Effect of acute administration of nicotine on in vivo release of noradrenaline in the hippocampus of freely moving rats: a dose-response and antagonist study

Memory enhancing effects of nicotine, cocaine, and their conditioned stimuli; effects of beta-adrenergic and dopamine D2 receptor antagonists

BACKGROUND

There is evidence that post-training exposure to nicotine, cocaine, and their conditioned stimuli (CS), enhance memory consolidation in rats. The present study assessed the effects of blocking noradrenergic and dopaminergic receptors on nicotine and cocaine unconditioned and conditioned memory modulation.

METHODS

Males Sprague-Dawley rats tested on the spontaneous object recognition task received post-sample exposure to 0.4 mg/kg nicotine, 20 mg/kg cocaine, or their CSs, in combination with 5-10 mg/kg propranolol (PRO; beta-adrenergic antagonist) or 0.2-0.6 mg/kg pimozide (PIM; dopamine D2 receptor antagonist). The CSs were established by confining rats in a chamber (the CS +) after injections of 0.4 mg/kg nicotine, or 20 mg/kg cocaine, for 2 h and in another chamber (the CS -) after injections of vehicle, repeated over 10 days (5 drug/CS + and 5 vehicle/CS - pairings in total). Object memory was tested 72 h post sample in drug-free animals.

RESULTS

Co-administration of PRO or PIM blocked the memory-enhancing effects of post-training injections of nicotine, cocaine, and, importantly, exposure to their CSs.

CONCLUSIONS

These data suggest that nicotine, cocaine as well as their conditioned stimuli share actions on overlapping noradrenergic and dopaminergic systems to modulate memory consolidation.

Cocaine, nicotine, and their conditioned contexts enhance consolidation of object memory in rats

To test the hypothesis that drugs of abuse and their conditioned stimuli (CSs) enhance memory consolidation, the effects of post-training exposure to cocaine and nicotine were compared to the effects of post-training exposure to contextual stimuli that were paired with the effects of these drugs. Using the object recognition (OR) task, it was first demonstrated that both 10 and 20 mg/kg cocaine, and 0.2 and 0.4 mg/kg nicotine, enhanced recognition memory when administered immediately after, but not 6 h after the sample phase. To establish the drug CSs, rats were confined for 2 h in a chamber (the CS+) after injections of 20 mg/kg cocaine, or 0.4 mg/kg nicotine, and in another chamber (the CS−) after injections of vehicle. This was repeated over 10 d (5 drug/CS+ and 5 vehicle/CS− pairings in total). At the end of this conditioning period, when tested in a drug-free state, rats displayed conditioned hyperactivity in the CS+ relative to the CS−. More important, immediate, but not delayed, post-sample exposure to the cocaine CS+, or nicotine CS+, enhanced OR memory. Therefore, this study reports for the first time that contextual stimuli paired with cocaine and nicotine, like the drugs themselves, have the ability to enhance memory consolidation.

Modulatory effects of the basolateral amygdala α2-adrenoceptors on nicotine-induced anxiogenic-like behaviours of rats in the elevated plus maze

The present study was designed to clarify whether α2-adrenoceptors of the basolateral amygdala (BLA) are involved in nicotine-induced anxiogenic-like behaviours. Adult male Wistar rats were bilaterally cannulated in the BLA and anxiety-like behaviours were assessed in an elevated plus maze (EPM) task. Systemic intraperitoneal (i.p.) administration of nicotine (0.3, 0.5 and 0.7 mg/kg) dose-dependently decreased open arm time (%OAT) and open arm entry (%OAE), indicating the anxiogenic-like effect of nicotine. The activation of the BLA α2-adrenoceptors by the injection of α2-receptor agonist, clonidine (0.1, 0.3 and 0.5 μg/rat) into the BLA (intra-BLA) reversed nicotine-induced anxiogenic-like behaviours. It is important to note that intra-BLA injection of a higher dose of clonidine (0.5 μg/rat) by itself increased %OAT, but not %OAE which showed an anxiolytic effect of the agonist. On the other hand, intra-BLA injection of different doses of α2-adrenoceptor antagonist, yohimbine (1, 3 and 5 μg/rat) in combination with an ineffective dose of nicotine (0.3 mg/kg) decreased %OAT and %OAE, suggesting a potentiative effect of the antagonist on nicotine response. In addition, intra-BLA injection of the same doses of yohimbine did not alter %OAT and %OAE. Interestingly, intra-BLA injection of yohimbine (0.5 and 1 μg/rat) significantly reversed the inhibitory effect of clonidine on nicotine-induced anxiogenic-like behaviours. It should be considered that the drug treatments had no effect on locomotor activity in all experiments. Taken together, it can be concluded that nicotine produces anxiogenic-like behaviours which may be mediated through the BLA α2-adrenoceptor mechanism.

Translational utility of rodent hippocampal auditory gating in schizophrenia research: a review and evaluation

Impaired gating of the auditory evoked P50 potential is one of the most pharmacologically well-characterized features of schizophrenia. This deficit is most commonly modeled in rodents by implanted electrode recordings from the hippocampus of the rodent analog of the P50, the P20-N40. The validity and effectiveness of this tool, however, has not been systematically reviewed. Here, we summarize findings from studies that have examined the effects of pharmacologic modulation on gating of the rodent hippocampal P20-N40 and the human P50. We show that drug effects on the P20-N40 are highly predictive of human effects across similar dose ranges. Furthermore, mental status (for example, anesthetized vs alert) does not appear to diminish the predictive capacity of these recordings. We then discuss hypothesized neuropharmacologic mechanisms that may underlie gating effects for each drug studied. Overall, this review supports continued use of hippocampal P20-N40 gating as a translational tool for schizophrenia research.

Nicotine-induced impairments of spatial cognition and long-term potentiation in adolescent male rats

The aim of the present study was to investigate whether cognitive behavioral impairment, induced by nicotine in offspring rats, was associated with the alteration of hippocampal short-term potentiation (STP) and long-term potentiation (LTP) and to discuss the potential underlying mechanism. Young adult offspring rats were randomly divided into three groups. The groups include: control group (CC), nicotine group 1 (NC), in which their mothers received nicotine from gestational day 3 (GD3) to GD18, and nicotine group 2 (CN), in which young adult offspring rats received nicotine from postnatal day 42 (PD42) to PD56. Morris water maze (MWM) test was performed and then field excitatory postsynaptic potentials elicited by the stimulation of perforant pathway were recorded in the hippocampal dentate gyrus region. The results of the MWM test showed that learning and memory were impaired by either prenatal or postnatal nicotine exposure. In addition, it was found that there was no statistical difference of the MWM data between both nicotine treatments. In the electrophysiological test, LTP and STP were significantly inhibited in both NC and CN groups in comparison with the CC group. Notably, STP in CN group was also lower than that in the NC group. These findings suggested that both prenatal and postnatal exposure to nicotine induced learning and memory deficits, while the potential mechanism might be different from each other due to their dissimilar impairments of synaptic plasticity.

Nicotine and clozapine cross-prime the locus coeruleus noradrenergic system to induce long-lasting potentiation in the rat hippocampus

A priming-challenge schedule of nicotine treatment causes long-lasting potentiation (LLP), a form of synaptic plasticity closely associated with the norepinephrine (NE) neurotransmitter system, at the medial perforant path (MPP)-dentate gyrus (DG) synapse in the rat hippocampus. Previous reports revealed that nicotine activates the locus coeruleus (LC) noradrenergic (NAergic) system and this mechanism may underlie its beta-adrenoceptor sensitive LLP effects. Clozapine, an atypical antipsychotic, is also known to activate the LC. Interactions between nicotine and clozapine are of interest because of the prevalence of smoking in patients with schizophrenia and increasing interest in the use of nicotinic receptor ligands as cognitive enhancers. Rats were subchronically primed with nicotine, clozapine or saline. Twenty-one to twenty-eight days later, the effects of the nicotine, clozapine or saline challenge on the evoked field excitatory postsynaptic potentials (fEPSP) at the MPP-DG monosynaptic pathway were recorded as a measure of LLP. We confirmed the hypothesis that a challenge dose of either nicotine or clozapine induces LLP exclusively in nicotine- and clozapine-primed rats, and not in saline-primed rats, thus indicating a cross-priming effect. Moreover, unilateral suppression of LC using lidocaine abolished the LLP induced by nicotine in clozapine-primed rats. Furthermore, systemic treatment with clonidine (an α2 adrenoceptor agonist that reduces NAergic activity via autoreceptors) prior to the challenge doses blocked the nicotine/clozapine-induced LLP in nicotine- and clozapine-primed rats. These findings may add to understanding of the cognitive enhancing effects of nicotine.

