Basolateral amygdala, nicotinic cholinergic receptors, and nicotine: Pharmacological effects and addiction in animal models and humans.

The amygdala is involved in processing incoming information about rewarding stimuli and emotions that denote danger such as anxiety and fear. Bi-directional neural connections between basolateral amygdala (BLA) and brain regions such as nucleus accumbens, prefrontal cortex, hippocampus, and hindbrain regions regulate motivation, cognition, and responses to stress. Altered local regulation of BLA excitability is pivotal to the behavioral disturbances characteristic of posttraumatic stress disorder, and relapse to drug use induced by stress. Herein, we review the physiological regulation of BLA by cholinergic inputs, emphasizing the role of BLA nicotinic receptors. We review BLA-dependent effects of nicotine on cognition, motivated behaviors, and emotional states, including memory, taking and seeking drugs, and anxiety and fear in humans and animal models. The alterations in BLA activity observed in animal studies inform human behavioral and brain imaging research by enabling a more exact understanding of altered BLA function. Converging evidence indicates that cholinergic signaling from basal forebrain projections to local nicotinic receptors is an important physiological regulator of BLA and that nicotine alters BLA function. In essence, BLA is necessary for behavioral responses to stimuli that evoke anxiety and fear; reinstatement of cue-induced drug seeking; responding to second-order cues conditioned to abused drugs; reacquisition of amplified nicotine self-administration due to chronic stress during abstinence; and to promote responding for natural reward.

Neurogenetic determinants and mechanisms of addiction to nicotine and smoked tobacco.

The single most preventable cause of disease, disability, and death in the United States is tobacco use. Decades of study show that the risk of becoming addicted to smoked cigarettes varies greatly amongst individuals and is heritable, yet environmental factors are also important contributors. In this review, we consider a wide range of methodologies and key published reports that have defined the inheritance of different stages of nicotine-dependent smoking behavior, including preference, initiation, regular use, withdrawal and dependence as well as cessation and relapse. Major findings from both animal and human studies are discussed. Current findings converge primarily on the role of nicotinic cholinergic receptor subunits, although other neurotransmitter systems as well as nicotine metabolism enzymes are implicated. Various stages of nicotine addiction may share common genetic mechanisms, yet several lines of evidence indicate that each stage also has its own unique genetic determinants. Studies on the heritability of smoking initiation demonstrate substantial evidence for gene-environment interaction, although the precise molecular genetic mechanism(s) remains unknown. Considering the relatively few genes identified so far and the small to modest fraction of the variance in risk for a particular smoking phenotype (e.g., smoking initiation in late adolescence) attributable to these genes, a large gap remains to be filled in order to account for the heritability of key phenotypes involved in each stage of addiction to smoked tobacco. Looking forward, new research strategies involving both human and animal studies will produce the fundamental genetic insights that are the foundation for the precision medical treatment of individuals addicted to smoked tobacco.

Full-gestational exposure to nicotine and ethanol augments nicotine self-administration by altering ventral tegmental dopaminergic function due to NMDA receptors in adolescent rats.

In adult rats, we have shown full-gestational exposure to nicotine and ethanol (Nic + EtOH) augmented nicotine self-administration (SA) (increased nicotine intake) compared to pair-fed (PF) offspring. Therefore, we hypothesized that full-gestational exposure to Nic + EtOH disrupts control of dopaminergic (DA) circuitry by ventral tegmental area (VTA) NMDA receptors, augmenting nicotine SA and DA release in nucleus accumbens (NAcc) of adolescents. Both NAcc DA and VTA glutamate release were hyper-responsive to intra-VTA NMDA in Nic + EtOH offspring versus PF (p = 0.03 and 0.02, respectively). Similarly, DA release was more responsive to i.v. nicotine in Nic + EtOH offspring (p = 0.02). Local DL-2-Amino-5-phosphonopentanoic acid sodium salt (AP5) (NMDA receptor antagonist) infusion into the VTA inhibited nicotine-stimulated DA release in Nic + EtOH and PF offspring. Nicotine SA was augmented in adolescent Nic + EtOH versus PF offspring (p = 0.000001). Daily VTA microinjections of AP5 reduced nicotine SA by Nic + EtOH offspring, without affecting PF (p = 0.000032). Indeed, nicotine SA in Nic + EtOH offspring receiving AP5 was not different from PF offspring. Both VTA mRNA transcripts and NMDA receptor subunit proteins were not altered in Nic + EtOH offspring. In summary, adolescent offspring exposed to gestational Nic + EtOH show markedly increased vulnerability to become dependent on nicotine. This reflects the enhanced function of a subpopulation of VTA NMDA receptors that confer greater nicotine-induced DA release in NAcc. We hypothesized that concurrent gestational exposure to nicotine and ethanol would disrupt the control of VTA dopaminergic circuitry by NMDA receptors. Resulting in the augmented nicotine self-administration (SA) in adolescent offspring.

