Nicotinic-serotonergic drug interactions and attentional performance in rats

Nicotine reduces social dominance and neutralizes experience-dependent effects during social conflicts in zebrafish (Danio rerio)

Nicotine, a psychoactive pollutant, binds to nicotinic acetylcholine receptors and disrupts the cholinergic modulation and reward systems of the brain, leading to attention deficit, memory loss, and addiction. However, whether nicotine affects social behaviors remains unknown. We assessed the effects of nicotine on the fighting behavior of zebrafish. Adult zebrafish treated with 5 μM nicotine were used in dyadic fighting tests with size-matched control siblings. The results indicate that nicotine treatment not only significantly reduced the likelihood of winning but also impaired the winner-loser effects (winner and loser fish did not show higher winning and losing tendencies in the second fight, respectively, after treatment.) Nicotine led to a considerable increase in c-fos-positive signals in the interpeduncular nucleus (IPN) of the brain, indicating that nicotine induces neural activity in the habenula (Hb)-IPN circuit. We used transgenic fish in which the Hb-IPN circuit was silenced to verify whether nicotine impaired the winner-loser effect through the Hb-IPN pathway. Nicotine-treated fish in which the medial part of the dorsal Hb was silenced did not have a higher winning rate, and nicotine-treated fish in which the lateral part of the dorsal Hb was silenced did not have a higher loss rate. This finding suggests that nicotine impairs the winner-loser effect by modulating the Hb-IPN circuit. Therefore, in these zebrafish, nicotine exposure impaired social dominance and neutralized experience-dependent effects in social conflicts, and it may thereby disturb the social hierarchy and population stability of such fish.

Nicotine's effect on cognition, a friend or foe?

Tobacco smoking is a preventable cause of morbidity and mortality throughout the world. Smoking comes in form of absorption of many compounds, among which nicotine is the main psychoactive component of tobacco and its positive and negative reinforcement effects are proposed to be the key mechanism for the initiation and maintenance of smoking. Growing evidence suggests that the cognitive enhancement effects of nicotine may also contribute to the difficulty of quitting smoking, especially in individuals with psychiatric disorders. In this review, we first introduce the beneficial effect of nicotine on cognition including attention, short-term memory and long-term memory. We next summarize the beneficial effect of nicotine on cognition under pathological conditions, including Alzheimer's disease, Parkinson's disease, Schizophrenia, Stress-induced Anxiety, Depression, and drug-induced memory impairment. The possible mechanism underlying nicotine's effect is also explored. Finally, nicotine's detrimental effect on cognition is discussed, including in the prenatal and adolescent periods, and high-dose nicotine- and withdrawal-induced memory impairment is emphasized. Therefore, nicotine serves as both a friend and foe. Nicotine-derived compounds could be a promising strategy to alleviate neurological disease-associated cognitive deficit, however, due to nicotine's detrimental effect, continued educational programs and public awareness campaigns are needed to reduce tobacco use among pregnant women and smoking should be quitted even if it is e-cigarette, especially for the adolescents.

Attention-enhancing effects of propranolol and synergistic effects with nicotine

Nicotine increases the output of every major neurotransmitter. In previous studies designed to identify the secondary neurotransmitter systems mediating nicotine's attention-enhancing effects in a rat model, the β-adrenoceptor antagonist propranolol blocked these effects. The present study was designed to test whether this mechanism held true in humans, thus guiding development of novel nicotinic agonists for cognitive enhancement. Twenty-six nonsmokers completed a nicotine (7 mg/24 h transdermally) x propranolol (40 mg p.o., body weight-adjusted) interaction study. Over four test days, each participant received double-placebo, nicotine only, propranolol only, and nicotine plus propranolol in randomized sequence before cognitive testing. No drug effects were seen in a visuospatial attention task. In the Rapid Visual Information Processing Task, performed in two 15-min blocks, neither drug alone significantly affected hit rate, but both drugs combined acted synergistically to alleviate its decrement over time in the first block and displayed additive beneficial effects in the second. In a change detection task, propranolol enhanced accuracy and reduced reaction time independent of nicotine presence. Propranolol also enhanced subjective self-reports of vigor. Overall, the findings were contrary to those hypothesized. Propranolol displayed beneficial effects on cognition, especially on sustaining performance over time. β-adrenoceptor activation by nicotine-induced noradrenaline release appeared to limit performance-enhancing effects of nicotine, because they were unmasked by β-adrenoceptor antagonism. The results suggest that cognitive effects of changes in β-adrenoceptor tone are context-dependent; contrary to rodent paradigms, human cognitive paradigms require no physical orienting in space but prolonged periods of remaining stationary while sustaining predictable processing demands.

Acute and Chronic Nicotine Exposures Differentially Affect Central Serotonin 2A Receptor Function: Focus on the Lateral Habenula

Nicotine addiction is a serious public health problem causing millions of deaths worldwide. Serotonin (5-hydroxytryptamine; 5-HT) is involved in central nervous system (CNS) nicotine effects, and it has been suggested as a promising pharmacological target for smoking cessation. In this regard, what is particularly interesting are the 5-HT_2A receptors (5-HT_2ARs) and the lateral habenula (LHb), a central area in nicotine addiction that we showed to be under a strong 5-HT_2AR-modulation. Single-cell extracellular recording of LHb neurons was used to study the 5-HT_2AR function by intravenously administrating the potent agonist TCB-2. Acute nicotine (2 mg/kg, intraperitoneal, i.p.) and chronic nicotine (6 mg/kg/day for 14 days) differently affected both the 5-HT_2AR-immuno reactive (IR) neuron number and the 5-HT_2AR immunostaining area in the different brain areas studied. After acute nicotine, TCB-2 cumulative doses (5–640 µg/kg, intravenous, i.v.) bidirectionally affected the activity of 74% of LHb recorded neurons. After chronic nicotine treatment, TCB-2 was only capable of decreasing the LHb firing rate. The expression of 5-HT_2AR under acute and chronic nicotine exposure was studied in the LHb and in other brain areas involved in nicotine effects in rats by using immunohistochemistry. These data reveal that acute and chronic nicotine differentially affect the 5-HT_2AR function in different brain areas and this might be relevant in nicotine addiction and its treatment.

