A novel nicotinic agonist facilitates induction of long-term potentiation in the rat hippocampus

Nicotinic receptor-mediated enhancement of long-term potentiation involves activation of metabotropic glutamate receptors and ryanodine-sensitive calcium stores in the dentate gyrus

Little is known about the mechanisms underlying the enhancement of long-term potentiation (LTP) by nicotine. In the present study, the mechanisms of nicotinic enhancement of LTP were investigated in the rat dentate gyrus in vitro. Acute application of nicotine enhanced LTP induction, an action requiring activation of alpha7 nicotinic acetylcholine receptors (nAChRs), as it was blocked by the nAChR antagonist methyl-lycaconitine, mimicked by the acetylcholine receptor agonist choline and absent in mutant mice null for alpha7 nAChR. Nicotinic enhancement of LTP was both dependent on N-methyl-D-aspartate receptor activation, as no LTP was induced in the presence of nicotine and an N-methyl-D-aspartate receptor antagonist, and expressed post-synaptically, as no change in paired-pulse ratio accompanied nicotinic enhancement of LTP. The nicotinic-enhanced component of LTP, unlike control LTP, was dependent on activation of metabotropic glutamate receptors (mGluRs), being inhibited by the group I/II antagonist LY341495 and the mGluR5 antagonist MPEP, and also dependent on influx of Ca via L-type Ca channels and release from ryanodine (RyR)-sensitive intracellular stores, being prevented by nifedipine and RyR, respectively. It is suggested that nicotinic activation of the Ca-permeable alpha7 nAChRs fills RyR Ca stores and release of Ca from such stores by high-frequency stimulation via Ca-induced Ca release and activation of mGluRs induces an additional component of LTP which summates with control LTP. Chronic application of nicotine in vivo also enhanced LTP induction in slices and was dependent on activation of mGluRs and Ca release from RyR-sensitive intracellular stores, although acutely applied nicotine was not required for such enhanced LTP.

An autoradiographic analysis of rat brain nicotinic receptor plasticity following dietary choline modification

Choline is known to be involved with numerous physiological functions of the nervous system and also acts as a direct acting agonist of alpha7 nicotinic acetylcholine receptors (nAChRs). The purpose of this study was to conduct a brain region-specific evaluation of changes in nAChR subtype expression following dietary choline modification. In addition, we assessed changes in body weight, food/water intake, as well as changes in spatial learning (Morris Water Maze) in response to dietary choline modification. Male Sprague-Dawley rats were exposed to standard, choline supplemented or choline deficient diets for periods of 14 or 28 days. Choline supplemented animals gained significantly less weight over the course of the experiment, in spite of the fact that there were minimal differences in food consumption between the dietary regimens. Spatial memory did not differ between animals maintained on a standard rat diet, and the choline supplemented food. Brains of the animals kept on the diets for 14 and 28 days were used for quantitative autoradiographic analysis of nicotinic receptor subtypes using 125I-Bungarotoxin (alpha7) and 125I-Epibatidine (non-alpha7). There were no significant differences in nicotinic receptor binding or physiologic parameters measured between animals fed standard and choline deficient diets. However 2 weeks of dietary choline supplementation caused significant up-regulation of alpha7 receptors without significant effect on the density of non-alpha7 nAChRs. Increases in BTX binding predominantly occurred in cortical and hippocampal brain regions and ranged between 14 and 30% depending on the brain region. The results of our study suggest that choline acts as a selective agonist at alpha7 nicotinic cholinergic receptors in the rat central nervous system.

Blockade of nicotinic acetylcholine receptors suppresses hippocampal long-term potentiation in wild-type but not ApoE4 targeted replacement mice

Both impaired nicotinic neurotransmission and the inheritance of apoE4 are associated with increased risk for Alzheimer disease (AD) as well as other deficiencies in memory-related behavior. Long-term potentiation (LTP), a cellular model of memory, is known to be altered by nicotinic agents. Recent studies also support an emergent role for apoE in LTP. We compared the effects of mecamylamine, a nonspecific antagonist of nicotinic acetylcholine receptors (nAChRs), on basal synaptic transmission and LTP in hippocampal slices from wild-type (wt) mice and targeted replacement mice expressing human apoE4 (apoE4-TR). Field excitatory postsynaptic potentials (EPSPs) were recorded in the dentate gyrus (DG) in response to medial perforant path activation, and theta burst stimulation was used to induce LTP. Bath application of mecamylamine (3 microM) did not alter input-output relationships or paired pulse depression in either mouse strain. Under control conditions, apoE4-TR mice showed significantly less LTP than wt mice (17.5% +/- 3.2%, n = 9, vs. 30.1% +/- 3.9%, n = 11, P < 0.02). Mecamylamine reduced LTP in wt mice to a level that was similar to control levels for apoE4-TR mice (15.7% +/- 3.4%, n = 9), whereas apoE4-TR showed no further reduction of LTP (16.6% +/- 3.7%, n = 8) by mecamylamine. Thus mice expressing human apoE4 differ from wt mice both in their capacity for LTP and in the effect on LTP of nicotinic cholinergic blockade. It is possible that nicotinic neurotransmission is already compromised in apoE4-TR mice and, hence, that interference with the integrity of this cholinergic system represents a mechanism by which inheritance of the apoE4 allele contributes to cognitive risk.

α7 Nicotinic acetylcholine receptor as a target to rescue deficit in hippocampal LTP induction in β-amyloid infused rats

Continuous intracerebroventricular infusion of beta-amyloid peptide 1-40 (Abeta(1-40)) in animal models induces learning and memory impairment associated with dysfunction of the cholinergic neuronal system, which has been considered to be a pathological model of Alzheimer's disease [Nitta, A., Itoh, A., Hasegawa, T., Nabeshima, T., 1994. Beta-amyloid protein-induced Alzheimer's disease animal model. Neurosci. Lett. 170, 63-66.]. Here, using a real-time optical recording technique, we demonstrate that basal synaptic transmission and several forms of synaptic plasticity, including long-term potentiation (LTP), post-tetanic potentiation (PTP) and paired-pulse facilitation (PPF) are deficient at the Schaffer collateral-CA1 synapse in hippocampal slices from Abeta-infused brain. Throughout this study, an effort was made to address whether the alpha7 nicotinic acetylcholine receptor (alpha7nAChR), which is believed to be a primary target of Abeta [Wang, H.Y., Lee, D.H., Davis, C.B., Shank, R.P., 2000a. Amyloid peptide Abeta (1-42) binds selectively and with picomolar affinity to alpha 7 nicotinic acetylcholine receptors. J. Neurochem. 75, 1155-1161.], is responsible for the deficits in synaptic plasticity observed in the Abeta-infused rats. First, we found that Abeta-infusion markedly depressed the response of alpha7nAChR to a selective alpha7nAChR agonist [3-(2,4-dimethoxybenzylidene)-anabaseine] (DMXB). Second, blockade of alpha7nAChR with either methyllycaconitine (MLA) or alpha-bungarotoxin (alpha-BTX) in control rats inhibited LTP induction, suggesting that the activation of alpha7nAChR is required for LTP induction. Finally, pre-treatment of the slices from Abeta-infused rats with 10 microM DMXB rescued CA1 synapses from the deficit in LTP and PPF. These results suggest that Abeta-impaired LTP and PPF arise as a consequence of dysfunctional alpha7nAChR, and that alpha7nAChR may be an important target to help ameliorate AD patient cognitive deficits.

Dynorphin A (2-13) improves mecamylamine-induced learning impairment accompanied by reversal of reductions in acetylcholine release in rats

Accumulating evidence indicates that the endogenous opioid peptides dynorphin A (1-17) and synthetic dynorphin A (1-13) interact not only with opioid receptors but also with as yet poorly characterized non-opioid binding sites. Dynorphin A (1-13) improved impairments of learning and memory via not only kappa-opioid receptor-mediated, but also 'non-opioid' mechanisms. In the present study, the effects of des-tyrosine(1) dynorphin A (2-13) as a non-opioid metabolite of dynorphin A, and dynorphin A (1-13) on mecamylamine-induced impairment of the acquisition of learning in rats were investigated using a step-through type passive avoidance task. Further, hippocampal acetylcholine release was examined using in vivo microdialysis. Mecamylamine significantly shortened the step-through latency when given 30 min before the acquisition trial. Not only dynorphin A (1-13) but also dynorphin A (2-13) attenuated the mecamylamine-induced impairment of the acquisition of learning. The effect of dynorphin A (2-13) was not blocked by pre-treatment with nor-binaltorphimine (nor-BNI), a selective kappa-opioid receptor antagonist. Dynorphin A (2-13) completely abolished the decrease in the extracellular acetylcholine concentration induced by mecamylamine and this effect was not blocked by nor-BNI. Taken together with our previous findings, the present results may indicate that dynorphin A (2-13) improves impairment of learning and/or memory in 'non-opioid' mechanisms and dynorphin A (1-13) ameliorates impairment of the acquisition of learning via not only kappa-opioid receptor-mediated mechanisms but also 'non-opioid' mechanisms, by regulating the release of extracellular acetylcholine.

