Activation of α7 nicotinic acetylcholine receptors increases intracellular cAMP levels via activation of AC1 in hippocampal neurons

Macrophage α7nAChR alleviates the inflammation of neonatal necrotizing enterocolitis through mTOR/NLRP3/IL-1β pathway

BACKGROUND

Neonatal necrotizing enterocolitis (NEC) is one of the most prevalent and severe intestinal emergencies in newborns. The inflammatory activation of macrophages is associated with the intestinal injury of NEC. The neuroimmune regulation mediated by α7 nicotinic acetylcholine receptor (α7nAChR) plays an important role in regulating macrophage activation and inflammation progression, but in NEC remains unclear. This study aims to explore the effect of macrophage α7nAChR on NEC.

METHODS

Mice NEC model were conducted with high-osmolarity formula feeding, hypoxia, and cold stimulation. The α7nAChR agonist PNU-282987 and mTOR inhibitor rapamycin were treated by intraperitoneal injections in mice. The expression and distribution of macrophages, α7nAChR, and phospho-mammalian target of rapamycin (p-mTOR) in the intestines of NEC patients and mice was assessed using immunohistochemistry, immunofluorescence, and flow cytometry. The expression of NLRP3, activated caspase-1 and IL-1β in mice intestines was detected by flow cytometry, western blot or ELISA. In vitro, the mouse RAW264.7 macrophage cell line was also cultured followed by various treatments. Expression of p-mTOR, NLRP3, activated caspase-1, and IL-1β in macrophages was determined.

RESULTS

Macrophages accumulated in the intestines and the expression of α7nAChR in the mucosal and submucosal layers of the intestines was increased in both the NEC patients and mice. The p-mTOR and CD68 were increased and co-localized in intestines of NEC patients. In vitro, α7nAChR agonist PNU-282987 significantly reduced the increase of NLRP3, activated caspase-1, and IL-1β in macrophages. PNU-282987 also significantly reduced the increase of p-mTOR. The effect was blocked by AMPK inhibitor compound C. The expression of NLRP3, activated caspase-1, and IL-1β was inhibited after mTOR inhibitor rapamycin treatment. In NEC model mice, PNU-282987 reduced the expression of p-mTOR, NLRP3, activated caspase-1, and IL-1β in the intestine. Meanwhile, rapamycin significantly attenuated NLRP3 activation and the release of IL-1β. Moreover, the proportion of intestinal macrophages and intestinal injury decreased after PNU-282987 treatment.

CONCLUSION

Macrophage α7nAChR activation mitigates NLRP3 inflammasome activation by modulating mTOR phosphorylation, and subsequently alleviates intestinal inflammation and injury in NEC.

Hydroxynorketamine, but not ketamine, acts via α7 nicotinic acetylcholine receptor to control presynaptic function and gene expression

Ketamine is clinically used fast-acting antidepressant. Its metabolite hydroxynorketamine (HNK) shows a robust antidepressant effect in animal studies. It is unclear, how these chemically distinct compounds converge on similar neuronal effects. While KET acts mostly as N-methyl-d-aspartate receptor (NMDAR) antagonist, the molecular target of HNK remains enigmatic. Here, we show that KET and HNK converge on rapid inhibition of glutamate release by reducing the release competence of synaptic vesicles and induce nuclear translocation of pCREB that controls expression of neuroplasticity genes connected to KET- and HNK-mediated antidepressant action. Ro25-6981, a selective antagonist of GluN2B, mimics effect of KET indicating that GluN2B-containing NMDAR might mediate the presynaptic effect of KET. Selective antagonist of α7 nicotinic acetylcholine receptors (α7nAChRs) or genetic deletion of Chrna7, its pore-forming subunit, fully abolishes HNK-induced synaptic and nuclear regulations, but leaves KET-dependent cellular effects unaffected. Thus, KET or HNK-induced modulation of synaptic transmission and nuclear translocation of pCREB can be mediated by selective signaling via NMDAR or α7nAChRs, respectively. Due to the rapid metabolism of KET to HNK, it is conceivable that subsequent modulation of glutamatergic and cholinergic neurotransmission affects circuits in a cell-type-specific manner and contributes to the therapeutic potency of KET. This finding promotes further exploration of new combined medications for mood disorders.

Role of Alpha-7-Nicotinic Acetylcholine Receptor in Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disorder affecting millions worldwide. One of the leading hypotheses for the underlying cause of AD is a reduction in nicotinic receptor levels in the brain. Among the nicotinic receptors, the alpha-7-nicotinic acetylcholine receptor (α7nAChR) has received particular attention due to its involvement in cognitive function.α7nAChR is a ligand-gated ion channel that is primarily found in the hippocampus and prefrontal cortex, areas of the brain responsible for learning, memory, and attention. Studies have shown that α7nAChR dysfunction is a key contributor to the pathogenesis of AD. The receptor is involved in regulating amyloidbeta (Aβ) production, a hallmark of AD pathology. Many drugs have been investigated as α7nAChR agonists or allosteric modulators to improve cognitive deficits in AD. Clinical studies have shown promising results with α7nAChR agonists, including improved memory and cognitive function. Although several studies have shown the significance of the α7 nAChR in AD, little is known about its function in AD pathogenesis. As a result, in this review, we have outlined the basic information of the α7 nAChR's structure, functions, cellular responses to its activation, and its role in AD's pathogenesis.

Apoptosis in the neuroprotective effect of α7 nicotinic receptor in neurodegenerative models

The α7 subtype of nicotinic receptors (α7 nAChRs) is one of the most abundant nicotinic receptor subtypes in the central nervous system (CNS) and both neurons and nonneuronal cells express α7 nAChRs. When activated, α7 nAChRs become permeable to cations and promote cellular responses such as anti-apoptotic signaling by modulating the caspases and proteins of the Bcl-2 family. Neuroprotection is an important function of these receptors, promoting neuronal survival under pathological conditions, including situations of stress and neuronal degeneration. Studies have demonstrated the relationship between the activation of these receptors and the reduction of neuronal or glial cell injury, by controlling apoptotic processes in different models, including neurodegenerative diseases such as Alzheimer's disease. Therefore, one of the most important signaling pathways activated by α7 nAChRs is the PI3K/Akt signaling cascade, which promotes the stimulation of anti-apoptotic molecules of the Bcl-2 family, Bcl-2 and Bcl-xl, and reduces the expression of caspases and proapoptotic molecules, resulting in cell survival. In Alzheimer's models, the literature shows that α7 nAChR activation attenuates Aβ-induced neurotoxicity through modulation of different intrinsic apoptotic pathways via PI3K/Akt and mitogen-activated protein kinase (MAPK). In this review, we provide an up-to-date summary of the current evidence on the relationship between the activation of α7 nAChRs, a subtype of nicotinic acetylcholine receptor, and its role in neuroprotection by modulating apoptotic pathways.

