Tamara L. Doremus-Fitzwater

Motivational Systems in Adolescence: Possible Implications for Age Differences in Substance Abuse and Other Risk-Taking Behaviors.

Adolescence is an evolutionarily conserved developmental phase characterized by hormonal, physiological, neural and behavioral alterations evident widely across mammalian species. For instance, adolescent rats, like their human counterparts, exhibit elevations in peer-directed social interactions, risk-taking/novelty seeking and drug and alcohol use relative to adults, along with notable changes in motivational and reward-related brain regions. After reviewing these topics, the present paper discusses conditioned preference and aversion data showing adolescents to be more sensitive than adults to positive rewarding properties of various drugs and natural stimuli, while less sensitive to the aversive properties of these stimuli. Additional experiments designed to parse specific components of reward-related processing using natural rewards have yielded more mixed findings, with reports of accentuated positive hedonic sensitivity during adolescence contrasting with studies showing less positive hedonic affect and reduced incentive salience at this age. Implications of these findings for adolescent substance abuse will be discussed.

Social and non-social anxiety in adolescent and adult rats after repeated restraint

Adolescence is associated with potentially stressful challenges, and adolescents may differ from adults in their stress responsivity. To investigate possible age-related differences in stress responsiveness, the consequences of repeated restraint stress (90 min/day for 5 days) on anxiety, as indexed using the elevated plus-maze (EPM) and modified social interaction (SI) tests, were assessed in adolescent and adult Sprague-Dawley male and female rats. Control groups at each age included non-stressed and socially deprived animals, with plasma corticosterone (CORT) levels also measured in another group of rats on days 1 and 5 of stress (sampled 0, 30, 60, 90, and 120 min following restraint onset). While repeatedly restrained animals exhibited similar anxiety levels compared to non-stressed controls in the EPM, restraint stress increased anxiety at both ages in the SI test (as indexed by reduced social investigation and social preference). Daily weight gain measurements, however, revealed more marked stress-related suppression of body weight in adolescents versus adults. Analysis of stress-induced increases in CORT likewise showed that adolescents demonstrated less habituation than adults, embedded within typical sex differences in CORT magnitude (females greater than males) and age differences in CORT recovery (adolescents slower than adults). Despite no observable age-related differences in the behavioral response to restraint, adolescents were more sensitive to the repeated stressor in terms of physiological indices of attenuated weight gain and habituation of stress-induced CORT.

Repeated restraint stress alters sensitivity to the social consequences of ethanol in adolescent and adult rats

Human adolescents consume alcohol largely to enhance social interactions. Adolescent, but not adult rats likewise exhibit ethanol-induced social facilitation under low-stress circumstances. Since the relationship between stress and ethanol sensitivity across ontogeny still has yet to be well explored, the present study sought to characterize possible age-associated differences in the influence of stressor exposure on ethanol-induced changes in social behavior in adolescent [postnatal days (P) 30-36] and adult (P65-71) male and female Sprague-Dawley rats. Animals were repeatedly restrained (90min/day) for 5days, followed by examination of ethanol-induced (0, 0.25, 0.5, 0.75, or 1.0g/kg) alterations in social behaviors on the last day. Results revealed typical age-related differences in sensitivity to ethanol among controls, with adolescents being uniquely sensitive to low-dose ethanol stimulation of social investigation and play fighting, but less sensitive than adults to the social suppression emerging at higher doses. At both ages, stressor exposure decreased sensitivity to social inhibitory effects of ethanol, while augmenting expression of ethanols social facilitatory effects. Ethanol also attenuated the stress-related suppression of social motivation at both ages. These results suggest that repeated stressor exposure diminishes age-related differences in the social consequences of ethanol, with stress enhancing ethanol-induced social facilitation across age.

Amphetamine-induced incentive sensitization of sign-tracking behavior in adolescent and adult female rats

Age-specific behavioral and neural characteristics may predispose adolescents to initiate and escalate use of alcohol and drugs. Adolescents may avidly seek novel experiences, including drugs of abuse, because of enhanced incentive motivation for drugs and natural rewards, perhaps especially when that incentive motivation is sensitized by prior drug exposure. Using a Pavlovian conditioned approach (PCA) procedure, sign-tracking (ST) and goal-tracking (GT) behavior was examined in amphetamine-sensitized and control adolescent and adult female Sprague-Dawley rats, with expression of elevated ST behavior used to index enhanced incentive motivation for reward-associated cues. Rats were first exposed to a sensitizing regimen of amphetamine injections (3.0 mg/kg/ml d-amphetamine per day) or given saline (0.9% wt/vol) once daily for 4 days. Expression of ST and GT was then examined over 8 days of PCA training consisting of 25 pairings of an 8-s presentation of an illuminated lever immediately followed by response-independent delivery of a banana-flavored food pellet. Results showed that adults clearly displayed more ST behavior than adolescents, reflected via both more contacts with, and shorter latencies to approach, the lever. Prior amphetamine sensitization increased ST (but not GT) behaviors regardless of age. Thus, when indexed via ST, incentive motivation was found to be greater in adults than adolescents, with a prior history of amphetamine exposure generally sensitizing incentive motivation for cues predicting a food reward regardless of age.

