Second-by-second analysis of alpha 7 nicotine receptor regulation of glutamate release in the prefrontal cortex of awake rats

Advancements in Brain Research: The In Vivo/In Vitro Electrochemical Detection of Neurochemicals

Neurochemicals, crucial for nervous system function, influence vital bodily processes and their fluctuations are linked to neurodegenerative diseases and mental health conditions. Monitoring these compounds is pivotal, yet the intricate nature of the central nervous system poses challenges. Researchers have devised methods, notably electrochemical sensing with micro-nanoscale electrodes, offering high-resolution monitoring despite low concentrations and rapid changes. Implantable sensors enable precise detection in brain tissues with minimal damage, while microdialysis-coupled platforms allow in vivo sampling and subsequent in vitro analysis, addressing the selectivity issues seen in other methods. While lacking temporal resolution, techniques like HPLC and CE complement electrochemical sensing's selectivity, particularly for structurally similar neurochemicals. This review covers essential neurochemicals and explores miniaturized electrochemical sensors for brain analysis, emphasizing microdialysis integration. It discusses the pros and cons of these techniques, forecasting electrochemical sensing's future in neuroscience research. Overall, this comprehensive review outlines the evolution, strengths, and potential applications of electrochemical sensing in the study of neurochemicals, offering insights into future advancements in the field.

The role of astrocytic α7 nicotinic acetylcholine receptors in Alzheimer disease

The ongoing search for therapeutic interventions in Alzheimer disease (AD) has highlighted the complexity of this condition and the need for additional biomarkers, beyond amyloid-β (Aβ) and tau, to improve clinical assessment. Astrocytes are brain cells that control metabolic and redox homeostasis, among other functions, and are emerging as an important focus of AD research owing to their swift response to brain pathology in the initial stages of the disease. Reactive astrogliosis - the morphological, molecular and functional transformation of astrocytes during disease - has been implicated in AD progression, and the definition of new astrocytic biomarkers could help to deepen our understanding of reactive astrogliosis along the AD continuum. As we highlight in this Review, one promising biomarker candidate is the astrocytic α7 nicotinic acetylcholine receptor (α7nAChR), upregulation of which correlates with Aβ pathology in the brain of individuals with AD. We revisit the past two decades of research into astrocytic α7nAChRs to shed light on their roles in the context of AD pathology and biomarkers. We discuss the involvement of astrocytic α7nAChRs in the instigation and potentiation of early Aβ pathology and explore their potential as a target for future reactive astrocyte-based therapeutics and imaging biomarkers in AD.

Negative and positive allosteric modulators of the α7 nicotinic acetylcholine receptor regulates the ability of adolescent binge alcohol exposure to enhance adult alcohol consumption

Rationale and Objectives: Ethanol acts directly on the α7 Nicotinic acetylcholine receptor (α7). Adolescent-binge alcohol exposure (ABAE) produces deleterious consequences during adulthood, and data indicate that the α7 receptor regulates these damaging events. Administration of an α7 Negative Allosteric Modulator (NAM) or the cholinesterase inhibitor galantamine can prophylactically prevent adult consequences of ABAE. The goals of the experiments were to determine the effects of co-administration of ethanol and a α7 agonist in the mesolimbic dopamine system and to determine if administration of an α7 NAM or positive allosteric modulator (PAM) modulates the enhancement of adult alcohol drinking produced by ABAE. Methods: In adult rats, ethanol and the α7 agonist AR-R17779 (AR) were microinjected into the posterior ventral tegmental area (VTA), and dopamine levels were measured in the nucleus accumbens shell (AcbSh). In adolescence, rats were treated with the α7 NAM SB-277011-A (SB) or PNU-120596 (PAM) 2 h before administration of EtOH (ABAE). Ethanol consumption (acquisition, maintenance, and relapse) during adulthood was characterized. Results: Ethanol and AR co-administered into the posterior VTA stimulated dopamine release in the AcbSh in a synergistic manner. The increase in alcohol consumption during the acquisition and relapse drinking during adulthood following ABAE was prevented by administration of SB, or enhanced by administration of PNU, prior to EtOH exposure during adolescence. Discussion: Ethanol acts on the α7 receptor, and the α7 receptor regulates the critical effects of ethanol in the brain. The data replicate the findings that cholinergic agents (α7 NAMs) can act prophylactically to reduce the alterations in adult alcohol consumption following ABAE.

Effect of Cigarette Smoke Exposure and Aspirin Treatment on Neurotransmitters’ Tissue Content in Rats’ Hippocampus and Amygdala

Cigarette smoke withdrawal can cause anxiety-like behavior and modulate neurotransmitter-related proteins in the brain. We examined the effects of cigarette smoke with and without aspirin treatment on the concentrations of neurotransmitters, including dopamine, serotonin, glutamate, glutamine, and GABA in the amygdala and hippocampus. Sprague-Dawley rats were randomly assigned to four different groups: (1) control group exposed only to standard room air, (2) cigarette smoke exposed group treated with saline vehicle, (3) cigarette smoke exposed group treated with aspirin (30 mg/kg), and (4) control group treated only with aspirin (30 mg/kg). Cigarette smoke exposure was performed for 2 h/day, 5 days/week, for 31 days. Behavioral testing was carried out weekly, 24 h after cigarette smoke exposure, during acute withdrawal. At the end of week 4, rats were given either distilled water (1 mL) or aspirin 45 min before cigarette exposure for 11 days. Dopamine, serotonin, glutamate, glutamine, and GABA were extracted from both the amygdala and hippocampus and were separated and quantified using a developed and validated HPLC-MS/MS method. Cigarette smoke withdrawal induced anxiety behaviors, and aspirin treatment reduced this effect. Cigarette smoke exposure increased tissue content of dopamine, serotonin, glutamate, glutamine, and GABA, and aspirin treatment reversed this effect. Cigarette smoke caused an increase in tissue content of several neurotransmitters as well as anxiety-like behavior, and these effects were normalized by aspirin treatment.

Enhancing the resolution of behavioral measures: Key observations during a forty year career in behavioral neuroscience

This manuscript reviews several key observations from the research program of Professor John P. Bruno that are believed to have significantly advanced our understanding of the brain's mediation of behavior. This review focuses on findings within several important research areas in behavioral neuroscience, including a) age-dependent neurobehavioral plasticity following brain damage; b) the role of the cortical cholinergic system in attentional processing and cognitive flexibility; and c) the design and validation of animal models of cognitive deficits in schizophrenia. In selecting these observations, emphasis was given to examples in which the heuristic potency was increased by maximizing the resolution and microanalysis of behavioral assays in the same fashion as one typically refines neuronal manipulations. Professor Bruno served the International Behavioral Neuroscience Society (IBNS) as an IBNS Fellow (1995-present) and President of the IBNS (2001-02).

Lumateperone-mediated effects on prefrontal glutamatergic receptor-mediated neurotransmission: A dopamine D1 receptor dependent mechanism

Lumateperone is a novel drug approved for the treatment of schizophrenia in adults and depressive episodes associated with bipolar depression in adults, as monotherapy and as adjunctive therapy with lithium or valproate treatment in the United States. Lumateperone simultaneously modulates key neurotransmitters, such as serotonin, dopamine, and glutamate, implicated in serious mental illness. In patients with schizophrenia, lumateperone was shown to improve positive symptoms along with negative and depressive symptoms, while also enhancing prosocial behavior. Moreover, in patients with bipolar I or II disorder, lumateperone improved depressive symptoms as well. To further understand the mechanisms related to lumateperone's clinical response, the aim of this study was to investigate the effect of lumateperone on dopaminergic- and glutamatergic signaling in the rat medial prefrontal cortex (mPFC). We used the conditioned avoidance response (CAR) test to determine the antipsychotic-like effect of lumateperone, electrophysiology in vitro to study lumateperone's effects on NMDA- and AMPA-induced currents in the mPFC, and the neurochemical techniques microdialysis and amperometry to measure dopamine- and glutamate release in the rat mPFC. Our results demonstrate that lumateperone; i) significantly suppressed CAR in rats, indicating an antipsychotic-like effect, ii) facilitated NMDA and AMPA receptor-mediated currents in the mPFC, in a dopamine D₁-dependent manner, and iii) significantly increased dopamine and glutamate release in the rat mPFC. To the extent that these findings can be translated to humans, the ability of lumateperone to activate these pathways may contribute to its demonstrated effectiveness in safely improving symptoms related to neuropsychiatric disorder including mood alterations.

