Side Groups Convert the α7 Nicotinic Receptor Agonist Ether Quinuclidine into a Type I Positive Allosteric Modulator.

The quinuclidine scaffold has been extensively used for the development of nicotinic acetylcholine receptor (nAChR) agonists, with hydrophobic substituents at position 3 of the quinuclidine framework providing selectivity for α7 nAChRs. In this study, six new ligands (4-9) containing a 3-(pyridin-3-yloxy)quinuclidine moiety (ether quinuclidine) were synthesized to gain a better understanding of the structural-functional properties of ether quinuclidines. To evaluate the pharmacological activity of these ligands, two-electrode voltage-clamp and single-channel recordings were performed. Only ligand 4 activated α7 nAChR. Ligands 5 and 7 had no effects on α7 nAChR, but ligands 6, 8, and 9 potentiated the currents evoked by ACh. Ligand 6 was the most potent and efficacious of the potentiating ligands, with an estimated EC50 for potentiation of 12.6 ± 3.32 µM and a maximal potentiation of EC20 ACh responses of 850 ± 120%. Ligand 6 increased the maximal ACh responses without changing the kinetics of the current responses. At the single-channel level, the potentiation exerted by ligand 6 was evidenced in the low micromolar concentration range by the appearance of prolonged bursts of channel openings. Furthermore, computational studies revealed the preference of ligand 6 for an intersubunit site in the transmembrane domain and highlighted some putative key interactions that explain the different profiles of the synthesized ligands. Notably, Met276 in the 15' position of the transmembrane domain 2 almost abolished the effects of ligand 6 when mutated to Leu. We conclude that ligand 6 is a novel type I positive allosteric modulator (PAM-I) of α7 nAChR.

Cannabidiol as a modulator of α7 nicotinic receptors.

Cannabidiol (CBD), an important terpenoid compound from marijuana with no psychoactive effects, has become of great pharmaceutical interest for several health conditions. As CBD is a multitarget drug, there is a need to establish the molecular mechanisms by which CBD may exert therapeutic as well as adverse effects. The α7 nicotinic acetylcholine receptor (α7 nAChR) is a cation-permeable ACh-gated channel present in the nervous system and in non-neuronal cells. It is involved in different pathological conditions, including neurological and neurodegenerative disorders, inflammation, and cancer. By high-resolution single-channel recordings and confocal microscopy, we here reveal how CBD modulates α7 nAChR ionotropic and metabotropic functions. CBD leads to a profound concentration-dependent decrease of α7 nAChR single-channel activity with an IC50 in the sub-micromolar range. The inhibition of α7 nAChR activity, which takes place through a membrane pathway, is neither mediated by receptor phosphorylation nor overcome by positive allosteric modulators and is compatible with CBD stabilization of resting or desensitized α7 nAChR conformational states. CBD modulation is complex as it also leads to the later appearance of atypical, low-frequency α7 nAChR channel openings. At the cellular level, CBD inhibits the increase in intracellular calcium triggered by α7 nAChR activation, thus decreasing cell calcium responses. The modulation of α7 nAChR is of pharmacological relevance and should be considered in the evaluation of CBD potential therapeutic uses. Thus, our study provides novel molecular information of CBD multiple actions and targets, which is required to set the basis for prospective applications in human health.

The nematode serotonin-gated chloride channel MOD-1: A novel target for anthelmintic therapy.

Anthelmintics are used to treat human and veterinary parasitic diseases and to reduce crop and livestock production loss associated with parasitosis. The free-living nematode Caenorhabditis elegans, a model system for anthelmintic drug discovery, has a serotonin (5-HT)-gated chloride channel, MOD-1, which belongs to the Cys-loop receptor family and modulates locomotory and behavioral functions. Since MOD-1 is unique to nematodes, it is emerging as an attractive anthelmintic drug target, but details of MOD-1 function are unclear. Here, we revealed novel aspects of MOD-1 function from the molecular level to the organism level and identified compounds targeting this receptor, which may provide new directions for anthelmintic drug discovery. We used whole-cell current recordings from heterologously expressed MOD-1 to show that tryptamine (Tryp), a weak partial agonist of vertebrate serotonin type 3 (5-HT3) receptors, efficaciously activates MOD-1. A screen for modulators revealed that GABAergic ligands piperazine (PZE) and muscimol reduce 5-HT-elicited currents, thus identifying novel MOD-1 allosteric inhibitors. Next, we performed locomotor activity assays, and we found 5-HT and Tryp rapidly decrease worm motility, which is reversible only at low 5-HT concentrations. Mutants lacking MOD-1 are partially resistant to both drugs, demonstrating its role in locomotion. Acting as an antagonist of MOD-1, we showed PZE reduces the locomotor effects of exogenous 5-HT. Therefore, Tryp- and PZE-derived compounds, acting at MOD-1 through different molecular mechanisms, emerge as promising anthelmintic agents. This study enhances our knowledge of the function and drug selectivity of Cys-loop receptors and postulates MOD-1 as a potential target for anthelmintic therapy.

