Luis A. Natividad

Enhanced vulnerability to the rewarding effects of nicotine during the adolescent period of development

This study compared the rewarding and aversive effects of nicotine in adolescent, adult, and adult rats preexposed to nicotine during adolescence. Prior to conditioning, the rats were tested for their initial preference for either of 2 distinct compartments. Adolescent and adult rats then received various nicotine doses in their initially non-preferred side on one day and saline in the other side on alternate days. This 2-day procedure was repeated over 8 consecutive days. Following conditioning, rats were re-tested for their preference. Another cohort of adolescent and adult rats were conditioned with various doses of D-amphetamine. Nicotine produced CPP in an inverted U-shaped manner in both age groups. However, adolescents displayed a larger upward shift in CPP that was significant across a wider dose range relative to adults. There were no developmental differences to CPP produced by D-amphetamine. In a final study, adolescents were prepared with pumps that delivered nicotine for 14 days. These rats were conditioned later as adults using the same procedures used previously. Pre-exposure to nicotine during adolescence diminished the aversive effects produced by the highest nicotine dose in naive adults. Taken together, these studies provide a basis for enhanced vulnerability to nicotine during adolescence.

Female rats display dose-dependent differences to the rewarding and aversive effects of nicotine in an age-, hormone-, and sex-dependent manner

IntroductionThe objective of this study was to examine age-, hormone-, and sex-dependent differences to the behavioral effects of nicotine using place-conditioning procedures in female rats.MethodsAnimals received nicotine in their initially non-preferred side and saline on alternate days in their initially preferred side. Following four conditioning trials, rats were re-tested for their preference. To examine developmental differences, we compared the effects of various nicotine doses in female and male adolescent and adult rats. To examine whether our developmental differences are specific to nicotine, we included adolescent and adult females that were conditioned with various amphetamine doses. To examine the influence of hormones on the behavioral effects of nicotine, we compared the effects of various nicotine doses in intact females that were tested during different phases of the estrous cycle and in separate females that were ovariectomized.ResultsThe rewarding effects of nicotine were observed at a lower nicotine dose in adolescents versus adults. Amphetamine produced similar rewarding effects across age groups in females. The shifts in preference produced by nicotine were similar across the different phases of estrous. Females lacking ovarian hormones did not display rewarding effects of nicotine at any dose. The rewarding effects of nicotine were enhanced in adult female versus male rats. An intermediate nicotine dose produced rewarding effects in adolescent male but not female rats, suggesting that developmental differences to nicotine may be enhanced in males.ConclusionIn females, nicotine reward is enhanced during adolescence and is facilitated by the presence of ovarian hormones.

Nicotine withdrawal produces a decrease in extracellular levels of dopamine in the nucleus accumbens that is lower in adolescent versus adult male rats

The behavioral effects of nicotine withdrawal are lower in adolescent versus adult rats. However, the neurochemical mechanisms that mediate these developmental differences are unknown. Previous studies have shown that extracellular levels of dopamine in the nucleus accumbens (NAcc) are reduced in adult rats experiencing withdrawal. This study compared dopamine levels in the NAcc of male adolescent and adult rats experiencing nicotine withdrawal. Animals were prepared with subcutaneous pumps that delivered an equivalent nicotine dose in these age groups. Following 13 days of nicotine exposure, rats were implanted unilaterally with microdialysis probes into the NAcc and ipsilateral ventral tegmental area (VTA). The next day, dialysate levels were collected following systemic administration of the nicotinic‐receptor antagonist mecamylamine to precipitate withdrawal. Mecamylamine produced an average % decrease in NAcc dopamine that was lower in adolescents (20%) versus adults (44%). Similar developmental differences were observed with the dopaminergic (DOPAC and HVA) but not serotonergic (5‐HIAA) metabolites. A follow‐up study compared NAcc dopamine in adolescent and adult rats receiving intra‐VTA administration of bicuculline, which reduces γ‐aminobutyric acid (GABA) inhibition of dopamine transmission. The results revealed that blockade of GABAA receptors in the VTA produced a two‐fold increase in NAcc dopamine of adults but not adolescents. These results provide a potential mechanism involving dopamine that mediates developmental differences in nicotine withdrawal. Specifically, they suggest that GABA systems are underdeveloped during adolescence and this reduced inhibition of dopamine neurons in the VTA may lead to reduced decreases in NAcc dopamine of young animals experiencing withdrawal. Synapse 64:136–145, 2010.

