Frances M. Leslie

Age-dependent effects of nicotine on locomotor activity and conditioned place preference in rats

RationaleMost adult smokers start smoking during their adolescence. This adolescent initiation may be due to multiple factors, but little evidence is available regarding whether their brains are differentially sensitive to the addictive effects of nicotine during adolescence.ObjectiveTo test the hypothesis that adolescents are more sensitive than adults to nicotine’s rewarding actions.MethodsAn unbiased, counterbalanced, place-conditioning procedure was used to examine drug-induced reward and locomotor activity. Early adolescent (postnatal dayxa028), late adolescent (P38) and adult (P90) rats received either saline or nicotine (0.125, 0.25 or 0.5xa0mg/kg, s.c.) and were tested for place conditioning.ResultsDuring early adolescence, a single nicotine injection (0.5xa0mg/kg) induced significant conditioned place preference (CPP). In contrast, during late adolescence or adulthood, nicotine did not induce CPP after either one or four conditioning trials. Initial locomotor responses to acute nicotine administration during the first conditioning trial also differed with age, with no effect at P28, but substantial inhibitory responses at all doses studied (0.125–0.5xa0mg/kg) at later ages. Although not differing in their initial locomotor response to nicotine, there was a significantly greater tolerance/sensitization during the second and subsequent drug exposures in late adolescents than in adults.ConclusionsThese findings provide evidence that adolescent brain is differentially sensitive to both the acute and repeated effects of nicotine relative to adult brain. Furthermore, there are significant differences in nicotine sensitivity between early and late phases of adolescence.

Monoamine Oxidase Inhibitors Allow Locomotor and Rewarding Responses to Nicotine

Although nicotine is generally considered to be the main compound responsible for the addictive properties of tobacco, experimental data indicate that nicotine does not exhibit all the characteristics of other abused substances, such as psychostimulants and opiates. For example, nicotine is only a weak locomotor enhancer in rats and generally fails to induce a locomotor response in mice. This observation contradicts the general consensus that all drugs of abuse release dopamine in the nucleus accumbens, a subcortical structure, and thus increase locomotor activity in rodents. Because tobacco smoke contains monoamine oxidase inhibitors (MAOIs) and decreases MAO activity in smokers, we have combined MAOIs with nicotine to determine whether it is possible to obtain a locomotor response to nicotine in C57Bl6 mice. Among 15 individual or combined MAOIs, including harmane, norharmane, moclobemide, selegiline, pargyline, clorgyline, tranylcypromine and phenelzine, only irreversible inhibitors of both MAO-A and -B (tranylcypromine, phenelzine, and clorgyline+selegiline) allowed a locomotor response to nicotine. The locomotor stimulant interaction of tranylcypromine and nicotine was absent in β2-nicotinic acetylcholine receptor subunit knockout mice. Finally, it was found that, whereas naïve rats did not readily self-administer nicotine (10u2009μg/kg/injection), a robust self-administration of nicotine occurred when animals were pretreated with tranylcypromine (3u2009mg/kg). Our data suggest that MAOIs contained in tobacco and tobacco smoke act in synergy with nicotine to enhance its rewarding effects.

Degradation of dynorphin-(1–13) by membrane-bound rat brain enzymes

Abstract Under the incubation conditions of the opioid receptor binding assay, dynorphin-(1–13) is metabolized by membrane-bound exopeptidases which cleave the NH2-terminal tyrosine and the COOH-terminal lysine. Enzymatic attack, particularly that of carboxypeptidase, is extremely rapid. Following a 5 min incubation at 22°C with wellwashed brain membranes (0.5 ml; 1 g original wet weight per 100 ml), less than 50% of dynorphin-(1–13) (20 nM) remains intact. Various methods of stabilization of dynorphin-(1–13) have been examined. Degradation can be reduced, but not eliminated, by the use of metabolic inhibitors or by lowering the incubation temperature to 0°C. Substitution of an amide or methyl ester function at the carboxyl terminus of the peptide provides complete protection not only from carboxypeptidase attack but also from aminopeptidase attack. Carboxyl-substituted compounds are limited in their usefulness as stable analogues, however, since they are not resistant to hydrolysis in the presence of physiological salts.

