Neuronal bungarotoxin blocks the nicotinic stimulation of endogenous dopamine release from rat striatum

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

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

Nicotinic receptor-based therapeutics and candidates for smoking cessation

Tobacco dependence is the most preventable cause of death and is a chronic, relapsing disorder in which compulsive tobacco use persists despite known negative health consequences. All currently available cessation agents (nicotine, varenicline and bupropion) have limited efficacy and are associated with high relapse rates, revealing a need for more efficacious, alternative pharmacotherapies. The major alkaloid in tobacco, nicotine, activates nicotinic receptors (nAChRs) which increase brain extracellular dopamine producing nicotine reward leading to addiction. nAChRs are located primarily presynaptically and modulate synaptic activity by regulating neurotransmitter release. Subtype-selective nAChR antagonists that block reward-relevant mesocorticolimbic and nigrostriatal dopamine release induced by nicotine may offer advantages over current therapies. An innovative approach is to provide pharmacotherapies which are antagonists at nAChR subtypes mediating nicotine evoked dopamine release. In addition, providing multiple medications with a wider array of targets and mechanisms should provide more treatment options for individuals who are not responsive to the currently available pharmacotherapies. This review summarizes the currently available smoking cessation therapies and discusses emerging potential therapeutic approaches employing pharmacological agents which act as antagonists at nicotinic receptors.

Nicotine-induced upregulation of nicotinic receptors: underlying mechanisms and relevance to nicotine addiction

A major hurdle in defining the molecular biology of nicotine addiction has been characterizing the different nicotinic acetylcholine receptor (nAChR) subtypes in the brain and how nicotine alters their function. Mounting evidence suggests that the addictive effects of nicotine, like other drugs of abuse, occur through interactions with its receptors in the mesolimbic dopamine system, particularly ventral tegmental area (VTA) neurons, where nicotinic receptors act to modulate the release of dopamine. The molecular identity of the nicotinic receptors responsible for drug seeking behavior, their cellular and subcellular location and the mechanisms by which these receptors initiate and maintain addiction are poorly defined. In this commentary, we review how nicotinic acetylcholine receptors (nAChRs) are upregulated by nicotine exposure, the potential posttranslational events that appear to cause it and how upregulation is linked to nicotine addiction.

Tris-azaaromatic quaternary ammonium salts: Novel templates as antagonists at nicotinic receptors mediating nicotine-evoked dopamine release

A series of tris-azaaromatic quaternary ammonium salts has been synthesized and evaluated for their ability to inhibit neuronal nicotinic acetylcholine receptors (nAChRs) mediating nicotine-evoked [(3)H]dopamine release from superfused rat striatal slices and for inhibition of [(3)H]nicotine and [(3)H]methyllycaconitine binding to whole rat brain membranes. The 3-picolinium compound 1,3,5-tri-{5-[1-(3-picolinium)]-pent-1-ynyl}benzene tribromide (tPy3PiB), 3b, exhibited high potency and selectivity for nAChR subtypes mediating nicotine-evoked [(3)H]dopamine release with an IC(50) of 0.2 nM and I(max) of 67%.

Presynaptic nicotinic receptors and the modulation of transmitter release

Nicotine is increasingly recognized to promote transmitter release in the brain by a direct action on presynaptic terminals. Pharmacological evidence indicates that this action is mediated by nicotinic receptors. From their sensitivity to mecamylamine, neosurugatoxin and neuronal bungarotoxin these presynaptic receptors can be distinguished from alpha-bungarotoxin-sensitive muscle-type nicotinic receptors, and can be correlated with [3H] nicotine binding sites in the brain. The release of many transmitters in different brain regions is susceptible to stimulation by nicotine, but this effect is not ubiquitous. However, lesioning and subcellular fractionation studies suggest that the majority of brain nicotine receptors are located presynaptically, so that a direct influence of nicotine on transmitter release assumes considerable importance. Although the sensitivity of presynaptic receptors is such that they are likely to be partially activated by doses of nicotine obtained by smoking, the desensitization-induced up-regulation of nicotinic binding sites that follows chronic nicotine treatment raises questions about their functional status during tobacco usage. Chronic administration of the agonist (+)anatoxin-a also up-regulated [3H] nicotine binding sites, and led to increased nicotine-evoked transmitter release in vitro. This could have implications for the involvement of these receptors during withdrawal.

Introduction of unsaturation into the N-n-alkyl chain of the nicotinic receptor antagonists, NONI and NDNI: effect on affinity and selectivity

N-n-octylnicotinium iodide (NONI) and N-n-decylnicotinium iodide (NDNI) are selective nicotinic receptor (nAChR) antagonists mediating nicotine-evoked striatal dopamine (DA) release, and inhibiting [3H]nicotine binding, respectively. This study evaluated effects of introducing unsaturation into the N-n-alkyl chains of NONI and NDNI on inhibition of [3H]nicotine and [3H]methyllycaconitine binding (alpha4beta2* and alpha7* nAChRs, respectively), (86)Rb+ efflux and [3H]DA release (agonist or antagonist effects at alpha4beta2* and alpha6beta2*-containing nAChRs, respectively). In the NONI series, introduction of a C3-cis- (NONB3c), C3-trans- (NONB3t), C7-double-bond (NONB7e), or C3-triple-bond (NONB3y) afforded a 4-fold to 250-fold increased affinity for [3H]nicotine binding sites compared with NONI. NONB7e and NONB3y inhibited nicotine-evoked 86Rb+ efflux, indicating alpha4beta2* antagonism. NONI analogs exhibited a 3-fold to 8-fold greater potency inhibiting nicotine-evoked [3H]DA overflow compared with NONI (IC50 = 0.62 microM; Imax = 89%), with no change in Imax, except for NONB3y (Imax = 50%). In the NDNI series, introduction of a C4-cis- (NDNB4c), C4-trans-double-bond (NDNB4t), or C3-triple-bond (NDNB3y) afforded a 4-fold to 80-fold decreased affinity for [3H]nicotine binding sites compared with NDNI, whereas introduction of a C9 double-bond (NDNB9e) did not alter affinity. NDNB3y and NDNB4t inhibited nicotine-evoked 86Rb+ efflux, indicating antagonism at alpha4beta2* nAChRs. Although NDNI had no effect, NDNB4t and NDNB9e potently inhibited nicotine-evoked [3H]DA overflow (IC50 = 0.02-0.14 microM, Imax = 90%), as did NDNB4c (IC50 = 0.08 microM; Imax = 50%), whereas NDNB3y showed no inhibition. None of the analogs had significant affinity for alpha7* nAChRs. Thus, unsaturated NONI analogs had enhanced affinity at alpha4beta2*- and alpha6beta2*-containing nAChRs, however a general reduction of affinity at alpha4beta2* and an uncovering of antagonist effects at alpha6beta2*-containing nAChRs were observed with unsaturated NDNI analogs.

