Nicotine modulates evoked GABAergic transmission in the brain

Evaluation of novel epibatidine analogs in the rat nicotine drug discrimination assay and in the rat chronic constriction injury neuropathic pain model

Nicotine is the primary psychoactive component responsible for maintaining tobacco dependence in humans. Chronic pain is often a consequence of tobacco-related pathologies, and the development of a dual therapeutic that could treat chronic pain and tobacco dependence would be advantageous. Epibatidine reliably substitutes for nicotine in the drug discrimination assay, and is a potent analgesic, but has a side-effect profile that limits its therapeutic potential. Thus, considerable efforts to produce epibatidine derivatives are underway. Here we tested three epibatidine derivatives, 2'-fluoro-3'-(4-nitrophenyl)deschloroepibatidine (RTI-7527-102; i.e., RTI-102), 2'-fluorodeschloroepibatidine (RTI-7527-36; i.e., RTI-36), and 3'-(3″-dimethylaminophenyl)-epibatidine (RTI-7527-76; i.e., RTI-76) in both the rat nicotine drug discrimination assay as well as in the rat chronic constriction injury (CCI) of the sciatic nerve neuropathic pain model. Male and female Sprague-Dawley rats were trained on a fixed-ratio 10 schedule to discriminate nicotine (0.32 mg/kg base) from vehicle. All compounds dose-dependently substituted for nicotine, without significant decreases in response rates. In the discrimination assay the rank order potency was RTI-36 > nicotine > RTI-102 > RTI-76. Evidence suggests the α4β2* subtype is particularly important to nicotine-related abuse potential. Thus, here we utilized the antagonist dihydro-β-erythroidine (DHβE) to examine relative β2 subunit contribution. DHβE (3.2 mg/kg, s.c.) antagonized the discriminative stimulus effects of nicotine. However, relative to antagonism of nicotine, DHβE produced less antagonism of RTI-102 and RTI-76 and greater antagonism of RTI-36. It is likely that at nicotinic receptor subunits RTI-102, RTI-76 and RTI-36 possess differing activity. To confirm that the full discriminative stimulus of these compounds was due to nAChR activity beyond the β2 subunit, we examined these compounds in the presence of the non-selective nicotinic receptor antagonist mecamylamine. Mecamylamine (0.56 mg/kg, s.c.) pretreatment abolished nicotine-paired lever responding for all compounds. In a separate cohort, male and female Sprague-Dawley rats underwent CCI surgery and tested for CCI-induced mechanical allodynia via the von Frey assay. Each compound produced CCI-induced mechanical allodynia reversal. RTI-36 displayed higher potency than either RTI-102 or RTI-76. These novel epibatidine analogs may prove to be useful tools in the fight against nicotine dependence as well as novel neuropathic pain analgesics.

Understanding the neurobiological effects of drug abuse: Lessons from zebrafish models

Drug abuse and brain disorders related to drug comsumption are public health problems with harmful individual and social consequences. The identification of therapeutic targets and precise pharmacological treatments to these neuropsychiatric conditions associated with drug abuse are urgently needed. Understanding the link between neurobiological mechanisms and behavior is a key aspect of elucidating drug abuse-related targets. Due to various molecular, biochemical, pharmacological, and physiological features, the zebrafish (Danio rerio) has been considered a suitable vertebrate for modeling complex processes involved in drug abuse responses. In this review, we discuss how the zebrafish has been successfully used for modeling neurobehavioral phenotypes related to drug abuse and review the effects of opioids, cannabinoids, alcohol, nicotine, and psychedelic drugs on the central nervous system (CNS). Moreover, we summarize recent advances in zebrafish-based studies and outline potential advantages and limitations of the existing zebrafish models to explore the neurochemical bases of drug abuse and addiction. Finally, we discuss how the use of zebrafish models may present fruitful approaches to provide valuable clinically translatable data.

Nicotine Addiction: Neurobiology and Mechanism

Nicotine, primary component of tobaco produces craving and withdrawal effect both in humans and animals. Nicotine shows a close resemblance to other addictive drugs in molecular, neuroanatomical and pharmacological, particularly the drugs which enhances the cognitive functions. Nicotine mainly shows its action through specific nicotinic acetylcholine receptors located in brain. It stimulates presynaptic acetylcholine receptors thereby enhancing Ach release and metabolism. Dopaminergic system is also stimulated by it, thus increasing the concentration of dopamine in nuclear accumbens. This property of nicotine according to various researchers is responsible for reinforcing behavioral change and dependence of nicotine. Various researchers have also depicted that some non dopaminergic systems are also involved for rewarding effect of nicotinic withdrawal.

Neurological systems such as GABAergic, serotonergic, noradrenergic, and brain stem cholinergic may also be involved to mediate the actions of nicotine. Further, the neurobiological pathway to nicotine dependence might perhaps be appropriate to the attachment of nicotine to nicotinic acetylcholine receptors, peruse by stimulation of dopaminergic system and activation of general pharmacological changes that might be responsible for nicotine addiction. It is also suggested that MAO A and B both are restrained by nicotine. This enzyme helps in degradation dopamine, which is mainly responsible for nicotinic actions and dependence. Various questions remain uninsurable to nicotine mechanism and require more research. Also, various genetic methods united with modern instrumental analysis might result for more authentic information for nicotine addiction.

Whole tobacco smoke extracts to model tobacco dependence in animals

Smoking tobacco is highly addictive and a leading preventable cause of death. The main addictive constituent is nicotine; consequently it has been administered to laboratory animals to model tobacco dependence. Despite extensive use, this model might not best reflect the powerful nature of tobacco dependence because nicotine is a weak reinforcer, the pharmacology of smoke is complex and non-pharmacological factors have a critical role. These limitations have led researchers to expose animals to smoke via the inhalative route, or to administer aqueous smoke extracts to produce more representative models. The aim was to review the findings from molecular/behavioural studies comparing the effects of nicotine to tobacco/smoke extracts to determine whether the extracts produce a distinct model. Indeed, nicotine and tobacco extracts yielded differential effects, supporting the initiative to use extracts as a complement to nicotine. Of the behavioural tests, intravenous self-administration experiments most clearly revealed behavioural differences between nicotine and extracts. Thus, future applications for use of this behavioural model were proposed that could offer new insights into tobacco dependence.

Brain proton magnetic resonance spectroscopy of alcohol use disorders

This chapter critically reviews brain proton magnetic resonance spectroscopy ((1)H MRS) studies performed since 1994 in individuals with alcohol use disorders (AUD). We describe the neurochemicals that can be measured in vivo at the most common magnetic field strengths, summarize our knowledge about their general brain functions, and briefly explain some basic human (1)H MRS methods. Both cross-sectional and longitudinal research of individuals in treatment and of treatment-naïve individuals with AUD are discussed and interpreted on the basis of reported neuropathology. As AUDs are highly comorbid with chronic cigarette smoking and illicit substance abuse, we also summarize reports on their respective influences on regional proton metabolite levels. After reviewing research on neurobiologic correlates of relapse and genetic influences on brain metabolite levels, we finish with suggestions on future directions for (1)H MRS studies in AUDs. The review demonstrates that brain metabolic alterations associated with AUDs as well as their cognitive correlates are not simply a consequence of chronic alcohol consumption. Future MR research of AUDs in general has to be better prepared - and supported - to study clinically complex relationships between personality characteristics, comorbidities, neurogenetics, lifestyle, and living environment, as all these factors critically affect an individual's neurometabolic profile. (1)H MRS is uniquely positioned to tackle these complexities by contributing to a comprehensive biopsychosocial profile of individuals with AUD: it can provide non-invasive biochemical information on select regions of the brain at comparatively low overall cost for the ultimate purpose of informing more efficient treatments of AUDs.