Dysregulation of kappa-opioid receptor systems by chronic nicotine modulate the nicotine withdrawal syndrome in an age-dependent manner

RationaleMechanisms that mediate age differences during nicotine withdrawal are unclear.ObjectiveThis study compared kappa-opioid receptor (KOR) activation in naïve and nicotine-treated adolescent and adult rats using behavioral and neurochemical approaches to study withdrawal.MethodsThe behavioral models used to assess withdrawal included conditioned place and elevated plus maze procedures. Deficits in dopamine transmission in the nucleus accumbens (NAcc) were examined using microdialysis procedures. Lastly, the effects of KOR stimulation and blockade on physical signs produced upon removal of nicotine were examined in adults.ResultsNicotine-treated adults displayed a robust aversion to an environment paired with a KOR agonist versus naïve adults. Neither of the adolescent groups displayed a place aversion. KOR activation produced an increase in anxiety-like behavior that was highest in nicotine-treated adults versus all other groups. KOR activation produced a decrease in NAcc dopamine that was largest in nicotine-treated adults versus all other groups. Lastly, KOR activation facilitated physical signs of withdrawal upon removal of nicotine and KOR blockade reduced this effect.ConclusionChronic nicotine enhanced the affective, anxiogenic, and neurochemical effects produced by KOR activation in adult rats. Our data suggest that chronic nicotine elicits an increase in KOR function, and this may contribute to nicotine withdrawal since KOR activation facilitated and KOR blockade prevented withdrawal signs upon removal of nicotine. Given that chronic nicotine facilitated the neurochemical effects of KOR agonists in adults but not in adolescents, it is suggested that KOR regulation of mesolimbic dopamine may contribute to age differences in nicotine withdrawal.

Behavioral, Biochemical, and Molecular Indices of Stress are Enhanced in Female Versus Male Rats Experiencing Nicotine Withdrawal

Stress is a major factor that promotes tobacco use and relapse during withdrawal. Although women are more vulnerable to tobacco use than men, the manner in which stress contributes to tobacco use in women versus men is unclear. Thus, the goal of this study was to compare behavioral and biological indices of stress in male and female rats during nicotine withdrawal. Since the effects of nicotine withdrawal are age-dependent, this study also included adolescent rats. An initial study was conducted to provide comparable nicotine doses across age and sex during nicotine exposure and withdrawal. Rats received sham surgery or an osmotic pump that delivered nicotine. After 14u2009days of nicotine, the pumps were removed and controls received a sham surgery. Twenty-four hours later, anxiety-like behavior and plasma corticosterone were assessed. The nucleus accumbens (NAcc), amygdala, and hypothalamus were examined for changes in corticotropin-releasing factor (CRF) gene expression. In order to differentiate the effects of nicotine withdrawal from exposure to nicotine, a cohort of rats did not have their pumps removed. The major finding is that during nicotine withdrawal, adult females display higher levels of anxiety-like behavior, plasma corticosterone, and CRF mRNA expression in the NAcc relative to adult males. However, during nicotine exposure, adult males exhibited higher levels of corticosterone and CRF mRNA in the amygdala relative to females. Adolescents displayed less nicotine withdrawal than adults. Moreover, adolescent males displayed an increase in anxiety-like behavior and an up-regulation of CRF mRNA in the amygdala during nicotine exposure and withdrawal. These findings are likely related to stress produced by the high doses of nicotine that were administered to adolescents to produce equivalent levels of cotinine as adults. In conclusion, these findings suggest that intense stress produced by nicotine withdrawal may contribute to tobacco use in women.

Adolescence is a period of development characterized by short- and long-term vulnerability to the rewarding effects of nicotine and reduced sensitivity to the anorectic effects of this drug.