Nicotine-induced switch in the nicotinic cholinergic mechanisms of facilitation of long-term potentiation induction

Nicotine facilitates the induction of long-term potentiation (LTP) in the hippocampal CA1 region. The present study reveals the potential mechanisms underlying this effect of nicotine. Timed ACh-mediated activation of alpha7 nicotinic acetylcholine receptors (nAChRs) on pyramidal cells is known to promote LTP induction. Nicotine could suppress this timing-dependent mechanism by desensitizing nAChRs. Timed ACh-mediated activation of alpha7 nAChRs on feedforward interneurons can prevent LTP induction by inhibiting pyramidal cells. Nicotine diminished this ACh-mediated inhibition by desensitizing alpha7 nAChRs, thereby reducing the inhibitory influence on pyramidal cells. In addition to these desensitizing effects, nicotine activated presynaptic non-alpha7 nAChRs on feedforward interneurons to decrease the evoked release of gamma-aminobutyric acid (GABA) onto pyramidal cells. Furthermore, nicotine increased the frequency of spontaneous inhibitory postsynaptic currents (IPSCs) in pyramidal cells, and concomitantly caused a reduction in the size of responses to focal GABA application onto the dendrites of pyramidal cells, suggesting that the nicotine-induced increase in interneuronal activity leads ultimately to a use-dependent depression of evoked IPSCs in pyramidal cells. These nicotine-induced suppressions of inhibition of pyramidal cells were accompanied by enhanced N-methyl-D-aspartate (NMDA) responses in pyramidal cells. Thus, our results suggest that nicotine promotes the induction of LTP by diminishing inhibitory influences on NMDA responses while suppressing the ACh-mediated mechanisms. These ACh-independent mechanisms probably contribute to the nicotine-induced cognitive enhancement observed in the presence of cholinergic deficits, such as those in Alzheimer's disease patients.

Paired-pulse potentiation of alpha7-containing nAChRs in rat hippocampal CA1 stratum radiatum interneurones

Diverse subtypes of nicotinic acetylcholine receptors (nAChRs), including fast-desensitizing alpha7-containing receptors, are expressed in the CNS. While nAChRs appear to regulate cognitive processing and synaptic plasticity, little is known to date about how this regulation occurs, particularly in brain regions known to be important for cognition. By combining patch-clamp electrophysiology with local photolysis of caged carbachol to rapidly activate the alpha7-containing nAChRs in rat hippocampal CA1 stratum radiatum interneurones in slices, we describe a novel transient up-regulation of channel function. The nAChRs were activated using a paired-pulse uncaging protocol, where the duration of the UV laser pulses (5-25 ms) and the interval between pulses (200 ms to 30 s) were varied. At relatively long interpulse intervals, we observed a strong (> 75%) decrease in the amplitude of the second response due to desensitization. However, when two pulses were applied at a 200 ms interval, a > 3-fold increase in the amplitude of the second response was observed, a phenomenon referred to here as paired-pulse potentiation. Interestingly, this potentiation appeared to be regulated by [Ca2+]i, and/or Ca2+-dependent processes, as it was significantly enhanced by dialysing cells with either the Ca2+ chelator BAPTA, or with peptide inhibitors of either calcineurin or PKC, and was attenuated by dialysing cells with the CaMKII inhibitor KN-93. No potentiation was observed using caged GABA or glutamate, indicating some specificity for nAChRs. Thus, rat hippocampal alpha7-containing nAChRs possess a newly described phenomenon of paired-pulse potentiation that may be involved in regulating synaptic plasticity in the hippocampus.

Ca2+ permeability of nicotinic acetylcholine receptors in rat hippocampal CA1 interneurones

Neuronal nicotinic acetylcholine receptors (nAChRs) are widely expressed in the brain where they are involved in a variety of physiological processes, including cognition and development. The nAChRs are ligand-gated cationic channels, and different subtypes are known to be differentially permeable to Ca2+; the alpha7-containing nAChRs are generally considered to be the most permeable. Ca2+ can activate and regulate a variety of signal transduction cascades, and the influx of Ca2+ through these receptors may have implications for synaptic plasticity. To determine the Ca2+ permeability of the nAChRs in rat hippocampal interneurones in the slice, which contain diverse subtypes of alpha7- and non-alpha7-containing nAChRs, we combined patch-clamp electrophysiology recordings with conventional fura-2 fluorescence imaging techniques. We estimated the relative Ca2+ permeability of the channels by determining the ratio of the increase in [Ca2+]i level (Delta[Ca2+]i) in the soma to the integrated transmembrane current (charge, Q) induced by the activation of the nAChRs, and compared this ratio to the highly Ca2+ permeable NMDA subtype of glutamate receptor channel. In all cells tested, the Delta[Ca2+]i/Q ratio was significantly larger (i.e. more than twice as big) for responses activated by NMDA than for alpha7-containing nAChRs in interneurones; the activation of the non-alpha7 nAChRs did not produce any significant increase in [Ca2+]i. Interestingly, the Ca2+ permeability of native alpha7 nAChRs in PC12 cells was significantly larger than in hippocampal interneurones, and not significantly different from NMDA receptors. Therefore, the alpha7-containing nAChRs in rat hippocampal interneurones are significantly less permeable to Ca2+ than not only NMDA receptors but also alpha7 nAChRs in PC12 cells.

Tobacco components stimulate Akt-dependent proliferation and NFkappaB-dependent survival in lung cancer cells

Retrospective studies have shown that patients with tobacco-related cancers who continue to smoke after their diagnoses have lower response rates and shorter median survival compared with patients who stop smoking. To provide insight into the biologic basis for these clinical observations, we tested whether two tobacco components, nicotine or the tobacco-specific carcinogen, 4-(methylnitrosoamino)-1-(3-pyridyl)-1-butanone (NNK), could activate the Akt pathway and increase lung cancer cell proliferation and survival. Nicotine or NNK, rapidly and potently, activated Akt in non-small cell lung cancer (NSCLC) or small cell lung cancer (SCLC) cells. Nicotinic activation of Akt increased phosphorylation of multiple downstream substrates of Akt in a time-dependent manner, including GSK-3, FKHR, tuberin, mTOR and S6K1. Since nicotine or NNK bind to cell surface nicotinic acetylcholine receptors (nAchR), we used RT-PCR to assess expression of nine alpha and three beta nAchR subunits in five NSCLC cell lines and two types of primary lung epithelial cells. NSCLC cells express multiple nAchR subunits in a cell line-specific manner. Agonists of alpha3/alpha4 or alpha7 subunits activated Akt in a time-dependent manner, suggesting that tobacco components utilize these subunits to activate Akt. Cellular outcomes after nicotine or NNK administration were also assessed. Nicotine or NNK increased proliferation of NSCLC cells in an Akt-dependent manner that was closely linked with changes in cyclin D1 expression. Despite similar induction of proliferation, only nicotine decreased apoptosis caused by serum deprivation and/or chemotherapy. Protection conferred by nicotine was NFkappaB-dependent. Collectively, these results identify tobacco component-induced, Akt-dependent proliferation and NFkappaB-dependent survival as cellular processes that could underlie the detrimental effects of smoking in cancer patients.

Epibatidine induces long-term potentiation (LTP) via activation of alpha4beta2 nicotinic acetylcholine receptors (nAChRs) in vivo in the intact mouse dentate gyrus: both alpha7 and alpha4beta2 nAChRs essential to nicotinic LTP

Activation of nicotinic acetylcholine receptors (nAChRs) induces nocotinic long-term potentiation (LTPn) in vivo in the mouse dentate gyrus. We have found that alpha4beta2 nAChRs activated by epibatidine induce LTPn, the full size of which requires the involvement of alpha4beta2 and alpha7 nAChRs, in the intact mouse dentate gyrus using extracellular recording techniques. Intraperitoneal application of epibatidine, a potent alpha4beta2 nAChR agonist, at 0.3 - 3.0 mug/kg induced a long-lasting increase similar to LTPn induced by choline, a selective alpha7 nAChR agonist, and at 10 mug/kg caused a transient increase followed by a depression. The LTPn induced by epibatidine at 3.0 mug/kg or choline at 30 mg/kg was significantly suppressed by pre-treatment but not post-treatment with mecamylamine (0.5 mg/kg, i.p.), a non-selective neuronal nicotinic antagonist. Post-application of nicotine at 3.0 mg/kg enhanced epibatidine-induced LTPn to the same level of nicotine-induced LTPn, but post-application of epibatidine had no effect on nicotine-induced LTPn. Epibatidine-induced LTPn was additionally increased by post-application of choline, and vice versa, reaching the same level of nicotine-induced LTPn. The present study revealed that epibatidine induced the LTPn via alpha4beta2 nAChRs and that both alpha7 and alpha4beta2 nAChRs were essential for full-sized LTPn, suggesting that both nAChRs play an important role in synaptic plasticity.