Brain regions mediating α3β4 nicotinic antagonist effects of 18-MC on nicotine self-administration

18-Methoxycoronaridine (18-MC), a putative anti-addictive agent, has been shown to decrease the self-administration of several drugs of abuse in rats. 18-MC is a potent antagonist at α3β4 nicotinic receptors. Consistent with high densities of α3β4 nicotinic receptors being located in the medial habenula and the interpeduncular nucleus, 18-MC has been shown to act in these regions to decrease both morphine and methamphetamine self-administration. The present study was conducted to determine if 18-MC's effect on nicotine self-administration is mediated by acting in these same brain regions. Because moderate densities of α3β4 receptors occur in the dorsolateral tegmentum, ventral tegmental area, and basolateral amygdala, these brain areas were also examined as potential sites of action of 18-MC. Local administration of 18-MC into either the medial habenula, the basolateral amygdala or the dorsolateral tegmentum decreased nicotine self-administration. Surprisingly, local administration of 18-MC into the interpeduncular nucleus increased nicotine self-administration while local administration of 18-MC into the ventral tegmental area had no effect on nicotine self-administration. Similar effects were produced by local administration of either mecamylamine or conotoxin AuIB. These data are consistent with the hypothesis that 18-MC decreases nicotine self-administration by indirectly modulating the dopaminergic mesolimbic pathway via blockade of α3β4 nicotinic receptors in the medial habenula, basolateral amygdala, and dorsolateral tegmentum. The data also suggest that an action of 18-MC in the interpeduncular nucleus may attenuate aversive and/or depressive effects of nicotine.

Recent advances in gene manipulation and nicotinic acetylcholine receptor biology

Pharmacological and immunological methods have been valuable for both identifying some native nicotinic acetylcholine receptor (nAChR) subtypes that exist in vivo and determining the neurobiological and behavioral role of certain nAChR subtypes. However, these approaches suffer from shortage of subtype specific ligands and reliable immunological reagents. Consequently, genetic approaches have been developed to complement earlier approaches to identify native nAChR subtypes and to assess the contribution of nAChRs to brain function and behavior. In this review we describe how assembly partners, knock-in mice and targeted lentiviral re-expression of genes have been utilized to improve our understanding of nAChR neurobiology. In addition, we summarize emerging genetic tools in nAChR research.

Nicotine excites hypothalamic arcuate anorexigenic proopiomelanocortin neurons and orexigenic neuropeptide Y neurons: similarities and differences

Two of the biggest health problems facing us today are addiction to nicotine and the increased prevalence of obesity. Interestingly, nicotine attenuates obesity, but the underlying mechanism is not clear. Here we address the hypothesis that if weight-reducing actions of nicotine are mediated by anorexigenic proopiomelanocortin (POMC) neurons of the hypothalamic arcuate nucleus, nicotine should excite these cells. Nicotine at concentrations similar to those found in smokers, 100-1,000 nM, excited POMC cells by mechanisms based on increased spike frequency, depolarization of membrane potential, and opening of ion channels. This was mediated by activation of both α7 and α4β2 nicotinic receptors; by itself, this nicotine-mediated excitation could explain weight loss caused by nicotine. However, in control experiments nicotine also excited the orexigenic arcuate nucleus neuropeptide Y (NPY) cells. Nicotine exerted similar actions on POMC and NPY cells, with a slightly greater depolarizing action on POMC cells. Immunocytochemistry revealed cholinergic axons terminating on both cell types. Nicotine actions were direct in both cell types, with nicotine depolarizing the membrane potentials and reducing input resistance. We found no differences in the relative desensitization to nicotine between POMC and NPY neurons. Nicotine inhibited excitatory synaptic activity recorded in NPY, but not POMC, cells. Nicotine also excited hypocretin/orexin neurons that enhance cognitive arousal, but the responses were smaller than in NPY or POMC cells. Together, these results indicate that nicotine has a number of similar actions, but also a few different actions, on POMC and NPY neurons that could contribute to the weight loss associated with smoking.

Biological basis of tobacco addiction: Implications for smoking-cessation treatment

Tobacco use became common all over the world after discovery of Americas. Tobacco, a plant carries in its leaves an alkaloid called nicotine, which is responsible not only for several pathophysiological changes in the body but also develops tolerance to its own action with repeated use. Studies suggest that the alpha-4 beta-2 nicotine acetylcholine receptor subtype is the main receptor that mediates nicotine dependence. Nicotine acts on these receptors to facilitate neurotransmitter release (dopamine and others), producing pleasure and mood modulation. Repeated exposure to nicotine develops neuroadaptation of the receptors, resulting in tolerance to many of the effects of nicotine. Withdrawal symptoms appear on stoppage of tobacco use, which are characterized by irritability, anxiety, increased eating, dysphoria, and hedonic dysregulation, among others. Smoking is also reinforced by conditioning. Pharmacotherapies for smoking cessation should reduce withdrawal symptoms and block the reinforcing effects of nicotine obtained from smoking without causing excessive adverse effects.

Nicotinic receptors and stages of nicotine dependence

Smoking is one of the leading causes of preventable death, where nicotine has been identified as the primary addictive constituent of tobacco. Consequently, there have been extensive investigations into the neuroadaptations that occur as nicotine dependence develops, where numerous neurological systems have been implicated. The focus of this review was on nicotinic acetylcholine receptor neuroadaptations that occur during the development of nicotine dependence. This focus was selected because (1) the nicotinic receptors are the primary binding sites for both nicotine and the most efficacious pharmacological smoking cessation treatments and (2) the receptors are located throughout the brain with considerable neuromodulatory ability. However, there was difficulty associated in outlining the role of nicotinic receptors in the development of nicotine dependence because it comprises a series of stages involving different neurological systems rather than a single state. To address this issue, the review adopts a novel approach and considers the role of nicotinic receptor subtypes at separate stages of the nicotine dependence cycle. This information was then used to examine the nicotinic receptor-related therapeutic mechanisms of three main pharmacological smoking cessation treatments.

Prenatal nicotine exposure and development of nicotinic and fast amino acid-mediated neurotransmission in the control of breathing

There is mounting evidence that neonatal animals exposed to nicotine in the prenatal period exhibit a variety of anatomic and functional abnormalities that adversely affect their respiratory and cardiovascular control systems, but how nicotine causes these developmental alterations is unknown. The principle that guides our work is that PNE impairs the ability of nicotinic acetylcholine receptors (nAChRs) to modulate the pre-synaptic release of both inhibitory (particularly GABA) and excitatory (glutamate) neurotransmitters, leading to marked alterations in the density and/or function of receptors on the (post-synaptic) membrane of respiratory neurons. Such changes could lead to impaired ventilatory responses to sensory afferent stimulation, and altered breathing patterns, including central apneic events. In this brief review we summarize the work that lead to the development of this hypothesis, and introduce some new data that support and extend it.

