Chronic exposure approaches with a minipump or nicotine patch at higher doses have also demonstrated decreased exploratory activity, decreased food consumption under anxiety-related conditions, and deficits in contextual condition to shock-associated cues in Sprague-Dawley rats . In mice, adolescent exposure to high dose minipump has also been shown to disrupt contextual fear condition, but not cued fear conditioning . However, since studies have shown that of those adolescents age 12–17 who smoke, the majority smoke one or less than one cigarette per day, the current studies focused on a rewarding dose with once daily exposure as an investigative goal. Thus, the lack of difference in the behavioral measures with nicotine exposure in the current studies may be attributed to this relatively lower dose administered. Along these lines, it should be noted that this dose was selected based on the rewarding effects of doses in this range, as assessed with the brain reward threshold measure, and behavioral effects elicited in adolescent mice, and thus,vertical growing system the current results have particular relevance to experimental patterns of drug consumption found in youth. With adolescent cannabinoid agonist exposure, findings derived from prior rat studies have been somewhat variable.
In one study, adolescent male and female rats treated with the cannabinoid agonist, CP 55,940, exhibited overall increased time on the open-arm of the elevated plus maze, but these effects were not maintained when examining males and females independently, suggesting these differences may have been confounded by baseline differences between the sexes. Since CP 55,940 has high affinity for both the CB1 and CB2 receptors, as well as GPR55, the lack of differences within each sex for drug condition may also have been due to actions on alternate signaling pathways or differences in agonist actions. Interestingly, male Sprague-Dawley rats treated with WIN, the CB1 and CB2 specific agonist, during adolescence exhibited increased depressive-like behaviors in the forced swim and sucrose consumption tests. In our mouse studies, we did not find any differences in these measures with the low dose of WIN and opposing effects at the moderate dose of WIN, indicating that species differences in metabolism and/or genetic heritability factors likely mediate the effects of cannabinoids on adolescent neurodevelopment. Finally, adolescent WIN exposure has also been found to increase palatable food intake and alter attribution of incentive salience for food reward in adult male Long Evans rats. The increase in natural reward-related effects with adolescent exposure is consistent with our findings at the moderate WIN dose in mice, suggesting cannabinoid exposure during adolescence similarly alters brain reward pathways to enhance subsequent responsiveness to natural reward.
Interestingly, Schoch and colleagues also demonstrated increased expression of the endocannabinoids anandamide and oleoylethanolamine in the nucleus accumbens only during a food restricted state with adolescent WIN exposure in rats. Thus, dependent on the availability of food and level of satiety, changes in neural systems regulating reward-related behaviors may be differentially affected in the presence of cannabinoids. Along these lines, it is interesting to note that in the current study, mice were at a satiated level during sucrose consumption, during which time the opposing differences were found in males and females exposed to adolescent WIN. However, during conditions of food restriction, such as during operant food training in the current study, group differences only emerged for males in the reversal task. Thus, altered endocannabinoid signaling may account for this effect during the food restricted state, whereas other mechanisms likely underlie the behavioral differences observed in the anxiety and natural reward-related measures. Cannabinoid and nicotinic acetylcholine receptors exhibit overlapping expression within brain regions implicated in reward-related and affective behaviors, including the prefrontal cortex, ventral tegmental area, nucleus accumbens, medial habenula, interpeduncular nucleus and hippocampus. On the cellular level, both receptors types are expressed on presynaptic terminals and function to modulate release of various neurotransmitters. For instance, with acute administration, both drugs increase extracellular dopamine in the nucleus accumbens and prefrontal cortex, and adolescent cannabinoid or nicotine exposure have also been shown to affect cholinergic, serotonergic and noradrenergic signaling mechanisms.
