Chronic nicotine exposure during adolescence differentially influences calcium-binding proteins in rat anterior cingulate cortex

E-cigarettes may serve as a gateway to conventional cigarettes and other addictive drugs

Electronic cigarettes (e-cigarettes) are devices that allow the user to inhale nicotine in a vapor, and are primarily marketed as a means of quitting smoking and a less harmful replacement for traditional cigarette smoking. However, further research is needed to determine if vaping nicotine via e-cigarettes can be effective. Conversely, nicotine has been considered a gateway drug to alcohol and other addictive drugs and e-cigarettes containing nicotine may have the same effects. Previous reports have shown that e-cigarette use may open the gate for the use of other drugs including conventional cigarettes, cannabis, opioids, etc. The increasing prevalence of e-cigarettes, particularly among youth and adolescents in the last decade have led to an increase in the dual use of e-cigarettes with alcohol, cannabis, and other illicit drug use like heroin and 3-4-methylenedioxymethamphetamine (MDMA). The advent of e-cigarettes as a device to self-administer addictive agents such as cocaine and synthetic cathinones may bring about additional adverse health effects associated with their concurrent use. This review aims to briefly describe e-cigarettes and their different generations, and their co-use with other addictive drugs as well as the use of the device as a tool to self-administer addictive drugs, such as cocaine, etc.

Abnormal white matter tracts of insula in smokers

Nicotine addiction is characterized as a neural circuit dysfunction, particularly with regard to the alterations in central reward pathways. The insula, a cortical region that is thought to play a central role in this reward circuitry, has been implicated in the maintenance of nicotine addiction. However, it remains largely unclear about the white matter (WM) microstructural alterations of insula in nicotine addiction and whether the WM alterations of insula could predict smoking cessation outcomes. In this study, 58 male nicotine-dependent smokers and 34 matched male nonsmoking controls were recruited. After a 12-week smoking cessation treatment with varenicline, 38 smokers relapsed, and 20 did not relapse. Diffusion tensor imaging and probabilistic tractography were used to investigate the differences of WM tracts of insula between smokers and nonsmokers. Relative to nonsmokers, in the left hemisphere, smokers showed lower fractional anisotropy (FA) in the fiber tracts of anterior insula cortex-to-nucleus accumbens and posterior insula cortex-to-nucleus accumbens; in the right hemisphere, smokers showed higher FA, and lower axial diffusivity (AD), radial diffusivity (RD), and mean diffusivity (MD) in the fiber tracts of anterior insula cortex-to-medial orbitofrontal cortex, posterior insula cortex-to-medial orbitofrontal cortex, and posterior insula cortex-to-nucleus accumbens. However, there were no differences of WM diffusion properties between relapsers and nonrelapsers. This study is the first using probabilistic tractography to exclusively clarify the precise roles of insular WM tracts in smokers, which may provide new insights into the underlying neurobiology of nicotine addiction.

The Influence of Cannabis and Nicotine Co-use on Neuromaturation: A Systematic Review of Adolescent and Young Adult Studies

Accumulating evidence suggests that the use of cannabis and nicotine and tobacco-related products (NTPs) during the adolescent years has harmful effects on the developing brain. Yet, few studies have focused on the developing brain as it relates to the co-administration of cannabis and NTPs, despite the high prevalence rates of co-use in adolescence. This review aims to synthesize the existing literature on neurocognitive, structural neuroimaging, and functional neuroimaging outcomes associated with cannabis and NTP co-use. A systematic search of peer-reviewed articles resulted in a pool of 1107 articles. Inclusion criteria were 1) data-based study; 2) age range of 13 to 35 years or, for preclinical studies, nonadult subjects; 3) cannabis and NTP group jointly considered; and 4) neurocognitive, structural neuroimaging, or functional neuroimaging as an outcome measure. Twelve studies met inclusion criteria. Consistent with the literature, cannabis and nicotine were found to have independent effects on cognition. The available research on the co-use of cannabis and NTPs demonstrates a potential nicotine-related masking effect on cognitive deficits associated with cannabis use, yet there is little research on co-use and associations with neuroimaging indices. In neuroimaging studies, there is preliminary evidence for hippocampal volume differences in co-users and a lack of evidence for co-use differences related to nucleus accumbens activity during reward processing. Notably, no structural neuroimaging studies were found to examine the combined effects of nicotine and cannabis in adolescent-only populations. Further research, including longitudinal studies, is warranted to investigate the influence of cannabis and NTP co-use on maturation.

The effects of nicotine and cannabis co-use during adolescence and young adulthood on white matter cerebral blood flow estimates

RATIONALE

Co-use of cannabis and nicotine is common among adolescents/young adults and is associated with poorer psychological and physical outcomes, compared with single substance use. Little is known about the impact of co-use on the developing brain.

OBJECTIVES

Preliminary investigation of the effects of nicotine on white matter (WM) cerebral blood flow (CBF) in adolescents/young adults and its potential moderation by cannabis use.

METHODS

Adolescent/young adult (16-22 years old) nicotine and tobacco product users (NTP; N = 37) and non-nicotine users (non-NTP; N = 26) underwent a neuroimaging session comprised of anatomical, optimized pseudo-continuous arterial spin labeling, and diffusion tensor imaging scans. Groups were compared on whole-brain WM CBF estimates and their relation to past-year cannabis use. Follow-up analyses assessed correlations between identified CBF clusters and corresponding fractional anisotropy (FA) values.