Strain dependency of the effects of nicotine and mecamylamine in a rat model of attention

RATIONALE

Processes of attention have a heritable component, suggesting that genetic predispositions may predict variability in the response to attention-enhancing drugs. Among lead compounds with attention-enhancing properties are nicotinic acetylcholine receptor (nAChR) agonists.

OBJECTIVES

This study aims to test, by comparing three rat strains, whether genotype may influence the sensitivity to nicotine in the 5-choice serial reaction time task (5-CSRTT), a rodent model of attention.

METHODS

Strains tested were Long Evans (LE), Sprague Dawley (SD), and Wistar rats. The 5-CSRTT requires responses to light stimuli presented randomly in one of five locations. The effect of interest was an increased percentage of responses in the correct location (accuracy), the strongest indicator of improved attention.

RESULTS

Nicotine (0.05-0.2 mg/kg s.c.) reduced omission errors and response latency and increased anticipatory responding in all strains. In contrast, nicotine dose-dependently increased accuracy in Wistar rats only. The nAChR antagonist mecamylamine (0.75-3 mg/kg s.c.) increased omissions, slowed responses, and reduced anticipatory responding in all strains. There were no effects on accuracy, which was surprising giving the clear improvement with nicotine in the Wistar group.

CONCLUSIONS

The findings suggest strain differences in the attention-enhancing effects of nicotine, which would indicate that genetic predispositions predict variability in the efficacy of nAChR compounds for enhancing attention. The absence of effect of mecamylamine on response accuracy may suggest a contribution of nAChR desensitization to the attention-enhancing effects of nicotine.

Heterogeneity across brain regions and neurotransmitter interactions with nicotinic effects on memory function

Nicotinic acetylcholine receptors have been shown in many studies to be critically involved in memory function. The precise roles these receptors play depend on the receptor subtype, their anatomic localization, their interactions with other parts of the neural systems underlying cognition and the particular domain of cognitive function. Nicotinic agonists can significantly improve learning, memory, and attention. Nicotinic receptors in the hippocampus are innervated by cholinergic projections from the medial septum and diagonal band. Local infusions of either α7 or α4β2 nicotinic antagonists into either the dorsal or ventral hippocampus produce amnestic effects in rats navigating about a radial arm maze. There is cholinergic innervation of nicotinic receptors in other components of the limbic system as well. In the basolateral amygdala and the anterior thalamus, similar amnestic effects of nicotinic α7 and α4β2 antagonists are seen. Interestingly, there are no additive amnestic effects observed in these limbic areas when α7 and α4β2 receptor antagonists are combined. The particular expression patterns of α7 and α4β2 nicotinic receptors in these limbic and cortical areas may explain this nonadditivity, but further research is needed to determine the specific cause of this phenomenon. Nicotinic receptor mechanisms in the limbic system play an important role in cognitive impairment for a variety of neurological disorders, including Alzheimer's disease and schizophrenia. Alzheimer's disease results in a dramatic decrease in hippocampal nicotinic receptor density, affecting α4β2 receptor expression most prominently. In schizophrenia, there are anomalies in α7 nicotinic receptor expression, which seem to be crucial for the cognitive impairment of the disorder. Chronic nicotine exposure, such as seen with tobacco use, results in an increase in nicotinic receptor density in the limbic system. This effect appears to be related to the desensitization of nicotinic receptors seen after agonist application. Open questions remain concerning the role of desensitization versus activation of nicotinic receptors in cognitive improvement.

Nicotinic receptors and attention

Facilitation of different attentional functions by nicotinic acetylcholine receptor (nAChR) agonists may be of therapeutic potential in disease conditions such as Alzheimer's disease or schizophrenia. For this reason, the neuronal mechanisms underlying these effects have been the focus of research in humans and in preclinical models. Attention-enhancing effects of the nonselective nAChR agonist nicotine can be observed in human nonsmokers and in laboratory animals, suggesting that benefits go beyond a reversal of withdrawal deficits in smokers. The ultimate aim is to develop compounds acting with greater selectivity than nicotine at a subset of nAChRs, with an effects profile narrowly matching the targeted cognitive deficits and minimizing unwanted effects. To date, compounds tested clinically target the nAChR subtypes most abundant in the brain. To help pinpoint more selectively expressed subtypes critical for attention, studies have aimed at identifying the secondary neurotransmitter systems whose stimulation mediates the attention-enhancing properties of nicotine. Evidence indicates that noradrenaline and glutamate, but not dopamine release, are critical mediators. Thus, attention-enhancing nAChR agents could spare the system central to nicotine dependence. Neuroimaging studies suggest that nAChR agonists act on a variety of brain systems by enhancing activation, reducing activation, and enhancing deactivation by attention tasks. This supports the notion that effects on different attentional functions may be mediated by distinct central mechanisms, consistent with the fact that nAChRs interact with a multitude of brain sites and neurotransmitter systems. The challenge will be to achieve the optimal tone at the right subset of nAChR subtypes to modulate specific attentional functions, employing not just direct agonist properties, but also positive allosteric modulation and low-dose antagonism.