The long-term effects of prenatal nicotine exposure on neurologic development

A large body of documented evidence has found that smoking during pregnancy is harmful to both the mother and the fetus. Prenatal exposure to nicotine in various forms alters neurologic development in experimental animals and may increase the risk for neurologic conditions in humans. There is a positive association between maternal smoking and sudden infant death syndrome (SIDS); however, the connection between nicotine addiction, depression, attention disorders, and learning and behavior problems in humans is not straightforward. Nicotine's action on the production and function of neurotransmitters makes it a prime suspect in the pathology of these diseases. Nicotine accentuates neurotransmitter function in adults but desensitizes these functions in prenatally exposed infants and children. This desensitization causes an abnormal response throughout the lifespan. Furthermore, nicotine use by adolescents and adults can alleviate some of the symptoms caused by these neurotransmitter problems while they increase the risk for nicotine addiction. Although nicotine replacement drugs are used by pregnant women, there is no clear indication that they improve outcomes during pregnancy, and they may add to the damage that occurs to the developing neurologic system in the fetus. Understanding the effects of nicotine exposure is important in providing safe care for pregnant women, children, and families and for developing appropriate smoking cessation programs during pregnancy.

Inhibition of monoamine oxidases desensitizes 5-HT1A autoreceptors and allows nicotine to induce a neurochemical and behavioral sensitization

Although nicotine is generally considered to be the main compound responsible for addictive properties of tobacco, experimental data indicate that nicotine does not exhibit all the characteristics of other substances of abuse. We recently showed that a pretreatment with mixed irreversible monoamine oxidases inhibitors (MAOIs), such as tranylcypromine, triggers a locomotor response to nicotine in mice and allows maintenance of behavioral sensitization to nicotine in rats. Moreover, we showed by microdialysis in mice that behavioral sensitization induced by compounds belonging to main groups of drugs of abuse, such as amphetamine, cocaine, morphine, or alcohol, was underlain by sensitization of noradrenergic and serotonergic neurons. Here, this neurochemical sensitization was tested after nicotine, tranylcypromine, or a mixture of both compounds. Data indicate that, whereas neither repeated nicotine nor repeated tranylcypromine alone has any effect by itself, a repeated treatment with a mixture of nicotine and tranylcypromine induces both behavioral sensitization and sensitization of noradrenergic and serotonergic neurons. The development of neurochemical and behavioral sensitizations is blocked by prazosin and SR46349B [(1Z,2E)-1-(2-fluoro-phenyl)-3-(4-hydroxyphenyl)-prop-2-en-one-O-(2-dimethylamino-ethyl)-oxime hemifumarate], two antagonists of alpha1b-adrenergic and 5-HT(2A) receptors, respectively, but not by SCH23390 [R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride], a D(1) receptor antagonist. Finally, we found that pretreatments with WAY 100635 [N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclo-hexane carboxamide trihydrochloride], a 5-HT(1A) receptor antagonist, can also induce a behavioral and neurochemical sensitization to repeated nicotine. Complementary experiments with 8-OHDPAT (8-hydroxy-dipropylamino-tetralin), a 5-HT(1A) receptor agonist, and analysis of 5-HT(1A) receptors expression in the dorsal raphe nucleus after a tranylcypromine injection indicate that MAOIs contained in tobacco desensitize 5-HT(1A) autoreceptors to trigger the strong addictive properties of tobacco.

Acquired appetitive responding to intravenous nicotine reflects a Pavlovian conditioned association

Recent research examining Pavlovian appetitive conditioning has extended the associative properties of nicotine from the unconditioned stimulus or reward to include the role of a conditional stimulus (CS), capable of acquiring the ability to evoke a conditioned response. To date, published research has used presession extravascular injections to examine nicotine as a contextual CS in that appetitive Pavlovian drug discrimination task. Two studies in the current research examined whether a nicotine CS can function discretely, multiple times within a session using passive iv infusions. In Experiment 1, rats readily acquired a discrimination in conditioned responding between nicotine and saline infusions when nicotine was selectively paired with sucrose presentations. In Experiment 2, rats were either trained with nicotine paired with sucrose or explicitly unpaired with sucrose. The results showed that rats trained with explicitly unpaired nicotine and sucrose did not increase dipper entries after the infusions. Nicotine was required to be reliably paired with sucrose for control of conditioned responding to develop. Implications of these findings are discussed in relation to tobacco addiction, learning theory, and pharmacology.

Theta driving both inhibits and potentiates the effects of nicotine on dentate gyrus responses

The medial septal area of conscious rats was stimulated through previously implanted electrodes at a frequency of 7.7 Hz for 20 min each day for 7 days to evoke rhythmic slow activity in CA1 at a similar frequency to spontaneous theta. Two weeks later in the anaesthetized rats the effects of a single subcutaneous injection of nicotine (0.4 mg/kg) on fEPSPs, evoked in the dentate gyrus by separate stimulation of the MPP and LPP, were studied and compared with those obtained in controls. Nicotine increased the firing of locus coeruleus neurons and the slope of the fEPSPs evoked by LPP stimulation, but not by MPP stimulation. Prior theta driving considerably increased the effect of nicotine on the responses evoked by stimulation of the MPP and abolished the nicotine-induced potentiation of the responses evoked by stimulation of the LPP. The results are attributed to theta driving increasing the amount of noradrenaline released by nicotine and to noradrenaline producing a beta-adrenoceptor long-lasting potentiation at the medial perforant path synapse and a long-lasting depression at the lateral perforant path synapse.

Guidelines on nicotine dose selection for in vivo research

RATIONALE

This review provides insight for the judicious selection of nicotine dose ranges and routes of administration for in vivo studies. The literature is replete with reports in which a dosaging regimen chosen for a specific nicotine-mediated response was suboptimal for the species used. In many cases, such discrepancies could be attributed to the complex variables comprising species-specific in vivo responses to acute or chronic nicotine exposure.

OBJECTIVES

This review capitalizes on the authors' collective decades of in vivo nicotine experimentation to clarify the issues and to identify the variables to be considered in choosing a dosaging regimen. Nicotine dose ranges tolerated by humans and their animal models provide guidelines for experiments intended to extrapolate to human tobacco exposure through cigarette smoking or nicotine replacement therapies. Just as important are the nicotine dosaging regimens used to provide a mechanistic framework for acquisition of drug-taking behavior, dependence, tolerance, or withdrawal in animal models.

RESULTS

Seven species are addressed: humans, nonhuman primates, rats, mice, Drosophila, Caenorhabditis elegans, and zebrafish. After an overview on nicotine metabolism, each section focuses on an individual species, addressing issues related to genetic background, age, acute vs chronic exposure, route of administration, and behavioral responses.

CONCLUSIONS

The selected examples of successful dosaging ranges are provided, while emphasizing the necessity of empirically determined dose-response relationships based on the precise parameters and conditions inherent to a specific hypothesis. This review provides a new, experimentally based compilation of species-specific dose selection for studies on the in vivo effects of nicotine.

The genetics of nicotine dependence: Relationship to pancreatic cancer

Smoking of tobacco products continues to be a major cause of worldwide health problems. Epidemiological studies have shown that tobacco smoking is the greatest risk factor for the development of pancreatic cancer. Smokers who are able to quit smoking can reduce their risk of pancreatic cancer by nearly 50% within two years, however, their risk of developing pancreatic cancer remains higher than that of non-smokers for 10 years. Nicotine is the major psychoactive substance in tobacco, and is responsible for tobacco dependence and addiction. Recent evidence suggests that individuals have genetically based differences in their ability to metabolize nicotine, as well as genetic differences in the psychological reward pathways that may influence individual response to smoking initiation, dependence, addiction and cessation. Numerous associations have been reported between smoking behavior and genetic polymorphisms in genes that are responsible for nicotine metabolism. In addition, polymorphisms in genes that encode neurotransmitters and transporters that function in psychological reward pathways have been implicated in differences in smoking behavior. However, there is a large degree of between-study variability that demonstrates the need for larger, well-controlled case-control studies to identify target genes and deduce mechanisms that account for the genetic basis of inter-individual differences in smoking behavior. Understanding the genetic factors that increase susceptibility to tobacco addiction may result in more effective tobacco cessation programs which will, in turn, reduce the incidence of tobacco related disease, including pancreatic cancer.

