aconitine and cytisine

The extent to which non-α4β2 versus α4β2* nAChRs contribute to the behavioral effects of varenicline and other nAChR agonists is unclear.. The purpose of this study was to characterize the discriminative stimulus effects of varenicline and nicotine using various nAChR agonists and antagonists to elucidate possible non-α4β2 nAChR mechanisms.. Separate groups of male C57BL/6J mice were trained to discriminate varenicline (3.2 mg/kg) or nicotine (1 mg/kg). Test drugs included mecamylamine; the nAChR agonists epibatidine, nicotine, cytisine, varenicline, and RTI-102; the β2-containing nAChR antagonist dihydro-β-erythroidine (DHβE); the α7 nAChR agonist PNU-282987; the α7 antagonist methyllycaconitine (MLA); the α3β4 antagonist 18-methoxycoronaridine (18-MC); and the non-nAChR drugs midazolam and cocaine.. In nicotine-trained mice, maximum nicotine-appropriate responding was 95% nicotine, 94% epibatidine, 63% varenicline, 58% cytisine, and less than 50% for RTI-102, PNU-282987, midazolam, and cocaine. In varenicline-trained mice, maximum varenicline-appropriate responding was 90% varenicline, 86% epibatidine, 74% cytisine, 80% RTI-102, 50% cocaine, and 50% or less for nicotine, PNU-282987, and midazolam. Drugs were studied to doses that abolished operant responding. Mecamylamine antagonized the discriminative stimulus effects, but not the rate-decreasing effects, of nicotine and varenicline. DHβE antagonized the discriminative stimulus and rate-decreasing effects of nicotine but not varenicline in either the nicotine or varenicline discrimination assays. The discriminative stimulus, but not the rate-decreasing, effects of epibatidine were antagonized by DHβE regardless of the training drug.. These results suggest that α4β2* nAChRs differentially mediate the discriminative stimulus effects of nicotine and varenicline, and suggest that varenicline has substantial non-α4β2 nAChR activity.

Topics: Aconitine; Alkaloids; alpha7 Nicotinic Acetylcholine Receptor; Animals; Azocines; Benzamides; Bridged Bicyclo Compounds; Bridged Bicyclo Compounds, Heterocyclic; Cocaine; Dihydro-beta-Erythroidine; Discrimination, Psychological; Dopamine Uptake Inhibitors; Hypnotics and Sedatives; Ibogaine; Male; Mecamylamine; Mice; Mice, Inbred C57BL; Midazolam; Nerve Tissue Proteins; Nicotine; Nicotinic Agonists; Nicotinic Antagonists; Pyridines; Quinolizines; Receptors, Nicotinic; Varenicline

Neuronal nicotinic acetylcholine receptors (nAChRs) play a modulatory role in cognition, and zebrafish provide a preclinical model to study learning and memory.. We investigated the effect of nicotine (NIC) and some new cytisine-derived partial agonists (CC4 and CC26) on spatial memory in zebrafish using a rapid assay on T-maze task. The role of α4/α6β2 and the α7 nAChRs in NIC-induced memory enhancement was evaluated using selective nAChR antagonists.. Low and high doses of NIC, cytisine (CYT), CC4 and CC26 respectively improved and worsened the mean running time, showing an inverted U dose-response function. The effective dose (ED50) (×10⁻⁵ mg/kg) was 0.4 for CC4, 4.5 for CYT, 140 for NIC and 200 for CC26. NIC-induced cognitive enhancement was reduced by the selective nAChR subtype antagonists: methyllycaconitine (MLA) for α7, α-conotoxin (MII) for α6β2, dihydro-β-erythroidine (DhβE) for α4β2, the nonselective antagonist mecamylamine (MEC) and the muscarinic antagonist scopolamine (SCOP), with DhβE being more active than MLA or MII. All the partial agonists blocked the cognitive enhancement. The improvement with the maximal active dose of each partial agonist was blocked by low doses of DhβE (0.001 mg/kg) and MII (0.01 mg/kg). MLA reduced the effects of CC26 and CC4 at doses of 0.01 and 1 mg/kg, respectively, but did not antagonize CYT-induced memory improvement at any of the tested dose. No change in swimming activity was observed.. Our findings demonstrate that zebrafish make a useful model for the rapid screening of the effect of new α4β2 nAChR compounds on spatial memory.

