Distinct regional expression of nicotinic acetylcholine receptor genes in chick brain

Nicotinic acetylcholine receptors in a songbird brain

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels that mediate fast synaptic transmission and cell signaling, which contribute to learning, memory, and the execution of motor skills. Birdsong is a complex learned motor skill in songbirds. Although the existence of 15 nAChR subunits has been predicted in the avian genome, their expression patterns and potential contributions to song learning and production have not been comprehensively investigated. Here, we cloned all the 15 nAChR subunits (ChrnA1-10, B2-4, D, and G) from the zebra finch brain and investigated the mRNA expression patterns in the neural pathways responsible for the learning and production of birdsong during a critical period of song learning. Although there were no detectable hybridization signals for ChrnA1, A6, A9, and A10, the other 11 nAChR subunits were uniquely expressed in one or more major subdivisions in the song nuclei of the songbird brain. Of these 11 subunits, ChrnA3-5, A7, and B2 were differentially regulated in the song nuclei compared with the surrounding anatomically related regions. ChrnA5 was upregulated during the critical period of song learning in the lateral magnocellular nucleus of the anterior nidopallium. Furthermore, single-cell RNA sequencing revealed ChrnA7 and B2 to be the major subunits expressed in neurons of the vocal motor nuclei HVC and robust nucleus of the arcopallium, indicating the potential existence of ChrnA7-homomeric and ChrnB2-heteromeric nAChRs in limited cell populations. These results suggest that relatively limited types of nAChR subunits provide functional contributions to song learning and production in songbirds.

Brain regional heterogeneity and toxicological mechanisms of organophosphates and carbamates

The brain is a well-organized, yet highly complex, organ in the mammalian system. Most investigators use the whole brain, instead of a selected brain region(s), for biochemical analytes as toxicological endpoints. As a result, the obtained data is often of limited value, since their significance is compromised due to a reduced effect, and the investigators often arrive at an erroneous conclusion(s). By now, a plethora of knowledge reveals the brain regional variability for various biochemical/neurochemical determinants. This review describes the importance of brain regional heterogeneity in relation to cholinergic and noncholinergic determinants with particular reference to organophosphate (OP) and carbamate pesticides and OP nerve agents.

Low level nicotine: a novel approach to reduce osteoporosis incidence

Osteoporosis is a serious health problem worldwide and leads to a significant burden on society. Unfortunately, efforts to control osteoporosis are largely unsuccessful. Lowering an individual's risk for osteoporosis must focus not only on treatment but also on modification of risk factors. One of the common risk factors is smoking tobacco. Here, we review the clinical evidence on nicotine consumption and osteoporosis, and propose a possible protective mechanism. It should be note that there is no strong clinical evidence that proves nicotine is detrimental. Studies also indicated that the prime criminal for osteoporosis is smoking not nicotine. Moreover, low level nicotine has preventive efforts on osteoporosis by stimulating osteoblasts proliferation and differentiation. We present a hypothesis that low level nicotine may be a novel approach to reduce osteoporosis incidence.

Nicotine modulates bone metabolism-associated gene expression in osteoblast cells

Smoking has a broad range of physiological effects, such as being a risk factor in osteoporosis, bone fracture incidence, and increased nonunion rates. Recent studies showed that nicotine has effects at the cellular level in human osteoblast cells. To identify possible mechanisms underlying nicotine-induced changes in osteogenic metabolism, we defined changes in proliferation and osteocalcin, type I collagen, and alkaline phosphatase gene expression after treating human osteosarcoma cells (MG63), with various concentration of nicotine. Nicotine affects cell proliferation in a biphasic manner, including toxic and antiproliferative effects at high levels of nicotine and stimulatory effects at low levels. Moreover, low levels of nicotine upregulated osteocalcin, type I collagen, and alkaline phosphatase gene expression. The increased cell proliferation and gene upregulation induced by nicotine were inhibited by addition of the nicotinic receptor antagonist D: -tubocurarine. High nicotine concentrations downregulated the investigated genes. Our results demonstrate, for the first time, that the addition of nicotine concentrations analogous to those acquired by a light to moderate smoker yields increased osteoblast proliferation and bone metabolism, whereas the addition of nicotine concentrations analogous to heavy smokers leads to the opposite effect. The inhibition of these effects by D: -tubocurarine suggests that nicotine acts via the nicotinic acetylcholine receptor (nAChR).

Nicotine suppresses bone sialoprotein gene expression

BACKGROUND AND OBJECTIVE

Tobacco smoking is a risk factor for periodontitis and osteoporosis. Nicotine is a major component of tobacco, and has been reported to inhibit proliferation and differentiation of osteoblasts. Bone sialoprotein (BSP) is a mineralized tissue-specific protein expressed by differentiated osteoblasts that appears to function in the initial mineralization of bone. The purpose of this study was to determine the effects of nicotine on bone metabolism.

MATERIAL AND METHODS

We used rat osteobast-like UMR106 and ROS 17/2.8 cells and rat stromal bone marrow RBMC-D8 cells. To determine the molecular basis of the transcriptional regulation of the BSP gene by nicotine, we conducted Northern hybridization, transient transfection analyses with chimeric constructs of the BSP gene promoter linked to a luciferase reporter gene and gel mobility shift assays.

RESULTS

Nicotine (250 microg/mL) decreased the BSP mRNA levels at 12 and 24 h in UMR106 and ROS 17/2.8 cells. From transient transfection assays using various sized BSP promoter-luciferase constructs, nicotine decreased the luciferase activities of the construct, including the promoter sequence nucleotides -116 to +60, in UMR106 and RBMC-D8 cells. Nicotine decreased the nuclear protein binding to the cAMP response element (CRE), fibroblast growth factor 2 response element (FRE) and homeodomain protein-binding site (HOX) at 12 and 24 h.

CONCLUSION

This study indicates that nicotine suppresses BSP transcription mediated through CRE, FRE and HOX elements in the proximal promoter of the rat BSP gene.

Opioid receptor activation attenuates nicotinic enhancement of spontaneous GABA release in lateral spiriform nucleus of the chick

We examined the effects of opioids on the nicotinic enhancement of spontaneous GABA release from presynaptic terminals in the lateral spiriform nucleus (SpL) of the chick. Whole cell recordings from SpL neurons in brain slices were used to monitor spontaneous GABA release. Nicotine (1 microM) produced an 8-fold increase in the frequency of GABA events without changing their amplitude, consistent with an increase of GABA release from presynaptic terminals. L-enkephalin (1 microM) blocked these effects of nicotine on presynaptic GABA release, and the opioid antagonist naloxone (100 nM) antagonized the actions of L-enkephalin. The selective mu agonist DAMGO (300 nM) also attenuated the nicotine-mediated enhancement of GABA release, and the mu selective antagonist CTOP (1 microM) blocked the actions of DAMGO. In contrast, the kappa opioid agonist U50488 (3 microM) and the delta opioid agonist DPDPE (1 microM) had no effect. The results demonstrate that presynaptic release of GABA in the SpL can be regulated by both nicotinic agonists and mu opioids. While mu opioids have little effect on GABA release by themselves, they are able to block the marked enhancement of GABA release normally produced by nicotine. Since both cholinergic and enkephalinergic nerves are present in the SpL, the interactions of these two neurotransmitter systems may serve to precisely regulate GABA release in this brain region.

Neurotransmitter regulation of neural development: acetylcholine and nicotinic receptors

Several neurotransmitter systems have been related to developmental processes during the past decade. In this review, we discuss the evidence that the nicotinic acetylcholine receptors could have an additional function during development that may be unrelated to their role in cholinergic neurotransmission in the vertebrate brain. Both temporal expression data and in vitro and in vivo studies with nicotinic agonists and antagonists have provided direct support for a role of nicotinic receptors in neural developmental processes such as neurite outgrowth and differentiation. A similar picture has emerged for other neurotransmitter and receptor systems as well, which generates a new view of neural processes during both development and mature life.

