Characterization of N-[3H]methylcarbamylcholine binding sites and effect of N-methylcarbamylcholine on acetylcholine release in rat brain

Nicotinic modulation of hippocampal cell signaling and associated effects on learning and memory

The hippocampus is a key brain structure involved in synaptic plasticity associated with long-term declarative memory formation. Importantly, nicotine and activation of nicotinic acetylcholine receptors (nAChRs) can alter hippocampal plasticity and these changes may occur through modulation of hippocampal kinases and transcription factors. Hippocampal kinases such as cAMP-dependent protein kinase (PKA), calcium/calmodulin-dependent protein kinases (CAMKs), extracellular signal-regulated kinases 1 and 2 (ERK1/2), and c-jun N-terminal kinase 1 (JNK1), and the transcription factor cAMP-response element-binding protein (CREB) that are activated either directly or indirectly by nicotine may modulate hippocampal plasticity and in parallel hippocampus-dependent learning and memory. Evidence suggests that nicotine may alter hippocampus-dependent learning by changing the time and magnitude of activation of kinases and transcription factors normally involved in learning and by recruiting additional cell signaling molecules. Understanding how nicotine alters learning and memory will advance basic understanding of the neural substrates of learning and aid in understanding mental disorders that involve cognitive and learning deficits.

Nicotine modulation of fear memories and anxiety: Implications for learning and anxiety disorders

Anxiety disorders are a group of crippling mental diseases affecting millions of Americans with a 30% lifetime prevalence and costs associated with healthcare of $42.3 billion. While anxiety disorders show high levels of co-morbidity with smoking (45.3% vs. 22.5% in healthy individuals), they are also more common among the smoking population (22% vs. 11.1% in the non-smoking population). Moreover, there is clear evidence that smoking modulates symptom severity in patients with anxiety disorders. In order to better understand this relationship, several animal paradigms are used to model several key symptoms of anxiety disorders; these include fear conditioning and measures of anxiety. Studies clearly demonstrate that nicotine mediates acquisition and extinction of fear as well as anxiety through the modulation of specific subtypes of nicotinic acetylcholine receptors (nAChRs) in brain regions involved in emotion processing such as the hippocampus. However, the direction of nicotine's effects on these behaviors is determined by several factors that include the length of administration, hippocampus-dependency of the fear learning task, and source of anxiety (novelty-driven vs. social anxiety). Overall, the studies reviewed here suggest that nicotine alters behaviors related to fear and anxiety and that nicotine contributes to the development, maintenance, and reoccurrence of anxiety disorders.

Nicotinic receptors, memory, and hippocampus

Nicotinic acetylcholine receptors (nAChRs) modulate the neurobiological processes underlying hippocampal learning and memory. In addition, nicotine's ability to desensitize and upregulate certain nAChRs may alter hippocampus-dependent memory processes. Numerous studies have examined the effects of nicotine on hippocampus-dependent learning, as well as the roles of low- and high-affinity nAChRs in mediating nicotine's effects on hippocampus-dependent learning and memory. These studies suggested that while acute nicotine generally acts as a cognitive enhancer for hippocampus-dependent learning, withdrawal from chronic nicotine results in deficits in hippocampus-dependent memory. Furthermore, these studies demonstrated that low- and high-affinity nAChRs functionally differ in their involvement in nicotine's effects on hippocampus-dependent learning. In the present chapter, we reviewed studies using systemic or local injections of acute or chronic nicotine, nAChR subunit agonists or antagonists; genetically modified mice; and molecular biological techniques to characterize the effects of nicotine on hippocampus-dependent learning.

Altered acetylcholine release in the hippocampus of dystrophin-deficient mice

Mild cognitive impairments have been described in one-third of patients with Duchenne muscle dystrophy (DMD). DMD is characterized by progressive and irreversible muscle degeneration caused by mutations in the dystrophin gene and lack of the protein expression. Previously, we have reported altered concentrations of α7- and β2-containing nicotinic acetylcholine receptors (nAChRs) in hippocampal membranes of dystrophic (mdx) mice. This suggests that alterations in the central cholinergic synapses are associated with dystrophin deficiency. In this study, we examined the release of acetylcholine (ACh) and the level of the vesicular ACh transporter (VAChT) using synaptosomes isolated from brain regions that normally have a high density of dystrophin (cortex, hippocampus and cerebellum), in control and mdx mice at 4 and 12months of age. ACh release evoked by nicotinic stimulation or K(+) depolarization was measured as the tritium outflow from superfused synaptosomes preloaded with [(3)H]-choline. The results showed that the evoked tritium release was Ca(2+)-dependent and mostly formed by [(3)H]-ACh. β2-containing nAChRs were involved in agonist-evoked [(3)H]-ACh release in control and mdx preparations. In hippocampal synaptosomes from 12-month-old mdx mice, nAChR-evoked [(3)H]-ACh release increased by 57% compared to age-matched controls. Moreover, there was a 98% increase in [(3)H]-ACh release compared to 4-month-old mdx mice. [(3)H]-ACh release evoked by K(+) depolarization was not altered, while the VAChT protein level was decreased (19%) compared to that of age-matched controls. In cortical and cerebellar preparations, there was no difference in nAChR-evoked [(3)H]-ACh release and VAChT levels between mdx and age-matched control groups. Our previous findings and the presynaptic alterations observed in the hippocampi of 12-month-old mdx mice indicate possible dysfunction of nicotinic cholinergic synapses associated with dystrophin deficiency. These changes may contribute to the cognitive and behavioral abnormalities described in dystrophic mice and patients with DMD.

Cellular, molecular, and genetic substrates underlying the impact of nicotine on learning

Addiction is a chronic disorder marked by long-lasting maladaptive changes in behavior and in reward system function. However, the factors that contribute to the behavioral and biological changes that occur with addiction are complex and go beyond reward. Addiction involves changes in cognitive control and the development of disruptive drug-stimuli associations that can drive behavior. A reason for the strong influence drugs of abuse can exert on cognition may be the striking overlap between the neurobiological substrates of addiction and of learning and memory, especially areas involved in declarative memory. Declarative memories are critically involved in the formation of autobiographical memories, and the ability of drugs of abuse to alter these memories could be particularly detrimental. A key structure in this memory system is the hippocampus, which is critically involved in binding multimodal stimuli together to form complex long-term memories. While all drugs of abuse can alter hippocampal function, this review focuses on nicotine. Addiction to tobacco products is insidious, with the majority of smokers wanting to quit; yet the majority of those that attempt to quit fail. Nicotine addiction is associated with the presence of drug-context and drug-cue associations that trigger drug seeking behavior and altered cognition during periods of abstinence, which contributes to relapse. This suggests that understanding the effects of nicotine on learning and memory will advance understanding and potentially facilitate treating nicotine addiction. The following sections examine: (1) how the effects of nicotine on hippocampus-dependent learning change as nicotine administration transitions from acute to chronic and then to withdrawal from chronic treatment and the potential impact of these changes on addiction, (2) how nicotine usurps the cellular mechanisms of synaptic plasticity, (3) the physiological changes in the hippocampus that may contribute to nicotine withdrawal deficits in learning, and (4) the role of genetics and developmental stage (i.e., adolescence) in these effects.

Cholinergic modulation of slow cortical rhythm in urethane-anesthetized rats

Slow cortical rhythm (SCR) is characterized by rhythmic cycling of active (UP) and silent (DOWN) states in cortical cells. In urethane anesthesia, SCR appears as alternation of almost isoelectrical EEG periods and low-frequency, high-amplitude large shifts with superimposed high-frequency activity in the local field potentials (LFPs). Dense cholinergic projection reaches the cortex from the basal forebrain (BF), and acetylcholine (ACh) has been demonstrated to play a crucial role in the regulation of cortical activity. In the present experiments, cholinergic drugs were administered topically to the cortical surface of urethane-anesthetized rats to examine the direct involvement of ACh and the BF cholinergic system in the SCR. SCR was recorded by a 16-pole vertical electrode array from the hindlimb area of the somatosensory cortex. Multiple unit activity (MUA) was recorded from layer V to VI in close proximity of the recording array. Neither a low dose (10 mM solution) of the muscarinic antagonist atropine or the nicotinic agonist nicotine (1 mM solution) had any effect on SCR. In contrast, the higher dose (100 mM solution) of atropine, the cholinergic agonist carbachol (32 mM solution), and the cholinesterase inhibitor physostigmine (13 mM solution) all decreased the number of UP states, delta power (0-3 Hz) and MUA. These results suggest that cholinergic system may influence SCR through muscarinic mechanisms during urethane anesthesia. Cholinergic activation obstructs the mechanisms responsible for local or global synchronization seen during SCR as this rhythm was disrupted or aborted. Muscarinic antagonism can evoke similar changes when high dose of atropine is applied.