The Effects of Glutamate and GABA Receptor Antagonists on Nicotine-induced Neurotransmitter Changes in Cognitive Areas*

In the present study, we tested the effects of glutamate and GABA receptor antagonists on nicotine-induced neurotransmitter changes in the hippocampal (dorsal and ventral) and cortical (medial temporal and prefrontal) brain areas of conscious freely moving rats via microdialysis. Both the antagonists and nicotine were administered intracerebrally. The antagonists tested were NMDA, AMPA-kainate, and metabotropic glutamate receptor subtype antagonists (MK801, CNQX, and LY 341495, respectively) and GABA(A) and GABA(B) receptor subtype antagonists (bicuculline and hydroxysaclofen, respectively). We assayed nicotine-induced changes in dopamine (DA), norepinephrine (NE), serotonin (5-HT), and their metabolites. We found with the antagonists, both decreases and increases in nicotine-induced neurotransmitter responses. In the presence of nicotine all the antagonists (except LY 341495) caused a decrease in DA levels in the regions tested. NE levels were decreased in the cortex by all antagonists. In the hippocampus, GABA antagonists decreased NE levels, as did the metabotropic glutamate antagonist, LY 341495, while the other glutamate antagonists increased NE levels. The results of the 5-HT assay were more variable and dependent on the region and antagonist examined; increases were found slightly more often than decreases. The changes in metabolites were not often parallel with changes in their associated neurotransmitters, indicating that the antagonists also affect the metabolism of the neurotransmitters. The effect of the antagonists in the absence of nicotine was mostly to decrease the level of neurotransmitters, although increases were seen in a few cases. The results suggest that the excitatory glutamatergic- and inhibitory GABAergic-amino acid receptors are both involved in mediating nicotine-induced neurotransmitter responses, and their inhibitory or stimulatory effects are receptor subtype and brain region dependent.

Role of the cannabinoid system in the effects induced by nicotine on anxiety-like behaviour in mice

RATIONALE

Acute behavioural effects and motivational responses induced by nicotine can be modulated by the endocannabinoid system supporting the existence of a physiological interaction between these two systems.

OBJECTIVES

The present study was designed to examine the possible involvement of the cannabinoid system in the anxiolytic- and anxiogenic-like responses induced by nicotine in mice.

METHODS

Animals were only exposed once to nicotine. The acute administration of low (0.05) or high (0.8 mg/kg, s.c.) doses of nicotine produced opposite effects in the elevated plus-maze, i.e. anxiolytic- and anxiogenic-like responses, respectively. The effects of the pretreatment with the CB1 cannabinoid receptor antagonist, rimonabant (0.25, 0.5 and 1 mg/kg, i.p.), and the cannabinoid agonist, delta9-tetrahydrocannabinol (delta9-THC, 0.1 mg/kg, ip), were evaluated on the anxiolytic- and anxiogenic-like responses induced by nicotine.

RESULTS

Rimonabant completely abolished nicotine-induced anxiolytic-like effects and increased the anxiogenic-like responses of nicotine, suggesting an involvement of CB1 receptors in these behavioural responses. On the other hand, delta9-THC failed to modify nicotine anxiolytic-like responses but attenuated its anxiogenic-like effects. In addition, the association of non-effective doses of delta9-THC and nicotine produced clear anxiolytic-like responses.

CONCLUSIONS

These results demonstrate that the endogenous cannabinoid system is involved in the regulation of nicotine anxiety-like behaviour in mice and provide new findings to support the use of cannabinoid antagonists in the treatment of tobacco addiction.

Involvement of the opioid system in the effects induced by nicotine on anxiety-like behaviour in mice

RATIONALE

Recent studies have revealed the participation of the endogenous opioid system in several behavioural responses induced by nicotine including antinociception, rewarding properties, and physical drug dependence.

OBJECTIVES

The present study was designed to examine the possible involvement of the various opioid receptors in the anxiolytic- and anxiogenic-like responses induced by nicotine in mice.

METHODS

The acute administration of low (0.05) or high (0.8 mg/kg) doses of nicotine subcutaneously produced opposite effects in the elevated plus maze, i.e. anxiolytic- and anxiogenic-like responses, respectively. Animals were only exposed once to nicotine. The effects of the pretreatment with the mu-opioid receptor antagonist, beta-funaltrexamine (5 mg/kg), the delta-opioid antagonist, naltrindole (2.5 mg/kg) and the kappa-opioid antagonist, nor-binaltorphimine (2.5 mg/kg) intraperitoneally were evaluated on the anxiolytic- and anxiogenic-like responses induced by nicotine.

RESULTS

beta-funaltrexamine, but not nor-binaltorphimine or naltrindole, abolished nicotine-induced anxiolytic-like effects, suggesting an involvement of mu-opioid receptors in this behavioural response. On the other hand, naltrindole, but not nor-binaltorphimine or beta-funaltrexamine, increased the anxiogenic-like responses of nicotine, suggesting an involvement of delta-receptors in this behavioural effect.

CONCLUSIONS

These results demonstrate that the endogenous opioid system is involved in the effects induced by nicotine on anxiety-like behaviour and provide new findings to further clarify the interaction between these two neurochemical systems.

Nicotinic acetylcholine receptor-mediated [3H]dopamine release from hippocampus

The mechanism of nicotinic acetylcholine receptor (nAChR)-induced hippocampal dopamine (DA) release was investigated using rat hippocampal slices. nAChRs involved in hippocampal DA and norepinephrine (NE) release were investigated using prototypical agonists and antagonists and several relatively novel compounds: ABT-594 [(R)-5-(2-azetidinylmethoxy)-2-chloropyridine], (+/-)-UB-165 [(2-chloro-5-pyridyl)-9-azabicyclo [4.2.1]non2-ene], and MG 624 [N,N,N-triethyl-2-[4-(2 phenylethenyl)phenoxy]-ethanaminium iodine]. (+/-)-Epibatidine, (+/-)-UB-165, anatoxin-a, ABT-594, (-)-nicotine, 1,1-dimethyl-4-phenyl-piperazinium iodide, and (-)-cytisine (in decreasing order of potency) evoked [(3)H]DA release in a mecamylamine-sensitive manner. Aside from (+/-)-UB-165, all the agonists displayed full efficacy relative to 100 microM (-)-nicotine in [(3)H]DA release. In contrast, (+/-)-UB-165 was a partial agonist, evoking 58% of 100 microM (-)-nicotine response. Mecamylamine, MG 624, hexamethonium, d-tubocurare, and dihydro-beta-erythroidine (in decreasing order of potency), but not alpha-conotoxin-MII, methyllycaconitine, alpha-conotoxin-ImI, or alpha-bungarotoxin, attenuated 100 microM (-)-nicotine-evoked [(3)H]DA release in a concentration-dependent manner. (+/-)-UB-165, ABT-594, and MG 624 exhibited different pharmacologic profiles in the [(3)H]NE release assay when compared with their effect on [(3)H]DA release. ABT-594 was 4.5-fold more potent, and (+/-)-UB-165 was a full agonist in contrast to its partial agonism in [(3)H]DA release. MG 624 potently and completely blocked NE release evoked by 100 microM (-)-nicotine and 10 microM (+/-)-UB-165, whereas it only partially inhibited (-)-nicotine-evoked [(3)H]DA release. In conclusion, we provide evidence that [(3)H]DA can be evoked from the hippocampus and that the pharmacologic profile for nAChR-evoked hippocampal [(3)H]DA release suggests the involvement of alpha3beta4(*) and at least one other nAChR subtype, thus distinguishing it from that of nAChR-evoked hippocampal [(3)H]NE release.

The role of alpha-adrenoceptor mechanism(s) in morphine-induced conditioned place preference in female mice

It has been shown that the alpha-adrenergic system is involved in some effects of opioids, including analgesia and reward. Gender differences also exist between males and females in response to alpha-adrenergic agents. This study was designed to determine the effects of alpha-adrenoceptor agonists and antagonists on the acquisition or expression of morphine-induced conditioned place preference (CPP) in female mice. The experiments showed that subcutaneous injections of morphine (0.5-8 mg/kg) induced CPP in a dose-dependent manner in mice. Intrapritoneal administration of the alpha-1-adrenoceptor agonist, phenylephrine (0.03, 0.1 and 0.3 mg/kg), and alpha-2 adrenoceptor agonist, clonidine (0.0001, 0.0005 and 0.001 mg/kg), as well as alpha-1-adrenoceptor antagonist, prazosin (0.01, 0.05 and 0.1 mg/kg) or alpha-2 adrenoceptor antagonist, yohimbine (0.005, 0.01 and 0.05 mg/kg) did not induce motivational effects and also did not alter locomotor activity in the animals. In the second set of experiments, the drugs were used before testing on Day 5, to test their effects on the expression of morphine-induced CPP. Intrapritoneal administration of phenylephrine and clonidine decreased the expression of morphine-induced CPP. In contrast, after application of prazosin or yohimbine, the expression of morphine-induced CPP was increased. Administration of lower (0.03 mg/kg) and higher doses of phenylephrine (0.1 and 0.3 mg/kg) during acquisition of morphine CPP decreased and increased the morphine CPP, respectively. Similarly, the administration of prazosin and clonidine decreased while yohimbine increased the morphine CPP. It may be concluded that alpha-adrenoceptor mechanism(s) influence morphine-induced CPP in female mice.