Thus, in consideration of the effects of nicotine and cannabinoids on several neurotransmitter systems and the behavioral findings from the current studies, future studies will need to dissect the differential impact of single or co-drug exposure during adolescence on neural signaling mechanisms. In conclusion, activation of cannabinoid receptors with or without nicotine led to differential sex-specific effects on anxiety- and reward-related behaviors during adulthood. Together, these studies provide evidence that adolescent exposure to drugs of abuse may lead to alterations in affective and cognitive behaviors during adulthood. These data support the conclusion that consumption of cannabis by youth may alter later cognitive function, and thus, policy approaches should be considered to discourage and/or restrict substance use by this vulnerable population.Nicotine dependence is among the largest preventable causes of disease and death worldwide. Further, poly drug use, including that of nicotine and cannabis, may lead to interactive effects on brain neurocircuitries. Thus, this study represents the first to begin deciphering the coconsumption effects of nicotine and cannabinoids during adolescent development on later dependence and/or resistance to achieving abstinence. According to a 2015 nationwide survey, 32.3% of high school students self-reported prior cigarette use, whereas 44.9% reported using vaporized nicotine products. Of further concern, 38.6% of these students reported using cannabis. Given that recreational cannabis use was illegal in most states at the time of this survey, the number of adolescents exposed to this drug will likely only increase through both primary use and secondhand exposure as the drug becomes more readily accessible. This is supported by the finding that 44% of 12th graders in a recent 2018 nationwide survey reported using cannabis in their lifetime. Further, the practice of mulling, combining tobacco with cannabis to smoke as a joint, has been reported as frequently occurring in adolescent users, with highest incidence among daily cigarette smokers in some populations. Furthermore, individuals who reported smoking cannabis and tobacco cigarettes consumed more cigarettes than those smoking cigarettes alone. Together, these findings have introduced increasing concerns regarding the interaction between the drugs and the effects of early adolescent exposure on later drug-taking behaviors. Nicotine, the main psychoactive component in tobacco and e-cigarettes, acts in the brain on neuronal nicotinic acetylcholine receptors , and the psychoactive effects of cannabis have been attributed to action on the cannabinoid 1 receptor . The CB1Rs are also targeted by other abused drugs, such as synthetic “spice” cannabinoid agonists for which the majority belong to the aminoalkylindole class, including WIN55,212-2. The nAChRs and CB1Rs exhibit overlapping expression patterns within brain regions implicated in drug reinforcement and aversion, including the prefrontal cortex, ventral tegmental area, nucleus accumbens, medial habenula, interpeduncular nucleus, and hippocampus . On the cellular level, CB1Rs and nAChRs are expressed on presynaptic axon terminals, and both function to modulate release of neurotransmitters. Reciprocal outcomes are found in their actions and behavioral effects. Exogenous cannabinoids can modulate cholinergic neurotransmission in the brain, and similarly, nicotine administration alters endogenous cannabinoid signaling. Further, similar effects are found with neurotransmitter release; for instance, administration of either nicotine or the CB1R agonist, WIN55,212-2,how to dry cannabis increases extracellular dopamine in the nucleus accumbens and prefrontal cortex. These findings provide evidence to support the notion that exogenously derived cannabinoid or cholinergic modulation of neurotransmission during adolescence may lead to various altered drug-associated behaviors along the continuum of the dependence processes. In humans, tobacco exposure during development has been associated with increased drug use during adulthood. However, given the nature of human studies, it is unclear as to whether the early life exposure increases vulnerability, or whether a preexisting neural state and/or environmental factors prompted consumption of the drug products.