Involvement of nicotinic and muscarinic receptors in the endogenous cholinergic modulation of the balance between excitation and inhibition in the young rat visual cortex

This study aims to clarify how endogenous release of cortical acetylcholine (ACh) modulates the balance between excitation and inhibition evoked in visual cortex. We show that electrical stimulation in layer 1 produced a significant release of ACh measured intracortically by chemoluminescence and evoked a composite synaptic response recorded intracellularly in layer 5 pyramidal neurons of rat visual cortex. The pharmacological specificity of the ACh neuromodulation was determined from the continuous whole-cell voltage clamp measurement of stimulation-locked changes of the input conductance during the application of cholinergic agonists and antagonists. Blockade of glutamatergic and gamma-aminobutyric acid (GABAergic) receptors suppressed the evoked response, indicating that stimulation-induced release of ACh does not directly activate a cholinergic synaptic conductance in recorded neurons. Comparison of cytisine and mecamylamine effects on nicotinic receptors showed that excitation is enhanced by endogenous evoked release of ACh through the presynaptic activation of alpha(*)beta4 receptors located on glutamatergic fibers. DHbetaE, the selective alpha4beta2 nicotinic receptor antagonist, induced a depression of inhibition. Endogenous ACh could also enhance inhibition by acting directly on GABAergic interneurons, presynaptic to the recorded cell. We conclude that endogenous-released ACh amplifies the dominance of the inhibitory drive and thus decreases the excitability and sensory responsiveness of layer 5 pyramidal neurons.

Brain regions mediating alpha3beta4 nicotinic antagonist effects of 18-MC on methamphetamine and sucrose self-administration

The novel iboga alkaloid congener 18-methoxycoronaridine (18-MC) is a putative anti-addictive agent that has been shown, in rats, to decrease the self-administration of several drugs of abuse. Previous work has established that 18-MC is a potent antagonist at alpha3beta4 nicotinic receptors. Because high densities of alpha3beta4 nicotinic receptors occur in the medial habenula and the interpeduncular nucleus and moderate densities occur in the dorsolateral tegmentum, ventral tegmental area, and basolateral amygdala, the present study was conducted to determine if 18-MC could act in these brain areas to modulate methamphetamine self-administration in rats. Local administration of 18-MC into either the medial habenula, the interpeduncular area or the basolateral amygdala decreased methamphetamine self-administration. Similar results were produced by local administration into the same brain areas of two other alpha3beta4 nicotinic antagonists, mecamylamine and alpha-conotoxin AuIB. Local administration of 18-MC, or the other antagonists, into the dorsolateral tegmentum or the ventral tegmental area had no effect on methamphetamine self-administration. In contrast, local administration of 18-MC and the other antagonists decreased sucrose self-administration when administered into the dorsolateral tegmentum or basolateral amygdala but had no effect when infused into the medial habenula, interpeduncular nucleus, or ventral tegmental area. These data are consistent with the hypothesis that 18-MC decreases methamphetamine self-administration by indirectly modulating the dopaminergic mesolimbic pathway via blockade of alpha3beta4 nicotinic receptors in the habenulo-interpeduncular pathway and the basolateral amygdala. The data also suggest that the basolateral amygdala along with a different pathway involving alpha3beta4 receptors in the dorsolateral tegmentum mediate the effect of 18-MC on sucrose self-administration.

Regulation of synaptic transmission and plasticity by neuronal nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are widely expressed throughout the central nervous system and participate in a variety of physiological functions. Recent advances have revealed roles of nAChRs in the regulation of synaptic transmission and synaptic plasticity, particularly in the hippocampus and midbrain dopamine centers. In general, activation of nAChRs causes membrane depolarization and directly and indirectly increases the intracellular calcium concentration. Thus, when nAChRs are expressed on presynaptic membranes their activation generally increases the probability of neurotransmitter release. When expressed on postsynaptic membranes, nAChR-initiated calcium signals and depolarization activate intracellular signaling mechanisms and gene transcription. Together, the presynaptic and postsynaptic effects of nAChRs generate and facilitate the induction of long-term changes in synaptic transmission. The direction of hippocampal nAChR-mediated synaptic plasticity - either potentiation or depression - depends on the timing of nAChR activation relative to coincident presynaptic and postsynaptic electrical activity, and also depends on the location of cholinergic stimulation within the local network. Therapeutic activation of nAChRs may prove efficacious in the treatment of neuropathologies where synaptic transmission is compromised, as in Alzheimer's or Parkinson's disease.

Mechanisms of pathogenesis in the Sudden Infant Death Syndrome

The likely processes of the Sudden Infant Death Syndrome (SIDS) were identified many years ago (apnea, failed arousal, failed autoresuscitation, etc.). The neurophysiological basis of these processes and the neurophysiological reasons some infants die of SIDS and others do not are, however, only emerging now. We reviewed recent studies that have shed light on the way in which epidemiological risk factors, genetics, neurotransmitter receptor defects and neonatal cardiorespiratory reflex responses interact to lead to sudden death during sleep in a small number of normal appearing infants. As a result of this review and analysis, we hypothesize that the neurophysiological basis of SIDS resides in a persistence of fetal reflex responses into the neonatal period, amplification of inhibitory cardiorespiratory reflex responses and reduced excitatory cardiorespiratory reflex responses. The hypothesis we developed explores the ways in which multiple subtle abnormalities interact to lead to sudden death and emphasizes the difficulty of ante-mortem identification of infants at risk for SIDS, although identification of infants at risk remains an essential goal of SIDS research.

The Effects of Glutamate and GABA Receptor Antagonists on Nicotine-induced Neurotransmitter Changes in Cognitive Areas*

In the present study, we tested the effects of glutamate and GABA receptor antagonists on nicotine-induced neurotransmitter changes in the hippocampal (dorsal and ventral) and cortical (medial temporal and prefrontal) brain areas of conscious freely moving rats via microdialysis. Both the antagonists and nicotine were administered intracerebrally. The antagonists tested were NMDA, AMPA-kainate, and metabotropic glutamate receptor subtype antagonists (MK801, CNQX, and LY 341495, respectively) and GABA(A) and GABA(B) receptor subtype antagonists (bicuculline and hydroxysaclofen, respectively). We assayed nicotine-induced changes in dopamine (DA), norepinephrine (NE), serotonin (5-HT), and their metabolites. We found with the antagonists, both decreases and increases in nicotine-induced neurotransmitter responses. In the presence of nicotine all the antagonists (except LY 341495) caused a decrease in DA levels in the regions tested. NE levels were decreased in the cortex by all antagonists. In the hippocampus, GABA antagonists decreased NE levels, as did the metabotropic glutamate antagonist, LY 341495, while the other glutamate antagonists increased NE levels. The results of the 5-HT assay were more variable and dependent on the region and antagonist examined; increases were found slightly more often than decreases. The changes in metabolites were not often parallel with changes in their associated neurotransmitters, indicating that the antagonists also affect the metabolism of the neurotransmitters. The effect of the antagonists in the absence of nicotine was mostly to decrease the level of neurotransmitters, although increases were seen in a few cases. The results suggest that the excitatory glutamatergic- and inhibitory GABAergic-amino acid receptors are both involved in mediating nicotine-induced neurotransmitter responses, and their inhibitory or stimulatory effects are receptor subtype and brain region dependent.

