Studies on the possible role of brain 5-HT systems and adrenocortical activity in behavioural responses to nicotine and diazepam in an elevated X-maze

Opposing effects of acute and repeated nicotine exposure on boldness in zebrafish

Nicotine is an addictive compound that activates neuronal nicotinic acetylcholine receptors (nAChRs) and causes behavioural effects that vary with dose, schedule of administration, and animal model. In zebrafish (Danio rerio), acute doses of nicotine have been consistently found to have anxiolytic properties, whereas, chronic exposure elicits anxiogenic effects. To date, however, studies on repeated nicotine administration and the effects of nicotine withdrawal have not been well explored using this model. In this study, we administered nicotine with three different dosing regimens: 1. Single exposures of a "high" dose (25, 50, 100, or 400 mg/L) for 3 minutes. 2. Single exposures to a "low" dose (2.5, 5, or 20 mg/L) for one hour. 3. Repeated one-hour exposure to a "low" dose (2.5, 5, or 20 mg/L) for 21 days. The novel object approach test was used to examine boldness based on the tendency of the fish to explore a novel object. Acutely, nicotine significantly increased the time spent approaching the object with both three-minute and onehour durations of exposure, indicating increased boldness. Conversely, after repeated nicotine exposure for 21 days, fish spent less time approaching the object suggesting a decrease in boldness. Distance moved was unaffected one hour after repeated nicotine exposure, yet decreased after a two-day withdrawal period. Our work suggests that nicotine can have opposing effects on boldness that vary based on dosage and schedule of exposure.

High fat feeding is associated with stimulation of the hypothalamic-pituitary-adrenal axis and reduced anxiety in the rat

Previous studies have shown that diet-induced obesity is associated with insulin resistance and impaired feedback control of the hypothalamic-pituitary-adrenal (HPA) axis. The objective of this study was to test the hypothesis that hyper-secretion of glucocorticoid, evoked by feeding rats a high fat (HF) diet for 12 weeks, also influences behavioural and neural responses to the elevated plus-maze (EPM) test of anxiety. HF-fed animals exhibited anxiolytic-like behaviour in the EPM but were also hyperactive in this test. Covariant analysis established that the anxiolytic-like behaviour was not secondary to the increase in activity. The HF diet significantly increased basal levels of plasma corticosterone. The groups exposed to the EPM also displayed increased plasma corticosterone levels compared to the relevant control group, although the increment was smaller in the HF-fed animals. Glucocorticoid receptor (GR) immunoreactivity in the cytoplasmic fraction of parietal cortex and hypothalamus and the particulate fraction of the parietal cortex were increased by HF feeding. The behavioural changes evoked by HF feeding did not correlate significantly with changes in GR immunoreactivity in each treatment group or 5-HT turnover in the brain areas studied. It is concluded that anxiolytic properties evoked in the EPM by high fat feeding are unlikely to be related to the changes in HPA function seen in animals fed this diet.

Age-related effects of acute nicotine on behavioural and neuronal measures of anxiety

Adolescence is considered a period of enhanced vulnerability to the initiation of tobacco use. Biological differences in the sensitivity of adolescents and adults to the anxiogenic and anxiolytic effects of nicotine may contribute to this heightened vulnerability. Here, we investigated the age-dependency of the effects of acute nicotine on anxiety-related behaviour and neurotransmission. In Experiment 1, male adolescent (P33-37) and adult (P65-69) Long-Evans rats received nicotine (0, 0.4 or 0.8 mg/kg,s.c.) prior to testing using two measures of anxiety, the elevated plus maze (EPM) and light-dark (LD) transition box. In Experiment 2, in situ hybridization was used to assess, in different male adolescent and adult rats, CRF mRNA expression in the BNST, PVN and CeA in response to acute nicotine. In the EPM, adult rats displayed less anxious behaviour than adolescents. Nicotine (0.4, 0.8 mg/kg) increased open and closed arm entries in adolescent rats, suggesting increased general activity, but it did not affect behaviour in the LD box. CRF mRNA expression was elevated in PVN of adolescent rats, relative to adults. Nicotine, however, had no effect on CRF mRNA expression in the BNST, PVN or CeA. The present findings suggest that adolescents are more sensitive to the general activational, rather than anxiety-related, effects of nicotine, and that CRF mRNA expression in stress-related brain regions does not correlate with these effects. This work further characterizes the age-related differences in the anxiety-related effects of nicotine, and provides insight into potential factors influencing vulnerability to tobacco abuse.

