Functional Changes in the Adult Mouse Retina using an Alpha7 Nicotinic Acetylcholine Receptor Agonist after Blast Exposure

Currently, there is a lack of treatments for retinal neurotrauma. To address this issue, this study uses an alpha7 nAChR agonist, PNU-282987, to determine it effects on functional activity in the retina shortly after a traumatic blast exposure. The objectives of this research include: (1) examination of the cellular and functional damage associated with ocular blast exposure, and (2) evaluation of structural and functional changes that occur post PNU-282987 treatment. Significant ocular blast damage was induced in adult mice after exposure to a single blast of 35 psi to the left eye. Blast-exposed transgenic mice expressing tdTomato Müller glia were treated daily with eyedrops containing PNU-282987 for 4 weeks following the blast exposure. Antibody staining studies in these transgenic mice was conducted to examine lineage tracing and electroretinograms (ERGs) were obtained to examine functional changes. Blast exposure caused a significant loss of cells in all retinal layers after 4 weeks. Immunohistochemical analysis demonstrated tdTomato-positive labeled photoreceptors and retinal ganglion cells in blast-exposed mice treated with PNU-282987. ERG recordings were taken from control animals, from blast-damaged animals and from animals exposed to blast followed by 4 weeks of PNU-282987 treatment. Scotopic ERG recordings from blast-exposed mice had significantly decreased amplitudes of a-wave, b-wave, oscillatory potentials and flicker frequencies, which were prevented after PNU-282987 treatment. In photopic experiments, the PhNR response was reduced significantly after blast exposure but the decrease was prevented after treatment with PNU-282987. These are the first experiments that demonstrate preservation of retinal function after blast exposure using an alpha7 nAChR agonist.

Stimulation of α7 nAChR leads to regeneration of damaged neurons in adult mammalian retinal disease models

The adult mammal lacks the ability to regenerate neurons lost to retinal damage or disease in a meaningful capacity. However, previous studies from this laboratory have demonstrated that PNU-282987, an α7 nicotinic acetylcholine receptor agonist, elicits a robust neurogenic response in the adult murine retina. With eye drop application of PNU-282987, Müller glia cells re-enter the cell cycle and produce progenitor-like cells that can differentiate into various types of retinal neurons. In this study, we analyzed the regenerative capability of PNU-282987 in two retinal disease models and identified the source of newly regenerated neurons. Wild-type mice and mice with a transgenic Müller-glia lineage tracer were manipulated to mimic loss of retinal cells associated with glaucoma or photoreceptor degeneration. Following treatment with PNU-282987, the regenerative response of retinal neurons was quantified and characterized. After onset of photoreceptor degeneration, PNU-282987 was able to successfully regenerate both rod and cone photoreceptors. Quantification of this response demonstrated significant regeneration, restoring photoreceptors to near wild-type density. In mice that had glaucoma-like conditions induced, PNU-282987 treatment led to a significant increase in retinal ganglion cells. Retrograde labeling of optic nerve axon fibers demonstrated that newly regenerated axons projected into the optic nerve. Lineage tracing analysis demonstrated that these new neurons were derived from Müller glia. These results demonstrate that PNU-282987 can induce retinal regeneration in adult mice following onset of retinal damage. The ability of PNU-282987 to regenerate retinal neurons in a robust manner offers a new direction for developing novel and potentially transformative treatments to combat neurodegenerative disease.

BrdU Positive Cells Induced in a Genetic Mouse Model of Glaucoma

Previous studies have shown that eye drop application of the selective α7 nicotinic acetylcholine receptor agonist, PNU-282987, induces neurogenesis of RGCs in adult wild-type rodents. This study was designed to test the hypothesis that PNU-282987 reverses the loss of RGCs associated with glaucoma. A DBA/2J mouse model that auto-induces a glaucoma-like condition in adulthood was used for these studies. Short-term effects using PNU-282987 and BrdU eye drop treatments were examined, as well as the effects of early treatment and the effects in a chronic early treatment group in DBA/2J mice aged 3, 6 and 10 months. With and without treatment, retinas were removed, fixed, immunostained and RGC counts were assessed. IOP measurements were obtained weekly using a Tonolab tonometer. Results showed an average typical loss of BrdU positive RGCs by 29% by 10 months of age in this DBA/2J colony corresponding with a significant increase in IOP. However, the two-week short term application of PNU-282987 and BrdU induced a significant 21% increase in RGCs for DBA/2J mice at all ages. Chronic early PNU-282987 treatment produced a similarly significant increase in RGCs, while acute early treatment had no effect on RGC numbers. IOP measurements were not affected with PNU-282987 treatment. These studies demonstrated that 2-week treatment with PNU-282987, as well as chronic long-term treatment, induced a significant increase in the number of RGCs in the DBA/2J retina, counteracting the effects of the DBA/2J genetic glaucoma-like condition. These results suggest a potential future treatment of degenerative retinal diseases with PNU-282987.

