Nicotinic alpha 7 receptors protect against glutamate neurotoxicity and neuronal ischemic damage

α7nACh receptor, a promising target to reduce BBB damage by regulating inflammation and autophagy after ischemic stroke

Increased blood-brain barrier (BBB) permeability can lead to cerebral vasogenic edema and hemorrhagic transformation (HT) after reperfusion with tissue plasminogen activator (tPA), the only United States Food and Drug Administration (FDA)-approved treatment for acute ischemia stroke (AIS). The therapeutic benefits of tPA after AIS are partially outweighed by a more than a six-fold increase in the risk of symptomatic intracerebral hemorrhage. Therefore, strategies to protect the integrity of BBB are urgently needed to reduce HT and vasogenic edema after tPA thrombolysis or endovascular thrombectomy. Interestingly, an NIH study showed that smokers treated with tPA had a significantly lower prevalence of brain hemorrhage than nonsmokers, suggesting that cigarette smoking may protect patients treated with tPA from the side effects of cerebral hemorrhage. Importantly, we recently showed that treatment with nicotine reduces AIS-induced BBB damage and that modulating α7nAChR by modulation could reduce ischemia/reperfusion-induced BBB damage, suggesting that α7nAChR could be a potential target to reduce BBB after AIS. In this review, we first provide an overview of stroke and the impact of α7nAChR activation on BBB damage. Next, we discuss the features and mechanism of BBB destruction after AIS. We then discuss the effect of nicotine effect on BBB integrity as well as the mechanism underlying those effects. Finally, we discuss the side effects and potential strategies for modulating α7nAChR to reduce AIS-induced BBB damage.

Effect of Intrahippocampal Administration of α7 Subtype Nicotinic Receptor Agonist PNU-282987 and Its Solvent Dimethyl Sulfoxide on the Efficiency of Hypoxic Preconditioning in Rats

We have previously suggested a key role of the hippocampus in the preconditioning action of moderate hypobaric hypoxia (HBH). The preconditioning efficiency of HBH is associated with acoustic startle prepulse inhibition (PPI). In rats with PPI > 40%, HBH activates the cholinergic projections of hippocampus, and PNU-282987, a selective agonist of α7 nicotinic receptors (α7nAChRs), reduces the HBH efficiency and potentiating effect on HBH of its solvent dimethyl sulfoxide (DMSO, anticholinesterase agent) when administered intraperitoneally. In order to validate the hippocampus as a key structure in the mechanism of hypoxic preconditioning and research a significance of α7nAChR activation in the hypoxic preconditioning, we performed an in vivo pharmacological study of intrahippocampal injections of PNU-282987 into the CA1 area on HBH efficiency in rats with PPI ≥ 40%. We found that PNU-282987 (30 μM) reduced HBH efficiency as with intraperitoneal administration, while DMSO (0.05%) still potentiated this effect. Thus, direct evidence of the key role of the hippocampus in the preconditioning effect of HBH and some details of this mechanism were obtained in rats with PPI ≥ 40%. The activation of α7nAChRs is not involved in the cholinergic signaling initiated by HBH or DMSO via any route of administration. Possible ways of the potentiating action of DMSO on HBH efficiency and its dependence on α7nAChRs are discussed.

Possible Engagement of Nicotinic Acetylcholine Receptors in Pathophysiology of Brain Ischemia-Induced Cognitive Impairment

Post-stroke disabilities like cognitive impairment impose are complex conditions with great economic burdens on health care systems. For these comorbidities, no effective therapies have been identified yet. Nicotinic acetylcholine receptors (nAChRs) are multifunctional receptors participating in various behavioral and neurobiological functions. During brain ischemia, the increased glutamate accumulation leads to neuronal excitotoxicity as well as mitochondrial dysfunction. These abnormalities then cause the increased levels of oxidants, which play key roles in neuronal death and apoptosis in the infarct zone. Additionally, recall of cytokines and inflammatory factors play a prominent role in the exacerbation of ischemic injury. As well, neurotrophic factors' insufficiency results in synaptic dysfunction and cognitive impairments in ischemic brain. Of note, nAChRs through various signaling pathways can participate in therapeutic approaches such as cholinergic system's stimulation, and reduction of excitotoxicity, inflammation, apoptosis, oxidative stress, mitochondrial dysfunction, and autophagy. Moreover, the possible roles of nAChRs in neurogenesis, synaptogenesis, and stimulation of neurotrophic factors expression have been reported previously. On the other hand, the majority of the above-mentioned mechanisms were found to be common in both brain ischemia pathogenesis and cognitive function tuning. Therefore, it seems that nAChRs might be known as key regulators in the control of ischemia pathology, and their modulation could be considered as a new avenue in the multi-target treatment of post-stroke cognitive impairment.

Therapeutic efficacy of α7 ligands after acute ischaemic stroke is linked to conductive states of α7 nicotinic ACh receptors

BACKGROUND AND PURPOSE

Targeting α7 nicotinic ACh receptors (nAChRs) in neuroinflammatory disorders including acute ischaemic stroke holds significant therapeutic promise. However, therapeutically relevant signalling mechanisms remain unidentified. Activation of neuronal α7 nAChRs triggers ionotropic signalling, but there is limited evidence for it in immunoglial tissues. The α7 ligands which are effective in reducing acute ischaemic stroke damage promote α7 ionotropic activity, suggesting a link between their therapeutic effects for treating acute ischaemic stroke and activation of α7 conductive states.

EXPERIMENTAL APPROACH

This hypothesis was tested using a transient middle cerebral artery occlusion (MCAO) model of acute ischaemic stroke, NS6740, a known selective non-ionotropic agonist of α7 nAChRs and 4OH-GTS-21, a partial α7 agonist. NS6740-like ligands exhibiting low efficacy/potency for ionotropic activity will be referred to as non-ionotropic agonists or "metagonists".

KEY RESULTS

4OH-GTS-21, used as a positive control, significantly reduced neurological deficits and brain injury after MCAO as compared to vehicle and NS6740. By contrast, NS6740 was ineffective in identical assays and reversed the effects of 4OH-GTS-21 when these compounds were co-applied. Electrophysiological recordings from acute hippocampal slices obtained from NS6740-injected animals demonstrated its remarkable brain availability and protracted effects on α7 nAChRs as evidenced by sustained (>8 h) alterations in α7 ionotropic responsiveness.

CONCLUSION AND IMPLICATIONS

These results suggest that α7 ionotropic activity may be obligatory for therapeutic efficacy of α7 ligands after acute ischaemic stroke yet, highlight the potential for selective application of α7 ligands to disease states based on their mode of receptor activation.

