Anatoxin-a-evoked [3H]dopamine release from rat striatal synaptosomes

Toxic Effects Produced by Anatoxin-a under Laboratory Conditions: A Review

The presence of cyanotoxins and its bioaccumulation in the food chain is an increasingly common problem worldwide. Despite the toxic effects produced by Anatoxin-a (ATX-a), this neurotoxin has been less studied compared to microcystins (MCs) and cylindrospermopsin (CYN). Studies conducted under laboratory conditions are of particular interest because these provide information which are directly related to the effects produced by the toxin. Currently, the World Health Organization (WHO) considers the ATX-a toxicological database inadequate to support the publication of a formal guideline reference value. Therefore, the aim of the present work is to compile all of the in vitro and in vivo toxicological studies performed so far and to identify potential data gaps. Results show that the number of reports is increasing in recent years. However, more in vitro studies are needed, mainly in standardized neuronal cell lines. Regarding in vivo studies, very few of them reflect conditions occurring in nature and further studies with longer periods of oral exposure would be of interest. Moreover, additional toxicological aspects of great interest such as mutagenicity, genotoxicity, immunotoxicity and alteration of hormonal balance need to be studied in depth.

Cyanotoxins and the Nervous System

Cyanobacteria are capable of producing a wide range of bioactive compounds with many considered to be toxins. Although there are a number of toxicological outcomes with respect to cyanobacterial exposure, this review aims to examine those which affect the central nervous system (CNS) or have neurotoxicological properties. Such exposures can be acute or chronic, and we detail issues concerning CNS entry, detection and remediation. Exposure can occur through a variety of media but, increasingly, exposure through air via inhalation may have greater significance and requires further investigation. Even though cyanobacterial toxins have traditionally been classified based on their primary mode of toxicity, increasing evidence suggests that some also possess neurotoxic properties and include known cyanotoxins and unknown compounds. Furthermore, chronic long-term exposure to these compounds is increasingly being identified as adversely affecting human health.

Targeting the cholinergic system in Parkinson's disease

Motor control in the striatum is an orchestra played by various neuronal populations. Loss of harmony due to dopamine deficiency is considered the primary pathological cause of the symptoms of Parkinson’s disease (PD). Recent progress in experimental approaches has enabled us to examine the striatal circuitry in a much more comprehensive manner, not only reshaping our understanding of striatal functions in movement regulation but also leading to new opportunities for the development of therapeutic strategies for treating PD. In addition to dopaminergic innervation, giant aspiny cholinergic interneurons (ChIs) within the striatum have long been recognized as a critical node for balancing dopamine signaling and regulating movement. With the roles of ChIs in motor control further uncovered and more specific manipulations available, striatal ChIs and their corresponding receptors are emerging as new promising therapeutic targets for PD. This review summarizes recent progress in functional studies of striatal circuitry and discusses the translational implications of these new findings for the treatment of PD.

Cyanobacterial bioactive metabolites-A review of their chemistry and biology

Cyanobacterial blooms occur when algal densities exceed baseline population concentrations. Cyanobacteria can produce a large number of secondary metabolites. Odorous metabolites affect the smell and flavor of aquatic animals, whereas bioactive metabolites cause a range of lethal and sub-lethal effects in plants, invertebrates, and vertebrates, including humans. Herein, the bioactivity, chemistry, origin, and biosynthesis of these cyanobacterial secondary metabolites were reviewed. With recent revision of cyanobacterial taxonomy by Anagnostidis and Komárek as part of the Süβwasserflora von Mitteleuropa volumes 19(1-3), names of many cyanobacteria that produce bioactive compounds have changed, thereby confusing readers. The original and new nomenclature are included in this review to clarify the origins of cyanobacterial bioactive compounds. Due to structural similarity, the 157 known bioactive classes produced by cyanobacteria have been condensed to 55 classes. This review will provide a basis for more formal procedures to adopt a logical naming system. This review is needed for efficient management of water resources to understand, identify, and manage cyanobacterial harmful algal bloom impacts.

Cyanobacterial bioactive metabolites-A review of their chemistry and biology

Cyanobacterial blooms occur when algal densities exceed baseline population concentrations. Cyanobacteria can produce a large number of secondary metabolites. Odorous metabolites affect the smell and flavor of aquatic animals, whereas bioactive metabolites cause a range of lethal and sub-lethal effects in plants, invertebrates, and vertebrates, including humans. Herein, the bioactivity, chemistry, origin, and biosynthesis of these cyanobacterial secondary metabolites were reviewed. With recent revision of cyanobacterial taxonomy by Anagnostidis and Komárek as part of the Süβwasserflora von Mitteleuropa volumes 19(1-3), names of many cyanobacteria that produce bioactive compounds have changed, thereby confusing readers. The original and new nomenclature are included in this review to clarify the origins of cyanobacterial bioactive compounds. Due to structural similarity, the 157 known bioactive classes produced by cyanobacteria have been condensed to 55 classes. This review will provide a basis for more formal procedures to adopt a logical naming system. This review is needed for efficient management of water resources to understand, identify, and manage cyanobacterial harmful algal bloom impacts.

Striatal cholinergic interneurons and Parkinson's disease

Giant, aspiny cholinergic interneurons (ChIs) have long been known to be key nodes in the striatal circuitry controlling goal-directed actions and habits. In recent years, new experimental approaches, like optogenetics and monosynaptic rabies virus mapping, have expanded our understanding of how ChIs contribute to the striatal activity underlying action selection and the interplay of dopaminergic and cholinergic signaling. These approaches also have begun to reveal how ChI function is distorted in disease states affecting the basal ganglia, like Parkinson's disease (PD). This review gives a brief overview of our current understanding of the functional role played by ChIs in striatal physiology and how this changes in PD. The translational implications of these discoveries, as well as the gaps that remain to be bridged, are discussed as well.

Protection by a transdermal patch containing eserine and pralidoxime chloride for prophylaxis against (±)-Anatoxin A poisoning in rats

The prophylactic and neuroprotective impact of a transdermal patch containing eserine and pralidoxime chloride (2-PAM) against (±)-Anatoxin A poisoning was investigated using Wistar strain albino rats. Rats were smooth-shaved on the dorsal side, attached with a drug-in-adhesive matrix type prophylactic transdermal patch for 72 h and challenged with subcutaneous injection of three doses (1.0, 1.5 and 2.0×LD50) of (±)-Anatoxin A. The LD50 value of (±)-Anatoxin A was determined to be 1.25mg/kg, and at this particular dose (1.0×LD50) of toxin induced severe clinical symptom including extreme seizures in rats, resulting acute brain injuries in discrete brain regions, leading to 100% mortality within 5 min. The anticonvulsant effect, antiarrythmic effect, nerve conduction study, clinical observations and mortality, neuroprotective effect as well as skin histopathology of the prophylactic transdermal patch against (±)-Anatoxin A poisoning were investigated systematically. It was found that seizures, tachycardia, nerve damage, clinical symptoms, brain injuries and mortality induced by such lethal toxin were effectively prevented by the prophylactic patch treatment up to certain LD50 level. Hence, it could be a choice of potential therapeutic regimen against such lethal poisoning.