RESULTS

Group by cannabis effects were observed in five clusters (voxel-wise alpha < 0.001, cluster-wise alpha < 0.05; ≥ 11 contiguous voxels): non-NTP exhibited positive correlations between CBF and cannabis use in all clusters, whereas no significant relationships were observed for NTP. Greater CBF extracted from one cluster (including portions of right superior longitudinal fasciculus) was associated with reduced FA for non-NTP group only.

CONCLUSIONS

This is the first investigation of WM health as indexed by CBF, and its association with FA, in adolescents/young adults with nicotine and/or cannabis use. Results suggest that cannabis use by itself may be related to increased CBF in WM fiber tracts demonstrating poorer structural intergrity, yet the occurrence of even infrequent NTP use (greater than once per month) appears to diminish this relationship.

Increased thalamic volume and decreased thalamo-precuneus functional connectivity are associated with smoking relapse

The thalamus, with the highest density of nicotinic acetylcholine receptor (nAChR) in the brain, plays a central role in thalamo-cortical circuits that are implicated in nicotine addiction. However, little is known about whether the thalamo-cortical circuits are potentially predictive of smoking relapse. In the current study, a total of 125 participants (84 treatment-seeking male smokers and 41 age-matched male nonsmokers) were recruited. Structural and functional magnetic resonance images (MRI) were acquired from all participants. After a 12-week smoking cessation treatment with varenicline, the smokers were then divided into relapsers (n = 54) and nonrelapsers (n = 30). Then, we compared thalamic volume and seed-based thalamo-cortical resting state functional connectivity (rsFC) prior to the cessation treatment among relapsers, nonrelapsers and nonsmokers to investigate the associations between thalamic structure/function and smoking relapse. Increased thalamic volume was detected in smokers relative to nonsmokers, and in relapsers relative to nonrelapsers, especially on the left side. Moreover, decreased left thalamo-precuneus rsFC was detected in relapsers relative to nonrelapsers. Additionally, a logistic regression analysis showed that the thalamic volume and thalamo-precuneus rsFC predicted smoking relapse with an accuracy of 75.7%. These novel findings indicate that increased thalamic volume and decreased thalamo-precuneus rsFC are associated with smoking relapse, and these thalamic measures may be used to predict treatment efficacy of nicotine addiction and serve as a potential biomarker for personalized medicine.

Higher nicotinic receptor availability in the cingulo-insular network is associated with lower cardiac parasympathetic tone

The dorsal anterior cingulate cortex (dACC) and the anterior insula (AI) constitute the salience network and form as well the major cortical components of the central autonomic nervous system. These two cortical regions have the highest density in α4β2 nicotinic acetylcholine receptors (nAChRs) within the whole cortex.The aim of the study was to test the association between nAChRs density/availability in the salience network and the heart rate variability in humans. We selected subjects from a previous positron emission tomography (PET) imaging study in epilepsy with 18F-FA-85380, a specific marker for α4β2 nAChRs, including 10 healthy controls, 10 patients with nonlesional focal epilepsy and 8 patients with idiopathic generalized epilepsy. Participants underwent a 10 min-resting electrocardiogram as they were lying still in a semi-supine position while watching an emotionally neutral video. We tested the association between parasympathetic tone and the regional brain nAChR availability, as measured by 18F-F-A-85380 binding potential (BP), using linear regression. We observed an association between higher nAChRs availability in the bilateral dACC and the right dorsal AI/frontal operculum and a lower parasympathetic tone, without significant effect of the clinical group on this relation. Our study is the first one to show a neurochemical correlate to the parasympathetic role of the anterior cingulate cortex and the AI. The nicotinic system, which plays a major role in the peripheral autonomic nervous system intervening both in the parasympathetic and sympathetic chains, seems also to play a role in the central autonomic nervous system.

Nicotinic receptor abnormalities as a biomarker in idiopathic generalized epilepsy

PURPOSE

Mutations of cholinergic neuronal nicotinic receptors have been identified in the autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE), associated with changes on PET images using [¹⁸F]-F-85380-A (F-A-85380), an α4β2 nicotinic receptor ligand. The aim of the present study was to evaluate potential changes in nicotinic receptor availability in other types of epilepsy.

METHODS

We included 34 male participants, 12 patients with idiopathic generalized epilepsy (IGE), 10 with non-lesional diurnal focal epilepsy, and 12 age-matched healthy controls. All patients underwent PET/CT using F-A-85380 and [¹⁸F]-fluorodeoxyglucose (FDG), 3D T1 MRI and diffusion tensor imaging (DTI). F-A-85380 and FDG images were compared with the control group using a voxel-wise (SPM12) and a volumes of interest (VOI) analysis.

RESULTS

In the group of patients with IGE, the voxel-wise and VOI analyses showed a significant increase of F-A-85380 ratio index of binding potential (BP_RI, corresponding to the receptor availability) in the anterior cingulate cortex (ACC), without structural changes on MRI. At an individual level, F-A-85380 BP_RI increase in the ACC could distinguish IGE patients from controls and from patients with focal epilepsy with good accuracy.