Alpha7 nicotinic-N-methyl-D-aspartate hypothesis in the treatment of schizophrenia and beyond

Development of novel treatments for positive, cognitive, and negative symptoms continue to be a high-priority area of schizophrenia research and a major unmet clinical need. Given that all randomized controlled trials (RCTs) conducted to date failed with one add-on medication/mechanism of action, future RCTs with the same approach are not warranted. Even if the field develops a medication for cognition, others are still needed to treat negative and positive symptoms. Therefore, fixing one domain does not completely solve the problem. Also, targeting the cholinergic system, glutamatergic system, and cholinergic plus alpha7 nicotinic and N-methyl-D-aspartate (NMDA) receptors failed independently. Hence, targeting other less important pathophysiological mechanisms/targets is unlikely to be successful. Meta-analyses of RCTs targeting major pathophysiological mechanisms have found some efficacy signal in schizophrenia; thus, combination treatments with different mechanisms of action may enhance the efficacy signal. The objective of this article is to highlight the importance of conducting RCTs with novel combination treatments in schizophrenia to develop antischizophrenia treatments. Positive RCTs with novel combination treatments that target the alpha7 nicotinic and NMDA receptors simultaneously may lead to a disease-modifying therapeutic armamentarium in schizophrenia. Novel combination treatments that concurrently improve the three domains of psychopathology and several prognostic and theranostic biomarkers may facilitate therapeutic discovery in schizophrenia.

Opposite Pathways of Cholinergic Mechanisms of Hypoxic Preconditioning in the Hippocampus: Participation of Nicotinic α7 Receptors and Their Association with the Baseline Level of Startle Prepulse Inhibition

(1) Background. A one-time moderate hypobaric hypoxia (HBH) has a preconditioning effect whose neuronal mechanisms are not studied well. Previously, we found a stable correlation between the HBH efficiency and acoustic startle prepulse inhibition (PPI). This makes it possible to predict the individual efficiency of HBH in animals and to study its potential adaptive mechanisms. We revealed a bi-directional action of nicotinic α7 receptor agonist PNU-282987 and its solvent dimethyl sulfoxide on HBH efficiency with the level of PPI > or < 40%. (2) The aim of the present study was to estimate cholinergic mechanisms of HBH effects in different brain regions. (3) Methods: in rats pretested for PPI, we evaluated the activity of synaptic membrane-bound and water-soluble choline acetyltransferase (ChAT) in the sub-fractions of ‘light’ and ‘heavy’ synaptosomes of the neocortex, hippocampus and caudal brainstem in the intact brain and after HBH. We tested the dose-dependent influence of PNU-282987 on the HBH efficiency. (4) Results: PPI level and ChAT activity correlated negatively in all brain structures of the intact animals, so that the values of the latter were higher in rats with PPI < 40% compared to those with PPI > 40%. After HBH, this ChAT activity difference was leveled in the neocortex and caudal brainstem, while for membrane-bound ChAT in the ‘light’ synaptosomal fraction of hippocampus, it was reversed to the opposite. In addition, a pharmacological study revealed that PNU-282987 in all used doses and its solvent displayed corresponding opposite effects on HBH efficiency in rats with different levels of PPI. (5) Conclusion: We substantiate that in rats with low and high PPI two opposite hippocampal cholinergic mechanisms are involved in hypoxic preconditioning, and both are implemented by forebrain projections via nicotinic α7 receptors. Possible causes of association between general protective adaptation, HBH, PPI, forebrain cholinergic system and hippocampus are discussed.

Nicotinic Acetylcholine Receptors of PC12 Cells

Nicotinic acetylcholine receptors (nAChRs) have gained much attention in the scientific community since they play a significant role in multiple physiological and pathophysiological processes. Multiple approaches to study the receptors exist, with characterization of the receptors' functionality at a single cellular level using cell culturing being one of them. Derived from an adrenal medulla tumor, PC12 cells express nicotinic receptor subunits and form functional nicotinic receptors. Thus, the cells offer a convenient environment to address questions related to the functionality of the receptors. The review summarizes the findings on nicotinic receptors' expression and functions which were conducted using PC12 cells. Specific focus is given to α3-containing receptors as well as α7 receptor. Critical evaluation of findings is provided alongside insights into what can still be learned about nAChRs, using PC12 cells.

Treatment duration affects cytoprotective efficacy of positive allosteric modulation of α7 nAChRs after focal ischemia in rats

To minimize irreversible brain injury after acute ischemic stroke (AIS), the time to treatment (i.e., treatment delay) should be minimized. However, thus far, all cytoprotective clinical trials have failed. Analysis of literature identified short treatment durations (≤72 h) as a common motif among completed cytoprotective clinical trials. Here, we argue that short cytoprotective regimens even if given early after AIS may only slow down the evolution of ischemic brain injury and fail to deliver sustained long-term solutions leading to relapses that may be misinterpreted for conceptual failure of cytoprotection. In this randomized blinded study, we used young adult male rats subjected to transient 90 min suture middle cerebral artery occlusion (MCAO) and treated with acute vs. sub-chronic regimens of PNU120596, a prototypical positive allosteric modulator of α7 nicotinic acetylcholine receptors with anti-inflammatory cytoprotective properties to test the hypothesis that insufficient treatment durations may reduce therapeutic benefits of otherwise efficacious cytoprotectants after AIS. A single acute treatment 90 min after MCAO significantly reduced brain injury and neurological deficits 24 h later, but these effects vanished 72 h after MCAO. These relapses were avoided by utilizing sub-chronic treatments. Thus, extending treatment duration augments therapeutic efficacy of PNU120596 after MCAO. Furthermore, sub-chronic treatments could offset the negative effects of prolonged treatment delays in cases where the acute treatment window after MCAO was left unexploited. We conclude that a combination of short treatment delays and prolonged treatment durations may be required to maximize therapeutic effects of PNU120596, reduce relapses and ensure sustained therapeutic efficacy after AIS. Similar concepts may hold for other cytoprotectants including those that failed in clinical trials.

Impact of Bone Fracture on Ischemic Stroke Recovery

Stroke is one of the most devastating complications of bone fracture, occurring in up to 4% of patients after surgical repair for hip fracture. Bone fracture and ischemic stroke have many common risk factors. The impact of bone fracture on stroke recovery has not drawn much attention in the research field. Bone fracture could occur in stroke patients at different times during the recovery phase, which steepens the trajectory of cognitive decline, greatly affects the quality of life, and causes a heavy burden on healthcare resources. In this paper, we reviewed the growing information on the pathophysiological mechanisms by which bone fracture may affect ischemic stroke recovery process.

Inflammation in stroke: the role of cholinergic, purinergic and glutamatergic signaling

The inflammatory response is a major factor in stroke pathophysiology and contributes to secondary neuronal damage in both acute and chronic stages of the ischemic injury. Recent work in experimental cerebral ischemia has demonstrated the involvement of neurotransmitter signaling in the modulation of neuroinflammation. The present review discusses recent findings on the therapeutic potential and diagnostic perspectives of cholinergic, purinergic and glutamatergic receptors and transporters in experimental stroke. It provides evidence of the role of neurotransmission signaling as a promising inflammatory biomarker in stroke. Finally, recent molecular imaging studies using positron emission tomography of cholinergic receptors and glutamatergic transporters are outlined along with their potential as novel anti-inflammatory therapy to reduce the outcome of cerebral ischemia.

