alpha-Bungarotoxin binding sites in rat hippocampal and cortical cultures: initial characterisation, colocalisation with alpha 7 subunits and up-regulation by chronic nicotine treatment

The brain alpha7 nicotinic receptor may be an important therapeutic target for the treatment of Alzheimer's disease: studies with DMXBA (GTS-21)

A large decrease in brain nicotinic receptor levels occurs in Alzheimer's disease, relative to muscarinic and other receptors. Neurons possessing high affinity nicotinic receptors seem particularly vulnerable. The low affinity nicotinic receptors which selectively bind alpha-bungarotoxin are not significantly affected. The major nicotinic receptor subtype which binds this toxin is a homo-oligomer composed of alpha7 subunits. Due to its exceptionally high calcium ion selectivity, this particular receptor can be considered as a ligand-gated calcium channel. Alpha7 receptors are found in regions of the brain which are important for cognition, including cerebral cortex and hippocampus. Hippocampal receptors are largely confined to GABAergic interneurons. Alpha7 receptors seem less likely than alpha4-beta2 receptors to be up-regulated in number and down-regulated in function as a result of chronic agonist exposure. A family of nicotinic agonists based upon the marine animal toxin anabaseine have been synthesized and investigated. One of these compounds, DMXBA [3-(2,4-dimethoxybenzylidene)-anabaseine; code name GTS-21] has displayed promising characteristics during phase I clinical tests. In the rat DMXBA is selectively agonistic upon alpha7 nicotinic receptors. In addition it is a moderately potent antagonist at alpha4-beta2 receptors. DMXBA enhances a variety of cognitive behaviors in mice, monkeys, rats and rabbits. It also displays neuroprotective activity upon cultured neuronal cells exposed to beta-amyloid or deprived of NGF. The compound is much less toxic than nicotine and does not affect autonomic and skeletal muscle systems at doses which enhance cognitive behavior. Phase I clinical tests indicate that large doses can be safely administered orally without adverse effects. Psychological tests on healthy young male subjects indicate a positive effect of DMXBA on some measures of cognition. While DMXBA is a much weaker partial agonist on human alpha7 receptors than upon rat alpha7 receptors, its 4-hydroxy metabolite has been shown to have excellent efficacy on both receptors. Thus, some of the physiological and behavioral effects of GTS-21 may be due to the actions of this primary metabolite.

Prenatal nicotine exposure: effects on locomotor activity and central [125I]alpha-BT binding in rats

Maternal smoking during pregnancy or in utero exposure of the fetus to nicotine may result in learning difficulties and hyperactivity in the child. To elucidate possible involvement of the alpha(7) nicotinic receptor subtype in these behavioral impairments, pregnant dams were treated with nicotine (9 mg/kg/day) via osmotic minipumps throughout gestation. Male offspring were weaned at postnatal day 18, and were tested for locomotor activity at postnatal days 20-24. Pups were sacrificed on postnatal day 36-38 and 18 discrete brain areas were analyzed for [125I]alpha-bungarotoxin (alpha-BT) binding by quantitative autoradiography. Prenatal nicotine caused an elevation in locomotor activity (vertical movements) in offspring. [125I]alpha-BT binding was significantly reduced in the hippocampal CA1 region (29%), dentate gyrus (22%), and medial geniculate nucleus (29%). These findings suggest that some of the behavioral abnormalities induced by prenatal nicotine exposure may be due to a reduction of alpha(7) nicotinic receptors in discrete brain regions.

Amino acid determinants of alpha 7 nicotinic acetylcholine receptor surface expression

Transient transfection has not been a successful method to express the alpha7 nicotinic acetylcholine receptor such that these receptors are detected on the cell surface. This is not the case for all ligand-gated ion channels. Transient transfection with the 5-hydroxytryptamine type 3 subunit cDNA results in detectable surface receptor expression. Cell lines stably expressing the alpha7 nicotinic acetylcholine receptor produce detectable, albeit variable, levels of surface receptor expression. alpha7 nicotinic acetylcholine receptor surface expression is dependent, at least in part, on cell-specific factors. In addition to factors provided by the cells used for receptor expression, we hypothesize that the surface expression level in transfected cells is an intrinsic property of the receptor protein under study. Employing a set of alpha7-5-hydroxytryptamine type 3 chimeric receptor subunit cDNAs, we expressed these constructs in a transient transfection system and quantified surface receptor expression. We have identified amino acids that control receptor distribution between surface and intracellular pools; surface receptor expression can be manipulated without affecting the total number of receptors. These determinants function independently of the cell line used for expression and the transfection method employed. How these surface expression determinants in the alpha7 nicotinic acetylcholine receptor might influence synaptic efficacy is discussed.