Nicotinic acetylcholine receptors in attention circuitry: the role of layer VI neurons of prefrontal cortex

Cholinergic modulation of prefrontal cortex is essential for attention. In essence, it focuses the mind on relevant, transient stimuli in support of goal-directed behavior. The excitation of prefrontal layer VI neurons through nicotinic acetylcholine receptors optimizes local and top-down control of attention. Layer VI of prefrontal cortex is the origin of a dense feedback projection to the thalamus and is one of only a handful of brain regions that express the α5 nicotinic receptor subunit, encoded by the gene chrna5. This accessory nicotinic receptor subunit alters the properties of high-affinity nicotinic receptors in layer VI pyramidal neurons in both development and adulthood. Studies investigating the consequences of genetic deletion of α5, as well as other disruptions to nicotinic receptors, find attention deficits together with altered cholinergic excitation of layer VI neurons and aberrant neuronal morphology. Nicotinic receptors in prefrontal layer VI neurons play an essential role in focusing attention under challenging circumstances. In this regard, they do not act in isolation, but rather in concert with cholinergic receptors in other parts of prefrontal circuitry. This review urges an intensification of focus on the cellular mechanisms and plasticity of prefrontal attention circuitry. Disruptions in attention are one of the greatest contributing factors to disease burden in psychiatric and neurological disorders, and enhancing attention may require different approaches in the normal and disordered prefrontal cortex.

Effects of the neurotensin NTS₁ receptor agonist PD149163 on visual signal detection in rats

Antipsychotic drugs provide limited efficacy for cognitive impairment in schizophrenia. Recent studies have found that the neurotensin NTS1 receptor agonist and putative atypical antipsychotic drug PD149163 reverses deficits in sensory-gating and novel object recognition, suggesting that this compound may have the potential to improve cognitive functioning in schizophrenia. The present study sought to extend these investigations by evaluating the effects of PD149163 on sustained attention using a visual signal detection operant task in rats. PD149163, the atypical antipsychotic drug clozapine, and the dopamine D2/3 receptor antagonist raclopride all significantly decreased percent "hit" accuracy, while none of these compounds altered "correct rejections" (compared to vehicle control). Clozapine and raclopride significantly increased response latency, while high doses of PD149163 and raclopride significantly increased trial omissions. Nicotine, which was tested as a positive control, significantly improved overall performance in this task and did not affect response latency or trial omissions. The present findings suggest that neurotensin NTS1 receptor agonists, like antipsychotic drugs, may inhibit sustained attention in this task despite having different pharmacological mechanisms of action.

Effects of chronic sazetidine-A, a selective α4β2 neuronal nicotinic acetylcholine receptors desensitizing agent on pharmacologically-induced impaired attention in rats

RATIONALE

Nicotine and nicotinic agonists have been shown to improve attentional function. Nicotinic receptors are easily desensitized, and all nicotinic agonists are also desensitizing agents. Although both receptor activation and desensitization are components of the mechanism that mediates the overall effects of nicotinic agonists, it is not clear how each of the two opposed actions contributes to attentional improvements. Sazetidine-A has high binding affinity at α4β2 nicotinic receptors and causes a relatively brief activation followed by a long-lasting desensitization of the receptors. Acute administration of sazetidine-A has been shown to significantly improve attention by reversing impairments caused by the muscarinic cholinergic antagonist scopolamine and the NMDA glutamate antagonist dizocilpine.

METHODS

In the current study, we tested the effects of chronic subcutaneous infusion of sazetidine-A (0, 2, or 6 mg/kg/day) on attention in Sprague-Dawley rats. Furthermore, we investigated the effects of chronic sazetidine-A treatment on attentional impairment induced by an acute administration of 0.02 mg/kg scopolamine.

RESULTS

During the first week period, the 6-mg/kg/day sazetidine-A dose significantly reversed the attentional impairment induced by scopolamine. During weeks 3 and 4, the scopolamine-induced impairment was no longer seen, but sazetidine-A (6 mg/kg/day) significantly improved attentional performance on its own. Chronic sazetidine-A also reduced response latency and response omissions.

CONCLUSIONS

This study demonstrated that similar to its acute effects, chronic infusions of sazetidine-A improve attentional performance. The results indicate that the desensitization of α4β2 nicotinic receptors with some activation of these receptors may play an important role in improving effects of sazetidine-A on attention.

Behavioral animal models to assess pro-cognitive treatments for schizophrenia

Cognitive dysfunction is a core aspect of schizophrenia that constitutes a major obstacle toward reintegration of patients into society. Although multiple cognitive deficits are evident in schizophrenia patients, no medication is currently approved for their amelioration. Although consensus clinical test batteries have been developed for the assessment of putative cognition enhancers in patients with schizophrenia, parallel animal tests remain to be validated. Having no approved treatment for cognitive symptoms means no positive control can be used to examine pharmacological predictive validity of animal models. Thus, focus has been placed on animal paradigms that have demonstrable construct validity for the cognitive domain being assessed.This review describes the growing arsenal of animal paradigms under development that have putative construct validity to cognitive domains affected in schizophrenia. We discuss (1) the construct validity of the paradigms; (2) compounds developed to investigate putative treatment targets; and (3) manipulations used to first impair task performance. Focus is placed on the paradigm design, including how the use of multivariate assessments can provide evidence that main effects of treatment are not confounded by extraneous effects.

Selective nicotinic receptor antagonists: effects on attention and nicotine-induced attentional enhancement

RATIONALE

The question of the subtype(s) of the nicotinic acetylcholine receptor (nAChR) mediating the attention-enhancing effects of nicotine is still unsettled. While early studies pointed towards subtypes other than the homomeric α7 nAChR, pro-cognitive effects of α7 nAChR agonists have since been demonstrated.

