alpha7*nAChRs. The I-V relationship of the choline-induced current showed a strong inward-going rectification. Electrically evoked excitatory postsynaptic currents (eEPSCs) could be recorded. At -60 mV, eEPSCs peaked at -26.2 pA and decayed monoexponentially with a mean time constant of 8.5 ms. The current-voltage relationship for eEPSCs exhibited a strong inward rectification and a reversal potential close to 0 mV, compatible with a non-selective cationic current. The appearance of eEPSCs was entirely suppressed by the application of 100 microM ACh or nicotine. Choline (10 mM) and 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP; 100 microM) both reduced the amplitude of eEPSCs, whereas cytisine (100 microM) had no effect. Strychnine (1 microM) and alpha-BgT (50 nM) both suppressed the eEPSCs. Blocking the P2X purinergic and 5-HT(3) receptors had no effect on eEPSCs. DMPP induced four types of current, which differed in their onset and desensitization rate. The most frequently encountered responses were insensitive to the action of strychnine and alpha-BgT, and were reproduced by ACh and nicotine but not by cytisine. We conclude that SPNs of the lamina X express several classes of nAChRs and in particular alpha-BgT-sensitive nAChRs. This is the first demonstration in a mammalian spinal cord preparation of a fast cholinergic neurotransmission in which alpha-BgT-sensitive nicotinic receptors are involved.

Effects of the plant alkaloid tetrandrine on human nicotinic acetylcholine receptors

Functional effects of the well-characterized antagonist of L-type Ca(2+) channels tetrandrine on recombinant human gamma-aminobutyric acid type A (GABA(A)) (alpha1beta2gamma2s) receptor or human alpha7, alpha4beta2, alpha1beta1deltagamma and alpha1beta1delta epsilon nicotinic acetylcholine receptors expressed in Xenopus oocytes were examined using two-electrode voltage clamp. Tetrandrine inhibited the function of acetylcholine nicotinic receptors, but it had no effect on GABA(A) receptors. Potency of inhibition was influenced by the receptor subtype and the rank order was alpha4beta2>alpha7>alpha1beta1deltagamma congruent with alpha1beta1delta epsilon. Functional inhibition of alpha4beta2 and alpha1beta1deltagamma receptors was noncompetitive, but only inhibition of alpha1beta1deltagamma receptors was voltage-dependent. Binding of 125I-alpha-bungarotoxin to alpha1beta1deltagamma or 3H-cytisine to alpha4beta2 receptors was also inhibited by tetrandrine, but inhibition was noncompetitive and required concentrations higher than those needed to inhibit receptor function. Inhibition of both alpha7 receptor function and binding of 125I-alpha-bungarotoxin to alpha7 receptor were mixed competitive/noncompetitive and occurred at a similar concentration range.

Effects of clomipramine on neuronal nicotinic acetylcholine receptors

The action of the tricyclic antidepressant clomipramine on membrane currents elicited by acetylcholine was studied in Xenopus oocytes expressing neuronal alpha2beta4 nicotinic acetylcholine receptors. Clomipramine inhibited the acetylcholine responses rapidly and reversibly, with a similar IC(50) when the oocytes were preincubated with clomipramine (1.3+/-0.2 microM) or when they were exposed simultaneously with acetylcholine and clomipramine (1.5+/-0.3 microM). The EC(50) was 39.9+/-2.1 microM for acetylcholine alone and 65.7+/-3.6 microM for acetylcholine in the presence of 2 microM clomipramine. The inhibitory effect of clomipramine was weakly voltage-dependent, with an electric distance of approximately 0.14. Moreover, clomipramine increased the rate of decay of currents elicited by acetylcholine. From all of these, we conclude that clomipramine reversibly and noncompetitively regulates neuronal alpha2beta4 nicotinic acetylcholine receptors by blocking the open receptor-channel complex at a site close to the extracellular vestibule of the channel. The actions of clomipramine on neuronal nicotinic acetylcholine receptors may play an important role in the treatment of mental depression and other mood disorders.