Cholinergic Hypofunction in Presbycusis-Related Tinnitus With Cognitive Function Impairment: Emerging Hypotheses

Presbycusis (age-related hearing loss) is a potential risk factor for tinnitus and cognitive deterioration, which result in poor life quality. Presbycusis-related tinnitus with cognitive impairment is a common phenotype in the elderly population. In these individuals, the central auditory system shows similar pathophysiological alterations as those observed in Alzheimer's disease (AD), including cholinergic hypofunction, epileptiform-like network synchronization, chronic inflammation, and reduced GABAergic inhibition and neural plasticity. Observations from experimental rodent models indicate that recovery of cholinergic function can improve memory and other cognitive functions via acetylcholine-mediated GABAergic inhibition enhancement, nicotinic acetylcholine receptor (nAChR)-mediated anti-inflammation, glial activation inhibition and neurovascular protection. The loss of cholinergic innervation of various brain structures may provide a common link between tinnitus seen in presbycusis-related tinnitus and age-related cognitive impairment. We hypothesize a key component of the condition is the withdrawal of cholinergic input to a subtype of GABAergic inhibitory interneuron, neuropeptide Y (NPY) neurogliaform cells. Cholinergic denervation might not only cause the degeneration of NPY neurogliaform cells, but may also result in decreased AChR activation in GABAergic inhibitory interneurons. This, in turn, would lead to reduced GABA release and inhibitory regulation of neural networks. Reduced nAChR-mediated anti-inflammation due to the loss of nicotinic innervation might lead to the transformation of glial cells and release of inflammatory mediators, lowering the buffering of extracellular potassium and glutamate metabolism. Further research will provide evidence for the recovery of cholinergic function with the use of cholinergic input enhancement alone or in combination with other rehabilitative interventions to reestablish inhibitory regulation mechanisms of involved neural networks for presbycusis-related tinnitus with cognitive impairment.

In vivo imaging of Α7 nicotinic receptors as a novel method to monitor neuroinflammation after cerebral ischemia

In vivo positron emission tomography (PET) imaging of nicotinic acetylcholine receptors (nAChRs) is a promising tool for the imaging evaluation of neurologic and neurodegenerative diseases. However, the role of α7 nAChRs after brain diseases such as cerebral ischemia and its involvement in inflammatory reaction is still largely unknown. In vivo and ex vivo evaluation of α7 nAChRs expression after transient middle cerebral artery occlusion (MCAO) was carried out using PET imaging with [¹¹ C]NS14492 and immunohistochemistry (IHC). Pharmacological activation of α7 receptors was evaluated with magnetic resonance imaging (MRI), [¹⁸ F]DPA-714 PET, IHC, real time polymerase chain reaction (qPCR) and neurofunctional studies. In the ischemic territory, [¹¹ C]NS14492 signal and IHC showed an expression increase of α7 receptors in microglia and astrocytes after cerebral ischemia. The role played by α7 receptors on neuroinflammation was supported by the decrease of [¹⁸ F]DPA-714 binding in ischemic rats treated with the α7 agonist PHA 568487 at day 7 after MCAO. Moreover, compared with non-treated MCAO rats, PHA-treated ischemic rats showed a significant reduction of the cerebral infarct volumes and an improvement of the neurologic outcome. PHA treatment significantly reduced the expression of leukocyte infiltration molecules in MCAO rats and in endothelial cells after in vitro ischemia. Despite that, the activation of α7 nAChR had no influence to the blood brain barrier (BBB) permeability measured by MRI. Taken together, these results suggest that the nicotinic α7 nAChRs play a key role in the inflammatory reaction and the leukocyte recruitment following cerebral ischemia in rats.

Revisiting nicotine’s role in the ageing brain and cognitive impairment

Brain ageing is a complex process which in its pathologic form is associated with learning and memory dysfunction or cognitive impairment. During ageing, changes in cholinergic innervations and reduced acetylcholinergic tonus may trigger a series of molecular pathways participating in oxidative stress, excitotoxicity, amyloid-β toxicity, apoptosis, neuroinflammation, and perturb neurotrophic factors in the brain. Nicotine is an exogenous agonist of nicotinic acetylcholine receptors (nAChRs) and acts as a pharmacological chaperone in the regulation of nAChR expression, potentially intervening in age-related changes in diverse molecular pathways leading to pathology. Although nicotine has therapeutic potential, paradoxical effects have been reported, possibly due to its inverted U-shape dose-response effects or pharmacokinetic factors. Additionally, nicotine administration should result in optimum therapeutic effects without imparting abuse potential or toxicity. Overall, this review aims to compile the previous and most recent data on nicotine and its effects on cognition-related mechanisms and age-related cognitive impairment.

Huperzine A as a neuroprotective and antiepileptic drug: a review of preclinical research

Huperzine A (HupA) is an acetylcholinesterase (AChE) inhibitor extracted from Huperzia Serrata, a firmoss, which has been used for various diseases in traditional Chinese medicine for fever and inflammation. More recently, it has been used in Alzheimer's disease and other forms of dementia with a presumed mechanism of action via central nicotinic and muscarinic receptors. HupA is marketed as a dietary supplement in the U.S. This article reviews newly proposed neuroprotective and anticonvulsant HupA properties based on animal studies. HupA exerts its effects mainly via α7nAChRs and α4β2nAChRs, thereby producing a potent anti-inflammatory response by decreasing IL-1β, TNF-α protein expression, and suppressing transcriptional activation of NF-κB signaling. Thus, it provides protection from excitotoxicity and neuronal death as well as increase in GABAergic transmission associated with anticonvulsant activity.

Boosting Endogenous Resistance of Brain to Ischemia

Most survivors of ischemic stroke remain physically disabled and require prolonged rehabilitation. However, some stroke victims achieve a full neurological recovery suggesting that the human brain can defend itself against ischemic injury, but the protective mechanisms are unknown. This study used selective pharmacological agents and a rat model of cerebral ischemic stroke to detect endogenous brain protective mechanisms that require activation of α7 nicotinic acetylcholine receptors (nAChRs). This endogenous protection was found to be (1) limited to less severe injuries; (2) significantly augmented by intranasal administration of a positive allosteric modulator of α7 nAChRs, significantly reducing brain injury and neurological deficits after more severe ischemic injuries; and (3) reduced by inhibition of calcium/calmodulin-dependent kinase-II. The physiological role of α7 nAChRs remains largely unknown. The therapeutic activation of α7 nAChRs after cerebral ischemia may serve as an important physiological responsibility of these ubiquitous receptors and holds a significant translational potential.

The Dietary Flavonoid Rhamnetin Inhibits Both Inflammation and Excitotoxicity During Ethanol Withdrawal in Rat Organotypic Hippocampal Slice Cultures

BACKGROUND

Ethanol (EtOH) causes neurotoxicity via several mechanisms including neuroinflammation (during EtOH exposure), and excitotoxicity (during EtOH withdrawal [EWD]). Alpha7 nicotinic acetylcholine receptor (nAChR) selective agonists have the potential to reduce both. The aim of this study was to evaluate the anti-inflammatory and neuroprotective potential of rhamnetin, a dietary flavonoid with alpha7 nAChR selective activity, in an in vitro model of EtOH-induced neurotoxicity.

METHODS

The anti-inflammatory and neuroprotective properties of rhamnetin were assessed in neonatal organotypic hippocampal slice cultures undergoing EWD (or not) and challenged with N-methyl-D-aspartate (NMDA) and/or lipopolysaccharide (LPS). Neurotoxicity was determined using propidium iodide uptake, and the inflammatory response was evaluated by measuring the release of tumor necrosis factor (TNF)-alpha (NO; quantified by ELISA) and nitric oxide (quantified by the Griess reaction) into culture media.