Alpha-7 nicotinic receptor agonists: potential new candidates for the treatment of schizophrenia

RATIONALE AND OBJECTIVE

Auditory sensory gating, a biological measurement of the ability to suppress the evoked response to the second of two auditory stimuli, is diminished in people with schizophrenia. Deficits in sensory gating are associated with attentional impairment, and may contribute to cognitive symptoms and perceptual disturbances. This inhibitory process, which involves the alpha(7) nicotinic receptor mediated release of gamma-aminobutyric acid (GABA) by hippocampal interneurons, represents a potential new target for therapeutic intervention in schizophrenia.

METHOD

This paper will review several lines of evidence implicating the nicotinic-cholinergic, and specifically, the alpha(7) nicotinic receptor system in the pathology of schizophrenia and the evidence that alpha(7) nicotinic receptor agonists may ameliorate some of these deficits.

RESULTS

Impaired auditory sensory gating has been linked to the alpha(7) nicotinic receptor gene on the chromosome 15q14 locus. Single nucleotide polymorphisms of the promoter region of this gene are more frequent in people with schizophrenia. Although nicotine can acutely reverse diminished auditory sensory gating in people with schizophrenia, this effect is lost on a chronic basis due to receptor desensitization. Clozapine is able to reverse auditory sensory gating impairment, probably through an alpha(7) nicotinic receptor mechanism, in both humans and animal models with repeated dosing. The alpha(7) nicotinic agonist 3-2,4 dimethoxybenzylidene anabaseine (DMXBA) can also enhance auditory sensory gating in animal models. DMXBA is well tolerated in humans and improves several cognitive measures.

CONCLUSION

Alpha-7 nicotinic receptor agonists appear to be reasonable candidates for the treatment of cognitive and perceptual disturbances in schizophrenia.

Functional mapping and Ca2+ regulation of nicotinic acetylcholine receptor channels in rat hippocampal CA1 neurons

Diverse subtypes of nicotinic acetylcholine receptors (nAChRs), including fast-desensitizing alpha7-containing receptors thought to be Ca2+-permeable, are expressed in the CNS, where they appear to regulate cognitive processing and synaptic plasticity. To understand the physiological role of nAChRs in regulating neuronal excitability, it is important to know the distribution of functional receptors along the surface of neurons, whether they can increase [Ca2+]i, and/or are regulated by Ca2+. We mapped the distribution of receptors on the membrane of rat hippocampal CA1 stratum radiatum interneurons and pyramidal cells in acute slices by recording nAChR-mediated currents elicited by local UV laser-based photolysis of caged carbachol in patch-clamped neurons. The local application (approximately 7 microm patches) allowed mapping of functional nAChRs along the soma and dendritic tree, whereas the fast uncaging minimized the effects of desensitization of alpha7-containing nAChRs and allowed us to measure the kinetics of responses. The alpha7-containing nAChRs were the predominant subtype on interneurons, and were located primarily at perisomatic sites (<70 microm from the soma; in contrast to the more uniform distribution of glutamate receptors); no currents were detectable on pyramidal neurons. The activation of nAChRs increased [Ca2+]i, indicating that these native receptors in acute slices are significantly Ca2+-permeable, consistent with previous observations made with recombinant receptors. In addition, they exhibited strong desensitization, the rate of recovery from which was controlled by [Ca2+]i. Our results demonstrate the strategic location and Ca2+ regulation of alpha7-containing nAChRs, which may contribute to understanding their involvement in hippocampal plasticity.

The inhibition of long-term potentiation in the rat dentate gyrus by pro-inflammatory cytokines is attenuated in the presence of nicotine

Nicotine has previously been shown to affect both long-term potentiation (LTP) and long-term depression and to reverse age-related impairments of LTP in the hippocampus. Levels of proinflammatory cytokines are known to be elevated with age and to inhibit LTP. In the present study we have investigated the effects of three pro-inflammatory cytokines on nicotine-enhanced LTP in the rat hippocampus in vitro. In the presence of nicotine the inhibitory effect of interleukin-1 beta, interleukin-18 and tumour necrosis factor-alpha on LTP was eliminated. Furthermore, significant depotentiation of established LTP could not be obtained in slices treated with nicotine. These experiments demonstrate that nicotine can reverse the inhibitory effects of pro-inflammatory cytokines on LTP.

Functional properties of homomeric, human alpha 7-nicotinic acetylcholine receptors heterologously expressed in the SH-EP1 human epithelial cell line

alpha 7-Nicotinic acetylcholine receptors (alpha 7-nAChRs) are broadly distributed in the central nervous system, where they play important roles in chemical and electrical signaling, and perhaps in neurite outgrowth, synaptic plasticity, and neuronal death/survival. To help elucidate their normal and pathophysiological roles, we have heterologously expressed human alpha 7-nAChR in transfected SH-EP1 human epithelial cells. Reverse transcription-polymerase chain reaction and mRNA fluorescence in situ hybridization analyses demonstrate expression of human alpha 7 subunits as messenger RNA. Patch-clamp recordings exploiting a novel strategy to prevent functional rundown of whole-cell peak current responses to repeated acute challenges with nicotinic agonists show successful expression of functional alpha 7-nAChR that mediate inward currents characterized by rapid phases of activation and inactivation. Concentration-response curves show that nicotine, acetylcholine, and choline are efficacious agonists at human alpha 7-nAChRs. Current-voltage relationships show inward rectification for agonist-induced currents. Human alpha 7-nAChRs exhibit some sensitivity to alpha 7-nAChR antagonists alpha-bungarotoxin (Bgt) or methyllycaconitine (MLA) when applied coincidentally with agonist, but much higher affinity block occurs when cells and alpha 7-nAChRs are pre-exposed to antagonists for 2 min before challenge with agonist. Both Bgt and MLA are competitive inhibitors of alpha 7-nAChR function. Whole-cell current peak amplitudes and half-times for inactivation of alpha 7-nAChR functional responses to nicotine are dramatically reduced in the absence of extracellular Ca2+, suggestive of high Ca2+ permeability of the alpha 7-nAChR channel. Thus, heterologously expressed human alpha 7-nAChR in mammalian cells have properties of native alpha 7-nAChR or of alpha 7-nAChR heterologously expressed in other systems and serve as excellent models for studies of molecular bases of alpha 7-nAChR function.

Nicotine enhances the depressive actions of A beta 1-40 on long-term potentiation in the rat hippocampal CA1 region in vivo

Hippocampal long-term potentiation (LTP) is a form of synaptic plasticity used as a cellular model of memory. Beta amyloid (A beta) is involved in Alzheimer's disease (AD), a neurode-generative disorder leading to cognitive deficits. Nicotine is also claimed to act as a cognitive enhancer. A beta is known to bind with high affinity to the alpha 7-nicotinic acetylcholine receptor (nAChR). Here we have investigated the effect of intracerebroventricular (i.c.v.) injection of the endogenous peptide A beta 1-40 on LTP in area CA1 of urethananesthetized rats. We also examined the effect of A beta 12-28 (i.c.v.), which binds with high affinity to the alpha 7-nAChR and the specific alpha 7-nAChR antagonist methyllycaconitine (MLA) on LTP. We found that A beta 12-28 had no effect on LTP, whereas MLA depressed significantly LTP, suggesting that activation of the alpha 7-nAChR is a requirement for LTP. Within the in vivo environment, where other factors may compete with A beta 12-28 for binding to alpha 7-nAChR, it does not appear to modulate LTP. To determine if the depressive action of A beta 1-40 on LTP could be modulated by nicotine, these agents were also co-applied. Injection of 1 or 10 nmol A beta 1-40 caused a significant depression of LTP, whereas nicotine alone (3 mg/kg) had no effect on LTP. Co-injection of nicotine with A beta 1-40 1 h prior to LTP induction caused a further significant depression of LTP compared with A beta 1-40 alone. These results demonstrate that nicotine enhances the deficit in LTP produced by A beta 1-40. This then suggests that nicotine may exacerbate the depressive actions of A beta on synaptic plasticity in AD.

