Lack of stereoselectivity in ability of nicotine to release dopamine from rat synaptosomal preparations

Central administration of nicotine suppresses tracheobronchial cough in anesthetized cats

We tested the hypothesis that nicotine, which acts peripherally to promote coughing, might inhibit reflex cough at a central site. Nicotine was administered via the vertebral artery [intra-arterial (ia)] to the brain stem circulation and by microinjections into a restricted area of the caudal ventral respiratory column in 33 pentobarbital anesthetized, spontaneously breathing cats. The number of coughs induced by mechanical stimulation of the tracheobronchial airways; amplitudes of the diaphragm, abdominal muscle, and laryngeal muscles EMGs; and several temporal characteristics of cough were analyzed after administration of nicotine and compared with those during control and recovery period. (-)Nicotine (ia) reduced cough number, cough expiratory efforts, blood pressure, and heart rate in a dose-dependent manner. (-)Nicotine did not alter temporal characteristics of the cough motor pattern. Pretreatment with mecamylamine prevented the effect of (-)nicotine on blood pressure and heart rate, but did not block the antitussive action of this drug. (+)Nicotine was less potent than (-)nicotine for inhibition of cough. Microinjections of (-)nicotine into the caudal ventral respiratory column produced similar inhibitory effects on cough as administration of this isomer by the ia route. Mecamylamine microinjected in the region just before nicotine did not significantly reduce the cough suppressant effect of nicotine. Nicotinic acetylcholine receptors significantly modulate functions of brain stem and in particular caudal ventral respiratory column neurons involved in expression of the tracheobronchial cough reflex by a mecamylamine-insensitive mechanism.

In vivo brain imaging of human exposure to nicotine and tobacco

While most cigarette smokers endorse a desire to quit smoking, only 14-49% will achieve abstinence after 6 months or more of treatment. A greater understanding of the effects of smoking on brain function may result in improved pharmacological and behavioral interventions for this condition. Research groups have examined the effects of acute and chronic nicotine/cigarette exposure on brain activity using functional imaging; the purpose of this chapter is to synthesize findings from such studies and present a coherent model of brain function in smokers. Responses to acute administration of nicotine/smoking include reduced global brain activity; activation of the prefrontal cortex, thalamus, and visual system; activation of the thalamus and visual cortex during visual cognitive tasks; and increased dopamine (DA) concentration in the ventral striatum/nucleus accumbens. Responses to chronic nicotine/cigarette exposure include decreased monoamine oxidase (MAO) A and B activity in the basal ganglia and a reduction in alpha4beta2 nicotinic acetylcholine receptor (nAChR) availability in the thalamus and putamen (accompanied by an overall upregulation of these receptors). These findings indicate that smoking enhances neurotransmission through cortico-basal ganglia-thalamic circuits by direct stimulation of nAChRs, indirect stimulation via DA release or MAO inhibition, or a combination of these and possibly other factors. Activation of this circuitry may be responsible for the effects of smoking seen in tobacco-dependent smokers, such as improvements in attentional performance, mood, anxiety, and irritability.

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.

Nicotine-induced phosphorylation of ERK in mouse primary cortical neurons: evidence for involvement of glutamatergic signaling and CaMKII

Extracellular signal-regulated kinase (ERK) is activated in vivo in a number of brain areas by nicotine and other drugs of abuse. Here we show that nicotine stimulation of cultured mouse cortical neurons leads to a robust induction of ERK phosphorylation that is dependent on nicotine concentration and duration of exposure. Calcium/calmodulin-dependent protein kinase II activity is necessary for nicotine-induced ERK phosphorylation and neither cAMP-dependent protein kinase or protein kinase C appear to be involved. Activity of glutamate receptors, L-type voltage-gated calcium channels, and voltage-gated sodium channels are also required for nicotine-induced ERK phosphorylation. Nicotine-induced ERK phosphorylation was inhibited by high concentrations of mecamylamine, however it was not blocked by other broad nicotinic acetylcholine receptor (nAChR) inhibitors (including hexamethonium and chlorisondamine) or nAChR subtype selective inhibitors (such as methyllycaconitine, alpha-bungarotoxin, dihydro-beta-erythroidine, and alpha-conotoxin Au1B). In accord with these pharmacological results, nicotine-induced ERK phosphorylation was normal in primary cultures made from beta2 or alpha7 nAChR subunit knockout mice. The alpha3/beta4 nAChR agonist cytisine did not induce ERK phosphorylation suggesting that alpha3/beta4 nAChRs were not involved in this process. Taken together, these data define a necessary role for glutamatergic signaling and calcium/calmodulin-dependent protein kinase II in nicotine-induced ERK phosphorylation in cortical neurons and do not provide evidence for the involvement of classical nAChRs.