In vivo neurochemical characterization of clothianidin induced striatal dopamine release

Clothianidin (CLO) is a neonicotinoid insecticide with selective action on nicotinic acetylcholine receptors. The aim of this study was to determine the neurochemical basis for CLO-induced striatal dopamine release using the microdialysis technique in freely moving and conscious rats. Intrastriatal administration of CLO (3.5mM), produced an increase in both spontaneous (2462 ± 627% with respect to basal values) and KCl-evoked (4672 ± 706% with respect to basal values) dopamine release. This effect was attenuated in Ca(2+)-free medium, and was prevented in reserpine pre-treated animals or in presence of tetrodotoxin (TTX). To investigate the involvement of dopamine transporter (DAT), the effect of CLO was observed in presence of nomifensine. The coadministration of CLO and nomifensine produced an additive effect on striatal dopamine release. The results suggest that the effect of CLO on striatal dopamine release is predominantly mediated by an exocytotic mechanism, Ca(2+), vesicular and TTX-dependent and not by a mechanism mediated by dopamine transporter.

Muscarinic and nicotinic acetylcholine receptor agonists and allosteric modulators for the treatment of schizophrenia

Muscarinic and nicotinic acetylcholine (ACh) receptors (mAChRs and nAChRs) are emerging as important targets for the development of novel treatments for the symptoms associated with schizophrenia. Preclinical and early proof-of-concept clinical studies have provided strong evidence that activators of specific mAChR (M(1) and M(4)) and nAChR (α(7) and α(2)β(4)) subtypes are effective in animal models of antipsychotic-like activity and/or cognitive enhancement, and in the treatment of positive and cognitive symptoms in patients with schizophrenia. While early attempts to develop selective mAChR and nAChR agonists provided important preliminary findings, these compounds have ultimately failed in clinical development due to a lack of true subtype selectivity and subsequent dose-limiting adverse effects. In recent years, there have been major advances in the discovery of highly selective activators for the different mAChR and nAChR subtypes with suitable properties for optimization as potential candidates for clinical trials. One novel strategy has been to identify ligands that activate a specific receptor subtype through actions at sites that are distinct from the highly conserved ACh-binding site, termed allosteric sites. These allosteric activators, both allosteric agonists and positive allosteric modulators, of mAChR and nAChR subtypes demonstrate unique mechanisms of action and high selectivity in vivo, and may provide innovative treatment strategies for schizophrenia.

Mechanisms involved in nicotinic acetylcholine receptor-induced neurotransmitter release from sympathetic nerve terminals in the mouse vas deferens

Prejunctional nicotinic acetylcholine receptors (nAChRs) amplify postganglionic sympathetic neurotransmission, and there are indications that intraterminal Ca²⁺ stores might be involved. However, the mechanisms by which nAChR activation stimulates neurotransmitter release at such junctions is unknown. Rapid local delivery (picospritzing) of the nAChR agonist epibatidine was combined with intracellular sharp microelectrode recording to monitor spontaneous and field-stimulation-evoked neurotransmitter release from sympathetic nerve terminals in the mouse isolated vas deferens. Locally applied epibatidine (1 µM) produced ‘epibatidine-induced depolarisations’ (EIDs) that were similar in shape to spontaneous excitatory junction potentials (SEJPs) and were abolished by nonselective nAChR antagonists and the purinergic desensitizing agonist α,β-methylene ATP. The amplitude distribution of EIDs was only slightly shifted towards lower amplitudes by the selective α7 nAChR antagonists α-bungarotoxin and methyllcaconitine, the voltage-gated Na⁺ channel blocker tetrodotoxin or by blocking voltage-gated Ca²⁺ channels with Cd²⁺. Lowering the extracellular Ca²⁺ concentration reduced the frequency of EIDs by 69%, but more surprisingly, the Ca²⁺-induced Ca²⁺ release blocker ryanodine greatly decreased the amplitude (by 41%) and the frequency of EIDs by 36%. Ryanodine had no effect on electrically-evoked neurotransmitter release, paired-pulse facilitation, SEJP frequency, SEJP amplitude or SEJP amplitude distribution. These results show that activation of non-α7 nAChRs on sympathetic postganglionic nerve terminals induces high-amplitude junctional potentials that are argued to represent multipacketed neurotransmitter release synchronized by intraterminal Ca²⁺-induced Ca²⁺ release, triggered by Ca²⁺ influx directly through the nAChR. This nAChR-induced neurotransmitter release can be targeted pharmacologically without affecting spontaneous or electrically-evoked neurotransmitter release.

Presynaptic nicotinic α7 and non-α7 receptors stimulate endogenous GABA release from rat hippocampal synaptosomes through two mechanisms of action

Background

Although converging evidence has suggested that nicotinic acetylcholine receptors (nAChR) play a role in the modulation of GABA release in rat hippocampus, the specific involvement of different nAChR subtypes at presynaptic level is still a matter of debate. In the present work we investigated, using selective α7 and α4β2 nAChR agonists, the presence of different nAChR subtypes on hippocampal GABA nerve endings to assess to what extent and through which mechanisms they stimulate endogenous GABA release.

Methodology/Findings

All agonists elicited GABA overflow. Choline (Ch)-evoked GABA overflow was dependent to external Ca²⁺, but unaltered in the presence of Cd²⁺, tetrodotoxin (TTX), dihydro-β-erythroidine (DHβE) and 1-(4,4-Diphenyl-3-butenyl)-3-piperidinecarboxylic acid hydrochloride SKF 89976A. The effect of Ch was blocked by methyllycaconitine (MLA), α-bungarotoxin (α-BTX), dantrolene, thapsigargin and xestospongin C, suggesting that GABA release might be triggered by Ca²⁺ entry into synaptosomes through the α7 nAChR channel with the involvement of calcium from intracellular stores. Additionally, 5-Iodo-A-85380 dihydrochloride (5IA85380) elicited GABA overflow, which was Ca²⁺ dependent, blocked by Cd²⁺, and significantly inhibited by TTX and DHβE, but unaffected by MLA, SKF 89976A, thapsigargin and xestospongin C and dantrolene. These findings confirm the involvement of α4β2 nAChR in 5IA85380-induced GABA release that seems to occur following membrane depolarization and opening calcium channels.

Conclusions/Significance

Rat hippocampal synaptosomes possess both α7 and α4β2 nAChR subtypes, which can modulate GABA release via two distinct mechanisms of action. The finding that GABA release evoked by the mixture of sub-maximal concentration of 5IA85380 plus sub-threshold concentrations of Ch was significantly larger than that elicited by the sum of the effects of the two agonists is compatible with the possibility that they coexist on the same nerve terminals. These findings would provide the basis for possible selective pharmacological strategies to treat neuronal disorders that involve the dysfunction of hippocampal cholinergic system.

