Nicotinic and muscarinic receptors in the rat prefrontal cortex: differential roles in working memory, response selection and effortful processing

Long-lasting spatial memory deficits and impaired hippocampal plasticity following unilateral vestibular loss

Unilateral vestibular loss (UVL) induces a characteristic vestibular syndrome composed of various posturo-locomotor, oculomotor, vegetative and perceptivo-cognitive symptoms. Functional deficits are progressively recovered over time during vestibular compensation, that is supported by the expression of multiscale plasticity mechanisms. While the dynamic of post-UVL posturo-locomotor and oculomotor deficits is well characterized, the expression over time of the cognitive deficits, and in particular spatial memory deficits, is still debated. In this study we aimed at investigating spatial memory deficits and their recovery in a rat model of unilateral vestibular neurectomy (UVN), using a wide spectrum of behavioral tasks. In parallel, we analyzed markers of hippocampal plasticity involved in learning and memory. Our results indicate the UVN affects all domains of spatial memory, from working memory to reference memory and object-in-place recognition. These deficits are associated with long-lasting impaired plasticity in the ipsilesional hippocampus. These results highlight the crucial role of symmetrical vestibular information in spatial memory and contribute to a better understanding of the cognitive disorders observed in vestibular patients.

Standardized Extract (HemoHIM) Protects against Scopolamine-Induced Amnesia in a Murine Model

HemoHIM is a medicinal herbal preparation of Angelica gigas Nakai (Apiaceae), Cnidium officinale Makino (Umbelliferae), and Paeonia lactiflora Pallas (Paeoniaceae) designed for immune regulation. In the present study, the memory-enhancing effects of a standardized extract (HemoHIM) on scopolamine-induced memory impairment in a murine model was investigated. To induce amnesia, scopolamine (1 mg/kg) was intraperitoneally (i.p.) injected into mice 30 min before the start of behavioral tests. The Y-maze, novel object recognition test (NORT), and passive avoidance task (PAT) were used to evoke memory functions. HemoHIM significantly improved scopolamine-induced memory impairment in ICR mice, which was evidenced by an improvement of spontaneous alternation in the Y-maze, recognition index in NORT, and latency time in PAT. To elucidate the possible mechanism, the cholinergic activity and mRNA levels of choline acetyltransferase (ChAT), muscarinic acetylcholine receptor (mAchR), brain-derived neurotrophic factor (BDNF), and cAMP response element-binding protein (CREB) were measured using reverse transcription (RT-PCR) and western blot analyses, respectively. HemoHIM treatment attenuated the scopolamine-induced hyperactivation of acetylcholinesterase (AchE) activity. In addition, ChAT, mAchR, and CREB mRNA levels were increased in the hippocampus compared with the scopolamine group. Furthermore, HemoHIM treatment resulted in elevated BDNF protein expression. These results indicate that HemoHIM may exert antiamnesic activity by increasing Ach and inhibiting AchE in the hippocampus. In addition, HemoHIM has therapeutic potential by upregulating ChAT, mAchR, and BDNF, which is apparently mediated by activation of the CREB and ERK signaling pathways.

The Free-movement pattern Y-maze: A cross-species measure of working memory and executive function

Numerous neurodegenerative and psychiatric disorders are associated with deficits in executive functions such as working memory and cognitive flexibility. Progress in developing effective treatments for disorders may benefit from targeting these cognitive impairments, the success of which is predicated on the development of animal models with validated behavioural assays. Zebrafish offer a promising model for studying complex brain disorders, but tasks assessing executive function are lacking. The Free-movement pattern (FMP) Y-maze combines aspects of the common Y-maze assay, which exploits the inherent motivation of an organism to explore an unknown environment, with analysis based on a series of sequential two-choice discriminations. We validate the task as a measure of working memory and executive function by comparing task performance parameters in adult zebrafish treated with a range of glutamatergic, cholinergic and dopaminergic drugs known to impair working memory and cognitive flexibility. We demonstrate the cross-species validity of the task by assessing performance parameters in adapted versions of the task for mice and Drosophila, and finally a virtual version in humans, and identify remarkable commonalities between vertebrate species' navigation of the maze. Together, our results demonstrate that the FMP Y-maze is a sensitive assay for assessing working memory and cognitive flexibility across species from invertebrates to humans, providing a simple and widely applicable behavioural assay with exceptional translational relevance.

Estrous cycle stage gates sex differences in prefrontal muscarinic control of fear memory formation

The association of a sensory cue and an aversive footshock that are separated in time, as in trace fear conditioning, requires persistent activity in prelimbic cortex during the cue-shock interval. The activation of muscarinic acetylcholine receptors has been shown to facilitate persistent firing of cortical cells in response to brief stimulation, and muscarinic antagonists in the prefrontal cortex impair working memory. It is unknown, however, if the acquisition of associative trace fear conditioning is dependent on muscarinic signaling in the prefrontal cortex. Here, we delivered the muscarinic receptor antagonist scopolamine to the prelimbic cortex of rats prior to trace fear conditioning and tested their memories of the cue and training context the following day. The effect of scopolamine on working memory performance was also tested using a spatial delayed non-match to sample task. Male and female subjects were included to examine potential sex differences in the modulation of memory formation, as we have previously observed for pituitary adenylate cyclase-activating polypeptide signaling in the prefrontal cortex (Kirry et al., 2018). We found that pre-training administration of intra-prelimbic scopolamine impaired the formation of cued and contextual fear memories in males, but not females at a dose that impairs spatial working memory in both sexes. Fear memory formation in females was impaired by a higher dose of scopolamine and this impairment was gated by estrous cycle stage: scopolamine failed to impair memory in rats in the diestrus or proestrus stages of the estrous cycle. These findings add to the growing body of evidence that the prefrontal cortex is sexually dimorphic in learning and memory and additionally suggest that males and females differentially engage prefrontal neuromodulatory systems in support of learning.

Scopolamine Impairs Appetitive But Not Aversive Trace Conditioning: Role of the Medial Prefrontal Cortex

The muscarinic acetylcholine receptor is an important modulator of medial prefrontal cortex (mPFC) functions, such as the working memory required to bridge a trace interval in associative leaning. Aversive and appetitive trace conditioning procedures were used to examine the effects of scopolamine (0.1 and 0.5 mg/kg, i.p.) in male rats. Follow-up experiments tested the effects of microinfusion of 0.15 μg of scopolamine (0.075 μg of in 0.5 μl/side) in infralimbic (IL) versus prelimbic regions of rat mPFC, in appetitive trace and locomotor activity (LMA) procedures. Systemic scopolamine was without effect in an aversive trace conditioning procedure, but impaired appetitive conditioning at a 2 s trace interval. This effect was demonstrated as reduced responding during presentations of the conditioned stimulus (CS) and during the interstimulus interval (ISI). There was no such effect on responding during food (unconditioned stimulus, US) responding or in the intertrial interval (ITI). In contrast, systemic scopolamine dose-relatedly increased LMA. Trace conditioning was similarly impaired at the 2 s trace (shown as reduced responding to the CS and during the ISI, but not during US presentations or in the ITI) after infusion in mPFC, whereas LMA was increased (after infusion in IL only). Therefore, our results point to the importance of cholinergic modulation in mPFC for trace conditioning and show that the observed effects cannot be attributed to reduced activity.SIGNIFICANCE STATEMENT Events are very often separated in time, in which case working memory is necessary to condition their association in "trace conditioning." The present study used conditioning variants motivated aversively with foot shock and appetitively with food. The drug scopolamine was used to block muscarinic acetylcholine receptors involved in working memory. The results show that reduced cholinergic transmission in medial prefrontal cortex (mPFC) impaired appetitive trace conditioning at a 2 s trace interval. However, scopolamine was without effect in the aversive procedure, revealing the importance of procedural differences to the demonstration of the drug effect. The finding that blockade of muscarinic receptors in mPFC impaired trace conditioning shows that these receptors are critical modulators of short-term working memory.