5-Hydroxyindole potentiates human alpha 7 nicotinic receptor-mediated responses and enhances acetylcholine-induced glutamate release in cerebellar slices

The effects of 5-hydroxyindole (5-HI) have been investigated on human alpha 7 nicotinic acetylcholine receptors (nAChRs) expressed in Xenopus oocytes and GH4 cells, on native alpha 7 nAChRs expressed by IMR-32 cells and on alpha 7 nAChR-mediated events in mossy fibre-granule cell synapses in rat cerebellar slices. In oocytes expressing alpha 7 nAChRs, 5-HI potentiated sub-maximal, 60 micro M ACh-induced ion currents in a concentration-dependent manner, the threshold effective concentration being 30 micro M. 5-HI itself did not act as an agonist on alpha 7 nAChRs. A maximum potentiation of 12 times the control was observed at 20 mM 5-HI. The effect of 1 mM 5-HI on the concentration-response curve for ACh revealed that 5-HI increased the potency as well as the efficacy of ACh on alpha 7 nAChRs. 5-HI also potentiated alpha 7-mediated increases in intracellular free calcium levels in both mammalian cells heterologously expressing human alpha 7 nAChRs and in human IMR-32 neuroblastoma cells expressing native alpha 7 nAChRs. At mossy fibre-granule cell synapses, application of 1 mM ACh induced glutamate-evoked excitatory post-synaptic currents (EPSCs). Co-application of 1 mM 5-HI with 1 mM ACh further increased the frequency of the EPSCs. The ACh-induced release, as well as the 5-HI-induced enhancement of release, were blocked by 1-10 nM methyllycaconitine or 200 nM alpha-bungarotoxin, demonstrating that both effects were mediated by presynaptic alpha 7 nAChRs. The results demonstrate that responses mediated by alpha 7 nAChRs are strongly potentiated by 5-HI.

Structure-activity relationships of benzylidene anabaseines in nicotinic acetylcholine receptors of cockroach nerve cords

Ten analogues of 6'-chloro-3-benzylideneanabaseine (CBA) bearing substituents at the ortho- and the para-positions of the phenyl group were synthesized, together with two related compounds. The affinity of the synthesized compounds for nicotinic acetylcholine receptors (nAChRs) in the nerve cord of the American cockroach (Periplaneta americana L.) was examined by the radioligand binding assay using [(3)H]epibatidine (EPI), a nAChR agonist. All 12 tested compounds inhibited [(3)H]EPI binding, showing K(i) values ranging from 14.6 to 6830nM. The potency variation of para-substituted CBA analogues was explained by the steric (Delta B(1)) and electronic (sigma(p)) parameters of the para-substituents, or by the steric parameter and the charge of the N1 nitrogen atom (qN(1)). Among the CBA analogues, only two compounds containing a dimethylamino group and a methoxy group at the para-position showed high insecticidal activity against the German cockroach (Blattella germanica) when injected after pretreatment with metabolic inhibitors. High-affinity analogues of CBA might be suitable probes for use in classifying and characterizing insect nAChR subtypes.

Alpha4beta2 nicotinic acetylcholine receptor activation ameliorates impairment of spontaneous alternation behavior in stroke-prone spontaneously hypertensive rats, an animal model of attention deficit hyperactivity disorder

The objective of the present study was to elucidate the role of nicotine in impairment of spontaneous alternation behavior of juvenile stroke-prone spontaneously hypertensive rats (SHRSP), an animal model of attention deficit hyperactivity disorder (ADHD). Spontaneous alternation behavior assessed by a Y-maze task was significantly lower, and total arm entries were significantly higher in SHRSP than in genetic control Wistar-Kyoto rats. Nicotine (0.1-1 mg/kg, s.c.) dose dependently improved the spontaneous alternation deficit without affecting total arm entries in SHRSP. Nicotine-induced (1 mg/kg, s.c.) improvement was significantly abolished by the centrally acting nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine (1 mg/kg, i.p.), but not by peripherally acting hexamethonium (5 mg/kg, i.p.), suggesting that nicotine-induced improvement is mediated via central nAChR. The alpha4beta2 nAChR antagonist dihydro-beta-erythroidine (3-10 mg/kg, i.p.) dose dependently counteracted nicotine-induced improvement of spontaneous alternation in SHRSP, whereas the alpha7 nAChR antagonist methyllycaconitine (3-10 mg/kg, i.p.) did not. In addition, the alpha4beta2 nAChR agonist RJR-2403 (N-methyl-4-(3-pyridinyl)-3-butene-1-amine; 1-10 mg/kg, s.c.) dose dependently and significantly improved the spontaneous alternation deficit. These findings revealed that nicotine improved spontaneous alternation behavior in SHRSP via the activation of alpha4beta2, but not alpha7, nAChR. Thus, the alpha4beta2 nAChR mechanism might be responsible for the spontaneous alternation deficit in juvenile SHRSP, an animal model of ADHD. This evidence indicates the possibility that selective alpha4beta2 nAChR agonists might be useful for treating attentional dysfunction in ADHD.

Benzylidene anabaseines act as high-affinity agonists for insect nicotinic acetylcholine receptors

Benzylidene anabaseines are agonists selective for vertebrate alpha7-nicotinic acetylcholine receptor (nAChR), while they exhibit antagonist activity toward vertebrate alpha4beta2-nAChR. To investigate the effects of benzylidene anabaseines on insect nAChRs, we performed [3H]epibatidine-binding assays and neurophysiological experiments using American cockroach (Periplaneta americana) nerve-cord preparations. Of the six compounds tested, 3-benzylideneanabaseine (BA) and 6'-chloro-3-benzylideneanabaseine (CBA) displayed the highest potency in the binding assays, with K(i)s of 35.0 and 21.2nM, respectively. The introduction of a nitro group at the 4-position of the phenyl group led to a decrease in affinity by two orders of magnitude, while that of a chlorine atom at the 6'-position had little effect on affinity. In neurophysiological experiments, BA at 3.3 microg/ml increased the spike frequency observed with the nerve preparation, as observed with nicotine at 16.6 microg/ml. These findings suggest that benzylidene anabaseines act as high-affinity agonists in P. americana nAChRs and that they might therefore prove useful as probes for insect nAChRs.

Hippocampal alpha 7 and alpha 4 beta 2 nicotinic receptors and working memory

Nicotine and other nicotinic receptor agonists have been found in a variety of studies to improve memory, while nicotinic receptor blockade can impair memory. The critical neural mechanisms for nicotinic involvement with memory are still under investigation. Initial evidence supports the involvement of the ventral hippocampus. Lesions in this area block nicotine-induced memory improvement and mecamylamine-induced impairment. Local ventral hippocampal application of the nicotinic channel blocker mecamylamine impairs memory in the 8-arm radial maze. Both alpha 4 beta 2 and alpha 7 nicotinic receptors seem to be involved. Ventral hippocampal infusions of high doses of the alpha 4 beta 2 nicotinic antagonist dihydro-beta-erythrodine (DH beta E) and the alpha 7 nicotinic antagonist methyllycaconitine (MLA) impair memory performance on the 8-arm radial maze. However, high doses of these drugs may limit specificity and they cause preconvulsant effects, which in themselves may affect memory. The current study used the more challenging 16-arm radial maze to determine the effects of lower doses of these drugs on memory and to differentiate effects on working and reference memory. Adult female Sprague-Dawley rats were trained on a working and reference memory task in the 16-arm radial maze and then were implanted with bilateral chronic guide cannulae directed to the ventral hippocampus. After recovery from surgery, the rats received acute intrahippocampal infusions of dose combinations of DH beta E and MLA. In the first study, DH beta E (0 and 6.75 microg/side) and MLA (0, 6.75, 13.5 and 27 microg/side) were administered in a counter-balanced order. In the second study, lower doses of DH beta E (0, 1.6375, 3.275 and 6.75 microg/side) were administered alone or with MLA (0 and 6.75 microg/side) in a counter-balanced order. In the first study, DH beta E caused a significant increase in both working and reference memory errors. MLA at a dose of 27 microg/side caused a significant increase in working memory errors, but this dose had no significant effect on reference memory errors. Interestingly, no additive effects were seen with combined administration of DH beta E and MLA in this study, and at the doses used, no effects were seen on response latency. In the second study, lower doses of DH beta E did not cause a significant deficit in working memory performance. Co-administration of MLA with these subthreshold doses did precipitate a memory impairment. The current results confirm the specificity of the memory deficits caused by these drugs. These results support the involvement of alpha 4 beta 2 and alpha 7 nicotinic receptors in the ventral hippocampus as being critical for memory function.

Clustering of nicotinic acetylcholine receptors: from the neuromuscular junction to interneuronal synapses

Fast and accurate synaptic transmission requires high-density accumulation of neurotransmitter receptors in the postsynaptic membrane. During development of the neuromuscular junction, clustering of acetylcholine receptors (AChR) is one of the first signs of postsynaptic specialization and is induced by nerve-released agrin. Recent studies have revealed that different mechanisms regulate assembly vs stabilization of AChR clusters and of the postsynaptic apparatus. MuSK, a receptor tyrosine kinase and component of the agrin receptor, and rapsyn, an AChR-associated anchoring protein, play crucial roles in the postsynaptic assembly. Once formed, AChR clusters and the postsynaptic membrane are stabilized by components of the dystrophin/utrophin glycoprotein complex, some of which also direct aspects of synaptic maturation such as formation of postjunctional folds. Nicotinic receptors are also expressed across the peripheral and central nervous system (PNS/CNS). These receptors are localized not only at the pre- but also at the postsynaptic sites where they carry out major synaptic transmission. In neurons, they are found as clusters at synaptic or extrasynaptic sites, suggesting that different mechanisms might underlie this specific localization of nicotinic receptors. This review summarizes the current knowledge about formation and stabilization of the postsynaptic apparatus at the neuromuscular junction and extends this to explore the synaptic structures of interneuronal cholinergic synapses.