Alpha 7 nicotinic acetylcholine receptor modulates lymphocyte activation

AIMS

Even though the presence of alpha7 nicotinic receptor (nAChR) in lymphocytes has been demonstrated, its functional role still remains elusive. The aim of our study was to characterize alpha7 nAChRs in human lymphocytes upon phytohemagglutinin (PHA) stimulation.

MAIN METHODS

Lymphocytes were activated with the mitogen PHA. alpha7 nAChRs were studied by reverse transcription-polymerase chain reaction (RT-PCR), real time PCR, flow cytometry and confocal laser scanning microscopy. The effects of nicotinic drugs on PHA-induced proliferation was evaluated by the [(3)H]-thymidine incorporation assay.

KEY FINDINGS

We show that the expression of functional alpha7 receptors increases after PHA stimulation. The activation of peripheral lymphocytes by PHA increases 2.2-fold the alpha7 subunit mRNA expression and 4-fold the binding of the antagonist alpha-bungarotoxin (alpha-BTX) with respect to non activated lymphocytes. By measuring the increase of intracellular calcium in response to nicotine we determine that alpha7 receptors in lymphocytes are functional. Nicotinic drugs differentially modulate T cell activation. While nicotine tends to inhibit proliferative responses, specific alpha7 antagonists, such as alpha-BTX and methyllycaconitine, enhance cell division.

SIGNIFICANCE

This study reveals that the alpha7 receptor modulates lymphocyte activation and contributes to clarifying the role of the non neuronal cholinergic system in the immune response.

Spatial and intracellular relationships between the alpha7 nicotinic acetylcholine receptor and the vesicular acetylcholine transporter in the prefrontal cortex of rat and mouse

The alpha 7 subunit of the nicotinic acetylcholine receptor (alpha7nAChR) is expressed in the prefrontal cortex (PFC), a brain region where these receptors are implicated in cognitive function and in the pathophysiology of schizophrenia. Activation of this receptor is dependent on release of acetylcholine (ACh) from axon terminals that contain the vesicular acetylcholine transporter (VAChT). Since rat and mouse models are widely used for studies of specific abnormalities in schizophrenia, we sought to determine the subcellular location of the alpha7nAChR with respect to VAChT storage vesicles in axon terminals in the PFC in both species. For this, we used dual electron microscopic immunogold and immunoperoxidase labeling of antisera raised against the alpha7nAChR and VAChT. In both species, the alpha7nAChR-immunoreactivity ((-)ir) was principally identified within dendrites and dendritic spines, receptive to axon terminals forming asymmetric excitatory-type synapses, but lacking detectable alpha7nAChR or VAChT-ir. Quantitative analysis of the rat PFC revealed that of alpha7nAChR-labeled neuronal profiles, 65% (299/463) were postsynaptic structures (dendrites and dendritic spine) and only 22% (104/463) were axon terminals or small unmyelinated axons. In contrast, VAChT was principally localized to varicose vesicle-filled axonal profiles, without recognized synaptic specializations (n=240). Of the alpha7nAChR-labeled axons, 47% (37/79) also contained VAChT, suggesting that ACh release is autoregulated through the presynaptic alpha7nAChR. The VAChT-labeled terminals rarely formed synapses, but frequently apposed alpha7nAChR-containing neuronal profiles. These results suggest that in rodent PFC, the alpha7nAChR plays a major role in modulation of the postsynaptic excitation in spiny dendrites in contact with VAChT containing axons.

Tetrahydroaminoacridine (THA) in Alzheimer's disease: an assessment of attentional and mnemonic function using CANTAB

A placebo-controlled cross-over trial (n = 89) investigated the use of a chronic dose of the cholinesterase inhibitor THA, as a treatment for dementia of the Alzheimer type (DAT). Effects on both subjective clinical rating scales and objective computerised tests were assessed. In regard to the former, analysis of the three main clinical outcome measures showed statistically significant effects of the drug on the Mini-Mental State Examination (MMSE) and the Abbreviated Mental Test Score (AMTS), but not on the Activities of Daily Living scale (ADL). Using the objective computerised CANTAB tests, sensitive to specific aspects of memory and attention, evidence was found for improvements in attentional function rather than memory, in patients with mild to moderate DAT. Although these improvements were significant, they were small and restricted to certain tests of attentional function. Nevertheless, they add to the growing body of evidence that the cholinergic system is involved in the control of attentional processes; and are substantiated by the findings of a second study examining the effects of an acute dose of nicotine on attentional and mnemonic performance in patients with DAT. This study found significant improvements in cognitive performance in patients receiving nicotine, in objective tests of attention but not of short-term memory. These data will clearly provide important comparative data for future investigations of putative cognitive enhancing drugs in DAT sufferers.

Role of presynaptic nicotinic acetylcholine receptors in the regulation of gastrointestinal motility

Presynaptic nicotinic acetylcholine receptors (nAChRs) located on cholinergic terminals facilitate the release of acetylcholine (ACh), thereby constituting a fail-safe mechanism at strategic locations, such as the neuromuscular junction, where reliable transmission is vital. Accumulating data indicate that myenteric neurons in the enteric nervous system possess not only somatodendritic nAChRs, which mediate cholinergic transmission between neurons, but also presynaptic nAChRs. Functional evidence shows that these receptors mediate a positive feedback with respect to ACh release from myenteric motoneurons, and might therefore play an important role in the regulation of gastrointestinal motility. These presynaptic nAChRs were found to be more sensitive to nicotinic ligands than somatodendritic nAChRs and could therefore be primary targets of exogenous compounds, such as nicotine. This interaction might provide a neurochemical basis for the effect of smoking on gastrointestinal motility. Another important human pharmacological implication is based on our recent observation that monoamine uptake inhibitor-type antidepressant drugs are able to inhibit presynaptic nAChRs in the enteric nervous system. The disruption of the nAChR-mediated positive feedback modulation by antidepressants might explain the frequent occurrence of constipation, a common side effect, attributed to these drugs. Clarification of the role of presynaptic nAChRs in feedback mechanisms in the enteric nervous system might be instrumental in the development of new drugs affecting gastrointestinal motility.

EFFECT of nicotine on hippocampal nicotinic acetylcholine alpha7 receptor and NMDA receptor subunits 2A and 2B expression in young and old rats

Nicotinic acetylcholine (nAChR) and N-methyl-D-aspartate receptors (NMDARs) play critical roles in memory function. This study administered chronic nicotine to determine the alterations of N-methyl-D-aspartate receptor subunit 2A and 2B (NR2A, NR2B) and the alterations of alpha7nAChR receptor. It was determined that the effectivity of nicotine and the data support that nicotine increases hippocampal NR2A and B expression. Additionally, the role of nicotine in the cognitive improvement was not supported by the antioxidative mechanisms or the authors observed no effect of nicotine on lipid peroxidation at the hippocampus.

Co-expression of alpha7 and beta2 nicotinic acetylcholine receptor subunit mRNAs within rat brain cholinergic neurons

Nicotine enhances cognitive and attentional processes through stimulation of the basal forebrain cholinergic system. Although muscarinic cholinergic autoreceptors have been well characterized, pharmacological characterization of nicotinic autoreceptors has proven more difficult. The present study used double-labeling in situ hybridization to determine expression of nicotinic acetylcholine receptor (nAChR) subunit mRNAs within basal forebrain cholinergic neurons in order to gain information about possible nAChR autoreceptor properties. Cholinergic cells of the mesopontine tegmentum and striatal interneurons were also examined, as were septohippocampal GABAergic neurons that interact with cholinergic neurons to regulate hippocampal activity. alpha7 and beta2 nAChR mRNAs were found to be co-expressed in almost all cholinergic cells and in the majority of GABAergic neurons examined. alpha4 nAChR mRNA expression was restricted to cholinergic cells of the nucleus basalis magnocellularis, and to non-cholinergic cells of the medial septum and mesopontine tegmentum. These data suggest possible regional differences in the pharmacological properties of nicotinic autoreceptors on cholinergic cells. Whereas most cholinergic cells express rapidly desensitizing alpha7 homomers or alpha7beta2 heteromers, cortical projection neurons may also express a pharmacologically distinct alpha4beta2 nAChR subtype. There may also be differential nAChR regulation of cholinergic and non-cholinergic cells within the mesopontine tegmentum that are implicated in acquisition of nicotine self-administration.