OBJECTIVES

This study tested whether the performance-enhancing effects of nicotine in a rodent model of attention could be reversed by the α4β2, α4β4, α3β2, and α2β2 nAChR antagonist dihydro-β-erythroidine (DHβE), or the α7 antagonist methyllycaconitine (MLA).

METHODS

In repeated tests, 12 rats trained to perform the 5-choice serial reaction time task were systemically injected with nicotine or vehicle in the presence of increasing doses of DHβE or MLA.

RESULTS

DHβE did not antagonize the attention-enhancing effects of nicotine reflected by measures of accuracy and omission errors, suggesting that its previously reported antagonism of nicotine effects on latency and anticipatory responses specifically reflected the stimulant effects of nicotine. MLA dose-dependently reversed the reduction in omission errors by nicotine. In the absence of nicotine, low doses of MLA (0.4 and 1.3 mg/kg) not previously tested on attention improved response accuracy, resulting in an inverted U-shape dose-response function.

CONCLUSIONS

nAChR subtypes involved in the performance-enhancing effects of nicotine appear to vary depending on the function assessed. Our findings suggest a greater involvement of α7 nAChRs in the effects of nicotine on attention than first suggested by preclinical studies, with different optimal receptor tones for aspects of stimulus detection and response readiness to task stimuli.

The monoamine oxidase (MAO) inhibitor tranylcypromine enhances nicotine self-administration in rats through a mechanism independent of MAO inhibition

Our current study aims to evaluate the mechanisms of tranylcypromine (TCP)-mediated enhancement of nicotine self-administration. We replicated our previous findings which demonstrate that 1 h pretreatment with TCP (3 mg/kg, i.p.) enhances nicotine self-administration (7.5 μg/kg/inj, i.v.) when compared with vehicle-treated rodents. We tested whether TCP-mediated enhancement of nicotine self-administration was due to MAO inhibition or off-target effects by (i) extending the TCP pretreatment time from 1 to 20 h, and (ii) evaluating the role of the individual TCP stereoisomers in nicotine self-administration studies. While 20 h and (-)TCP pretreatment induced significant inhibition of MAO (60-90%), animals found nicotine only weakly reinforcing. Furthermore, while both (+) and (±)TCP treatment induced nearly 100% MAO inhibition, (+)TCP pretreated animals took longer to acquire nicotine self-administration compared to (±)TCP pretreated animals. Stable nicotine self-administration in (+)TCP pretreated animals was influenced by nicotinic receptor activation but not nicotine-paired cues. The opposite was found in (±)TCP pretreated animals. Treatment with (-) or (±)TCP increased dopamine and serotonin overflow, while the (+) and (±)TCP treatment enhanced monoamine overflow subsequent to nicotine. Together, our data suggests TCP enhancement of nicotine self-administration are mediated through mechanisms independent of MAO inhibition, including nicotine-paired cues and monoamine uptake inhibition.

Sazetidine-A, a selective α4β2 nicotinic acetylcholine receptor ligand: effects on dizocilpine and scopolamine-induced attentional impairments in female Sprague-Dawley rats

BACKGROUND

Neuronal nicotinic receptor systems have been shown to play key roles in cognition. Nicotine and nicotinic analogs improve attention and nicotinic antagonists impair it. This study was conducted to investigate the role of α4β2 nicotinic receptors in sustained attention using a novel selective α4β2 nicotinic receptor ligand, sazetidine-A.

METHODS

Female rats were trained to perform the signal detection task to a stable baseline of accuracy. The rats were injected with saline, sazetidine-A (0.01, 0.03, and 0.1 mg/kg), dizocilpine (0.05 mg/kg), or their combination; or, in another experiment, the rats were injected with the same doses of sazetidine-A, scopolamine (0.02 mg/kg), or their combination.

RESULTS

Percent hit and percent correct rejection showed that dizocilpine caused significant (p < 0.025) impairments in performance, which were significantly reversed by each of the sazetidine-A doses. Response omissions were significantly (p < 0.05) increased by dizocilpine, and this was also significantly reversed by each of the sazetidine-A doses. None of the sazetidine-A doses had significant effects on hit, correct rejection, or response omissions when given alone. Scopolamine also caused significant (p < 0.0005) impairments in percent hit and percent correct rejection and increased response omissions, which were significantly attenuated by all the sazetidine-A doses for percent hit and response omissions and by the highest dose of sazetidine-A for percent correct rejection. Both scopolamine and dizocilpine significantly (p < 0.0005) increased response latency, an effect which was significantly attenuated by sazetidine-A coadministration.

CONCLUSIONS

These studies imply an important role for α4β2 nicotinic receptors in improving sustained attention under conditions that disrupt it. Very low doses of sazetidine-A or drugs with a similar profile may provide therapeutic benefit for reversing attentional impairment in patients suffering from mental disorders and/or cognitive impairment.

Pharmacological enhancement of memory and executive functioning in laboratory animals

Investigating how different pharmacological compounds may enhance learning, memory, and higher-order cognitive functions in laboratory animals is the first critical step toward the development of cognitive enhancers that may be used to ameliorate impairments in these functions in patients suffering from neuropsychiatric disorders. Rather than focus on one aspect of cognition, or class of drug, in this review we provide a broad overview of how distinct classes of pharmacological compounds may enhance different types of memory and executive functioning, particularly those mediated by the prefrontal cortex. These include recognition memory, attention, working memory, and different components of behavioral flexibility. A key emphasis is placed on comparing and contrasting the effects of certain drugs on different cognitive and mnemonic functions, highlighting methodological issues associated with this type of research, tasks used to investigate these functions, and avenues for future research. Viewed collectively, studies of the neuropharmacological basis of cognition in rodents and non-human primates have identified targets that will hopefully open new avenues for the treatment of cognitive disabilities in persons affected by mental disorders.