Gene variants of brain dopamine pathways and smoking-induced dopamine release in the ventral caudate/nucleus accumbens

CONTEXT

Preclinical studies demonstrate that nicotine administration leads to dopamine release in the ventral striatum. However, human studies reveal considerable interindividual variability in the extent of smoking-induced dopamine release.

OBJECTIVE

To determine whether common gene variants of the brain dopamine pathway explain this observed phenotypic variability in humans.

DESIGN

Blood samples were drawn to determine gene variants of dopamine system components, and positron emission tomography scanning with the radiotracer raclopride labeled with radioactive carbon (11C) was performed to measure smoking-induced dopamine release.

SETTING

Academic brain imaging center.

PARTICIPANTS

Forty-five tobacco-dependent smokers.

INTERVENTIONS

Subjects either smoked a cigarette (n = 35) or did not smoke (n = 10) during positron emission tomography scanning.

MAIN OUTCOME MEASURES

Gene variants of dopamine system components (the dopamine transporter variable nucleotide tandem repeat, D2 receptor Taq A1/A2, D4 receptor variable nucleotide tandem repeat, and catechol-O-methyltransferase Val158Met polymorphisms) and change in [11C]raclopride binding potential in the ventral caudate/nucleus accumbens on positron emission tomography scans.

RESULTS

For subjects who smoked during scanning, those with at least one 9 allele of the dopamine transporter variable nucleotide tandem repeat, fewer than 7 repeats of the D4 variable nucleotide tandem repeat, and the Val/Val catechol-O-methyltransferase genotype had greater decreases in binding potential (an indirect measure of dopamine release) with smoking than those with the alternate genotypes. An overall decrease in ventral caudate/nucleus accumbens binding potential in those who smoked compared with those who did not smoke was also found but was smaller in magnitude than previously reported.

CONCLUSIONS

Smokers with genes associated with low resting dopamine tone have greater smoking-induced (phasic) dopamine release than those with alternate genotypes. These findings suggest that dopamine system genotype variabilities explain a significant proportion of the interindividual variability in smoking-induced dopamine release and indicate that smoking-induced dopamine release has a genetic predisposition.

Functional brain imaging of tobacco use and dependence

While most cigarette smokers endorse a desire to quit smoking, only about 14% to 49% will achieve abstinence after 6 months or more of treatment. A greater understanding of the effects of smoking on brain function may (in conjunction with other lines of research) result in improved pharmacological (and behavioral) interventions. Many research groups have examined the effects of acute and chronic nicotine/cigarette exposure on brain activity using functional imaging; the purpose of this paper is to synthesize findings from such studies and present a coherent model of brain function in smokers. Responses to acute administration of nicotine/smoking include: a reduction in global brain activity; activation of the prefrontal cortex, thalamus, and visual system; activation of the thalamus and visual cortex during visual cognitive tasks; and increased dopamine (DA) concentration in the ventral striatum/nucleus accumbens. Responses to chronic nicotine/cigarette exposure include decreased monoamine oxidase (MAO) A and B activity in the basal ganglia and a reduction in alpha4beta2 nicotinic acetylcholine receptor (nAChR) availability in the thalamus and putamen. Taken together, these findings indicate that smoking enhances neurotransmission through cortico-basal ganglia-thalamic circuits either by direct stimulation of nAChRs, indirect stimulation via DA release or MAO inhibition, or a combination of these factors. Activation of this circuitry may be responsible for the effects of smoking seen in tobacco dependent subjects, such as improvements in attentional performance, mood, anxiety, and irritability.

Nicotine attenuates place aversion induced by naloxone in single-dose, morphine-treated rats

RATIONALE

Acute physical dependence refers to the withdrawal syndrome precipitated by an opioid antagonist administered several hours after either a single dose or a short-term infusion of an opioid agonist.

