The unusual nature of epibatidine responses at the alpha4beta2 nicotinic acetylcholine receptor

The identification of an equatorial frog toxin, epibatidine, as a potent non-morphinic analgesic, selective for neuronal nicotinic acetylcholine receptors, provoked a marked renewal in our understanding of pain and its mechanisms. In this work we have examined the effects of epibatidine at the major brain rat alpha4beta2 nicotinic acetylcholine receptor expressed in a cell line. Fast drug applications obtained with a modified liquid filament system were used for the analyses of the currents evoked by acetylcholine, nicotine and epibatidine. Characterized by a slow onset and offset, epibatidine responses were of smaller amplitude to those evoked by acetylcholine or nicotine. About a thousand times more sensitive to epibatidine than acetylcholine, the alpha4beta2 receptor also displayed a more pronounced apparent desensitization to this compound. Finally, overnight exposure to 1 nM epibatidine failed to produce the functional upregulation observed with nicotine. These data indicate that, at the rat alpha4beta2 receptor, epibatidine acts as a partial agonist causing a pronounced inhibition of agonist evoked currents at concentrations that do not activate the receptors.

Neuronal nicotinic acetylcholine receptors: from the gene to the disease

The neuronal nicotinic acetylcholine receptors are excitatory ligand-gated channels. Widely expressed throughout the peripheral and central nervous system, their properties depend upon their subunit composition. Furthermore, genetic studies have revealed a high degree of variation at the genomic level and alternative splicing of the mRNAs coding for these integral membrane proteins. In particular, genes coding for alpha4 and alpha7 subunits harbour a high degree of polymorphisms. Although well characterised at their molecular and functional level, the role of these receptors in the central nervous system remains obscure. Despite accumulating evidence for the participation of nicotinic receptors in disorders of the central nervous system including nicotinic addiction, Parkinson's disease, Alzheimer's disease and Tourette's syndrome, the exact role of these receptors is still speculative. Because most of these phenotypes are complex and genetically heterogeneous, the investigation is difficult. However, in the past few years, significant progress has been made in understanding the contribution of nicotinic acetylcholine receptors to the origin of epilepsies and schizophrenia. By concentrating on the latest results gained for these diseases, we discuss in this review the possible relationships between neuronal nicotinic receptors and neurological and psychiatric disorders.

Chemical modification of epibatidine causes a switch from agonist to antagonist and modifies its selectivity for neuronal nicotinic acetylcholine receptors

BACKGROUND

Studies of ligand gated channels strongly rely on the availability of compounds that can activate or inhibit with high selectivity one set or a subset of defined receptors. The alkaloid epibatidine (EPB), originally isolated from the skin of an Ecuadorian poison frog, is a very specific agonist for the neuronal nicotinic acetylcholine receptors (nAChRs). We used EPB derivatives to investigate the pharmacophore of nAChR subtypes.

RESULTS

Optically pure enantiomers of EPB analogues were synthesised. Analogues were obtained altered in the aromatic part: the chlorine was eliminated and the relative position of the pyridyl nitrogen changed. Voltage clamp electrophysiology was performed with these compounds on neuronal nAChRs reconstituted in Xenopus oocytes. The EPB derivatives show different activities towards the various nAChR subtypes.

CONCLUSIONS

Small changes in the molecular structure of EPB produce marked changes in its capacity to activate the nAChRs. Subtype specificity can be obtained by changing the position of or by eliminating the pyridyl nitrogen.

[Evaluation of inpatient satisfaction. Comparison of different survey methods]

OBJECTIVES

To assess the impact of patient satisfaction survey method on response rate, data quality and satisfaction.

PATIENTS AND METHODS

Four modes of data collection were assessed during a randomized trial that included 400 inpatients discharged from a teaching hospital.

RESULTS

The response rate was 58% within the mail survey group (72% with follow-up letter) versus 73% and 81% within the home and telephone interview group (p < 0.01). 69% of the mailed questionnaires contained no missing values versus 94% and 96% for home and telephone interview modes (p < 0.01). The global satisfaction scale score was greater within the mail survey groups (8.1/10 without follow-up letter and 7.9 with follow-up letter) than within the telephone interview group (7.8) and the face to face interview group (7.3), (p < 0.05).