Depressive characteristics of FSL rats: involvement of central nicotinic receptors

Antidepressant effects of acute or chronic nicotine treatments in swim test immobility of Flinders sensitive line (FSL) rats, an animal model of depression, were recently demonstrated (Tizabi et al. Psychopharmacology 142:193, 1999). In the present study we sought to determine whether the antidepressant effects of nicotine could be blocked by the nicotinic antagonist, mecamylamine (MEC). Moreover, the effects of chronic nicotine treatment on [3H]cytisine binding in discrete brain regions of FSL and their control Flinders resistant line (FRL) rats were also evaluated. Adult male FSL rats were treated with MEC (0.5 mg/kg) 20 min prior to an acute or chronic nicotine administration. MEC by itself did not affect the immobility in swim test. However, it completely blocked the acute or chronic nicotine effects. Daily nicotine injection (0.4 mg/kg/day for 14 days) resulted in an increase in [3H]cytisine binding primarily in the FRL rats. An increase in nicotinic receptor binding following chronic nicotine administration is believed to reflect desensitization of these receptors. These findings, coupled with previous observation of higher basal nicotinic receptors in FSL rats, further support the involvement of central nicotinic receptors in depressive characteristics of these rats. Moreover, the data suggest therapeutic potential for selective nicotinic receptor agonists in depressive disorders.

alpha7 receptor-selective agonists and modes of alpha7 receptor activation

The alpha7-selective agonists 3-(2, 4-dimethoxybenzylidene)-anabaseine (GTS-21), also known as DMXB, and 3-(4-hydroxy,2-methoxybenzylidene)anabaseine (4OH-GTS-21) produce a variety of behavioral and cytoprotective effects that may be related to the activation of either large transient currents at high concentrations or small sustained currents at lower agonist concentrations. We are using acutely dissociated hypothalamic neurons, which express a central nervous system (CNS) alpha7-type receptor, to test a model for the concentration-dependent desensitization of alpha7-mediated responses. Our results confirm that 4OH-GTS-21 is a potent activator of neuronal alpha7 nicotinic-acetylcholine receptor. The rapid application of agonist leads to a brief period of maximal receptor-activation followed by desensitization. Rise rates, decay rates, and the degree to which current was desensitized were all concentration-dependent. Following the initial peak response to a 300-microM 4OH-GTS-21 application, current is reduced to baseline values within about 100 ms. Application of 30 microM 4OH-GTS-21 produced both a transient peak current and a sustained current that decayed only slowly after the removal of agonist. In the case of a 300-microM 4OH-GTS-21 application, after agonist was removed, we saw a rebound response up to the level of the 30-microM sustained current. The data, therefore, suggest that a sufficient level of agonist occupation can be retained on the receptor to promote activation for up to several hundred milliseconds.

Expression of a neuronal nicotinic acetylcholine receptor in insect and mammalian host cell systems

Different mammalian and insect somatic host cell systems were tested in their ability to express, fold, and assemble alpha7-type neuronal acetylcholine receptor (AChR) both at the transcriptional and translational level. For this purpose we employed clonal cell lines derived from the neural crest, such as PC12 cells from a rat adrenal pheochromocytoma, and GH3 cells isolated from a rat pituitary tumor, as well as non-neuronal cells such as NIH-3T3 fibroblasts from embryonic NIH Swiss mouse and Sf9 cells from ovary tissue of the Spodoptera frugiperda butterfly. Total RNA, isolated from either transfected or non-transfected PC12, GH3 or 3T3 cells, or recombinant AcNPV-infected and mock-infected Sf9 cells was analyzed by Northern blot. PC12 cells, which endogenously express alpha7 AChR, and all its heterologous alpha7-transfectant clones, exhibited variable but generally high amounts of a single transcript. GH3 and NIH-3T3 transfectant clones and recombinant AcNPV-infected Sf9 cells expressed variable levels of alpha7-mRNA, with a single transcript that co-migrated with the 28S rat rRNA. Only the neural crest-derived cell lines appeared to functionally express the alpha7 AChR, as measured by their [125I]alpha-bungarotoxin binding ability. The results suggest that heterologous expression of alpha7 is regulated not at the transcriptional, but at the postranslational level and that not all host cell systems appear to express the cellular factors needed for the correct postranslational modifications leading to mature and functional alpha7 AChR. Furthermore, the results suggest that tightly controlled expression mechanisms have evolved in parallel with this ancient cholinergic sequence.

Distributions of nicotinic acetylcholine receptor alpha7 and beta2 subunits on cultured hippocampal neurons

The hippocampus receives cholinergic afferents and expresses neuronal nicotinic acetylcholine receptors. In particular, the alpha7 and beta2 nicotinic subunits are highly expressed in the hippocampus. There has been controversy about the location, distribution and roles of neuronal nicotinic acetylcholine receptors [Role L. W. and Berg D. K. (1996) Neuron 16, 1077-1085; Wonnacott S. (1997) Trends Neurosci. 20, 92-98]. Using immunocytochemistry and patch-clamp techniques, we examined the density and distribution of nicotinic receptors on rat hippocampal neurons in primary tissue culture. The density and distribution of alpha7 subunits change with days in culture. Before 10 days in culture, alpha7 expression, monitored immunocytochemically, is low and nicotinic currents are small or absent. In older cultures, about two-thirds of the neurons express nicotinic currents, and alpha7 appears in small patches on the soma and out along the neuronal processes. These patches of alpha7 subunits on the surface of the neuronal processes often co-localize with the presynaptic marker, synaptotagmin. The other most common nicotinic subunit, beta2, stays confined mainly to the soma and proximal processes, and beta2 is distributed more uniformly and is not specifically localized at presynaptic areas. The two subunits, alpha7 and beta2, have different expression patterns on the surface of the cultured hippocampal neurons. Taken together with previous physiological studies, the results indicate that alpha7 subunits can be found at presynaptic terminals, and at these locations, these calcium-permeable channels may influence transmitter release.