Topics: Aconitine; Alkaloids; Animals; Azocines; Cholinergic Agents; Conotoxins; Dihydro-beta-Erythroidine; Dose-Response Relationship, Drug; Maze Learning; Mecamylamine; Memory; Muscarinic Antagonists; Neurons; Nicotine; Nicotinic Agonists; Nicotinic Antagonists; Quinolizines; Receptors, Nicotinic; Scopolamine; Swimming; Zebrafish; Zebrafish Proteins

Nicotinic cholinergic receptors mediate autonomic transmission at ganglia. However, whether different subtypes of nicotinic cholinergic receptors expressed in autonomic ganglia elicit distinct roles in mediating sympathetic and parasympathetic regulations remain to be defined. In this study, we observed that different subtypes of nicotinic receptors were responsible for the sympathetic and parasympathetic cardiovascular responses. In urethane anesthetized mice, intravenous injection with cytisine, a non-selective nicotinic agonist, induced a brief but pronounced decrease in heart rate, followed by increases in heart rate and arterial blood pressure. The bradycardic response was blocked by atropine, and the pressor response was blocked by prazosin, confirming that these responses were parasympathetic and sympathetic activities, respectively. Hexamethonium, a ganglionic blocker, blocked both sympathetic and parasympathetic responses. Pretreatment with methyllycaconitine citrate, a selective alpha7 nicotinic receptor antagonist, significantly attenuated cytisine-induced sympathetic response with little effect on the parasympathetic response. In contrast, pretreatment with dihydro-beta-erythroidine hydrobromide, a selective alpha4beta2 nicotinic receptor antagonist, blocked cytisine-induced parasympathetic response but not the sympathetic response. Pretreatment with dihydro-beta-erythroidine hydrobromide also blocked baroreflex associated parasympathetic bradycardic response. Moreover, treatment with nicotine induced a bradycardic response without a significant pressor response, which was also attenuated by dihydro-beta-erythroidine hydrobromide. Collectively, these data suggest that different nicotinic receptors play distinct roles in sympathetic and parasympathetic ganglia. Specifically, activations of alpha7 and alpha4beta2 nicotinic receptors are involved in cytisine-induced cardiovascular sympathetic and parasympathetic responses, respectively.

Topics: Aconitine; Adrenergic alpha-Antagonists; Alkaloids; Analysis of Variance; Animals; Atropine; Azocines; Blood Pressure; Bradycardia; Cardiovascular Physiological Phenomena; Dihydro-beta-Erythroidine; Heart Rate; Hexamethonium; Male; Mice; Muscarinic Antagonists; Nicotinic Agonists; Nicotinic Antagonists; Prazosin; Quinolizines; Receptors, Nicotinic

Dopaminergic (DAergic) neuronal activity in the ventral tegmental area (VTA) is thought to contribute generally to pleasure, reward, and drug reinforcement and has been implicated in nicotine dependence. nAChRs expressed in the VTA exhibit diverse subunit compositions, but the functional and pharmacological properties are largely unknown. Here, using patch-clamp recordings in single DAergic neurons freshly dissociated from rat VTA, we clarified three functional subtypes of nAChRs (termed ID, IID and IIID receptors) based on whole-cell current kinetics and pharmacology. Kinetic analysis demonstrated that comparing to ID, IID receptor-mediated current had faster activation and decay constant and IIID receptor-mediated current had larger current density. Pharmacologically, ID receptor-mediated current was sensitive to the alpha4beta2-nAChR agonist RJR-2403 and antagonist dihydro-beta-erythroidine (DHbetaE); IID receptor-mediated current was sensitive to the selective alpha7-nAChR agonist choline and antagonist methyllycaconitine (MLA); while IIID receptor-mediated current was sensitive to the beta4-containing nAChR agonist cytisine and antagonist mecamylamine (MEC). The agonist concentration-response relationships demonstrated that IID receptor-mediated current exhibited the highest EC(50) value compared to ID and IIID receptors, suggesting a relatively low agonist affinity of type IID receptors. These results suggest that the type ID, IID and IIID nAChR-mediated currents are predominately mediated by activation of alpha4beta2-nAChR, alpha7-nAChR and a novel nAChR subtype(s), respectively. Collectively, these findings indicate that the VTA DAergic neurons express diversity and multiplicity of functional nAChR subtypes. Interestingly, each DAergic neuron predominantly expresses only one particularly functional nAChR subtype, which may have distinct but important roles in regulation of VTA DA neuronal function, DA transmission and nicotine dependence.