Molecular and cellular mechanisms of action of nicotine in the CNS

Nicotine achieves its psychopharmacological effects by interacting with nicotinic acetylcholine receptors (nAChRs) in the brain. There are numerous subtypes of nAChR that differ in their properties, including their sensitivity to nicotine, permeability to calcium and propensity to desensitise. The nAChRs are differentially localised to different brain regions and are found on presynaptic terminals as well as in somatodendritic regions of neurones. Through their permeability to cations, these ion channel proteins can influence both neuronal excitability and cell signalling mechanisms, and these various responses can contribute to the development or maintenance of dependence. However, many questions and uncertainties remain in our understanding of these events and their relevance to tobacco addiction. In this chapter, we briefly overview the fundamental characteristics of nAChRs that are germane to nicotine's effects and then consider the cellular responses to acute and chronic nicotine, with particular emphasis on dopamine systems because they have been the most widely studied in the context of nicotine dependence. Where appropriate, methodological aspects are critically reviewed.

Mammalian nicotinic acetylcholine receptors: from structure to function

The classical studies of nicotine by Langley at the turn of the 20th century introduced the concept of a "receptive substance," from which the idea of a "receptor" came to light. Subsequent studies aided by the Torpedo electric organ, a rich source of muscle-type nicotinic receptors (nAChRs), and the discovery of alpha-bungarotoxin, a snake toxin that binds pseudo-irreversibly to the muscle nAChR, resulted in the muscle nAChR being the best characterized ligand-gated ion channel hitherto. With the advancement of functional and genetic studies in the late 1980s, the existence of nAChRs in the mammalian brain was confirmed and the realization that the numerous nAChR subtypes contribute to the psychoactive properties of nicotine and other drugs of abuse and to the neuropathology of various diseases, including Alzheimer's, Parkinson's, and schizophrenia, has since emerged. This review provides a comprehensive overview of these findings and the more recent revelations of the impact that the rich diversity in function and expression of this receptor family has on neuronal and nonneuronal cells throughout the body. Despite these numerous developments, our understanding of the contributions of specific neuronal nAChR subtypes to the many facets of physiology throughout the body remains in its infancy.

Effects of weekly exposure to anatoxin-a and nicotine on operant performance of rats

This study examined the effects of acute and weekly administration of anatoxin-a and nicotine on operant performance. Anatoxin-a is a potent nicotinic receptor agonist produced by cyanobacteria, which are found in fresh waters throughout the world. Anatoxin-a is a potential human health hazard and has been responsible for numerous deaths of wildlife, livestock and domestic animals. Remarkably little is known, however, about the effects of anatoxin-a on behavior. Nicotine, the psychomotor stimulant in tobacco, has many well-documented behavioral effects, which often diminish (i.e. tolerance develops) when it is given daily. Male Long Evans rats initially were trained to respond under a multiple variable-ratio 30-response variable-interval 60-s (mult VR-30 VI 60-s) schedule of food reinforcement. They were then divided into 12 groups of 8 that received four weekly subcutaneous injections of anatoxin-a (0.05-0.2 mg/kg), nicotine (0.125-1.8 mg/kg), or vehicle 5-min prior to testing. When initially administered, each compound decreased response rates and reinforcement rates in both components of the multiple schedule. Substantial tolerance developed to the disruptive effects of nicotine with weekly administration. Tolerance also developed to the effects of anatoxin-a, although to a lesser degree; the highest dose severely decreased performance with little evidence of recovery. In conjunction with prior findings, these results suggest the behavioral effects of anatoxin-a and nicotine are similar, but not identical, and that relatively infrequent (episodic) administration can produce tolerance.

Toxicology and detection methods of the alkaloid neurotoxin produced by cyanobacteria, anatoxin-a

Freshwater resources are under stress due to naturally occurring conditions and human impacts. One of the consequences is the proliferation of cyanobacteria, microphytoplankton organisms that are capable to produce toxins called cyanotoxins. Anatoxin-a is one of the main cyanotoxins. It is a very potent neurotoxin that was already responsible for some animal fatalities. In this review we endeavor to divulgate much of the internationally published information about toxicology, occurrence and detection methods of anatoxin-a. Cyanobacteria generalities, anatoxin-a occurrence and production as well as anatoxin-a toxicology and its methods of detection are the aspects focused in this review. Remediation of anatoxin-a occurrence will be addressed with a public health perspective. Final remarks call the attention for some important gaps in the knowledge about this neurotoxin and its implication to public health. Alterations of aquatic ecosystems caused by anatoxin-a is also addressed. Although anatoxin-a is not the more frequent cyanotoxin worldwide, it has to be regarded as a health risk that can be fatal to terrestrial and aquatic organisms because of its high toxicity.

Catecholamine outflow from mouse and rat brain slice preparations evoked by nicotinic acetylcholine receptor activation and electrical field stimulation

BACKGROUND AND PURPOSE

Mice with targeted deletions of neuronal nicotinic acetylcholine receptor (nAChR) subunit genes are valuable models to study nAChR function such as catecholamine outflow by presynaptic receptor activation. Contrary to the rat, our present knowledge on presynaptic nAChRs in mice primarily relies on observations made with synaptosomes. We have now used brain slices to investigate nicotine-induced catecholamine outflow in wild type (WT) and nAChR (beta2 and alpha5) knockout mice for a comparison with rat brain slice preparations.

EXPERIMENTAL APPROACH

Brain slices from rat and mouse hippocampus, parieto-occipital neocortex, and corpus striatum were loaded with either [3H]-noradrenaline or [3H]-dopamine. We provoked catecholamine outflow by electrical field stimulation and nicotinic agonists.

KEY RESULTS

When set in relation to electrical field stimulation, nicotine-evoked catecholamine release was sizeable in the striatum but low in the neocortex of both rats and mice. [3H]-noradrenaline outflow was, on the other hand, substantial in the rat but low in the mouse hippocampus. About 10% (or less) of nicotine-induced catecholamine release persisted in the presence of tetrodotoxin in all our preparations.

CONCLUSIONS AND IMPLICATIONS

Targeted deletion of the beta2 subunit gene essentially abolished the effect of nicotine, indicating that this subunit is an essential constituent of nAChRs that indirectly (via action potentials) induce catecholamine release from hippocampal and striatal slices in mice. The impact of nAChRs in catecholaminergic projection areas differs between species and has thus to be considered when extrapolating results from animal models to human conditions.

Developmental regulation of nicotinic acetylcholine receptors within midbrain dopamine neurons

We have combined anatomical and functional methodologies to provide a comprehensive analysis of the properties of nicotinic acetylcholine receptors (nAChRs) on developing dopamine (DA) neurons of Sprague-Dawley rats. Double-labeling in situ hybridization was used to examine the expression of nAChR subunit mRNAs within developing midbrain DA neurons. As brain maturation progressed there was a change in the pattern of subunit mRNA expression within DA neurons, such that alpha3 and alpha4 subunits declined and alpha6 mRNA increased. Although there were strong similarities in subunit mRNA expression in substantia nigra (SNc) and ventral tegmental area (VTA), there was higher expression of alpha4 mRNA in SNc than VTA at gestational day (G) 15, and of alpha5, alpha6 and beta3 mRNAs during postnatal development. Using a superfusion neurotransmitter release paradigm to functionally characterize nicotine-stimulated release of [(3)H]DA from striatal slices, the properties of the nAChRs on DA terminals were also found to change with age. Functional nAChRs were detected on striatal terminals at G18. There was a decrease in maximal release in the first postnatal week, followed by an increase in nicotine efficacy and potency during the second and third postnatal weeks. In the transition from adolescence (postnatal days (P) 30 and 40) to adulthood, there was a complex pattern of functional maturation of nAChRs in ventral, but not dorsal, striatum. In males, but not females, there were significant changes in both nicotine potency and efficacy during this developmental period. These findings suggest that nAChRs may play critical functional roles throughout DA neuronal maturation.