Nicotinic α4β2 Cholinergic Receptor Influences on Dorsolateral Prefrontal Cortical Neuronal Firing during a Working Memory Task

The primate dorsolateral prefrontal cortex (dlPFC) subserves top-down regulation of attention and working memory abilities. Depletion studies show that the neuromodulator acetylcholine (ACh) is essential to dlPFC working memory functions, but the receptor and cellular bases for cholinergic actions are just beginning to be understood. The current study found that nicotinic receptors comprised of α4 and β2 subunits (α4β2-nAChR) enhance the task-related firing of delay and fixation cells in the dlPFC of monkeys performing a working memory task. Iontophoresis of α4β2-nAChR agonists increased the neuronal firing and enhanced the spatial tuning of delay cells, neurons that represent visual space in the absence of sensory stimulation. These enhancing effects were reversed by coapplication of a α4β2-nAChR antagonist, consistent with actions at α4β2-nAChR. Delay cell firing was reduced when distractors were presented during the delay epoch, whereas stimulation of α4β2-nAChR protected delay cells from these deleterious effects. Iontophoresis of α4β2-nAChR agonists also enhanced the firing of fixation cells, neurons that increase firing when the monkey initiates a trial, and maintain firing until the trial is completed. These neurons are thought to contribute to sustained attention and top-down motor control and have never before been the subject of pharmacological inquiry. These findings begin to build a picture of the cellular actions underlying the beneficial effects of ACh on attention and working memory. The data may also help to explain why genetic insults to α4 subunits are associated with working memory and attentional deficits and why α4β2-nAChR agonists may have therapeutic potential.SIGNIFICANCE STATEMENT The acetylcholine (ACh) arousal system in the brain is needed for robust attention and working memory functions, but the receptor and cellular bases for its beneficial effects are poorly understood in the newly evolved primate brain. The current study found that ACh stimulation of nicotinic receptors comprised of α4 and β2 subunits (α4β2-nAChR) enhanced the firing of neurons in the primate prefrontal cortex that subserve top-down attentional control and working memory. α4β2-nAChR stimulation also protected neuronal responding from the detrimental effects of distracters presented during the delay epoch, when information is held in working memory. These results illuminate how ACh strengthens higher cognition and help to explain why genetic insults to the α4 subunit weaken cognitive and attentional abilities.

Loss of M1 Receptor Dependent Cholinergic Excitation Contributes to mPFC Deactivation in Neuropathic Pain

In chronic pain, the medial prefrontal cortex (mPFC) is deactivated and mPFC-dependent tasks such as attention and working memory are impaired. We investigated the mechanisms of mPFC deactivation in the rat spared nerve injury (SNI) model of neuropathic pain. Patch-clamp recordings in acute slices showed that, 1 week after the nerve injury, cholinergic modulation of layer 5 (L5) pyramidal neurons was severely impaired. In cells from sham-operated animals, focal application of acetylcholine induced a left shift of the input/output curve and persistent firing. Both of these effects were almost completely abolished in cells from SNI-operated rats. The cause of this impairment was an ∼60% reduction of an M1-coupled, pirenzepine-sensitive depolarizing current, which appeared to be, at least in part, the consequence of M1 receptor internalization. Although no changes were detected in total M1 protein or transcript, both the fraction of the M1 receptor in the synaptic plasma membrane and the biotinylated M1 protein associated with the total plasma membrane were decreased in L5 mPFC of SNI rats. The loss of excitatory cholinergic modulation may play a critical role in mPFC deactivation in neuropathic pain and underlie the mPFC-specific cognitive deficits that are comorbid with neuropathic pain.SIGNIFICANCE STATEMENT The medial prefrontal cortex (mPFC) undergoes major reorganization in chronic pain. Deactivation of mPFC output is causally correlated with both the cognitive and the sensory component of neuropathic pain. Here, we show that cholinergic excitation of commissural layer 5 mPFC pyramidal neurons is abolished in neuropathic pain rats due to a severe reduction of a muscarinic depolarizing current and M1 receptor internalization. Therefore, in neuropathic pain rats, the acetylcholine (ACh)-dependent increase in neuronal excitability is reduced dramatically and the ACh-induced persisting firing, which is critical for working memory, is abolished. We propose that the blunted cholinergic excitability contributes to the functional mPFC deactivation that is causal for the pain phenotype and represents a cellular mechanism for the attention and memory impairments comorbid with chronic pain.

Soluble Immune Response Suppressor (SIRS): Reassessing the immunosuppressant potential of an elusive peptide

A previously studied immunosuppressive cytokine, Soluble Immune Response Suppressor (SIRS), may have relevance to current studies of immune suppression in a variety of human disease states. Despite extensive efforts using experimental models, mainly in mice, much remains to be discovered as to how autoimmune cells in mice and humans escape normal regulation and, conversely, how tumor cells evade evoking an immune response. It is the contention of this commentary that the literature pre-2000 contain results that might inform current studies. The broadly immunosuppressive protein, SIRS, was studied extensively from the 1970s to 1990s and culminated in the determination of the n-terminal 21mer sequence of this 15kDa protein which had high homology to the short neurotoxins from sea snakes, that are canonical members of the three finger neurotoxin superfamily (3FTx). It was not until 2007 that the prophylactic administration of the synthetic N-terminal peptide of the SIRS 21mer, identical to the published sequence, was reported to inhibit or delay the development of two autoimmune diseases in mice: experimental allergic encephalomyelitis (EAE) and type I diabetes (T1D). These findings were consistent with other studies of the 3FTx superfamily as important probes in the study of mammalian pharmacology. It is the perspective of this commentary that SIRS, SIRS peptide and the anti-peptide mAb, represent useful, pharmacologically-active probes for the study of the immune response as well as in the potential treatment of autoimmune, inflammatory diseases and cancer.