Brain-selective stimulation of nicotinic receptors by TC-1734 enhances ACh transmission from frontoparietal cortex and memory in rodents

The authors have described the effect of TC-1734, a brain-selective nicotinic acetylcholine receptor (nAChR) agonist, on acetylcholine (ACh) release in the frontoparietal cortex of rats and on cognitive function in mice. Oral administration of TC-1734 (5, 10 and 20 mg/kg) stimulated ACh release in a dose-dependent manner, as measured by transversal microdialysis. The maximal effect on the amplitude of ACh release was observed at a dose of 10 mg/kg (about 70% above baseline), whereas the maximal effect on the duration of ACh release was observed at the dose of 20 mg/kg. By contrast, oral administration of nicotine (1, 2.5 and 5 mg/kg) did not stimulate ACh release in a dose-dependent manner but produced the same maximal effect on the amplitude of ACh release (about 50% above baseline) at all the doses tested. The ability of both TC-1734 (10 mg/kg) and nicotine (1 mg/kg) to increase ACh levels was antagonized by mecamylamine (1 mg/kg s.c.), suggesting a specific nicotine receptor-mediated effect of both agonists. No tolerance to TC-1734- and nicotine-stimulated ACh release was observed after repeated treatment with TC-1734 (10 mg/kg) or nicotine (1 mg/kg) for 4 days. TC-1734 (1 mg/kg p.o.) improved memory in the object recognition test in mice, and this effect was antagonized by mecamylamine (2.5 mg/kg i.p.). Taken together, these results show that TC-1734 stimulates nAChR in the brain to induce an increase of ACh release in the cortex of rats and enhance memory in mice.

Consenso sobre o tratamento da dependência de nicotina

Têm sido atribuídas à dependência de nicotina 20% das mortes nos EUA. Estudos têm mostrado que 30% a 50% das pessoas que começam a fumar escalam para um uso problemático. Nos últimos 20 anos, a educação e a persuasão não foram suficientes para promover uma mudança política, cultural e social relacionada ao comportamento de fumar. As intervenções para interromper o uso de tabaco ainda não estão integradas às rotinas dos serviços de saúde no mundo. A falta de estratégias de integração, de tempo disponível para acoplar ações assistenciais mais específicas e mesmo a percepção dos profissionais de saúde de que os tratamentos para a dependência de nicotina são pouco efetivos são algumas das barreiras apontadas. Assim, elaborar um consenso sobre a dependência de nicotina teve como objetivos: • levantar dados epidemiológicos relevantes relacionados ao uso do tabaco no mundo e no Brasil; • revisar as ações gerais e centrais da nicotina; • elaborar um protocolo de triagem mínimo para serviços de atenção primária à saúde; • recomendar diretrizes básicas de avaliação, diagnóstico e tratamento para todos os níveis de atenção à saúde em relação à dependência de nicotina; • fornecer sugestões para a abordagem de grupos especiais de pacientes: adolescentes, gestantes, idosos, pacientes em regime de internação, obesos e pacientes com comorbidades psiquiátricas, cardiovasculares e respiratórias.

Neurobiology of the nicotine withdrawal syndrome

The aversive aspects of withdrawal from chronic nicotine exposure are thought to be an important motivational factor contributing to the maintenance of the tobacco habit in human smokers. Much emphasis has been placed on delineating the underlying neurobiological mechanisms mediating different components of the nicotine withdrawal syndrome. Recent studies have shown that both central and peripheral populations of nicotinic acetylcholine receptors (nAChRs) are involved in mediating somatic signs of nicotine withdrawal as measured by the rodent nicotine abstinence scale. However, only central populations of nAChRs are involved in mediating affective aspects of nicotine withdrawal, as measured by elevations in brain-stimulation reward thresholds and conditioned place aversion. Nicotine interacts with several neurotransmitter systems, including acetylcholine, dopamine, opioid peptides, serotonin, and glutamate systems. Evidence so far suggests that these neurotransmitters play a role in nicotine dependence and withdrawal processes. The available evidence also suggests that different underlying neurochemical deficits mediate somatic and affective components of nicotine withdrawal. The aim of the present review is to discuss preclinical findings concerning the neuroanatomical and neurochemical substrates involved in these different aspects of nicotine withdrawal.

A unifying hypothesis of Alzheimer's disease. IV. Causation and sequence of events

Contrary to common concepts, the brain in Alzheimer's disease (AD) does not follow a suicide but a rescue program. Widely shared features of metabolism in starvation, hibernation and various conditions of energy deprivation, e.g. ischemia, allow the definition of a deprivation syndrome which is a phylogenetically conserved adaptive response to energetic stress. It is characterized by hypometabolism, oxidative stress and adjustments of the glucose-fatty acid cycle. Cumulative evidence suggests that the brain in aging and AD actively adapts to the progressive fuel deprivation. The counterregulatory mechanisms aim to preserve glucose for anabolic needs and promote the oxidative utilization of ketone bodies. The agent mediating the metabolic switch is soluble Abeta which inhibits glucose utilization and stimulates ketone body utilization at various levels. These processes, which are initiated during normal aging, include inhibition of pro-glycolytic neurohormones, cholinergic transmission, and pyruvate dehydrogenase, the key transmitter and effector systems regulating glucose metabolism. Hormonal and effector systems which promote ketone body utilization, such as glucocorticosteroid and galanin activity, GABAergic transmission, nitric oxide, lipid transport, Ca2+ elevation, and ketone body metabolizing enzymes, are enhanced. A multitude of risk factors feed into this pathophysiological cascade at a variety of levels. Taking into account its pleiotropic regulatory actions in the deprivation response, a new name for Abeta is suggested: deprivin. On the other hand, cumulative evidence, taken together compelling, suggests that senile plaques are the dump rather than the driving force of AD. Moreover, the neurotoxic action of fibrillar Abeta is a likely in vitro artifact but does not contribute significantly to the in vivo pathophysiological events. This archaic program, conserved from bacteria to man, aims to ensure the survival of a deprived organism and controls such divergent processes as sporulation, hibernation, aging and aging-related diseases. In contrast to the immature brain, ketone body utilization of the aged brain is no longer sufficient to meet the energetic demands and is later supplemented by lactate, thus recapitulating in reverse order the sequential fuel utilization of the immature brain. The transduction pathways which operate to switch metabolism also convey the programming and balancing of the de-/redifferentiation/apoptosis cell cycle decisions. This encompasses the reiteration of developmental processes such as transcription factor activation, tau hyperphosphorylation, and establishment of growth factor independence by means of Ca2+ set point shift. Thus, the increasing energetic insufficiency results in the progressive centralization of metabolic activity to the neuronal soma, leading to pruning of the axonal/dendritic trees, loss of neuronal polarity, downregulation of neuronal plasticity and, eventually, depending on the Ca2+ -energy-redox homeostasis, degeneration of vulnerable neurons. Finally, it is outlined that genetic (e.g. Down's syndrome, APP and presenilin mutations and apoE4) and environmental risk factors represent progeroid factors which accelerate the aging process and precipitate the manifestation of AD as a progeroid systemic disease. Aging and AD are related to each other by threshold phenomena, corresponding to stage 2, the stage of resistance, and stage 3, exhaustion, of a metabolic stress response.

Nicotine and its interaction with beta-amyloid protein: a short review

Two features of Alzheimer's disease (AD) are beta-amyloid protein (betaAP) deposition and a severe cholinergic deficit. beta-Amyloid protein is a 39- to 43-amino acid transmembrane fragment of a larger precursor molecule, amyloid precursor protein. It is a major constituent of senile plaque, a neuropathologic hallmark of AD, and has been shown to be neurotoxic in vivo and in vitro. The cholinergic neurotransmission system is seen as the primary target of AD. However, other systems are also found to show functional deficit. An association between cholinergic deficit and betaAP is suggested by a negative correlation between cigarette smoking and AD. Evidence hitherto suggests that betaAP causes neuronal death possibly via apoptosis by disrupting calcium homeostasis, which may involve direct activation or enhancement of ligand-gated or voltage-dependent calcium channels. Selective second messengers such as protein kinases are triggered that signal neuronal death. Nicotine or acetylcholinesterase inhibitors can partially prevent the neurotoxicity of betaAP in vivo and in vitro. However, the exact mechanism by which nicotine provides its protective effects is not fully understood, but clearly there are protective roles for nicotine. Here, some aspects of betaAP neurotoxicity and nicotinic intervention as a protective agent are discussed.