Mediation of glutamatergic receptors and nitric oxide on striatal dopamine release evoked by anatoxin-a. An in vivo microdialysis study

In this work, the involvement of ionotropic glutamatergic receptors and nitric oxide on striatal dopamine release induced by anatoxin-a was investigated in conscious and freely-moving rats. To study the participation of glutamatergic receptors, the effects of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate receptors antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and N-methyl-D-aspartate (NMDA) receptor antagonists, dizocilpine (MK-801) and d(-)-2-amino-5-phosphonopentanoic acid (APV), were examined. The perfusion of 3.5 mM anatoxin-a increased the extracellular dopamine levels to 701% relative to the basal. When CNQX was administered with 3.5 mM anatoxin-a, the increase of dopamine levels was 29% smaller than that observed with anatoxin-a alone. When MK-801 and APV were administered, the effect of anatoxin-a was attenuated 26% and 25% respectively in terms of that observed with anatoxin-a alone. And with CNQX plus MK-801, the effect of anatoxin-a was 53% inhibited in terms of the effect of anatoxin-a alone. These results suggest that the striatal dopamine release induced by anatoxin-a is partly mediated by activation of both ionotropic glutamatergic receptors. Since the neuronal form of nitric oxide synthase (nNOS) produces nitric oxide (NO) primarily in response to activation of NMDA receptors, it was tested if NO could play any role in the effect of anatoxin-a. Treatment with NOS inhibitors, L-nitro-arginine methyl ester (L-NAME) and d(-)-2-amino-5-phosphonopentanoic acid (7-NI), induced decreased anatoxin-a effects of 22% and 26% respectively. In conclusion, the present in vivo results demonstrate that anatoxin-a induced an indirect activation of ionotropic glutamatergic receptors (NMDA and AMPA/kainite receptors), which stimulate striatal dopamine release. On the other hand, activation of NMDA receptors may elicit NO increased levels enhancing dopamine release.

In vivo neurochemical characterization of Anatoxin-a evoked dopamine release from striatum

Anatoxin-a (AnTx) is a natural neurotoxin, which acts as a potent and stereoselective agonist at the nicotinic acetylcholine receptors. The in vivo actions of the AnTx on dopamine (DA) release are scarcely characterized. The aim of this study was to determine the neurochemical bases for AnTx-induced striatal DA release, using the brain microdialysis technique, in freely moving rats. Local application of AnTx (3.5 mM) through the microdialysis probe produced an increase in striatal DA levels (701 +/- 51% with respect to basal values). The effect of infusion of AnTx in Ca(2+)-free Ringer medium, in Na(+)-free Ringer medium and with TTX in the medium, was inhibited. Also, reserpine pre-treatment blocked the action of AnTx on striatal DA levels. To investigate the involvement of the DA transporter, the effects of AnTx were observed in the presence of nomifensine. The coadministration of AnTx and nomifensine evoked an additive effect on striatal DA levels. The latter results show that the DA release is not mediated by a decreased DA uptake. Taken as a whole, these results suggest that the effects of AnTx are predominantly mediated by an exocytotic mechanism, Ca(2+)-, Na(+)- and TTX-dependent, and not by a mechanism mediated by the DA transporter.

In vivo Effects of the Anatoxin-a on Striatal Dopamine Release

Anatoxin-a is an important neurotoxin that acts a potent nicotinic acetylcholine receptor agonist. This characteristic makes anatoxin-a an important tool for the study of nicotinic receptors. Anatoxin-a has been used extensively in vitro experiments, however anatoxin-a has never been studied by in vivo microdialysis studies. This study test the effect of anatoxin-a on striatal in vivo dopamine release by microdialysis.The results of this work show that anatoxin-a evoked dopamine release in a concentration-dependent way. Atropine had not any effect on dopamine release evoked by 3.5 mM anatoxin-a. However, perfusion of nicotinic antagonists mecamylamine and alpha-bungarotoxin induced a total inhibition of the striatal dopamine release. Perfusion of alpha7*-receptors antagonists, metillycaconitine or alpha-bungarotoxin, partially inhibits the release of dopamine stimulated by anatoxin-a. These results show that anatoxin-a can be used as an important nicotinic agonist in the study of nicotinic receptor by in vivo microdialysis technique and also support further in vivo evidences that alpha7*nicotinic AChRs are implicated in the regulation of striatal dopamine release.

Presynaptic α7 and non-α7 nicotinic acetylcholine receptors modulate [3H]d-aspartate release from rat frontal cortex in vitro

The presynaptic nicotinic modulation of glutamatergic transmission in the CNS has been associated with activation of the alpha7 subtype of nicotinic acetylcholine receptor (nAChR) in sub-cortical regions, whereas in the frontal cortex, non-alpha7 nAChRs have been implicated. The aim of this investigation was to directly characterise nAChR-evoked release of excitatory amino acids from rat frontal cortex, by monitoring the release of [3H]D-aspartate from superfused synaptosomes or minces. Co-administration of a nAChR agonist with a depolarising stimulus enhanced [3H]D-aspartate release above the effect of depolarising agent alone. This enhancement was blocked by the nicotinic antagonist mecamylamine. Other experiments revealed that in the absence of a depolarising stimulus, the nAChR agonists nicotine, epibatidine and anatoxin-a could evoke the release of [3H]D-aspartate in a Ca2+- and concentration-dependant manner. Differential sensitivity to the alpha7- and beta2*-selective nAChR antagonists alpha-bungarotoxin (alpha-Bgt) and dihydro-beta-erythroidine (DHbetaE) implicated two nAChR subtypes (alpha7 and beta2*), and this was supported by using the subtype-selective agonists choline (10 mM; alpha7 selective, blocked by alpha-Bgt but not by DHbetaE) and 5-Iodo-A-85380 (10 nM; beta2*-selective, blocked by DHbetaE but not by alpha-Bgt). Immunocytochemistry showed that alpha-Bgt labelling was associated with structures immunopositive for vesicular glutamate transporters, in both frontal cortex sections and synaptosome preparations, supporting the presence of alpha7 nAChR on glutamatergic terminals in rat frontal cortex.