Cognitive status of young and older cigarette smokers: data from the international brain database

Previous studies that have examined the impact of cigarette smoking on cognition have revealed mixed results; some studies report no impact and others report detrimental effects, especially in older individuals. Few studies, however, have examined the effects of cigarette smoking on both young and old healthy individuals using highly robust and standardized methods of cognitive assessment. This study draws on an international database to contrast cognitive differences between younger and older individuals who regularly smoke cigarettes and non-smokers. Data were sampled from 1000 highly screened healthy individuals free of medical or psychiatric health complications. A cohort of 62 regular smokers (n = 45 < 45 years of age; n = 1745 years) with a Fagerstrom nicotine dependency score of 1 or more were identified and matched to a cohort of 62 healthy nonsmokers (n = 43 < 45 years; n = 1945 years) on demographic variables and estimated intelligence. Performances on cognitive measures of attention, reaction time, cognitive flexibility, psychomotor speed, and memory were considered for analysis. As a group, smokers performed more poorly than nonsmokers on one measure of executive function. A significant age and smoking status interaction was identified with older smokers performing more poorly than older nonsmokers and younger smokers on a measure of long-delayed recall of new information. Cigarette smoking is associated with isolated and subtle cognitive difficulties among very healthy individuals.

The interaction of neuroactive steroids and GABA in the development of neuropsychiatric disorders in women

A growing literature suggests that hormonal fluctuations occurring across the menstrual cycle, during and after pregnancy, and during the menopausal transition are associated with onset of affective disorders or exacerbation of existing disorders. This influence of the neuroendocrine system on psychiatric disorders is thought to be mediated by an abnormality in central nervous system response to neuroactive steroids such as estradiol, progesterone, and the progesterone derivative allopregnanolone (ALLO). This interplay is considerably complex as neuroactive steroids modulate the function of multiple neurotransmitter systems throughout various stages of development. While one could choose to study any number of steroid-neurotransmitter interactions, our group in addition to others has focused our investigative efforts on unraveling the contribution of neuroactive steroids to psychiatric syndromes and disorders via their modulation of gamma aminobutyric acid (GABA), the brain's major inhibitory neurotransmitter. The goal of this article is two-fold: to synthesize the clinical and preclinical research focusing on the interplay between neuroactive steroids and GABA as they relate to neuropsychiatric and substance use disorders in women and to integrate data from our laboratory using proton magnetic resonance spectroscopy into this context.

Modulation of GABAergic synaptic transmission by terminal nicotinic acetylcholine receptors in the central autonomic nucleus of the neonatal rat spinal cord

Using patch clamp recordings from an in vitro spinal cord slice preparation of neonatal rats (9-15days old), we characterized the GABAergic synaptic transmission in sympathetic preganglionic neurones (SPN) of the central autonomic nucleus (CA) of lamina X. Local applications of isoguvacine (100microM), a selective agonist at GABA(A) receptors, induced in all cells tested a chloride current which was abolished by bicuculline, a competitive antagonist at GABA(A) receptors. In addition, 25% of the recorded cells displayed spontaneous tetrodotoxin-insensitive and bicuculline-sensitive chloride miniature inhibitory postsynaptic currents (mIPSCs). Acetylcholine (100microM) increased the frequency of GABAergic mIPSCs without affecting their amplitudes or their kinetic properties indicating a presynaptic site of action. The presynaptic effect of ACh was restricted to GABAergic neurones synapsing onto sympathetic preganglionic neurones. The facilitatory effect of ACh was abolished in the absence of external calcium or in the presence of 100microM cadmium added to the bath solution. Choline 10mM, an agonist at alpha7 nicotinic acetylcholine receptors (nAChRs) or muscarine (10microM), a muscarinic receptor agonist, did not reproduce the presynaptic effect of ACh. The presynaptic effect of ACh was blocked by 1microM of dihydro-beta-erythroidine (DHbetaE), an antagonist of non-alpha7 nAChRs but was insensitive to alpha7 nAChRs antagonists (strychnine, alpha-bungarotoxin and methyllycaconitine) or to the muscarinic receptor antagonist atropine (10microM). It was concluded that SPNs of the central autonomic nucleus displayed a functional GABAergic transmission which is facilitated by terminal non alpha7 nAChRs.

Cholinergic modulation of appetite-related synapses in mouse lateral hypothalamic slice

Nicotine administration reduces appetite and alters feeding patterns; a major deterrent to smoking cessation is hyperphagia and resultant weight gain. We demonstrate here that lateral hypothalamic (LH) circuits involving melanin-concentrating hormone (MCH) neurons are subject to cholinergic modulation that may be related to the effects of nicotine on appetite control. Cholinergic input to the perifornical LH area of the mouse is confirmed by examination of immunostaining for vesicular acetylcholine (ACh) transporter (VAT) in conjunction with antibodies to MCH and the vesicular GABA transporter (vGABAT). vAChT-positive neurons border the LH, and VAT-positive projections are detected throughout the perifornical area. MCH-positive dendrites appear studded with vGABAT-positive contacts, consistent with recordings of GABAergic inputs to LH/MCH neurons identified by their location, morphology, electrophysiological profile, and MCH expression. Activation of presynaptic nicotinic ACh receptors (nAChRs) enhances GABAergic transmission. GABAergic transmission is potentiated by (1) direct nicotine application, (2) increasing local ACh concentration, and (3) stimulation of cholinergic projections. Based on pharmacological studies and comparisons of wild-type versus alpha7 nAChR subunit mutant mice, we propose that alpha7*-nAChRs are required for the modulation of GABAergic inputs in LH. Prenatal exposure to nicotine elicits a persistent elevation of GABAergic transmission in the LH of postnatal pups. Furthermore, GABAergic inputs to LH of prenatal nicotine-exposed pups are insensitive to subsequent nicotine challenge. Our studies support the hypothesis that nicotine administration or elevated cholinergic tone enhance inhibition of perifonical LH/MCH neurons via activation of presynaptic alpha7*-nAChRs.

Nicotine-ethanol interactions in flash-evoked potentials and behavior of Long-Evans rats

Although nicotine and ethanol are often used together, little is known about their combined effects on visual system electrophysiology. This experiment examined the separate and combined effects of nicotine and ethanol on flash-evoked potentials (FEPs) recorded from both the visual cortex (VC) and superior colliculus (SC) of chronically implanted male Long-Evans rats. There were four treatment conditions administered on separate days: either saline or ethanol (2.0 g/kg, i.p.) was given 10 min before either saline or nicotine (1.0 mg/kg, s.c.). FEPs were recorded at 5, 20, and 40 min following the second injection. In the VC, ethanol significantly decreased the amplitude of most components, but increased P46. Peaks P22 and N53 were unchanged. Nicotine enhanced most component amplitudes, but decreased N29 and P234, while P22 and N139 were unchanged. In the SC, ethanol depressed the amplitude of all components studied. In contrast, nicotine significantly depressed only P27 and N48. Latencies of most components in both structures were increased by ethanol, nicotine, and the combination treatment, although a nicotine-induced enhancement of the effects of ethanol on latencies was not typically observed. Each drug treatment also produced significant hypothermia, with the combination treatment resulting in the greatest hypothermia. Ethanol, either alone or in combination with nicotine, significantly reduced body movements during the FEP recording sessions. In subsequent open-field observations, ethanol, but not nicotine, significantly increased the number of squares crossed, while the combination treatment produced the greatest increase in movement. Nicotine significantly increased rearing behavior, but both ethanol and the combination treatment eliminated rearings. Overall, data suggesting that nicotine can counteract some of the effects of ethanol was demonstrated in varying degrees in the amplitude of VC components N39, P46, N53, N65, and P88, the latency of VC component N53, the amplitude of SC component N59, and the latency of SC components N48 and N54. In contrast, a nicotine-induced enhancement of the effects of ethanol was found for only the latency of VC components N39, P88, and P234, body temperature, and open-field ambulation.

