A microdialysis study of the effects of the nicotinic agonist RJR-2403 on cortical release of acetylcholine and biogenic amines

The α4β2 nicotinic acetylcholine receptor modulates autism-like behavioral and motor abnormalities in pentylenetetrazol-kindled mice

Epilepsy is associated with several psychiatric disorders, including cognitive impairment, autism and attention deficit/hyperactivity disorder (ADHD). However, the psychopathology of epilepsy is frequently unrecognized and untreated in patients. In the present study, we investigated the effects of ABT-418, a neuronal nicotinic acetylcholine receptor agonist, on pentylenetetrazol (PTZ)-kindled mice with behavioral and motor abnormalities. PTZ-kindled mice displayed impaired motor coordination (in the rotarod test), anxiety (in the elevated plus maze test) and social approach impairment (in the three-chamber social test) compared with control mice. ABT-418 treatment (0.05 mg/kg, intraperitoneally) alleviated these behavioral abnormalities in PTZ-kindled mice. Immunolabeling of tissue sections demonstrated that expression of the α4 nicotinic acetylcholine receptor subunit in the medial habenula was similar in control and PTZ-kindled mice. However, expression was significantly decreased in the piriform cortex in PTZ-kindled mice. In addition, we examined the expression of the synaptic adhesion molecule neuroligin 3 (NLG3). NLG3 expression in the piriform cortex was significantly higher in PTZ-kindled mice compared with control mice. Collectively, our findings suggest that ADHD-like or autistic-like behavioral abnormalities associated with epilepsy are closely related to the downregulation of the α4 nicotinic receptor and the upregulation of NLG3 in the piriform cortex. In summary, this study indicates that ABT-418 might have therapeutic potential for attentional impairment in epileptic patients with psychiatric disorders such as autism and ADHD.

Nicotine withdrawal upregulates nitrergic and galaninergic activity in the rat dorsal raphe nucleus and locus coeruleus

The dorsal raphe nucleus (DRN), a major source of forebrain serotonin, mediates various neural functions including anxiety. The nucleus locus coeruleus (LC) is likewise involved in mediating central components of the stress response and anxiety. An anxiety-reducing effect is widely believed to underlie many cases of nicotine dependence. While much is known about nicotine-serotonin interactions, little is known about how nicotine engages the DRN non-serotonergic domain in specific physiological functions that influence organismal behavior. The aim of this study was to determine how chronic nicotine withdrawal influences neuronal nitric oxide (NO) synthase (nNOS) and galanin immunoreactivity in the DRN and LC of adult rats. Compared with saline, nicotine increased nicotinamide adenine dinucleotide phosphate diaphorase profiles within distinct DRN subregions and also enhanced intensity in nNOS and galanin cell bodies in the rostral DRN as well as galanin in the LC. Nicotine-induced nNOS/galanin staining of somata was abundant in the rostral ventromedial DRN. Galanin-positive terminals surrounded nNOS-containing cell bodies in the DRN lateral wing subregions. These observations suggest that the DRN NOS-galanin domain and galanin in the LC are engaged in the organism's neural adaptation to chronic nicotine exposure. Hence NO and galanin synthesized or released within the DRN and LC or at the respective target sites might regulate the whole animal behavioral response to nicotine exposure.

Age-dependent effects of initial exposure to nicotine on serotonin neurons

Adolescence is a critical vulnerable period during which exposure to nicotine greatly enhances the possibility to develop drug addiction. Growing evidence suggests that serotonergic (5-HT) neurotransmission may contribute to the initiation and maintenance of addictive behavior. As the dorsal raphe (DR) and median raphe (MnR) nuclei are the primary 5-HT source to the forebrain, the current study tested the hypothesis that there are age-dependent effects of acute nicotine administration on activation of 5-HT neurons within these regions. Both adolescent (Postnatal day 30) and adult (Postnatal day 70) male Sprague-Dawley rats received subcutaneous injection of either saline or nicotine (0.2, 0.4, or 0.8 mg/kg). Subsequently, the number of 5-HT cells that were double-labeled for Fos and tryptophan hydroxylase was counted in seven subregions within the DR and the entire MnR. The results show that acute nicotine injection induces Fos expression in 5-HT neurons in a region-specific manner. In addition, adolescents show broader regional activations at either a lower (0.2 mg/kg) and a higher (0.8 mg/kg) dose of nicotine, displaying a unique U-shape response curve across doses. In contrast, 5-HT cells with activated Fos expression were restricted to fewer regions in adults, and the patterns of expression were more consistent across doses. The results reveal dose-dependent effects of nicotine during adolescence with apparent sensitization at different ends of the dosage spectrum examined compared to adults. These data indicate that initial exposure to nicotine may have unique effects in adolescence on the ascending 5-HT system, with the potential for consequences on the affective-motivational qualities of the drug and the subsequent propensity for repeated use.

