Prenatal exposure to ethanol increases ethanol consumption: a conditioned response?

The interaction of genetic sex and prenatal alcohol exposure on health across the lifespan

Prenatal alcohol exposure (PAE) can reprogram the development of cells and tissues, resulting in a spectrum of physical and neurobehavioral teratology. PAE immediately impacts fetal growth, but its effects carry forward post-parturition, into adolescence and adulthood, and can result in a cluster of disabilities, collectively termed Fetal Alcohol Spectrum Disorders. Emerging preclinical and clinical research investigating neurological and behavioral outcomes in exposed offspring point to genetic sex as an important modifier of the effects of PAE. In this review, we discuss the literature on sex differences following PAE, with studies spanning the fetal period through adulthood, and highlight gaps in research where sex differences are likely, but currently under-investigated. Understanding how sex and PAE interact to affect offspring health outcomes across the lifespan is critical for identifying the full complement of PAE-associated secondary conditions, and for refining targeted interventions to improve the quality of life for individuals with PAE.

Prenatal alcohol exposure and attention-deficit/hyperactivity disorder independently predict greater substance use in young adulthood

BACKGROUND

The degree to which prenatal alcohol exposure (PAE) may influence alcohol and drug use in adulthood is difficult to determine. That is because PAE is highly correlated with environmental factors, including low socioeconomic status and exposure to parental drinking, and with behavioral problems, such as, attention-deficit/hyperactivity disorder (ADHD), which are correlated with alcohol use and abuse.

METHODS

Participants were 121 young adults from our Detroit Longitudinal Cohort study. Mothers were recruited during pregnancy and interviewed about their alcohol consumption using a timeline follow-back procedure. At 19 years, their offspring were interviewed regarding current and past use of alcohol, cigarettes, and other illicit drugs.

RESULTS

PAE was associated with greater alcohol, cannabis, and cigarette use. PAE, assessed using overall alcohol intake during pregnancy and alcohol dose per occasion, was associated with larger quantities of alcohol per occasion and greater alcohol tolerance in early adulthood. These effects persisted after control for demographic background, sex, age and education of participant, home environment, other prenatal drug exposure, and postnatal alcohol and drug use by the primary caregiver. Whereas ADHD predicted average alcohol consumed/month during young adulthood, PAE predicted alcohol dose/drinking occasion, and the effect on dose/occasion was not mediated by ADHD.

CONCLUSIONS

The effects of PAE on alcohol and cannabis use in young adulthood are not attributable to being reared in an environment that is socioeconomically disadvantaged or in one in which there is extensive maternal drinking. Furthermore, PAE was related to enhanced alcohol tolerance in young adults, a risk factor for alcohol use disorder later in life. Although ADHD was associated with greater alcohol consumption in early adulthood, it did not mediate the effect of PAE on offspring's alcohol use.

Prenatal opioid exposure reprograms the behavioural response to future alcohol reward

As the opioid crisis has continued to grow, so has the number of infants exposed to opioids during the prenatal period. A growing concern is that prenatal exposure to opioids may induce persistent neurological changes that increase the propensity for future addictions. Although alcohol represents the most likely addictive substance that the growing population of prenatal opioid exposed will encounter as they mature, no studies to date have examined the effect of prenatal opioid exposure on future sensitivity to alcohol reward. Using a recently developed mouse model of prenatal methadone exposure (PME), we investigated the rewarding properties of alcohol and alcohol consumption in male and female adolescent PME and prenatal saline exposed (PSE) control animals. Conditioned place preference to alcohol was disrupted in PME offspring in a sex-dependent manner with PME males exhibiting resistance to the rewarding properties of alcohol. Repeated injections of alcohol revealed enhanced sensitivity to the locomotor-stimulating effects of alcohol specific to PME females. PME males consumed significantly more alcohol over 4 weeks of alcohol access relative to PSE males and exhibited increased resistance to quinine-adulterated alcohol. Further, a novel machine learning model was developed to employ measured differences in alcohol consumption and drinking microstructure to reliably predict prenatal exposure. These findings indicate that PME alters the sensitivity to alcohol reward in adolescent mice in a sex-specific manner and suggests prenatal opioid exposure may induce persistent effects on reward neurocircuitry that can reprogram offspring behavioural response to alcohol later in life.

Investigating the effects of chronic perinatal alcohol and combined nicotine and alcohol exposure on dopaminergic and non-dopaminergic neurons in the VTA

The ventral tegmental area (VTA) is the origin of dopaminergic neurons and the dopamine (DA) reward pathway. This pathway has been widely studied in addiction and drug reinforcement studies and is believed to be the central processing component of the reward circuit. In this study, we used a well-established rat model to expose mother dams to alcohol, nicotine-alcohol, and saline perinatally. DA and non-DA neurons collected from the VTA of the rat pups were used to study expression profiles of miRNAs and mRNAs. miRNA pathway interactions, putative miRNA-mRNA target pairs, and downstream modulated biological pathways were analyzed. In the DA neurons, 4607 genes were differentially upregulated and 4682 were differentially downregulated following nicotine-alcohol exposure. However, in the non-DA neurons, only 543 genes were differentially upregulated and 506 were differentially downregulated. Cell proliferation, differentiation, and survival pathways were enriched after the treatments. Specifically, in the PI3K/AKT signaling pathway, there were 41 miRNAs and 136 mRNAs differentially expressed in the DA neurons while only 16 miRNAs and 20 mRNAs were differentially expressed in the non-DA neurons after the nicotine-alcohol exposure. These results depicted that chronic nicotine and alcohol exposures during pregnancy differentially affect both miRNA and gene expression profiles more in DA than the non-DA neurons in the VTA. Understanding how the expression signatures representing specific neuronal subpopulations become enriched in the VTA after addictive substance administration helps us to identify how neuronal functions may be altered in the brain.

Early-life exposure to alcohol and the risk of alcohol-induced liver disease in adulthood

Alcohol consumption remains prevalent among pregnant and nursing mothers despite the well-documented adverse effects this may have on the offspring. Moderate-to-high levels of alcohol consumption in pregnancy result in fetal alcohol syndrome (FAS) disorders, with brain defects being chief among the abnormalities. Recent findings indicate that while light-to-moderate levels may not cause FAS, it may contribute to epigenetic changes that make the offspring prone to adverse health outcomes including metabolic disorders and an increased propensity in the adolescent-onset of drinking alcohol. On the one hand, prenatal alcohol exposure (PAE) causes epigenetic changes that affect lipid and glucose transcript regulating genes resulting in metabolic abnormalities. On the other hand, it can program offspring for increased alcohol intake, enhance its palatability, and increase acceptance of alcohol's flavor through associative learning, making alcohol a plausible second hit for the development of alcohol-induced liver disease. Adolescent drinking results in alcohol dependence and abuse in adulthood. Adolescent drinking results in alcohol dependence and abuse in adulthood. Alterations on the opioid system, particularly, the mu-opioid system, has been implicated in the mechanism that induces increased alcohol consumption and acceptance. This review proposes a mechanism that links PAE to the development of alcoholism and eventually to alcoholic liver disease (ALD), which results from prolonged alcohol consumption. While PAE may not lead to ALD development in childhood, there are chances that it may lead to ALD in adulthood.

Unraveling the molecular mechanisms involved in alcohol intake and withdrawal in adolescent mice exposed to alcohol during early life stages

Alcohol interferes with foetal development and prenatal alcohol exposure can lead to adverse effects known as foetal alcohol spectrum disorders. We aimed to assess the underlying neurobiological mechanisms involved in alcohol intake and withdrawal in adolescent mice exposed to alcohol during early life stages, in discrete brain areas. Pregnant C57BL/6 female mice were exposed to binge alcohol drinking from gestation to weaning. Subsequently, alcohol seeking and taking behaviour were evaluated in male adolescent offspring, as assessed in the two-bottle choice and oral self-administration paradigms. Brain area samples were analysed to quantify AMPAR subunits GluR1/2 and pCREB/CREB expression following alcohol self-administration. We measured the expression of mu and kappa opioid receptors both during acute alcohol withdrawal (assessing anxiety alterations by the EPM test) and following reinstatement in the two-bottle choice paradigm. In addition, alcohol metabolism was analysed by measuring blood alcohol concentrations under an acute dose of 3 g/kg alcohol. Our findings demonstrate that developmental alcohol exposure enhances alcohol intake during adolescence, which is associated with a decrease in the pCREB/CREB ratio in the hippocampus, prefrontal cortex and striatum, while the GluR1/GluR2 ratio showed a decrease in the hippocampus. Moreover, PLAE mice showed behavioural alterations, such as increased anxiety-like responses during acute alcohol withdrawal, and higher BAC levels. No significant changes were identified for mu and kappa opioid receptors mRNA expression. The current study highlights that early alcohol exposed mice increased alcohol consumption during late adolescence. Furthermore, a diminished CREB signalling and glutamatergic neuroplasticity are proposed as underpinning neurobiological mechanisms involved in the sensitivity to alcohol reinforcing properties.