Adolescent Maturation of Cocaine-Sensitive Neural Mechanisms

Both clinical and animal studies have shown that adolescents undergo a late maturation of the central nervous system, which may underlie adolescent typical behaviors. In particular, decreased behavioral response to cocaine has been found in adolescents as compared to adults. In the present study, cocaine was used as a tool to explore adolescent brain maturation. Juvenile (postnatal day (P) 27), adolescent (P37), and adult (P90) male Sprague–Dawley rats were treated acutely with cocaine (750u2009μg/kg/injection × 2, i.v.), and c-fos mRNA expression, a marker of neuronal activation, was evaluated by in situ hybridization. Cocaine-induced c-fos mRNA was similar across ages in the dorsal caudate putamen (CPu), nucleus accumbens, and lateral bed nucleus of the stria terminalis. In contrast, there was a diminished response in juvenile/adolescent ventral CPu and in juvenile central nucleus of the amygdala, and an increased response in juvenile/adolescent cortex. Further studies evaluated the mechanism of the late maturation of cocaine response in ventral CPu. No significant age differences were observed in regional dopamine (DA) transporter binding. Although striatal DA content was significantly reduced at P27 as compared to adult, there was no difference between dorsal and ventral subregions. In contrast, basal- and cocaine-induced extracellular DA overflow, as measured by in vivo microdialysis, was lower in juvenile ventral CPu than in the adults. This age difference was not observed in dorsal CPu. These findings suggest that impulse activity in DA afferents to ventral CPu is immature in adolescents. In conclusion, the present study showed that cocaine-sensitive neuronal circuits continue to mature during adolescence.

Involvement of alpha1-adrenergic receptors in tranylcypromine enhancement of nicotine self-administration in rat

RationaleThe mechanisms mediating tobacco addiction remain elusive. Nicotine, the psychoactive component in tobacco, is generally believed to be the main cause of reward and addiction. However, tobacco smoke contains thousands of constituents, some of which may interact with nicotine to enhance reward. It has previously been shown that monoamine oxidase (MAO) inhibition, known to result from smoking, can enhance nicotine self-administration. The aim of the present study was to evaluate the role of noradrenergic systems in mediating this enhancement of nicotine reward.ObjectiveThe objective of this study was to test the hypothesis that MAO inhibitor pretreatment enhances nicotine self-administration by activation of noradrenergic pathways that regulate dopamine release in the nucleus accumbens (NAc).MethodsThe effect of prazosin (0.0625–0.5xa0mg/kg, i.p.), a specific α1-adrenergic receptor antagonist, was examined on male rats pretreated with tranylcypromine (3xa0mg/kg), an irreversible inhibitor of MAO A and B. Acquisition of nicotine (10xa0μg kg−1 inj−1, i.v.) self-administration behavior was examined over a 5-day period. Nicotine (60xa0μg kg−1 inj−1, i.v.)-induced increase in NAc extracellular dopamine levels was examined by in vivo microdialysis in non-self-administering animals.ResultsWe have shown that (1) tranylcypromine enhances nicotine self-administration, (2) prazosin pretreatment blocks both the acquisition and the expression of nicotine self-administration, and (3) prazosin pretreatment diminishes nicotine-induced dopamine release in the NAc.ConclusionThese data indicate that the stimulation of α1-adrenergic receptors is critical for tranylcypromine enhancement of nicotine reward and suggest a critical interplay between the noradrenergic and dopaminergic systems in tobacco addiction.

Age-dependent effects of low-dose nicotine treatment on cocaine-induced behavioral plasticity in rats

RationaleEpidemiological evidence of early adolescent tobacco use, prior to that of marijuana and other illicit drugs, has led to the hypothesis that nicotine is a “gateway” drug that sensitizes reward pathways to the addictive effects of other psychostimulants.ObjectiveTo test this hypothesis, we have compared the effect of a brief, low-dose nicotine pretreatment of adolescent and adult rats on subsequent locomotor response to acute and chronic cocaine.Materials and methodsAdolescents, aged postnatal day (P) 28, and adults, aged P86, were given four daily injections of saline or nicotine (0.06xa0mg/kg, i.v.). At P32 and P90, rats were given acute injections of cocaine (0, 0.4 or 1.0xa0mg/kg, i.v.) and monitored for locomotor activity in either a habituated or novel test environment. To examine cocaine sensitization, rats were treated for 3xa0days with saline or cocaine (0.4xa0mg/kg, i.v.), and, after 1xa0day of withdrawal, were given a challenge dose of cocaine (0.4xa0mg/kg, i.v.).ResultsNicotine pretreatment did not affect acute, drug-induced locomotor activity at either age. However, age differences in cocaine response were observed, with adolescent animals showing enhanced locomotor activity in the novel environment. Adolescent controls did not exhibit cocaine-induced locomotor sensitization, whereas adults did. Nicotine pretreatment during adolescence promoted the development and expression of a sensitized response to repeated cocaine exposure similar to that observed in saline-pretreated adult controls.ConclusionsThese findings show that brief pretreatment with nicotine, in a low dose comparable to that inhaled in 2–4 cigarettes, enhances cocaine-induced behavioral plasticity in adolescent rats.

Gestational exposure to nicotine and monoamine oxidase inhibitors influences cocaine-induced locomotion in adolescent rats.

