Nicotinic mechanisms influencing synaptic plasticity in the hippocampus

Identification of Psychoplastogenic Tropanes Lacking Muscarinic Activity

Tropane-containing small molecules like scopolamine are a promising class of psychoplastogens. However, their potent antagonism of all muscarinic receptor subtypes presents the potential for undesirable anticholinergic side effects. In an effort to decouple their neuroplasticity-promoting effects from their muscarinic activity, we performed phenotypic structure-activity relationship studies across a variety of structurally distinct subclasses of tropanes. We discovered several novel tropanes capable of significantly increasing cortical neuronal growth while exhibiting drastically reduced activity at all muscarinic receptor subtypes compared to scopolamine.

Ferulic acid improves cognitive impairment by regulating jumonji C domain-containing protein 6 and synaptophysin in the hippocampus in neonatal and juvenile rats with intrauterine hypoxia during pregnancy

To investigate the impacts of ferulic acid (FA) on jumonji C domain-containing protein 6 (JMJD6) and synaptophysin in the tissues of the hippocampus in neonatal and juvenile rats with intrauterine hypoxia-induced cognitive impairment. The Sprague-Dawley pregnant rats were randomly divided into three groups: control, hypoxia, and hypoxia + FA. On day 14 of pregnancy, the intrauterine hypoxia model was created by placing pregnant rats in the hypoxic and low-pressure experimental chamber for 2 hr a day for 3 days. In the hypoxia + FA group, pregnant rats were injected intraperitoneally with 4% FA, once a day for 7 days. The hypoxia group was treated with equal amounts of saline. After delivery, JMJD6 and synaptophysin mRNA and proteins in the hippocampus regions were detected by in situ hybridization and western blotting. The Morris water maze was used to evaluate cognitive function. The neonatal and juvenile rats in the hypoxia group had significantly increased expression of JMJD6 and decreased expression of synaptophysin protein and synaptophysin I mRNA in the hippocampus than those in the control group. Meanwhile, hypoxia also clearly prolonged the escape latency and shortened the stay time in the target quadrant. FA decreased the expression of JMJD6 and increased the expression of synaptophysin and improved cognitive function compared with those in the hypoxia group. FA probably ameliorated the cognitive impairment by regulating JMJD6 and synaptophysin in the hippocampus of neonatal and juvenile rats who had intrauterine hypoxia during pregnancy.

Amelioration of Nicotine-Induced Conditioned Place Preference Behaviors in Mice by an FABP3 Inhibitor

We previously demonstrated that fatty acid-binding protein 3 null (FABP3−/−) mice exhibit resistance to nicotine-induced conditioned place preference (CPP). Here, we confirm that the FABP3 inhibitor, MF1 ((4-(2-(1-(2-chlorophenyl)-5-phenyl-1H-pyrazol-3-yl)phenoxy) butanoic acid), successfully reduces nicotine-induced CPP scores in mice. MF1 (0.3 or 1.0 mg/kg) was orally administered 30 min before nicotine, and CPP scores were assessed in the conditioning, withdrawal, and relapse phases. MF1 treatment decreased CPP scores in a dose-dependent manner. Failure of CPP induction by MF1 (1.0 mg/kg, p.o.) was associated with the inhibition of both CaMKII and ERK activation in the nucleus accumbens (NAc) and hippocampal CA1 regions. MF1 treatment reduced nicotine-induced increases in phosphorylated CaMKII and cAMP-response element-binding protein (CREB)-positive cells. Importantly, the increase in dopamine D2 receptor (D2R) levels following chronic nicotine exposure was inhibited by MF1 treatment. Moreover, the quinpirole (QNP)-induced increase in the level of CaMKII and ERK phosphorylation was significantly inhibited by MF1 treatment of cultured NAc slices from wild type (WT) mice; however, QNP treatment had no effect on CaMKII and ERK phosphorylation levels in the NAc of D2R null mice. Taken together, these results show that MF1 treatment suppressed D2R/FABP3 signaling, thereby preventing nicotine-induced CPP induction. Hence, MF1 can be used as a novel drug to block addiction to nicotine and other drugs by inhibiting the dopaminergic system.

Hippocampal cholinergic receptors and the mTOR participation in fear-motivated inhibitory avoidance extinction memory

Evidence has demonstrated the hippocampal cholinergic system and the mammalian target of rapamycin (mTOR) participation during the memory formation of aversive events. This study assessed the role of these systems in the hippocampus for the extinction memory process by submitting male Wistar rats to fear-motivated step-down inhibitory avoidance (IA). The post-extinction session administration of the nicotinic and muscarinic cholinergic receptor antagonists, mecamylamine and scopolamine, respectively, both at doses of 2 µg/µl/side, and rapamycin, an mTOR inhibitor (0.02 µg/µl/side), into the CA1 region of the dorsal hippocampus, impaired the IA extinction memory. Furthermore, the nicotinic and muscarinic cholinergic receptor agonists, nicotine and muscarine, respectively, had a dose-dependent effect on the IA extinction memory when administered intra-CA1, immediately after the extinction session. Nicotine (0.6 µg/µl/side) and muscarine (0.02 µg/µl/side), respectively, had no effect, while the higher doses (6 and 2 µg/µl/side, respectively) impaired the IA extinction memory. Interestingly, the co-administration of muscarine at the lower dose blocked the impairment that was induced by rapamycin. This effect was not observed when nicotine at the lower dose was co-administered. These results have demonstrated the participation of the cholinergic receptors and mTOR in the hippocampus for IA extinction, and that the cholinergic agonists had a dose-dependent effect on the IA extinction memory. This study provides insights related to the behavioural aspects and the neurobiological properties underlying the early stage of fear-motivated IA extinction memory consolidation and suggests that there is hippocampal muscarinic receptor participation independent of mTOR in this memory process.

Impact of Prenatal Nicotine Exposure on Placental Function and Respiratory Neural Network Development

Smoking during pregnancy is associated with multiple undesirable outcomes in infants, such as low birth weight, increased neonatal morbidity and mortality, and catastrophic conditions like sudden infant death syndrome (SIDS). Nicotine, the most addictive and teratogenic substance in tobacco smoke, reaches and crosses the placenta and can be accumulated in the amniotic fluid and distributed by fetal circulation, altering the cholinergic transmission by acting on the nicotinic acetylcholine receptors (nAChRs) expressed from very early gestational stages in the placenta and fetal tissue. Because nAChRs influence the establishment of feto-maternal circulation and the emergence of neuronal networks, prenatal nicotine exposure can lead to multiple alterations in newborns. In this mini-review, we discuss the undeniable effects of nicotine in the placenta and the respiratory neural network as examples of how prenatal nicotine and smoking exposition can affect brain development because dysfunction in this network is involved in SIDS etiology.

Memory enhancing effects of nicotine, cocaine, and their conditioned stimuli; effects of beta-adrenergic and dopamine D2 receptor antagonists

BACKGROUND

There is evidence that post-training exposure to nicotine, cocaine, and their conditioned stimuli (CS), enhance memory consolidation in rats. The present study assessed the effects of blocking noradrenergic and dopaminergic receptors on nicotine and cocaine unconditioned and conditioned memory modulation.

