Nicotinic receptors in mammalian brain: localization and relation to cholinergic innervation

Regional variation in cholinergic terminal activity determines the non-uniform occurrence of cortical slow waves during REM sleep in mice

Sleep consists of two basic stages: non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. NREM sleep is characterized by slow high-amplitude cortical electroencephalogram (EEG) signals, while REM sleep is characterized by desynchronized cortical rhythms. Despite this, recent electrophysiological studies have suggested the presence of slow waves (SWs) in local cortical areas during REM sleep. Electrophysiological techniques, however, have been unable to resolve the regional structure of these activities because of relatively sparse sampling. Here, we map functional gradients in cortical activity during REM sleep using mesoscale imaging in mice and show local SW patterns occurring mainly in somatomotor and auditory cortical regions with minimum presence within the default mode network. The role of the cholinergic system in local desynchronization during REM sleep is also explored by calcium imaging of cholinergic activity within the cortex and analyzing structural data. We demonstrate weaker cholinergic projections and terminal activity in regions exhibiting frequent SWs during REM sleep.

Nicotine Gateway Effects on Adolescent Substance Use

Given the rise in teenage use of electronic nicotine delivery systems ("vaping") in congruence with the increasing numbers of drug-related emergencies, it is critical to expand the knowledge of the physical and behavioral risks associated with developmental nicotine exposure. A further understanding of the molecular and neurochemical underpinnings of nicotine's gateway effects allows emergency clinicians to advise patients and families and adjust treatment accordingly, which may minimize the use of tobacco, nicotine, and future substances. Currently, the growing use of tobacco products and electronic cigarettes among teenagers represents a major public health concern. Adolescent exposure to tobacco or nicotine can lead to subsequent abuse of nicotine and other substances, which is known as the gateway hypothesis. Adolescence is a developmentally sensitive time period when risk-taking behaviors, such as sensation seeking and drug experimentation, often begin. These hallmark behaviors of adolescence are largely due to maturational changes in the brain. The developing brain is particularly vulnerable to the harmful effects of drugs of abuse, including tobacco and nicotine products, which activate nicotinic acetylcholine receptors (nAChRs). Disruption of nAChR development with early nicotine use may influence the function and pharmacology of the receptor subunits and alter the release of reward-related neurotransmitters, including acetylcholine, dopamine, GABA, serotonin, and glutamate. In this review, we emphasize that the effects of nicotine are highly dependent on timing of exposure, with a dynamic interaction of nAChRs with dopaminergic, endocannabinoid, and opioidergic systems to enhance general drug reward and reinforcement. We analyzed available literature regarding adolescent substance use and nicotine's impact on the developing brain and behavior using the electronic databases of PubMed and Google Scholar for articles published in English between January 1968 and November 2018. We present a large collection of clinical and preclinical evidence that adolescent nicotine exposure influences long-term molecular, biochemical, and functional changes in the brain that encourage subsequent drug abuse.

Treatment duration affects cytoprotective efficacy of positive allosteric modulation of α7 nAChRs after focal ischemia in rats

To minimize irreversible brain injury after acute ischemic stroke (AIS), the time to treatment (i.e., treatment delay) should be minimized. However, thus far, all cytoprotective clinical trials have failed. Analysis of literature identified short treatment durations (≤72 h) as a common motif among completed cytoprotective clinical trials. Here, we argue that short cytoprotective regimens even if given early after AIS may only slow down the evolution of ischemic brain injury and fail to deliver sustained long-term solutions leading to relapses that may be misinterpreted for conceptual failure of cytoprotection. In this randomized blinded study, we used young adult male rats subjected to transient 90 min suture middle cerebral artery occlusion (MCAO) and treated with acute vs. sub-chronic regimens of PNU120596, a prototypical positive allosteric modulator of α7 nicotinic acetylcholine receptors with anti-inflammatory cytoprotective properties to test the hypothesis that insufficient treatment durations may reduce therapeutic benefits of otherwise efficacious cytoprotectants after AIS. A single acute treatment 90 min after MCAO significantly reduced brain injury and neurological deficits 24 h later, but these effects vanished 72 h after MCAO. These relapses were avoided by utilizing sub-chronic treatments. Thus, extending treatment duration augments therapeutic efficacy of PNU120596 after MCAO. Furthermore, sub-chronic treatments could offset the negative effects of prolonged treatment delays in cases where the acute treatment window after MCAO was left unexploited. We conclude that a combination of short treatment delays and prolonged treatment durations may be required to maximize therapeutic effects of PNU120596, reduce relapses and ensure sustained therapeutic efficacy after AIS. Similar concepts may hold for other cytoprotectants including those that failed in clinical trials.

The Insula and Taste Learning

The sense of taste is a key component of the sensory machinery, enabling the evaluation of both the safety as well as forming associations regarding the nutritional value of ingestible substances. Indicative of the salience of the modality, taste conditioning can be achieved in rodents upon a single pairing of a tastant with a chemical stimulus inducing malaise. This robust associative learning paradigm has been heavily linked with activity within the insular cortex (IC), among other regions, such as the amygdala and medial prefrontal cortex. A number of studies have demonstrated taste memory formation to be dependent on protein synthesis at the IC and to correlate with the induction of signaling cascades involved in synaptic plasticity. Taste learning has been shown to require the differential involvement of dopaminergic GABAergic, glutamatergic, muscarinic neurotransmission across an extended taste learning circuit. The subsequent activation of downstream protein kinases (ERK, CaMKII), transcription factors (CREB, Elk-1) and immediate early genes (c-fos, Arc), has been implicated in the regulation of the different phases of taste learning. This review discusses the relevant neurotransmission, molecular signaling pathways and genetic markers involved in novel and aversive taste learning, with a particular focus on the IC. Imaging and other studies in humans have implicated the IC in the pathophysiology of a number of cognitive disorders. We conclude that the IC participates in circuit-wide computations that modulate the interception and encoding of sensory information, as well as the formation of subjective internal representations that control the expression of motivated behaviors.

Sleep loss and recovery after administration of drugs related to different arousal systems in rats

Sleep is homeostatically regulated suggesting a restorative function. Sleep deprivation is compensated by an increase in length and intensity of sleep. In this study, suppression of sleep was induced pharmacologically by drugs related to different arousal systems. All drugs caused non-rapid eye movement (NREM) sleep loss followed by different compensatory processes. Apomorphine caused a strong suppression of sleep followed by an intense recovery. In the case of fluoxetine and eserine, recovery of NREM sleep was completed by the end of the light phase due to the biphasic pattern demonstrated for these drugs first in the present experiments. Yohimbine caused a long-lasting suppression of NREM sleep, indicating that either the noradrenergic system has the utmost strength among the examined systems, or that restorative functions occurring normally during NREM sleep were not blocked. Arousal systems are involved in the regulation of various wakefulness-related functions, such as locomotion and food intake. Therefore, it can be hypothesized that activation of the different systems results in qualitatively different waking states which might affect subsequent sleep differently. These differences might give some insight into the homeostatic function of sleep in which the dopaminergic and noradrenergic systems may play a more important role than previously suggested.