Vigilancia epidemiológica de COVID-19 mediante testeos agrupados focalizada en trabajadores de salud asintomáticos en Bahía Blanca / Epidemiological surveillance of COVID-19 through group testing focused on asymptomatic health workers in Bahía Blanca

INTRODUCCIÓN: Uno de los desafíos más relevantes al comienzo de la pandemia consistió en implementar estrategias dirigidas a prevenir la transmisión del virus SARS-CoV-2 y mitigar el impacto de la COVID-19. El propósito de este estudio fue contribuir a reducir la transmisión comunitaria a través de una iniciativa interinstitucional, cuyos objetivos fueron validar un método de detección de ARN del SARS-CoV-2 e implementar y evaluar la vigilancia en trabajadores de salud asintomáticos de instituciones de salud pública de Bahía Blanca. MÉTODOS: Se validó una prueba de detección del gen E del coronavirus mediante RT-PCR a partir de ARN aislado de hisopados nasofaríngeos. Para aumentar la capacidad de testeo se validó la detección del ARN viral en muestras agrupadas (pooles). Se realizó un estudio de cohorte prospectiva entre el 15/09/20 y el 15/09/21. RESULTADOS: La sensibilidad y especificidad de la prueba en muestras individuales fue del 95% (IC 95%: 85%-100%). La sensibilidad de la detección en pooles fue del 73% (IC 95%: 46%-99%) y la especificidad, del 100%. A lo largo de la vigilancia se incluyeron 855 trabajadores y 1764 hisopados, con una incidencia acumulada anual de 2,3% (IC 95%: 1,2%-3,4%). Se detectaron 20 casos asintomáticos positivos. DISCUSIÓN: El tamizaje de trabajadores de salud asintomáticos en la pandemia contribuyó a reducir el riesgo de brotes hospitalarios. Asimismo, se generó un marco de trabajo interdisciplinario aplicable a otros problemas de salud.

COVID-19 epidemiological surveillance through pool testing focused on asymptomatic healthcare workers in the city of Bahía Blanca / Vigilancia epidemiológica de covid-19 mediante testeos agrupados focalizada en trabajadores de salud asintomáticos en bahía blanca

RESUMEN INTRODUCCIÓN: Uno de los desafíos más relevantes al comienzo de la pandemia consistió en implementar estrategias dirigidas a prevenir la transmisión del virus SARS-CoV-2 y mitigar el impacto de la COVID-19. El propósito de este estudio fue contribuir a reducir la transmisión comunitaria a través de una iniciativa interinstitucional, cuyos objetivos fueron validar un método de detección de ARN del SARS-CoV-2 e implementar y evaluar la vigilancia en trabajadores de salud asintomáticos de instituciones de salud pública de Bahía Blanca. MÉTODOS: Se validó una prueba de detección del gen E del coronavirus mediante RT-PCR a partir de ARN aislado de hisopados nasofaríngeos. Para aumentar la capacidad de testeo se validó la detección del ARN viral en muestras agrupadas (pooles). Se realizó un estudio de cohorte prospectiva entre el 15/09/20 y el 15/09/21. RESULTADOS: La sensibilidad y especificidad de la prueba en muestras individuales fue del 95% (IC 95%: 85%-100%). La sensibilidad de la detección en pooles fue del 73% (IC 95%: 46%-99%) y la especificidad, del 100%. A lo largo de la vigilancia se incluyeron 855 trabajadores y 1764 hisopados, con una incidencia acumulada anual de 2,3% (IC 95%: 1,2%-3,4%). Se detectaron 20 casos asintomáticos positivos. DISCUSIÓN: El tamizaje de trabajadores de salud asintomáticos en la pandemia contribuyó a reducir el riesgo de brotes hospitalarios. Asimismo, se generó un marco de trabajo interdisciplinario aplicable a otros problemas de salud.

