Effects of nicotine on spatial memory deficits in rats with septal lesions

Septohippocampal cholinergic system at the intersection of stress and cognition: Current trends and translational implications

Deficits in hippocampus-dependent memory processes are common across psychiatric and neurodegenerative disorders such as depression, anxiety and Alzheimer's disease. Moreover, stress is a major environmental risk factor for these pathologies and it exerts detrimental effects on hippocampal functioning via the activation of hypothalamic-pituitary-adrenal (HPA) axis. The medial septum cholinergic neurons extensively innervate the hippocampus. Although, the cholinergic septohippocampal pathway (SHP) has long been implicated in learning and memory, its involvement in mediating the adaptive and maladaptive impact of stress on mnemonic processes remains less clear. Here, we discuss current research highlighting the contributions of cholinergic SHP in modulating memory encoding, consolidation and retrieval. Then, we present evidence supporting the view that neurobiological interactions between HPA axis stress response and cholinergic signalling impact hippocampal computations. Finally, we critically discuss potential challenges and opportunities to target cholinergic SHP as a therapeutic strategy to improve cognitive impairments in stress-related disorders. We argue that such efforts should consider recent conceptualisations on the dynamic nature of cholinergic signalling in modulating distinct subcomponents of memory and its interactions with cellular substrates that regulate the adaptive stress response.

Critical roles of nicotinic acetylcholine receptors in olfactory memory formation and retrieval in crickets

Acetylcholine (ACh) is a major excitatory neurotransmitter in the insect central nervous system, and insect neurons express several types of ACh receptors (AChRs). AChRs are classified into two subgroups, muscarinic AChRs and nicotinic AChRs (nAChRs). nAChRs are also divided into two subgroups by sensitivity to α-bungarotoxin (α-BGT). The cricket Gryllus bimaculatus is one of the useful insects for studying the molecular mechanisms in olfactory learning and memory. However, the roles of nAChRs in olfactory learning and memory of the cricket are still unknown. In the present study, to investigate whether nAChRs are involved in cricket olfactory learning and memory, we tested the effects of two different AChR antagonists on long-term memory (LTM) formation and retrieval in a behavioral assay. The two AChR antagonists that we used are mecamylamine (MEC), an α-BGT-insensitive nAChR antagonist, and methyllycaconitine (MLA), an α-BGT-sensitive nAChR antagonist. In crickets, multiple-trial olfactory conditioning induced 1-day memory (LTM), whereas single-trial olfactory conditioning induced 1-h memory (mid-term memory, MTM) but not 1-day memory. Crickets injected with MEC 20 min before the retention test at 1 day after the multiple-trial conditioning exhibited no memory retrieval. This indicates that α-BGT-insensitive nAChRs participate in memory retrieval. In addition, crickets injected with MLA before the multiple-trial conditioning exhibited MTM but not LTM, indicating that α-BGT-sensitive nAChRs participate in the formation of LTM. Moreover, injection of nicotine (an nAChR agonist) before the single-trial conditioning induced LTM. Finally, the nitric oxide (NO)-cGMP signaling pathway is known to participate in the formation of LTM in crickets, and we conducted co-injection experiments with an agonist or inhibitor of the nAChR and an activator or inhibitor of the NO-cGMP signaling pathway. The results suggest that nAChR works upstream of the NO-cGMP signaling system in the LTM formation process.

Optogenetic suppression of the medial septum impairs working memory maintenance

Spatial working memory (SWM) is the ability to encode, maintain, and retrieve spatial information over a temporal gap, and relies on a network of structures including the medial septum (MS), which provides critical input to the hippocampus. Although the role of the MS in SWM is well-established, up until recently, we have been unable to use temporally precise circuit manipulation techniques to examine the specific role of the MS in SWM, particularly to distinguish between encoding, maintenance, and retrieval. Here, we test the hypothesis that the MS supports the maintenance of spatial information over a temporal gap using precisely timed optogenetic suppression delivered during specific portions of three different tasks, two of which rely on SWM and one that does not. In experiment 1, we found that MS optogenetic suppression impaired choice accuracy of a SWM dependent conditional discrimination task. Moreover, this deficit was only observed when MS suppression was delivered during the cue-sampling, but not the cue-retrieval, portion of the trial. There was also no deficit when MS neurons were optogenetically suppressed as rats performed a SWM-independent variant of the task. In experiment 2, we tested whether MS suppression affected choice accuracy on a delayed nonmatch to position (DNMP) task when suppression was limited to the sample, delay, and choice phases of the task. We found that MS suppression delivery during the delay phase of the DNMP task, but not during the sample or choice phases, impaired choice accuracy. Our results collectively suggest that the MS plays an important role in SWM by maintaining task-relevant information over a temporal delay.

Empagliflozin nanoparticles attenuates type2 diabetes induced cognitive impairment via oxidative stress and inflammatory pathway in high fructose diet induced hyperglycemic mice

There is snowballing evidence that type 2 diabetes (T2D) predisposes to neuropathophysiological alterations including oxidative stress and triggered inflammatory responses in brain that eventually culminates into cognitive impairment.Accumulating evidences suggest that SGLT2 inhibitor can be a promising intervention for cognitive decline in T2DM. In the present paper, the potential effects of Empagliflozin (EMPA), a SGLT2 inhibitor, against T2D induced cognitive dysfunction have been explored. The effect of EMPA on array of inflammatory mediators including Interleukin-6(IL-6), Interleukin -1β (IL-1β), and Tumour necrosis factor-α(TNF-α)), neuronal proteins including glycogen synthase kinase-3β (GSK- 3β), Phosphorylated tau (p-tau), amyloid beta (Aβ) (1-40, 1-42) and altered oxidative parameters including SOD, catalase, TBARS was determined in the high fructose diet induced hyperglycaemic mice. The obtained results were compared with EMPA nanoparticles (Nps) formulated in our laboratory and found that EMPA Nps significantly showed reduced levels of inflammatory mediators and oxidative stress. Further, decrease in levels of p-tau, Aβ (1-40) and Aβ (1-42) were also observed with EMPA nanoparticles.Thus, the study has demonstrated that EMPA Nps could be a promising therapy to alleviate the progression of cognitive decline in T2D.

The Cholinergic System Modulates Memory and Hippocampal Plasticity via Its Interactions with Non-Neuronal Cells

Degeneration of central cholinergic neurons impairs memory, and enhancement of cholinergic synapses improves cognitive processes. Cholinergic signaling is also anti-inflammatory, and neuroinflammation is increasingly linked to adverse memory, especially in Alzheimer's disease. Much of the evidence surrounding cholinergic impacts on the neuroimmune system focuses on the α7 nicotinic acetylcholine (ACh) receptor, as stimulation of this receptor prevents many of the effects of immune activation. Microglia and astrocytes both express this receptor, so it is possible that some cholinergic effects may be via these non-neuronal cells. Though the presence of microglia is required for memory, overactivated microglia due to an immune challenge overproduce inflammatory cytokines, which is adverse for memory. Blocking these exaggerated effects, specifically by decreasing the release of tumor necrosis factor α (TNF-α), interleukin 1β (IL-1β), and interleukin 6 (IL-6), has been shown to prevent inflammation-induced memory impairment. While there is considerable evidence that cholinergic signaling improves memory, fewer studies have linked the "cholinergic anti-inflammatory pathway" to memory processes. This review will summarize the current understanding of the cholinergic anti-inflammatory pathway as it relates to memory and will argue that one mechanism by which the cholinergic system modulates hippocampal memory processes is its influence on neuroimmune function via the α7 nicotinic ACh receptor.