Baseline impulsive choice predicts the effects of nicotine and nicotine withdrawal on impulsivity in rats

Impulsive choice, a form of impulsivity, is associated with tobacco smoking in humans. Trait impulsivity may be a vulnerability factor for smoking, or smoking may lead to impulsive behaviors. We investigated the effects of 14-day nicotine exposure (6.32mg/kg/day base, subcutaneous minipumps) and spontaneous nicotine withdrawal on impulsive choice in low impulsive (LI) and high impulsive (HI) rats. Impulsive choice was measured in the delayed reward task in which rats choose between a small immediate reward and a large delayed reward. HI and LI rats were selected from the highest and lowest quartiles of the group before exposure to nicotine. In non-selected rats, nicotine or nicotine withdrawal had no effect on impulsive choice. In LI rats, chronic nicotine exposure decreased preference for the large reward with larger effects at longer delays, indicating increased impulsive choice. Impulsive choices for the smaller immediate rewards continued to increase during nicotine withdrawal in LI rats. In HI rats, nicotine exposure and nicotine withdrawal had no effect on impulsive choice, although there was a tendency for decreased preference for the large reward at short delays. These results indicate that nicotine- and nicotine withdrawal-induced increases in impulsive choice depend on trait impulsivity with more pronounced increases in impulsive choice in LI compared to HI subjects. Increased impulsivity during nicotine exposure may strengthen the addictive properties of nicotine and contribute to compulsive nicotine use.

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.

Nicotinic activation of laterodorsal tegmental neurons: implications for addiction to nicotine

Identifying the neurological mechanisms underlying nicotine reinforcement is a healthcare imperative, if society is to effectively combat tobacco addiction. The majority of studies of the neurobiology of addiction have focused on dopamine (DA)-containing neurons of the ventral tegmental area (VTA). However, recent data suggest that neurons of the laterodorsal tegmental (LDT) nucleus, which sends cholinergic, GABAergic, and glutamatergic-containing projections to DA-containing neurons of the VTA, are critical to gating normal functioning of this nucleus. The actions of nicotine on LDT neurons are unknown. We addressed this issue by examining the effects of nicotine on identified cholinergic and non-cholinergic LDT neurons using whole-cell patch clamp and Ca(2+)-imaging methods in brain slices from mice (P12-P45). Nicotine applied by puffer pipette or bath superfusion elicited membrane depolarization that often induced firing and TTX-resistant inward currents. Nicotine also enhanced sensitivity to injected current; and, baseline changes in intracellular calcium were elicited in the dendrites of some cholinergic LDT cells. In addition, activity-dependent calcium transients were increased, suggesting that nicotine exposure sufficient to induce firing may lead to enhancement of levels of intracellular calcium. Nicotine also had strong actions on glutamate and GABA-releasing presynaptic terminals, as it greatly increased the frequency of miniature EPSCs and IPSCs to both cholinergic and non-cholinergic neurons. Utilization of nicotinic acetylcholine receptors (nAChR) subunit antagonists revealed that presynaptic, inhibitory terminals on cholinergic neurons were activated by receptors containing alpha 7, beta2, and non-alpha 7 subunits, whereas, presynaptic glutamatergic terminals were activated by nAChRs that comprised non-alpha 7 subunits. We also found that direct nicotinic actions on cholinergic LDT neurons were mediated by receptors containing alpha 7, beta2, and non-alpha 7 subunits. These findings led us to suggest that nicotine exposure from smoking will enhance both the excitability and synaptic modulation of cholinergic and non-cholinergic LDT neurons, and increase their signature neurotransmitter outflow to target regions, including the VTA. This may reinforce the direct actions of this drug within reward circuitry and contribute to encoding stimulus saliency.

Properties of distinct ventral tegmental area synapses activated via pedunculopontine or ventral tegmental area stimulation in vitro

Anatomical studies indicate that synaptic inputs from many cortical and subcortical structures converge on neurons of the ventral tegmental area (VTA). Although in vitro electrophysiological studies have examined synaptic inputs to dopamine (DA) and non-DA neurons in the VTA, they have largely relied upon local electrical stimulation to activate these synapses. This provides little information regarding the distinct properties of synapses originating from different brain areas. Using whole-cell recordings in parasagittal rat brain slices that preserved subcortical axons from the pedunculopontine nucleus (PPN) to the VTA, we compared these synapses with those activated by intra-VTA stimulation. PPN-evoked currents demonstrated longer latencies than intra-VTA-evoked currents, and both VTA and PPN responses were mediated by GABA(A) and AMPA receptors. However, unlike VTA-evoked currents, PPN currents were exclusively mediated by glutamate in 25-40% of the VTA neurons. Consistent with a cholinergic projection from the PPN to the VTA, nicotinic acetylcholine receptors (nAChR) were activated by endogenous acetylcholine released during PPN, but not VTA, stimulation. This was seen as a reduction of PPN-evoked, and not VTA-evoked, synaptic currents by the alpha7-nAChR antagonist methyllycaconitine (MLA) and the agonist nicotine. The beta2-nAChR subunit antagonist dihydro-beta-erythroidine had no effect on VTA- or PPN-evoked synaptic currents. The effects of MLA on PPN-evoked currents were unchanged by the GABA(A) receptor blocker picrotoxin, indicating that alpha7-nAChRs presynaptically modulated glutamate and not GABA release. These differences in physiological and pharmacological properties demonstrate that ascending PPN and presumed descending inputs to VTA utilize distinct mechanisms to differentially modulate neuronal activity and encode cortical and subcortical information.