ABSTRACT INTRODUCTION: One of the most important challenges at the beginning of the pandemic was to implement strategies to prevent SARS-CoV-2 virus transmission and reduce the impact of COVID-19. The purpose of this study was to contribute to the reduction of community transmission through an interinstitutional initiative, aimed at validating a SARS-CoV-2 RNA detection method, and at implementing and assessing the surveillance of asymptomatic infected healthcare workers (HCWs) at public health institutions in the city of Bahía Blanca. METHODS: To validate a coronavirus RNA detection method, RNA was extracted from nasopharyngeal swabs and identification of the viral E gene was done by RT-PCR. Validation of sample pooling was performed to increase the testing capacity. A prospective cohort study was conducted from 15 September 2020 to 15 September 2021. RESULTS: The sensitivity and specificity of the test in individual samples was 95% (CI 95%: 85%-100%). The sensitivity of the pooling strategy was 73% (CI 95%: 46%-99%) and the specificity was 100%. A total of the 855 HCWs were included in the surveillance and 1764 swabs were performed, with an annual cumulative incidence of 2.3% (CI 95%: 1,2%-3,4%), and 20 positive asymptomatic cases were detected. DISCUSSION: The screening of asymptomatic HCWs during the pandemic contributed to reduce the risk of outbreaks in hospital settings. Moreover, an interdisciplinary team framework applicable to other health problems was generated.

A conserved arginine with non-conserved function is a key determinant of agonist selectivity in α7 nicotinic ACh receptors.

BACKGROUND AND PURPOSE: The α7 and α4ß2* ("*" denotes possibly assembly with another subunit) nicotinic acetylcholine receptors (nAChRs) are the most abundant nAChRs in the mammalian brain. These receptors are the most targeted nAChRs in drug discovery programmes for brain disorders. However, the development of subtype-specific agonists remains challenging due to the high degree of sequence homology and conservation of function in nAChRs. We have developed C(10) variants of cytisine, a partial agonist of α4ß2 nAChR that has been used for smoking cessation. The C(10) methyl analogue used in this study displays negligible affinity for α7 nAChR, while retaining high affinity for α4ß2 nAChR. EXPERIMENTAL APPROACH: The structural underpinning of the selectivity of 10-methylcytisine for α7 and α4ß2 nAChRs was investigated using molecular dynamic simulations, mutagenesis and whole-cell and single-channel current recordings. KEY RESULTS: We identified a conserved arginine in the ß3 strand that exhibits a non-conserved function in nAChRs. In α4ß2 nAChR, the arginine forms a salt bridge with an aspartate residue in loop B that is necessary for receptor expression, whereas in α7 nAChR, this residue is not stabilised by electrostatic interactions, making its side chain highly mobile. This lack of constrain produces steric clashes with agonists and affects the dynamics of residues involved in agonist binding and the coupling network. CONCLUSION AND IMPLICATIONS: We conclude that the high mobility of the ß3-strand arginine in the α7 nAChR influences agonist binding and possibly gating network and desensitisation. The findings have implications for rational design of subtype-selective nAChR agents.

Caenorhabditis elegans muscle Cys-loop receptors as novel targets of terpenoids with potential anthelmintic activity.