Using the MATRICS to guide development of a preclinical cognitive test battery for research in schizophrenia

Cognitive deficits in schizophrenia are among the core symptoms of the disease, correlate with functional outcome, and are not well treated with current antipsychotic therapies. In order to bring together academic, industrial, and governmental bodies to address this great 'unmet therapeutic need', the NIMH sponsored the Measurement and Treatment Research to Improve Cognition in Schizophrenia (MATRICS) initiative. Through careful factor analysis and consensus of expert opinion, MATRICS identified seven domains of cognition that are deficient in schizophrenia (attention/vigilance, working memory, reasoning and problem solving, processing speed, visual learning and memory, verbal learning and memory, and social cognition) and recommended a specific neuropsychological test battery to probe these domains. In order to move the field forward and outline an approach for translational research, there is a need for a "preclinical MATRICS" to develop a rodent test battery that is appropriate for drug development. In this review, we outline such an approach and review current rodent tasks that target these seven domains of cognition. The rodent tasks are discussed in terms of their validity for probing each cognitive domain as well as a brief overview of the pharmacology and manipulations relevant to schizophrenia for each task.

Chronic nicotine improves cognitive performance in a test of attention but does not attenuate cognitive disruption induced by repeated phencyclidine administration

RATIONALE

Nicotine-induced cognitive enhancement may be a factor maintaining tobacco smoking, particularly in psychiatric populations suffering from cognitive deficits. Schizophrenia patients exhibit higher smoking rates compared with the general population, suggesting that attempts to self-medicate cognitive schizophrenia deficits may underlie these high smoking levels.

OBJECTIVES

The present study explored pro-cognitive effects of nicotine in a model of schizophrenia-like cognitive dysfunction to test this self-medication hypothesis.

MATERIALS AND METHODS

We investigated whether chronic nicotine (3.16 mg/kg/day, base) would attenuate the performance disruption in the five-choice serial reaction time task (5-CSRTT, a task assessing various cognitive modalities, including attention) induced by repeated administration of phencyclidine (PCP), an N-methyl-D-aspartate receptor antagonist that induces cognitive deficits relevant to schizophrenia.

RESULTS

Chronic nicotine administration shortened 5-CSRTT response latencies under baseline conditions. Nicotine-treated rats also made more correct responses and fewer omissions than vehicle-treated rats. Replicating previous studies, repeated PCP administration (2 mg/kg, 30 min before behavioral testing for two consecutive days followed 2 weeks later by five consecutive days of PCP administration) decreased accuracy and increased response latencies, premature responding, and timeout responding. Chronic nicotine did not attenuate these PCP-induced disruptions.

CONCLUSIONS

Chronic nicotine had pro-cognitive effects by itself, supporting the hypothesis that cognitive enhancement may contribute to tobacco smoking. At the doses of nicotine and PCP used, however, no support was found for the hypothesis that the beneficial effects of nicotine on cognitive deficits induced by repeated PCP administration, assessed in the 5-CSRTT, are larger than nicotine effects in the absence of PCP.

Nicotine self-medication of cognitive-attentional processing

This article selectively reviews research concerning nicotine's effects on cognition, including the neurobiological mechanism for these effects, task and experimental features that may be important for elucidating these effects, and why these effects may have amplified motivational significance among smokers with cognitive deficit. Nicotine has effects on various cognitive processes, though most studies in humans have focused on the amelioration of cognitive deficits experienced during drug withdrawal. The direct cognitive-enhancing effect of nicotine remains a controversial topic. The relationship between attentional and non-attentional cognitive effects of nicotine is discussed in the context of cognitive self-medication. Further research should include theory-driven examination of cognitive effects of nicotine, and develop targeted smoking cessation programs based on an improved understanding of the role of cognitive self-medication in high-risk individuals.

Attenuation of pharmacologically-induced attentional impairment by methylphenidate in rats

Methylphenidate is widely used as a treatment option for attention deficit hyperactivity disorder. In animal models of attentional impairment, it is an important validation to determine whether this clinically effective treatment attenuates deficits. The purpose of the current study was to determine whether methylphenidate can diminish attentional impairment induced by three pharmacological agents with different mechanisms of action: scopolamine, mecamylamine, and dizocilpine. Female rats were trained on an operant visual signal detection task. Ten min before the test, the rats were injected subcutaneously with methylphenidate (0, 0.1, 0.3 mg/kg), scopolamine (0, 0.005, 0.01 mg/kg), mecamylamine (0, 2, 4 mg/kg), dizocilpine (0, 0.025, 0.05 mg/kg) or combinations of methylphenidate with these drugs. In each of the experiments, all rats received every treatment in a repeated measures counterbalanced order. Correction rejection accuracy was impaired by all three of the antagonists and these effects were attenuated by methylphenidate. Both scopolamine at 0.01 and dizocilpine at 0.05 mg/kg significantly impaired percent correct rejection choice accuracy, an effect that was ameliorated by methylphenidate. Mecamylamine (4 mg/kg) impaired attentional performance by reducing percent hit and percent correct rejection. Co-administration of methylphenidate failed to significantly affect the mecamylamine-induced attentional impairment. Methylphenidate alone at 0.3 mg/kg significantly improved percent hit choice accuracy only in low-performing rats in one experiment, an effect which was reversed by scopolamine. These data show that methylphenidate effectively reverses the attentional impairment caused by scopolamine and dizocilpine. These findings further validate the operant visual signal detection task for assessing attentional impairments and their reversal.