Neurochemical alterations produced by daily nicotine exposure in periadolescent vs. adult male rats

Chronic treatment with nicotine differentially alters behavior in adolescent rats compared to adult rats. It is not known, however, whether the effects of nicotine on the neurochemical pathways with which it interacts differ in adolescents vs. adults. In the current study, the effects of a 7-day treatment with nicotine on nicotinic, dopaminergic, and serotonergic neurochemistry were examined in the caudate putamen and nucleus accumbens in periadolescent vs. adult male rats. Nicotine treatment increased dopamine transporter densities and decreased serotonin transporter densities in periadolescent rats. There was no change in nicotinic acetylcholine receptor densities or dopamine D1 or D2 receptor densities in nicotine-pretreated periadolescent rats. In adult rats pretreated with nicotine, there was an increase in nicotinic acetylcholine densities, but no change in dopamine transporter, dopamine D1 or D2 receptor, or serotonin transporter densities. Overall, these findings show that periadolescent rats have neurochemical adaptations to nicotine different from adult rats. These alterations may explain, at least in part, the differential behavioral effects of chronic nicotine in adult and adolescent male rats.

Subtype-selective nicotinic receptor antagonists: potential as tobacco use cessation agents

N-n-Alkylpicolinium and N,N'-alkyl-bis-picolinium analogues were assessed in nicotinic receptor (nAChR) assays. The most potent and subtype-selective analogue, N,N'-dodecyl-bis-picolinium bromide (bPiDDB), inhibited nAChRs mediating nicotine-evoked [(3)H]dopamine release (IC(50)=5 nM; I(max) of 60%), and did not interact with alpha4beta2* or alpha7* nAChRs. bPiDDB represents the current lead compound for development as a tobacco use cessation agent.

N-n-alkylnicotinium analogs, a novel class of nicotinic receptor antagonists: interaction with alpha4beta2* and alpha7* neuronal nicotinic receptors

The current study demonstrates that N-n-alkylnicotinium analogs with increasing n-alkyl chain lengths from 1 to 12 carbons have varying affinity (Ki = 90 nM-20 microM) for S-(-)-[3H]nicotine binding sites in rat striatal membranes. A linear relationship was observed such that increasing n-alkyl chain length provided increased affinity for the alpha4beta2* nicotinic acetylcholine receptor (nAChR) subtype, with the exception of N-n-octylnicotinium iodide (NONI). The most potent analog was N-n-decylnicotinium iodide (NDNI; Ki = 90 nM). In contrast, none of the analogs in this series exhibited high affinity for the [3H]methyllycaconitine binding site, thus indicating low affinity for the alpha7* nAChR. The C8 analog, NONI, had low affinity for S-(-)-[3H]nicotine binding sites but was a potent inhibitor of S-(-)-nicotine-evoked [3H]dopamine (DA) overflow from superfused striatal slices (IC50 = 0.62 microM), thereby demonstrating selectivity for the nAChR subtype mediating S-(-)-nicotine-evoked [3H]DA overflow (alpha3alpha6beta2* nAChRs). Importantly, the N-n-alkylnicotinium analog with highest affinity for the alpha4beta2* subtype, NDNI, lacked the ability to inhibit S-(-)-nicotine-evoked [3H]DA overflow and, thus, appears to be selective for alpha4beta2* nAChRs. Furthermore, the present study demonstrates that the interaction of these analogs with the alpha4beta2* subtype is via a competitive mechanism. Thus, selectivity for the alpha4beta2* subtype combined with competitive interaction with the S-(-)-nicotine binding site indicates that NDNI is an excellent candidate for studying the structural topography of alpha4beta2* agonist recognition binding sites, for identifying the antagonist pharmacophore on the alpha4beta2* nAChR, and for defining the role of this subtype in physiological function and pathological disease states.

Nicotine and nicotinic receptors; relevance to Parkinson's disease

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

Bupropion inhibits nicotine-evoked [(3)H]overflow from rat striatal slices preloaded with [(3)H]dopamine and from rat hippocampal slices preloaded with [(3)H]norepinephrine

Bupropion, an efficacious antidepressant and smoking cessation agent, inhibits dopamine and norepinephrine transporters (DAT and NET, respectively). Recently, bupropion has been reported to noncompetitively inhibit alpha3beta2, alpha3beta4, and alpha4beta2 nicotinic acetylcholine receptors (nAChRs) expressed in Xenopus oocytes or established cell lines. The present study evaluated bupropion-induced inhibition of native alpha3beta2* and alpha3beta4* nAChRs using functional neurotransmitter release assays, nicotine-evoked [(3)H]overflow from superfused rat striatal slices preloaded with [(3)H]dopamine ([(3)H]DA), and nicotine-evoked [(3)H]overflow from hippocampal slices preloaded with [(3)H]norepinephrine ([(3)H]NE). The mechanism of inhibition was evaluated using Schild analysis. To eliminate the interaction of bupropion with DAT or NET, nomifensine or desipramine, respectively, was included in the superfusion buffer. A high bupropion concentration (100 microM) elicited intrinsic activity in the [(3)H]DA release assay. However, none of the concentrations (1 nM-100 microM) examined evoked [(3)H]NE overflow and, thus, were without intrinsic activity in this assay. Moreover, bupropion inhibited both nicotine-evoked [(3)H]DA overflow (IC(50) = 1.27 microM) and nicotine-evoked [(3)H]NE overflow (IC(50) = 323 nM) at bupropion concentrations well below those eliciting intrinsic activity. Results from Schild analyses suggest that bupropion competitively inhibits nicotine-evoked [(3)H]DA overflow, whereas evidence for receptor reserve was obtained upon assessment of bupropion inhibition of nicotine-evoked [(3)H]NE overflow. Thus, bupropion acts as an antagonist at alpha3beta2* and alpha3beta4* nAChRs in rat striatum and hippocampus, respectively, across the same concentration range that inhibits DAT and NET function. The combination of nAChR and transporter inhibition produced by bupropion may contribute to its clinical efficacy as a smoking cessation agent.