Inactivation of phosphodiesterase-4B gene in rat nucleus accumbens shell by CRISPR/Cas9 or positive allosteric modulation of the protein affects the motivation to chronically self-administer nicotine.

Large scale human genome wide association studies (GWAS) have identified a growing pool of genes associated with cigarette smoking. One of the most prominent, phosphodiesterase-4B (PDE4B), has been associated with multiple smoking phenotypes. Although PDE4B modulates the half-life of neuronal cAMP, its precise role in smoking behaviors is unknown. To address this knowledge gap, we used a reverse translational approach. We inactivated PDE4B in bilateral medial nucleus accumbens shell (NAcs) neurons by injecting AAV containing a specific gRNA in female transgenic Cas9+ Long Evans rats. These rats then were given 23-h chronic access to nicotine intravenous self-administration (IVSA) under a schedule of increasing fixed ratios (FR). With the increased effort required at FR7, nicotine SA (i.e. active presses and drug infusions) declined significantly in controls, whereas it was maintained in the mutagenized group. A progressive ratio (PR) study also showed significantly greater cumulative nicotine infusions in the PDE4B-edited group. Hence, we hypothesized that enhanced PDE4B protein activity would reduce nicotine IVSA. A positive allosteric modulator, 2-(3-(4-chloro-3-fluorophenyl)-5-ethyl-1H-1,2,4-triazol-1-yl)-N-(3,5-dichlorobenzyl)acetamide (MR-L2), was microinfused into NAcs bilaterally at FR3 or FR5; in both cohorts, MR-L2 acutely reduced nicotine IVSA. In summary, these studies show that the activity of PDE4B regulates the capacity of NAcs to maintain nicotine IVSA in face of the cost of increasing work. This finding and the results of the PR study indicate that PDE4B affects the motivation to obtain nicotine. These reverse translational studies in rats provide insight into the motivational effects of NAcs PDE4B that advance our understanding of the smoking behaviors mapped in human GWAS.

A revamped rat reference genome improves the discovery of genetic diversity in laboratory rats.

The seventh iteration of the reference genome assembly for Rattus norvegicus-mRatBN7.2-corrects numerous misplaced segments and reduces base-level errors by approximately 9-fold and increases contiguity by 290-fold compared with its predecessor. Gene annotations are now more complete, improving the mapping precision of genomic, transcriptomic, and proteomics datasets. We jointly analyzed 163 short-read whole-genome sequencing datasets representing 120 laboratory rat strains and substrains using mRatBN7.2. We defined ∼20.0 million sequence variations, of which 18,700 are predicted to potentially impact the function of 6,677 genes. We also generated a new rat genetic map from 1,893 heterogeneous stock rats and annotated transcription start sites and alternative polyadenylation sites. The mRatBN7.2 assembly, along with the extensive analysis of genomic variations among rat strains, enhances our understanding of the rat genome, providing researchers with an expanded resource for studies involving rats.

Inactivation of phosphodiesterase-4B gene in rat nucleus accumbens shell by CRISPR/Cas9 modulates the motivation to chronically self-administer nicotine.