Nicotinic acetylcholine receptors: from structure to brain function

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels and can be divided into two groups: muscle receptors, which are found at the skeletal neuromuscular junction where they mediate neuromuscular transmission, and neuronal receptors, which are found throughout the peripheral and central nervous system where they are involved in fast synaptic transmission. nAChRs are pentameric structures that are made up of combinations of individual subunits. Twelve neuronal nAChR subunits have been described, alpha2-alpha10 and beta2-beta4; these are differentially expressed throughout the nervous system and combine to form nAChRs with a wide range of physiological and pharmacological profiles. The nAChR has been proposed as a model of an allosteric protein in which effects arising from the binding of a ligand to a site on the protein can lead to changes in another part of the molecule. A great deal is known about the structure of the pentameric receptor. The extracellular domain contains binding sites for numerous ligands, which alter receptor behavior through allosteric mechanisms. Functional studies have revealed that nAChRs contribute to the control of resting membrane potential, modulation of synaptic transmission and mediation of fast excitatory transmission. To date, ten genes have been identified in the human genome coding for the nAChRs. nAChRs have been demonstrated to be involved in cognitive processes such as learning and memory and control of movement in normal subjects. Recent data from knockout animals has extended the understanding of nAChR function. Dysfunction of nAChR has been linked to a number of human diseases such as schizophrenia, Alzheimer's and Parkinson's diseases. nAChRs also play a significant role in nicotine addiction, which is a major public health concern. A genetically transmissible epilepsy, ADNFLE, has been associated with specific mutations in the gene coding for the alpha4 or beta2 subunits, which leads to altered receptor properties.

Nicotine and nicotinic receptors; relevance to Parkinson's disease

The development of nicotinic agonists for therapy in neurodegenerative disorders such as Parkinson's disease is an area currently receiving considerable attention. The rationale for such work stems from findings that reveal a loss of nicotinic receptors in Parkinson's disease brains. These results, coupled with reports that nicotine treatment relieves some of the symptoms of this disorder, provides support for the contention that nicotine and/or nicotinic agonists may be beneficial for acute symptomatic treatment. Moreover, the observation that there is a decreased incidence of Parkinson's disease with tobacco use, possibly due to the nicotine in tobacco products, may imply that such drugs are useful for long-term neuroprotection. However, there are multiple nicotinic receptor populations in the brain with different functional properties. Identification of the subtypes involved in nigrostriatal dopaminergic activity is therefore critical for the rational use of selective therapeutic agents for symptomatic treatment and/or neuroprotection. Accumulating evidence, both in rodents and nonhuman primates now indicate that alpha6* nicotinic receptors are present on nigrostriatal dopaminergic neurons, and furthermore, that receptors containing this subunit may be most vulnerable to nigrostriatal damage, at least in nonhuman primates. These data suggest that nicotinic receptor ligands directed to alpha6* nicotinic receptors might be particularly relevant for Parkinson's disease therapeutics.

Presynaptic localisation of the nicotinic acetylcholine receptor beta2 subunit immunoreactivity in rat nigrostriatal dopaminergic neurones

Nicotinic acetylcholine receptors (nAChR) are widely distributed in the central nervous system, where they exert a modulatory influence on synaptic transmission. For the striatum, pharmacological evidence supports the presence of presynaptic alpha3beta2* and alpha4beta2* nAChR that modulate dopamine release from nigrostriatal terminals. The objective of this study was to examine the precise subcellular distribution of the nAChR beta2 subunit in these neurones and its localisation at presynaptic sites. Double immunolabelling with tyrosine hydroxylase (TH) at the confocal level revealed that the cell bodies and axon terminals (synaptosomes) of nigrostriatal neurones were also immunoreactive for the nAChR beta2 subunit. Double-preembedding electron microscopy confirmed that beta2-immunogold labelling was enriched in TH-positive terminals in the dorsal striatum. Quantitative analysis of doubly immunogold-labelled sections in postembedding electron microscopy showed that 86% of TH-positive axonal boutons are also labelled for the nAChR beta2 subunit, whereas 45% of beta2 subunit-immunolabeled boutons do not contain TH. Thus the beta2 subunit is localised within at least two populations of axon terminals in the dorsal striatum. In these structures, 15% of beta2 subunit immunoreactivity was at the plasma membrane but was rarely associated with synapses. These findings are compatible with functional presynaptic beta2-containing nAChR that may be stimulated physiologically by acetylcholine that diffuses from synaptic or nonsynaptic sites of acetylcholine release. These results demonstrate the presynaptic localisation of an nAChR subunit in nigrostriatal dopaminergic neurones, providing morphological evidence for the presynaptic nicotinic modulation of dopamine release.

Neurotransmitter release mechanisms in sympathetic neurons: past, present, and future perspectives

In 1969, Paton and Vizi described the inhibitory actions of noradrenaline on acetylcholine release from the innervation of the guinea-pig ileum longitudinal muscle. They concluded "that acetylcholine output by the nervous networks of the longitudinal strip is under the normal control of the sympathetic by a species of presynaptic inhibition mediated by <==> receptors". This work was carried out in the Pharmacology Department at Oxford University. Clearly, a period in the 'Dreaming Spires' of Oxford sufficiently inspired Sylvester to take up a life long career in scientific research. He has published more than 300 papers on a wide range of topics but clearly has a strong interest in neurotransmitter release mechanisms and recently, non-synaptic interactions between neurons. It seems fitting therefore to write a brief review on the continuing studies on neurotransmitter release mechanisms in sympathetic neurons in a volume honoring the now distinguished Professor Vizi.

Nicotine-evoked transmitter release from synaptosomes: functional association of specific presynaptic acetylcholine receptors and voltage-gated calcium channels

It has previously been shown that nicotine-evoked dopamine release from rat striatal synaptosomes and nicotine-evoked norepinephrine release from hippocampal synaptosomes are mediated by distinct nicotinic acetylcholine receptor (nAChR) subtypes. In the present study, the functional association of these nicotinic receptors with specific subtypes of voltage-gated calcium channels was examined. Cd(2+) (200 microM), as well as omega-conotoxin MVIIC (5 microM), blocks approximately 85% of nicotine-evoked dopamine release from striatal synaptosomes, indicating a major involvement of calcium channels. Furthermore, the toxin-susceptibility suggests that these calcium channels contain alpha(1A) and/or alpha(1B) subunits. Inhibition of nicotine-evoked dopamine release by conotoxins alpha-MII and omega-GVIA is additive and indicates that presynaptic alpha3beta2 nAChRs are functionally coupled to alpha(1A), but not alpha(1B), calcium channel subtypes. Conversely, insensitivity to alpha-AuIB and sensitivity to omega-MVIIC indicate that non-alpha3beta2/alpha3beta4-containing nAChRs are functionally coupled to alpha(1B)-containing calcium channels. In contrast, Cd(2+) blocks only 65% of nicotine-evoked norepinephrine release from hippocampal synaptosomes, indicating that a substantial fraction of this release occurs through mechanisms not involving calcium channels. This Cd(2+)-insensitive component of release is blocked by alpha-AuIB and therefore appears to be triggered by Ca(2+) flowing directly through the channels of presynaptic alpha3beta4 nAChRs. Thus, these data indicate that different presynaptic termini can have distinctive functional associations of specific nAChRs and voltage-gated calcium channels.