The anthelmintic treatment of nematode infections remains the pillar of worm control in both human and veterinary medicine. Since control is threatened by the appearance of drug resistant nematodes, there is a need to develop novel compounds, among which phytochemicals constitute potential anthelmintic agents. Caenorhabditis elegans has been pivotal in anthelmintic drug discovery and in revealing mechanisms of drug action and resistance. By using C. elegans, we here revealed the anthelmintic actions of three plant terpenoids -thymol, carvacrol and eugenol- at the behavioral level. Terpenoids produce a rapid paralysis of worms with a potency rank order carvacrol > thymol > eugenol. In addition to their paralyzing activity, they also inhibit egg hatching, which would, in turn, lead to a broader anthelmintic spectrum of activity. To identify drug targets, we performed an in vivo screening of selected strains carrying mutations in receptors involved in worm locomotion for determining resistance to the paralyzing effect of terpenoids. The assays revealed that two Cys-loop receptors with key roles in worm locomotion -Levamisole sensitive nicotinic receptor (L-AChR) and GABA(A) (UNC-49) receptor- are involved in the paralyzing effects of terpenoids. To decipher the mechanism by which terpenoids affect these receptors, we performed electrophysiological studies using a primary culture of C. elegans L1 muscle cells. Whole cell recordings from L1 cells demonstrated that terpenoids decrease macroscopic responses of L-AChR and UNC-49 receptor to their endogenous agonists, thus acting as inhibitors. Single-channel recordings from L-AChR revealed that terpenoids decrease the frequency of opening events, probably by acting as negative allosteric modulators. The fact that terpenoids act at different receptors may have important advantages regarding efficacy and development of resistance. Thus, our findings give support to the use of terpenoids as either an alternative or a complementary anthelmintic strategy to overcome the ever-increasing resistance of parasites to classical anthelmintic drugs.

Flavonoids as positive allosteric modulators of α7 nicotinic receptors.

The use of positive allosteric modulators (PAM) of α7 nicotinic receptors is a promising therapy for neurodegenerative, inflammatory and cognitive disorders. Flavonoids are polyphenolic compounds showing neuroprotective, anti-inflammatory and pro-cognitive actions. Besides their well-known antioxidant activity, flavonoids trigger intracellular pathways and interact with receptors, including α7. To reveal how the beneficial actions of flavonoids are linked to α7 function, we evaluated the effects of three representative flavonoids -genistein, quercetin and the neoflavonoid 5,7-dihydroxy-4-phenylcoumarin- on whole-cell and single-channel currents. All flavonoids increase the maximal currents elicited by acetylcholine with minimal effects on desensitization and do not reactivate desensitized receptors, a behaviour consistent with type I PAMs. At the single-channel level, they increase the duration of the open state and produce activation in long-duration episodes with a rank order of efficacy of genistein > quercetin ≥ neoflavonoid. By using mutant and chimeric α7 receptors, we demonstrated that flavonoids share transmembrane structural determinants with other PAMs. The α7-PAM activity of flavonoids results in decreased cell levels of reactive oxygen species. Thus, allosteric potentiation of α7 may be an additional mechanism underlying neuroprotective actions of flavonoids, which may be used as scaffolds for designing new therapeutic agents.

Molecular function of α7 nicotinic receptors as drug targets.

Nicotinic acetylcholine receptors (nAChRs) are pentameric ligand-gated ion channels involved in many physiological and pathological processes. In vertebrates, there are seventeen different nAChR subunits that combine to yield a variety of receptors with different pharmacology, function, and localization. The homomeric α7 receptor is one of the most abundant nAChRs in the nervous system and it is also present in non-neuronal cells. It plays important roles in cognition, memory, pain, neuroprotection, and inflammation. Its diverse physiological actions and associated disorders have made of α7 an attractive novel target for drug modulation. Potentiation of the α7 receptor has emerged as a novel therapeutic strategy for several neurological diseases, such as Alzheimer's and Parkinson's diseases, and inflammatory disorders. In contrast, increased α7 activity has been associated with cancer cell proliferation. The presence of different drug target sites offers a great potential for α7 modulation in different pathological contexts. In particular, compounds that target allosteric sites offer significant advantages over orthosteric agonists due to higher selectivity and a broader spectrum of degrees and mechanisms of modulation. Heterologous expression of α7, together with chaperone proteins, combined with patch clamp recordings have provided important advances in our knowledge of the molecular basis of α7 responses and their potential modulation for pathological processes. This review gives a synthetic view of α7 and its molecular function, focusing on how its unique activation and desensitization features can be modified by pharmacological agents. This fundamental information offers insights into therapeutic strategies.

Diazepam treatment reduces inflammatory cells and mediators in the central nervous system of rats with experimental autoimmune encephalomyelitis.

Benzodiazepines are psychoactive drugs and some of them also affect immune cells. We here characterized the inflammatory and infiltrating immune cells in the central nervous system (CNS) during the acute phase of experimental autoimmune encephalomyelitis (EAE) in animals treated with Diazepam. Also, we evaluated the expression of Translocator Protein (18kDa) (TSPO), which is a biomarker of neuroinflammatory diseases. The results indicate that Diazepam exerts protective effects on EAE development, decreasing the incidence of the disease and reducing the number of inflammatory cells in CNS, with a concomitant decrease of TSPO levels in brain tissue and CNS inflammatory CD11b+ cells.