Cortical gamma-aminobutyric acid levels and the recovery from ethanol dependence: preliminary evidence of modification by cigarette smoking

BACKGROUND

Gamma-aminobutyric acid (GABA)ergic adaptations contribute to the neurobiology of ethanol dependence and withdrawal. Clinical data suggest that tobacco smoking attenuates alcohol withdrawal symptoms. This study's objective was to measure time-dependent cortical GABA levels with sobriety in ethanol-dependent patients with mild to moderate withdrawal severity, controlling for alcoholism-related neurotoxicity and smoking.

METHODS

Proton magnetic resonance spectroscopy (MRS) was used to measure occipital cortical N-acetylaspartate (NAA), glutamate plus glutamine, and GABA in 12 ethanol-dependent men at approximately 1 week and 1 month of medication-free sobriety on an inpatient unit. Eight healthy men were studied once. The tissue composition of the MRS volume was determined.

RESULTS

Adjusting for less white matter in patients, GABA differed insignificantly between ethanol-dependent patients (smokers plus nonsmokers) and healthy subjects. In early sobriety, nonsmoking patients had more GABA than did smoking patients, but by 1 month, GABA decreased in nonsmokers without changing in smokers. Smoking was associated with increased glutamate plus glutamine in patients and healthy subjects, adjusting for NAA levels.

CONCLUSIONS

These data do not show that deficits in cortical GABA contribute directly to acute ethanol withdrawal. If smoking prevents withdrawal-related changes in cortical GABA systems, it may contribute to comorbidity of alcoholism and tobacco smoking.

Functional characterization of mouse alpha4beta2 nicotinic acetylcholine receptors stably expressed in HEK293T cells

Mouse alpha4beta2 nicotinic acetylcholine receptors (nAchRs) were stably expressed in HEK293T cells. The function of this stable cell line, termed mmalpha4beta2, was assessed using an aequorin-based luminescence method that measures agonist-evoked changes in intracellular calcium. Agonist-elicited changes in intracellular calcium were due primarily to direct entry of calcium through the alpha4beta2 channel, although release of calcium from intracellular stores contributed approximately 28% of the agonist-evoked response. Agonist pharmacologies were very similar between the mmalpha4beta2 cells and most cell lines that stably express human alpha4beta2 nAchRs. Based on agonist profiles and sensitivity to the antagonist dihydro-beta-erythroidine (DHbetaE), the predominant alpha4beta2 nAchR expressed in the mmalpha4beta2 cells exhibits a pharmacology that most resembles the DHbetaE-sensitive component of 86Rb+ efflux from mouse brain synaptosomes. However, when evaluated with the aequorin assay, the mmalpha4beta2 nAchR was found to be atypically sensitive to blockade by the presumed alpha7-selective antagonist methyllycaconitine (MLA), exhibiting an IC50 value of 31 +/- 0.1 nm. Similar IC50 values have been reported for the MLA inhibition of nicotine-stimulated dopamine release, a response that is mediated by beta2-subunit-containing nAchRs and not alpha7-subunit-containing nAchRs. Consequently, at low nanomolar concentrations, MLA may not be as selective for alpha7-containing nAchRs as previously thought.

Prenatal nicotine exposure increases the strength of GABA(A) receptor-mediated inhibition of respiratory rhythm in neonatal rats

Infants born to mothers that smoke while pregnant have a relatively high incidence of central respiratory control abnormalities. Recent studies have shown that prenatal nicotine exposure increases GABA release and the frequency of GABAergic currents, leading to an up-regulation of GABA(A) receptors in central neurones. Activation of GABA(A) receptors inhibits ventilatory activity, with intense activation causing apnoea. These observations lead us to hypothesize that prenatal nicotine exposure alters GABAergic control of respiratory motor pattern in the early neonatal period. Osmotic minipumps were implanted in pregnant Sprague-Dawley rats on the fifth day of gestation, and filled with nicotine (6 mg kg(-1) day(-1), 2.5 microl h(-1)) or physiological saline (2.5 microl h(-1)). Brainstem-spinal cord preparations from 1- to 3-day-old neonates were studied under in vitro conditions. Electrical activity was recorded from the fourth cervical ventral root (C4 VR), which contains the axons of phrenic motoneurones. Bath application of GABA(A) receptor agonists muscimol (250 microM) or pentobarbital sodium (60 microM) to the brainstem led to consistent, reversible and significant reductions in C4 VR burst frequency. In saline-exposed animals, frequency (bursts min(-1)) fell from 6.8 +/- 0.4 to a nadir of 2.8 +/- 0.5 with muscimol, and from 6.5 +/- 0.3 to a nadir of 2.9 +/- 0.3 for pentobarbital; in nicotine-exposed animals, frequency fell from 6.3 +/- 0.4 to 1.0 +/- 0.4 with muscimol and from 6.4 +/- 0.2 to 1.7 +/- 0.4 with pentobarbital (P < 0.05 in all cases). The decrease in C4 VR frequency was significantly greater in nicotine-exposed compared to saline-exposed preparations with both muscimol and pentobarbital (P < 0.001 for both). There were no changes in the amplitude of C4 VR bursts under any condition. The GABA(A) receptor antagonist bicuculline methiodide (8 microM) did not change C4 VR frequency or amplitude in either group, although it was effective in reversing the effects of muscimol. These experiments demonstrate that prenatal nicotine exposure alters the GABAergic regulation of respiratory rhythm in a reduced preparation. The results may lead to a better understanding of the perturbed breathing pattern observed in neonates that are exposed to nicotine in utero.

Baclofen interactions with nicotine in rats: effects on memory

Nicotine has been shown in numerous previous studies to significantly improve memory on the radial-arm maze, yet the critical mechanisms underlying this effect are not fully characterized. Nicotine stimulates the release of a number of neurotransmitters important for memory function including (gamma-aminobutyric acid) GABA. The importance of nicotinic-GABA interactions regarding memory is currently unknown. The purpose of the current study was to determine the interactive effects of nicotine and the GABA agonist baclofen on working memory function as measured by choice accuracy in the radial-arm maze. Female Sprague-Dawley rats trained to asymptotic performance levels on a win-shift eight-arm radial maze task were used for assessment of nicotine-baclofen interactions. Low doses of baclofen improved memory performance while higher doses impaired it. Nicotine, as seen before, improved memory performance. Nicotine also significantly reversed the higher dose baclofen-induced deficit. These data show the importance of both nicotinic and GABA systems in working memory function and the interactions between these two transmitter receptor systems. This not only provides information concerning the neural bases of cognitive performance, it also lends insight into new combination treatments for memory impairment.