Contribution of the α5 nAChR Subunit and α5SNP to Nicotine-Induced Proliferation and Migration of Human Cancer Cells

Nicotine in tobacco is known to induce tumor-promoting effects and cause chemotherapy resistance through the activation of nicotinic acetylcholine receptors (nAChRs). Many studies have associated the α5 nicotinic receptor subunit (α5), and a specific polymorphism in this subunit, with (i) nicotine administration, (ii) nicotine dependence, and (iii) lung cancer. The α5 gene CHRNA5 mRNA is upregulated in several types of cancer, including lung, prostate, colorectal, and stomach cancer, and cancer severity is correlated with smoking. In this study, we investigate the contribution of α5 in the nicotine-induced cancer hallmark functions proliferation and migration, in breast, colon, and prostate cancer cells. Nine human cell lines from different origins were used to determine nAChR subunit expression levels. Then, selected breast (MCF7), colon (SW480), and prostate (DU145) cancer cell lines were used to investigate the nicotine-induced effects mediated by α5. Using pharmacological and siRNA-based experiments, we show that α5 is essential for nicotine-induced proliferation and migration. Additionally, upon downregulation of α5, nicotine-promoted expression of EMT markers and immune regulatory proteins was impaired. Moreover, the α5 polymorphism D398N (α5SNP) caused a basal increase in proliferation and migration in the DU145 cell line, and the effect was mediated through G-protein signaling. Taken together, our results indicate that nicotine-induced cancer cell proliferation and migration are mediated via α5, adding to the characterization of α5 as a putative therapeutical target.

α7 nicotinic acetylcholine receptors in the medial prefrontal cortex control rewarding but not aversive memory expression in a dopamine-sensitive manner

Emotional learning involves the association between sensory cues and rewarding or aversive stimuli, and this stored information can be recalled during memory retrieval. In this process, the medial prefrontal cortex (mPFC) plays an essential role. We have previously shown that the antagonism of α7 nicotinic acetylcholine receptors (nAChRs) by methyllycaconitine (MLA) in the mPFC blocked cue-induced cocaine memory retrieval. However, little is known about the involvement of prefrontal α7 nAChRs in the retrieval of aversive memories. Here, by using pharmacology and different behavioral tasks, we found that MLA did not affect aversive memory retrieval, indicating a differential effect of cholinergic prefrontal control of appetitive and aversive memories. Despite being shown that acetylcholine modulates dopamine release in the mPFC, it remains unknown if those modulatory systems act together to control reward-based behavior. We examined that question and found that dopamine type 1 receptor (D1R) activation prevented MLA-induced blockade of cocaine CPP retrieval. Our results suggest that α7 nAChRs and D1R signaling interact in the mPFC to modulate cocaine-associated memory retrieval.

Targeting α7-nAChR by galantamine mitigates reserpine-induced fibromyalgia-like symptoms in rats: Involvement of cAMP/PKA, PI3K/AKT, and M1/M2 microglia polarization

Fibromyalgia (FM) is a pain disorder marked by generalized musculoskeletal pain accompanied by depression, fatigue, and sleep disturbances. Galantamine (Gal) is a positive allosteric modulator of neuronal nicotinic acetylcholine receptors (nAChRs) and a reversible inhibitor of cholinesterase. The current study aimed to explore the therapeutic potential of Gal against reserpine (Res)-induced FM-like condition along with investigating the α7-nAChR's role in Gal-mediated effects. Rats were injected with Res (1 mg/kg/day; sc) for 3 successive days then Gal (5 mg/kg/day; ip) was given alone and with the α7-nAChR blocker methyllycaconitine (3 mg/kg/day; ip), for the subsequent 5 days. Galantamine alleviated Res-induced histopathological changes and monoamines depletion in rats' spinal cord. It also exerted analgesic effect along with ameliorating Res-induced depression and motor-incoordination as confirmed by behavioral tests. Moreover, Gal produced anti-inflammatory effect through modulating AKT1/AKT2 and shifting M1/M2 macrophage polarization. The neuroprotective effects of Gal were mediated through activating cAMP/PKA and PI3K/AKT pathways in α7-nAChR-dependent manner. Thus, Gal can ameliorate Res-induced FM-like symptoms and mitigate the associated monoamines depletion, neuroinflammation, oxidative stress, apoptosis, and neurodegeneration through α7-nAChR stimulation, with the involvement of cAMP/PKA, PI3K/AKT, and M1/M2 macrophage polarization.

Loss of GABA co-transmission from cholinergic neurons impairs behaviors related to hippocampal, striatal, and medial prefrontal cortex functions

Introduction: Altered signaling or function of acetylcholine (ACh) has been reported in various neurological diseases, including Alzheimer's disease, Tourette syndrome, epilepsy among others. Many neurons that release ACh also co-transmit the neurotransmitter gamma-aminobutyrate (GABA) at synapses in the hippocampus, striatum, substantia nigra, and medial prefrontal cortex (mPFC). Although ACh transmission is crucial for higher brain functions such as learning and memory, the role of co-transmitted GABA from ACh neurons in brain function remains unknown. Thus, the overarching goal of this study was to investigate how a systemic loss of GABA co-transmission from ACh neurons affected the behavioral performance of mice. Methods: To do this, we used a conditional knock-out mouse of the vesicular GABA transporter (vGAT) crossed with the ChAT-Cre driver line to selectively ablate GABA co-transmission at ACh synapses. In a comprehensive series of standardized behavioral assays, we compared Cre-negative control mice with Cre-positive vGAT knock-out mice of both sexes. Results: Loss of GABA co-transmission from ACh neurons did not disrupt the animal's sociability, motor skills or sensation. However, in the absence of GABA co-transmission, we found significant alterations in social, spatial and fear memory as well as a reduced reliance on striatum-dependent response strategies in a T-maze. In addition, male conditional knockout (CKO) mice showed increased locomotion. Discussion: Taken together, the loss of GABA co-transmission leads to deficits in higher brain functions and behaviors. Therefore, we propose that ACh/GABA co-transmission modulates neural circuitry involved in the affected behaviors.