GABAA receptor plasticity in Jurkat T cells.

GABAA receptors (GABAAR) mediate inhibitory neurotransmission in the human brain. Neurons modify subunit expression, cellular distribution and function of GABAAR in response to different stimuli, a process named plasticity. Human lymphocytes have a functional neuronal-like GABAergic system with GABAAR acting as inhibitors of proliferation. We here explore if receptor plasticity occurs in lymphocytes. To this end, we analyzed human T lymphocyte Jurkat cells exposed to different physiological stimuli shown to mediate plasticity in neurons: GABA, progesterone and insulin. The exposure to 100 µM GABA differently affected the expression of GABAAR subunits measured at both the mRNA and protein level, showing an increase of α1, ß3, and γ2 subunits but no changes in δ subunit. Exposure of Jurkat cells to different stimuli produced different changes in subunit expression: 0.1 µM progesterone decreased δ and 0.5 µM insulin increased ß3 subunits. To identify the mechanisms underlying plasticity, we evaluated the Akt pathway, which is involved in the phosphorylation of ß subunits and receptor translocation to the membrane. A significant increase of phosphorylated Akt and on the expression of ß3 subunit in membrane occurred in cells exposed 15 h to GABA. To determine if plastic changes are translated into functional changes, we performed whole cell recordings. After 15 h GABA-exposure, a significantly higher percentage of cells responded to GABA application when compared to 0 and 40 h exposure, thus indicating that the detected plastic changes may have a role in GABA-modulated lymphocyte function. Our results reveal that lymphocyte GABAAR are modified by different stimuli similarly and by similar mechanisms to those in neurons. This property is of significance for the development of future therapies involving pharmacological modulation of the immune response.

New insights into the structural bases of activation of Cys-loop receptors.

Neurotransmitter receptors of the Cys-loop superfamily mediate rapid synaptic transmission throughout the nervous system, and include receptors activated by ACh, GABA, glycine and serotonin. They are involved in physiological processes, including learning and memory, and in neurological disorders, and they are targets for clinically relevant drugs. Cys-loop receptors assemble either from five copies of one type of subunit, giving rise to homomeric receptors, or from several types of subunits, giving rise to heteromeric receptors. Homomeric receptors are invaluable models for probing fundamental relationships between structure and function. Receptors contain a large extracellular domain that carries the binding sites and a transmembrane region that forms the ion pore. How the structural changes elicited by agonist binding are propagated through a distance of 50Å to the ion channel gate is central to understanding receptor function. Depending on the receptor subtype, occupancy of either two, as in the prototype muscle nicotinic receptor, or three binding sites, as in homomeric receptors, is required for full activation. The conformational changes initiated at the binding sites are propagated to the gate through the interface between the extracellular and transmembrane domains. This region forms a network that relays structural changes from the binding site towards the pore, and also contributes to open channel lifetime and rate of desensitization. Thus, this coupling region controls the beginning and duration of a synaptic response. Here we review recent advances in the molecular mechanism by which Cys-loop receptors are activated with particular emphasis on homomeric receptors.

Functional relationships between agonist binding sites and coupling regions of homomeric Cys-loop receptors.

Each subunit in a homopentameric Cys-loop receptor contains a specialized coupling region positioned between the agonist binding domain and the ion conductive channel. To determine the contribution of each coupling region to the stability of the open channel, we constructed a receptor subunit (α7-5-HT(3A)) with both a disabled coupling region and a reporter mutation that alters unitary conductance, and coexpressed normal and mutant subunits. The resulting receptors show single-channel current amplitudes that are quantized according to the number of reporter mutations per receptor, allowing correlation of the number of intact coupling regions with mean open time. We find that each coupling region contributes an equal increment to the stability of the open channel. However, by altering the numbers and locations of active coupling regions and binding sites, we find that a coupling region in a subunit flanked by inactive binding sites can still stabilize the open channel. We also determine minimal requirements for channel opening regardless of stability and find that channel opening can occur in a receptor with one active coupling region flanked by functional binding sites or with one active binding site flanked by functional coupling regions. The overall findings show that, whereas the agonist binding sites contribute interdependently and asymmetrically to open-channel stability, the coupling regions contribute independently and symmetrically.