Ketanserin, a 5-HT2 receptor antagonist, decreases nicotine self-administration in rats

Nicotine intake constitutes a principal mechanism for tobacco addiction. In addition to primary effects on nicotinic acetylcholine receptors, nicotine has cascading effects, which may also underlie its neurobehavioral actions. Nicotine induces serotonin (5-HT) release, which has not classically been thought to be involved in tobacco addiction as dopamine has. However, addiction can be characterized more as a disorder of compulsion than a disorder of enjoyment. 5-HT mechanisms play key roles in compulsive disorders. Nicotine-induced 5-HT release may be a key to tobacco addiction. Ketanserin, a 5-HT2a and 5-HT2c receptor antagonist, significantly attenuates nicotine effects on attention and memory. These studies were conducted to determine if ketanserin would reduce nicotine self-administration in rats. Male Sprague-Dawley rats (N=12) were given initial food pellet training and then 10 sessions of nicotine self-administration training (0.03 mg/kg/infusion, i.v.). Then the rats were administered ketanserin (1 or 2 mg/kg, s.c.) or the saline vehicle. Ketanserin (2 mg/kg) significantly decreased nicotine self-administration. This did not seem to be due to sedative or amnestic effects of ketanserin. In a second study, the effects of repeated administration of 2 mg/kg ketanserin (N=11) vs. saline injections (N=10) were examined. In the initial phase, the acute effectiveness of ketanserin in significantly reducing nicotine self-administration was replicated. The effect became attenuated during the following several sessions, but the significant effect became re-established during the final phases of this two-week study. 5-HT mechanisms play critical roles in the maintenance of nicotine self-administration. Better understanding of those roles may help lead to new 5-HT-based treatments for tobacco addiction.

Chronic nicotine and dizocilpine effects on nicotinic and NMDA glutamatergic receptor regulation: interactions with clozapine actions and attentional performance in rats

Blockade of NMDA glutamate receptors with dizocilpine (MK-801) has been shown to cause substantial cognitive deficits and has been used to model symptoms of schizophrenia. Nicotine or nicotinic agonists, in contrast, may enhance cognitive or attentional functions and be of therapeutic potential in schizophrenia. Nicotinic-glutamatergic interactions, therefore, may have important implications in cognitive functions and antipsychotic treatments. Clozapine, a widely used antipsychotic drug, has been shown in some studies to be effective in ameliorating the cognitive deficits associated with schizophrenia. However, there is some evidence to suggest that clozapine similar to haloperidol may impair sustained attention in rats. In this study, we sought to determine whether chronic nicotine or dizocilpine may modify the effects of acute clozapine on attentional parameters and whether the behavioral effects would correlate with nicotinic or NMDA receptor densities in discrete brain regions. Adult female rats trained on an operant visual signal detection task were given 4 weeks of nicotine (5 mg/kg/day), dizocilpine (0.15 mg/kg/day), the same doses of both nicotine and dizocilpine as a mixture, or saline by osmotic minipump. While on chronic treatment, rats received acute injections of various doses of clozapine (0, 0.625, 1.25, 2.5 mg/kg, sc) 10 min prior to tests on attentional tasks. The pumps were removed on day 28 and 24 h later the animals were sacrificed for measurements of receptor densities in specific brain regions. The percent correct hit as a measure of sustained attention was significantly impaired by clozapine in a dose-related manner. Neither chronic nicotine nor dizocilpine affected this measure on their own or modified the effects of clozapine. Both nicotine and dizocilpine affected the receptor bindings in a region specific manner and their combination further modified the effects of each other in selective regions. Attentional performance was inversely correlated with alpha-bungarotoxin binding in the frontal cortex only. In conclusion, the data suggest attentional impairments with clozapine alone and no modification of this effect with nicotine or dizocilpine. Moreover, cortical low affinity nicotinic receptors may have a role in attentional functions.

Nicotine and clozapine effects on attentional performance impaired by the NMDA antagonist dizocilpine in female rats

Cognitive impairment is very prevalent in schizophrenia and is currently undertreated in most patients. Attentional deficit is one of the hallmark symptoms of schizophrenia. Antipsychotic drugs, which can be quite effective in combating hallucinations are often ineffective in reducing cognitive impairment and can potentiate cognitive impairment. Previously, we found that the antipsychotic drug clozapine impaired, while nicotine improved, the accuracy of rats performing a visual signal detection attentional task in normal rats. For the current study, in a model of cognitive impairment of schizophrenia with the NMDA antagonist dizocilpine (0.05 mg/kg), we examined the effects of clozapine and nicotine on significantly impaired attentional hit accuracy. This dizocilpine-induced impairment was significantly (p<0.05) reversed by either clozapine (1.25 mg/kg) or nicotine (0.025 mg/kg). Interestingly, when clozapine and nicotine were given together, they blocked each other's beneficial effects. When the effective doses of 1.25 mg/kg clozapine and 0.025 mg/kg nicotine were given together the combination no longer significantly reversed the dizocilpine-induced hit-accuracy impairment. Given that the great majority of people with schizophrenia smoke, the potential beneficial effects of clozapine on attentional function may be largely blocked by self-administered nicotine. In addition, there are promising results concerning the development of nicotinic treatments to reverse cognitive deficits including attentional impairment. This is supported in the current study by the reversal of the dizocilpine-induced attentional impairment by nicotine. However, in schizophrenia the efficacy of nicotinic treatments may be limited by co-treatment with antipsychotic drugs like clozapine. It will be important to determine which of the complex effects of clozapine and nicotine are key in reversing attentional impairment and how they block each other's effects for the development of therapy to combat the attentional impairment of schizophrenia.

The serotonin 2C receptor agonist Ro-60-0175 attenuates effects of nicotine in the five-choice serial reaction time task and in drug discrimination

RATIONALE

There is evidence that serotonin(2C) (5-HT(2C)) receptors can modulate some behavioural effects of nicotine, but the generality of this action is not known.