CONCLUSION

Mail survey with follow-up letter constitutes an operational method despite lower data quality and overestimation of patients' satisfaction scores.

Neuronal nicotinic acetylcholine receptors from Drosophila: two different types of alpha subunits coassemble within the same receptor complex

Although neuronal nicotinic acetylcholine receptors from insects have been reconstituted in vitro more than a decade ago, our knowledge about the subunit composition of native receptors as well as their functional properties still remains limited. Immunohistochemical evidence has suggested that two alpha subunits, alpha-like subunit (ALS) and Drosophila alpha2 subunit (Dalpha2), are colocalized in the synaptic neuropil of the Drosophila CNS and therefore may be subunits of the same receptor complex. To gain further understanding of the composition of these nicotinic receptors, we have examined the possibility that a receptor may imbed more than one alpha subunit using immunoprecipitations and electrophysiological investigations. Immunoprecipitation experiments of fly head extracts revealed that ALS-specific antibodies coprecipitate Dalpha2, and vice versa, and thereby suggest that these two alpha subunits must be contained within the same receptor complex, a result that is supported by investigations of reconstituted receptors in Xenopus oocytes. Discrimination between binary (ALS/beta2 or Dalpha2/beta2) and ternary (ALS/Dalpha2/beta2) receptor complexes was made on the basis of their dose-response curve to acetylcholine as well as their sensitivity to alpha-bungarotoxin or dihydro-beta-erythroidine. These data demonstrate that the presence of the two alpha subunits within a single receptor complex confers new receptor properties that cannot be predicted from knowledge of the binary receptor's properties.

[Neuropathologic consequences of status epilepticus]

Although the results of a number of histopathological studies have unequivocally shown that epileptic seizures are frequently associated with neurodegenerative mechanisms or other alterations in the brain structure, as it is not possible to carry out longitudinal studies the interpretation of these observations remains speculative. It is not clear whether these brain alterations are the cause or the result of epileptic seizures. It is now widely accepted that epileptic seizures correspond to the synchronization of neuronal activity and increase in their discharge frequency. It has been shown that this abnormal firing activity can lead to increase in intracellular calcium concentration in the nerve cells. A rise in intracellular calcium levels can trigger other processes within the cell, or be cytotoxic if these levels are sufficiently high. In this study, in the light of the most recent findings on cell death (apoptosis), the authors have examined the mechanisms whereby epileptic seizures can result in neuronal cell loss.

A new look at the neuronal nicotinic acetylcholine receptor pharmacophore

Interest in the field of nicotinic receptors has been recently stimulated both by the discovery of the potential therapeutic effects of new agonists, and by the discovery of an association between nicotinic receptor mutations and human neurological diseases. Expression of human receptors in an exogenous system allows their study in isolation. Receptors reconstituted by pairwise injection of either alpha4 or alpha3 with beta2 or beta4 subunits displayed important differences between the resulting receptor subtypes. These results were further compared with those obtained with alpha3:alpha4 fusion proteins. The modifications of either the ligand-binding site in the N-terminal domain or in the ionic pore domain were found to affect the pharmacological properties of the receptors. Finally, the analysis of non-natural derivatives of epibatidine demonstrates how an agonist can be modified to be selective at one receptor subtype or to become an antagonist. These data are well explained on the basis of a three-state allosteric model.

Magnocellular neurons of the rat supraoptic nucleus are endowed with functional nicotinic acetylcholine receptors

Acetylcholine can stimulate the release of vasopressin. In organ-cultured hypothalamo-neurohypophyseal systems, acetylcholine enhanced vasopressin release by acting in or near the supraoptic nucleus Extracellular recordings suggested that acetylcholine can increase supraoptic neuron excitability. These effects could be mimicked, in part, by nicotine or blocked by nicotinic antagonists, suggesting that they might be mediated by nicotinic acetylcholine receptors. Autoradiography indicated that alpha-bungarotoxin binding sites are present in the supraoptic nucleus; however, neither acetylcholine nor nicotine binding sites could be detected. Thus, the existence, let alone the nature, of nicotinic receptors in the supraoptic nucleus has so far remained elusive. The present work attempts to determine: (i) whether functional nicotinic receptors are present in this nucleus; (ii) whether they are located on neurosecretory magnocellular cells or at presynaptic sites; (iii) what their pharmacological and biophysical properties are; (iv) whether they influence the activity of all or only part of supraoptic neurons. Whole-cell recordings were performed in hypothalamic slices or in acutely dissociated supraoptic neurons and the effect of nicotinic agonists was tested under voltage-clamp conditions. Autoradiography was done in coronal hypothalamic sections, using [3H]epibatidine and [125I]alpha-bungarotoxin as ligands. Our results indicate that supraoptic neurons possess functional nicotinic receptors containing the alpha7 subunit.

