Electrophysiological perspectives on the therapeutic use of nicotinic acetylcholine receptor partial agonists.

Partial agonist therapies rely variously on two hypotheses: the partial agonists have their effects through chronic low-level receptor activation or the partial agonists work by decreasing the effects of endogenous or exogenous full agonists. The relative significance of these activities probably depends on whether acute or chronic effects are considered. We studied nicotinic acetylcholine receptors (nAChRs) expressed in Xenopus laevis oocytes to test a model for the acute interactions between acetylcholine (ACh) and weak partial agonists. Data were best-fit to a basic competition model that included an additional factor for noncompetitive inhibition. Partial agonist effects were compared with the nAChR antagonist bupropion in prolonged bath application experiments that were designed to mimic prolonged drug exposure typical of therapeutic drug delivery. A primary effect of prolonged application of nicotine was to decrease the response of all nAChR subtypes to acute applications of ACh. In addition, nicotine, cytisine, and varenicline produced detectable steady-state activation of α4ß2* [(α4)(2)(ß2)(3), (α4)(3)(ß2)(2), and (α4)(2)(ß2)(2)α5)] receptor subtypes that was not seen with other test compounds. Partial agonists produced no detectable steady-state activation of α7 nAChR, but seemed to show small potentiation of ACh-evoked responses; however, "run-up" of α7 ACh responses was also sometimes observed under control conditions. Potential off-target effects of the partial agonists therefore included the modulation of α7 responses by α4ß2 partial agonists and decreases in α4ß2* responses by α7-selective agonists. These data indicate the dual effects expected for α4ß2* partial agonists and provide models and insights for utility of partial agonists in therapeutic development.

Dissociating beta-amyloid from alpha 7 nicotinic acetylcholine receptor by a novel therapeutic agent, S 24795, normalizes alpha 7 nicotinic acetylcholine and NMDA receptor function in Alzheimer's disease brain.

Alzheimer's disease (AD) is characterized by synaptic dysfunction and cardinal neuropathological features including amyloid plaques and neurofibrillary tangles. Soluble amyloid-beta (Abeta) can suppress synaptic activities by interacting with alpha7 nicotinic acetylcholine receptors (alpha7nAChRs). Here, we show that alpha7nAChR and NMDA glutamatergic receptor (NMDAR) activities are severely compromised in synaptosomes prepared from AD and Abeta(1-42) (Abeta42)-exposed control frontal cortex slices from postmortem tissue. Whereas Abeta(12-28) prevents Abeta42 from binding to alpha7nAChRs, 2-[2-(4-bromophenyl)-2-oxoethyl]-1-methyl pyridinium (S 24795), a novel alpha7nAChR partial agonist, facilitates release of Abeta42 from Abeta42-alpha7nAChR and -Abeta42 complexes. S 24795 interacts with alpha7nAChR and Abeta(15-20) region of the Abeta42 to enable partial recovery of the alpha7nAChR and NMDAR channel function. These findings suggest that the Abeta-alpha7nAChR interaction may be an upstream pathogenic event in AD and demonstrate that some recovery of neuronal channel activities may be achieved in AD brains by removing Abeta from alpha7nAChRs.

The partial alpha7 nicotine acetylcholine receptor agonist S 24795 enhances long-term potentiation at CA3-CA1 synapses in the adult mouse hippocampus.

The effects of S 24795, a newly developed partial agonist at alpha7 nAChRs, were tested on synaptic transmission and plasticity using extracellular field excitatory postsynaptic potentials (fEPSPs) evoked in the CA1 region by Schaffer collateral stimulation in hippocampal slices obtained from adult mice. S 24795 reduced the amplitude of the fEPSPs in a concentration-dependent manner with an IC(50) of 127 microM and a Hill coefficient of 1.1. The reduction in amplitude of the fEPSPs started at S 24795 concentrations higher than 3muM and reached 71% of controls at 300 microM. This effect was mediated by alpha7 nAChRs since it was blocked by nAChR antagonists and was not observed in alpha7 -/- mice. This effect was probably due to a reduction in glutamate release from presynaptic terminals since it was associated with a significant increase in the paired pulse ratio. In addition, S 24795 (100 microM) significantly reduced the frequency, but not the amplitude of spontaneous excitatory postsynaptic currents, recorded in the whole cell configuration of the patch clamp technique (in voltage clamp mode), further supporting a presynaptic site of action of S 24795. In addition, S 24795 at 3 microM, a concentration that did not affect basic synaptic transmission, potentiated LTP. This effect was mediated by alpha7 nAChRs since it was prevented by MLA (10 nM) and was absent in alpha7 -/- mice. Galantamine an allosteric modulator of nAChRs, at the concentrations of 0.3-3 microM, failed to potentiate LTP. In view of its powerful effect on LTP and on cognitive function, S 24795 can be considered a novel useful tool for the treatment of AD patients and other senile forms of dementia.