Memantine has a nicotinic neuroprotective pathway in acute hippocampal slices after an NMDA insult

Memantine is a non-competitive antagonist with a moderate affinity to the N-methyl-d-Aspartate (NMDA) receptor. The present study assessed memantine's neuroprotective activity using electrophysiology of ex-vivo hippocampal slices. Interestingly, a nicotinic component was necessary for memantine's neuroprotection (NP). Memantine demonstrated a bell-shaped dose-response curve of NP against NMDA. Memantine was neuroprotective at concentrations below 3 μM, but the NP declined at higher concentrations (>3 μM) when memantine inhibits the NMDA receptor. Additional evidence that memantine NP is mediated by an alternate mechanism independent of the inhibition of the NMDA receptor is supported by its ability to protect neurons when applied before or after the NMDA insult and in the presence of D(-)-2-Amino-5-phosphonopentanoic acid (APV), the standard NMDA receptor inhibitor. We found several similarities between the memantine NP mechanism and the neuroprotective nicotinic drug, the 4R cembranoid. Memantine's NP requires the release of acetylcholine, the activation of α4β2, and is independent of MEK/MAPK signaling. Both 4R and memantine require the activation of PI3K/AKT for NP against NMDA-mediated excitotoxicity, although at different concentrations. In conclusion, our studies show memantine is neuroprotective through a nicotinic pathway, similar to the nicotinic drug 4R. This information leads to a better understanding of memantine's mechanisms of action and explains its dose-dependent effectiveness in Alzheimer's and other neurological disorders.

Axonal α7* nicotinic acetylcholine receptors modulate glutamatergic signaling and synaptic vesicle organization in ventral hippocampal projections

Modulation of the release of glutamate by activation of presynaptic nicotinic acetylcholine receptors (nAChRs) is one of the most prevalent mechanism of nicotinic facilitation of glutamatergic transmission in cortico-limbic circuits. By imaging gene chimeric co-cultures from mouse, we examined the role of α7* nAChRs mediated cholinergic modulation of glutamate release and synaptic vesicle organization in ventral hippocampal projections. We directly visualized exogenous and endogenous cholinergic facilitation of glutamate release in this specialized preparation of circuits in vitro. Disrupting α7* nAChRs mediated cholinergic signaling genetically or pharmacologically diminished cholinergic facilitation of glutamate release at presynaptic terminals. Alteration of α7* nAChRs mediated cholinergic signaling along glutamatergic axons also decreased functional synaptic vesicle clustering to presynaptic terminals. These findings suggest that presynaptic α7* nAChRs contribute to cholinergic modulation of glutamate release and synaptic vesicle organization.

Differential signalling induced by α7 nicotinic acetylcholine receptors in hippocampal dentate gyrus in vitro and in vivo

The activation of α7 nicotinic acetylcholine receptors (nAChRs) has been shown to improve hippocampus-dependent learning and memory. α7 nAChRs are densely expressed among several different cell types in the hippocampus, with high Ca²⁺  permeability, although it is unclear if α7 nAChRs mobilize differential signalling mechanisms among distinct neuronal populations. To address this question, we compared α7 nAChR agonist-induced responses (i.e. calcium and cAMP changes) between granule cells and GABAergic neurons in the hippocampal dentate gyrus both in vitro and in vivo. In cultured organotypic hippocampal slices, we observed robust intracellular calcium and cAMP increases in dentate granule cells upon activation of α7 nAChRs. In contrast, GABAergic interneurons displayed little change in either calcium or cAMP concentration after α7 nAChR activation, even though they displayed much larger α7 nAChR current responses than those of dentate granule cells. We found that this was due to smaller α7 nAChR-induced Ca²⁺ rises in GABAergic interneurons. Thus, the regulation of the Ca²⁺ transients in different cell types resulted in differential subsequent intracellular signalling cascades and likely the ultimate outcome of α7 nAChR activation. Furthermore, we monitored neuronal activities of dentate granule cells and GABAergic interneurons in vivo via optic fibre photometry. We observed enhancement of neuronal activities after nicotine administration in dentate granule cells, but not in GABAergic neurons, which was absent in α7 nAChR-deficient granule cells. In summary, we reveal a mechanism for α7 nAChR-mediated increase of neuronal activity via cell type-specific intracellular signalling pathways. KEY POINTS: α7 nicotinic acetylcholine receptors (nAChRs) are widely distributed throughout the central nervous system and regulate a variety of brain functions including learning and memory. Understanding the cellular signalling mechanisms of their activations among different neuronal populations is important for delineating their actions in cognitive function, and developing effective treatment strategies for cognitive deficits. We report that α7 nAChR activation leads to Ca²⁺ and cAMP increases in granule cells (but not in GABAergic interneurons) in hippocampal dentate gyrus in vitro, a key region for pattern separation during learning. We also found that nicotine enhanced granule cell (but not in GABAergic interneurons) activity in an α7 nAChR-dependent manner via in vivo fibre photometry recording. Based on our findings, we propose that differential responses to α7 nAChR activation between granule cells and GABAergic interneurons is responsible for the increase of excitation by α7 nAChR agonists in hippocampal circuits synergistically.

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

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

Nicotinic acetylcholine receptors in chemotherapeutic drugs resistance: An emerging targeting candidate

There is no definitive cure for cancer, and most of the current chemotherapy drugs have limited effects due to the development of drug resistance and toxicity at high doses. Therefore, there is an ongoing need for identifying the causes of chemotherapeutic resistance, and it will be possible to develop innovative treatment approaches based on these novel targeting candidates. Cigarette smoking is known to be one of the main causes of resistance to chemotherapeutic agents. Nicotine as a component of cigarette smoke is an exogenous activator of nicotinic acetylcholine receptors (nAChRs). It can inhibit apoptosis, increase cell proliferation and cell survival, reducing the cytotoxic effects of chemotherapy drugs and cause a reduced therapeutic response. Recent studies have demonstrated that nAChRs and their downstream signaling pathways have considerable implications in different cancer's initiation, progression, and chemoresistance. In some previous studies, nAChRs have been targeted to obtain better efficacies for chemotherapeutics. Besides, nAChRs-based therapies have been used in combination with chemotherapy drugs to reduce the side effects. This strategy requires lower doses of chemotherapy drugs compared to the conditions that must be used alone. Here, we discussed the experimental and clinical studies that show the nAChRs involvement in response to chemotherapy agents. Also, controversies relating to the effects of nAChR on chemotherapy-induced apoptosis are in our focus in this review article. Delineating the complex influences of nAChRs would be of great interest in establishing new effective chemotherapy regimens.