Reboxetine: functional inhibition of monoamine transporters and nicotinic acetylcholine receptors

The present study determined whether repeated administration of the antidepressant and selective norepinephrine (NE) uptake inhibitor reboxetine resulted in an adaptive modification of the function of the NE transporters (NETs), serotonin (5-HT) transporters, or dopamine (DA) transporters. Because antidepressants may be effective tobacco smoking cessation agents and because antidepressants have recently been shown to interact with nicotinic acetylcholine receptors (nAChRs), the interaction of reboxetine with nAChRs was also evaluated. Repeated administration of reboxetine (10 mg/kg i.p., twice daily for 14 days) did not alter the potency or selectivity of reboxetine inhibition of [(3)H]NE, [(3)H]DA, or [(3)H]5-HT uptake into striatal or hippocampal synaptosomes (IC(50) values = 8.5 nM, 89 microM, and 6.9 microM, respectively). In a separate series of experiments, reboxetine did not inhibit (K(i) > 1 microM) [(3)H]methyllycaconitine, [(3)H]cytisine, or [(3)H]epibatidine binding to rat whole brain membranes. However, at concentrations that did not exhibit intrinsic activity, reboxetine potently inhibited (IC(50) value = 7.29 nM) nicotine-evoked [(3)H]NE overflow from superfused hippocampal slices via a noncompetitive mechanism. In the latter experiments, the involvement of NET was eliminated by inclusion of desipramine (10 microM) in the superfusion buffer. Reboxetine also inhibited (IC(50) value = 650 nM) nicotine-evoked (86)Rb(+) efflux at reboxetine concentrations that did not exhibit intrinsic activity in this assay. Thus, in addition to inhibition of NET function, reboxetine inhibits nAChR function, suggesting that it may have potential as a smoking cessation agent.

N-n-alkylnicotinium analogs, a novel class of nicotinic receptor antagonist: inhibition of S(-)-nicotine-evoked [(3)H]dopamine overflow from superfused rat striatal slices

The structure of the S(-)-nicotine molecule was modified via N-n-alkylation of the pyridine-N atom to afford a series of N-n-alkylnicotinium iodide salts with carbon chain lengths varying between C(1) and C(12). The ability of these analogs to evoke [(3)H] overflow and inhibit S(-)-nicotine-evoked [(3)H] overflow from [(3)H]dopamine ([(3)H]DA)-preloaded rat striatal slices was determined. At high concentrations, analogs with chain lengths > or =C(6) evoked [(3)H] overflow. Specifically, N-n-decylnicotinium iodide (NDNI; C(10)) evoked significant [(3)H] overflow at 1 microM, and N-n-dodecylnicotinium iodide (NDDNI; C(12)) at 10 microM, whereas N-n-octylnicotinium iodide (NONI; C(8)), N-n-heptylnicotinium iodide (NHpNI; C(7)), and N-n-hexylnicotinium iodide (C(6)) evoked [(3)H] overflow at 100 microM. Thus, intrinsic activity at these concentrations prohibited assessment of inhibitory activity. The most potent N-n-alkylnicotinium analog to inhibit S(-)-nicotine-evoked [(3)H] overflow was NDDNI, with an IC(50) value of 9 nM. NHpNI, NONI, and N-n-nonylnicotinium iodide (C(9)) also inhibited S(-)-nicotine-evoked [(3)H] overflow with IC(50) values of 0.80, 0.62, and 0.21 microM, respectively. In comparison, the competitive neuronal nicotinic acetylcholine receptor (nAChR) antagonist, dihydro-beta-erythroidine, had an IC(50) of 1.6 microM. A significant correlation of N-n-alkyl chain length with analog-induced inhibition was observed, with the exception of NDNI, which was devoid of inhibitory activity. The mechanism of N-n-alkylnicotinium-induced inhibition of the high-affinity, low-capacity component of S(-)-nicotine-evoked [(3)H] overflow was determined via Schild analysis, using the representative analog, NONI. Linear Schild regression and slope not different from unity suggested that NONI competitively interacts with a single nAChR subtype to inhibit S(-)-nicotine-evoked [(3)H]DA release (K(i) value = 80.2 nM). Thus, modification of the S(-)-nicotine molecule converts this agonist into an antagonist at nAChRs, mediating S(-)-nicotine-evoked DA release in striatum.

Neuronal nicotinic receptor and psychiatric disorders: functional and behavioral effects of nicotine

Both retrospective and prospective clinical studies have demonstrated positive associations of smoking with psychiatric disorders such as schizophrenia, depression and anxiety. Neuronal nicotinic acetylcholine receptors (nAChR) belong to a family of ligand-gated ion channels that are widely distributed in the brain. The pre-synaptically located nAChR, which are composed of alpha3 or alpha4 subunits in combination with beta2 subunit on axon terminals, modulate the multiple transmission release. Several studies indicated which individual nicotinic receptor subtype is responsible for mediating each of the behavioral effects of nicotine. A reduced number of alpha7 nicotinic receptor subtypes in the hippocampus were reported in schizophrenic patients. In addition, it was assumed that nicotine provided useful therapeutic treatment for a variety of cognitive impairments including those found in Alzheimer's disease, schizophrenia and attention deficit hyperactive disorder. Both alpha7 and alpha4beta2 nicotinic receptors in the hippocampus are involved in these phenomena. In the genetic depressive rats, nicotine showed antidepressant-like effects in forced swim models of depression, suggesting the involvement of alpha4beta2 nicotinic receptor in this phenomenon. Thus, it appears likely that pre-synaptic nAChR on monoaminergic fibers are composed of alpha3 or alpha4 subunits in combination with the beta2 subunit, and these subunit compositions mediate dopaminergic and noradrenergic release, and glutamate is mainly controlled by the alpha7 subunit. All these findings suggest that nicotine and other nicotinic drugs warrant further study for possible clinical prescription to psychiatric disorders.

Nornicotine, a nicotine metabolite and tobacco alkaloid: desensitization of nicotinic receptor-stimulated dopamine release from rat striatum

Nornicotine, a major tobacco alkaloid and nicotine metabolite, accumulates in rat brain in pharmacologically relevant concentrations following repeated nicotine administration. Nornicotine-evoked striatal dopamine release is Ca2+-dependent, stereoselective and sensitive to nicotinic receptor antagonists, indicating nicotinic receptor-mediation. The present study determined if S-(-)-nornicotine desensitizes nicotinic receptors and if cross-desensitization to S-(-)-nicotine occurs. S-(-)-Nicotine (10 and 100 nM) diminished [3H]overflow from [3H]dopamine-preloaded rat striatal slices following subsequent superfusion with 10 microM S-(-)-nicotine (46% and 74%, respectively) or 10 microM S-(-)-nornicotine (59% and 81%, respectively). S-(-)-Nornicotine (1 and 10 microM) diminished the response to subsequent superfusion with 10 microM S-(-)-nornicotine (85% and 97%, respectively) or 10 microM S-(-)-nicotine (82% and 88%, respectively). Thus, similar to S-(-)-nicotine, S-(-)-nornicotine desensitizes nicotinic receptors. but with approximately 12-fold lower potency. Cross-desensitization suggests involvement of common nicotinic receptor subtypes. Therefore, S-(-)-nicotine metabolites, such as nornicotine, have neuropharmacologically relevant effects.