Improvement of contextual memory by S 24795 in aged mice: comparison with memantine.

RESULTS: In comparison with 5-month-old mice, 18- to 19-month-old mice exhibited a severe and specific memory impairment in a contextual serial discrimination (CSD) task involving the learning and remembering of two successive spatial discriminations carried out on two distinct floors. This impairment was specific, as spatial memory, simultaneously tested on a simple discrimination (SD) task, was not affected in these aged mice. This deficit was completely reversed by 9-day per os administration of S 24795, a partial agonist of alpha 7 nicotinic receptors, at either 0.3 or 1.0 mg/kg. Memantine, an NMDA receptor antagonist, also had a memory-enhancing effect at a dose of 3.0 mg/kg, but not at 0.3 mg/kg. CONCLUSIONS: The memory-enhancing effect of S 24795 was due to a strong enhancement of contextual memory as indicated by a decrease in interference rate, whereas memantine enhanced spatial/semantic memory. S 24795 was more effective than memantine and also appears to be more specific to flexible forms of memory, one of the first cognitive domains (i.e. episodic memory) affected in Alzheimer's disease.

Partial agonist and neuromodulatory activity of S 24795 for alpha7 nAChR responses of hippocampal interneurons.

S 24795 evoked methyllycaconitine-sensitive inward currents in voltage-clamped hippocampal interneurons with maximum amplitude about 14% that of ACh-evoked responses. Experiments with rat alpha7 receptors expressed in Xenopus oocytes confirmed that S 24795 is a partial agonist of alpha7 nAChR with an EC(50) of 34+/-11 microM and I(max) of approximately 10% relative to ACh. When 60 microM ACh was co-applied to alpha7-expressing oocytes along with increasing concentrations of S 24795, there was a progressive decrease in response compared to the responses to 60 microM ACh alone (IC(50) 45+/-9 microM). The positive allosteric modulator 5-hydroxyindole potentiated ACh- and S 24795-evoked responses of alpha7 receptors in both oocytes and hippocampal interneurons. In hippocampal slice experiments, depending on the ACh concentrations in the application pipette and the ratio of ACh to S 24795, co-application of S 24795 with ACh variously increased, decreased, or had no effect on responses, compared to ACh alone. In order to estimate the effective dilution factor for the pressure application experiments, we tested alpha7 receptors in oocytes with ACh alone and in co-application with S 24795 at the same ratios as in the slice experiments, but at varying dilution factors. The pattern of interaction seen in the slice experiments was most closely matched under the conditions of a 3:100 dilution, suggesting that the pipette solution was diluted approximately 30-fold at the site of action. This dilution factor was consistent with the potency of ACh and S 24795 in the oocyte expression system (EC(50)s approximately 30 microM).

Increased Aß42-α7-like nicotinic acetylcholine receptor complex level in lymphocytes is associated with apolipoprotein E4-driven Alzheimer's disease pathogenesis.

BACKGROUND: The apolipoprotein E ε4 (APOE4) genotype is a prominent late-onset Alzheimer's disease (AD) risk factor. ApoE4 disrupts memory function in rodents and may contribute to both plaque and tangle formation. METHODS: Coimmunoprecipitation and Western blot detection were used to determine: 1) the effects of select fragments from the apoE low-density lipoprotein (LDL) binding domain and recombinant apoE subtypes on amyloid beta (Aß)42-α7 nicotinic acetylcholine receptor (α7nAChR) interaction and tau phosphorylation in rodent brain synaptosomes; and 2) the level of Aß42-α7nAChR complexes in matched controls and patients with mild cognitive impairment (MCI) and dementia due to AD with known APOE genotypes. RESULTS: In an ex vivo study using rodent synaptosomes, apoE141-148 of the apoE promotes Aß42-α7nAChR association and Aß42-induced α7nAChR-dependent tau phosphorylation. In a single-blind study, we examined lymphocytes isolated from control subjects, patients with MCI and dementia due to AD with known APOE genotypes, sampled at two time points (1 year apart). APOE ε4 genotype was closely correlated with heightened Aß42-α7nAChR complex levels and with blunted exogenous Aß42 effects in lymphocytes derived from AD and MCI due to AD cases. Similarly, plasma from APOE ε4 carriers enhanced the Aß42-induced Aß42-α7nAChR association in rat cortical synaptosomes. The progression of cognitive decline in APOE ε4 carriers correlated with higher levels of Aß42-α7nAChR complexes in lymphocytes and greater enhancement by their plasma of Aß42-induced Aß42-α7nAChR association in rat cortical synaptosomes. CONCLUSIONS: Our data suggest that increased lymphocyte Aß42-α7nAChR-like complexes may indicate the presence of AD pathology especially in APOE ε4 carriers. We show that apoE, especially apoE4, promotes Aß42-α7nAChR interaction and Aß42-induced α7nAChR-dependent tau phosphorylation via its apoE141-148 domain. These apoE-mediated effects may contribute to the APOE ε4-driven neurodysfunction and AD pathologies.