Learning experiences comprising central ethanol exposure in rat neonates: Impact upon respiratory plasticity and the activity of brain catalase

Fetal ethanol exposure represents a risk factor for sudden infant death syndrome, and the respiratory effects of fetal ethanol exposure promote hypoxic ischemic consequences. This study analyzes central ethanol's effects upon breathing plasticity during an ontogenetic stage equivalent to the human third gestational trimester. Ethanol's unconditioned breathing effects and their intervention in learning processes were examined. Since central ethanol is primarily metabolized via the catalase system, we also examined the effects of early history with the drug upon this system. During postnatal days 3, 5, and 7 (PDs 3-7), pups were intracisternally administered with vehicle or ethanol (300 mg%). They were tested in a plethysmograph scented or not scented with ethanol odor. The state of intoxication attenuated the onset of apneas, a phenomenon that is suggestive of ethanol's anxiolytic effects given the state of arousal caused by the novel environment and the stress of ethanol administration. At PD9, pups were evaluated when sober under sequential air conditions (initial-normoxia, hypoxia, and recovery-normoxia), with or without the presence of ethanol odor. Initial apneic episodes increased when ethanol intoxication was previously associated with the odor. Pups then ingested ethanol, and brain catalase activity was determined. Pre-exposure to ethanol intoxication paired with the odor of the drug resulted in heightened enzymatic activity. Central ethanol exposure appears to exert antianxiety effects that attenuate apneic disruptions. However, during withdrawal, the cues associated with such effects elicit an opposite reaction. The activity of the catalase system was also dependent upon learning processes that involved the association of environmental stimuli and ethanol intoxication.

Preclinical methodological approaches investigating of the effects of alcohol on perinatal and adolescent neurodevelopment

Despite much evidence of its economic and social costs, alcohol use continues to increase. Much remains to be known as to the effects of alcohol on neurodevelopment across the lifespan and in both sexes. We provide a comprehensive overview of the methodological approaches to ethanol administration when using animal models (primarily rodent models) and their translational relevance, as well as some of the advantages and disadvantages of each approach. Special consideration is given to early developmental periods (prenatal through adolescence), as well as to the types of research questions that are best addressed by specific methodologies. The zebrafish is used increasingly in alcohol research, and how to use this model effectively as a preclinical model is reviewed as well.

Fetal Alcohol Programming of Subsequent Alcohol Affinity: A Review Based on Preclinical, Clinical and Epidemiological Studies

The anatomo-physiological disruptions inherent to different categories of the Fetal Alcohol Spectrum Disorder do not encompass all the negative consequences derived from intrauterine ethanol (EtOH) exposure. Preclinical, clinical and epidemiological studies show that prenatal EtOH exposure also results in early programming of alcohol affinity. This affinity has been addressed through the examination of how EtOH prenatally exposed organisms recognize and prefer the drug’s chemosensory cues and their predisposition to exhibit heightened voluntary EtOH intake during infancy and adolescence. In altricial species these processes are determined by the interaction of at least three factors during stages equivalent to the 2nd and 3rd human gestational trimester: (i) fetal processing of the drug’s olfactory and gustatory attributes present in the prenatal milieu; (ii) EtOH’s recruitment of central reinforcing effects that also imply progressive sensitization to the drug’s motivational properties; and (iii) an associative learning process involving the prior two factors. This Pavlovian learning phenomenon is dependent upon the recruitment of the opioid system and studies also indicate a significant role of EtOH’s principal metabolite (acetaldehyde, ACD) which is rapidly generated in the brain via the catalase system. The central and rapid accumulation of this metabolite represents a major factor involved in the process of fetal alcohol programming. According to recent investigations, it appears that ACD exerts early positive reinforcing consequences and antianxiety effects (negative reinforcement). Finally, this review also acknowledges human clinical and epidemiological studies indicating that moderate and binge-like drinking episodes during gestation result in neonatal recognition of EtOH’s chemosensory properties coupled with a preference towards these cues. As a whole, the studies under discussion emphasize the notion that even subteratogenic EtOH exposure during fetal life seizes early functional sensory and learning capabilities that pathologically shape subsequent physiological and behavioral reactivity towards the drug.

Prenatal Alcohol Exposure as a Case of Involuntary Early Onset of Alcohol Use: Consequences and Proposed Mechanisms From Animal Studies

Prenatal alcohol exposure has been found to be an important factor determining later consumption of this drug. In humans, despite the considerable diversity of variables that might influence alcohol consumption, longitudinal studies show that maternal alcohol intake during gestation is one of the best predictors of later alcohol use from adolescence to young adulthood. Experimental studies with animals also provide abundant evidence of the effects of prenatal alcohol exposure on later alcohol intake. In addition to increased consumption, other effects include enhanced palatability and attractiveness of alcohol flavor as well as sensitization to its sensory and reinforcing effects. Most of these outcomes have been obtained after exposing rats to binge-like administrations of moderate alcohol doses during the last gestational period when the fetus is already capable of detecting flavors in the amniotic fluid and learning associations with aversive or appetitive consequences. On this basis, it has been proposed that one of the mechanisms underlying the increased acceptance of alcohol after its prenatal exposure is the acquisition (by the fetus) of appetitive learning via an association between the sensory properties of alcohol and its reinforcing pharmacological effects. It also appears that this prenatal appetitive learning is mediated by the activation of the opioid system, with fetal brain acetaldehyde playing an important role, possibly as the main chemical responsible for its activation. Here, we review and analyze together the results of all animal studies testing these hypotheses through experimental manipulation of the behavioral and neurochemical elements of the assumed prenatal association. Understanding the mechanisms by which prenatal alcohol exposure favors the early initiation of alcohol consumption, along with its role in the causal pathway to alcohol disorders, may allow us to find strategies to mitigate the behavioral effects of this early experience with the drug. We propose that prenatal alcohol exposure is regarded as a case of involuntary early onset of alcohol use when designing prevention policies. This is particularly important, given the notion that the sooner alcohol intake begins, the greater the possibility of a continued history of alcohol consumption that may lead to the development of alcohol use disorders.

Early exposure to environmental enrichment modulates the effects of prenatal ethanol exposure upon opioid gene expression and adolescent ethanol intake

Prenatal ethanol exposure (PEE) promotes ethanol consumption in the adolescent offspring accompanied by the transcriptional regulation of kappa opioid receptor (KOR) system genes. This study analysed if environmental enrichment (EE, from gestational day 20 to postnatal day 26) exerts protective effects upon PEE-modulation of gene expression, ethanol intake and anxiety responses. Pregnant rats were exposed to PEE (0.0 or 2.0 g/kg ethanol, gestational days 17-20) and subsequently the dam and offspring were reared under EE or standard conditions. PEE upregulated KOR mRNA levels in amygdala (AMY) and prodynorphin (PDYN) mRNA levels in ventral tegmental area (VTA) with the latter effect associated with lower DNA methylation at the gene promoter. These effects were normalized by exposure to EE. PEE modulated BDNF mRNA levels in VTA and Nucleus accumbens (AcbN), and EE mitigated the changes in AcbN. EE induced a protective effect on ethanol intake and preference, an effect more noticeable in males than in females, and in prenatal vehicle-treated (PV) than in PEE rats. The male offspring drank significantly less ethanol than the female offspring. The latter result suggests that the protective effect of EE on ethanol drinking may only emerge at lower levels of drinking. In the dams, PEE induced an upregulation of PDYN and KOR in AcbN. PDYN gene expression was normalized by exposure to EE. These results suggest that EE is a promising treatment to inhibit the effects of PEE. The results confirm that PEE effects are mediated by alterations in the transcriptional regulation of KOR system genes.

Effects of Fetal Substance Exposure on Offspring Substance Use

Prenatal exposure to alcohol and drugs is associated with physical, cognitive, and behavioral problems across the offspring's lifespan and an increased risk of alcohol and drug use in adolescent and young adult offspring. These prenatal effects continue to be evident after control for demographic background and parental alcohol and drug use. Behavior problems in childhood and adolescence associated with prenatal exposures may serve as a mediator of the prenatal exposure effects on offspring substance use.

Prenatal ethanol exposure attenuates sensitivity to the aversive effects of ethanol in adolescence and increases adult preference for a 5% ethanol solution in males, but not females

The present set of experiments investigated the effects of a moderate dose of ethanol (2 g/kg; 20% v/v intragastrically) during late gestation (G17-20 [gestational day]) on ethanol-induced conditioned taste aversion (CTA) in adolescence, and on ethanol consumption during adolescence and early adulthood. In experiment 1, male and female Sprague-Dawley rats were given 30-min access to a sweetened "supersaccharin" (SS) solution or sodium chloride (NaCl), followed by an intraperitoneal injection of 20% ethanol (0, 1, 1.25, or 1.5 g/kg) for three conditioning/test sessions. Among animals conditioned with SS, prenatally ethanol-exposed males exhibited attenuated ethanol-induced CTA relative to males prenatally gavaged with water or non-manipulated, whereas prenatal treatment had no effect on CTA in females. Among animals conditioned with NaCl, there were no exposure group differences in males, with modest evidence for attenuated CTA in prenatally ethanol-exposed females. In experiment 2, the effects of prenatal ethanol exposure on ethanol consumption in adolescents (P35 ± 1 day [postnatal day]) and adults (P56-60) were explored. At the beginning of the dark cycle, pair-housed rats were given three bottles containing 0, 5, and 10% ethanol for 18 h every other day (i.e., Monday, Wednesday, Friday) for 3 weeks. Relative to water controls, adult males prenatally exposed to ethanol showed greater preference and more intake (g/kg) of 5% ethanol, while showing lower intake of 10% ethanol. These intake and preference differences were not evident in adolescent males. Among females at both ages, ethanol-exposed animals showed lower preference and intake (g/kg) of 5% ethanol than their water-exposed controls. Thus, moderate ethanol exposure during late gestation produced a largely male-specific attenuation in the aversive effects of ethanol during adolescence that could contribute to later increases in preference and intake of a 5% ethanol solution, although this emergent effect was not evident in adolescence (or in females), but only manifested in adulthood.