Revisiting nicotine’s role in the ageing brain and cognitive impairment

Brain ageing is a complex process which in its pathologic form is associated with learning and memory dysfunction or cognitive impairment. During ageing, changes in cholinergic innervations and reduced acetylcholinergic tonus may trigger a series of molecular pathways participating in oxidative stress, excitotoxicity, amyloid-β toxicity, apoptosis, neuroinflammation, and perturb neurotrophic factors in the brain. Nicotine is an exogenous agonist of nicotinic acetylcholine receptors (nAChRs) and acts as a pharmacological chaperone in the regulation of nAChR expression, potentially intervening in age-related changes in diverse molecular pathways leading to pathology. Although nicotine has therapeutic potential, paradoxical effects have been reported, possibly due to its inverted U-shape dose-response effects or pharmacokinetic factors. Additionally, nicotine administration should result in optimum therapeutic effects without imparting abuse potential or toxicity. Overall, this review aims to compile the previous and most recent data on nicotine and its effects on cognition-related mechanisms and age-related cognitive impairment.

Nicotine and networks: Potential for enhancement of mood and cognition in late-life depression

Late-life depression is characterized by both lower mood and poor cognitive performance, symptoms that often do not fully respond to current antidepressant medications. Nicotinic acetylcholine receptor (nAChR) agonists such as nicotine may serve as a novel therapeutic approach for this population. Both preclinical and preliminary clinical studies suggest that nAChR agonists can improve depressive behavior in animal models and improve mood in depressed individuals. Substantial literature also supports that nAChR agonists benefit cognitive performance, particularly in older populations. These potential benefits may be mediated by the effects of nAChR stimulation on neural network function and connectivity. Functional neuroimaging studies detail effects of nAChR agonists on the default mode network, central-executive network, and salience network that may oppose or reverse network changes seen in depression. We propose that, given the existent literature and the clinical presentation of late-life depression, nicotine or other nAChR agonists may have unique therapeutic benefits in this population and that clinical trials examining nicotine effects on mood, cognition, and network dynamics in late-life depression are justified.

Unsolved Mysteries: The Hippocampus

The continuing explosion of scientific interest in the hippocampus began in the 1950s, initiated in large part by the recognition of the importance of the observations of hippocampectomized monkeys made by Klüver and Bucy and the remarkable memory loss of patient H. M. following temporal lobe surgery. Subsequent to these studies, research and theories about the hippocampus grew exponentially in number and diversity. As yet, no theory of hippocampal function explains all of the phenomena discovered in the clinic or laboratory. In this article, experimental results that have been forgotten or ignored in most theories are presented. Adequate theories of hippocampal function must account for known, reliable postsurgical behavioral observations and consider the conditions under which anomalies are noted. Comprehensive theories will require new approaches in which the interactions of the hippocampus with the central nervous system are understood.

Zinc Chloride and Lead Acetate-Induced Passive Avoidance Memory Retention Deficits Reversed by Nicotine and Bucladesine in Mice

It is very important to investigate the neurotoxic effects of metals on learning and memory processes. In this study, we tried to investigate the effects and time course properties of oral administration of zinc chloride (25, 50, and 75 mg/kg, for 2 weeks), lead acetate (250, 750, 1,500, and 2,500 ppm for 4, 6 and 8 weeks), and their possible mechanisms on a model of memory function. For this matter, we examined the intra-peritoneal injections of nicotine (0.25, 0.5, 1, and 1.5 mg/kg) and bucladesine (50, 100, 300, and 600 nM/mouse) for 4 days alone and in combination with mentioned metals in the step-through passive avoidance task. Control animals received saline, drinking water, saline, and DMSO (dimethyl sulfoxide)/deionized water (1:9), respectively. At the end of each part of studies, animals were trained for 1 day in step-through task. The avoidance memory retention alterations were evaluated 24 and 48 h later in singular and combinational studies. Zinc chloride (75 mg/kg) oral gavage for 2 weeks decreased latency times compared to control animals. Also, lead acetate (750 ppm oral administrations for 8 weeks) caused significant lead blood levels and induced avoidance memory retention impairments. Four-days intra-peritoneal injection of nicotine (1 mg/kg) increased latency time compared to control animals. Finally, findings of this research showed that treatment with intra-peritoneal injections of nicotine (1 mg/kg) and/or bucladesine (600 nM/mouse) reversed zinc chloride- and lead acetate-induced avoidance memory retention impairments. Taken together, these results showed the probable role of cholinergic system and protein kinase A pathways in zinc chloride- and lead acetate-induced avoidance memory alterations.

Cannabinoid receptor CB1 is involved in nicotine-induced protection against Aβ1-42 neurotoxicity in HT22 cells

Emerging evidences suggest that nicotine exerts a neuroprotective effect on Alzheimer's disease (AD), yet the precise mechanism is not fully elucidated. Here, HT22 cells were exposed to amyloid beta protein fragment (Aβ)1-42 to mimic the pathological process of neuron in AD. We hypothesized that cannabinoid receptor CB1 is involved in the nicotine-induced neuroprotection against Aβ1-42 injury in HT22 cells. CB1 expression in HT22 cells was investigated by immunocytochemistry and Western blot. The injury of HT22 cells was evaluated by cellular morphology, cell viability, and lactate dehydrogenase (LDH) release. The apoptosis of HT22 cells was assessed by flow cytometry and expressions of Bcl-2 and Bax. The results demonstrated that nicotine markedly upregulated CB1 expression, increased cell viability, ameliorated cellular morphology, decreased LDH release, and reduced the apoptotic rate of HT22 cells exposed to Aβ1-42 for 24 h, while the blockade of CB1 or the inhibition of protein kinase C (PKC) partially reversed the neuroprotection. Furthermore, the blockade of CB1 reversed nicotine-induced PKC activation in HT22 cells exposed to Aβ1-42. These results suggest that CB1 is involved in the nicotine-induced neuroprotection against Aβ1-42 neurotoxicity, and the neuroprotection may be dependent on the activation of PKC.

Nicotinic and muscarinic agonists and acetylcholinesterase inhibitors stimulate a common pathway to enhance GluN2B-NMDAR responses

Nicotinic and muscarinic ACh receptor agonists and acetylcholinesterase inhibitors (AChEIs) can enhance cognitive function. However, it is unknown whether a common signaling pathway is involved in the effect. Here, we show that in vivo administration of nicotine, AChEIs, and an m1 muscarinic (m1) agonist increase glutamate receptor, ionotropic, N-methyl D-aspartate 2B (GluN2B)-containing NMDA receptor (NR2B-NMDAR) responses, a necessary component in memory formation, in hippocampal CA1 pyramidal cells, and that coadministration of the m1 antagonist pirenzepine prevents the effect of cholinergic drugs. These observations suggest that the effect of nicotine is secondary to increased release of ACh via the activation of nicotinic ACh receptors (nAChRs) and involves m1 receptor activation through ACh. In vitro activation of m1 receptors causes the selective enhancement of NR2B-NMDAR responses in CA1 pyramidal cells, and in vivo exposure to cholinergic drugs occludes the in vitro effect. Furthermore, in vivo exposure to cholinergic drugs suppresses the potentiating effect of Src on NMDAR responses in vitro. These results suggest that exposure to cholinergic drugs maximally stimulates the m1/guanine nucleotide-binding protein subunit alpha q/PKC/proline-rich tyrosine kinase 2/Src signaling pathway for the potentiation of NMDAR responses in vivo, occluding the in vitro effects of m1 activation and Src. Thus, our results indicate not only that nAChRs, ACh, and m1 receptors are on the same pathway involving Src signaling but also that NR2B-NMDARs are a point of convergence of cholinergic and glutamatergic pathways involved in learning and memory.