CONCLUSIONS

We observed focal changes of density/availability of nicotinic receptors in IGE, namely an increase in the ACC. These data suggest that the modulation of α4β2 nicotinic receptors plays a role not only in ADNFLE, but also in other genetic epileptic syndromes such as IGE and could serve as a biomarker of epilepsy syndromes with a genetic background.

Suppressive and enhancing effects of nicotine on food-seeking behavior

The present study examined how systemic low doses of nicotine affect the microstructure of reinforced food-seeking behavior in rats. Rats were first given an acute saline or nicotine treatment (0.1-0.6mg/kg, with an inter-injection interval of at least 48h), and then a chronic saline or nicotine treatment (0.3mg/kg/day for 10 consecutive days). Immediately after each injection, rats were required to press a lever five times to obtain food that was available at unpredictable times (on average every 80s) with constant probability. Acute nicotine dose-dependently suppressed behavior prior to the delivery of the first reinforcer, but enhanced food-reinforced behavior afterwards. These effects were primarily observed in the time it took rats to initiate food-seeking behavior. Enhancing effects were also observed in the microstructure of food-seeking behavior, with lower nicotine doses (0.1-0.3mg/kg) increasing the rate at which response bouts were initiated, and higher doses (0.3-0.6mg/kg) increasing within-bout response rates. A pre-feeding control suggests that changes in appetite alone cannot explain these effects. Over the course of chronic nicotine exposure, tolerance developed to the suppressive, but not to the enhancing effects of nicotine on food-seeking behavior. These results suggest that (a) lower doses of nicotine enhance the reward value of food and/or food-associated stimuli, (b) higher doses of nicotine enhance motoric activity, and (c) ostensive sensitization effects of nicotine on behavior partially reflect a tolerance to its transient suppressive motoric effects.

A Novel Multisensory Integration Task Reveals Robust Deficits in Rodent Models of Schizophrenia: Converging Evidence for Remediation via Nicotinic Receptor Stimulation of Inhibitory Transmission in the Prefrontal Cortex

Atypical multisensory integration is an understudied cognitive symptom in schizophrenia. Procedures to evaluate multisensory integration in rodent models are lacking. We developed a novel multisensory object oddity (MSO) task to assess multisensory integration in ketamine-treated rats, a well established model of schizophrenia. Ketamine-treated rats displayed a selective MSO task impairment with tactile-visual and olfactory-visual sensory combinations, whereas basic unisensory perception was unaffected. Orbitofrontal cortex (OFC) administration of nicotine or ABT-418, an α₄β₂ nicotinic acetylcholine receptor (nAChR) agonist, normalized MSO task performance in ketamine-treated rats and this effect was blocked by GABA_A receptor antagonism. GABAergic currents were also decreased in OFC of ketamine-treated rats and were normalized by activation of α₄β₂ nAChRs. Furthermore, parvalbumin (PV) immunoreactivity was decreased in the OFC of ketamine-treated rats. Accordingly, silencing of PV interneurons in OFC of PV-Cre mice using DREADDs (Designer Receptors Exclusively Activated by Designer Drugs) selectively impaired MSO task performance and this was reversed by ABT-418. Likewise, clozapine-N-oxide-induced inhibition of PV interneurons in brain slices was reversed by activation of α₄β₂ nAChRs. These findings strongly imply a role for prefrontal GABAergic transmission in the integration of multisensory object features, a cognitive process with relevance to schizophrenia. Accordingly, nAChR agonism, which improves various facets of cognition in schizophrenia, reversed the severe MSO task impairment in this study and appears to do so via a GABAergic mechanism. Interactions between GABAergic and nAChR receptor systems warrant further investigation for potential therapeutic applications. The novel behavioral procedure introduced in the current study is acutely sensitive to schizophrenia-relevant cognitive impairment and should prove highly valuable for such research.

SIGNIFICANCE STATEMENT

Adaptive behaviors are driven by integration of information from different sensory modalities. Multisensory integration is disrupted in patients with schizophrenia, but little is known about the neural basis of this cognitive symptom. Development and validation of multisensory integration tasks for animal models is essential given the strong link between functional outcome and cognitive impairment in schizophrenia. We present a novel multisensory object oddity procedure that detects selective multisensory integration deficits in a rat model of schizophrenia using various combinations of sensory modalities. Moreover, converging data are consistent with a nicotinic-GABAergic mechanism of multisensory integration in the prefrontal cortex, results with strong clinical relevance to the study of cognitive impairment and treatment in schizophrenia.

White matter integrity in young smokers: a tract-based spatial statistics study

Previous diffusion tensor imaging (DTI) studies revealed contradictory effects of smoking on fractional anisotropy (FA). Multiple DTI-derived indices may help to deduce the pathophysiological type of white matter (WM) changes and provide more specific biomarkers of WM neuropathology in the whole brain of young smokers. Twenty-three young smokers and 22 age-, education- and gender-matched healthy non-smoking controls participated in this study. Tract-based spatial statistics was employed to investigate the WM microstructure in young smokers by integrating multiple indices, including FA, mean diffusivity (MD), radial diffusivity (RD) and axial diffusivity (AD). Compared with healthy non-smoking controls, young smokers showed significantly increased FA with increased AD and decreased RD in several brain regions, while no difference in MD was observed. Specifically, the overlapped WM regions with increased FA, increased AD and decreased RD were found in the right posterior limb of the internal capsule, the right external capsule and the right superior corona radiata. Additionally, average FA and RD values in the WM regions mentioned earlier were significantly correlated with pack-years and Fagerström Test for Nicotine Dependence, while no correlation in AD was found. The WM tracts with increased FA may be more associated with RD, rather than AD in young smokers. We suggested that WM properties of several fibres in young smokers may be the biomarker as the cumulative effect and severity of nicotine dependence.

