α7 nicotinic acetylcholine receptors and their role in cognition

ABT-089, but not ABT-107, ameliorates nicotine withdrawal-induced cognitive deficits in C57BL6/J mice

Nicotine withdrawal produces cognitive deficits that can predict relapse. Amelioration of these cognitive deficits emerges as a target in current smoking cessation therapies. In rodents, withdrawal from chronic nicotine disrupts contextual fear conditioning (CFC), whereas acute nicotine enhances this hippocampus-specific learning and memory. These modifications are mediated by β2-subunit-containing (β2*) nicotinic acetylcholine receptors in the hippocampus. We aimed to test ABT-089, a partial agonist of α4β2*, and ABT-107, an α7 nicotinic acetylcholine receptor agonist, for amelioration of cognitive deficits induced by withdrawal from chronic nicotine in mice. Mice underwent chronic nicotine administration (12.6 mg/kg/day or saline for 12 days), followed by 24 h of withdrawal. At the end of withdrawal, mice received 0.3 or 0.6 mg/kg ABT-089 or 0.3 mg/kg ABT-107 (doses were determined through initial dose-response experiments and prior studies) and were trained and tested for CFC. Nicotine withdrawal produced deficits in CFC that were reversed by acute ABT-089, but not ABT-107. Cued conditioning was not affected. Taken together, our results suggest that modulation of hippocampal learning and memory using ABT-089 may be an effective component of novel therapeutic strategies for nicotine addiction.

The human CHRNA7 and CHRFAM7A genes: A review of the genetics, regulation, and function

The human α7 neuronal nicotinic acetylcholine receptor gene (CHRNA7) is ubiquitously expressed in both the central nervous system and in the periphery. CHRNA7 is genetically linked to multiple disorders with cognitive deficits, including schizophrenia, bipolar disorder, ADHD, epilepsy, Alzheimer's disease, and Rett syndrome. The regulation of CHRNA7 is complex; more than a dozen mechanisms are known, one of which is a partial duplication of the parent gene. Exons 5-10 of CHRNA7 on chromosome 15 were duplicated and inserted 1.6 Mb upstream of CHRNA7, interrupting an earlier partial duplication of two other genes. The chimeric CHRFAM7A gene product, dupα7, assembles with α7 subunits, resulting in a dominant negative regulation of function. The duplication is human specific, occurring neither in primates nor in rodents. The duplicated α7 sequence in exons 5-10 of CHRFAM7A is almost identical to CHRNA7, and thus is not completely queried in high throughput genetic studies (GWAS). Further, pre-clinical animal models of the α7nAChR utilized in drug development research do not have CHRFAM7A (dupα7) and cannot fully model human drug responses. The wide expression of CHRNA7, its multiple functions and modes of regulation present challenges for study of this gene in disease. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

A phase 2 randomized, controlled trial of the α7 agonist ABT-126 in mild-to-moderate Alzheimer's dementia

Introduction

The safety and efficacy of the novel α7 nicotinic acetylcholine receptor agonist ABT-126 were investigated in subjects with mild-to-moderate Alzheimer's dementia (AD).

Methods

Subjects not currently receiving acetylcholinesterase inhibitors were randomized to ABT-126 (5 or 25 mg once daily), donepezil 10 mg once daily, or placebo for 12 weeks. The primary efficacy end point was the change from baseline to final evaluation in the 11-item Alzheimer's Disease Assessment Scale-cognitive subscale (ADAS-Cog) total score.

Results

A total of 274 subjects were randomized. Although the study did not meet its primary end point, trends toward improvement were seen with ABT-126 25 mg (least squares mean [standard error] difference from placebo −1.19 [0.90]; one-sided P = .095) and donepezil (−1.43 [0.90]; one-sided P = .057) on the 11-item ADAS-Cog total score change from baseline to the final evaluation. ABT-126 5 mg was numerically similar to placebo. An exposure-response analysis indicated a statistically significant relationship between ABT-126 exposure and the change from baseline in ADAS-Cog, with no evidence of a plateau. No clinically meaningful differences in safety were observed among treatment groups.