Serotonergic mechanism underlying tranylcypromine enhancement of nicotine self-administration

Although nicotine is generally considered to be the main psychoactive component of tobacco, growing evidence highlights the importance of nonnicotine compounds in smoking reinforcement. Monoamine oxidase (MAO) inhibition is a major consequence of smoking and MAO inhibitors, such as tranylcypromine, increase nicotine reinforcement. Tranylcypromine has multiple pharmacological effects, increasing monoamine release for a few hours immediately after its administration and blocking MAO activity for several days. To assess the relative role of these two actions, adult male rats were tested in consecutive daily 3-h sessions for self-administration of nicotine (3 μg kg⁻¹) inj⁻¹, i.v.) either 20 or 1 h following administration of tranylcypromine (3 mg kg⁻¹). Both paradigms were shown to produce highly significant inhibition of MAO activity. However, whereas animals readily acquired self-administration when pretreated with tranylcypromine 1 h prior to testing, they did not with the longer pretreatment interval. Such animals did immediately acquire nicotine self-administration when the tranylcypromine pretreatment interval was switched to 1 h prior to testing on Day 4, indicating that an acute effect of the MAO inhibitor was responsible for enhanced nicotine reinforcement. Several lines of evidence implicate serotonin (5-HT) as the mediator of this enhancement: (1) Tranyclypromine-enhanced nicotine reinforcement was blocked by the 5-HT₂ receptor antagonists, ritanserin and ketanserin; (2) parachloroamphetamine (PCA), a 5-HT releaser, also enhanced nicotine self-administration in animals in which MAO activity was inhibited; (3) pretreatment with tranylcypromine increased PCA-induced 5-HT overflow in the nucleus accumbens. These findings suggest that MAO inhibition enhances serotonergic transmission, which serves a critical role in the reinforcing effects of nicotine.

Chronic effect of nicotine on serotonin transporter mRNA in the raphe nucleus of rats: reversal by co-administration of bupropion

AIM

Epidemiologic studies suggest the existence of a biological link between nicotine withdrawal and depression. To investigate the neuronal mechanisms of the precipitation of depression during smoking cessation, an animal model of nicotine withdrawal was used, and the expression of serotonin transporter (5HTT), abnormality of which is implicated in the pathogenesis of depression, was investigated. The effect of co-administration of bupropion, which has been clinically shown to ameliorate nicotine withdrawal symptoms, was also investigated in this model.

METHODS

Male Wistar rats were implanted with a minipump s.c., which delivered nicotine at a rate of 6 mg/kg per day for 12 days (days 1-12). Rats given chronic nicotine were killed on day 13, or 2 days after the removal of minipump (withdrawal day 2). In a separate experiment, bupropion (15 or 30 mg/kg per day) was injected into the nicotine infused rats on days 2-12. The expression of mRNA for 5HTT in the dorsal raphe was determined on in situ hybridization.

RESULTS

Chronic nicotine infusion resulted in the reduction of 5HTT mRNA expression, which lasted through withdrawal day 2. Co-administration of bupropion, however, significantly antagonized this reduction.

CONCLUSIONS

Chronic nicotine infusion reduces the synthesis of 5HTT protein, which may consequently precipitate depression during nicotine withdrawal, but co-administration of bupropion may ameliorate withdrawal symptoms by counteracting nicotine's effect on 5HTT.