OBJECTIVES

We examined the mechanism of nicotine-induced attenuation of naloxone-precipitated withdrawal syndrome when used to produce an aversive motivational state in a place-conditioning paradigm.

METHODS

The effect of nicotine was investigated through place aversion induced by naloxone in morphine-pretreated rats. Additionally, the mechanism of nicotine action in this model was explored specifically in relation to the dopaminergic system through the use of dopamine receptor antagonist and agonist.

RESULTS

Place avoidance behavior was potently elicited by naloxone (0.5 mg/kg s.c.) 24 h after a single exposure to morphine (10 mg/kg s.c.). Avoidance behavior was attenuated by pretreatment with a 0.2-mg/kg dose of nicotine 15 min prior to naloxone administration. The effect of nicotine was completely blocked by mecamylamine, but not hexamethonium. The dopamine receptor antagonists haloperidol (0.05, 0.1 mg/kg, s.c.), SCH23390 (0.1 mg/kg, s.c.), raclopride (1.0 mg/kg, s.c.) and eticlopride (0.1 mg/kg, s.c.) showed effects similar to mecamylamine. Additionally, the dopamine receptor agonist apomorphine (0.03, 0.1, 0.3 mg/kg, s.c.) inhibited naloxone-induced place aversion in morphine-treated rats.

CONCLUSION

The inhibitory effect of nicotine on place aversion induced by naloxone-precipitated morphine withdrawal may involve a dopaminergic portion of the central nervous system.

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.

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.

Dopamine and nicotine, but not serotonin, modulate the crustacean ventilatory pattern generator

Dopamine (DA) causes a dose-dependent increase in the frequency of motor neuron bursts [virtual ventilation (fR)] produced by deafferented crab ventilatory pattern generators (CPGv). Domperidone, a D2-specific DA antagonist, by itself reversibly depresses fR and also blocks the stimulatory effects of DA. Serotonin (5HT) has no direct effects on this CPGv. Nicotine also causes dramatic dose-dependent increases in the frequency of motor bursts from the CPGv. The action is triphasic, beginning with an initial reversal of burst pattern typical of reversed-mode ventilation, followed by a 2- to 3-min period of depression and then a long period of elevated burst rate. Acetylcholine chloride (ACh) alone is ineffective, but in the presence of eserine is moderately stimulatory. The inhibitory effects of nicotine are only partially blocked by curare. The excitatory action of nicotine is blocked by prior perfusion of domperidone, but not by SKF-83566.HCl, a D1-specific DA antagonist. SKF-83566 had no effects on the ongoing pattern of firing. These observations support the hypothesis that dopaminergic pathways are involved in the maintenance of the CPGv rhythm and that the acceleratory effects of nicotine may involve release of DA either directly or via stimulation of atypical ACh receptors at intraganglionic sites.

Dopamine Efflux from Striatum After Chronic Nicotine: Evidence for Autoreceptor Desensitization

We examined the effect of chronic nicotine treatment on dopaminergic activity by measuring the effects of D1 and D2 dopamine (DA) receptor agonists and antagonists on tritium release from mouse striatum preloaded with [3H]DA. The radioactivity released during superfusion was separated on alumina columns and the distribution and efflux of [3H]DA and its main 3H-labeled metabolites were quantified. After preloading by incubation with [3H]DA, the electrical stimulation-evoked tritium overflow was higher in striatum prepared from nicotine-treated mice, whereas in vitro addition of nicotine caused a similar increase in tritium release from striatum of untreated and chronic nicotine-treated mice. The overflow of [3H]DA and its 3H-metabolites exhibited similar distribution patterns in [3H]DA-preloaded striatum dissected from untreated and chronic nicotine-pretreated mice, indicating that repeated injections with nicotine did not alter the metabolism of [3H]DA taken up by the tissue. (-)-Quinpirole, a selective agonist for D2 DA receptors, and apomorphine, a nonselective D1/D2 agonist, inhibited the electrical stimulation-induced tritium efflux from striatum of untreated mice, whereas (+/-)-sulpiride, a D2 DA receptor antagonist, enhanced the evoked release of tritium. These changes in tritium efflux effected by (-)-quinpirole and (+/-)-sulpiride reflected changes in [3H]DA release and not in DA metabolism, as shown by separation of the released radioactivity on alumina columns. The D1 receptor agonist (+/-)-SKF-38393 did not affect the tritium overflow, whereas the D1 receptor antagonist (+)-SCH-23390 exerted a stimulatory action but only at a high concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic effects of nicotine on mesolimbic dopaminergic system in rats

