Acute alcohol intoxication paired with appetitive reinforcement: effects upon ethanol intake in infant rats

Does Maternal Diet Influence Future Infant Taste and Odor Preferences? A Critical Analysis

The dietary choices a mother makes during pregnancy offer her developing fetus its earliest exposure to the family's culinary preferences. This comprehensive literature review synthesizes five decades of research, which has provided valuable insights into fetal flavor learning. Converging evidence across various species supports the functionality of fetal chemoreceptive systems by the end of gestation, enabling the detection of an extensive array of chemosensory cues derived from the maternal diet and transmitted to the amniotic fluid. The fetus effectively encodes these flavors, resulting in their enhanced acceptance after birth. While existing studies predominantly concentrate on fetal learning about odor volatiles, limited evidence suggests a capacity for learning about gustatory (i.e., taste) properties. Examining whether these prenatal odor, taste, and flavor experiences translate into enduring shifts in dietary behaviors beyond weaning remains a crucial avenue for further investigation.

Effects of escalating versus fixed ethanol exposure on ∆FosB expression in the mesocorticolimbic pathway in adolescent and adult rats

Background: We have reported induction of ∆FosB in adolescent rats that drank less ethanol than adults yet exhibited a progressive increase in ethanol intake.Objective: To test the hypothesis that an escalating pattern of ethanol exposure is more effective to induce ∆FosB expression [at prelimbic cortex (PrL), nucleus accumbens core and shell, striatum, basolateral amygdala (BLA) and central amygdala (CeC)] than a pattern equated for number of exposures yet employing a fixed ethanol dose.Methods: Adolescent and adult (Exp. 1, n = 48) male and female (n = 24 of each sex) or only adult male (Exp. 2, n = 36) Wistar rats were intermittently intubated with vehicle, escalating (from 0.5 to 2.5 g/kg) or fixed (2.0 g/kg) doses of ethanol, across 18 sessions. ∆FosB induction was assessed using immunohistochemistry. Ethanol intake, anxiety and risk-taking were assessed (in adults only) via two-bottles tests and the multivariate concentric square field.Results: Both patterns heightened ∆FosB levels similarly in adolescents and adults and in males and females. Fixed dosing induced ∆FosB in all areas (p < .05) except the CeC, whereas the escalating pattern induced ∆FosB in the PrL and BLA only (p < .05). Ethanol intake was initially lower in ethanol pre-exposed subjects than in control subjects (p < .05). Rats exposed to the fixed pattern exhibited enhanced risk-taking behavior (p < .05).Conclusions: The results agree with studies showing ethanol-mediated induction of ∆FosB in reward areas and indicate that, following ethanol intubations, this induction is similar in adolescents and adults. The induction of ∆FosB seems not necessarily associated with susceptibility for ethanol intake.

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.

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.