Topics: Acetylcholine; Aconitine; Action Potentials; Alkaloids; Animals; Azocines; Brain; Choline; Dihydro-beta-Erythroidine; Dopamine; Dose-Response Relationship, Drug; Gene Expression; Kinetics; Mecamylamine; Neurons; Nicotine; Nicotinic Agonists; Nicotinic Antagonists; Patch-Clamp Techniques; Protein Subunits; Quinolizines; Rats; Rats, Wistar; Receptors, Nicotinic; Reverse Transcriptase Polymerase Chain Reaction; Tyrosine 3-Monooxygenase; Ventral Tegmental Area

alpha7-Nicotinic acetylcholine receptors (alpha7-nAChR) have been implicated in a range of cognitive deficits in schizophrenia. Therefore we examined alpha7-nAChR knockout (KO), heterozygote (HT) and wildtype (WT) littermate mice in the 5-CSR (a rodent model of sustained attention) and odour span (a novel mouse working memory paradigm) tasks, and related performance to nAChR density. Whilst there was no difference between groups in baseline 5-CSR task performance, alpha7-nAChR KO's exhibited significantly higher omission levels compared to WT mice on increasing the attentional load, with HT mice performing at an intermediate level. Furthermore, alpha7-nAChR KO mice were significantly impaired in the odour span task when compared to WT mice, in a pattern consistent with impaired attention. These behavioural deficits were associated with the loss of alpha7-nAChRs, as alpha4beta2-nAChR density was unaltered in these mice. Thus these studies intimate that the attentional impairment in alpha7-nAChR transgenic mice maybe core to other deficits in cognition.

Topics: Aconitine; Alkaloids; alpha7 Nicotinic Acetylcholine Receptor; Animals; Animals, Newborn; Attention Deficit Disorder with Hyperactivity; Azocines; Behavior, Animal; Choice Behavior; Cognition Disorders; Dose-Response Relationship, Drug; Mice; Mice, Knockout; Nicotinic Antagonists; Protein Binding; Quinolizines; Reaction Time; Receptors, Nicotinic; Tritium

Selective and brain penetrating pharmacological antagonists for use in clarifying a role of alpha7 nicotinic acetylcholine receptors (nAChR) in behavioral paradigms are presently unavailable. Studies in alpha7 knock-out mice (KO) have not revealed convincing changes in behavioral phenotype, in particular measures of cognition that include contextual fear conditioning and spatial memory, which may be due to compensatory developmental changes. Therefore, an antisense oligonucleotide (aON) targeted toward the 3'- and 5'-UTR coding regions of the rat alpha7 nicotinic acetylcholine receptor was used. Following central injection of aON into the lateral ventricle of Long Evans rats for 6 days, treated rats exhibited a significant 42% and 25% decrease in alpha7 nAChR densities in hippocampus and cortex, respectively, as measured by [(3)H]-methyllycaconitine (MLA) binding. There was no change in alpha4beta2 densities measured by [(3)H]-cytisine binding. Acquisition of Morris Water Maze (MWM) performance, a measure of spatial memory, was impaired in aON-treated rats. In addition, a reduction in target platform crossings during a subsequent probe-trial was observed. These data demonstrate the ability of this aON to reduce hippocampal and cortical alpha7 nicotinic receptor densities associated with impaired MWM performance and support the specific involvement of the alpha7 nAChR in spatial learning and memory, a phenotype not affected in alpha7 KO mice.

Topics: Aconitine; Alkaloids; alpha7 Nicotinic Acetylcholine Receptor; Animals; Azocines; Behavior, Animal; Cerebral Cortex; Dose-Response Relationship, Drug; Drug Interactions; Hippocampus; Male; Maze Learning; Memory Disorders; Nicotinic Antagonists; Oligodeoxyribonucleotides, Antisense; Protein Binding; Quinolizines; Rats; Rats, Long-Evans; Receptors, Nicotinic; Space Perception; Time Factors; Tritium

An alpha7 nicotinic acetylcholine receptor sequence was cloned from Rhesus monkey (Macaca mulatta). This clone differs from the mature human alpha7 nicotinic acetylcholine receptor in only four amino acids, two of which are in the extracellular domain. The monkey alpha7 nicotinic receptor was characterized in regard to its functional responses to acetylcholine, choline, cytisine, and the experimental alpha7-selective agonists 4OH-GTS-21, TC-1698, and AR-R17779. For all of these agonists, the EC(50) for activation of monkey receptors was uniformly higher than for human receptors. In contrast, the potencies of mecamylamine and MLA for inhibiting monkey and human alpha7 were comparable. Acetylcholine and 4OH-GTS-21 were used to probe the significance of the single point differences in the extracellular domain. Mutants with the two different amino acids in the extracellular domain of the monkey receptor changed to the corresponding sequence of the human receptor had responses to these agonists that were not significantly different in EC(50) from wild-type human alpha7 nicotinic receptors. Monkey alpha7 nicotinic receptors have a serine at residue 171, while the human receptors have an asparagine at this site. Monkey S171N mutants were more like human alpha7 nicotinic receptors, while mutations at the other site (K186R) had relatively little effect. These experiments point toward the basic utility of the monkey receptor as a model for the human alpha7 nicotinic receptor, albeit with the caveat that these receptors will vary in their agonist concentration dependency. They also point to the potential importance of a newly identified sequence element for modeling the specific amino acids involved with receptor activation.