Nicotinic regulation of c-fos and osteopontin expression in human-derived osteoblast-like cells and human trabecular bone organ culture

Long-term in vivo studies have highlighted smoking as a risk factor in postmenopausal osteoporosis, bone fracture incidence, and increased nonunion rates. In contrast, there are few data postulating the effects of smoking at the cellular level in human skeletal tissue. In this study, we present novel evidence demonstrating that the nicotinic receptor alpha4 subunit is present in human primary bone cells by using reverse transcriptase-polymerase chain reaction (RT-PCR). In addition, we demonstrate direct cellular effects of nicotine on primary human bone cells and blockage of these effects with a nicotinic receptor antagonist, D-tubocurarine. Nicotine effects on cell proliferation were biphasic with toxic, antiproliferative effects at high levels of nicotine (>1 mmol/L) and stimulatory effects at very low levels (0.01-10 micromol/L) after 72 h. This nicotine-induced increase in cell proliferation was inhibited in a dose-dependent manner by the addition of D-tubocurarine. In addition, proliferation effects from low-level treatment correlated with an upregulation of expression of the AP-1 transcription factor, c-fos, within 1 h, which was blocked by incubation with D-tubocurarine. To determine in situ bone cell responses within their trabecular matrix, cores of human bone isolated from biopsies were perfused with 0.1 micromol/L nicotine for 24 h. Western analysis of proteins isolated from the cores highlighted an increase in osteopontin, a bone matrix protein implicated in regulating resorption, which was partially inhibited by the addition of D-tubocurarine. To conclude, our results suggest that nicotine has a direct effect on human bone cells in modulating proliferation, upregulation of the c-fos transcription factor, and the synthesis of the bone matrix protein, osteopontin.

A comparative non-radioactive in situ hybridization and immunohistochemical study of the distribution of alpha7 and alpha8 subunits of the nicotinic acetylcholine receptors in visual areas of the chick brain

The distribution of mRNA transcripts corresponding to the alpha7 and alpha8 subunits of the nicotinic acetylcholine receptors (nAChRs) was studied in selected structures of the chick visual system with non-radioactive in situ hybridization and immunohistochemical techniques. The results indicated that the alpha7 and alpha8 nAChR transcripts are widely distributed in the brain, exhibiting differential expression in some structures but also some degree of co-localization. The pattern of localization of alpha7 and alpha8 nAChR transcripts was highly correlated with immunohistochemical data, with very few instances of possible mismatches between the distribution of mRNAs and their corresponding proteins.

Multiorgan autonomic dysfunction in mice lacking the beta2 and the beta4 subunits of neuronal nicotinic acetylcholine receptors

Transcripts for the beta2 and the beta4 nicotinic acetylcholine receptor (nAChR) subunits are found throughout the CNS and the peripheral nervous system. These two beta subunits can form heteromultimeric channels with any of the alpha2, alpha3, alpha4, or alpha5 subunits in heterologous expression systems. Nonetheless, the subunit composition of native nAChRs and the role of different nAChR subtypes in vivo remain unclear. We prepared null mutations for the beta2 and the beta4 genes and bred beta2-/-beta4-/- mice by mating mice of identical beta2-/-beta4+/- or beta2+/-beta4-/- genotype. The beta2-/- and the beta4-/- single-mutant mice grow to adulthood with no visible phenotypic abnormalities. The beta2-/-beta4-/- double mutants survive to birth but have impaired growth and increased perinatal mortality. They also present enlarged bladders with dribbling urination and develop urinary infection and bladder stones. The ocular pupils are widely dilated and do not constrict in response to light. Histological studies revealed no significant abnormalities of brain or peripheral tissues except for hyperplasia in the bladder mucosa of beta4-/- and beta2-/-beta4-/- mutants. Bladder strips from beta2-/-beta4-/- mice did not respond to nicotine but contracted when stimulated with a muscarinic agonist or electric field stimulation. Bladder strips from beta4 mutants did not respond to nicotine despite the absence of major bladder dysfunction in vivo. Acetylcholine-activated whole-cell currents were absent in superior cervical ganglion neurons from beta2-/-beta4-/- mice and reduced in neurons from beta4-/- mice. Although there is apparent redundancy and a superficially normal phenotype in beta2-/- and beta4-/- mice, physiological studies indicate major deficits in the beta4-/- mice. Our previous description of a similar phenotype in alpha3-/- mice and the current data suggest that the alpha3 and the beta4 subunits are major components in autonomic nAChRs. The phenotype of the beta2-/-beta4-/- and alpha3-/- mice resembles the autosomal recessive megacystis-microcolon-hypoperistalsis syndrome in humans.

Fast excitatory nicotinic transmission in the chick lateral spiriform nucleus

The lateral spiriform nucleus (SpL) in the chick mesencephalon contains functional nicotinic receptors and receives a cholinergic fiber projection. We now use double-label immunohistochemistry to demonstrate that choline acetyltransferase-immunopositive fibers in the SpL and in the cholinergic fiber tract lateral to the nucleus are associated with fibers expressing the alpha5 and/or alpha3 nicotinic receptor subunits as determined by mAb35 immunoreactivity. This morphological evidence suggests that there might be synapses between the cholinergic fibers and the dendrites of SpL neurons. Whole-cell recordings from SpL neurons in current-clamp mode revealed EPSPs evoked by stimulation of the cholinergic fiber tract lateral to the SpL. These EPSPs increased in amplitude in the presence of bicuculline. Further addition of the nicotinic antagonist dihydro-beta-erythroidine (DHbetaE) to the buffer significantly attenuated them. Almost all of the remaining EPSP was blocked by 6,7-dinitroquinoxaline-2,3-dione. In the presence of an antagonist cocktail that isolated the nicotinic responses, a fast, monosynaptic nicotinic EPSP or EPSC was evoked. In some neurons, the nicotinic EPSP resulted in the generation of an action potential. The nicotinic nature of the evoked response was confirmed by blockade of the EPSPs or EPSCs with nicotinic antagonists, including DHbetaE, D-tubocurare, and mecamylamine. The nicotinic response was insensitive to low concentrations (10-100 nM) of methyllycaconitine, indicating that typical alpha7-containing receptors were not involved. The results demonstrate that endogenously released acetylcholine generates EPSPs that can elicit action potentials by acting at postsynaptic nicotinic receptors on SpL neurons.

Functional alpha6-containing nicotinic receptors are present in chick retina

Despite the fact that the neuronal chick alpha6 subunit was first cloned several years ago and recently has been shown to form acetylcholine (ACh)-activated channels in heterologous systems, no information is yet available concerning the structure and function of the alpha6-containing nicotinic receptors in neuronal tissues. Using subunit-specific antibodies directed against two different epitopes of the chick alpha6 subunit, we performed immunoprecipitation experiments on immunopurified alpha6-containing receptors radiolabeled with the nicotinic agonist [3H]epibatidine (Epi): almost all of the alpha6 receptors contained the beta4 subunit, 51% the beta3 subunit, 42% the alpha3 subunit, and 7.5% the beta2 subunit. Western blot analyses of the purified receptors confirmed the presence of the alpha3, beta3, beta2, and beta4 subunits, and the absence of the alpha4, alpha5, and alpha7 subunits. The alpha6-containing receptors bind [3H]Epi (Kd = 35 pM) and a number of other nicotinic agonists with very high affinity, the rank order being Epi >> cytisine > nicotine > 1, 1-dimethyl-4-phenylpiperazinium > acetylcholine > carbamylcholine. The alpha6 receptors also have a distinct antagonist pharmacological profile with a rank order of potency of alpha-conotoxin MII > methyllycaconitine > dihydro-beta-erythroydine > MG624 > d-tubocurarine > decamethonium > hexamethonium. When reconstituted in lipid bilayers, the alpha6-containing receptors form functional cationic channels with a main conductance state of 48 pS. These channels are activated by nicotinic agonists in a dose-dependent manner, and blocked by the nicotinic antagonist d-tubocurarine.