Conditioned effects of ethanol on the immune system

Several studies indicate that the immune system can be subjected to classical conditioning. Acute ethanol intoxication significantly modulates several pro-inflammatory cytokines (e.g. interleukins-1 and 6 [IL-1β and IL-6, respectively] and tumor necrosis factor alpha [TNFα])) in several brain areas, including amygdala (AMG), paraventricular nucleus (PVN), and hippocampus (HPC). It is unknown, however, whether cues associated with ethanol can elicit conditioned alterations in cytokine expression. The present study analyzed, in male Sprague-Dawley rats, whether ethanol-induced changes in the central cytokine response may be amenable to conditioning. In Experiments 1 and 2, the rats were given one or two pairings between a distinctive odor (conditional stimulus, CS) and the post-absorptive effects of a high (3.0 or 4.0 g/kg, Experiments 1 and 2, respectively) ethanol dose. Neither of these experiments revealed conditioning of IL-6, IL-1β, or TNFα, as measured via mRNA levels. Yet, re-exposure to the lemon-odor CS in Experiment 1 significantly increased C-Fos levels in the PVN. In Experiment 3, the rats were given four pairings between an odor CS and a moderate ethanol dose (2.0 g/kg), delivered intraperitoneally (i.p.) or intragastrically (i.g.). Re-exposure to the odor CS significantly increased IL-6 levels in HPC and AMG, an effect only evident in paired rats administered ethanol i.p. Overall, this study suggests that ethanol exposure can regulate the levels of IL-6 at HPC and AMG via classical conditioning mechanisms. These ethanol-induced, conditioned alterations in cytokine levels may ultimately affect the intake and motivational effects of ethanol. Impact statement This study examines, across three experiments, whether odor cues associated with ethanol exposure can condition changes in cytokine expression. The analysis of ethanol-induced conditioning of immune responses is a novel niche that can help understand the transition from social drinking to alcohol abuse and dependence. Ethanol-induced conditioning of the immune system could likely exacerbate neuroinflammation and drug-related toxicity, which in turn may facilitate further engagement in ethanol intake. The main new finding of the present study was that, after four pairings of ethanol's unconditioned effects and a distinctive odor, the latter CS increased IL-6 levels in HPC and AMG. This suggests that ethanol's effects upon IL-6 in HPC and AMG may come under conditioned control, particularly after repeated pairings between distinctive odor cues and ethanol's effects. This article advances our knowledge of conditioned increases in cytokine responses, which should help understand the mechanisms underlying alcohol use, abuse, and relapse.

The Interaction of Ethanol Ingestion and Social Interaction with an Intoxicated Peer on the Odor-Mediated Response to the Drug in Adolescent Rats

BACKGROUND

Using a social transmission of food preference paradigm in rats, we previously demonstrated that ethanol (EtOH) exposure during adolescence, as either an observer (interaction with an intoxicated conspecific) or demonstrator (intragastric infusion with EtOH), altered the reflexive odor-mediated responses to the drug. The 2 modes of exposure were equivalent in the magnitude of their effects. Human adolescents, however, are likely to experience the drug in a social setting as both an EtOH observer and demonstrator. That is, both interacting with an intoxicated peer and experiencing EtOH's postingestive consequences in conjunction with hematogenic olfaction. Therefore, we tested whether combined adolescent exposure as both an observer and demonstrator differed from either form of individual experience.

METHODS

Beginning on postnatal day (P) 29, naïve rats received EtOH or water exposures in a social interaction paradigm as either an observer, a demonstrator, or combined experience (where each animal in the interaction was, itself, an observer and demonstrator). Exposures occurred 4 times, once every 48 hours. On P37, the reflexive behavioral response to EtOH odor was tested, using whole-body plethysmography.

RESULTS

The odor-mediated responses of adolescent EtOH observers, demonstrators, and combined exposure animals all significantly differed from controls. Compared to controls, however, the magnitude of the behavioral effect was greatest in the combined exposure animals. Moreover, combined exposure as both an EtOH observer and demonstrator significantly differed from either form of individual EtOH experience.

CONCLUSIONS

EtOH's component chemosensory qualities are known to be central contributors to its acceptance and increases in the acceptability of EtOH's odor, resulting from a social transmission experience, are predictive of enhanced EtOH avidity in adolescence. Our findings demonstrate that combined exposure as an observer and demonstrator, within a socially relevant framework, may represent a higher risk scenario for increased EtOH avidity in adolescence (and by extension adult persistence) as compared to the individual effects of direct ingestion or social 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.

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.

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.

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.

The interaction of gestational and postnatal ethanol experience on the adolescent and adult odor-mediated responses to ethanol in observer and demonstrator rats

BACKGROUND

Gestational ethanol exposure enhances the adolescent reflexive sniffing response to ethanol odor. Postnatal exposures of naïve animals as either an observer (i.e., conspecific) or demonstrator (i.e., intoxicated peer) using a social transmission of food odor preference paradigm also yields enhanced odor-mediated responses. Studies on the interaction of fetal and postnatal exposures using the social transmission paradigm have been limited to the responses of observers. When combined, the enhanced response is greater than either form of exposure alone and, in observer females, yields adult persistence. The absence of a male effect is noteworthy, given that chemosensory mechanisms are suggested to be an important antecedent factor in the progression of ethanol preference. Observers gain odor information on the breath of the demonstrator through social interaction. Demonstrators experience the pharmacologic properties of ethanol along with retronasal and hematogenic olfaction. Thus, we tested whether augmentation of the fetal ethanol-induced behavioral response with postnatal exposure as a demonstrator differed from that as an observer. We also examined whether re-exposure as a demonstrator yields persistence in both sexes.