Characterization of the effects of bupropion on the reinforcing properties of nicotine and food in rats

Bupropion is an atypical antidepressant drug that is the only nonnicotine-based prescription medicine approved for smoking cessation by the Food and Drug Administration. The aim of the present experiments was to investigate the effects of bupropion (5-40 mg/kg) on the reinforcing properties of nicotine and food in rats. The effects of bupropion were studied under two schedules of reinforcement: a fixed ratio 5 time-out 20-sec (FR5 TO20 s) and a progressive ratio (PR). Rats were trained to respond for nicotine (0.01 or 0.03 mg/kg/infusion, free base) or food under the FR5 TO20 s schedule. Pretreatment with the highest dose of bupropion (40 mg/kg) resulted in a significant reduction (approximately 50%) of nicotine intake in rats self-administering 0.03 mg/kg/infusion of nicotine. The same dose of bupropion also decreased (approximately 40%) the self-administration of 0.01 mg/kg/infusion of nicotine, but this effect did not reach statistical significance. Pretreatment with bupropion slightly (approximately 15%) reduced responding for food under the FR5 TO20 s schedule. Finally, pretreatment with bupropion did not affect the self-administration of nicotine (0.03 mg/kg/infusion) under a PR schedule, but dose-dependently increased responding for food under the same PR schedule. These findings indicate that a high dose of bupropion decreases the reinforcing properties of nicotine as measured under an FR schedule, while having no apparent effects on breaking points for nicotine under a PR schedule that reflects both the reinforcing properties and the motivation to obtain nicotine.

Cholinergic nicotinic receptors: competitive ligands, allosteric modulators, and their potential applications

Discovery of the important role played by nicotinic acetylcholine receptors (nAChRs) in several CNS disorders has called attention to these membrane proteins and to ligands able to modulate their functions. The existence of different subtypes at multiple levels has complicated the understanding of this receptor's physiological role, but at the same time has increased the efforts to discover selective compounds in order to improve the pharmacological characterization of this kind of receptor and to make the possible therapeutical use of its modulators safer. This review focuses on the structure of new ligands for nAChRs, agonists, antagonists and allosteric modulators, and on their possible applications.

Developmental regulation of nicotinic acetylcholine receptor-mediated [3H]norepinephrine release from rat cerebellum

Presynaptic modulation of synaptic transmission is the primary function of central nicotinic acetylcholine receptors (nAChRs) in developing and adult brain. nAChR activation regulates release of various neurotransmitters, including norepinephrine (NA). Given evidence that NA may serve a critical functional role in cerebellar development, we have undertaken studies to determine whether nAChRs modulate NA release in developing cerebellum. In vitro experiments using cerebellar slices examined the effects of nAChR stimulation on release of radiolabeled NA ([3H]NA). Our data indicate the presence of functional nAChRs on NA terminals in immature cerebellum and subsequent developmental regulation of receptor properties. During postnatal week one, the maximally effective dose of nicotine released 35.0 +/- 1.2% of cerebellar [3H]NA stores. There was a subsequent decline in maximal nicotine-stimulated NA release until postnatal day 30, when Emax values were statistically indistinguishable from adult. Although the efficacy of nicotine changed substantially throughout development, EC50 values did not differ significantly (EC50 = 4.4-12.0 micro m). Pharmacological analysis indicated that this developmental shift in maximum nicotine effect reflects a change in the properties of the nAChRs. These data support recent findings of a possible functional role of nAChRs in regulating cerebellar ontogeny, and provides further support for the role of NA as a neurotrophic factor during development.

Nicotine-induced changes in cerebrocortical neuroactive steroids and plasma corticosterone concentrations in the rat

Nicotine, one of the most widely used psychotropic substances, is able to induce both anxiolytic and anxiogenic effects. The effect of this drug on the brain and plasma concentrations of neuroactive steroids was examined in the rat. Anxiolytic doses of nicotine (0.03-0.3 mg/kg) had no significant effect, whereas administration of anxiogenic doses (0.5 to 2 mg/kg) produced a dose- and time-dependent increase in the cerebrocortical concentrations of pregnenolone, progesterone, and allopregnanolone, with the greatest observed effects (+180%, +223%, and +124%, respectively) apparent at the dose of 2 mg/kg. In contrast, nicotine (1-2 mg/kg) decrease by 31% and 38%, respectively, the concentration of 3alpha,21-dihydroxy-5alpha-pregnan-20-one (allotetrahydrodeoxycorticosterone, or THDOC) in the cerebral cortex. Nicotine also increased the plasma concentrations of pregnenolone and progesterone, whereas failed to affect significantly those of allopregnanolone or THDOC. Nicotine induced a dose- and time-dependent increase in the plasma concentration of corticosterone, indicating that this drug activates the hypothalamic-pituitary-adrenal (HPA) axis. These results suggest that the changes in emotional behavior elicited by nicotine, similar to those induced by stressful stimuli or other anxiogenic drugs, are associated with an increase in neuroactive steroids content of the brain.

Functional alterations in CNS catecholamine systems in adolescence and adulthood after neonatal chlorpyrifos exposure

Chlorpyrifos (CPF), one of the most widely used pesticides, is a neurobehavioral teratogen in animals. We administered CPF to neonatal rats on postnatal days (PN) 1-4 (1 mg/kg) or PN11-14 (5 mg/kg), regimens devoid of overt systemic toxicity. We then examined the impact on catecholaminergic systems in adolescence (PN30) and adulthood (PN60), assessing basal neurotransmitter content and transmitter utilization rates (turnover) in brain regions comprising the major noradrenergic and dopaminergic projections. Although CPF had only sporadic effects on basal norepinephrine and dopamine content, it profoundly suppressed norepinephrine turnover across multiple regions, indicative of net reductions in presynaptic activity. Dopamine turnover showed less consistent effects, with subnormal turnover in some regions and activation in others. We also evaluated whether CPF exposure altered the ability of catecholamine systems to respond to acute cholinergic stimulation, elicited by administration of a single challenge dose of nicotine. In the normal brain, nicotine increases the utilization of norepinephrine and dopamine. With only a few exceptions, animals receiving neonatal CPF exposure showed lasting desensitization of the nicotine response; not only was the activation by nicotine blunted in the CPF group, but in some regions the nicotine response was reversed, eliciting a reduction in transmitter turnover. These results indicate that neonatal CPF exposure produces widespread deficiencies in catecholaminergic synaptic function that persist into adulthood, and that are best revealed by dynamic measures of synaptic activity and responsiveness, as opposed to static markers like basal transmitter levels. The effects seen here are likely to contribute to alterations in behavioral performance that persist or emerge long after the termination of CPF exposure.

Alterations in serotonin transporter expression in brain regions of rats exposed neonatally to chlorpyrifos

Chlorpyrifos (CPF), one of the most widely-used organophosphate pesticides, is a suspected neuroteratogen. We administered CPF to neonatal rats on postnatal days (PN) 1-4 (1 mg/kg) or PN11-14 (5 mg/kg), treatments devoid of overt toxicity. At the end of the treatment period (PN5 and 15, respectively) and 5-7 days later, we then examined the effects on paroxetine (PXT) binding to the presynaptic 5HT high-affinity transporter, a marker for serotonin (5HT) projections. In males, we found a persistent decrease in PXT binding across the two different treatment regimens, with deficits apparent in a brain region containing 5HT terminal fields (forebrain) as well as in a region containing 5HT cell bodies (brainstem). In contrast, females given the early treatment regimen (PN1-4) showed deficits in the brainstem but transient elevations in the forebrain; the later treatment regimen (PN11-14) had no significant effect on PXT binding in females. These data are consistent with earlier work showing brainstem cell injury resulting from neonatal CPF exposure, and indicate specific damage to 5HT neurons, with a consequent loss of transporter expression in both terminal fields and perikarya. In females, the damage may be temporarily offset by initial trophic effects in the terminal region, consequent to the cholinergic stimulation evoked by cholinesterase inhibition via the active metabolite, CPF oxon. The gender-selective effects on 5HT systems are likely to contribute to similar gender dimorphism in behavioral performance. Because the CPF effects involve 5HT, a neurotransmitter intimately involved in the control of mood, we suggest the need to evaluate behaviors that typify animal models of depression.