Large scale human genome wide association studies (GWAS) have identified a growing pool of genes associated with cigarette smoking. One of the most prominent, phosphodiesterase-4B (PDE4B), has been associated with multiple smoking phenotypes. Although PDE4B modulates the half-life of neuronal cAMP, its precise role in smoking behaviors is unknown. To address this knowledge gap, we used a reverse translational approach. We inactivated PDE4B in bilateral medial nucleus accumbens shell (NAcs) neurons by injecting AAV containing a specific gRNA in female transgenic Cas9+ Long Evans rats. These rats then were given 23-hour chronic access to nicotine intravenous self-administration (IVSA) under a schedule of increasing fixed ratios (FR). With the increased effort required at FR7, nicotine SA (i.e. active presses and drug infusions) declined significantly in controls, whereas it was maintained in the mutagenized group. A progressive ratio (PR) study also showed significantly greater cumulative nicotine infusions in the mutant group. Hence, we hypothesized that enhanced PDE4B protein activity would reduce nicotine IVSA. A positive allosteric modulator,2-(3-(4-chloro-3-fluorophenyl)-5-ethyl-1H-1,2,4-triazol-1-yl)-N-(3,5-dichlorobenzyl)acetamide (MR-L2), was microinfused into NAcs bilaterally at FR3 or FR5; in both cohorts, MR-L2 acutely reduced nicotine IVSA. In summary, these studies show that the activity of PDE4B regulates the capacity of NAcs to maintain nicotine IVSA in face of the cost of increasing work. This finding and the results of the PR study indicate that PDE4B affects the motivation to obtain nicotine. These reverse translational studies in rats provide insight into the motivational effects of NAcs PDE4B that advance our understanding of the smoking behaviors mapped in human GWAS.

Sex and heredity are determinants of oral oxycodone self-administration in 36 Inbred Rat Strains: correlations with behavioral tests of anxiety and novelty-seeking.

Most individuals affected in the national epidemic of oxycodone abuse began taking oral oxycodone by prescription. We studied vulnerability to oxycodone intake in a rat model of oral drug self-administration (SA), since pharmacokinetics affect abuse potential. Females (33 inbred strains) and males (26) obtained oxycodone at increasing concentrations in operant sessions (FR5; 1-16-h) followed by extinction and reinstatement. Active spout licks were greater in females than males during 4-h and 16-h sessions (p< 0.001 for all). Across all stages of oxycodone SA, intake/session was greater in females (p<0.001). Both sexes escalated intake during 16-h extended access vs 4-h sessions (p<2e-16). Intake and active licks varied greatly by strain. The heritability (h2) of active licks/4-h at increasing oxycodone dose was larger in males (h2 females: 0.30-0.39 vs. males: 0.41-0.53). Under a progressive ratio schedule, breakpoints differed by strain (p<2e-16) and by sex in some strains (p=0.018). For cue-induced reinstatement, active licks were greater in females than males (p<0.001). Behavior in naive rats was assessed using elevated plus maze (EPM), open field (OF) and novel object interaction. (NOI) tests. We correlated these behaviors with 28 parameters of oxycodone SA. EPM-defining traits were most commonly associated with SA in both sexes, whereas more OF and NOI traits were SA-associated in males. Overall, sex and heredity are major determinants of the motivation to take and seek oxycodone, which escalates during extended access. The correlation of EPM, a measure of anxiety, with multiple SA parameters indicates the influence of pleiotropic genes.

A revamped rat reference genome improves the discovery of genetic diversity in laboratory rats.

The seventh iteration of the reference genome assembly for Rattus norvegicus-mRatBN7.2-corrects numerous misplaced segments and reduces base-level errors by approximately 9-fold and increases contiguity by 290-fold compared to its predecessor. Gene annotations are now more complete, significantly improving the mapping precision of genomic, transcriptomic, and proteomics data sets. We jointly analyzed 163 short-read whole genome sequencing datasets representing 120 laboratory rat strains and substrains using mRatBN7.2. We defined ~20.0 million sequence variations, of which 18.7 thousand are predicted to potentially impact the function of 6,677 genes. We also generated a new rat genetic map from 1,893 heterogeneous stock rats and annotated transcription start sites and alternative polyadenylation sites. The mRatBN7.2 assembly, along with the extensive analysis of genomic variations among rat strains, enhances our understanding of the rat genome, providing researchers with an expanded resource for studies involving rats.

Nicotine modulates multiple regions in the limbic stress network regulating activation of hypophysiotrophic neurons in hypothalamic paraventricular nucleus.