PERK regulates Gq protein-coupled intracellular Ca2+ dynamics in primary cortical neurons

PERK (EIF2AK3) is an ER-resident eIF2α kinase required for behavioral flexibility and metabotropic glutamate receptor-dependent long-term depression via its translational control. Motivated by the recent discoveries that PERK regulates Ca²⁺ dynamics in insulin-secreting β-cells underlying glucose-stimulated insulin secretion, and modulates Ca²⁺ signals-dependent working memory, we explored the role of PERK in regulating G_q protein-coupled Ca²⁺ dynamics in pyramidal neurons. We found that acute PERK inhibition by the use of a highly specific PERK inhibitor reduced the intracellular Ca²⁺ rise stimulated by the activation of acetylcholine, metabotropic glutamate and bradykinin-2 receptors in primary cortical neurons. More specifically, acute PERK inhibition increased IP₃ receptor mediated ER Ca²⁺ release, but decreased receptor-operated extracellular Ca²⁺ influx. Impaired G_q protein-coupled intracellular Ca²⁺ rise was also observed in genetic Perk knockout neurons. Taken together, our findings reveal a novel role of PERK in neurons, which is eIF2α-independent, and suggest that the impaired working memory in forebrain-specific Perk knockout mice may stem from altered G_q protein-coupled intracellular Ca²⁺ dynamics in cortical pyramidal neurons.

PERK Regulates Working Memory and Protein Synthesis-Dependent Memory Flexibility

PERK (EIF2AK3) is an ER-resident eIF2α kinase required for memory flexibility and metabotropic glutamate receptor-dependent long-term depression, processes known to be dependent on new protein synthesis. Here we investigated PERK’s role in working memory, a cognitive ability that is independent of new protein synthesis, but instead is dependent on cellular Ca²⁺ dynamics. We found that working memory is impaired in forebrain-specific Perk knockout and pharmacologically PERK-inhibited mice. Moreover, inhibition of PERK in wild-type mice mimics the fear extinction impairment observed in forebrain-specific Perk knockout mice. Our findings reveal a novel role of PERK in cognitive functions and suggest that PERK regulates both Ca²⁺ -dependent working memory and protein synthesis-dependent memory flexibility.

Phospholipase C-β1 Hypofunction in the Pathogenesis of Schizophrenia

Schizophrenia is a mental disorder that is characterized by various abnormal symptoms. Previous studies indicate decreased expression of phospholipase C-β1 (PLC-β1) in the brains of patients with schizophrenia. PLC-β1-null (PLC-β1(-/-)) mice exhibit multiple endophenotypes of schizophrenia. Furthermore, a study of PLC-β1 knockdown in the medial prefrontal cortex of mice has shown a specific behavioral deficit, impaired working memory. These results support the notion that disruption of PLC-β1-linked signaling in the brain is strongly involved in the pathogenesis of schizophrenia. In this review, we broadly investigate recent studies regarding schizophrenia-related behaviors as well as their various clinical and biological correlates in PLC-β1(-/-) and knockdown mouse models. This will provide a better understanding of the pathological relevance of the altered expression of PLC-β1 in the brains of patients with schizophrenia. Evidence accumulated will shed light on future in-depth studies, possibly in human subjects.

Nicotinic, glutamatergic and dopaminergic synaptic transmission and plasticity in the mesocorticolimbic system: focus on nicotine effects

Cigarette smoking is currently the leading cause of preventable deaths and disability throughout the world, being responsible for about five million premature deaths/year. Unfortunately, fewer than 10% of tobacco users who try to stop smoking actually manage to do so. The main addictive agent delivered by cigarette smoke is nicotine, which induces psychostimulation and reward, and reduces stress and anxiety. The use of new technologies (including optogenetics) and the development of mouse models characterised by cell-specific deletions of receptor subtype genes or the expression of gain-of-function nAChR subunits has greatly increased our understanding of the molecular mechanisms and neural substrates of nicotine addiction first revealed by classic electrophysiological, neurochemical and behavioural approaches. It is now becoming clear that various aspects of nicotine dependence are mediated by close interactions of the glutamatergic, dopaminergic and γ-aminobutyric acidergic systems in the mesocorticolimbic system. This review is divided into two parts. The first provides an updated overview of the circuitry of the ventral tegmental area, ventral striatum and prefrontal cortex, the neurotransmitter receptor subtypes expressed in these areas, and their physiological role in the mesocorticolimbic system. The second will focus on the molecular, functional and behavioural mechanisms involved in the acute and chronic effects of nicotine on the mesocorticolimbic system.

Differential contribution of TRPM4 and TRPM5 nonselective cation channels to the slow afterdepolarization in mouse prefrontal cortex neurons

In certain neurons from different brain regions, a brief burst of action potentials can activate a slow afterdepolarization (sADP) in the presence of muscarinic acetylcholine receptor agonists. The sADP, if suprathreshold, can contribute to persistent non-accommodating firing in some of these neurons. Previous studies have characterized a Ca²⁺-activated non-selective cation (CAN) current (I_CAN) that is thought to underlie the sADP. I_CAN depends on muscarinic receptor stimulation and exhibits a dependence on neuronal activity, membrane depolarization and Ca²⁺-influx similar to that observed for the sADP. Despite the widespread occurrence of sADPs in neurons throughout the brain, the molecular identity of the ion channels underlying these events, as well as I_CAN, remains uncertain. Here we used a combination of genetic, pharmacological and electrophysiological approaches to characterize the molecular mechanisms underlying the muscarinic receptor-dependent sADP in layer 5 pyramidal neurons of mouse prefrontal cortex. First, we confirmed that in the presence of the cholinergic agonist carbachol a brief burst of action potentials triggers a prominent sADP in these neurons. Second, we confirmed that this sADP requires activation of a PLC signaling cascade and intracellular calcium signaling. Third, we obtained direct evidence that the transient receptor potential (TRP) melastatin 5 channel (TRPM5), which is thought to function as a CAN channel in non-neural cells, contributes importantly to the sADP in the layer 5 neurons. In contrast, the closely related TRPM4 channel may play only a minor role in the sADP.

DRD2/CHRNA5 interaction on prefrontal biology and physiology during working memory

BACKGROUND

Prefrontal behavior and activity in humans are heritable. Studies in animals demonstrate an interaction between dopamine D2 receptors and nicotinic acetylcholine receptors on prefrontal behavior but evidence in humans is weak. Therefore, we hypothesize that genetic variation regulating dopamine D2 and nicotinic acetylcholine receptor signaling impact prefrontal cortex activity and related cognition. To test this hypothesis in humans, we explored the interaction between functional genetic variants in the D2 receptor gene (DRD2, rs1076560) and in the nicotinic receptor α5 gene (CHRNA5, rs16969968) on both dorsolateral prefrontal cortex mediated behavior and physiology during working memory and on prefrontal gray matter volume.

METHODS

A large sample of healthy subjects was compared for genotypic differences for DRD2 rs1076560 (G>T) and CHNRA5 rs16969968 (G>A) on prefrontal phenotypes, including cognitive performance at the N-Back task, prefrontal physiology with BOLD fMRI during performance of the 2-Back working memory task, and prefrontal morphometry with structural MRI.

RESULTS

We found that DRD2 rs1076560 and CHNRA5 rs16969968 interact to modulate cognitive function, prefrontal physiology during working memory, and prefrontal gray matter volume. More specifically, CHRNA5-AA/DRD2-GT subjects had greater behavioral performance, more efficient prefrontal cortex activity at 2Back working memory task, and greater prefrontal gray matter volume than the other genotype groups.