Nicotinic agonists stimulate acetylcholine release from mouse interpeduncular nucleus: a function mediated by a different nAChR than dopamine release from striatum

Acetylcholine release stimulated by nicotinic agonists was measured as radioactivity released from perfused synaptosomes prepared from mouse interpeduncular nucleus (IPN) that had been loaded with [(3)H]choline. Agonist-stimulated release was dependent upon external calcium and over 90% of released radioactivity was acetylcholine. The release process was characterized by dose response curves for 13 agonists and inhibition curves for six antagonists. alpha-Conotoxin MII did not inhibit this release, while alpha-conotoxin AuIB inhibited 50% of agonist-stimulated release. Comparison of this process with [(3)H]dopamine release from mouse striatal synaptosomes indicated that different forms of nicotinic acetylcholine receptors (nAChRs) may mediate these processes. This was confirmed by assays using mice homozygous for the beta 2 subunit null mutation. The deletion of the beta 2 subunit had no effect on agonist-stimulated acetylcholine release, but abolished agonist-stimulated release of dopamine from striatal synaptosomes. Mice heterozygous for the beta 2 subunit null mutation showed decreased dopamine release evoked by L-nicotine with no apparent change in EC(50) value, as well as similar decreases in both transient and persistent phases of release with no changes in desensitization rates.

Evidence for a complex influence of nicotinic acetylcholine receptors on hippocampal serotonin release

The effects of nicotine on 5-hydroxytryptamine (5-HT) release from serotonergic nerve endings in rat dorsal hippocampal slices were studied. Nicotine (50-500 microM:) caused a concentration-dependent increase in 5-HT release. This effect was antagonised by mecamylamine (0.5 microM:), indicating an action at nicotinic receptors. Nicotine-evoked 5-HT release was not affected by tetrodotoxin (3 microM:), cadmium chloride (0.1 mM:), or the absence of Ca(2+) or Na(+) in the superfusion medium. Unexpectedly, higher concentrations of mecamylamine alone (1-50 microM:) increased 5-HT release. This suggested the presence of inhibitory input to 5-HT neurones and that these inhibitory neurones possess tonically active nicotinic receptors. The effect of mecamylamine (50 microM:) on 5-HT release was reduced by the muscarinic M(1) receptor agonist, McN-A-343 (100 microM:), but pirenzepine (0.005-1 microM:), which blocks M(1) receptors, alone increased 5-HT release. Hippocampal serotonergic neurones are known to possess both excitatory nicotinic receptors and inhibitory M(1) receptors. Although there may be several explanations for our results, one possible explanation is that nicotine stimulates 5-HT release by activating nicotinic heteroreceptors on 5-HT terminals. Mecamylamine (0.5 microM:) antagonises this effect, but higher concentrations increase 5-HT release indirectly by blocking the action of endogenous acetylcholine on nicotinic receptors situated on cholinergic neurones that provide muscarinic inhibitory input to 5-HT neurones.

No involvement of nicotinic receptors in the facilitation of acetylcholine outflow in mouse cortex in the presence of neostigmine and atropine

The role of nicotinic and muscarinic receptors in the modulation of acetylcholine release was studied using field stimulated mouse cortex slices incubated with [(3)H]-choline. Both acetylcholine (100 microM) and the cholinesterase inhibitor neostigmine (100 microM) inhibited the stimulation-induced (S-I) outflow of radioactivity but in the presence of atropine (0.3 microM) an enhancement was seen, which may be indicative of facilitatory nicotinic receptors. Mecamylamine (100 microM) was unable to antagonize the enhancement seen in the presence of acetylcholine and atropine. The nicotinic agonist dimethylphenylpiperazinium (30 microM) did not facilitate S-I outflow of radioactivity. A range of nicotinic blockers had no effect on the enhancement seen in the presence of neostigmine and atropine, nor did indomethacin, the 5HT(3) antagonist MDL 7222 nor the NMDA antagonist MK-801. The inability to block this effect suggests that nicotinic receptors are not involved. We postulate, at least for neostigmine, that the facilitation is an artefact because of the use of [(3)H]-choline as a radiotracer whereby the efflux of radioactivity is enhanced because the radiolabelled acetylcholine is not metabolized to choline and therefore flows out of the tissue more readily.

Nicotine, brain nicotinic receptors, and neuropsychiatric disorders

Neuronal nicotinic acetylcholine receptors (nAChRs) represent a large family of ligand-gated cation channels with diverse structures and properties. In contrast to the muscular nAChRs, the physiological functions of neuronal nAChRs are not well defined to date. Behavioral studies indicate that brain nAChRs participate in complex functions such as attention, memory, and cognition, whereas clinical data suggest their involvement in the pathogenesis of certain neuropsychiatric disorders (Alzheimer's and Parkinson's diseases, Tourette's syndrome, schizophrenia, depression, etc.). For the majority of these disorders, the use of nAChRs' agonists may represent either a prophylactic (especially for Alzheimer's and Parkinson's diseases) or a symptomatic treatment. The possible mechanisms underlying these beneficial effects as well as the characteristics and potential therapeutic use of new, subtype-selective nAChRs agonists are presented.

Nicotine for Alzheimer's disease

BACKGROUND

Nicotine is a cholinergic agonist that acts, not only post-synaptically, but also releases pre-synaptic acetylcholine, and in animal models has been shown to reverse spatial memory decline in rats with lesion in the medial septal nucleus and to show recovery on memory in aged monkeys. Nicotine also has effects on other transmitters like serotonin (5HT), dopamine, or GABA. On the other hand, because nicotine has serious adverse effects, especially concerning cardiovascular risks in elderly people, and also on sleep and behavior, there are several important reasons to conduct a systematic review to assess the clinical efficacy and safety of nicotine in patients with AD.

OBJECTIVES

The aim of this review is to determine whether there is evidence of beneficial effect, and to assess its safety profile, when nicotine is used for Alzheimer's disease.

SEARCH STRATEGY

The Cochrane Controlled Trials Register (Issue 2, 99) was searched using the terms nicotin* and alzheimer*. Three references to trials were deemed suitable for inclusion but are awaiting consideration while the investigators are contacted.

SELECTION CRITERIA

All unconfounded, double-blind, randomized trials in which treatment with nicotine patches or administration of nicotine intravenously was administered for more than a day and compared to placebo in people with Alzheimer's disease.

DATA COLLECTION AND ANALYSIS

As no trials were suitable for inclusion, no data have been extracted or pooled in a meta-analysis. One trial is awaiting consideration, and if included in an update of this review, any available data will be incorporated.

MAIN RESULTS

The poor quality of the trials did not allow any synthesis of results across studies. However, the data available in trials considered are compatible with nicotine producing harm, no change or improvement.

REVIEWER'S CONCLUSIONS

This review is not able to provide reliable evidence that nicotine is a useful treatment for Alzheimer's disease.

Pharmacological characterization of nicotine-induced acetylcholine release in the rat hippocampus in vivo: evidence for a permissive dopamine synapse

In this study, the mechanism of nicotine-induced hippocampal acetylcholine (ACh) release in awake, freely moving rats was examined using in vivo microdialysis. Systemic administration of nicotine (0.4 mg kg(-1), s.c.) increased the levels of ACh in hippocampal dialysates. The nicotine-induced hippocampal ACh release was sensitive to the pretreatment of neuronal nicotinic acetylcholine receptor (nAChR) antagonists mecamylamine (3.0 mg kg(-1), s.c.) and dihydro-beta-erythrodine (DHbetaE; 4.0 mg kg(-1), s.c.) as well as systemic administration of the dopamine (DA) D1 receptor antagonist SCH-23390 (R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-benzaz epine; 0.3 mg kg(-1), s.c.). Local perfusion of mecamylamine (100 microM), DHbetaE (100 microM) or SCH-23390 (10 microM) through microdialysis probe did not increase basal hippocampal ACh release. Hippocampal ACh release elicited by systemic administration of nicotine (0.4 mg kg(-1), s.c.) was antagonized by local perfusion of SCH-23390 (10 microM), but not by MEC (100 microM) or DHbetaE (100 microM). Direct perfusion of nicotine (1 mM, but not 0.1 mM) increased hippocampal ACh levels; however, this effect was relatively insensitive to blockade by co-perfusion of either mecamylamine (100 microM) or SCH-23390 (10 microM). These results suggest that nicotine-induced hippocampal ACh release occurs by two distinct mechanisms: (1) activation of nAChRs outside the hippocampus leading to DA release and subsequent ACh release involving a permissive DA synapse, and (2) direct action of nicotine within the hippocampus leading to ACh release via non-DA-ergic mechanism.