OBJECTIVE

To analyse the influence of the 5-HT(2C) agonist Ro-60-0175 on responses to nicotine in the five-choice serial reaction time task (5-CSRTT) and on its discriminative stimulus effect; these procedures constitute models for attention-enhancing and subjective effects of nicotine, respectively.

MATERIALS AND METHODS

In the 5-CSRTT, rats were trained to obtain food reinforcers by detecting light stimuli and then challenged with Ro-60-0175 (0.3-0.8 mg/kg) and nicotine (0.2 mg/kg). For drug discrimination studies, rats were trained to discriminate nicotine (0.2 mg/kg) from saline in a two-lever procedure using a tandem schedule of food reinforcement.

RESULTS

In the 5-CSRTT, nicotine positively influenced most response indices, confirming previous results. Ro-60-0175 increased response latencies and omission errors and reduced anticipatory responding but had little effect on response accuracy; importantly, it counteracted the effects of nicotine on response speed and omission errors. Pentobarbitone (10-14 mg/kg) also slowed performance of the 5-CSRTT but did not weaken the nicotine-induced enhancement of performance. In the drug discrimination procedure, Ro-60-0175 was not generalised with nicotine but shifted the nicotine dose-response curve to the right in a dose-related manner.

CONCLUSIONS

The data suggest that selective occupancy of 5-HT(2C) receptors can attenuate some effects of nicotine in the 5-CSRTT and weaken the nicotine discriminative stimulus; these effects cannot be explained by a sedative action of Ro-60-0175.

Nicotinic interactions with antipsychotic drugs, models of schizophrenia and impacts on cognitive function

People with schizophrenia often have substantial cognitive impairments, which may be related to nicotinic receptor deficits, (alpha7 and alpha4beta2), documented in the brains of people with schizophrenia. The large majority of people with schizophrenia smoke cigarettes. Thus, nicotinic interactions with antipsychotic drugs are widespread. Complementary co-therapies of novel nicotinic ligands are being developed to add to antipsychotic therapy to treat the cognitive impairment of schizophrenia. Thus, it is critical to understand the interaction between nicotinic treatments and antipsychotic drugs. Nicotinic interactions with antipsychotic drugs, are complex since both nicotine and antipsychotics have complex actions. Nicotine stimulates and desensitizes nicotinic receptors of various subtypes and potentiates the release of different neurotransmitters. Antipsychotics also act on a verity of receptor systems. For example, clozapine acts as an antagonist at a variety of neurotransmitter receptors such as those for dopamine, serotonin, norepinepherine and histamine. In a series of studies, we have found that in normally functioning rats, moderate doses of clozapine impair working memory and that clozapine blocks nicotine-induced memory and attentional improvement. Clozapine and nicotine can attenuate each other's beneficial effects in reversing the memory impairment caused by the psychototmimetic drug dizocilpine. A key to the clozapine-induced attenuation of nicotine-induced cognitive improvement appears to be its 5HT(2) antagonist properties. The selective 5HT(2) antagonist ketanserin has a similar action of blocking nicotine-induced memory and attentional improvements. It is important to consider the interactions between nicotinic and antipsychotic drugs to develop the most efficacious treatment for cognitive improvement in people with schizophrenia.

Different effects of ionotropic and metabotropic glutamate receptor antagonists on attention and the attentional properties of nicotine

Distinct lines of evidence indicate that glutamate plays a primary role in modulating cognitive functions. Notably, competitive glutamate receptor antagonists acting at ionotropic N-methyl-d-aspartate (NMDA) or metabotropic glutamate 5 (mGlu5) receptors impair cognitive performance. Conversely, nicotine and other psychostimulants stimulate glutamatergic mechanisms and can act as cognitive enhancers. Hence we analysed the role of glutamate in performance of an attentional task and in nicotine-induced enhancement of attention by using the rodent five-choice serial reaction time task (5-CSRTT). Rats were trained to criterion performance and were then pre-dosed with either vehicle, the NMDA receptor antagonist (+)3-(2-carboxypiperazin-4-propyl)-1-propenyl-1-phosphonic acid (CPP, 0.3-2.0 mg/kg) or the mGlu5 antagonist 2-methyl-6-phenylethynyl-pyridine (MPEP, 1.0-9.0 mg/kg) and challenged with nicotine (0.2 mg/kg). Nicotine improved attentional performance, an effect that was weakened by doses of CPP that themselves had little impact on performance; importantly, CPP dose-dependently blunted the ability of nicotine to improve response accuracy, the major measure of signal detection in the paradigm. MPEP dose-dependently impaired signal detection under conditions with a high attentional load, an effect that was reversed by nicotine; thus, MPEP did not block nicotine-induced attentional enhancement. Co-administration of either CPP or MPEP with nicotine also produced a general slowing of performance characterised by increases in omission errors and response latencies and reduced anticipatory responding. It is concluded that activation of NMDA receptors may be an important determinant of the effects of nicotine in the 5-CSRTT. Stimulation of nicotinic receptors may also reverse attentional deficits associated with the impaired function of the glutamate network.