Effects of ginsenosides, active components of ginseng, on nicotinic acetylcholine receptors expressed in Xenopus oocytes

We investigated the effects of ginsenosides, the active ingredient of ginseng, on neuronal or muscle-type nicotinic acetylcholine receptor channel activity expressed in Xenopus oocytes after injection of cRNA encoding bovine neuronal alpha3beta4, alpha7 or human muscle alphabetadeltavarepsilon subunits. Treatment with acetylcholine elicited an inward peak current (I(ACh)) in oocytes expressing nicotinic acetylcholine receptor subtypes. Cotreatment with ginsenoside Rg2 and acetylcholine inhibited I(ACh) in oocytes expressing with alpha3beta4 or alphabetadeltavarepsilon but not in oocytes expressing alpha7 nicotinic acetylcholine receptors. The inhibition of I(ACh) by ginsenoside Rg2 was reversible and dose-dependent. The half-inhibitory concentrations (IC50) of ginsenoside Rg2 were 60.2+/-14.1 and 15.7+/-3.5 microM in oocytes expressing alpha3beta4 and alphabetadeltavarepsilon nicotinic acetylcholine receptors, respectively. The inhibition of I(ACh) by ginsenoside Rg2 was voltage-independent and noncompetitive. Other ginsenosides besides ginsenoside Rg2 also inhibited I(ACh) in oocytes expressing alpha3beta4 or alphabetadeltavarepsilon nicotinic acetylcholine receptors. The order of potency for the inhibition of I(ACh) was ginsenoside Rg2>Rf>Re>Rg1>Rc>Rb2>Rb1 in oocytes expressing alpha3beta4 nicotinic acetylcholine receptors and was ginsenoside Rg2>Rf>Rg1>Re>Rb1>Rc>Rb2 in oocytes expressing alphabetadeltavarepsilon nicotinic acetylcholine receptors. These results indicate that ginsenosides might regulate nicotinic acetylcholine receptors in a differential manner and this regulation might be one of the pharmacological actions of Panax ginseng.

Inhibition of skeletal muscle nicotinic receptors by the atypical antipsychotic clozapine

We have previously observed that certain atypical antipsychotic drugs reduce the amplitude and duration of miniature end-plate currents (EPCs) at the frog neuromuscular junction (Effects of atypical antipsychotics on vertebrate neuromuscular transmission, Nguyen, Q.-T., Yang, J., Miledi, R. Neuropharmacology 42, 2002, 670-676), therefore suggesting that these drugs act on nicotinic acetylcholine receptors. In this study we examined the effects of the atypical antipsychotic clozapine on nicotinic receptors of frog neuromuscular end-plates or in Xenopus oocytes expressing the alpha(1)beta(1)gamma delta mouse skeletal muscle nicotinic receptor. At neuromuscular junctions, postsynaptic currents were reduced by micromolar concentrations of clozapine. This compound also acted presynaptically by increasing the quantal content of EPCs of muscles without noticeably affecting paired-pulse facilitation. In oocytes, clozapine inhibited alpha(1)beta(1)gamma delta receptors with an IC(50) of 10 microM and a Hill coefficient of 1. Blockage of alpha(1)beta(1)gamma delta receptors by clozapine bears several hallmarks of open-channel blockers, including faster response decays, strong voltage dependence of the block, large rebound currents upon wash, and reduction of peak responses even at saturating concentrations of acetylcholine. However, clozapine increased the EC(50) for acetylcholine and its blocking effect was enhanced by preincubation. These results suggest that clozapine antagonizes muscle nicotinic receptors by blocking open channels, and possibly also by another mechanism which still remains to be investigated.