The role of nicotinic acetylcholine receptors in the mechanisms of anesthesia

Nicotinic acetylcholine receptors are members of the ligand-gated ion channel superfamily, that includes also gamma-amino-butiric-acid(A), glycine, and 5-hydroxytryptamine(3) receptors. Functional nicotinic acetylcholine receptors result from the association of five subunits each contributing to the pore lining. The major neuronal nicotinic acetylcholine receptors are heterologous pentamers of alpha4beta2 subunits (brain), or alpha3beta4 subunits (autonomic ganglia). Another class of neuronal receptors that are found both in the central and peripheral nervous system is the homomeric alpha7 receptor. The muscle receptor subtypes comprise of alphabetadeltagamma (embryonal) or alphabetadeltaepsilon (adult) subunits. Although nicotinic acetylcholine receptors are not directly involved in the hypnotic component of anesthesia, it is possible that modulation of central nicotinic transmission by volatile agents contributes to analgesia. The main effect of anesthetic agents on nicotinic acetylcholine receptors is inhibitory. Volatile anesthetics and ketamine are the most potent inhibitors both at alpha4beta2 and alpha3beta4 receptors with clinically relevant IC(50) values. Neuronal nicotinic acetylcholine receptors are more sensitive to anesthetics than their muscle counterparts, with the exception of the alpha7 receptor. Several intravenous anesthetics such as barbiturates, etomidate, and propofol exert also an inhibitory effect on the nicotinic acetylcholine receptors, but only at concentrations higher than those necessary for anesthesia. Usual clinical concentrations of curare cause competitive inhibition of muscle nicotinic acetylcholine receptors while higher concentrations may induce open channel blockade. Neuronal nAChRs like alpha4beta2 and alpha3beta4 are inhibited by atracurium, a curare derivative, but at low concentrations the alpha4beta2 receptor is activated. Inhibition of sympathetic transmission by clinically relevant concentrations of some anesthetic agents is probably one of the factors involved in arterial hypotension during anesthesia.

Ventral hippocampal alpha 7 nicotinic receptor blockade and chronic nicotine effects on memory performance in the radial-arm maze

Chronic nicotine administration has been shown to significantly improve working memory. Nicotinic involvement in memory function critically involves the ventral hippocampus. Local ventral hippocampal infusions of the nicotinic antagonists mecamylamine, dihydro-beta-erythroidine (DH beta E) and methyllycaconitine (MLA) significantly impair working memory. The impairment caused by hippocampal infusion of the alpha 4 beta 2 antagonist DH beta E is reversed by chronic systemic nicotine. This study determined the interaction of chronic systemic nicotine with acute ventral hippocampal infusions of the alpha 7 antagonist MLA. Adult female Sprague-Dawley rats were trained on an 8-arm radial maze working memory task. Then they underwent ventral hippocampal cannulation and received sc implants of minipumps delivering nicotine (0 or 5 mg/kg/day for 28 days). Acute ventral hippocampal infusions of MLA (0, 4.88, 14.64 and 43.92 microg/side) were given during 3-4 weeks of chronic nicotine. MLA caused a significant dose-related memory impairment. In the rats not receiving nicotine, the 14.64 and 43.92 microg/side MLA doses caused significant memory impairment. Chronic systemic nicotine exposure did not block the MLA-induced memory impairment. Comparing the current results with MLA with previous results with DH beta E, equimolar ventral hippocampal DH beta E more effectively impaired memory than MLA, but the DH beta E-induced impairment was more effectively reversed by chronic systemic nicotine administration.

Ultrastructural distribution of the alpha7 nicotinic acetylcholine receptor subunit in rat hippocampus

Acetylcholine (ACh) is an important neurotransmitter in the mammalian brain; it is implicated in arousal, learning, and other cognitive functions. Recent studies indicate that nicotinic receptors contribute to these cholinergic effects, in addition to the established role of muscarinic receptors. In the hippocampus, where cholinergic involvement in learning and memory is particularly well documented, alpha7 nicotinic acetylcholine receptor subunits (alpha7 nAChRs) are highly expressed, but their precise ultrastructural localization has not been determined. Here, we describe the results of immunogold labeling of serial ultrathin sections through stratum radiatum of area CA1 in the rat. Using both anti-alpha7 nAChR immunolabeling and alpha-bungarotoxin binding, we find that alpha7 nAChRs are present at nearly all synapses in CA1 stratum radiatum, with immunolabeling present at both presynaptic and postsynaptic elements. Morphological considerations and double immunolabeling indicate that GABAergic as well as glutamatergic synapses bear alpha7 nAChRs, at densities approaching those observed for glutamate receptors in CA1 stratum radiatum. Postsynaptically, alpha7 nAChRs often are distributed at dendritic spines in a perisynaptic annulus. In the postsynaptic cytoplasm, immunolabeling is associated with spine apparatus and other membranous structures, suggesting that alpha7 nAChRs may undergo dynamic regulation, with insertion into the synapse and subsequent internalization. The widespread and substantial expression of alpha7 nAChRs at synapses in the hippocampus is consistent with an important role in mediating and/or modulating synaptic transmission, plasticity, and neurodegeneration.

Differential effects of chronic drug treatment on alpha3* and alpha7 nicotinic receptor binding sites, in hippocampal neurones and SH-SY5Y cells

1. The aim of this study was to compare the effects of chronic treatment (for 4 or 7 days) with nicotinic drugs and 20 mM KCl on numbers of surface alpha7 nicotinic AChR, identified by [(125)I]-alpha bungarotoxin (alpha-Bgt) binding, in primary hippocampal cultures and SH-SY5Y cells. Numbers of alpha3* nicotinic AChR were also examined in SH-SY5Y cells, using [(3)H]-epibatidine, which is predicted to label the total cellular population of predominantly alpha3beta2* nicotinic AChR under the conditions used. 2. All the nicotinic agonists examined, the antagonists d-tubocurarine and methyllycaconitine, and KCl, upregulated [(125)I]-alpha Bgt binding sites by 20 - 60% in hippocampal neurones and, where examined, SH-SY5Y cells. 3. Upregulation of [(125)I]-alpha-Bgt binding sites by KCl was prevented by co-incubation with the L-type Ca2+ channel blocker verapamil or the Ca2+-calmodulin dependent kinase II (CaM-kinase II) inhibitor KN-62. Upregulation of [(125)I]-alpha-Bgt binding sites by nicotine or 3,[(4-dimethylamino) cinnamylidene] anabaseine maleate (DMAC) was insensitive to these agents. 4. [(3)H]-Epibatidine binding sites in SH-SY5Y cells were not affected by KCl but were upregulated in a verapamil-insensitive manner by nicotine and DMAC. KN-62 itself provoked a 2 fold increase in [(3)H]-epibatidine binding. The inactive analogue KN-04 had no effect, suggesting that CaM-kinase II plays a role in regulating numbers of alpha3* nicotinic AChR. 5. These data indicate that numbers of alpha3* and alpha7 nicotinic AChR are modulated differently. Nicotinic agonists and KCl upregulate alpha7 nicotinic AChR through distinct cellular mechanisms, the latter involving L-type Ca2+ channels and CaM-kinase II. In contrast, alpha3* nicotinic AChR are not upregulated by KCl. This difference may reflect the distinct physiological roles proposed for alpha7 nicotinic AChR.