Involvement of protein kinase C in the presynaptic nicotinic modulation of [(3)H]-dopamine release from rat striatal synaptosomes

1. Presynaptic nicotinic ACh receptors modulate transmitter release in the brain. Here we report their interactions with protein kinase C (PKC) with respect to [(3)H]-dopamine release from rat striatal synaptosomes, monitored by superfusion. 2. Two specific PKC inhibitors, Ro 31-8220 (1 microM) and D-erythro-sphingosine (10 microM) significantly reduced (by 51 and 26% respectively) [(3)H]-dopamine release stimulated by anatoxin-a (AnTx), a potent and selective agonist of nicotinic ACh receptors. The inactive structural analogue of Ro 31-8220, bisindolylmaleimide V (1 microM) had no effect. 3. Two phorbol esters, PDBu (1 microM) and PMA (1 microM) potentiated AnTx-evoked [(3)H]-dopamine release by 50 - 80%. This was Ca(2+)-dependent and prevented by PKC inhibitors. In the absence of nicotinic agonist, phorbol esters enhanced basal release through a PKC-independent mechanism. 4. A (86)Rb(+) efflux assay of nicotinic ACh receptor function confirmed that Ro 31-8220 has no nonspecific effect on presynaptic nicotinic ACh receptors. 5. These results suggest that PKC is activated by nicotinic ACh receptor stimulation and mediates a component of AnTx-evoked [(3)H]-dopamine release. In addition, independent activation of PKC can further amplify the response, offering a potential mechanism for receptor crosstalk.

alpha-bungarotoxin-sensitive nicotinic receptors indirectly modulate [(3)H]dopamine release in rat striatal slices via glutamate release

Nicotinic agonists elicit the release of dopamine from striatal synaptosomes by acting on presynaptic nicotinic acetylcholine receptors (nAChRs) on dopamine nerve terminals. Both alpha3beta2* and alpha4beta2 nAChR subtypes (but not alpha7* nAChRs) have been implicated. Here, we compared nAChR-evoked [(3)H]dopamine release from rat striatal synaptosome and slice preparations by using the nicotinic agonist anatoxin-a. In the more integral slice preparation, the concentration-response curve for anatoxin-a-evoked [(3)H]dopamine release was best fitted to a two-site model, giving EC(50) values of 241 nM and 5.1 microM, whereas only the higher-affinity component was observed in synaptosome preparations (EC(50) = 134 nM). Responses to a high concentration of anatoxin-a (25 microM) in slices (but not in synaptosomes) were partially blocked by ionotropic glutamate receptor antagonists (kynurenic acid, 6,7-dinitroquinoxaline-2,3-dione) and by alpha7*-selective nAChR antagonists (alpha-bungarotoxin, alpha-conotoxin-ImI, methyllycaconitine) in a nonadditive manner. In contrast, the alpha3beta2-selective nAChR antagonist alpha-conotoxin-MII partially inhibited [(3)H]dopamine release from both slice and synaptosome preparations, stimulated with both low (1 microM) and high (25 microM) concentrations of anatoxin-a. Antagonism by alpha-conotoxin-MII was additive with that of alpha7*-selective antagonists. These data support a model in which alpha7* nAChRs on striatal glutamate terminals elicit glutamate release, which in turn acts at ionotropic glutamate receptors on dopamine terminals to stimulate dopamine release. In addition, non-alpha7* nAChRs on dopamine terminals also stimulate dopamine release. These observations have implications for the complex cholinergic modulation of inputs onto the major efferent neurons of the striatum.

Nicotinic modulation of [(3)H]D-aspartate outflow from cultured cerebellar granule cells

The effect of nicotine on basal and electrically evoked (20 Hz for 20 sec) [(3)H]D-aspartate efflux (assumed as an index of transmitter release) was studied in rat cerebellar granule primary cultures. Nicotine (10-100 nM) increased the basal efflux two to three times and concentration-dependently enhanced the electrically evoked efflux up to ten times. Higher drug concentration (1 microM) underwent rapid desensitization. Facilitation of the efflux was similarly reduced by the nicotinic acetylcholine receptor antagonists, alpha-bungarotoxin and mecamylamine, suggesting the involvement of at least two receptor subtypes containing and lacking alpha(7) subunits, respectively. Since the increased efflux induced by nicotine in granule cells kept at rest or depolarized by KCl 15 mM was antagonized by tetrodotoxin, the involvement of sodium channels by receptors located at preterminal sites was suggested. Taken together, these findings emphasize the role of the cholinergic input in granule cell function and in glutamatergic signaling.

UB-165: a novel nicotinic agonist with subtype selectivity implicates the alpha4beta2* subtype in the modulation of dopamine release from rat striatal synaptosomes

Presynaptic nicotinic acetylcholine receptors (nAChRs) on striatal synaptosomes stimulate dopamine release. Partial inhibition by the alpha3beta2-selective alpha-conotoxin-MII indicates heterogeneity of presynaptic nAChRs on dopamine terminals. We have used this alpha-conotoxin and UB-165, a novel hybrid of epibatidine and anatoxin-a, to address the hypothesis that the alpha-conotoxin-MII-insensitive subtype is composed of alpha4 and beta2 subunits. UB-165 shows intermediate potency, compared with the parent molecules, at alpha4beta2* and alpha3-containing binding sites, and resembles epibatidine in its high discrimination of these sites over alpha7-type and muscle binding sites. (+/-)-Epibatidine, (+/-)-anatoxin-a, and (+/-)-UB-165 stimulated [(3)H]-dopamine release from striatal synaptosomes with EC(50) values of 2.4, 134, and 88 nM, and relative efficacies of 1:0.4:0.2, respectively. alpha-Conotoxin-MII inhibited release evoked by these agonists by 48, 56, and 88%, respectively, suggesting that (+/-)-UB-165 is a very poor agonist at the alpha-conotoxin-MII-insensitive nAChR subtype. In assays of (86)Rb(+) efflux from thalamic synaptosomes, a model of an alpha4beta2* nAChR response, (+/-)-UB-165 was a very weak partial agonist; the low efficacy of (+/-)-UB-165 at alpha4beta2 nAChR was confirmed in Xenopus oocytes expressing various combinations of human nAChR subunits. In contrast, (+/-)-UB-165 and (+/-)-anatoxin-a were similarly efficacious and similarly sensitive to alpha-conotoxin-MII in increasing intracellular Ca(2+) in SH-SY5Y cells, a functional assay for native alpha3-containing nAChR. These data support the involvement of alpha4beta2* nAChR in the presynaptic modulation of striatal dopamine release and illustrate the utility of exploiting a novel partial agonist, together with a selective antagonist, to dissect the functional roles of nAChR subtypes in the brain.