Differential effect of nicotinic agonists on the [3H]norepinephrine release from rat hippocampal slices

The aim of this study was to investigate the mechanisms involved in the effect of nicotinic agonists on the [3H]norepinephrine ([3H]NE) release from rat hippocampal slices. The stimulatory effect of nicotine, cytisine, epibatidine and anatoxin-A was completely blocked by the nicotinic antagonist mecamylamine (10 microM). In contrast, the effect of dimethylphenylpiperazinium (DMPP) was only partially inhibited by mecamylamine but was completely blocked by the NE uptake inhibitor desipramine (DMI, 10 microM). Finally, the effect of lobeline was not affected by mecamylamine and was only partially blocked by DMI. Our data indicate that the majority of nicotinic agonists increase the release of [3H]NE exclusively via stimulation of nicotinic acetylcholine receptors (nAChRs). DMPP, in addition to the stimulation of nAChRs, also evokes a carrier-mediated release. Lobeline has no stimulatory effect on nAChRs, induces a carrier-mediated release and has a further action of unidentified mechanism. Our results suggest that special caution is required for the interpretation of data, when DMPP or lobeline are used as nicotinic agonists.

Interactive effects of nicotine and alcohol co-administration on expression of inducible transcription factors in mouse brain

Nicotine and alcohol are abused substances that are often used concurrently. Despite their combined usage, little is known about how they interact to produce changes in behavior and neural activity. Two experiments were conducted to identify interactions on both behavior and neural targets resulting from the co-administration of nicotine and alcohol. In Experiment 1, male C57BL/6J mice were administered saline, alcohol (2.4 g/kg, i.p.), nicotine (0.5 mg/kg, i.p.) or an alcohol/nicotine mixture and returned to their home cage. In Experiment 2, a higher dose of nicotine (1.0 mg/kg, i.p.) was included and animals were exposed to a novel environment. Several behavioral measures were analysed during novelty exposure. Immunohistochemical detection of inducible transcription factors (c-Fos and Egr1) was used in both experiments to identify changes in neural activation. Behavioral results suggested that the drugs were interacting in the production of behaviors. In particular, alcohol produced locomotor stimulation while it suppressed counts of rearing and leaning. When co-administered, nicotine appeared to counteract the alcohol-enhanced locomotor activity. Several brain regions were observed to have altered transcription factor expression in response to the different drug treatments, including amygdalar, hippocampal and cortical subregions. In a subset of these brain areas, nicotine and alcohol counteracted one another in the expression of transcription factors. These results identify several interactive target sites within the hippocampus, extended amygdala and cortical regions. The interactions appear to be a result of antagonizing actions of nicotine and alcohol. Finally, the results suggest that the combined use of nicotine and alcohol may offset the effects of the drug administered independently.

Involvement of adrenergic and cholinergic systems in nicotine-induced anxiogenesis in mice

In the present study, the effect of alpha-adrenoceptor agents on response to nicotine in an anxiety model (elevated plus-maze) in mice was investigated. Administered nicotine reduced indices of anti-anxiety behaviour (percent open-arm time (%open-arm time) and percent open-arm entries (%open-arm entry)) and increased indices of anxiety behaviour (protected stretched attention posture and percent of protected head dipping (%protected dipping)), indicating that nicotine elicits an anxiogenic response. This response to the drug was obtained 7 min but not 30 min after drug injection and with doses of 0.25 and 0.5 mg/kg. Nicotinic receptor antagonists mecamylamine (0.5 and 1 mg/kg) and hexamethonium (5 and 10 mg/kg) reduced the response induced by nicotine (0.25 mg/kg). Mecamylamine (1 mg/kg; decreased %open-arm entry and increased protected stretched attention posture) and hexamethonium (10 mg/kg; decreased %open-arm time) showed an anxiogenic-like profile. A muscarinic receptor antagonist, atropine (2.5 and 5 mg/kg), did not alter the nicotine response but elicited an anxiogenic effect by itself. The alpha(1)-adrenoceptor antagonist prazosin (0.25 and 0.5 mg/kg), but not the alpha(1)-adrenoceptor agonist, phenylephrine (4 and 6 mg/kg), reversed the nicotine effect. Single administration of phenylephrine (6 mg/kg) increased %open-arm time, while prazosin did not alter the anxiety behaviour. The alpha(2)-adrenoceptor agonist clonidine (0.001 and 0.01 mg/kg), induced complete immobility when administered in combination with nicotine. However, an alpha(2)-adrenoceptor antagonist, yohimbine (0.5 and 1 mg/kg), appeared to reverse the nicotine response, but did not show interaction with nicotine's effect. Clonidine did not elicit any effect, but yohimbine (1 mg/kg) increased %open-arm entry and %open-arm time by itself. It can be concluded that certain doses of nicotine (0.25 and 0.5 mg/kg) 7 min after their injection induce an anxiogenic effect through nicotinic mechanism(s), and that involvement of alpha(1)- but not alpha(2)-adrenoceptors in the response to nicotine seems likely.

Previous exposure to footshock stress attenuates nicotine-induced serotonin release in rat striatum during the subsequent stress

We have analyzed the effects of chronic or repeated footshock stress on the release of serotonin (5-hydroxytryptamine: 5-HT) in the striatum of rats that received nicotine by using a microdialysis technique. Neither local infusion of nicotine alone nor stress application alone changed 5-HT release. Local infusion of 1 mM nicotine to the striatum, however, significantly increased 5-HT release in the striatum to 145.9 +/- 30.8 pg/dialysate during simultaneous stress application. These increases of extracellular 5-HT release induced by the combination of nicotine and stress application were also observed in rats that had received daily chronic footshock. However, the previously administered footshock induced the reduced release of 5-HT from the striatum to 33.5 +/- 8. 6 (repeated footshock) and 10.0 +/- 3.6 pg/dialysate (daily footshock) when footshock was given together with nicotine infusion. These results suggest that previous exposure to stress attenuated the nicotine-induced 5-HT release in the striatum during the subsequent stress.

Nicotine, brain nicotinic receptors, and neuropsychiatric disorders

Neuronal nicotinic acetylcholine receptors (nAChRs) represent a large family of ligand-gated cation channels with diverse structures and properties. In contrast to the muscular nAChRs, the physiological functions of neuronal nAChRs are not well defined to date. Behavioral studies indicate that brain nAChRs participate in complex functions such as attention, memory, and cognition, whereas clinical data suggest their involvement in the pathogenesis of certain neuropsychiatric disorders (Alzheimer's and Parkinson's diseases, Tourette's syndrome, schizophrenia, depression, etc.). For the majority of these disorders, the use of nAChRs' agonists may represent either a prophylactic (especially for Alzheimer's and Parkinson's diseases) or a symptomatic treatment. The possible mechanisms underlying these beneficial effects as well as the characteristics and potential therapeutic use of new, subtype-selective nAChRs agonists are presented.

Effect of R-(-)-alpha-methylhistamine and thioperamide on in vivo release of norepinephrine in the rat hippocampus

1. The modifications of hippocampal release of norepinephrine following the administration of R-(-)-alpha-methylhistamine and thioperamide, respectively agonist and antagonist of histamine H3 receptors, were assessed in freely moving rats by microdialysis. 2. Both the systemic (2 mg/kg i.p.) and local (100 microM via the probe) administration of thioperamide caused no modifications of basal release, indicating that the histaminergic system is not tonically involved in regulating the hippocampal noradrenergic activity. 3. R-(-)-alpha-methylhistamine (1 and 100 microM) produced a slight, short-lasting and dose-dependent reduction of norepinephrine release antagonized by local perfusion (100 microM) and prevented by systemic administration of thioperamide 2 mg/kg. 4. The results seem to indicate that the modulation of norepinephrine release through presynaptic H3-receptors in the rat hippocampus plays a minor role in the memory-enhancing effects of thioperamide.

Differential effects of nicotine against stress-induced changes in dopaminergic system in rat striatum and hippocampus

A number of studies have shown an increase in nicotine self-administration among smokers when exposed to stress. Since it is well known that nicotine or stress alter the dopaminergic system, we examined the effect of chronic nicotine administration on the dopamine level and its metabolism in the striatum and the hippocampus during stressful conditions in rats. Nicotine (0.4 mg/kg, i.p. for 14 days) increased the dopamine level in the striatum (P<0. 05) and decreased it in the hippocampus (P<0.05) in comparison with the effect of saline. Three hours of water-immersion restraint stress sharply elevated the dopamine level (P<0.05) and reduced the 3-methoxytyramine level (P ranged from 0.05 to 0.001 depending on the area and time point) in both brain regions studied, while dihydroxyphenylacetic acid and homovanilic acid levels were not altered. Nicotine pretreatment attenuated some of these changes in a region- and time-dependent manner. However, stress induced a decrease in dopamine turnover in the hippocampus (P<0.05) but not in the striatum, and nicotine failed to prevent this effect. Stress-induced alterations gradually returned toward normal during the 48-h observation period, and in some cases this was facilitated by nicotine. Thus, we demonstrated differential, region- and time-dependent protective effects of chronic nicotine administration against stress-induced changes in dopamine levels and release in brain regions critically affected by stress.