Structural basis of activation of cys-loop receptors: the extracellular-transmembrane interface as a coupling region.

Cys-loop receptors mediate rapid transmission throughout the nervous system by converting a chemical signal into an electric one. They are pentameric proteins with an extracellular domain that carries the transmitter binding sites and a transmembrane region that forms the ion pore. Their essential function is to couple the binding of the agonist at the extracellular domain to the opening of the ion pore. How the structural changes elicited by agonist binding are propagated through a distance of 50 A to the gate is therefore central for the understanding of the receptor function. A step forward toward the identification of the structures involved in gating has been given by the recently elucidated high-resolution structures of Cys-loop receptors and related proteins. The extracellular-transmembrane interface has attracted attention because it is a structural transition zone where beta-sheets from the extracellular domain merge with alpha-helices from the transmembrane domain. Within this zone, several regions form a network that relays structural changes from the binding site toward the pore, and therefore, this interface controls the beginning and duration of a synaptic response. In this review, the most recent findings on residues and pairwise interactions underlying channel gating are discussed, the main focus being on the extracellular-transmembrane interface.

Number and locations of agonist binding sites required to activate homomeric Cys-loop receptors.

Homo-pentameric Cys-loop receptors contain five identical agonist binding sites, each formed at a subunit interface. To determine the number and locations of binding sites required to generate a stable active state, we constructed a receptor subunit with a mutation that disables the agonist binding site and a reporter mutation that alters unitary conductance and coexpressed mutant and nonmutant subunits. Although receptors with a range of different subunit compositions are produced, patch-clamp recordings reveal that the amplitude of each single-channel opening event reports the number and, for certain subunit combinations, the locations of subunits with intact binding sites. We find that receptors with three binding sites at nonconsecutive subunit interfaces exhibit maximal mean channel open time, receptors with binding sites at three consecutive or two nonconsecutive interfaces exhibit intermediate open time, and receptors with binding sites at two consecutive or one interface exhibit brief open time. Macroscopic recordings after rapid application of agonist reveal that channel activation slows and the extent of desensitization decreases as the number of binding sites per receptor decreases. The overall results provide a framework for defining mechanisms of activation and drug modulation for homo-pentameric Cys-loop receptors.

Interaction of bupropion with muscle-type nicotinic acetylcholine receptors in different conformational states.

To characterize the binding sites and the mechanisms of inhibition of bupropion on muscle-type nicotinic acetylcholine receptors (AChRs), structural and functional approaches were used. The results established that bupropion (a) inhibits epibatidine-induced Ca(2+) influx in embryonic muscle AChRs, (b) inhibits adult muscle AChR macroscopic currents in the resting/activatable state with approximately 100-fold higher potency compared to that in the open state, (c) increases the desensitization rate of adult muscle AChRs from the open state and impairs channel opening from the resting state, (d) inhibits binding of [(3)H]TCP and [(3)H]imipramine to the desensitized/carbamylcholine-bound Torpedo AChR with higher affinity compared to the resting/alpha-bungarotoxin-bound AChR, (e) binds to the Torpedo AChR in either state mainly by an entropy-driven process, and (f) interacts with a binding domain located between the serine (position 6') and valine (position 13') rings, by a network of van der Waals, hydrogen bond, and polar interactions. Collectively, our data indicate that bupropion first binds to the resting AChR, decreasing the probability of ion channel opening. The remnant fraction of open ion channels is subsequently decreased by accelerating the desensitization process. Bupropion interacts with a luminal binding domain shared with PCP that is located between the serine and valine rings, and this interaction is mediated mainly by an entropy-driven process.

The interface between extracellular and transmembrane domains of homomeric Cys-loop receptors governs open-channel lifetime and rate of desensitization.