Topics: Acetylcholine; Aconitine; Algorithms; Alkaloids; alpha7 Nicotinic Acetylcholine Receptor; Amino Acid Sequence; Anabasine; Animals; Azocines; Bridged Bicyclo Compounds; Choline; Dose-Response Relationship, Drug; Female; Humans; Macaca mulatta; Mecamylamine; Membrane Potentials; Microinjections; Molecular Sequence Data; Mutation; Nicotinic Antagonists; Oocytes; Pyridines; Quinolizines; Receptors, Nicotinic; RNA, Complementary; Sequence Alignment; Sequence Homology, Amino Acid; Structural Homology, Protein; Xenopus laevis

Whole-cell patch clamp recordings were performed on neurones in the lamina X of rat spinal cord slices in order to characterize glycinergic synaptic currents and their modulation by nicotinic acetylcholine receptors. In the presence of TTX, bicuculline and kynurenic acid, glycine-induced currents and miniature glycinergic postsynaptic currents (mIPSCs) were recorded. These currents reversed near the chloride ion equilibrium potential and were blocked by strychnine (1 microM). A selective nicotinic acetylcholine receptor (nAChR) agonist 1,1-dimethyl-4-phenyl-piperazinium (DMPP), increased the frequency of glycinergic mIPSCs without altering significantly their amplitude distributions or their kinetic properties. The effects of DMPP were mimicked by different nAChRs agonists with the following apparent order of potency: ACh > DMPP > nicotine > cytisine. The effect of DMPP on mIPSCs was blocked by both d-tubocurarine and hexamethonium, and was reduced by dihydro-beta-erythroidine and methyllycaconitine (MLA), antagonists of non alpha7- and alpha7-containing nAChRs, respectively. In the absence of TTX, strychnine-sensitive glycinergic electrically evoked postsynaptic currents (eIPSCs) could be recorded. DMPP blocked the appearance of electrically evoked IPSCs while still inducing the appearance of spontaneous glycine IPSCs. These data demonstrate that neurones surrounding the central canal of the spinal cord present a glycinergic synaptic transmission which is modulated by terminal nAChRs.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Aconitine; Alkaloids; Anesthetics, Local; Animals; Animals, Newborn; Azocines; Bicuculline; Dihydro-beta-Erythroidine; Dimethylphenylpiperazinium Iodide; Drug Interactions; Excitatory Amino Acid Antagonists; Glycine; Glycine Agents; Hexamethonium; In Vitro Techniques; Kynurenic Acid; Membrane Potentials; Neurons; Nicotinic Agonists; Nicotinic Antagonists; Patch-Clamp Techniques; Quinolizines; Rats; Rats, Wistar; Receptors, Nicotinic; Spinal Cord; Strychnine; Tetrodotoxin; Time Factors; Tubocurarine

We have developed one-step syntheses of halogenated derivatives of (-)-cytisine featuring a halogen substituent at positions 3, 5 or 3 and 5 of the 2-pyridone fragment, and prepared the novel bioisosteric thiocytisine by oxygen-sulphur exchange. The affinities of these pyridone-modified analogs of (-)-cytisine for (alpha 4)(2)(beta 2)(3) and alpha 7* nAChRs in rat forebrain membranes were determined by competition with (+/-)-[(3)H]epibatidine and [(3)H]MLA, respectively. The 3-halocytisines 7 possess subnanomolar affinities for (alpha 4)(2)(beta 2)(3) nAChRs, higher than those found for (-)-cytisine as well as for the 5-halocytisines 8 and 3,5-dihalocytisines 6. In contrast to the parent alkaloid the 3-halogenated species display much a higher affinity for the alpha 7* nAChR subtype. The most potent molecule was 3-bromocytisine (7b) with preferential selectivity (200-fold) for the (alpha 4)(2)(beta 2)(3) subtype [K(i)=10 pM (alpha 4 beta 2) and 2.0 nM (alpha 7*)]. Replacement of the lactam with a thiolactam pharmacophore to thiocytisine (12) resulted in a subnanomolar affinity for the (alpha 4)(2)(beta 2)(3) nAChR subtype (K(i)=0.832 nM), but in a drastic decrease of affinity for the alpha 7* subtype; thiocytisine (12) has a K(i) value of 4000 nM (alpha 7*), giving a selectivity of 4800-fold for the neuronal (alpha 4)(2)(beta 2)(3)-nAChR and thus displaying the best affinity-selectivity profile in the series under consideration.