Parabrachial origin of calcitonin gene-related peptide-immunoreactive axons innervating Meynert's basal nucleus

Meynert's basal nucleus is innervated by calcitonin gene-related peptide (CGRP)-immunoreactive axons synapsing with cholinergic principal cells. Origin of CGRP-immunopositive axons was studied in the albino rat. Since beaded axons containing the nicotinic acetylcholine receptor (nAChR) are also present in the basal nucleus, the microstructural arrangement raises the question whether or not an interaction between CGRP and nAChR exists like in the neuromuscular junction. We found that electrolytic lesion of the parabrachial nucleus results in degeneration of CGRP-immunoreactive axons in the ipsilateral nucleus basalis and induces shrinkage of principal cholinergic neurons while the contralateral nucleus basalis remains intact. Electrolytic lesions in the thalamus, caudate-putamen, and hippocampus did not induce alterations in Meynert's basal nucleus. Disappearance of CGRP after lesions of the parabrachial nucleus does not impair presynaptic nAChR in the basal nucleus, suggesting that, unlike in the neuromuscular junction, CGRP is not involved in the maintenance of nAChR in the basal forebrain. It is concluded that the parabrachial nucleus is involved in the activation of the nucleus basalis-prefrontal cortex system, essential in gnostic and mnemonic functions.

Neurochemistry and pharmacology of the major hippocampal transmitter systems: synaptic and nonsynaptic interactions

Hippocampus plays a crucial role in important brain functions (e.g. memory, learning) thus in the past two decades this brain region became a major objective of neuroscience research. During this period large number of anatomical, neurochemical and electrophysiological data have been accumulated. While excellent reviews have been published on the anatomy and electrophysiology of hippocampal formation, the neurochemistry of this area has not been thoroughly surveyed. Therefore the aim of this review is to summarize the neurochemical and pharmacological data on the release of the major neurotransmitters found in the hippocampal region: glutamate (GLU), gamma-amino butyric acid (GABA), acetylcholine (ACh), noradrenaline (NA) and serotonin (5-HT). In addition, this review analyzes the synaptic and nonsynaptic interactions between hippocampal neuronal elements and overviews how auto- and heteroreceptors are involved in the presynaptic modulation of transmitter release. The presented data clearly show that transmitters released from axon terminals without synaptic contact play an important role in the fine tuning of communication between neurons within a neuronal circuit.

Nerve terminal currents induced by autoreception of acetylcholine release

The activation of autoreceptors is known to be important in the modulation of presynaptic transmitter secretion in peripheral and central neurons. Using whole-cell recordings made from the free growth cone of myocyte-contact motoneurons of Xenopus cell cultures, we have observed spontaneous nerve terminal currents (NTCs). These spontaneous NTCs are blocked by d-tubocurarine (d-TC) and alpha-bungarotoxin (alpha-BuTx), indicating that endogenously released acetylcholine (ACh) can produce substantial membrane depolarization in the nerve terminals. Local application of NMDA to the growth cone increased the frequency of spontaneous NTCs. When the electrical stimulations were applied at the soma to initiate evoked-release of ACh, evoked ACh-induced potentials were recorded in the nerve terminals, which were inhibited by d-TC and hexamethonium but not by atropine. Replacement of normal Ringer's solution with high-Mg2+, low-Ca2+ solution also reversibly inhibited evoked ACh-induced potentials. The possible regulatory role of presynaptic nicotinic autoreceptors on the synaptic transmission was also examined. When the innervated myocyte was whole-cell voltage-clamped to record synaptic currents, application of hexamethonium inhibited the amplitude of evoked synaptic currents at a higher degree than that of iontophoretic ACh-induced currents. Furthermore, hexamethonium markedly reduced the frequency of spontaneous synaptic currents at high-activity synapses. Pretreatment of neurons with alpha-BuTx also inhibited the evoked synaptic currents in manipulated synapses. These results suggest that ACh released spontaneously or by electrical stimulation may act on the presynaptic nicotinic autoreceptors of the same nerve terminals to produce membrane potential change and to regulate synaptic transmission.

Pharmacological characterization of nicotinic receptor-mediated acetylcholine release in rat brain--an in vivo microdialysis study

In vivo microdialysis was used to investigate nicotinic receptor-mediated acetylcholine release in the hippocampus, frontal cortex, and striatum of freely moving rats. Intraperitoneal administration of (-)-nicotine increased the release of acetylcholine in the hippocampus and frontal cortex but not in the striatum. (-)-Nicotine exhibited a bell-shaped dose-response relationship, and showed attenuation of response at the highest dose (5.0 mg/kg i.p.) in both the hippocampus and frontal cortex. In the hippocampus, (-)-nicotine (1.0 mg/kg i.p.)-induced increase of acetylcholine release was blocked by pretreatment with the centrally acting nicotinic receptor channel blocker, mecamylamine (1.0 mg/kg i.p.), but not by hexamethonium (5.0 mg/kg i.p.), suggesting that the effects of (-)-nicotine were mediated by the central nicotinic receptor. (S)-3-methyl-5-(1-methyl-2-pyrrolidinyl)isoxazole (ABT-418, 1.0 and 5.0 mg/kg i.p.), reported to be a selective agonist for alpha4beta2 nicotinic receptor subunits, also enhanced the release of acetylcholine in the hippocampus, while 3-(2,4-dimethoxybenzlidene)-anabaseine (GTS-21, 1.0 and 5.0 mg/kg i.p.), which has high affinity for the alpha7 nicotinic receptor subunit, was without effect. The natural alkaloids isolated from plants, (-)-cytisine and (-)-lobeline, had little effect on acetylcholine release from the hippocampus. A competitive antagonist for alpha4beta2 subunits of the nicotinic receptor, dihydro-beta-erythroidine, and a partial agonist for the beta2 subunit-containing nicotinic receptor, (-)-cytisine, inhibited (-)-nicotine-induced increase of acetylcholine release from the hippocampus, whereas a selective antagonist for the alpha7 subunit, methyllycaconitine, and a partial agonist for the alpha3 subunit-containing nicotinic receptor, (-)-lobeline, did not. These results indicate that there are certain differences among brain regions in the response of nicotinic receptor-mediated acetylcholine release and that (-)-nicotine-induced acetylcholine release in the rat hippocampus may be attributed to activation of the alpha4beta2 nicotinic receptor subunits.

Activation of midbrain presumed dopaminergic neurones by muscarinic cholinergic receptors: an in vivo electrophysiological study in the rat

1. Extracellular single-unit recording and iontophoresis were used to examine the effects of different cholinoceptor agonists and antagonists on the firing rate and firing pattern of A9 and A10 presumed dopaminergic neurones in the anaesthetized rat. 2. Administration of low currents (1-5 nA) of the selective muscarinic agonists oxotremorine M (Oxo M) and muscarine and of the non-selective muscarinic/nicotinic agonist carbamylcholine (CCh) produced a dose-dependent increase in firing rate in most of the A9 and A10 presumed dopaminergic neurones tested. Oxo M-induced activation could be completely blocked by iontophoretic application of the muscarinic antagonist butyl-scopolamine or systemic administration of the muscarinic antagonist scopolamine (300 microg kg(-1), i.v.). 3. Iontophoretic application of the selective nicotinic agonist methylcarbamylcholine (MCCh), but not nicotine, induced a consistent increase in firing rate. Surprisingly, the excitatory effect of MCCh was significantly reduced by the selective muscarinic antagonist scopolamine (300 microg kg(-1), i.v.), but not by the selective nicotinic antagonist mecamylamine (2.2 mg kg(-1), i.v.). Mecamylamine (3 mg kg(-1), i.v.) was also ineffective in reducing the CCh-induced activation of presumed dopamine neurones, suggesting that both CCh and MCCh increased the activity of dopamine neurones via an interaction with muscarinic receptors. 4. Iontophoretic application of the endogenous agonist acetylcholine (ACh) had no or little effect on the firing activity of A10 presumed dopaminergic neurones. However, concomitant application of neostigmine, a potent cholinesterase inhibitor, with acetylcholine induced a substantial activation of these neurones. This activation consisted of two components; one, which was prevalent, was scopolamine (300 microg kg(-1), i.v.)-sensitive, and the other was mecamylamine (2 mg kg(-1), i.v.)-sensitive. 5. In addition to their effect on firing activity, Oxo M, muscarine and concomitant neostigmine/ACh caused a significant increase in burst firing of A10 neurones, but not of A9 neurones. 6. These data suggest that dopamine cells, both in the A9 and A10 regions, possess functional muscarinic receptors, the activation of which can increase their firing rate and, for A10 neurones, their amount of burst activity. These cholinoceptors would be able to influence the activity of the midbrain dopamine system greatly and may play a role in, and/or be a therapeutic target for, brain disorders in which dopamine is involved (e.g., Parkinson's disease, drug addiction and schizophrenia).