Maternal manipulation during late gestation (GDs 17-20) enhances ethanol consumption and promotes changes and opioid mRNA expression in infant rats

Fetal ethanol experience generates learning and memories capable to increase ethanol consummatory behaviors during infancy. Opioid system seems to be involved in mediating those alcohol-related behaviors. In this work, we proposed to study the impact of prenatal exposure to a moderate ethanol dose, upon ingestion of the drug and possible ethanol-induced molecular changes on opioid precursor peptides (POMC, Pro-enk and Pro-DYN) and receptors (MOR, DOR and KOR) mRNA expression, in hypothalamus. Pregnant rats received during gestational days (GDs) 17-20, a daily intragastric (i.g.) administration with 2g/kg ethanol or water. A third group of dams was left undisturbed during pregnancy (Unmanipulated group). Intake test was conducted at postnatal days (PDs) 14-15. Three groups of pups were performed: control (no intake test), water (vehicle) and 5% ethanol. At the end of intake test blood samples were taken to quantify blood ethanol concentrations (BECs) and hypothalamus sections were obtained to perform qRT-PRC assessment of opioid precursor peptides and receptors. The analysis of the consummatory responses (% of consumption) and pharmacokinetic profiles (BECs) suggested that maternal manipulation induced by i.g. intubations, during the last four days of gestation (whenever ethanol or water), are sufficient to induce infantile ethanol intake during infancy. Gene expression from the hypothalamus of unmanipulated group revealed that infantile ingestive experiences with ethanol can down-regulate expression of mRNA Pro-Dyn and up-regulate mRNA expression of MOR and KOR. Finally, MOR mRNA expression was attenuated by prenatal i.g. manipulation in pups exposed to 5% ethanol.

Hypothalamic CCL2/CCR2 Chemokine System: Role in Sexually Dimorphic Effects of Maternal Ethanol Exposure on Melanin-Concentrating Hormone and Behavior in Adolescent Offspring

Clinical and animal studies show that ethanol exposure and inflammation during pregnancy cause similar behavioral disturbances in the offspring. While ethanol is shown to stimulate both neuroimmune and neurochemical systems in adults, little is known about their anatomical relationship in response to ethanol in utero and whether neuroimmune factors mediate ethanol's effects on neuronal development and behavior in offspring. Here we examined in female and male adolescent rats a specific population of neurons concentrated in lateral hypothalamus, which coexpress the inflammatory chemokine C-C motif ligand 2 (CCL2) or its receptor CCR2 with the orexigenic neuropeptide, melanin-concentrating hormone (MCH), that promotes ethanol drinking behavior. We demonstrate that maternal administration of ethanol (2 g/kg/d) from embryonic day 10 (E10) to E15, while having little impact on glia, stimulates expression of neuronal CCL2 and CCR2, increases density of both large CCL2 neurons colocalizing MCH and small CCL2 neurons surrounding MCH neurons, and stimulates ethanol drinking and anxiety in adolescent offspring. We show that these neuronal and behavioral changes are similarly produced by maternal administration of CCL2 (4 or 8 μg/kg/d, E10-E15) and blocked by maternal administration of a CCR2 antagonist INCB3344 (1 mg/kg/d, E10-E15), and these effects of ethanol and CCL2 are sexually dimorphic, consistently stronger in females. These results suggest that this neuronal CCL2/CCR2 system closely linked to MCH neurons has a role in mediating the effects of maternal ethanol exposure on adolescent offspring and contributes to the higher levels of adolescent risk factors for alcohol use disorders described in women.SIGNIFICANCE STATEMENT Ethanol consumption and inflammatory agents during pregnancy similarly increase alcohol intake and anxiety in adolescent offspring. To investigate how neurochemical and neuroimmune systems interact to mediate these disturbances, we examined a specific population of hypothalamic neurons coexpressing the inflammatory chemokine CCL2 and its receptor CCR2 with the neuropeptide, melanin-concentrating hormone. We demonstrate in adolescent offspring that maternal administration of CCL2, like ethanol, stimulates these neurons and increases ethanol drinking and anxiety, and these effects of ethanol are blocked by maternal CCR2 antagonist and consistently stronger in females. This suggests that neuronal chemokine signaling linked to neuropeptides mediates effects of maternal ethanol exposure on adolescent offspring and contributes to higher levels of adolescent risk factors for alcohol use disorders in women.

Effects of alcohol and psychostimulants on the vasopressin system: behavioral implications

Drug addiction is a chronic brain disease characterized by a compulsion to seek drugs, a loss of control with respect to drug consumption, and negative emotional states, including increased anxiety and irritability during withdrawal. Central vasopressin (AVP) and its receptors are involved in controlling social behavior, anxiety and reward, all of which are altered by drugs of abuse. Hypothalamic AVP neurons influence the stress response by modulating the hypothalamic-pituitary-adrenal (HPA) axis. The extrahypothalamic AVP system, however, is commonly associated with social recognition, motivational and anxiety responses. The specific relationship between AVP and drugs of abuse has been rarely reviewed. Here, we provide an overview of the interaction between the brain AVP system and psychostimulants and alcohol. We focus on the effects of alcohol and psychostimulants on AVP regulation of the HPA axis, their effect on the brain AVP system and their behavioral implications, the influence of the AVP system on addictive behaviors, AVP's organizational effects on the brain and consequently on behavior, and we highlight clinical studies on the relation between the AVP system and drug addiction. Finally, we discuss the data to address areas that need further research to support clinical trials and prevent drug-related disorders. This article is protected by copyright. All rights reserved.

Prenatal ethanol exposure modifies locomotor activity and induces selective changes in Met-enk expression in adolescent rats

Several studies suggest that prenatal ethanol exposure (PEE) facilitates ethanol intake. Opioid peptides play a main role in ethanol reinforcement during infancy and adulthood. However, PEE effects upon motor responsiveness elicited by an ethanol challenge and the participation of opioids in these actions remain to be understood. This work assessed the susceptibility of adolescent rats to prenatal and/or postnatal ethanol exposure in terms of behavioral responses, as well as alcohol effects on Met-enk expression in brain areas related to drug reinforcement. Motor parameters (horizontal locomotion, rearings and stereotyped behaviors) in pre- and postnatally ethanol-challenged adolescents were evaluated. Pregnant rats received ethanol (2g/kg) or water during gestational days 17-20. Adolescents at postnatal day 30 (PD30) were tested in a three-trial activity paradigm (habituation, vehicle and drug sessions). Met-enk content was quantitated by radioimmunoassay in several regions: ventral tegmental area [VTA], nucleus accumbens [NAcc], prefrontal cortex [PFC], substantia nigra [SN], caudate-putamen [CP], amygdala, hypothalamus and hippocampus. PEE significantly reduced rearing responses. Ethanol challenge at PD30 decreased horizontal locomotion and showed a tendency to reduce rearings and stereotyped behaviors. PEE increased Met-enk content in the PFC, CP, hypothalamus and hippocampus, but did not alter peptide levels in the amygdala, VTA and NAcc. These findings suggest that PEE selectively modifies behavioral parameters at PD30 and induces specific changes in Met-enk content in regions of the mesocortical and nigrostriatal pathways, the hypothalamus and hippocampus. Prenatal and postnatal ethanol actions on motor activity in adolescents could involve activation of specific neural enkephalinergic pathways.

The Role of Acetaldehyde in the Increased Acceptance of Ethanol after Prenatal Ethanol Exposure

Recent studies show that acetaldehyde, the first metabolite in the oxidation of ethanol, can be responsible for both, the appetitive and the aversive effects produced by ethanol intoxication. More specifically, it has been hypothesized that acetaldehyde produced in the periphery by the liver is responsible for the aversive effects of ethanol, while the appetitive effects relate to the acetaldehyde produced centrally through the catalase system. On the other hand, from studies in our and other laboratories, it is known that ethanol exposure during the last gestational days (GD) consistently enhances the postnatal acceptance of ethanol when measured during early ontogeny in the rat. This increased liking of ethanol is a conditioned appetitive response acquired by the fetus by the association of ethanol’s flavor and an appetitive reinforcer. Although this reinforcer has not yet been fully identified, one possibility points to acetaldehyde produced centrally in the fetus as a likely candidate. This hypothesis is supported by data showing that very early in the rat’s ontogeny brain catalases are functional, while the liver’s enzymatic system is still immature. In this study, rat dams were administered on GD 17–20 with water or ethanol, together with an acetaldehyde-sequestering agent (D-penicillamine). The offspring’s responses to ethanol was then assessed at different postnatal stages with procedures adequate for each developmental stage: on day 1, using the “odor crawling locomotion test” to measure ethanol’s odor attractiveness; on day 5, in an operant conditioning procedure with ethanol as the reinforcer; and on day 14 in an ethanol intake test. Results show that the absence of acetaldehyde during prenatal ethanol exposure impeded the observation of the increased acceptance of ethanol at any age. This seems to confirm the crucial role of acetaldehyde as a reinforcer in the appetitive learning occurring during prenatal ethanol exposure.