METHODS

Pregnant dams were fed an ethanol containing or control liquid diet throughout gestation. Progeny received four ethanol or water exposures: one every 48 hours through either intragastric infusion or social interaction with the infused peer beginning on P29. The reflexive behavioral sniffing response to ethanol odor was tested at postnatal (P) day 37 or P90, using whole-body plethysmography.

RESULTS

When tested in either adolescence or adulthood - fetal ethanol exposed adolescent ethanol observers and demonstrators significantly differed in their odor-mediated response to ethanol odor both between themselves and from their respective water controls. Nonetheless, adolescent ethanol re-exposure as a demonstrator, like an observer, enhanced the reflexive sniffing response to ethanol odor at both testing ages by augmenting the known effects of prior fetal ethanol experience. At each age, the magnitude of the enhanced odor response in demonstrators was similar to that of observers. Interestingly, only re-exposure as a demonstrator resulted in persistence of the behavioral response into adulthood in both sexes.

CONCLUSIONS

The method of ethanol re-exposure plays an important role in prolonging the odor-mediated effects of fetal exposure. While ethanol odor-specific exposure through social interaction is important, additional factors such as the pairing of retronasal and hematogenic olfaction with ethanol's intoxicating properties appear necessary to achieve persistence in both sexes.

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.

Ethanol-induced preferences or aversions as a function of age in preweanling rats

The reinforcing value of a relatively high ethanol dose (3 g/kg) administered intragastrically was investigated in preweanling rats. Ethanol intoxication on Postnatal Days 7-8 increased ethanol intake and enhanced the drug's palatability when tested 3 days later. In contrast, intoxication on Days 10-11 decreased ethanol intake and increased aversive responses to the drug. When reducing the delay between intoxication and test to 24 hr, the taste aversion was enhanced, whereas the preference was unaffected. Results suggest an ontogenetic change in pups' perception of the reinforcing value of ethanol that coincides with the end of a sensitive period for learning preferences in rats younger than 9 days old (T. L. Roth & R. M. Sullivan, 2003; R. M. Sullivan, M. Landers, B. Yeaman, & D. A. Wilson, 2000).

Binge ethanol exposure in late gestation induces ethanol aversion in the dam but enhances ethanol intake in the offspring and affects their postnatal learning about ethanol

Previous studies show that exposure to 1 or 2g/kg of ethanol during the last days of gestation increases ethanol acceptance in infant rats. We tested whether prenatal exposure to 3g/kg, a relatively high ethanol dose, generates an aversion to ethanol in both the dam and offspring, and whether this prenatal experience affects the expression of learning derived from ethanol exposure postnatally. The answer was uncertain, because postnatal administration of a 3-g/kg ethanol dose induces an aversion to ethanol after postnatal day (PD) 10 but increases ethanol acceptance when administered during the first postnatal week. In the present study, pregnant rats received intragastric administrations of water or ethanol (3g/kg) on gestation days 17-20. On PDs 7-8 or 10-11, the offspring were administered water or ethanol (3g/kg). Intake of ethanol and water, locomotor activity in an open field, and ethanol odor preference were evaluated in the pups, whereas the mothers were evaluated in terms of ethanol intake. Results indicated an aversion to ethanol in dams that had been administered ethanol during gestation, despite a general increase in ethanol intake observed in their pups relative to controls. The prenatal ethanol exposure also potentiated the increase in ethanol intake observed after intoxication on PDs 7-8. Ethanol intoxication on PDs 10-11 reduced ethanol consumption; this ethanol aversion was still evident in infant rats exposed prenatally to ethanol despite their general increase in ethanol intake. No effects of prenatal ethanol exposure were observed in terms of motor activity or odor preference. It is concluded that prenatal exposure to ethanol, even in a dose that induces ethanol aversion in the gestating dam, increases ethanol intake in infant rats and that this experience modulates age-related differences in subsequent postnatal learning about ethanol.