Ceftriaxone as a Novel Therapeutic Agent for Hyperglutamatergic States: Bridging the Gap Between Preclinical Results and Clinical Translation

Dysregulation of glutamate homeostasis is a well-established core feature of neuropsychiatric disorders. Extracellular glutamate concentration is regulated by glutamate transporter 1 (GLT-1). The discovery of a beta-lactam antibiotic, ceftriaxone (CEF), as a safe compound with unique ability to upregulate GLT-1 sparked the interest in testing its efficacy as a novel therapeutic agent in animal models of neuropsychiatric disorders with hyperglutamatergic states. Indeed, more than 100 preclinical studies have shown the efficacy of CEF in attenuating the behavioral manifestations of various hyperglutamatergic brain disorders such as ischemic stroke, amyotrophic lateral sclerosis (ALS), seizure, Huntington’s disease, and various aspects of drug use disorders. However, despite rich and promising preclinical data, only one large-scale clinical trial testing the efficacy of CEF in patients with ALS is reported. Unfortunately, in that study, there was no significant difference in survival between placebo- and CEF-treated patients. In this review, we discussed the translational potential of preclinical efficacy of CEF based on four different parameters: (1) initiation of CEF treatment in relation to induction of the hyperglutamatergic state, (2) onset of response in preclinical models in relation to onset of GLT-1 upregulation, (3) mechanisms of action of CEF on GLT-1 expression and function, and (4) non-GLT-1-mediated mechanisms for CEF. Our detailed review of the literature brings new insights into underlying molecular mechanisms correlating the preclinical efficacy of CEF. We concluded here that CEF may be clinically effective in selected cases in acute and transient hyperglutamatergic states such as early drug withdrawal conditions.

Effects of Chronic Inhalation of Electronic Cigarette Vapor Containing Nicotine on Neurobehaviors and Pre/Postsynaptic Neuron Markers

Nicotine-exposed animal models exhibit neurobehavioral changes linked to impaired synaptic plasticity. Previous studies highlighted alterations in neurotransmitter levels following nicotine exposure. Vesicular glutamate transporter (VGLUT1) and vesicular gamma-aminobutyric acid (GABA) transporter (VGAT) are essential for the transport and release of glutamate and GABA, respectively, from presynaptic neurons into synapses. In our work, an e-cigarette device was used to deliver vapor containing nicotine to C57BL/6J mice for four weeks. Novel object recognition, locomotion, and Y-maze tests were performed to investigate the behavioral parameters. Protein studies were conducted to study the hippocampal expression of VGLUT1, VGAT, and postsynaptic density protein 95 (PSD95) as well as brain cytokine markers. Long-term memory and locomotion tests revealed that e-cigarette aerosols containing nicotine modulated recognition memory and motor behaviors. We found that vapor exposure increased VGLUT1 expression and decreased VGAT expression in the hippocampus. No alterations were found in PSD95 expression. We observed that vapor-containing nicotine exposure altered certain brain cytokines such as IFNβ-1 and MCP-5. Our work provides evidence of an association between neurobehavioral changes and altered hippocampal VGLUT1 and VGAT expression in mice exposed to e-cigarette vapors containing nicotine. Such exposure was also associated with altered neurobehaviors, which might affect neurodegenerative diseases.

Astrocytic Glutamatergic Transmission and Its Implications in Neurodegenerative Disorders

Several neurodegenerative disorders involve impaired neurotransmission, and glutamatergic neurotransmission sets a prototypical example. Glutamate is a predominant excitatory neurotransmitter where the astrocytes play a pivotal role in maintaining the extracellular levels through release and uptake mechanisms. Astrocytes modulate calcium-mediated excitability and release several neurotransmitters and neuromodulators, including glutamate, and significantly modulate neurotransmission. Accumulating evidence supports the concept of excitotoxicity caused by astrocytic glutamatergic release in pathological conditions. Thus, the current review highlights different vesicular and non-vesicular mechanisms of astrocytic glutamate release and their implication in neurodegenerative diseases. As in presynaptic neurons, the vesicular release of astrocytic glutamate is also primarily meditated by calcium-mediated exocytosis. V-ATPase is crucial in the acidification and maintenance of the gradient that facilitates the vesicular storage of glutamate. Along with these, several other components, such as cystine/glutamate antiporter, hemichannels, BEST-1, TREK-1, purinergic receptors and so forth, also contribute to glutamate release under physiological and pathological conditions. Events of hampered glutamate uptake could promote inflamed astrocytes to trigger repetitive release of glutamate. This could be favorable towards the development and worsening of neurodegenerative diseases. Therefore, across neurodegenerative diseases, we review the relations between defective glutamatergic signaling and astrocytic vesicular and non-vesicular events in glutamate homeostasis. The optimum regulation of astrocytic glutamatergic transmission could pave the way for the management of these diseases and add to their therapeutic value.

Relationships and Interactions between Ionotropic Glutamate Receptors and Nicotinic Receptors in the CNS

Although ionotropic glutamate receptors and nicotinic receptors for acetylcholine (ACh) have usually been studied separately, they are often co-localized and functionally inter-dependent. The objective of this review is to survey the evidence for interactions between the two receptor families and the mechanisms underlying them. These include the mutual regulation of subunit expression, which change the NMDA:AMPA response balance, and the existence of multi-functional receptor complexes which make it difficult to distinguish between individual receptor sites, especially in vivo. This is followed by analysis of the functional relationships between the receptors from work on transmitter release, cellular electrophysiology and aspects of behavior where these can contribute to understanding receptor interactions. It is clear that nicotinic receptors (nAChRs) on axonal terminals directly regulate the release of glutamate and other neurotransmitters, α7-nAChRs generally promoting release. Hence, α7-nAChR responses will be prevented not only by a nicotinic antagonist, but also by compounds blocking the indirectly activated glutamate receptors. This accounts for the apparent anticholinergic activity of some glutamate antagonists, including the endogenous antagonist kynurenic acid. The activation of presynaptic nAChRs is by the ambient levels of ACh released from pre-terminal synapses, varicosities and glial cells, acting as a 'volume neurotransmitter' on synaptic and extrasynaptic sites. In addition, ACh and glutamate are released as CNS co-transmitters, including 'cholinergic' synapses onto spinal Renshaw cells. It is concluded that ACh should be viewed primarily as a modulator of glutamatergic neurotransmission by regulating the release of glutamate presynaptically, and the location, subunit composition, subtype balance and sensitivity of glutamate receptors, and not primarily as a classical fast neurotransmitter. These conclusions and caveats should aid clarification of the sites of action of glutamate and nicotinic receptor ligands in the search for new centrally-acting drugs.

Prefrontal α7nAChR Signaling Differentially Modulates Afferent Drive and Trace Fear Conditioning Behavior in Adolescent and Adult Rats

Increased level of kynurenic acid is thought to contribute to the development of cognitive deficits in schizophrenia through an α7nAChR-mediated mechanism in the prefrontal cortex (PFC). However, it remains unclear to what extent disruption of PFC α7nAChR signaling impacts afferent transmission and its modulation of behavior. Using male rats, we found that PFC infusion of methyllycaconitine (MLA; α7nAChR antagonist) shifts ventral hippocampal-induced local field potential (LFP) suppression to LFP facilitation, an effect only observed in adults. Hippocampal stimulation can also elicit a GluN2B-mediated LFP potentiation (when PFC GABA_AR is blocked) that is insensitive to MLA. Conversely, PFC infusion of MLA diminished the gain of amygdalar transmission, which is already enabled by postnatal day (P)30. Behaviorally, the impact of prefrontal MLA on trace fear-conditioning and extinction was also age related. While freezing behavior during conditioning was reduced by MLA only in adults, it elicited opposite effects in adolescent and adult rats during extinction as revealed by the level of reduced and increased freezing response, respectively. We next asked whether the late-adolescent onset of α7nAChR modulation of hippocampal inputs contributes to the age-dependent effect of MLA during extinction. Data revealed that the increased freezing behavior elicited by MLA in adult rats could be driven by a dysregulation of the GluN2B transmission in the PFC. Collectively, these results indicate that distinct neural circuits are recruited during the extinction of trace fear memory in adolescents and adults, likely because of the late-adolescent maturation of the ventral hippocampal-PFC functional connectivity and its modulation by α7nAChR signaling.SIGNIFICANCE STATEMENT Abnormal elevation of the astrocyte-derived metabolite kynurenic acid in the prefrontal cortex (PFC) is thought to impair cognitive functions in schizophrenia through an α7nAChR-mediated mechanism. Here, we found that prefrontal α7nAChR signaling is recruited to control the gain of hippocampal and amygdalar afferent transmission in an input-specific, age-related manner during the adolescent transition to adulthood. Behaviorally, prefrontal α7nAChR modulation of trace fear memory was also age-related, likely because of the late-adolescent maturation of the ventral hippocampal pathway and its recruitment of PFC GABAergic transmission enabled by local α7nAChR signaling. Collectively, these results reveal that distinct α7nAChR-sensitive neural circuits contribute to regulate behavior responses in adolescents and adults, particularly those requiring proper integration of hippocampal and amygdalar inputs by the PFC.