Nociceptin/orphanin FQ reverses mecamylamine-induced learning and memory impairment as well as decrease in hippocampal acetylcholine release in the rat

Nociceptin/orphanin FQ is an endogenous neuropeptide that plays important roles in several physiological functions including pain, anxiety, locomotion, learning, and memory. We previously reported that low doses of nociceptin improved the scopolamine-induced impairment of learning and memory in the passive avoidance test and the spontaneous Y-maze alternation task in mice. In the present study, the effects of nociceptin on learning and memory impairment as well as the decrease in acetylcholine release induced by mecamylamine were investigated in rats. Mecamylamine (49 micromol/kg, s.c.), a nicotinic acetylcholine receptor antagonist, impaired learning and memory in the step-through type passive avoidance test and decreased acetylcholine release in the hippocampus, as determined by in vivo microdialysis. The administration of nociceptin (10 fmol/rat, i.c.v.) reversed the impairment of learning and memory and blocked the decrease in acetylcholine release induced by mecamylamine. This ameliorating effect on the mecamylamine-induced impairment of learning and memory was not blocked by [NPhe(1)]nociceptin(1-13)NH(2) (1 nmol/rat, i.c.v.), an opioid receptor-like 1 (NOP) receptor antagonist. These results suggest that nociceptin improves the impairment of learning and memory as well as decrease in acetylcholine release induced by mecamylamine, and that these effects may not be mediated by NOP receptors.

Exposure to nicotine and sensitization of nicotine-induced behaviors

Evidence for an important link between sensitization of midbrain dopamine (DA) neuron reactivity and enhanced self-administration of amphetamine and cocaine has been reported. To the extent that exposure to nicotine also sensitizes nucleus accumbens DA reactivity, it is likely that it will also impact subsequent drug taking. It is thus necessary to gain an understanding of the long-term effects of exposure to nicotine on nicotinic acetylcholine receptors (nAChRs), neuronal excitability and behavior. A review of the literature is presented in which different regimens of nicotine exposure are assessed for their effects on upregulation of nAChRs, induction of LTP in interconnected midbrain nuclei and development of long-lasting locomotor and DA sensitization. Exposure to nicotine upregulates nAChRs and nAChR currents and produces LTP of excitatory inputs to midbrain DA neurons. These effects appear in the hours to days following exposure. Exposure to nicotine also leads to long-lasting sensitization of nicotine's nucleus accumbens DA and locomotor activating effects. These effects appear days to weeks after drug exposure. A model is proposed in which nicotine exposure regimens that produce transient nAChR upregulation and LTP consequently produce long-lasting sensitization of midbrain DA neuron reactivity and nicotine-induced behaviors. These neuroadaptations are proposed to constitute critical components of the mechanisms underlying the initiation, maintenance and escalation of drug use.

Acute intermittent nicotine treatment induces fibroblast growth factor-2 in the subventricular zone of the adult rat brain and enhances neuronal precursor cell proliferation

Over the past years, evidence has accumulated that stem cells are present in the adult brain, and generate neurons and/or glia from two active germinal zones: the subventricular zone (SVZ) of the lateral ventricles and the subgranular zone (SGZ) of the dentate gyrus of the hippocampus. This study shows that acute intermittent nicotine treatment significantly enhances neuronal precursor cell proliferation in the SVZ of adult rat brain, but not in the SGZ of the hippocampus, and pre-treatment with mecamylamine, a nonselective nAChR antagonist, blocks the enhanced precursor proliferation by nicotine. This effect is supported by up-regulation of fibroblast growth factor-2 (FGF-2) mRNA in the SVZ and the expression of its receptor FGFR-1 in cells of SVZ showing precursor cells profile. It is also demonstrated that the nicotine effect on neuronal precursor proliferation is mediated by FGF-2 via fibroblast growth factor receptor 1 (FGFR-1) activation by showing that i.c.v. pre-treatment with anti-FGF-2 antibodies or with FGFR-1 inhibitor 3-[(3-(2-carboxyethyl)-4-methylpyrrol-2-yl)methylene]-2-indolinone (SU5402) blocks nicotine-induced precursor cell proliferation. This nicotine enhancement of neuronal precursor cell proliferation was not accompanied by an increase in the number of apoptotic cells. Taken together the present findings revealed the existence in the SVZ of the adult rat brain of a trophic mechanism mediated by FGF-2 and its receptor and regulated by nAchR activation. This possibility of in vivo regulation of neurogenesis in the adult brain by exogenous factors may aid to develop treatments stimulating neurogenesis with potential therapeutic implications.

Effects of acetylcholine on coding of taste information in the primary gustatory cortex in rats

Acetylcholine (ACh) receptors are widely distributed throughout the cerebral cortex in rats. Recently, cholinergic innervation of the gustatory cortex (GC) was reported to be involved in certain taste learning in rats. Here, the effects of iontophoretic application of ACh on the response properties of GC neurons were studied in urethane-anesthetized rats. ACh affected spontaneous discharges in a small fraction of taste neurons (11 of 86 neurons tested), but influenced taste responses in 27 of 43 neurons tested. No correlations with ACh susceptibility were noted for spontaneous discharges and taste responses. Among the 27 neurons, ACh facilitated taste responses in 13, inhibited taste responses in 13 and either facilitated or inhibited taste responses depending on the stimuli in 1. Furthermore, ACh affected the responses to best stimuli that produced the largest responses among four basic tastants (best responses) in 7 of 27 taste neurons, to non-best responses in 9, and to both best and non-best responses in 11. ACh mostly inhibited the best responses (13 of 18 neurons). Thus, ACh often decreased the response selectivity to the four basic tastants and changed the response profile. Atropine, a general antagonist of muscarinic receptors, antagonized ACh actions on taste responses or displayed the opposite effects on taste responses to ACh actions in two-thirds of the neurons tested. These findings indicate that ACh mostly modulates taste responses through muscarinic receptors, and suggest that ACh shifts the state of the neuron network in the GC, in terms of the response selectivities and response profiles.

Nicotine use in schizophrenia: the self medication hypotheses

The behavioural and cognitive effects of nicotine in schizophrenia have received much interest in recent years. The rate of smoking in patients with schizophrenia is estimated to be two- to four-fold the rate seen in the general population. Furthermore such patients favour stronger cigarettes and may also extract more nicotine from their cigarettes than other smokers. The question has been raised whether the widespread smoking behaviour seen in this patient group is in fact a manifestation of a common underlying physiology, and that these patients smoke in an attempt to self-medicate. We present an overview of the explanations for elevated rates of smoking in schizophrenia, with particular emphasis on the theories relating this behaviour to sensory gating and cognitive deficits in this disorder that have been viewed as major support for the self-medication hypotheses.