Activation of α7-nAChRs Promotes the Clearance of α-Synuclein and Protects Against Apoptotic Cell Death Induced by Exogenous α-Synuclein Fibrils

Misfolding and abnormal aggregation of α-synuclein (αSyn) have been shown to increase the risk of developing Parkinson's disease (PD). Finding some way to reduce the aggregation of αSyn is particularly important for the treatment of PD. The main route in prion-like αSyn spreading is the cholinergic innervated vagus nervous system and central cholinergic neurons. Since the degenerative changes and death of cholinergic neurons also run through the pathological process of PD, we hypothesize an involvement of the cholinergic system in αSyn aggregation. The α7 nicotinic acetylcholine receptors (α7-nAChRs) are one of the most abundant nAChRs in the mammalian brain. Using nicotine and a selective α7-nAChRs agonist PNU-282987, we found a protective effect of α7-nAChRs on the cell damage induced by αSyn-PFF (preformed fibrils) through inhibiting apoptotic cell death. We further discovered an additive effect of α7-nAChRs on the clearance of αSyn in normal and αSyn stably transduced SH-SY5Y cells. Moreover, using α7-nAChRs knockout mice, we noticed that α7-nAChRs deficiency increased the deposition of αSyn and aggravated the loss of dopaminergic neurons in a chronic MPTP mouse model of PD. Our findings for the first time indicated that α7-nAChRs activation exhibited a neuroprotective effect on αSyn pathology and aggregation by promoting the clearance of αSyn.

PNU-282987 Attenuates Intestinal Epithelial Barrier Dysfunction in LPS-Induced Endotoxemia

PNU-282987, the α7 acetylcholine receptor(α7nAchR) agonist, has been repeatedly reported to play a key role in anti-inflammatory action of multiple disease. However, little is known about its effect on LPS-induced intestinal epithelial barrier dysfunction. This study investigated the protective effects and mechanisms of PNU-282987 on intestinal epithelial barrier dysfunction in lipopolysaccharide(LPS)-induced endotoxemic rats. Endotoxemia models were induced by intraperitoneal injection of 10 mg/kg LPS. In the endotoxemic group, results showed increases in ileum mucosal permeability, ultrastructural damage of tight junction and redistribution of zonula occludens-1, apoptosis of intestinal epithelial cells and caspase-3 activation. These changes were significantly improved after PNU-282987 administration(P < 0.05). Pretreatment with α-bungarotoxin before PNU-282987 administration reversed the effects of PNU-282987(P < 0.05). These results indicate that PNU-282987 exerts protective effects on intestinal epithelial barrier dysfunction in LPS-induced endotoxemic rats, and its mechanism may involve the improvement of zonula occludens-1 and inhibition of enterocyte apoptosis in an α7nAchR-dependent manner.

Protective Effect of the α7 Nicotinic Receptor Agonist PNU-282987 on Dopaminergic Neurons Against 6-Hydroxydopamine, Regulating Anti-neuroinflammatory and the Immune Balance Pathways in Rat

Neuroinflammation and inner immune dysfunction are increasingly accepted as important components of the etiopathogenesis of Parkinson’s disease (PD). According to emerging evidence, a7 nicotinic acetylcholine receptor (α7nAChR), a ligand-gated ion channel, plays an important role in inflammatory reactions and is also expressed on the surface of T cells. In particular, regulatory T cells (Tregs) are critical for the maintenance of immunological tolerance. In the present study, we investigated the roles of α7nAChR in inhibiting inflammation and maintaining the immune balance in rats with 6-hydroxydopamine (6-OHDA)-induced lesions and the possible mechanisms regulating the proportion of Tregs in vivo. Adult male Wistar rats (n = 90) were subjected to a unilateral injection of 6-OHDA into the left medial forebrain bundle, and PNU-282987, an α7nAChR agonist, was intraperitoneally injected 2 h prior to the induction of lesions by 6-OHDA and again at days 1, 7, and 13 postlesion. Behavioral tests and immunohistochemical staining to detect the expression of tyrosine hydroxylase (TH) in the bilateral substantial nigra (SN) were performed. Subsequently, CD4+ T lymphocytes and the expression of forkhead/winged helix transcription factor p3 (Foxp3, which is a marker of Treg cells) in the SN were also assessed using immunofluorescence staining. The expression of glial fibrillary acidic protein (GFAP) in the SN was determined by performing immunohistochemical staining. Additionally, the protein levels of α7nAChR, extracellular signal-regulated kinase (Erk) phosphorylated-Erk (p-Erk) and Foxp3 in the ventral midbrain were determined using Western blotting, and the relative expression of the TNF-α, IL-1β, and IL-10 mRNAs were detected using real-time quantitative reverse transcription-polymerase chain reaction (RT-PCR). We found that PNU-282987 significantly improved the motor deficits induced by 6-OHDA, reduced the loss of TH in the SN, suppressed the overactivation of GFAP+ cells and expression of related inflammatory cytokines, and increased the number of Foxp3+ cells. In addition, we also showed that PNU-282987 significantly increased the protein expression of the a7nAchR, p-Erk, and Foxp3 in 6-OHDA-lesioned rats (p < 0.05). These results indicated that α7nAChR activation could exert an anti-inflammatory effect and participate in the process of modulating the immune balance during 6-OHDA-induced injury, potentially through the α7nAChR/p-Erk/Foxp3 signaling pathway.