Genetic variation in the serotonin pathway and smoking cessation with nicotine replacement therapy: new data from the Patch in Practice trial and pooled analyses

The serotonin pathway has been implicated in nicotine dependence and may influence smoking cessation. Therefore, 792 cigarette smokers from the Patch in Practice trial were genotyped for the tryptophan hydroxylase (TPH1 A779C), serotonin transporter (SLC6A45-HTTLPR), and 5-HT1A (HTR1A C-1019G) polymorphisms. Cox regression analysis did not demonstrate significant effects of any of the three genotypes on relapse to smoking: TPH1 (Reference AA; AC: hazard ratio (HR) 0.99, 95% confidence interval (CI) 0.78, 1.24, p=0.90; CC: HR 0.93, 95% CI 0.73, 1.18, p=0.55); 5-HTTLPR (Reference LL; SL: HR 1.01, 95% CI 0.85, 1.20, p=0.90; SS: HR 1.13, 95% CI 0.91, 1.39, p=0.27); HTR1A (Reference CC; CG: HR 1.04, 95% CI 0.86, 1.25, p=0.70; GG: HR 1.01, 95% CI 0.82, 1.24, p=0.93). Moreover, pooled analyses of data from all three extant pharmacogenetic NRT trials (N=1398) found no significant effect of 5-HTTLPR genotype on continuous abstinence at 12-week (Reference LL; SL: odds ratio (OR)=1.25, 95% CI 0.89, 1.74, p=0.19; SS: OR=1.31, 95% CI 0.86, 1.98, p=0.21) or 26-week follow-up (Reference LL; SL: OR=0.93, 95% CI 0.64, 1.33, p=0.68; SS: OR=1.00, 95% CI 0.63, 1.58, p=1.00). These data do not support a statistically or clinically significant moderating effect of these specific 5-HT pathway genetic variants on smoking cessation. However, the possibility remains that other variants in these or other 5-HT genes may influence NRT efficacy for smoking cessation in treatment seeking smokers.

Acute effects of nicotine on restraint stress-induced anxiety-like behavior, c-Fos expression, and corticosterone release in mice

Clinical evidence suggests that nicotine reduces anxiety in stressful situations. In the present study, we investigated the effect of nicotine on restraint-enhanced anxiety-like behavior, c-Fos expression, an index of neuronal activation in the brain, and plasma corticosterone. Two-hour restraint stress-enhanced anxiety-like behavior in the elevated plus-maze (EPM) and nicotine hydrogen tartrate (0.25 mg/kg, i.p.) attenuated the stress-induced changes. Pretreatment with the centrally acting nicotinic antagonist, mecamylamine (2 mg/kg), blocked nicotine's effects. In addition, restraint led to significant increases of c-Fos expression in several brain regions related to stress or anxiety including paraventricular hypothalamic nucleus (PVN), lateral hypothalamic area (LH), central amygdaloid nucleus (CeA), medial amygdaloid nucleus (MeA) and cingulate and retrosplenial cortices (Cg/RS), paraventricular thalamic nucleus (PVT), and basolateral amygdaloid nucleus (BLA). Nicotine attenuated the restraint-induced expression of c-Fos in the PVN, LH, CeA, MeA, and Cg/RS, while leaving the BLA and PVT unaffected. In contrast, nicotine did not reverse the increased levels of plasma corticosterone induced by restraint. These findings suggest that nicotine may modify the stress-induced behavioral changes via regulating the neuronal activation in selected brain regions rather than affecting hypothalamo-pituitary-adrenocortical axis hormone responses.

Effects of nicotine on elevated plus maze and locomotor activity in male and female adolescent and adult rats

Over 4500 adolescents start smoking every day in the United States. Of these, one-third will die prematurely from smoking-related diseases. The current experiment examined the effects of repeated-acute nicotine administration (saline, 0.1, 0.5, or 1.0 mg/kg daily) on elevated plus maze (EPM) and locomotor behaviors of 160 adolescent and adult male and female Sprague-Dawley rats. Nicotine's effects depended on age and sex of animal. On the EPM, nicotine exerted anxiolytic effects (increased percentage of time in the open arms) in adolescent males, but exerted anxiogenic effects (decreased percentage of time in the open arms) in adolescent females and in adult males and females. For adults, peak locomotor activity occurred at the 0.5-mg/kg dosage, and the 1.0-mg/kg dosage reduced activity below the saline level on Day 1 and below the 0.5-mg/kg level on Days 1, 3, and 5. For adolescents, peak locomotor activity occurred at the 1.0-mg/kg dosage and there were no activity-depressant effects. These findings suggest there are age differences in sensitivity to nicotine that may affect vulnerability to long-term tobacco use.

Selective stimulation of serotonin2c receptors blocks the enhancement of striatal and accumbal dopamine release induced by nicotine administration