Altered human brain anatomy in chronic smokers: a review of magnetic resonance imaging studies

Cigarette smoking is becoming more prevalent in developing countries, such as China, and is the largest single cause of preventable death worldwide. New emerging reports are highlighting how chronic cigarette smoking plays a role in neural dysfunctions, such as cognitive decline. Basic animal experimental studies have shown that rats undergo persistent pathological brain changes after being given chronic levels of nicotine. What is perhaps less appreciated is the fact that chronic cigarette smoking induces subtle anatomical changes in the human brain. Consequently, this chapter aims to summarize and integrate the existing magnetic resonance imaging studies on both gray- and white-matter marcostructural and microstructural changes. The reviewed studies demonstrate that chronic cigarette smoking results in discrete and localized alterations in brain region tissue (both the gray and white matter of different brain regions), which may, in part, be responsible for different neural dysfunctions. In addition, we further discuss the possible pathological and neurobiological mechanisms of these nicotinic effects on the brain tissue. We will also address the limitations of the current studies on this issue and identify opportunities for future research.

Smoking and the developing brain: altered white matter microstructure in attention-deficit/hyperactivity disorder and healthy controls

Brain white matter (WM) tracts, playing a vital role in the communication between brain regions, undergo important maturational changes during adolescence and young adulthood, a critical period for the development of nicotine dependence. Attention-deficit/hyperactivity disorder (ADHD) is associated with increased smoking and widespread WM abnormalities, suggesting that the developing ADHD brain might be especially vulnerable to effects of smoking. This study aims to investigate the effect of smoking on (WM) microstructure in adolescents and young adults with and without ADHD. Diffusion tensor imaging was performed in an extensively phenotyped sample of nonsmokers (n = 95, 50.5% ADHD), irregular smokers (n = 41, 58.5% ADHD), and regular smokers (n = 50, 82.5% ADHD), aged 14-24 years. A whole-brain voxelwise approach investigated associations of smoking, ADHD and their interaction, with WM microstructure as measured by fractional anisotropy (FA) and mean diffusivity (MD). Widespread alterations in FA and MD were found for regular smokers compared to irregular and nonsmokers, mainly located in the corpus callosum and WM tracts surrounding the basal ganglia. Several regions overlapped with regions of altered FA for ADHD versus controls, albeit in different directions. Irregular and nonsmokers did not differ, and ADHD and smoking did not interact. Results implicate that smoking and ADHD have independent effects on WM microstructure, and possibly do not share underlying mechanisms. Two mechanisms may play a role in the current results. First, smoking may cause alterations in WM microstructure in the maturing brain. Second, pre-existing WM microstructure differences possibly reflect a risk factor for development of a smoking addiction.

New mechanisms and perspectives in nicotine withdrawal

Diseases associated with tobacco use constitute a major health problem worldwide. Upon cessation of tobacco use, an unpleasant withdrawal syndrome occurs in dependent individuals. Avoidance of the negative state produced by nicotine withdrawal represents a motivational component that promotes continued tobacco use and relapse after smoking cessation. With the modest success rate of currently available smoking cessation therapies, understanding mechanisms involved in the nicotine withdrawal syndrome are crucial for developing successful treatments. Animal models provide a useful tool for examining neuroadaptative mechanisms and factors influencing nicotine withdrawal, including sex, age, and genetic factors. Such research has also identified an important role for nicotinic receptor subtypes in different aspects of the nicotine withdrawal syndrome (e.g., physical vs. affective signs). In addition to nicotinic receptors, the opioid and endocannabinoid systems, various signal transduction pathways, neurotransmitters, and neuropeptides have been implicated in the nicotine withdrawal syndrome. Animal studies have informed human studies of genetic variants and potential targets for smoking cessation therapies. Overall, the available literature indicates that the nicotine withdrawal syndrome is complex, and involves a range of neurobiological mechanisms. As research in nicotine withdrawal progresses, new pharmacological options for smokers attempting to quit can be identified, and treatments with fewer side effects that are better tailored to the unique characteristics of patients may become available. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

Amiloride-sensitive sodium currents in fungiform taste cells of rats chronically exposed to nicotine

Many studies have demonstrated that chronic exposure to nicotine, one of the main components of tobacco smoke, has profound effects on the functionality of the mammalian taste system. However, the mechanisms underlying nicotine action are poorly understood. In particular no information is available on the chronic effect of nicotine on the functioning of taste cells, the peripheral detectors which transduce food chemicals into electrical signals to the brain. To address this issue, I studied the membrane properties of rat fungiform taste cells and evaluated the effect of long-term exposure to nicotine on the amiloride-sensitive sodium currents (ASSCs). These currents are mediated by the epithelial sodium channels (ENaC) thought to be important, at least in part, in the transduction of salty stimuli. Patch-clamp recording data indicated that ASSCs in taste cells from rats chronically treated with nicotine had a reduced amplitude compared to controls. The pharmacological and biophysical analysis of ASSCs revealed that amplitude reduction was not dependent on changes in amiloride sensitivity or channel ionic permeability, but likely derived from a decrease in the activity of ENaCs. Since these channels are considered to be sodium receptors in taste cells, my results suggest that chronic exposure to nicotine hampers the capability of these cells to respond to sodium ions. This might represent a possible cellular mechanism underlying the reduced taste sensitivity to salt typically found in smokers.