E-cigarette aerosols containing nicotine modulate nicotinic acetylcholine receptors and astroglial glutamate transporters in mesocorticolimbic brain regions of chronically exposed mice

Nicotine exposure increases the release of glutamate in part through stimulatory effects on pre-synaptic nicotinic acetylcholine receptors (nAChRs). To assess the impact of chronic electronic (e)-cigarette use on these drug dependence pathways, we exposed C57BL/6 mice to three types of inhalant exposures for 3 months; 1) e-cigarette aerosol generated from liquids containing nicotine (ECN), 2) e-cigarette aerosol generated from liquids containing vehicle chemicals without nicotine (Veh), and 3) air only (AC). We investigated the effects of daily e-cigarette exposure on protein levels of α7 nAChR and α4/β2 nAChR, gene expression and protein levels of astroglial glutamate transporters, including glutamate transporter-1 (GLT-1) and cystine/glutamate antiporter (xCT), in the frontal cortex (FC), striatum (STR) and hippocampus (HIP). We found that chronic inhalation of ECN increased α4/β2 nAChR in all brain regions, and increased α7 nAChR expression in the FC and STR. The total GLT-1 relative mRNA and protein expression were decreased in the STR. Moreover, GLT-1 isoforms (GLT-1a and GLT-1b) were downregulated in the STR in ECN group. However, inhalation of e-cigarette aerosol downregulated xCT expression in STR and HIP compared to AC and Veh groups. ECN group had increased brain-derived neurotrophic factor in the STR compared to control groups. Finally, mass spectrometry detected high concentrations of the nicotine metabolite, cotinine, in the FC and STR in ECN group. This work demonstrates that chronic inhalation of nicotine within e-cigarette aerosols significantly alters the expression of nAChRs and astroglial glutamate transporters in specific mesocorticolimbic brain regions.

α7 nicotinic receptors contributes to glutamatergic activity in the hippocampus of patients with mesial temporal lobe epilepsy with hippocampal sclerosis (MTLE-HS)

Hyperglutamatergic activity in the hippocampus is a major feature of patients with mesial temporal lobe epilepsy with hippocampal sclerosis (MTLE-HS). Here we investigated whether tonic α7 nicotinic receptor (nAChR) activity could contribute to enhanced glutamatergic activity in the hippocampus of patients with MTLE-HS. Results showed that frequency and amplitude of glutamatergic events recorded from pyramidal neurons in the hippocampal samples obtained from patients with MTLE-HS were altered by α7 nAChR antagonist, methyllycaconitine, suggesting α7 nAChRs may influence hyperexcitability in MTLE-HS.

Amyloid Beta-Related Alterations to Glutamate Signaling Dynamics During Alzheimer's Disease Progression

Alzheimer’s disease (AD) ranks sixth on the Centers for Disease Control and Prevention Top 10 Leading Causes of Death list for 2016, and the Alzheimer’s Association attributes 60% to 80% of dementia cases as AD related. AD pathology hallmarks include accumulation of senile plaques and neurofibrillary tangles; however, evidence supports that soluble amyloid beta (Aβ), rather than insoluble plaques, may instigate synaptic failure. Soluble Aβ accumulation results in depression of long-term potentiation leading to cognitive deficits commonly characterized in AD. The mechanisms through which Aβ incites cognitive decline have been extensively explored, with a growing body of evidence pointing to modulation of the glutamatergic system. The period of glutamatergic hypoactivation observed alongside long-term potentiation depression and cognitive deficits in later disease stages may be the consequence of a preceding period of increased glutamatergic activity. This review will explore the Aβ-related changes to the tripartite glutamate synapse resulting in altered cell signaling throughout disease progression, ultimately culminating in oxidative stress, synaptic dysfunction, and neuronal loss.

In Vivo Electrochemical Sensors for Neurochemicals: Recent Update

In vivo electrochemical sensing based on implantable microelectrodes is a strong driving force of analytical neurochemistry in brain. The complex and dynamic neurochemical network sets stringent standards of in vivo electrochemical sensors including high spatiotemporal resolution, selectivity, sensitivity, and minimized disturbance on brain function. Although advanced materials and novel technologies have promoted the development of in vivo electrochemical sensors drastically, gaps with the goals still exist. This Review mainly focuses on recent attempts on the key issues of in vivo electrochemical sensors including selectivity, tissue response and sensing reliability, and compatibility with electrophysiological techniques. In vivo electrochemical methods with bare carbon fiber electrodes, of which the selectivity is achieved either with electrochemical techniques such as fast-scan cyclic voltammetry and differential pulse voltammetry or based on the physiological nature will not be reviewed. Following the elaboration of each issue involved in in vivo electrochemical sensors, possible solutions supported by the latest methodological progress will be discussed, aiming to provide inspiring and practical instructions for future research.

Effects of Chronic Inhalation of Electronic Cigarette Vapor Containing Nicotine on Neurotransmitters in the Frontal Cortex and Striatum of C57BL/6 Mice

Electronic (E)-cigarettes are the latest form of nicotine delivery device and are highly popular in the general population. It is currently unknown whether vaping E-cigarettes (E-CIGs) leads to nicotine addiction. Alterations in the levels of the neurotransmitters in the mesocorticolimbic areas have been reported to mediate the initiation and development of nicotine addiction. Therefore, to determine whether E-CIGs activate the same addiction pathways as conventional cigarettes, we investigated for the effects of daily inhalation of nicotine (24 mg/ml)-containing E-CIG vapor for 6 months on the concentrations of these neurotransmitters in the frontal cortex (FC) and striatum (STR) of male C57BL/6 mice as compared to control group that was exposed to air only. We reported here that 6-month E-CIG vapor containing nicotine inhalation decreased dopamine concentration only in the STR. There were no changes in serotonin concentrations in the FC or STR. Chronic E-CIG exposure also increased glutamate concentration in the STR alone, while glutamine concentrations were increased in both the FC and STR. We found that E-CIG exposure also decreased GABA concentration only in the FC. These data suggest that chronic E-CIG use alters homeostasis of several neurotransmitters in the mesocorticolimbic areas, which may result in the development of nicotine dependence in E-CIG users.

Astrocytic Mechanisms Involving Kynurenic Acid Control Δ9-Tetrahydrocannabinol-Induced Increases in Glutamate Release in Brain Reward-Processing Areas

The reinforcing effects of Δ⁹-tetrahydrocannabinol (THC) in rats and monkeys, and the reinforcement-related dopamine-releasing effects of THC in rats, can be attenuated by increasing endogenous levels of kynurenic acid (KYNA) through systemic administration of the kynurenine 3-monooxygenase inhibitor, Ro 61-8048. KYNA is a negative allosteric modulator of α7 nicotinic acetylcholine receptors (α7nAChRs) and is synthesized and released by astroglia, which express functional α7nAChRs and cannabinoid CB1 receptors (CB1Rs). Here, we tested whether these presumed KYNA autoreceptors (α7nAChRs) and CB1Rs regulate glutamate release. We used in vivo microdialysis and electrophysiology in rats, RNAscope in situ hybridization in brain slices, and primary culture of rat cortical astrocytes. Acute systemic administration of THC increased extracellular levels of glutamate in the nucleus accumbens shell (NAcS), ventral tegmental area (VTA), and medial prefrontal cortex (mPFC). THC also reduced extracellular levels of KYNA in the NAcS. These THC effects were prevented by administration of Ro 61-8048 or the CB1R antagonist, rimonabant. THC increased the firing activity of glutamatergic pyramidal neurons projecting from the mPFC to the NAcS or to the VTA in vivo. These effects were averted by pretreatment with Ro 61-8048. In vitro, THC elicited glutamate release from cortical astrocytes (on which we demonstrated co-localization of the CB1Rs and α7nAChR mRNAs), and this effect was prevented by KYNA and rimonabant. These results suggest a key role of astrocytes in interactions between the endocannabinoid system, kynurenine pathway, and glutamatergic neurotransmission, with ramifications for the pathophysiology and treatment of psychiatric and neurodegenerative diseases.