Nicotine intake affects CNS responses to stressors. We reported that nicotine self-administration (SA) augmented the hypothalamo-pituitary-adrenal (HPA) stress response, in part because of the altered neurotransmission and neuropeptide expression within hypothalamic paraventricular nucleus (PVN). Limbic-PVN interactions involving medial prefrontal cortex, amygdala, and bed nucleus of the stria terminalis (BST) greatly impact the HPA stress response. Therefore, we investigated the effects of nicotine SA on stress-induced neuronal activation in limbic-PVN network, using c-Fos protein immunohistochemistry and retrograde tracing. Nicotine decreased stress-induced c-Fos in prelimbic cortex (PrL), anteroventral BST (avBST), and peri-PVN, but increased c-Fos induction in medial amygdala (MeA), locus coeruleus, and PVN. Fluoro-gold (FG) was injected into avBST or PVN, as GABAergic neurons in avBST projecting to PVN corticotrophin-releasing factor neurons relay information from both PrL glutamatergic and MeA GABAergic neurons. The stress-induced c-Fos expression in retrograde-labeled FG+ neurons was decreased in PrL by nicotine, but increased in MeA, and also reduced in avBST. Therefore, within limbic-PVN network, nicotine SA exerts selective regional effects on neuronal activation by stress. These findings expand the mechanistic framework by demonstrating altered limbic-BST-PVN interactions underlying the disinhibition of PVN corticotrophin-releasing factor neurons, an essential component of the amplified HPA response to stress by nicotine.

Sex and heredity are determinants of drug intake in a novel model of rat oral oxycodone self-administration.

The steady rise in prescription opioids such as oxycodone has led to a virulent epidemic of widespread abuse and deaths in the United States; approximately 80% of affected individuals initiate the habitual use of oxycodone by using prescription oral oxycodone. Given the importance of drug pharmacokinetics in determining abuse potential, we designed an oral operant oxycodone self-administration (SA) procedure in rats to model drug intake by most human users/abusers of oxycodone. Key aspects of the model include limited initial drug intake followed by increasing drug concentrations during extended 4-h sessions on alternating days. Sex and genetic predisposition are major determinants of human opiate abuse. Therefore, we studied females in seven inbred strains (WLI, WMI, LEW, DSS, F344, BN and SHR) and both sexes in three of these strains. All strains increased intake across serially increasing doses (0.025-0.2 mg/ml; p < 0.001): the range of intakes at the final concentration of oxycodone was 0.72 ± 0.17-4.84 ± 1.42 mg/kg (mean ± SEM) - a 6.7-fold difference across strains. In LEW, WLI and WMI strains, oxycodone intake increased significantly across all sessions in both sexes. However, in LEW and WMI male rats but not WLI, daily oxycodone intake was significantly lower across all 4-h sessions than females (p < 0.005). The estimated heritability in oxycodone intake was in the range of 0.21-0.41. In summary, our novel operant oral oxycodone SA model captures the strong abuse potential of oral oxycodone and shows dose, sex and strain-specific drug intake that is significantly dependent on heredity.

Nicotine self-administration diminishes stress-induced norepinephrine secretion but augments adrenergic-responsiveness in the hypothalamic paraventricular nucleus and enhances adrenocorticotropic hormone and corticosterone release.

Chronic nicotine self-administration augments the thalamo-pituitary-adrenal (HPA) responses to stress. Altered neuropeptide expression within corticotropin-releasing factor (CRF) neurons in the hypothalamic paraventricular nucleus (PVN) contributes to this enhanced HPA response to stress. Herein, we determined the role of norepinephrine, a primary regulator of CRF neurons, in the responses to footshock during nicotine self-administration. On day 12-15 of self-administration, microdialysis showed nicotine reduced PVN norepinephrine release by footshock (< 50% of saline). Yet, the reduction in footshock-induced adrenocorticotropic hormone and corticosterone secretion because of intra-PVN prazosin (alpha(1) adrenergic antagonist) was significantly greater in rats self-administering nicotine (2-fold) than saline. Additionally, PVN phenylephrine (alpha(1) agonist) stimulated adrenocorticotropic hormone and corticosterone release to a similar extent in unstressed rats self-administering nicotine or saline. Nicotine self-administration also decreased footshock-induced c-Fos expression in the nucleus of the solitary tract-A2/C2 catecholaminergic neurons that project to the PVN. Therefore, footshock-induced nucleus of the solitary tract activation and PVN norepinephrine input are both attenuated by nicotine self-administration, yet PVN CRF neurons are more responsive to alpha(1) stimulation, but only during stress. This plasticity in noradrenergic regulation of PVN CRF neurons provides a new mechanism contributing to the HPA sensitization to stress by nicotine self-administration and smoking.