Adolescent ethanol experience alters immediate and long-term behavioral responses to ethanol odor in observer and demonstrator rats

Background

The social transmission of food preference paradigm centers on the finding that observers obtain dietary information through olfactory cues on the breath of a demonstrator peer that has ingested a novel substance. This phenomenon plays a role in ethanol acceptability. Historically, studies using this technique have focused on observer animals in order to study the social transmission process. With respect to ethanol, studies of acute intoxication have shown that the pharmacologic properties of ethanol and hematogenic olfaction can influence the subsequent ethanol odor-mediated responses of the intoxicated animals. These acute studies, however, demonstrate odor aversion. The present study compared the effect of adolescent ethanol exposure, via the social transmission paradigm, on the behavioral response to ethanol odor in both observer and demonstrator animals in adolescence (postnatal day (P) 37) and the persistence of these effects into adulthood (P90).

Methods

Beginning on P29, naïve rats received four ethanol or water exposures: one every 48 hours through either direct intragastric infusion or social interaction with an infused peer. The reflexive sniffing response of observers and demonstrators to ethanol odor was tested at P37 or P90 using whole-body plethysmography.

Results

The behavioral response of adolescent ethanol observers and demonstrators significantly differed between themselves and from their respective water controls. Ethanol and water observers both displayed a greater response to ethanol odor than their respective demonstrator counterparts. Compared to controls, both modes of ethanol exposure produced similar magnitudes of enhancement. At P90, both forms of exposure displayed similar responses to ethanol odor and similar magnitudes of enhancement. Only demonstrators displayed equivalent enhanced responses in both sexes.

Conclusion

In contrast to previous studies showing odor aversion following acute ethanol intoxication, within the context of the social transmission paradigm, adolescent demonstrators like observers showed an enhanced behavioral response to ethanol odor. The differential enhanced odor response between observers and demonstrators, despite similar net enhancements relative to controls, suggests the presence of a stress effect from the infusion technique. This finding contrasts previous suggestions that intragastric infusions create minimal stress: an important consideration when conducting ethanol research. This stress effect appears to ameliorate by adulthood.

Assessing appetitive, aversive, and negative ethanol-mediated reinforcement through an immature rat model

The motivational effects of drugs play a key role during the transition from casual use to abuse and dependence. Ethanol reinforcement has been successfully studied through Pavlovian and operant conditioning in adult rats and mice genetically selected for their ready acceptance of ethanol. Another model for studying ethanol reinforcement is the immature (preweanling) rat, which consumes ethanol and exhibits the capacity to process tactile, odor and taste cues and transfer information between different sensorial modalities. This review describes the motivational effects of ethanol in preweanling, heterogeneous non-selected rats. Preweanlings exhibit ethanol-mediated conditioned taste avoidance and conditioned place aversion. Ethanol's appetitive effects, however, are evident when using first- and second-order conditioning and operant procedures. Ethanol also devalues the motivational representation of aversive stimuli, suggesting early negative reinforcement. It seems that preweanlings are highly sensitive not only to the aversive motivational effects of ethanol but also to its positive and negative (anti-anxiety) reinforcement potential. The review underscores the advantages of using a developing rat to evaluate alcohol's motivational effects.