Inhibition of nicotinic acetylcholine receptors by bicuculline

A study was made on the effects of bicuculline, the classical gamma-aminobutyric acid-A receptor antagonist, on heteromeric mouse muscle alphabetagammadelta, heteromeric neuronal rat alpha2beta4 and alpha4beta2 and homomeric human alpha7 nicotinic acetylcholine receptors (nAChRs), expressed in Xenopus oocytes. Bicuculline reduced the ACh-induced currents in a rapid and reversible way, with IC50 values of 34+/-1.5 microM for mouse muscle alphabetagammadelta and 12.4+/-0.7 and 18+/-1 microM for rat neuronal alpha2beta4 and alpha4beta2 nAChRs, respectively. Therefore, the three types of heteromeric receptors are inhibited by bicuculline but the neuronal alpha2beta4 and alpha4beta2 receptors were more sensitive than the muscle alphabetagammadelta receptor. The Hill coefficients for ACh-current inhibition were close to one for all types of receptors, suggesting a single site of action for bicuculline inhibition of nAChRs. Bicuculline shifted the ACh-dose-current response curve to the right and the maximal current was reduced, a reduction that for the heteromeric receptors was not overcome by high concentrations of ACh. The effect of bicuculline was examined at different membrane potentials, and the ACh-current-membrane potential relationships obtained indicate that the inhibition by bicuculline is voltage-dependent for muscle alphabetagammadelta and neuronal alpha2beta4 and alpha4beta2 nAChRs. All these results are consistent with the notion that bicuculline blocks the heteromeric muscle and neuronal nAChRs in a non-competitive way. Studies were also made on the wild type (wt alpha7) and mutant leu-to-threo (L248T) homomeric human neuronal alpha7-nAChRs. In sharp contrast to the heteromeric ACh receptors examined, bicuculline blocked in a competitive way the homomeric wt alpha7-nAChRs, as evidenced by a parallel shift of the bicuculline dose-ACh-current inhibition on raising the ACh concentration. Moreover, similar to the effects of serotonin on wt and mutant alpha7 ACh receptors, the mutation converted bicuculline from an antagonist into a competitive agonist. All this suggests that bicuculline may serve as a lead molecule to design new anticholinergic substances.

A critical role of the strychnine-sensitive glycinergic system in spontaneous retinal waves of the developing rabbit

In the developing vertebrate retina, spontaneous electric activity occurs rhythmically in the form of propagating waves and is believed to play a critical role in activity-dependent visual system development, including the establishment of precise retinal and geniculate circuitry. To elucidate how spontaneous retinal waves encode specific developmental cues at various developmental stages, it is necessary to understand how the waves are generated and regulated. Using Ca(2+) imaging and patch clamp in a flat-mount perinatal rabbit retinal preparation, this study demonstrates that, in addition to the cholinergic system, a strychnine-sensitive system in the inner retina plays an obligatory and developmentally regulated role in the initiation and propagation of spontaneous retinal waves. This system, which is believed to be the glycinergic network, provided an excitatory drive during early retinal development. It then became inhibitory after postnatal day 1 (P1) to P2, an age when a number of coordinated transitions in neurotransmitter systems occurred concomitantly, and finally contributed to the complete inhibition and disappearance of spontaneous waves after P7-P9. This glycinergic contribution was notably distinct from that of the ionotropic GABAergic system, which was found to exert an inhibitory but nonessential influence on the early wave formation. Blocking glycine- and GABA-gated anion currents had opposing effects on spontaneous retinal waves between embryonic day 29 and P0, suggesting that Cl(-) transporters, particularly R(+)-butylindazone-sensitive K-Cl cotransporters, may have a synapse- and/or cell type-specific distribution pattern, in addition to an age-dependent expression pattern in the inner retina. Overall, the results revealed an important reliance of spontaneous retinal waves on dynamic and coordinated interactions among multiple, nonredundant neurotransmitter systems.

A critical role of the strychnine-sensitive glycinergic system in spontaneous retinal waves of the developing rabbit

In the developing vertebrate retina, spontaneous electric activity occurs rhythmically in the form of propagating waves and is believed to play a critical role in activity-dependent visual system development, including the establishment of precise retinal and geniculate circuitry. To elucidate how spontaneous retinal waves encode specific developmental cues at various developmental stages, it is necessary to understand how the waves are generated and regulated. Using Ca(2+) imaging and patch clamp in a flat-mount perinatal rabbit retinal preparation, this study demonstrates that, in addition to the cholinergic system, a strychnine-sensitive system in the inner retina plays an obligatory and developmentally regulated role in the initiation and propagation of spontaneous retinal waves. This system, which is believed to be the glycinergic network, provided an excitatory drive during early retinal development. It then became inhibitory after postnatal day 1 (P1) to P2, an age when a number of coordinated transitions in neurotransmitter systems occurred concomitantly, and finally contributed to the complete inhibition and disappearance of spontaneous waves after P7-P9. This glycinergic contribution was notably distinct from that of the ionotropic GABAergic system, which was found to exert an inhibitory but nonessential influence on the early wave formation. Blocking glycine- and GABA-gated anion currents had opposing effects on spontaneous retinal waves between embryonic day 29 and P0, suggesting that Cl(-) transporters, particularly R(+)-butylindazone-sensitive K-Cl cotransporters, may have a synapse- and/or cell type-specific distribution pattern, in addition to an age-dependent expression pattern in the inner retina. Overall, the results revealed an important reliance of spontaneous retinal waves on dynamic and coordinated interactions among multiple, nonredundant neurotransmitter systems.