Neuronal nicotinic receptors: from structure to pathology

Neuronal nicotinic receptors (NAChRs) form a heterogeneous family of ion channels that are differently expressed in many regions of the central nervous system (CNS) and peripheral nervous system. These different receptor subtypes, which have characteristic pharmacological and biophysical properties, have a pentameric structure consisting of the homomeric or heteromeric combination of 12 different subunits (alpha2-alpha10, beta2-beta4). By responding to the endogenous neurotransmitter acetylcholine, NAChRs contribute to a wide range of brain activities and influence a number of physiological functions. Furthermore, it is becoming evident that the perturbation of cholinergic nicotinic neurotransmission can lead to various diseases involving nAChR dysfunction during development, adulthood and ageing. In recent years, it has been discovered that NAChRs are present in a number of non-neuronal cells where they play a significant functional role and are the pathogenetic targets in several diseases. NAChRs are also the target of natural ligands and toxins including nicotine (Nic), the most widespread drug of abuse. This review will attempt to survey the major achievements reached in the study of the structure and function of NAChRs by examining their regional and cellular localisation and the molecular basis of their functional diversity mainly in pharmacological and biochemical terms. The recent availability of mice with the genetic ablation of single or double nicotinic subunits or point mutations have shed light on the role of nAChRs in major physiological functions, and we will here discuss recent data relating to their behavioural phenotypes. Finally, the role of NAChRs in disease will be considered in some details.

The beta3 nicotinic receptor subunit: a component of alpha-conotoxin MII-binding nicotinic acetylcholine receptors that modulate dopamine release and related behaviors

Nigrostriatal dopaminergic neurons express many nicotinic acetylcholine receptor (nAChR) subunits capable of forming multiple nAChR subtypes. These subtypes are expressed differentially along the neuron and presumably mediate diverse responses. beta3 subunit mRNA has restricted expression but is abundant in the substantia nigra and ventral tegmental areas. To investigate the potential role(s) of nicotinic receptors containing the beta3 subunit in dopaminergic tracts, we generated mice with a null mutation in the beta3 gene. We were thereby able to identify a population of beta3-dependent alpha-conotoxin MII-binding nAChRs that modulate striatal dopamine release. Changes were also observed in locomotor activity and prepulse inhibition of acoustic startle, behaviors that are controlled, in part, by nigrostriatal and mesolimbic dopaminergic activity, respectively, suggesting that beta3-containing nAChRs modulate these behaviors.

Modulation of cellular activity and synaptic transmission in the ventral tegmental area

The mesolimbic dopamine system, of which the cell bodies are located in the ventral tegmental area, has been implicated in the physiology of reward and the related pathophysiology of drug abuse. This area has been a site of significant interest to study the effects of drugs of abuse and neurotransmitter systems implicated in the rewarding effects of these compounds. One important aspect of synaptic transmission is the ability of synapses to strengthen or weaken their connection as a consequence of synaptic activity. Recently, it has become apparent that this phenomenon is also present in the ventral tegmental area and that this may bear important functional consequences for the ways in which drugs of abuse assert their effect. Here, we will review the effects of neurotransmitter systems and drugs of abuse on cellular activity and synaptic transmission in the ventral tegmental area.

Executive and social behaviors under nicotinic receptor regulation

Nicotine enhances several cognitive and psychomotor behaviors, and nicotinic antagonists cause impairments in tasks requiring cognitive effort. To explore the contribution of nicotinic receptors to complex cognitive functions, we developed an automated method to investigate sequential locomotor behavior in the mouse and an analysis of social behavior. We show that, in the beta2-/- mutant, the high-order spatiotemporal organization of locomotor behavior, together with conflict resolution and social interaction, is selectively dissociated from low-level, more automatic motor behaviors. Such deficits in executive functions resemble the rigid and asocial behavior found in some psychopathological disorders such as autism and attention deficit hyperactivity disorder.

Participation of the cholinergic system in the excessive grooming behavior induced by neuropeptide (N) glutamic acid (E) isoleucine (I) amide (NEI)

The neuropeptide (N) glutamic acid (E) isoleucine (I) amide (NEI) injected into the ventral tegmental area (VTA) or intraventricularly (icv) induces excessive grooming behavior (EGB) and motor activity (MA). Here, we studied whether the cholinergic system is involved in the NEI-induced behavior. The present results demonstrate that atropine, a general muscarinic antagonist, injected icv previous to NEI, suppresses the behavior provoked by icv injections of the peptide, whereas the prior icv injection of dyhidro-beta-erythroidine, a general nicotinic antagonist, did not affect the EGB and MA induced by the peptide. From the experimental evidence, it is suggested that NEI may act specifically on a cholinergic afferent to dopaminergic cells. Also, the results appear to indicate that a neural target, different from the dopamine system, may be activated by the peptide to elicit behavioral changes, such as EGB.

Prolonged stimulation of presynaptic nicotinic acetylcholine receptors in the rat interpeduncular nucleus has differential effects on transmitter release

Alterations in nicotinic acetylcholine (nAChR) receptor number can be induced by chronic exposure to nicotine possibly by stabilization of the desensitized state(s) of the receptor. Since within the central nervous system (CNS), many nAChRs are localized presynaptically, we have investigated the physiological consequences of prolonged nicotine applications on spontaneous transmitter release. In the presence of glutamate receptor antagonists, bicuculline-sensitive spontaneous GABA inhibitory synaptic currents (IPSCs) could be readily resolved in whole-cell recordings from neurons in the interpeduncular nucleus (IPN) maintained as brain slices. Nicotine (300nM) caused a marked enhancement in the frequency of spontaneous events. During a 15min exposure to nicotine, the time course of changes in IPSC frequency could be divided into two groups. In most neurons, there was a fast increase in event frequency followed by a decline to a lower steady-state level that remained above baseline. In the remaining neurons, the effect of nicotine was more slowly developing and outlasted the application. Interestingly, the rapid effect was associated with a shift to higher amplitude events, whereas, no change in the IPSC amplitude histogram was observed during the slow onset effect. These data show that prolonged stimulation of presynaptic nicotinic receptors can have different outcomes that could potentially contribute to the diverse effects of nicotine on central information processing.

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.

Absence of alpha7-containing neuronal nicotinic acetylcholine receptors does not prevent nicotine-induced seizures

Nicotine is the primary addictive component in tobacco, and at relatively low doses it affects cardiovascular responses, locomotor activity, thermoregulation, learning, memory, and attention. At higher doses nicotine produces seizures. The mechanisms underlying the convulsive effects of nicotine are not known, but studies conducted on a number of inbred strains of mice have indicated a positive correlation between the number of alpha-bungarotoxin (alpha-BTX) binding sites in the hippocampus and the sensitivity to nicotine-induced seizures. Because alpha7-containing neuronal nicotinic acetylcholine receptors (nAChRs) represent the major binding site for alpha-BTX, mice lacking the alpha7 nAChR subunit were predicted to be less sensitive to the convulsive effects of nicotine. To test this hypothesis, we injected nicotine intraperitoneally in alpha7 mutant mice and found that the dose-response curve for nicotine-induced seizures was similar in the alpha7 +/+, alpha7 +/- and alpha7 -/- mice. The retained sensitivity to the convulsant effects of nicotine could not be explained by the presence of cholinergic compensatory mechanisms such as increases in mRNA levels for other nAChR subunits, or changes in binding levels or affinity for nicotinic ligands such as epibatidine and nicotine. These findings indicate that alpha7 may not be necessary for the mechanisms underlying nicotine-induced seizures.