Nicotine blocks ethanol and diazepam impairment of air righting and ethanol impairment of maze performance

Results of our previous research in rats demonstrate the following: (a) Angiotensin II (Ang II) inhibits long term potentiation (LTP) in dentate granule cell-perforant path synapses and that this inhibition can be blocked by losartan, an Ang II AT1 receptor antagonist; (b) both ethanol and diazepam inhibit LTP induction and this inhibition can be blocked by losartan; (c) impairment of air righting by ethanol and diazepam (DZ) and eight-arm radial maze performance by ethanol can be blocked by pretreatment with losartan: (d) inhibition of dentate granule cell LTP by Ang II can also be prevented by pretreatment with nicotine. Therefore, it seemed reasonable to hypothesize that ethanol and diazepam impairment of air righting and maze performance might also be blocked by pretreatment with nicotine. The purpose of the present study was to determine the effects of nicotine 0.1, 0.2, 0.3, and 0.4 mg/kg subcutaneously (SC) on 2.0 g/kg ethanol per os (PO) and 1.0 and 2.0 mg/kg DZ intraperitoneally (i.p.) induced impairment of air righting; and to determine if the impaired maze performance due to 2.0 g/kg ethanol PO could be prevented by pretreatment with 0.4 mg/kg of nicotine, SC. Results confirm the hypothesis that moderate doses of ethanol, 2.0 g/kg PO, and DZ, 1.0 mg/kg i.p. impair air righting and that the impairment can be prevented by pretreatment with nicotine SC. Nicotine was not effective in blocking the 2.0 mg/kg DZ impairment of air righting. Nicotine, 0.4 mg/kg SC, prevented the impaired maze performance induced by 2.0 g/kg ethanol PO.

Glutamate and GABA release are enhanced by different subtypes of presynaptic nicotinic receptors in the lateral geniculate nucleus

The functional role of nicotinic acetylcholine receptors (nAChRs) in the ventral lateral geniculate nucleus (LGNv) was examined in chick brain slices. Whole-cell patch-clamp recordings of neurons in the LGNv revealed the presence of bicuculline-resistant spontaneous postsynaptic currents (PSCs), which were subsequently blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), an AMPA receptor antagonist. Carbachol and other nicotinic agonists produced marked increases in the frequency of the glutamatergic spontaneous PSCs in the presence of tetrodotoxin, whereas they had little or no effect on current amplitude. The nicotinic receptor antagonist dihydro-beta-erythroidine (DHbetaE) blocked the carbachol-induced enhancement of spontaneous glutamatergic PSCs. alpha-bungarotoxin (alpha-BgTx) selectively blocked the nAChR-mediated enhancement of spontaneous glutamatergic PSCs but did not prevent nAChR-mediated enhancement of spontaneous GABAergic PSCs in the LGNv. Methyllycaconitine and strychnine, other blockers of nAChRs containing the alpha7 subunit, failed to inhibit carbachol's increase of spontaneous glutamatergic and GABAergic PSCs. These results demonstrate that the LGNv neurons receive both glutamatergic and GABAergic inputs and that the release of these transmitters can be modulated by different presynaptic nAChRs. Thus, the regulation of synaptic efficacy in the brain by presynaptic nAChRs can be complex, involving multiple neurotransmitters acting on the same neuron.

Nicotinic receptor subtypes in the developing chick brain: appearance of a species containing the alpha4, beta2, and alpha5 gene products

Increasing evidence suggests nicotinic receptors regulate developmental events in the nervous system. We used [3H]epibatidine and 125I-alpha-bungarotoxin, together with subunit-specific monoclonal antibodies, to distinguish and quantify nicotinic receptor subtypes in developing chick brain. The results show that more than three fourths of the epibatidine-binding receptors at both early and late embryonic stages contain alpha4 and beta2 subunits, representing receptors previously distinguished by high affinity nicotine binding. A fraction of these also contain the alpha5 gene product, which is consistent with studies on transfected cells showing that the alpha4, beta2, and alpha5 gene products coassemble to produce epibatidine-binding receptors. A small portion of the receptors contain alpha3 and beta4 subunits, assembled in part with either alpha4 or beta2 subunits. The most abundant nicotinic receptors, however, at both early and late embryonic stages are those having high affinity for alpha-bungarotoxin rather than epibatidine. Most contain alpha7 subunits, whereas about half contain alpha8 subunits as well. The sharpest developmental increase between embryonic days 8 and 17/18 occurs with receptors containing alpha5 subunits, whereas receptors containing alpha3 or beta4 subunits undergo no specific increase. The three major receptor species (containing alpha4 and beta2 but not alpha5 subunits; alpha7 subunits; or alpha7 and alpha8 subunits) each increase approximately 3-fold during the same period. The results indicate greater receptor complexity than appreciated previously; they provide information about the rules governing subunit assembly in neuronal nicotinic receptors and draw attention to the role of alpha5 subunits in late development.

Effects of the alpha subunit on imidacloprid sensitivity of recombinant nicotinic acetylcholine receptors

1. Imidacloprid is a new insecticide with selective toxicity for insects over vertebrates. Recombinant (alpha4beta2) chicken neuronal nicotinic acetylcholine receptors (AChRs) and a hybrid nicotinic AChR formed by co-expression of a Drosophila melanogaster neuronal alpha subunit (SAD) with the chicken beta2 subunit were heterologously expressed in Xenopus oocytes by nuclear injection of cDNAs. The agonist actions of imidacloprid and other nicotinic AChR ligands ((+)-epibatidine, (-)-nicotine and acetylcholine) were compared on both recombinant nicotinic AChRs by use of two-electrode, voltage-clamp electrophysiology. 2. Imidacloprid alone of the 4 agonists behaved as a partial agonist on the alpha4beta2 receptor; (+)-epibatidine, (-)-nicotine and acetylcholine were all full, or near full, agonists. Imidacloprid was also a partial agonist of the hybrid Drosophila SAD chicken beta2 receptor, as was (-)-nicotine, whereas (+)-epibatidine and acetylcholine were full agonists. 3. The EC50 of imidacloprid was decreased by replacing the chicken alpha4 subunit with the Drosophila SAD alpha subunit. This alpha subunit substitution also resulted in an increase in the EC50 for (+)-epibatidine, (-)-nicotine and acetylcholine. Thus, the Drosophila (SAD) alpha subunit contributes to the greater apparent affinity of imidacloprid for recombinant insect/vertebrate nicotinic AChRs. 4. Imidacloprid acted as a weak antagonist of ACh-mediated responses mediated by SADbeta2 hybrid receptors and as a weak potentiator of ACh responses mediated by alpha4beta2 receptors. This suggests that imidacloprid has complex effects upon these recombinant receptors, determined at least in part by the alpha subunit.

Influence of subunit composition on desensitization of neuronal acetylcholine receptors at low concentrations of nicotine

The influence of alpha and beta subunits on the properties of nicotine-induced activation and desensitization of neuronal nicotinic acetylcholine receptors (nAChRs) expressed in Xenopus oocytes was examined. Receptors containing alpha4 subunits were more sensitive to activation by nicotine than alpha3-containing receptors. At low concentrations of nicotine, nAChRs containing beta2 subunits reached near-maximal desensitization more rapidly than beta4-containing receptors. The concentration of nicotine producing half-maximal desensitization was influenced by the particular alpha subunit expressed; similar to results for activation, alpha4-containing receptors were more sensitive to desensitizing levels of nicotine than alpha3-containing receptors. The alpha subunit also influenced the rate of recovery from desensitization; this rate was approximately inversely proportional to the apparent nicotine affinity for the desensitized state. The homomeric alpha7 receptor showed the lowest sensitivity to nicotine for both activation and desensitization; alpha7 nAChRs also demonstrated the fastest desensitization kinetics. These subunit-dependent properties remained in the presence of external calcium, although subtle, receptor subtype-specific effects on both the apparent affinities for activation and desensitization and the desensitization kinetics were noted. These data imply that the subunit composition of various nAChRs determines the degree to which receptors are desensitized and/or activated by tobacco-related levels of nicotine. The subtype-specific balance between receptor activation and desensitization should be considered important when the cellular and behavioral actions of nicotine are interpreted.