METHODS

Males Sprague-Dawley rats tested on the spontaneous object recognition task received post-sample exposure to 0.4 mg/kg nicotine, 20 mg/kg cocaine, or their CSs, in combination with 5-10 mg/kg propranolol (PRO; beta-adrenergic antagonist) or 0.2-0.6 mg/kg pimozide (PIM; dopamine D2 receptor antagonist). The CSs were established by confining rats in a chamber (the CS +) after injections of 0.4 mg/kg nicotine, or 20 mg/kg cocaine, for 2 h and in another chamber (the CS -) after injections of vehicle, repeated over 10 days (5 drug/CS + and 5 vehicle/CS - pairings in total). Object memory was tested 72 h post sample in drug-free animals.

RESULTS

Co-administration of PRO or PIM blocked the memory-enhancing effects of post-training injections of nicotine, cocaine, and, importantly, exposure to their CSs.

CONCLUSIONS

These data suggest that nicotine, cocaine as well as their conditioned stimuli share actions on overlapping noradrenergic and dopaminergic systems to modulate memory consolidation.

Developmental nicotine exposure impairs memory and reduces acetylcholine levels in the hippocampus of mice

Nicotine is a strong psychoactive and addictive compound found in tobacco. Use of nicotine in the form of smoking, vaping or other less common methods during pregnancy has been shown to be related to poor health conditions, including cognitive problems, in babies and children. However, mechanisms of such cognitive deficits are not fully understood. In this study we analyzed hippocampus dependent cognitive deficits using a mouse model of developmental nicotine exposure. Pregnant dams were exposed to nicotine and experiments were performed in one month old offspring. Our results show that nicotine exposure did not affect locomotor behavior in mice. Hippocampus dependent working memory and object location memory were diminished in nicotine exposed mice. Furthermore, acetylcholine levels in the hippocampus of nicotine exposed mice were reduced along with reduced activity of acetylcholinesterase enzyme. Analysis of transcripts for proteins that are known to regulate acetylcholine levels revealed a decline in mRNA levels of high affinity choline transporters in the hippocampus of nicotine exposed mice but those of vesicular acetylcholine transporter, choline acetyltransferase, and α7-nicotinic acetylcholine receptors were not altered. These results suggest that developmental nicotine exposure impairs hippocampus dependent memory forms and this effect is likely mediated by altered cholinergic function.

Nicotinic receptor targeting in physiological and environmental vulnerability: A whole of biosphere perspective

We propose a biosphere model of convergent interactions between nicotine and neonicotinoids (neonics) within a related framework of nicotinic receptor targeting agents (NrTA) across the globe. We explore how rising global trends in the use nicotine as well as neonics impacts vulnerability, within and across species, and posit that evolutionary conservation at the nicotinic acetylcholine receptor (nAChR) provides an operational strategy map for pathogens and disease. Furthermore, we examine the effects of NrTA exposure on balance within extant and developing ecological niches, food chains, and human societies. We advocate for a global strategy for biomonitoring across agriculture, wildlife, and human centers. Such a strategy would relate emergent pathogenic and infectious diseases, amongst others, along a tractable biological stress pathway. This new framework aims to better prepare society in the face of emergent pandemics through 1. identifying primary chemical drivers that can impact emergent diseases; 2. outlining data-driven strategy options for health and environmental policy decision makers.

Regulation of habenular G-protein gamma 8 on learning and memory via modulation of the central acetylcholine system

Guanine nucleotide binding protein (G protein) gamma 8 (Gng8) is a subunit of G proteins and expressed in the medial habenula (MHb) and interpeduncular nucleus (IPN). Recent studies have demonstrated that Gng8 is involved in brain development; however, the roles of Gng8 on cognitive function have not yet been addressed. In the present study, we investigated the expression of Gng8 in the brain and found that Gng8 was predominantly expressed in the MHb-IPN circuit of the mouse brain. We generated Gng8 knockout (KO) mice by CRISPR/Cas9 system in order to assess the role of Gng8 on cognitive function. Gng8 KO mice exhibited deficiency in learning and memory in passive avoidance and Morris water maze tests. In addition, Gng8 KO mice significantly reduced long-term potentiation (LTP) in the hippocampus compared to that of wild-type (WT) mice. Furthermore, we observed that levels of acetylcholine (ACh) and choline acetyltransferase (ChAT) in the MHb and IPN of Gng8 KO mice were significantly decreased, compared to WT mice. The administration of nAChR α4β2 agonist A85380 rescued memory impairment in the Gng8 KO mice, suggesting that Gng8 regulates cognitive function via modulation of cholinergic activity. Taken together, Gng8 is a potential therapeutic target for memory-related diseases and/or neurodevelopmental diseases.

The effect of electronic cigarettes exposure on learning and memory functions: behavioral and molecular analysis

Objective: Electronic cigarettes (ECIGs) are battery-powered devices that emit vaporized solutions for the user to inhale. ECIGs are marketed as a less harmful alternative to combustible cigarettes. The current study examined the effects of ECIG aerosol exposure on learning and memory, expression of brain derived neurotrophic factor (BDNF), and the activity of antioxidant enzymes in the hippocampus.Methods: Male Wistar rats were exposed to ECIG aerosol, by a whole-body exposure system, 1 h/day for 1 week, 4 weeks, and 12 weeks. Spatial learning and memory were tested using the Radial Arm Water Maze (RAWM). Hippocampal BDNF protein level, and oxidative stress biomarkers (GPx, SOD, GSH, GSSG, GSH/GSSG ratio) were also assessed.Results: ECIG aerosol exposure for 4 and 12 weeks impaired both short- and long- term memory and induced reductions in the hippocampus BDNF, SOD and GPx activities, and GSH/GSSG ratio (p < 0.05). No changes in any examined biomarkers were observed after 1-week exposure to ECIG aerosol (p > 0.05).Conclusions: ECIG aerosol exposure impaired functional memory and elicited changes in brain chemistry that are consistent with reduced function and oxidative stress.

Role of the purinergic P2Y2 receptor in hippocampal function in mice

OBJECTIVE

The aim of this study is to investigate the role of the purinergic P2Y2 receptor in learning and memory processes.

MATERIALS AND METHODS

Behavioral, electrophysiological, and biochemical tests of memory function were conducted in P2Y2 receptor knockout (P2Y2R-KO) mice, and the findings were compared to those of wild-type mice with the help of unpaired Student's t-test.

RESULTS

The findings of the behavioral Y-maze test showed that the P2Y2R-KO mice had impaired memory and cognitive function. Electrophysiological studies on paired-pulse facilitation showed that glutamate release was higher in the P2Y2R-KO mice than in the WT mice. Furthermore, PCR and Western blot analysis revealed that the mRNA and protein expression of acetylcholinesterase E (AChE) and alpha-7 nicotinic acetylcholine receptors (α7 nAChRs) were increased in the hippocampus of P2Y2R-KO mice.

CONCLUSIONS

The findings of this study indicate that P2Y2 receptors are important regulators of both glutamatergic and cholinergic systems in the hippocampus.