Cotinine halts the advance of Alzheimer's disease-like pathology and associated depressive-like behavior in Tg6799 mice

Alzheimer's disease (AD) is associated with cognitive and non-cognitive symptoms for which there are currently no effective therapies. We have previously reported that cotinine, a natural product obtained from tobacco leaves, prevented memory loss and diminished amyloid-β (Aβ) plaque pathology in transgenic 6799 mice (Tg6799 mice) when treated prior to the development of the pathology. We have also shown that cotinine reduces depressive-like behavior in normal and chronically stressed C57BL/6 mice. Here, we extend our previous studies by investigating the effects of cotinine on the progression of AD-like pathology, depressive-like behavior, and the mechanisms underlying its beneficial effects in Tg6799 mice when left untreated until after a more advanced stage of the disease's development. The results show that vehicle-treated Tg6799 mice displayed an accentuated loss of working memory and an abundant Aβ plaque pathology that were accompanied by higher levels of depressive-like behavior as compared to control littermates. By contrast, prolonged daily cotinine treatment to Tg6799 mice, withheld until after a mid-level progression of AD-like pathology, reduced Aβ levels/plaques and depressive-like behavior. Moreover, this treatment paradigm dramatically improved working memory as compared to control littermates. The beneficial effects of cotinine were accompanied by an increase in the expression of the active form of protein kinase B and the postsynaptic density protein 95 in the hippocampi and frontal cortices of Tg6799 mice. This suggests that cotinine halts the progression of AD-like pathology while reducing depressive-like behavior by stimulating signaling pathways supporting synaptic plasticity in Tg6799 mice. The potential use of cotinine to treat cognitive and non-cognitive symptoms of AD is discussed.

Radiosynthesis and first evaluation in mice of [(18)F]NS14490 for molecular imaging of α7 nicotinic acetylcholine receptors

[(18)F]NS14490, a new potential radiotracer for neuroimaging of α7 nicotinic acetylcholine receptors (α7 nAChRs), was synthesized and evaluated in vitro and in vivo. Radioligand binding studies using [(3)H]methyllycaconitine and NS14490 as competitor showed a good target affinity (K(i,α7) = 2.5 nM) and a high selectivity towards other nAChRs. Radiosynthesis of [(18)F]NS14490 was performed by two different labelling procedures: a two-step synthesis using a prosthetic group, which led to 7% labelling yield, and the convenient direct nucleophilic substitution of the corresponding tosylate precursor, which resulted in 70% labelling yield. After optimisation of the isolation, purification and formulation process, biodistribution studies were performed in CD-1 mice. The brain uptake of [(18)F]NS14490 was comparably low (0.16% ID g(-1) wet weight at 5 min p.i.). The radiotracer showed a high metabolic stability in plasma and brain. Also, the target specificity was proven by pre-administration of a highly affine α7 ligand providing a rationale basis for further in vivo evaluation.

Augmentation of cognitive function by NS9283, a stoichiometry-dependent positive allosteric modulator of α2- and α4-containing nicotinic acetylcholine receptors

BACKGROUND AND PURPOSE

Positive allosteric modulation of α4β2 nicotinic acetylcholine (nACh) receptors could add a new dimension to the pharmacology and therapeutic approach to these receptors. The novel modulator NS9283 was therefore tested extensively.

EXPERIMENTAL APPROACH

Effects of NS9283 were evaluated in vitro using fluorescence-based Ca(2+) imaging and electrophysiological voltage clamp experiments in Xenopus oocytes, mammalian cells and thalamocortical neurons. In vivo the compound was tested in models covering a range of cognitive domains in mice and rats.

KEY RESULTS

NS9283 was shown to increase agonist-evoked response amplitude of (α4)(3) (β2)(2) nACh receptors in electrophysiology paradigms. (α2)(3) (β2)(2) , (α2)(3) (β4)(2) and (α4)(3) (β4)(2) were modulated to comparable extents, but no effects were detected at α3-containing or any 2α : 3β stoichiometry nACh receptors. Native nACh receptors in thalamocortical neurons similarly displayed DHβE-sensitive currents that were receptive to modulation. NS9283 had favourable effects on sensory information processing, as shown by reversal of PCP-disrupted pre-pulse inhibition. NS9283 further improved performance in a rat model of episodic memory (social recognition), a rat model of sustained attention (five-choice serial reaction time task) and a rat model of reference memory (Morris water maze). Importantly, the effects in the Morris water maze could be fully reversed with mecamylamine, a blocker of nACh receptors.

CONCLUSIONS AND IMPLICATIONS

These results provide compelling evidence that positive allosteric modulators acting at the (α4)(3) (β2)(2) nACh receptors can augment activity across a broad range of cognitive domains, and that α4β2 nACh receptor allosteric modulation therefore constitutes a promising therapeutic approach to symptomatic treatment of cognitive impairment.

Nicotine-induced neuroprotection against ischemic injury involves activation of endocannabinoid system in rats

Nicotine has been reported to exert certain protective effect in the Parkinson's and Alzheimer's diseases. Whether it has a similar action in focal cerebral ischemia was unclear. In the present study, rats received either an injection of (-)-nicotine hydrogen tartrate salt (1.2 mg/kg, i.p.) or the vehicle 2 h before the 120 min middle cerebral artery occlusion. Neurological deficits and histological injury were assessed at 24 h after reperfusion. The content of endocannabinoids and the expression of cannabinoid receptor CB1 in brain tissues were determined at different time points after nicotine administration. Results showed that nicotine administration ameliorated neurological deficits and reduced infarct volume induced by cerebral ischemia in the rats. The neuroprotective effect was partially reversed by CB1 blockage. The content of the endocannabinoids N-arachidonylethanolamine and 2-arachidonoylglycerol, as well as the expression of cannabinoid receptor CB1 were up-regulated in brain tissues after nicotine delivery. These results suggest that endogenous cannabinoid system is involved in the nicotine-induced neuroprotection against transient focal cerebral ischemia.

Nicotine uses neuron-glia communication to enhance hippocampal synaptic transmission and long-term memory

Nicotine enhances synaptic transmission and facilitates long-term memory. Now it is known that bi-directional glia-neuron interactions play important roles in the physiology of the brain. However, the involvement of glial cells in the effects of nicotine has not been considered until now. In particular, the gliotransmitter D-serine, an endogenous co-agonist of NMDA receptors, enables different types of synaptic plasticity and memory in the hippocampus. Here, we report that hippocampal long-term synaptic plasticity induced by nicotine was annulled by an enzyme that degrades endogenous D-serine, or by an NMDA receptor antagonist that acts at the D-serine binding site. Accordingly, both effects of nicotine: the enhancement of synaptic transmission and facilitation of long-term memory were eliminated by impairing glial cells with fluoroacetate, and were restored with exogenous D-serine. Together, these results show that glial D-serine is essential for the long-term effects of nicotine on synaptic plasticity and memory, and they highlight the roles of glial cells as key participants in brain functions.