Consumption of Substances of Abuse during Pregnancy Increases Consumption in Offspring: Possible Underlying Mechanisms

Correlative human observational studies on substances of abuse have been highly dependent on the use of rodent models to determine the neuronal and molecular mechanisms that control behavioral outcomes. This is particularly true for gestational exposure to non-illicit substances of abuse, such as excessive dietary fat, ethanol, and nicotine, which are commonly consumed in our society. Exposure to these substances during the prenatal period has been shown in offspring to increase their intake of these substances, induce other behavioral changes, and affect neurochemical systems in several brain areas that are known to control behavior. More importantly, emerging studies are linking the function of the immune system to these neurochemicals and ingestion of these abused substances. This review article will summarize the prenatal rodent models used to study developmental changes in offspring caused by prenatal exposure to dietary fat, ethanol, or nicotine. We will discuss the various techniques used for the administration of these substances into rodents and summarize the published outcomes induced by prenatal exposure to these substances. Finally, this review will cover some of the recent evidence for the role of immune factors in causing these behavioral and neuronal changes.

Pharmacological characterization of the nociceptin/orphanin FQ receptor on ethanol-mediated motivational effects in infant and adolescent rats

Activation of nociceptin/orphanin FQ (NOP) receptors attenuates ethanol drinking and prevents relapse in adult rodents. In younger rodents (i.e., infant rats), activation of NOP receptors blocks ethanol-induced locomotor activation but does not attenuate ethanol intake. The aim of the present study was to extend the analysis of NOP modulation of ethanol's effects during early ontogeny. Aversive and anxiolytic effects of ethanol were measured in infant and adolescent rats via conditioned taste aversion and the light-dark box test; whereas ethanol-induced locomotor activity and ethanol intake was measured in adolescents only. Before these tests, infant rats were treated with the natural ligand of NOP receptors, nociceptin (0.0, 0.5 or 1.0 μg) and adolescent rats were treated with the specific agonist Ro 64-6198 (0.0, 0.1 or 0.3 mg/kg). The activation of NOP receptors attenuated ethanol-induced anxiolysis in adolescents only, and had no effect on ethanol's aversive effects. Administration of Ro 64-6198 blocked ethanol-induced locomotor activation but did not modify ethanol intake patterns. The attenuation of ethanol stimulating and anxiolytic effect by activation of NOP receptors indicates a modulatory role of this receptor on ethanol effects, which is expressed early in ontogeny.

Prenatal exposure to ethanol stimulates hypothalamic CCR2 chemokine receptor system: Possible relation to increased density of orexigenic peptide neurons and ethanol drinking in adolescent offspring

Clinical and animal studies indicate that maternal consumption of ethanol during pregnancy increases alcohol drinking in the offspring. Possible underlying mechanisms may involve orexigenic peptides, which are stimulated by prenatal ethanol exposure and themselves promote drinking. Building on evidence that ethanol stimulates neuroimmune factors such as the chemokine CCL2 that in adult rats is shown to colocalize with the orexigenic peptide, melanin-concentrating hormone (MCH) in the lateral hypothalamus (LH), the present study sought to investigate the possibility that CCL2 or its receptor CCR2 in LH is stimulated by prenatal ethanol exposure, perhaps specifically within MCH neurons. Our paradigm of intraoral administration of ethanol to pregnant rats, at low-to-moderate doses (1 or 3g/kg/day) during peak hypothalamic neurogenesis, caused in adolescent male offspring twofold increase in drinking of and preference for ethanol and reinstatement of ethanol drinking in a two-bottle choice paradigm under an intermittent access schedule. This effect of prenatal ethanol exposure was associated with an increased expression of MCH and density of MCH(+) neurons in LH of preadolescent offspring. Whereas CCL2(+) cells at this age were low in density and unaffected by ethanol, CCR2(+) cells were dense in LH and increased by prenatal ethanol, with a large percentage (83-87%) identified as neurons and found to colocalize MCH. Prenatal ethanol also stimulated the genesis of CCR2(+) and MCH(+) neurons in the embryo, which co-labeled the proliferation marker, BrdU. Ethanol also increased the genesis and density of neurons that co-expressed CCR2 and MCH in LH, with triple-labeled CCR2(+)/MCH(+)/BrdU(+) neurons that were absent in control rats accounting for 35% of newly generated neurons in ethanol-exposed rats. With both the chemokine and MCH systems believed to promote ethanol consumption, this greater density of CCR2(+)/MCH(+) neurons in the LH of preadolescent rats suggests that these systems function together in promoting alcohol drinking during adolescence.

Prenatal ethanol increases ethanol intake throughout adolescence, alters ethanol-mediated aversive learning, and affects μ but not δ or κ opioid receptor mRNA expression

Animal models of prenatal ethanol exposure (PEE) have indicated a facilitatory effect of PEE on adolescent ethanol intake, but few studies have assessed the effects of moderate PEE throughout adolescence. The mechanisms underlying this facilitatory effect remain largely unknown. In the present study, we analysed ethanol intake in male and female Wistar rats with or without PEE (2.0 g/kg, gestational days 17-20) from postnatal days 37 to 62. The results revealed greater ethanol consumption in PEE rats than in controls, which persisted throughout adolescence. By the end of testing, ethanol ingestion in PEE rats was nearly 6.0 g/kg. PEE was associated with insensitivity to ethanol-induced aversion. PEE and control rats were further analysed for levels of μ, δ and κ opioid receptor mRNA in the infralimbic cortex, nucleus accumbens shell, and ventral tegmental area. Similar levels of mRNA were observed across most areas and opioid receptors, but μ receptor mRNA in the ventral tegmental area was significantly increased by PEE. Unlike previous studies that assessed the effects of PEE on ethanol intake close to birth, or in only a few sessions during adolescence, the present study observed a facilitatory effect of PEE that lasted throughout adolescence. PEE was associated with insensitivity to the aversive effect of ethanol, and increased levels of μ opioid receptor transcripts. PEE is a prominent vulnerability factor that probably favors the engagement of adolescents in risky trajectories of ethanol use.

Endogenous opioids as substrates for ethanol intake in the neonatal rat: The impact of prenatal ethanol exposure on the opioid family in the early postnatal period

BACKGROUND

Despite considerable knowledge that prenatal ethanol exposure can lead to devastating effects on the developing fetus, alcohol consumption by pregnant women remains strikingly prevalent. Both clinical and basic research has suggested that, in addition to possible physical, behavioral, and cognitive deficits, gestational exposure to alcohol may lead to an increased risk for the development of later alcohol-related use and abuse disorders. The current work sought to characterize alterations in endogenous opioid signaling peptides and gene expression produced by ethanol exposure during the last days of gestation.

METHODS

Experimental subjects were 4-, 8-, and 12-day old infant rats obtained from pregnant females that were given daily intubations of 0, 1, or 2g/kg ethanol during the last few days of gestation (GDs 17-20). Using real-time RT-PCR, western blotting analysis, and enzyme immunoassays, we examined mRNA and protein for three opioid receptors and ligands in the nucleus accumbens, ventral tegmental area, and hypothalamus.

RESULTS

Three main trends emerged - (1) mRNA for the majority of factors was found to upregulate across each of the three postnatal ages assessed, indicative of escalating ontogenetic expression of opioid-related genes; (2) prenatal ethanol significantly reduced many opioid peptides, suggesting a possible mechanism by which prenatal exposure can affect future responsiveness towards ethanol; and (3) the nucleus accumbens emerged as a key site for ethanol-dependent effects, suggesting a potential target for additional assessment and intervention towards understanding the ethanol's ability to program the developing brain.

CONCLUSION

We provide a global assessment of relatively long-term changes in both opioid gene expression and protein following exposure to only moderate amounts of ethanol during a relatively short window in the prenatal period. These results suggest that, while continuing to undergo ontogenetic changes, the infant brain is sensitive to prenatal ethanol exposure and that such exposure may lead to relatively long-lasting changes in the endogenous opioid system within the reward circuitry. These data indicate a potential mechanism and target for additional assessments of ethanol's ability to program the brain, affecting later responsiveness towards the drug.

Prenatal exposure to vanilla or alcohol induces crawling after these odors in the neonate rat: The role of mu and kappa opioid receptor systems

Rat fetuses can perceive chemosensory stimuli derived from their mother's diet, and they may learn about those stimuli. In previous studies we have observed that prenatal exposure to alcohol during the last days of gestation increases the acceptance and liking of an alcohol flavor in infant and adolescent rats. While these results were not found after prenatal exposure to vanilla, cineole or anise, suggesting that the pharmacological properties of alcohol, mediated by the opioid system, underlie the effects observed with this drug. Considering that other studies report enhanced acceptance of non-alcohol flavors experienced prenatally when subjects were tested before infancy, we explore the possibility of observing similar results if testing 1-day old rats exposed prenatally to vanilla. Using an "odor-induced crawling" testing procedure, it was observed that neonates exposed prenatally to vanilla or alcohol crawl for a longer distance towards the experienced odor than to other odors or than control pups. Blocking mu, but not kappa opioid receptors, reduced the attraction of vanilla odor to neonates exposed to vanilla in utero, while the response to alcohol in pups exposed prenatally to this drug was affected by both antagonists. Results confirm that exposure to a non-alcohol odor enhances postnatal responses to it, observable soon after birth, while also suggesting that the mu opioid receptor system plays an important role in generating this effect. The results also imply that with alcohol exposure, the prenatal opioid system is wholly involved, which could explain the longer retention of the enhanced attraction to alcohol following prenatal experience with the drug.