Peptidergic innervation and the nicotinic acetylcholine receptor in the primate basal nucleus

Peptidergic innervation and localization of the neuronal nicotinic acetylcholine receptor (nAChR) was studied in the basal forebrain of Macaca fascicularis in order to provide microstructural proofs for the theory (Changeux et al., 1992) that calcitonin gene-related peptide (CGRP) is responsible for the maintenance of the acetylcholine receptor. Distribution and localization of five neuropeptides, namely substance P (SP), CGRP, neuropeptide Y (NPY), vasoactive intestinal polypeptide (VIP) neurotensin (NT), and the neuropeptides parvalbumin (PV) and the alpha-bungarotoxin- (alpha-BTX-) binding protein was studied by means of light- and electron microscopic pre-embedding immunocytochemistry. Immunohistochemical double staining revealed that large cholinergic principal nerve cells in the basal forebrain, corresponding to cell group Ch4 constituting Meynert's basal nucleus (BNM), and exerting intense choline acetyltransferase (ChAT) immunoreactivity, are synaptically innervated by axons displaying CGRP immunoreactivity. While SP, NPY, PV and CGRP establish dense networks in BNM, innervation by NT and VIP is sparse. Biotinylated alpha-BTX visualizes beaded axons that surround dendrites and perikarya of cholinergic principal cells. Electron microscopic organization of the neuropil in BNM is characterized by a glomerular (or rather cartridge-like) arrangement of axons surrounding dendrites of non-cholinergic principal nerve cells. At least one of the axons establishing the glomerulus (cartridge) exerts CGRP immunopositivity while alpha-BTX-immunopositive axons, presynaptic to dendrites of principal cells, are attached to the glomeruli (cartridges) from outside. As alpha-BTX-binding indicates localization of the alpha7 subunit of the neuronal nAChR, the microtopographical arrangement supports the idea that, in a manner similar to that in the neuromuscular junction, CGRP might contribute to the maintenance of nAChR also in BNM. Our results suggest that presynaptic nAChR-s are involved in the regulation of acetylcholine release from a feed-forward amplification mechanism of cholinergic principal cells of BNM.

Epibatidine binding sites and activity in the spinal cord

Epibatidine has been shown to be the most potent nicotinic agonist in several neuronal nicotinic receptor preparations. Similar to other nicotinic agonists, intrathecal (-)-epibatidine elicits dose-dependent increases in pressor, heart rate and pain responses in rats, as well as an increase in latency to withdraw from a noxious thermal stimulus. The latter response requires higher doses and is of shorter duration, suggesting interaction with multiple subtypes of spinal nicotinic receptors. In the present study, we relate the binding properties of (+/-)-[3H]epibatidine in spinal cord membrane preparations to the cardiovascular and behavioral responses. Unlike (-)-[3H]cytisine or (-)-[3H]nicotine, (+/-)-[3H]epibatidine reveals two sites; the ratio of high affinity to low affinity sites is 2:1. The rank ordering of potencies of the nicotinic agonists in displacing (+/-)-[3H]epibatidine binding from spinal cord membranes correlates with the potencies in eliciting cardiovascular and behavioral responses upon spinal administration. The nicotinic receptor antagonists, alpha-lobeline, dihydro-beta-erythroidine and methyllycaconitine, also displayed similar rank ordering of potencies in displacing (+/-)-[3H]epibatidine, (-)-[3H]cytisine or (-)-[3H]nicotine binding to spinal nicotinic receptors. Virtually all the nicotinic analogs exhibited a Hill coefficient of less than one in competing with (+/-)-[3H]epibatidine to spinal cord membranes indicating their interaction with at least two classes of binding sites. Intrathecal (-)-epibatidine, in addition to eliciting an initial and subsequently a sustained pressor and tachycardic response, also exhibited a transient intervening bradycardia which coincided temporally with the duration of the analgesia. Repeated administration of (-)-epibatidine desensitized its responses as well as the cardiovascular and behavioral responses to spinal nicotine and cytisine. Intrathecal alpha-lobeline showed selectivity for blocking the analgesic response, whereas methyllycaconitine exhibited selectivity for the pressor and irritation responses. The NMDA receptor antagonist, AP-5, inhibited the pressor, tachycardic and irritation responses elicited by intrathecal (-)-epibatidine, confirming a role for spinal excitatory amino acids released by the nicotinic agonist.

Presynaptic nicotinic ACh receptors

Nicotinic ACh (nACh) receptors in the CNS are composed of a diverse array of subunits and have a range of pharmacological properties. However, despite the fact that they are ligand-gated cation channels, their physiological functions have not been determined. This has led to increased interest in presynaptic nACh receptors that act to modulate the release of transmitter from presynaptic terminals.

Pharmacological characterization of a nicotinic autoreceptor in rat hippocampal synaptosomes

The modulation of [3H]ACh release by nicotinic compounds was studied in superfused rat hippocampal synaptosomes loaded with [3H]choline, (-)-Nicotine (0.1-10 microM) evoked a dose-dependent increase in [3H]ACh release; higher concentrations were less effective. Nicotine-evoked release was Ca(2+)-dependent, and blocked by the nicotinic antagonists dihydro-beta-erythroidine, mecamylamine, and pempidine. The alpha 7-selective antagonist methyllycaconitine did not inhibit nicotine-evoked release when tested at 1 microM, although at 10 microM some attenuation of the response was observed. Six agonists tested were equally efficacious in stimulating [3H]ACh release, as judged by the maximum responses, and gave the following EC50 values: (+/-)-epibatidine 0.12 microM; (+)-anatoxin-a 0.14 microM; (-)-nicotine 0.99 microM; (-)-cytisine 1.06 microM; ABT-418 2.6 microM; isoarecolone 43 microM. Each agonist generated a "bell-shaped" dose response curve, suggesting desensitisation at higher concentrations. This is supported by analysis of repetitive stimulation with (-)-nicotine and (-)-cytisine: S2/S1 ratios declined sharply with increasing concentration, whereas subsequent KC1-evoked release remained constant. These results are discussed in terms of possible nicotinic receptor subtypes that might be present on hippocampal nerve terminals.

A novel nicotinic-cholinergic role in behavioral sensitization to amphetamine-induced stereotypy in mice

Cholinergic antagonists were used to investigate the role of the cholinergic system in amphetamine- and cocaine-induced behavioral sensitization to stereotypy in mice. Systemically, mecamylamine (1 mg/kg) and dihydro-beta-erythroidine (2 mg/kg) - nicotinic antagonists - and atropine (2 mg/kg) - a muscarinic antagonist - were ineffective against psychostimulant-induced stereotypy in naive animals. The nicotinic antagonists, however, blocked both the induction and expression of sensitization to amphetamine; in contrast, atropine was ineffective. All three drugs were ineffective against either the induction or expression of cocaine sensitization. Intrastriatally, the nicotinic antagonists blocked induction but not expression of amphetamine-induced sensitization. The results suggest that the nicotinic system participates in sensitization induced by amphetamine but not cocaine; that the nicotinic component of the amphetamine response in sensitized animals is novel as compared to the response in naive animals; and that the striatum is a locus for the nicotinic involvement in induction but not expression. The data add support to the inference that behavioral sensitization represents not only a quantitative but a qualitative change in response to amphetamine.

Comparative ontogenic profile of cholinergic markers, including nicotinic and muscarinic receptors, in the rat brain

The ontogenic profiles of several cholinergic markers were assessed in the rat brain by using quantitative in vitro receptor autoradiography. Brain sections from animals at different stages of development were processed with [3H]AH5183 (vesamicol; vesicular acetylcholine transport sites), [3H]N-methylcarbamylcholine (alpha(4)beta(2) nicotinic receptor sites), [3H]hemicholinium-3 (high-affinity choline uptake sites), [3H]3-quinuclidinyl benzilate (total population of muscarinic receptor sites), [3H]4-DAMP (muscarinic M1/M3 receptor sites), [3H]pirenzepine (muscarinic M1 receptor sites), and [3H]AF-DX 116 and [3H]AF-DX 384 (muscarinic M2 receptor sites) as radiolabeled probes. The results revealed that, by the end of the prenatal period (embryonic day 20), the densities of nicotinic receptor and vesicular acetylcholine transport sites already represented a considerable proportion of those observed in adulthood (postnatal day 60) in different laminae of the frontal, parietal, and occipital cortices, in the layers of Ammon's horn fields and the dentate gyrus of the hippocampal formation, as well as in the amygdaloid body, the olfactory tubercle, and the striatum. In contrast, at that stage, the densities of total muscarinic, M1/M3, M1, and possibly M2 receptor and high-affinity choline uptake sites represent only a small proportion of levels seen in the adult. Differences were also observed in the postnatal ontogenic profiles of nicotinic, muscarinic, vesamicol, and high-affinity choline uptake sites. For example, between postnatal weeks 3 and 5, the levels of M1/M3 and M1 sites were at least as high as in the adult, whereas M2 and high-affinity choline uptake site densities appeared to be delayed and to reach adult values only after postnatal week 5. With regard to cholinergic innervation in the developing rat brain, the present findings suggest a temporal establishment of several components of the cholinergic systems. The first components are the vesicular acetylcholine transporter and nicotinic sites; these are followed by M1/M3 and M1 sites and, finally, by M2 and high-affinity choline uptake sites.

