The effects of corticotropin-releasing factor on the cortical EEG are reduced following adolescent nicotine exposure

Development of network oscillations through adolescence in male and female rats

The primary aim of this research was to study the developmental trajectory of oscillatory synchronization in neural networks of normal healthy rats during adolescence, corresponding to the vulnerable age of schizophrenia prodrome in human. To monitor the development of oscillatory networks through adolescence we used a "pseudo-longitudinal" design. Recordings were performed in terminal experiments under urethane anesthesia, every day from PN32 to PN52 using rats-siblings from the same mother, to reduce individual innate differences between subjects. We found that hippocampal theta power decreased and delta power in prefrontal cortex increased through adolescence, indicating that the oscillations in the two different frequency bands follow distinct developmental trajectories to reach the characteristic oscillatory activity found in adults. Perhaps even more importantly, theta rhythm showed age-dependent stabilization toward late adolescence. Furthermore, sex differences was found in both networks, more prominent in the prefrontal cortex compared with hippocampus. Delta increase was stronger in females and theta stabilization was completed earlier in females, in postnatal days PN41-47, while in males it was only completed in late adolescence. Our finding of a protracted maturation of theta-generating networks in late adolescence is overall consistent with the findings of longitudinal studies in human adolescents, in which oscillatory networks demonstrated a similar pattern of maturation.

The impact of adolescent nicotine exposure on alcohol use during adulthood: The role of neuropeptides

Nicotine and alcohol abuse and co-dependence represent major public health crises. Indeed, previous research has shown that the prevalence of alcoholism is higher in smokers than in non-smokers. Adolescence is a susceptible period of life for the initiation of nicotine and alcohol use and the development of nicotine-alcohol codependence. However, there is a limited number of pharmacotherapeutic agents to treat addiction to nicotine or alcohol alone. Notably, there is no effective medication to treat this comorbid disorder. This chapter aims to review the early nicotine use and its impact on subsequent alcohol abuse during adolescence and adulthood as well as the role of neuropeptides in this comorbid disorder. The preclinical and clinical findings discussed in this chapter will advance our understanding of this comorbid disorder's neurobiology and lay a foundation for developing novel pharmacotherapies to treat nicotine and alcohol codependence.

Cannabis use before age 15 and subsequent executive functioning

BACKGROUND

Many studies have suggested that adolescence is a period of particular vulnerability to neurocognitive effects associated with substance misuse. However, few large studies have measured differences in cognitive performance between chronic cannabis users who started in early adolescence (before age 15) with those who started later.

AIMS

To examine the executive functioning of individuals who started chronic cannabis use before age 15 compared with those who started chronic cannabis use after 15 and controls.

METHOD

We evaluated the performance of 104 chronic cannabis users (49 early-onset users and 55 late-onset users) and 44 controls who undertook neuropsychological tasks, with a focus on executive functioning. Comparisons involving neuropsychological measures were performed using generalised linear model analysis of variance (ANOVA).

RESULTS

The early-onset group showed significantly poorer performance compared with the controls and the late-onset group on tasks assessing sustained attention, impulse control and executive functioning.

CONCLUSIONS

Early-onset chronic cannabis users exhibited poorer cognitive performance than controls and late-onset users in executive functioning. Chronic cannabis use, when started before age 15, may have more deleterious effects on neurocognitive functioning.

A psychobiological framework of the substrates that mediate nicotine use during adolescence

Adolescents are especially likely to initiate tobacco use and are more vulnerable to long-term nicotine dependence. A unifying hypothesis is proposed based largely on animals studies that adolescents, as compared to adults, experience enhanced short-term positive and reduced aversive effects of nicotine, as well as less negative effects during nicotine withdrawal. Thus, during adolescence the strong positive effects of nicotine are inadequately balanced by negative effects that contribute to nicotine dependence in adults. This review provides a neural framework to explain developmental differences within the mesolimbic pathway based on the established role of dopamine in addiction. This pathway originates in the ventral tegmental area (VTA) and terminates in the nucleus accumbens (NAcc) where dopamine is increased by nicotine but decreased during withdrawal. During adolescence, excitatory glutamatergic systems that facilitate dopamine are overdeveloped, whereas inhibitory GABAergic systems are underdeveloped. Thus, it is hypothesized that adolescents display enhanced nicotine reward and reduced withdrawal via enhanced excitation and reduced inhibition of VTA cell bodies that release dopamine in the NAcc. Although this framework focuses on adolescents and adults, it may also apply to the understanding of enhanced vulnerability to nicotine in adults that were previously exposed to nicotine during adolescence. The hypothesis presented in this review suggests that the clinical diagnostic criteria developed for nicotine dependence in adults, based primarily on withdrawal, may be inappropriate during adolescence when nicotine withdrawal does not appear to play a major role in nicotine use. Furthermore, treatment strategies involving nicotine replacement may be harmful for adolescents because it may cause enhanced vulnerability to nicotine dependence later in adulthood.