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.

Activation of α7 Nicotinic Acetylcholine Receptor Protects Against 1-Methyl-4-Phenylpyridinium-Induced Astroglial Apoptosis

Astrocytes, as the largest population of glial subtype, play crucial roles in normal brain function and pathological conditions, such as Parkinson's disease (PD). Restoring the functions of astrocyte is a promising new therapeutic target for PD. Astrocytes can express multiple types of neurotransmitter receptors, including functional α7 nicotinic acetylcholine receptor (α7nAChR). Previously, we found that a non-selective α7nAChR agonist nicotine exerted a protective effect against H₂O₂-induced astrocyte apoptosis via an α7nAChR-dependent pathway. However, the molecular mechanism of the antiapoptotic response of astroglial α7nAChR has not been studied. In the present study, using pharmacological inhibition and genetic knockout of α7nAChR, we assessed the antiapoptotic effects of an α7nAChR agonist PNU-282987 in primary cultured astrocytes treated with 1-methyl-4-phenylpyridinium (MPP⁺). PNU-282987 promoted the viability of astrocytes, alleviated MPP⁺ induced apoptosis, and decreased the number of GFAP⁺/TUNEL⁺ cells. Meanwhile, PNU-282987 upregulated the expression of the antiapoptotic protein Bcl-2 and downregulated the expression of the apoptotic protein Bax and cleaved-caspase-3. Moreover, the suppression of the JNK-p53-caspase-3 signaling may underlie the neuroprotective property of PNU-282987. Therefore, PNU-282987 ameliorates astroglial apoptosis induced by MPP⁺ through α7nAChR-JNK-p53 signaling. Our findings suggest that PNU-282987 may be a potential drug for restoring astroglial functions in the treatment of PD.

Eye Drops for Delivery of Bioactive Compounds and BrdU to Stimulate Proliferation and Label Mitotically Active Cells in the Adult Rodent Retina

Eye drop treatments are typically used to apply drugs to the anterior structures of the eye. Recently, however, studies have demonstrated that eye drops can reach the retina in the back of the eye if pharmacological agents are carried in appropriate vehicles. Here, we introduce an eye drop procedure to deliver a drug (PNU-282987), in combination with BrdU, to stimulate cell cycle re-entry and label dividing cells in the retinas of adult rodents. This procedure avoids potential systemic complications of repeated intraperitoneal injections, as well as the retinal damage that is induced by repeated intravitreal injections. Although the delivery of PNU-282987 and BrdU is the focus of this article, many different proliferating compounds could be delivered to the retina using this procedure.

Cholinergic agonist reverses H1-induced memory deficit in mice

This study investigated the effects of bilateral intraamygdalar microinjections of PNU-282987, a nicotinic cholinergic agonist, on anxiety and the reversal of amnesia induced by chlorpheniramine (CPA), an H1 histaminergic antagonist, in mice subjected to the elevated plusmaze (EPM). Two experiments were performed with seventy-nine adult male Swiss mice. The isolated microinjections of PNU-282987 did not produce effects on emotional memory; however, the combined microinjections of PNU-282987 and CPA were able to reverse the deficit in memory induced by CPA (ANOVA, p<0.05). Taken together, these results suggest that intraamygdalar injections of PNU-282987 did not induce effects on anxiety and emotional memory per se; however, concurrent microinjections of PNU-282987 and CPA-reverse amnesia induced-CPA which is suggestive of an interaction between the histaminergic and cholinergic systems in the modulation of emotion memory acquisition in mice.

Behavioral effects of different enriched environments in mice treated with the cholinergic agonist PNU-282987

Environmental enrichment is an experimental model in which rodents are housed in complex environments that favor lower levels of anxiety-like behavior. PNU-282987 (PNU) is a α7 nicotinic acetylcholine receptor agonist with beneficial effects on learning though its effects on anxiety are unclear. Our main aim was to carry out a study of its effects in NMRI (n=96) mice reared in different environments: environmental enrichment (EE), Marlau™ cages (MC) and standard environment (SE). After a 4-month period, mice received acute treatment of PNU (2.5, 5 and 10mg/kg) and were evaluated in the elevated plus-maze (EPM) and hole-board (HB). In the EPM, both EE and MC reared mice showed an increase in percentage of entries into open arms while those from EE group differed from SE in time spent on open arms. Mice treated with 2.5 and 10 mg/kg of PNU devoted less time to rearing into open arms. In the HB task, MC mice displayed higher exploratory activity reflected in more head-dips (HD) during the first minute than EE and SE, whereas EE displayed low exploration levels reflected in total HD (5 min). Further research is needed in order to clarify the behavioral effects of this nicotinic agonist in interaction with different environmental conditions. This article is part of a Special Issue entitled: insert SI title.