Prenatal ethanol exposure alters met-enkephalin expression in brain regions related with reinforcement: possible mechanism for ethanol consumption in offspring

The endogenous opioid system is involved in ethanol reinforcement. Ethanol-induced changes in opioidergic transmission have been extensively studied in adult organisms. However, the impact of ethanol exposure at low or moderate doses during early ontogeny has been barely explored. We investigated the effect of prenatal ethanol exposure on alcohol intake and Methionine-enkephalin (Met-enk) content in rat offspring. Met-enk content was assessed in the ventral tegmental area [VTA], nucleus accumbens [NAcc], prefrontal cortex [PFC], substantia nigra [SN], caudate-putamen [CP], amygdala, hypothalamus and hippocampus. Pregnant rats were treated with ethanol (2g/kg) or water during GDs 17-20. At PDs 14 and 15, preweanlings were evaluated in an intake test (5% and 10% ethanol, or water). Met-enk content in brain regions of infants prenatally exposed to ethanol was quantitated by radioimmunoassay. Ethanol consumption was facilitated by prenatal experience with the drug, particularly in females. Met-enk content in mesocorticolimbic regions - PFC and NAcc - was increased as a consequence of prenatal exposure to ethanol. Conversely, Met-enk levels in the VTA were reduced by prenatal ethanol manipulation. Prenatal ethanol also increased peptide levels in the medial-posterior zone of the CP, and strongly augmented Met-enk content in the hippocampus and hypothalamus. These findings show that prenatal ethanol exposure stimulates consumption of the drug in infant rats, and induces selective changes in Met-enk levels in regions of the mesocorticolimbic and nigrostriatal systems, the hypothalamus and hippocampus. Our results support the role of mesocorticolimbic enkephalins in ethanol reinforcement in offspring, as has been reported in adults.

Prenatal alcohol exposure increases postnatal acceptability of nicotine odor and taste in adolescent rats

Human studies indicate that alcohol exposure during gestation not only increases the chance for later alcohol abuse, but also nicotine dependence. The flavor attributes of both alcohol and nicotine can be important determinants of their initial acceptance and they both share the component chemosensory qualities of an aversive odor, bitter taste and oral irritation. There is a growing body of evidence demonstrating epigenetic chemosensory mechanisms through which fetal alcohol exposure increases adolescent alcohol acceptance, in part, by decreasing the aversion to alcohol's bitter and oral irritation qualities, as well as its odor. Given that alcohol and nicotine have noteworthy chemosensory qualities in common, we investigated whether fetal exposure to alcohol increased the acceptability of nicotine's odor and taste in adolescent rats. Study rats were alcohol-exposed during fetal development via the dams' liquid diet. Control animals received ad lib access to an iso-caloric, iso-nutritive diet throughout gestation. Odorant-induced innate behavioral responses to nicotine odor (Experiment 1) or orosensory-mediated responses to nicotine solutions (Experiment 2) were obtained, using whole-body plethysmography and brief access lick tests, respectively. Compared to controls, rats exposed to fetal alcohol showed an enhanced nicotine odor response that was paralleled by increased oral acceptability of nicotine. Given the common aversive component qualities imbued in the flavor profiles of both drugs, our findings demonstrate that like postnatal alcohol avidity, fetal alcohol exposure also influences nicotine acceptance, at a minimum, by decreasing the aversion of both its smell and taste. Moreover, they highlight potential chemosensory-based mechanism(s) by which fetal alcohol exposure increases the later initial risk for nicotine use, thereby contributing to the co-morbid expression with enhanced alcohol avidity. Where common chemosensory mechanisms are at play, our results suggest broader implications related to the consequence of fetal exposure with one substance of abuse and initial acceptability of others.

New evidence of ethanol's anxiolytic properties in the infant rat

Ethanol induces appetitive, aversive, and anxiolytic effects that are involved in the development of ethanol use and dependence. Because early ethanol exposure produces later increased responsiveness to ethanol, considerable effort has been devoted to analysis of ethanol's appetitive and aversive properties during early ontogeny. Yet, there is a relative scarcity of research related to the anxiolytic effects of ethanol during early infancy, perhaps explained by a lack of age-appropriate tests. The main aim of this study was to validate a model for the assessment of ethanol's anxiolytic effects in the infant rat (postnatal days 13-16). The potentially anxiolytic effects of ethanol tested included: i) amelioration of conditioned place aversion, ii) ethanol intake in the presence of an aversive conditioned stimulus, iii) the inhibitory behavioral effect in an anxiogenic environment, and iv) innate aversion to a brightly illuminated area in a modified light/dark paradigm. Ethanol doses employed across experiments were 0.0, 0.5, and 2.0 g/kg. Results indicated that a low ethanol dose (0.5 g/kg) was effective in attenuating expression of a conditioned aversion. Ethanol intake, however, was unaffected by simultaneous exposure to an aversive stimulus. An anxiogenic environment diminished ethanol-induced locomotor stimulation. Finally, animals given 0.5 g/kg ethanol and evaluated in a light/dark box showed increased time spent in the illuminated area and increased latency to escape from the brightly lit compartment than rats treated with a higher dose of ethanol or vehicle. These new results suggest that ethanol doses as low as 0.5 g/kg are effective in ameliorating an aversive and/or anxiogenic state in preweanling rats. These behavioral preparations can be used to assess ethanol's anxiolytic properties during early development.

Prenatal ethanol increases sucrose reinforcement, an effect strengthened by postnatal association of ethanol and sucrose

Late prenatal exposure to ethanol recruits sensory processing of the drug and of its motivational properties, an experience that leads to heightened ethanol affinity. Recent studies indicate common sensory and neurobiological substrates between this drug and sweet tastants. Using a recently developed operant conditioning technique for infant rats, we examined the effects of prenatal ethanol history upon sucrose self-administration (postnatal days, PDs 14-17). Prior to the last conditioning session, a low (0.5 g/kg) or a high (2.5 g/kg) ethanol dose were paired with sucrose. The intention was to determine if ethanol would inflate or devalue the reinforcing capability of the tastant and if these effects are dependent upon prenatal ethanol history. Male and female pups prenatally exposed to ethanol (2.0 g/kg) responded more when reinforced with sucrose than pups lacking this antenatal experience. Independently of prenatal status, a low ethanol dose (0.5 g/kg) enhanced the reinforcing capability of sucrose while the highest dose (2.5 g/kg) seemed to ameliorate the motivational properties of the tastant. During extinction (PD 18), two factors were critical in determining persistence of responding despite reinforcement omission. Pups prenatally exposed to ethanol that subsequently experienced the low ethanol dose paired with sucrose, showed higher resistance to extinction. The effects here reported were not associated with differential blood alcohol levels across prenatal treatments. These results indicate that fetal ethanol experience promotes affinity for a natural sweet reinforcer and that low doses of ethanol are also capable of enhancing the positive motivational consequences of sucrose when ethanol and sucrose are paired during infancy.

Prenatal exposure to ethanol during late gestation facilitates operant self-administration of the drug in 5-day-old rats

Prenatal ethanol exposure modifies postnatal affinity to the drug, increasing the probability of ethanol use and abuse. The present study tested developing rats (5-day-old) in a novel operant technique to assess the degree of ethanol self-administration as a result of prenatal exposure to low ethanol doses during late gestation. On a single occasion during each of gestational days 17-20, pregnant rats were intragastrically administered ethanol 1 g/kg, or water (vehicle). On postnatal day 5, pups were tested on a novel operant conditioning procedure in which they learned to touch a sensor to obtain 0.1% saccharin, 3% ethanol, or 5% ethanol. Immediately after a 15-min training session, a 6-min extinction session was given in which operant behavior had no consequence. Pups were positioned on a smooth surface and had access to a touch-sensitive sensor. Physical contact with the sensor activated an infusion pump, which served to deliver an intraoral solution as reinforcement (Paired group). A Yoked control animal evaluated at the same time received the reinforcer when its corresponding Paired pup touched the sensor. Operant behavior to gain access to 3% ethanol was facilitated by prenatal exposure to ethanol during late gestation. In contrast, operant learning reflecting ethanol reinforcement did not occur in control animals prenatally exposed to water only. Similarly, saccharin reinforcement was not affected by prenatal ethanol exposure. These results suggest that in 5-day-old rats, prenatal exposure to a low ethanol dose facilitates operant learning reinforced by intraoral administration of a low-concentration ethanol solution. This emphasizes the importance of intrauterine experiences with ethanol in later susceptibility to drug reinforcement. The present operant conditioning technique represents an alternative tool to assess self-administration and seeking behavior during early stages of development.