The effects of acute and repeated nicotine administration on the extracellular levels of dopamine (DA) in the corpus striatum and the nucleus accumbens were studied in conscious, freely moving rats by in vivo microdialysis. Acute intraperitoneal (i.p.) injection of nicotine (1 mg/kg) increased DA outflow both in the corpus striatum and the nucleus accumbens. Repeated daily injection of nicotine (1 mg/kg, i.p.) for 10 consecutive days caused a significant increase in basal DA outflow both in the corpus striatum and the nucleus accumbens. Acute challenge with nicotine (1 mg/kg, i.p.) in animals treated repeatedly with this drug enhanced DA extracellular levels in both brain areas. However, the effect of nicotine was potentiated in the nucleus accumbens, but not in the corpus striatum. To test the hypothesis that stimulation of 5-HT (5-hydroxytryptamine, serotonin)(2C) receptors could affect nicotine-induced DA release, the selective 5-HT(2C) receptor agonist RO 60-0175 was used. Pretreatment with RO 60-0175 (1 and 3 mg/kg, i.p.) dose-dependently prevented the enhancement in DA release elicited by acute nicotine in the corpus striatum, but was devoid of any significant effect in the nucleus accumbens. RO 60-0175 (1 and 3 mg/kg, i.p.) dose-dependently reduced the stimulatory effect on striatal and accumbal DA release induced by an acute challenge with nicotine (1 mg/kg, i.p.) in rats treated repeatedly with this alkaloid. However, only the effect of 3 mg/kg RO 60-0175 reached statistical significance. The inhibitory effect of RO 60-0175 on DA release induced by nicotine in the corpus striatum and the nucleus accumbens was completely prevented by SB 242084 (0.5 mg/kg, i.p.) and SB 243213 (0.5 mg/kg, i.p.), two selective antagonists of 5-HT(2C) receptors. It is concluded that selective activation of 5-HT(2C) receptors can block the stimulatory action of nicotine on central DA function, an effect that might be relevant for the reported antiaddictive properties of RO 60-0175.

Smoking, stress, and negative affect: correlation, causation, and context across stages of smoking

This transdisciplinary review of the literature addresses the questions, Do stress and negative affect (NA) promote smoking? and Does smoking genuinely relieve stress and NA? Drawing on both human and animal literatures, the authors examine these questions across three developmental stages of smoking--initiation, maintenance, and relapse. Methodological and conceptual distinctions relating to within- and between-subjects levels of analyses are emphasized throughout the review. Potential mechanisms underlying links between stress and NA and smoking are also reviewed. Relative to direct-effect explanations, the authors argue that contextual mediator-moderator approaches hold greater potential for elucidating complex associations between NA and stress and smoking. The authors conclude with recommendations for research initiatives that draw on more sophisticated theories and methodologies.

Effect of nicotine and nicotinic receptors on anxiety and depression

Nicotine has been shown to have effects on anxiety and depression in both human and animal studies. These studies suggest that nicotinic acetylcholine receptors (nAChRs) can modulate the function of pathways involved in stress response, anxiety and depression in the normal brain, and that smoking can result in alterations of anxiety level and mood. The effects of nicotine are complex however, and nicotine treatment can be either anxiolytic or anxiogenic depending on the anxiety model tested, the route of nicotine administration and the time course of administration. The paradoxical effects of nicotine on emotionality are likely due to the broad expression of nAChRs throughout the brain, the large number of nAChR subtypes that have been identified and the ability of nicotine treatment to both activate and desensitize nAChRs. Activation of nAChRs has been shown to modulate many systems associated with stress response including stress hormone pathways, monoaminergic transmission and release of classical neurotransmitters throughout the brain. Local administration studies in animals have identified brain areas that may be involved in the anxiogenic and anxiolytic actions of nicotine including the lateral septum, the dorsal raphe nuclei, the mesolimbic dopamine system and the hippocampus. The ensemble of studies to date suggest that under certain conditions nicotine can act as an anxiolytic and an antidepressant, but that following chronic use, adaptations to nicotine can occur resulting in increased anxiety and depression following withdrawal.

Involvement of serotonin in nicotine dependence: processes relevant to positive and negative regulation of drug intake

The neurobiological substrate of nicotine dependence has been the subject of extensive preclinical and clinical research. Many experimental reports have implicated the brain serotonin (5-HT) systems in processes relevant to nicotine dependence, but the specific role of this neurotransmitter system largely remains to be elucidated. This review will focus on the role of 5-HT in the acute and chronic effects of nicotine. In particular, the evidence for a role of 5-HT neurotransmission in brain processes thought to be involved in positive and negative control of nicotine use will be examined, and potential clinical implications discussed.

Effects of 5-HT1A and 5-HT2 receptor agonists on the behavioral and neurochemical consequences of repeated nicotine treatment

This study investigated the effects of repeated daily (15 days) treatment with nicotine, alone or in combination with the 5-HT1A/7 receptor agonist (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) or the 5-HT2 receptor agonist (+/-)-2,5-dimethoxy-4-iodoamphetamine (DOI) on locomotor sensitization, mesolimbic dopamine neurochemistry and on behavioral inhibition in the rat. Acute nicotine elevated the extracellular dopamine levels in the nucleus accumbens and stimulated locomotor activity, effects that were sensitized after repeated nicotine treatment. Repeated nicotine administration also produced nicotine-induced behavioral disinhibition in the elevated plus-maze. Treatment with DOI counteracted the expression of the nicotine-induced locomotor and neurochemical sensitization, but had no effect on nicotine-induced behavioral disinhibition. Treatment with 8-OH-DPAT decreased the expression of nicotine-induced behavioral disinhibition, but had no effect on locomotor or neurochemical sensitization. Taken together, these findings suggest that the 5-HT1A and the 5-HT2 receptor subtypes are differentially involved in the effects of repeated nicotine on locomotor sensitization, behavioral inhibition and mesolimbic dopamine neurochemistry.