Nicotinic Acetylcholine Receptor Agonists for the Treatment of Alzheimer's Dementia: An Update

A significant portion of the clinical phenotype observed in Alzheimer's disease (AD) occurs through nicotinic acetylcholine receptors (nAChRs). Degeneration of cholinergic neurons, combined with aberrant nAChR expression and activation partially through amyloid-beta peptide (Aβ)-nAChR leads to upregulation of pro-inflammatory pathways and subsequently the progressive cognitive decline of AD. Interestingly, the cholinergic anti-inflammatory pathway is also mediated through nAChR particularly α7 nAChR. Thus, agonists of these receptors will likely exert pro-cognitive benefits through multiple mechanisms including stimulating the cholinergic pathway, modulating inflammation, and buffering the effects of amyloid. Despite this promising theoretical use, trials thus far have been complicated by adverse effects or minimal improvement. This review will provide an update on several pharmacological nAChR agonists tested in clinical trials and reasons that further investigation of nAChR agonists is merited.

IMPLICATIONS

nAChRs have consistently presented a promising theoretical use in the treatment of AD; however, trials thus far have been complicated by adverse effects or minimal improvement. This review will provide an update on several pharmacological nAChR agonists trialed and reasons that further investigation of nAChR agonists is merited.

NAChR α4β2 Subtype and their Relation with Nicotine Addiction, Cognition, Depression and Hyperactivity Disorder

BACKGROUND

Neuronal α4β2 nAChRs are receptors involved in the role of neurotransmitters regulation and release, and this ionic channel participates in biological process of memory, learning and attention. This work aims to review the structure and functioning of the α4β2 nAChR emphasizing its role in the treatment of associated diseases like nicotine addiction and underlying pathologies such as cognition, depression and attention-deficit hyperactivity disorder.

METHODS

The authors realized extensive bibliographic research using the descriptors "Nicotine Receptor α4β2" and "cognition", "depression", "attention-deficit hyperactivity disorder", besides cross-references of the selected articles and after analysis of references in the specific literature.

RESULTS

As results, it was that found 179 relevant articles presenting the main molecules with affinity to nAChR α4β2 related to the cited diseases. The α4β2 nAChR subtype is a remarkable therapeutic target since this is the most abundant receptor in the central nervous system.

CONCLUSION

In summary, this review presents perspectives on the pharmacology and therapeutic targeting of α4β2 nAChRs for the treatment of cognition and diseases like nicotine dependence, depression and attention-deficit hyperactivity disorder.

Sex differences in the nicotinic excitation of principal neurons within the developing hippocampal formation

The hippocampal formation (HF) plays an important role to facilitate higher order cognitive functions. Cholinergic activation of heteromeric nicotinic acetylcholine receptors (nAChRs) within the HF is critical for the normal development of principal neurons within this brain region. However, previous research investigating the expression and function of heteromeric nAChRs in principal neurons of the HF is limited to males or does not differentiate between the sexes. We used whole-cell electrophysiology to show that principal neurons in the CA1 region of the female mouse HF are excited by heteromeric nAChRs throughout postnatal development, with the greatest response occurring during the first two weeks of postnatal life. Excitability responses to heteromeric nAChR stimulation were also found in principal neurons in the CA3, dentate gyrus, subiculum, and entorhinal cortex layer VI (ECVI) of young postnatal female HF. A direct comparison between male and female mice found that principal neurons in ECVI display greater heteromeric nicotinic passive and active excitability responses in females. This sex difference is likely influenced by the generally more excitable nature of ECVI neurons from female mice, which display a higher resting membrane potential, greater input resistance, and smaller afterhyperpolarization potential of medium duration (mAHP). These findings demonstrate that heteromeric nicotinic excitation of ECVI neurons differs between male and female mice during a period of major circuitry development within the HF, which may have mechanistic implications for known sex differences in the development and function of this cognitive brain region.

Nicotine affects ethanol-conditioned taste, but not place, aversion in a simultaneous conditioning procedure

The conditioned taste aversion (CTA) induced by ethanol is a key factor limiting ethanol intake. Nicotine, a drug co-consumed with ethanol, may decrease this aversion by modulating the unconditioned effects of ethanol or by disrupting the association between ethanol and its associated cues. This study analyzed ethanol-induced CTA and conditioned place aversion (CPA) in Long-Evans rats with subchronic exposure to nicotine. The rats were treated with nicotine (0.0 or 0.4 mg/kg) three times before conditioning (on lickometer training sessions 3, 4, and 5) and across conditioning days. During the conditioning the rats were given ethanol (1.3 g/kg) preceded and followed by presentation of a taste (NaCl) and tactile (rod or hole floors) conditioned stimulus (CS+), respectively. On CS- conditioning days, the rats were given vehicle and exposed to alternative stimuli. Three CTA and CPA testing sessions were then conducted. It was found that nicotine reduced ethanol-induced CTA and enhanced locomotor activity, but did not significantly modify the magnitude of ethanol-induced CPA. The effects of nicotine on CTA were observed during both conditioning and testing sessions, and were specific to the NaCl CS+, having no effect on reactivity to water. The dissociation between the effect of nicotine on ethanol-induced CTA and CPA suggests that nicotine does not alter ethanol's motivational properties by generally increasing its positive rewarding effects, nor does it blunt all aversive-like responses to this drug. Instead, nicotine may impede ethanol-induced CTA induced by ethanol by disrupting the neural underpinnings of this specific form of associative learning.

Similar nicotinic excitability responses across the developing hippocampal formation are regulated by small-conductance calcium-activated potassium channels

The hippocampal formation forms a cognitive circuit that is critical for learning and memory. Cholinergic input to nicotinic acetylcholine receptors plays an important role in the normal development of principal neurons within the hippocampal formation. However, the ability of nicotinic receptors to stimulate principal neurons across all regions of the developing hippocampal formation has not been determined. We show in this study that heteromeric nicotinic receptors mediate direct inward current and depolarization responses in principal neurons across the hippocampal formation of the young postnatal mouse. These responses were found in principal neurons of the CA1, CA3, dentate gyrus, subiculum, and entorhinal cortex layer VI, and they varied in magnitude across regions with the greatest responses occurring in the subiculum and entorhinal cortex. Despite this regional variation in the magnitude of passive responses, heteromeric nicotinic receptor stimulation increased the excitability of active principal neurons by a similar amount in all regions. Pharmacological experiments found this similar excitability response to be regulated by small-conductance calcium-activated potassium (SK) channels, which exhibited regional differences in their influence on neuron activity that offset the observed regional differences in passive nicotinic responses. These findings demonstrate that SK channels play a role to coordinate the magnitude of heteromeric nicotinic excitability responses across the hippocampal formation at a time when nicotinic signaling drives the development of this cognitive brain region. This coordinated input may contribute to the normal development, synchrony, and maturation of the hippocampal formation learning and memory network. NEW & NOTEWORTHY This study demonstrates that small-conductance calcium-activated potassium channels regulate similar-magnitude excitability responses to heteromeric nicotinic acetylcholine receptor stimulation in active principal neurons across multiple regions of the developing mouse hippocampal formation. Given the importance of nicotinic neurotransmission for the development of principal neurons within the hippocampal formation, this coordinated excitability response is positioned to influence the normal development, synchrony, and maturation of the hippocampal formation learning and memory network.