Topics: Aconitine; Alkaloids; Animals; Azocines; Binding, Competitive; Brain; Bridged Bicyclo Compounds, Heterocyclic; Bridged-Ring Compounds; Cell Membrane; Drug Evaluation, Preclinical; Halogens; Ligands; Nicotinic Agonists; Pyridines; Quinolizines; Rats; Rats, Sprague-Dawley; Receptors, Nicotinic; Structure-Activity Relationship

Although nicotinic agonists can modulate sensory transmission, particularly nociceptive signaling, remarkably little is known about the functional expression of nicotinic acetylcholine receptors (nAChRs) on primary sensory neurons. We have utilized molecular and electrophysiological techniques to characterize the functional diversity of nAChR expression on mammalian dorsal root ganglion (DRG) neurons. RT-PCR analysis of subunit mRNA in DRG tissue revealed the presence of nAChR subunits alpha2-7 and beta2-beta4. Using whole cell patch-clamp recording and rapid application of nicotinic agonists, four pharmacologically distinct categories of nicotinic responses were identified in cultured DRG neurons. Capacitance measurements were used to divide neurons into populations of large and small cells, and the prevalence of nicotinic responses was compared between groups. Category I (alpha7-like) responses were seen in 77% of large neurons and 32% of small neurons and were antagonized by 10 nM methyllycaconitine citrate (MLA) or or 50 nM alpha-bungarotoxin (alpha-BTX). Category II (alpha3beta4-like) responses were seen in 16% of large neurons and 9% of small neurons and were antagonized by 20 microM mecamylamine but not 10 nM MLA or 1 microM DHbetaE. Category II responses had a higher sensitivity to cytisine than nicotine. Two other types of responses were identified in a much smaller percentage of neurons and were classified as either category III (alpha4beta2-like) or category IV (subtype unknown) responses. Both the alpha7-like and alpha3beta4-like responses could be desensitized by prolonged applications of the analgesic epibatidine.

Topics: Acetylcholine; Aconitine; Alkaloids; alpha7 Nicotinic Acetylcholine Receptor; Animals; Azocines; Bridged Bicyclo Compounds, Heterocyclic; Bungarotoxins; Cell Size; Cells, Cultured; DNA Primers; Electrophysiology; Ganglia, Spinal; Gene Expression; Insecticides; Neurons, Afferent; Nicotine; Nicotinic Agonists; Pyridines; Quinolizines; Rats; Rats, Sprague-Dawley; Receptors, Nicotinic; RNA, Messenger; Vasodilator Agents

We have recently reported evidence that a very high affinity interaction between the beta-amyloid peptide Abeta(1-42) and the alpha7 nicotinic acetylcholine receptor (alpha7nAChR) may be a precipitating event in the formation of amyloid plaques in Alzheimer's disease. In the present study, the kinetics for the binding of Abeta(1-42) to alpha7nAChR and alpha4beta2nAChR were determined using the subtype-selective nicotinic receptor ligands [(3)H]methyllycaconitine and [(3)H]cytisine. Synaptic membranes prepared from rat and guinea pig cerebral cortex and hippocampus were used as the source of receptors. Abeta(1-42) bound to the alpha7nAChR with exceptionally high affinity, as indicated by K(i) values of 4.1 and 5.0 pM for rat and guinea pig receptors, respectively. When compared with the alpha7nAChR, the affinity of Abeta(1-42) for the alpha4beta2nAChR was approximately 5,000-fold lower, as indicated by corresponding K(i) values of 30 and 23nM. The results of this study support the concept that an exceptionally high affinity interaction between Abeta(1-42) and alpha7nAChR could serve as a precipitating factor in the formation of amyloid plaques and thereby contribute to the selective degeneration of cholinergic neurons that originate in the basal forebrain and project to the cortex and hippocampus.