Nicotinic receptor binding sites in rat primary neuronal cells in culture: characterization and their regulation by chronic nicotine

We have characterized high affinity neuronal nicotinic acetylcholine receptors labeled by [3H]cytisine in primary neuronal cell cultures from fetal rat brains. After 15 days in culture, the highest density of [3H]cytisine binding sites (Bmax approximately 57 fmol/mg protein) was found in cells from the brainstem, which includes the following subcortical brain areas: the septum, thalamus, hypothalamus, midbrain, pons and medulla. A lower density of sites was found in cells from the cerebral cortex, hippocampus, and caudate nucleus. [3H]Cytisine binds to receptors in primary cells from the brainstem and cerebral cortex with a Kd of approximately 0. 5 nM, and the binding is inhibited by the agonists nicotine, acetylcholine, and epibatidine with IC50 values of 1 to 20 nM, and by carbachol and the antagonist dihydro-beta-erythroidine with IC50 values of 0.5 to 1.5 microM. Chronic treatment of neuronal cultures with nicotine for 7 days differentially affected the number of nicotinic receptors in cells from different brain areas; it significantly increased the number of nicotinic binding sites in cells from the cerebral cortex, hippocampus, and caudate, but not in cells from the brainstem. The nicotine-induced increase of receptors in cerebral cortical cultures was not blocked by either mecamylamine or dihydro-beta-erythroidine. These results indicate that primary cultures of rat neuronal cells provide a good model system in which to study and compare the properties and regulation of native neuronal nicotinic acetylcholine receptors.

Effects of nicotine and stress on startle amplitude and sensory gating depend on rat strain and sex

We recently reported that 14 days of nicotine administration (12 mg/kg/day) reduced acoustic startle reflex amplitude and impaired prepulse inhibition (PPI) of startle in male and female Long-Evans rats. These findings contrasted with reports of nicotine-induced enhancement of startle and PPI in Sprague-Dawley (a different strain) male rats. The present experiment administered 0, 6, or 12 mg/kg/day nicotine via osmotic minipump for 14 days to 120 Sprague-Dawley rats (male and female) and to 120 Long-Evans rats (male and female) and examined ASR and PPI. Half of the subjects also were stressed by immobilization once each day to examine nicotine-stress interactions. Nicotine enhanced ASR and PPI responses of Sprague-Dawley rats but impaired these responses in Long-Evans rats, regardless of sex. Effects of stress were complex and depended on strain, sex, and drug dose. These findings indicate that effects of nicotine on measures of reactivity (ASR) and sensory gating (PPI) depend on genotype and that nicotine stress interactions depend on genotype, sex, and nicotine dosage.

Dopamine and nicotinic receptor binding and the levels of dopamine and homovanillic acid in human brain related to tobacco use

Reports of a reduction in the risk of developing Parkinson's disease and Alzheimer's disease in tobacco smokers, together with the loss of high-affinity nicotine binding in these diseases, suggest that consequences of nicotinic cholinergic transmission may be neuroprotective. Changes in brain dopaminergic parameters and nicotinic receptors in response to tobacco smoking have been assessed in this study of autopsy samples from normal elderly individuals with known smoking histories and apolipoprotein E genotype. The ratio of homovanillic acid to dopamine, an index of dopamine turnover, was reduced in elderly smokers compared with age matched non-smokers (P<0.05) in both the caudate and putamen. Dopamine levels were significantly elevated in the caudate of smokers compared with non-smokers (P<0.05). However there was no significant change in the numbers of dopamine (D1, D2 and D3) receptors or the dopamine transporter in the striatum, or for dopamine D1 and D2 receptors in the hippocampus in smokers compared with non-smokers or ex-smokers. The density of high-affinity nicotine binding was higher in smokers than non-smokers in the hippocampus, entorhinal cortex and cerebellum (elevated by 51-221%) and to a lesser extent in the striatum (25-55%). The density of high-affinity nicotine binding in ex-smokers was similar to that of the non-smokers in all the areas investigated. The differences in high-affinity nicotine binding between smokers and the non- and ex-smokers could not be explained by variation in apolipoprotein E genotype. There were no differences in alpha-bungarotoxin binding, measured in hippocampus and cerebellum, between any of the groups. These findings suggest that chronic cigarette smoking is associated with a reduction of the firing of nigrostriatal dopaminergic neurons in the absence of changes in the numbers of dopamine receptors and the dopamine transporter. Reduced dopamine turnover associated with increased numbers of high-affinity nicotine receptors is consistent with attenuated efficacy of these receptors in smokers. A decrease in striatal dopamine turnover may be a mechanism of neuroprotection in tobacco smokers that could delay basal ganglia pathology. The current findings are also important in the interpretation of measurements of nicotinic receptors and dopaminergic parameters in psychiatric conditions such as schizophrenia, in which there is a high prevalence of cigarette smoking.

