Differential effects of pair housing on voluntary nicotine consumption: a comparison between male and female adolescent rats

Sex Differences in Dopamine Release in Nucleus Accumbens and Dorsal Striatum Determined by Chronic Fast Scan Cyclic Voltammetry: Effects of social housing and repeated stimulation

We investigated sex differences in dopamine (DA) release in the nucleus accumbens (NAc) and dorsolateral striatum (DLS) using a chronic 16-channel carbon fiber electrode and fast-scan cyclic voltammetry (FSCV). Electrical stimulation (ES; 60Hz) induced DA release was recorded in the NAc of single or pair-housed male and female rats. When core (NAcC) and shell (NAcS) were recorded simultaneously, there was greater ES DA release in NAcC of pair-housed females compared with single females and males. Housing did not affect ES NAc DA release in males. In contrast, there was significantly more ES DA release from the DLS of female rats than male rats. This was true prior to and after treatment with methamphetamine. Furthermore, in castrated (CAST) males and ovariectomized (OVX) females, there were no sex differences in ES DA release from the DLS, demonstrating the hormone dependence of this sex difference. However, in the DLS of both intact and gonadectomized rats, DA reuptake was slower in females than in males. Finally, DA release following ES of the medial forebrain bundle at 60Hz was studied over four weeks. ES DA release increased over time for both CAST males and OVX females, demonstrating sensitization. Using this novel 16-channel chronic FSCV electrode, we found sex differences in the effects of social housing in the NAcS, sex differences in DA release from intact rats in DLS, sex differences in DA reuptake in DLS of intake and gonadectomized rats, and we report sensitization of ES-induced DA release in DLS in vivo.

Adolescent Intermittent Ethanol Drives Modest Neuroinflammation but Does Not Escalate Drinking in Male Rats

During adolescence, the brain is highly susceptible to alcohol-induced damage and subsequent neuroimmune responses, effects which may enhance development of an alcohol use disorder (AUD). Neuroimmune reactions are implicated in adolescent alcohol exposure escalating adulthood drinking. Therefore, we investigated whether intermittent alcohol exposure in male, adolescent rats (AIE) escalated adult drinking via two-bottle choice (2BC). We also examined the influence of housing environment across three groups: standard (group-housed with enrichment during 2BC), impoverished (group-housed without enrichment during 2BC), or isolation (single-housed without bedding or enrichment throughout). In the standard group immediately after AIE/saline and after 2BC, we also examined the expression of microglial marker, Iba1, reactive astrocyte marker, vimentin, and neuronal cell death dye, FluoroJade B (FJB). We did not observe an escalation of adulthood drinking following AIE, regardless of housing condition. Further, only a modest neuroimmune response occurred after AIE in the standard group: no significant microglial reactivity or neuronal cell death was apparent using this model, although some astrocyte reactivity was detected in adolescence following AIE that resolved by adulthood. These data suggest that the lack of neuroimmune response in adolescence in this model may underlie the lack of escalation of alcohol drinking, which could not be modified through isolation stress.

Determining nicotine-related behavior changes in juvenile female rats through long-term maternal nicotine exposure

Nicotine replacement therapy (NRT) has been developed as a drug therapy for smoking cessation and has been considered a safe alternative to smoking during pregnancy. However, the effects of long-term nicotine exposure via NRT on the fetus are still being debated. Here, we determined the effects of long-term maternal nicotine exposure in gestation and lactation on nicotine-related behavior and drug vulnerability in dams and offspring rats. To expose long-term nicotine, on gestation day 14, pregnant rats were implanted with osmotic minipumps releasing nicotine tartrate (6 mg/kg/day, subcutaneously, equivalent to 2 mg nicotine-freebase) for 28 days. The concentration of cotinine in blood was 373.0 ± 109.0 ng/ml in dams and 12.50 ± 1.19 ng/ml in offspring rats. In dams, we found no significant differences in anxiety-like behaviors and various maternal behaviors such as touching, sniffing, pup licking, laying on pups, and retrieval between saline- and nicotine-exposed groups. Adolescent offspring female rats showed no significant differences in anxiety-like behavior and forced alcohol consumption between saline- and nicotine-exposed groups. Nicotine-exposed offspring rats showed more increased nicotine aversion than saline-exposed groups, but the effect was disturbed in the forced alcohol consumption condition on the first day of the nicotine consumption test. Taken together, these results suggest that, in the last gestation and lactation period corresponding to the second and third trimester of human pregnancy, long-term maternal nicotine exposure has a minor effect on dam and female offspring health and does not involve serious pathological changes in rat offspring, despite the presence of nicotine in their blood.

Adolescent neurodevelopment and substance use: Receptor expression and behavioral consequences

Adolescence is the transitional period between childhood and adulthood, during which extensive brain development occurs. Since this period also overlaps with the initiation of drug use, it is important to consider how substance use during this time might produce long-term neurobiological alterations, especially against the backdrop of developmental changes in neurotransmission. Alcohol, cannabis, nicotine, and opioids all produce marked changes in the expression and function of the neurotransmitter and receptor systems with which they interact. These acute and chronic alterations also contribute to behavioral consequences ranging from increased addiction risk to cognitive or neuropsychiatric behavioral dysfunctions. The current review provides an in-depth overview and update of the developmental changes in neurotransmission during adolescence, as well as the impact of drug exposure during this neurodevelopmental window. While most of these factors have been studied in animal models, which are the focus of this review, future longitudinal studies in humans that assess neural function and behavior will help to confirm pre-clinical findings. Furthermore, the neural changes induced by each drug should also be considered in the context of other contributing factors, such as sex. Further understanding of these consequences can help in the identification of novel approaches for preventing and reversing the neurobiological effects of adolescent substance use.

Alteration of adolescent aversive nicotine response and anxiety-like behavior in nicotine-exposed rats during late lactation period

Early nicotine exposure is an important cause of further habitual tobacco smoking. Although nicotine has not only rewarding but also aversive properties, the effects of early nicotine exposure on the distinct properties of nicotine are not well known. To reveal the effects of early adolescent nicotine exposure on further persistent tobacco smoking, we demonstrated developmental changes in nicotine-related appetitive and aversive behaviors of rats exposed to nicotine during the late lactation period. Sprague-Dawley rats were injected with saline or nicotine (2, 6 and 12mg/kg). We performed a two bottle free-choice test using escalating doses of nicotine (25, 50 and 100μg/ml), saccharin and quinine and the open field test in both adolescent and adult rats. The rats' aversive response to nicotine was increased according to the increase in nicotine concentration. Adolescent rats showed higher nicotine preference and consumption behaviors than did adult rats at an aversive dose of nicotine. Nicotine-exposed rats increased adolescent nicotine consumption when the nicotine concentration was 12mg/kg. We observed significant increases in anxious behaviors in adolescent nicotine-injected rats compared to saline-injected rats, but there were no alterations in adult rats. In both adolescent and adult rats, saccharin and quinine intake were not significantly different between groups. Taken together, it suggests that repeated nicotine exposure in late lactation period affect changes in aversive nicotine responses and anxious behaviors during adolescence but there is no difference in adults.