Topics: Aconitine; Alkaloids; alpha7 Nicotinic Acetylcholine Receptor; Amyloid beta-Peptides; Animals; Azocines; Binding, Competitive; Cerebral Cortex; Guinea Pigs; Hippocampus; Kinetics; Male; Nicotine; Peptide Fragments; Quinolizines; Radioligand Assay; Rats; Rats, Long-Evans; Receptors, Muscarinic; Receptors, N-Methyl-D-Aspartate; Receptors, Nicotinic; Synaptic Membranes; Tritium

Tetracycline-regulated expression of recombinant nicotinic acetylcholine receptors (nAChR) composed of human alpha7 subunits is achieved in native nAChR-null SH-EP1 human epithelial cells. alpha7 subunits are heterologously expressed as messenger RNA and as components of 125I-labeled alpha-bungarotoxin (I-Bgt)-binding nAChR ( approximately 10 pmol per milligram of membrane protein) at levels sensitive to the amount of tetracycline in cell growth medium. I-Bgt-binding alpha7-nAChR appear on the cell surface pool and in intracellular pools. The pharmacological profile for drug competition toward I-Bgt binding to these recombinant alpha7-nAChR matches that of human native alpha7-nAChR naturally expressed in SH-SY5Y human neuroblastoma cells (rank order potency methyllycaconitine>1, 1-dimethyl-4-phenylpiperazinium>(-)nicotine>cytisine>carbamylch oli ne> /=d-tubocurarine). Chronic exposure to nicotine induces up-regulation of human recombinant alpha7-nAChR (80% up-regulation at 10 microM nicotine) just as it does native alpha7-nAChR in other human cell lines. These studies confirm expression of nAChR as homooligomers of human alpha7 subunits from transgenes, establish a native nAChR-null background for such expression, and demonstrate that this expression can be regulated to facilitate studies of human alpha7-nAChR.

Topics: Acetylcholine; Aconitine; Alkaloids; alpha7 Nicotinic Acetylcholine Receptor; Azocines; Blotting, Northern; Bungarotoxins; Carbachol; Dimethylphenylpiperazinium Iodide; DNA, Complementary; Gene Expression; Humans; Insecticides; Iodine Radioisotopes; Neuroblastoma; Neurons; Nicotine; Nicotinic Agonists; Plasmids; Protein Synthesis Inhibitors; Quinolizines; Receptors, Nicotinic; Recombinant Proteins; RNA, Messenger; Tetracycline; Transfection; Tubocurarine; Tumor Cells, Cultured; Up-Regulation

1. Heterologous expression studies of the alpha5 subunit of the neuronal acetylcholine receptor (nAChR) gene family have demonstrated that it can participate in the function of ACh-gated channels if co-expressed with another alpha- and a beta-subunit. Previous studies also indicate prominent expression of alpha5 in both central and peripheral nervous systems. The participation of alpha5 in native nAChRs and its functional role in these channels is, however, unknown. 2. In this study, we present evidence that alpha5 has a role in at least two distinct subtypes of nAChR complexes expressed by embryonic chick sympathetic neurones. 3. alpha5 contributes not only to agonist but also to antagonist sensitivity of natively expressed nAChR channels. Functional deletion of the alpha5 subunit by antisense oligonucleotide treatment removes the nAChRs with relatively low affinity to ACh and cytisine. Deletion of alpha5 also eliminates channels that are blocked by the alpha7-specific antagonist methyllycaconitine (MLA) while increasing the percentage of current carried by nAChRs that are sensitive to alpha-bungarotoxin (alpha-BgTx). 4. Single channel analyses indicate that functional deletion of alpha5 results in the deletion of both the 'brief' and 'long' open duration, 50 pS subtypes of nAChR channels while increasing the expression of the 18 pS, alpha-BgTx-sensitive native nAChRs normally detected in sympathetic neurones at later developmental stages. 5. The biophysical and pharmacological profiles of native nAChRs revealed by this study and previous work are discussed in the context of a proposed model of the nAChR channels expressed by chick sympathetic neurones throughout development.