4R-cembranoid confers neuroprotection against LPS-induced hippocampal inflammation in mice

BACKGROUND

Chronic brain inflammation has been implicated in the pathogenesis of various neurodegenerative diseases and disorders. For example, overexpression of pro-inflammatory cytokines has been associated with impairments in hippocampal-dependent memory. Lipopolysaccharide (LPS) injection is a widely used model to explore the pathobiology of inflammation. LPS injection into mice causes systemic inflammation, neuronal damage, and poor memory outcomes if the inflammation is not controlled. Activation of the alpha-7 nicotinic receptor (α7) plays an anti-inflammatory role in the brain through vagal efferent nerve signaling. 4R-cembranoid (4R) is a natural compound that crosses the blood-brain barrier, induces neuronal survival, and has been shown to modulate the activity of nicotinic receptors. The purpose of this study is to determine whether 4R reduces the deleterious effects of LPS-induced neuroinflammation and whether the α7 receptor plays a role in mediating these beneficial effects.

METHODS

Ex vivo population spike recordings were performed in C57BL/6J wild-type (WT) and alpha-7-knockout (α7KO) mouse hippocampal slices in the presence of 4R and nicotinic receptor inhibitors. For in vivo studies, WT and α7KO mice were injected with LPS for 2 h, followed by 4R or vehicle for 22 h. Analyses of IL-1β, TNF-α, STAT3, CREB, Akt1, and the long-term novel object recognition test (NORT) were performed for both genotypes. In addition, RNA sequencing and RT-qPCR analyses were carried out for 12 mRNAs related to neuroinflammation and their modification by 4R.

RESULTS

4R confers neuroprotection after NMDA-induced neurotoxicity in both WT and α7KO mice. Moreover, hippocampal TNF-α and IL-1β levels were decreased with 4R treatment following LPS exposure in both strains of mice. 4R restored LPS-induced cognitive decline in NORT. There was a significant increase in the phosphorylation of STAT3, CREB, and Akt1 with 4R treatment in the WT mouse hippocampus following LPS exposure. In α7KO mice, only pAkt levels were significantly elevated in the cortex. 4R significantly upregulated mRNA levels of ORM2, GDNF, and C3 following LPS exposure. These proteins are known to play a role in modulating microglial activation, neuronal survival, and memory.

CONCLUSION

Our results indicate that 4R decreases the levels of pro-inflammatory cytokines; improves memory function; activates STAT3, Akt1, and CREB phosphorylation; and upregulates the mRNA levels of ORM2, GDNF, and C3. These effects are independent of the α7 nicotinic receptor.

The Neuroprotective Beta Amyloid Hexapeptide Core Reverses Deficits in Synaptic Plasticity in the 5xFAD APP/PS1 Mouse Model

Alzheimer's disease (AD) is the most common cause of dementia in the aging population. Evidence implicates elevated soluble oligomeric Aβ as one of the primary triggers during the prodromic phase leading to AD, effected largely via hyperphosphorylation of the microtubule-associated protein tau. At low, physiological levels (pM-nM), however, oligomeric Aβ has been found to regulate synaptic plasticity as a neuromodulator. Through mutational analysis, we found a core hexapeptide sequence within the N-terminal domain of Aβ (N-Aβcore) accounting for its physiological activity, and subsequently found that the N-Aβcore peptide is neuroprotective. Here, we characterized the neuroprotective potential of the N-Aβcore against dysfunction of synaptic plasticity assessed in ex vivo hippocampal slices from 5xFAD APP/PS1 mice, specifically hippocampal long-term potentiation (LTP) and long-term depression (LTD). The N-Aβcore was shown to reverse impairment in synaptic plasticity in hippocampal slices from 5xFAD APP/PS1 model mice, both for LTP and LTD. The reversal by the N-Aβcore correlated with alleviation of downregulation of hippocampal AMPA-type glutamate receptors in preparations from 5xFAD mice. The action of the N-Aβcore depended upon a critical di-histidine sequence and involved the phosphoinositide-3 (PI3) kinase pathway via mTOR (mammalian target of rapamycin). Together, the present findings indicate that the non-toxic N-Aβcore hexapeptide is not only neuroprotective at the cellular level but is able to reverse synaptic dysfunction in AD-like models, specifically alterations in synaptic plasticity.

S-oxiracetam Facilitates Cognitive Restoration after Ischemic Stroke by Activating α7nAChR and the PI3K-Mediated Pathway

S-oxiracetam (S-ORC), a nootropic drug, was used to protect against ischemic stroke by lessening the blood brain barrier dysfunction and inhibiting neuronal apoptosis. However, the potential effects of S-ORC in the recovery of cognitive functions after ischemic stroke and the underlying mechanisms remains unclear. In this study, middle cerebral artery occlusion/reperfusion (MCAO/R) in rats was used as the animal model. By using Y-maze test, Morris water maze, triphenyl tetrazolium chloride (TTC) staining, terminal deoxynucleotidyl transferase-mediated 2'-deoxyuridine 5'-triphosphate (dUTp) nick end labeling (TUNEL) assay, hematoxylin and eosin, immunohistochemical staining and western blot to evaluate the protective effect of S-ORC on cognitive recovery, we were able to confirm that S-ORC ameliorated spatial learning impairment, tissue loss, and hippocampal neuronal apoptosis and injury induced by MCAO/R in rats. These cognitive effects were achieved by restoring the normal function of synaptophysin and increasing PSD95 expression in the hippocampus. Furthermore, we found that methyllycaconitine, the antagonist of α7 nicotinic acetylcholine receptor (α7nAChR), and LY294002, the inhibitor of phosphoinositide 3-kinase (PI3K), were able to block the cognitive effects of S-ORC after MCAO/R in rats. In conclusion, α7nAChR and PI3K are key molecules that mediated the signaling pathway leading to S-ORC-induced cognitive restoration after MCAO/R.