[General physicians' viewpoint on the evaluation of health care quality in the hospital]

The principal objective of this work was to determine whether the evaluation of general practitioners' satisfaction vis-à-vis a health care establishment impacted that establishment to improve the quality of the services it provides. Ten doctors were randomly selected and 100% agreed to respond to a semi-directive interview. Two independent doctors analysed each corpus and identified 15 themes, of which 5 systematically came up: the organisation of the care of patients, the availability of hospital doctors, the transmission of information, the speed of emergency admissions, and the patient-hospital doctor relationship. The opinion of private practice doctors usefully complements the evaluation of patient satisfaction used to identify the weaknesses of an establishment. It is proposed to include this evaluation in the procedure of continued improvement of the quality of care within establishments.

A scorpion alpha-like toxin that is active on insects and mammals reveals an unexpected specificity and distribution of sodium channel subtypes in rat brain neurons

Several scorpion toxins have been shown to exert their neurotoxic effects by a direct interaction with voltage-dependent sodium channels. Both classical scorpion alpha-toxins such as Lqh II from Leiurus quiquestratus hebraeus and alpha-like toxins as toxin III from the same scorpion (Lqh III) competitively interact for binding on receptor site 3 of insect sodium channels. Conversely, Lqh III, which is highly toxic in mammalian brain, reveals no specific binding to sodium channels of rat brain synaptosomes and displaces the binding of Lqh II only at high concentration. The contrast between the low-affinity interaction and the high toxicity of Lqh III indicates that Lqh III binding sites distinct from those present in synaptosomes must exist in the brain. In agreement, electrophysiological experiments performed on acute rat hippocampal slices revealed that Lqh III strongly affects the inactivation of voltage-gated sodium channels recorded either in current or voltage clamp, whereas Lqh II had weak, or no, effects. In contrast, Lqh III had no effect on cultured embryonic chick central neurons and on sodium channels from rat brain IIA and beta1 subunits reconstituted in Xenopus oocytes, whereas sea anemone toxin ATXII and Lqh II were very active. These data indicate that the alpha-like toxin Lqh III displays a surprising subtype specificity, reveals the presence of a new, distinct sodium channel insensitive to Lqh II, and highlights the differences in distribution of channel expression in the CNS. This toxin may constitute a valuable tool for the investigation of mammalian brain function.

Molecular basis of the charge selectivity of nicotinic acetylcholine receptor and related ligand-gated ion channels

Nicotinic acetylcholine receptors are homo- or heteropentameric proteins belonging to the superfamily of receptor channels including the glycine and GABA-A receptors. Affinity labelling and mutagenesis experiments indicated that the M2 transmembrane segment of each subunit lines the ion channel and is coiled into an alpha-helix. Comparison of the M2 sequence of the cation-selective alpha 7 nicotinic receptor to that of the anion-selective alpha 1 glycine receptor identified amino acids involved in charge selectivity. Mutations of the alpha 7 homo-oligomeric receptor within (or near) M2, namely E237A, V251T and a proline insertion P236' were shown to convert the ionic selectivity of alpha 7 from cationic to anionic. Systematic analysis of each of these three mutations supports the notion that the conversion of ionic selectivity results from a local structural reorganization of the 234-238 loop. The 234-238 coiled loop, previously shown to lie near the narrowest portion of the channel, is thus proposed to contribute directly to the charge selectivity filter. A possible functional analogy with the voltage-gated ion channels and related receptors is discussed.