Impact of overactive bladder on the brain: central sequelae of a visceral pathology

Neural circuits that allow for reciprocal communication between the brain and viscera are critical for coordinating behavior with visceral activity. At the same time, these circuits are positioned to convey signals from pathologic events occurring in viscera to the brain, thereby providing a structural basis for comorbid central and peripheral symptoms. In the pons, Barrington's nucleus and the norepinephrine (NE) nucleus, locus coeruleus (LC), are integral to a circuit that links the pelvic viscera with the forebrain and coordinates pelvic visceral activity with arousal and behavior. Here, we demonstrate that a prevalent bladder dysfunction, produced by partial obstruction in rat, has an enduring disruptive impact on cortical activity through this circuit. Within 2 weeks of partial bladder obstruction, the activity of LC neurons was tonically elevated. LC hyperactivity was associated with cortical electroencephalographic activation that was characterized by decreased low-frequency (1-3 Hz) activity and prominent theta oscillations (6-8 Hz) that persisted for 4 weeks. Selective lesion of the LC-NE system significantly attenuated the cortical effects. The findings underscore the potential for significant neurobehavioral consequences of bladder disorders, including hyperarousal, sleep disturbances, and disruption of sensorimotor integration, as a result of central noradrenergic hyperactivity. The results further imply that pharmacological manipulation of central NE function may alleviate central sequelae of these visceral disorders.

Neurobiological processes in adolescent addictive disorders

The purpose of this review is to summarize the neurobiological factors involved in the etiology of adolescent addiction and present evidence implicating various mechanisms in its development. Adolescents are at heightened risk for experimentation with substances, and early experimentation is associated with higher rates of SUD in adulthood. Both normative (e.g., immature frontal-limbic connections, immature frontal lobe development) and non-normative (e.g., lowered serotonergic function, abnormal hypothalamic-pituitary-adrenal axis function) neurobiological developmental factors can predispose adolescents to a heightened risk for SUD. In addition, a normative imbalance in the adolescent neurobiological motivational system may be caused by the relative underdevelopment of suppressive mechanisms when compared to stimulatory systems. These neurobiological liabilities may correspond to neurobehavioral impairments in decision-making, affiliation with deviant peers and externalizing behavior; these and other cognitive and behavioral traits converge with neurobiological factors to increase SUD risk. The progression to SUD acts as an amplifying feedback loop, where the development of SUD results in reciprocal impairments in neurobehavioral and neurobiological processes. A clearer understanding of adolescent neurobiology is a necessary step in the development of prevention and treatment interventions for adolescent SUD.

Differences in ERP topographies during color matching of smoking-related and neutral pictures in smokers and non-smokers

The concept of a specific memory network that drives addictive behavior has often been discussed in relation to the phenomenon of sudden relapse into addiction after years of abstinence. But there is still a lack of data that shows a link between drug-related cue processing and specific changes of behavior in addicts. In the present study we investigated the relationship between smoking-related picture processing, performance in a color matching task, and ERP topographies. Fifteen smokers and 19 non-smoking participants performed a color matching task including monochromic pictures with smoking-related and neutral content. Smokers and non-smokers showed remarkable differences between stimulus category-related ERP topographies. Furthermore, both smokers and non-smokers showed increased reaction times during color matching when the picture contents were related to smoking behavior. The results are discussed with respect to different drug-cue-related patterns of information processing in smokers and non-smokers.

Age of first marijuana use and the occurrence of marijuana use disorders in Southwest California Indians

In several national surveys a younger age of substance usage has been associated with a higher likelihood of the development of dependence. Some studies have suggested that age at first use is primarily an environmentally driven variable, whereas others suggest that it may be partially mediated by a general vulnerability to exhibit problem behaviors. Although Native Americans, overall, have the highest prevalence of substance dependence of any US ethnic group, the relationship of age of first marijuana use on the development of dependence in Native American populations is relatively unknown. Demographic information and DSM-III-R diagnoses were obtained from 525 Southwest California Indian adults residing on contiguous reservations. Multinomial logistic regression was used to investigate the relationship between age of first use and marijuana use disorders. Early marijuana use was found to be strongly associated with abuse and dependence in this population, even in the presence of several other risk factors including externalizing diagnoses. These data suggest that effective environmental prevention efforts at reducing early marijuana use may be an important strategy to lower the prevalence of use disorders in this high risk population.

Adolescent reward system perseveration due to nicotine: studies with methylphenidate

Previous data suggests adolescent nicotine exposure permanently disrupts reward systems. Behavioral pharmacological methods were used to assess the effects of adolescent nicotine exposure on methylphenidate (MPD) sensitivity and reward from adolescence to adulthood. For experiment one, testing was performed on adult mice exposed to nicotine (0.3 and 3.0 mg/kg, SC, M-F, b.i.d.) or saline during adolescence (PND 25-57). After a 28-day drug-free, time-off period, the locomotor effects (30 min, 30 cm traveled) of MPD (5, 10, 20, and 40 mg/kg, IP) were measured. Thereafter, mice underwent condition-place-preference testing (CPP). MPD (20 mg/kg) was paired with the subject's non-preferred side and saline with the preferred side. Conditioning sessions were conducted for 8 days with a drug-free post-test on the day following the final conditioning session. A second experiment was conducted to determine if adolescent mice respond differently to MPD compared to adult mice. The study compared adolescent mice (PND 25-46) to adult mice (PND 77-98) using identical MPD testing procedures as in experiment one. Adult subjects exposed to nicotine during adolescence behaved remarkably similar to adolescent subjects receiving only MPD. Both nicotine exposed subjects and naïve adolescents exhibited increased response to MPD's motor activating effects and a decreased response to MPD's rewarding effects. Taken together it appears that adolescent nicotine exposure retards the development of reward systems, thus, maintaining an adolescent state indefinitely, which could result in increased vulnerability to substance abuse problems throughout adulthood.