Cellular responses and functions of α7 nicotinic acetylcholine receptor activation in the brain: a narrative review

The α7 nicotinic acetylcholine receptor (α7nAChR) has been studied for many years since its discovery. Although many functions and characteristics of brain α7nAChR are widely understood, much remains to be elucidated. The α7nAChR is widely expressed in the central nervous system, not only in neurons but also in astrocytes, microglia, and endothelial cells. α7nAChR can be activated by endogenous agonist like acetylcholine or exogenous agonists like nicotine and PNU282987. Its agonists can be divided into selective agonists and non-selective agonists. The activation of α7nAChR results in a series of physiological processes which have both short-term and long-term effects on cells, for example, calcium influx, neurotransmitter release, synaptic plasticity, and excitatory transmission. It also induces other downstream events, such as inflammation, autophagy, necrosis, transcription, and apoptosis. The cellular responses to α7nAChR activation vary according to cell types and conditions. For example, α7nAChR activation in pyramidal neurons leads to long-term potentiation, while α7nAChR activation in GABAergic interneurons leads to long-term depression. Studies have also shown some contradictory phenomena, which requires further study for clarification. Herein, the cellular responses of α7nAChR activation are summarized, and the functions of α7nAChR in neurons and non-neuronal cells are discussed. We also summarized contradictory conclusions to show where we stand and where to go for future studies.

The Spinal α7-Nicotinic Acetylcholine Receptor Contributes to the Maintenance of Cancer-Induced Bone Pain

Introduction

Cancer-induced bone pain (CIBP) is acknowledged as a multifactorial chronic pain that tortures advanced cancer patients, but existing treatment strategies for CIBP have not been satisfactory yet. Investigators have demonstrated that the activation of α7-nAChRs exerts analgesic effects in some chronic pain models. However, the role of spinal α7-nAChRs in CIBP remains unknown. This study was designed to investigate the role of α7-nAChRs in a well-established CIBP model induced by Walker 256 rat mammary gland carcinoma cells.

Methods

The paw withdrawal threshold (PWT) of the ipsilateral hind paw was measured using von Frey filament. The expressions of spinal α7-nAChRs and NF-κB were measured with Western blotting analysis. Immunofluorescence was employed to detect the expression of α7-nAChRs and co-expressed of α7-nAChRs with NeuN or GFAP or Iba1.

Results

Experiment results showed that the expression of spinal α7-nAChRs was significantly downregulated over time in CIBP rats, and in both CIBP rats and sham rats, most of the α7-nAChRs located in neurons. Behavioral data suggested PNU-282,987, a selective α7-nAChRs agonist, dose-dependently produced analgesic effect and positive allosteric modulator could intensify its effects. Further, repeated administration of PNU-282,987 reversed the expression of α7-nAChRs, inhibited the nuclear factor kappa B (NF-κB) signaling pathway, and attenuates CIBP-induced mechanical allodynia state as well.

Conclusion

These results suggest that the reduced expression of spinal α7-nAChRs contributes to the maintenance of CIBP by upregulating NF-κB expression, which implying a novel pharmacological therapeutic target for the treatment of CIBP.

Loss of α7 nicotinic acetylcholine receptors in GABAergic neurons causes sex-dependent decreases in radial glia-like cell quantity and impairments in cognitive and social behavior

The dentate gyrus (DG) is a unique brain structure in that neurons can be generated postnatally and integrated within existing circuitry throughout life. The maturation process of these newly generated neurons (granule cells) is modulated by nicotinic acetylcholine receptors (nAChRs) through a variety of mechanisms such as neural stem pool proliferation, cell survival, signal modulation, and dendritic integration. Disrupted nAChR signaling has been implicated in neuropsychiatric and neurodegenerative disorders, potentially via alterations in DG neurogenesis. GABAergic interneurons are known to express nAChRs, predominantly the α7 subtype, and have been shown to shape development, integration, and circuit reorganization of DG granule cells. Therefore, we examined histological and behavioral effects of knocking out α7 nAChRs in GABAergic neurons. Deletion of α7 nAChRs resulted in a reduction of radial glia-like cells within the subgranular zone of the DG and a concomitant trend towards decreased immature neurons, specifically in male mice, as well as sex-dependent changes in several behaviors, including social recognition and spatial learning. Overall, these findings suggest α7 nAChRs expressed in GABAergic neurons play an important role in regulating the adult neural stem cell pool and behavior in a sex-dependent manner. This provides important insight into the mechanisms by which cholinergic dysfunction contributes to the cognitive and behavioral changes associated with neurodevelopmental and neurodegenerative disorders.

Machine Learning Neuroprotective Strategy Reveals a Unique Set of Parkinson Therapeutic Nicotine Analogs

AIMS

Present a novel machine learning computational strategy to predict the neuroprotection potential of nicotine analogs acting over the behavior of unpaired signaling pathways in Parkinson's disease.

BACKGROUND

Dopaminergic replacement has been used for Parkinson's Disease (PD) treatment with positive effects on motor symptomatology but low progression and prevention effects. Epidemiological studies have shown that nicotine consumption decreases PD prevalence through neuroprotective mechanisms activation associated with the overstimulation of signaling pathways (SP) such as PI3K/AKT through nicotinic acetylcholine receptors (e.g α7 nAChRs) and over-expression of anti-apoptotic genes such as Bcl-2. Nicotine analogs with similar neuroprotective activity but decreased secondary effects remain as a promissory field.

OBJECTIVE

The objective of this study is to develop an interdisciplinary computational strategy predicting the neuroprotective activity of a series of 8 novel nicotine analogs over Parkinson's disease.

METHODS

We present a computational strategy integrating structural bioinformatics, SP manual reconstruction, and deep learning to predict the potential neuroprotective activity of 8 novel nicotine analogs over the behavior of PI3K/AKT. We performed a protein-ligand analysis between nicotine analogs and α7 nAChRs receptor using geometrical conformers, physicochemical characterization of the analogs and developed manually curated neuroprotective datasets to analyze their potential activity. Additionally, we developed a predictive machine-learning model for neuroprotection in PD through the integration of Markov Chain Monte-Carlo transition matrix for the 2 SP with synthetic training datasets of the physicochemical properties and structural dataset.

RESULTS

Our model was able to predict the potential neuroprotective activity of seven new nicotine analogs based on the binomial Bcl-2 response regulated by the activation of PI3K/AKT.

CONCLUSION

Hereby, we present a robust novel strategy to assess the neuroprotective potential of biomolecules based on SP architecture. Our theoretical strategy can be further applied to the study of new treatments related to SP deregulation and may ultimately offer new opportunities for therapeutic interventions in neurodegenerative diseases.