RESULTS

As predicted, rhamnetin reduced LPS-induced release of TNF-alpha and NO both under control conditions and during EWD. Additionally, rhamnetin had no effect on NMDA-induced neurotoxicity under control conditions, but significantly reduced NMDA toxicity during EWD. In contrast, rhamnetin had no effect on neurotoxicity induced by NMDA and LPS combined despite reducing TNF-alpha and NO levels under these conditions.

CONCLUSIONS

Rhamnetin is anti-inflammatory and neuroprotective during EWD and therefore has potential value in treating neurotoxicity caused by EtOH.

Comparing α7 nicotinic acetylcholine receptor binding, amyloid-β deposition, and mitochondria complex-I function in living brain: A PET study in aged monkeys

This study was aimed to assess the correlations among α7 nicotinic acetylcholine receptor (α7-nAChR) binding, amyloid-β (Aβ) deposition, and mitochondrial complex I (MC-I) activity in the brain of aged monkeys (Macaca mulatta). Positron emission tomography (PET) measurements with [(11) C](R)-MeQAA, [(11) C]PIB, and [(18) F]BCPP-EF were conducted in monkeys in a conscious condition. [(11) C](R)-MeQAA binding was analyzed by a simplified reference tissue model to calculate nondisplaceable binding potential (BPND), [(11) C]PIB uptake was calculated by standard uptake value ratio (SUVR), and [(18) F]BCPP-EF binding was determined by Logan graphical analysis to calculate total distribution volume (VT) with arterial blood sampling. Higher brain uptake was determined in the thalamus, hippocampus, striatum, and cortical regions for [(11) C](R)-MeQAA, while being lower in the cerebellum. Significant age-related reduction of [(11) C](R)-MeQAA binding to α7-nAChR was determined only in the occipital cortex. The plot of Vt of [(18) F]BCPP-EF against BPND of [(11) C](R)-MeQAA indicated a significant negative correlation in the hippocampus and cortical regions in aged animals. Plotting of SUVR of [(11) C]PIB against BPND of [(11) C](R)-MeQAA showed a positive correlation. The in vivo binding of [(11) C](R)-MeQAA could reflect the upregulation of α7-nAChR induced by neurodegenerative damage determined by Aβ deposition as well as impaired MC-I activity in living brain.

In vivo PET imaging of the α4β2 nicotinic acetylcholine receptor as a marker for brain inflammation after cerebral ischemia

PET imaging of nicotinic acetylcholine receptors (nAChRs) could become an effective tool for the diagnosis and therapy evaluation of neurologic diseases. Despite this, the role of nAChRs α4β2 receptors after brain diseases such as cerebral ischemia and its involvement in inflammatory reaction is still largely unknown. To investigate this, we performed in parallel in vivo magnetic resonance imaging (MRI) and positron emission tomography (PET) with 2[(18)F]-fluoro-A85380 and [(11)C]PK11195 at 1, 3, 7, 14, 21, and 28 d after middle cerebral artery occlusion (MCAO) in rats. In the ischemic territory, PET with 2[(18)F]-fluoro-A85380 and [(11)C]PK11195 showed a progressive binding increase from days 3-7, followed by a progressive decrease from days 14-28 after cerebral ischemia onset. Ex vivo immunohistochemistry for the nicotinic α4β2 receptor and the mitochondrial translocator protein (18 kDa) (TSPO) confirmed the PET findings and demonstrated the overexpression of α4β2 receptors in both microglia/macrophages and astrocytes from days 7-28 after experimental ischemic stroke. Likewise, the role played by α4β2 receptors on neuroinflammation was supported by the increase of [(11)C]PK11195 binding in ischemic rats treated with the α4β2 antagonist dihydro-β-erythroidine hydrobromide (DHBE) at day 7 after MCAO. Finally, both functional and behavioral testing showed major impaired outcome at day 1 after ischemia onset, followed by a recovery of the sensorimotor function and dexterity from days 21-28 after experimental stroke. Together, these results suggest that the nicotinic α4β2 receptor could have a key role in the inflammatory reaction underlying cerebral ischemia in rats.

Gender differences in the effect of tobacco use on brain phosphocreatine levels in methamphetamine-dependent subjects

BACKGROUND

A high prevalence of tobacco smoking has been observed in methamphetamine users, but there have been no in vivo brain neurochemistry studies addressing gender effects of tobacco smoking in methamphetamine users. Methamphetamine addiction is associated with increased risk of depression and anxiety in females. There is increasing evidence that selective analogues of nicotine, a principal active component of tobacco smoking, may ease depression and improve cognitive performance in animals and humans.

OBJECTIVES

To investigate the effects of tobacco smoking and gender on brain phosphocreatine (PCr) levels, a marker of brain energy metabolism reported to be reduced in methamphetamine-dependent subjects.

METHODS

Thirty female and 27 male methamphetamine-dependent subjects were evaluated with phosphorus-31 magnetic resonance spectroscopy ((31)P-MRS) to measure PCr levels within the pregenual anterior cingulate, which has been implicated in methamphetamine neurotoxicity.

RESULTS

Analysis of covariance revealed that there were statistically significant slope (PCr versus lifetime amount of tobacco smoking) differences between female and male methamphetamine-dependent subjects (p = 0.03). In females, there was also a statistically significant interaction between lifetime amounts of tobacco smoking and methamphetamine in regard to PCr levels (p = 0.01), which suggests that tobacco smoking may have a more significant positive impact on brain PCr levels in heavy, as opposed to light to moderate, methamphetamine-dependent females.

CONCLUSION

These results indicate that tobacco smoking has gender-specific effects in terms of increased anterior cingulate high energy PCr levels in methamphetamine-dependent subjects. Cigarette smoking in methamphetamine-dependent women, particularly those with heavy methamphetamine use, may have a potentially protective effect upon neuronal metabolism.

Positive allosteric modulation of alpha-7 nicotinic receptors promotes cell death by inducing Ca(2+) release from the endoplasmic reticulum