Neurotoxicity of channel mutations in heterologously expressed alpha7-nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChR) composed of chick alpha7 subunits mutated to threonine at amino acid valine-251 in the putative channel-lining M2 domain were expressed heterologously in several neuron-like and non-neuronal mammalian cell lines. Expression of mutant alpha7-nAChR is toxic to neuron-like cells of the human neuroblastoma cell lines SH-SY5Y and IMR-32, but not to several other cell types. Growth in the presence of the alpha7-nAChR antagonist methyllycaconitine (MLA) protects against neurotoxicity, as does gradual downregulation of functional, mutant alpha7-nAChR in surviving transfected SH-SY5Y cells. Relative to wild-type alpha7-nAChR, functional alpha7-nAChR mutants show a higher affinity for agonists, slower rates of desensitization, and sensitivity to dihydro-beta-erythroidine (DHbetaE) as an agonist, but they retain sensitivity to MLA as a competitive antagonist. These findings demonstrate that expression of hyperfunctional, mutant forms of Ca2+-permeable alpha7-nAChR is toxic to neuron-like cells.

alpha 7 nicotinic receptor transduces signals to phosphatidylinositol 3-kinase to block A beta-amyloid-induced neurotoxicity

Multiple lines of evidence, from molecular and cellular to epidemiological, have implicated nicotinic transmission in the pathogenesis of Alzheimer's disease (AD). Here we show the signal transduction mechanism involved in nicotinic receptor-mediated protection against beta-amyloid-enhanced glutamate neurotoxicity. Nicotine-induced protection was suppressed by an alpha7 nicotinic receptor antagonist (alpha-bungarotoxin), a phosphatidylinositol 3-kinase (PI3K) inhibitor (LY294002 and wortmannin), and a Src inhibitor (PP2). Levels of phosphorylated Akt, an effector of PI3K, and Bcl-2 were increased by nicotine. The alpha7 nicotinic receptor was physically associated with the PI3K p85 subunit and Fyn. These findings indicate that the alpha7 nicotinic receptor transduces signals to PI3K in a cascade, which ultimately contributes to a neuroprotective effect. This might form the basis of a new treatment for AD.

Nicotine accelerates reversal of long-term potentiation and enhances long-term depression in the rat hippocampal CA1 region

In the hippocampal CA1 region, low-frequency stimulation (LFS; 200 pulses at 1 Hz) causes reversal of long-term potentiation (depotentiation, DP) and long-term depression (LTD), both of which are thought to be the cellular substrate of learning and memory. Because nicotine enhances learning and memory, we examined if nicotine modulates DP and LTD in the hippocampal CA1 region. Bath application of nicotine during LFS accelerated DP, that is, potentiated synaptic responses in hippocampal CA1 neurons returned to pre-tetanic control levels more rapidly in the presence of nicotine. Because a similar acceleration of DP was observed using the alpha7 nicotinic acetylcholine receptor (nAChR)-selective antagonist methyllcaconitine (MLA), the nicotine effect appeared to be at least partly mediated by nicotine-induced desensitization of alpha7 nAChRs. Delivery of LFS in the presence of nicotine or MLA also depressed synaptic responses in a naive pathway and facilitated LTD, that is, the magnitude of LTD was larger when the drug was present during LFS. Thus, these results demonstrate that nicotine facilitates DP and LTD, which may represent, at least in part, the cellular mechanism underlying nicotine-induced cognitive enhancement.

Nicotinic receptor-mediated protection against beta-amyloid neurotoxicity

Multiple lines of evidence, from molecular and cellular to epidemiologic, have implicated nicotinic transmission in the pathology of Alzheimer's disease. In this review we present evidence for nicotinic receptor-mediated protection against beta-amyloid and glutamate neurotoxicity, and the signal transduction involved in this mechanism. The data are based mainly on our studies using rat-cultured primary neurons. Nicotine-induced protection was blocked by an alpha7 nicotinic receptor antagonist, a phosphatidylinositol 3-kinase inhibitor, and an Src inhibitor. Levels of phosphorylated Akt, an effector of phosphatidylinositol 3-kinase; Bcl-2; and Bcl-x were increased by nicotine administration. From these experimental data, our hypothesis for the mechanism of nicotinic receptor-mediated survival signal transduction is that the alpha7 nicotinic receptor stimulates the Src family, which activates phosphatidylinositol 3-kinase to phosphorylate Akt, which subsequently transmits the signal to upregulate Bcl-2 and Bcl-x. Upregulation of Bcl-2 and Bcl-x could prevent cells from neuronal death induced by beta-amyloid and glutamate. These findings suggest that an early diagnosis of Alzheimer's disease and protective therapy with nicotinic receptor stimulation could delay the progress of Alzheimer's disease.

Involvement of alpha7 nicotinic acetylcholine receptors in activation of tyrosine hydroxylase and dopamine beta-hydroxylase gene expression in PC12 cells

Nicotine treatment increases intracellular free Ca(2+) concentration [Ca(2+)](i), stimulates catecholamine release, and elevates gene expression for the catecholamine biosynthetic enzymes tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (DBH). However, the type of nicotinic acetylcholine receptors (nAChRs) mediating these events is unclear. The nAChR receptor antagonists alpha-bungarotoxin (alphaBTX) and methyllycaconitine greatly reduced the nicotine-triggered initial transient rise in [Ca(2+)](i) and prevented the second prolonged elevation of [Ca(2+)](i), suggesting the involvement of alpha7 nAChRs. Two specific alpha7 nicotinic agonists, 3-(2,4-dimethoxybenzilidene)anabaseine (DMXB) and E, E-3-(cinnamylidene)anabaseine (3-CA), were found to elicit a small, delayed increase in [Ca(2+)](i) with kinetics and magnitude similar to the second elevation observed with nicotine. This increase was inhibited by the inositol trisphosphate receptor antagonist xestospongin C. Exposure to 3-CA or DMXB for 6 or 24 h elevated TH and DBH mRNA levels two- to fourfold over control levels. These agonists were more effective than nicotine alone in increasing TH and DBH gene expression and significantly elevated [Ca(2+)](i) for up to 6 h. The increase in [Ca(2+)](i) or the elevation in TH mRNA by 3-CA was completely inhibited by alphaBTX. This study, for the first time, implicates stimulation of alpha7 nAChRs in the activation of TH and DBH gene expression.

Presynaptic nicotinic acetylcholine receptors as a functional target of nefiracetam in inducing a long-lasting facilitation of hippocampal neurotransmission

Nefiracetam (1-10 microM), a nootropic (or cognition-enhancing) agent, persistently potentiated currents through Torpedo acetylcholine (ACh) receptors expressed in Xenopus oocytes as a result of interacting with a protein kinase C pathway and the ensuing protein kinase C phosphorylation of the receptors. A similar effect was found in neuronal nicotinic ACh receptors (alpha4beta2 and alpha7). In contrast, the other nootropic agents such as piracetam and aniracetam had no potentiating action on the receptors. A sustained enhancement in the activity of nicotinic ACh receptors induced by nefiracetam caused a marked increase in the glutamate release, leading to a long-term potentiation-like facilitation of hippocampal synaptic transmissions. One of the consistent neuropathologic features of the Alzheimer brain is a loss of nicotinic ACh receptors. This fact, together with the results of our study, raises the possibility that the loss of nicotinic ACh receptors may be a key factor in the decline of cognitive function observed in Alzheimer disease and that agents targeting neuronal nicotinic ACh receptors like nefiracetam could, therefore, be of great therapeutic importance.

Activation of nicotinic acetylcholine receptors induces long-term potentiation in vivo in the intact mouse dentate gyrus

The present study was conducted to clarify the role of nicotinic ACh receptors (nAChRs) on long-term potentiation (LTP) in vivo in the intact mouse dentate gyrus using extracellular recording techniques. Intraperitoneal application of nicotine at a dose of 3.0 mg/kg but not 0.03 or 0.3 mg/kg produced a gradually developing, long-lasting increase for 120 min similar to tetanic LTP. Nicotine at a dose of 9. 0 mg/kg caused a temporary increase followed by depression. The long-lasting potentiation induced by nicotine at 3.0 mg/kg, which was named nicotinic long-term potentiation (LTPn), and tetanic LTP were significantly suppressed by pretreatment with mecamylamine (0.5 mg/kg i.p.), a nonselective nicotinic antagonist, but not affected by postapplication of mecamylamine. Interestingly, choline, a selective alpha7 nAChR agonist, at 3.0-90 mg/kg, induced the long-lasting potentiation similar to LTPn in a dose-dependent manner in vivo in the intact mouse dentate gyrus. The long-lasting potentiation induced by choline (30 mg/kg i.p.) was additionally increased by postapplication of nicotine (3.0 mg/kg i.p.) or tetanic stimulation. The present study revealed that systemic application of nicotine or choline induced the long-lasting potentiation in vivo in the intact mouse dentate gyrus, suggesting that alpha7 nAChRs may contribute to the induction of LTP by nicotine, and supporting in vivo animal studies that nicotine improves learning and memory performance.

Strategies that support declining cholinergic neurotransmission in Alzheimer's disease patients

With an increased public awareness of Alzheimer's disease (AD), the last two decades have witnessed an immense research effort directed towards discovering the cause of AD with the ultimate hope of developing safe and effective pharmacological treatments. Biochemical and histopathological changes of neurotransmitter markers in the brains of AD patients both at postmortem and neurosurgical cerebral biopsy have demonstrated an association between a decline in learning and memory, and a deficit in cholinergic and associated excitatory amino acid neurotransmission. These observations illustrate the selective neurotransmitter pathology of AD. Accordingly, although there is presently no 'cure' for AD, a large number of potential therapeutic strategies have emerged that are designed to correct the loss of cholinergic neurotransmission in AD. Such strategies include increasing acetylcholine synthesis, enhancing its presynaptic synthesis and/or release, potentiating its pre- or postsynaptic action, obstructing its metabolism, or by influencing the function of cholinergic neurones themselves by administration of nerve growth factor or by trans-synaptic modulation. Indeed, the cholinesterase (ChE) inhibitors have gained the most attention. Adverse events limited the usage of the first ChE inhibitors, but more recently introduced, well-tolerated compounds, for example donepezil, have confirmed efficacy in delaying the deterioration of symptoms of AD, a valuable treatment target considering the progressive nature of the disease. Other agents which affect the cholinergic system, directly or indirectly, that are in the early stages of development for the treatment of AD are also discussed.