Effects of serotonergic manipulations on the behavioral sensitization and disinhibition associated with repeated amphetamine treatment

This study investigated the effects of repeated amphetamine treatment on locomotor activity and behavioral inhibition in the elevated plus-maze, and the influence of serotonin (5-HT) neurotransmission on these behaviors. Acute administration of amphetamine (1.0 mg/kg subcutaneously [SC]) stimulated locomotor activity, which was attenuated by acute citalopram (5.0 mg/kg SC) pretreatment. Repeated daily treatment with amphetamine (15 days) sensitized the rats to the amphetamine-induced locomotor stimulation. Acute pretreatment with the 5-HT precursor l-5-hydroxytryptophan (5-HTP; 25 mg/kg IP) or chronic treatment with the selective 5-HT reuptake inhibitor citalopram (5.0 mg/kg SC, twice daily), did not alter the expression of amphetamine-induced locomotor sensitization. In the elevated plus-maze, animals subjected to repeated amphetamine treatment expressed behavioral disinhibition after amphetamine exposure (1.0 mg/kg SC; -35 min), which was antagonized both by acute 5-HTP and chronic citalopram treatment. In summary, these findings suggest that behavioral sensitization to amphetamine is associated with amphetamine-induced behavioral disinhibition, and that acute 5-HTP as well as chronic citalopram treatment counteract the expression of amphetamine-induced behavioral disinhibition, but not locomotor sensitization. It appears likely that the antagonistic effects of 5-HTP and citalopram on behavioral disinhibition derive from a drug-induced facilitation of brain 5-HT neurotransmission.

The effects of nicotine on neural pathways implicated in depression: a factor in nicotine addiction?

The prevalence of tobacco smoking varies considerably between different groups within the community, tobacco smoking being particularly prevalent in patients with depressive disorder. This review will focus on results, derived from animal studies, which suggest that, in addition to its primary reinforcing properties, nicotine also exerts effects in stressful environments, which may account for its enhanced addictive potential in depressed patients. It focuses on the evidence that depression sensitises patients to the adverse effects of stressful stimuli, and that this can be relieved by drugs that stimulate dopamine release in the forebrain. This mechanism, it is proposed, contributes to the increased craving to smoke in abstinent smokers exposed to such stimuli, because they become conditioned to use this property of nicotine to produce rapid alleviation of the adverse effects of the stress. The review also explores the possibility that chronic exposure to nicotine elicits changes in 5-HT formation and release in the hippocampus which are depressogenic. It is postulated that smokers are protected from the consequences of these changes, while they continue to smoke, by the antidepressant properties of nicotine. However, they contribute to the symptoms of depression experienced by many smokers when they first quit the habit.

The pharmacology of (-)-nicotine and novel cholinergic channel modulators

Advances in the understanding of the molecular biology and pharmacology of nAChRs may provide targets for the development of novel and selective modulators of nAChRs in the brain. This contention is supported by the dissimilar behavioral effects observed following systemic administration of currently available nicotinic ligands. The concept of multiple subtypes of nAChRs is not unique, as evidenced by the pharmacology of other ligand-gated ion channels, such as GABA-A receptor, which also exist in multiple subtypes. At present, with respect to the nAChRs, relatively few of the subtypes identified have been cloned from human tissue and pharmacologically evaluated, but several groups are focusing their research efforts in this direction. With a thorough understanding of the pharmacological and functional characteristics of more of the putative human nAChR subtypes, this knowledge will facilitate the discovery of more efficacious and less toxic ChCMs that may provide potential novel therapeutic agents for a variety of CNS conditions.

Neural mechanisms underlying nicotine dependence

There is little doubt that many habitual smokers find it difficult to quit the habit because they have become addicted to the nicotine present in the smoke. This paper addresses some of the pharmacological mechanisms underlying this addiction and discusses how an understanding of these mechanisms may contribute to the more effective use of nicotine replacement therapy during smoking cessation. It considers critically the evidence that the "rewarding" properties of nicotine, which serve to reinforce drug-seeking behaviour, are related to stimulation of the mesolimbic dopamine system of the brain. The critique focuses specifically on the evidence that many central nicotinic receptors, including those which mediate the effects of the drug on dopamine secretion, are readily desensitized by chronic exposure to agonist and that hypotheses which assume that nicotine inhaled from tobacco smoke invariably results in stimulation of the receptors must be treated with caution. Nicotinic receptors in the brain are, however, heterogeneous in nature with different molecular structures and pharmacologies. It is concluded that the reinforcing properties of nicotine sought by smokers may reflect both stimulation and desensitization of the different nicotinic receptor populations, and that smokers may adjust their smoking habits to achieve the balance of receptor stimulation and desensitization which they find most reinforcing. It seems likely that the efficacy of the different nicotine formulations during the treatment of smoking cessation may also reflect their ability to stimulate or desensitize brain nicotinic receptors.