Discovery and Development of Non-Dopaminergic Agents for the Treatment of Schizophrenia: Overview of the Preclinical and Early Clinical Studies

Schizophrenia is a chronic psychiatric disorder that affects about 1 in 100 people around the world and results in persistent emotional and cognitive impairments. Untreated schizophrenia leads to deterioration in quality of life and premature death. Although the clinical efficacy of dopamine D2 receptor antagonists against positive symptoms of schizophrenia supports the dopamine hypothesis of the disease, the resistance of negative and cognitive symptoms to these drugs implicates other systems in its pathophysiology. Many studies suggest that abnormalities in glutamate homeostasis may contribute to all three groups of schizophrenia symptoms. Scientific considerations also include disorders of gamma-aminobutyric acid-ergic and serotonergic neurotransmissions as well as the role of the immune system. The purpose of this review is to update the most recent reports on the discovery and development of non-dopaminergic agents that may reduce positive, negative, and cognitive symptoms of schizophrenia, and may be alternative to currently used antipsychotics. This review collects the chemical structures of representative compounds targeting metabotropic glutamate receptor, gamma-aminobutyric acid type A receptor, alpha 7 nicotinic acetylcholine receptor, glycine transporter type 1 and glycogen synthase kinase 3 as well as results of in vitro and in vivo studies indicating their efficacy in schizophrenia. Results of clinical trials assessing the safety and efficacy of the tested compounds have also been presented. Finally, attention has been paid to multifunctional ligands with serotonin receptor affinity or phosphodiesterase inhibitory activity as novel strategies in the search for dedicated medicines for patients with schizophrenia.

Behavior and Hippocampal Epac Signaling to Nicotine CPP in Mice

Tobacco use is a major challenge to public health in the United States and across the world. Many studies have demonstrated that adult men and women differ in their responses to tobacco smoking, however neurobiological studies about the effect of smoking on males and females were limited. Exchange protein directly activated by cAMP (Epac) signaling participates in drug addictive behaviors. In this study, we examined the hippocampal Epac signaling in nicotine-induced place conditioning mice. Nicotine at 0.2 mg/kg and 0.4 mg/kg induced a conditioned place preference (CPP) in male and female mice, respectively. After CPP, male mice presented less anxiety-like behavior as demonstrated by an open-field test. The hippocampal Epac2 protein was elevated in both male and female nicotine place conditioning mice. However, Rap1 protein was elevated and CREB phosphorylation was reduced in female nicotine place conditioning mice. Our data provide direct evidence that hippocampal Epac signaling is altered in nicotine-induced CPP mice. Pharmacology manipulation Epac signaling may open a new avenue for the treatment of nicotine abuse and dependence.

Neuroimmune Communication in Health and Disease

The immune and nervous systems are tightly integrated, with each system capable of influencing the other to respond to infectious or inflammatory perturbations of homeostasis. Recent studies demonstrating the ability of neural stimulation to significantly reduce the severity of immunopathology and consequently reduce mortality have led to a resurgence in the field of neuroimmunology. Highlighting the tight integration of the nervous and immune systems, afferent neurons can be activated by a diverse range of substances from bacterial-derived products to cytokines released by host cells. While activation of vagal afferents by these substances dominates the literature, additional sensory neurons are responsive as well. It is becoming increasingly clear that although the cholinergic anti-inflammatory pathway has become the predominant model, a multitude of functional circuits exist through which neuronal messengers can influence immunological outcomes. These include pathways whereby efferent signaling occurs independent of the vagus nerve through sympathetic neurons. To receive input from the nervous system, immune cells including B and T cells, macrophages, and professional antigen presenting cells express specific neurotransmitter receptors that affect immune cell function. Specialized immune cell populations not only express neurotransmitter receptors, but express the enzymatic machinery required to produce neurotransmitters, such as acetylcholine, allowing them to act as signaling intermediaries. Although elegant experiments have begun to decipher some of these interactions, integration of these molecules, cells, and anatomy into defined neuroimmune circuits in health and disease is in its infancy. This review describes these circuits and highlights continued challenges and opportunities for the field.

Microphthalmia-associated transcription factor up-regulates acetylcholinesterase expression during melanogenesis of murine melanoma cells

Acetylcholinesterase (AChE) hydrolyzes the neurotransmitter acetylcholine in neurons. However, AChE has been proposed to also have nonneuronal functions in different cell types. Here, we report that AChE is expressed in melanocytes and melanoma cells, and that the tetrameric (G4) form is the major AChE isoform in these cells. During melanogenesis of B16F10 murine melanoma cells, AChE levels decreased markedly. The differentiation of melanoma cells led to (i) an increase in melanin and tyrosinase, (ii) a change in intracellular cAMP levels, and (iii) a decrease in microphthalmia-associated transcription factor (MITF). We hypothesized that the regulation of AChE during melanogenesis is mediated by two transcription factors: cAMP-response element-binding protein (CREB) and MITF. In melanoma cells, exogenous cAMP suppressed AChE expression and the promoter activity of the ACHE gene. This suppression was mediated by a cAMP-response element (CRE) located on the ACHE promoter, as mutation of CRE relieved the suppression. In melanoma, MITF overexpression induced ACHE transcription, and mutation of an E-box site in human ACHE promoter blocked this induction. An AChE inhibitor greatly enhanced acetylcholine-mediated responses of melanogenic gene expression levels in vitro; however, this enhancement was not observed in the presence of agonists of the muscarinic acetylcholine receptor. These results indicate that ACHE transcription is regulated by cAMP-dependent signaling during melanogenesis of B16F10 cells, and the effect of this enzyme on melanin production suggests that it has a potential role in skin pigmentation.

Beta-hydroxybutyrate Promotes the Expression of BDNF in Hippocampal Neurons under Adequate Glucose Supply

Neurobiological evidence suggests that the ketone metabolite β-hydroxybutyrate (BHBA) exerts many neuroprotective functions for the brain. The previous study revealed that BHBA could promote the expression of brain-derived neurotrophic factor (BDNF) at glucose inadequate condition. Here we demonstrated that BHBA administration induced the expression of BDNF in the hippocampus of mice fed with normal diet. In vitro experiment results also showed that 0.02-2 mM BHBA significantly increased BDNF expression in both the primary hippocampal neurons and the hippocampus neuron cell line HT22 under adequate glucose supply. Bdnf transcription induced by BHBA stimulus was mediated through the cAMP/PKA-triggered phosphorylation of CREB (S133) and the subsequent up-regulation of histone H3 Lysine 27 acetylation (H3K27ac) binding at Bdnf promoters I, II, IV, and VI. Moreover, BHBA stimulus induced a decrease in tri-methylation of H3K27 (H3K27me3) binding at the Bdnf promoters II and VI and the elevation of H3K27me3-specific demethylase JMJD3, which also contributed to the activation of Bdnf transcription. These results demonstrated that BHBA within the physiological range could promote BDNF expression in neurons via a novel signaling function. Moreover, BHBA might possess more broad epigenetic regulatory activities, which affected both the acetylation and demethylation of H3K27. Our findings reinforce the beneficial effect of BHBA on the central nervous system (CNS) and suggest that BHBA administration with no need for energy restriction might also be a promising intervention to improve the neuronal activity and ameliorate the degeneration of CNS.