Validated multi-step approach for in vivo recording and analysis of optogenetically evoked glutamate in the mouse globus pallidus

Precise quantification of extracellular glutamate concentrations upon neuronal activation is crucial for the understanding of brain function and neurological disorders. While optogenetics is an outstanding method for the correlation between distinct neurons and their role in circuitry and behavior, the electrochemically inactive nature of glutamate has proven challenging for recording upon optogenetic stimulations. This difficulty is due to the necessity for using enzyme-coated microelectrodes and the risk for light-induced artifacts. In this study, we establish a method for the combination of in vivo optogenetic stimulation with selective measurement of glutamate concentrations using enzyme-coated multielectrode arrays and amperometry. The glutamatergic subthalamic nucleus (STN), which is the main electrode target site in deep brain stimulation treatment of advanced Parkinson's disease, has recently proven opotogenetically targetable in Pitx2-Cre-transgenic mice and was here used as model system. Upon stereotactic injection of viral Channelrhodopsin2-eYFP constructs into the STN, amperometric recordings were performed at a range of optogenetic stimulation frequencies in the globus pallidus, the main STN target area, in anesthetized mice. Accurate quantification was enabled through a multi-step analysis approach based on self-referencing microelectrodes and repetition of the experimental protocol at two holding potentials, which allowed for the identification, isolation and removal of photoelectric and photoelectrochemical artifacts. This study advances the field of in vivo glutamate detection with combined optogenetics and amperometric recordings by providing a validated analysis framework for application in a wide variety of glutamate-based approaches in neuroscience.

Amperometric Self-Referencing Ceramic Based Microelectrode Arrays for D-Serine Detection

D-serine is the major D-amino acid in the mammalian central nervous system. As the dominant co-agonist of the endogenous synaptic NMDA receptor, D-serine plays a role in synaptic plasticity, learning, and memory. Alterations in D-serine are linked to neuropsychiatric disorders including schizophrenia. Thus, it is of increasing interest to monitor the concentration of D-serine in vivo as a relevant player in dynamic neuron-glia network activity. Here we present a procedure for amperometric detection of D-serine with self-referencing ceramic-based microelectrode arrays (MEAs) coated with D-amino acid oxidase from the yeast Rhodotorulagracilis (RgDAAO). We demonstrate in vitro D-serine recordings with a mean sensitivity of 8.61 ± 0.83 pA/µM to D-serine, a limit of detection (LOD) of 0.17 ± 0.01 µM, and a selectivity ratio of 80:1 or greater for D-serine over ascorbic acid (mean ± SEM; n = 12) that can be used for freely moving studies.

A computerized exposure system for animal models to optimize nicotine delivery into the brain through inhalation of electronic cigarette vapors or cigarette smoke

Pre-clinical studies investigated the effects of chronic exposure to nicotine on lungs, kidneys and brains using animal models. Most of these studies delivered nicotine into the circulatory and central nervous systems (CNS) through intraperitoneal injection or oral consumption methods. Few studies used inhalation machine system for nicotine delivery into brains in rodents to mimic human exposure to cigarettes. However, finding a more accurate and clinically relevant method of nicotine delivery is critical. A computerized inhalation machine has been designed (SciReq) and is currently employed in several institutions. The computerized machine delivers electronic (e)-cigarette vapor as well as tobacco smoke to rodents using marketed e-cigarette devices or tobacco cigarettes. This provides evidence about clinical effects of nicotine delivery by traditional methods (combustible cigarettes) and new methodologies (e-cigarettes) in physiological systems. Potential neurobiological mechanisms for the development of nicotine dependence have been determined recently in mice exposed to e-cigarette vapors in our laboratory using SciReq system. In this review article, the discussion focuses on the efficiency and practical applicability of using this computerized inhalation exposure system in inducing significant changes in brain protein expression and function as compared to other nicotine delivery methods. The SciReq inhalation system utilized in our laboratory and others is a method of nicotine delivery to the CNS, which has physiological relevance and mimics human inhalant exposures. Translation of the effects of inhaled nicotine on the CNS into clinical settings could provide important health considerations.

Oral administration of a specific kynurenic acid synthesis (KAT II) inhibitor attenuates evoked glutamate release in rat prefrontal cortex

Cognitive deficits represent core symptoms in schizophrenia (SZ) and predict patient outcome; however, they remain poorly treated by current antipsychotic drugs. Elevated levels of the endogenous alpha7 nicotinic receptor negative allosteric modulator and NMDA receptor antagonist, kynurenic acid (KYNA), are commonly seen in post-mortem tissue and cerebrospinal fluid of patients with SZ. When acutely or chronically elevated in rodents, KYNA produces cognitive deficits similar to those seen in the disease, making down-regulation of KYNA, via inhibition of kynurenine aminotransferase II (KAT II), a potential treatment strategy. We determined, in adult Wistar rats, if the orally available KAT II inhibitor BFF816 a) prevents KYNA elevations in prefrontal cortex (PFC) after a systemic kynurenine injection and b) reverses the kynurenine-induced attenuation of evoked prefrontal glutamate release caused by stimulation of the nucleus accumbens shell (NAcSh). Systemic injection of kynurenine (25 or 100 mg/kg, i.p.) increased KYNA levels in PFC (532% and 1104% of baseline, respectively). NMDA infusions (0.15 μg/0.5 μL) into NAcSh raised prefrontal glutamate levels more than 30-fold above baseline. The two doses of kynurenine reduced evoked glutamate release in PFC (by 43% and 94%, respectively, compared to NMDA alone). Co-administration of BFF816 (30 or 100 mg/kg, p.o.) with kynurenine (25 mg/kg, i.p.) attenuated the neosynthesis of KYNA and dose-dependently restored NMDA-stimulated glutamate release in the PFC (16% and 69%, respectively). The ability to prevent KYNA neosynthesis and to normalize evoked glutamate release in PFC justifies further development of KAT II inhibitors for the treatment of cognitive deficits in SZ.

Effects of chronic inhalation of electronic cigarettes containing nicotine on glial glutamate transporters and α-7 nicotinic acetylcholine receptor in female CD-1 mice

Alteration in glutamate neurotransmission has been found to mediate the development of drug dependence, including nicotine. We and others, through using western blotting, have reported that exposure to drugs of abuse reduced the expression of glutamate transporter-1 (GLT-1) as well as cystine/glutamate antiporter (xCT), which consequently increased extracellular glutamate concentrations in the mesocorticolimbic area. However, our previous studies did not reveal any changes in glutamate/aspartate transporter (GLAST) following exposure to drugs of abuse. In the present study, for the first time, we investigated the effect of chronic exposure to electronic (e)-cigarette vapor containing nicotine, for one hour daily for six months, on GLT-1, xCT, and GLAST expression in frontal cortex (FC), striatum (STR), and hippocampus (HIP) in outbred female CD1 mice. In this study, we also investigated the expression of alpha-7 nicotinic acetylcholine receptor (α-7 nAChR), a major pre-synaptic nicotinic receptor in the glutamatergic neurons, which regulates glutamate release. We found that inhalation of e-cigarette vapor for six months increased α-7 nAChR expression in both FC and STR, but not in the HIP. In addition, chronic e-cigarette exposure reduced GLT-1 expression only in STR. Moreover, e-cigarette vapor inhalation induced downregulation of xCT in both the STR and HIP. We did not find any significant changes in GLAST expression in any brain region. Finally, using liquid chromatography-tandem mass spectrometry (LC-MS/MS) techniques, we detected high concentrations of nicotine and cotinine, a major metabolite of nicotine, in the FC tissues of e-cigarette exposed mice. These data provide novel evidence about the effects of chronic nicotine inhalation on the expression of key glial glutamate transporters as well as α-7 nAChR. Our work may suggest that nicotine exposure via chronic inhalation of e-cigarette vapor may be mediated in part by alterations in the glutamatergic system.