Analogues of carbacholine: synthesis and relationship between structure and affinity for muscarinic and nicotinic acetylcholine receptors

A series of acyclic and heterocyclic analogues of carbacholine (1) was synthesized using N-methylcarbacholine (MCC, 2), N,N-dimethylcarbacholine (DMCC, 3), and the corresponding tertiary amine (4) as leads. Whereas nicotinic acetylcholine receptor affinity was determined using [3H]nicotine as the radioactive ligand, [3H]oxotremorine-M ([3H]Oxo-M) and [3H]quinuclidinyl benzilate ([3H]QNB), in some cases supplemented with [3H]pirenzepine ([3H]PZ), were used as radioligands for muscarinic acetyicholine receptors on rat brain membranes. On the basis of receptor binding data, nicotinic/muscarinic (N/M) selectivity factors were determined, and muscarinic receptor efficacy (M agonist index) and M1 selectivity (M2/M1 index) estimated. In most cases, quaternized analogues showed higher affinity than the corresponding tertiary amines for muscarinic and, in particular, nicotinic receptor sites. Among the new compounds, N,N-diethylcarbacholine (9e) (IC50 = 0.046 microM), (S)-1-methyl-2-(N,N- dimethyl-aminocarbonyloxymethyl)pyrrolidine (17k) (IC50 = 0.068 microM), and the corresponding quaternized analogue, 18k (IC50 = 0.018 microM) showed the highest nicotinic receptor affinity. The tertiary amine, 17k showed much higher nicotinic receptor affinity than the acyclic analogue, 4 (IC50 = 5.7 microM), and the N/M selectivity factor determined for 17k (150) is an order of magnitude lower than that of nicotine (1400). THe N/M selectivity factors for MCC (2) and DMCC (3), previously reported to be highly selective nicotinic receptor ligands, were shown to be 6.5 and 60, respectively, the latter value being comparable with that of 18k (89).

Sensitivity of rat frontal cortical neurones to nicotine is increased by chronic administration of nicotine and by lesions of the nucleus basalis magnocellularis: comparison with numbers of [3H]nicotine binding sites

The effects of chronic nicotine treatment and of unilateral AMPA lesion of the nucleus basalis magnocellularis (nbm) on the sensitivity of frontal cortical neurones to iontophoretically applied nicotine were studied. Chronic nicotine treatment increased the number of [3H]nicotine binding sites from 2.9 to 3.9 pmol g-1 wet weight, and increased the proportion of cortical neurones responding to nicotine from 32.3% to 60.0%. After unilateral nbm lesions, the densities of AChE-positive fibers and [3H]nicotine binding sites were reduced by approximately 97% and 55%, respectively, and the proportion of neurones responding to nicotine increased from 32.3% to 53.8%. The two treatments, chronic nicotine administration and nbm lesion, also increased the size of individual neuronal responses, prolonged their duration, and shortened the response latency. Responses to glutamate were unaffected by either procedures. The results show that the increase in [3H]nicotine binding produced by chronic nicotine administration is associated with an increased response to iontophoretically applied nicotine, suggesting that the receptor upregulation induced by the chronic treatment were functional. Less easily explained is the association between increased sensitivity of frontal cortical neurons to nicotine after nbm lesion with a decreased receptor density. It is suggested that a substantial proportion of nicotinic receptors are located presynaptically, and that their loss after lesion concealed an upregulation at postsynaptic sites.

Immunocytochemical localization of nicotinic acetylcholine receptor in rat cerebral cortex

Localization of nicotinic acetylcholine receptor (nAChR) alpha 4 subunits was investigated in rat cerebral cortex using a monoclonal antibody against alpha 4 subunits. The antibody depleted more than 70% of the [3H]methylcarbamylcholine choline binding activity of the solubilized membrane fraction. By light microscopy alpha 4-like immunoreactivity (alpha 4-LI) was found through layers II to VI. The immunostaining was the most prominent in cell bodies and apical dendrites of pyramidal cells in layer V. By electron microscopy most immunoreaction products were observed in the rough endoplasmic reticulum, cytoplasmic matrix and synaptic membranes. Alpha 4-LI was detected in the postsynaptic membranes of neuronal cell bodies and apical dendrites. These findings suggest that alpha 4-containing subtypes serve as one possibly the postsynaptic nAChR in rat cerebral cortex.

Comparative laminar distribution of various autoradiographic cholinergic markers in adult rat main olfactory bulb

To provide anatomical information on the complex effects of acetylcholine (ACh) in the olfactory bulb (OB), the distribution of different cholinergic muscarinic and nicotinic receptor sub-types was studied by quantitative in vitro autoradiography. The muscarinic M1-like and M2-like sub-types, as well as the nicotinic bungarotoxin-insensitive (alpha 4 beta 2-like) and bungarotoxin-sensitive (alpha 7-like) receptors were visualized using [3H]pirenzepine, [3H]AF-DX 384, [3H]cytisine and [125I] alpha-bungarotoxin (BTX), respectively. In parallel, labelling patterns of [3H]vesamicol (vesicular acetylcholine transport sites) and [3H]hemicholinium-3 (high-affinity choline uptake sites), two putative markers of cholinergic nerve terminals, were investigated. Specific labelling for each cholinergic radioligand is distributed according to a characteristic laminar and regional pattern within the OB revealing the lack of a clear overlap between cholinergic afferents and receptors. The presynaptic markers, [3H]vesamicol and [3H]hemicholinium-3, demonstrated similar laminar pattern of distribution with two strongly labelled bands corresponding to the glomerular layer and the area around the mitral cell layer. Muscarinic M1-like and M2-like receptor sub-types exhibited unique distribution with their highest levels seen in the external plexiform layer (EPL). Intermediate M1-like and M2-like binding densities were found throughout the deeper bulbar layers. In the glomerular layer, the levels of muscarinic receptor subtypes were low, the level of M2-like sites being higher than M1. Both types of nicotinic receptor sub-types displayed distinct distribution pattern. Whereas [125I] alpha-BTX binding sites were mostly concentrated in the superficial bulbar layers, [3H]cytisine binding was found in the glomerular layers, as well as the mitral cell layer and the underlying laminae. An interesting feature of the present study is the visualization of two distinct cholinoceptive glomerular subsets in the posterior OB. The first one exhibited high levels of both [3H]vesamicol and [3H]hemicholinium-3 sites. It corresponds to the previously identified atypical glomeruli and apparently failed to express any of the cholinergic receptors under study. In contrast, the second subset of glomeruli is not enriched with cholinergic nerve terminal markers but displayed high amounts of [3H]cytisine/nicotinic binding sites. Taken together, these results suggest that although muscarinic receptors have been hypothesized to be mostly involved in cholinergic olfactory processing and short-term memory in the OB, nicotinic receptors, especially of the cytisine/ alpha 4 beta 2 sub-type, may have important roles in mediating olfactory transmission of efferent neurons as well as in a subset of olfactory glomeruli.

Cholinergic markers in aged cognitively impaired Long-Evans rats

Aged Long-Evans rats (24-25 months old) were classified into cognitively impaired or unimpaired subgroups based on their performances in the Morris Swim Maze task compared to young controls. Using quantitative in vitro receptor autoradiography, we investigated the status of various cholinergic markers in these two groups and in young adults (six months) animals. The apparent density of [3H]pirenzepine (muscarinic M1) sites was similar in the three groups of rats in various cortical areas, subfields of the hippocampus, medial septum and striatum. Similarly, choline acetyltransferase activity and the density of [3H]hemicholinium-3 (high-affinity choline uptake) and [3H]cytisine (nicotinic) binding sites were also unchanged in the brain regions studied between the aged cognitively impaired, unimpaired and young adult rats. In contrast, significant increases in [3H]AF-DX 384 (muscarinic M2) binding density were observed in various cortical areas and in the molecular layer of the dentate gyrus of aged cognitively impaired versus unimpaired rats and in few cortical regions of old as compared to young animals. Therefore, a selective alteration in the regulation of putative M2 receptor sites is apparent, particularly in the aged cognitively impaired rats. Increases in M2 binding sites could lead to a decrease in the capacity to release acetylcholine, as some of the M2 receptors are believed to act as negative autoreceptors. This could influence cognitive functions as selective M2 blockers have recently been reported to facilitate spatial memory in aged impaired rats [Doods et al. (1993) Life Sci. 52, 497-503: Quirion et al. (1995) J. Neurosci. 15, 1455-1462.