Increased CRF-like and NPY-like immunoreactivity in adult rats exposed to nicotine during adolescence: relation to anxiety-like and depressive-like behavior

OBJECTIVE

Recently, animal models have been developed that demonstrate that adolescent nicotine exposure produces neurobehavioral changes which persist into adulthood. This study further examined the impact of adolescent nicotine exposure on anxiety-like and depressive-like behavior, as well as on levels of corticotropin-releasing factor (CRF) and neuropeptide Y (NPY) in this model.

METHODS

Male adolescent rats (35-40 days old) were administered nicotine using Nicoderm CQ patches (Smith-Kline Beecham). Behavior in the elevated plus maze (EPM) and forced swim test (FST) was assessed 2-3 weeks after exposure ended. Brain levels of CRF and NPY were then assessed 5-6 weeks after behavioral tests were completed. In addition, blood and brain levels of nicotine resulting from nicotine treatment were examined.

RESULTS

After 5 days of exposure to 5 mg/kg/day nicotine, blood levels of nicotine averaged 66+/-5 ng/ml and brain nicotine levels averaged 52+/-4 ng/g. Rats exposed to nicotine displayed an anxiety-like profile in the EPM (i.e., decreased time spent in the open arms) and an antidepressant-like profile in the FST (i.e., less time spent immobile). Rats exposed to nicotine also had increased hypothalamic and frontal cortical CRF, increased hypothalamic and hippocampal NPY, and a decreased ratio of NPY to CRF in the amygdala.

CONCLUSIONS

This study demonstrates that adolescent nicotine exposure produces lasting increases in anxiety-like behavior and may reduce depressive-like behavior. These behavioral changes also occurred in concert with alterations in CRF and NPY systems. Thus, lasting neurobehavioral changes associated with adolescent nicotine exposure may be related to allostatic changes in stress peptide systems.

Differential behavioral effects of nicotine exposure in adolescent and adult rats

RATIONALE

Although the detrimental effects of nicotine in early brain development and the addictive properties in adulthood are well known, little is known about the neurobiological effects of nicotine in adolescence. An important question is whether adolescents and adults differ in the development of nicotine sensitization and drug-cue conditioning.

OBJECTIVE

To examine the behavioral effects of multiple, repeated injections of nicotine on both sensitization and drug-cue conditioning in the adolescent rat, and to compare this profile with the adult rat.

METHODS

Sixteen male adolescent (28 day) and 16 young adult (70 day) rats were given injections of either saline or nicotine and tested for motor activity for 90 min for ten consecutive days. Following 4 days of no testing, animals were given a sham injection and placed in the testing apparatus for 90 min. A dose-response curve for nicotine was also generated using two additional groups of ten adolescent and ten adult male rats.

RESULTS

Adolescent rats, unlike adults, did not exhibit signs of nicotine-cue conditioning, and displayed less robust sensitization to the locomotor effects of nicotine than adults. Dose-response testing revealed differences in adolescent responsivity to nicotine in measures of rearing, but not ambulation. Initial exposure to nicotine resulted in increased sensitivity to the motor-activating effects of nicotine but less sensitivity to the depressant effects of nicotine in rearing in adolescents.

CONCLUSIONS

Adolescent animals display different long-term neuroadaptive responses to nicotine than adult animals, possibly related to immature or still-developing plasticity mechanisms in the prefrontal cortex.

Long-term neurobehavioral effects of alcohol or nicotine exposure in adolescent animal models

Adolescent alcohol and nicotine abuse is common, but its neurodevelopmental consequences remain unclear. This laboratory utilized adolescent rodent models to assess the hypothesis that adolescents are highly susceptible to the effects of alcohol and nicotine. Rats were exposed to ethanol for 10-14 days using an intermittent vapor inhalation paradigm. Rats were continuously exposed to nicotine for 5 days using Nicoderm CQ transdermal patches. Alcohol or nicotine exposure altered neurobehavioral function when assessed after 3-7 weeks of abstinence. Alcohol-induced changes include increased electroencephalographic (EEG) frequency, decreased amplitude of the cortical N1 and hippocampal P3 event-related potential (ERP) components, enhanced anxiety-like behavior, and enhanced depressive-like behavior. Nicotine-induced changes include decreased slow-wave cortical EEG power, increased cortical N1 ERP amplitude, decreased motor activity, and increased anxiety-like behavior. These findings support the hypothesis that adolescents are uniquely susceptible to the effects of chronic alcohol and nicotine exposure.