Kynurenic Acid in Schizophrenia: A Systematic Review and Meta-analysis

Kynurenic acid (KYNA) is an endogenous antagonist of N-methyl-D-aspartate and α7 nicotinic acetylcholine receptors that is derived from astrocytes as part of the kynurenine pathway of tryptophan degradation. Evidence suggests that abnormal KYNA levels are involved in the pathophysiology of schizophrenia. However, this has never been assessed through a meta-analysis. A literature search was conducted through Ovid using Embase, Medline, and PsycINFO databases (last search: December 2016) with the search terms: (kynuren* or KYNA) and (schizophreni* or psychosis). English language studies measuring KYNA levels using any method in patients with schizophrenia and healthy controls (HCs) were identified. Standardized mean differences (SMDs) were calculated to determine differences in KYNA levels between groups. Subgroup analyses were separately performed for nonoverlapping participant samples, KYNA measurement techniques, and KYNA sample source. The influences of patients' age, antipsychotic status (%medicated), and sex (%male) on study SMDs were assessed through a meta-regression. Thirteen studies were deemed eligible for inclusion in the meta-analysis. In the main analysis, KYNA levels were elevated in the patient group. Subgroup analyses demonstrated that KYNA levels were increased in nonoverlapping participant samples, and centrally (cerebrospinal fluid and brain tissue) but not peripherally. Patients' age, %medicated, and %male were each positively associated with study SMDs. Overall, KYNA levels are increased in patients with schizophrenia, specifically within the central nervous system. An improved understanding of KYNA in patients with schizophrenia may contribute to the development of novel diagnostic approaches and therapeutic strategies.

Attenuating Nicotine Reinforcement and Relapse by Enhancing Endogenous Brain Levels of Kynurenic Acid in Rats and Squirrel Monkeys

The currently available antismoking medications have limited efficacy and often fail to prevent relapse. Thus, there is a pressing need for newer, more effective treatment strategies. Recently, we demonstrated that enhancing endogenous levels of kynurenic acid (KYNA, a neuroinhibitory product of tryptophan metabolism) counteracts the rewarding effects of cannabinoids by acting as a negative allosteric modulator of α7 nicotinic receptors (α7nAChRs). As the effects of KYNA on cannabinoid reward involve nicotinic receptors, in the present study we used rat and squirrel monkey models of reward and relapse to examine the possibility that enhancing KYNA can counteract the effects of nicotine. To assess specificity, we also examined models of cocaine reward and relapse in monkeys. KYNA levels were enhanced by administering the kynurenine 3-monooxygenase (KMO) inhibitor, Ro 61-8048. Treatment with Ro 61-8048 decreased nicotine self-administration in rats and monkeys, but did not affect cocaine self-administration. In rats, Ro 61-8048 reduced the ability of nicotine to induce dopamine release in the nucleus accumbens shell, a brain area believed to underlie nicotine reward. Perhaps most importantly, Ro 61-8048 prevented relapse-like behavior when abstinent rats or monkeys were reexposed to nicotine and/or cues that had previously been associated with nicotine. Ro 61-8048 was also effective in monkey models of cocaine relapse. All of these effects of Ro 61-8048 in monkeys, but not in rats, were reversed by pretreatment with a positive allosteric modulator of α7nAChRs. These findings suggest that KMO inhibition may be a promising new approach for the treatment of nicotine addiction.

Temporal Rewiring of Striatal Circuits Initiated by Nicotine

Drug addiction has been conceptualized as maladaptive recruitment of integrative circuits coursing through the striatum, facilitating drug-seeking and drug-taking behavior. The aim of this study was to define temporal neuroadaptations in striatal subregions initiated by 3 weeks of intermittent nicotine exposure followed by protracted abstinence. Enhanced rearing activity was assessed in motor activity boxes as a measurement of behavioral change induced by nicotine (0.36 mg/kg), whereas electrophysiological field potential recordings were performed to evaluate treatment effects on neuronal activity. Dopamine receptor mRNA expression was quantified by qPCR, and nicotine-induced dopamine release was measured in striatal subregions using in vivo microdialysis. Golgi staining was performed to assess nicotine-induced changes in spine density of medium spiny neurons. The data presented here show that a brief period of nicotine exposure followed by abstinence leads to temporal changes in synaptic efficacy, dopamine receptor expression, and spine density in a subregion-specific manner. Nicotine may thus initiate a reorganization of striatal circuits that continues to develop despite protracted abstinence. We also show that the response to nicotine is modulated in previously exposed rats even after 6 months of abstinence. The data presented here suggests that, even though not self-administered, nicotine may produce progressive neuronal alterations in brain regions associated with goal-directed and habitual performance, which might contribute to the development of compulsive drug seeking and the increased vulnerability to relapse, which are hallmarks of drug addiction.

Prenatal kynurenine exposure in rats: age-dependent changes in NMDA receptor expression and conditioned fear responding

RATIONALE

Levels of kynurenic acid (KYNA), an endogenous negative modulator of alpha 7 nicotinic acetylcholine receptors (α7nAChRs) and antagonist at glutamatergic N-methyl-D-aspartate receptors (NMDARs), are elevated in the brain of patients with schizophrenia (SZ). In rats, dietary exposure to KYNA's immediate precursor kynurenine during the last week of gestation produces neurochemical and cognitive deficits in adulthood that resemble those seen in patients with SZ.

OBJECTIVES

The present experiments examined whether prenatal kynurenine exposure results in age-dependent changes in the kynurenine pathway (KP), expression of selected receptors, and cognitive function.

METHODS

Pregnant dams were fed unadulterated mash (progeny = ECON) or mash containing kynurenine (100 mg/day; progeny = EKYN) from embryonic day (ED) 15 to 22. Male offspring were assessed as juveniles, i.e., prior to puberty (postnatal day [PD] 32), or as adults (PD70) for brain KYNA levels, α7nAChR and NMDAR gene expression, and performance on a trace fear conditioning (TFC) task.

RESULTS

KYNA levels were comparable between juvenile ECON and EKYN rats, whereas EKYN adults exhibited a ~3-fold increase in brain KYNA relative to ECONs. NR2A expression was persistently reduced (30-40 %) in EKYN rats at both ages. Compared to ECON adults, there was a 50 % reduction in NR1, and a trend toward decreased α7nAChR expression, in adult EKYN rats. Surprisingly, juvenile EKYN rats performed significantly better in the TFC paradigm than controls, whereas adult EKYN animals showed the predicted deficits.

CONCLUSIONS

Collectively, our results provide evidence that KP changes in the fetal brain alter neuronal development and cause age-dependent effects on neurochemistry and cognitive performance.

Alpha7 nicotinic acetylcholine receptor agonists and PAMs as adjunctive treatment in schizophrenia. An experimental study

Nicotine has been found to improve cognition and reduce negative symptoms in schizophrenia and a genetic and pathophysiological link between the α7 nicotinic acetylcholine receptors (nAChRs) and schizophrenia has been demonstrated. Therefore, there has been a large interest in developing drugs affecting the α7 nAChRs for schizophrenia. In the present study we investigated, in rats, the effects of a selective α7 agonist (PNU282987) and a α7 positive allosteric modulator (PAM; NS1738) alone and in combination with the atypical antipsychotic drug risperidone for their utility as adjunct treatment in schizophrenia. Moreover we also investigated their utility as adjunct treatment in depression in combination with the SSRI citalopram. We found that NS1738 and to some extent also PNU282987, potentiated a subeffective dose of risperidone in the conditioned avoidance response test. Both drugs also potentiated the effect of a sub-effective concentration of risperidone on NMDA-induced currents in pyramidal cells of the medial prefrontal cortex. Moreover, NS1738 and PNU282987 enhanced recognition memory in the novel object recognition test, when given separately. Both drugs also potentiated accumbal but not prefrontal risperidone-induced dopamine release. Finally, PNU282987 reduced immobility in the forced swim test, indicating an antidepressant-like effect. Taken together, our data support the utility of drugs targeting the α7 nAChRs, perhaps especially α7 PAMs, to potentiate the effect of atypical antipsychotic drugs. Moreover, our data suggest that α7 agonists and PAMs can be used to ameliorate cognitive symptoms in schizophrenia and depression.