Nicotinic receptor abnormalities in Alzheimer's disease

Loss of cortical nicotinic acetylcholine receptors with high affinity for agonists (20-50%) in patients with Alzheimer's disease is a common finding. Recent immunochemical analyses indicate that this deficit is predominantly associated with the loss of alpha4 subunits (30-50%), although modest reductions of alpha3 may occur in some individuals (25-29%). No reduction of beta2 subunit protein expression or levels of alpha3 and alpha4 messenger RNA has been reported. Decline in cortical [(125)I]alpha-bungarotoxin binding and alpha7 protein expression does not appear to be as extensive or widespread as the loss of alpha4 (0-40%), with no reduction in messenger RNA expression. In the thalamus, there was a trend for reduced [(3)H]nicotine binding in the majority of nuclei (0-20%) in Alzheimer's disease; however, there was a significant decline in [(125)I]alpha-bungarotoxin binding in the reticular nucleus. In the striatum [(3)H]nicotine binding was reduced in Alzheimer's disease, and although neuroleptic medication accentuated this change, it occurred in those free of neuroleptics. Changes in nicotinic acetylcholine receptors in Alzheimer's disease are distinct from those in normal aging and are likely to contribute to clinical features and possibly neuropathology.

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.

Long-term potentiation of excitatory inputs to brain reward areas by nicotine

Nicotine reinforces smoking behavior by activating nicotinic acetylcholine receptors (nAChRs) in the midbrain dopaminergic (DA) reward centers, including the ventral tegmental area (VTA). Although nicotine induces prolonged excitation of the VTA in vivo, the nAChRs on the DA neurons desensitize in seconds. Here, we show that activation of nAChRs on presynaptic terminals in the VTA enhances glutamatergic inputs to DA neurons. Under conditions where the released glutamate can activate NMDA receptors, long-term potentiation (LTP) of the excitatory inputs is induced. Both the short- and the long-term effects of nicotine required activation of presynaptic alpha7 subunit-containing nAChRs. These results can explain the long-term excitation of brain reward areas induced by a brief nicotine exposure. They also show that nicotine alters synaptic function through mechanisms that are linked to learning and memory.

Neuronal nicotinic acetylcholine receptors: from the gene to the disease

The neuronal nicotinic acetylcholine receptors are excitatory ligand-gated channels. Widely expressed throughout the peripheral and central nervous system, their properties depend upon their subunit composition. Furthermore, genetic studies have revealed a high degree of variation at the genomic level and alternative splicing of the mRNAs coding for these integral membrane proteins. In particular, genes coding for alpha4 and alpha7 subunits harbour a high degree of polymorphisms. Although well characterised at their molecular and functional level, the role of these receptors in the central nervous system remains obscure. Despite accumulating evidence for the participation of nicotinic receptors in disorders of the central nervous system including nicotinic addiction, Parkinson's disease, Alzheimer's disease and Tourette's syndrome, the exact role of these receptors is still speculative. Because most of these phenotypes are complex and genetically heterogeneous, the investigation is difficult. However, in the past few years, significant progress has been made in understanding the contribution of nicotinic acetylcholine receptors to the origin of epilepsies and schizophrenia. By concentrating on the latest results gained for these diseases, we discuss in this review the possible relationships between neuronal nicotinic receptors and neurological and psychiatric disorders.

Interaction between alpha-MSH and acetylcholinergic system upon striatal cAMP and IP(3) levels

The interaction between the neuropeptide alpha-MSH and the acetylcholinergic system as reflected by changes in cAMP and inositol 1-3-5 triphosphate(IP(3))production was investigated in an in vitro model of striatal slices. The possible involvement of D(1) receptors in cholinergic and alpha-MSH- stimulated cAMP and IP(3) production in slices of rat striatum was also examined, because it has been demonstrated that acetylcholinergic drugs induce endogenous dopamine release in the striatum. alpha-MSH, pilocarpine(PL) and the selective muscarinic M1 agonist McN-A-343 increased cAMP and IP(3) striatal levels, effects blocked by the D(1) antagonist SCH-23390, except for the effects of alpha-MSH on IP(3). The muscarinic M(2) antagonist gallamine (GL) brought about an increase in cAMP levels, an effect blocked by SCH-23390. The M(1) antagonist pirenzepine (Pz) induced a decrease both in cAMP and IP(3) content, and the nicotinic antagonist di-hydro-beta-eritroidine(DBE) only diminished cAMP production. When alpha-MSH and cholinergic agents were simultaneously added, cAMP and IP(3) levels were modified with respect to the values reached when these agents were added alone. An interaction between the acetylcholinergic system and alpha-MSH through M(1) and nicotinic receptors was also observed. These results suggest that the intracellular signaling pathways related to cAMP and IP(3) production gated by alpha-MSH and these cholinergic receptors are probably related. alpha-MSH striatum cAMP IP(3) muscarinic and nicotinic receptors an in vitro model.

Smoking and schizophrenia: abnormal nicotinic receptor expression

Biological and genetic evidence suggests a role for the neuronal nicotinic receptors in the neuropathophysiology of schizophrenia. Nicotine normalizes an auditory evoked potential deficit seen in subjects who suffer from the disease. Nicotinic receptors with both high and low affinity for nicotine are decreased in postmortem brain of schizophrenics compared to control subjects. The chromosomal locus of the human alpha-7 gene (15q14) is linked to the gating deficit with a lod of 5.3, and antagonists of the alpha-7 receptor (alpha-bungarotoxin and methyllycaconitine) induce a loss of gating in rodents. We have cloned the human alpha-7 gene and found it to be partially duplicated proximal to the full-length gene. The duplication is expressed in both the brain and in peripheral blood cells of normal subjects, but is missing in some schizophrenic subjects. The results of these studies suggest the presence of abnormal expression and function of the neuronal nicotinic receptor gene family in schizophrenia.

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.