Low-dose nicotine facilitates spatial memory in ApoE-knockout mice in the radial arm maze

Here, we investigated the effects of nicotine on spatial memory in ApoE-knockout (ApoE-KO) and wild-type (WT) mice in a radial arm maze. Training occurred on three consecutive days and the test was performed on day 4, with one trial per day. Then on day 4, animals were administered nicotine (0.1, 0.25, 0.5, and 1.0 mg/kg) or the antagonist of nicotinic receptors (nAChRs) mecamylamine (MEC 2 mg/kg) alone or together with 0.1 mg/kg nicotine. The number of errors in the first eight choices was recorded. The results were that 0.1 mg/kg nicotine decreased errors in ApoE-KO mice, while 0.1 and 0.25 mg/kg nicotine reduced errors in WT mice, indicating that lower doses of nicotine elicit a memory improvement. In contrast, 1.0 mg/kg nicotine increased errors in WT mice, but not in ApoE-KO mice. MEC alone had no noticeable effect on errors in either strain of mice. However, co-administration of 0.1 mg/kg nicotine and MEC increased errors and reduced the effects of nicotine in WT mice, but not in ApoE-KO mice. Our study found a biphasic effect of nicotine in WT mice: it improves spatial memory at lower doses and impairs it at a higher dose. In ApoE-KO mice, nicotine improves memory at a low dose and has no effect at a higher dose, suggesting that the ApoE deficiency may influence the efficacy of nicotine. Moreover, a reversal of nicotinic effects with MEC was seen in WT mice, indicating the likelihood of the involvement of nAChRs in the spatial-memory response to nicotine.

Nicotine attenuates spatial learning deficits induced by sodium metavanadate

Learning can be severely impaired as a consequence of exposure to environmental pollutants. Vanadium (V), a metalloid which is widely distributed in the environment, has been shown to exert toxic effects on a variety of biological systems including the nervous system. However, studies exploring the impact of vanadium on learning are limited. Herein, we investigated the effects of oral administration of sodium metavanadate (SMV) (15, 20 and 25mg/kg/day for 2weeks) on spatial learning using Morris water maze (MWM). Our results showed that pre-training administration of sodium metavanadate impaired learning in Morris water maze. Analyzing the role of cholinergic system in SMV-induced learning deficit, we found that bilateral intra-hippocampal infusion of nicotine (1μg/side) during training could significantly diminish the SMV-induced learning impairment. We next examined the expression of choline acetyltransferase (ChAT) and vesicular acetylcholine transporter (VAChT) as cholinergic markers in CA1 region of hippocampus as well as in medial septal area (MSA). Our molecular analyses showed that vanadium administration decreased ChAT and VAChT protein expression, an effect that was attenuated by nicotine. Altogether, our results confirmed the toxic effects of SMV on spatial acquisition, while also pointing to the neuroprotective effects of nicotine on SMV-induced impairments in learning capabilities. These findings might open a new avenue for the prevention of vanadium adverse effects on spatial learning and memory through activation of cholinergic signaling pathway.

The quantitative evaluation of cholinergic markers in spatial memory improvement induced by nicotine-bucladesine combination in rats

We previously showed that post-training intra-hippocampal infusion of nicotine-bucladesine combination enhanced spatial memory retention in the Morris water maze. Here we investigated the role of cholinergic markers in nicotine-bucladesine combination-induced memory improvement. We assessed the expression of choline acetyltransferase (ChAT) and vesicular acetylcholine transporter (VAChT) in CA1 region of the hippocampus and medial septal area (MSA) of the brain. Post-training bilateral infusion of a low concentration of either nicotine or bucladesine into the CA1 region of the hippocampus did not affect spatial memory significantly. Quantitative immunostaining analysis of optical density in CA1 regions and evaluation of immunopositive neurons in medial septal area of brain sections from all combination groups revealed a significant increase (P<0.001) in the ChAT and VAChT immunoreactivity. The maximum increase was observed with combination of 10-microM/side bucladesine and 0.5 microg/side nicotine and in a concentration dependent manner. Also, increase in the optical density and amount of ChAT and VAChT immunostaining correlated with the decrease in escape latency and traveled distance in rats treated with nicotine and low dose of bucladesine. Taken together, these results suggest that significant increases of ChAT and VAChT protein expressions in the CA1 region and medial septal area are the possible mechanisms of spatial memory improvement induced by nicotine-bucladesine combination.

Alteration of dopaminergic markers in gastrointestinal tract of different rodent models of Parkinson's disease

Parkinson's disease (PD) is a progressive neurological disorder that is often associated with various gastrointestinal (GI) symptoms. The link between the alteration of dopaminergic system and the symptoms of the GI tract in PD is complicated. To determine the changes in the dopaminergic system in the GI tract in PD, two kinds of rodent PD models were used in the present study. One was 6-hydroxydopamine (6-OHDA) -treated rats in which 6-OHDA was microinjected in the bilateral substantia nigra (SN). The other was 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) -treated mice in which MPTP was injected intraperitoneally. Immunofluorescence, reverse transcription (RT)-real time polymerase chain reaction (PCR) and Western blot were used to evaluate and compare the levels of mRNA and protein expression of tyrosine hydroxylase (TH) and dopamine transporter (DAT) in the GI tract between normal and rodent PD models, as well as between 6-OHDA-treated rats and MPTP-treated mice. The results indicated that TH- and DAT-positive cells were widely distributed in the GI tract. There were significant differences in TH and DAT expression in the GI tract between normal and PD models, as well as between 6-OHDA-treated rats and MPTP-treated mice. The protein levels of TH and DAT in the GI tract were significantly increased in 6-OHDA-treated rats, but the protein level of TH was significantly decreased in MPTP-treated mice. In addition, there was visible atrophy of gastric epithelial parietal cells in MPTP-treated mice, although the protein level of DAT was not significantly changed. The different alterations of dopaminergic system in the GI tract of the two kinds of PD models might underline the differences in GI symptoms in PD patients and might be correlated with the disease severity and disease process affecting the GI tract.

Nicotine gates long-term potentiation in the hippocampal CA1 region via the activation of alpha2* nicotinic ACh receptors

Hippocampal CA1 pyramidal cells receive two major excitatory synaptic inputs via the Schaffer collateral (SC) and temporoammonic (TA) pathways. Nicotine promotes induction of long-term potentiation (LTP) in the SC path; however, it is not known whether the modulatory effect of nicotine on LTP induction is pathway-specific. Here we show that nicotine suppresses LTP induction in the TA path. Interestingly, these opposing effects of nicotine were absent or greatly reduced in alpha2 nicotinic acetylcholine receptor (nAChR)-knockout (KO) mice, suggesting that an alpha2-containing nAChR subtype mediates these effects. Optical imaging with a voltage-sensitive dye revealed significantly stronger membrane depolarization in the presence of nicotine in the SC path, facilitating spread of excitatory neural activity along both the somatodendritic and the CA1 proximodistal axes. These effects of nicotine were also absent in alpha2 nAChR-KO mice, suggesting that the enhanced optical signal is related to the nicotine-induced facilitation of LTP induction. In contrast, in the TA path nicotine terminated depolarization more quickly and increased the delayed hyperpolarization in the termination zone of the TA path input. These inhibitory effects of nicotine were absent in alpha2 nAChR-KO mice and, thus, most probably underlie the nicotine-induced suppression of LTP induction. Our results suggest that nicotine influences the local balance between excitation and inhibition, gates LTP, and directs information flow through the hippocampal circuits via the activation of alpha2* nAChRs. These effects of nicotine may represent the cellular basis of nicotine-mediated cognitive enhancement.