Rats were pretreated with saline or nicotine (1.5 mg/kg/day) by subcutaneously implanting each animal with an Alzet osmotic minipump which continuously released saline or nicotine (1.5 mg/kg/day) for 14 days. The behavioral and biochemical effects of nicotine on the dopaminergic neuronal system in rat nucleus accumbens were examined. It was found that chronic nicotine treatment increased the affinity of L-[3H]nicotine binding site in the nucleus accumbens. This treatment also potentiated the ability of (+)-amphetamine, but not high potassium, to stimulate formation and release of [3H]dopamine in tissue slices from rat nucleus accumbens. Chronic nicotine treatment did not alter the characteristics of [3H]spiperone binding site, the rate of dopamine turnover and the concentrations of gamma-aminobutyric acid in the nucleus accumbens.

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.

Chronic administration of nicotine fails to alter the MPTP-induced neurotoxicity in mice

1. The effects of chronic (14 day) administration of nicotine on 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) (15 mg/kg, s.c.)-induced neurotoxicity in C57BL/6 mice were examined. 2. Nicotine pretreatment failed to alter the deficit in locomotor activity and the reduction in striatal levels of dopamine produced by MPTP. 3. Our results do not support a therapeutic action of nicotine in a Parkinsonian animal model.

Cholinergic-dopaminergic interactions in cognitive performance

Both acetylcholinergic (ACh) and dopaminergic (DA) systems have been found to be crucial for the maintenance of accurate cognitive performance. In a series of studies examining those aspects of cognitive function revealed by the radial-arm maze, we have found that these two neurotransmitter systems interact in a complex fashion. Choice accuracy deficits in the radial-arm maze can be induced by blockade of either muscarinic- or nicotinic-ACh receptors. The choice accuracy deficit induced by blockade of muscarinic receptors with scopolamine can be reversed by the DA receptor blocker, haloperidol. The specific DA D1 blocker SCH 23390 also has this effect, whereas the specific D2 blocker raclopride does not, implying that it is D1 blockade that is critical for reversing the scopolamine effect. On the other hand, the choice accuracy deficit induced by nicotinic blockade with mecamylamine is potentiated by haloperidol. This effect is also seen with the D2 antagonist raclopride, but not with the D1 antagonist SCH 23390, implying that it is the D2 receptor which is important for the potentiation of the mecamylamine effect. The relevance of the D2 receptor for nicotinic actions on cognitive function is emphasized by the finding that the selective D2 agonist LY 171555 reverses the choice accuracy deficit caused by mecamylamine. Nicotinic and muscarinic blockade are synergistic in the deficit they produce. Antagonist doses subthreshold when given alone produce a pronounced impairment when given together. This latter deficit can be reversed by the D2 agonist LY 171555. These studies have outlined the complex nature of ACh-DA interactions with regard to cognitive function. Possible neural circuits for these interactions are discussed. The effectiveness of these selective DA treatments in reversing cognitive deficits due to ACh underactivation suggests a novel approach to treating cognitive dysfunction in syndromes such as Alzheimer's disease.

Nicotine releases stereoselectively and Ca2(+)-dependently endogenous 3,4-dihydroxyphenylalanine from rat striatal slices

In superfused slices of rat striatum, nicotine-evoked release of endogenous 3,4-dihydroxyphenylalanine (DOPA) was studied in comparison with that of dopamine (DA). (+/-)-Nicotine (0.1-10 microM) constantly and repetitively released DOPA and DA over a similar time course in a concentration-dependent manner. The ratio of DOPA and DA evoked was approximately 1:2-3. The turnover rate of DOPA was about 300 times higher compared to DA. (+/-)-Nicotine (10 microM)-induced DOPA release was mecamylamine (20 microM)-sensitive, Ca2(+)-dependent and tetrodotoxin (0.3 microM)-insensitive. The (+)-isomer induced no DOPA release. These characteristics of DOPA release were almost the same as those of DA. Nicotine evokes endogenous DOPA via nicotinic cholinergic receptors in a manner similar to the transmitter DA. These findings further support a probable role of DOPA as a neuroactive substance in the rat central nervous system.