Stress-Induced Synaptic Dysfunction and Neurotransmitter Release in Alzheimer's Disease: Can Neurotransmitters and Neuromodulators be Potential Therapeutic Targets?

The communication between neurons at synaptic junctions is an intriguing process that monitors the transmission of various electro-chemical signals in the central nervous system. Albeit any aberration in the mechanisms associated with transmission of these signals leads to loss of synaptic contacts in both the neocortex and hippocampus thereby causing insidious cognitive decline and memory dysfunction. Compelling evidence suggests that soluble amyloid-β (Aβ) and hyperphosphorylated tau serve as toxins in the dysfunction of synaptic plasticity and aberrant neurotransmitter (NT) release at synapses consequently causing a cognitive decline in Alzheimer's disease (AD). Further, an imbalance between excitatory and inhibitory neurotransmission systems induced by impaired redox signaling and altered mitochondrial integrity is also amenable for such abnormalities. Defective NT release at the synaptic junction causes several detrimental effects associated with altered activity of synaptic proteins, transcription factors, Ca2+ homeostasis, and other molecules critical for neuronal plasticity. These detrimental effects further disrupt the normal homeostasis of neuronal cells and thereby causing synaptic loss. Moreover, the precise mechanistic role played by impaired NTs and neuromodulators (NMs) and altered redox signaling in synaptic dysfunction remains mysterious, and their possible interlink still needs to be investigated. Therefore, this review elucidates the intricate role played by both defective NTs/NMs and altered redox signaling in synaptopathy. Further, the involvement of numerous pharmacological approaches to compensate neurotransmission imbalance has also been discussed, which may be considered as a potential therapeutic approach in synaptopathy associated with AD.

TRPV1 regulates excitatory innervation of OLM neurons in the hippocampus

TRPV1 is an ion channel activated by heat and pungent agents including capsaicin, and has been extensively studied in nociception of sensory neurons. However, the location and function of TRPV1 in the hippocampus is debated. We found that TRPV1 is expressed in oriens-lacunosum-moleculare (OLM) interneurons in the hippocampus, and promotes excitatory innervation. TRPV1 knockout mice have reduced glutamatergic innervation of OLM neurons. When activated by capsaicin, TRPV1 recruits more glutamatergic, but not GABAergic, terminals to OLM neurons in vitro. When TRPV1 is blocked, glutamatergic input to OLM neurons is dramatically reduced. Heterologous expression of TRPV1 also increases excitatory innervation. Moreover, TRPV1 knockouts have reduced Schaffer collateral LTP, which is rescued by activating OLM neurons with nicotine—via α2β2-containing nicotinic receptors—to bypass innervation defects. Our results reveal a synaptogenic function of TRPV1 in a specific interneuron population in the hippocampus, where it is important for gating hippocampal plasticity.

The role of TRPV1 in the CNS is not fully understood. Here the authors show that TRPV1 is expressed specifically in somatostatin-positive OLM interneurons of the hippocampus, where it promotes excitatory innervation of these cells.

Compromised neuroplasticity in cigarette smokers under nicotine withdrawal is restituted by the nicotinic α4β2-receptor partial agonist varenicline

Nicotine modulates neuroplasticity and improves cognitive functions in animals and humans. In the brain of smoking individuals, calcium-dependent plasticity induced by non-invasive brain stimulation methods such as transcranial direct current stimulation (tDCS) and paired associative stimulation (PAS) is impaired by nicotine withdrawal, but partially re-established after nicotine re-administration. In order to investigate the underlying mechanism further, we tested the impact of the α₄β₂-nicotinic receptor partial agonist varenicline on focal and non-focal plasticity in smokers during nicotine withdrawal, induced by PAS and tDCS, respectively. We administered low (0.3 mg) and high (1.0 mg) single doses of varenicline or placebo medication before stimulation over the left motor cortex of 20 healthy smokers under nicotine withdrawal. Motor cortex excitability was monitored by single-pulse transcranial magnetic stimulation-induced motor evoked potential amplitudes for 36 hours after plasticity induction. Stimulation-induced plasticity was absent under placebo medication, whereas it was present in all conditions under high dose. Low dose restituted only tDCS-induced non-focal plasticity, producing no significant impact on focal plasticity. High dose varenicline also prolonged inhibitory plasticity. These results are comparable to the impact of nicotine on withdrawal-related impaired plasticity in smokers and suggest that α₄β₂ nicotinic receptors are relevantly involved in plasticity deficits and restitution in smokers.

Postsynaptic nicotinic acetylcholine receptors facilitate excitation of developing CA1 pyramidal neurons

The hippocampus plays a key role in learning and memory. The normal development and mature function of hippocampal networks supporting these cognitive functions depends on afferent cholinergic neurotransmission mediated by nicotinic acetylcholine receptors. Whereas it is well-established that nicotinic receptors are present on GABAergic interneurons and on glutamatergic presynaptic terminals within the hippocampus, the ability of these receptors to mediate postsynaptic signaling in pyramidal neurons is not well understood. We use whole cell electrophysiology to show that heteromeric nicotinic receptors mediate direct inward currents, depolarization from rest and enhanced excitability in hippocampus CA1 pyramidal neurons of male mice. Measurements made throughout postnatal development provide a thorough developmental profile for these heteromeric nicotinic responses, which are greatest during the first 2 wk of postnatal life and decrease to low adult levels shortly thereafter. Pharmacological experiments show that responses are blocked by a competitive antagonist of α4β2* nicotinic receptors and augmented by a positive allosteric modulator of α5 subunit-containing receptors, which is consistent with expression studies suggesting the presence of α4β2 and α4β2α5 nicotinic receptors within the developing CA1 pyramidal cell layer. These findings demonstrate that functional heteromeric nicotinic receptors are present on CA1 pyramidal neurons during a period of major hippocampal development, placing these receptors in a prime position to play an important role in the establishment of hippocampal cognitive networks.

Astrocytic and microglial nicotinic acetylcholine receptors: an overlooked issue in Alzheimer's disease

It is increasingly recognized that astrocytes and microglia-associated dysfunction contribute to AD pathology. In addition, glial nicotinic acetylcholine receptors (nAChRs) play a role in AD-related phenomena, such as neuron survival, synaptic plasticity, and memory. From mechanistic point of view, the glial regulation of pro-inflammatory cytokines, as common contributors in AD, is modulated by nAChRs. Astrocytic and microglial nAChRs contribute to Aβ metabolism, including Aβ phagocytosis and degradation as well as Aβ-related oxidative stress and neurotoxicity. These receptors are also involved in neurotransmission and gliotransmission through indirect interaction with N-Methyl-D-aspartate (NMDA) and a-amino-3-hydroxy-5-methyl-4 isoxazolepropionic acid (AMPA) receptors as well as gamma-aminobutyric acid (GABA) and intracellular calcium regulation. In addition, glial nAChRs participate in trophic factors-induced neuroprotection. This review gathers the most recent advances along with the previous data on astrocytic and microglial nAChRs role in AD pathogenesis.

Differential modulation of GABAA and NMDA receptors by α7-nicotinic receptor desensitization in cultured rat hippocampal neurons

AIM

To explore the modulatory effect of desensitized α7-containing nicotinic receptors (α7-nAChRs) on excitatory and inhibitory amino acid receptors in cultured hippocampal neurons and to identify the mechanism underlying this effect.