CONCLUSIONS

The present data extend previous studies in animals and enhance our understanding of dopamine and acetylcholine signaling in the human prefrontal cortex, demonstrating interactions elicited by working memory that are modulated by genetic variants in DRD2 and CHRNA5.

Nonaggressive and adapted social cognition is controlled by the interplay between noradrenergic and nicotinic receptor mechanisms in the prefrontal cortex

Social animals establish flexible behaviors and integrated decision-making processes to adapt to social environments. Such behaviors are impaired in all major neuropsychiatric disorders and depend on the prefrontal cortex (PFC). We previously showed that nicotinic acetylcholine receptors (nAChRs) and norepinephrine (NE) in the PFC are necessary for mice to show adapted social cognition. Here, we investigated how the cholinergic and NE systems converge within the PFC to modulate social behavior. We used a social interaction task (SIT) in C57BL/6 mice and mice lacking β2*nAChRs (β2(-/-) mice), making use of dedicated software to analyze >20 social sequences and pinpoint social decisions. We performed specific PFC NE depletions before SIT and measured monoamines and acetylcholine (ACh) levels in limbic corticostriatal circuitry. After PFC-NE depletion, C57BL/6 mice exhibited impoverished and more rigid social behavior and were 6-fold more aggressive than sham-lesioned animals, whereas β2(-/-) mice showed unimpaired social behavior. Our biochemical measures suggest a critical involvement of DA in SIT. In addition, we show that the balance between basal levels of monoamines and of ACh modulates aggressiveness and this modulation requires functional β2*nAChRs. These findings demonstrate the critical interplay between prefrontal NE and nAChRs for the development of adapted and nonaggressive social cognition.

Prefrontal cortex HCN1 channels enable intrinsic persistent neural firing and executive memory function

In many cortical neurons, HCN1 channels are the major contributors to Ih, the hyperpolarization-activated current, which regulates the intrinsic properties of neurons and shapes their integration of synaptic inputs, paces rhythmic activity, and regulates synaptic plasticity. Here, we examine the physiological role of Ih in deep layer pyramidal neurons in mouse prefrontal cortex (PFC), focusing on persistent activity, a form of sustained firing thought to be important for the behavioral function of the PFC during working memory tasks. We find that HCN1 contributes to the intrinsic persistent firing that is induced by a brief depolarizing current stimulus in the presence of muscarinic agonists. Deletion of HCN1 or acute pharmacological blockade of Ih decreases the fraction of neurons capable of generating persistent firing. The reduction in persistent firing is caused by the membrane hyperpolarization that results from the deletion of HCN1 or Ih blockade, rather than a specific role of the hyperpolarization-activated current in generating persistent activity. In vivo recordings show that deletion of HCN1 has no effect on up states, periods of enhanced synaptic network activity. Parallel behavioral studies demonstrate that HCN1 contributes to the PFC-dependent resolution of proactive interference during working memory. These results thus provide genetic evidence demonstrating the importance of HCN1 to intrinsic persistent firing and the behavioral output of the PFC. The causal role of intrinsic persistent firing in PFC-mediated behavior remains an open question.

Cortical control of VTA function and influence on nicotine reward

Tobacco use is a major public health problem. Nicotine acts on widely distributed nicotinic acetylcholine receptors (nAChRs) in the brain and excites dopamine (DA) neurons in the ventral tegmental area (VTA). The elicited increase of DA neuronal activity is thought to be an important mechanism for nicotine reward and subsequently the transition to addiction. However, the current understanding of nicotine reward is based predominantly on the data accumulated from in vitro studies, often from VTA slices. Isolated VTA slices artificially terminate communications between neurons in the VTA and other brain regions that may significantly alter nicotinic effects. Consequently, the mechanisms of nicotinic excitation of VTA DA neurons under in vivo conditions have received only limited attention. Building upon the existing knowledge acquired in vitro, it is now time to elucidate the integrated mechanisms of nicotinic reward on intact systems that are more relevant to understanding the action of nicotine or other addictive drugs. In this review, we summarize recent studies that demonstrate the impact of prefrontal cortex (PFC) on the modulation of VTA DA neuronal function and nicotine reward. Based on existing evidence, we propose a new hypothesis that PFC-VTA functional coupling serves as an integration mechanism for nicotine reward. Moreover, addiction may develop due to nicotine perturbing the PFC-VTA coupling and thereby eliminating the PFC-dependent cognitive control over behavior.

Impact of prefrontal cortex in nicotine-induced excitation of ventral tegmental area dopamine neurons in anesthetized rats

Systemic administration of nicotine increases dopaminergic (DA) neuron firing in the ventral tegmental area (VTA), which is thought to underlie nicotine reward. Here, we report that the medial prefrontal cortex (mPFC) plays a critical role in nicotine-induced excitation of VTA DA neurons. In chloral hydrate-anesthetized rats, extracellular single-unit recordings showed that VTA DA neurons exhibited two types of firing responses to systemic nicotine. After nicotine injection, the neurons with type-I response showed a biphasic early inhibition and later excitation, whereas the neurons with type-II response showed a monophasic excitation. The neurons with type-I, but not type-II, response exhibited pronounced slow oscillations (SOs) in firing. Pharmacological or structural mPFC inactivation abolished SOs and prevented systemic nicotine-induced excitation in the neurons with type-I, but not type-II, response, suggesting that these VTA DA neurons are functionally coupled to the mPFC and nicotine increases firing rate in these neurons in part through the mPFC. Systemic nicotine also increased the firing rate and SOs in mPFC pyramidal neurons. mPFC infusion of a non-α7 nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine blocked the excitatory effect of systemic nicotine on the VTA DA neurons with type-I response, but mPFC infusion of nicotine failed to excite these neurons. These results suggest that nAChR activation in the mPFC is necessary, but not sufficient, for systemic nicotine-induced excitation of VTA neurons. Finally, systemic injection of bicuculline prevented nicotine-induced firing alterations in the neurons with type-I response. We propose that the mPFC plays a critical role in systemic nicotine-induced excitation of VTA DA neurons.

Α4β2 and α7 nicotinic acetylcholine receptor binding predicts choice preference in two cost benefit decision-making tasks

Nicotinic receptors have been linked to a wide range of cognitive and behavioral functions, but surprisingly little is known about their involvement in cost benefit decision making. The goal of these experiments was to determine how nicotinic acetylcholine receptor (nAChR) expression is related to two forms of cost benefit decision making. Male Long Evans rats were tested in probability- and delay-discounting tasks, which required discrete trial choices between a small reward and a large reward associated with varying probabilities of omission and varying delays to reward delivery, respectively. Following testing, radioligand binding to α4β2 and α7 nAChR subtypes in brain regions implicated in cost benefit decision making was examined. Significant linear relationships were observed between choice of the large delayed reward in the delay discounting task and α4β2 receptor binding in both the dorsal and ventral hippocampus. Additionally, trends were found suggesting that choice of the large costly reward in both discounting tasks was inversely related to α4β2 receptor binding in the medial prefrontal cortex and nucleus accumbens shell. Similar trends suggested that choice of the large delayed reward in the delay discounting task was inversely related to α4β2 receptor binding in the orbitofrontal cortex, nucleus accumbens core, and basolateral amygdala, as well as to α7 receptor binding in the basolateral amygdala. These data suggest that nAChRs (particularly α4β2) play both unique and common roles in decisions that require consideration of different types of reward costs.