Pharmacological activity of N-methyl-carbamylcholine, a novel acetylcholine receptor agonist with selectivity for nicotinic receptors

N-Methyl-carbamylcholine (also called N-methyl-carbachol) is an analogue of the mixed muscarinic-nicotinic acetylcholine receptor agonist, carbachol. Previous studies have provided evidence that radiolabelled N-methyl-carbachol can bind selectively to nicotinic acetylcholine receptors in rat brain. To determine whether N-methyl-carbachol acts as an agonist or an antagonist at nicotine and/or muscarinic receptor sites, the present study examined the pharmacological activity of this compound on some cholinergically innervated tissues. N-Methyl-carbachol, like carbachol, depolarized rat isolated sympathetic ganglia and these effects were inhibited by a nicotinic antagonist, d-tubocurarine, but not by a muscarinic antagonist, atropine. Exposure of rat sympathetic ganglia to N-methyl-carbachol blocked the compound action potential generated in ganglia by stimulation of the pre-ganglionic trunk; this effect of N-methyl-carbachol was likely due to desensitization of the nicotinic response. N-Methyl-carbochol, like carbachol, stimulated the release of [3H]noradrenaline from cultured adrenal medullary cells that had been pre-loaded with [3H]noradrenaline; these effects were largely inhibited by a nicotinic antagonist, mecamylamine, while atropine produced less blockade. N-Methyl-carbachol contracted the frog isolated rectus abdominis muscle and the effect was completely blocked by d-tubocurarine. By contrast, contracture of the rectus abdominis produced by carbachol was partially inhibited by either atropine or d-tubocurarine. N-Methyl-carbachol, like carbachol, contracted the rat isolated ileum and these effects were completely blocked by atropine; however, N-methyl-carbachol was about 42 times less potent than carbachol for this effect. Intravenous injection of N-methyl-carbachol, like nicotine, to the rat produced a transient decrease followed by a more sustained rise in blood pressure while carbachol produced only a sustained decrease in blood pressure. The effects of N-methyl-carbachol and nicotine on blood pressure were blocked by pretreatment of the animal with a nicotinic antagonist, hexamethonium. N-methyl-carbachol, like nicotine, stimulated the release of [3H]dopamine from rat striatal synaptosomes, pre-loaded with [3H]dopamine; release induced by either N-methyl-carbachol or nicotine was inhibited by mecamylamine but not by atropine. In rat cerebral cortical slices pre-loaded with [3H]inositol, carbachol, but not N-methyl-carbachol, stimulated the accumulation of [3H]inositol-1-phosphate, an effect blocked by atropine but not by mecamylamine.(ABSTRACT TRUNCATED AT 400 WORDS)

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

Nicotinic receptors in the brain are receiving increased attention due in part to the recent cloning of receptor subunits and to postmortem studies revealing alterations in receptor density associated with Alzheimer's disease. The peptide neurotoxin neuronal bungarotoxin (NBT) has been shown to block nicotinic cholinergic responses in autonomic ganglia and in retinal ganglion cells. These findings suggest that NBT may be a useful probe for studying nicotinic receptors in the brain. Therefore, we have investigated the effects of NBT on the nicotine-mediated enhancement of endogenous dopamine release from rat striatal slices. It was found that the transient increase in dopamine release caused by 100 microM nicotine was completely blocked by 100 nM NBT, indicating that NBT is a functional nicotinic antagonist in this system.