Nicotine and nicotinic system in hypoglutamatergic models of schizophrenia

Schizophrenia is a complex neuropsychiatric disorder with devastating consequences. It is characterized by thought fragmentation, hallucination and delusion, collectively referred to as positive symptoms. In addition, mood changes or affective disorders, referred to as negative symptoms, as well as cognitive impairments can be manifested in these patients. Arguably, modeling such a disorder in its entirety in animals might not be feasible. Despite this limitation, various models with significant construct, predictive and some face validity have been developed. One such model, based on hypoglutamatergic hypothesis of schizophrenia, makes use of administering NMDA receptor antagonists and evaluating behavioral paradigms such as sensorimotor gating. Because of very high incidence of smoking among schizophrenic patients, it has been postulated that some of these patients may actually be self medicating with tobacco's nicotine. Research on nicotinic-glutamatergic interactions using various animal models has yielded conflicting results. In this review, some of these models and possible confounding factors are discussed. Overall, a therapeutic potential for nicotinic agonists in schizophrenia can be suggested. Moreover, it is evident that various experimental paradigms or models of schizophrenia symptoms need to be combined to provide a wider spectrum of the behavioral phenotype, as each model has its inherent limitations.

Alpha4 containing nicotinic receptors are positioned to mediate postsynaptic effects on 5-HT neurons in the rat dorsal raphe nucleus

Nicotinic acetylcholine receptors containing the alpha4 and beta2 subunits constitute the most abundant high-affinity binding site of nicotine in the brain and are critical for the addictive qualities of nicotine. 5-HT neurotransmission is thought to be an important contributor to nicotine addiction. Therefore in this study it was examined how alpha4-containing receptors are positioned to modulate the function of 5-HT neurons using ultrastructural analysis of immunolabeling for the alpha4 receptor subunit in the dorsal raphe nucleus (DR), a primary source of forebrain 5-HT in the rat. Of 150 profiles labeled for the alpha4 subunit, 140 or 93% consisted of either soma or dendrites, these were often small-caliber (distal) dendrites <1.5 microm in diameter (63/150 or 42%). The majority (107/150 or 71%) of profiles containing labeling for alpha4 were dually labeled for the synthetic enzyme for 5-HT, tryptophan hydroxylase (TPH). Within dendrites immunogold labeling for alpha4 was present on the plasma membrane or near postsynaptic densities. However, labeling for alpha4 was commonly localized to the cytoplasmic compartment often associated with smooth endoplasmic reticulum, plausibly representing receptors in transit to or from the plasma membrane. Previous studies have suggested that nicotine presynaptically regulates activity onto 5-HT neurons, however alpha4 immunolabeling was detected in only 10 axons in the DR or 7% of profiles sampled. This finding suggest that alpha4 containing receptors are minor contributors to presynaptic regulation of synaptic activity onto 5-HT neurons, but rather alpha4 containing receptors are positioned to influence 5-HT neurons directly at postsynaptic sites.

Uncoupling between noradrenergic and serotonergic neurons as a molecular basis of stable changes in behavior induced by repeated drugs of abuse

A challenge in drug dependence is to delineate long-term behavioral and neurochemical modifications induced by drugs of abuse. In rodents, drugs of abuse induce locomotor hyperactivity, and repeating injections enhance this response. This effect, called behavioral sensitization, persists many months after the last administration, thus mimicking long-term sensitivity to drugs observed in human addicts. Although addictive properties of drugs of abuse are generally considered to be mediated by an increased release of dopamine in the ventral striatum, recent pharmacological and genetic experiments indicate an implication of alpha1b-adrenergic receptors in behavioral and rewarding responses to psychostimulants and opiates. Later on, it was shown that not only noradrenergic but also serotonergic systems, through 5-HT(2A) receptors, were controlling behavioral effects of drugs of abuse. More recently, experiments performed in animals knockout for alpha1b-adrenergic or 5-HT(2A) receptors indicated that noradrenergic and serotonergic neurons, besides their activating effects, inhibit each other by means of the stimulation of alpha1b-adrenergic and 5-HT(2A) receptors and that this mutual inhibition vanishes in wild type mice with repeated injections of psychostimulants, opiates or alcohol. Uncoupling induced by repeated treatments with drugs of abuse installs a stable sensitization of noradrenergic and serotonergic neurons, thus explaining an increased reactivity of dopaminergic neurons and behavioral sensitization. We propose that noradrenergic/serotonergic uncoupling is a common stable neurochemical consequence of repeated drugs of abuse which may also occur during chronic stressful situations and facilitate the onset of mental illness. Drug consumption would facilitate an artificial re-coupling of these neurons, thus bringing a temporary relief.