Modulatory effects of S 38232, a non alpha-7 containing nicotine acetylcholine receptor agonist on network activity in the mouse hippocampus.

Extracellular field potentials (fEPSPs) and whole cell patch-clamp recordings were used to test the effect of S 38232, a newly developed potent non-alpha7 nicotinic acetylcholine receptors (nAChR) agonist, on synaptic transmission in hippocampal slices obtained from adult mice. S 38232 increased the amplitude of fEPSPs, evoked in stratum radiatum by Schaffer collateral stimulation. This effect was potentiated by picrotoxin, suggesting that S 38232 exerts at least in part its effect on GABAergic interneurons. The action of S 38232 was mediated by non-alpha7 containing nAChRs since it was prevented by DHbetaE (1muM) but not by alpha-BTX (100nM). A similar potentiating effect on fEPSPs was observed when nicotine (1muM) was applied to hippocampal slices obtained from alpha7 -/- mice in the presence of picrotoxin. The potentiating effect of S 38232 was probably presynaptic in origin since it was associated with a significant reduction in paired-pulse ratio. In addition, in patch clamp experiments, S 38232 enhanced the frequency (but not the amplitude) of spontaneous excitatory and inhibitory postsynaptic currents (sEPSCs, sIPSCs) recorded from CA1 principal cells. Moreover, it enhanced the frequency of miniature IPSCs but not EPSCs, suggesting that it was acting on nAChRs located on presynaptic/pre-terminal regions of GABAergic interneurons. The effect of S 38232 on GABAergic signaling was concentration-dependent with an EC(50) of 43muM. In conclusions, we present evidence that the new nicotine ligand S 38232, by selectively activating non-alpha7 nAChRs located on principal cells and GABAergic interneurons, influences network activity and information processing in the hippocampus.

Enhancement of attentional performance by selective stimulation of alpha4beta2(*) nAChRs: underlying cholinergic mechanisms.

Impairments in attention are a major component of the cognitive symptoms of neuropsychiatric and neurodegenerative disorders. Using an operant sustained attention task (SAT), including a distractor condition (dSAT), we assessed the putative pro-attentional effects of the selective alpha4beta2(*) nicotinic acetylcholine receptor (nAChR) agonist S 38232 in comparison with the non-selective agonist nicotine. Neither drug benefited SAT performance. However, in interaction with the increased task demands implemented by distractor presentation, the selective agonist, but not nicotine, enhanced the detection of signals during the post-distractor recovery period. This effect is consistent with the hypothesis that second-long increases in cholinergic activity ('transients') mediate the detection of cues and that nAChR agonists augment such transients. Electrochemical recordings of prefrontal cholinergic transients evoked by S 38232 and nicotine indicated that the alpha4beta2(*) nAChR agonist evoked cholinergic transients that were characterized by a faster rise time and more rapid decay than those evoked by nicotine. Blockade of the alpha7 nAChR 'sharpens' nicotine-evoked transients; therefore, we determined the effects of co-administration of nicotine and the alpha7 nAChR antagonist methyllycaconitine on dSAT performance. Compared with vehicle and nicotine alone, this combined treatment significantly enhanced the detection of signals. These results indicate that compared with nicotine, alpha4beta2(*) nAChR agonists significantly enhance attentional performance and that the dSAT represents a useful behavioral screening tool. The combined behavioral and electrochemical evidence supports the hypothesis that nAChR agonist-evoked cholinergic transients, which are characterized by rapid rise time and fast decay, predict robust drug-induced enhancement of attentional performance.