Effects of alpha-erabutoxin, alpha-bungarotoxin, alpha-cobratoxin and fasciculin on the nicotine-evoked release of dopamine in the rat striatum in vivo

Snake neurotoxins (NTX) have proven to be valuable tools for the characterisation of muscular nicotinic acetylcholine receptor structure and function. It is very likely that they could also be utilised to identify subtypes of neuronal nicotinic receptors controlling specific functions within the central nervous system. In this study we examined the effects of long alpha NTX (alpha-bungarotoxin, alpha-Bgt, and alpha-cobratoxin, alpha-Cbt) and short alpha NTX (alpha-erabutoxin a, alpha-Ebt) as well as the anticholinesterase toxin fasciculin-2 (FAS), on the nicotine-evoked release of dopamine (DA) in the striatum, using the in vivo push-pull technique. The short toxins alpha-Ebt and FAS blocked the extracellular increase of DA evoked by nicotine at 4.2 microM concentrations and alpha-Ebt was more potent, as reflected by the blockade at the lower dose of 0.42 microM. In contrast, the long toxins showed a different profile of action. Alpha-Cbt did not show any blockade of the nicotine-evoked release of DA at the doses studied while alpha-Bgt did block it only at the higher dose (4.2 microM) These results indicate that short neurotoxins show a stronger interaction with striatal nicotinic receptors subtypes controlling DA release when compared to the long ones. This interaction of short neurotoxin polypeptides and presynaptic receptors may permit the further elucidation of the particular nicotinic receptor populations responsible for the modulation of striatal DA release.

Nicotinic receptor-induced apoptotic cell death of hippocampal progenitor cells

Nicotine has many effects on CNS functions, presumably through its action on neuronal nicotinic acetylcholine receptors (AChRs). One subclass of AChRs that binds the snake venom toxin alpha-bungarotoxin (alpha-Bgt-AChRs) has been shown to modulate neurotransmission in the brain. We now show that alpha-Bgt-AChR activation by low doses of nicotine results in apoptotic cell death of both primary and immortalized hippocampal progenitor cells. In HC2S2-immortalized hippocampal progenitors, nicotine is cytotoxic to undifferentiated cells, whereas it spares the same cells once differentiation has been induced. The activation of alpha-Bgt-AChRs by nicotine results in the induction of the tumor suppressor protein p53 and the cdk inhibitor p21. The cytotoxic effect of nicotine is dependent on extracellular calcium levels and is probably attributable to the poor ability of undifferentiated progenitors to buffer calcium loads. The major calcium buffer in these cells, calbindin D28K, is present only after differentiation has been induced. Furthermore transfection of undifferentiated cells with calbindin results in dramatic protection against the cytotoxic effects of nicotine. These results show that nicotine abuse could have significant effects on the survival of progenitor populations in the developing and adult brain and also suggest an endogenous role for alpha-Bgt-AChRs in neuronal development and differentiation.

Nicotinic acetylcholine receptors in rat cochlear nucleus: [125I]-alpha-bungarotoxin receptor autoradiography and in situ hybridization of alpha 7 nAChR subunit mRNA

The cochlear nucleus (CN) is the first site in the central nervous system (CNS) for processing auditory information. Acetylcholine in the CN is primarily extrinsic and is an important neurotransmitter in efferent pathways thought to provide CNS modulation of afferent signal processing. Although muscarinic acetylcholine receptors have been studied in the CN, the role of nicotinic receptors has not. We examined the distribution of one nicotinic acetylcholine receptor subtype, the alpha-bungarotoxin receptor (alpha Bgt), in the CN. Quantitative autoradiography was used to localize receptors and in situ hybridization was used to localize alpha 7 mRNA in CN neurons that express the alpha Bgt receptor. Binding sites for alpha Bgt are abundant in the anterior ventral, posterior ventral, and dorsal divisions of the CN, and receptor density is low in the granule cell layer and interstitial nucleus. Heterogeneity in CN subregions is described. Four distinct patterns of alpha Bgt binding were observed: (1) binding over and around neuronal cell bodies, (2) receptors locally surrounding neurons, (3) dense punctate binding in the dorsal CN (DCN) not associated with neuronal cell bodies, and (4) diffuse fields of alpha Bgt receptors prominent in the DCN molecular layer, a field underlying the granule cell layer and in the medial sheet. The perikaryial receptors are abundant in the ventral CN (VCN) and are always associated with neurons expressing mRNA for the receptor. Other neurons in the VCN also express alpha 7 mRNA, but without alpha Bgt receptor expression associated with the cell body. In general, alpha Bgt receptor distribution parallels cholinergic terminal distribution, except in granule cell regions rich in cholinergic markers but low in alpha Bgt receptors. The findings indicate that alpha Bgt receptors are widespread in the CN but are selectively localized on somata, proximal dendrites, or distal dendrites depending on the specific CN subregion. The data are consistent with the hypothesis that descending cholinergic fibers modulate afferent auditory signals by regulating intracellular Ca2+ through alpha Bgt receptors.