A genetic comparison of behavioral actions of ethanol and nicotine in the mirrored chamber

Human alcoholics are almost invariably heavy users of tobacco, perhaps because both ethanol and nicotine may have anxiolytic activity. However, studies in humans have not uniformly detected anxiolytic effects because significant individual differences in anxiolytic actions of these agents seem to exist. One factor that seems to contribute to these individual differences is tolerance to ethanol. Individuals who are more sensitive to depressant actions of alcohol seem to show anxiolytic actions more readily. Consequently, we examined the relative sensitivities of the ethanol-sensitive (to the anesthetic actions of ethanol) long-sleep (LS) and ethanol-resistant short-sleep (SS) mouse lines to diazepam, ethanol, nicotine, and ethanol-nicotine combinations in the mirrored chamber test. This test measures approach-conflict behavior. Ethanol and nicotine evoked changes in mirrored chamber activities that resembled those elicited by diazepam. These effects were seen at doses that did not markedly affect locomotor activity, thereby suggesting that these changes in behavior represent anxiolytic actions. The LS-SS mice did not differ in sensitivity to diazepam, but the SS were more uniformly responsive to the other drugs. Only the SS showed clear evidence for interactions between ethanol and nicotine. If the changes in mirrored chamber behavior elicited by ethanol, nicotine, and combinations of the two drugs occur because of anxiety reduction, it seems that the SS mouse line is more responsive to anxiolytic actions of these drugs.

Nicotinic receptor agonists exhibit anxiolytic-like effects on the elevated plus-maze test

The effects of nicotinic receptor agonists on the elevated plus-maze test of anxiety were investigated in CD1 mice after intraperitoneal injections. Nicotine and lobeline, but not cytisine, exhibited a significant increase in the time spent by the mice in the open arms, a measure of anxiolytic activity. Nicotine also increased the total number of arm entries, a measure of general activity, but this effect was secondary to its anxiolytic-like properties. Nicotinic receptor antagonists on their own did not modify the behavior of mice in the maze. The effect of nicotine was mediated by central nicotinic receptors as it was blocked by the centrally-acting nicotinic antagonists mecamylamine and chlorisondamine, but not by hexamethonium (a peripherally acting blocker). Cotinine, the major metabolite of nicotine, was evaluated at different times after systemic injections and had no effect in the plus-maze. The anxiolytic-like profile induced by nicotinic receptor stimulation was not associated with potentiation of alcohol effects, a liability associated with the benzodiazepine therapy. This study demonstrates the anxiolytic-like properties of nicotine and lobeline in mice, and suggests that central nicotinic receptors are involved in the expression of emotional behavior.

Nicotinic systems and cognitive function

Nicotinic acetylcholine receptors have been found to be important for maintaining optimal performance on a variety of cognitive tasks. In humans, nicotine-induced improvement of rapid information processing is particularly well documented. In experimental animals nicotine has been found to improve learning and memory on a variety of tasks, while the nicotinic antagonist mecamylamine has been found to impair memory performance. Nicotine has been found to be effective in attenuating memory deficits resulting from lesions of the septohippocampal pathway or aging in experimental animals. Nicotinic receptors are decreased in the cortex of patients with Alzheimer's disease. Preliminary studies have found that some aspects of the cognitive deficit in Alzheimer's disease can be attenuated by nicotine. Nicotine may prove to be useful therapeutic treatment for this and other types of dementia.

Conditioned tolerance to the anorectic and corticosterone-elevating effects of nicotine

We have shown that tolerance to the behavioral effects of nicotine is partially dependent on conditioned environmental cues that predict drug delivery. The present research extends this finding to physiological effects of nicotine by assessing both the appetite-suppressing and adrenocortical-activating effects of nicotine, as measured by plasma corticosterone (CORT). In the first study, male rats on a 22-h food deprivation schedule were injected daily with 0.33 or 0.66 mg/kg (free base) of nicotine bitartrate or saline in a distinctive environment and tested for milk intake. Nicotine initially suppressed milk intake and tolerance developed over 10 days. Changing cues associated with drug administration partially reversed tolerance since injection of nicotine in a new environment reduced milk intake of tolerant animals. Similarly, animals who repeatedly received nicotine in one environment exhibited CORT levels lower than rats injected for the first time, and this tolerance also was partially reversed when administration occurred in the new environment. The second experiment indicated that the increased CORT of Experiment 1 was not a stress response associated with injecting animals in a different environment. These results indicate that tolerance to both behavioral and neuroendocrine effects of nicotine is influenced by conditioning.

The influence of stress on psychopharmacological responses to nicotine

This essay considers the mechanisms which may mediate the apparent anxiolytic properties of nicotine and which are thought to be responsible for the 'calming' effect of tobacco smoke experienced by many smokers. It summarizes the evidence that, in many tests for anxiolytic activity, the effects of nicotine do not resemble those of established anxiolytic drugs such as diazepam and concludes that it is likely that neural systems other than those which mediate the responses to the benzodiazepines are responsible for the putative anxiolytic properties of nicotine. Circumstantial evidence which suggests that the increase in mesolimbic dopamine secretion evoked by nicotine may not only be rewarding per se but may also contribute to the ability of the drug to alleviate the effects of stress is presented. The essay also summarizes results which suggest that chronic nicotine evokes changes in the mesolimbic dopamine system which resemble those seen in animals treated chronically with antidepressant drugs and proposes that the mechanisms which mediate the ability of antidepressant drugs to alleviate the effects of stress may also mediate the apparent anxiolytic properties of nicotine. The possible consequences of this hypothesis for future research are considered.