Distribution of the alpha 2, alpha 3, and alpha 5 nicotinic acetylcholine receptor subunits in the chick brain

Nicotinic acetylcholine receptors (nAChRs) are ionotropic receptors comprised of alpha and beta subunits. These receptors are widely distributed in the central nervous system, and previous studies have revealed specific patterns of localization for some nAChR subunits in the vertebrate brain. In the present study we used immunohistochemical methods and monoclonal antibodies to localize the alpha 2, alpha 3, alpha 5 nAChR subunits in the chick mesencephalon and diencephalon. We observed a differential distribution of these three subunits in the chick brain, and showed that the somata and neuropil of many central structures contain the alpha 5 nAChR subunit. The alpha 2 and alpha 3 subunits, on the other hand, exhibited a more restricted distribution than alpha 5 and other subunits previously studies, namely alpha 7, alpha 8 and beta 2. The patterns of distribution of the different nAChR subunits suggest that neurons in many brain structures may contain several subtypes of nAChRs and that in a few regions one particular subtype may determine the cholinergic nicotinic responses.

Contribution of CNS nicotine metabolites to the neuropharmacological effects of nicotine and tobacco smoking

Nicotine, the principal alkaloid in tobacco products, is generally accepted to be the active pharmacological agent responsible for CNS effects resulting from tobacco use. Arguments are presented in this commentary which take issue with this popular dogma, by providing evidence that nicotine metabolites may also be responsible for the CNS effects commonly attributed to nicotine. CNS effects attributed to nicotine include reinforcing effects, mood elevation, arousal, locomotor stimulant effects, and learning and memory enhancement. The reinforcing and locomotor stimulant effects of nicotine have been suggested to be the result of activation of CNS dopaminergic systems, and nicotine-induced modulation of dopaminergic neurotransmission has been studied in detail. Nicotine acts at a family of nicotinic receptor subtypes composed of multiple subunits; however, the exact composition of the subunits in native nicotinic receptors and the functional significance of the receptor subtype diversity are currently unknown. This nicotinic subtype diversity increases the complexity of the potential mechanisms of action of nicotine and its metabolites. Although peripheral metabolism of nicotine has been studied extensively, metabolism in the CNS has not been investigated to any great extent. Recently, studies from our laboratory have demonstrated that several nicotine metabolites are present in the CNS after acute nicotine administration. Moreover, nicotine metabolites are pharmacologically active in neurochemical and behavioral assays. Thus, CNS effects resulting from nicotine exposure may not be due solely to nicotine, but may result, at least in part, from the actions of nicotine metabolites.

The ventral lateral geniculate nucleus and the intergeniculate leaflet: interrelated structures in the visual and circadian systems

The ventral lateral geniculate nucleus (vLGN) and the intergeniculate leaflet (IGL) are retinorecipient subcortical nuclei. This paper attempts a comprehensive summary of research on these thalamic areas, drawing on anatomical, electrophysiological, and behavioral studies. From the current perspective, the vLGN and IGL appear closely linked, in that they share many neurochemicals, projections, and physiological properties. Neurochemicals commonly reported in the vLGN and IGL are neuropeptide Y, GABA, enkephalin, and nitric oxide synthase (localized in cells) and serotonin, acetylcholine, histamine, dopamine and noradrenalin (localized in fibers). Afferent and efferent connections are also similar, with both areas commonly receiving input from the retina, locus coreuleus, and raphe, having reciprocal connections with superior colliculus, pretectum and hypothalamus, and also showing connections to zona incerta, accessory optic system, pons, the contralateral vLGN/IGL, and other thalamic nuclei. Physiological studies indicate species differences, with spectral-sensitive responses common in some species, and varying populations of motion-sensitive units or units linked to optokinetic stimulation. A high percentage of IGL neurons show light intensity-coding responses. Behavioral studies suggest that the vLGN and IGL play a major role in mediating non-photic phase shifts of circadian rhythms, largely via neuropeptide Y, but may also play a role in photic phase shifts and in photoperiodic responses. The vLGN and IGL may participate in two major functional systems, those controlling visuomotor responses and those controlling circadian rhythms. Future research should be directed toward further integration of these diverse findings.

The molecular biology of neuronal nicotinic acetylcholine receptors

The molecular cloning of genes encoding neuronal nicotinic acetylcholine receptors (nAChRs) has made possible a better understanding of the pharmacology and toxicology of cholinergic compounds. Neuronal nAChRs are related in structure to the nAChRs present at the neuromuscular junction. They are composed of multiple subunits designated either alpha and beta. Eight alpha and three beta subunit genes have been cloned. The alpha subunits contain the ligand binding sites, whereas beta subunits are structural subunits that contribute to the function of the receptor. A large number of nAChRs can be formed from different combinations of alpha and beta subunits. Different combinations of alpha and beta subunits can produce receptors in vitro with distinct ion conducting properties. Each subunit gene is expressed in a distinct pattern in the nervous system. The expression of at least some of the nAChR subunit genes is regulated during development and by cell-cell interactions. Each neuronal nAChR subtype has a distinct pharmacology. Both alpha and beta subunits contribute to the pharmacological properties of each subtype. The expression of multiple nAChR subtypes may allow for precise control of neurotransmission mediated by acetylcholine in diverse populations of neurons.

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)

Presumptive presynaptic nicotinic acetylcholine receptors in the chick tectum: effects of lesions of the lateral spiriform nucleus

There are indications that nicotinic acetylcholine receptor subunits in the superficial layers of the chick tectum (Cajal's layers 1-7) may be transported from the retina. However, nicotinic receptor subunits are detectable by immunohistochemistry in all layers of the optic tectum. In this study, we performed unilateral electrolytic lesions of the lateral spiriform nucleus, which projects to the deep layers of the tectum and contains high amounts of nicotinic receptors in its perikarya. Following lesions of the lateral spiriform nucleus, both the alpha 5 and the beta 2 subunits were markedly depleted in the neuropil of the deep layers of the ipsilateral optic tectum (layers 8-13). No changes were observed in somata that contain either subunit. The present results suggest that most of the nicotinic acetylcholine receptor subunits in the chick optic tectum occur in axonal systems and could then constitute presynaptic receptors.

Differential regulation of neuronal nicotinic acetylcholine receptor subunit gene promoters by Brn-3 POU family transcription factors

The regulatory region of the neuronal nicotinic acetylcholine (nACh) receptor alpha 2 subunit gene is activated by the Brn-3b POU family transcription factor but not by the closely related factors Brn-3a and Brn-3c. This pattern of regulation has not previously been observed for other neuronally expressed genes, several of which, such as those encoding alpha-internexin or SNAP-25, are activated by Brn-3a and Brn-3c but repressed by Brn-3b. The alpha 3 nACh receptor subunit gene is also shown to be activated by Brn-3a but is repressed by Brn-3b and Brn-3c. In contrast, the Brn-3 POU family transcription factors have no effects on either the alpha 7 or beta 4 nACh receptor subunit genes. The actions of Brn-3b on the alpha 2 subunit are thus in contrast to the inhibitory actions of Brn-3b on several promoters that are activated by Brn-3 alpha. The different actions of the Brn-3 POU factors on the range of nACh receptor genes tested suggests that the novel stimulation of the alpha 2 subunit by Brn-3b is specific to this subunit and not a general feature of nACh receptor genes.