Nicotine, but neither the α4β2 ligand RJR2403 nor an α7 nAChR subtype selective agonist, protects against a partial 6-hydroxydopamine lesion of the rat median forebrain bundle

Although previous studies suggest nicotine protects against a 6-hydroxydopamine (6-OHDA)-induced lesion of the nigrostriatal tract in rats, it is not known whether functional motor recovery occurs or which nicotinic acetylcholine receptor (nAChR) subtypes mediate this effect. These issues were investigated by comparing the effects of the subtype-specific nAChR agonists, RJR2403 (alpha4beta2 preferring) and (R)-N-(1-azabicyclo[2.2.2.]oct-3-yl)(5-(2-pyridyl)thiopene-2-carboxamide (Compound A; alpha7-selective) and nicotine given 30 min prior to and daily for 14 days after a partial 6-OHDA lesion. In vehicle treated animals, 6-OHDA (6 microg) produced a 65 +/- 1.8% loss of striatal tyrosine hydroxylase (TH) immunoreactivity in the lesion versus intact hemisphere. This loss was reduced in animals treated with nicotine (0.6 and 0.8 mg kg(-1)), reaching significance at the higher dose (36.6 +/- 3.7% loss; P < 0.01 versus vehicle). Treatment with nicotine (0.6 and 0.8 mg kg(-1)) also significantly reduced the number of amphetamine-induced rotations compared to vehicle treatment. In contrast, treatment with RJR2403 (0.2 and 0.4 mg kg(-1)) or Compound A (10 and 20 mg kg(-1)) reduced neither the degree of amphetamine-induced rotations nor the loss of striatal TH immunoreactivity. These data suggest that whilst nicotine is neuroprotective in this partial lesion model, activation of neither the alpha4beta2 nor alpha7 subtypes alone is sufficient to provide protection.

Irreversible blockade of monoamine oxidases reveals the critical role of 5-HT transmission in locomotor response induced by nicotine in mice

Although nicotine is generally considered as the main compound responsible for addictive properties of tobacco, some experimental data indicate that nicotine does not exhibit all the characteristics of other substances of misuse such as psychostimulants and opiates. For example, nicotine generally fails to induce locomotor response in mice and self-administration of nicotine is difficult to obtain in rats. We have shown recently that a pretreatment with mixed irreversible monoamine oxidase inhibitors (MAOIs), such as tranylcypromine, triggers a locomotor response to nicotine in mice and induces a robust self-administration of nicotine in rats. We show here that when mice were pretreated with enhancers of extracellular levels of noradrenaline, dopamine or serotonin (D-amphetamine, GBR12783 or para-chloro-amphetamine, respectively) and injected with nicotine (1 mg/kg), only those animals pretreated with para-chloro-amphetamine exhibited a specific locomotor response to nicotine. These data indicate a critical role of serotonin in nicotine-induced locomotor activity in mice. This was further confirmed in microdialysis experiments showing that nicotine induces an increase in extracellular serotonin levels in the ventral striatum in mice pretreated with tranylcypromine. This effect of nicotine on extracellular serotonin levels was absent in mice lacking the beta2-subunit of the nicotinic acetylcholine receptor. Our data suggest that mixed irreversible MAOIs contained in tobacco facilitate the effects of nicotine on serotonin release, thus allowing the locomotor and rewarding effects of nicotine.