Nicotinic-antipsychotic drug interactions and cognitive function

In summary, neuronal nicotinic systems are important for a variety of aspects of cognitive function impacted by antipsychotic drugs. It has been demonstrated that antipsychotic drugs have memory and attentional impairing effects when given to unimpaired subjects. Nicotine can reduce some of these impairments, but antipsychotic drug administration can also attenuate nicotine effects. We have found that nicotinic agonists selective for alpha7 and alpha4beta2 receptor subtypes significantly improve learning and memory. Serotonergic actions of antipsychotic drugs may decrease efficacy of nicotinic co-treatments. When the antipsychotic drug clozapine and nicotine are administered to subjects with cognitive impairments caused by NMDA glutamate receptor blockade or hippocampal dysfunction they can significantly attenuate the attentional and memory impairments. Nicotine has been shown in our studies to reverse the memory impairment caused by acute clozapine-induced memory improvement. Acute risperidone and haloperidol has been shown to attenuate nicotine-induced memory improvement. We have determined the role of hippocampal alpha7 and alpha4beta2 nicotinic receptors in the neural basis of nicotinic antipsychotic interactions. Local acute and chronic hippocampal infusion of either nicotinic alpha7 or alpha4beta2 antagonists cause significant spatial working memory impairment. Chronic hippocampal nicotinic antagonist infusions have served as a model of persistent decreases in nicotinic receptor level seen in schizophrenia and Alzheimer's disease. Clozapine attenuated the memory deficit caused by chronic suppression of hippocampal alpha4beta2 receptors while the amnestic effects of clozapine were potentiated by chronic suppression of hippocampal alpha7 receptors. Nicotinic co-treatment may be a useful adjunct in the treatment of schizophrenia, to attenuate cognitive impairment of schizophrenia. Nicotine as well as selective nicotinic alpha7 and alpha4beta2 receptor agonists significantly improve working memory and attentional function. Nicotine treatment was found to be effective in attenuating the attentional and memory impairments caused by the psychototmimetic NMDA antagonist dizocilpine (MK-801), a model of the cognitive impairment of schizophrenia. Studies of the interactions of antipsychotic drugs with nicotinic agents provided quite useful information concerning possible co-treatment of people with schizophrenia with nicotinic therapy. Nicotine was found to significantly attenuate the memory impairments caused by the antipsychotic drugs clozapine and olanzapine. Interestingly, nicotine-induced cognitive improvement was significantly attenuated by the antipsychotic drug clozapine. One of the principal effects of clozapine is to block 5HT2 receptors. Ketanserin a 5HT2 antagonist significantly attenuated nicotine-induced improvements in attention and memory. Thus it appears that antipsychotic drugs with actions blocking 5HT2 receptors may limit the efficacy of nicotinic co-treatments for cognitive enhancement.

Nicotinic effects on cognitive function: behavioral characterization, pharmacological specification, and anatomic localization

RATIONALE

Nicotine has been shown in a variety of studies in humans and experimental animals to improve cognitive function. Nicotinic treatments are being developed as therapeutic treatments for cognitive dysfunction.

OBJECTIVES

Critical for the development of nicotinic therapeutics is an understanding of the neurobehavioral bases for nicotinic involvement in cognitive function.

METHODS

Specific and diverse cognitive functions affected by nicotinic treatments are reviewed, including attention, learning, and memory. The neural substrates for these behavioral actions involve the identification of the critical pharmacologic receptor targets, in particular brain locations, and how those incipient targets integrate with broader neural systems involved with cognitive function.

RESULTS

Nicotine and nicotinic agonists can improve working memory function, learning, and attention. Both alpha4beta2 and alpha7 nicotinic receptors appear to be critical for memory function. The hippocampus and the amygdala in particular have been found to be important for memory, with decreased nicotinic activity in these areas impairing memory. Nicotine and nicotinic analogs have shown promise for inducing cognitive improvement. Positive therapeutic effects have been seen in initial studies with a variety of cognitive dysfunctions, including Alzheimer's disease, age-associated memory impairment, schizophrenia, and attention deficit hyperactivity disorder.

CONCLUSIONS

Discovery of the behavioral, pharmacological, and anatomic specificity of nicotinic effects on learning, memory, and attention not only aids the understanding of nicotinic involvement in the basis of cognitive function, but also helps in the development of novel nicotinic treatments for cognitive dysfunction. Nicotinic treatments directed at specific receptor subtypes and nicotinic cotreatments with drugs affecting interacting transmitter systems may provide cognitive benefits most relevant to different syndromes of cognitive impairment such as Alzheimer's disease, schizophrenia, and attention deficit hyperactivity disorder. Further research is necessary in order to determine the efficacy and safety of nicotinic treatments of these cognitive disorders.

Ketanserin attenuates nicotine-induced working memory improvement in rats

Nicotinic systems have been shown in numerous studies to be important for spatial working memory. Nicotinic systems are certainly not acting alone in the basis of memory function, but act in concert with a variety of other neural systems. Important for these interactions is nicotinic induced release of a variety of neurotransmitters involved in memory function including serotonin (5-HT). We have found that the 5-HT2 receptor antagonist, ketanserin, effectively attenuated nicotine-induced attentional improvement. The current study explored the interaction between nicotinic and serotonergic systems in the performance of a spatial working memory task in the radial-arm maze. Female Sprague-Dawley rats were trained on the win-shift working memory task on the 8-arm radial maze. After 18 sessions of acquisition training the rats were given acute doses of nicotine (0.2 and 0.4 mg/kg), ketanserin (0.5, 1 and 2 mg/kg) or combinations of the two. The vehicle served as the control. As seen in previous studies, nicotine caused a significant improvement in working memory performance as indexed by the number of correct arm entries before the first error (entries to repeat). Ketanserin at the doses tested did not cause a significant effect on choice accuracy, but it did significantly attenuate the improvement caused by the 0.2 mg/kg nicotine dose. The higher 0.4 mg/kg nicotine dose was nearly sufficient to overcome the ketanserin effect. This study shows that, as with attentional function, nicotine-induced working memory improvement is attenuated by the 5-HT2 antagonist ketanserin. Given that many antipsychotic drugs have substantial 5-HT2 antagonist effects, these atypical antipsychotic drugs may reduce the cognitive improvements caused by nicotinic treatments.