Cholinergic depletion in nucleus accumbens impairs mesocortical dopamine activation and cognitive function in rats

In rats, selective depletion of the cholinergic interneurons in the ventral striatum (nucleus accumbens or N.Acc.) results in heightened behavioural sensitivity to amphetamine and impaired sensorimotor gating processes, suggesting a hyper-responsiveness to dopamine (DA) activity in the N.Acc. We hypothesized that local cholinergic depletion may also trigger distal functional alterations, particularly in prefrontal cortex (PFC). Adult male Sprague-Dawley rats were injected bilaterally in the N.Acc. with an immunotoxin targeting choline acetyltransferase. Two weeks later, cognitive function was assessed using the delayed alternation paradigm in the T-maze. The rats were then implanted with voltammetric recording electrodes in the ventromedial PFC to measure in vivo extracellular DA release in response to mild tail pinch stress. The PFC was also examined for density of tyrosine hydroxylase (TH)-labelled varicosities. In another cohort of control and lesioned rats, we measured post mortem tissue content of DA. Depletion of cholinergic neurons (restricted to N.Acc.) significantly impaired delayed alternation performance across delay intervals. While (basal) post mortem indices of PFC DA function were unaffected by N.Acc. lesions, in vivo mesocortical DA activation was markedly reduced; this deficit correlated significantly with cognitive impairments. TH-labelled varicosities however, were unaffected in cortical layer V relative to controls. These data suggest that selective depletion of cholinergic interneurons in N.Acc. triggers widespread functional impairments in mesocorticolimbic DA function and cognition. The possible relevance of these findings is also discussed in relation to schizophrenia, where reduced density of cholinergic neurons in ventral striatum has been reported.

5-HT denervation of the adult rat prefrontal cortex induces changes in the expression of α4 and α7 nicotinic acetylcholine receptor subtypes

INTRODUCTION

Nicotinic acetylcholine receptors (nAChRs) are widely expressed throughout several brain regions. Formation of the α4β2 and α7 subtypes in particular is involved in the organisation of different types of memory. Furthermore, due to their location, these receptors can control the release of various types of neurotransmitters and contribute to synaptic plasticity.

METHODS

Rats were divided into three groups, an experimental group (E), a sham-operated group, (S) and an intact group (T). In group E, stereotactic guidance was used to induce a chemical lesion with 1 μ/μL of 5,7-dihydroxytryptamine (5,7-DHT) in the anteroventral part of the dorsal raphe nucleus (DRN). In the sham-operated group (S), animals underwent surgery including delivery of the same excipient solution to the same site. The intact group (T) received no treatment whatsoever. Twenty days after surgery, animals in all groups were euthanised by decapitation to evaluate the expression of α4 and α7 nAChRs by means of molecular biology techniques.

RESULTS

5-HT denervation of the rat PFC differentially modified the expression of α4 and α7 receptors: while α4 receptor expression increased, α7 expression decreased.

CONCLUSION

Expression differences observed between the two subtypes may be due to their separate locations. The α4 subtype is found in postsynaptic locations and may be related to adaptive changes in postsynaptic cells, while the location of α7 is presynaptic. This explains why the lesion and the elimination of 5-HT fibres in the CPF would cause a decrease in α7 expression.

Medial prefrontal cortex is selectively involved in response selection using visual context in the background

The exact roles of the medial prefrontal cortex (mPFC) in conditional choice behavior are unknown and a visual contextual response selection task was used for examining the issue. Inactivation of the mPFC severely disrupted performance in the task. mPFC inactivations, however, did not disrupt the capability of perceptual discrimination for visual stimuli. Normal response selection was also observed when nonvisual cues were used as conditional stimuli. The results strongly suggest that the mPFC is not necessarily involved in the inhibition of response or flexible response selection in general, but is rather critical when response selection is required conditionally using visual context in the background.

Prefrontal neuromodulation by nicotinic receptors for cognitive processes

RATIONALE

The prefrontal cortex (PFC) mediates executive functions, a set of control processes that optimize performance on cognitive tasks. It enables appropriate decision-making and mediates adapted behaviors, all processes impaired in psychiatric or degenerative disorders. Key players of normal functioning of the PFC are neurotransmitter (NT) systems arising from subcortical nuclei and targeting PFC subareas and, also, neuronal nicotinic acetylcholine receptors (nAChRs). These ion channels, located on multiple cell compartments in all brain areas, mediate direct cholinergic transmission and modulate the release of NTs that cross onto PFC neurons or interneurons.

OBJECTIVE

We compiled current knowledge concerning the role of nAChRs in NT release, focusing on the PFC. We point out plausible mechanisms of interaction among PFC circuits implicated in executive functions and emphasized the role of β2-containing nAChRs, the high-affinity receptors for acetylcholine (ACh). These receptors are more directly implicated in behavioral flexibility either when located on PFC neurons or in the monoaminergic or cholinergic systems targeting the PFC.

RESULTS

We shed light on potentially crucial roles played by nAChRs in complex interactions between local and afferent NTs. We show how they could act on cognition via PFC networks.

CONCLUSIONS

nAChRs are crucial for decision-making, during integration of emotional and motivational features, both mediated by different NT pathways in the PFC. We review the knowledge recently gained on cognitive functions in mice and our current understanding of PFC NT modulation. The combination of these data is expected to provide new hypotheses concerning the role of AChRs in cognitive processes.

Nicotine improves working memory span capacity in rats following sub-chronic ketamine exposure

Ketamine, an NMDA-receptor antagonist, produces cognitive deficits in humans in a battery of tasks involving attention and memory. Nicotine can enhance various indices of cognitive performance, including working memory span capacity measured using the odor span task (OST). This study examined the effects of a sub-chronic ketamine treatment to model cognitive deficits associated with schizophrenia, and to evaluate the effectiveness of nicotine, antipsychotic clozapine, and the novel mGlu2/3 agonist, LY404039, in restoring OST performance. Male hooded Lister rats were trained in the OST, a working memory task involving detection of a novel odor from an increasing number of presented odors until they exhibited asymptotic levels of stable performance. Sub-chronic ketamine exposure (10 and 30 mg/kg i.p. for 5 consecutive days) produced a dose-dependent impairment that was stable beyond 14 days following exposure. In one cohort, administration of graded doses of nicotine (0.025-0.1 mg/kg) acutely restored the performance in ketamine-treated animals, while significant improvements in odor span were observed in control subjects. In a second cohort of rats, acute tests with clozapine (1-10 mg/kg) and LY404039 (0.3-10 mg/kg) failed to reverse ketamine-induced deficits in doses that were observed to impair performance in the control groups. These data suggest that sub-chronic ketamine exposure in the OST presents a valuable method to examine novel treatments to restore cognitive impairments associated with neuropsychiatric disorders such as schizophrenia. Moreover, it highlights a central role for neuronal nicotinic receptors as viable targets for intervention that may be useful adjuncts to the currently prescribed anti-psychotics.