Kynurenines and Glutamate: Multiple Links and Therapeutic Implications

Glutamate is firmly established as the major excitatory neurotransmitter in the mammalian brain and is actively involved in most aspects of neurophysiology. Moreover, glutamatergic impairments are associated with a wide variety of dysfunctional states, and both hypo- and hyperfunction of glutamate have been plausibly linked to the pathophysiology of neurological and psychiatric diseases. Metabolites of the kynurenine pathway (KP), the major catabolic route of the essential amino acid tryptophan, influence glutamatergic activity in several distinct ways. This includes direct effects of these "kynurenines" on ionotropic and metabotropic glutamate receptors or vesicular glutamate transport, and indirect effects, which are initiated by actions at various other recognition sites. In addition, some KP metabolites affect glutamatergic functions by generating or scavenging highly reactive free radicals. This review summarizes these phenomena and discusses implications for brain physiology and pathology.

nAChR dysfunction as a common substrate for schizophrenia and comorbid nicotine addiction: Current trends and perspectives

INTRODUCTION

The prevalence of tobacco use in the population with schizophrenia is enormously high. Moreover, nicotine dependence is found to be associated with symptom severity and poor outcome in patients with schizophrenia. The neurobiological mechanisms that explain schizophrenia-nicotine dependence comorbidity are not known. This study systematically reviews the evidence highlighting the contribution of nicotinic acetylcholine receptors (nAChRs) to nicotine abuse in schizophrenia.

METHODS

Electronic data bases (Medline, Google Scholar, and Web of Science) were searched using the selected key words that match the aims set forth for this review. A total of 276 articles were used for the qualitative synthesis of this review.

RESULTS

Substantial evidence from preclinical and clinical studies indicated that dysregulation of α7 and β2-subunit containing nAChRs account for the cognitive and affective symptoms of schizophrenia and nicotine use may represent a strategy to remediate these symptoms. Additionally, recent meta-analyses proposed that early tobacco use may itself increase the risk of developing schizophrenia. Genetic studies demonstrating that nAChR dysfunction that may act as a shared vulnerability factor for comorbid tobacco dependence and schizophrenia were found to support this view. The development of nAChR modulators was considered an effective therapeutic strategy to ameliorate psychiatric symptoms and to promote smoking cessation in schizophrenia patients.

CONCLUSIONS

The relationship between schizophrenia and smoking is complex. While the debate for the self-medication versus addiction vulnerability hypothesis continues, it is widely accepted that a dysfunction in the central nAChRs represent a common substrate for various symptoms of schizophrenia and comorbid nicotine dependence.

Integration of inhibitory and excitatory effects of α7 nicotinic acetylcholine receptor activation in the prelimbic cortex regulates network activity and plasticity

Cognitive and attentional processes governed by the prefrontal cortex (PFC) are influenced by cholinergic innervation. Here we have explored the role of α7 nicotinic acetylcholine receptors (nAChRs) as mediators of cholinergic signalling in the dorsomedial (prelimbic) PFC, using mouse brain slice electrophysiology. Activation of α7 nAChRs located on glutamatergic terminals and cell soma of GABAergic interneurons increased excitation and inhibition, respectively, in layer V of the prelimbic cortex. These actions were distinguished by their differential dependence on local acetylcholine (ACh): potentiation of endogenous cholinergic signalling with the positive allosteric modulator, PNU-120596, enhanced spontaneous excitatory events, an effect that was further increased by inhibition of acetylcholinesterase. In contrast, α7 nicotinic modulation of inhibitory signalling required addition of exogenous agonist (PNU-282987) as well as PNU-120596, and was unaffected by acetylcholinesterase inhibition. Thus α7 nAChRs can bi-directionally regulate network activity in the prelimbic cortex, depending on the magnitude and localisation of cholinergic signalling. This bidirectional influence is manifest in dual effects of α7 nAChRs on theta-burst-induced long-term potentiation (LTP) in layer V of the prelimbic cortex. Antagonism of α7 nAChRs significantly decreased LTP implicating a contribution from endogenous ACh, consistent with the ability of local ACh to enhance glutamatergic signalling. Exogenous agonist plus potentiator also decreased LTP, indicative of the influence of this drug combination on inhibitory signalling. Thus α7 nAChRs make a complex contribution to network activity and synaptic plasticity in the prelimbic cortex.

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α7 nAChRs exist at glutamatergic nerve terminals in the prelimbic cortex.

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α7 nAChRs exist at GABAergic cell bodies in the prelimbic cortex.

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Tonic ACh preferentially activates α7 nAChRs at glutamatergic nerve terminals.

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α7 nAChRs exert bidirectional control of LTP in the prelimbic cortex.

Targeting glutamate homeostasis for potential treatment of nicotine dependence

Several studies demonstrated that impairment in glutamatergic neurotransmission is linked to drug dependence and drug-seeking behavior. Increased extracellular glutamate concentration in mesocorticolimbic regions has been observed in animals developing nicotine dependence. Changes in glutamate release might be associated with stimulatory effect of nicotinic acetylcholine receptors (nAChRs) via nicotine exposure. We and others have shown increased extracellular glutamate concentration, which was associated with down regulation of the major glutamate transporter, glutamate transporter 1 (GLT-1), in brain reward regions of animals exposed to drug abuse, including nicotine and ethanol. Importantly, studies from our laboratory and others showed that upregulation of GLT-1 expression in the mesocorticolimbic brain regions may have potential therapeutic effects in drug dependence. In this review article, we discussed the effect of antagonizing presynaptic nAChRs in glutamate release, the upregulatory effect in GLT-1 expression and the role of glutamate receptors antagonists in the treatment of nicotine dependence.

Pharmacological Mechanisms of Cortical Enhancement Induced by the Repetitive Pairing of Visual/Cholinergic Stimulation

Repetitive visual training paired with electrical activation of cholinergic projections to the primary visual cortex (V1) induces long-term enhancement of cortical processing in response to the visual training stimulus. To better determine the receptor subtypes mediating this effect the selective pharmacological blockade of V1 nicotinic (nAChR), M1 and M2 muscarinic (mAChR) or GABAergic A (GABA_AR) receptors was performed during the training session and visual evoked potentials (VEPs) were recorded before and after training. The training session consisted of the exposure of awake, adult rats to an orientation-specific 0.12 CPD grating paired with an electrical stimulation of the basal forebrain for a duration of 1 week for 10 minutes per day. Pharmacological agents were infused intracortically during this period. The post-training VEP amplitude was significantly increased compared to the pre-training values for the trained spatial frequency and to adjacent spatial frequencies up to 0.3 CPD, suggesting a long-term increase of V1 sensitivity. This increase was totally blocked by the nAChR antagonist as well as by an M2 mAChR subtype and GABA_AR antagonist. Moreover, administration of the M2 mAChR antagonist also significantly decreased the amplitude of the control VEPs, suggesting a suppressive effect on cortical responsiveness. However, the M1 mAChR antagonist blocked the increase of the VEP amplitude only for the high spatial frequency (0.3 CPD), suggesting that M1 role was limited to the spread of the enhancement effect to a higher spatial frequency. More generally, all the drugs used did block the VEP increase at 0.3 CPD. Further, use of each of the aforementioned receptor antagonists blocked training-induced changes in gamma and beta band oscillations. These findings demonstrate that visual training coupled with cholinergic stimulation improved perceptual sensitivity by enhancing cortical responsiveness in V1. This enhancement is mainly mediated by nAChRs, M2 mAChRs and GABA_ARs. The M1 mAChR subtype appears to be involved in spreading the enhancement of V1 cortical responsiveness to adjacent neurons.

The role of the serotonin receptor subtypes 5-HT1A and 5-HT7 and its interaction in emotional learning and memory

Serotonin [5-hydroxytryptamine (5-HT)] is a multifunctional neurotransmitter innervating cortical and limbic areas involved in cognition and emotional regulation. Dysregulation of serotonergic transmission is associated with emotional and cognitive deficits in psychiatric patients and animal models. Drugs targeting the 5-HT system are widely used to treat mood disorders and anxiety-like behaviors. Among the fourteen 5-HT receptor (5-HTR) subtypes, the 5-HT_1AR and 5-HT₇R are associated with the development of anxiety, depression and cognitive function linked to mechanisms of emotional learning and memory. In rodents fear conditioning and passive avoidance (PA) are associative learning paradigms to study emotional memory. This review assesses the role of 5-HT_1AR and 5-HT₇R as well as their interplay at the molecular, neurochemical and behavioral level. Activation of postsynaptic 5-HT_1ARs impairs emotional memory through attenuation of neuronal activity, whereas presynaptic 5-HT_1AR activation reduces 5-HT release and exerts pro-cognitive effects on PA retention. Antagonism of the 5-HT_1AR facilitates memory retention possibly via 5-HT₇R activation and evidence is provided that 5HT₇R can facilitate emotional memory upon reduced 5-HT_1AR transmission. These findings highlight the differential role of these 5-HTRs in cognitive/emotional domains of behavior. Moreover, the results indicate that tonic and phasic 5-HT release can exert different and potentially opposing effects on emotional memory, depending on the states of 5-HT_1ARs and 5-HT₇Rs and their interaction. Consequently, individual differences due to genetic and/or epigenetic mechanisms play an essential role for the responsiveness to drug treatment, e.g., by SSRIs which increase intrasynaptic 5-HT levels thereby activating multiple pre- and postsynaptic 5-HTR subtypes.