Topics: Acetylcholine; Aconitine; Alkaloids; Animals; Antisense Elements (Genetics); Azocines; Bungarotoxins; Chick Embryo; Dose-Response Relationship, Drug; Electrophysiology; Ganglia, Sympathetic; Gene Deletion; Insecticides; Ion Channel Gating; Mutagenesis; Neurons; Nicotinic Agonists; Nicotinic Antagonists; Quinolizines; Receptors, Nicotinic

Ionotropic, nicotinic receptors have previously been shown to mediate both inhibitory (Cl-dependent) and excitatory (cationic) cholinergic responses in Aplysia neurons. We have used fast perfusion methods of agonist and antagonist application to reevaluate the effects on these receptors of a wide variety of cholinergic compounds, including a number of recently isolated and/or synthesized alpha toxins [alpha-conotoxin (alphaCTx)] from Conus snails. These toxins have been shown in previous studies to discriminate between the many types of nicotinic receptors now known to be expressed in vertebrate muscle, neuroendocrine, and neuronal cells. One of these toxins (alphaCTx ImI from the worm-eating snail Conus imperialis) revealed that two kinetically and pharmacologically distinct elements underlie the ACh-induced Cl-dependent response in Aplysia neurons: one element is a rapidly desensitizing current that is blocked by the toxin; the other is a slowly desensitizing current that is unaffected by the toxin. The two kinetically defined elements were also found to be differentially sensitive to different agonists. Finally, the proportion of the rapidly desensitizing element to the sustained element was found to be cell-specific. These observations led to the conclusion that two distinct nicotinic receptors mediate Cl currents in Aplysia neurons. The receptor mediating the rapidly desensitizing Cl-dependent response shows a strong pharmacological resemblance to the vertebrate alpha-bungarotoxin-sensitive, alpha7-containing receptor, which is permeable to calcium and mediates a rapidly desensitizing excitatory response.

Topics: Acetylcholine; Aconitine; Alkaloids; alpha7 Nicotinic Acetylcholine Receptor; Animals; Aplysia; Azocines; Bungarotoxins; Cations; Chlorides; Choline; Cholinergic Antagonists; Conotoxins; Dihydro-beta-Erythroidine; Electric Conductivity; Glycine Agents; Insecticides; Ion Channel Gating; Mollusk Venoms; Nicotinic Agonists; Nicotinic Antagonists; Oligopeptides; Patch-Clamp Techniques; Peptides; Quinolizines; Receptors, Nicotinic; Strychnine; Vertebrates

Presynaptic nicotinic acetylcholine receptors (nAChRs) are present in many regions of the brain and potentially serve as targets for the pharmacological action of nicotine in vivo. To investigate their mechanism of action, we performed patch-clamp recordings in relay neurons from slices of thalamus sensory nuclei. In these nuclei, nAChR activation facilitated the release of the inhibitory neurotransmitter GABA. Micromolar concentrations of nicotinic agonists increased the frequency of miniature GABAergic synaptic currents and decreased the failure rate of evoked synaptic currents. These actions of nicotinic agonists were not observed in knock-out mice lacking the beta 2 nAChR subunit gene. Nicotinic effects were dependent on extracellular calcium ions, and they persisted when calcium was replaced by strontium or barium but not by magnesium. Furthermore, in high extracellular calcium concentrations, nicotinic agonists evoked an increase in spontaneous release lasting for minutes after removal of the agonist. This supports the view that presynaptic nAChRs facilitate the release of neurotransmitter by increasing the calcium concentrations in presynaptic nerve endings. With use of cadmium and nickel ions as selective blockers, it was found that in different sensory nuclei the presynaptic influx of calcium could result either from the activation of voltage-dependent calcium channels or from a direct influx through nAChR channels. Finally, we propose that the nicotinic facilitation of GABAergic transmission may contribute to the increase of signal-to-noise ratio observed in the thalamus in vivo during arousal.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Aconitine; Action Potentials; Alkaloids; Animals; Arousal; Atropine; Azocines; Bacterial Toxins; Bridged Bicyclo Compounds, Heterocyclic; Calcium; Calcium Channel Blockers; Calcium Channels; Carbachol; Cations, Divalent; Cyanobacteria Toxins; Dihydro-beta-Erythroidine; Dimethylphenylpiperazinium Iodide; GABA Antagonists; gamma-Aminobutyric Acid; Ion Transport; Lobeline; Marine Toxins; Mice; Mice, Inbred C57BL; Mice, Knockout; Microcystins; Neurons, Afferent; Nicotine; Nicotinic Agonists; Nicotinic Antagonists; Patch-Clamp Techniques; Potassium; Pyridazines; Pyridines; Quinolizines; Receptors, GABA-A; Receptors, Nicotinic; Tetrodotoxin; Thalamic Nuclei

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.