Emergent pharmacology of conscious experience: new perspectives in substance addiction

We here review experimental findings relevant for the pharmacology of conscious experience, an issue largely neglected in pharmacological research. First, we focus on self-awareness, a pivotal component of conscious experience and its integration within the global neuronal network (GNW), a theoretical concept that unifies convergent approaches on the neural bases of conscious processing. We report recent evidence to show that self-awareness mobilizes a paralimbic circuitry of γ synchrony, and that such synchrony is, in particular, regulated by GABA interneurons under the control of acetylcholine and dopamine. Recent data illustrate that these neurotransmitters establish a causal relationship with the control of self-awareness. The hypothesis is presented that not only is self-awareness chemically regulated, but the reverse may be true. Long-term deficit in self-control of drug intake would result in compulsive substance use, accompanied, in particular, with lesions of the paralimbic circuitry of self-awareness, leading to aggravation of substance abuse, resulting in addiction in a vicious circle. Finally, we propose that the emergent pharmacology of conscious experience may provide new perspectives, not only in substance addiction but also in the many other pathological conditions with deficient self-awareness.

Cholinergic receptor subtypes and their role in cognition, emotion, and vigilance control: an overview of preclinical and clinical findings

RATIONALE

The cholinergic system has long been linked to cognitive processes. Two main classes of acetylcholine (ACh) receptors exist in the human brain, namely muscarinic and nicotinic receptors, of which several subtypes occur.

OBJECTIVES

This review seeks to provide an overview of previous findings on the influence of cholinergic receptor manipulations on cognition in animals and humans, with particular emphasis on the role of selected cholinergic receptor subtypes. Furthermore, the involvement of these receptor subtypes in the regulation of emotion and brain electrical activity as measured by electroencephalography (EEG) shall be addressed since these domains are considered to be important modulators of cognitive functioning.

RESULTS

In regard to cognition, the muscarinic receptor subtypes have been implicated mainly in memory functions, but have also been linked to attentional processes. The nicotinic α7 receptor subtype is involved in working memory, whereas the α4β2* subtype has been linked to tests of attention. Both muscarinic and nicotinic cholinergic mechanisms play a role in modulating brain electrical activity. Nicotinic receptors have been strongly associated with the modulation of depression and anxiety.

CONCLUSIONS

Cholinergic receptor manipulations have an effect on cognition, emotion, and brain electrical activity as measured by EEG. Changes in cognition can result from direct cholinergic receptor manipulation or from cholinergically induced changes in vigilance or affective state.

Involvement of the cholinergic system in conditioning and perceptual memory

The cholinergic systems play a pivotal role in learning and memory, and have been the centre of attention when it comes to diseases containing cognitive deficits. It is therefore not surprising, that the cholinergic transmitter system has experienced detailed examination of its role in numerous behavioural situations not least with the perspective that cognition may be rescued with appropriate cholinergic 'boosters'. Here we reviewed the literature on (i) cholinergic lesions, (ii) pharmacological intervention of muscarinic or nicotinic system, or (iii) genetic deletion of selective receptor subtypes with respect to sensory discrimination and conditioning procedures. We consider visual, auditory, olfactory and somatosensory processing first before discussing more complex tasks such as startle responses, latent inhibition, negative patterning, eye blink and fear conditioning, and passive avoidance paradigms. An overarching reoccurring theme is that lesions of the cholinergic projection neurones of the basal forebrain impact negatively on acquisition learning in these paradigms and blockade of muscarinic (and to a lesser extent nicotinic) receptors in the target structures produce similar behavioural deficits. While these pertain mainly to impairments in acquisition learning, some rare cases extend to memory consolidation. Such single case observations warranted replication and more in-depth studies. Intriguingly, receptor blockade or receptor gene knockout repeatedly produced contradictory results (for example in fear conditioning) and combined studies, in which genetically altered mice are pharmacological manipulated, are so far missing. However, they are desperately needed to clarify underlying reasons for these contradictions. Consistently, stimulation of either muscarinic (mainly M(1)) or nicotinic (predominantly α7) receptors was beneficial for learning and memory formation across all paradigms supporting the notion that research into the development and mechanisms of novel and better cholinomimetics may prove useful in the treatment of neurodegenerative or psychiatric disorders with cognitive endophenotypes.

Molecular mechanisms of working memory

Working memory is a process for temporary active maintenance of information and the role of prefrontal cortex in this memory has been known since the pioneering experiments of Fulton in the early 20th century. Sustained firing of prefrontal neurons during the delay period is considered the neural correlate of working memory. Evidence in literature suggests the involvement of areas beyond the frontal lobe and illustrate that working memory involves parallel, distributed neuronal networks. Prefrontal cortex is part of a complex neural circuit that includes both cortical and subcortical components and many of these regions play vital roles in working memory function. In this article, we review the current understanding of the neural mechanisms of memory maintenance in the brain.

A comparison of scopolamine and biperiden as a rodent model for cholinergic cognitive impairment

RATIONALE

The nonselective muscarinic antagonist scopolamine hydrobromide (SCOP) is employed as the gold standard for inducing memory impairments in healthy humans and animals. However, its use remains controversial due to the wide spectrum of behavioral effects of this drug.

OBJECTIVE

The present study investigated whether biperiden (BIP), a muscarinic m1 receptor antagonist, is to be preferred over SCOP as a pharmacological model for cholinergic memory deficits in rats. This was done by comparing the effects of SCOP and BIP using a battery of operant tasks: fixed ratio (FR5) and progressive ratio (PR10) schedules of reinforcement, an attention paradigm and delayed nonmatching to position task.

RESULTS

SCOP induced diffuse behavioral disruption, which included sensorimotor responding (FR5, 0.3 and 1 mg/kg), food motivation (PR10, 1 mg/kg), attention (0.3 mg/kg, independent of stimulus duration), and short-term memory (delayed nonmatching to position (DNMTP), 0.1 and 0.3 mg/kg, delay-dependent but also impairment at the zero second delay). BIP induced relatively more selective deficits, as it slowed sensorimotor responding (FR5, 10 mg/kg) and disrupted short-term memory (DNMTP, 3 mg/kg, delay-dependent but no impairment at the zero second delay). BIP had no effect on food motivation (PR10) or attention.

CONCLUSION

Muscarinic m1 antagonists should be considered an interesting alternative for SCOP as a pharmacological model for cholinergic mnemonic deficits in animals.