Effects of age and acute ethanol on glutamatergic neurotransmission in the medial prefrontal cortex of freely moving rats using enzyme-based microelectrode amperometry

Ethanol abuse during adolescence may significantly alter development of the prefrontal cortex which continues to undergo structural remodeling into adulthood. Glutamatergic neurotransmission plays an important role during these brain maturation processes and is modulated by ethanol. In this study, we investigated glutamate dynamics in the medial prefrontal cortex of freely moving rats, using enzyme-based microelectrode amperometry. We analyzed the effects of an intraperitoneal ethanol injection (1 g/kg) on cortical glutamate levels in adolescent and adult rats. Notably, basal glutamate levels decreased with age and these levels were found to be significantly different between postnatal day (PND) 28-38 vs PND 44-55 (p<0.05) and PND 28-38 vs adult animals (p<0.001). We also observed spontaneous glutamate release (transients) throughout the recordings. The frequency of transients (per hour) was significantly higher in adolescent rats (PND 28-38 and PND 44-55) compared to those of adults. In adolescent rats, post-ethanol injection, the frequency of glutamate transients decreased within the first hour (p<0.05), it recovered slowly and in the third hour there was a significant rebound increase of the frequency (p<0.05). Our data demonstrate age-dependent differences in extracellular glutamate levels in the medial prefrontal cortex and suggest that acute ethanol injections have both inhibitory and excitatory effects in adolescent rats. These effects of ethanol on the prefrontal cortex may disturb its maturation and possibly limiting individuals´ control over addictive behaviors.

Elevated levels of kynurenic acid during gestation produce neurochemical, morphological, and cognitive deficits in adulthood: implications for schizophrenia

The levels of kynurenic acid (KYNA), an endogenous negative modulator of alpha7 nicotinic acetylcholine receptors (α7nAChRs), are elevated in the brains of patients with schizophrenia (SZ). We reported that increases of brain KYNA in rats, through dietary exposure to its precursor kynurenine from embryonic day (ED)15 to postnatal day (PD) 21, result in neurochemical and cognitive deficits in adulthood. The present experiments focused on the effects of prenatal exposure to elevated kynurenine on measures of prefrontal excitability known to be impaired in SZ. Pregnant dams were fed a mash containing kynurenine (100 mg/day; progeny = EKYNs) from ED15 until ED22. Controls were fed an unadulterated mash (progeny = ECONs). The dietary loading procedure elevated maternal and fetal plasma kynurenine (2223% and 693% above controls, respectively) and increased fetal KYNA (forebrain; 500% above controls) on ED21. Elevations in forebrain KYNA disappeared after termination of the loading (PD2), but KYNA levels in the prefrontal cortex (PFC) were unexpectedly increased again when measured in adults (PD56-80; 75% above controls). We also observed changes in several markers of prefrontal excitability, including expression of the α7nAChR (22% and 17% reductions at PD2 and PD56-80), expression of mGluR2 (31% and 24% reductions at ED21 and PD56-80), dendritic spine density (11-14% decrease at PD56-80), subsensitive mesolimbic stimulation of glutamate release in PFC, and reversal/extra-dimensional shift deficits in the prefrontally-mediated set-shifting task. These results highlight the deleterious impact of elevated KYNA levels during sensitive periods of early development, which model the pathophysiological and cognitive deficits seen in SZ.

Electrochemical techniques for subsecond neurotransmitter detection in live rodents

Alterations in neurotransmission have been implicated in numerous neurodegenerative and neuropsychiatric disorders, including Alzheimer disease, Parkinson disease, epilepsy, and schizophrenia. Unfortunately, few techniques support the measurement of real-time changes in neurotransmitter levels over multiple days, as is essential for ethologic and pharmacodynamic testing. Microdialysis is commonly used for these research paradigms, but its poor temporal and spatial resolution make this technique inadequate for measuring the rapid dynamics (milliseconds to seconds) of fast signaling neurotransmitters, such as glutamate and acetylcholine. Enzymatic microelectrode arrays (biosensors) coupled with electrochemical recording techniques have demonstrated fast temporal resolution (less than 1 s), excellent spatial resolution (micron-scale), low detection limits (≤200 nM), and minimal damage (50 to 100 μm) to surrounding brain tissue. Here we discuss the benefits, methods, and animal welfare considerations of using platinum microelectrodes on a ceramic substrate for enzyme-based electrochemical recording techniques for real-time in vivo neurotransmitter recordings in both anesthetized and awake, freely moving rodents.

Thalamic glutamate decreases with cigarette smoking

RATIONALE

Findings from animal studies and human PET imaging indicate that nicotine and cigarette smoking affect glutamate (Glu) and related neurochemical markers in the brain and imply that smoking reduces extracellular Glu. As Glu release is mediated by nicotinic acetylcholine receptors (nAChRs), which are present at high concentrations in the thalamus, we examined the effects of smoking on thalamic Glu.

OBJECTIVE

To determine the effects of tobacco smoking on thalamic glutamate levels.

METHODS

Thalamic Glu levels were measured in vivo in 18 smokers and 16 nonsmokers using proton magnetic resonance spectroscopic imaging ((1)H MRSI) at 1.5 T.

RESULTS

Mean Glu levels did not differ significantly between the subject groups. However, within smokers, Glu levels were negatively correlated with self-reports of both cigarettes/day over the last 30 days (r = -0.64, p = 0.006) and pack-years of smoking (r = -0.66, p = 0.005).

CONCLUSIONS

Consistent with expectations based on preclinical studies, within smokers, cigarettes/day and pack-years are associated with reduced Glu in thalamus, a brain region rich in nAchRs. These results encourage work on candidate glutamatergic therapies for smoking cessation and suggest a noninvasive metric for their action in the brain.

Transient inactivation of the ventral hippocampus in neonatal rats impairs the mesolimbic regulation of prefrontal glutamate release in adulthood

Cognitive deficits in schizophrenia (SZ) reflect maturational disruptions within a neural system that includes the ventral hippocampus (VH), nucleus accumbens (NAc), basal forebrain, and prefrontal cortex (PFC). A better understanding of these changes may reveal drug targets for more efficacious cognition enhancers. We have utilized an animal model in which the above distributed system is altered, during a sensitive period of development, by transiently inactivating the VH and its efferent projections. We determined the ability of NAc shell activation to evoke prefrontal glutamate release in adult male Wistar rats that had received saline (Sal) or tetrodotoxin (TTX) as neonates (PD7) or as adolescents (PD32). The nucleus accumbens shell (NAcSh) was activated by NMDA infusions (0.05-0.30 μg/0.5 μL). Basal and evoked glutamate levels were measured amperometrically using a glutamate-sensitive microelectrode. There were no differences in basal glutamate levels among the groups tested (overall 1.41 ± 0.26 uM). However, the dose-related stimulation of prefrontal glutamate levels seen in control rats treated with saline on PD7 (4.31 ± 0.22 μM after 0.15 μg) was markedly attenuated in rats treated with TTX on PD7 (0.45 ± 0.12 μM after 0.15 μg). This effect was age-dependent as infusions of TTX on PD32 did not alter the NMDA-induced increases in glutamate release (4.10 ± 0.37 μM after 0.15 μg). Collectively, these findings reveal that transient inactivation of VH transmission, during a sensitive period of development, leads to a functional mesolimbic-cortical disconnection that produces neurochemical and ultimately cognitive impairments resembling those seen in SZ.