Discussion

Although the ABT-126 25 mg dose did not demonstrate statistically significant improvement, results of the exposure-response analysis suggest that higher doses may produce better efficacy, and the safety profile of ABT-126 in this study supports additional studies with higher doses in subjects with mild-to-moderate AD.

Clinical trial register number

NCT00948909.

α7 and β2 Nicotinic Acetylcholine Receptor Subunits Form Heteromeric Receptor Complexes that Are Expressed in the Human Cortex and Display Distinct Pharmacological Properties

The existence of α7β2 nicotinic acetylcholine receptors (nAChRs) has recently been demonstrated in both the rodent and human brain. Since α7-containing nAChRs are promising drug targets for schizophrenia and Alzheimer’s disease, it is critical to determine whether α7β2 nAChRs are present in the human brain, in which brain areas, and whether they differ functionally from α7 nAChR homomers. We used α-bungarotoxin to affinity purify α7-containing nAChRs from surgically excised human temporal cortex, and found that α7 subunits co-purify with β2 subunits, indicating the presence of α7β2 nAChRs in the human brain. We validated these results by demonstrating co-purification of β2 from wild-type, but not α7 or β2 knock-out mice. The pharmacology and kinetics of human α7β2 nAChRs differed significantly from that of α7 homomers in response to nAChR agonists when expressed in Xenopus oocytes and HEK293 cells. Notably, α7β2 heteromers expressed in HEK293 cells display markedly slower rise and decay phases. These results demonstrate that α7 subunits in the human brain form heteromeric complexes with β2 subunits, and that human α7β2 nAChR heteromers respond to nAChR agonists with a unique pharmacology and kinetic profile. α7β2 nAChRs thus represent an alternative mechanism for the reported clinical efficacy of α7 nAChR ligands.

The muscarinic system, cognition and schizophrenia

An increasing body of evidence has implicated the central muscarinic system as contributing to a number of symptoms of schizophrenia and serving as a potential target for pharmaceutical interventions. A theoretical review is presented that focuses on the central muscarinic system's contribution to the cognitive symptoms of schizophrenia. The aim is to bridge the void between pertinent neuropsychological and neurobiological research to provide an explanatory account of the role that the central muscarinic system plays in the symptoms of schizophrenia. First, there will be a brief overview of the relevant neuropsychological schizophrenia literature, followed by a concise introduction to the central muscarinic system. Subsequently, we will draw from animal, neuropsychological and pharmacological literature, and discuss the findings in relation to cognition, schizophrenia and the muscarinic system. Whilst unifying the multiple domains of research into a concise review will act as a useful line of enquiry into the central muscarinic systems contribution to the symptoms of schizophrenia, it will be made apparent that more research is needed in this field.

Structure-activity relationship studies of SEN12333 analogues: determination of the optimal requirements for binding affinities at α7 nAChRs through incorporation of known structural motifs

Alpha7 nicotinic acetylcholine receptors (nAChRs) have implications in the regulation of cognitive processes such as memory and attention and have been identified as a promising therapeutic target for the treatment of the cognitive deficits associated with schizophrenia and Alzheimer's disease (AD). Structure affinity relationship studies of the previously described α7 agonist SEN12333 (8), have resulted in the identification of compound 45, a potent and selective agonist of the α7 nAChR with enhanced affinity and improved physicochemical properties over the parent compound (SEN12333, 8).