Effects of diltiazem on human nicotinic acetylcholine and GABA(A) receptors

Effects of the L-type calcium channel antagonist diltiazem on recombinant human GABA(A) receptor (alpha1beta2gamma2s) or on muscle (alpha1beta1deltagamma and alpha1beta1delta(epsilon)) or neuronal (alpha7 and alpha4beta2) nicotinic acetylcholine receptors expressed in Xenopus oocytes were examined using two-electrode voltage-clamp. Diltiazem inhibited the function of both muscle and neuronal nicotinic receptors, but it had no effect on GABA(A) receptors. The extent of functional inhibition of nicotinic receptors depended on the receptor subtype, and the order of inhibition potency by diltiazem was alpha7>alpha4beta2 approximately alpha1beta1deltagamma approximately alpha1beta1delta(epsilon). Inhibition of alpha7 receptor function was non-competitive and voltage-independent, and it occurred at concentrations far lower than those needed to inhibit (never completely) binding of (125)I-alpha-bungarotoxin to heterologously expressed alpha7 receptors in mammalian cells. Pre-incubation in diltiazem before concomitant application with acetylcholine increased inhibition of function and slowed recovery from inhibition. Verapamil, a phenylalkylamine antagonist of L-type Ca(2+) channels also fully inhibited alpha7 receptor function and partially inhibited (125)I-alpha-bungarotoxin binding to alpha7 receptors, but was less potent than diltiazem. Effects on both alpha7 receptor function and (125)I-alpha-bungarotoxin binding by verapamil plus diltiazem suggest separate sites for verapamil and diltiazem on alpha7 receptors. These results provide further evidence that L-type Ca(2+) channel drugs inhibit ligand-gated cationic channels and suggest that caution should be applied when using these compounds to study systems in which L-type Ca(2+) channels and ligand-gated cationic channels co-exist.

Strychnine activates neuronal alpha7 nicotinic receptors after mutations in the leucine ring and transmitter binding site domains

Recent work has shown that strychnine, the potent and selective antagonist of glycine receptors, is also an antagonist of nicotinic acetylcholine (AcCho) receptors including neuronal homomeric alpha7 receptors, and that mutating Leu-247 of the alpha7 nicotinic AcCho receptor-channel domain (L247Talpha7; mut1) converts some nicotinic antagonists into agonists. Therefore, a study was made of the effects of strychnine on Xenopus oocytes expressing the chick wild-type alpha7 or L247Talpha7 receptors. In these oocytes, strychnine itself did not elicit appreciable membrane currents but reduced the currents elicited by AcCho in a reversible and dose-dependent manner. In sharp contrast, in oocytes expressing L247Talpha(7) receptors with additional mutations at Cys-189 and Cys-190, in the extracellular N-terminal domain (L247T/C189-190Salpha7; mut2), micromolar concentrations of strychnine elicited inward currents that were reversibly inhibited by the nicotinic receptor blocker alpha-bungarotoxin. Single-channel recordings showed that strychnine gated mut2-channels with two conductance levels, 56 pS and 42 pS, and with kinetic properties similar to AcCho-activated channels. We conclude that strychnine is a modulator, as well as an activator, of some homomeric nicotinic alpha7 receptors. After injecting oocytes with mixtures of cDNAs encoding mut1 and mut2 subunits, the expressed hybrid receptors were activated by strychnine, similar to the mut2, and had a high affinity to AcCho like the mut1. A pentameric symmetrical model yields the striking conclusion that two identical alpha7 subunits may be sufficient to determine the functional properties of alpha7 receptors.