Nicotinic acetylcholine receptor antagonistic activity of monoamine uptake blockers in rat hippocampal slices

The aim of our study was to investigate the effect of different monoamine uptake blockers on the nicotine-evoked release of [3H]noradrenaline ([3H]NA) from rat hippocampal slices. We found that desipramine (DMI), nisoxetine, cocaine, citalopram, and nomifensine inhibit the nicotine-evoked release of [3H]NA with an IC50 of 0.36, 0.59, 0.81, 0.93, and 1.84 microM, respectively. These IC50 values showed no correlation with the inhibitory effect (Ki) of monoamine uptake blockers on the neuronal NA transporter (r = 0.17, slope = 0.02), indicating that the NA uptake system is not involved in the process. In whole-cell patch clamp experiments neither drug blocked Na+ currents at 1 microM in sympathetic neurons from rat superior cervical ganglia, and only DMI produced a pronounced inhibition (52% decrease) at 10 microM. Comparison of the effect of DMI and tetrodotoxin (TTX) on the electrical stimulation- and nicotine-evoked release of [3H]NA showed that DMI, in contrast to TTX, inhibits only the nicotine-induced response, indicating that the target of DMI is not the Na+ channel. Our data suggest that monoamine uptake blockers with different chemical structure and selectivity are able to inhibit the nicotinic acetylcholine receptors in the CNS. Because these compounds are widely used in the therapy of depressed patients, our findings may have great importance in the evaluation of their clinical effects.

Neonatal chlorpyrifos exposure alters synaptic development and neuronal activity in cholinergic and catecholaminergic pathways

After routine home application of chlorpyrifos (CPF), infant and child exposures can exceed acceptable levels. We treated neonatal rats daily on postnatal days (PN) 1-4 (1 mg/kg) or days 11-14 (5 mg/kg), treatments that evoked no overt signs of toxicity. Effects on the development of cholinergic neuronal function were assessed using choline acetyltransferase (ChAT) activity and hemicholinium-3 (HC-3) binding as indices of synaptic proliferation and synaptic activity, respectively. In the forebrain, early CPF treatment caused a decrease in ChAT without affecting HC-3 binding; late treatment decreased HC-3 binding without affecting ChAT. In the brainstem, early treatment had no effect on either parameter but late treatment decreased both ChAT and HC-3 binding. Effects of CPF were not limited to development of cholinergic synapses but also involved catecholamine pathways. For norepinephrine or dopamine, either early or late CPF treatment evoked an increase in synaptic activity (transmitter turnover). The cerebellum, a region with sparse cholinergic innervation, was affected the most. Effects on catecholamine systems were unrelated to the magnitude or temporal pattern of cholinesterase inhibition. Our results suggest that CPF exposure during the postnatal period of synaptogenesis elicits widespread disruption of cholinergic and catecholaminergic pathways. As this is the period in which patterns of synaptic responsiveness is programmed by neural input, the period of developmental vulnerability to CPF is likely to extend into childhood.

Pharmacological characterization of nicotine-induced acetylcholine release in the rat hippocampus in vivo: evidence for a permissive dopamine synapse

In this study, the mechanism of nicotine-induced hippocampal acetylcholine (ACh) release in awake, freely moving rats was examined using in vivo microdialysis. Systemic administration of nicotine (0.4 mg kg(-1), s.c.) increased the levels of ACh in hippocampal dialysates. The nicotine-induced hippocampal ACh release was sensitive to the pretreatment of neuronal nicotinic acetylcholine receptor (nAChR) antagonists mecamylamine (3.0 mg kg(-1), s.c.) and dihydro-beta-erythrodine (DHbetaE; 4.0 mg kg(-1), s.c.) as well as systemic administration of the dopamine (DA) D1 receptor antagonist SCH-23390 (R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-benzaz epine; 0.3 mg kg(-1), s.c.). Local perfusion of mecamylamine (100 microM), DHbetaE (100 microM) or SCH-23390 (10 microM) through microdialysis probe did not increase basal hippocampal ACh release. Hippocampal ACh release elicited by systemic administration of nicotine (0.4 mg kg(-1), s.c.) was antagonized by local perfusion of SCH-23390 (10 microM), but not by MEC (100 microM) or DHbetaE (100 microM). Direct perfusion of nicotine (1 mM, but not 0.1 mM) increased hippocampal ACh levels; however, this effect was relatively insensitive to blockade by co-perfusion of either mecamylamine (100 microM) or SCH-23390 (10 microM). These results suggest that nicotine-induced hippocampal ACh release occurs by two distinct mechanisms: (1) activation of nAChRs outside the hippocampus leading to DA release and subsequent ACh release involving a permissive DA synapse, and (2) direct action of nicotine within the hippocampus leading to ACh release via non-DA-ergic mechanism.

Cholinergic receptors in heart and brainstem of rats exposed to nicotine during development: implications for hypoxia tolerance and perinatal mortality

Cigarette smoking during pregnancy increases the incidence of perinatal mortality and Sudden Infant Death Syndrome (SIDS). We have evaluated prenatal or postnatal nicotine exposure in developing rats to examine the potential role of altered neurotransmitter receptor expression in these processes. Pregnant rats received continuous infusions of nicotine throughout gestation, at doses mimicking the plasma levels found in smokers. After birth, cardiac M2-muscarinic cholinergic receptors, which are responsible for inhibitory autonomic actions, were enhanced in the nicotine group, coincidentally with decreases in stimulatory beta-adrenergic receptors that have been demonstrated previously. Studies of adenylyl cyclase activity confirmed that the changes in receptor binding represented functional alterations: the stimulatory response to isoproterenol was obtunded by prenatal nicotine exposure, whereas the inhibitory response to carbachol was enhanced. Elevations of M2-muscarinic receptor binding were not generalized to all tissues, as the same prenatal nicotine treatment elicited a reduction in these receptors in the brainstem, an effect that has also been noted in infants who died of SIDS; we found no effects of prenatal nicotine on brainstem M1-receptor binding. Postnatal administration of nicotine produced similar brainstem receptor effects when treatment was conducted during the first postnatal week but not thereafter; postnatal nicotine treatment did not affect cardiac M2-receptor binding. Thus, during a critical developmental period, nicotine exposure produces cardiac and brainstem receptor imbalances that favor inhibitory responses, effects that can contribute to morbidity and mortality evoked by hypoxic episodes, such as those experienced during parturition, sleep apnea or airway obstruction.

The effects of nicotine on spatial and non-spatial expectancies in a covert orienting task

The present study examined how nicotine influences shifts of visuo spatial attention in casual smokers at each of three delays after smoking one cigarette: immediately, 1 h and 24 h. Informative peripheral cues were used to exogenously orient attention to the location where an increase or decrease in the size of a peripheral object was most likely to occur. One size change was more likely to occur than the other and the task was choice (expansion/contraction) reaction time. The performance decrement obtained when the target appeared at an uncued location was smallest in sessions run immediately after smoking (when nicotine levels were highest), suggesting that nicotine may increase the ease with which attention can be disengaged from a cued location. This finding confirms previous research which suggests a specific role for the basal forebrain cholinergic system in visual orienting. In contrast, nicotine was not found to affect non-spatial expectancies based on stimulus-response (expansion/contraction) probability. These findings, together with recent converging evidence, strongly support the proposition that different attentional operations are mediated by different neural subsystems.

The role of monoamine neurotransmitter systems in the nicotine discriminative stimulus

Nicotine serves as a reinforcer and induces a robust discriminative stimulus which is primarily mediated by neuronal nicotinic receptors. As a secondary effect of nicotinic stimulation, nicotine elicits an enhanced release of the biogenic amine neurotransmitters dopamine, norepinephrine and serotonin. In particular, compounds with dopaminergic activity have been reported to modify both the reinforcing and discriminative stimulus properties of nicotine. The present study examined a number of dopaminergic, noradrenergic and serotonergic compounds for their effectiveness in reproducing or modifying the stimulus properties of nicotine in rats. The non-selective dopamine agonists amphetamine, cocaine and apomorphine produced partial substitution for nicotine, while the selective D2/D3 agonists bromocriptine and 7-OH-DPAT and the dopamine autoreceptor antagonist (+)-AJ-76 had little effect. The substitution of amphetamine for nicotine was not blocked by haloperidol, suggesting a minimal role for D2 receptors in the nicotine-like discriminative effects of stimulants. The selective D1 agonist SKF 81,297 produced partial substitution for nicotine (45% maximum), but further experiments with the D1 antagonist SCH 23,390 and with rats trained in a three-way discrimination procedure failed to support a primary role for this receptor in the substitution of dopaminergic drugs for nicotine. Finally, tests of compounds with effects on noradrenergic or serotonergic neurotransmission did not yield strong evidence for the involvement of these systems. Taken together, these data support earlier suggestions that activation of dopamine receptor subtypes plays a role in the nicotine-like stimulus properties of abused stimulants, but do not clearly identify a single subtype that is uniquely responsible.