Protein kinase C regulation of dopamine transporter initiated by nicotinic receptor activation in slices of rat prefrontal cortex

We previously reported that activation of nicotinic receptors causes an enhancement in amphetamine-stimulated release of dopamine via its transporter from slices of prefrontal cortex, but no such enhancement of release from slices of nucleus accumbens or striatum. The nicotinic receptors mediating the enhancement most likely contain alpha4 and beta2 subunits based upon pharmacological characterization. In this study, we sought to characterize the second messenger systems associated with the nicotine-mediated response. Sodium channel involvement was confirmed by the observation that tetrodotoxin blocked nicotine-mediated enhancement, whereas veratridine or elevated K(+) mimicked the enhancement seen with nicotine. Inclusion of EGTA blocked nicotine-mediated enhancement, suggesting that, even though no exogenous Ca(2+) was added, endogenous stores were required for the enhancement. The enhancement by nicotine was also abolished by the L-type voltage-dependent calcium channel (VDCC) antagonist nitrendipine, but not by the N-type VDCC antagonist omega-conotoxin GVIA. Finally, inhibition of protein kinase C also abolished the nicotine-mediated enhancement of amphetamine-stimulated dopamine release, whereas inhibitors of Ca(2+)/calmodulin kinase II did not. These findings establish that nicotine can exert selective effects on dopamine transporter activity in prefrontal cortex, an area involved in cognition and learning.

Ca(2+) changes induced by different presynaptic nicotinic receptors in separate populations of individual striatal nerve terminals

Presynaptic nicotinic acetylcholine receptors likely play a modulatory role in the nerve terminal. Using laser-scanning confocal microscopy, we have characterized physiological responses obtained on activation of presynaptic nicotinic receptors by measuring calcium changes in individual nerve terminals (synaptosomes) isolated from the rat corpus striatum. Nicotine (500 nM) induced Ca(2+) changes in a subset (10-25%) of synaptosomes. The Ca(2+) responses were dependent on extracellular Ca(2+) and desensitized very slowly (several minutes) on prolonged exposure to agonist. The nicotine-induced Ca(2+) responses were dose-dependent and were completely blocked by dihydro-beta-erythroidine (5 microM), differentially affected by mecamylamine (10 microM) and alpha-conotoxin MII (100 nM), and not affected by alpha-bungarotoxin (500 nM). Immunocytochemical studies using well-characterized monoclonal antibodies revealed the presence of the alpha4 and alpha3/alpha5 nicotinic subunits. The nicotine-induced responses were unaffected by prior depolarization or by a mixture of Ca(2+) channel toxins including omega-conotoxin MVIIC (500 nM), omega-conotoxin GVIA (500 nM) and agatoxin TK (200 nM). Our results indicate that nicotinic receptors present on striatal nerve terminals induce Ca(2+) entry largely without involving voltage-gated Ca(2+) channels, most likely by direct permeation via the receptor channel itself. In addition, at least two subpopulations of presynaptic nicotinic receptors reside on separate terminals in the striatum, suggesting distinct modulatory roles.

Nicotinic receptor-mediated regulation of dopamine transporter activity in rat prefrontal cortex

The objective of this study was to determine whether nicotine could selectively influence dopamine levels in the prefrontal cortex as compared with other dopaminergic areas of brain. Using a superfusion system, we found that nicotine and other agonists at nicotinic acetylcholine receptors enhanced the release of radiolabeled dopamine that was stimulated by 10 microM amphetamine from slices prepared from rat prefrontal cortex. In contrast, nicotine had no effect on amphetamine-stimulated [(3)H]dopamine release from slices of nucleus accumbens nor striatum. Under the conditions used, which included no added calcium to exclude contribution by exocytotic release, nicotine had no effect on basal release of [(3)H]dopamine. The enhancement by nicotine was concentration-dependent, reaching a maximum at 5 microM, and producing less release at higher concentrations. Enhancement by nicotine was fully reversed by 30 microM dihydro-beta-erythroidine, and by 10 microM mecamylamine, but was not affected by alpha-bungarotoxin. The potencies of nicotine, epibatidine, cytisine, and A85380 to enhance amphetamine-stimulated dopamine release, as well as the sensitivity of nicotine enhanced release to antagonists, are consistent with mediation via a high-affinity nicotinic acetylcholine receptor containing alpha 4 and beta 2 subunits, the major species of nicotinic receptor in forebrain. Since low dopaminergic activity in prefrontal cortex is correlated with cognitive deficits in schizophrenia, our findings may help explain why these deficits are improved in schizophrenics by smoking or nicotine administration.

Lobeline inhibits nicotine-evoked [(3)H]dopamine overflow from rat striatal slices and nicotine-evoked (86)Rb(+) efflux from thalamic synaptosomes

The present study evaluated the interaction of lobeline with neuronal nicotinic acetylcholine receptors using two in vitro assays, [(3)H] overflow from [(3)H]dopamine ([(3)H]DA)-preloaded rat striatal slices and (86)Rb(+) efflux from rat thalamic synaptosomes. To assess agonist interactions, the effect of lobeline was determined and compared to S(-)-nicotine. To assess antagonist interactions, the ability of lobeline to inhibit the effect of S(-)-nicotine was determined. Both S(-)-nicotine (0.1-1 microM) and lobeline (>1.0 microM) evoked [(3)H] overflow from superfused [(3)H]DA-preloaded striatal slices. However, lobeline-evoked [(3)H] overflow is mecamylamine-insensitive, indicating that this response is not mediated by nicotinic receptors. Moreover, at concentrations (<1.0 microM) which did not evoke [(3)H] overflow, lobeline inhibited S(-)-nicotine (0.1-10 microM)-evoked [(3)H] overflow, shifting the S(-)-nicotine concentration-response curve to the right. S(-)-Nicotine (30 nM-300 microM) increased (EC(50) value=0.2 microM) (86)Rb(+) efflux from thalamic synaptosomes. In contrast, lobeline (1 nM-10 microM) did not evoke (86)Rb(+) efflux, and the lack of intrinsic activity indicates that lobeline is not an agonist at this nicotinic receptor subtype. Lobeline completely inhibited (IC(50) value=0.7 microM) (86)Rb(+) efflux evoked by 1 microM S(-)-nicotine, a concentration which maximally stimulated (86)Rb(+) efflux. Thus, the results of these in vitro experiments demonstrate that lobeline inhibits the effects of S(-)-nicotine, and suggest that lobeline acts as a nicotinic receptor antagonist.