Exposure to prenatal nicotine transiently increases neuronal nicotinic receptor subunit alpha7, alpha4 and beta2 messenger RNAs in the postnatal rat brain

This study determined the effects of prenatal nicotine exposure (2 mg/kg/day) in Sprague Dawley CD rats via subcutaneously implanted osmotic minipumps, during gestational days 7-21, on postnatal levels of neuronal nicotinic receptor alpha4, alpha7 and beta2 subunit messenger RNAs. Northern analysis of postnatal day 1, 7, 14 and 28 hippocampal/septal and cortical total RNA using alpha-[32P]dCTP-labeled alpha4, alpha7 and beta2 complementary DNA probes identified a single (5.7-kb) alpha7 messenger RNA, three (2.4-, 3.8- and 8.0-kb) alpha4 messenger RNAs and four (3.7-, 5.0-, 7.5- and 10.0-kb) beta2 messenger RNAs. In comparison to prenatal saline, prenatal nicotine produced several significantly higher messenger RNA levels (cortical: 5.7-kb alpha7, 2.4-, 3.8- and 8.0-kb alpha4, 10.0-kb beta2; hippocampal/septal: 2.4- and 8.0-kb alpha4); these increases occurred predominantly on, but were not restricted to, postnatal day 14. Effects of nicotine were generally resolved by postnatal day 28. Collapsing the data across sex and age, a significant treatment effect indicated that hippocampal/septal and cortical 8.0-kb alpha4 messenger RNA levels and 10.0-kb beta2 messenger RNA levels were significantly higher following prenatal nicotine exposure. This is the first study indicating that prenatal nicotine produces alterations in developing postnatal rat neuronal nicotinic receptor messenger RNA levels, possibly by premature stimulation of neuronal nicotinic receptors. These results further implicate the teratogenic potential of nicotine in postnatal neuronal development.

Up-regulation of human alpha7 nicotinic receptors by chronic treatment with activator and antagonist ligands

This study examined the binding and functional properties of human alpha7 neuronal nicotinic acetylcholine receptors stably expressed in human embryonic kidney (HEK) 293 cells following chronic treatment with nicotinic receptor ligands. Treatment of cells with (-)-nicotine (100 microM) for 120 h increased the Bmax values of [125I]alpha-bungarotoxin binding 2.5-fold over untreated cells. This effect was concentration-dependent (EC50) = 970 microM) and a 6-fold upregulation was observed with the maximal concentration of (-)-nicotine tested. Also, treatment of cells with ligands of varying intrinsic activities including (+/-)-epibatidine, (2,4)-dimethoxybenzylidene anabaseine (GTS-21) and 1,1-dimethyl-4-phenyl piperazinium iodide (DMPP) also upregulated [125I]alpha-bungarotoxin binding. A concentration-dependent upregulation of binding sites was also observed following treatment with the alpha7 nicotinic receptor antagonist, methyllycaconitine (EC50 = 92 microM) with a maximal upregulation of about 7-fold. Functionally, the peak amplitude of the whole-cell currents recorded by fast application of (-)-nicotine after chronic treatment of cells with concentrations of (-)-nicotine (1000 microM) or methyllycaconitine (10 microM) that elicited similar increases in binding levels (3.5-fold) resulted in increases of 2-fold (505 +/- 21 pA) and 6-fold (1820 +/- 137 pA) respectively in whole cell current amplitude compared to untreated cells (267 +/- 24 pA). These studies clearly demonstrate that long-term exposure to both activator and antagonist ligands can increase the density of alpha7 nicotinic receptors and can differentially enhance nicotinic receptor function.

Nicotinic acetylcholine receptors in the rat and primate nucleus tractus solitarius and on rat and human inferior vagal (nodose) ganglia: evidence from in vivo microdialysis and [125I]alpha-bungarotoxin autoradiography

The nucleus tractus solitarius is a key brain centre involved in the regulation of numerous autonomic functions. The present study has employed in vitro autoradiography and in vivo microdialysis to investigate the presence and function of nicotinic acetylcholine receptors located in the medial nucleus tractus solitarius of the rat. Autoradiography using [125I]alpha-bungarotoxin (0.5 nM) enabled visualization of binding sites on sections of rat and monkey brainstem. Specific binding was highest in the medial nucleus tractus solitarius. The presence of binding sites was also apparent on sections of rat nodose ganglia/vagus nerve and human inferior vagal ganglia. Subsequent to unilateral ligation of the vagus nerve in the rat, an accumulation of binding sites was visualized adjacent to the ligature. Unilateral nodose ganglionectomy in the rat caused an approximate 97% reduction in [125I]alpha-bungarotoxin binding site density in the medial nucleus tractus solitarius from 814 +/- 183 to 27 +/- 2 d.p.m./mm2. Microdialysis results indicated that local administration of nicotine (1 mM) into the nucleus tractus solitarius of the rat resulted in increases of extracellular L-glutamate of 146 +/- 9% of basal levels. This effect was not reproducible following a second stimulation and was also blocked by prior and co-administration of the nicotinic acetylcholine receptor antagonist mecamylamine (100 microM). Extracellular levels of L-aspartate exhibited a similar pattern although results were not significant. Taken together, these results are supportive of the presence of a population of [125I]alpha-bungarotoxin binding sites located presynaptically with respect to vagal afferent terminals in the medial nucleus tractus solitarius of the rat. It is possible that these binding sites are the site of action of locally administered nicotine on extracellular levels of L-glutamate, the favoured neurotransmitter at primary baroreceptor afferent fibres. These data are discussed in relation to the functional pharmacology of acetylcholine and nicotinic acetylcholine receptors in this region of the brain.