Transient suppression of late-stage neuronal progenitor cell differentiation in the hippocampal dentate gyrus of rat offspring after maternal exposure to nicotine

To examine the developmental exposure effect of nicotine (NIC) on hippocampal neurogenesis, pregnant Sprague-Dawley rats were treated with (-)-NIC hydrogen tartrate salt through drinking water at 2, 10 or 50 ppm from gestational day 6 to day 21 after delivery. On postnatal day (PND) 21, immunohistochemically doublecortin (Dcx)(+) cells increased at ≥10 ppm in the dentate subgranular zone (SGZ) as examined in male offspring; however, dihydropyrimidinase-like 3 (TUC4)(+) cells decreased at 2 ppm, and T box brain 2 (Tbr2)(+) cells were unchanged at any dose. Double immunohistochemistry revealed decreases in TUC4(+)/Dcx(+) and TUC4(+)/Dcx(-) cells, an increase in TUC4(-)/Dcx(+) cells at 2 and 10 ppm and an increase in Tbr2(-)/Dcx(+) cells at 50 ppm, suggesting an increase in type-3 progenitor cells at ≥2 ppm and decrease in immature granule cells at 2 and 10 ppm. The number of mature neuron-specific NeuN(-) progenitor cells expressing nicotinic acetylcholine receptor α7 in the SGZ and mRNA levels of Chrna7 and Chrnb2 in the dentate gyrus was unchanged at any dose, suggesting a lack of direct nicotinic stimulation on progenitor cells. In the dentate hilus, glutamic acid decarboxylase 67(+) interneurons increased at ≥10 ppm. All changes disappeared on PND 77. Therefore, maternal exposure to NIC reversibly affects hippocampal neurogenesis targeting late-stage differentiation in rat offspring. An increase in interneurons suggested that their activation affected granule cell differentiation. The lowest observed adverse effect level was at 2 ppm (0.091 mg/kg/day as a free base) by the affection of hippocampal neurogenesis at ≥2 ppm.

Cigarette smoking and white matter microstructure

RATIONALE

Diffusion tensor imaging has been used before in testing associations between cigarette smoking and white matter integrity, with inconsistent results. Published reports indicate higher fractional anisotropy (FA, a measure of linear water diffusion) in some brain regions and lower FA in others in adult smokers compared to nonsmokers. Adolescent smokers exhibited elevated FA at several brain regions and a positive correlation of FA in the genu corpus callosum with exposure to smoking (pack-years).

OBJECTIVE

To help resolve prior discrepancies, we studied adults, sampling multiple brain regions, and testing for relationships to clinical features of nicotine dependence and exposure to smoking.

METHODS

Brain MRI scans (1.5 T) were acquired, and FA and apparent diffusion coefficient (ADC, a measure of random diffusion) were assayed in corpus callosum and prefrontal white matter, corona radiata, internal capsule, cingulum bundle, and hippocampal perforant fibers in 18 smokers (33.7 ± 7.9 years of age) and 18 age- and gender-matched nonsmokers.

RESULTS

ADC showed no group difference, but smokers had higher (4.3-21.1%) FA than nonsmokers. The differences were significant in right prefrontal white matter, cingulum, and genu corpus callosum. FA in several regions was negatively correlated with nicotine dependence or cigarettes/day.

CONCLUSIONS

Combined with earlier findings, these results suggest a model of changing trajectories whereby FA is higher with tobacco exposure during adolescence and declines with continued smoking in adulthood. This notion is supported by the observation that, at multiple sampling sites, participants who had started smoking earlier in life had higher FA than those who had started later.

Bilateral fronto-parietal integrity in young chronic cigarette smokers: a diffusion tensor imaging study

BACKGROUND

Cigarette smoking continues to be the leading cause of preventable morbidity and mortality in China and other countries. Previous studies have demonstrated gray matter loss in chronic smokers. However, only a few studies assessed the changes of white matter integrity in this group. Based on those previous reports of alterations in white matter integrity in smokers, the aim of this study was to examine the alteration of white matter integrity in a large, well-matched sample of chronic smokers and non-smokers.

METHODOLOGY/PRINCIPAL FINDINGS

Using in vivo diffusion tensor imaging (DTI) to measure the differences of whole-brain white matter integrity between 44 chronic smoking subjects (mean age, 28.0±5.6 years) and 44 healthy age- and sex-matched comparison non-smoking volunteers (mean age, 26.3±5.8 years). DTI was performed on a 3-Tesla Siemens scanner (Allegra; Siemens Medical System). The data revealed that smokers had higher fractional anisotropy (FA) than healthy non-smokers in almost symmetrically bilateral fronto-parietal tracts consisting of a major white matter pathway, the superior longitudinal fasciculus (SLF).