The lifetimes of activated postsynaptic receptor channels contribute to the efficiency of synaptic transmission. Here we show that structural differences within the interface dividing extracellular and transmembrane domains of homomeric alpha7 and 5-HT(3A) receptors account for the large differences in open-channel lifetime and time of desensitization onset between these contrasting members of the Cys-loop receptor superfamily. For alpha7 receptors, agonist-evoked single-channel currents appear mainly as isolated brief openings (tau(o) = 0.35 ms), whereas macroscopic currents after a step pulse of agonist desensitize rapidly (tau(d) = 0.4 ms). In contrast for 5-HT(3A) receptors, agonist-evoked single-channel currents appear as clusters of many long openings in quick succession (tau(cluster) = 1.2 s), whereas macroscopic currents desensitize slowly (tau(d) = 1.1 s). A chimeric alpha7-5HT(3A) receptor exhibits functional properties intermediate between those of the parent receptors, but the functional signatures of each parent are reconstituted after substituting the major loops within the interface of the extracellular and transmembrane domains from the corresponding parent receptor. Furthermore, these structural loops contribute to open-channel lifetime and time of desensitization onset in a nonadditive manner. The results suggest that desensitization is the major determinant of the lifetimes of activated alpha7 and 5-HT(3A) receptors and that functional differences between the two receptors arise primarily through structural differences at the interface between extracellular and transmembrane domains.

Tricyclic antidepressants inhibit homomeric Cys-loop receptors by acting at different conformational states.

Tricyclic antidepressants not only inhibit monoamine reuptake but also modulate Cys-loop receptors. However, it is not understood how this modulation is involved in their therapeutic effects. We analyzed the mechanisms of inhibition of homomeric 5-HT(3A) and alpha7-5HT(3A) receptors by tricyclic antidepressants at the single-channel and macroscopic current levels. These drugs reduce agonist-evoked currents in a noncompetitive and concentration-dependent manner. When they act on the open state, the reduction is similar for both receptors and it is voltage-dependent, thus suggesting an open-channel block process in which the blocked channel can either close or remain stabilized. By acting on the resting state, tricyclic antidepressants reduce the peak current in a voltage-independent manner, with a potency 6-fold higher for 5-HT(3A) than for alpha7-5HT(3A) (IC(50): 6 microM and 1 microM for alpha7-5HT(3A) and 5-HT(3A), respectively). Thus, tricyclic antidepressants may act on closed channels at the unshared extracellular domain from where they inhibit channel opening. Single alpha7-5HT(3A) channels in the continued presence of tricyclic antidepressants show: i) reduced open durations, compatible with open-channel block; ii) reduced burst durations, compatible with closing of blocked channels; and iii) reduced frequency of opening events, compatible with both impaired opening and stabilization of a closed state. In summary, our study reveals that tricyclic antidepressants inhibit homomeric Cys-loop receptors by acting through different mechanisms at open and closed conformational states and probably at two different domains, namely, the pore in the open state and the extracellular domain in the closed state.

Single-channel kinetic analysis of chimeric alpha7-5HT3A receptors.

The receptor chimera alpha7-5HT3A has served as a prototype for understanding the pharmacology of alpha7 neuronal nicotinic receptors, yet its low single channel conductance has prevented studies of the activation kinetics of single receptor channels. In this study, we show that introducing mutations in the M3-M4 cytoplasmic linker of the chimera alters neither the apparent affinity for the agonist nor the EC50 but increases the amplitude of agonist-evoked single channel currents to enable kinetic analysis. Channel events appear as single brief openings flanked by long closings or as bursts of several openings in quick succession. Both the open and closed time distributions are described as the sum of multiple exponential components, but these do not change over a wide range of acetylcholine (ACh), nicotine, or choline concentrations. Bursts elicited by a saturating concentration of ACh contain brief and long openings and closings, and a cyclic scheme containing two open and two closed states is found to adequately describe the data. The analysis indicates that once fully occupied, the receptor opens rapidly and efficiently, and closes and reopens several times before it desensitizes. Channel closing and desensitization occur at similar rates and account for the invariant open and closed time distributions.

Relationship between alpha 7 nAChR and apoptosis in human lymphocytes.

The presence of nicotinic receptors (nAChRs) in blood cells has been demonstrated. However, little is known about their functional roles. We have detected mRNA of alpha7 nAChR in peripheral human lymphocytes and determined that its expression is highly variable among individuals and within the same individual at different times. Upregulation of alpha7 is systematically observed after incubation of lymphocytes with nicotine or alpha-bungarotoxin. In addition, the incubation with these drugs decreases the percentage of apoptotic cells induced by the exposure to cortisol. Our results suggest that alpha7 nAChRs are involved in the modulation of cortisol-induced apoptosis.