Neuroprotective effects of 2,4-dimethoxybenzylidene anabaseine (DMXB) and tetrahydroaminoacridine (THA) in neocortices of nucleus basalis lesioned rats

The nicotinic alpha7 agonist dimethoxybenzilidene anabaseine (DMXB) and cholinesterase inhibitor tetrahydroaminoacridine (THA) were investigated in a trans-synaptic model for neocortical atrophy and degeneration following nucleus basalis lesions. Bilateral lesions reduced parietal neuronal density in layers II-V 8 months later. DMXB administered i.p. daily to rats for 3 months attenuated this loss in layers II-V at a 1 mg/kg i.p. dose. A lower, 0.2 mg/kg i.p. dose, was neuroprotective in layer IV only. THA (1 mg/kg i.p.) also protected against neocortical Nissl-staining deficits.

Peptidergic innervation and the nicotinic acetylcholine receptor in the primate basal nucleus

Peptidergic innervation and localization of the neuronal nicotinic acetylcholine receptor (nAChR) was studied in the basal forebrain of Macaca fascicularis in order to provide microstructural proofs for the theory (Changeux et al., 1992) that calcitonin gene-related peptide (CGRP) is responsible for the maintenance of the acetylcholine receptor. Distribution and localization of five neuropeptides, namely substance P (SP), CGRP, neuropeptide Y (NPY), vasoactive intestinal polypeptide (VIP) neurotensin (NT), and the neuropeptides parvalbumin (PV) and the alpha-bungarotoxin- (alpha-BTX-) binding protein was studied by means of light- and electron microscopic pre-embedding immunocytochemistry. Immunohistochemical double staining revealed that large cholinergic principal nerve cells in the basal forebrain, corresponding to cell group Ch4 constituting Meynert's basal nucleus (BNM), and exerting intense choline acetyltransferase (ChAT) immunoreactivity, are synaptically innervated by axons displaying CGRP immunoreactivity. While SP, NPY, PV and CGRP establish dense networks in BNM, innervation by NT and VIP is sparse. Biotinylated alpha-BTX visualizes beaded axons that surround dendrites and perikarya of cholinergic principal cells. Electron microscopic organization of the neuropil in BNM is characterized by a glomerular (or rather cartridge-like) arrangement of axons surrounding dendrites of non-cholinergic principal nerve cells. At least one of the axons establishing the glomerulus (cartridge) exerts CGRP immunopositivity while alpha-BTX-immunopositive axons, presynaptic to dendrites of principal cells, are attached to the glomeruli (cartridges) from outside. As alpha-BTX-binding indicates localization of the alpha7 subunit of the neuronal nAChR, the microtopographical arrangement supports the idea that, in a manner similar to that in the neuromuscular junction, CGRP might contribute to the maintenance of nAChR also in BNM. Our results suggest that presynaptic nAChR-s are involved in the regulation of acetylcholine release from a feed-forward amplification mechanism of cholinergic principal cells of BNM.

Nicotinic and muscarinic cholinergic receptor binding in the human hippocampal formation during development and aging

High-affinity nicotine, alpha-bungarotoxin (alpha BT) and muscarinic receptor binding was measured in the human hippocampal formation in a series of 57 cases aged between 24 weeks gestation and 100 years. Changes in nicotine receptor binding during development and aging were more striking than differences in alpha BT and muscarinic binding. Nicotine binding was higher at the late foetal stage than at any other subsequent time in all areas investigated. In the hippocampus a fall in binding then occurred within the first six months of life, with little or no subsequent fall during aging, whereas in the entorhinal cortex and the presubiculum the major loss of nicotine binding occurred after the fourth decade. alpha BT binding was significantly elevated in the CA 1 region, but in no other region of the hippocampus, in the late foetus, and there was also a fall in alpha BT binding in the entorhinal cortex during aging from the second decade. The modest changes in total muscarinic binding, which appeared to reflect those in M1 and M3 + 4 rather than M2 binding, were a rise in the entorhinal cortex between the foetal stage and childhood and a tendency for receptors to fall with age in the hippocampus and subicular complex. These findings implicate mechanisms controlling the expression of nicotinic receptors to a greater extent than muscarinic receptors in postnatal development and aging in the human hippocampus.

Chronic nicotine treatment up-regulates alpha3 and alpha7 acetylcholine receptor subtypes expressed by the human neuroblastoma cell line SH-SY5Y

Chronic exposure to nicotine has been reported to increase the number of nicotinic acetylcholine receptors (AChRs) in brain. The mechanism of up-regulation for the alpha4beta2 AChR subtype, which accounts for the majority of high affinity nicotine binding in mammalian brain, has previously been shown to involve a decrease in the rate of alpha4beta2 AChR turnover. Here, we report an investigation of the extent and mechanism of nicotine-induced up-regulation of alpha3 AChRs and alpha7 AChR subtypes expressed in the human neuroblastoma cell line SH-SY5Y. Up-regulation of human alpha3 AChRs and alpha7 AChRs, unlike alpha4beta2 AChRs, requires much higher nicotine concentrations than are encountered in smokers; the extent of increase of surface AChRs is much less; and the mechanisms of up-regulation are different than with alpha4beta2 AChRs. The mechanisms of up-regulation may be different for alpha3 AChRs or alpha7 AChRs. Chronic treatment with nicotine or carbamylcholine, but not d-tubocurarine, mecamylamine, or dihydro-beta-erythroidine, induced a 500-600% increase in the number of alpha3 AChRs but only a 30% increase in alpha7 AChRs. Chronic nicotine treatment did not increase affinity for nicotine or increase the amount of RNA for alpha3 or alpha7 subunits. The effect of nicotine on up-regulation of alpha7 AChRs was partially blocked by either d-tubocurarine or mecamylamine. The effect of nicotine treatment on the number of alpha3 AChRs was only slightly blocked by the antagonists d-tubocurarine, mecamylamine, or dihydro-beta-erythroidine at concentrations that efficiently block alpha3 AChR function. Most of the nicotine-induced increase in alpha3 AChRs was found to be intracellular. The alpha3 AChRs, which accumulate intracellularly, were shown to have been previously exposed on the cell surface by their susceptibility to antigenic modulation. The data suggest that chronic exposure to nicotine may induce a conformation of cell surface alpha3 AChRs that at least in this cell line are consequently internalized but not immediately destroyed.