METHODS

Whole-cell patch-clamp recordings were performed on cultured rat hippocampal neurons to measure α7-nAChR currents and to determine the role of desensitized α7-nAChRs on brain amino acid receptor activity.

RESULTS

Pulse and perfusion applications of the α7-nAChR agonist choline were applied to induce different types of α7-nAChR desensitization in cultured hippocampal neurons. After a brief choline pulse, α7-nAChR was desensitized as a result of receptor activation, which reduced the response of the A type γ-aminobutyric acid (GABAA) receptor to its agonist, muscimol, and enhanced the response of the NMDA receptor to its agonist NMDA. By contrast, the responses of glycine or AMPA receptors to their agonists, glycine or AMPA, respectively, were not affected. Pretreatment with the α7-nAChR antagonist methyllycaconitine (MLA, 10 nmol/L) blocked the choline-induced negative modulation of the GABAA receptor and the positive modulation of the NMDA receptor. The regulation of the GABAA and NMDA receptors was confirmed using another type of α7-nAChR desensitization, which was produced by a low concentration of choline perfusion. The negative modulation of the GABAA receptor was characterized by choline-duration dependency and intracellular calcium dependency, but the positive modulation of the NMDA receptor was not associated with cytoplasmic calcium.

CONCLUSION

Brain GABAA and NMDA receptors are modulated negatively and positively, respectively, by desensitized α7-nAChR as a result of choline pretreatment in cultured hippocampal neurons.

The Oncogenic Functions of Nicotinic Acetylcholine Receptors

Nicotinic acetylcholine receptors (nAChRs) are ion channels that are expressed in the cell membrane of all mammalian cells, including cancer cells. Recent findings suggest that nAChRs not only mediate nicotine addiction in the brain but also contribute to the development and progression of cancers directly induced by nicotine and its derived carcinogenic nitrosamines whereas deregulation of the nAChRs is observed in many cancers, and genome-wide association studies (GWAS) indicate that SNPs nAChRs associate with risks of lung cancers and nicotine addiction. Emerging evidences suggest nAChRs are posited at the central regulatory loops of numerous cell growth and prosurvival signal pathways and also mediate the synthesis and release of stimulatory and inhibitory neurotransmitters induced by their agonists. Thus nAChRs mediated cell signaling plays an important role in stimulating the growth and angiogenic and neurogenic factors and mediating oncogenic signal transduction during cancer development in a cell type specific manner. In this review, we provide an integrated view of nAChRs signaling in cancer, heightening on the oncogenic properties of nAChRs that may be targeted for cancer treatment.

Modulation of AMPA receptor mediated current by nicotinic acetylcholine receptor in layer I neurons of rat prefrontal cortex

Layer I neurons in the prefrontal cortex (PFC) exhibit extensive synaptic connections with deep layer neurons, implying their important role in the neural circuit. Study demonstrates that activation of nicotinic acetylcholine receptors (nAChRs) increases excitatory neurotransmission in this layer. Here we found that nicotine selectively increased the amplitude of AMPA receptor (AMPAR)-mediated current and AMPA/NMDA ratio, while without effect on NMDA receptor-mediated current. The augmentation of AMPAR current by nicotine was inhibited by a selective α7-nAChR antagonist methyllycaconitine (MLA) and intracellular calcium chelator BAPTA. In addition, nicotinic effect on mEPSC or paired-pulse ratio was also prevented by MLA. Moreover, an enhanced inward rectification of AMPAR current by nicotine suggested a functional role of calcium permeable and GluA1 containing AMPAR. Consistently, nicotine enhancement of AMPAR current was inhibited by a selective calcium-permeable AMPAR inhibitor IEM-1460. Finally, the intracellular inclusion of synthetic peptide designed to block GluA1 subunit of AMPAR at CAMKII, PKC or PKA phosphorylation site, as well as corresponding kinase inhibitor, blocked nicotinic augmentation of AMPA/NMDA ratio. These results have revealed that nicotine increases AMPAR current by modulating the phosphorylation state of GluA1 which is dependent on α7-nAChR and intracellular calcium.

Nicotine recruits glutamate receptors to postsynaptic sites

Cholinergic neurons project throughout the nervous system and activate nicotinic receptors to modulate synaptic function in ways that shape higher order brain function. The acute effects of nicotinic signaling on long-term synaptic plasticity have been well-characterized. Less well understood is how chronic exposure to low levels of nicotine, such as those encountered by habitual smokers, can alter neural connections to promote addiction and other lasting behavioral effects. We show here that chronic exposure of hippocampal neurons in culture to low levels of nicotine recruits AMPA and NMDA receptors to the cell surface and sequesters them at postsynaptic sites. The receptors include GluA2-containing AMPA receptors, which are responsible for most of the excitatory postsynaptic current mediated by AMPA receptors on the neurons, and include NMDA receptors containing GluN1 and GluN2B subunits. Moreover, we find that the nicotine treatment also increases expression of the presynaptic component synapsin 1 and arranges it in puncta juxtaposed to the additional AMPA and NMDA receptor puncta, suggestive of increases in synaptic contacts. Consistent with increased synaptic input, we find that the nicotine treatment leads to an increase in the excitatory postsynaptic currents mediated by AMPA and NMDA receptors. Further, the increases skew the ratio of excitatory-to-inhibitory input that the cell receives, and this holds both for pyramidal neurons and inhibitory neurons in the hippocampal CA1 region. The GluN2B-containing NMDA receptor redistribution at synapses is associated with a significant increase in GluN2B phosphorylation at Tyr1472, a site known to prevent GluN2B endocytosis. These results suggest that chronic exposure to low levels of nicotine not only alters functional connections but also is likely to change excitability levels across networks. Further, it may increase the propensity for synaptic plasticity, given the increase in synaptic NMDA receptors.

The multiple roles of the α7 nicotinic acetylcholine receptor in modulating glutamatergic systems in the normal and diseased nervous system

Neuronal nicotinic acetylcholine receptors (nAChRs) play an important role in a variety of modulatory and regulatory processes including neurotransmitter release and synaptic transmission in various brain regions of the central nervous system (CNS). Glutamate is the principal excitatory neurotransmitter in the brain and the glutamatergic system participates in the pathophysiology of several neuropsychiatric disorders. Underpinning the importance of nAChRs, many studies demonstrated that nAChRs containing the α7 subunit facilitate glutamate release. Here, we review the currently available body of experimental evidence pertaining to α7 subunit containing nAChRs in their contribution to the modulation of glutamatergic neurotransmission, and we highlight the role of α7 in synaptic plasticity, the morphological and functional maturation of the glutamatergic system and therefore its important contribution in the modulation of neural circuits of the CNS.

Activities of nicotinic acetylcholine receptors modulate neurotransmission and synaptic architecture

The cholinergic system is involved in a broad spectrum of brain function, and its failure has been implicated in Alzheimer's disease. Acetylcholine transduces signals through muscarinic and nicotinic acetylcholine receptors, both of which influence synaptic plasticity and cognition. However, the mechanisms that relate the rapid gating of nicotinic acetylcholine receptors to persistent changes in brain function have remained elusive. Recent evidence indicates that nicotinic acetylcholine receptors activities affect synaptic morphology and density, which result in persistent rearrangements of neural connectivity. Further investigations of the relationships between nicotinic acetylcholine receptors and rearrangements of neural circuitry in the central nervous system may help understand the pathogenesis of Alzheimer's disease.