Topics: Aconitine; Alkaloids; Animals; Azocines; Calcium; Calcium Channels; Cells, Cultured; Dose-Response Relationship, Drug; Fura-2; Hippocampus; Membrane Potentials; Nicotine; Quinolizines; Rats; Rats, Inbred Strains; Time Factors

Abnormal central cholinergic activity has been reported to be responsible in part for the pathogenesis of high blood pressure in spontaneously hypertensive rats (SHR). Administration of cholinergic agonists in brain and spinal cord results in exaggerated pressor responses in SHR. Studies to date have focused largely on the muscarinic cholinergic system. Recently, we demonstrated that intrathecal administration of nicotinic agonists results in pressor, tachycardic, and irritation responses. In the present study we examine the cardiovascular and behavioral responses to nicotine and cytisine administered intrathecally in La Jolla strain (LJ) SHRLJ and age-matched Wistar-Kyoto (WKYLJ) rats. Nicotinic agonists produced augmented pressor, heart rate, and irritation responses in SHRLJ compared with normotensive rats. In both SHRLJ and WKYLJ rats, cytisine elicited a greater nociceptive response and greater spinobulbar component to the pressor response than nicotine. SHRLJ and WKYLJ rats also differ in that the SHRLJ strain shows a diminished tendency for desensitization to cytisine. As in Sprague-Dawley rats, in SHRLJ and WKYLJ rats the cardiovascular and behavioral responses to intrathecal nicotine were significantly inhibited by mecamylamine, dihydro-beta-erthyroidine, and methyllycaconitine. However, methyllycaconitine, which effectively blocked cytisine-elicited cardiovascular and behavioral responses in Sprague-Dawley and WKYLJ rats, was unable to inhibit the maximal rise in cystine-elicited blood pressure, heart rate, and irritation responses in SHRLJ. In contrast to the heightened cardiovascular and behavioral responses, the number of nicotinic binding sites in spinal cord membranes was significantly decreased in the hypertensive rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Topics: Aconitine; Alkaloids; Animals; Azocines; Blood Pressure; Dihydro-beta-Erythroidine; Dose-Response Relationship, Drug; Heart Rate; Hypertension; Injections, Spinal; Male; Mecamylamine; Motor Activity; Nicotine; Nicotinic Agonists; Quinolizines; Rats; Rats, Inbred SHR; Rats, Inbred WKY; Spinal Cord; Stereotyped Behavior

We have examined the role of the spinal nicotinic receptors in mediating cardiovascular and behavioral responses in conscious rats. Intrathecal administration of nicotinic agonists to the lumbosacral region of the spinal cord caused a dose-dependent increase in systolic blood pressure, heart rate and a nociceptive (behavioral) response. The order of potencies for the pressor response was l-nicotine > or = cytisine > N-methylcarbamylcholine > or = dimethylphenylpiperazinium > d-nicotine. However, cytisine was the most potent in producing the heart rate increase and nociceptive response. Unlike the other agonists, cytisine also exhibited marked desensitization of the three responses upon repeated administration. The effects of nicotine were antagonized in a dose-dependent fashion by mecamylamine, hexamethonium, alpha-lobeline, dihydro-beta-erythroidine and methyllycaconitine. By contrast, cytisine-induced responses were blocked effectively by mecamylamine and methyllycaconitine, but not by alpha-lobeline or dihydro-beta-erythroidine. However, when alpha-lobeline or dihydro-beta-erythroidine antagonism of the pressor response to cytisine was monitored during the initial minute following intrathecal administration, both antagonists significantly inhibited the response. The competitive ganglionic blocker, trimethaphan, or the elapid alpha-toxin, alpha-bungarotoxin, when administered intrathecally, had no effect on nicotine- or cytisine-elicited responses. The cardiovascular responses to intrathecal nicotine and cytisine have two components. The first is likely mediated through direct sympathetic output and desensitizes rapidly to cytisine, the second is coupled indirectly to the nociceptive response and shows a diminished capacity for rapid desensitization. Agonist and antagonist specificities indicate that the spinal nicotinic receptor differs from those in ganglia and those characterized in brain to date. Although antagonist specificity of the blockade of nicotine and cytisine elicited responses differ, this may be due to the unique desensitization capacity of cytisine rather than an action mediated by distinct receptor subtypes.

Topics: Aconitine; Alkaloids; Animals; Atropine; Azocines; Behavior, Animal; Blood Pressure; Bungarotoxins; Cardiovascular System; Dihydro-beta-Erythroidine; Ganglionic Stimulants; Heart Rate; Injections, Spinal; Lobeline; Male; Mecamylamine; Nicotine; Nicotinic Antagonists; Nociceptors; Quinolizines; Rats; Rats, Sprague-Dawley; Receptors, Nicotinic; Sensitivity and Specificity; Spinal Cord; Trimethaphan