Ionotropic and Metabotropic Mechanisms of Allosteric Modulation of α7 Nicotinic Receptor Intracellular Calcium

The pharmacological targeting of the α7 nicotinic acetylcholine receptor (α7) is a promising strategy in the development of new drugs for neurologic diseases. Because α7 receptors regulate cellular calcium, we investigated how the prototypical type II-positive allosteric modulator PNU120596 affects α7-mediated calcium signaling. Live imaging experiments show that PNU120596 augments ryanodine receptor-driven calcium-induced calcium release (CICR), inositol-induced calcium release (IICR), and phospholipase C activation by the α7 receptor. Both influx of calcium through the α7 nicotinic acetylcholine receptor (nAChR) channel as well as the binding of intracellular G proteins were involved in the effect of PNU120596 on intracellular calcium. This is evidenced by the findings that chelation of extracellular calcium, expression of α7D44A or α7345-348A mutant subunits, or blockade of calcium store release compromised the ability of PNU120596 to increase intracellular calcium transients generated by α7 ligand activation. Spatiotemporal stochastic modeling of calcium transient responses corroborates these results and indicates that α7 receptor activation enables calcium microdomains locally and to lesser extent in the distant cytosol. From the model, allosteric modulation of the receptor activates CICR locally via ryanodine receptors and augments IICR through enhanced calcium influx due to prolonged α7 nAChR opening. These findings provide a new mechanistic framework for understanding the effect of α7 receptor allosteric modulation on both local and global calcium dynamics.

CA1 pyramidal neuron gene expression mosaics in the Ts65Dn murine model of Down syndrome and Alzheimer's disease following maternal choline supplementation

Although there are changes in gene expression and alterations in neuronal density and afferent inputs in the forebrain of trisomic mouse models of Down syndrome (DS) and Alzheimer's disease (AD), there is a lack of systematic assessments of gene expression and encoded proteins within individual vulnerable cell populations, precluding translational investigations at the molecular and cellular level. Further, no effective treatment exists to combat intellectual disability and basal forebrain cholinergic neurodegeneration seen in DS. To further our understanding of gene expression changes before and following cholinergic degeneration in a well-established mouse model of DS/AD, the Ts65Dn mouse, we assessed RNA expression levels from CA1 pyramidal neurons at two adult ages (∼6 months of age and ∼11 months of age) in both Ts65Dn and their normal disomic (2N) littermates. We further examined a therapeutic intervention, maternal choline supplementation (MCS), which has been previously shown to lessen dysfunction in spatial cognition and attention, and have protective effects on the survival of basal forebrain cholinergic neurons in the Ts65Dn mouse model. Results indicate that MCS normalized expression of several genes in key gene ontology categories, including synaptic plasticity, calcium signaling, and AD-associated neurodegeneration related to amyloid-beta peptide (Aβ) clearance. Specifically, normalized expression levels were found for endothelin converting enzyme-2 (Ece2), insulin degrading enzyme (Ide), Dyrk1a, and calcium/calmodulin-dependent protein kinase II (Camk2a), among other relevant genes. Single population expression profiling of vulnerable CA1 pyramidal neurons indicates that MCS is a viable therapeutic for long-term reprogramming of key transcripts involved in neuronal signaling that are dysregulated in the trisomic mouse brain which have translational potential for DS and AD.

Beyond the Channel: Metabotropic Signaling by Nicotinic Receptors

The α7 nicotinic acetylcholine receptor (nAChR) is a ligand-gated ion channel (LGIC) that plays an important role in cellular calcium signaling and contributes to several neurological diseases. Agonist binding to the α7 nAChR induces fast channel activation followed by inactivation and prolonged desensitization while triggering long-lasting calcium signaling. These activities foster neurotransmitter release, synaptic plasticity, and somatodendritic regulation in the brain. We discuss here the ability of α7 nAChRs to operate in ionotropic (α7ⁱ) and metabotropic (α7^m) modes, leading to calcium-induced calcium release (CICR) and G protein-associated inositol trisphosphate (IP₃)-induced calcium release (IICR), respectively. Metabotropic activity extends the spatial and temporal aspects of calcium signaling by the α7 channel beyond its ionotropic limits, persisting into the desensitized state. Delineation of the ionotropic and metabotropic properties of the α7 nAChR will provide definitive indicators of moment-to-moment receptor functional status that will, in turn, spearhead new drug development.

Precisely Timed Nicotinic Activation Drives SST Inhibition in Neocortical Circuits

Sleep, waking, locomotion, and attention are associated with cell-type-specific changes in neocortical activity. The effect of brain state on circuit output requires understanding of how neuromodulators influence specific neuronal classes and their synapses, with normal patterns of neuromodulator release from endogenous sources. We investigated the state-dependent modulation of a ubiquitous feedforward inhibitory motif in mouse sensory cortex, local pyramidal (Pyr) inputs onto somatostatin (SST)-expressing interneurons. Paired whole-cell recordings in acute brain slices and in vivo showed that Pyr-to-SST synapses are remarkably weak, with failure rates approaching 80%. Pharmacological screening revealed that cholinergic agonists uniquely enhance synaptic efficacy. Brief, optogenetically gated acetylcholine release dramatically enhanced Pyr-to-SST input, via nicotinic receptors and presynaptic PKA signaling. Importantly, endogenous acetylcholine release preferentially activated nicotinic, not muscarinic, receptors, thus differentiating drug effects from endogenous neurotransmission. Brain state- and synapse-specific unmasking of synapses may be a powerful way to functionally rewire cortical circuits dependent on behavioral demands.