Hippocampal changes produced by overexpression of the human CHRNA5/A3/B4 gene cluster may underlie cognitive deficits rescued by nicotine in transgenic mice

Addiction involves long-lasting maladaptive changes including development of disruptive drug-stimuli associations. Nicotine-induced neuroplasticity underlies the development of tobacco addiction but also, in regions such as the hippocampus, the ability of this drug to enhance cognitive capabilities. Here, we propose that the genetic locus of susceptibility to nicotine addiction, the CHRNA5/A3/B4 gene cluster, encoding the α5, α3 and β4 subunits of the nicotinic acetylcholine receptors (nAChRs), may influence nicotine-induced neuroadaptations. We have used transgenic mice overexpressing the human cluster (TgCHRNA5/A3/B4) to investigate hippocampal structure and function in genetically susceptible individuals. TgCHRNA5/A3/B4 mice presented a marked reduction in the dendrite complexity of CA1 hippocampal pyramidal neurons along with an increased dendritic spine density. In addition, TgCHRNA5/A3/B4 exhibited increased VGLUT1/VGAT ratio in the CA1 region, suggesting an excitatory/inhibitory imbalance. These hippocampal alterations were accompanied by a significant impairment in short-term novelty recognition memory. Interestingly, chronic infusion of nicotine (3.25 mg/kg/d for 7 d) was able to rescue the reduced dendritic complexity, the excitatory/inhibitory imbalance and the cognitive impairment in TgCHRNA5/A3/B4. Our results suggest that chronic nicotine treatment may represent a compensatory strategy in individuals with altered expression of the CHRNA5/A3/B4 region.

The role of nicotinic receptors in shaping and functioning of the glutamatergic system: a window into cognitive pathology

The involvement of the cholinergic system in learning, memory and attention has long been recognized, although its neurobiological mechanisms are not fully understood. Recent evidence identifies the endogenous cholinergic signaling via nicotinic acetylcholine receptors (nAChRs) as key players in determining the morphological and functional maturation of the glutamatergic system. Here, we review the available experimental and clinical evidence of nAChRs contribution to the establishment of the glutamatergic system, and therefore to cognitive function. We provide some clues of the putative underlying molecular mechanisms and discuss recent human studies that associate genetic variability of the genes encoding nAChR subunits with cognitive disorders. Finally, we discuss the new avenues to therapeutically targeting nAChRs in persons with cognitive dysfunction for which the α7-nAChR subunit is an important etiological mechanism.

Role of neuronal nicotinic acetylcholine receptors (nAChRs) on learning and memory in zebrafish

RATIONALE

Neuronal nicotinic acetylcholine receptors (nAChRs) play a modulatory role in cognition, and zebrafish provide a preclinical model to study learning and memory.

OBJECTIVES

We investigated the effect of nicotine (NIC) and some new cytisine-derived partial agonists (CC4 and CC26) on spatial memory in zebrafish using a rapid assay on T-maze task. The role of α4/α6β2 and the α7 nAChRs in NIC-induced memory enhancement was evaluated using selective nAChR antagonists.

RESULTS

Low and high doses of NIC, cytisine (CYT), CC4 and CC26 respectively improved and worsened the mean running time, showing an inverted U dose-response function. The effective dose (ED50) (×10⁻⁵ mg/kg) was 0.4 for CC4, 4.5 for CYT, 140 for NIC and 200 for CC26. NIC-induced cognitive enhancement was reduced by the selective nAChR subtype antagonists: methyllycaconitine (MLA) for α7, α-conotoxin (MII) for α6β2, dihydro-β-erythroidine (DhβE) for α4β2, the nonselective antagonist mecamylamine (MEC) and the muscarinic antagonist scopolamine (SCOP), with DhβE being more active than MLA or MII. All the partial agonists blocked the cognitive enhancement. The improvement with the maximal active dose of each partial agonist was blocked by low doses of DhβE (0.001 mg/kg) and MII (0.01 mg/kg). MLA reduced the effects of CC26 and CC4 at doses of 0.01 and 1 mg/kg, respectively, but did not antagonize CYT-induced memory improvement at any of the tested dose. No change in swimming activity was observed.

CONCLUSIONS

Our findings demonstrate that zebrafish make a useful model for the rapid screening of the effect of new α4β2 nAChR compounds on spatial memory.

A novel mechanism for nicotinic potentiation of glutamatergic synapses

Selective strengthening of specific glutamatergic synapses in the mammalian hippocampus is critical for encoding new memories. This is most commonly achieved by input-specific Hebbian-type plasticity involving glutamate-dependent coincident presynaptic and postsynaptic depolarization. Our results demonstrate a novel mechanism by which nicotinic signaling, independently of coincident fast glutamatergic transmission, increases synaptic strength in the hippocampus. Electrophysiological recordings from rat hippocampal neurons in culture revealed that 1-3 h of exposure to 1 μm nicotine, even with action potentials being blocked, produced increases in both the frequency and amplitude of miniature EPSCs. Possible mechanisms were analyzed both in mouse organotypic slice culture and in rat cell culture by inducing the cells to express super-ecliptic pHluorin-tagged GluA1-containing AMPA receptors, which fluoresce only on the cell surface. Pharmacological and genetic manipulation of the cells, in combination with fluorescence-recovery-after-photobleaching experiments, revealed that nicotine, acting through α7-containing nicotinic acetylcholine receptors on the postsynaptic neuron, induces the stabilization and accumulation of GluA1-containing AMPA receptors on dendritic spines. The process relies on intracellular calcium signaling, PDZ [postsynaptic density-95 (PSD-95)/Discs large (Dlg)/zona occludens-1 (ZO-1)] interactions with members of the PSD-95 family, and lateral diffusion of the GluA1 receptors on the cell surface. These findings define a new avenue by which nicotinic signaling modulates synaptic mechanisms thought to subserve learning and memory.

Chronic administration of nicotine enhances NMDA-activated currents in the prefrontal cortex and core part of the nucleus accumbens of rats

Nicotine is an addictive substance of tobacco. It has been suggested that nicotine acts on glutamatergic (N-methyl-d-aspartate, NMDA) neurotransmission affecting dopamine release in the mesocorticolimbic system. This effect is reflected in neuroadaptative changes that can modulate neurotransmission in the prefrontal cortex (PFC) and nucleus accumbens (NAcc) core (cNAcc) and shell (sNAcc) regions. We evaluated the effect of chronic administration of nicotine (4.23 mg/kg/day for 14 days) on NMDA activated currents in dissociated neurons from the PFC, and NAcc (from core and shell regions). We assessed nicotine blood levels by mass spectrophotometry and we confirmed that nicotine increases locomotor activity. An electrophysiological study showed an increase in NMDA currents in neurons from the PFC and core part of the NAcc in animals treated with nicotine compared to those of control rats. No change was observed in neurons from the shell part of the NAcc. The enhanced glutamatergic activity observed in the neurons of rats with chronic administration of nicotine may explain the increased locomotive activity also observed in such rats. To assess one of the possible causes of increased NMDA currents, we used magnesium, to block NMDA receptor that contains the NR2B subunit. If there is a change in percent block of NMDA currents, it means that there is a possible change in expression of NMDA receptor subunits. Our results showed that there is no difference in the blocking effect of magnesium on the NMDA currents. The magnesium lacks of effect after nicotinic treatment suggests that there is no change in expression of NR2B subunit of NMDA receptors, then, the effect of nicotine treatment on amplitude of NMDA currents may be due to an increase in the quantity of receptors or to a change in the unitary conductance, rather than a change in the expression of the subunits that constitute it.