Effects of buspirone and chlordiazepoxide on plasma catecholamine and corticosterone levels in stressed and nonstressed rats

The effects of intragastric administration of the prototypical benzodiazepine (BDZ) anxiolytic drug chlordiazepoxide (CDP) and the non-BDZ anxiolytic agent buspirone (BUSP) on basal and stress-elevated plasma noradrenaline (NA), adrenaline (A) and corticosterone (CS) contents were investigated. Acute dosing of CDP (1-27 mg/kg) produced dose-related increases in basal CS secretion but was without effect on basal NA levels. The high dose of CDP caused a slight short-term A increase. Dose-dependent increases in plasma A, NA and CS contents were observed after acute treatment with BUSP (2 and 20 mg/kg). A medium dose of CDP (9 mg/kg) attenuated the stress-induced CS and A elevations. High doses of CDP that elevated basal CS release prevented a further increase of CS by stress and inhibited the NA and A response to stress. BUSP (2 and 20 mg/kg) was not effective in decreasing the stress-elicited rise of CS, NA or A. Conversely, the 20 mg/kg dose of BUSP enhanced the stress-induced A response. Repeated administration of CDP (9 mg/kg/day for six days) produced tolerance to the elevation of basal CS triggered by acute CDP treatment, but increased the efficacy of the drug's CS and A attenuating action in stressed rats. Repeated administration of BUSP (2 mg/kg/day for six days) also produced tolerance to the acute BUSP-induced effect on basal CS release, but did not affect the stress-induced CS, NA and A responses. It is concluded that the clinically effective anxiolytic BUSP does not have the BDZ-like property to inhibit stress-induced elevations in CS, NA and A. Furthermore, the present data support other evidence that activation of 5-HT1A receptor mechanisms increases plasma catecholamine and corticosterone concentrations.

Sites of action of ondansetron to inhibit withdrawal from drugs of abuse

The cerebral site of action of the selective 5-HT3 receptor antagonist ondansetron to influence the behavioural consequences of withdrawal from subchronic treatment with diazepam, ethanol, nicotine or cocaine was studied in the light/dark exploration test in the mouse. The aversive response to the light compartment of the test box was reduced during a subchronic treatment with peripherally administered diazepam, ethanol, nicotine and cocaine, but was exacerbated following withdrawal from the 4 treatments. The behavioural consequences of withdrawal from diazepam (10 mg/kg IP b.i.d. 14 days), ethanol (8%/w/v drinking water for 14 days), nicotine (0.1 mg/kg IP b.i.d. 14 days) or cocaine (1.0 mg/kg IP b.i.d. 14 days) were antagonised by ondansetron injected into the amygdala and dorsal raphe nucleus (1-10 ng); injections of ondansetron (10 ng) into the median raphe nucleus, the nucleus accumbens and striatum were ineffective. It is concluded that the amygdala and dorsal raphe nucleus may be sites of action for ondansetron to antagonise the aversive behaviour caused by withdrawal from 4 common drugs of abuse in a mouse model, and that 5-HT projections from the dorsal raphe nucleus may be involved in aversive behaviour.

A comparison of the effects of nicotine and (+)-amphetamine on rat behaviour in an unsignalled Sidman avoidance schedule

In agreement with the results of previous studies, the withdrawal of nicotine from rats trained on an unsignalled Sidman avoidance schedule under the influence of the drug (0.4 mg kg-1 given subcutaneously 3 min before each training session) was associated with a reduction in lever-pressing responses (P less than 0.05) and an increase in the number of shocks received (P less than 0.01). The number of shocks received by the withdrawn rats was also greater (P less than 0.05) than the number of shocks received by rats trained and tested with saline, whereas the number of lever-pressing responses recorded for saline-treated rats was not influenced significantly by the drug used during training. The subcutaneous administration of (+)-amphetamine (0.5 mg kg-1 30 min before the test session) stimulated lever-pressing in rats trained with saline or nicotine and abolished the increase in the number of shocks received by the nicotine-withdrawn rats, but had no significant effect on the number of shocks received by rats trained with saline. The number of shocks received by the rats trained on the schedule with (+)-amphetamine but tested after an injection of saline was also greater (P less than 0.05) than the number of shocks received by rats trained and tested with saline. It is concluded that the disruption in shock avoidance performance observed for the nicotine- and (+)-amphetamine-withdrawn rats may reflect the development of dependence upon the stimulant properties of these drugs.