In rodents, adolescent nicotine exposure results in increased time spent in an environment associated with nicotine during adulthood, suggesting an enhanced rewarding effect of nicotine following prior exposure. In an oral self-administration study, rats that drank a nicotine solution during late adolescence into early adulthood exhibited either a similar level or diminished nicotine drinking behavior in later adulthood. However, high variability in the amount of nicotine consumed has been found in such oral drinking paradigms,potentially due to activation of nAChRs expressed in the tongue and/or post consummatory gastrointestinal effects. In contrast, the intravenous nicotine self-administration procedure is generally accepted as having greater translational relevance to human behavior, as stable responding and titration of intake are found across doses. In one study in rats, nicotine exposure during PND 25–42 did not alter later nicotine self-administration behavior during early adulthood, but it should be noted that the subjects in this study were individually housed and shipped during PND 20–21; factors that could have elicited stressful conditions during the adolescent period. In contrast, another study found a decrease in the motivation to self-administer nicotine during adulthood; in this paradigm, subjects had variable access to a range of nicotine doses for self-administration, including high aversive doses, beginning at PND 34, and prior to adult testing, which may have subsequently biased the resultant level pressing behavior. Here, we sought to examine whether adolescent exposure to nicotine and/or a cannabinoid agonist would alter intravenous nicotine self-administration during adulthood in male and female mice. The current investigations focus on the coexposure condition, which is commonly found in human subjects, and the resulting effects on later nicotine intake. Adolescent mice were exposed to a moderate or low dose of the cannabinoid receptor agonist, WIN55,212-2, and/or nicotine and then were assessed for nicotine reinforcement behaviors during adulthood. Drug exposure occurred during PND 38–49, which corresponds to mid-adolescence in rodents or ~13–17 years of age in humans. Given the previously established differential responses for males and females with drug-related effects and baseline receptor expression across development,male and female mice were examined in a within-sex manner. Finally, we also examined whether acute or repeated administration of the cannabinoid agonist during adulthood would alter nicotine self administration dependent on the prior adolescent exposure condition. The goal of this study was to determine if an interaction effect would occur during adulthood, in consideration of each adolescent exposure condition. Together, these studies provide evidence that adolescent drug exposure alters nicotine reinforcement in a sex-dependent manner and prevents the dampening effects of a cannabinoid on nicotine intake during adulthood in both sexes.Male and female wild-type C57BL/6J mice were derived from breeders in our laboratory animal facilities; in total, 54 male and 63 female mice were examined in these studies. Mice were maintained in an environmentally controlled vivarium on a 12-hour reversed light/dark cycle. Food and water were provided ad libitum until behavioral training commenced. During food and nicotine self-administration, subjects were mildly food restricted to 85%–90% of their free-feeding body weight, and water was provided ad libitum. All experiments were conducted in strict accordance with the NIH Guide for the Careand Use of Laboratory Animals and were approved by the Institutional Animal Care and Use Committee at the University of California, Irvine.The cannabinoid receptor agonist WIN55,212-2 mesylate was dissolved in vehicle containing 1% dimethyl sulfoxide, 1% Tween-80, and 98% saline . The doses of WIN55,212-2 administered were 0.2 or 2 mg/kg, intraperitoneally . The moderate dose of WIN was selected based on prior studies demonstrating altered neural function with adolescent exposure in mice and rats, and the low dose of WIN was selected based on evidence from adolescent WIN self-administration in rats. -Nicotine hydrogen tartrate salt was dissolved in 0.9% sterile saline and adjusted to pH 7.4. Nicotine was administered at a dose of 0.36 mg/kg, subcutaneous ; this dose is considered to be within the rewarding range of the dose–response function that also elicits a behavioral response in adolescent C57BL/6J mice. Peripheral injections were administered at a volume of 10 mL/kg.Beginning on PND 38, the first set of male and female mice were randomly subdivided into four experimental groups: Control , NIC , WIN , and NIC/WIN . Saline and vehicle were the solutions used to dissolve nicotine and WIN, respectively. Mice received once daily injections for 12 consecutive days from PND 38 to PND 49. This timeframe is considered mid-adolescence in rodents, corresponding to ~13–17 in human years. This represents a dynamic developmental period for both the endogenous nicotinic acetylcholine and cannabinoid systems; for instance, the highest level of CB1 receptor expression is found during this period. The daily injection schedule was selected to model an experimental pattern of adolescent exposure as previously described. Body weight was recorded prior to each injection.