Protection against β-amyloid neurotoxicity by a non-toxic endogenous N-terminal β-amyloid fragment and its active hexapeptide core sequence

High levels (μM) of beta amyloid (Aβ) oligomers are known to trigger neurotoxic effects, leading to synaptic impairment, behavioral deficits, and apoptotic cell death. The hydrophobic C-terminal domain of Aβ, together with sequences critical for oligomer formation, is essential for this neurotoxicity. However, Aβ at low levels (pM-nM) has been shown to function as a positive neuromodulator and this activity resides in the hydrophilic N-terminal domain of Aβ. An N-terminal Aβ fragment (1-15/16), found in cerebrospinal fluid, was also shown to be a highly active neuromodulator and to reverse Aβ-induced impairments of long-term potentiation. Here, we show the impact of this N-terminal Aβ fragment and a shorter hexapeptide core sequence in the Aβ fragment (Aβcore: 10-15) to protect or reverse Aβ-induced neuronal toxicity, fear memory deficits and apoptotic death. The neuroprotective effects of the N-terminal Aβ fragment and Aβcore on Aβ-induced changes in mitochondrial function, oxidative stress, and apoptotic neuronal death were demonstrated via mitochondrial membrane potential, live reactive oxygen species, DNA fragmentation and cell survival assays using a model neuroblastoma cell line (differentiated NG108-15) and mouse hippocampal neuron cultures. The protective action of the N-terminal Aβ fragment and Aβcore against spatial memory processing deficits in amyloid precursor protein/PSEN1 (5XFAD) mice was demonstrated in contextual fear conditioning. Stabilized derivatives of the N-terminal Aβcore were also shown to be fully protective against Aβ-triggered oxidative stress. Together, these findings indicate an endogenous neuroprotective role for the N-terminal Aβ fragment, while active stabilized N-terminal Aβcore derivatives offer the potential for therapeutic application.

Neuro-immune interactions in the cholinergic anti-inflammatory reflex

Communication between the nervous and immune systems can significantly alter immune cell function in a number of inflammatory diseases. Elegant studies have defined a basic functional circuit in a "cholinergic anti-inflammatory pathway" that highlights a unique role for the vagus nerve, and has brought about a resurgence in the field of neuro-immunology. This research has further identified that in addition to tonic signals that can restrain immune cell activation; the anti-inflammatory reflex arc is amiable to targeted stimulation as a therapeutic modality. The success of vagal electrical neural stimulation in a plethora of pre-clinical inflammation models has spurred the development of "electroceuticals" or neurostimulatory devices in the treatment of chronic inflammation. This development has begun despite addressing of fundamental questions such as the functional neural circuitry being crudely mapped and unresolved mechanisms of action of acetylcholine on target immune cells. Perhaps fortuitously, rapid advances in neuroscience techniques may allow us to begin to answer some of these longstanding questions and clarify recent controversies.

Antidepressant therapies inhibit inflammation and microglial M1-polarization

Macrophages and their counterparts in the central nervous system, the microglia, detect and subsequently clear microbial pathogens and injured tissue. These phagocytic cells alter and adapt their phenotype depending on their prime activity, i.e., whether they participate in acute defence against pathogenic organisms ('M1'-phenotype) or in clearing damaged tissues and performing repair activities ('M2'-phenotype). Stimulation of pattern recognition receptors by viruses (vaccines), bacterial membrane components (e.g., LPS), alcohol, or long-chain saturated fatty acids promotes M1-polarization. Vaccine or LPS administration to healthy human subjects can result in sickness symptoms and low mood. Alcohol abuse and abdominal obesity are recognized as risk factors for depression. In the M1-polarized form, microglia and macrophages generate reactive oxygen and nitrogen radicals to eradicate microbial pathogens. Inadvertently, also tetrahydrobiopterin (BH4) may become oxidized. This is an irreversible reaction that generates neopterin, a recognized biomarker for depression. BH4 is a critical cofactor for the synthesis of dopamine, noradrenaline, and serotonin, and its loss could explain some of the symptoms of depression. Based on these aspects, the suppression of M1-polarization would limit the inadvertent catabolism of BH4. In the current review, we evaluate the evidence that antidepressant treatments (monoamine reuptake inhibitors, PDE4 inhibitors, lithium, valproate, agomelatine, tianeptine, electroconvulsive shock, and vagus nerve stimulation) inhibit LPS-induced microglia/macrophage M1-polarization. Consequently, we propose that supplementation with BH4 could limit the reduction in central monoamine synthesis and might represent an effective treatment for depressed mood.

Modulatory effects of α7 nAChRs on the immune system and its relevance for CNS disorders

The clinical development of selective alpha-7 nicotinic acetylcholine receptor (α7 nAChR) agonists has hitherto been focused on disorders characterized by cognitive deficits (e.g., Alzheimer's disease, schizophrenia). However, α7 nAChRs are also widely expressed by cells of the immune system and by cells with a secondary role in pathogen defense. Activation of α7 nAChRs leads to an anti-inflammatory effect. Since sterile inflammation is a frequently observed phenomenon in both psychiatric disorders (e.g., schizophrenia, melancholic and bipolar depression) and neurological disorders (e.g., Alzheimer's disease, Parkinson's disease, and multiple sclerosis), α7 nAChR agonists might show beneficial effects in these central nervous system disorders. In the current review, we summarize information on receptor expression, the intracellular signaling pathways they modulate and reasons for receptor dysfunction. Information from tobacco smoking, vagus nerve stimulation, and cholinesterase inhibition is used to evaluate the therapeutic potential of selective α7 nAChR agonists in these inflammation-related disorders.

The effect of α7 nicotinic receptor activation on glutamatergic transmission in the hippocampus

Nicotinic acetylcholine receptors (nAChRs) are expressed widely in the CNS, and mediate both synaptic and perisynaptic activities of endogenous cholinergic inputs and pharmacological actions of exogenous compounds (e.g., nicotine and choline). Behavioral studies indicate that nicotine improves such cognitive functions as learning and memory, however the cellular mechanism of these actions remains elusive. With help from newly developed biosensors and optogenetic tools, recent studies provide new insights on signaling mechanisms involved in the activation of nAChRs. Here we will review α7 nAChR's action in the tri-synaptic pathway in the hippocampus. The effects of α7 nAChR activation via either exogenous compounds or endogenous cholinergic innervation are detailed for spontaneous and evoked glutamatergic synaptic transmission and synaptic plasticity, as well as the underlying signaling mechanisms. In summary, α7 nAChRs trigger intracellular calcium rise and calcium-dependent signaling pathways to enhance glutamate release and induce glutamatergic synaptic plasticity.