Positive allosteric modulation of α7 isoform of nicotinic acetylcholine receptors (α7-nAChRs) is emerging as a promising therapeutic approach for central nervous system disorders such as schizophrenia or Alzheimer's disease. However, its effect on Ca(2+) signaling and cell viability remains controversial. This study focuses on how the type II positive allosteric modulator (PAM II) PNU120596 affects intracellular Ca(2+) signaling and cell viability. We used human SH-SY5Y neuroblastoma cells overexpressing α7-nAChRs (α7-SH) and their control (C-SH). We monitored cytoplasmic and endoplasmic reticulum (ER) Ca(2+) with Fura-2 and the genetically encoded cameleon targeting the ER, respectively. Nicotinic inward currents were measured using patch-clamp techniques. Viability was assessed using methylthiazolyl blue tetrazolium bromide or propidium iodide staining. We observed that in the presence of a nicotinic agonist, PNU120596 (i) reduced viability of α7-SH but not of C-SH cells; (ii) significantly increased inward nicotinic currents and cytosolic Ca(2+) concentration; (iii) released Ca(2+) from the ER by a Ca(2+) -induced Ca(2+) release mechanism only in α7-SH cells; (iv) was cytotoxic in rat organotypic hippocampal slice cultures; and, lastly, all these effects were prevented by selective blockade of α7-nAChRs, ryanodine receptors, or IP3 receptors. In conclusion, positive allosteric modulation of α7-nAChRs with the PAM II PNU120596 can lead to dysregulation of ER Ca(2+) , overloading of intracellular Ca(2+) , and neuronal cell death. This study focuses on how the type II positive allosteric modulator PNU120596 (PAM II PNU12) affects intracellular Ca(2+) signaling and cell viability. Using SH-SY5Y neuroblastoma cells overexpressing α7-nAChRs (α7-SH) and their control (C-SH), we find that PAM of α7-nAChRs with PNU120596: (i) increases inward calcium current (ICa ) and cytosolic Ca(2+) concentration ([Ca(2+) ]cyt ); (ii) releases Ca(2+) from the ER ([Ca(2+) ]ER ) by a Ca(2+) -induced Ca(2+) release mechanism; and (iv) reduces cell viability. These findings were corroborated in rat hippocampal organotypic cultures. [Ca(2+) ]cyt , cytosolic Ca(2+) concentration; [Ca(2+) ]ER , endoplasmic reticulum Ca(2+) concentration; α7 nAChR, α7 isoform of nicotinic acetylcholine receptors; α7-SH, SH-SY5Y stably overexpressing α7 nAChRs cells; C-SH, control SH-SY5Y cells; Nic, nicotine; PNU12, PNU120596.

Natural products from marine organisms with neuroprotective activity in the experimental models of Alzheimer's disease, Parkinson's disease and ischemic brain stroke: their molecular targets and action mechanisms

Continuous increases in the incidence of neurodegenerative diseases, such as Alzheimer's disease (AD), Parkinson's disease (PD), and brain stroke demand the urgent development of therapeutics. Marine organisms are well-known producers of natural products with diverse structures and pharmacological activities. Therefore, researchers have endeavored to identify marine natural products with neuroprotective effects. In this regard, this review summarizes therapeutic targets for AD, PD, and ischemic brain stroke and marine natural products with pharmacological activities on the targets according to taxonomies of marine organisms. Furthermore, several marine natural products on the clinical trials for the treatment of neurological disorders are discussed.

Activation of α-7 nicotinic acetylcholine receptor reduces ischemic stroke injury through reduction of pro-inflammatory macrophages and oxidative stress

Activation of α-7 nicotinic acetylcholine receptor (α-7 nAchR) has a neuro-protective effect on ischemic and hemorrhagic stroke. However, the underlying mechanism is not completely understood. We hypothesized that α-7 nAchR agonist protects brain injury after ischemic stroke through reduction of pro-inflammatory macrophages (M1) and oxidative stress. C57BL/6 mice were treated with PHA568487 (PHA, α-7 nAchR agonist), methyllycaconitine (MLA, nAchR antagonist), or saline immediately and 24 hours after permanent occlusion of the distal middle cerebral artery (pMCAO). Behavior test, lesion volume, CD68⁺, M1 (CD11b⁺/Iba1⁺) and M2 (CD206/Iba1⁺) microglia/macrophages, and phosphorylated p65 component of NF-kB in microglia/macrophages were quantified using histological stained sections. The expression of M1 and M2 marker genes, anti-oxidant genes and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase were quantified using real-time RT-PCR. Compared to the saline-treated mice, PHA mice had fewer behavior deficits 3 and 7 days after pMCAO, and smaller lesion volume, fewer CD68⁺ and M1 macrophages, and more M2 macrophages 3 and 14 days after pMCAO, whereas MLA's effects were mostly the opposite in several analyses. PHA increased anti-oxidant genes and NADPH oxidase expression associated with decreased phosphorylation of NF-kB p65 in microglia/macrophages. Thus, reduction of inflammatory response and oxidative stress play roles in α-7 nAchR neuro-protective effect.

The therapeutic promise of positive allosteric modulation of nicotinic receptors

In the central nervous system, deficits in cholinergic neurotransmission correlate with decreased attention and cognitive impairment, while stimulation of neuronal nicotinic acetylcholine receptors improves attention, cognitive performance and neuronal resistance to injury as well as produces robust analgesic and anti-inflammatory effects. The rational basis for the therapeutic use of orthosteric agonists and positive allosteric modulators (PAMs) of nicotinic receptors arises from the finding that functional nicotinic receptors are ubiquitously expressed in neuronal and non-neuronal tissues including brain regions highly vulnerable to traumatic and ischemic types of injury (e.g., cortex and hippocampus). Moreover, functional nicotinic receptors do not vanish in age-, disease- and trauma-related neuropathologies, but their expression and/or activation levels decline in a subunit- and brain region-specific manner. Therefore, augmenting the endogenous cholinergic tone by nicotinic agents is possible and may offset neurological impairments associated with cholinergic hypofunction. Importantly, because neuronal damage elevates extracellular levels of choline (a selective agonist of α7 nicotinic acetylcholine receptors) near the site of injury, α7-PAM-based treatments may augment pathology-activated α7-dependent auto-therapies where and when they are most needed (i.e., in the penumbra, post-injury). Thus, nicotinic-PAM-based treatments are expected to augment the endogenous cholinergic tone in a spatially and temporally restricted manner creating the potential for differential efficacy and improved safety as compared to exogenous orthosteric nicotinic agonists that activate nicotinic receptors indiscriminately. In this review, I will summarize the existing trends in therapeutic applications of nicotinic PAMs.

A positive allosteric modulator of α7 nAChRs augments neuroprotective effects of endogenous nicotinic agonists in cerebral ischaemia

BACKGROUND AND PURPOSE

Activation of α7 nicotinic acetylcholine receptors (nAChRs) can be neuroprotective. However, endogenous choline and ACh have not been regarded as potent neuroprotective agents because physiological levels of choline/ACh do not produce neuroprotective levels of α7 activation. This limitation may be overcome by the use of type-II positive allosteric modulators (PAMs-II) of α7 nAChRs, such as 1-(5-chloro-2,4-dimethoxyphenyl)-3-(5-methylisoxazol-3-yl)-urea (PNU-120596). This proof-of-concept study presents a novel neuroprotective paradigm that converts endogenous choline/ACh into potent neuroprotective agents in cerebral ischaemia by inhibiting α7 nAChR desensitization using PNU-120596.

EXPERIMENTAL APPROACH

An electrophysiological ex vivo cell injury assay (to quantify the susceptibility of hippocampal neurons to acute injury by complete oxygen and glucose deprivation; COGD) and an in vivo middle cerebral artery occlusion model of ischaemia were used in rats.

KEY RESULTS

Choline (20-200 μM) in the presence, but not absence of 1 μM PNU-120596 significantly delayed anoxic depolarization/injury of hippocampal CA1 pyramidal neurons, but not CA1 stratum radiatum interneurons, subjected to COGD in acute hippocampal slices and these effects were blocked by 20 nM methyllycaconitine, a selective α7 antagonist, thus, activation of α7 nAChRs was required. PNU-120596 alone was ineffective ex vivo. In in vivo experiments, both pre- and post-ischaemia treatments with PNU-120596 (30 mg·kg(-1) , s.c. and 1 mg·kg(-1) , i.v., respectively) significantly reduced the cortical/subcortical infarct volume caused by transient focal cerebral ischaemia. PNU-120596 (1 mg·kg(-1) , i.v., 30 min post-ischaemia) remained neuroprotective in rats subjected to a choline-deficient diet for 14 days prior to experiments.