Chronic nicotine-induced switch in Src-family kinase signaling for long-term potentiation induction in hippocampal CA1 pyramidal cells

Here, we show that chronic nicotine exposure induces changes in Src signaling for the modulation of N-methyl-D-aspartate receptor (NMDAR) function and LTP induction in CA1 pyramidal cells. Activation of muscarinic receptors normally potentiates NMDAR responses in pyramidal cells via a Gq/protein kinase C (PKC)/proline-rich tyrosine kinase 2/Src signaling cascade. However, muscarinic, PKC and Src stimulation had no effect on NMDAR responses after chronic nicotine treatment. The lack of effect was apparently due to enhanced tyrosine phosphorylation, and therefore further stimulation of the signaling cascade caused no effect on NMDAR responses. Interestingly, another Src-family kinase potentiated NMDAR responses after, but not before, chronic nicotine treatment. In control pyramidal cells, Src inhibitor peptides prevented tetanus-induced long-term potentiation (LTP). Conversely, in chronic nicotine-exposed cells, the inhibitor was ineffective in blocking tetanus-induced LTP. Furthermore, in control pyramidal cells, applying exogenous Src and administration of an endogenous Src-family kinase activator increased alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptor (AMPAR)-mediated responses. This increase was blocked by Src inhibitor peptides and occluded tetanus-induced LTP, as reported previously. In contrast, in chronic nicotine-treated pyramidal cells, applying exogenous Src had no effect on AMPAR-mediated responses and a tetanus-induced LTP. Interestingly, however, administration of an endogenous Src-family kinase activator enhanced AMPAR-mediated responses, which occluded tetanus-induced LTP. This enhancement was not prevented by co-application of Src inhibitor peptides. Thus, it appears that chronic nicotine exposure recruits another member of the Src-family for the regulation of NMDAR function and LTP induction. The nicotine-induced distinct signaling cascades may be involved in long-lasting memories of nicotine misuse.

Post-training intrahippocampal infusion of nicotine-bucladesine combination causes a synergistic enhancement effect on spatial memory retention in rats

We previously had shown that bilateral intrahippocampal infusion of 1 microg nicotine (but not 0.5 microg dose) led to an improvement in spatial memory retention in the Morris water maze task in male rats. We also reported that a similar type of bilateral infusion of H89, a protein kinase AII (PKA II) inhibitor, caused a deficit in spatial memory retention. In the present study, we wished to test the hypothesis that intrahippocampal infusion of dibutyryl cyclic AMP (DB-cAMP also called bucladesine), a membrane permeable selective activator of PKA, into the CA1 region can cause an improvement in spatial memory in this maze task. Indeed, bilateral infusion of 10 and 100 microM bucladesine (but not 1 and 5 microM doses) led to a significant reduction in escape latency and travel distance (showing an improvement in spatial memory) compared to the control. Also, bilateral infusion of 0.5 microg nicotine or 1 microM bucladesine alone did not lead to an improvement in spatial memory. However, such bilateral infusion of bucladesine at 1 and 5 microM concentrations infused within minutes after 0.5 microg nicotine infusion improved spatial memory retention. Taken together, our data suggest that intrahippocampal bucladesine infusions improve spatial memory retention in male rats and that bucladesine can interact synergistically with nicotine to improve spatial memory.

Nicotine-induced enhancement of synaptic plasticity at CA3-CA1 synapses requires GABAergic interneurons in adult anti-NGF mice

The hippocampus, a key structure for learning and memory processes, receives an important cholinergic innervation and is densely packed with a variety of nicotinic acetylcholine receptors (nAChRs) localized on principal cells and interneurons. Activation of these receptors by nicotine or endogenously released acetylcholine enhances activity-dependent synaptic plasticity processes. Deficits in the cholinergic system produce impairment of cognitive functions that are particularly relevant during senescence and in age-related neurodegenerative pathologies. In particular, Alzheimer's disease (AD) is characterized by a selective loss of cholinergic neurons in the basal forebrain and nAChRs in particular regions controlling memory processes such as the cortex and the hippocampus. Field excitatory postsynaptic potentials were recorded in order to examine whether nicotine was able to regulate induction of long-term potentiation at CA3-CA1 synapses in hippocampal slices from adult anti-NGF transgenic mice (AD 11), a comprehensive animal model of AD, in which cholinergic deficits due to nerve growth factor depletion are accompanied by progressive Alzheimer-like neurodegeneration. Both AD 11 and wild-type (WT) mice exhibited short- and long-lasting synaptic plasticity processes that were boosted by nicotine. The effects of nicotine on WT and AD 11 mice were mediated by both alpha7- and beta2-containing nAChRs. In the presence of GABA(A) receptor antagonists, nicotine failed to boost synaptic plasticity in AD 11 but not in WT mice, indicating that in anti-NGF transgenic mice GABAergic interneurons are able to compensate for the deficit in cholinergic modulation of glutamatergic transmission. This compensation may occur at different levels and may involve the reorganization of the GABAergic circuit. However, patch-clamp whole-cell recordings from principal cells failed to reveal any change in spontaneous release of GABA following pressure application of nicotine to nearby GABAergic interneurons. Together, these experiments indicate that in AD 11 mice a rearrangement of the GABAergic circuit can 'rescue' nicotine-induced potentiation of synaptic plasticity. This may be relevant for developing proper therapeutic tools useful for the treatment of AD.

Neuroprotective effects of behavioural training and nicotine on age-related deficits in spatial learning

Studies in humans and animals show a clear decline in spatial memory with age and several approaches have been adopted to alleviate this impairment. The purpose of our review is to assess the studies that have suggested the possible neuroprotective actions of behavioural training and nicotine-applied both independently and in conjunction-on age-related deficits in spatial learning. Both spatial pretraining and nonspatial experiences influence an animal's performance in spatial tasks. In aged rats, the experience of training in the water maze task increases the number of newly generated neurons in the hippocampus. The neuroprotective effects of nicotine have been demonstrated in both in-vitro and in-vivo models, although the molecular mechanisms underlying these actions are not yet fully understood. It had been concluded in different studies that nicotine can improve, impair or have no effect on performance in the water maze. Neurobiological data also suggest an interaction between nicotine and prior experience in complex tasks, although few studies have raised the question of whether nicotine treatment and training in spatial tasks may contribute in an interactive manner to alleviate spatial cognition impairment associated with the ageing process. Different findings suggest that past experience could be a confounding variable in longitudinal studies that aim to evaluate the neuroprotective effects of nicotine on age-related deficits in spatial learning.

The effects of chronic nicotine on spatial learning and bromodeoxyuridine incorporation into the dentate gyrus of the rat

RATIONALE

Nicotine is reported to improve learning and memory in experimental animals. Improved learning and memory has also been related to increased neurogenesis in the dentate gyrus (DG) of the hippocampal formation. Surprisingly, recent studies suggest that self-administered nicotine depresses cell proliferation in the DG.