A molecular link between inward rectification and calcium permeability of neuronal nicotinic acetylcholine alpha3beta4 and alpha4beta2 receptors

Many nicotinic acetylcholine receptors (nAChRs) expressed by central neurons are located at presynaptic nerve terminals. These receptors have high calcium permeability and exhibit strong inward rectification, two important physiological features that enable them to facilitate transmitter release. Previously, we showed that intracellular polyamines act as gating molecules to block neuronal nAChRs in a voltage-dependent manner, leading to inward rectification. Our goal is to identify the structural determinants that underlie the block by intracellular polyamines and govern calcium permeability of neuronal nAChRs. We hypothesize that two ring-like collections of negatively charged amino acids (cytoplasmic and intermediate rings) near the intracellular mouth of the pore mediate the interaction with intracellular polyamines and also influence calcium permeability. Using site-directed mutagenesis and electrophysiology on alpha(4)beta(2) and alpha(3)beta(4) receptors expressed in Xenopus oocytes, we observed that removing the five negative charges of the cytoplasmic ring had little effect on either inward rectification or calcium permeability. However, partial removal of negative charges of the intermediate ring diminished the high-affinity, voltage-dependent interaction between intracellular polyamines and the receptor, abolishing inward rectification. In addition, these nonrectifying mutant receptors showed a drastic reduction in calcium permeability. Our results indicate that the negatively charged glutamic acid residues at the intermediate ring form both a high-affinity binding site for intracellular polyamines and a selectivity filter for inflowing calcium ions; that is, a common site links inward rectification and calcium permeability of neuronal nAChRs. Physiologically, this molecular mechanism provides insight into how presynaptic nAChRs act to influence transmitter release.

Effects of nicotine and chlorisondamine on cerebral glucose utilization in immobilized and freely-moving rats

Chlorisondamine blocks central nicotinic receptors for many weeks via an unknown mechanism. Intracerebroventricular administration of [(3)H]-chlorisondamine in rats results in an anatomically restricted and persistent intracellular accumulation of radioactivity. The initial aim of the present study was to test whether nicotinic receptor antagonism by chlorisondamine is also anatomically restricted. Male adult rats were pretreated several times with nicotine to avoid the disruptive effects of the drug seen in drug-naïve animals. They then received chlorisondamine (10 microg i. c.v.) or saline, and local cerebral glucose utilization (LCGU) was measured 4 weeks later after acute nicotine (0.4 mg kg(-1) s.c.) or saline administration. During testing, rats were partially immobilized. Nicotine significantly increased LCGU in the anteroventral thalamus and in superior colliculus. Chlorisondamine completely blocked the first of these effects. Chlorisondamine significantly reduced LCGU in the lateral habenula, substantia nigra pars compacta, ventral tegmental area, and cerebellar granular layer. The second experiment was of similar design, but the rats were not pre-exposed to nicotine, and were tested whilst freely-moving. Acute nicotine significantly increased LCGU in anteroventral thalamus, superior colliculus, medial habenula and dorsal lateral geniculate. Overall, however, nicotine significantly decreased LCGU. Most or all of the central effects of nicotine on LCGU were reversed by chlorisondamine given 4 weeks beforehand. These findings suggest that chlorisondamine blocks nicotinic effects widely within the brain. They also indicate that in freely-moving rats, nicotine can reduce or stimulate cerebral glucose utilization, depending on the brain area. British Journal of Pharmacology (2000) 129, 147 - 155

Multiple and opposing roles of cholinergic transmission in the main olfactory bulb

The main olfactory bulb is a critical relay step between the olfactory epithelium and the olfactory cortex. A marked feature of the bulb is its massive innervation by cholinergic inputs from the basal forebrain. In this study, we addressed the functional interaction between cholinergic inputs and intrinsic bulbar circuitry. Determining the roles of acetylcholine (ACh) requires the characterization of cholinergic effects on both neural excitability and synaptic transmission. For this purpose, we used electrophysiological techniques to localize and characterize the diverse roles of ACh in mouse olfactory bulb slices. We found that cholinergic inputs have a surprising number of target receptor populations that are expressed on three different neuronal types in the bulb. Specifically, nicotinic acetylcholine receptors excite both the output neurons of the bulb, i.e., the mitral cells, as well as interneurons located in the periglomerular regions. These nicotine-induced responses in interneurons are short lasting, whereas responses in mitral cells are long lasting. In contrast, muscarinic receptors have an inhibitory effect on the firing rate of interneurons from a deeper layer, granule cells, while at the same time they increase the degree of activity-independent transmitter release from these cells onto mitral cells. Cholinergic signaling thus was found to have multiple and opposing roles in the olfactory bulb. These dual cholinergic effects on mitral cells and interneurons may be important in modulating olfactory bulb output to central structures required for driven behaviors and may be relevant to understanding mechanisms underlying the perturbations of cholinergic inputs to cortex that occur in Alzheimer's disease.

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.

Immunohistochemical localization of nicotinic beta2 and alpha4 receptor subunits in normal human brain and individuals with Lewy body and Alzheimer's disease: preliminary observations

Optimum immunohistochemical methods were established to immuno-localize nicotinic acetylcholine receptor alpha4 and beta2 subunits in temporal cortex and substantia nigra of normal aged and diseased human brain. In normal aged brain, fibers were immunoreactive for both the alpha4 and beta2 subunits of the nicotinic receptor in the temporal cortex and the substantia nigra. In the cortex of normal aged brain, rare neurofibrillary tangles occurring could be identified with either anti-alpha4 or anti-beta2 antibodies, but existing senile plaques were demonstrable with neither. In Alzheimer's disease temporal cortex, there were diminished numbers of nicotinic receptor subunit immunoreactive fibers, and there were appreciable numbers of neuropil threads, neurofibrillary tangles and senile plaques immunoreactive with both the alpha4 and beta2 antibodies.

Nicotinic receptor-induced apoptotic cell death of hippocampal progenitor cells

Nicotine has many effects on CNS functions, presumably through its action on neuronal nicotinic acetylcholine receptors (AChRs). One subclass of AChRs that binds the snake venom toxin alpha-bungarotoxin (alpha-Bgt-AChRs) has been shown to modulate neurotransmission in the brain. We now show that alpha-Bgt-AChR activation by low doses of nicotine results in apoptotic cell death of both primary and immortalized hippocampal progenitor cells. In HC2S2-immortalized hippocampal progenitors, nicotine is cytotoxic to undifferentiated cells, whereas it spares the same cells once differentiation has been induced. The activation of alpha-Bgt-AChRs by nicotine results in the induction of the tumor suppressor protein p53 and the cdk inhibitor p21. The cytotoxic effect of nicotine is dependent on extracellular calcium levels and is probably attributable to the poor ability of undifferentiated progenitors to buffer calcium loads. The major calcium buffer in these cells, calbindin D28K, is present only after differentiation has been induced. Furthermore transfection of undifferentiated cells with calbindin results in dramatic protection against the cytotoxic effects of nicotine. These results show that nicotine abuse could have significant effects on the survival of progenitor populations in the developing and adult brain and also suggest an endogenous role for alpha-Bgt-AChRs in neuronal development and differentiation.