Interactions of anxiolytic and antidepressant drugs with hormones of the hypothalamic-pituitary-adrenal axis

Changes in hormones of the hypothalamic-pituitary-adrenal (HPA) axis in patients suffering from anxiety and depressive disorders are reviewed, and the changes that occur when animals are exposed to test situations used preclinically to model anxiety or depression. The effects of exogenous administration of HPA hormones both clinically and in animal tests is discussed and the effects of clinically used anxiolytics and antidepressants on hormones of the HPA axis. The final section discusses stress-induced changes in the CNS.

Ethologically-based animal models of anxiety disorders

An overview of ethologically-based animal models suitable for investigating the pharmacological treatment of anxiety disorders is presented. The DSM-IIIR classification provides a framework for the discussion. The limitations of the models in current use are considered. It is suggested that there is a need for a greater emphasis on animal models of anxiety with an etiological basis.

The psychopharmacology of 5-HT3 receptors

The review presents evidence that 5-HT3 receptors within the brain may contribute to the control of behavior. 5-HT3 receptor antagonists GR38032F, zacopride, ICS 205-930 and other agents are very potent in reducing mesolimbic dopamine hyperactivity caused by the injection of amphetamine or infusion of dopamine into the rat nucleus accumbens and amygdala, and the ventral striatum of the marmoset. Such actions are distinguished from those of neuroleptic agents by a failure to reduce normal levels of activity or to induce a rebound hyperactivity after discontinuation of treatment. Indeed, the 5-HT3 receptor antagonists can prevent the neuroleptic-induced rebound hyperactivity. Further evidence that 5-HT3 receptors moderate limbic dopamine function is shown by their ability to reduce both the behavioral hyperactivity and changes in limbic dopamine metabolism caused by DiMe-C7 injection into the ventral tegmental area. The 5-HT3 receptor antagonists also have an anxiolytic profile in the social interaction test in the rat, the light/dark exploration test in the mouse, the marmoset human threat test and behavioral observations in the cynomolgus monkey. They differ from the benzodiazepines by an absence of effect in the rat water lick conflict test and a withdrawal syndrome. Importantly, the 5-HT3 receptor antagonists are highly effective to prevent the behavioral syndrome following withdrawal from treatment with diazepam, nicotine, cocaine and alcohol. Intracerebral injection techniques in the mouse indicate that the dorsal raphe nucleus and amygdala may be important sites of 5-HT3 receptor antagonist action to inhibit aversive behavior. Studies with GR38032F indicate an additional effect in reducing alcohol consumption in the marmoset. The identification and distribution of 5-HT3 receptors in the brain using a number of 5-HT3 receptor ligands, [3H]65630, [3H]zacopride and [3H]ICS 205-930 correlates between studies, and the 5-HT3 recognition sites in cortical, limbic and other areas meet the criteria for 5-HT3 receptors to mediate the above behavioral effects. Thus the use of 5-HT3 receptor antagonists reveals an important role for 5-hydroxytryptamine in the control of disturbed behavior in the absence of effect on normal behavior. The profile of action of the 5-HT3 receptor antagonists has generated a major clinical interest in their potential use for schizophrenia, anxiety and in the control of drug abuse.

Multiple behavioral effects of diazepam in rhesus monkeys

The acute effects of diazepam (Valium) were assessed using a battery of complex food-reinforced operant tasks that included responding in delayed matching to sample (DMTS, n = 5), conditioned position response (CPR, n = 7) progressive ratio (PR, n = 8), temporal response differentiation (TRD, n = 4), and incremental repeated acquisition (IRA, n = 9) tests. Diazepam (0.25-4.0 mg/kg IV) produced significant dose-dependent decreases in the number of reinforcers obtained in the TRD and IRA tasks only. TRD accuracy was significantly decreased at doses of 0.25, 1.0, 2.0, and 4.0 mg/kg when compared to vehicle injections. Significant decreases in IRA accuracy generally did not occur at doses below 1.0 mg/kg. DMTS accuracy was decreased at 0.5 mg/kg for some time delays but showed no clear dose-delay interaction. Performance in the CPR and PR tests showed no significant effects of diazepam exposure over the dose range tested. These results indicate that diazepam selectively disrupts performance of operant tasks in monkeys designed to model human correlates of time perception, learning ability and visual attention/short-term memory while not affecting tasks designed to model motivation and position/color discrimination.

Therapeutic and hypothermic properties of diazepam altered by a diazepam-chlorpromazine association

Rats were injected (IP) with diazepam (2.5 mg/kg) and chlorpromazine (10.0 mg/kg) with a 30-min interval between the two injections. After 10-12 repeated drug pairings of this type, the thermic, muscle relaxant, and anxiolytic responses of the animals to diazepam alone were tested. These tests revealed: 1) an enhanced hypothermia (rectal temperature), 2) an attenuated muscle relaxant effect (inclined plane test), and 3) a potentiated anxiolytic effect (plus-maze test). Although various interdrug associations have previously been demonstrated using other measures of conditioning, this is the first instance in which changes in the therapeutic effects of a drug (in this case, muscle relaxation and anxiety reduction) have been obtained with this procedure.