RationaleMany pregnant women continue to smoke, despite a strong association between maternal smoking and neurobehavioral deficits in the offspring. Although gestational nicotine (GN) treatment in rodents is used as the primary animal model of maternal smoking, tobacco smoke contains more than 4,000 constituents, including monoamine oxidase inhibitors (MAOIs).ObjectivesThe aim of this study was to determine whether there are interactions between the effects of gestational exposure to nicotine and MAOIs on cocaine-induced locomotor sensitization in adolescent rats.Materials and methodsPregnant rats were implanted on dayxa04 of gestation with osmotic minipumps delivering saline, nicotine (3xa0mg/kg per day), the MAOIs clorgyline and deprenyl (1 and 0.25xa0mg/kg per day, respectively), or nicotine/clorgyline/deprenyl (GMN). Adolescent female offspring were tested for cocaine-induced locomotor sensitization. Animals were treated with saline or cocaine (5 or 15xa0mg/kg, intraperitoneally) daily from postnatal (P) daysxa032–36 and challenged with cocaine (15xa0mg/kg) on P51 (dayxa020).ResultsGroup differences were observed in chronic but not acute effects of cocaine. Whereas gestational MAOI treatment, with or without nicotine, increased ambulatory response to cocaine on dayxa05, the opposite was found for vertical activity. Different adaptive responses were observed on cocaine challenge day. GNM animals exhibited enhanced locomotor activity in the cocaine-associated environment before cocaine challenge on dayxa020. In contrast, only GN animals exhibited significant locomotor sensitization to the cocaine challenge.ConclusionsGestational nicotine and MAOIs both influence brain development. Such interactions may sensitize adolescents to drug abuse and should be considered in animal models of maternal smoking.

Age influences the effects of nicotine and monoamine oxidase inhibition on mood-related behaviors in rats

RationaleEpidemiological studies have demonstrated a comorbidity of smoking with depression and anxiety, particularly during adolescence. However, few animal studies have considered possible synergistic interactions between nicotine and other tobacco smoke constituents, such as monoamine oxidase (MAO) inhibitors, in the regulation of mood.ObjectivesThe aim of the study was to test the hypothesis that nicotine combined with the irreversible MAO inhibitor, tranylcypromine, will differentially affect depression- and anxiety-related behaviors in adolescent and adult rats.MethodsNicotine (0, 0.05, 0.2xa0mg/kg, s.c.) and tranylcypromine (3xa0mg/kg, i.p.) were tested separately, or together, on male rats aged postnatal days 30 and 68, in three mood-related behavioral tests: forced swim test (FST), elevated plus maze (EPM), and open field.ResultsNicotine (0.2xa0mg/kg) in adults significantly decreased floating time in the FST and increased time spent in the open arm of the EPM, with no change in locomotor activity. Tranylcypromine pretreatment combined with nicotine (0.2xa0mg/kg) significantly increased locomotor activity and time spent in the center of the open field. Whereas nicotine alone had no significant effect on adolescents, it significantly increased locomotor activity and decreased floating time in the FST when combined with tranylcypromine pretreatment.ConclusionsThere is an age-dependent effect of nicotine, alone and in combination with MAO inhibition, on mood-related behaviors. Whereas nicotine alone induces mood improvement in adults, it has no effect on adolescents. Nicotine combined with tranylcypromine has unique, age-dependent effects. Thus, experimental studies of smoking should consider both age and other tobacco constituents, such as MAO inhibitors, as critical factors.

Opioid Receptors and the Developing Nervous System

Three endogenous opioid peptide families have been identified, including those derived from the proenkephalin, prodynorphin and proopiomelanocortin genes (Imura et al., 1985, for review). Peptides derived from these genes interact with three major classes of opioid receptor, mu, delta and kappa (Leslie, 1987). These receptors are widely distributed throughout the central nervous system and mediate the biological effects of endogenous opioid peptides, as well as of certain non-peptide analgesic drugs such as morphine and ketocyclazocine. In addition, epsilon (Schulz et al., 1979; Chang et al., 1984) and zeta (Zagon et al., 1989) receptor types have been described, which have high affinity and selectivity for the opioid peptides beta-endorphin and enkephalin, respectively.

Nicotine-induced brain metabolism associated with anger provocation

Cortico-limbic brain activity associated with anger may be susceptible to nicotine and, thus, may contribute to smoking initiation and nicotine addiction. The purpose of the study was to identify the brain regions that are most reactive to nicotine and show the greatest association with anger task performance. Twenty adult nonsmokers (9 women, 11 men) participated in two laboratory sessions to assess brain metabolism with fluoro deoxy-glucose Positron Emission Topography (FDG-PET) in response to nicotine and placebo patches during an anger provocation task. Outcome variables for the anger provocation task were reaction time, intensity and length of retaliation. Reaction time was associated with nicotine-induced changes in the left thalamus. Length of retaliation was associated with a functionally linked set of cortical and subcortical structures such as right frontal lobe, right anterior cingulate (BA 24), right uncus, left parietal lobe, left BA 11, left cingulate, left BA 25, left amygdala, left BA 30, left BA 38 and BA 9. These findings reveal the underlying brain circuitry targeted by nicotine during anger provocation.