Nicotine alters flash-evoked potentials in Long–Evans rats

This experiment examined the effects of nicotine on flash-evoked potentials (FEPs) recorded from both the visual cortex (VC) and the superior colliculus (SC) of chronically implanted male Long-Evans rats. FEPs were recorded at 5, 20, 40, and 60 min following subcutaneous injections of saline, and of 0.4, 0.7, and 1.0 mg/kg nicotine on separate days. In the VC, the amplitude of components N(39), N(53), N(67), and P(88) increased, while the amplitude of components N(30) and P(235) decreased following nicotine administration. P(22), P(47), and N(153) were unchanged. In the SC, components P(27), N(48), and N(53) were reduced in amplitude, while P(37) and N(57) were unaffected by nicotine. Many peak latencies in the VC and SC were increased by nicotine, often at all three doses. However, effects of nicotine on FEPs were both dose- and time-dependent. When body temperature was recorded 65 min after drug administration, significant hypothermia was found with both the 0.7- and 1.0-mg/kg nicotine doses. The 1.0-mg/kg dose of nicotine resulted in a significant increase in movement during the recording sessions, but not in subsequent open-field observations. The results demonstrate that nicotinic acetylcholine receptors (nAChRs) play a differential role in the production/modulation of the various components of FEPs.

Exposure to low concentrations of nicotine during cranial nerve development inhibits apoptosis and causes cellular hypertrophy in the ventral oculomotor nuclei of the chick embryo

Maternal cigarette use during pregnancy is associated with increased incidence of neural impairments in offspring, but nicotine's unique contribution to any neuropathology remains unclear, and nicotine's neurodevelopmental effects assessed in animal models vary with concentration. During ontogenesis, the chick oculomotor complex (OMN) is regulated by central nervous system (CNS) afferent-derived and target-derived trophic factors, allowing assessment of nicotine's potential interference in receptor-mediated CNS trophic phenomena, unconfounded by myriad other compounds in cigarette smoke. In the current study, 100 ng nicotine applied daily in ovo to yolk during embryonic days (E) 1-7 mimicked maternal plasma nicotine concentrations during fetal cranial nerve development. Nicotine-treated embryos exhibited a 15% decrease in whole body weight and 7% decrease in brain weight at E16. However, at E16, nicotine-treated embryos had 37% and 15% increases in the combined ventromedial+lateral (v) OMN motoneuron density and soma area, respectively, effects not observed in the optic tectum, in which nicotine cholinergic receptor expression is delayed until E8-12. Incorporation of tritiated thymidine into whole brain DNA demonstrated that the nicotine treatment did not cause increased rates of whole brain mitosis, suggesting that the dosage regimen did not elicit a cytotoxic, wound-healing, response of differentiating cells. As determined by DNA fragment-labeling assay during the normal period of cell death, vOMN apoptosis occurs maximally on E11 during a normal period of declining cell density, and a dose-response study demonstrated 78% E11 vOMN apoptotic suppression at approximately 0.30 microM cumulative yolk nicotine with an inhibition threshold between 0.10 and 0.20 microM. These results suggest that plasma nicotine concentrations resulting from tobacco use or nicotine replacement therapy (NRT) are sufficient to inhibit motoneuron apoptosis and enhance neuronal growth.

A study of the acute effect of smoking on cerebral blood flow using 99mTc-ECD SPET

Cigarette smoking is known to be associated with atherosclerosis, is an important risk factor for stroke and has other serious effects. The aim of this study was to evaluate the acute effect of cigarette smoking on cerebral blood flow using statistical parametric mapping (SPM). Ten healthy volunteers with a smoking habit were studied using technetium-99m-labelled ethylcysteinate dimer single-photon emission tomography (SPET). We evaluated the regional cerebral blood flow under the smoking and resting states. The regional cerebral blood flow on smoking-activated SPET was significantly decreased in the whole brain as compared with that on resting SPET. Our findings therefore suggest that one of the acute effects of cigarette smoking is to induce a diffuse decrease in cerebral blood flow.

Nicotinic receptors mediate increased GABA release in brain through a tetrodotoxin-insensitive mechanism during prolonged exposure to nicotine

The effects of nicotine on the spontaneous release of GABA from nerve terminals in the chick lateral spiriform nucleus were examined using whole cell patch-clamp recording in brain slices. Exposure to 1 microM nicotine produced an early immediate increase in the frequency of spontaneous postsynaptic GABAergic currents. This effect was blocked in the presence of 0.5 microM tetrodotoxin. However, a prolonged application of 0.1-1 microM nicotine (>3 min) caused a tetrodotoxin-insensitive increase in the frequency of spontaneous GABAergic currents. This late tetrodotoxin-insensitive effect was blocked by the nicotinic antagonists dihydro-beta-erythroidine (30 microM) and mecamylamine (10 microM), but not by methyllycaconitine (50-100 nM), indicating that activation of high affinity nicotine receptors was mainly responsible for this effect. This enhancement was inhibited by the high threshold Ca(2+) channel blocker Cd(2+) (100 microM), but not by dantrolene or ryanodine. The tetrodotoxin-insensitive enhancement of the frequency of GABA currents by nicotine was reduced by inhibition of cAMP-dependent protein kinase with HA1004 (30 microM), but not by inhibition of protein kinase C with staurosporine (1 microM), and was facilitated by forskolin (10 microM) or bromo-cAMP (50 microM). The results indicate that nicotine-enhanced GABA release can operate through both tetrodotoxin-sensitive and -insensitive mechanisms in a single brain region and that a second messenger cascade may be involved in the tetrodotoxin-insensitive enhancement by nicotine.

Cholinergic modulation of purinergic and GABAergic co-transmission at in vitro hypothalamic synapses

The lateral hypothalamus (LH) is an important center for the integration of autonomic and limbic information and is implicated in the modulation of visceral motor and sensory pathways, including those underlying feeding and arousal behaviors. LH neurons in vitro release both ATP and GABA. The control of ATP and GABA co-transmission in LH may underlie the participation of LH in basic aspects of arousal and reinforcement. LH neurons receive cholinergic input from the pedunculopontine and laterodorsal tegmental nuclei as well as from cholinergic interneurons within the LH per se. This study presents evidence for nicotinic acetylcholine receptor (nAChR)-mediated enhancement of GABAergic, but not of purinergic, transmission despite the co-transmission of ATP and GABA at LH synapses in vitro. Facilitation of GABAergic transmission by nicotine is inhibited by antagonists of (alphabeta)*-containing nAChRs, but is unaffected by an alpha7-selective antagonist, consistent with a nAChR-mediated enhancement of GABA release mediated by non-alpha7-containing nAChRs. Activation of muscarinic ACh receptors enhances the release of ATP while concomitantly depressing GABAergic transmission. The independent modulation of ATP/GABAergic transmission may provide a new level of synaptic flexibility in which individual neurons utilize more than one neurotransmitter but retain independent control over their synaptic activity.