Presynaptic nicotinic receptors modulating dopamine release in the rat striatum

The modulation of striatal dopamine release by presynaptic nicotinic acetylcholine receptors is well documented for both synaptosomes and slices. Because the latter retain local anatomical integrity, we have compared [3H]dopamine release evoked by the nicotinic receptor agonists (-)-nicotine and (+/-)-anatoxin-a from striatal synaptosome and slice preparations in parallel. At higher agonist concentrations, mecamylamine-sensitive [3H]dopamine release was greater from slices, indicative of an additional component, and this increase was abolished by glutamate receptor antagonists. To begin to examine the localisation of specific nicotinic acetylcholine receptor subtypes in the striatum, immunogold electron microscopy was undertaken with the beta2-specific monoclonal antibody 270. In striatal sections, gold particles were associated with symmetric synapses (dopaminergic) but were absent from asymmetric synapses (glutamatergic). Surface labelling of striatal synaptosomes with gold particles was also demonstrated. Taken together, these results are consistent with dopamine release mediated by beta2-containing nicotinic acetylcholine receptors on dopamine terminals, while non-beta2-containing nicotinic acetylcholine receptors may enhance dopamine release indirectly by releasing glutamate from neighbouring terminals.

Nicotine, brain nicotinic receptors, and neuropsychiatric disorders

Neuronal nicotinic acetylcholine receptors (nAChRs) represent a large family of ligand-gated cation channels with diverse structures and properties. In contrast to the muscular nAChRs, the physiological functions of neuronal nAChRs are not well defined to date. Behavioral studies indicate that brain nAChRs participate in complex functions such as attention, memory, and cognition, whereas clinical data suggest their involvement in the pathogenesis of certain neuropsychiatric disorders (Alzheimer's and Parkinson's diseases, Tourette's syndrome, schizophrenia, depression, etc.). For the majority of these disorders, the use of nAChRs' agonists may represent either a prophylactic (especially for Alzheimer's and Parkinson's diseases) or a symptomatic treatment. The possible mechanisms underlying these beneficial effects as well as the characteristics and potential therapeutic use of new, subtype-selective nAChRs agonists are presented.

Ion channels in presynaptic nerve terminals and control of transmitter release

The primary function of the presynaptic nerve terminal is to release transmitter quanta and thus activate the postsynaptic target cell. In almost every step leading to the release of transmitter quanta, there is a substantial involvement of ion channels. In this review, the multitude of ion channels in the presynaptic terminal are surveyed. There are at least 12 different major categories of ion channels representing several tens of different ion channel types; the number of different ion channel molecules at presynaptic nerve terminals is many hundreds. We describe the different ion channel molecules at the surface membrane and inside the nerve terminal in the context of their possible role in the process of transmitter release. Frequently, a number of different ion channel molecules, with the same basic function, are present at the same nerve terminal. This is especially evident in the cases of calcium channels and potassium channels. This abundance of ion channels allows for a physiological and pharmacological fine tuning of the process of transmitter release and thus of synaptic transmission.

Lobeline inhibits Ca2+ current in cultured neurones from rat sympathetic ganglia

The effect of lobeline was studied on the voltage-activated Ca2+ current in sympathetic neurones from the rat superior cervical ganglia using the whole-cell variant of the patch-clamp technique. Lobeline (10-300 microM) inhibited the Ca2+ current evoked by voltage steps from -80 mV (holding potential) to 0 mV (test potential) in a dose dependent manner. The inhibitory effects of noradrenaline (10 microM) and lobeline (100 microM) were compared using a prepulse protocol with high (+80 mV) depolarization. Within the same cell depolarizing prepulses decreased the inhibitory effect of noradrenaline but did not change the extent of lobeline inhibition. Addition of GTPgammaS (300 microM) to the pipette solution did not prevent the inhibitory effect of lobeline (100 microM) but greatly reduced that of noradrenaline (100 microM). Our experiments suggest, that the weak nicotinic agonist lobeline exerts a direct blocking effect on Ca2+ channels at concentrations commonly used to release transmitters.

Postsynaptic nicotinic receptors on dopaminergic neurons in the substantia nigra pars compacta of the rat

Previous studies have shown that application of nicotinic agonists in the substantia nigra pars compacta increases the firing rate of dopaminergic neurons. We have used intracellular recordings to show that the response of these neurons to nicotine is postsynaptic, since it persists in the presence of low-calcium buffer containing tetrodotoxin. Burst firing in the presence of nicotine was not observed. The presence of postsynaptic nicotinic receptors was confirmed by immunohistochemical localization of the alpha4 nicotinic receptor subunit on dendrites in the substantia nigra pars compacta. The majority of tyrosine hydroxylase-immunopositive neurons in the substantia nigra pars compacta were also immunopositive for the alpha4 subunit. Immunohistochemical localization of the alpha4 and beta2 subunits in adjacent brain sections produced similar patterns of staining. Electron micrographs clearly indicated the presence of alpha4 subunit at postsynaptic densities. The predominant role of nicotinic receptors in the central nervous system has been suggested to be the presynaptic modulation of neurotransmitter release [McGehee D. S. and Role L. W. (1995) A. Rev. Physiol. 57, 521-546]. Although several postsynaptic nicotinic responses have also been reported in the literature, it is unclear as to whether the postsynaptic nicotinic receptors mediating responses to exogenously applied agonists are involved in synaptic transmission. From our electrophysiological and immunohistochemical results, we conclude that alpha4-containing nicotinic receptors are found at synapses on dopaminergic neurons. These synapses are similar to the cholinergic synapses described at these neurons, suggesting that nicotinic receptors are important in modulating the excitability of dopaminergic neurons by direct synaptic transmission.

Human neuronal nicotinic receptors

Nicotine is a very widely used drug of abuse, which exerts a number of neurovegetative, behavioural and psychological effects by interacting with neuronal nicotinic acetylcholine receptors (NAChRs). These receptors are distributed widely in human brain and ganglia, and form a family of ACh-gated ion channels of different subtypes, each of which has a specific pharmacology and physiology. As human NAChRs have been implicated in a number of human central nervous system disorders (including the neurodegenerative Alzheimer's disease, schizophrenia and epilepsy), they are suitable potential targets for rational drug therapy. Much of our current knowledge about the structure and function of NAChRs comes from studies carried out in other species, such as rodents and chicks, and information concerning human nicotinic receptors is still incomplete and scattered in the literature. Nevertheless, it is already evident that there are a number of differences in the anatomical distribution, physiology, pharmacology, and expression regulation of certain subtypes between the nicotinic systems of humans and other species. This review will attempt to survey the major achievements reached in the study of the structure and function of NAChRs by examining the molecular basis of their functional diversity viewed mainly from pharmacological and biochemical perspectives. It will also summarize our current knowledge concerning the structure and function of the NAChRs expressed by other species, and the newly discovered drugs used to classify their numerous subtypes. Finally, the role of NAChRs in behaviour and pathology will be considered.

Presynaptic nicotinic ACh receptors

Nicotinic ACh (nACh) receptors in the CNS are composed of a diverse array of subunits and have a range of pharmacological properties. However, despite the fact that they are ligand-gated cation channels, their physiological functions have not been determined. This has led to increased interest in presynaptic nACh receptors that act to modulate the release of transmitter from presynaptic terminals.