CONCLUSION/SIGNIFICANCE

We found the almost symmetrically bilateral fronto-parietal whiter matter changes in a relatively large sample of chronic smokers. These findings support the hypothesis that chronic cigarette smoking involves alterations of bilateral fronto-parietal connectivity.

Poststroke subgranular and rostral subventricular zone proliferation in a mouse model of neonatal stroke

Stroke in the neonatal brain is an understudied cause of neurologic morbidity. Recently we have characterized a new immature mouse model of stroke utilizing unilateral carotid ligation alone to produce infarcts and acute seizures in postnatal day 12 (P12) CD-1 mice. In this study, the amount of poststroke neural progenitor proliferation was examined in the subgranular (SGZ) of the dentate gyrus and the subventricular zone (SVZ) 7, 14, and 21days after ischemia (DAI). A single IP injection (50 mg/kg) of bromodeoxyuridine (BrdU) given 2 hr before perfusion fixation labeled newborn cells. Early cell phenotypes were quantified by colabeling with GFAP, nestin, and DCX. Control mice revealed an age-dependent decrease in neural proliferation, with an approximately 50% drop in BrdU-labeled cell counts at P33 compared with P19 both in the SGZ and in the SVZ. Significant reduction in the amount of neural proliferation in the ipsilateral injured SGZ of ligated mice correlated with both the severity of the stroke-injury and the acute seizure scores. Similar correlations were not detected contralaterally. Contralateral SGZ neural proliferation was initially lowered at 7 DAI but normalized by 21 DAI. In both injured and control brains, approximately 90% of newborn SGZ cells colabeled with nestin, approximately 30% colabeled with GFAP, and a few colabeled with DCX. In contrast, poststroke SVZ cell proliferation was enhanced ipsi- more than contralaterally at 7 DAI. In the SVZ, the enhanced neural proliferation normalized to control levels by P33. In conclusion, the neural cell proliferation was differentially altered in the SGZ vs. SVZ after neonatal stroke.

Cellular events in nicotine addiction

In the 25 years since the observation that chronic exposure to nicotine could regulate the number and function of high affinity nicotine binding sites in the brain there has been a major effort to link alterations in nicotinic acetylcholine receptors (nAChRs) to nicotine-induced behaviors that drive the addiction to tobacco products. Here we review the proposed roles of various nAChR subtypes in the addiction process, with emphasis on how they are regulated by nicotine and the implications for understanding the cellular neurobiology of addiction to this drug.

Differential alterations in the relations among GABAergic, catecholaminergic and calcium binding protein expression in the olfactory bulb of amphetamine-administered mouse

This study explores cellular responses of distinct layers of the main olfactory bulb (OB) to amphetamine (Amph), by examining the expression of glutamic acid decarboxylase67 (GAD67), calcium binding proteins (CaBP) and tyrosine hydroxylase (TH). Immunocytochemical analysis was performed on OB sections prepared from adult mice at 0.5 h or 4 h after receiving one intraperitoneal injection or multiple (2 doses/day, 7 doses in total) injections of saline or Amph, 5 mg/kg. In the glomerular layer, though the expression of TH and GAD67 was unaltered by the single Amph injection, at 0.5 h post-repeated Amph exposure the levels of TH-immunopositive somata and processes/punctates, and GAD67-somata/punctates were increased by 48-147%, compared with respective saline controls. By contrast, at 4 h post-repeated Amph GAD67 levels were lower than saline, and TH similar to saline. For the repetitively saline-injected groups, TH and GAD67 levels were higher at 4h than 0.5 h, suggesting an injection-associated stress response. Double staining revealed that at 0.5h post-repeated Amph exposure, the percentage of TH-soma number that expressed GAD67 was raised to 46%, compared with 30% of the corresponding saline, and thus implies an activation of dopaminergic neurons to become GABAergic. In the external plexiform layer, the numbers of CaBP, parvalbumin or calretinin-somata were increased at 0.5 h/4 h or 4 h post-acute Amph injection; double staining disclosed that at 4 h post-acute Amph, 66% or 47% of GAD67-somata contained parvalbumin or calretinin, being greater than 43% or 28% of the saline. In the granule somata, Amph probably inhibits expression of GAD67 by decreasing phosphorylation of CREB (pCREB). The up-regulation of CaBPs, GAD67 and TH at 0.5/4 h post-acute or 0.5 h post-repeated Amph could implicate protective roles and synaptic plasticity against Amph, whereas decreases of GAD67 and pCREB at 4 h post-repeated Amph may indicate toxicity of Amph.

Benes F. N-methyl-D-aspartate receptor and calbindin-containing neurons in the anterior cingulate cortex in schizophrenia and bipolar disorder

BACKGROUND

Glutamatergic modulation of gamma-aminobutyric acid (GABA) interneurons via the NR2A subunit of the N-methyl-D-aspartate (NMDA) receptor in the cerebral cortex contributes to the pathophysiology of schizophrenia and bipolar disorder. Previously, we found that, in the anterior cingulate cortex (ACCx), the number of GABA cells that expressed the messenger RNA (mRNA) for the NMDA NR2A subunit was significantly decreased in subjects with schizophrenia and bipolar disorder and that this decrease occurred most prominently in layer 2. In this study, we hypothesized that the subset of GABA interneurons that contained the calcium-binding protein calbindin (CB), by virtue of their preferential localization to layer 2, might be particularly affected.