Exposure to ethanol on prenatal days 19-20 increases ethanol intake and palatability in the infant rat: involvement of kappa and mu opioid receptors

Prenatal exposure to ethanol on gestation Days 19-20, but not 17-18, increases ethanol acceptance in infant rats. This effect seems to be a conditioned response acquired prenatally, mediated by the opioid system, which could be stimulated by ethanol's pharmacological properties (mu-opioid receptors) or by a component of the amniotic fluid from gestation-day 20 (kappa-inducing factor). The latter option was evaluated administering non-ethanol chemosensory stimuli on gestation Days 19-20 and testing postnatal intake and palatability. However, prenatal exposure to anise or vanilla increased neither intake nor palatability of these tastants on postnatal Day 14. In experiment 2, the role of ethanol's pharmacological effect was tested by administering ethanol and selective antagonists of mu and kappa opioid receptors prenatally. Blocking the mu-opioid receptor system completely reversed the effects on intake and palatability, while antagonizing kappa receptors only partially reduced the effects on palatability. This suggests that the pharmacological effect of ethanol on the fetal mu opioid system is the appetitive reinforcer, which induces the prenatally conditioned preference detected in the preweanling period.

Acetaldehyde involvement in ethanol's postabsortive effects during early ontogeny

Clinical and biomedical studies sustains the notion that early ontogeny is a vulnerable window to the impact of alcohol. Experiences with the drug during these stages increase latter disposition to prefer, use or abuse ethanol. This period of enhanced sensitivity to ethanol is accompanied by a high rate of activity in the central catalase system, which metabolizes ethanol in the brain. Acetaldehyde (ACD), the first oxidation product of ethanol, has been found to share many neurobehavioral effects with the drug. Cumulative evidence supports this notion in models employing adults. Nevertheless very few studies have been conducted to analyze the role of ACD in ethanol postabsorptive effects, in newborns or infant rats. In this work we review recent experimental literature that syndicates ACD as a mediator agent of reinforcing aspects of ethanol, during early ontogenetic stages. We also show a meta-analytical correlational approach that proposes how differences in the activity of brain catalase across ontogeny, could be modulating patterns of ethanol consumption.

Acetaldehyde reinforcement and motor reactivity in newborns with or without a prenatal history of alcohol exposure

Animal models have shown that early ontogeny seems to be a period of enhanced affinity to ethanol. Interestingly, the catalase system that transforms ethanol (EtOH) into acetaldehyde (ACD) in the brain, is more active in the perinatal rat compared to adults. ACD has been found to share EtOH's behavioral effects. The general purpose of the present study was to assess ACD motivational and motor effects in newborn rats as a function of prenatal exposure to EtOH. Experiment 1 evaluated if ACD (0.35 μmol) or EtOH (0.02 μmol) supported appetitive conditioning in newborn pups prenatally exposed to EtOH. Experiment 2 tested if prenatal alcohol exposure modulated neonatal susceptibility to ACD's motor effects (ACD dose: 0, 0.35 and 0.52 μmol). Experiment 1 showed that EtOH and ACD supported appetitive conditioning independently of prenatal treatments. In Experiment 2, latency to display motor activity was altered only in neonates prenatally treated with water and challenged with the highest ACD dose. Prenatal EtOH experience results in tolerance to ACD's motor activity effects. These results show early susceptibility to ACD's appetitive effects and attenuation of motor effects as a function of prenatal history with EtOH, within a stage in development where brain ACD production seems higher than later in life.

Participation of the nociceptin/orphanin FQ receptor in ethanol-mediated locomotor activation and ethanol intake in preweanling rats

Activation of nociceptin/orphanin FQ (NOP) receptors seems to attenuate ethanol-induced reinforcement in adult rodents. Since early ethanol exposure results in later increased responsiveness to ethanol, it is important to analyze NOP receptor modulation of ethanol-related behaviors during early ontogeny. By measuring NOP involvement in ethanol intake and ethanol-induced locomotor activation, we analyzed the specific participation of NOP receptors on these ethanol-related behaviors in two-week-old rats. In each experiment animals were pre-treated with the endogenous ligand for this receptor (nociceptin/orphanin FQ at 0.0, 0.5, 1.0 or 2.0 μg) or a selective NOP antagonist (J-113397 at 0.0, 0.5, 2.0 or 5.0 mg/kg). Results indicated that activation of the nociceptin receptor system had no effect on ethanol or water intake, while blockade of the NOP receptor has an unspecific effect on consummatory behavior: J-113397 increased ethanol (at a dose of 0.5 mg/kg) and water intake (at 0.5 and 5.0 mg/kg). Ethanol-mediated locomotor stimulation was attenuated by activation of the NOP system (nociceptin at 1.0 and 2.0 μg). Nociceptin had no effect on basal locomotor activity. Blockade of NOP receptors did not modify ethanol-induced locomotor activation. Contrary to what has been reported for adult rodents, nociceptin failed to suppress intake of ethanol in infants. Attenuation of ethanol-induced stimulation by activation of NOP receptor system suggests an early role of this receptor in this ethanol-related behavior.

Prenatal ethanol exposure increases ethanol intake and reduces c-Fos expression in infralimbic cortex of adolescent rats

Prenatal ethanol exposure significantly increases later predisposition for alcohol intake, but the mechanisms associated with this phenomenon remain hypothetical. This study analyzed (Experiment 1) ethanol intake in adolescent inbred WKAH/Hok Wistar rats prenatally exposed to ethanol (2.0g/kg) or vehicle, on gestational days 17-20. Subsequent Experiments (2, 3 and 4) tested several variables likely to underlie the effect of gestational ethanol on adolescent ethanol preference, including ethanol-induced locomotor activation (LMA), ethanol-induced emission of ultrasonic vocalizations (USVs) after exposure to a rough exteroceptive stimulus, and induction of the immediate early gene C-fos in brain areas associated with processing of reward stimuli and with the retrieval and extinction of associative learning. Prenatal ethanol induced a two-fold increase in ethanol intake. Adolescents exhibited significant ethanol-induced LMA, emitted more aversive than appetitive USVs, and postnatal ethanol administration significantly exacerbated the emission of USVs. These effects, however, were not affected by prenatal ethanol. Adolescents prenatally exposed to ethanol as fetuses exhibited reduced neural activity in infralimbic cortex (but not in prelimbic cortex or nucleus accumbens core or shell), an area that has been implicated in the extinction of drug-mediated associative memories. Ethanol metabolism was not affected by prenatal ethanol. Late gestational exposure to ethanol significantly heightened drinking in the adolescent offspring of an inbred rat strain. Ethanol-induced LMA and USVs were not associated with differential ethanol intake due to prenatal ethanol exposure. Prenatal ethanol, however, altered basal neural activity in the infralimbic prefrontal cortex. Future studies should analyze the functionality of medial prefrontal cortex after prenatal ethanol and its potential association with predisposition for heightened ethanol intake.

Gestational naltrexone ameliorates fetal ethanol exposures enhancing effect on the postnatal behavioral and neural response to ethanol

The association between gestational exposure to ethanol and adolescent ethanol abuse is well established. Recent animal studies support the role of fetal ethanol experience-induced chemosensory plasticity as contributing to this observation. Previously, we established that fetal ethanol exposure, delivered through a dam's diet throughout gestation, tuned the neural response of the peripheral olfactory system of early postnatal rats to the odor of ethanol. This occurred in conjunction with a loss of responsiveness to other odorants. The instinctive behavioral response to the odor of ethanol was also enhanced. Importantly, there was a significant contributory link between the altered response to the odor of ethanol and increased ethanol avidity when assessed in the same animals. Here, we tested whether the neural and behavioral olfactory plasticity, and their relationship to enhanced ethanol intake, is a result of the mere exposure to ethanol or whether it requires the animal to associate ethanol's reinforcing properties with its odor attributes. In this later respect, the opioid system is important in the mediation (or modulation) of the reinforcing aspects of ethanol. To block endogenous opiates during prenatal life, pregnant rats received daily intraperitoneal administration of the opiate antagonist naltrexone from gestational day 6-21 jointly with ethanol delivered via diet. Relative to control progeny, we found that gestational exposure to naltrexone ameliorated the enhanced postnatal behavioral response to the odor of ethanol and postnatal drug avidity. Our findings support the proposition that in utero ethanol-induced olfactory plasticity (and its relationship to postnatal intake) requires, at least in part, the associative pairing between ethanol's odor quality and its reinforcing aspects. We also found suggestive evidence that fetal naltrexone ameliorated the untoward effects of gestational ethanol exposure on the neural response to non-fetal-exposure odorants. Thus, gestational naltrexone may also have a neuroprotective and/or neuroproliferative impact on olfactory development.