Nicotinic receptors regulate the release of glycine onto lamina X neurones of the rat spinal cord

Whole-cell patch clamp recordings were performed on neurones in the lamina X of rat spinal cord slices in order to characterize glycinergic synaptic currents and their modulation by nicotinic acetylcholine receptors. In the presence of TTX, bicuculline and kynurenic acid, glycine-induced currents and miniature glycinergic postsynaptic currents (mIPSCs) were recorded. These currents reversed near the chloride ion equilibrium potential and were blocked by strychnine (1 microM). A selective nicotinic acetylcholine receptor (nAChR) agonist 1,1-dimethyl-4-phenyl-piperazinium (DMPP), increased the frequency of glycinergic mIPSCs without altering significantly their amplitude distributions or their kinetic properties. The effects of DMPP were mimicked by different nAChRs agonists with the following apparent order of potency: ACh > DMPP > nicotine > cytisine. The effect of DMPP on mIPSCs was blocked by both d-tubocurarine and hexamethonium, and was reduced by dihydro-beta-erythroidine and methyllycaconitine (MLA), antagonists of non alpha7- and alpha7-containing nAChRs, respectively. In the absence of TTX, strychnine-sensitive glycinergic electrically evoked postsynaptic currents (eIPSCs) could be recorded. DMPP blocked the appearance of electrically evoked IPSCs while still inducing the appearance of spontaneous glycine IPSCs. These data demonstrate that neurones surrounding the central canal of the spinal cord present a glycinergic synaptic transmission which is modulated by terminal nAChRs.

Mutations of the neuronal nicotinic acetylcholine receptors and their association with ADNFLE

Elucidating the origin of epileptic seizures represents one of the many ways by which today's scientists are approaching this devastating neurological disorder. Although epilepsies have several different origins ranging from head trauma to genetically transmissible affections, common neuronal network dysfunction can be recognised between these many forms of the disease. Thus, understanding the basic mechanisms underlying some genetically transmissible epilepsies should bring new and important knowledge that is readily applicable to other types of epilepsies. In this work we review our current knowledge of one genetically transmissible form of nocturnal epilepsy, the ADNFLE. In the light of the most recent findings obtained on five mutants of the neuronal nicotinic acetylcholine receptors associated with ADNFLE, we discuss the effects of these spontaneous genome alterations on the receptor function. The only common trait identified so far between these mutant receptors is an increase in acetylcholine sensitivity. Based on our understanding of the receptor distribution in the different brain areas, their development and the neuronal network circuitry, we hypothesise that increased acetylcholine sensitivity causes an unbalance in the fine tuning of the cortico-reticular thalamic and thalamo-cortical loops. In addition, we illustrate how spontaneous mutations in the gene coding for a receptor provoke a change in its pharmacological profile and thereby might account for the inter-individual therapeutic sensitivity.

Frequent occurrence of nicotinic acetylcholine receptors in GABAergic neurons of the chick visual system

Double-labeling immunohistochemical methods were used to investigate the occurrence of the alpha8 and alpha5 nicotinic receptor subunits in presumptive GABAergic neurons of the chick nervous system. Nicotinic receptor immunoreactivity was often found in cells exhibiting GABA-like immunoreactivity, especially in the visual system. The alpha8 subunit appeared to be present in presumptive GABAergic cells of the ventral lateral geniculate nucleus, nucleus of the basal optic root of the accessory optic system, and the optic tectum, among several other structures. The alpha5 subunit was also found in GABA-positive neurons, as observed in the lentiform nucleus of the mesencephalon and other pretectal nuclei. The numbers of alpha8- and alpha5-positive neurons that were also GABA-positive represented high percentages of the total number of neurons containing nicotinic receptor labeling in several brain areas, which indicates that most of the alpha8 and alpha5 nicotinic receptor subunits are present in GABAergic cells. Taken together with data from other studies, our results indicate an important role of the nicotinic acetylcholine receptors in the functional organization of GABAergic circuits in the visual system.

The therapeutic potential of nicotinic acetylcholine receptor agonists for pain control

Due to the limitations of currently available analgesics, a number of novel alternatives are currently under investigation, including neuronal nicotinic acetylcholine receptor (nAChR) agonists. During the 1990s, the discovery of the antinociceptive properties of the potent nAChR agonist epibatidine in rodents sparked interest in the analgesic potential of this class of compounds. Although epibatidine also has several mechanism-related toxicities, the identification of considerable nAChR diversity suggested that the toxicities and therapeutic actions of the compound might be mediated by distinct receptor subtypes. Consistent with this view, a number of novel nAChR agonists with antinociceptive activity and improved safety profiles in preclinical models have now been identified, including A-85380, ABT-594, DBO-83, SIB-1663 and RJR-2403. Of these, ABT-594 is the most advanced and is currently in Phase II clinical evaluation. Nicotinically-mediated antinociception has been demonstrated in a variety of rodent pain models and is likely mediated by the activation of descending inhibitory pathways originating in the brainstem with the predominant high-affinity nicotine site in brain, the alpha4beta2 subtype, playing a critical role. Thus, preclinical findings suggest that nAChR agonists have the potential to be highly efficacious treatments in a variety of pain states. However, clinical proof-of-principle studies will be required to determine if nAChR agonists are active in pathological pain.

Distinct muscarinic receptors enhance spontaneous GABA release and inhibit electrically evoked GABAergic synaptic transmission in the chick lateral spiriform nucleus

The effects of muscarinic agonists on GABAergic synaptic transmission were examined using whole-cell patch-clamp recording in chick brain slices containing the lateral spiriform nucleus. Bath application of muscarine (10 microM) both increased the frequency of spontaneous GABAergic postsynaptic currents and reduced the amplitude of evoked GABAergic polysynaptic postsynaptic currents elicited by focal afferent fiber electrical stimulation. Both of these muscarinic actions were reversible and dose-dependent. Two M(1) antagonists, telenzepine and pirenzipine, and to a lesser extent the M(2) antagonist methoctramine, protected against muscarine's inhibition of the evoked polysynaptic currents. Other M(2) antagonists (tripitramine and gallamine) as well as the M(3) antagonist 4-DAMP mustard (4-diphenylacetoxy-N-(2-chloroethyl)-piperidine hydrochloride) and an M(4) antagonist (tropicamide) provided little or no protection against muscarine in this assay. In contrast, 4-diphenylacetoxy-N-(2-chloroethyl)-piperidine hydrochloride, tropicamide and telenzepine, but not pirenzepine, methoctramine, tripitramine and gallamine, blocked muscarine's enhancement of spontaneous GABAergic currents. McN-A-343 [(4-hydroxy-2-butynyl)-1-trimethylammonium-m-chlorocarbanilate chloride] and CDD-0097 (5-propargyloxycarbonyl-1,4,5,6-tetrahydropyrimidine hydrochloride), two M(1) agonists, mimicked muscarine's inhibition of the evoked polysynaptic GABAergic currents but did not mimic muscarine's enhancement of spontaneous GABAergic currents. Both actions of muscarine persisted when slices were pretreated with pertussis toxin or N-ethylmaleimide, which inactivate G-proteins coupled to M(2) and M(4) receptors while leaving G-proteins coupled to M(1), M(3) and M(5) receptors intact. Muscarine had no significant effect on the amplitude of the direct postsynaptic current elicited by exogenous GABA in the presence of tetrodotoxin. The results demonstrate that distinct muscarinic receptors oppositely modulate GABAergic transmission in the lateral spiriform nucleus. The receptor mediating the inhibition of evoked GABAergic polysynaptic currents is pharmacologically similar to an M(1) receptor, while the enhancement of spontaneous GABAergic currents appears to be mediated by an M(3) receptor.