METHODS

We simultaneously labeled the mRNA for the NMDA NR2A subunit with [(35)S] and the mRNA for CB with digoxigenin with an immunoperoxidase procedure.

RESULTS

We found that, in the normal human ACCx, only approximately 10% of all CB-containing cells expressed NR2A mRNA. However, compared with the normal control subjects and subjects with bipolar disorder, the density of CB+/NR2A+ neurons in layer 2 was increased by 41% to 44 % in subjects with schizophrenia, whereas the amount of NR2A mRNA/CB+ neurons was unchanged.

CONCLUSIONS

These observations suggest that, in schizophrenia, a number of CB-containing cells that normally do not express NR2A might become NR2A-expressing or, perhaps not mutually exclusively, the number of CB-expressing cells might be increased and these cells express NR2A. The findings of this study highlight the notion that glutamatergic innervation of subsets of GABA cells might be differentially altered in schizophrenia and bipolar disorder.

Chronic cigarette smoking and the microstructural integrity of white matter in healthy adults: a diffusion tensor imaging study

Results from recent studies suggest that chronic cigarette smoking is associated with increased white matter volume in the brain as determined by in vivo neuroimaging. We used diffusion tensor imaging to examine the microstructural integrity of the white matter in 10 chronic smokers and 10 nonsmokers. All individuals were healthy, without histories of medical or psychiatric illness. Fractional anisotropy (FA) and trace were measured in the genu, body, and splenium of the corpus callosum. FA provides a measure of directional versus nondirectional water diffusion, whereas trace provides a measure of nondirectional water diffusion. Lower FA and higher trace values are considered to reflect less brain integrity. Voxel-based morphometry was used to define volumes in each of these regions of the corpus callosum. Chronic smokers exhibited significantly higher FA in the body and whole corpus callosum and a strong trend for higher FA in the splenium compared with nonsmokers. FA did not differ between groups in the genu, and neither trace nor white matter volumes differed between groups in any of the regions of interest. When subdivided by Fagerström score (low vs. high), the low Fagerström group exhibited significantly higher FA in the body of the corpus callosum compared with the high Fagerström group and the nonsmokers. These results suggest that, among healthy adults, lower exposure to cigarette smoking is associated with increased microstructural integrity of the white matter compared with either no exposure or higher exposure. Additional studies are needed to further explore differences in white matter integrity between smokers and nonsmokers.

Long-term effects of nicotine on rat fungiform taste buds

Nicotine, an alkaloid found in tobacco smoke, has been recognized as capable of inducing changes in taste functionality in conditions of chronic exposure. The mechanisms underlying these sensory alterations, however, are currently unknown. We addressed this issue by studying the long-term effects of nicotine on the anatomical features of taste buds, the peripheral end-organs of taste, in rat fungiform papillae. Nicotine was administered to rats via drinking water over a period of 3 weeks, which represents a standard method to achieve chronic drug exposure in laboratory animals. We found that prolonged administration of nicotine induced a significant reduction in the size of fungiform taste buds, without affecting their total number on the rat tongue. Morphometric measurements as well as evaluations of taste cell membrane capacitance suggested that the reduced size of taste organs was determined by a decrease in the number of cells per taste bud. In addition, chronic treatment with nicotine caused an increase in the relative density of cells expressing gustducin, a specific G protein alpha-subunit found in some taste cells and involved in bitter/sweet transduction. Interestingly, changes in the expression pattern of gustducin turned out to be more pronounced in periadolescent/adolescent than in adult rats. As a whole, our data indicate that long-term nicotine administration induces significant changes in the anatomical properties of taste buds in rat fungiform papillae. These changes could have a profound impact on the sensory information relayed to the brain; therefore, they may be responsible, at least in part, for the alterations in taste functionality observed during chronic nicotine exposure, a condition found in regular smokers.

Serotonin2C receptor localization in GABA neurons of the rat medial prefrontal cortex: implications for understanding the neurobiology of addiction

Serotonin (5-HT) action via the 5-HT(2C) receptor (5-HT(2C)R) provides an important modulatory influence over neurons of the prefrontal cortex (PFC), which is critically involved in disorders of executive function including substance use disorders. In the present study, we investigated the distribution of the 5-HT(2C)R in the rat prelimbic prefrontal cortex (PrL), a subregion of the medial prefrontal cortex (mPFC), using a polyclonal antibody raised against the 5-HT(2C)R. The expression of 5-HT(2C)R immunoreactivity (IR) was highest in the deep layers (layers V/VI) of the mPFC. The 5-HT(2C)R-IR was typically most intense at the periphery of cell bodies and the initial segment of cell processes. Approximately 50% of the 5-HT(2C)R-IR detected was found in glutamate decarboxylase, isoform 67 (GAD 67)-positive neurons. Of the subtypes of GABA interneurons identified by expression of several calcium-binding proteins, a significantly higher percentage of neurons expressing IR for parvalbumin also expressed 5-HT(2C)R-IR than did the percentage of neurons expressing calbindin-IR or calretinin-IR that also expressed 5-HT(2C)R-IR. Since parvalbumin is located in basket and chandelier GABA interneurons which project to cell body and initial axon segments of pyramidal cells, respectively, these results raise the possibility that the 5-HT(2C)R in the mPFC acts via the parvalbumin-positive GABAergic interneurons to regulate the output of pyramidal cells in the rat mPFC.