S 24795 limits beta-amyloid-alpha7 nicotinic receptor interaction and reduces Alzheimer's disease-like pathologies.

BACKGROUND: Beta-amyloid (Abeta) enables Alzheimer's disease (AD) plaque and neurofibrillary pathogenesis. Soluble Abeta promotes intraneuronal Abeta aggregates and tau phosphorylation by interacting with alpha7 nicotinic receptors (alpha7nAChRs). The current study assessed whether the novel alpha7nAChR partial agonist 2-(2-(4-bromophenyl)-2-oxoethyl)-1-methyl pyridinium (S 24795) could reduce AD-like pathologies by interfering with Abeta-alpha7nAChR interaction. METHODS: We compared the in vitro effect of S 24795, memantine, galantamine, and Abeta(12-28) on Abeta(42)-alpha7nAChR interaction in rat hippocampal synaptosomes. We further evaluated the effect of S 24795 on Abeta(42)-induced tau phosphorylation with rat hippocampal synaptosomes in vitro. Effects of S 24795 on Abeta(42) immunostaining, Abeta(42)-alpha7nAChR interaction, and/or Abeta(42)-mediated reduction of calcium (Ca(2+)) influx through alpha7nAChR and N-methyl-d-aspartate receptor (NMDAR) were assessed in Abeta(42)-incubated organotypic brain slices and intracerebroventricularly (ICV) Abeta(42)-injected mouse brain. RESULTS: Preincubation with S 24795 in vitro reduces Abeta(42)-alpha7nAChR interaction and Abeta(42)-induced tau phosphorylation. In organotypic brain slice cultures and in an ICV Abeta(42) injection in vivo model, S 24795 reduces Abeta(42)-alpha7nAChR association and Abeta(42) immunostaining. S 24795 also normalizes Ca(2+) fluxes through both alpha7nAChR and NMDAR channels in Abeta(42)-infused mouse brains and Abeta(42)-exposed organotypic cortical slices. Unlike S 24795 and Abeta(12-28), galantamine or memantine minimally affect Abeta(42)-alpha7nAChR coupling and Abeta(42)-mediated reduction of alpha7nAChR- and NMDAR-mediated Ca(2+) influx. INTERPRETATION: Drugs like S 24795 that disrupt Abeta(42)-alpha7nAChR interaction might alleviate Abeta(42)-mediated synaptic dysfunction and block AD-like pathologies.

Positive modulation of alpha7 nAChR responses in rat hippocampal interneurons to full agonists and the alpha7-selective partial agonists, 4OH-GTS-21 and S 24795.

One approach for the identification of therapeutic agents for Alzheimer's disease has focused on the research of alpha7 nAChR-selective agonists such as the partial agonists 3-(4-hydroxy,2-methoxybenzylidene)anabaseine (4OH-GTS-21) and, more recently, 2-[2-(4-bromophenyl)-2-oxoethyl]-1-methyl pyridinium (S 24795). An alternative approach for targeting alpha7 nAChR has been the development of positive modulators for this receptor. In this study we examined the interactions between full or partial agonists and positive modulators of alpha7 nAChRs in situ in brain tissue. Three positive modulators were used, 5-hydroxyindole (5-HI), 1-(5-chloro-2,4-dimethoxy-phenyl)-3-(5-methyl-isoxanol-3-yl)-urea (PNU-120596), and genistein. Whole-cell recordings were performed in stratum radiatum interneurons from rat brain slices. Hippocampal interneurons were stimulated by ACh, choline, S 24795, or 4OH-GTS-21, before and after bath perfusion with the positive modulators. 5-HI was not effective at potentiating 200 microM 4OH-GTS-21-evoked responses, however 5-HI induced a sustained potentiation of responses evoked by 30 microM 4OH-GTS-21. When 1 mM ACh and 200 microM 4OH-GTS-21 were applied alternately alpha7-mediated responses to both agonists were reduced, suggesting that high concentration of 4OH-GTS-21 produces residual inhibition or desensitization and that 5-HI is not effective at overcoming receptor desensitization. Similar results were obtained with alpha7 receptors expressed in Xenopus oocytes. Interestingly, responses evoked by S 24795 were potentiated by 5-HI but not by genistein. Additionally, PNU-120596 was able to potentiate alpha7-mediated responses, regardless of the nature of the agonist. We demonstrated that the potentiation of alpha7 nAChR response would depend on the nature and the effective concentration of the agonist involved and its particular interaction with the positive modulator.