Acetylcholine and attention

Historically, ACh has been implicated in learning and short-term memory functions. However, more recent studies have provided support for a role of cortical ACh in attentional effort, orienting and the detection of behavioral significant stimuli. The current review article summarizes studies in animals and humans which have investigated the role of ACh in attention and cognition. An attempt has been made to differentiate between brain regions involved in attentional processes versus those important for other cognitive functions. To this purpose, various experimental methods and interventions were used. Animal behavioral studies have injected the selective immunotoxin IgG-saporin to induce specific cholinergic lesions, employed electrochemical techniques such as microdialysis, or have administered cholinergic compounds into discrete parts of the brain. Human studies that give some indication on the link between central cholinergic signaling and cognition are obviously confined to less invasive, imaging methods such as fMRI. The brain areas that are deemed most important for intact attentional processing in both animals and humans appear to be the (pre)frontal, parietal and somatosensory (especially visual) regions, where ACh plays a vital role in the top-down control of attentional orienting and stimulus discrimination. In contrast, cholinergic signaling in the septohippocampal system is suggested to be involved in memory processes. Thus, it appears that the role of ACh in cognition is different per brain region and between nicotinic versus muscarinic receptor subtypes.

Nicotinic actions on neuronal networks for cognition: general principles and long-term consequences

Nicotine enhances cognitive performance in humans and laboratory animals. The immediate positive actions of nicotine on learning, memory and attention are well-documented. Several brain areas involved in cognition, such as the prefrontal cortex, have been implicated. Besides acute effects on these brain areas and on brain function, a picture is emerging showing that long-term consequences of nicotine exposure during adolescence can be detrimental for cognitive performance. The majority of adult smokers started the habit during adolescence. Our knowledge on the types of nicotinic receptors in the brain areas that are candidates for mediating nicotine's effects is increasing. However, much less is known about the underlying cellular mechanisms. A series of recent studies have uncovered exciting features of the mechanisms by which nicotine alters prefrontal cortex neuronal activity, synaptic plasticity, gene expression and cognitive function, and how these changes may have a lasting effect on the developing brain. In this review, we discuss these exciting findings and identify several common principles by which nicotinic receptor activation modulates cortical circuits involved in cognition. Understanding how nicotine induces long-term changes in neuronal circuits and alters plasticity in the prefrontal cortex is essential to determining how these mechanisms interact to alter cognition.

Chronic underactivity of medial frontal cortical beta2-containing nicotinic receptors increases clozapine-induced working memory impairment in female rats

Nicotinic receptor decreases in the frontal cortex and hippocampus are important mediators of cognitive impairment in both schizophrenia and Alzheimer's disease. Drug treatments for these diseases should take into account the impacts of compromised brain function on drug response. This study investigated the impact of compromised nicotinic receptor activity in the frontal cortex in rats on memory function. Since both Alzheimer's disease and schizophrenia can involve psychosis, antipsychotic drugs are often given. The impacts of antipsychotic drugs on cognitive function have been found to be quite variable. It is the hypothesis of this and previous studies that the cognitive effects of antispychotic drugs on cognitive function depend on the integrity of brain systems involved in cognition. Previously in studies of the hippocampus, we found that chronic inhibition of beta2-containing nicotinic receptors with dihydro-beta-erythrodine (DHbetaE) impaired working memory and that this effect was attenuated by the antipsychotic drug clozapine. In contrast, chronic hippocampal alpha7 nicotinic receptor blockade with methyllycaconitine (MLA) potentiated the clozapine-induced memory impairment which is seen in rats without compromised nicotinic receptor activity. The current study determined medial frontal cortical alpha7 and beta2-containing nicotinic receptor involvement in memory and the interactions with antipsychotic drug therapy with clozapine. Chronic DHbetaE and MLA infusion effects and interactions with systemic clozapine were assessed in female rats tested for memory on the radial-arm maze. Antipsychotic drug interactions with chronic systemic nicotine were investigated because nicotinic procognitive treatment has been proposed. The same local infusion DHbetaE dose that impaired memory with hippocampal infusion did not impair memory when infused in the medial frontal cortex. Frontal DHbetaE infusion potentiated clozapine-induced memory impairment, whereas previously the memory impairment caused by hippocampal DHbetaE infusion was attenuated by clozapine. Frontal cortical MLA infusions at a dose that previously was found to potentiate the clozapine-induced memory impairment with hippocampal infusion had no significant effect when infused into the medial frontal cortex. The location and subtype of nicotinic receptor underactivity are critical determinates for clozapine effects on memory. Patients with hippocampal beta2-containing nicotinic receptor loss may be well treated with clozapine therapy, while those with frontal cortical beta2-containing receptor loss may have a potentiated memory impairment caused by clozapine.

NMDA receptor blockade augmented nicotine-evoked dopamine release from rat prefrontal cortex slices

Nicotine evokes dopamine release through activation of nicotinic acetylcholine receptors, and tobacco cigarette smoking is more prevalent among individuals diagnosed with schizophrenia. Blockade of ionotropic glutamate (NMDA) receptors can induce changes in central dopamine and glutamate circuits, which models the symptoms of schizophrenia. The effect of the NMDA receptor antagonist, ketamine, on the effect of nicotine in rat prefrontal cortex was examined using a slice superfusion assay in which cortical slices were preloaded with [(3)H] dopamine. A wide range of ketamine concentrations (0.1-300 microM) did not evoke [(3)H] overflow from slices, indicating that NMDA receptor blockade did not induce dopamine release. Ketamine, at concentrations that model the symptoms of schizophrenia (1-10 microM), augmented the effect of nicotine (1-100 microM) to evoke [(3)H] overflow from slices and decreased the threshold nicotine concentration to evoke [(3)H] overflow. This indicates that NMDA receptor blockade increased the potency and efficacy of nicotine to evoke dopamine release from prefrontal cortex slices, suggesting that ketamine induced hypersensitivity to nicotine. The present results support a role for nicotinic acetylcholine receptors in the pathophysiology and treatment of schizophrenia.

Differential effects of muscarinic receptor blockade in prelimbic cortex on acquisition and memory formation of an odor-reward task

The present experiments determined the consequences of blocking muscarinic cholinergic receptors of the prelimbic (PL) cortex in the acquisition and retention of an odor-reward associative task. Rats underwent a training test (five trials) and a 24-h retention test (two retention trials and two relearning trials). In the first experiment, rats were bilaterally infused with scopolamine (20 or 5 microg/site) prior to training. Although scopolamine rats showed acquisition equivalent to PBS-injected controls, they exhibited weakened performance in the 24-h retention test measured by number of errors. In the second experiment, rats were injected with scopolamine (20 microg/site) immediately or 1 h after training and tested 24 h later. Scopolamine rats injected immediately showed severe amnesia detected in two performance measures (errors and latencies), demonstrating deficits in retention and relearning, whereas those injected 1 h later showed good 24-h test performance, similar to controls. These results suggest that muscarinic transmission in the PL cortex is essential for early memory formation, but not for acquisition, of a rapidly learned odor discrimination task. Findings corroborate the role of acetylcholine in consolidation processes and the participation of muscarinic receptors in olfactory associative tasks.