This study compared nicotine intake and changes in food intake and weight gain in naïve adolescent, naïve adult, and adult rats that were exposed to nicotine during adolescence. An extended intravenous self-administration (IVSA) model was used whereby rats had 23-hour access to saline or increasing doses of nicotine (0.03, 0.06, and 0.09 mg/kg/0.1 mL infusion) for 4-day intervals separated by 3-day periods of abstinence. Rats began IVSA as adolescents (PND 32-34) or adults (PND 75). A separate group of rats was exposed to nicotine via osmotic pumps (4.7 mg/kg) for 14 days during adolescence and then began nicotine IVSA as adults (PND 75). The rats that completed the nicotine IVSA regimen were also tested for nicotine-seeking behavior during extinction. The results revealed that nicotine intake was highest in adolescents followed by adults that were pre-exposed to nicotine during adolescence as compared to naïve adults. A similar pattern of nicotine-seeking behavior was observed during extinction. In contrast to nicotine intake, naïve adults displayed robust appetite and weight suppressant effects of nicotine, an effect that was absent in adolescents and adults that were pre-exposed to nicotine during adolescence. Our findings suggest that adolescence is a unique period of enhanced vulnerability to the reinforcing effects of nicotine. Although adolescents gain weight faster than adults, the food intake and weight suppressant effects of nicotine are reduced during adolescence. Importantly, our findings suggest that adolescent nicotine exposure produces long-lasting consequences that enhance nicotine reward and promote tolerance to the anorectic effects of this drug.

Increased impulsivity in rats as a result of repeated cycles of alcohol intoxication and abstinence

Impulsivity is a risk factor for alcoholism, and long‐term alcohol exposure may further impair impulse control in a manner that propels problematic alcohol use. The present study employed the rat 5‐choice serial reaction time task (5‐CSRTT) to measure behavioral inhibition and attentional capacity during abstinence from repeated 5‐day cycles of alcohol liquid diet consumption. Task performance was not disrupted following the first cycle of alcohol exposure; however, evidence of impaired behavioral inhibition emerged following the third cycle of alcohol exposure. In comparison with controls, alcoholic rats exhibited deficits in inhibitory control during cognitively challenging 5‐CSRTT tests employing variable intertrial interval (varITI). This behavioral disruption was not present during early abstinence (3 days) but was evident by 7 days of abstinence and persisted for at least 34 days. Interestingly, renewed alcohol consumption ameliorated these disruptions in impulse control, although deficient behavioral inhibition re‐emerged during subsequent abstinence. Indices of increased impulsivity were no longer present in tests conducted after 49 days of abstinence. Alcohol‐related impairments in impulse control were not evident in sessions employing highly familiar task parameters regardless of the abstinence period, and control experiments confirmed that performance deficits during the challenge sessions were unlikely to result from alcohol‐related disruption in the adaptation to repeated varITI testing. Together, the current findings demonstrate that chronic intermittent alcohol consumption results in decreased behavioral inhibition in rats that is temporally similar to clinical observations of disrupted impulsive control in abstinent alcoholics performing tasks of behavioral inhibition.

Constitutive Increases in Amygdalar Corticotropin-Releasing Factor and Fatty Acid Amide Hydrolase Drive an Anxious Phenotype