Nicotine self-administration differentially modulates glutamate and GABA transmission in hypothalamic paraventricular nucleus to enhance the hypothalamic-pituitary-adrenal response to stress.

The mechanisms by which chronic nicotine self-administration augments hypothalamo-pituitary-adrenal (HPA) responses to stress are only partially understood. Nicotine self-administration alters neuropeptide expression in corticotropin-releasing factor (CRF) neurons within paraventricular nucleus (PVN) and increases PVN responsiveness to norepinephrine during mild footshock stress. Glutamate and GABA also modulate CRF neurons, but their roles in enhanced HPA responsiveness to footshock during chronic self-administration are unknown. We show that nicotine self-administration augmented footshock-induced PVN glutamate release, but further decreased GABA release. In these rats, intra-PVN kynurenic acid, a glutamate receptor antagonist, blocked enhanced adrenocorticotropic hormone and corticosterone responses to footshock. In contrast, peri-PVN kynurenic acid, which decreases activity of GABA afferents to PVN, enhanced footshock-induced corticosterone secretion only in control rats self-administering saline. Additionally, in rats self-administering nicotine, footshock-induced elevation of corticosterone was significantly less than in controls after intra-PVN saclofen (GABA-B receptor antagonist). Therefore, the exaggerated reduction in GABA release by footshock during nicotine self-administration disinhibits CRF neurons. This disinhibition combined with enhanced glutamate input provides a new mechanism for HPA sensitization to stress by chronic nicotine self-administration. This mechanism, which does not preserve homeostatic plasticity, supports the concept that smoking functions as a chronic stressor that sensitizes the HPA to stress.

Allosteric Modulation of GABAA Receptors in Rat Basolateral Amygdala Blocks Stress-Enhanced Reacquisition of Nicotine Self-Administration.

Stress is a major determinant of relapse to smoked tobacco. In a rat model, repeated stress during abstinence from nicotine self-administration (SA) results in enhanced reacquisition of nicotine SA, which is dependent on the basolateral amygdala (BLA). We postulate that repeated stress during abstinence causes hyperexcitability of the BLA principal output neurons (PNs) due to disinhibition of the PNs from reduced inhibitory regulation by local GABAergic interneurons. To determine if enhanced GABAergic regulation of the BLA PNs can lessen the effects of stress on nicotine intake, positive allosteric modulators (PAMs) of GABAA receptors were infused into the BLA immediately prior to reacquisition of nicotine SA. Three selective PAMs [NS 16085 (binds the benzodiazepine site on α2/α3 GABAA); DCUK-OEt (binds a novel, benzodiazepine site on α1 or α5, ß2 or ß3, γ2 or δ GABAA); DS2 (binds exclusively to δ GABAA] with varied GABAA subunit specificities abolished the stress-induced amplification of nicotine taking during reacquisition. These studies indicate that highly selective PAMS targeting α3 or δ subunit-containing GABAA in the BLA may be effective in ameliorating the stress-induced relapse to smoked tobacco during abstinence from cigarettes.

Nicotine self-administration differentially regulates hypothalamic corticotropin-releasing factor and arginine vasopressin mRNAs and facilitates stress-induced neuronal activation.