Single-channel recording in brain slices reveals heterogeneity of nicotinic receptors on individual neurons within the chick lateral spiriform nucleus

We examined the functional properties of central nicotinic acetylcholine receptors at the single-channel level using tight-seal, voltage-clamp techniques. Single-channel currents were recorded from cell-attached patches on lateral spiriform neurons in chick brain slices. These neurons are known to express functional nicotinic receptors that are insensitive to the antagonists alpha-bungarotoxin and kappa-bungarotoxin, and which exhibit a high affinity for nicotine and other nicotinic agonists. Single-channel openings were observed in 84% of patches (n = 118) when the nicotinic agonists acetylcholine (1-100 microM), carbamylcholine (3-100 microM), or nicotine (3-10 microM) were present in the patch pipette. In contrast, single-channels were markedly reduced in number or entirely absent when the nicotinic antagonist dihydro-beta-erythroidine was present along with acetylcholine (n = 7) or when no agonist was present in the pipette (n = 22). Single-channel openings displayed inward rectification at depolarized potentials, and were dependent on extracellular sodium. Between 1 and 30 microM acetylcholine, a dose-response relationship was observed between agonist concentration and single-channel open probability during the first minute following seal formation. Multiple classes of single nicotinic channels, with calculated mean slope conductances of 15, 31, 40, and approximately 70 pS, were observed in membrane patches on different neurons within the lateral spiriform nucleus, and even within single patches on individual neurons. We conclude that neurons within the lateral spiriform nucleus express functionally heterogeneous nicotinic receptors and that in some neurons different nicotinic receptor subtypes are present in close proximity to each other on the same cell surface.

Nicotinic acetylcholine receptors in separate brain regions exhibit different affinities for methyllycaconitine

The family of nicotinic acetylcholine receptors contains numerous subtypes. Since the subunit compositions of most native neuronal nicotinic receptors are unknown, an important method for distinguishing subtypes of functional neuronal receptors is based on pharmacological criteria, such as affinity for snake toxins. We have now examined the affinities of native chick nicotinic receptors for methyllycaconitine, a toxin purified from Delphinium. We find that methyllycaconitine is a potent antagonist at central nicotinic receptors located on Edinger-Westphal neurons, producing nearly complete functional blockade of nicotinic responses at 10 nM. In marked contrast, methyllycaconitine is 1000-fold less potent at blocking nicotinic responses in the lateral spiriform nucleus. Methyllycaconitine inhibits kappa-bungarotoxin-sensitive nicotinic receptors in ciliary ganglia at 0.5-1.0 microM. Radioligand binding studies also reveal heterogeneity in the affinity of the toxin for nicotinic receptors. Methyllycaconitine binds most avidly to [125I] alpha-bungarotoxin sites in brain (Ki = 5.4 nM), and is 200-fold less potent at muscle nicotinic receptors (IC50 = 1.1 microM). The least potent binding of the toxin is to [3H]nicotine sites in brain (Ki = 3.7 microM). Methyllycaconitine is thus a useful pharmacological tool for distinguishing certain subtypes of native nicotinic receptors. The relatively low affinity of the toxin for nicotinic receptors in the lateral spiriform nucleus is consistent with the known properties of these receptors, which include a high affinity for [3H]nicotine and a lack of sensitivity to alpha- and kappa-bungarotoxin. On the basis of high affinity for methyllycaconitine and insensitivity to alpha-bungarotoxin, the nicotinic receptors in the Edinger-Westphal nucleus are unlike any previously described nicotinic receptor subtype.

Promoter elements conferring neuron-specific expression of the beta 2-subunit of the neuronal nicotinic acetylcholine receptor studied in vitro and in transgenic mice

Several genes encoding subunits of the neuronal nicotinic acetylcholine receptors have been cloned and regulatory elements involved in the transcription of the alpha 2 and alpha 7-subunit genes have been described. Yet, the detailed mechanisms governing the neuron-specific transcription and the spatio-temporal expression pattern of these genes remain largely uninvestigated. The beta 2-subunit is the most widely expressed neuronal nicotinic receptor subunit in the nervous system. We have studied the structural and regulatory properties of the 5' sequence of this gene. A fragment of 1163 bp of upstream sequence is sufficient to drive the cell-specific transcription of a reporter gene in both transient transfection assays and in transgenic mice. Deletion analysis and site-directed mutagenesis of this promoter reveal two negative elements and one positive element. The positively-acting sequence includes one functional E-box. One of the repressor elements is located in the transcribed region and is the NRSE/RE1 sequence already described in promoters of neuronal genes. In this paper, we describe the neuron-specific promoter of the gene encoding the neuronal nicotinic acetylcholine receptor beta 2-subunit.

A novel regulatory element of a nicotinic acetylcholine receptor gene interacts with a DNA binding activity enriched in rat brain

Nicotinic acetylcholine receptors are ligand-gated ion channels that play a critical role in signal transmission in the nervous system. The genes encoding the various subunits that comprise functional acetylcholine receptors are expressed in distinct temporal and spatial patterns. Studies to understand the molecular mechanisms underlying the differential expression of the receptor subunit genes have led to the identification, in this report, of a 19-base pair cis-acting element that is required for transcriptional activation of the rat beta 4 subunit gene. Screening of computer data bases with the 19-base pair element revealed the sequence to be unique among known transcriptional regulatory elements. Loss of this element resulted in drastically reduced beta 4 promoter activity in transfected cholinergic SN17 cells. Furthermore, this element specifically interacts with nuclear proteins prepared from both SN17 cells and adult rat brain. UV cross-linking experiments indicated the presence, in SN17 nuclear extracts, of a prominent protein species (approximately 50 kDa) that interacts specifically with the 19-base pair element. These results lead us to hypothesize that interactions between the 50-kDa protein and the novel 19-base pair element are necessary for transcriptional activation of the beta 4 subunit gene.

Characterization of the nicotinic acetylcholine receptor beta 3 gene. Its regulation within the avian nervous system is effected by a promoter 143 base pairs in length

Genomic and cDNA clones encoding the chicken neuronal nicotinic acetylcholine receptor beta 3 subunit were isolated and sequenced. The beta 3 gene consists of six protein-encoding exons and the deduced protein has the structural features found in all other members of the neuronal nicotinic acetylcholine receptor subunit family. Although they are undetectable in most brain compartments, beta 3 mRNAs are relatively abundant in the developing retina and in the trigeminal ganglion. In situ hybridization and immunohistochemical analysis demonstrated that in retina, beta 3 transcripts and protein are confined to subpopulations of cells in the inner nuclear and ganglion cell layers. Beta 3 is expressed in the proximal and distal regions of the developing trigeminal ganglion, i.e. in both placode- and neural crest-derived neurons. Transient transfection assays in cells freshly dissociated from selected regions of the central nervous system at different developmental stages allowed the identification of genetic elements involved in the neuronal-selective expression of the beta 3 gene. A promoter fragment 143 base pairs in length and containing TATA, CAAT, and other consensus sequences is sufficient to restrict reporter gene expression to a subpopulation of retinal neurons. This promoter is totally inactive upon transfection into neuronal and non-neuronal cells from other regions of the central nervous system.

Effects of retinal lesions upon the distribution of nicotinic acetylcholine receptor subunits in the chick visual system

Immunohistochemistry was used in this study to evaluate the effects of retinal lesions upon the distribution of neuronal nicotinic acetylcholine receptor subunits in the chick visual system. Following unilateral retinal lesions, the neuropil staining with an antibody against the beta 2 receptor subunit, a major component of alpha-bungarotoxin-insensitive nicotinic receptors, was dramatically reduced or completely eliminated in all of the contralateral retinorecipient structures. On the other hand, neuropil staining with antibodies against two alpha-bungarotoxin-sensitive receptor subunits, alpha 7 and alpha 8, was only slightly affected after retinal lesions. Decreased neuropil staining for alpha 7-like immunoreactivity was only observed in the nucleus of the basal optic root and layers 2-4 and 7 of the optic tectum. For alpha 8-like immunoreactivity, slight reduction of neuropil staining could be observed in the ventral geniculate complex, griseum tecti, nucleus lateralis anterior, nucleus lentiformis mesencephali, layers 4 and 7 of the tectum, and nucleus suprachiasmaticus. Taken together with previous data on the localization of nicotinic receptors in the retina, the present results indicate that the beta 2 subunit is transported from retinal ganglion cells to their central targets, whereas the alpha 7 and alpha 8 subunit immunoreactivity appears to have a central origin. The source of these immunoreactivities could be, at least in part, the stained perikarya that were observed to contain alpha 7 and alpha 8 subunits in all retinorecipient areas. In agreement with this hypothesis, the beta 2 subunit of the nicotinic acetylcholine receptors was not frequently found in perikarya of the same areas.