High therapeutic potential of positive allosteric modulation of α7 nAChRs in a rat model of traumatic brain injury: proof-of-concept

There are currently no clinically efficacious drug therapies to treat brain damage secondary to traumatic brain injury (TBI). In this proof-of-concept study, we used a controlled cortical impact model of TBI in young adult rats to explore a novel promising approach that utilizes PNU-120596, a previously reported highly selective Type-II positive allosteric modulator (α7-PAM) of α7 nicotinic acetylcholine receptors (nAChRs). α7-PAMs enhance and prolong α7 nAChR activation, but do not activate α7 nAChRs when administered without an agonist. The rational basis for the use of an α7-PAM as a post-TBI treatment is tripartite and arises from: (1) the intrinsic ability of brain injury to elevate extracellular levels of choline (a ubiquitous cell membrane-building material and a selective endogenous agonist of α7 nAChRs) due to the breakdown of cell membranes near the site and time of injury; (2) the ubiquitous expression of functional α7 nAChRs in neuronal and glial/immune brain cells; and (3) the potent neuroprotective and anti-inflammatory effects of α7 nAChR activation. Therefore, both neuroprotective and anti-inflammatory effects can be achieved post-TBI by targeting only a single player (i.e., the α7 nAChR) using α7-PAMs to enhance the activation of α7 nAChRs by injury-elevated extracellular choline. Our data support this hypothesis and demonstrate that subcutaneous administration of PNU-120596 post-TBI in young adult rats significantly reduces both brain cell damage and reactive gliosis. Therefore, our results introduce post-TBI systemic administration of α7-PAMs as a promising therapeutic intervention that could significantly restrict brain injury post-TBI and facilitate recovery of TBI patients.

Development of (18)F-labeled radiotracers for neuroreceptor imaging with positron emission tomography

Positron emission tomography (PET) is an in vivo molecular imaging tool which is widely used in nuclear medicine for early diagnosis and treatment follow-up of many brain diseases. PET uses biomolecules as probes which are labeled with radionuclides of short half-lives, synthesized prior to the imaging studies. These probes are called radiotracers. Fluorine-18 is a radionuclide routinely used in the radiolabeling of neuroreceptor ligands for PET because of its favorable half-life of 109.8 min. The delivery of such radiotracers into the brain provides images of transport, metabolic, and neurotransmission processes on the molecular level. After a short introduction into the principles of PET, this review mainly focuses on the strategy of radiotracer development bridging from basic science to biomedical application. Successful radiotracer design as described here provides molecular probes which not only are useful for imaging of human brain diseases, but also allow molecular neuroreceptor imaging studies in various small-animal models of disease, including genetically-engineered animals. Furthermore, they provide a powerful tool for in vivo pharmacology during the process of pre-clinical drug development to identify new drug targets, to investigate pathophysiology, to discover potential drug candidates, and to evaluate the pharmacokinetics and pharmacodynamics of drugs in vivo.

The α7 nicotinic receptor agonist ABT-107 decreases L-Dopa-induced dyskinesias in parkinsonian monkeys

Previous studies in Parkinsonian rats and monkeys have shown that β2-selective nicotinic acetylcholine receptor (nAChR) agonists reduce l-Dopa-induced dyskinesias (LIDs), a serious complication of l-Dopa therapy for Parkinson's disease. Since rodent studies also suggested an involvement of α7 nAChRs in LIDs, we tested the effect of the potent, selective α7 agonist ABT-107 [5-(6-[(3R)-1-azabicyclo[2.2.2]oct-3-yloxy] pyridazin-3-yl)-1H-indole]. MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine)-lesioned monkeys were gavaged with l-Dopa/carbidopa (10 and 2.5 mg/kg, respectively) twice daily, which resulted in stable LIDs. A dose-response study (0.03-1.0 mg/kg) showed that oral ABT-107 decreased LIDs by 40-60%. LIDs returned to control levels only after a 6-week ABT-107 washout, suggesting that long-term molecular changes were involved. Subsequent readministration of ABT-107 decreased LIDs by 50-60%, indicating that tolerance did not develop. ABT-107 had no effect on Parkinsonism or cognitive performance. We next tested ABT-107 together with the β2 agonist ABT-894 [(3-(5,6-dichloro-pyridin-3-yl)-1(S),5 (S)-3,6-diazabicyclo[3.2.0]heptane], previously shown to reduce LIDs in Parkinsonian monkeys. In one study, the monkeys were first given oral ABT-894 (0.01 mg/kg), which maximally decreased LIDs by 50-60%; they were then also treated with 0.1 mg/kg ABT-107, a dose that maximally reduced LIDs. The effect of combined treatment on LIDs was similar to that with either drug alone. Comparable results were observed in a group of monkeys first treated with ABT-107 and then also given ABT-894. Thus, α7 and β2 nAChR-selective drugs may function via a final common mechanism to reduce LIDs. The present results suggest that drugs targeting either α7 or β2 nAChRs may be useful as antidyskinetic agents in Parkinson's disease.