Nicotine increases stress-induced serotonin release by stimulating nicotinic acetylcholine receptor in rat striatum

We used a microdialysis technique to analyze the effects of footshock stress on the release of serotonin (5-hydroxytryptamine: 5-HT) in the striatum or prefrontal cortex (PFC) in rats that were pretreated with nicotine. Neither nicotine administration alone nor stress application alone changed 5-HT release. During stress application, however, both chronic nicotine administration and local infusion of nicotine to the striatum significantly increased 5-HT release in the striatum, though not in the PFC. These increases in 5-HT release were eradicated by a local infusion of mecamylamine. Release of 5-HT increased in the striatum during stress application when nicotine was injected to the striatum, while nicotinic injection to the dorsal raphe nucleus did not increase 5-HT release in the striatum. The present study demonstrates that nicotine induced a release of 5-HT upon stress application by stimulating presynaptic nicotinic receptors in the striatum.

Noradrenergic control of central oxytocin release during lactation in rats

Noradrenergic systems regulate the systemic release of oxytocin (OT) in lactating rats. However, a role for norepinephrine (NE) in release of OT within the magnocellular nuclei during suckling has not been established. These studies were designed to determine 1) if suckling induces NE release in the supraoptic (SON) and paraventricular (PVN) nuclei of conscious rats and 2) the role of NE in the central, intranuclear release of OT within these nuclei. Female Holtzman rats were implanted with microdialysis probes adjacent to the PVN or SON on lactation days 8-12. The following day, the pups were isolated from the dams for 4 h. Microdialysis probes were perfused with artificial cerebrospinal fluid (ACSF) or with ACSF containing an alpha- or a beta-adrenergic receptor antagonist. Dialysate was collected before, during, and after suckling and analyzed for NE or OT. In an additional experiment, an alpha- or beta-adrenergic agonist was administered via the microdialysis probes into the PVN in nonsuckled, lactating rats. Extracellular NE increased in the PVN during suckling but was not detectable in the SON. OT concentrations in dialysates from the PVN and SON significantly increased during suckling. Blockade of either alpha-(in both PVN and SON) or beta- (PVN) adrenergic receptors prevented the suckling-induced increase in central OT release. OT release was increased in nonsuckled, lactating rats by central application of either an alpha- or beta-adrenergic agonist. These data demonstrate that intranuclear NE release is increased in the PVN by suckling and that subsequent stimulation of both alpha- and beta-noradrenergic receptors mediates intranuclear OT release.

Desensitization and resensitization of norepinephrine release in the rat hippocampus with repeated nicotine administration

Desensitization of norepinephrine release was investigated with repeated intravenous (i.v.) infusions of nicotine and in vivo microdialysis of the hippocampus. At 100 min intervals, rats received three infusions of one of the following doses of nicotine: 0.045, 0.09 or 0.135 mg/kg. Doses of 0.09 mg/kg or higher increased norepinephrine release (F= 2.41, P < 0.05). However, the norepinephrine response to the second or third infusion was significantly reduced, compared to the first. The extent of desensitization and rate of resensitization was investigated further by administering consecutive infusions of nicotine (0.135 mg/kg) 40, 60, 100 or 200 min apart. Less norepinephrine was released after a second nicotine infusion given 40 to 100 min later, but this was not reduced further by a third infusion. Norepinephrine release was unchanged with a 200 min inter-infusion interval. Therefore, in the hippocampus, maximal desensitization of nicotine-stimulated norepinephrine release occurs as early as 40 min and persists for at least 100 min; thereafter, resensitization becomes the dominant process.

Cerebrospinal fluid neuroendocrinology of alcohol misusers

Cerebrospinal fluid (CSF) neurohumors reflect central nervous system physiology in a way that peripheral indices can not. We reviewed clinical studies of CSF biogenic amines and neurohormones in alcohol misusers during various stages of withdrawal or abstinence and found them difficult to compare because of highly variable experimental methods, reliance on single time collections (lumbar punctures) that fail to control for potential stress-induced effects of the procedure, lack of control for tobacco use, and a paucity of non-alcoholmisusing controls. However, taken together, the data thus far show that a variety of neuroactive substances are reduced in concentration in the CSF of some alcohol misusers. Low CSF levels of corticotropinreleasing hormone, beta-endorphin, norepinephrine, diazepam-binding inhibitor, 5-hydroxyindoleacetic acid and somatostatin have all been reported. Whether the decreased CSF levels of these neurohormones and neurotransmitters are a cause or consequence of alcoholism has not been determined. In fact, further studies using serial or continuous CSF sampling techniques with homogeneous, better-characterized patients and normal volunteers are still needed to establish the precise CSF neurochemical abnormalities in alcohol misusers.

Nicotine induces long-lasting potentiation in the dentate gyrus of nicotine-primed rats

A challenge dose of nicotine (0.4 mg kg-1 s.c.) produced a long-lasting potentiation (LLP) of field EPSPs evoked in the dentate gyrus by stimulation of the medial perforant path of urethane-anaesthetized rats primed four weeks previously with seven daily injections of nicotine (0.8 mg kg-1 s.c.). The same dose of nicotine did not alter EPSPs evoked in rats primed with saline vehicle. Systemic injection of either mecamylamine or propranolol, 30 min before nicotine challenge, prevented induction of LLP without affecting baseline EPSPs but neither drug affected LLP when given after it was established. Since the nicotine-priming regime is known to increase tyrosine hydroxylase activity and hippocampal noradrenaline release in response to nicotine challenge, the results suggest that systemic injection of nicotine released sufficient noradrenaline in nicotine-primed rats to induce a beta-adrenoceptor-mediated LLP.

Nicotinic acetylcholine receptor antagonist effect of fluoxetine in rat hippocampal slices

We compared the effect of mecamylamine and fluoxetine on the hippocampal noradrenaline (NA) release evoked by nicotine in vitro. Nicotine (100 microM) increased the basal release of [3H]NA from rat hippocampal slices. This effect was blocked by the potent nicotinic antagonist mecamylamine in a dose-dependent manner (IC50 = 0.19 microM). The selective serotonin reuptake inhibitor (SSRI) fluoxetine also antagonised the response to nicotine in a dose-dependent manner with a similar strength (IC50 = 0.57 microM). Our data indicate that fluoxetine has nicotinic acetylcholine receptor antagonist effect in the central nervous system. The possible clinical significance of this finding is discussed.

Adrenocorticotropin response and nicotine-induced norepinephrine secretion in the rat paraventricular nucleus are mediated through brainstem receptors

Nicotine is a potent stimulus for the secretion of ACTH, and norepinephrinergic neurons originating in the brainstem are involved. Prior reports using in vivo microdialysis in alert rats have shown that nicotine, administered i.p. or into the fourth ventricle, stimulated the release of norepinephrine (NE) into the hypothalamic paraventricular nucleus (PVN), the site of neurons containing CRH. In the present studies, rats received an i.v. infusion of nicotine into the jugular vein on alternate days during their active (dark) phase; therefore, direct correlations between the levels of NE microdialyzed from the PVN and plasma ACTH could be made in each animal. Nicotine administered i.v. (0.045-0.135 mg/kg) elicited dose-dependent increases in both NE and ACTH (P < 0.01). A significant correlation was found between nicotine-stimulated NE release in the PVN and ACTH secretion (r = 0.91, P < 0.01). To address whether the site(s) of action of nicotine was on presynaptic receptors on NE terminals in the PVN or on receptors on neurons in brainstem regions accessible from the fourth ventricle, the nicotinic cholinergic antagonist, mecamylamine (0.1-4.8 microg), was microinjected directly into the PVN or into the fourth ventricle before nicotine infusion. Fourth-ventricular administration of mecamylamine (1.6 microg) or higher, before i.v. nicotine (0.09 mg/kg), completely blocked both NE release in the PVN (IC50 = 0.64 microg) and ACTH secretion (IC50 = 0.40 microg) (P < 0.01, compared with vehicle before nicotine), whereas it was ineffective when injected directly into the PVN. The results demonstrate that the nicotinic cholinergic receptors in the brainstem, rather than presynaptic receptors within the PVN itself, mediate nicotine-stimulated PVN NE release and ACTH secretion.