Prenatal ethanol exposure leads to greater ethanol-induced appetitive reinforcement

Prenatal ethanol significantly heightens later alcohol consumption, but the mechanisms that underlie this phenomenon are poorly understood. Little is known about the basis of 'this effect of prenatal ethanol on the sensitivity to ethanol's reinforcing effects. One possibility is that prenatal ethanol exposure makes subjects more sensitive to the appetitive effects of ethanol or less sensitive to ethanol's aversive consequences. The present study assessed ethanol-induced second-order conditioned place preference (CPP) and aversion and ethanol-induced conditioned taste aversion (CTA) in infant rats prenatally exposed to ethanol (2.0 g/kg) or vehicle (water) or left untreated. The involvement of the κ opioid receptor system in ethanol-induced CTA was also explored. When place conditioning occurred during the ascending limb of the blood-ethanol curve (Experiment 1), the pups exposed to ethanol in utero exhibited greater CPP than untreated controls, with a shift to the right of the dose-response curve. Conditioning during a later phase of intoxication (30-45 min post-administration; Experiment 2) resulted in place aversion in control pups exposed to vehicle during late gestation but not in pups that were exposed to ethanol in utero. Ethanol induced a reliable and similar CTA (Experiment 3) in the pups treated with vehicle or ethanol during gestation, and CTA was insensitive to κ antagonism. These results suggest that brief exposure to a moderate ethanol dose during late gestation promotes ethanol-mediated reinforcement and alters the expression of conditioned aversion by ethanol. This shift in the motivational reactivity to ethanol may be an underlying basis of the effect of prenatal ethanol on later ethanol acceptance.

Role of mu, delta and kappa opioid receptors in ethanol-reinforced operant responding in infant rats

We recently observed that naloxone, a non-specific opioid antagonist, attenuated operant responding to ethanol in infant rats. Through the use of an operant conditioning technique, we aimed to analyze the specific participation of mu, delta, and kappa opioid receptors on ethanol reinforcement during the second postnatal week. In Experiment 1, infant rats (PDs 14-17) were trained to obtain 5, 7.5, 10, or 15% ethanol, by operant nose-poking. Experiment 2 tested blood ethanol levels (BELs) attained by operant behavior. In Experiment 3, at PDs 16-18, rats received CTOP (mu antagonist: 0.1 or 1.0 mg/kg), naltrindole (delta antagonist: 1.0 or 5.0 mg/kg) or saline before training. In Experiment 4, rats received nor-binaltorphimine (kappa antagonist: 10.0 or 30.0 mg/kg, a single injection after completion of PD 15 operant training), spiradoline mesylate (kappa agonist: 1.0 or 5.0 mg/kg; at PDs 16-18) or saline (PDs 16-18), before the conditioning. Experiments 5 and 6 assessed possible side effects of opioid drugs in locomotor activity (LA) and conditioned taste aversion (CTA). Ethanol at 7.5 and 10% promoted the highest levels of operant responding. BELs were 12-15 mg/dl. In Experiment 3 naltrindole (dose-response effect) and CTOP (the lowest dose) were effective in decreasing operant responding. Nor-binaltorphimine at 10.0 mg/kg and spiradoline at 5.0 mg/kg also blocked ethanol responding. The effects of opioid drugs on ethanol reinforcement cannot be explained by effects on LA or CTA. Even though particular aspects of each opioid receptor require further testing, a fully functional opioid system seems to be necessary for ethanol reinforcement, during early ontogeny.

Fetal ethanol exposure attenuates aversive oral effects of TrpV1, but not TrpA1 agonists in rats

In humans, fetal ethanol exposure is highly predictive of adolescent ethanol use and abuse. Prior work in our labs indicated that fetal ethanol exposure results in stimulus-induced chemosensory plasticity in the taste and olfactory systems of adolescent rats. In particular, we found that increased ethanol acceptability could be attributed, in part, to an attenuated aversion to ethanol's aversive odor and quinine-like bitter taste quality. Here, we asked whether fetal ethanol exposure also alters the oral trigeminal response of adolescent rats to ethanol. We focused on two excitatory ligand-gated ion channels, TrpV1 and TrpA1, which are expressed in oral trigeminal neurons and mediate the aversive orosensory response to many chemical irritants. To target TrpV1, we used capsaicin, and to target TrpA1, we used allyl isothiocyanate (or mustard oil). We assessed the aversive oral effects of ethanol, together with capsaicin and mustard oil, by measuring short-term licking responses to a range of concentrations of each chemical. Experimental rats were exposed in utero by administering ethanol to dams through a liquid diet. Control rats had ad libitum access to an iso-caloric iso-nutritive liquid diet. We found that fetal ethanol exposure attenuated the oral aversiveness of ethanol and capsaicin, but not mustard oil, in adolescent rats. Moreover, the increased acceptability of ethanol was directly related to the reduced aversiveness of the TrpV1-mediated orosensory input. We propose that fetal ethanol exposure increases ethanol avidity not only by making ethanol smell and taste better, but also by attenuating ethanol's capsaicin-like burning sensations.

Naloxone attenuation of ethanol-reinforced operant responding in infant rats in a re-exposure paradigm

RATIONALE

Early ethanol exposure promotes ethanol reinforcement, mediated perhaps by ethanol's motivational effects. The opioid system mediates ethanol reinforcement, at least in part.

OBJECTIVES

Modulation of consummatory and seeking behaviors by the opioid system was tested in terms of ethanol or sucrose operant self-administration.

METHODS

Wistar-derived infant rats were tested in an operant conditioning task. (1) Infants were trained on postnatal days (PDs) 14-17 to obtain 5% sucrose and 3.75% ethanol or water, and evaluated in an extinction session at PD 18. (2) Ethanol (3.75%) was used as reinforcer. At PDs 16-17, 6 h before operant task, pups were re-exposed to ethanol after naloxone injection (0 or 1 mg/kg). (3) Sucrose (5%) acted as reinforcer. Pups were re-exposed to sucrose after naloxone injection. (4) A PD 18 re-exposure trial in which pups were injected with naloxone and re-exposed to ethanol was added.

RESULTS

Sucrose and ethanol promoted higher levels of operant responding than water during training and extinction. Re-exposure to ethanol preceded by naloxone decreased nose-poking. A similar profile was observed towards sucrose. No seeking behavior was observed in pups re-exposed to ethanol following naloxone injection during PDs 16-18.

CONCLUSIONS

Self-administration of ethanol was established in terms of operant responding in preweanling rats with no previous exposure to the drug. Pairing of naloxone with ethanol, at a point separate in time from operant responding, reduced ethanol reinforcement. This indicated participation of the opioid system in ethanol reinforcement. This effect seems not to be unique to ethanol but also is observable when sucrose acts as reinforcer.

Adolescent alcohol exposure alters the central brain circuits known to regulate the stress response

Adolescent alcohol exposure (AAE) may exert long-term effects on the adult brain. Here, we tested the hypothesis that the brain regions affected include the rat hypothalamic-pituitary-adrenal (HPA) axis. Specifically, we examined the consequences of AAE [postnatal days (PND) 28-42] on the HPA axis-related brain circuitry of male rats challenged with an intragastric (ig) administration of alcohol in young adulthood (PND 61-62). Adolescent rats were exposed to alcohol vapors, while controls did not receive the drug. The mean blood alcohol level in adolescence on PND 40 was 212.8±5.7 mg %. Using immunohistochemistry and in situ hybridization procedures, we measured signals for c-fos and corticotropin releasing factor (CRF) in the paraventricular nucleus (PVN) of the hypothalamus, as well as signals for c-fos and phenylethanolamine N-methyltransferase (PNMT) in the adrenergic brain stem regions (C1 and C2). PVN CRF mRNA expression was significantly blunted in AAE rats tested at PND 61-62, compared to their controls. These animals also displayed a significant increase in the mean number of PNMT-ir cells/brain stem section in the C2 area. Collectively, these results suggest that exposure to alcohol vapors during adolescence exerts long-term effects on the ability of the PVN to mount a response to an acute alcohol administration in young adulthood, possibly mediated by medullary catecholamine input to the PVN.

Prenatal and postnatal ethanol experiences modulate consumption of the drug in rat pups, without impairment in the granular cell layer of the main olfactory bulb

The effect of moderate exposure to ethanol during late gestation was studied in terms of its interaction with moderate exposure during nursing from an intoxicated dam. A further issue was whether behavioral effects of ethanol, especially the enhanced ethanol intake known to occur after moderate ethanol prenatally or during nursing, depend upon teratological effects that may include death of neurons in the main olfactory bulb (MOB). During gestational days 17-20 rats were given 0, 1 or 2g/kg ethanol doses intragastrically (i.g.). After parturition these dams were given a dose of 2.5g/kg ethanol i.g. each day and allowed to perform regular nursing activities. During postnatal days (PDs) 15 and 16, ethanol intake of pups was assessed along with aspects of their general activity. In a second experiment pups given the same prenatal treatment as above were tested for blood ethanol concentration (BEC) in response to an ethanol challenge on PD6. A third experiment (Experiment 2b) assessed stereologically the number of cells in the granular cell layer of the MOB on PD7, as a function of analogous pre- and postnatal ethanol exposures. Results revealed that ethanol intake during the third postnatal week was increased by prenatal as well as postnatal ethanol exposure, with a few interesting qualifications. For instance, pups given 1g/kg prenatally did not have increased ethanol intake unless they also had experienced ethanol during nursing. There were no effects of ethanol on either BECs or conventional teratology (cell number). This increases the viability of an explanation of the effects of prenatal and early postnatal ethanol on later ethanol intake in terms of learning and memory.