Pharmacological modulation of cholinergic brain activity and its reflection in special EEG frequency ranges from various brain areas in the freely moving rat (Tele-Stereo-EEG)

Due to the electrochemical nature of the communication structure of the brain an intimate relationship between neurotransmitter activity on one side and field potentials (EEG) on the other side can be suspected. In order to learn more about this relationship pharmacological manipulation of the cholinergic transmitter system by means of agonistic and antagonistic receptor active drugs was used. Continuous recording of electrical activity from four selected brain areas (frontal cortex, hippocampus, striatum and reticular formation) in freely moving day-night converted Fisher rats was used (Tele-Stereo-EEG). Frequency analysis of telemetrically transmitted data and special definitions of frequency ranges were used to analyse the data from 45 min pre-drug and 180 min postdrug periods. Pharmacological modulation of brain activity by the selective nicotinic agonist metanicotine as well as the alpha 7 selective nicotinic antagonist methyllycaconitine revealed major changes in delta (1-4.5 Hz) and alpha2 (9-12.5 Hz) frequencies. In general blockade of the cholinergic system resulted in electrical power increases and activation in decreases. Unspecific modulation of cholinergic activity by using the cholinesterase inhibitors physostigmine, tacrine and galantamine led to alpha1 frequency (7-9.5 Hz) changes in addition to the delta and alpha2 changes. These three drugs produced a nearly identical pattern of frequency changes. Theta (4.75-6.75 Hz) and beta1 frequencies (12.75-18.5 Hz) changed to a lesser degree. A peculiar finding arose with respect to the effects of the antagonistic drugs scopolamine and biperiden since biperiden-besides the massive increase of delta and alpha1 power in common-induced a general decrease of alpha2 frequencies within all brain areas opposite to the effect of scopolamine. This special property of biperiden gives a plausible explanation for its efficacy in Parkinsonian patients since decreases of alpha2 waves in this model indicate enhancement of dopaminergic transmission [Dimpfel, W., Spüler, M., Koch, R., Schatton, W., 1987. Radioelectroencephalographic comparison of memantine with receptor-specific drugs acting on dopaminergic transmission in freely moving rats. Neuropsychobiology 18, 212-218]. Except for the effects of methyllycaconitine (p<10%) all results were statistically significant at least at the p<5% level. The results are best explained by assuming that electrical delta activity reflects cholinergic transmitter control followed by and closely linked to changes in dopaminergic transmission as indicated by additional concomitant changes in alpha2 electrical power.

Effects on serotonin of (-)nicotine and dimethylphenylpiperazinium in the dorsal raphe and nucleus accumbens of freely behaving rats

The aim of this study was to investigate the neurochemical mechanism underlying the effect of nicotine and dimethylphenylpiperazinium (DMPP) on 5-hydroxytryptamine (5-HT) release in the dorsal raphe nucleus and nucleus accumbens of freely behaving rats. For comparison, lobeline, cytisine and RJR-2403 were also investigated. It was found that all drugs, when infused locally, evoked an increase of 5-HT in the dorsal raphe nucleus. However, the magnitudes of the 5-HT increase were comparatively different between the drugs in the ranking of their potency: DMPP>RJR 2403>>nicotine>lobeline>cytisine. Both methyllycaconitine, a nicotinic acetylcholine receptor (nAChR) antagonist and methyllycaconitine, a selective alpha7-containing nAChR antagonist blocked the effects of nicotine and DMPP, suggesting that alpha7 subunit mediated the increases in 5-HT. However, DMPP was reported to increase 5-HT using non-nAChR mechanism [Lendvai B, Sershen H, Lajtha A, Santha E, Baranyi M, Vizi ES (1996) Differential mechanisms involved in the effect of nicotinic agonists DMPP and lobeline to release [3H]5-HT from rat hippocampal slices. Neuropharmacology 35:1769-1777]. To test if 5-HT carriers were involved, a selective 5-HT reuptake inhibitor citalopram (1 microM) was infused into the dorsal raphe nucleus before administration of nicotine or DMPP. As a result, citalopram significantly blocked the effect of DMPP, whereas it had no influence on nicotine. Finally, the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) was used to test whether the increases in 5-HT were depolarization-dependent. Administration of 8-OH-DPAT (0.1 mg/kg, s.c.) produced significant decreases in 5-HT in the animals treated with nicotine. In contrast, the effect of DMPP was not altered by 8-OH-DPAT, suggesting that the increases in 5-HT were independent of cell membrane depolarization. In conclusion, there are different mechanisms involved in nicotine- and DMPP-evoked increases in 5-HT. This is consistent with prior work suggesting DMPP may primarily act on 5-HT carriers.