Ca(2+)-sensitive inhibition by Pb(2+) of alpha7-containing nicotinic acetylcholine receptors in hippocampal neurons

In the present study the patch-clamp technique was applied to cultured hippocampal neurons to determine the kinetics as well as the agonist concentration- and Ca(2+)-dependence of Pb(2+)-induced inhibition of alpha7 nicotinic receptors (nAChRs). Evidence is provided that more than two-thirds of the inhibition by Pb(2+) (3-30 microM) of alpha7 nAChR-mediated whole-cell currents (referred to as type IA currents) develops rapidly and is fully reversible upon washing. The estimated values for tau(onset) and tau(recovery) were 165 and 240 ms, respectively. The magnitude of the effect of Pb(2+) was the same regardless of whether acetylcholine or choline was the agonist. Pre-exposure of the neurons for 800 ms to Pb(2+) (30 microM) decreased the amplitude and accelerated the decay phase of currents evoked by moderate to high agonist concentrations. In contrast, only the amplitude of currents evoked by low agonist concentrations was reduced when the neurons were exposed simultaneously to Pb(2+) and the agonists. Taken together with the findings that Pb(2+) reduces the frequency of opening and the mean open time of alpha7 nAChR channels, these data suggest that Pb(2+) accelerates the rate of receptor desensitization. An additional reduction of type IA current amplitudes occurred after 2-min exposure of the neurons to Pb(2+). This effect was not reversible upon washing of the neurons and was most likely due to an intracellular action of Pb(2+). Pb(2+)-induced inhibition of alpha7 nAChRs, which was hindered by the enhancement of extracellular Ca(2+) concentrations, may contribute to the neurotoxicity of the heavy metal.

The brain alpha7 nicotinic receptor may be an important therapeutic target for the treatment of Alzheimer's disease: studies with DMXBA (GTS-21)

A large decrease in brain nicotinic receptor levels occurs in Alzheimer's disease, relative to muscarinic and other receptors. Neurons possessing high affinity nicotinic receptors seem particularly vulnerable. The low affinity nicotinic receptors which selectively bind alpha-bungarotoxin are not significantly affected. The major nicotinic receptor subtype which binds this toxin is a homo-oligomer composed of alpha7 subunits. Due to its exceptionally high calcium ion selectivity, this particular receptor can be considered as a ligand-gated calcium channel. Alpha7 receptors are found in regions of the brain which are important for cognition, including cerebral cortex and hippocampus. Hippocampal receptors are largely confined to GABAergic interneurons. Alpha7 receptors seem less likely than alpha4-beta2 receptors to be up-regulated in number and down-regulated in function as a result of chronic agonist exposure. A family of nicotinic agonists based upon the marine animal toxin anabaseine have been synthesized and investigated. One of these compounds, DMXBA [3-(2,4-dimethoxybenzylidene)-anabaseine; code name GTS-21] has displayed promising characteristics during phase I clinical tests. In the rat DMXBA is selectively agonistic upon alpha7 nicotinic receptors. In addition it is a moderately potent antagonist at alpha4-beta2 receptors. DMXBA enhances a variety of cognitive behaviors in mice, monkeys, rats and rabbits. It also displays neuroprotective activity upon cultured neuronal cells exposed to beta-amyloid or deprived of NGF. The compound is much less toxic than nicotine and does not affect autonomic and skeletal muscle systems at doses which enhance cognitive behavior. Phase I clinical tests indicate that large doses can be safely administered orally without adverse effects. Psychological tests on healthy young male subjects indicate a positive effect of DMXBA on some measures of cognition. While DMXBA is a much weaker partial agonist on human alpha7 receptors than upon rat alpha7 receptors, its 4-hydroxy metabolite has been shown to have excellent efficacy on both receptors. Thus, some of the physiological and behavioral effects of GTS-21 may be due to the actions of this primary metabolite.

alpha7 receptor-selective agonists and modes of alpha7 receptor activation

The alpha7-selective agonists 3-(2, 4-dimethoxybenzylidene)-anabaseine (GTS-21), also known as DMXB, and 3-(4-hydroxy,2-methoxybenzylidene)anabaseine (4OH-GTS-21) produce a variety of behavioral and cytoprotective effects that may be related to the activation of either large transient currents at high concentrations or small sustained currents at lower agonist concentrations. We are using acutely dissociated hypothalamic neurons, which express a central nervous system (CNS) alpha7-type receptor, to test a model for the concentration-dependent desensitization of alpha7-mediated responses. Our results confirm that 4OH-GTS-21 is a potent activator of neuronal alpha7 nicotinic-acetylcholine receptor. The rapid application of agonist leads to a brief period of maximal receptor-activation followed by desensitization. Rise rates, decay rates, and the degree to which current was desensitized were all concentration-dependent. Following the initial peak response to a 300-microM 4OH-GTS-21 application, current is reduced to baseline values within about 100 ms. Application of 30 microM 4OH-GTS-21 produced both a transient peak current and a sustained current that decayed only slowly after the removal of agonist. In the case of a 300-microM 4OH-GTS-21 application, after agonist was removed, we saw a rebound response up to the level of the 30-microM sustained current. The data, therefore, suggest that a sufficient level of agonist occupation can be retained on the receptor to promote activation for up to several hundred milliseconds.

Molecular and cellular mechanisms of cognitive function: implications for psychiatric disorders

Recent studies on the molecular and cellular basis of learning and memory have brought us closer than ever to understanding the mechanisms of synaptic plasticity and their relevance to memory formation. Genetic approaches have played a central role in these new findings because the same mutant mice can be studied with molecular, cellular, circuit, and behavioral tools. Therefore, the results can be used to construct models that cut across levels of analytical complexity, forging connections from the biochemistry of the modified protein to the behavior of the mutant mice. These findings are not only improving our understanding of learning and memory, they are also enriching our understanding of cognitive disorders, such as neurofibromatosis type I. Mechanisms underlying long-term changes in synaptic function are likely to be at the heart of many cognitive and emotional processes in humans. Therefore, molecular and cellular insights into learning and memory undoubtedly will have a profound impact on the understanding and treatment of psychiatric disorders.

Nicotinic receptors in the brain: correlating physiology with function

Nicotinic ACh receptors (nAChRs) have been implicated in a variety of brain functions, including neuronal development, learning and memory formation, and reward. Although there are substantial data indicating that nAChR subunits are found in many brain regions, the precise cellular roles of these subunits in neuronal functions have remained elusive. Until recently, nAChRs were thought primarily to serve a modulatory role in the brain by regulating neurotransmitter release from nerve terminals. However, new evidence has revealed that nAChRs also function in a postsynaptic role by mediating fast ACh-mediated synaptic transmission in the hippocampus and in the sensory cortex, and are found at somatodendritic as well as nerve terminal sites in the reward system. It is possible that presynaptic and postsynaptic nAChRs mediate changes in the efficacy of synaptic transmission in these brain regions. These changes could underlie the proposed functions of nAChRs in cognitive functions of the hippocampus and cerebral cortex, in neuronal development in the sensory cortex, and in reward.

Modulatory role of presynaptic nicotinic receptors in synaptic and non-synaptic chemical communication in the central nervous system

Neuronal nicotinic acetylcholine receptors (nAChRs) belong to a family of ligand-gated channels closely related to but distinct from the muscle nAChRs. Recent progress in neurochemical and pharmacological methods supports the hypothesis of presynaptically located nAChRs on axon terminals and indicates that the major effect of nAChR is the modulation rather than processing of fast synaptic transmission. Strong neurochemical evidence indicate that the most important function of presynaptic nAChRs in either synaptic or non-synaptic localization is to increase transmitter release initiated by axonal firing, or directly induce Na(+) and Ca(2+) influx followed by a depolarization sufficient to activate local voltage-sensitive Ca(2+) channels, as a result transmitter of vesicular origin will be released. Therefore, it is somewhat expected that nicotine-induced transmitter release of different monoamines including norepinephrine (NE), dopamine (DA), serotonin (5-HT) can be tetrodotoxin (TTX)- and [Ca(2+)](o)-sensitive. However, some of the nAChR agonists at higher concentrations (1, 1-dimethyl-4-phenylpiperazinium (DMPP) and lobeline), besides their effects on presynaptic nAChRs, are able to inhibit the uptake of NE and 5-HT into nerve terminals, thereby their transmitter releasing effects are extended in time and space. The effect on the uptake process is different from classical nicotinic actions, not being sensitive to nAChR antagonism, but can be prevented by selective uptake blockers or reduced temperature. Considering neurochemical, pharmacological and electrophysiological evidence it seems likely that presynaptic nAChRs on monoaminergic fibers are composed of alpha3 or alpha4 subunits in combination with the beta2 subunit. This is supported by the observation that nicotinic agonists have no presynaptic effect on transmitter release in knockout mice lacking the beta2 nAChR subunit gene. The essential brain function lies not only in impulse transmission within a hard-wired neuronal circuitry but also within synaptic and non-synaptic communication subjected to presynaptic modulation. Since the varicose noradrenergic, dopaminergic, serotonergic, glutamatergic and cholinergic axon terminals mainly do not make synaptic contact, but their varicosities are equipped with nAChRs and these non-synaptically localized receptors are of high affinity, it is suggested that nicotine inhaled during smoking might exert its behavioral, psychological, neurological and neuroendocrinological effects via these receptors.