OBJECTIVE

To test the hypothesis that the effects of nicotine on cell proliferation in the DG and learning and memory depend upon the nicotine dose administered.

METHODS

Rats were chronically infused from subcutaneous osmotic mini pumps with nicotine (0.25 or 4 mg kg(-1) day(-1)) or the saline vehicle for 10 days. Half the rats in each treatment group were trained to locate a hidden platform in a water maze task on days 4-7; a probe trial was performed on day 8. The remaining rats remained in their home cages. The effects of nicotine and of training in the water maze task on cell genesis in the DG were determined by measuring 5-bromo-2'-deoxyuridine (BrDU) uptake using fluorescence immunohistochemistry.

RESULTS

Training in the water maze task increased cell proliferation in the DG. Infusions of nicotine at 4 mg kg(-1) day(-1), but not 0.25 mg kg(-1) day(-1), decreased cell proliferation in both untrained animals and animals trained in the maze and impaired spatial learning.

CONCLUSIONS

The data suggest that learning in the water maze task is impaired by higher doses of nicotine tested, and that this response may be related to reduced cell genesis in the DG.

Nicotinic effects on cognitive function: behavioral characterization, pharmacological specification, and anatomic localization

RATIONALE

Nicotine has been shown in a variety of studies in humans and experimental animals to improve cognitive function. Nicotinic treatments are being developed as therapeutic treatments for cognitive dysfunction.

OBJECTIVES

Critical for the development of nicotinic therapeutics is an understanding of the neurobehavioral bases for nicotinic involvement in cognitive function.

METHODS

Specific and diverse cognitive functions affected by nicotinic treatments are reviewed, including attention, learning, and memory. The neural substrates for these behavioral actions involve the identification of the critical pharmacologic receptor targets, in particular brain locations, and how those incipient targets integrate with broader neural systems involved with cognitive function.

RESULTS

Nicotine and nicotinic agonists can improve working memory function, learning, and attention. Both alpha4beta2 and alpha7 nicotinic receptors appear to be critical for memory function. The hippocampus and the amygdala in particular have been found to be important for memory, with decreased nicotinic activity in these areas impairing memory. Nicotine and nicotinic analogs have shown promise for inducing cognitive improvement. Positive therapeutic effects have been seen in initial studies with a variety of cognitive dysfunctions, including Alzheimer's disease, age-associated memory impairment, schizophrenia, and attention deficit hyperactivity disorder.

CONCLUSIONS

Discovery of the behavioral, pharmacological, and anatomic specificity of nicotinic effects on learning, memory, and attention not only aids the understanding of nicotinic involvement in the basis of cognitive function, but also helps in the development of novel nicotinic treatments for cognitive dysfunction. Nicotinic treatments directed at specific receptor subtypes and nicotinic cotreatments with drugs affecting interacting transmitter systems may provide cognitive benefits most relevant to different syndromes of cognitive impairment such as Alzheimer's disease, schizophrenia, and attention deficit hyperactivity disorder. Further research is necessary in order to determine the efficacy and safety of nicotinic treatments of these cognitive disorders.

Post-training intrahippocampal infusion of nicotine prevents spatial memory retention deficits induced by the cyclo-oxygenase-2-specific inhibitor celecoxib in rats

Recently, we demonstrated that intrahippocampal infusion of the cyclo-oxygenase (COX)-2-specific inhibitor celecoxib impaired spatial memory retention in the Morris water maze. In the present work, we investigated the effects of nicotine, infused in the rat dorsal hippocampus several minutes after infusion of celecoxib, on memory retention in the Morris water maze. Rats were trained for 3 days; each day included two blocks, and each block contained four trials. Test trials were conducted 48 h after surgery. As expected, bilateral intrahippocampal infusion of celecoxib (19 microg/side; 0.1 m) increased escape latency and travel distance in rats, indicating significant impairment of spatial memory retention. We also examined the effects of bilateral infusion of nicotine (0.5, 1.0 and 2.0 microg/side) on memory retention. Infusion of 1 microg nicotine significantly decreased escape latency and travel distance but not swimming speed, compared with controls, suggesting memory retention enhancement by nicotine at this concentration. In separate experiments, bilateral infusion of nicotine, infused 5 min after 0.1 m (19 microg/side) celecoxib infusion, was associated with escape latency, travel distance and swimming speed profiles very similar to those in control animals. Brain tissue sections from several of these animals were subjected to immunohistochemical staining analysis with anti-COX-2 antibodies. Quantification analysis by optical density measurements showed that the celecoxib infusion reduced the immunoreactivity of COX-2-containing neurons in the CA1 area of the hippocampus compared with controls, although this reduction was not significant. However, infusion of a combination of celecoxib and nicotine significantly increased this immunoreactivity compared with levels in control and celecoxib-infused groups. These results suggest that nicotine prevented or reversed the adverse effects of celecoxib on spatial memory retention and protected or restored the immunostaining pattern of COX-2 neurons in the rat dorsal hippocampus.

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.

Effects of histamine H3 receptor antagonists in two models of spatial learning

Despite the well-described attention and short-term memory enhancing effects of H3 receptor antagonists, and evidence to suggest a close relationship between central histaminergic and cholinergic systems, there is a paucity of evidence for a role for H3 receptor blockade in spatial learning. To address this, we investigated two H3 receptor antagonists in a visual discrimination water maze in rats, and in a Barnes circular maze in mice. Thioperamide and ciproxifan significantly attenuated a scopolamine-induced deficit in the water maze task, while only ciproxifan showed a modest attenuation in the Barnes maze. Taken together, these data suggest a role for H3 receptors in spatial learning that appears to be task-dependent.

APP23 mice as a model of Alzheimer's disease: an example of a transgenic approach to modeling a CNS disorder

Animal models are considered essential in research ensuing elucidation of human disease processes and subsequently, testing of potential therapeutic strategies. This is especially true for neurodegenerative disorders, in which the first steps in pathogenesis are often not accessible in human patients. Alzheimer's disease is vastly becoming a major medical and socioeconomic problem in our aging society. Valid animal models for this uniquely human condition should exhibit histopathological, biochemical, cognitive, and behavioral alterations observed in Alzheimer's disease patients. Major progress has been made since the understanding of the genetic basis of Alzheimer's disease and the development and improvement of transgenic mouse models. All present Alzheimer's disease models developed are partial but nevertheless essential in further unraveling the nature and spatial and temporal development of the complex molecular pathology underlying this condition. One of the more recent transgenic attempts to model Alzheimer's disease is the APP23 transgenic mouse. This article describes the development and assessment of this human amyloid precursor protein overexpression model. We summarize histopathological and biochemical, cognitive and behavioral observations made in heterozygous APP23 mice, thereby emphasizing the model's contribution to clarification of neurodegenerative disease mechanisms. In addition, the first therapeutic interventions in the APP23 model are included.