Basal and exercise-induced sympathetic nervous activity and lipolysis in adipose tissue of patients with anorexia nervosa

BACKGROUND

The sympathetic nervous system plays an important role in the regulation of adipose tissue (AT) lipolysis, which is a key step in the metabolic processes leading to the decrease of fat mass. The present study was designed to determine in vivo basal and exercise-stimulated lipolysis and concentrations of catecholamines, the major hormones controlling lipolysis, in subcutaneous abdominal AT in patients with anorexia nervosa (AN), characterized by self-induced starvation and excessive exercises resulting in severe malnutrition and fat store loss. The results of local catecholamines and glycerol levels were compared with those in plasma in both experimental groups.

MATERIAL AND METHODS

An in vivo microdialysis technique was used for the assessment of norepinephrine, dihydroxyphenylalanine, dihydroxyphenylacetic acid and glycerol concentrations in subcutaneous AT of 10 women with AN (body mass index: 15.57 +/- 0.55 kg m(-2)) and 10 age-matched controls (body mass index: 21.56 +/- 0.41 kg m(-2)). Both the AN patients and the control subjects underwent a 1.5 W kg(-1) exercise test.

RESULTS

Basal AT norepinephrine concentrations were increased in the AN patients in comparison with the controls. Basal AT glycerol concentrations were similar in both groups. During exercise, a local increase in the AT norepinephrine and glycerol concentrations was observed in the AN patients only. In contrast to the controls, the basal AT dihydroxyphenylalanine and dihydroxyphenylacetic acid levels in the AN patients were high and remained unchanged during exercise. Basal and exercise-stimulated plasma norepinephrine, dihydroxyphenylalanine, dihydroxyphenylacetic acid and glycerol levels were not different in the AN patients and healthy controls.

CONCLUSION

Our study provides evidence of elevated baseline and exercise-induced sympathetic nervous activity and exercise-induced lipolysis in abdominal AT of AN patients.

TC-1734: an orally active neuronal nicotinic acetylcholine receptor modulator with antidepressant, neuroprotective and long-lasting cognitive effects

The development of selective ligands targeting neuronal nicotinic acetylcholine receptors to alleviate symptoms associated with neurodegenerative diseases presents the advantage of affecting multiple deficits that are the hallmarks of these pathologies. TC-1734 is an orally active novel neuronal nicotinic agonist with high selectivity for neuronal nicotinic receptors. Microdialysis studies indicate that TC-1734 enhances the release of acetylcholine from the cortex. TC-1734, by either acute or repeated administration, exhibits memory enhancing properties in rats and mice and is neuroprotective following excitotoxic insult in fetal rat brain in cultures and against alterations of synaptic transmission induced by deprivation of glucose and oxygen in hippocampal slices. At submaximal doses, TC-1734 produced additive cognitive effects when used in combination with tacrine or donepezil. Unlike (-)-nicotine, behavioral sensitization does not develop following repeated administration of TC-1734. Its pharmacokinetic (PK) profile (half-life of 2 h) contrasts with the long lasting improvement in working memory (18 h) demonstrating that cognitive improvement extends beyond the lifetime of the compound. The very low acute toxicity of TC-1734 and its receptor activity profile provides additional mechanistic basis for its suggested potential as a clinical candidate. TC-1734 was very well tolerated in acute and chronic oral toxicity studies in mice, rats and dogs. Phase I clinical trials demonstrated TC-1734's favorable pharmacokinetic and safety profile by acute oral administration at doses ranging from 2 to 320 mg. The bioavailability, pharmacological, pharmacokinetic, and safety profile of TC-1734 provides an example of a safe, potent and efficacious neuronal nicotinic modulator that holds promise for the management of the hallmark symptomatologies observed in dementia.