Phenotypic characterisation of rat striatal neurones in primary culture

The aim of this study was to determine to what extent the neuronal phenotypes present in primary cultures of rat striatal neurones correspond to those present in vivo. A large percentage of cultured striatal neurones contained relatively high levels of proenkephalin mRNA. In addition, a high level of expression was found for the prosomatostatin mRNA. Protachykinin mRNA and proneuropeptide Y mRNA were also expressed, but at a comparatively low level. No prodynorphin mRNA could be detected. Considerable numbers of neurones were also found to express NADPH-diaphorase activity, while a smaller number of neurones were positive for acetylcholinesterase. The NADPH-diaphorase and the acetylcholinesterase could be detected both in cell bodies, and in neuronal processes contacting groups of neighbouring neurones. Since nitric oxide does not require synaptic specialisations to exert its intercellular actions, this provides strong evidence that NADPH-positive neurones communicate with other cells in primary culture. These observations demonstrate that when striatal neurones are grown in primary culture, a range of neurochemical phenotypes are present which correspond closely to those present in the mature striatum in vivo. Together with the evidence for cell-cell interactions, this suggests that primary striatal cultures will provide a suitable model to study the molecular mechanisms controlling striatal function.

Developmental regulation of multiple nicotinic AChR channel subtypes in embryonic chick habenula neurons: contributions of both the alpha 2 and alpha 4 subunit genes

Habenula neurons from both early and late stage embryonic chickens express multiple subtypes of nicotinic acetylcholine receptor channels (nAChRs). The channel subtypes expressed by habenula neurons are similar in functional properties, but apparently distinct in subunit composition, from their peripheral counterparts in autonomic ganglia. Early in development, nicotine activates four classes of neuronal bungarotoxin (nBGT)-sensitive channels (approx. conductance = 15, 30, 50, 60pS) that are intermingled on the surface of habenula neuronal somata. In neurons removed from older animals, nAChR channel activity has increased 4- to 40-fold and channel subtypes have become spatially segregated from one another. Analysis of the profile of nAChR subunit gene expression by polymerase chain reaction indicates that several of the alpha-type subunit genes, including alpha 2,3,4,5,7, and alpha 8, as well as both beta 2 and beta 4, are expressed. Treatment of the neurons with subunit specific antisense oligonucleotides reveals that the alpha 2 and alpha 4 (but not alpha 3) subunits contribute to the functional profile of native nAChRs expressed by habenula neurons. Consideration of the functional properties and apparent subunit composition of autonomic ganglion nAChRs in the chick suggests that habenula neurons may utilize a very distinct set of subunit combinations to produce an array of nAChR channel subtypes similar in both conductance and pharmacological profile to those expressed by sympathetic neurons.

Distribution of nicotinic receptors in the human hippocampus and thalamus

Neuronal nicotinic acetylcholine receptors consist of different subunits, alpha and beta, with different subtype arrangement corresponding to distinct pharmacological and functional properties. The expression of alpha 3, alpha 7 and beta 2 mRNA in the human brain was studied by in situ hybridization and compared to [3H]nicotine, [3H]cytisine and [125I]alpha-bungarotoxin binding in contiguous sections. The beta 2 probe showed a strong hybridization signal in the granular layer of the dentate gyrus and in the CA2/CA3 region of the hippocampus and in the insular cortex, and a signal of lower intensity in the subicular complex and entorhinal cortex. The alpha 3 probe showed strong hybridization in the dorsomedial, lateral posterior, ventroposteromedial and reticular nuclei of the thalamus, and a weak signal in the hippocampal region and in the entorhinal, insular and cingular cortex. The amount of alpha 7 mRNA was high at the level of the dentate granular layer and the CA2/CA3 region of the hippocampus, in the caudate nucleus and in the pulvinar and ventroposterolateral nuclei of the thalamus. [3H]Nicotine and [3H]cytisine binding appeared to be identical in anatomical distribution and relative intensity. It was high in the thalamic nuclei, the putamen and in the hippocampal formation in the subicular complex and the stratum lacunosum moleculare. The level of [125I]alpha-bungarotoxin binding was particularly high in the hippocampus and in the pyramidal cells of the CA1 region, but was relatively low in the subicular complex. Our data indicate that in the human brain nicotinic receptor subtypes have discrete distributions, which are in part different from those of other species.

Nicotinic acetylcholine receptors in the ground squirrel retina: localization of the beta 4 subunit by immunohistochemistry and in situ hybridization

Immunohistochemical and in situ hybridization techniques were used to localize the beta 4 subunit of the neuronal nicotinic acetylcholine receptors (nAChRs) in the ground squirrel retina. The beta 4 nAChR subunit was detected in both transverse and horizontal sections of the retina using a subunit-specific antiserum and the avidin-biotin complex technique. Two bands of labeled processes were seen in the inner plexiform layer, corresponding approximately to the laminae where the cholinergic cells arborize. Labeled cells were found in the ganglion cell layer and the inner third of the inner nuclear layer. The cells in the ganglion cell layer were medium- to large-sized and were frequently observed to give rise to axon-like processes. Most of the labeled neurons in the inner nuclear layer were small presumptive amacrine cells, but a few medium-to-large cells were also labeled. These could constitute a different class of amacrine cells or displaced ganglion cells. The latter possibility is supported by the existence of nAChR-containing displaced ganglion cells in the avian retina. In situ hybridization with a 35S-labeled cRNA probe revealed the expression of mRNA coding for the nAChR beta 4 subunit in the ganglion cell layer and the inner third of the inner nuclear layer. This finding confirmed the immunohistochemical data of the cellular localization of beta 4 nAChR subunit. These results indicate that the beta 4 nAChR subunit is expressed by specific subtypes of neurons on the ground squirrel retina.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuronal expression of protease-nexin 1 mRNA in rat brain

Protease-nexin 1 (PN1), also known as glia-derived nexin, is a protease inhibitor secreted by cultured fibroblasts and glioma cells, with postulated roles in regeneration and the regulation of neurite outgrowth. In this study we have localised the sites of PN1 gene expression in rat brain using in situ hybridisation. As expected, cultured cortical astrocytes contained relatively high levels of PN1 mRNA. However, the mRNA localisation in rat brain suggested that the primary sites of synthesis in the CNS are neuronal. Relatively high levels of PN1 mRNA were found in the olfactory nerve layer of the olfactory bulb, in layer V of the cerebral cortex, in magnocellular neurones of the basal forebrain, and in scattered neurones of the striatum. The results show that PN1 gene expression occurs in discrete populations of neurones in the brain, and suggest that these neurones may therefore play a role in the local regulation of neurite outgrowth.

Nicotinic receptor activation facilitates GABAergic neurotransmission in the avian lateral spiriform nucleus

Whole-cell patch clamp recordings were performed in embryonic chick brain slices to characterize responses to nicotinic receptor activation in the mesencephalic lateral spiriform nucleus. Using intracellular recording, we previously reported the presence of functional high-affinity nicotinic sites in this nucleus that are insensitive to blockade with kappa- and alpha-bungarotoxin. We now report that nicotinic agonists not only produce an inward current in these cells, but also elicit a massive increase in the frequency of spontaneous postsynaptic currents without changing the amplitude distribution or risetime and decay kinetics of these events. The nicotinic receptor antagonist, dihydro-beta-erythroidine, blocks both the postsynaptic inward current and the enhancement of spontaneous postsynaptic currents. The spontaneous currents reverse at or near the chloride ion equilibrium potential and are completely blocked by 10 microM bicuculline, indicating that these events are likely to be GABAergic inhibitory postsynaptic currents. The nicotinic agonist-induced enhancement in inhibitory postsynaptic current frequency is blocked by 1.0 microM tetrodotoxin, demonstrating that the effect is mediated through the activation of voltage-dependent sodium channels. Nicotinic receptors are widely distributed in the central nervous system and in some cases are thought to modulate the release of various neurotransmitters. Our results show that activation of nicotinic receptors facilitates inhibitory neurotransmission in the avian lateral spiriform nucleus by increasing the frequency of spontaneous GABAergic postsynaptic currents. These data support a role for nicotinic receptors in the regulation of GABA release from nerve terminals in this nucleus.