Impairments of long-term potentiation in hippocampal slices of beta-amyloid-infused rats

In this study, we investigated the neuronal activity of hippocampal slices from the beta-amyloid protein-infused (300 pmol/day for 10-11 days) rats using the extracellular recording technique. Perfusion of nicotine (50 microM) reduced the amplitude of electrically evoked population spikes in the CA1 pyramidal cells of the vehicle control rats, but not in those of the beta-amyloid protein-infused rats, suggesting the impairment of nicotinic signaling in the beta-amyloid protein-infused rats. Long-term potentiation induced by tetanic stimulations in CA1 pyramidal cells, which was readily observed in the vehicle control rats, was also impaired in the beta-amyloid protein-infused rats. Nicotinic blockade by adding hexamethonium into the perfused solution inhibited long-term potentiation induction. Taken together, our previous and present results suggest that beta-amyloid protein infusion impairs the signal transduction mechanisms via nicotinic acetylcholine receptors. This dysfunction may be responsible, at least in part, for the impairment of long-term potentiation induction and may lead to learning deficits.

The alpha7 nicotinic acetylcholine receptor in neuronal plasticity

A growing body of evidence indicates that neuronal nicotinic acetylcholine receptors (nAChRs), in addition to promoting fast cholinergic transmission, may modulate other neuronal activities within the central nervous system (CNS). In particular, the alpha7 nAChR is highly permeable to Ca2+ and may serve a distinct role in regulating neuronal plasticity. By elevating intracellular Ca2+ levels in discrete neuronal locations, these ligand-gated ion channels may influence numerous physiological processes in developing and adult CNS. In this article, we review evidence that both pre- and postsynaptic alpha7 nAChRs modulate transmitter release in the brain and periphery through Ca2+-dependent mechanisms. The possible role of alpha7 nAChRs in regulating neuronal growth and differentiation in developing CNS is also evaluated. We consider an interaction between cholinergic and glutamatergic transmission and propose a hypothesis on the possible coregulation of intracellular Ca2+ by N-methyl-D-aspartate (NMDA) receptors and alpha7 nAChRs. Finally, the clinical significance of alterations in the normal function of alpha7 nAChRs is discussed as it pertains to prenatal nicotine exposure, schizophrenia, and epilepsy.

Metabolism and disposition of GTS-21, a novel drug for Alzheimer's disease

1. GTS-21, a novel drug for Alzheimer's disease, is currently under clinical development. In the current study, the metabolism and disposition of GTS-21 have been evaluated in rat and dog after single oral and intravenous administration. 2. Following oral administration of [14C]GTS-21 to rat, radioactivity was primarily excreted in the faeces (67%) via the bile with possible enterohepatic circulation. Urinary excretion of radioactivity in rat and dog was 20 and 19% respectively. 3. GTS-21 was rapidly and extensively absorbed after oral administration and rapidly cleared from plasma. The maximum concentration ratio of GTS-21 to total radioactivity in plasma was low, indicating first-pass or pre-systemic biotransformation. 4. In rat, GTS-21 showed linear pharmacokinetics over doses ranging from 1 to 10 mg/kg with an absolute bioavailability of 23%. In dog, the absolute bioavailability was 27% at an oral dose of 3 mg/kg. 5. GTS-21 was O-demethylated to yield compounds that were then subject to glucuronidation. Three of the metabolites in rat urine were isolated and characterized as 4-OH-GTS-21, 4-OH-GTS-21 glucuronide and 2-OH-GTS-21 glucuronide. The major urinary metabolites were 4-OH-GTS-21 glucuronide and 2-OH-GTS-21 glucuronide. 6. In vitro chemical inhibition of cytochrome P450 in human liver microsomes indicated that CYPIA2 and CYP2E1 were the isoforms primarily responsible for the O-demethylation of GTS-21, with some contribution from CYP3A.

A 'long-term-potentiation-like' facilitation of hippocampal synaptic transmission induced by the nootropic nefiracetam

Nefiracetam, a nootropic agent, enhanced the slope of field excitatory postsynaptic potentials in the CA1 region of rat hippocampal slices to about 170% of basal levels, being evident still at 4-h washing-out of the drug. A similar sustained enhancement (>/=16 h after i.m. injection with nefiracetam) was observed in the population spikes recorded from the granular cell layer of the intact mouse hippocampus. Saturation of the enhancement in the synaptic strength occluded potentiation obtained with long-term potentiation (LTP) induced by high-frequency (tetanic) stimulation, and vice versa. Interestingly, the facilitatory action of nefiracetam was blocked by either the nicotinic acetylcholine (ACh) receptor antagonists, alpha-bungarotoxin and mecamylamine, or the selective protein kinase C (PKC) inhibitor, GF109203X, but in contrast, it was not affected by D-2-amino-5-phosphonovaleric acid (APV), a selective N-methyl-D-aspartate (NMDA) receptor antagonist. The results of the present study suggest that nefiracetam, whereas the action is independent of NMDA receptors, induces an 'LTP-like' facilitation of hippocampal synaptic transmission as a consequence of modulation of nicotinic ACh receptors and PKC. This may represent a likely mechanism underlying the cognition-enhancing actions of nefiracetam.

Synaptic transmission at nicotinic acetylcholine receptors in rat hippocampal organotypic cultures and slices

1. Whole-cell clamp recordings of the compound synaptic current elicited by afferent stimulation of Schaffer collaterals showed that blockade of the NMDA, AMPA and GABAA receptor-mediated components by 6-nitro-7-sulphamoyl- benzo(f)quinoxaline-2,3-dione (NBQX), 3-((R)-2-carboxypiperazine-4-yl)propyl-1-phosphonate (R-CPP) and picrotoxin, respectively, left a small residual current in 39 out of 41 CA1 pyramidal neurones in organotypic cultures and 9 out of 16 CA1 cells in acutely prepared slices. 2. This current represented 2. 9 +/- 0.4 % of the compound evoked synaptic response in organoypic cultures and 1.4 +/- 0.5 % in slices. It was characterized by a slightly rectifying I-V curve and a reversal potential of 3.4 +/- 5. 1 mV. 3. This residual current was insensitive to blockers of GABAB, purinergic, muscarinic and 5-HT3 receptors, but it was essentially blocked by the nicotinic receptor antagonist d-tubocurarine (91 +/- 4 % blockade; 20 microM), and partly blocked by alpha-bungarotoxin (200 nM) and methyllycaconitine (10 nM), two antagonists with a higher selectivity for alpha7 subunit-containing nicotinic receptors (48 +/- 3 % and 55 +/- 11 % blockade, respectively). 4. The residual current was of synaptic origin, since it occurred after a small delay; its amplitude depended upon the stimulation intensity and it was calcium dependent and blocked by the sodium channel antagonist tetrodotoxin. 5. We conclude that afferent stimulation applied in the stratum radiatum evokes in some hippocampal neurones a small synaptic current mediated by activation of neuronal nicotinic receptors.

Effects of nicotinic receptor agonists on beta-amyloid beta-sheet formation

Previously we demonstrated that nicotinic acetylcholine receptor stimulation protects neurons against beta-amyloid (Abeta)-induced cytotoxicity. In the present study, the effects of nicotinic receptor agonists on the beta-sheet formation were investigated using a thioflavin T (ThT)-based fluorescence assay. Nicotine, cytisine (an alpha4beta2 agonist), and 3-(2,4)-dimethoxybenzylidene anabaseine (DMXB, an alpha7 agonist) did not reduce fluorescence intensity when these agents were added to the beta-sheet-formed Abeta. Simultaneous incubation of Abeta with nicotinic agonists also did not cause a reduction in fluorescence intensity. This data suggests that nicotinic receptor agonists do not influence the formation of the beta-sheet structure.