Ibogaine acts at the nicotinic acetylcholine receptor to inhibit catecholamine release

In an effort to determine mechanisms of action of the putative anti-addictive agent ibogaine, we have measured its effects on catecholamine release in a model neuronal system, cultured bovine chromaffin cells. Various modes of stimulating catecholamine release were used including nicotinic ACh receptor activation, membrane depolarization with elevated K+ and Na+ channel activation with veratridine. In addition, because ibogaine has been reported to interact with kappa opioid receptors, we tested whether kappa receptor antagonists could reverse ibogaine's effects on catecholamine release. Ibogaine, at low concentration (<10 microM) was found to selectively inhibit nicotinic receptor-mediated catecholamine release, while having no significant effect on release evoked by either veratridine or membrane depolarization with elevated K+. The inhibitory actions of ibogaine and the kappa agonists were not reversed by preincubation with the opioid antagonists nor-binaltorphimine or naltrexone, suggesting that these inhibitory effects are not mediated by the kappa opioid receptor. The effects of low dose (10 microM) ibogaine were rapidly reversible, while the inhibitory effects of higher ibogaine doses persisted for at least 19 h following ibogaine washout. The results provide evidence for a mechanism of action ibogaine at the nicotinic ACh receptor. The results are consistent with a model in which the initial high transient brain concentrations (100 microM) of ibogaine act at multiple cellular sites and then have a selective action at the nicotinic ACh receptor cation channel following its metabolism to lower brain concentrations. The present findings are relevant to potential anti-addictive actions of ibogaine and to the development of drugs to combat nicotine addiction.

Nicotine modifies the activity of ventral tegmental area dopaminergic neurons and hippocampal GABAergic neurons

While trying to mimic the dose and time course of nicotine as it is obtained by a smoker, we found the following results. The initial arrival of even a low concentration of nicotine increased the firing rate of dopaminergic neurons from the ventral tegmental area (VTA) and increased the spontaneous vesicular release of GABA from hippocampal neurons. Longer exposure to nicotine caused variable, but dramatic, desensitization of nicotinic receptors and diminished the effects of nicotine. The addictive properties of nicotine as well as its diverse effects on cognitive function could be mediated through differences in activation and desensitization of nicotinic receptors in various areas of the brain.

Brain nicotinic receptors: structure and regulation, role in learning and reinforcement

The introduction, in the late sixties, of the concepts and methods of molecular biology to the study of the nervous system had a profound impact on the field, primarily through the identification of its basic molecular components. These structures include, for example, the elementary units of the synapse: neurotransmitters, neuropeptides and their receptors, but also ionic channels, intracellular second messengers and the relevant enzymes, cell surface adhesion molecules, or growth and trophic factors [21,78,81, 52,79]. Attempts to establish appropriate causal relationships between these molecular components, the actual organisation of neural networks, and a defined behavior, nevertheless, still must overcome many difficulties. A first problem is the recognition of the minimum levels of organisation, from the molecular, cellular, or multicellular (circuit) to the higher cognitive levels, that determine the given physiological and/or behavioral performance under investigation. A common difficulty (and potential source of errors of interpretation) is to relate a cognitive function to a network organization which does not possess the required structural complexity and vice-versa. Another problem is to distinguish, among the components of the system, those which are actually necessary and those which, taken together, suffice for a given behavior to take place. Identification of such a minimal set of building blocks may receive decisive insights from the elaboration of neurally plausible formal models that bring together, within a single and coherent 'artificial organism', the neuronal network, the circulating activity, and the behavior they determine (see [42,43,45,72,30]). In this communication, we shall attempt, still in a preliminary fashion, to bring together: (1) our recent knowledge on the molecular biology of brain nicotinic receptors (nAChRs) and their allosteric properties and (2) integrated behaviors, such as cognitive learning, investigated for instance with delayed-response or passive avoidance tasks that are likely to involve nAChRs in particular at the level of reinforcement (or reward) mechanisms (see [18,29,135]).

Acetylcholine activates an alpha-bungarotoxin-sensitive nicotinic current in rat hippocampal interneurons, but not pyramidal cells

The effects of acetylcholine on both pyramidal neurons and interneurons in the area CA1 of the rat hippocampus were examined, using intracellular recording techniques in an in vitro slice preparation. In current-clamp mode, fast local application of acetylcholine (ACh) to the soma of inhibitory interneurons in stratum radiatum resulted in depolarization and rapid firing of action potentials. Under voltage-clamp, ACh produced fast, rapidly desensitizing inward currents that were insensitive to atropine but that were blocked by nanomolar concentrations of the nicotinic alpha7 receptor-selective antagonists alpha-bungarotoxin (alphaBgTx) and methyllycaconitine. Nicotinic receptor antagonists that are not selective for alpha7-containing receptors had little (mecamylamine) or no effect (dihydro-beta-erythroidine) on the ACh-induced currents. Glutamate receptor antagonists had no effect on the ACh-evoked response, indicating that the current was not mediated by presynaptic facilitation of glutamate release. However, the current could be desensitized almost completely by bath superfusion with 100 nM nicotine. In contrast to those actions on interneurons, application of ACh to the soma of CA1 pyramidal cells did not produce a detectable current. Radioligand-binding experiments with [125I]-alphaBgTx demonstrated that stratum radiatum interneurons express alpha7-containing nAChRs, and in situ hybridization revealed significant amounts of alpha7 mRNA. CA1 pyramidal cells did not show specific binding of [125I]-alphaBgTx and only low levels of alpha7 mRNA. These results suggest that, in addition to their proposed presynaptic role in modulating transmitter release, alpha7-containing nAChRs also may play a postsynaptic role in the excitation of hippocampal interneurons. By desensitizing these receptors, nicotine may disrupt this action and indirectly excite pyramidal neurons by reducing GABAergic inhibition.