Age-related differences in impulsivity among adolescent and adult Sprague-Dawley rats.

Adolescence is an ontogenetic period characterized by numerous hormonal, neural, and behavioral changes. In animal models, adolescents exhibit greater levels of novelty-seeking behavior and risk-taking relative to adults, behaviors associated in humans with increases in impulsivity and elevated propensities to engage in drug and alcohol seeking behaviors. The current series of experiments sought to explore possible age-related differences in impulsivity when indexed using delay discounting in adolescent (postnatal day [P] 25-27) and adult (P68-71) female (Experiment 1) and male (Experiment 2) Sprague-Dawley rats. In both experiments, adolescents exhibited significantly greater levels of impulsive-like behavior in this test relative to adults-even when data were adjusted to account for baseline differences in activity levels (i.e., general nose-poking behavior) across age. Taken together, these results extend to both sexes previous findings of adolescent-associated elevations in impulsivity observed among male mice using delay discounting, as well as among male rats using other procedures to index impulsivity. That these age differences were observed among both male and female rats suggests that impulsivity may be a pervasive feature of adolescence, and contributes to the expression of risky behaviors during this ontogenetic period.

Effects of pretest manipulation on elevated plus-maze behavior in adolescent and adult male and female Sprague-Dawley rats

The elevated plus-maze (EPM) is vulnerable to variations in pretest circumstances when testing adult rodents. Because of an increasing interest in adolescence, the present experiments examined the impact of pretest manipulations on anxiety levels in the EPM among adolescent and adult Sprague-Dawley rats of both sexes. In Exp. 1, animals removed from their home cage and immediately placed on the EPM were compared to rats tested following 30 min of social isolation, or following 30-min exposure to a novel context. These pretest manipulations only modestly decreased anxiety levels at both ages. In Exp. 2, more varied pretest conditions were examined: testing directly from the home cage; testing following 30 min of social isolation in a novel environment; or a large saline injection and rehousing 18 h prior to a 30-min period of social isolation in a novelty situation before testing. In adults, anxiety levels decreased linearly as pretest perturbation increased, whereas adolescents showed comparable levels of anxiety with both the moderate and large perturbations. As a result, observed age differences in anxiety differed as a function of pretest circumstances. Therefore, caution is urged when using the EPM for across-age comparisons of anxiolytic and anxiogenic effects of pharmacological or other manipulations.

Male adolescent rats display blunted cytokine responses in the CNS after acute ethanol or lipopolysaccharide exposure

Alcohol induces widespread changes in cytokine expression, with recent data from our laboratory having demonstrated that, during acute ethanol intoxication, adult rats exhibit consistent increases in interleukin (IL)-6 mRNA expression in several brain regions, while showing reductions in IL-1 and TNFα expression. Given evidence indicating that adolescence may be an ontogenetic period in which some neuroimmune processes and cells may not yet have fully matured, the purpose of the current experiments was to examine potential age differences in the central cytokine response of adolescent (P31-33days of age) and adult (69-71days of age) rats to either an acute immune (lipopolysaccharide; LPS) or non-immune challenge (ethanol). In Experiment 1, male Sprague-Dawley rats were given an intraperitoneal (i.p.) injection of either sterile saline, LPS (250μg/kg), or ethanol (4-g/kg), and then trunk blood and brain tissue were collected 3h later for measurement of blood ethanol concentrations (BECs), plasma endotoxin, and central mRNA expression of several immune-related gene targets. In Experiment 2, the response to intragastrically (i.g.) administered ethanol was examined and compared to animals given tap water (i.g.). Results showed that LPS stimulated robust increases in expression of IL-1, IL-6, TNFα, and IκBα in the hippocampus, PVN, and amygdala, and that these increases were generally less pronounced in adolescents relative to adults. Following an i.p. ethanol challenge, IL-6 and IκBα expression was significantly increased in both ages in the PVN and amygdala, and adults exhibited even greater increases in IκBα than adolescents. I.g. administration of ethanol also increased IL-6 and IκBα expression in all three brain regions, with hippocampal IL-6 elevated even more so in adults compared to adolescents. Furthermore, assessment of plasma endotoxin concentrations revealed (i) whereas robust increases in plasma endotoxin were observed in adults injected with LPS, no corresponding elevations were seen in adolescents after LPS; and (ii) neither adolescents nor adults demonstrated increases in plasma endotoxin concentrations following i.p. or i.g. ethanol administration. Analysis of BECs indicated that, for both routes of exposure, adolescents exhibited lower BECs than adults. Taken together, these data suggest that categorically different mechanisms are involved in the central cytokine response to antigen exposure versus ethanol administration. Furthermore, these findings confirm once again that acute ethanol intoxication is a potent activator of brain cytokines, and calls for future studies to identify the mechanisms underlying age-related differences in the cytokine response observed during ethanol intoxication.