In vivo pharmacological effects of dihydro-beta-erythroidine, a nicotinic antagonist, in mice

The comparative in vivo pharmacology of mecamylamine and dihydro-beta-erythroidine (DH beta E) in mice was studied. Modulation of the behavioral effects (antinociception, hypomotility, motor impairment and hypothermia) of nicotine in mice by DH beta E and mecamylamine were carried out. After SC administration, DH beta E and mecamylamine were nearly equipotent in blocking nicotine's effects except for antinociception, in which mecamylamine was clearly more potent. Intrathecal injection of DH beta E was also effective in blocking the antinociceptive effect of nicotine. In vivo interaction of DH beta E with calcium and calcium channels, involved in the central actions of nicotine, showed that intrathecal administration of DH beta E failed to reduce the antinociception induced by diverse drugs which increase intracellular calcium such as thapsigargin, (+/-)-BAYK 8644 and calcium, indicating that this antagonist does not affect calcium-dependent mechanisms involved in antinociception. On the other hand, mecamylamine blocked the antinociceptive effect of the calcium modulatory drugs, suggesting that it may be acting on calcium-dependent mechanisms involved in the intracellular signaling process. We conclude that DH beta E, a nicotinic neuromuscular antagonist, is able to block some of the central actions of nicotine after systemic and intrathecal administration. The mechanism of blockade is different from that of mecamylamine, a classical ganglionic antagonist, and may involve a direct action of DH beta E on nicotine receptor.

Lesion with the neurotoxin AF64A alters hippocampal cholinergic receptor function

The effect of selective lesion of cholinergic inputs to the hippocampus on the function of hippocampal cholinergic receptors was examined. Hippocampal cholinergic neurons were lesioned in the rat by administration of the selective cholinergic neurotoxin AF64A (ethylcholine mustard aziridinium). Cholinergic receptor function was examined by assessing the ability of cholinergic agonists and antagonists to modulate the evoked release of radiolabelled acetylcholine (ACh) from hippocampal slices. Nicotine enhanced release, with a bell-shaped dose-response curve. The dose-response curve and EC50 for nicotine was shifted 10-fold to the left in lesioned rats, suggesting an increased sensitivity to nicotine. However, there were no differences in either the number of affinity of nicotinic receptors as determined with binding studies. The muscarinic agonist oxotremorine inhibited the evoked release of ACh in control tissues, but had much less effect in AF64A-lesioned tissues. Binding to the M1 receptor subtype was not changed. However, the Kd for binding to the high affinity subtype of the M2 receptor was increased 10-fold, suggesting that the receptor has become less sensitive to stimulation. Loss of M2 function may allow an increase in the effect of stimulating nicotinic receptors that modulate ACh release.

Differences in ligand binding and phosphoinositide turnover between M1 muscarinic receptor gene transfected cells and mouse and rat brain membranes

The present study describes some unexpected receptor mediated effects of N-methylcarbamylcholine on mouse M1 muscarinic receptor gene transfected cell line (M1Y1) that were not evident from biochemical studies with mouse and rat brain tissue where N-methylcarbamylcholine exhibited only nicotinic properties. Although N-methylcarbamycholine was devoid of muscarinic properties in mouse and rat brain preparations, as determined by phosphoinositide turnover and inhibition of [3H]QNB binding, it exhibited significant muscarinic characteristics in the transfected M1Y1 cell line. At a concentration of 10(-6) M or greater, N-methylcarbamycholine caused a transient increase in intracellular Ca2+ of 50 s duration that was reversible by atropine or pirezepine. The Ca(2+)-transient was not elicited by other nicotinic agents such as nicotine and N,N-dimethylcarbamylcholine, a close analogue of N-methylcarbamylcholine, with comparable affinity for nicotinic receptors and devoid of muscarinic activity. N-Methylcarbamylcholine also stimulated phosphoinositide turnover in M1Y1 cells with an estimated EC50 value 10 times greater than that of carbachol, and the effect was blocked by atropine. Both carbachol and N-methylcarbamycholine inhibited [3H]QNB binding in a concentration-dependent manner; however, the IC50 for carbachol was over two orders of magnitude greater than that observed in mouse and rat brain membranes. In considering possible explanations for the differential characteristics of N-methylcarbamylcholine in mouse and rat brain as compared to the transfected M1Y1 cells, it was concluded that the difference may be attributable to differences in the receptor-transduction coupling efficiency and the microenvironment of the muscarinic receptors.

Localization of nicotinic cholinergic receptors in rat brain: autoradiographic studies with [3H]cytisine

There is a great deal of interest in the role of nicotinic acetylcholine receptors in the central nervous system, although their function is not well understood at present. Currently, central nicotinic receptors can be classified broadly as either alpha-bungarotoxin binding sites with low affinity for acetylcholine agonists, or as high-affinity agonist binding sites with low affinity for alpha-bungarotoxin. Neuronal nicotinic receptors with a high affinity for agonists are distributed widely in the central nervous system. Evidence from molecular biology and electrophysiology suggests that multiple nicotinic receptor types exist in the brain. In this study we have used the agonist [3H]cytisine as a ligand for autoradiography to generate a detailed quantitative map of the high-affinity agonist binding nicotinic receptor in the rat brain. Optimized binding conditions, characterization of the kinetic and equilibrium binding properties, and demonstration of the nicotinic pharmacology of this binding site in tissue sections confirm the usefulness of [3H]Cytisine as a ligand for nicotinic receptor autoradiography. [3H]Cytisine autoradiography provides excellent anatomic resolution with very low non-specific binding. This property has allowed us to describe variations in receptor density within subnuclei and gradients of receptor density in larger brain regions. Data from several studies suggest that the predominant high-affinity agonist binding nicotine receptor in the central nervous system is composed of the alpha 4 and beta 2 subunits. The data in the current study are consistent with the suggestion that [3H]cytisine labels only the alpha 4 beta 2 nicotinic receptor with high affinity, offering the possibility of localizing a specific nicotinic receptor subtype in the central nervous system. In summary, we characterize the optimum experimental conditions for the use of [3H]cytisine in tissue section autoradiography. [3H]Cytisine proves to be an excellent marker for nicotinic cholinergic receptors with a very high affinity and very low background. We provide a detailed quantitative characterization of nicotinic receptor density in the rat central nervous system and we find there are significant variations and gradients in receptor density within specific brain regions, including subregions previously thought to be homogeneous.

Pharmacological evaluation of methylcarbamylcholine-induced drinking behavior in rats

Methylcarbamylcholine (MCC), a structural analog of carbachol (an acetylcholine agonist), has been reported to be a specific nicotinic cholinergic receptor ligand. MCC produces a robust polydipsic response shortly following central administration. The purpose of the present study was to pharmacologically characterize this increase in drinking behavior. Male Wistar rats were implanted with intracerebroventricular (ICV) cannula guides directed at the left lateral ventricle. Following a recovery period, animals were injected ICV with saline or various doses of MCC (3-60 micrograms) and water consumption was quantified. MCC produced a dose-related, transient increase in water consumption that peaked at a dose of 30 micrograms. In contrast, nicotine, a potent nicotinic cholinergic receptor agonist, did not produce changes in drinking following ICV administration. MCC-induced increases in drinking were not blocked by pretreatment with several selective nicotinic receptor antagonists including dihydro-beta-erythriodine (DHBE), hexamethonium, and mecamylamine. However, pretreatment with the muscarinic antagonist atropine (0.01 or 1.0 microgram) completely abolished MCC-induced polydipsia. Following a chronic treatment regimen (MCC injected ICV twice daily for 10 days), no tolerance to MCC-induced changes in water consumption was observed. Previous studies have demonstrated that tolerance develops to nicotinic-receptor mediated responses following the identical chronic treatment paradigm. These results suggest that MCC-induced polydipsia is mediated through stimulation of muscarinic rather than nicotinic receptors.