Ethanol exposure during late gestation and nursing in the rat: effects upon maternal care, ethanol metabolism and infantile milk intake

Ethanol experiences, during late gestation as well as during nursing, modify the behavioral dynamics of the dam/pup dyad, and leads to heightened ethanol intake in the offspring. This study focuses on: a) behavioral and metabolic changes in intoxicated dams with previous exposure to ethanol during pregnancy and b) infantile consumption of milk when the dam is either under the effects of ethanol or sober. Pregnant rats received water, 1.0 or 2.0 g/kg ethanol, and were administered with water or ethanol during the postpartum period. Intoxication during nursing disrupted the capability of the dam to retrieve the pups and to adopt a crouching posture. These disruptions were attenuated when dams had exposure to ethanol during pregnancy. Ethanol experiences during gestation did not affect pharmacokinetic processes during nursing, whereas progressive postpartum ethanol experience resulted in metabolic tolerance. Pups suckling from intoxicated dams, with previous ethanol experiences, ingested more milk than did infants suckling from ethanol-intoxicated dams without such experience. Ethanol gestational experience results in subsequent resistance to the drug's disruptions in maternal care. Consequently, better maternal care by an intoxicated dam with ethanol experience during gestation facilitates access of pups to milk which could be contaminated with ethanol.

Fetal learning about ethanol and later ethanol responsiveness: evidence against "safe" amounts of prenatal exposure

Near-term fetuses of different mammalian species, including humans, exhibit functional sensory and learning capabilities. The neurobiological literature indicates that the unborn organism processes sensory stimuli present in the amniotic fluid, retains this information for considerable amounts of time, and is also capable of associating such stimuli with biologically relevant events. This research has stimulated studies aimed at the analysis of fetal and neonatal learning about ethanol, a topic that constitutes the core of the present review. Ethanol has characteristic sensory (olfactory, taste, and trigeminal) attributes and can exert pharmacologic reinforcing effects. The studies under examination support the hypothesis that low to moderate levels of maternal ethanol intoxication during late pregnancy set the opportunity for fetal learning about ethanol. These levels of prenatal ethanol exposure do not generate evident morphologic or neurobehavioral alterations in the offspring, but they exert a significant impact upon later ethanol-seeking and intake behaviors. Supported by preclinical and clinical findings, this review contributes to strengthening the case for the ability of prenatal ethanol exposure to have effects on the postnatal organism.

Ontogenetic difference in ethanol reinforcing properties: the role of the opioid system

Previous data indicate that ethanol intoxication (3 g/kg, intragastric) on postnatal day (PD) 7 and 8 increases ethanol acceptance, but on PD 10 and 11 generates an aversion in infant rats. We investigated the participation of the opioid system in these effects. Subcutaneous administration of naloxone (1 or 10 mg/kg) followed by ethanol intoxication on PD 7 and 8 prevented the increased ethanol intake effect observed in the younger pups, but when ethanol intoxication occurred on PD 10 and 11, naloxone treatment did not affect the aversion observed at this age. An aversion to ethanol was evidenced in the younger pups administered ethanol and naloxone, but only when exposed to ethanol odor during ethanol intoxication. Results indicate that the increased ethanol acceptance induced by ethanol intoxication in the younger pups is mediated by the opioid system, and that ethanol may also induce conditioned aversions at this early age.

Interactions between prenatal ethanol exposure and postnatal learning about ethanol in rat pups

Exposure to ethanol during the last days of gestation (1 or 2/kg) has been shown to induce greater ethanol intake as well as enhanced ethanol palatability in infant rats compared to pups without previous experience with the drug. This higher acceptance of ethanol seems to result from the prenatal association between the chemosensory aspects of ethanol and its reinforcing properties; the latter mediated at least in part by the opioid system. A series of experiments analyzed how this prenatal experience with ethanol affected postnatal conditioning of the flavor of ethanol. In the first experiment, pups exposed prenatally to ethanol (2g/kg during gestational days 17-20) were tested for acquisition and extinction of a conditioned aversion to the taste of ethanol (conditioned stimulus), employing lithium chloride as the aversive unconditioned stimulus. In the next two experiments, pups with the same prenatal exposure to ethanol were tested for the extinction of an aversion, employing three (Experiment 2) or one (Experiment 3) conditioning trials, using a high dose of ethanol (3g/kg) as the aversive agent. The first experiment showed that prenatal ethanol exposure delayed the postnatal acquisition of a conditioned aversion to the taste of ethanol. In the next two experiments, the same effect was observed during the extinction trials. These results provide additional support to the hypothesis of an appetitive associative memory established prenatally as a consequence of fetal ethanol exposure.