Phosphorylation of rat brain nicotinic acetylcholine receptor by cAMP-dependent protein kinase in vitro

The participation of protein kinases in phosphorylation of nicotinic acetylcholine receptor (nAChR) in electric organ and muscle has been precisely investigated in vitro and in vivo whereas phosphorylation of neuronal nAChR is not yet fully characterized. Here, we first report the in vitro phosphorylation of brain nAChR. nAChR purified from rat brains was phosphorylated in vitro by cAMP-dependent protein kinase (PKA), immunoprecipitated with monoclonal antibody against the receptor, and subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) followed by autoradiography. PKA specifically phosphorylated nAChR on the alpha 4 subunits, and H8, an inhibitor of PKA, inhibited completely the phosphorylation. Under the conditions used, a maximal stoichiometry of the phosphorylation by PKA was near to 1 mol of phosphate/mol of the alpha 4 subunits. The 32P-labeled subunits were digested with S. aureas V8 protease followed by SDS-PAGE autoradiography and the resultant phosphopeptide maps revealed three distinct phosphopeptide bands, one major band and two minor bands. Phosphoamino acid analysis of the 32P-labeled alpha 4 subunits showed that serine residues were exclusively phosphorylated. Based on these results, participation of PKA in the regulation of neuronal nAChR is discussed.

Control of receptor sensitivity at the mRNA level

Neurons are able to adjust the sensitivity of receptor-mediated processes according to the level of receptor activation. Extrapolating from our knowledge of other cellular proteins, regulation of receptor mRNA availability would provide a highly economical means of achieving this objective. Epidermal growth factor is able to induce long-lasting increases in its receptor binding by increasing receptor mRNA levels, and similar effects have been shown for other growth factors. Studies on G-protein-coupled receptors, in particular using adrenoceptor clones transfected into cultured cell lines, have shown that changes in receptor number are generally associated with an alteration in receptor mRNA content. At the neuromuscular junction, dramatic increases in nicotinic acetylcholine receptor number are achieved by activating receptor subunit gene transcription. Less information is available concerning the regulation of ligand-gated ion channels in the brain. Overall, the evidence suggests that receptor mRNA levels are frequently controlled by the degree of receptor stimulation. Receptor mRNA levels are therefore likely to be one of the most important control points for both homologous and heterologous regulation of receptor sensitivity.

Localization of NADPH-diaphorase in the brain of the chicken

NADPH-diaphorase, an enzyme catalyzed reaction thought to reflect the activity of nitric oxide synthase in the mammalian nervous system, was mapped in the brain of the chicken. Intensely stained neurons and fibers were found in most parts of the telencephalon, in particular in the neostriatum, paleostriatum augmentatum, olfactory tubercle, lobus parolfactorius, hyperstriatum accessorium, and hyperstriatum ventrale. Medial to the nucleus taeniae, an accumulation of stained cells was observed that appeared to merge with a band of stained neurons located dorsal to the occipitomesencephalic tract. These are considered to belong to the nucleus interstitialis of the dorsal olfactory projection. Further caudally, neurons with different staining intensities were found in the lateral hypothalamic area, lateral mammillary nucleus, periventricular organ, ventral tegmental area, medial spiriform nucleus, optic tectum, isthmooptic nucleus, mesencephalic trigeminal nucleus, interpeduncular nucleus, and central gray of the mesencephalon. A particularly dense cluster of NADPH-diaphorase positive neurons was located in the locus coeruleus. It is proposed that these might represent cholinergic cells intermingled with catecholaminergic neurons, thus forming the avian counterpart of the tegmental cholinergic nuclei of mammals. Several NADPH-diaphorase reactive neurons were seen in the parabrachial nucleus and medial and dorsal vestibular nucleus, as well as scattered in the reticular formation. In the caudal medulla, intensely stained cells were grouped around the central canal. Therefore the pattern of expression of NADPH-diaphorase, and thus possibly of nitric oxide synthase, within the avian and mammalian brain might be largely conserved.

Negative regulatory elements upstream of a novel exon of the neuronal nicotinic acetylcholine receptor alpha 2 subunit gene

The expression of the nicotinic acetylcholine receptor alpha 2 subunit gene is highly restricted to the Spiriform lateralis nucleus of the Chick diencephalon. As a first step toward understanding the molecular mechanism underlying this regulation, we have investigated the structural and regulatory properties of the 5' sequence of this gene. A strategy based on the ligation of an oligonucleotide to the first strand of the cDNA (SLIC) followed by PCR amplification was used. A new exon was found approximately 3kb upstream from the first coding exon, and multiple transcription start sites of the gene were mapped. Analysis of the flanking region shows many consensus sequences for the binding of nuclear proteins, suggesting that the 1 kb flanking region contains at least a portion of the promoter of the gene. We have analysed the negative regulatory elements present within this region and found that a silencer region located between nucleotide -144 and +76 is active in fibroblasts as well as in neurons. This silencer is composed of six tandem repeat Oct-like motifs (CCCCATGCAAT), but does not bind any member of the Oct family. Moreover these motifs were found to act as a silencer only when they were tandemly repeated. When two, four or five motifs were deleted, the silencer activity of the motifs unexpectedly became an enhancer activity in all cells we have tested.

Chicken neuronal acetylcholine receptor alpha 2-subunit gene exhibits neuron-specific expression in the brain and spinal cord of transgenic mice

Transgenic mice carrying the complete structural gene of the alpha 2 subunit of the chicken neuronal nicotinic acetylcholine receptor (nAChR) and 7 kilobase pairs (kbp) of 5' upstream and 3 kbp of 3' downstream sequences have been generated. The transgene was stably integrated in transgenic lines and transmitted to their progeny. Avian transgene expression was predominant in the central nervous system as detected by specific alpha 2-subunit cDNA amplification. Moreover, in at least two independent mouse lines, its expression appeared to be neuron-specific and reproducibly restricted to subregions in the brain and spinal cord, as revealed by in situ hybridization histochemistry. Most cranial motor nuclei were positive, and several of the alpha 2-subunit transgene-expressing structures corresponded to cholinergic areas in rodents. This study reveals that regulatory mechanisms giving rise to neuronal-specific gene expression have been conserved at least in part between birds and mammals.

Molluscan ligand-gated ion-channel receptors

In this chapter we introduce the reader to the structures of the different types of ligand-gated ion-channel receptor, and the numerous receptor subtypes that have recently been revealed to exist, in both invertebrate and vertebrate species, by the application of molecular biological methods. We then review some of the data in support of the existence, in molluscs, of such receptor/channel complexes for gamma-aminobutyric acid, glutamate and acetylcholine. Finally, recent results from our laboratory on the cloning and expression of complementary DNAs, from the pond-snail Lymnaea stagnalis, that encode GABA(A) and glutamate receptor subunits will be described.