Memory-improving activity of Melissa officinalis extract in naïve and scopolamine-treated rats

Melissa officinalis L. (Labiatae) traditionally used in treating neurological disorders has also been identified as a memory-enhancing herb. The extract of M. officinalis has a cholinergic property. The role of basal forebrain cholinergic neurons, the neurons that are destroyed in Alzheimer's disease (AD), in learning and memory, is also well known. The aim of this study is to investigate the role of cholinergic system on the memory improving activity of M. officinalis extract. The leaves of M. officinalis were extracted with ethanol 80% using the maceration method. Rats received intra-peritoneal injections of M. officinalis extract in different doses (50-400 mg/kg) alone or in combination with scopolamine (1 mg/kg) before being trained in a Morris water maze (MWM) in a single-day training protocol. After training, the acetylcholinesterase enzyme (AChE) activity was measured in the hippocampus. Administration of M. officinalis extract (200 mg/kg) could significantly enhance learning and memory of naïve rats (p<0.001) and significantly ameliorate scopolamine-induced learning deficit, but the effect of the extract was not dose dependent, and doses above 200 mg/kg could neither enhance memory in naïve rats nor reverse scopolamine-induced memory impairment. Also, inhibition of AChE activity was observed in both naïve and scopolamine-induced memory-impaired rats. These results suggest that M. officinalis can improve memory and that the cholinergic property of the extract may contribute to the memory-improving effects observed in this study. Then M. officinalis extract has potential therapeutic value in alleviating certain memory impairment observed in AD.

The role of astrocytes in the regulation of synaptic plasticity and memory formation

Astrocytes regulate synaptic transmission and play a role in the formation of new memories, long-term potentiation (LTP), and functional synaptic plasticity. Specifically, astroglial release of glutamate, ATP, and cytokines likely alters the survivability and functioning of newly formed connections. Among these pathways, regulation of glutamate appears to be most directly related to the promotion of LTP, which is highly dependent on the synchronization of synaptic receptors through the regulation of excitatory postsynaptic potentials. Moreover, regulation of postsynaptic glutamate receptors, particularly AMPA receptors, is dependent on signaling by ATP synthesized in astrocytes. Finally, cytokine signaling is also implicated in regulating LTP, but is likely most important in plasticity following tissue damage. Despite the role of these signaling factors in regulating LTP and functional plasticity, an integrative model of these factors has not yet been elucidated. In this review, we seek to summarize the current body of evidence on astrocytic mechanisms for regulation of LTP and functional plasticity, and provide an integrative model of the processes.

α7 Nicotinic receptor-mediated astrocytic gliotransmitter release: Aβ effects in a preclinical Alzheimer's mouse model

It is now recognized that astrocytes participate in synaptic communication through intimate interactions with neurons. A principal mechanism is through the release of gliotransmitters (GTs) such as ATP, D-serine and most notably, glutamate, in response to astrocytic calcium elevations. We and others have shown that amyloid-β (Aβ), the toxic trigger for Alzheimer's disease (AD), interacts with hippocampal α7 nicotinic acetylcholine receptors (nAChRs). Since α7nAChRs are highly permeable to calcium and are expressed on hippocampal astrocytes, we investigated whether Aβ could activate astrocytic α7nAChRs in hippocampal slices and induce GT glutamate release. We found that biologically-relevant concentrations of Aβ1-42 elicited α7nAChR-dependent calcium elevations in hippocampal CA1 astrocytes and induced NMDAR-mediated slow inward currents (SICs) in CA1 neurons. In the Tg2576 AD mouse model for Aβ over-production and accumulation, we found that spontaneous astrocytic calcium elevations were of higher frequency compared to wildtype (WT). The frequency and kinetic parameters of AD mice SICs indicated enhanced gliotransmission, possibly due to increased endogenous Aβ observed in this model. Activation of α7nAChRs on WT astrocytes increased spontaneous inward currents on pyramidal neurons while α7nAChRs on astrocytes of AD mice were abrogated. These findings suggest that, at an age that far precedes the emergence of cognitive deficits and plaque deposition, this mouse model for AD-like amyloidosis exhibits augmented astrocytic activity and glutamate GT release suggesting possible repercussions for preclinical AD hippocampal neural networks that contribute to subsequent cognitive decline.

Nicotine-induced impairments of spatial cognition and long-term potentiation in adolescent male rats

The aim of the present study was to investigate whether cognitive behavioral impairment, induced by nicotine in offspring rats, was associated with the alteration of hippocampal short-term potentiation (STP) and long-term potentiation (LTP) and to discuss the potential underlying mechanism. Young adult offspring rats were randomly divided into three groups. The groups include: control group (CC), nicotine group 1 (NC), in which their mothers received nicotine from gestational day 3 (GD3) to GD18, and nicotine group 2 (CN), in which young adult offspring rats received nicotine from postnatal day 42 (PD42) to PD56. Morris water maze (MWM) test was performed and then field excitatory postsynaptic potentials elicited by the stimulation of perforant pathway were recorded in the hippocampal dentate gyrus region. The results of the MWM test showed that learning and memory were impaired by either prenatal or postnatal nicotine exposure. In addition, it was found that there was no statistical difference of the MWM data between both nicotine treatments. In the electrophysiological test, LTP and STP were significantly inhibited in both NC and CN groups in comparison with the CC group. Notably, STP in CN group was also lower than that in the NC group. These findings suggested that both prenatal and postnatal exposure to nicotine induced learning and memory deficits, while the potential mechanism might be different from each other due to their dissimilar impairments of synaptic plasticity.

Differential responses of hippocampal neurons and astrocytes to nicotine and hypoxia in the fetal guinea pig

In utero exposure to cigarette smoke has severe consequences for the developing fetus, including increased risk of birth complications and behavioral and learning disabilities later in life. Evidence from animal models suggests that the cognitive deficits may be a consequence of in utero nicotine exposure in the brain during critical developmental periods. However, maternal smoking exposes the fetus to not only nicotine but also a hypoxic intrauterine environment. Thus, both nicotine and hypoxia are capable of initiating cellular cascades, leading to long-term changes in synaptic patterning that have the potential to affect cognitive functions. This study investigates the combined effect of in utero exposure to nicotine and hypoxia on neuronal and glial elements in the hippocampal CA1 field. Fetal guinea pigs were exposed in utero to normoxic or hypoxic conditions in the presence or absence of nicotine. Hypoxia increased the protein levels of matrix metalloproteinase-9 (MMP-9) and synaptophysin and decreased the neural density as measured by NeuN immunoreactivity (ir). Nicotine exposure had no effect on these neuronal parameters but dramatically increased the density of astrocytes immunopositive for glial fibrillary acidic protein (GFAP). Further investigation into the effects of in utero nicotine exposure revealed that both GFAP-ir and NeuN-ir in the CA1 field were significantly reduced in adulthood. Taken together, our data suggest that prenatal exposure to nicotine and hypoxia not only alters synaptic patterning acutely during fetal development, but that nicotine also has long-term consequences that are observed well into adulthood. Moreover, these effects most likely take place through distinct mechanisms.