BACKGROUNDnCorticotropin-releasing factor (CRF) mediates anxiogenic responses by activating CRF type 1 (CRF1) receptors in limbic brain regions. Anxiety is further modulated by the endogenous cannabinoid (eCB) system that attenuates the synaptic effects of stress. In the amygdala, acute stress activates the enzymatic clearance of the eCB N-arachidonoylethanolamine via fatty acid amide hydrolase (FAAH), although it is unclear whether chronic dysregulation of CRF systems induces maladaptive changes in amygdalar eCB signaling. Here, we used genetically selected Marchigian Sardinian P (msP) rats carrying an innate overexpression of CRF1 receptors to study the role of constitutive upregulation in CRF systems on amygdalar eCB function and persistent anxiety-like effects.nnnMETHODSnWe applied behavioral, pharmacological, and biochemical methods to broadly characterize anxiety-like behaviors and amygdalar eCB clearance enzymes in msP versus nonselected Wistar rats. Subsequent studies examined the influence of dysregulated CRF and FAAH systems in altering excitatory transmission in the central amygdala (CeA).nnnRESULTSnmsPs display an anxious phenotype accompanied by elevations in amygdalar FAAH activity and reduced dialysate N-arachidonoylethanolamine levels in the CeA. Elevations in CRF-CRF1 signaling dysregulate FAAH activity, and this genotypic difference is normalized with pharmacological blockade of CRF1 receptors. msPs also exhibit elevated baseline glutamatergic transmission in the CeA, and dysregulated CRF-FAAH facilitates stress-induced increases in glutamatergic activity. Treatment with an FAAH inhibitor relieves sensitized glutamatergic responses in msPs and attenuates the anxiety-like phenotype.nnnCONCLUSIONSnPathological anxiety and stress hypersensitivity are driven by constitutive increases in CRF1 signaling that dysregulate N-arachidonoylethanolamine signaling mechanisms and reduce neuronal inhibitory control of CeA glutamatergic synapses.

Adolescent rats are resistant to adaptations in excitatory and inhibitory mechanisms that modulate mesolimbic dopamine during nicotine withdrawal.

Adolescent smokers report enhanced positive responses to tobacco and fewer negative effects of withdrawal from this drug than adults, and this is believed to propel higher tobacco use during adolescence. Differential dopaminergic responses to nicotine are thought to underlie these age‐related effects, as adolescent rats experience lower withdrawal‐related deficits in nucleus accumbens (NAcc) dopamine versus adults. This study examined whether age differences in NAcc dopamine during withdrawal are mediated by excitatory or inhibitory transmission in the ventral tegmental area (VTA) dopamine cell body region. In vivo microdialysis was used to monitor extracellular levels of glutamate and gamma‐aminobutyric acid (GABA) in the VTA of adolescent and adult rats experiencing nicotine withdrawal. In adults, nicotine withdrawal produced decreases in VTA glutamate levels (44% decrease) and increases in VTA GABA levels (38% increase). In contrast, adolescents did not exhibit changes in either of these measures. Naïve controls of both ages did not display changes in NAcc dopamine, VTA glutamate, or VTA GABA following mecamylamine. These results indicate that adolescents display resistance to withdrawal‐related neurochemical processes that inhibit mesolimbic dopamine function in adults experiencing nicotine withdrawal. Our findings provide a potential mechanism involving VTA amino acid neurotransmission that modulates age differences during withdrawal.

Enhanced nicotine self-administration and suppressed dopaminergic systems in a rat model of diabetes

Patients with diabetes display a heightened propensity to use tobacco; however, it is unclear whether they experience enhanced rewarding effects of nicotine. Thus, this study examined the reinforcing effects of nicotine in a rodent model of diabetes involving administration of streptozotocin (STZ), a drug that is toxic to pancreatic insulin‐producing cells. The first study compared STZ‐ and vehicle‐treated rats that had 23‐hour access to intravenous self‐administration (IVSA) of nicotine or saline and concomitant access to food and water. In order to examine the contribution of dopamine to our behavioral effects, dopamine transporter (DAT), D1 and D2 receptor levels were compared in the nucleus accumbens (NAc) following 10 days of nicotine or saline IVSA. Dopamine levels in the NAc were also compared following nicotine administration. Lastly, nicotine metabolism and dose‐dependent effects of nicotine IVSA were assessed. The results revealed that STZ‐treated rats displayed enhanced nicotine intake and a robust increase in food and water intake relative to controls. Protein analysis revealed an increase in DAT and a decrease in D1 receptor levels in the NAc of STZ‐ versus vehicle‐treated rats regardless of IVSA condition. STZ‐treated rats also displayed suppressed NAc dopamine levels during baseline and in response to nicotine. STZ treatment did not alter our assessment of nicotine metabolism. Furthermore, STZ treatment increased nicotine IVSA in a dose‐dependent manner. Our findings suggest that STZ‐treatment increased the rewarding effects of nicotine. This suggests that strong reinforcing effects of nicotine may contribute to greater tobacco use in patients with diabetes.