Neuroregulatory effects of nicotine

The impact of nicotine on the central nervous system is, in an important sense, neuroregulatory, with cascading effects on physiological and biochemical function as well as on behavioral activity. Accordingly, the neurotransmitter and neuroendocrine effects of nicotine constitute a critical part of its biological action, which includes reinforcing as well as pathophysiological consequences. This review focuses on nicotine's effects on cholinergic and non-cholinergic nicotine receptors and on the responses of catecholamines, monoamines, hypophyseal hormones, and cortisol. The contribution of critical variables, such as timing and duration of neuroregulator release and the patterns that make up the total response, is still largely unknown, particularly with regard to the effects of environmental context, history of nicotine use, and mode of administration. The evidence suggests that by altering the bioavailability of the above-listed neuroregulators, nicotine serves as a pharmacological "coping response", providing immediate though temporary improvement in affect or performance in response to environmental demands. Much of what is known to date is based on studies involving the administration of agonists and antagonists under different environmental conditions. Newer technological approaches such as autoradiography and positron emission tomography show potential for determining the neuroregulatory patterns involved and specifying nicotine's locus of action relevant to its behavioral and physiological effects.

Effect of nicotine pretreatment on striatal dopaminergic system in rats

Rats were pretreated with saline or nicotine (1.5 mg/kg/day) by subcutaneously implanting each animal with an Alzet osmotic minipump for 1 or 14 days. Short-term (1-day) administration of nicotine to rats reduced the stimulatory effect of (+)-amphetamine on locomotor activity. This was correlated with an attenuation in the ability of (+)-amphetamine to stimulate [3H]dopamine formation from [3H]tyrosine in rat striatal slices of these nicotine-treated animals. In long-term (14-day) nicotine-pretreated animals, both the apomorphine- and (+)-amphetamine-induced locomotor activity were potentiated. This behavioral potentiation was associated with an increase in the total number of postsynaptic dopaminergic receptor binding sites in the striatum. The development of striatal dopamine receptor supersensitivity may be caused by a decrease in the rate of dopamine turnover in the striatum.

Hypothesis: a nicotine-dopamine interaction linking smoking with Parkinson's disease and tardive dyskinesia

1. Nicotine, an important pharmacological component of cigarette smoke, is known to have significant effects on central nervous system (CNS) dopaminergic function. Although acute doses of nicotine have been shown to facilitate dopamine release, recent data indicate that chronic nicotine treatment may actually decrease CNS dopamine turnover in the striatum. 2. A number of epidemiological investigations have demonstrated that individuals who are or who have been smokers are less likely to develop idiopathic Parkinson's disease (a disorder involving a deficit in nigrostriatal dopaminergic neurotransmission). In addition, there is preliminary evidence that individuals with tardive dyskinesia (a hyperkinetic movement disorder observed in some cases of chronic neuroleptic treatment and thought by some to be associated with striatal dopamine receptor supersensitivity) are more likely to be smokers. 3. A unitary hypothesis is presented, proposing that smoking in early adult life may decrease CNS catecholamine turnover, thereby protecting against free radical formation from catecholamine oxidation that in turn damages striatal neurons. These individuals are thereby "protected" from the later development of Parkinson's disease. In this hypothetical scheme, individuals who are given neuroleptics and who also are smokers may develop a greater degree of dopamine receptor supersensitivity due to combined receptor blockade by neuroleptics and a decrease in CNS dopamine turnover caused by nicotine, resulting in an increased prevalence of tardive dyskinesia in this group.

Receptor mechanisms of nicotine-induced locomotor hyperactivity in chronic nicotine-treated rats

Rats were pretreated with saline or nicotine (1.5 mg/kg per day) by subcutaneously implanting each animal with an Alzet osmotic mini-pump which continuously released saline or nicotine for 1, 5 and 14 days. At the end of each pretreatment period, animals were used for (i) determining their locomotor response to acutely injected nicotine (0.2 mg/kg, s.c.) and (ii) measuring the density of L-[3H]nicotine and [3H]spiperone binding sites in the striatum. We observed no changes in nicotine-induced locomotor response, striatal L-[3H]nicotine and [3H]spiperone binding in the animals pretreated with nicotine for 1 day. In rats which were pretreated with nicotine for 5 days, there was a significant increase in the nicotine-stimulated locomotor response which was associated with an increase in the number of L-[3H]nicotine binding sites and also with an elevated dopamine (DA) level in the striatum. The number of striatal [3H]spiperone binding sites was not affected. In animals pretreated with nicotine for 14 days, the nicotine-induced locomotor response remained to be potentiated. However, this response was correlated with an elevated number of striatal [3H]spiperone binding sites, whereas the number of striatal L-[3H]nicotine binding sites and the striatal DA level were normal. These results suggest that chronic nicotine-treated rats develop locomotor hyperactivity in response to nicotine initially due to increases of both the density of nicotinic receptors and DA concentration, followed by inducing DA receptor supersensitivity in the striatum.