CONCLUSIONS AND IMPLICATIONS

PNU-120596 and possibly other PAMs-II significantly improved neuronal survival in cerebral ischaemia by augmenting neuroprotective effects of endogenous choline/ACh.

A type-II positive allosteric modulator of α7 nAChRs reduces brain injury and improves neurological function after focal cerebral ischemia in rats

In the absence of clinically-efficacious therapies for ischemic stroke there is a critical need for development of new therapeutic concepts and approaches for prevention of brain injury secondary to cerebral ischemia. This study tests the hypothesis that administration of PNU-120596, a type-II positive allosteric modulator (PAM-II) of α7 nicotinic acetylcholine receptors (nAChRs), as long as 6 hours after the onset of focal cerebral ischemia significantly reduces brain injury and neurological deficits in an animal model of ischemic stroke. Focal cerebral ischemia was induced by a transient (90 min) middle cerebral artery occlusion (MCAO). Animals were then subdivided into two groups and injected intravenously (i.v.) 6 hours post-MCAO with either 1 mg/kg PNU-120596 (treated group) or vehicle only (untreated group). Measurements of cerebral infarct volumes and neurological behavioral tests were performed 24 hrs post-MCAO. PNU-120596 significantly reduced cerebral infarct volume and improved neurological function as evidenced by the results of Bederson, rolling cylinder and ladder rung walking tests. These results forecast a high therapeutic potential for PAMs-II as effective recruiters and activators of endogenous α7 nAChR-dependent cholinergic pathways to reduce brain injury and improve neurological function after cerebral ischemic stroke.

Functional screening of α7 nicotinic receptor ligands

BACKGROUND

The α7 nicotinic acetylcholine receptor, a ligand-gated ion channel, is an attractive drug discovery target in schizophrenia and Alzheimer's disease.

OBJECTIVE

We have evaluated the various approaches to discovering ligands targeting the α7 nicotinic receptor to define the current paradigm driving drug discovery efforts in this area.

METHODS

Assays using functional read-outs as a consequence of α7 nicotinic receptor activation have been reviewed.

CONCLUSION

Functional assays using fluorescence-based optical methods in combination with direct electrophysiological recordings of channel function currently provide an integrated approach to the discovery of α7 nicotinic receptor targeted ligands.

Enhancement of nicotinic receptors alleviates cytotoxicity in neurological disease models

The common pathological mechanisms among the spectrum of neurodegenerative diseases are supposed to be shared. Multiple lines of evidence, from molecular and cellular to epidemiological, have implicated nicotinic transmission in the pathology of the two most common neurodegenerative disorders, namely Alzheimer's disease (AD) and Parkinson's disease (PD). In this review article we present evidence of nicotinic acetylcholine receptor (nAChR)-mediated protection against neurotoxicity induced by β amyloid (Aβ), glutamate, rotenone, and 6-hydroxydopamine (6-OHDA) and the signal transduction involved in this mechanism. Our studies have clarified that survival signal transduction, the α7 nAChR/Src family/PI3K/AKT pathway and subsequent upregulation of Bcl-2 and Bcl-x, would lead to neuroprotection. In addition to the PI3K/AKT pathway, two other survival pathways, JAK2/STAT3 and MEK/ERK, are proposed by other groups. In rotenone- and 6-OHDA-induced PD models, nAChR-mediated neuroprotection was also observed, and the effect was blocked not only by α7 but also by α4β2 nAChR antagonists. We also document that nAChR stimulation blocks glutamate neurotoxicity in spinal cord motor neurons. These findings suggest that nAChR-mediated neuroprotection is achieved through subtypes of nAChRs and common signal cascades. An early diagnosis and protective therapy with nAChR stimulation could be effective in delaying the progression of neurodegenerative diseases such as AD, PD and amyotrophic lateral sclerosis.

Nicotine modifies in vivo and in vitro rat hippocampal amyloid precursor protein processing in young but not old rats

Previous studies have shown that administration of nicotine modifies the expression and secretion of amyloid precursor protein (APP) in various cell lines. The present study investigated the extent to which chronic subcutaneous nicotine administration influences APP levels and processing in cerebral cortex, striatum and hippocampus of young and old rat brains. The results showed that constant nicotine infusion (0.25 or 4.00mg/kg/day) increased the levels of particulate APP (APPp) but not secreted APP (APPs) in the hippocampus of young rats in vivo. This response to nicotine was not observed in the striatum or cerebral cortex of young rats or in any of the brain regions examined in old animals. Subsequent in vitro analysis demonstrated that nicotine enhanced the release of APPs from hippocampal slice preparations and that this increase was attenuated by mecamylamine, a non-selective nicotinic acetylcholine receptor (nAChR) antagonist. The in vitro effect of nicotine on APPs was age-related, being only detected from hippocampal slices derived from the young but not the older animals. These results suggest that nicotine modulates APP expression and secretion in the hippocampus and that the responses observed to the drug are age-dependent being only detected in younger rats.

α7 nicotinic ACh receptors as a ligand-gated source of Ca(2+) ions: the search for a Ca(2+) optimum

The spatiotemporal distribution of cytosolic Ca(2+) ions is a key determinant of neuronal behavior and survival. Distinct sources of Ca(2+) ions including ligand- and voltage-gated Ca(2+) channels contribute to intracellular Ca(2+) homeostasis. Many normal physiological and therapeutic neuronal functions are Ca(2+)-dependent, however an excess of cytosolic Ca(2+) or a lack of the appropriate balance between Ca(2+) entry and clearance may destroy cellular integrity and cause cellular death. Therefore, the existence of optimal spatiotemporal patterns of cytosolic Ca(2+) elevations and thus, optimal activation of ligand- and voltage-gated Ca(2+) ion channels are postulated to benefit neuronal function and survival. Alpha7 nicotinic -acetylcholine receptors (nAChRs) are highly permeable to Ca(2+) ions and play an important role in modulation of neurotransmitter release, gene expression and neuroprotection in a variety of neuronal and non-neuronal cells. In this review, the focus is placed on α7 nAChR-mediated currents and Ca(2+) influx and how this source of Ca(2+) entry compares to NMDA receptors in supporting cytosolic Ca(2+) homeostasis, neuronal function and survival.

Activation of functional α7-containing nAChRs in hippocampal CA1 pyramidal neurons by physiological levels of choline in the presence of PNU-120596

Background

The level of expression of functional α7-containing nicotinic acetylcholine receptors (nAChRs) in hippocampal CA1 pyramidal neurons is believed to be very low compared to hippocampal CA1 interneurons, and for many years this expression was largely overlooked. However, high densities of expression of functional α7-containing nAChRs in CA1 pyramidal neurons may not be necessary for triggering important cellular and network functions, especially if activation of α7-containing nAChRs occurs in the presence of positive allosteric modulators such as PNU-120596.