A systemically-available kynurenine aminotransferase II (KAT II) inhibitor restores nicotine-evoked glutamatergic activity in the cortex of rats

Kynurenic acid (KYNA) is a tryptophan metabolite that acts in the brain as an endogenous antagonist at multiple receptors, including glutamate and α7 nicotinic acetylcholine receptors. Increased levels of KYNA have been demonstrated in the brain of patients with a range of neurocognitive disorders, including schizophrenia, and are hypothesized to contribute to cognitive symptoms. Reducing KYNA levels by administering inhibitors of enzymes of the kynurenine pathway, particularly kynurenine aminotransferase II (KAT II), has been proposed as a treatment for such cognitive impairments. Here we report that administration of a systemically available KAT II inhibitor, PF-04859989, restores glutamate release events ("transients") evoked by pressure ejections of nicotine into the prefrontal cortex of rats exhibiting elevated KYNA levels. Nicotine-evoked glutamatergic transients can be reliably evoked and recorded after repeated pressure ejections of nicotine over 4-5 h. Systemic administration of l-kynurenine (100 mg/kg; i.p.) significantly increased frontal cortical KYNA levels and greatly attenuated the amplitude of nicotine-evoked glutamatergic transients. Systemic administration of PF-04859989 30 min prior to administration of l-kynurenine, but not when administered 30 min after l-kynurenine, restored glutamatergic transients recorded up to 75 min after the administration of the KAT II inhibitor. Furthermore, the KAT II inhibitor significantly reversed l-kynurenine-induced elevations of brain KYNA levels. The KAT II inhibitor did not affect nicotine-evoked glutamatergic transients in rats not pre-treated with l-kynurenine. Because PF-04859989 restores evoked glutamate signaling it therefore is a promising therapeutic compound for benefiting the cognitive symptoms of schizophrenia and other disorders associated with elevated brain KYNA levels.

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.

Localized infusions of the partial alpha 7 nicotinic receptor agonist SSR180711 evoke rapid and transient increases in prefrontal glutamate release

The ability of local infusions of the alpha 7 nicotinic acetycholine receptor (α7 nAChR) partial agonist SSR180711 to evoke glutamate release in prefrontal cortex was determined in awake rats using a microelectrode array. Infusions of SSR180711 produced dose-dependent increases in glutamate levels. The lower dose (1.0μg in 0.4μL) evoked a rapid rise (∼1.0s) in glutamate (1.41±0.30μM above baseline). The higher dose (5.0μg) produced a similarly rapid, yet larger increase (3.51±0.36μM above baseline). After each dose, the glutamate signal was cleared to basal levels within 7-18s. SSR180711-evoked glutamate was mediated by the α7 nAChR as co-infusion of the selective α7 nAChR antagonist α-bungarotoxin (10.0μM)+SSR1808711 (5.0μg) reduced the effect of 5.0μg alone by 87% (2.62 vs. 0.35μM). Finally, the clearance of the SSR180711 (5.0μg)-evoked glutamate was bidirectionally affected by drugs that inhibited (threo-beta-benzyl-oxy-aspartate (TβOA), 100.0μM) or facilitated (ceftriaxalone, 200mg/kg, i.p.) excitatory amino acid transporters. TβOA slowed both the clearance (s) and rate of clearance (μM/s) by 10-fold, particularly at the mid-late stages of the return to baseline. Ceftriaxone reduced the magnitude of the SSR180711-evoked increase by 65%. These results demonstrate that pharmacological stimulation of α7 nAChRs within the prefrontal cortex is sufficient to evoke rapid yet transient increases in glutamate levels. Such increases may underlie the cognition-enhancing effects of the drug in animals; further justifying studies on the use of α7 nAChR-positive modulators in treating cognition-impairing disorders in humans.

Evaluating the role of the alpha-7 nicotinic acetylcholine receptor in the pathophysiology and treatment of schizophrenia

The group of schizophrenia disorders affects approximately 1% of the population and has both genetic and environmental etiologies. Sufferers report various behavioral abnormalities including hallucinations and delusions (positive symptoms), reduced joy and amotivation (negative symptoms), plus inattention and poor learning (cognitive deficits). Despite the heterogeneous symptoms experienced, most patients smoke. The self-medication hypothesis posits that patients smoke to alleviate symptoms, consistent with evidence for nicotine-induced enhancement of cognition. While nicotine acts on multiple nicotinic acetylcholine receptors (nAChRs), the primary target of research is often the homomeric α7 nAChR. Given genetic linkages between schizophrenia and this receptor, its association with P50 sensory gating deficits, and its reduced expression in post-mortem brains, many have attempted to develop α7 nAChR ligands for treating schizophrenia. Recent evidence that ligands can be orthosteric agonists or positive allosteric modulators (PAMs) has revitalized the hope for treatment discovery. Herein, we present evidence regarding: (1) pathophysiological alterations of α7 nAChRs that might occur in patients; (2) mechanistic evidence for the normal action of α7 nAChRs; (3) preclinical studies using α7 nAChR orthosteric agonists and type I/II PAMs; and (4) where successful translational testing has occurred for particular compounds, detailing what is still required. We report that the accumulating evidence is positive, but that greater work is required using positron emission tomography to understand current alterations in α7 nAChR expression and their relationship to symptoms. Finally, cross-species behavioral tasks should be used more regularly to determine the predictive efficacy of treatments.

Nicotinic α7 receptors enhance NMDA cognitive circuits in dorsolateral prefrontal cortex

The cognitive function of the highly evolved dorsolateral prefrontal cortex (dlPFC) is greatly influenced by arousal state, and is gravely afflicted in disorders such as schizophrenia, where there are genetic insults in α7 nicotinic acetylcholine receptors (α7-nAChRs). A recent behavioral study indicates that ACh depletion from dlPFC markedly impairs working memory [Croxson PL, Kyriazis DA, Baxter MG (2011) Nat Neurosci 14(12):1510-1512]; however, little is known about how α7-nAChRs influence dlPFC cognitive circuits. Goldman-Rakic [Goldman-Rakic (1995) Neuron 14(3):477-485] discovered the circuit basis for working memory, whereby dlPFC pyramidal cells excite each other through glutamatergic NMDA receptor synapses to generate persistent network firing in the absence of sensory stimulation. Here we explore α7-nAChR localization and actions in primate dlPFC and find that they are enriched in glutamate network synapses, where they are essential for dlPFC persistent firing, with permissive effects on NMDA receptor actions. Blockade of α7-nAChRs markedly reduced, whereas low-dose stimulation selectively enhanced, neuronal representations of visual space. These findings in dlPFC contrast with the primary visual cortex, where nAChR blockade had no effect on neuronal firing [Herrero JL, et al. (2008) Nature 454(7208):1110-1114]. We additionally show that α7-nAChR stimulation is needed for NMDA actions, suggesting that it is key for the engagement of dlPFC circuits. As ACh is released in cortex during waking but not during deep sleep, these findings may explain how ACh shapes differing mental states during wakefulness vs. sleep. The results also explain why genetic insults to α7-nAChR would profoundly disrupt cognitive experience in patients with schizophrenia.

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.

Kynurenic acid inhibits glutamatergic transmission to CA1 pyramidal neurons via α7 nAChR-dependent and -independent mechanisms

Glutamatergic hypofunction and elevated levels of kynurenic acid (KYNA) in the brain are common features of patients with schizophrenia. In vivo studies indicate that in the hippocampus KYNA decreases glutamate levels, presumably via inhibition of α7 nicotinic receptors (nAChRs). Here we tested the hypothesis that basal synaptic glutamate activity in the hippocampus is regulated by tonically active α7 nAChRs and is sensitive to inhibition by KYNA. To this end, spontaneous excitatory postsynaptic currents (EPSCs), sensitive to AMPA receptor antagonist CNQX (10 μM), were recorded from CA1 pyramidal neurons at -70 mV in rat hippocampal slices. The α7 nAChR antagonists α-bungarotoxin (α-BGT, 100 nM) and methyllycaconitine (MLA, 1-50 nM), and the NMDA receptor antagonist 2-amino-5-phosphonovaleric acid (APV, 50 μM) reduced the frequency of EPSCs. MLA and α-BGT had no effect on miniature EPSCs (mEPSCs). The effect of MLA decreased in the presence of APV (50 μM), with 1 nM MLA becoming completely ineffective. KYNA (1-20 μM) suppressed the frequency of EPSCs, without affecting mEPSCs. The effect of KYNA decreased in the presence of MLA (1 nM) or α-BGT (100 nM), with 1 μM KYNA being devoid of any effect. In the presence of both MLA (10 nM) and APV (50 μM) higher KYNA concentrations (5-20 μM) still reduced the frequency of EPSCs. These results suggest that basal synaptic glutamate activity in CA1 pyramidal neurons is maintained in part by tonically active α7 nAChRs and NMDA receptors and is inhibited by micromolar concentrations of KYNA, acting via α7 nAChR-dependent and -independent mechanisms.