The actions of nicotine and cocaine in a mouse model of anxiety

The acute administration of nicotine (0.01-1.0 mg/kg IP) to the mouse increased the time spent and rearings and line crossings in the aversive brightly illuminated white area of a two compartment white/black test box, with a corresponding decrease in the black. This profile of change was maintained during twice daily administration (0.1 mg/kg IP) for 14 days. Eight to 96 hr following withdrawal of nicotine (14-day treatment), the behavioural profile was reversed to a preference for the black area: by 240 hr values had returned to control levels. In contrast to the effects of nicotine, an acute injection of cocaine (0.1-10 mg/kg IP) exacerbated the aversive response to the white area. However, similarly to nicotine, the administration of cocaine (1.0 mg/kg IP) twice daily for 14 days reduced the aversion to the white area and exacerbated the response following cocaine withdrawal. The effects of nicotine and cocaine to reduce and enhance responsiveness to the aversive properties of the white area are discussed in terms of an anxiolytic and anxiogenic response and the possibility of a serotonergic involvement.

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.

Anxiogenic properties of beta-CCE and FG 7142: a review of promises and pitfalls

The behavioral effects of the benzodiazepine (BZP)-receptor partial inverse agonists, beta-CCE and FG 7142, are reviewed and the claim that these compounds possess "anxiogenic" properties is examined. Results obtained from human studies and global observations in animals, as well as those from experiments on aggression in animals or from studies of pentylenetetrazole discrimination cannot be considered conclusive. Contradictory findings have been obtained in studies using animal testing procedures derived from BZP-sensitive models of anxiety and in newer experimental situations and these are discussed from various theoretical perspectives: (1) the ability of the models to measure increased anxiety; (2) the possible ability of the drugs to reveal latent anxiety which generalizes from a punished to an otherwise non-fearful component of a testing procedure ("spreading anxiety"); (3) anxiety produced by a pro- or pre-convulsant state. Finally, several hypotheses are considered to account for the behavioral effects of beta-CCE and FG 7142 without assuming anxiogenic properties. These include the possible existence of different forms of anxiety, rate dependency, and drug-induced motivational changes. It is concluded that available data are insufficient to strongly support the notion that FG 7142 and beta-CCE are the anxiogenic drugs "par excellence" they are often claimed to be.

The role of hippocampal 5-HT in the effects of nicotine on habituation to an X-maze

Neither the chronic administration (40 daily subcutaneous injections) of nicotine (0.4 mg/kg) nor its withdrawal (3 days) influenced avoidance of the more aversive open runways of an elevated X-maze in rats which had been habituated to the apparatus. Lesions of the serotonergic fibres innervating the hippocampus increased total activity (P less than 0.05). However, analysis of covariance indicated that reduced 5-hydroxytryptamine secretion in the hippocampus was probably not the mechanism by which nicotine stimulated total activity in these rats.

The effects of diazepam on brain 5-HT and 5-HIAA in stressed and unstressed rats

Diazepam, administered to rats at a high dose (25 mg/kg PO) has been shown to have no effect on the plasma corticosterone response to the stress of an elevated open platform. It did however, reduce the plasma corticosterone in rats repeatedly exposed to the apparatus. Diazepam-withdrawal from stress-habituated rats increased plasma corticosterone (p less than 0.01) whereas withdrawal of diazepam from unstressed rats had no effect on plasma corticosterone. It is concluded that this effect of diazepam-withdrawal may reflect the development of dependence upon the drug. Significant effects were not observed following the administration of a lower non-selective dose (5 mg/kg PO) of diazepam and, therefore, it is not clear whether dependence to its sedative, rather than the anxiolytic properties have been measured. Acute diazepam (25 mg/kg) increased (p less than 0.05) hippocampal 5-hydroxyindoleacetic acid; its withdrawal from unstressed rats after 40 days reduced (p less than 0.01) hypothalamic 5-hydroxytryptamine. There was no evidence that the effects of diazepam or its withdrawal on plasma corticosterone in stressed rats were associated directly with changes in brain 5-hydroxyindoles.

Behavioural and adrenocortical responses to nicotine measured in rats with selective lesions of the 5-hydroxytryptaminergic fibres innervating the hippocampus

The effects of acute and subchronic (7) injections of nicotine (0.4 mg kg-1, s.c.) and of selective lesions of the 5-hydroxytryptaminergic (5-HTergic) pathways innervating the hippocampus on the spontaneous behaviour of rats in an elevated X-maze composed of two open and two enclosed runways have been examined. Subchronic, but not acute, nicotine increased total spontaneous activity. Neither acute nor subchronic nicotine altered the ratio of open:closed runway entries. Destruction of the 5-HTergic pathways innervating the hippocampus with 5,7-dihydroxytryptamine caused a reduction in the ratio of open:enclosed runway entries. Acute, but not subchronic, nicotine caused a significant increase in plasma corticosterone. The lesion had no effects on the plasma levels of this hormone. No significant interactions between the lesion and the responses to nicotine were observed. The data failed to provide any evidence that hippocampal 5-HTergic systems may be implicated in the effects of nicotine on the spontaneous behaviour of the rat.