In vivo pharmacological characterization of (+/-)-4-[2-(1-methyl-2-pyrrolidinyl)ethyl]thiophenol hydrochloride (SIB-1553A), a novel cholinergic ligand: microdialysis studies

SIB-1553A ((+/-)-4-[2-(1-methyl-2-pyrrolidinyl)ethyl]thiophenol HCl) is a neuronal nicotinic acetylcholine receptor (nAChR) ligand which is active in rodent and primate models of cognition. In functional assays, SIB-1553A exhibits marked subtype selectivity for nAChRs as compared to nicotine. In addition SIB-1553A also exhibits affinities to histaminergic (H3) and serotonergic (5-HT1 and 5HT2) receptors and sigma binding sites. In the present investigation, we characterized SIB-1553A-induced neurotransmitter release in vivo. Following subcutaneous injection (s.c., 10 mg/kg), SIB-1553A rapidly entered the brain achieving concentration of approximately 20 microM 15 min post-injection and was eliminated from plasma with a terminal half-life of approximately 32 min. In freely moving rats, SIB-1553A (1-40 mg/kg, s.c.), markedly increased ACh release in the hippocampus and prefrontal cortex. In both regions, the magnitude of SIB-1553A-induced ACh release was greater than that seen with the prototypical nAChR agonist, nicotine (0.4 mg/kg, s.c.). Both isomers of SIB-1553A induced similar levels of increase in hippocampal ACh release. Increased hippocampal ACh release was also observed following oral administration of SIB-1553A (40 mg/kg) or after local perfusion into the hippocampus (1 mM). SIB-1553A-induced hippocampal ACh release was significantly attenuated by two nAChR antagonists, mecamylamine (MEC) and dihydro-beta-erythroidine (DHbetaE), and by the dopamine (DA) (D1) antagonist, SCH-23390, arguing that ACh release, in part, involves activation of nAChRs and a permissive DA synapse. In contrast to its robust effects on ACh release, SIB-1553A (40 mg/kg, s.c.) modestly increased striatal DA release (approximately 180% of baseline). Due to the proposed role of cholinergic pathways in learning and memory, the neurochemical profile of SIB-1553A suggests a potential for it to treat cognitive dysfunction.

Alpha4beta2 nicotinic acetylcholine receptor activation ameliorates impairment of spontaneous alternation behavior in stroke-prone spontaneously hypertensive rats, an animal model of attention deficit hyperactivity disorder

The objective of the present study was to elucidate the role of nicotine in impairment of spontaneous alternation behavior of juvenile stroke-prone spontaneously hypertensive rats (SHRSP), an animal model of attention deficit hyperactivity disorder (ADHD). Spontaneous alternation behavior assessed by a Y-maze task was significantly lower, and total arm entries were significantly higher in SHRSP than in genetic control Wistar-Kyoto rats. Nicotine (0.1-1 mg/kg, s.c.) dose dependently improved the spontaneous alternation deficit without affecting total arm entries in SHRSP. Nicotine-induced (1 mg/kg, s.c.) improvement was significantly abolished by the centrally acting nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine (1 mg/kg, i.p.), but not by peripherally acting hexamethonium (5 mg/kg, i.p.), suggesting that nicotine-induced improvement is mediated via central nAChR. The alpha4beta2 nAChR antagonist dihydro-beta-erythroidine (3-10 mg/kg, i.p.) dose dependently counteracted nicotine-induced improvement of spontaneous alternation in SHRSP, whereas the alpha7 nAChR antagonist methyllycaconitine (3-10 mg/kg, i.p.) did not. In addition, the alpha4beta2 nAChR agonist RJR-2403 (N-methyl-4-(3-pyridinyl)-3-butene-1-amine; 1-10 mg/kg, s.c.) dose dependently and significantly improved the spontaneous alternation deficit. These findings revealed that nicotine improved spontaneous alternation behavior in SHRSP via the activation of alpha4beta2, but not alpha7, nAChR. Thus, the alpha4beta2 nAChR mechanism might be responsible for the spontaneous alternation deficit in juvenile SHRSP, an animal model of ADHD. This evidence indicates the possibility that selective alpha4beta2 nAChR agonists might be useful for treating attentional dysfunction in ADHD.