Modulatory role of presynaptic nicotinic receptors in synaptic and non-synaptic chemical communication in the central nervous system

Neuronal nicotinic acetylcholine receptors (nAChRs) belong to a family of ligand-gated channels closely related to but distinct from the muscle nAChRs. Recent progress in neurochemical and pharmacological methods supports the hypothesis of presynaptically located nAChRs on axon terminals and indicates that the major effect of nAChR is the modulation rather than processing of fast synaptic transmission. Strong neurochemical evidence indicate that the most important function of presynaptic nAChRs in either synaptic or non-synaptic localization is to increase transmitter release initiated by axonal firing, or directly induce Na(+) and Ca(2+) influx followed by a depolarization sufficient to activate local voltage-sensitive Ca(2+) channels, as a result transmitter of vesicular origin will be released. Therefore, it is somewhat expected that nicotine-induced transmitter release of different monoamines including norepinephrine (NE), dopamine (DA), serotonin (5-HT) can be tetrodotoxin (TTX)- and [Ca(2+)](o)-sensitive. However, some of the nAChR agonists at higher concentrations (1, 1-dimethyl-4-phenylpiperazinium (DMPP) and lobeline), besides their effects on presynaptic nAChRs, are able to inhibit the uptake of NE and 5-HT into nerve terminals, thereby their transmitter releasing effects are extended in time and space. The effect on the uptake process is different from classical nicotinic actions, not being sensitive to nAChR antagonism, but can be prevented by selective uptake blockers or reduced temperature. Considering neurochemical, pharmacological and electrophysiological evidence it seems likely that presynaptic nAChRs on monoaminergic fibers are composed of alpha3 or alpha4 subunits in combination with the beta2 subunit. This is supported by the observation that nicotinic agonists have no presynaptic effect on transmitter release in knockout mice lacking the beta2 nAChR subunit gene. The essential brain function lies not only in impulse transmission within a hard-wired neuronal circuitry but also within synaptic and non-synaptic communication subjected to presynaptic modulation. Since the varicose noradrenergic, dopaminergic, serotonergic, glutamatergic and cholinergic axon terminals mainly do not make synaptic contact, but their varicosities are equipped with nAChRs and these non-synaptically localized receptors are of high affinity, it is suggested that nicotine inhaled during smoking might exert its behavioral, psychological, neurological and neuroendocrinological effects via these receptors.

Involvement of alpha6 nicotinic receptor subunit in nicotine-elicited locomotion, demonstrated by in vivo antisense oligonucleotide infusion

Enhanced locomotion in a habituated environment is a well documented effect of nicotine mediated by the mesotelencephalic dopaminergic system. The nicotinic receptor subunit alpha6 is, among other subunits, strongly expressed in the dopaminergic neurons of the mesencephalon. To examine the functional role of this subunit, we inhibited its expression in vivo using antisense oligonucleotides. In vitro treatments of embryonic mesencephalic neuron cultures demonstrated that the alpha6 antisense oligonucleotides caused a marked decrease in the level of alpha6 subunit protein. In vivo, 1 week infusion of alpha6 antisense oligonucleotides by osmotic mini-pump reduced the effect of nicotine on locomotor activity in habituated environment by 70%. These data support the notion that the effects of nicotine on the dopaminergic system involve alpha6 subunit containing nAChRs.

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.

6-hydroxydopamine lesion of rat nigrostriatal dopaminergic neurons differentially affects nicotinic acetylcholine receptor subunit mRNA expression

Nicotinic acetylcholine receptor (nAChR) subunit mRNA expression in the rat substantia nigra (SN) was assayed by semiquantitative RT-PCR following 6-hydroxydopamine (6-OHDA) lesion of nigrostriatal dopaminergic neurons. Six months after unilateral injection of 6-OHDA or saline into the SN, total RNA was isolated from ipsilateral and contralateral tissue samples. RT-PCR amplifications were performed with template titration using primers specific for sequences encoding 1. nAChR alpha 2-alpha 7 and beta 2-beta 4 subunits 2. Glutamic acid decarboxylase 3. Glyceraldehyde 3-phosphate dehydrogenase for normalization of template mass. PCR products specific for alpha 3, alpha 4, alpha 5, alpha 6, alpha 7, beta 2, beta 3, and glutamic acid decarboxylase were detected in the reactions containing SN RNA. This is the first evidence that alpha 7 may be expressed in the SN. alpha 2 and beta 4 PCR products were not detected in SN reactions, although they were observed in hippocampus and thalamus control reactions. A comparison of ipsilateral and contralateral SN RT-PCR reaction products showed substantial decreases in alpha 5, alpha 6, and beta 3 product yields following 6-OHDA, but not sham treatment. Neither the SN of sham-lesioned rats nor the thalamus of 6-OHDA-lesioned rats yielded similar results, indicating that the effects observed in 6-OHDA-treated SN were not caused by local mechanical damage or a nonspecific response, respectively. Effects of 6-OHDA treatment on alpha 3, alpha 4, alpha 7, beta 2, or glutamic acid decarboxylase product yields from SN samples were small or undetectable. The results suggest that alpha 5, alpha 6, and beta 3 subunit-encoding mRNAs are expressed at substantially higher levels in dopaminergic than in nondopaminergic cell bodies in the SN.

Neuronal nicotinic acetylcholine receptor in the hypothalamus: morphological diversity and neuroendocrine regulations

The subcellular localization and functional significance of neuronal nicotinic acetylcholine receptor alpha4-subunits were investigated in the rat hypothalamic supraoptic nucleus. A high level of alpha4 mRNA expression was found in the magnocellular neurons in the supraoptic nucleus. Strong immunoreactitivy for alpha4 in neurons of the supraoptic nucleus was detected in the rough endoplasmic reticulum and cytoplasmic matrix, although it was very weak in the Golgi apparatus, except for the transport vesicles. Immunoreactivity for alpha4 was detected in both the pre-synaptic axon terminals and post-synaptic axon terminals. A high level of signals for vasopressin mRNA was detected in the supraoptic nucleus after the animals were injected s.c. with nicotine. These findings suggest that alpha4-containing subtypes are synthesized in the rough endoplasmic reticulum and transported to the plasma membrane and serve as pre- and post-synaptic nicotinic acetylcholine receptors. Nicotine may up-regulate vasopressin gene expression in the supraoptic nucleus, acting through nicotinic acetylcholine receptors.