Methodology/Principal Findings

An approach previously developed for α7-containing nAChRs expressed in tuberomammillary neurons was applied to investigate functional CA1 pyramidal α7-containing nAChRs using rat coronal hippocampal slices and patch-clamp electrophysiology. The majority (∼71%) of tested CA1 pyramidal neurons expressed low densities of functional α7-containing nAChRs as evidenced by small whole-cell responses to choline, a selective endogenous agonist of α7 nAChRs. These responses were potentiated by PNU-120596, a novel positive allosteric modulator of α7 nAChRs. The density of functional α7-containing nAChRs expressed in CA1 pyramidal neurons (and thus, the normalized net effect of activation, i.e., response net charge per unit of membrane capacitance per unit of time) was estimated to be ∼5% of the density observed in CA1 interneurons. The results of this study demonstrate that despite low levels of expression of functional pyramidal α7-containing nAChRs, physiological levels of choline (∼10 µM) are sufficient to activate these receptors and transiently depolarize and even excite CA1 pyramidal neurons in the presence of PNU-120596. The observed effects are possible because in the presence of 10 µM choline and 1–5 µM PNU-120596, a single opening of an individual pyramidal α7-containing nAChR ion channel appears to transiently depolarize (∼4 mV) the entire pyramidal neuron and occasionally trigger action potentials.

Conclusions

1) The majority of hippocampal CA1 pyramidal neurons express functional α7-containing nAChRs. In the absence of PNU-120596, a positive allosteric modulator of α7 nAChRs, a lack of responsiveness of some hippocampal CA1 pyramidal neurons to focal application of 0.5–1 mM choline does not imply a lack of expression of functional α7-containing nAChRs in these neurons. Rather, it may indicate a lack of detection of α7-containing nAChR-mediated currents by patch-clamp electrophysiology. 2) PNU-120596 can serve as a powerful tool for detection and enhancement of responsiveness of low densities of functional α7-containing nAChRs such as those present in hippocampal CA1 pyramidal neurons. 3) In the presence of PNU-120596, physiological concentrations of choline activate functional CA1 pyramidal α7-containing nAChRs and produce step-like currents that cause repetitive step-like depolarizations, occasionally triggering bursts of action potentials in CA1 pyramidal neurons. Therefore, the results of this study suggest that in the presence of PNU-120596 and possibly other positive allosteric modulators, endogenous choline may persistently activate CA1 pyramidal α7-containing nAChRs, enhance the excitability of CA1 pyramidal neurons and thus act as a potent therapeutic agent with potential neuroprotective and cognition-enhancing properties.

Effect of resveratrol on gliotransmitter levels and p38 activities in cultured astrocytes

Accumulating evidence suggests that resveratrol may have beneficial effects against traumatic brain injury. However, its effect on the regulation of extracellular levels of gliotransmitter and on the activation of p38 MAPK in astrocytes is still unknown. We have examined whether resveratrol regulates extracellular levels of gliotransmitter as well as the activation of p38 MAPK in cultured astrocytes before and after stretch injury. The extracellular levels of glutamate, D-/L-serine and D-serine were apparently reduced by 100 μM resveratrol in control astrocyte cultures. The dramatic increase of glutamate and D-serine release induced by stretch injury was also clearly inhibited by resveratrol. Resveratrol mediates this response by reduction of release through inhibition of extracellular calcium influx and increment of gliotransmitter uptake through enhancement of amino acid transporter expressed in the membrane of astrocyte. In addition, resveratrol definitely reduced the activation of p38 MAPK in cultured astrocytes following stretch injury. AMPA receptor is involved in the activation of p38 following injury. Conversely, the levels of glutamine and glycine were not obviously affected by resveratrol before and after injury. Intracellular levels of glutamate and D-serine are not apparently changed by stretch injury. Collectively, our data suggest that resveratrol might play an important role in protection of the nervous system after injury by decreasing the extracellular levels of gliotransmitter and inhibiting activation of p38 MAPK following injury.

Role of alpha7 nicotinic acetylcholine receptor in calcium signaling induced by prion protein interaction with stress-inducible protein 1

The prion protein (PrP(C)) is a conserved glycosylphosphatidylinositol-anchored cell surface protein expressed by neurons and other cells. Stress-inducible protein 1 (STI1) binds PrP(C) extracellularly, and this activated signaling complex promotes neuronal differentiation and neuroprotection via the extracellular signal-regulated kinase 1 and 2 (ERK1/2) and cAMP-dependent protein kinase 1 (PKA) pathways. However, the mechanism by which the PrP(C)-STI1 interaction transduces extracellular signals to the intracellular environment is unknown. We found that in hippocampal neurons, STI1-PrP(C) engagement induces an increase in intracellular Ca(2+) levels. This effect was not detected in PrP(C)-null neurons or wild-type neurons treated with an STI1 mutant unable to bind PrP(C). Using a best candidate approach to test for potential channels involved in Ca(2+) influx evoked by STI1-PrP(C), we found that α-bungarotoxin, a specific inhibitor for α7 nicotinic acetylcholine receptor (α7nAChR), was able to block PrP(C)-STI1-mediated signaling, neuroprotection, and neuritogenesis. Importantly, when α7nAChR was transfected into HEK 293 cells, it formed a functional complex with PrP(C) and allowed reconstitution of signaling by PrP(C)-STI1 interaction. These results indicate that STI1 can interact with the PrP(C)·α7nAChR complex to promote signaling and provide a novel potential target for modulation of the effects of prion protein in neurodegenerative diseases.

Retrospect and prospect of active principles from Chinese herbs in the treatment of dementia

With an ageing population, dementia has become one of the world's primary health challenges. However, existing remedies offer limited benefits with certain side effects, which has prompted researchers to seek complementary and alternative therapies. China has long been known for abundant usage of various herbs. Some of these herbal decoctions are effective in stimulating blood circulation, supplementing vital energy and resisting aging, the lack of which are believed to underlie dementia. These herbs are regarded as new and promising sources of potential anti-dementia drugs. With the rapid evolution of life science and technology, numerous active components have been identified that are highly potent and multi-targeted with low toxicity, and therefore meet the requirements for dementia therapy. This review updates the research progress of Chinese herbs in the treatment of dementia, focusing on their effective principles.

Physiological concentrations of choline activate native alpha7-containing nicotinic acetylcholine receptors in the presence of PNU-120596 [1-(5-chloro-2,4-dimethoxyphenyl)-3-(5-methylisoxazol-3-yl)-urea]

The use of PNU-120596 [1-(5-chloro-2,4-dimethoxyphenyl)-3-(5-methylisoxazol-3-yl)-urea], a positive allosteric modulator of alpha7 nicotinic acetylcholine receptor (nAChR), may be beneficial for enhancing cholinergic therapies. However, the effects of PNU-120596 on activation of native alpha7-containing nAChRs by physiological concentrations of choline are not known and were investigated in this study using patch-clamp electrophysiology and histaminergic tuberomammillary neurons in hypothalamic slices. In the presence of PNU-120596, subthreshold (i.e., inactive) physiological concentrations of choline ( approximately 10 microM) elicited repetitive step-like whole-cell responses reminiscent of single ion channel openings that were reversibly blocked by 20 nM methyllycaconitine, a selective alpha7 nAChR antagonist. The effects of choline and PNU-120596 were synergistic as administration of 10 to 40 microM choline or 1 to 4 muM PNU-120596 alone did not elicit responses. In voltage clamp at -60 mV, the persistent activation of alpha7-containing nAChRs by 10 microM choline plus 1 microM PNU-120596 was estimated to produce a sustained influx of Ca(2+) ions at a rate of 8.4 pC/min ( approximately 0.14 pA). In the presence of PNU-120596 in current clamp, transient step-like depolarizations ( approximately 5 mV) enhanced neuronal excitability and triggered voltage-gated conductances; a single opening of an alpha7-containing nAChR channel appeared to transiently depolarize the entire neuron and facilitate spontaneous firing. Therefore, this study tested and confirmed the hypothesis that PNU-120596 enhances the effects of subthreshold concentrations of choline on native alpha7-containing nAChRs, allowing physiological levels of choline to activate these receptors and produce whole-cell responses in the absence of exogenous nicotinic agents. In certain neurological disorders, this activation may be therapeutically beneficial, more efficacious, and safer than treatments with nAChR agonists.