[Physicians' opinions and practices on the use of medical referrals at hospitals]

This work aims to take stock of the actual utilisation of medical standards (references), through a study of opinions and practices of the medical managers working in the clinical services of a university hospital centre. A survey through interviews was proposed to 103 medical managers, 101 responded to the questionnaire (38 Unit directors and 63 directors of "UF" units). Medical standards are used essentially as a teaching aid by 80% of doctors. Some of them (48%) make them available to prescribing doctors in the unit, and for 36% there exists an informational procedure for new residents. Evaluation studies concerning the implementation of medical standards remain rare (7 studies). Medical standards appear to be more useful for improving quality of care (90%) then for controlling health expenditures (72%). The majority of medical managers (72%) consider that certain standards should be opposable to hospitals. The medical managers of the university hospital centre are in favour of developing standards of clinical practice.

[Effectiveness of a program for improving drug prescriptions and hospitalization reporting at a university hospital]

OBJECTIVES

To evaluate the effectiveness of a method for improving the quality of care applied to the formulation of medical prescriptions and to the release of hospitalisation reports. The actions comprised the diffusion of results of audits and recommendations of good practice.

METHODS

For each of the 41 services, 30 visits from 1996 were selected at random. 3289 prescriptions and 1067 correspondence files were analyzed. The results were compared to those obtained from the previous two years.

RESULTS

Patient identification was complete in 44% of prescriptions, the identification of the prescribing doctor and his signature were present in 62% and 19% of cases respectively. 37% of medicines included all information. 7 indicators out of 12 for the quality of prescriptions improved (p < 0.0001). Files were found for 83% of hospital visits and 56% were sent (released) within a week. The practitioner was identified in 79% of cases, the main diagnosis in 96% and the treatment in 65% of cases. Five out of nine indicators of the quality of correspondence improved (p < 0.01). For each theme, the number of indicators improving was similar (p > 0.05). The services that improved for one theme didn't necessarily improve for the second (p > 0.05).

DISCUSSION

The evaluation of the programme, based on a strategy of quality assurance, shows modest progress. Given that the improvement of two themes for a given service are not correlated, the programme appears to sensitise professionals at an individual level rather than collectively. However, this programme is an important step for introducing a mode of continued improvement of quality.

Mutated nicotinic receptors responsible for autosomal dominant nocturnal frontal lobe epilepsy are more sensitive to carbamazepine

PURPOSE

The recent linkage between a genetically transmissible form of epilepsy (ADNFLE) and mutations within the alpha4 subunit, one component of the major brain neuronal nicotinic acetylcholine receptor (nAChR), raises the question of the role of this receptor in epileptogenesis. Although acting by different mechanisms, the two genetic alterations so far identified both render the nAChR less efficient. In view of the high sensitivity of ADNFLE to carbamazepine (CBZ), we studied the effects of this drug and of valproate (VPA) on the human alpha4beta2 nAChR and its mutations.

METHODS

The alpha4beta2 nAChRs from control and mutant alpha4 subunits were reconstituted in Xenopus oocytes and investigated by using a dual-electrode voltage clamp technique. Acetylcholine (ACh)-evoked currents recorded in the absence or presence of antiepileptic drugs (AEDs) were studied to analyze the mode of action of these compounds.

RESULTS

ACh-evoked currents at the human alpha4beta2 nAChR were readily and reversibly inhibited by approximately 100 microM CBZ. This compound was found to be a noncompetitive inhibitor of the nAChR, which probably acts by entering the channel and causing a blockade by steric hindrance. Dose-response inhibition curves determined on the control receptor and on ADNFLE-mutant receptors showed a greater sensitivity of the mutants to CBZ, with median inhibitory concentrations (IC50s) in the range of the antiepileptic plasma levels of CBZ. In contrast, VPA had nearly no effect on control and mutant nAChRs.

CONCLUSIONS

CBZ inhibits the neuronal alpha4beta2 nAChRs at pharmacologic concentrations, with ADNFLE mutants displaying about threefold higher sensitivity to this compound. The increased sensitivity of these mutant receptors supports the hypothesis that the antiepileptic activity of CBZ can, at least to some extent, be attributed to the nAChR inhibition.

Localization of mRNA for CHRNA7 in human fetal brain

The aim of this study was to determine the regional distribution in situ of the mRNA for the alpha 7 subunit of the neuronal nicotinic acetylcholine receptor in human fetal brain. We found high levels of alpha 7 gene expression in nuclei that receive sensory information, such as those of the neocortex and hippocampus, the thalamic nuclei, the reticular thalamic nucleus, the pontine nuclei and the superior olive complex. These data support a possible regulatory function for alpha 7-containing receptors in sensory processing, which may be involved in the pathological physiology of schizophrenia and autism. Early alpha 7 gene expression is also consistent with a morphogenetic role for alpha 7 receptors in central nervous system development.