Age-related differences in amphetamine sensitization: Effects of prior drug or stress history on stimulant sensitization in juvenile and adult rats

Repeated intermittent exposure to stimulants progressively increases a drugs effect, with stressors capable of producing cross-sensitization to stimulants. Studies examining such sensitization during development are few, however, with results mixed. In Experiment 1, juvenile (P22) and adult (P64) female Sprague-Dawley rats were administered (daily for 4days) 1.5mg/kg or 3.0mg/kg amphetamine (1.5A and 3.0A groups), or saline (SAL group). In a second experiment, rats were exposed to either repeated restraint (60min/day for 4days; RS group) or were left non-manipulated in the home cage (NM group). Animals from both experiments were then challenged with 1.5mg/kg of amphetamine and sensitization assessed via locomotion and stereotypy after a 2-day and 3-wk washout period. When compared to SAL animals, 3.0A juveniles and adults exhibited evidence of locomotor sensitization 2days post-drug exposure, but this sensitization did not persist to the 3-week challenge. Compared to NM animals, RS animals showed stress-induced locomotor sensitization both 2days and 3weeks post-stress exposure, regardless of age. These results demonstrate that repeated drug/stress exposures prior to stimulant challenge are sufficient to induce behavioral sensitization among both juveniles and adults, with these effects particularly long-lasting following repeated stressor exposure.

Sustained alterations in neuroimmune gene expression after daily, but not intermittent, alcohol exposure

Acute ethanol intoxication is associated with Rapid Alterations in Neuroimmune Gene Expression (RANGE), including increased Interleukin (IL)-6 and Nuclear factor of kappa light polypeptide gene enhancer in B-cells inhibitor, alpha (IκBα), and suppressed IL-1β and Tumor necrosis factor (TNF) α, yet little is known about adaptations in cytokines across the first few ethanol exposures. Thus, the present studies examined central cytokines during intoxication (3h post-ethanol) following 2, 4 or 6 intragastric ethanol challenges (4g/kg) delivered either daily or every-other-day (EOD). Subsequent analyses of blood ethanol concentrations (BECs) and corticosterone were performed to determine whether the schedule of ethanol delivery would alter the pharmacokinetics of, or general sensitivity to, subacute ethanol exposure. As expected, ethanol led to robust increases in IL-6 and IκBα gene expression in hippocampus, amygdala and bed nucleus of the stria terminalis (BNST), whereas IL-1β and TNFα were suppressed, thereby replicating our prior work. Ethanol-dependent increases in IL-6 and IκBα remained significant in all structures - even after 6 days of ethanol. When these doses were administered EOD, modest IL-6 increases in BNST were observed, with TNFα and IL-1β suppressed exclusively in the hippocampus. Analysis of BECs revealed a small but significant reduction in ethanol after 4 EOD exposures - an effect which was not observed when ethanol was delivered after 6 daily intubations. These findings suggest that ethanol-induced RANGE effects are not simply a function of ethanol load per se, and underscore the critical role that ethanol dosing interval plays in determining the neuroimmune consequences of alcohol.

A working model for the assessment of disruptions in social behavior among aged rats: The role of sex differences, social recognition, and sensorimotor processes

Aging results in a natural decline in social behavior, yet little is known about the processes underlying these changes. Engaging in positive social interaction is associated with many health benefits, including reduced stress reactivity, and may serve as a potential buffer against adverse consequences of aging. The goal of these studies was to establish a tractable model for the assessment of social behavior deficits associated with late aging. Thus, in Exp. 1, 1.5-, 3-, and 18-month-old male Fischer 344 (F344) rats were assessed for object investigation, and social interaction with a same-aged partner (novel/familiar), or a different-aged partner, thereby establishing working parameters for studies that followed. Results revealed that 18-month-old males exhibited reductions in social investigation and social contact behavior, with this age-related decline not influenced by familiarity or age of the social partner. Subsequently, Exp. 2 extended assessment of social behavior to both male and female F344 rats at multiple ages (3, 9, 18, and 24 months), after which a series of sensorimotor performance tests were conducted. In this study, both males and females exhibited late aging-related reductions in social interactions, but these changes were more pronounced in females. Additionally, sensorimotor performance was shown to be impaired in 24-month-olds, but not 18-month-olds, with this deficit more evident in males. Finally, Exp. 3 examined whether aging-related inflammation could account for declines in social behavior during late aging by administering naproxen (0, 7, 14, and 28 mg/kg; s.c.)-a non-steroidal anti-inflammatory drug-to 18-month-old females. Results from this study revealed that social behavior was unaffected by acute or repeated (6 days) naproxen, suggesting that aging-related social deficits in females may not be a consequence of a general aging-related inflammation and/or malaise. Together, these findings demonstrate that aging-related declines in social behavior are (i) specific to social stimuli and (ii) not indicative of a general state of aging-related debilitation. Thus, these findings establish working parameters for a highly tractable model in which the neural and hormonal mechanisms underlying aging-related declines in social behavior can be examined.