Nicotine enhances trace cued fear conditioning but not delay cued fear conditioning in C57BL/6 mice

Nicotine facilitates hippocampus-dependent contextual but not hippocampus-independent cued delay fear conditioning. To test if the effects of nicotine are specific to contextual fear conditioning or would extend to another hippocampus-dependent version of fear conditioning, we compared the effects of nicotine on cued delay and cued trace fear conditioning in male and female C57BL/6 mice. Unlike cued delay fear conditioning, cued trace fear conditioning is hippocampus dependent. Thus, if nicotine enhances hippocampus-dependent fear conditioning, nicotine should enhance trace fear conditioning. For both trace and delay conditioning, five 30 s, 85 dB white noise conditioned stimuli (CS) were paired with five 2 s, 0.5 mA footshock unconditioned stimuli (US). In the trace paradigm, a 30-s period was inserted between CS offset and US onset. The CS and US co-terminated in the delay paradigm. Testing occurred 24 h later. The data indicate that nicotine (given on training and testing day) enhances trace but not delay cued fear conditioning. No sex differences were found. For delay cued fear conditioning a high level of freezing to the CS was found. Thus, a second experiment examined if the lack of enhancement of delay cued fear conditioning by nicotine was due to a ceiling effect. The CS duration was decreased to 15 s and only one CS-US pairing was used for delay and trace cued fear conditioning. Although overall levels of freezing to the cue were lower in the second experiment, nicotine still enhanced trace fear conditioning but did not enhance delay fear conditioning. Taken together, the results of the present experiments suggest that nicotine enhances hippocampus-dependent versions of fear conditioning.

Nicotine improves learning and memory in rats: morphological evidence for acetylcholine involvement

It has been suggested that nicotine improves rapid information processing (learning and memory) tasks. However, it is not clear which aspects of cognition actually underlie these improvements because relatively less attention has been given to nicotinic cholinergic systems compared to muscarinic systems. The authors therefore studied the effects of nicotine on the learning and memory performance by a step-through passive avoidance task. Nicotine (0.4 mg/kg) was administered s.c. single dose (acute group), once a day for 3 days (subchronic group) or 21 days (chronic group). Nicotine treated and control rats were trained in one trial learning step-through passive avoidance task, where retention latencies were carried out 1 h, 24 h, and 3 days after learning trial. Treatment with nicotine before training session prolonged the latencies significantly (p < .01). Control group, acute, subacute and chronic nicotine treatment groups showed latencies 4.75 +/- 0.6, 69.4 +/- 14, 116.2 +/- 30, and 118.5 +/- 23 s, respectively. In addition, to prove the actual contribution of nicotinic cholinergic system in improvement of learning and memory processing, histological methods that permit the visualization and quantification of ACh levels were used. Electron microscopic evaluation revealed increased numbers of Ach-containing vesicles especially in hippocampus in chronic nicotine-treated rats; although frontal and temporal cortex in addition to hippocampus showed increment in Ach vesicles in a lesser extent in all nicotine treatment groups. These results indicate that long-term nicotine treatment can be important for improving cognitive function in regard to increased cholinergic activity.

Effects of nicotine and mecamylamine on cognition in rhesus monkeys

RATIONALE

Nicotine and other agonists of nicotinic cholinergic receptors (nAChR) have been shown to improve performance in specific memory domains in rodents and monkeys. Such beneficial effects are observed in preclinical models of age-related cognitive decline, stimulating interest in nAChR ligands as possible therapeutics. Prior work has typically focused on assays of spatial working memory in rodent studies and visual recognition memory in monkey studies.

OBJECTIVE

The current study was conducted to determine the role played by nAChRs in multiple types of memory in monkeys.

METHODS

Rhesus monkeys (n=6) were trained to perform a battery of six behavioral tasks and then serially challenged with acute doses of nicotine (3.2-56 microg/kg, i.m.) and the nAChR antagonist mecamylamine (0.32-1.78 mg/kg, i.m.).

RESULTS

Nicotine improved performance on tests designed to assay visual recognition memory, spatial working memory and visuo-spatial associative memory, while mecamylamine impaired visuo-spatial associative memory. Ballistic and fine motor performance was not significantly improved by nicotine but fine motor performance was impaired by mecamylamine.

CONCLUSIONS

Although nicotine may improve performance in multiple domains, effects on visuo-spatial associative memory is the most specifically attributable to nAChR signaling.

Evidence that muscarinic receptors are involved in nicotine-facilitated spatial memory

A delayed nonmatching-to-position task (DNMPT) in a water maze (nine trials per day, 10 days) was used to investigate the role of muscarinic acetylcholinergic receptor (mAChR) in nicotine-facilitated spatial memory in Wistar rats. The index of spatial memory was determined by the number of correct choices made in choice swim (CS) and the time taken to complete the swim in the correct CS. Diazepam (6 mg kg(-1) ip, daily) significantly impaired choice accuracy and increased the swim time for correct CS. In contrast, when nicotine (2 mg kg(-1) sc, twice daily) and diazepam were administered simultaneously in the last three sessions of training, the choice accuracies of CS were similar to controls and significantly higher than in the diazepam group. However, the swim times for correct CS were longer in the nicotine+diazepam group than in controls. Atropine (30 mg kg(-1) ip, daily) significantly decreased the choice accuracies of CS. The choice accuracies of CS swim times for correct CS in the atropine group did not differ significantly from those in the nicotine+atropine group, in which nicotine (2 mg kg(-1) sc, twice daily) and atropine were given simultaneously. These results show that nicotine improves memory performance when the functions of mAChRs are normal; when the mAChRs are blocked, nicotine does not enhance spatial memory. Therefore, mAChRs are involved in the spatial memory-enhancing effect of nicotine.

Long-term neuronal effects and disposition of ectopic preproNGF gene transfer into the rat septum

Although NGF gene therapy has been proposed to treat age- or disease-related brain cholinergic decline, little is known about the ectopic expression or function of this trophic factor after transduction in the brain especially over long intervals. The neuron-targeting, recombinant adeno-associated virus serotype 2 (rAAV2) vector was used to express mouse NGF with C-terminal myc-tag in septum using a full-length preproNGF sequence. While the predominant form of endogenous NGF immunoreactivity in septum was 31 kd of proNGF, almost all of the ectopic NGF-immunoreactivity attributable to the rAAV2-mediated transduction in this region was recovered as mature NGF. Transgene expression was found in both cholinergic and GABAergic neurons, with the number of transduced neurons dependent on vector dose. To determine the long-term effects of this NGF-expression on neuron function, fimbria-fornix (FF) lesions were conducted 6 months after NGF gene transfer. NGF gene transfer attenuated the lesion-induced loss of septal cholinergic but not GABAergic neurons, indicating that long-term expression did not eliminate this response, which has been noted over short intervals. The effects and dose dependency of NGF gene delivery on neuroprotection and neurotrophism were also examined. NGF transduction increased cholinergic cell size in the septum, but required a higher vector dose than neuroprotection. These results reveal potential long-term benefits as well as concerns for genetically modifying septal NGF gene expression to preserve neuronal viability and function.

Septohippocampal acetylcholine: involved in but not necessary for learning and memory?

The neurotransmitter acetylcholine (ACh) has been accorded an important role in supporting learning and memory processes in the hippocampus. Cholinergic activity in the hippocampus is correlated with memory, and restoration of ACh in the hippocampus after disruption of the septohippocampal pathway is sufficient to rescue memory. However, selective ablation of cholinergic septohippocampal projections is largely without effect on hippocampal-dependent learning and memory processes. We consider the evidence underlying each of these statements, and the contradictions they pose for understanding the functional role of hippocampal ACh in memory. We suggest that although hippocampal ACh is involved in memory in the intact brain, it is not necessary for many aspects of hippocampal memory function.