Parallel increases in [alpha-125I]bungarotoxin binding and alpha 7 nicotinic subunit immunoreactivity during the development of rat hippocampal neurons in culture

Previous studies have shown that hippocampal neurons cultured at high density express alpha-bungarotoxin binding sites and have alpha 7 nicotinic acetylcholine receptor subunit immunoreactivity [Barrantes, G.E., Rogers, A.T., Lindstrom, J. and Wonnacott, S., Brain Res., 672 (1995) 228-236]. We now examine both of these parameters in well-characterized hippocampal neurons cultured at sufficiently low densities to resolve individual neurons and their processes. The specific binding of [alpha-125I]bungarotoxin is first detectable after 3 days in culture and increases during the next 12 days in culture, reaching a maximum of approximately 30,000 binding sites per cell. This is accompanied, over the same timecourse, by an increase in immunoreactivity for two antibodies that specifically bind to the alpha 7 subunit. Both cell bodies and processes were labelled by 9 days in culture. The timecourse of alpha 7-type nicotinic receptor expression resembles that previously described for synapse formation in hippocampal cultures.

Nicotinic acetylcholine receptors on hippocampal neurons: distribution on the neuronal surface and modulation of receptor activity

The recent development of a technique that uses infrared microscopy for the visualization of well-defined areas on the surface of neurons, and a computerized system of micromanipulators led to the discovery that functional nicotinic acetylcholine receptors (nAChRs) are expressed at higher density on the dendrites than on the soma of rat hippocampal neurons. The finding that the expression of alpha-bungarotoxin-sensitive, alpha 7-bearing, nAChRs and dihydro-beta-erythroidine-sensitive, alpha 4 beta 2 nAChRs tends to increase along the dendritic length suggests that these receptors may be highly involved in the integration of synaptic functions in hippocampal neurons. The present report also discusses the finding that ligands such as the anticholinesterase galanthamine can modulate the nAChR activity by binding to a novel receptor site, and that 5-hydroxytryptamine (5-HT) may serve as an endogenous ligand for this site. The ability of 5-HT to modulate the nAChR function in vivo supports the concept that the overall CNS function is determined not only by the neuronal network established by the neuronal wiring, but also by a chemical network established by the ability of a single substance to act as the primary neurotransmitter in one system and as a co-transmitter in another system.

Pharmacology of neuronal nicotinic acetylcholine receptor subtypes

The search for the physiological function of nicotinic receptors on neurons in the brain began with their discovery. It was initially assumed that, as in ganglia and at the neuromuscular junction, nicotinic receptors would gate fast synaptic transmission in the brain. The best functional evidence now, however, points to a role in modifying the release of other transmitters. This does not preclude a postsynaptic role in transmission for nicotinic receptors in the brain, but attempts to locate such a synapse have not been successful. If fast nicotinic synapses are present in the brain, they are probably low in number and may be masked by other more prevalent synapses (such as glutamatergic) so identification will not be easy. The extent of diversity of nicotinic receptors is substantial. At the molecular level this is reflected in the number of different genes that encode receptor subunits and the multiple possible combinations of subunits that function in expression systems. From the cellular level there is a broad diversity of properties of native receptors in neurons. Some useful pharmacological tools allow the limited identification of subunits in native receptors. For example, block by alpha-bungarotoxin identifies alpha 7, alpha 8, or alpha 9 subunits; activation of a receptor by cytisine indicates an alpha 7 or beta 4 subunit; and neuronal bungarotoxin block identifies a beta 2 subunit. Despite the clues to identity gained by careful use of these agents, we have not been able to identify all the components of any native receptor based on pharmacological properties assessed from expression studies. When both pharmacological and biophysical properties of a receptor are taken into consideration, none of the combinations tested in oocytes mimics native receptors exactly. The reason for this discrepancy has been debated at length; it is possible that oocytes do not faithfully manufacture neuronal nicotinic receptors. For example, they may not correctly modify the protein after translation or they may allow a combination of subunits that do not occur in vivo. Another possibility is that correct combinations of subunits have not yet been tested in oocytes. Data from immunoprecipitation experiments suggest that many receptors contain three or more different subunits. Results from further experiments injecting combinations of three or more subunits into oocytes may be enlightening. The diversity of receptors may allow targeting of subtypes to specific locations. Nicotinic receptors are located presynaptically, preterminally, and on the cell soma. The function of the nicotinic receptors located on innervating axons is presumably to modify the release of other neurotransmitters. It is an attractive hypothesis that nicotinic receptors might be involved in modifying the weight of central synapses; however, in none of the regions where this phenomenon has been described is there any evidence for axoaxonal contacts. The presynaptic receptors described so far are pharmacologically unique; therefore, if there are different subtypes of nicotinic receptors modifying the release of different transmitters, they may provide a means of exogenously modifying the release of a particular transmitter with drugs. There are still many basic unanswered questions about nicotinic receptors in the brain. What are the compositions of native nicotinic receptors? What is their purpose on neurons? Although there is clearly a role presynaptically, what is the function of those located on the soma? Neuronal nicotinic receptors are highly permeable to calcium, unlike muscle nicotinic receptors, and this may have important implications for roles in synaptic plasticity and development. Finally, why is there such diversity? (ABSTRACT TRANCATED)