Nicotine and synaptic plasticity in prefrontal cortex

Nicotinic receptor activation enhances working memory and attention. The prefrontal cortex is a key brain area involved in working memory, and plasticity of excitatory synaptic transmission within the cortex is likely an important cellular mechanism of memory. A recent study has explored the cellular and synaptic basis of nicotine's effects on excitability within the prefrontal cortex. The findings suggest that nicotine enhances inhibitory synaptic inputs to layer V pyramidal cells, which suppresses induction of long-term potentiation (LTP). This inhibitory effect can be overcome by stimulating the pyramidal cells in bursts, which suggests a modification in the signal-to-noise ratio for synaptic input. Thus, the impact of strong stimuli on working memory would be enhanced when combined with nicotinic receptor activity. These findings may lead to novel and more effective treatments for memory disorders.

Molecular activity underlying working memory

The prefrontal cortex is necessary for directing thought and planning action. Working memory, the active, transient maintenance of information in mind for subsequent monitoring and manipulation, lies at the core of many simple, as well as high-level, cognitive functions. Working memory has been shown to be compromised in a number of neurological and psychiatric conditions and may contribute to the behavioral and cognitive deficits associated with these disorders. It has been theorized that working memory depends upon reverberating circuits within the prefrontal cortex and other cortical areas. However, recent work indicates that intracellular signals and protein dephosphorylation are critical for working memory. The present article will review recent research into the involvement of the modulatory neurotransmitters and their receptors in working memory. The intracellular signaling pathways activated by these receptors and evidence that indicates a role for G(q)-initiated PI-PLC and calcium-dependent protein phosphatase calcineurin activity in working memory will be discussed. Additionally, the negative influence of calcium- and cAMP-dependent protein kinase (i.e., calcium/calmodulin-dependent protein kinase II (CaMKII), calcium/diacylglycerol-activated protein kinase C (PKC), and cAMP-dependent protein kinase A (PKA)) activities on working memory will be reviewed. The implications of these experimental findings on the observed inverted-U relationship between D(1) receptor stimulation and working memory, as well as age-associated working memory dysfunction, will be presented. Finally, we will discuss considerations for the development of clinical treatments for working memory disorders.

Frontal cortical α7 and α4β2 nicotinic acetylcholine receptors in working and reference memory

The alpha7 and alpha4beta2 nicotinic acetylcholine receptor (nAchR) subtypes have been shown to be involved in memory. It is also known that losses of frontal cortical nAchRs are correlated to declining memory function in Alzheimer's disease, but the subtype-specific role of frontal cortical nAchRs in memory has not been well characterized. Hence, we sought to understand the role of frontal cortical alpha7 and alpha4beta2 nAchR subtypes in both working and reference memory by observing the effects of subtype specific agonists and antagonists on radial arm maze performance. It was found that alpha7 nAchRs in the frontal cortex are involved in working and reference memory, while alpha4beta2 nAchRs are only involved in working memory. Throughout the study, drug treatments did not affect motor functionality in the animals. Our data thus sheds further light on the frontal cortex as an important anatomical locus for nAchR-mediated memory function in the brain, and highlights the differing role of alpha7 and alpha4beta2 nAchRs in long and short term memory.

Distributed Network Actions by Nicotine Increase the Threshold for Spike-Timing-Dependent Plasticity in Prefrontal Cortex

Nicotine enhances attention and working memory by activating nicotinic acetylcholine receptors (nAChRs). The prefrontal cortex (PFC) is critical for these cognitive functions and is also rich in nAChR expression. Specific cellular and synaptic mechanisms underlying nicotine's effects on cognition remain elusive. Here we show that nicotine exposure increases the threshold for synaptic spike-timing-dependent potentiation (STDP) in layer V pyramidal neurons of the mouse PFC. During coincident presynaptic and postsynaptic activity, nicotine reduces dendritic calcium signals associated with action potential propagation by enhancing GABAergic transmission. This results from a series of presynaptic actions involving different PFC interneurons and multiple nAChR subtypes. Pharmacological block of nAChRs or GABA(A) receptors prevented nicotine's actions and restored STDP, as did increasing dendritic calcium signals with stronger postsynaptic activity. Thus, by activating nAChRs distributed throughout the PFC neuronal network, nicotine affects PFC information processing and storage by increasing the amount of postsynaptic activity necessary to induce STDP.

The Ferrier Lecture 1998. The molecular biology of consciousness investigated with genetically modified mice

The question is raised of the relevance of experimental work with the mouse and some of its genetically modified individuals in the study of consciousness. Even if this species does not go far beyond the level of 'minimal consciousness', it may be a useful animal model to examine the elementary building blocks of consciousness using the methods of molecular biology jointly with investigations at the physiological and behavioural levels. These building blocks which are anticipated to be universally shared by higher organisms (from birds to humans) may include: (i) the access to multiple states of vigilance, like wakefulness, sleep, general anaesthesia, etc.; (ii) the capacity for global integration of several sensory and cognitive functions, together with behavioural flexibility resulting in what is referred to as exploratory behaviour, and possibly a minimal form of intentionality. In addition, the contribution of defined neuronal nicotinic receptors species to some of these processes is demonstrated and the data discussed within the framework of recent neurocomputational models for access to consciousness.

Neuronal nicotinic acetylcholine receptors serve as sensitive targets that mediate beta-amyloid neurotoxicity

Alzheimer's disease (AD) is the most common form of brain dementia characterized by the accumulation of beta-amyloid peptides (Abeta) and loss of forebrain cholinergic neurons. Abeta accumulation and aggregation are thought to contribute to cholinergic neuronal degeneration, in turn causing learning and memory deficits, but the specific targets that mediate Abeta neurotoxicity remain elusive. Recently, accumulating lines of evidence have demonstrated that Abeta directly modulates the function of neuronal nicotinic acetylcholine receptors (nAChRs), which leads to the new hypothesis that neuronal nAChRs may serve as important targets that mediate Abeta neurotoxicity. In this review, we summarize current studies performed in our laboratory and in others to address the question of how Abeta modulates neuronal nAChRs, especially nAChR subunit function.

Preclinical research into cognition enhancers

The preclinical development of drugs to treat the cognitive symptoms of neuropsychiatric and neurological disorders is a formidable challenge. Evidence from a wide range of preclinical behavioral and neuropharmacological tests has formed the basis for predicting drug-induced cognition enhancement in normal volunteers and in patients with cognitive impairments. However, the limited validity of preclinical predictions of this enhancement in humans indicates that conventional screening for "broadly active" compounds represents a below-optimal research strategy. This article conceptualizes the evidence needed to improve the predictive validity of preclinical research designed to discover and characterize cognition enhancers. We suggest that the investigation of reciprocal relationships among molecular, cellular, behavioral and cognitive processes modulated by candidate drugs represents the core of such research. By contrast, the usefulness of simple and high-throughput screening tests for the detection of cognition enhancers might be restricted to advanced drug-finding programs that are guided by evidence of the modulation of neurocognitive relationships by cognition enhancers and that are informed by iterative preclinical-clinical cross-validation of research approaches. We stress the need for basic biopsychological research approaches in preclinical programs to find and characterize drugs to treat cognitive disorders.