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

Administration of a moderate dose (2 g/kg) of ethanol to the pregnant rat during the last days of gestation (17-20) has been observed to increase the offspring's ethanol consumption on postnatal day 14. This effect was not observed with a 1-g/kg dose (Experiment 1a). When pups were tested during adolescence (day 28) an augmented ethanol intake was observed in female rats exposed to the low dose of ethanol and in male rats exposed to the moderate one (Experiment 1b). The effect of increased ethanol intake in preweanling rats was reduced after naloxone was administered, together with ethanol, to the pregnant dam (Experiment 2). Postnatal reexposure to ethanol, together with a naloxone injection, also decreased ethanol intake in pups exposed prenatally to the drug (Experiment 3). All these results seem to support the hypothesis of a conditioned preference learned in utero as a consequence of the association between the orosensory characteristics of ethanol and its reinforcing properties, apparently mediated by the opioid system.

Social interaction with an intoxicated sibling can result in increased intake of ethanol by periadolescent rats

A novel procedure for enhancing voluntary intake of ethanol in periadolescent rats is described. The procedure is a modification of Galef et al.'s (e.g., Galef, Kennett, & Stein, 1985; Anim Learn Behave 13:25-30) demonstrator-observer procedure. Subjects were Sprague-Dawley rats, 28-35 days of age. The experimental subject (observer) interacted with a same-sex conspecific (demonstrator) previously administered (a) 1.5 g/kg ethanol, (b) an equal volume of water, or (c) 2.1% Sanka coffee intragastrically. Observers were tested with 24-hour access to ethanol and coffee solutions. Observers that had interacted with demonstrators administered ethanol ingested significantly more ethanol during the test than observers in the other two groups. In Experiment 2 demonstrators were administered one of several doses of ethanol (0.0, 1.0, 1.5, or 3.0 g/kg) and observers' ethanol intakes were assessed. Only those observers that interacted with 1.5 g/kg demonstrators increased their ingestion of ethanol, relative to water controls. The lower (1.0 g/kg) and higher (3.0 g/kg) dose groups did not show altered ethanol ingestion. These results are discussed with respect to threshold levels of respired ethanol cues and the ability of observers to detect these cues from demonstrators. The demonstrator-observer procedure appears to be effective for the social transmission of preferences for ethanol in periadolescent rats.

Ethanol perception and ingestion

Ethanol is a common dietary constituent, but knowledge of its chemosensory properties and their relationship to its ingestion is limited. Twenty-five male and 25 female, light-regular ethanol consumers participated in five test sessions. Sessions involved determination of taste, olfactory and nasal irritation thresholds for ethanol, taste thresholds for tetralone, suprathreshold intensity and quality judgements, determination of phenylthiocarbamide (PTC) taster status, acute oral exposure effect of ethanol on ratings for sucrose, NaCl, citric acid and quinine hydrochloride and hedonic judgments of beers. Questionnaires on personality traits, diet and health were also completed and body composition determined. Diet histories were obtained on three random days. A randomized subset of sensory tests and questionnaires were administered each session. The rank order of ethanol thresholds was taste>nasal irritancy>olfaction and the predominant taste near threshold was bitter. Except for a lower taste threshold in females, no other effects of gender, PTC taster status, ethanol use or body composition were observed. Acute exposure to ethanol suppressed the bitterness of quinine, but enhanced its bitter aftertaste. Ethanol use was not associated with differences in dietary, sensory or textural characteristics. Thus, among light to regular ethanol consumers, no substantive association was observed between sensory responses to ethanol and diet.