Stereoselective nicotine-induced release of dopamine from striatal synaptosomes: concentration dependence and repetitive stimulation

Using a sensitive perfusion system we have studied the nicotine-induced release of [3H]dopamine ([( 3H]DA) from striatal synaptosomes. Nicotine-evoked release was concentration dependent with an EC50 of 3.8 microM. The response to 1 microM nicotine was comparable to that to 16 mM K+; 10 microM veratridine evoked a larger response. All three stimuli were Ca2+ dependent but only the response to veratridine was blocked by tetrodotoxin. Repetitive stimulations by 1 microM (-)-nicotine (100 microliters) at 30-min intervals resulted in similar levels of [3H]DA release; higher concentrations of (-)-nicotine resulted in an attenuation of the response particularly following the third stimulation. This may reflect desensitisation or tachyphylaxis of the presynaptic nicotinic receptor. The action of nicotine was markedly stereoselective: a 100-fold higher concentration of (+)-nicotine was necessary to evoke the same level of response as 1 microM (-)-nicotine. It is proposed that these presynaptic nicotinic receptors on striatal terminals are equivalent to high-affinity nicotine binding sites described in mammalian brain.

Stimulation of [3H]dopamine release by nicotine in rat nucleus accumbens

The mesolimbic system of the brain has been shown to be involved in the reward properties of a number of agents. It is possible that release of monoamines by nicotine in this brain area could be related to the pleasurable aspects related to cigarette smoking. In this investigation, the effect of nicotine on the release of [3H]dopamine in the nucleus accumbens of the rat was studied. It was shown that nicotine produced a concentration-dependent increase in [3H]dopamine release at concentrations of 0.1 microM and above. The increase in release was found to be almost completely calcium dependent. The nicotine-induced release was only partially blocked by the nicotinic antagonists hexamethonium and d-tubocurarine. A number of cholinergic agonists, as well as other compounds, were tested for their capacity to mimic the effect of nicotine. At equimolar concentrations there was, at most, only 50% of the activity of nicotine. The results of this study demonstrate that nicotine stimulates the release of dopamine in the nucleus accumbens at concentrations similar to those in the blood of cigarette smokers. This suggests that the release of monoamines in specific nuclei of the mesolimbic system may be an important determinant of the desire to smoke cigarettes.

Dopamine-like action of nicotine: lack of tolerance and reverse tolerance

Rats with unilateral 6-hydroxydopamine lesions of the substantia nigra became briefly sedated and hypothermic after the acute injection of nicotine s.c. (0.4 or 0.8 mg/kg free base). When nicotine was repeated 5 days per week there was rapid tolerance for the sedation and slower tolerance for the hypothermia and the lesioned animals began to rotate ipsiversively after each injection. Stereotypic behavior was also noted. Rats injected with nicotine 5 days per week and nigrally lesioned on the 24th day rotated promptly on their first postoperative injection of nicotine. The nicotinic antagonist, mecamylamine (1.0 mg/kg i.p.), completely blocked the induced rotation. The appearance of rotation did not seem to depend on tolerance to sedation. The direction of rotation indicated enhancement of activity in the intact nigrostriatal system. However, 10 min after the acute injection of 0.8 mg/kg nicotine no change was found in the ratios of dopamine to its metabolites DOPAC and homovanillic acid in the substantia nigra, caudate-putamen, nucleus accumbens, olfactory tubercle, frontal cortex, or ventral tegmental area. Rats given 0.4 or 0.8 mg/kg nicotine 5 days per week and either lesioned prior to nicotine or lesioned during the third week rotated during the sixth week without any sign of tolerance. One day after the 30th injection in intact or lesioned rats the ratios of dopamine to its metabolites did not differ from those in saline controls on either the right or left side of any of the regions examined. There was no evidence of a change in dopamine metabolism after an acute challenge with nicotine or of a sustained change after repeated injection. The possibility remains that repeated nicotine modifies the dopaminergic response to nicotine without causing a sustained change in metabolism.