Interactive effects of nicotine and alcohol co-administration on expression of inducible transcription factors in mouse brain

Nicotine and alcohol are abused substances that are often used concurrently. Despite their combined usage, little is known about how they interact to produce changes in behavior and neural activity. Two experiments were conducted to identify interactions on both behavior and neural targets resulting from the co-administration of nicotine and alcohol. In Experiment 1, male C57BL/6J mice were administered saline, alcohol (2.4 g/kg, i.p.), nicotine (0.5 mg/kg, i.p.) or an alcohol/nicotine mixture and returned to their home cage. In Experiment 2, a higher dose of nicotine (1.0 mg/kg, i.p.) was included and animals were exposed to a novel environment. Several behavioral measures were analysed during novelty exposure. Immunohistochemical detection of inducible transcription factors (c-Fos and Egr1) was used in both experiments to identify changes in neural activation. Behavioral results suggested that the drugs were interacting in the production of behaviors. In particular, alcohol produced locomotor stimulation while it suppressed counts of rearing and leaning. When co-administered, nicotine appeared to counteract the alcohol-enhanced locomotor activity. Several brain regions were observed to have altered transcription factor expression in response to the different drug treatments, including amygdalar, hippocampal and cortical subregions. In a subset of these brain areas, nicotine and alcohol counteracted one another in the expression of transcription factors. These results identify several interactive target sites within the hippocampus, extended amygdala and cortical regions. The interactions appear to be a result of antagonizing actions of nicotine and alcohol. Finally, the results suggest that the combined use of nicotine and alcohol may offset the effects of the drug administered independently.

Differences in nicotine-induced dopamine release and nicotine pharmacokinetics between Lewis and Fischer 344 rats

Studies have shown a greater preference for the self-administration of drugs such as nicotine and cocaine in the Lewis rat strain than in the Fischer 344 strain. We examined some factors that could contribute to such a difference. The baseline level of extracellular dopamine in nucleus accumbens shell was about 3-times higher in Fischer rats than in Lewis rats (3.18 +/- 0.26 vs. 1.09 +/- 0.14 pg/ sample). Nicotine (50-100 microg/kg)-induced release of dopamine, expressed in absolute terms, was similar in the two strains. Dopamine release expressed in relative terms (as percent of baseline), however, was significantly greater in Lewis rats than in Fischer rats at 30 min after the first nicotine injection. We suggest that the relative increase is of more influence than the absolute level for determining preference; a lower physiological extracellular dopamine level thus represent a risk factor for increased preference. Amphetamine-induced dopamine release expressed in relative terms was not greater in the Lewis strain. In the initial time period of the microdialysis experiments, a sharper peak in nicotine-induced accumbal dopamine release in Lewis and a less but more sustained release in Fischer rats was observed. This release pattern paralleled the faster clearance of nicotine from blood of Lewis compared to Fischer rats. In tissue slices the electrically induced dopamine release was highest in the nucleus accumbens and lowest in the ventral tegmentum. A significant effect of nicotine was lowering the electrically induced release of dopamine in frontal cortex slices from Fischer brain and increasing this dopamine release in the ventral tegmentum of Lewis brain slices indicating that the ventral tegmentum, an area controlling dopamine release in the accumbens, is more responsive to nicotine in the Lewis rat. Nicotine levels tended to be more sustained in Fischer rats in different brain regions, although the difference in nicotine levels between the strains was not significant at any time period. Several factors contribute to nicotine preference, including the endogenous dopamine level, and the sensitivity of ventral tegmentum neurons to nicotine-induced dopamine release. Strain differences in pharmacokinetics of nicotine may also play a role.

Vesicular acetylcholine transporter in the rat nucleus accumbens shell: subcellular distribution and association with mu-opioid receptors

Cholinergic interneurons in the nucleus accumbens shell (AcbSh) are implicated in the reinforcing behaviors that develop in response to opiates active at mu-opioid receptors (MOR). We examined the electron microscopic immunocytochemical localization of the vesicular acetylcholine transporter (VAChT) and MOR to determine the functional sites for storage and release of acetylcholine (ACh), and potential interactions involving MOR in this region of rat brain. VAChT was primarily localized to membranes of small synaptic vesicles in axon terminals. Less than 10% of the VAChT-labeled terminals were MOR-immunoreactive. In contrast, 35% of the cholinergic terminals formed symmetric or punctate synapses with dendrites showing an extrasynaptic plasmalemmal distribution of MOR. Membranes of tubulovesicles in other selective dendrites were also VAChT-labeled, and almost half of these dendrites displayed plasmalemmal MOR immunoreactivity. The VAChT-labeled dendritic tubulovesicles often apposed unlabeled axon terminals that formed symmetric synapses. Our results indicate that in the AcbSh MOR agonists can modulate the release of ACh from vesicular storage sites in axon terminals as well as in dendrites where the released ACh may serve an autoregulatory function involving inhibitory afferents. These results also suggest, however, that many of the dendrites of spiny projection neurons in the AcbSh are dually influenced by ACh and opiates active at MOR, thus providing a cellular substrate for ACh in the reinforcement of opiates.

Differential localization of acetylcholinesterase in relation to pre- and postsynaptic nicotinic receptors in the chick brain

Immunofluorescence and confocal microscopy were combined to study the distribution of acetylcholinesterase in relation to the localization of the beta2 subunit of the nicotinic acetylcholine receptors in the chick brain. In several areas where the beta2 subunit is recognizably part of presynaptic receptors, the localization of acetylcholinesterase appeared not to overlap the localization of beta2. On the other hand, acetylcholinesterase and the beta2 subunit exhibited a strictly matching localization in areas where postsynaptic nicotinic receptors are known to be present. These data may represent a morphological substrate for possible differential actions of acetylcholinesterase at presynaptic and postsynaptic nicotinic sites.

Genetic clues to the molecular basis of tobacco addiction and progress towards personalized therapy

The molecular processes that underlie addiction are beginning to unfold. Genetically determined variations in dopaminergic neurotransmission predispose to nicotine dependence. In addition, tobacco use is likely to be governed by the rate at which smokers metabolize nicotine. Functional polymorphisms in CYTOCHROME P450 monooxygenases that metabolize nicotine have now been defined and it should soon be possible to identify fast nicotine metabolizers by DNA analysis. Here, we review the key neurotransmitter receptors and metabolic enzymes implicated in tobacco dependence. We explore the potential benefits of classifying smokers according to the molecular aetiology of their habit. One major benefit will be in planning effective strategies for smoking cessation. Methods of typing for alleles related to smoking behavior that might be suitable for use in clinical practice in the future will also be discussed

Modulation of fast synaptic transmission by presynaptic ligand-gated cation channels

There is now considerable evidence demonstrating that ligand-gated cation channels (i.e., P2X, nicotinic, kainate, NMDA, AMPA and 5-HT(3) receptors), in addition to mediating fast excitatory neurotransmission, may be located presynaptically on nerve terminals in the peripheral and central nervous systems where they function to modulate neurotransmitter release. This modulation can be facilitation, inhibition or both. In this article, we first outline the multiple mechanisms by which activation of presynaptic ligand-gated cation channels can modulate spontaneous and evoked neurotransmitter release, before reviewing in detail published electrophysiological studies of presynaptic P2X, nicotinic, kainate, NMDA, AMPA and 5-HT(3) receptors.