Distribution of nicotinic receptors in cynomolgus monkey brain and ganglia: localization of alpha 3 subunit mRNA, alpha-bungarotoxin and nicotine binding sites

The distribution of nicotinic receptors in the brain and ganglia of the Cynomolgus monkey was studied by in situ hybridization and receptor autoradiography. A 35S-labeled antisense riboprobe for the mRNA of the alpha 3 subunit of the human nicotinic receptor, [3H]L-nicotine and [125]alpha-bungarotoxin were used as markers. The highest levels of alpha 3-mRNA were observed in the hippocampus, the medial habenula, the lateral geniculate, the granular layer of the cerebellum, as well as in the pineal gland; moderate levels were found in other nuclei of the thalamus and in the deeper layers of the cerebral cortex. High-affinity binding sites for [3H]L-nicotine were observed mainly in the thalamus. The distribution of [125I]alpha-bungarotoxin binding sites was different from that observed for alpha 3-mRNA and [3H]L-nicotine; they were most abundant in a few specific thalamic nuclei, in the medial habenula and in lamina I of the cerebral cortex. The localization of these three markers was also investigated in the sympathetic, parasympathetic and sensory ganglia of the monkey. Intense labeling was observed for alpha 3-mRNA and for [125I]alpha-bungarotoxin in the sympathetic and parasympathetic ganglia, whereas no positive signal was seen in the ganglion of Gasser. [3H]L-nicotine binding was not detected in any of the ganglia examined. High levels of mRNA for the alpha 3 subunit of the nicotinic receptor were also detected in human sympathetic ganglia. Comparison between alpha 3-mRNA distribution and [3H]L-nicotine binding suggests that in the Cynomolgus monkey brain, the alpha 3 subunit may participate in the formation of more than one nicotinic receptor subtype: a high-affinity binding site for [3H]L-nicotine in the thalamus, and other sites with low affinity for nicotine in the medial habenula and cerebral cortex. Both the alpha 3-mRNA and the [125I]alpha-bungarotoxin are highly expressed in the sympathetic ganglia; however, since no information is presently available on the intraneuronal cellular localization, it cannot be established whether or not they are both present at synaptic sites.

Neurons of the chick brain and retina expressing both alpha-bungarotoxin-sensitive and alpha-bungarotoxin-insensitive nicotinic acetylcholine receptors: an immunohistochemical analysis

Immunohistochemical methods were used to study the possible co-localization of two alpha-bungarotoxin-sensitive (alpha 7 and alpha 8) and two alpha-bungarotoxin-insensitive (beta 2 and alpha 3) subunits of the nicotinic acetylcholine receptors in neurons of the chick brain and retina. Several structures contained neurons that were doubly-labeled with antibodies against the alpha 7 subunit and the beta 2 subunit. These structures included, for example, the interpeduncular nucleus, nucleus spiriformis lateralis, optic tectum, pretectal visual nuclei, and the lateral hypothalamus. Double-labeling with antibodies against the alpha 7 and alpha 8 subunits was also seen in several regions, which included the interpeduncular nucleus, visual pretectum, lateral hypothalamus, dorsal thalamus, and the habenular complex. In the retina, many cells in the inner nuclear layer were observed to contain alpha 8 and alpha 3 subunits, whereas neurons in the ganglion cell layer were seen to contain alpha 7 and alpha 8 or, less frequently, alpha 7 and alpha 3 subunits. These results indicate that alpha-bungarotoxin-sensitive and alpha-bungarotoxin-insensitive subunits of the nicotinic receptors are co-expressed by neurons of the chick brain and retina.

Localization of 3H-nicotine, 125I-kappa-bungarotoxin, and 125I-alpha-bungarotoxin binding to nicotinic sites in the chicken forebrain and midbrain

We have previously localized cholinergic cell bodies and fibers within the midbrain of the chicken with choline acetyltransferase immunohistochemistry. In a continuing effort to characterize the central cholinergic system, the present study examines the distribution of various nicotinic acetylcholine receptors in the forebrain and midbrain of the chicken. The binding of 3H-nicotine, 125I-kappa-bungarotoxin, and 125I-alpha-bungarotoxin was localized by film autoradiography in adjacent sections of the adult chicken brain, allowing a comparison of the distribution of different classes of nicotinic binding sites within the brain. Although all three ligands were often co-localized, there were areas that bound 3H-nicotine but not the 125I-neurotoxins, or vice versa. Very high densities of all three ligands were found in the hyperstriatum ventrale; the nucleus geniculatus lateralis, pars ventralis; the griseum tectale; the nucleus dorsolateralis anterior thalami; the nucleus lentiformis mesencephali, pars lateralis and pars medialis; the periventricular organ; and the stratum griseum et fibrosum superficiale, layer f of the optic tectum. The nucleus spiriformis lateralis had the highest levels of 3H-nicotine binding in the chicken brain, but it did not bind either of the two snake neurotoxins. On the other hand, high levels of both 125I-alpha-bungarotoxin and 125I-kappa-bungarotoxin binding were found in the nucleus semilunaris and the nucleus ovoidalis, but these areas contained little or no 3H-nicotine binding. No unique 125I-kappa-bungarotoxin sites, unrecognized by 125I-alpha-bungarotoxin, were identified by the low resolution autoradiography performed in this study. In general, nicotinic receptors were found in areas that have been reported to contain cholinergic cell bodies or fibers. Comparison of our results with the expression of neuronal nicotinic receptor subunits, as determined by in situ hybridization, suggests that many of the high affinity 3H-nicotine sites are localized presynaptically, as, for example, in the retinorecipient nuclei and the nucleus interpeduncularis. The lack of 125I-kappa-bungarotoxin binding in the presence of alpha-bungarotoxin indicates that the chicken brain has only very low levels of a unique kappa-bungarotoxin site. This is in marked contrast to chicken, frog, and rat autonomic ganglia, where a unique kappa-neurotoxin-sensitive receptor has been identified and shown to mediate nicotinic neurotransmission.

Diversity in primary structure and function of neuronal nicotinic acetylcholine receptor channels

Neuronal nicotinic acetylcholine receptors are oligomeric protein complexes whose component subunits are each encoded by a family of homologous genes. The current challenge is to determine the functional contributions of the related subunits to the receptor-linked ion channels they compose and to uncover the physiological impact of the distinct channel classes expressed in vivo. In the past year, new approaches to the analysis of these receptors have yielded important insights into their stoichiometry, pharmacology and functional properties.

Immunohistochemical localization of nicotinic acetylcholine receptor subunits in the mesencephalon and diencephalon of the chick (Gallus gallus)

Monoclonal antibodies against two alpha-bungarotoxin-binding subunits (alpha 7 and alpha 8) of the nicotinic acetylcholine receptors (nAChRs) were used as immunohistochemical probes to map their distribution in the chick diencephalon and mesencephalon. The distribution of the alpha 7 and alpha 8 nAChR subunits was compared to the distribution of immunoreactivity produced by a monoclonal antibody against the beta 2 structural subunit of the nAChRs. Structures that contained high numbers of alpha 7-like immunoreactive (LI) somata included the intergeniculate leaflet, nucleus intercalatus thalami, nucleus ovoidalis, organum paraventricularis, nucleus rotundus, isthmic nuclei, nucleus trochlearis, oculomotor complex, nucleus interstitio-pretecto-subpretectalis, stratum griseum centrale of the optic tectum, and nucleus semilunaris. Neuropil staining for alpha 7-LI was intense in the nucleus dorsomedialis hypothalami, nucleus geniculatus lateralis ventralis, griseum tecti, isthmic nuclei, nucleus lentiformis mesencephali, nucleus of the basal optic root, and stratum griseum et fibrosum superficiale of the tectum. High numbers of alpha 8-LI somata were found in the stratum griseum et fibrosum superficiale of the tectum and the nucleus interstitio-pretecto-subpretectalis, and intense neuropil staining for alpha 8-LI was found in the dorsal thalamus, nucleus geniculatus lateralis ventralis, lateral hypothalamus, griseum et fibrosum superficiale of the tectum. High numbers of beta 2-LI somata were found only in the nucleus spiriformis lateralis, whereas neuropil staining for beta 2-LI was intense in the nucleus geniculatus lateralis ventralis, nucleus suprachiasmaticus, nucleus lateralis anterior, nucleus habenularis lateralis, area pretectalis, griseum tecti, nucleus lentiformis mesencephalis, nucleus externus, and nucleus interpeduncularis, and in the stratum griseum centrale, stratum griseum et fibrosum superficiale, and stratum opticum of the tectum. These results indicate that there are major disparities in the localization of the alpha-bungarotoxin-binding alpha 7 and alpha 8 nAChR subunits and the beta 2 structural nAChR subunit in the chick diencephalon and mesencephalon. These nAChR subunits appear, however, to coexist in several regions of the chick brain.