Relationship between up-regulation of nicotine binding sites in rat brain and delayed cognitive enhancement observed after chronic or acute nicotinic receptor stimulation

(-)-Nicotine tartrate (2 mg/kg), and a nicotinic agonist, RJR 2403 (1.4 mg/kg), and antagonist, mecamylamine (1 mg/kg), were administered to separate groups of rats SC twice daily for 10 days. Two other groups received the same doses of nicotine or RJR 2403 for 1 day followed by saline for 9 days. Twenty-four hours after the final injection, the rats were compared to a 10-day saline-injected group on acquisition of a hidden platform position in the Morris water maze (20 trials, 30-min inter-trial interval). The rats were killed 48 h after the last drug injection and frontal, entorhinal and posterior cingulate cortex and dorsal and ventral hippocampus assayed for [3H]-nicotine binding density. Chronic nicotine significantly increased the number of frontal and entorhinal cortical and dorsal hippocampal, but not posterior cingulate cortical or ventral hippocampal, nicotinic receptors, and improved rate of learning. Chronic mecamylamine and RJR 2403 also significantly increased the number of nicotinic receptors in frontal cortex, though not other regions, but retarded rate of learning. Nicotine given for 1 day 11 days earlier marginally increased nicotinic receptors in entorhinal cortex (but not other regions) and significantly increased rate of learning, though significantly less than 10-day nicotine. Entorhinal cortical and dorsal hippocampal nicotinic receptor numbers were positively associated with rate of learning but not performance at asymptote. Thus cognitive enhancement after chronic nicotine is in part a delayed consequence of nicotine administration 11 days earlier, and may reflect regional changes in nicotinic receptor up-regulation.

Behaviour-related effects of nicotine on slow EEG waves in basal nucleus-lesioned rats

The basal magnocellular nucleus is assumed to play a crucial role in cholinergic activation of the cortical EEG. The aim of this study was to establish whether intraperitoneally applied nicotine may counteract the power asymmetry of the slow waves in the cortical EEG of both hemispheres after an unilateral lesion in the basal nucleus. In 17 rats the basal nucleus (substantia innominata/ventral pallidum) was unilaterally lesioned by ibotenic acid. The lesion produced unilateral power increases of all frequencies up to 20 Hz in the frontal EEG that increased with higher arousal level. Additionally, synchronized spike and wave discharges appeared in the frontal EEG. The results indicate that the basal nucleus suppresses especially the delta EEG waves in the frontal motor cortex during motor active behaviour. Nicotine (0.1 and 1 mg/kg) partially counteracts the power asymmetry of frontal slow waves (2-6 Hz) only during exploratory sniffing but not during grooming and waking immobility. Physostigmine (1 mg/kg) was also effective during exploratory sniffing. The results may indicate a role of nicotinic mechanisms in the information input component of exploratory behaviour.

Pharmacology of nicotine and its therapeutic use in smoking cessation and neurodegenerative disorders

During the last decade, nicotine has been used increasingly as an aid to smoking cessation and has been found to be a safe and efficacious treatment for the symptoms of nicotine withdrawal. This period has also seen significant advances in our understanding of the mechanisms underlying the psychopharmacological responses to nicotine, including, particularly, those that have been implicated in nicotine addiction. This paper reviews this decade of progress in the specific context of the therapeutic application of nicotine to the treatment of smoking cessation. Other putative future applications, particularly in the treatment of neurodegenerative disorders, are also reviewed.

Changes in the central and peripheral serotonergic system in rats exposed to water-immersion restrained stress and nicotine administration

The effects of water-immersion restraint stress (WS) on chronically nicotine-administered rats were studied in the blood and various regions of the brain. Serotonin (5-HT) levels increased in the hypothalamus, hippocampus, cortex and cerebellum following the administration of nicotine. 5-HT levels increased in all the brain regions following stress. Nicotine decreased stress-induced increased levels of 5-HT in the hippocampus and cerebellum. Nicotine administration alone increased 5-hydroxyindole acetic acid (5-HIAA) levels in the hippocampus and cerebellum. Stress alone also increased 5-HIAA levels in all the brain regions. In the cortex, 5-HT and 5-HIAA levels further increased following the administration of a combination of stress and nicotine compared to rats given stress alone. In the blood as well as in all the brain regions, except the cerebellum, stress or nicotine administration did not affect tryptophan levels. Stress given to nicotine-administered rats resulted in a decrease in tryptophan levels in the blood and plasma. Although 5-HT and 5-HIAA levels were not influenced by stress and/or nicotine administration, the 5-HIAA/5-HT ratio increased in the blood and plasma of rats administered with nicotine and exposed to stress. The effects of nicotine on the serotonergic system depend upon the kind of stress given together with the organs and brain regions involved.

Nicotine-induced hypothermia through an indirect dopaminergic mechanism

The effect of nicotine on core body temperature was studied in mice. Intraperitoneal (i.p.) injection of nicotine (0.5, 1 and 2 mg/kg) induced a dose-dependent hypothermia. The response was inhibited by reserpine (5 mg/kg), the centrally active nicotinic receptor antagonist mecamylamine (0.1-1 mg/kg) and the D-2 dopamine receptor antagonist sulpiride (25-100 mg/kg). The β-adrenoceptor antagonist propranolol (5 and 10 mg/kg) and the serotonergic blocker methysergide (5 and 10 mg/kg) did not inhibit but increased the nicotine response. The α-adrenoceptor antagonist phenoxybenzamine, the antimuscarinic agent atropine, the D-1 dopamine receptor antagonist SCH 23390, the peripheral dopamine antagonist domperidone and the peripheral nicotinic antagonist hexamethonium did not alter the nicotine-induced hypothermia. It is concluded that nicotine may cause a fall in core body temperature through a central dopaminergic mechanism.

Effect of nicotine, lobeline, and mecamylamine on sensory gating in the rat

In normal subjects, if an acoustic startle stimulus is immediately preceded by a small brief change in background noise intensity, the magnitude of the subsequent startle response is decreased. This prepulse inhibition (PPI) of an acoustic startle response has been shown to be associated with sensorimotor gating. PPI is disrupted in schizophrenic patients and has been linked to attentional disorders characteristic of this disease. We tested the effects of (-)-nicotine, (0.19, 0.62, and 1.9 mumol/kg IP) (equivalent to 0.03, 0.1, and 0.3 mg/kg base) and the nicotinic cholinergic receptor (nAChR) channel blocker, mecamylamine (5.0 and 50 mumol/kg IP) (equivalent to 1.0 and 10.0 mg/kg) on PPI of the acoustic startle response in the rat. Nicotine increased the PPI at the lowest prepulse signal levels but not at the stronger levels. Mecamylamine was without effect at 5.0 mumol/kg, but the 50 mumol/kg dose decreased the inhibition at both weak and strong prepulse (PP) levels. Mecamylamine (5.0 mumol/kg) pretreatment did not block the (-)-nicotine-induced increase in PPI. Lobeline (0.19, 0.62, 1.9, and 6.2 mumol/kg IP) (equivalent to 0.071, 0.23, 0.71, and 2.3 mg/kg) was without effect. These results are consistent with a mecamylamine-insensitive effect of nicotine to improve gating in normal rats. The nAChR subtype involved in producing nicotine's increase of PPI needs further investigation.

Are the cognitive-enhancing effects of nicotine in the rat with lesions to the forebrain cholinergic projection system mediated by an interaction with the noradrenergic system?

Experiments were conducted to test the hypothesis that the enhancing effect of nicotine on water maze performance in rats with lesions of the forebrain cholinergic projection systems (FCPS) is mediated by an interaction with the noradrenergic system, in particular the ascending dorsal noradrenergic bundle (DNAB) and its projection areas. Three groups of rats received lesions of either: i) the nucleus basalis (NBM) and medial septal area/diagonal band (MSA/DB) by infusion of alpha-amino-3-hydroxy-4-izoxazole propionic acid (AMPA) (FCPS group), ii) DNAB, by infusion of 6-hydroxydopamine (6-OHDA) (NOR group), or iii) both FCPS plus DNAB (COMB group). Control animals received vehicle. Choline acetyltransferase activity was reduced in the cortex and hippocampus of the FCPS and COMB groups and in the hippocampus of the NOR group. NA level was reduced in the cortex and hippocampus of the FCPS and COMB groups, but not the FCPS group. In a reference memory task, the performance of both the NOR and COMB groups, but not the NOR group, was significantly worse than that of controls; there was no effect of nicotine administration (0.1 mg/kg) on escape latency or other measures in this task. In a working memory task, FCPS and COMB rats took longer to find the submerged platform on the second and following trials, and there was a significant enhancement of performance by nicotine in both groups, but not in controls. These results indicate that the enhancing effects of nicotine in rats with FCPS lesions are not mediated by an interaction with the DNAB.