Nicotine uses neuron-glia communication to enhance hippocampal synaptic transmission and long-term memory

Nicotine enhances synaptic transmission and facilitates long-term memory. Now it is known that bi-directional glia-neuron interactions play important roles in the physiology of the brain. However, the involvement of glial cells in the effects of nicotine has not been considered until now. In particular, the gliotransmitter D-serine, an endogenous co-agonist of NMDA receptors, enables different types of synaptic plasticity and memory in the hippocampus. Here, we report that hippocampal long-term synaptic plasticity induced by nicotine was annulled by an enzyme that degrades endogenous D-serine, or by an NMDA receptor antagonist that acts at the D-serine binding site. Accordingly, both effects of nicotine: the enhancement of synaptic transmission and facilitation of long-term memory were eliminated by impairing glial cells with fluoroacetate, and were restored with exogenous D-serine. Together, these results show that glial D-serine is essential for the long-term effects of nicotine on synaptic plasticity and memory, and they highlight the roles of glial cells as key participants in brain functions.

Developmental nicotine exposure induced alterations in behavior and glutamate receptor function in hippocampus

In the developing brain, nicotinic acetylcholine receptors (nAChRs) are involved in cell survival, targeting, formation of neural and sensory circuits, and development and maturation of other neurotransmitter systems. This regulatory role is disrupted when the developing brain is exposed to nicotine, which occurs with tobacco use during pregnancy. Prenatal nicotine exposure has been shown to be a strong risk factor for memory deficits and other behavioral aberrations in the offspring. The molecular mechanisms underlying these neurobehavioral outcomes are not clearly elucidated. We used a rodent model to assess behavioral, neurophysiological, and neurochemical consequences of prenatal nicotine exposure in rat offspring with specific emphasis on the hippocampal glutamatergic system. Pregnant dams were infused with nicotine (6 mg/kg/day) subcutaneously from the third day of pregnancy until birth. Results indicate that prenatal nicotine exposure leads to increased anxiety and depressive-like effects and impaired spatial memory. Synaptic plasticity in the form of long-term potentiation (LTP), basal synaptic transmission, and AMPA receptor-mediated synaptic currents were reduced. The deficit in synaptic plasticity was paralleled by declines in protein levels of vesicular glutamate transporter 1 (VGLUT1), synaptophysin, AMPA receptor subunit GluR1, phospho(Ser845) GluR1, and postsynaptic density 95 (PSD-95). These results suggest that prenatal nicotine exposure by maternal smoking could result in alterations in the glutamatergic system in the hippocampus contributing to the abnormal neurobehavioral outcomes.

Selective cholinergic depletion in medial septum leads to impaired long term potentiation and glutamatergic synaptic currents in the hippocampus

Cholinergic depletion in the medial septum (MS) is associated with impaired hippocampal-dependent learning and memory. Here we investigated whether long term potentiation (LTP) and synaptic currents, mediated by alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA) and N-methyl-D-aspartate (NMDA) receptors in the CA1 hippocampal region, are affected following cholinergic lesions of the MS. Stereotaxic intra-medioseptal infusions of a selective immunotoxin, 192-saporin, against cholinergic neurons or sterile saline were made in adult rats. Four days after infusions, hippocampal slices were made and LTP, whole cell, and single channel (AMPA or NMDA receptor) currents were recorded. Results demonstrated impairment in the induction and expression of LTP in lesioned rats. Lesioned rats also showed decreases in synaptic currents from CA1 pyramidal cells and synaptosomal single channels of AMPA and NMDA receptors. Our results suggest that MS cholinergic afferents modulate LTP and glutamatergic currents in the CA1 region of the hippocampus, providing a potential synaptic mechanism for the learning and memory deficits observed in the rodent model of selective MS cholinergic lesioning.

Nicotinic receptors in the dorsal and ventral hippocampus differentially modulate contextual fear conditioning

Nicotine administration alters various forms of hippocampus-dependent learning and memory. Increasing work has found that the dorsal and ventral hippocampus differentially contribute to multiple behaviors. Thus, the present study examined whether the effects of nicotine in the dorsal and ventral hippocampus have distinct influences on contextual fear learning in male C57BL/6J mice. Direct infusion of nicotine into the dorsal hippocampus resulted in an enhancement of contextual fear learning, whereas nicotine infused into the ventral hippocampus resulted in deficits. Nicotine infusions into the ventral hippocampus did not alter hippocampus-independent cued fear conditioning or time spent in the open arm of the elevated plus maze, a measure of anxiety, suggesting that the effects are due to alterations in contextual learning and not other general processes. Finally, results from using direct infusions of MLA, a low-affinity α7 nicotinic acetylcholine receptor (nAChR) antagonist, in conjunction with systemic nicotine, provide evidence that α7-nAChRs in the ventral hippocampus mediate the detrimental effect of ventral hippocampal nicotine on contextual fear learning. These results suggest that with systemic nicotine administration, competition exists between the dorsal and ventral hippocampus for behavioral control over contextual learning.

Nicotine persistently activates ventral tegmental area dopaminergic neurons via nicotinic acetylcholine receptors containing α4 and α6 subunits

Nicotine is reinforcing because it activates dopaminergic (DAergic) neurons within the ventral tegmental area (VTA) of the brain's mesocorticolimbic reward circuitry. This increase in activity can occur for a period of several minutes up to an hour and is thought to be a critical component of nicotine dependence. However, nicotine concentrations that are routinely self-administered by smokers are predicted to desensitize high-affinity α4β2 neuronal nicotinic acetylcholine receptors (nAChRs) in seconds. Thus, how physiologically relevant nicotine concentrations persistently activate VTA DAergic neurons is unknown. Here we show that nicotine can directly and robustly increase the firing frequency of VTA DAergic neurons for several minutes. In mouse midbrain slices, 300 nM nicotine elicited a persistent inward current in VTA DAergic neurons that was blocked by α-conotoxin MII[H9A;L15A], a selective antagonist of nAChRs containing the α6 subunit. α-conotoxin MII[H9A;L15A] also significantly reduced the long-lasting increase in DAergic neuronal activity produced by low concentrations of nicotine. In addition, nicotine failed to significantly activate VTA DAergic neurons in mice that did not express either α4 or α6 nAChR subunits. Conversely, selective activation of nAChRs containing the α4 subunit in knock-in mice expressing a hypersensitive version of these receptors yielded a biphasic response to nicotine consisting of an acute desensitizing increase in firing frequency followed by a sustained increase that lasted several minutes and was sensitive to α-conotoxin MII[H9A;L15A]. These data indicate that nicotine persistently activates VTA DAergic neurons via nAChRs containing α4 and α6 subunits.