Guidelines on nicotine dose selection for in vivo research

RATIONALE

This review provides insight for the judicious selection of nicotine dose ranges and routes of administration for in vivo studies. The literature is replete with reports in which a dosaging regimen chosen for a specific nicotine-mediated response was suboptimal for the species used. In many cases, such discrepancies could be attributed to the complex variables comprising species-specific in vivo responses to acute or chronic nicotine exposure.

OBJECTIVES

This review capitalizes on the authors' collective decades of in vivo nicotine experimentation to clarify the issues and to identify the variables to be considered in choosing a dosaging regimen. Nicotine dose ranges tolerated by humans and their animal models provide guidelines for experiments intended to extrapolate to human tobacco exposure through cigarette smoking or nicotine replacement therapies. Just as important are the nicotine dosaging regimens used to provide a mechanistic framework for acquisition of drug-taking behavior, dependence, tolerance, or withdrawal in animal models.

RESULTS

Seven species are addressed: humans, nonhuman primates, rats, mice, Drosophila, Caenorhabditis elegans, and zebrafish. After an overview on nicotine metabolism, each section focuses on an individual species, addressing issues related to genetic background, age, acute vs chronic exposure, route of administration, and behavioral responses.

CONCLUSIONS

The selected examples of successful dosaging ranges are provided, while emphasizing the necessity of empirically determined dose-response relationships based on the precise parameters and conditions inherent to a specific hypothesis. This review provides a new, experimentally based compilation of species-specific dose selection for studies on the in vivo effects of nicotine.

Locomotor activity to nicotine and Fos immunoreactivity in the paraventricular nucleus of the hypothalamus in adolescent socially-stressed rats

We reported previously that social stressors in adolescence (SS: one-hour isolation and new cage partners daily for 16 days) increased locomotor activity to nicotine and to amphetamine in females, but not in males, when tested as adults. Here, we investigated whether effects of stressors in adolescence on locomotor responses to nicotine would be observed in both sexes if tested closer in time to the stressor exposure. We also tested whether social instability was necessary to alter nicotine's effects on locomotor activity by including a group that underwent daily isolation but was housed with the same partner (ISO). The locomotor-activating effects of nicotine were lower in SS rats compared to ISO and non-stressed control rats. In males, but not in females, there were effects of nicotine treatment and of stress condition on Fos immunoreactive (Fos-ir) cell counts in the paraventricular nucleus (PVN) of the hypothalamus: SS males had higher Fos-ir counts than did ISO and non-stressed control males, and higher Fos-ir counts in the PVN were found in repeated-nicotine groups than in acute-nicotine and saline groups. These results add to evidence that adolescents are uniquely vulnerable to stressors due to ongoing brain development, and also indicate that effects are sex- and stressor-specific.

Nicotine causes age-dependent changes in gene expression in the adolescent female rat brain

Humans often start smoking during adolescence. Recent results suggest that rodents may also be particularly vulnerable to nicotine dependence during adolescence. We examined the effect of chronic nicotine exposure on gene expression profiles during adolescence in female rats, who were dosed with nicotine (and control animals were dosed with saline) via subcutaneously implanted osmotic minipumps. Brain samples were collected at four ages: before puberty (postnatal day 25), at about the time of puberty in females (postnatal day 35), and after puberty (postnatal days 45 and 55). The expression of 7931 genes in three brain areas was measured using DNA microarrays. Quantitative RT-PCR was also employed to confirm the expression patterns of selected genes. We used a novel clustering technique (principal cluster analysis) to classify 162 nicotine-regulated genes into five clusters, of which only one (cluster A) showed similar patterns of gene expression across all three brain areas (ventral striatum, prefrontal cortex, and hippocampus). Three clusters of genes (A, B, and C) showed dramatic peaks in their nicotine responses at the same age (p35). The other two clusters (D1 and D2) showed smaller peaks and/or valleys in their nicotine responses at p35 and p45. Thus, the age of maximal gene expression response to nicotine in female rats corresponds approximately to the age of maximal behavioral response and the age of puberty.

Immediate and long-term behavioral effects of a single nicotine injection in adolescent and adult rats

We examined the acute rewarding as well as the long-term psychomotor altering effects of nicotine in early adolescent and adult male Sprague-Dawley rats. Place conditioning was used to examine nicotine-induced reward after a single drug pairing. A single pairing of nicotine with the initially non-preferred side of the place conditioning apparatus produced a conditioned place preference (CPP) in early adolescent but not adult animals. One month later, animals were given a nicotine challenge and locomotor activity observed in the open field to characterize age differences in the lasting alterations resulting from this single injection. Adult rats showed tolerance to the locomotor depressant effects of a low dose of nicotine whereas adolescent rats showed tolerance to a higher dose. Regardless of treatment group, animals tested during adolescence responded to the nicotine challenge with less hypoactivity when compared with animals tested as adults. The present results are in agreement with previous studies showing that early adolescent rats are more sensitive to nicotine's rewarding effects and are in accord with studies showing a unique profile of neurobehavioral alterations following nicotine exposure when compared with adults. Such findings are extended here by showing that these differences are seen following only a single pretreatment dose and persist for at least one month after pretreatment.