Brain-selective stimulation of nicotinic receptors by TC-1734 enhances ACh transmission from frontoparietal cortex and memory in rodents

The authors have described the effect of TC-1734, a brain-selective nicotinic acetylcholine receptor (nAChR) agonist, on acetylcholine (ACh) release in the frontoparietal cortex of rats and on cognitive function in mice. Oral administration of TC-1734 (5, 10 and 20 mg/kg) stimulated ACh release in a dose-dependent manner, as measured by transversal microdialysis. The maximal effect on the amplitude of ACh release was observed at a dose of 10 mg/kg (about 70% above baseline), whereas the maximal effect on the duration of ACh release was observed at the dose of 20 mg/kg. By contrast, oral administration of nicotine (1, 2.5 and 5 mg/kg) did not stimulate ACh release in a dose-dependent manner but produced the same maximal effect on the amplitude of ACh release (about 50% above baseline) at all the doses tested. The ability of both TC-1734 (10 mg/kg) and nicotine (1 mg/kg) to increase ACh levels was antagonized by mecamylamine (1 mg/kg s.c.), suggesting a specific nicotine receptor-mediated effect of both agonists. No tolerance to TC-1734- and nicotine-stimulated ACh release was observed after repeated treatment with TC-1734 (10 mg/kg) or nicotine (1 mg/kg) for 4 days. TC-1734 (1 mg/kg p.o.) improved memory in the object recognition test in mice, and this effect was antagonized by mecamylamine (2.5 mg/kg i.p.). Taken together, these results show that TC-1734 stimulates nAChR in the brain to induce an increase of ACh release in the cortex of rats and enhance memory in mice.

Nicotinic--serotonergic interactions in brain and behaviour

This review focuses on nicotinic--serotonergic interactions in the central nervous system (CNS). Nicotine increases 5-hydroxytryptamine (5-HT) release in the cortex, striatum, hippocampus, dorsal raphé nucleus (DRN), hypothalamus, and spinal cord. As yet, there is little firm evidence for nicotinic receptors on serotonergic terminals and thus nicotine's effects on 5-HT may not necessarily be directly mediated, but there is strong evidence that the 5-HT tone plays a permissive role in nicotine's effects. The effects in the cortex, hippocampus, and DRN involve stimulation of 5-HT(1A) receptors, and in the striatum, 5-HT(3) receptors. The 5-HT(1A) receptors in the DRN play a role in mediating the anxiolytic effects of nicotine and the 5-HT(1A) receptors in the dorsal hippocampus and lateral septum mediate its anxiogenic effects. The increased startle and anxiety during nicotine withdrawal is mediated by 5-HT(1A) and 5-HT(3) receptors. The locomotor stimulant effect of acute nicotine is mediated by 5-HT(1A) receptors and 5-HT(2) receptors may play a role in the expression of a sensitised response after chronic nicotine treatment. Unfortunately, the role of 5-HT(1A) receptors in mediating nicotine seeking has not yet been investigated and would seem an important area for future research. There is also evidence for nicotinic--serotonergic interactions in the acquisition of the water maze, passive avoidance, and impulsivity in the five-choice serial reaction task.

TC-2559: a novel orally active ligand selective at neuronal acetylcholine receptors

TC-2559 [(E)-N-Methyl-4-[3-(5-ethoxypyridin)yl]-3-buten-1-amine] is a novel nicotinic agonist markedly more selective than recently reported novel nicotinic receptor ligands (selectivity ratio for central nervous system (CNS) to peripheral nervous system (PNS)>4000). TC-2559 competes effectively with [3H]-nicotine binding (K(i)=5 nM) but not with [125I]-bungarotoxin (>50,000 nM). Dopamine release from striatal synaptosomes and ion flux from thalamic synaptosomes indicate that TC-2559 is potent and efficacious in the activation of CNS receptors and significantly reduced glutamate-induced neurotoxicity in vitro. TC-2559 has no detectable effects on muscle and ganglion-type nicotinic acetylcholine receptors at concentrations up to 1 mM. TC-2559 significantly attenuates scopolamine-induced cognitive deficits in a step-through passive avoidance task. Acute and repeated oral dosing of TC-2559 enhances performance in a radial arm maze task. In contrast to the effects of equimolar concentrations of (-) nicotine, TC-2559 does not induce hypothermia and locomotor activity is not enhanced following repeated daily administration of 14 days. TC-2559 has a markedly enhanced CNS-PNS selectivity ratio and an intra-CNS selectivity as evidenced by the improved cognition without increased locomotor activity. The in vitro and in vivo studies in the present study suggest that TC-2559 has the desired profile to be further evaluated as a potential therapeutic agent for neurodegenerative diseases.

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.