Acute nicotine is a potent stimulus for activation of the stress-responsive hypothalamic-pituitary-adrenal (HPA) axis, while chronic nicotine self-administration (SA) desensitizes the ACTH response to self-administered nicotine but cross-sensitizes to mild footshock stress (mFSS). To identify underlying mechanisms, we investigated (1) the effects of chronic nicotine SA on the coexpression of corticotropin-releasing factor (CRF) and arginine vasopressin (AVP) mRNAs, the primary hypothalamic neuropeptides regulating ACTH release, in the parvocellular division of paraventricular nucleus (pcPVN), and (2) mFSS-induced activation of these neurons during nicotine SA. Adult male Sprague Dawley rats were given 23 h/d unlimited access to self-administer nicotine (0.03 mg/kg per injection, i.v.) for 19 d. Brains were double labeled with fluorescence in situ hybridization of CRF and AVP mRNAs and triple labeled after mFSS exposure for CRF and AVP mRNAs and c-Fos protein. Chronic nicotine SA significantly increased AVP mRNA signal and the number of pcPVN AVP-positive (AVP(+)) neurons (twofold to threefold), reduced the number of CRF-positive (CRF(+)) neurons by approximately 60%, but increased pcPVN CRF(+)/AVP(+) neuronal number fivefold. Significantly, although chronic nicotine SA did not affect total c-Fos expression induced by mFSS in pcPVN CRF(+) neurons, the majority of the new CRF(+)/AVP(+) population was activated by this heterotypic stressor. These phenotypic neuronal alterations may provide the pivotal mechanism underlying the capacity of chronically self-administered nicotine to cross-sensitize the HPA response to specific stressors, suggesting that nicotine may augment HPA responsiveness to specific stressors in human smokers.

Stromal cell-derived factor 1-alpha (SDF)-induced human T cell chemotaxis becomes phosphoinositide 3-kinase (PI3K)-independent: role of PKC-theta.

Stromal cell-derived factor 1alpha (SDF-1alpha) is the exclusive ligand for the chemokine receptor CXCR4. This receptor plays a pivotal role in immune responses, the pathogenesis of infection such as HIV, and cellular trafficking. However, the signaling mechanisms regulating SDF-driven T cell migration are not well defined. In this study, we determined the role of PI3K and protein kinase C- theta (PKC-theta) in SDF-induced human T cell migration in fresh versus cultured T cells. Purified human T cells (fresh vs. 48 h in media, unstimulated or activated by anti-CD3+anti-CD28) were used. Western blots showed that SDF induced phospho-(p)-Akt [threonine (Thr)308 and serine 473], a proxy for PI3K activity, in fresh cells and p-PKC-theta in 48 h unstimulated cells. LY294002 (PI3K inhibitor) reduced SDF-induced chemotaxis in fresh cells by 51%, whereas it minimally affected chemotaxis in 48 h unstimulated or activated cells. However, a specific PKC-theta inhibitor, pseudosubstrate for PKC-theta, reduced chemotaxis in 48 h unstimulated and stimulated T cells by 72% and 87%, respectively. Thus, chemotaxis becomes independent of PI3K signaling in human T cells cultured for 48 h. Under these conditions, PKC-theta is phosphorylated (Thr538) by SDF, and chemotaxis becomes largely PKC-theta-dependent.

Neuroadaptive changes in the mesocortical glutamatergic system during chronic nicotine self-administration and after extinction in rats.

Nicotine self-administration causes adaptation in the mesocorticolimbic glutamatergic system, including the up-regulation of ionotropic glutamate receptor subunits. We therefore determined the effects of nicotine self-administration and extinction on NMDA-induced glutamate neurotransmission between the medial prefrontal cortex (mPFC) and ventral tegmental area (VTA). On day 19 of nicotine SA, both regions were microdialyzed for glutamate while mPFC was sequentially perfused with Kreb's Ringer buffer (KRB), 200 microM NMDA, KRB, 500 microM NMDA, KRB, and 100 mM KCl. Basal glutamate levels were unaffected, but nicotine self-administration significantly potentiated mPFC glutamate release to 200 microM NMDA, which was ineffective in controls. Furthermore, in VTA, nicotine self-administration significantly amplified glutamate responses to both mPFC infusions of NMDA. This hyper-responsive glutamate neurotransmission and enhanced glutamate subunit expression were reversed by extinction. Behavioral studies also showed that a microinjection of 2-amino-5-phosphonopentanoic acid (NMDA-R antagonist) into mPFC did not affect nicotine or sucrose self-administration. However, in VTA, NBQX (AMPA-R antagonist) attenuated both nicotine and sucrose self-administration. Collectively, these studies indicate that mesocortical glutamate neurotransmission adapts to chronic nicotine self-administration and VTA AMPA-R may be involved in the maintenance of nicotine self-administration.

Chronic nicotine self-administration augments hypothalamic-pituitary-adrenal responses to mild acute stress.