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.

Alpha-conotoxin MII blocks nicotine-stimulated dopamine release in rat striatal synaptosomes

Activation of presynaptic nicotinic acetylcholine receptors (nAChRs) can induce the release of neurotransmitters such as dopamine and norepinephrine in the CNS. Accumulating evidence suggests that distinct nAChR subtypes are involved; however, it has been difficult to determine the subunit composition of these receptors, in part because of the lack of a sufficient variety of selective nAChR ligands. We present experimental data that at least two different nAChR complexes are involved in dopamine release, one of which has an alpha3/beta2 subunit interface. The recently discovered peptide alpha-conotoxin MII is a potent and selective inhibitor of rat nAChRs containing an interface formed by alpha3 and beta2 subunits. We used this peptide to examine nicotine-stimulated release of dopamine from rat striatal synaptosomes and of norepinephrine from hippocampal synaptosomes. MII (100 nM) blocks 34-49% of the nicotine-stimulated dopamine release, but not dopamine release evoked by elevated [K+]. Furthermore, two peptides structurally related to alpha-conotoxin MII, namely alpha-conotoxin MI (selective for alpha1beta1gammadelta nAChRs) and alpha-conotoxin ImI (selective for alpha7-containing nAChRs), have no effect on nicotine-stimulated dopamine release. The results indicate that one third to half of the dopamine release in the striatal preparation is mediated by nAChRs with an alpha3/beta2 subunit interface. In contrast, </=10% of nicotine-stimulated release of norepinephrine from hippocampal synaptosomes is modulated by nAChRs with alpha3/beta2 subunit interfaces.

Presynaptic nicotinic ACh receptors

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

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)

Neuronal nicotinic receptor alpha 6 subunit mRNA is selectively concentrated in catecholaminergic nuclei of the rat brain

Although the neuronal nicotinic receptor alpha 6 subunit was cloned several years ago, its functional significance remains to be investigated. Here we describe an in situ hybridization study of the mRNA for this subunit in the adult rat central nervous system using oligonucleotide probes. Specific alpha 6 mRNA labelling was restricted to a few nuclei throughout the brain; it was particularly high in several catecholaminergic nuclei [the locus coeruleus (A6), the ventral tegmental area (A10) and the substantia nigra (A9)] at levels significantly higher than those found for any other known nicotinic receptor subunit mRNA. Labelling for alpha 6 mRNA was also detected at lower levels in the reticular thalamic nucleus, the supramammillary nucleus and the mesencephalic V nucleus. Some cells of the medial habenula (medioventral part) and of the interpeduncular nucleus (central and lateral parts) were also labelled. The distribution of alpha 6 mRNA was compared with the distribution of the other known nicotinic acetylcholine receptor subunit mRNAs. In several nuclei, the expression of alpha 6 was complementary to those of other alpha subunits. Moreover, some of the cell groups (such as the substantia nigra, the ventral tegmental area and the locus coeruleus) previously thought to contain mainly alpha 3 mRNA in fact were found to contain high levels of alpha 6 mRNA. Finally, we found extensive colocalization of alpha 6 and beta 3, indicating the possible existence of nicotinic receptor hetero-oligomers containing both subunits. The present results show that alpha 6 is the major nicotinic acetylcholine receptor alpha subunit expressed in dopaminergic cell groups of the mesencephalon and noradrenergic cells of the locus coeruleus. This suggests the involvement of the alpha 6 subunit in some of the major functions of central nicotinic circuits, including the modulation of locomotor behaviour and reward.

N- and C-terminal substance P fragments modulate striatal dopamine outflow through a cholinergic link mediated by muscarinic receptors

The present study investigated whether the modulatory effects of substance P and substance P fragments on striatal dopamine release involve a cholinergic link. Rat striatal slices were incubated with substance P, substance P(1-4), substance P(1-7), substance P(5-11) and substance P(8-11) in the absence or presence of various agents which modify cholinergic transmissions, and endogenous dopamine outflow was measured using high-performance liquid chromatography. The incubation of striatal slices with substance P and its N- and C-terminal fragments (1 nM) induced a significant overflow of endogenous dopamine. Neostigmine (150 nM) potentiated the effects of substance P and its fragments, whereas the incubation with hemicholinium-3 (50 microM) abolished the effects of the peptides on dopamine outflow. The acetylcholinesterase inhibitor and the inhibitor of choline uptake did not have intrinsic effects on dopamine outflow. The muscarinic antagonist atropine (1 microM) reversed completely the effects of substance P and its fragments, whereas the nicotinic antagonists dihydro-beta-erythroidine (0.5 microM) and pempidine (10 microM) were devoid of effects. None of the cholinergic antagonists modified dopamine outflow. The results suggest that substance P and several N- and C-terminal substance P fragments activate cholinergic neurons in striatal slices. The released acetylcholine induces an increased dopamine outflow, mediated by muscarinic receptors. These observations represent additional evidence which supports the functional interactions between substance P, acetylcholine and dopamine in the striatum. Furthermore, they show that substance P fragments may exert neuromodulatory effects through mechanisms similar to those underlying the effects of the parent peptide.

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

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

alpha-Bungarotoxin, kappa-bungarotoxin, alpha-cobratoxin and erabutoxin-b do not affect [3H]acetylcholine release from the rat isolated left hemidiaphragm

Endplate preparations of the rat left hemidiaphragm were incubated with [3H]choline to label neuronal transmitter stores. Nerve evoked release of newly-synthesized [3H]acetylcholine was measured in the absence of cholinesterase inhibitors to investigate whether snake venom neurotoxins by blocking presynaptic nicotinic autoreceptors affect evoked transmitter release. Contractions of the indirectly stimulated hemidiaphragm were recorded to characterize the blocking effect of alpha-neurotoxins at the post-synaptic nicotinic receptors. Neither the long chain neurotoxins alpha-cobratoxin (1 microgram ml-1) and alpha-bungarotoxin (5 microgram ml-1) nor the short chain neurotoxin erabutoxin-b (0.1, 1 and 10 micrograms ml-1) affected the nerve-evoked release of [3H]acetylcholine. kappa-Bungarotoxin (1 and 5 micrograms ml-1), a toxin preferentially blocking neuronal nicotinic receptors, did also not affect evoked [3H]acetylcholine release, whereas (+)-tubocurarine (1 microM) under identical conditions reduced the release by about 50%. alpha-Bungarotoxin, alpha-cobratoxin and erabutoxin-b concentration-dependently (0.01-0.6 micrograms ml-1) inhibited nerve-evoked contractions of the hemidiaphragm. All neurotoxins except erabutoxin-b enhanced the basal tritium efflux immediately when applied to the endplate preparation or to a non-innervated muscle strip labelled with [3H]choline. This effect was attributed to an enhanced efflux of [3H]phosphorylcholine, whereas the efflux of [3H]choline and [3H]acetylcholine was not affected. It is concluded that the alpha-neurotoxins and kappa-bungarotoxin do not block presynaptic nicotinic receptors of motor nerves. These nicotinic autoreceptors differ from nicotinic receptors localized at the muscle membrane and at autonomic ganglia.