Period 2 gene deletion abolishes beta-endorphin neuronal response to ethanol

BACKGROUND

Ethanol exposure during early life has been shown to permanently alter the circadian expression of clock regulatory genes and the beta-endorphin precursor proopiomelanocortin (POMC) gene in the hypothalamus. Ethanol also alters the stress- and immune-regulatory functions of beta-endorphin neurons in laboratory rodents. Our aim was to determine whether the circadian clock regulatory Per2 gene modulates the action of ethanol on beta-endorphin neurons in mice.

METHODS

Per2 mutant (mPer2(Brdml)) and wild type (C57BL/6J) mice were used to determine the effect of Per2 mutation on ethanol-regulated beta-endorphin neuronal activity during neonatal period using an in vitro mediobasal hypothalamic (MBH) cell culture model and an in vivo milk formula feeding animal model. The beta-endorphin neuronal activity following acute and chronic ethanol treatments was evaluated by measuring the peptide released from cultured cells or peptide levels in the MBH tissues, using enzyme-linked immunosorbent assay (ELISA).

RESULTS

Per2 mutant mice showed a higher basal level of beta-endorphin release from cultured MBH cells and a moderate increase in the peptide content in the MBH in comparison with control mice. However, unlike wild type mice, Per2 mutant mice showed no stimulatory or inhibitory beta-endorphin-secretory responses to acute and chronic ethanol challenges in vitro. Furthermore, Per2 mutant mice, but not wild type mice, failed to show the stimulatory and inhibitory responses of MBH beta-endorphin levels to acute and chronic ethanol challenges in vivo.

CONCLUSIONS

These results suggest for the first time that the Per2 gene may be critically involved in regulating beta-endorphin neuronal function. Furthermore, the data revealed an involvement of the Per2 gene in regulating beta-endorphin neuronal responses to ethanol.

The effect of taste familiarity on intake and taste reactivity in infant rats

With infant rats, unlike with adults, increased intake of a taste after mere exposure to this stimulus is not consistently found; this has sometimes been interpreted as a failure by the immature subject to recognize tastes as familiar. We studied the effect of preexposure to a tastant, measuring taste reactivity and intake in 14-day-old rats. Familiarity increased hedonic response to sucrose, but also increased aversive response to quinine and ethanol. With the sucrose-quinine compound, familiarity increased both the hedonic and the aversive reaction to the stimulus. In no case was a differential reactivity to water observed. Significant increased intake after familiarization was only found with quinine or the sucrose-quinine compound. Results indicate that in infant rats, and with the present parameters, taste familiarity enhances responsiveness to these stimuli, an effect not always accompanied by detectable changes in intake.

Participation of the endogenous opioid system in the acquisition of a prenatal ethanol-related memory: effects on neonatal and preweanling responsiveness to ethanol

The present study tested the involvement of the opioid system in the acquisition and expression of prenatal ethanol-related memories. We evaluated how this prenatal experience modulates ethanol self-administration in newborn rats, and preweanling's ingestion of the drug. During Gestational Days (GDs) 17-20, four groups of dams were treated with ethanol (2 g/kg) or water, followed immediately by naloxone (10 mg/kg) or saline administration. A fifth group received a similar dose of naloxone 20min before ethanol administration. On PD 1, pups were tested on an operant learning procedure to obtain milk or 3% ethanol. One hour later, an extinction session was performed. At Postnatal Days (PDs) 14 and 15, preweanlings representing each prenatal treatment were evaluated in an intake test with infusions of 5% ethanol or water. Prior to the intake test on PD14, preweanlings were administered naloxone (1 mg/kg), saline or remained untreated. In both tests, animals representative of both genders were utilized. One-day-old pups rapidly learned the operant behavior to gain access to milk. In contrast, only pups prenatally treated with ethanol (administered immediately before naloxone or saline injection) increased operant responding to gain access to ethanol. On an intake test at PDs 14 and 15, those animals prenatally exposed to naloxone 20 min before ethanol administration consumed significantly lower ethanol levels than the remaining prenatal ethanol groups. Postnatal treatment with naloxone diminished intake of all solutions at PD14. These results suggest that prenatal ethanol exposure facilitates neonatal operant learning reinforced by intraoral administration of ethanol and increases ethanol consumption during PDs 14-15. The endogenous opioid system apparently is involved in the acquisition of prenatal ethanol memories, which can modulate the reinforcing attributes of the drug in neonatal and preweanling rats.

Genetic and environmental influences on ethanol consumption: perspectives from preclinical research

BACKGROUND

Alcohol use disorders (abuse and dependence, AUD) are multifactorial phenomena, depending on the interplay of environmental and genetic variables.

METHOD

This review describes current developments in animal research that may help (a) develop gene therapies for the treatment of alcoholism, (b) understand the permissive role of stress on ethanol intake, and (c) elucidate why exposure to ethanol early in life is associated with a greater risk of AUD.

RESULTS

The polymorphisms found in liver alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH) affect the elimination of ethanol and the susceptibility to ethanol intake. A highly active ADH protects against alcoholism, an effect related to a presteady state burst in arterial acetaldehyde. Social stressors, such as repeated early maternal separation or social defeat, exert a permissive effect on ethanol intake, perhaps by altering the normal development of the hypothalamic-pituitary-adrenal axis. Ethanol exposure during gestation, infancy, or adolescence increases the likelihood of AUD later in life. Early perception of ethanol's positive and negative (anti-anxiety) reinforcing effects may play a role in this phenomenon.

CONCLUSIONS

The review underscores the advantages of using preclinical animal models of AUD and highlights points of intersection between the topics to help design a more integrated approach for the study of alcohol-related problems.

Differential role of mu, delta and kappa opioid receptors in ethanol-mediated locomotor activation and ethanol intake in preweanling rats

The opioid system modulates ethanol intake and reinforcement in adult and preweanling rodents. While adult heterogeneous rats normally do not show ethanol-mediated locomotor stimulation, preweanling rats show it quite clearly. We recently observed that naloxone, a non-specific opioid antagonist, attenuated ethanol-induced locomotor activation in preweanling rats. In the present study we tested the role of specific opioid receptors (mu, delta and kappa) in ethanol-mediated locomotor stimulation and ethanol intake. In Experiment 1 13-day-old rats received naloxonazine (mu antagonist: 0, 7.5 or 15 mg/kg), naltrindole (delta antagonist: 0, 2 or 4 mg/kg) or nor-binaltorphimine (kappa antagonist: 0, 2, 4 or 8 mg/kg) before an intragastric administration of ethanol (0 or 2.5 g/kg), and subsequent locomotor activity assessment. In Experiment 2, the same opioid antagonists were administered on postnatal days 13 and 14 before consumption of ethanol (6%), saccharin (0.05%) or distilled water. In Experiment 1 only naloxonazine reduced ethanol-mediated locomotor stimulation. None of the opioid antagonists affected locomotor activity in water controls. In Experiment 2 naloxonazine and naltrindole suppressed ingestion of all the solutions tested. Similar to what has been reported in adult rodents, mu-opioid receptors seem to modulate ethanol-activating effects during early ontogeny. Hence, there seems to be a partial overlap of neurochemical mechanisms involved in the rewarding and stimulating effects of ethanol in preweanling rats. Mu-receptor antagonists reduced both ethanol-induced activity and ethanol intake, but it is unclear whether the latter effect is specific to ethanol or only a reflection of an effect on consummatory behavior generally, since mu and delta receptor antagonists also suppressed ingestion of water and saccharin.

Ontogeny of the enhanced fetal-ethanol-induced behavioral and neurophysiologic olfactory response to ethanol odor

BACKGROUND

Studies report a fundamental relationship between chemosensory function and the responsiveness to ethanol, its component orosensory qualities, and its odor as a consequence of fetal ethanol exposure. Regarding odor, fetal exposed rats display enhanced olfactory neural and behavioral responses to ethanol odor at postnatal (P) day 15. Although these consequences are absent in adults (P90), the behavioral effect has been shown to persist into adolescence (P37). Given the developmental timing of these observations, we explored the decay in the response to ethanol odor by examining ages between P37 and young adulthood. Moreover, we sought to determine whether the P15 neurophysiologic effect persists, at least, to P40.

METHODS

Behavioral and olfactory epithelial (OE) responses of fetal ethanol exposed and control rats were tested at P40, P50, P60, or P70. Whole-body plethysmography was used to quantify each animal's innate behavioral response to ethanol odor. We then mapped the odorant-induced activity across the OE in response to different odorants, including ethanol, using optical recording methods.

RESULTS

Relative to controls, ethanol exposed animals showed an enhanced behavioral response to ethanol odor that, while significant at each age, decreased in magnitude. These results, in conjunction with previous findings, permitted the development of an ontologic odor response model of fetal exposure. The fitted model exemplifies that odor-mediated effects exist at birth, peak in adolescence and then decline, becoming absent by P90. There was no evidence of an effect on the odor response of the OE at any age tested.

CONCLUSIONS

Fetal exposure yields an enhanced behavioral response to ethanol odor that peaks in adolescence and wanes through young adulthood. This occurs absent an enhanced response of the OE. This latter finding suggests that by P40 the OE returns to an ethanol "neutral" status and that central mechanisms, such as ethanol-induced alterations in olfactory bulb circuitry, underlie the enhanced behavioral response. Our study provides a more comprehensive understanding of the ontogeny of fetal-ethanol-induced olfactory functional plasticity and the behavioral response to ethanol odor.