Pharmacological characterization of a nicotinic autoreceptor in rat hippocampal synaptosomes

The modulation of [3H]ACh release by nicotinic compounds was studied in superfused rat hippocampal synaptosomes loaded with [3H]choline, (-)-Nicotine (0.1-10 microM) evoked a dose-dependent increase in [3H]ACh release; higher concentrations were less effective. Nicotine-evoked release was Ca(2+)-dependent, and blocked by the nicotinic antagonists dihydro-beta-erythroidine, mecamylamine, and pempidine. The alpha 7-selective antagonist methyllycaconitine did not inhibit nicotine-evoked release when tested at 1 microM, although at 10 microM some attenuation of the response was observed. Six agonists tested were equally efficacious in stimulating [3H]ACh release, as judged by the maximum responses, and gave the following EC50 values: (+/-)-epibatidine 0.12 microM; (+)-anatoxin-a 0.14 microM; (-)-nicotine 0.99 microM; (-)-cytisine 1.06 microM; ABT-418 2.6 microM; isoarecolone 43 microM. Each agonist generated a "bell-shaped" dose response curve, suggesting desensitisation at higher concentrations. This is supported by analysis of repetitive stimulation with (-)-nicotine and (-)-cytisine: S2/S1 ratios declined sharply with increasing concentration, whereas subsequent KC1-evoked release remained constant. These results are discussed in terms of possible nicotinic receptor subtypes that might be present on hippocampal nerve terminals.

Presynaptic ionotropic receptors

Recent studies have provided new insights into the role of presynaptic ligand-gated ion channels in modifying synaptic transmission. Along with a growing list of different types of presynaptic ionotropic receptors and the cell types that express them, there have been advances in characterizing the molecular components of the receptors as well as the signaling processes that link receptor activation to changes in neurotransmitter release. Perhaps most striking is the recent convergence of data from biochemical, molecular and electrophysiological studies, implicating presynaptic ionotropic receptors in the effects of psychoactive and addictive drugs.

Release of [3H]-noradrenaline from rat hippocampal synaptosomes by nicotine: mediation by different nicotinic receptor subtypes from striatal [3H]-dopamine release

1. The aim of the present experiment was to characterize nicotine-evoked [3H]-noradrenaline ([3H]-NA) release from rat superfused hippocampal synaptosomes, using striatal [3H]-dopamine release for comparison. 2. (-)-Nicotine, cytisine, DMPP and acetylcholine (ACh) (with esterase inhibitor and muscarinic receptor blocker) increased NA release in a concentration-dependent manner (EC50 6.5 microM, 8.2 microM, 9.3 microM, and 27 microM, respectively) with similar efficacy. 3. Nicotine released striatal dopamine more potently than hippocampal NA (EC50 0.16 microM vs. 6.5 microM). (+)-Anatoxin-a also increased dopamine more potently than NA (EC50 0.05 microM vs. 0.39 microM), and maximal effects were similar to those of nicotine. Isoarecolone (10-320 microM) released dopamine more effectively than NA but a maximal effect was not reached. (-)-Lobeline (10-320 microM) evoked dopamine release, but the effect was large and delayed with respect to nicotine; NA release was not increased but rather depressed at high concentrations of lobeline. High K+ (10 mM) released and NA to similar extents. 4. Addition of the 5-hydroxytryptamine (5-HT) reuptake blocker, citalopram (1 microM) to hippocampal synaptosomes affected neither basal NA release nor nicotine-evoked release. 5. The nicotinic antagonist, mecamylamine (10 microM), virtually abolished NA and dopamine release evoked by high concentrations of nicotine, ACh, cytisine, isoarecolone, and anatoxin-a. Although NA release evoked by DMPP (100 microM) was entirely mecamylamine-sensitive, DMPP-evoked dopamine release was only partially blocked. Dopamine release evoked by lobeline (320 microM) was completely mecamylamine-insensitive. 6. The nicotinic antagonists dihydro-beta-erythroidine and methyllycaconitine inhibited nicotine-evoked dopamine release approximately 30 fold more potently than NA release. In contrast, the antagonist chlorisondamine, displayed a reverse sensitivity, whereas trimetaphan and mecamylamine did not preferentially block either response. None of these antagonists, given at a high concentration, significantly altered release evoked by high K+. 7. Blockade of nicotine-evoked transmitter release by methyllycaconitine and dihydro-beta-erythroidine was surmounted by a high concentration of nicotine (100 microM), but blockade by mecamylamine, chlorisondamine, and trimetaphan was insurmountable. 8. Nicotine-evoked NA release was unaffected by tetrodotoxin, whereas veratridine-evoked NA release was virtually abolished. 9. We conclude that presynaptic nicotinic receptors associated with striatal dopamine and hippocampal NA terminals differ pharmacologically. In situ hybridization studies suggest that nigrostriatal dopaminergic neurones express mainly alpha 4, alpha 5, and beta 2 nicotinic cholinoceptor subunits, whereas hippocampal-projecting noradrenaline (NA) neurones express alpha 3, beta 2 and beta 4 subunits. Pharmacological comparisons of recombinant receptors suggest that release of hippocampal NA may be modulated by receptors containing alpha 3 and beta 4 subunits.

Tetrodotoxin-sensitivity of nicotine-evoked dopamine release from rat striatum

Recent observations from synaptosome preparations have questioned the tetrodotoxin (TTX) insensitivity of nicotine-evoked release in the striatum, a characteristic previously considered diagnostic of presynaptically located nicotinic acetylcholine receptors (nAChRs). Therefore, we have undertaken a comparison of nicotine-evoked dopamine release in the presence of TTX from the rat striatum in vitro, using synaptosomes and brain slices, and in vivo, using microdialysis. In P2 and Percoll-purified synaptosome preparations, 1.5 microM TTX partially inhibited nicotine-evoked [3H]dopamine release by 54% and 37%, respectively, whereas in more intact preparations (brain slices and microdialysis) TTX completely inhibited mecamylamine-sensitive nicotine-stimulated dopamine release. These results suggest that caution should be exercised in the interpretation of TTX sensitivity of nicotine-evoked responses with regard to the location of nAChRs.

Nicotine increases intracellular calcium in rat hippocampal neurons via voltage-gated calcium channels

The effect of nicotinic receptor activation on intracellular calcium concentrations ([Ca2+]i) was quantitated in populations of cultured hippocampal neurons loaded with Fura-2. Nicotine (50 microM) and cytisine (50 microM) increased [Ca2+]i by 100%. This response was abolished in the presence of the nicotinic antagonist methyllycaconitine (MLA) whereas KCl-evoked increases in [Ca2+]i were insensitive to MLA. Glial cultures were unaffected by nicotine, although they did respond to glutamate with increased [Ca2+]i. In hippocampal neurons, responses to nicotinic agonists and KCl were dependent on the presence of extracellular Ca2+ and were similarly sensitive (85% inhibition) to CdCl2. These results are consistent with the presence of functional nicotinic receptors on hippocampal neurons. The receptors appear to elevate [Ca2+]i by promoting the influx of extracellular Ca2+ through voltage-gated calcium channels.