The effect of cholinoceptor agonists and neurotoxins on the release of vasopressin in the rat in relation to the subunit composition of the cholinoceptor

The effects of cholinoceptor agonists and neurotoxins on the release of vasopressin and oxytocin have been investigated in water-loaded rats under ethanol anaesthesia. Release of vasopressin was monitored by antidiuretic responses accompanied by increased urinary excretion of vasopressin. The rate of excretion of oxytocin-like radioimmunoreactivity was measured as an indicator of oxytocin release. Both nicotine and cytisine caused a preferential release of vasopressin. The release by nicotine was not inhibited by alpha- or neuronal-bungarotoxin. Neosurugatoxin blocked the release by cytisine. Comparison with the effects of these agents on combinations of alpha and beta subunits expressed in oocytes suggests that the central cholinoceptors mediating release of vasopressin are similar to those at autonomic ganglia and may contain a beta 4 subunit.

Genetic and environmental aspects of the role of nicotinic receptors in neurodegenerative disorders: emphasis on Alzheimer's disease and Parkinson's disease

As neurodegenerative disorders are better characterized, the importance of genetic and environmental interactions is becoming more evident. Among the neurodegenerative disorders, Alzheimer's disease and Parkinson's disease are both characterized by large losses of nicotinic binding sites in brain. In addition, losses in nicotinic receptors occur during normal aging. Chronic administration of nicotine in man or experimental animals increases the number of nicotinic receptors in brain. Nicotine has been shown to possess some neuroprotective properties for both cholinergic and dopaminergic neurons. These neuroprotective properties, when better understood, may provide important information on normal aging and neurodegenerative disorder related neuronal cell death. Understanding the functional aspects of neuronal nicotinic receptor subtypes may lead to successful therapeutic treatments or disease preventative strategies for neurodegenerative disorders.

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.

Stimulation by nicotine of the spontaneous release of [3H]gamma-aminobutyric acid in the substantia nigra and in the globus pallidus of the rat

The effect of (-)-nicotine on the spontaneous release of [3H]gamma-aminobutyric acid ([3H]GABA) was studied in vitro in rat substantia nigra (SN) and globus pallidus (GP) slices. In both structures, nicotine (10(-4) M) elicited a transient increase of [3H]GABA release lasting no more than 2.5 min. At the peak of the effect, a 18.5% and 25% increase of [3H]GABA was observed in GP and SN slices, respectively. At lower concentration (10(-5) M), nicotine produced a small but significant transient increase (+8%) in GP slices whereas this concentration was ineffective in SN slices. Pempidine (10(-5) M) totally antagonized the 10(-4) M nicotine-induced effect in SN and GP. The increase of [3H]GABA release elicited by 10(-4) M nicotine was abolished when Ca2+ concentration in the superfusion medium was lowered from 2.4 to 0.4 mM. To investigate a possible dopaminergic (DA) link in the response, we examined the sensitivity of the nicotine-induced effect to DA D1 (SCH23390) and D2 (sulpiride) receptor antagonists. In SN, SCH23390 (10(-6) M) abolished the 10(-4) M nicotine-induced effect. In GP, sulpiride (10(-5) M) failed to modify the response. Moreover, SCH23390 partially reversed the nicotine-induced effect (-37%) in GP. Taken together these results indicate that nicotine differentially modulate the [3H]GABA release in SN and GP. In SN, the nicotine-induced [3H]GABA release appears to be mediated by DA neurons. In GP, only a part of the nicotinic response involved a DA link. A possible direct stimulation of nicotinic receptors localized on striato-pallidal terminals is discussed.

An analysis of response to nicotine infusion using an automated radiotelemetry system

Previous studies from our laboratory have demonstrated that chronic nicotine infusion evokes tolerance to nicotine injected IP several hours after withdrawal from chronic infusion. This method may introduce problems related to withdrawal reactions and to stress associated with handling of the animals. The studies reported here measured tolerance to nicotine in mice using an automated radiotelemetry system. DBA/2 mice were infused intravenously with saline for 4 days followed by infusion of a 4 mg/kg per h dose of nicotine for 7 days. After the nicotine treatment, the mice were infused with saline for 7 days. The nicotine was infused continuously or in four 1 mg/kg pulses, two 2 mg/kg pulses or one 4 mg/kg pulse each hour. Home cage activity and body temperature were measured throughout the treatment periods using a radiotelemetry system. Nicotine infusion produced an abrupt decrease in body temperature and activity, but this effect was totally reversed within 12 h in the continuously infused and four infusions/h treatment groups. Mice that received one or two infusions/h also showed a rapid response to nicotine that was reversed as treatment proceeded, but nicotine continued to produce a measurable effect for several days. After nicotine withdrawal, temperature and activity returned to predrug infusion values in all of the groups except those infused once per hour. This group showed depressed activity for a minimum of 3 days after nicotine treatment stopped. Thus, the kinetics of nicotine administration affected the intensity of response during continued treatment as well as activity after cessation of chronic treatment.

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.

Desensitization of nicotine-stimulated [3H]dopamine release from mouse striatal synaptosomes

Potential desensitization of brain nicotinic receptors was studied using a [3H]dopamine release assay. Nicotine-stimulated [3H]dopamine release from mouse striatal synaptosomes was concentration-dependent with an EC50 of 0.33 +/- 0.13 microM and a Hill coefficient of 1.44 +/- 0.18. Desensitization by activating concentrations of nicotine had a similar EC50 and a half-time of 35 s. Concentrations of nicotine that evoked little release also induced a concentration-dependent desensitization (EC50 = 6.9 +/- 3.6 nM, t1/2 = 1.6-2.0 min, nH = 1.02 +/- 0.01). Both types of desensitization produced a maximum 75% decrease in [3H]dopamine release. Recovery from desensitization after exposure to low or activating concentrations of nicotine was time-dependent with half-times of 6.1 min and 12.4 min, respectively. Constants determined for binding of [3H]nicotine to striatal membrane at 22 degrees C included a KD of 3.7 +/- 0.5 nM, Bmax of 67.5 +/- 2.2 fmol/mg, and Hill coefficient of 1.07 +/- 0.06. Association of nicotine with membrane binding sites was biphasic with half-times of 9 s and 1.8 min. The fast rate process contributed 37% of the total reaction. Dissociation was a uniphasic process with a half-time of 1.6 min. Comparison of constants determined by the release and binding assays indicated that the [3H]-nicotine binding site could be the presynaptic receptor involved in [3H]dopamine release in mouse striatal synaptosomes.