Diffuse transmission by acetylcholine in the CNS

Recent immunoelectron microscopic studies have revealed a low frequency of synaptic membrane differentiations on ACh (ChAT-immunostained) axon terminals (boutons or varicosities) in adult rat cerebral cortex, hippocampus and neostriatum, suggesting that, besides synaptic transmission, diffuse transmission by ACh prevails in many regions of the CNS. Cytological analysis of the immediate micro-environment of these ACh terminals, as well as currently available immunocytochemical data on the cellular and subcellular distribution of ACh receptors, is congruent with this view. At least in brain regions densely innervated by ACh neurons, a further aspect of the diffuse transmission paradigm is envisaged: the existence of an ambient level of ACh in the extracellular space, to which all tissue elements would be permanently exposed. Recent experimental data on the various molecular forms of AChE and their presumptive role at the neuromuscular junction support this hypothesis. As in the peripheral nervous system, degradation of ACh by the prevalent G4 form of AChE in the CNS would primarily serve to keep the extrasynaptic, ambient level of ACh within physiological limits, rather than totally eliminate ACh from synaptic clefts. Long-lasting and widespread electrophysiological effects imputable to ACh in the CNS might be explained in this manner. The notions of diffuse transmission and of an ambient level of ACh in the CNS could also be of clinical relevance, in accounting for the production and nature of certain cholinergic deficits and the efficacy of substitution therapies.

Nicotine activates and desensitizes midbrain dopamine neurons

Tobacco use in developed countries is estimated to be the single largest cause of premature death. Nicotine is the primary component of tobacco that drives use, and like other addictive drugs, nicotine reinforces self-administration and place preference in animal studies. Midbrain dopamine neurons normally help to shape behaviour by reinforcing biologically rewarding events, but addictive drugs such as cocaine can inappropriately exert a reinforcing influence by acting upon the mesolimbic dopamine system. Here we show that the same concentration of nicotine achieved by smokers activates and desensitizes multiple nicotinic receptors thereby regulating the activity of mesolimbic dopamine neurons. Initial application of nicotine can increase the activity of the dopamine neurons, which could mediate the rewarding aspects of tobacco use. Prolonged exposure to even these low concentrations of nicotine, however, can cause desensitization of the nicotinic receptors, which helps to explain acute tolerance to nicotine's effects. The effects suggest a cellular basis for reports that the first cigarette of the day is the most pleasurable, whereas the effect of subsequent cigarettes may depend on the interplay between activation and desensitization of multiple nicotinic receptors.

Modulation of cholinergic nucleus basalis neurons by acetylcholine and N-methyl-D-aspartate

Known to exert an important modulatory influence on the cerebral cortex, the cholinergic neurons of the basal forebrain are modulated in turn by neurotransmitters which may include acetylcholine released from processes of brainstem or forebrain neurons. In the present study, we examined the effect of carbachol, a non-specific cholinergic agonist, either alone or in the presence of N-methyl-D-aspartate upon electrophysiologically identified cholinergic basalis neurons in guinea-pig basal forebrain slices. Carbachol produced a direct postsynaptic hyperpolarization, accompanied by a decrease in membrane resistance. Muscarine could mimic this hyperpolarizing effect, whereas nicotine produced a direct postsynaptic membrane depolarization. The interaction of carbachol with N-methyl-D-aspartate was subsequently tested since, in a prior study, N-methyl-D-aspartate was shown to induce rhythmic bursting in cholinergic cells when they were hyperpolarized by continuous injection of outward current. Applied simultaneously with N-methyl-D-aspartate in the absence of current injection, carbachol was also found to promote rhythmic bursting in half of the cells tested. Since the bursts under these conditions were markedly longer in duration than those observed in the presence of N-methyl-D-aspartate alone, it was hypothesized that carbachol might have another action, in addition to the membrane hyperpolarization. Using dissociated cells, it was found that brief applications of carbachol could indeed diminish the slow afterhyperpolarizations that follow single spikes, short bursts or long trains of action potentials in cholinergic basalis neurons. These results indicate that, through its dual ability to hyperpolarize cholinergic neurons and to reduce their afterhyperpolarizations, acetylcholine can promote the occurrence of rhythmic bursting in the presence of N-methyl-D-aspartate. Accordingly, whether derived from brainstem or local sources, acetylcholine may facilitate rhythmic discharge in cholinergic basalis neurons which could in turn impose a rhythmic modulation upon cortical activity during particular states across the sleep-waking cycle.

Influence of subunit composition on desensitization of neuronal acetylcholine receptors at low concentrations of nicotine

The influence of alpha and beta subunits on the properties of nicotine-induced activation and desensitization of neuronal nicotinic acetylcholine receptors (nAChRs) expressed in Xenopus oocytes was examined. Receptors containing alpha4 subunits were more sensitive to activation by nicotine than alpha3-containing receptors. At low concentrations of nicotine, nAChRs containing beta2 subunits reached near-maximal desensitization more rapidly than beta4-containing receptors. The concentration of nicotine producing half-maximal desensitization was influenced by the particular alpha subunit expressed; similar to results for activation, alpha4-containing receptors were more sensitive to desensitizing levels of nicotine than alpha3-containing receptors. The alpha subunit also influenced the rate of recovery from desensitization; this rate was approximately inversely proportional to the apparent nicotine affinity for the desensitized state. The homomeric alpha7 receptor showed the lowest sensitivity to nicotine for both activation and desensitization; alpha7 nAChRs also demonstrated the fastest desensitization kinetics. These subunit-dependent properties remained in the presence of external calcium, although subtle, receptor subtype-specific effects on both the apparent affinities for activation and desensitization and the desensitization kinetics were noted. These data imply that the subunit composition of various nAChRs determines the degree to which receptors are desensitized and/or activated by tobacco-related levels of nicotine. The subtype-specific balance between receptor activation and desensitization should be considered important when the cellular and behavioral actions of nicotine are interpreted.

Human neuronal nicotinic receptors

Nicotine is a very widely used drug of abuse, which exerts a number of neurovegetative, behavioural and psychological effects by interacting with neuronal nicotinic acetylcholine receptors (NAChRs). These receptors are distributed widely in human brain and ganglia, and form a family of ACh-gated ion channels of different subtypes, each of which has a specific pharmacology and physiology. As human NAChRs have been implicated in a number of human central nervous system disorders (including the neurodegenerative Alzheimer's disease, schizophrenia and epilepsy), they are suitable potential targets for rational drug therapy. Much of our current knowledge about the structure and function of NAChRs comes from studies carried out in other species, such as rodents and chicks, and information concerning human nicotinic receptors is still incomplete and scattered in the literature. Nevertheless, it is already evident that there are a number of differences in the anatomical distribution, physiology, pharmacology, and expression regulation of certain subtypes between the nicotinic systems of humans and other species. This review will attempt to survey the major achievements reached in the study of the structure and function of NAChRs by examining the molecular basis of their functional diversity viewed mainly from pharmacological and biochemical perspectives. It will also summarize our current knowledge concerning the structure and function of the NAChRs expressed by other species, and the newly discovered drugs used to classify their numerous subtypes. Finally, the role of NAChRs in behaviour and pathology will be considered.

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.