Nudicauline and elatine as potent norditerpenoid ligands at rat neuronal alpha-bungarotoxin binding sites: importance of the 2-(methylsuccinimido)benzoyl moiety for neuronal nicotinic acetylcholine receptor binding

Methyllycaconitine (MLA, 1) is a novel, potent probe for mammalian and insect nicotinic acetylcholine receptors (nAChR) and displays remarkable selectivity toward neuronal [125I]-alpha-bungarotoxin (alpha BgTX) binding sites that correspond to alpha 7-type nAChR in mammalian brain. We have shown that, among a number of selected norditerpenoid alkaloids, elatine (2) and nudicauline (3) are equipotent with, or better than, MLA (1) in binding to brain [125I]-alpha BgTX binding sites, with IC50 values of 6.1, 1.7, and 7.6 nM, respectively. The 2-((S)-methylsuccinimido)benzoyl moiety of these ligands is crucial for high-affinity binding, whereas structural modifications to the norditerpenoid core of the ligand can be tolerated without loss of activity or selectivity. In addition to MLA (1), elatine (2), and nudicauline (3), we have examined lycoctonine (4), inuline (6), lappaconitine (7), N-desacetyllappaconitine (8), delsoline (10), delcorine (11), deltaline (12), condelphine (13), and karacoline (14). This study therefore extends the range of norditerpenoids, other than MLA, which can be used to probe this important class of nAChR. All 12 alkaloids were assessed for activity at [3H]nicotine binding sites which are considered to represent alpha 4 beta 2 nAChR. Furthermore, the 1H and 13C NMR spectroscopic data of MLA and elatine have been critically compared.

Characterization of nicotinic receptors in immortalized hippocampal neurons

Nicotinic acetylcholine receptors, particularly nicotinic alpha-bungarotoxin (alpha-BGT) receptors, are present in relatively high concentrations in rat hippocampus. Because of the difficulties encountered in studying receptors using primary cells in culture, especially for biochemical work, we investigated the possibility of using an immortalized cell line from embryonic rat hippocampus (H19-7). RNase protection assays show that alpha 4, alpha 7 and beta 2 neuronal nicotinic receptor subunit mRNAs are present in differentiated but not undifferentiated H19-7 cells, while alpha 2, alpha 3, alpha 5 and beta 3 subunit mRNAs were not detectable under either condition. In line with these results, the present data demonstrate that the H19-7 cells express cell surface nicotinic alpha-BGT binding sites, which were maximal after seven days of differentiation in culture. The receptors were saturable, of high affinity (Kd = 1.30 nM and Bmax = 11.70 fmol/10(5) cells) and had a pharmacological profile similar to that observed for CNS alpha-BGT receptors. On the other hand, although alpha 4 and beta 2 neuronal nicotinic subunit mRNAs were present in differentiated H19-7 cells, no [3H]cytisine binding was observed. Because immortalized cell lines have the advantage that they provide a limitless supply of cells as compared to primary cell cultures, but yet are not malignant in origin, the present results may suggest that the H19-7 immortalized hippocampal cell line represent a useful CNS model system for examining alpha-BGT nicotinic receptors.

Effects of ethanol on ion channels

Ion channels play critical roles in nervous system function, from initiating rapid synaptic activity to propagation of action potentials. Studies have indicated that many of the effects of ethanol on the nervous system are likely caused by the actions of ethanol on ion channels. Ion channels are multimeric structures that gate ions through subtle changes in tertiary structure. Ethanol readily enters molecular sites within multimeric ion channels, modifying intermolecular forces and bonds that are important for the open-close-inactivation kinetic properties of channels. The diversity of channel composition caused by the multimeric structure results in subtypes of channels that have a spectrum of sensitivity to ethanol that translates into brain regional differences in ethanol sensitivity, in part caused by differences in ion channel subunit composition. Ethanol has been shown to affect both receptor-activated ion channels and voltage-gated ion channels. The acute intoxicating and incoordinating effects of ethanol are probably related to inhibition of subtypes of NMDA-glutamate receptor ion channels and potentiation of certain subtypes of GABAA receptor ion channels. Effects on these channels, as well as glycine, nicotinic cholinergic, serotonergic, and other ion channels, likely contribute to the euphoric, sedative, and other acute actions of ethanol. Changes in ion channel subunit composition, density, and properties probably also contribute to ethanol tolerance, dependence, withdrawal hyperexcitability, and neurotoxicity. A substantial number of studies have implicated glutamate NMDA receptor, GABAA, and L-type voltage-gated calcium channels in the adaptive changes in the brain during chronic ethanol exposure. The diversity of ion channels subunits, their prominent role in brain function, and ethanol action are likely to make them important contributors to alcoholism and alcohol abuse.

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.