Structural determinants for interaction of partial agonists with acetylcholine binding protein and neuronal alpha7 nicotinic acetylcholine receptor

The pentameric acetylcholine-binding protein (AChBP) is a soluble surrogate of the ligand binding domain of nicotinic acetylcholine receptors. Agonists bind within a nest of aromatic side chains contributed by loops C and F on opposing faces of each subunit interface. Crystal structures of Aplysia AChBP bound with the agonist anabaseine, two partial agonists selectively activating the alpha7 receptor, 3-(2,4-dimethoxybenzylidene)-anabaseine and its 4-hydroxy metabolite, and an indole-containing partial agonist, tropisetron, were solved at 2.7-1.75 A resolution. All structures identify the Trp 147 carbonyl oxygen as the hydrogen bond acceptor for the agonist-protonated nitrogen. In the partial agonist complexes, the benzylidene and indole substituent positions, dictated by tight interactions with loop F, preclude loop C from adopting the closed conformation seen for full agonists. Fluctuation in loop C position and duality in ligand binding orientations suggest molecular bases for partial agonism at full-length receptors. This study, while pointing to loop F as a major determinant of receptor subtype selectivity, also identifies a new template region for designing alpha7-selective partial agonists to treat cognitive deficits in mental and neurodegenerative disorders.

Smoking, nicotine and neuropsychiatric disorders

Tobacco smoking is an extremely addictive and harmful form of nicotine (NIC) consumption, but unfortunately also the most prevalent. Although disproportionately high frequencies of smoking and its health consequences among psychiatric patients are widely known, the neurobiological background of this epidemiological association is still obscure. The diverse neuroactive effects of NIC and some other major tobacco smoke constituents in the central nervous system may underlie this association. This present paper summarizes the pharmacology of NIC and its receptors (nAChR) based on a systematic review of the literature. The role of the brain's reward system(s) in NIC addiction and the results of functional and structural neuroimaging studies on smoking-related states and behaviors (i.e. dependence, craving, withdrawal) are also discussed. In addition, the epidemiological, neurobiological, and genetic aspects of smoking in several specific neuropsychiatric disorders are reviewed and the clinical relevance of smoking in these disease states addressed.

Chronic nicotine restores normal Aβ levels and prevents short-term memory and E-LTP impairment in Aβ rat model of Alzheimer's disease

Alzheimer's disease (AD) is a devastating neurodegenerative disorder characterized by increased deposition of beta-amyloid (Aβ) peptides and progressive cholinergic dysfunction in regions of the brain involved in learning and memory processing. In AD, progressive accumulation of Aβ peptide impairs nicotinic acetylcholine receptor (nAChR) function by an unknown mechanism believed to involve α(7)- and α(4)β(2)-nAChR blockade. The three approaches of the current study evaluated the effects of chronic nicotine treatment in the prevention of Aβ-induced impairment of learning and short-term memory. Rat AD model was induced by 14-day i.c.v. osmotic pump infusion of a 1:1 mixture of 300 pmol/day Aβ(1-40)/Aβ(1-42) or Aβ(40-1) (inactive peptide, control). The effect of nicotine (2 mg/(kg day)) on Aβ-induced spatial learning and memory impairments was assessed by evaluation of performance in the radial arm water maze (RAWM), in vivo electrophysiological recordings of early-phase long-term potentiation (E-LTP) in urethane-anesthetized rats, and immunoblot analysis to determine changes in the levels of beta-site amyloid precursor protein (APP)-cleaving enzyme (BACE), Aβ and memory-related proteins. The results indicate that 6 weeks of nicotine treatment reduced the levels of Aβ(1-40) and BACE1 peptides in hippocampal area CA1 and prevented Aβ-induced impairment of learning and short-term memory. Chronic nicotine also prevented the Aβ-induced inhibition of basal synaptic transmission and LTP in hippocampal area CA1. Furthermore, chronic nicotine treatment prevented the Aβ-induced reduction of α(7)- and α(4)-nAChR. These effects of nicotine may be due, at least in part, to upregulation of brain derived neurotropic factor (BDNF).

Nicotinic receptor-mediated neuroprotection in neurodegenerative disease models

Multiple lines of evidence, from molecular and cellular to epidemiological, have implicated nicotinic transmission in the pathology of Alzheimer's disease (AD) and Parkinson's disease (PD). This review article presents evidence for nicotinic acetylcholine receptor (nAChR)-mediated protection and the signal transduction involved in this mechanism. The data is based mainly on our studies using rat-cultured primary neurons. Nicotine-induced protection was blocked by an alpha7 nAChR antagonist, a phosphatidylinositol 3-kinase (PI3K) inhibitor, and an Src inhibitor. Levels of phosphorylated Akt, an effector of PI3K, Bcl-2 and Bcl-x were increased by nicotine administration. From these experimental data, our hypothesis for the mechanism of nAChR-mediated survival signal transduction is that the alpha7 nAChR stimulates the Src family, which activates PI3K to phosphorylate Akt, which subsequently transmits the signal to up-regulate Bcl-2 and Bcl-x. Up-regulation of Bcl-2 and Bcl-x could prevent cells from neuronal death induced by beta-amyloid (Abeta), glutamate and rotenone. These findings suggest that protective therapy with nAChR stimulation could delay the progress of neurodegenerative diseases such as AD and PD.

Natural antioxidants protect neurons in Alzheimer's disease and Parkinson's disease

"Modern" medicine and pharmacology require an effective medical drug with a single compound for a specific disease. This seams very scientific but usually has unavoidable side effects. For example, the chemical therapy to cancer can totally damage the immunological ability of the patient leading to death early than non-treatment. On the other hand, natural antioxidant drugs not only can cure the disease but also can enhance the immunological ability of the patient leading to healthier though they usually have several compounds or a mixture. For the degenerative disease such as Alzheimer's disease (AD) and Parkinson's disease (PD), natural antioxidant drugs are suitable drugs, because the pathogenesis of these diseases is complex with many targets and pathways. These effects are more evidence when the clinic trial is for long term treatment. The author reviews the studies on the protecting effects of natural antioxidants on neurons in neurodegenerative diseases, especially summarized the results about protective effect of green tea polyphenols on neurons against apoptosis of cellular and animal PD models, and of genestine and nicotine on neurons against A beta-induced apoptosis of hippocampal neuronal and transgenic mouse AD models.