Decline in response to nicotine in aged rat striatum: correlation with a decrease in a subpopulation of nicotinic receptors

Specific and reproducible changes involving the cholinergic and dopaminergic systems have been described in both the aging rodent and the human nervous system. Nevertheless, relatively little information is available on changes in nicotinic cholinergic receptors occurring in normal aging, and there have been few attempts to correlate alterations in receptor densities with changes in nicotinic actions. We have utilized the nicotine-mediated stimulation of endogenous dopamine efflux in a striatal slice preparation as a functional index of responsiveness to nicotine in aging. Following incubation with nicotine, this efflux was significantly lower in 25-month-old (aged) as opposed to 4-month-old (young) rats. In contrast, the release of striatal dopamine following a high-potassium stimulus was similar at both ages. Binding studies in young and aged animals did not reveal any significant change with age in the total number of striatal nicotinic receptors recognized by either [3H]nicotine or the neuronal nicotinic antagonist 125I-neuronal bungarotoxin. However, there was a nearly 80% decline in the subpopulation of striatal nicotinic receptors jointly recognized by both nicotine and neuronal bungarotoxin, but not by alpha-bungarotoxin. Quantitative autoradiography demonstrated declines with age in this receptor subtype in several brain regions examined. Decrements in this specific subpopulation of nicotinic receptors or in the nerve cells expressing these receptors may contribute to the functional declines that take place in the aging motor and visual systems.

A role for the basal ganglia in nicotinic modulation of the blink reflex

In humans and rats we found that nicotine transiently modifies the blink reflex. For blinks elicited by stimulation of the supraorbital branch of the trigeminal nerve, nicotine decreased the magnitude of the orbicularis oculi electromyogram (OOemg) and increased the latency of only the long-latency (R2) component. For blinks elicited by electrical stimulation of the cornea, nicotine decreased the magnitude and increased the latency of the single component of OOemg response. Since nicotine modified only one component of the supraorbitally elicited blink reflex, nicotine must act primarily on the central nervous system rather than at the muscle. The effects of nicotine could be caused by direct action on lower brainstem interneurons or indirectly by modulating descending systems impinging on blink interneurons. Since precollicular decerebration eliminated nicotine's effects on the blink reflex, nicotine must act through descending systems. Three lines of evidence suggest that nicotine affects the blink reflex through the basal ganglia by causing dopamine release in the striatum. First, stimulation of the substantia nigra mimicked the effects of nicotine on the blink reflex. Second, haloperidol, a dopamine (D2) receptor antagonist, blocked the effect of nicotine on the blink reflex. Third, apomorphine, a D2 receptor agonist, mimicked the effects of nicotine on the blink reflex.

Characterization of nicotinic receptor-mediated [3H]dopamine release from synaptosomes prepared from mouse striatum

This study establishes that presynaptic nicotinic receptors modulate dopamine release in the mouse striatum. Nicotinic agonists elicit a dose-dependent increase in the release of [3H]dopamine from synaptosomes prepared from mouse striatum. At low concentrations, this release is Ca2+ dependent, whereas at higher concentrations Ca(2+)-independent, mecamylamine-insensitive release was also observed. The Ca(2+)-dependent nicotine-evoked release was not blocked by alpha-bungarotoxin but was effectively blocked by neuronal bungarotoxin as well as several other nicotinic receptor antagonists. The relationship between potency for stimulation of release for agonists and potency for inhibition of release for antagonists was compared to the affinity of these compounds for the [3H]nicotine binding site. The overall correlation between release and binding potency was not high, but the drugs may be classified into separate groups, each of which has a high correlation with binding. This finding suggests either that more than one nicotinic receptor regulates dopamine release or that not all agonists interact with the same receptor in an identical fashion.

Alpha 4-2 beta 2 and other nicotinic acetylcholine receptor subtypes as targets of psychoactive and addictive drugs

1. Xenopus oocytes were injected with various muscle and neuronal nicotinic acetylcholine receptor (ACh receptor, cholinoceptor) subunit RNA combinations and their pharmacological properties studied using two-electrode voltage clamp. The functional expression of one of these combinations, rat alpha 4-2 beta 2, has not been previously described. The alpha 4-2 mRNA is a splicing variant transcribed from the alpha 4 gene. In the experiments reported here, the alpha 4-2 beta 2 subtype was functionally indistinguishable from the alpha 4-1 beta 2 subtype. 2. For each subtype, the relative potency of nicotine compared with acetylcholine was obtained by estimating the relative concentration of nicotine which would elicit the same current response as 0.1 microM Ach. The ratios of these concentrations (nicotine: ACh) for the mouse muscle ACh receptor-(alpha 1 beta 1 gamma delta) was 96.1:1. In contrast, the ratios for the rat neuronal subtypes were: alpha 2 beta 2, 1.01:1; alpha 3 beta 2, 2.01:1; alpha 4 beta 2, 0.76:1 and alpha 4-2 beta 2, 0.76:1. The much greater relative nicotine sensitivity of the neuronal subtypes as compared with muscle receptors illustrates their potential to mediate the psychoactive and addictive effects of nicotine. However, it does not appear that the differences in relative nicotinic sensitivity among the neuronal receptors themselves can be used as a simple discriminative tool in neuronal tissue. 3. The slopes of the log dose-log response curves at low ACh concentrations were all greater than 1 but less than 2, suggesting that at least two agonist binding sites mediate the functional response of each hetero-oligomer. 4. The response of all the neuronal subtypes to ACh could be inhibited by the psychoactive drugs mecamylamine, amitriptyline, phencyclidine, trifluoperazine and promethazine. With the exception of the very potent antagonist, mecamylamine, the degree of block of the peak current to ACh produced by 10 microM concentrations of these drugs was remarkably similar (around 50%). 5. The degree of inhibition produced when the antipsychotic drug, trifluoperazine, was co-applied with ACh increased as the duration of application increased. Such an effect was not observed with promethazine, a related phenothiazine derivative which does not have antipsychotic actions.