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

RATIONALE

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

OBJECTIVES

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

RESULTS

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

CONCLUSIONS

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

Sazetidine-A Activates and Desensitizes Native α7 Nicotinic Acetylcholine Receptors

The aim of this study was to investigate the ability of sazetidine-A, a novel partial agonist at α4β2 nicotinic acetylcholine receptors (nAChRs), to affect the function of native α7 nAChRs in SH-SY5Y cells and primary cortical cultures. The α7-selective positive allosteric modulator PNU-120596 was used to reveal receptor activation, measured as an increase in intracellular calcium using fluorescent indicators. In the absence of PNU-120596, sazetidine-A elicited mecamylamine-sensitive increases in fluorescence in SH-SY5Y cells (EC₅₀ 4.2 µM) but no responses from primary cortical neurons. In the presence on PNU-120596, an additional response to sazetidine-A was observed in SH-SY5Y cells (EC₅₀ 0.4 µM) and robust responses were recorded in 14 % of cortical neurons. These PNU-120596-dependent responses were blocked by methyllycaconitine, consistent with the activation of α7 nAChRs. Preincubtion with sazetidine-A concentration-dependently attenuated subsequent responses to the α7-selective agonist PNU-282987 in SH-SY5Y cells (IC₅₀ 476 nM) and cortical cultures. These findings support the ability of sazetidine-A to interact with α7 nAChRs, which may contribute to sazetidine-A’s actions in complex physiological systems.

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.

Kinetic properties and open probability of α7 nicotinic acetylcholine receptors

The alpha7 nicotinic acetylcholine receptor (nAChR) has some peculiar kinetic properties. From the literature of α7 nAChR-mediated currents we concluded that experimentally measured kinetic properties reflected properties of the solution exchange system, rather than genuine kinetic properties of the receptors. We also concluded that all experimentally measured EC50 values for agonists must inherently be inaccurate. The aim of this study was to assess the undistorted kinetic properties of α7 nAChRs, and to construct an improved kinetic model, which can also serve as a basis of modeling the effect of the positive allosteric modulator PNU-120596, as it is described in the accompanying paper. Agonist-evoked currents were recorded from GH4C1 cells stably transfected with pCEP4/rat α7 nAChR using patch-clamp and fast solution exchange. We used two approaches to circumvent the problem of insufficient solution exchange rate: extrapolation and kinetic modeling. First, using different solution exchange rates we recorded evoked currents, and extrapolated their amplitude and kinetics to instantaneous solution exchange. Second, we constructed a kinetic model that reproduced concentration-dependence and solution exchange rate-dependence of receptors, and then we simulated receptor behavior at experimentally unattainably fast solution exchange. We also determined open probabilities during choline-evoked unmodulated and modulated currents using nonstationary fluctuation analysis. The peak open probability of 10 mM choline-evoked currents was 0.033 ± 0.006, while in the presence of choline (10 mM) and PNU-120596 (10 μM), it was increased to 0.599 ± 0.058. Our kinetic model could adequately reproduce low open probability, fast kinetics, fast recovery and solution exchange rate-dependent kinetics.