Nicotine enhances contextual fear conditioning and ameliorates ethanol-induced deficits in contextual fear conditioning

Nicotine and ethanol are 2 commonly used and abused drugs that have divergent effects on learning. The present study examined the effects of acute nicotine (0.25 mg/kg), ethanol (1.0 g/kg), and ethanol-nicotine coadministration on fear conditioning in C57BL/6 mice. Mice were assessed for contextual and cued fear conditioning at 1 day and 1 week posttraining. Ethanol disrupted acquisition but not consolidation of contextual fear conditioning; nicotine enhanced contextual fear conditioning and ameliorated ethanol-associated deficits in contextual fear conditioning. Mecamylamine antagonized this effect. Fear conditioning was reassessed 1 week after initial testing with no drug administered. At the 1-week retest, mice previously treated with nicotine continued to show enhanced contextual fear, and mice previously treated with ethanol continued to show contextual fear deficits. Thus, nicotine both produces a long-lasting enhancement of contextual fear conditioning and protects against ethanol-associated deficits.

Knockout and knockin mice to investigate the role of nicotinic receptors in the central nervous system

The recent use of genetically engineered knockout (Ko) and knockin (Kin) animals for neurotransmitter receptor genes, in particular, nicotinic acetylcholine receptors (nAChRs) in the brain, has provided a powerful alternative to the classical pharmacological approach. These animal models are not only useful in order to reexamine and refine the results derived from pharmacological studies, but they do also provide a unique opportunity to determine the subunit composition of the nicotinic receptors which modulate various brain functions. Ultimately, this knowledge will be valuable in the process of designing new drugs that will mimic the effects of nicotine on several important pathologies or on smoking cessation therapies. In this review, we present recent data obtained from the studies of mutant animals that contributed to our understanding of the role and composition of nAChRs in the central nervous system (CNS). The advantages and pitfalls of Ko animal models will also be discussed.

Nicotine enhances contextual fear conditioning in C57BL/6J mice at 1 and 7 days post-training

Nicotine has been demonstrated to enhance learning processes. The present experiments extend these results to examine the effects of nicotine on acquisition and consolidation of contextual and cued fear conditioning, and the duration of nicotine's enhancement of conditioned fear. C57BL/6 mice were trained with two pairings of an auditory CS and a foot shock US. Multiple doses of nicotine were given before or immediately after training and on testing day (0.0, 0.050, 0.125, 0.250, and 0.375 mg/kg, i.p). Freezing to both the context and auditory CS was measured 24h after training and again 1 week after training. Mice did not receive nicotine for the 1-week retest. Nicotine (0.125 and 0.250 mg/kg) given on both training and testing days enhanced freezing to the context at 24h. In addition, elevated freezing to the context was seen 1 week post-training in mice previously treated with 0.125 and 0.250 mg/kg nicotine. Thus, nicotine-treated mice did show elevated levels of freezing when retested 1 week later, even though no nicotine was administered at the 1-week retest. Mice that received nicotine on training day or testing day only and mice that received nicotine with mecamylamine, a nicotinic receptor antagonist, were not different from saline-treated mice. In addition, post-training administration of nicotine did not enhance fear conditioning. The present results indicate that nicotine enhancement of contextual fear conditioning depends on administration of nicotine on training and test days but results in a long-lasting enhancement of memories of contextual fear conditioning that remains in the absence of nicotine.

Protein and mRNA levels of nicotinic receptors in brain of tobacco using controls and patients with Alzheimer's disease

The neuronal nicotinic receptors (nAChRs) are involved in several processes in brain including nicotine dependence and cognitive disorders. While the number of nAChRs in the brain of tobacco smokers is up-regulated, the receptors are reduced in the brain of patients with Alzheimer's disease (AD). The aim of this study was to investigate nAChR mRNA and protein levels in brain of smoking and non-smoking controls and AD patients. Western blotting and RT-PCR techniques were used to quantify different nAChR subunits in autopsy brain. The alpha4 and alpha7 but not the alpha3 nAChR protein levels were significantly increased in the temporal cortex of smoking (SC) compared with non-smoking controls (NSC). The alpha4-protein level was significantly higher in the temporal cortex of smoking AD (SAD) patients compared with non-smoking AD (NSAD). No changes in the alpha3, alpha4 or alpha7 subunits protein level were found in the hippocampus in any of the smoking groups. For both SADs and NSADs the protein levels for the alpha3 and alpha4 in temporal cortex and hippocampus and alpha7 in the hippocampus were significantly lower compared with non-smoking controls. No significant differences in alpha4 and alpha7 mRNA levels were detected in the hippocampus or temporal cortex of smokers compared with non-smokers. In conclusion this study showed an increased level of alpha4 and alpha7 nAChRs subunits in the temporal cortex of SC compared with NSC. This up-regulation was also seen in SAD although the protein levels of nAChR subunits were still lower in smoking AD brain compared with the NSC. The up-regulation of nAChRs in smoking groups and the loss of these receptors in AD patients were not correlated to any changes at the mRNA level suggesting that these changes may reflect post-transcriptional events.

Nicotine compensates for the loss of cholinergic function to enhance long-term potentiation induction

Long-term potentiation (LTP) in the hippocampal CA1 region is widely regarded to be the cellular substrate of learning and memory, and its induction critically depends on the activation of N-methyl-D-aspartate receptors (NMDARs). Nicotine reverses memory deficits caused by a lesion of the cholinergic system in animals. The mechanisms underlying this effect and the effect of nicotine on LTP after cholinergic degeneration are unknown. Here we show that cholinergic lesions impaired the induction of LTP, and nicotine reversed this effect and promoted the induction of LTP. The compensatory action of nicotine appears to be due to the enhancement of NMDAR responses mediated by nicotine-induced disinhibition of pyramidal cells. This may represent the cellular basis of nicotine-mediated cognitive enhancement observed in the presence of cholinergic deficits.

Ontogenetic quinpirole treatments produce spatial memory deficits and enhance skilled reaching in adult rats

There is a paucity of data on neurochemical abnormalities and associated effects on cognition and motor performance in rats ontogenetically treated with quinpirole, a rodent model of dopaminergic hyperfunction. The objective of the current study was to analyze the cognitive and motor effects produced by ontogenetic administration of quinpirole, a dopamine D(2)/D(3) receptor agonist. Past research from this laboratory has shown that ontogenetic quinpirole treatment sensitizes D(2) receptors and produces a variety of characteristic stereotypic behaviors in adult rats. In the current study, rats received quinpirole HCl (1 mg/kg/day) or saline from postnatal day (PD) 1 to PD 11 and went otherwise untreated until adulthood (PD 60). In Experiment 1, cognitive performance was assessed on the standard and matching-to-place versions of the Morris water task (MWT). In Experiment 2, skilled motor performance was assessed on the Whishaw reaching task and locomotor activity was also analyzed. We found that ontogenetically quinpirole-treated rats displayed a deficit on the probe trial given at the end of training of the standard version of the MWT but that there were no significant differences from control on the matching-to-place task. Additionally, rats treated in ontogeny with quinpirole showed significant enhancement in reaching accuracy on the Whishaw reaching task as well as increased locomotor activity relative to saline controls. These findings demonstrate that ontogenetic quinpirole treatments produce cognitive deficits, enhanced skilled reaching and hyperlocomotion. The behavioral changes produced by ontogenetic quinpirole treatment are consistent with dopaminergic hyperfunction, and possible mechanisms are discussed.