We investigated the effect of chronic nicotine self-administration (SA) on hypothalamic-pituitary-adrenal (HPA) hormonal responses to acute stressors. Adult male Sprague-Dawley rats were given access to nicotine (0.03 mg/kg) for 23 h per day for 20 days. On day 1 of acquisition of nicotine SA, plasma levels of both adrenocorticotropin and corticosterone were significantly increased 15-30 min after the first dose of nicotine. These hormonal changes were no longer significant on day 3, when adrenocorticotropin levels were <60 pg/ml and corticosterone levels were <110 ng/ml during the hour after the first dose of nicotine. Chronic nicotine SA (20 days) significantly augmented (2-3-fold) both hormonal responses to mild foot shock stress (0.6 mA, 0.5 s per shock, 5 shocks per 5 min), but did not affect hormonal responses to moderate shock (1.2 mA, 0.5 s per shock, 5 shocks per 5 min), lipopolysaccharide or immobilization. Similar data were obtained in Lewis rats. These results provide further support for the concept that chronic nicotine SA is a stressor. In alignment with the effects of other stressors, nicotine activated the HPA axis on the first day of SA, but desensitization occurred with repeated exposure. Furthermore, chronic nicotine SA selectively cross-sensitized the HPA response to a novel stressor. These observations suggest that nicotine may selectively increase the HPA response to stressors in human smokers.

Acquisition of nicotine self-administration in adolescent rats given prolonged access to the drug.

As most human tobacco use begins during adolescence and ongoing development of the adolescent central nervous system could affect acquisition of nicotine self-administration (SA), our established rat SA procedure was modified to study adolescent acquisition of SA with prolonged access to nicotine (23 h/day). Postnatal age 43-45 female Lewis rats, without prior shaping, conditioning, or food deprivation, were housed in operant chambers equipped with two levers; pressing the active lever triggered an i.v. injection of nicotine. By the 10th day of SA, rats receiving 7.5, 15, 30, or 60 microg/kg/injection nicotine (free base) obtained 23+/-16, 50+/-8, 65+/-8, or 48+/-5 injections (mean+/-SE), respectively. In the 30 microg/kg/injection group, active : inactive ratio was greater than 2 after SA day 4; 92% of injections occurred during the 12-h active (dark) phase of the light cycle. Main effects (analysis of variance) were shown for day and lever (ie active vs inactive) (p<0.001). Adolescent males showed similar dose-dependent nicotine SA. With the increasing workload imposed by raising the fixed ratio (FR), less nicotine was self-administered at FR 5 and 7 compared to FR 1 and 3. In comparison to adult females self-administering 30 microg/kg/injection of nicotine at FR 1, adolescents acquired nicotine SA at an accelerated rate (p<0.05) and received a greater number of injections (p<0.05) by day 10. In conclusion, when given prolonged access to the drug, both female and male adolescent Lewis rats rapidly acquire nicotine SA within the dosage range and FR constraints previously observed in adult Lewis rats. However, adolescent females acquired the behavior more rapidly and attained higher levels of stable nicotine SA than adults.

Upregulation of ionotropic glutamate receptor subunits within specific mesocorticolimbic regions during chronic nicotine self-administration.

Nicotine, an addictive substance, is the major psychoactive component in cigarette smoke. Both alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid and N-methyl-D-aspartate (NMDA) receptors are essential for the acute stimulative effects of nicotine on the mesocorticolimbic dopamine system, yet little is known about the effects of chronic nicotine treatment on glutamate receptors. Therefore, we used a model of chronic nicotine self-administration (SA), which emulates important aspects of nicotine intake by humans, to determine whether glutamate receptor subunits were affected. After 18 days of saline vs nicotine SA, ionotropic glutamate receptor subunit levels were determined in brain regions within the mesocorticolimbic system by Western blotting. In prefrontal cortex (PFC), the levels of NMDA receptor subunit 2A (NR2A) and NR2B were increased by 67% (p=0.04) and 83% (p=0.027), respectively. In the ventral tegmental area (VTA), glutamate receptor subunit 2/3 (GluR2/3) increased by 34% (p=0.011). Nicotine SA did not affect the expression of these subunits in dorsal striatum and nucleus accumbens. These findings suggest that chronic nicotine SA selectively increased the levels of ionotropic glutamate receptor subunits in a brain region-specific manner.