Presence of functional neuronal nicotinic acetylcholine receptors in brainstem motoneurons of the rat

In mammals, nicotinic acetylcholine receptors (nAChRs) play a crucial role in motor control. Muscle-type nAChRs mediate synaptic excitation of skeletal muscle by motoneurons, and nAChRs are present on Renshaw cells, where they produce recurrent inhibition of spinal motoneurons. We asked whether nAChRs are also present in motoneurons. Whole-cell recordings were performed on various motor nuclei in brainstem slices of young rats. Neurons were visualized using infrared (IR) videomicroscopy. Acetylcholine (ACh) or the nicotinic agonist, epibatidine, were delivered by pressure microinjection. Facial (VII), hypoglossal (XII) and vagal (X) motoneurons responded to ACh by generating a fast inward current. In VII motoneurons, the ACh effect was mimicked by epibatidine, and nicotine induced a slow inward current and desensitized the ACh-evoked current. In VII and XII motoneurons, the ACh-evoked current was blocked by the nicotinic antagonist dihydro-beta-erythroidine (DHbetaE), but was unaffected by methyllycaconitine (MLA), an alpha7-specific antagonist. By contrast, the ACh-induced current in X motoneurons was sensitive to MLA. Current-voltage relationships indicated that the currents mediated by either alpha7-containing (X) or non-alpha7-containing (VII, XII) nAChRs displayed inward rectification. In accordance with the electrophysiological data, autoradiography revealed that VII, X and XII nuclei of young rats contained binding sites for [3H]epibatidine; binding sites for [125I]alpha-bungarotoxin, a selective ligand of alpha7-containing nAChRs, were present in X nucleus but were almost undetectable in VII and XII nuclei. Thus, brainstem motoneurons of young rats possess functional nAChRs. They could promote fast synaptic coupling between motoneurons, and thus play a role in somatic and visceral motor functions.

[A quality reference manual for radiology]

The purpose of this article is to introduce a quality manual for radiology departments. A radiology department has implemented a quality improvement program since 1996. This manual was developed as a tool for quality improvement program. This manuscript, was based on foreign accreditation manuals as well as from French experiences and summarized. In addition, new criteria were added, especially in the field of interventional radiology. This reference book is first dedicated to the self-assessment of radiology departments. It can also be used for an external audit.

Neuronal nicotinic acetylcholine receptors: their properties and alterations in autosomal dominant nocturnal frontal lobe epilepsy

Identification of genes coding for the neuronal nicotinic acetylcholine receptors (nAChRs) has allowed rapid progress in the field of neuroscience. Determination of a high-affinity binding site for nicotine that correlates with the expression of mRNAs coding for nAChRs as well as protein expression is the best demonstration for localization of these receptors. Reconstitution of functional nAChRs in cells following cDNAs injection opened new ways to study these receptors in isolation. Furthermore, the recent linkage analysis between a form of autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) with a mutation in the gene coding for the alpha 4 subunit of the neuronal nAChRs constituted the first demonstration that alteration of these receptors may be at the origin of epileptic discharges. Physiological and pharmacological studies of these mutated receptors revealed that the two mutations so far identified in ADNFLE patient cause a loss of function. In this work we shall review, in the light of the latest findings, properties of control and mutated receptors and evaluate how their alteration can be at the origin of nocturnal seizures.

Neuronal nAChR stereoselectivity to non-natural epibatidine derivatives

The frog toxin epibatidine is one of the most powerful ligands of the neuronal nicotinic receptors and derivatives show promising possibilities for labeling in positron emission tomography studies. In an attempt to reduce epibatidine toxicity, new methyl derivatives were synthesized, tested in positron emission tomography imaging and in electrophysiology. labeling as well as physiological experiments highlighted the differences in sensitivity of the neuronal nicotinic acetylcholine receptors between two methyl enantiomers and the reduction in sensitivity caused by introducing the methyl group. At present, epibatidine derivatives seem the most promising compounds for in vivo labeling of neuronal nicotinic acetylcholine receptors.