Infant rats respond differently to alcohol after nursing from an alcohol-intoxicated dam

Our previous studies indicate that rat pups are able to detect the low levels of ethanol (175 mg %) found in the milk of a moderately intoxicated dam. The present study tested the effect of infantile interactions (including suckling) with ethanol-treated mothers on later behavioral responsiveness to ethanol's sensory properties. In Experiment 1, pups suckled from dams subjected to a 2.5 g/kg ethanol dose (i.g.) or water-treated females during postnatal days (PDs) 3, 5, 7, 9, 11, and 13. During PD 15, these pups were exposed to procedures to induce a conditioned aversion to the low level of ethanol (175 mg % in water), with lithium chloride as the unconditioned stimulus. Conditioning was more effective for pups with the prior ethanol experience within the nursing context. Greater responsiveness to ethanol in milk also was found for conditioning control pups that had interacted with intoxicated dams than for those that had interacted with water-treated dams. Experiment 2 determined that interaction with an intoxicated dam was sufficient for altered responsiveness to ethanol, in that the additional conditioning procedures of Experiment 1 were not needed for the effect. Generally, a relatively brief history of infantile interaction with ethanol-intoxicated dams increased later responsiveness to ethanol's orosensory properties. The results suggest that moderately intoxicated dams within the nursing context provide information to the progeny that may lead to the establishment of ethanol-related memories.

Interactions between perinatal and neonatal associative learning defined by contiguous olfactory and tactile stimulation

Tactile stimulation of the neonate, as performed by the mother during and after delivery, has been described as an effective unconditioned stimulus during early ontogeny (Leon, 1987; Ronca & Alberts, 1994). The present experiments examined the interaction between perinatal and neonatal learning determined by the explicit association between alcohol odor and vigorous body stimulation of the perinatal organism. In Experiment 1, rat fetuses were exposed to either alcohol or saline 10 min prior to cesarean delivery. The alcohol administration procedure here employed was sufficient to provide sensory contamination of the amniotic fluid but avoid fetal alcohol intoxication. Pups in the two prenatal treatments later experienced the smell of alcohol, tactile stimulation, or both stimuli explicitly paired or unpaired. Other postnatal groups were composed of pups that had no explicit experience with either experimental stimulus. Pups subjected to alcohol odor in utero displayed more overall motor activity in response to that odor than saline controls. The increased motor responses were further potentiated in pups that experienced additional postnatal alcohol odor paired with tactile stimulation. In Experiment 2, pups were exposed to alcohol in the amniotic fluid 10 or 30 min prior to birth. As previously demonstrated the memory acquired in utero appears highly dependent upon contingency between exposure to this particular scent and delivery procedures. Pups in both prenatal treatment groups were then exposed to alcohol odor paired or unpaired with tactile stimulation. Some control animals received no further experience with either stimuli. Those pups exposed to alcohol odor paired with tactile stimulation both pre- and postnatally later showed maximum motor activity elicited by the odor of alcohol. The results support the notion of fetal associative learning comprising alcohol's chemosensory cues and behaviorally activating stimuli. Furthermore, the conditioned response under analysis is potentiated whenever neonates are reexposed to contingent presentations of the elements that defined the original associative memory.

Chronic alcohol administration in the rat pup: effects upon later consumption of alcohol and other palatable solutions

Previous studies indicate that in rats, chronic alcohol exposure during the first weeks of life markedly affect subsequent ethanol consumption patterns. The present study examined the impact of different alcohol doses (0.5-3.0 g/kg), administered between postnatal days 6-12, upon subsequent infantile consumption of an ethanol solution as well as upon intake of various non-ethanol solutions (water, sucrose, quinine or sucrose mixed with quinine). Alcohol administration did not strongly affect consumption scores of water, sucrose or quinine. In contrast, 15-day-old pups pretreated with 2.0 and 3.0 g/kg alcohol doses showed significant increases in terms of alcohol consumption when compared to saline controls. Furthermore, a positive significant correlation was observed between alcohol intake patterns and alcohol dose administered during early ontogeny. Intake of sucrose mixed with quinine was also significantly and positively correlated with pretreatment ethanol dosage. Interestingly, this taste configuration has been shown to mimic psychophysical properties of ethanol in the rat. The results appear not to be explained by teratological effects of the drug upon sensory processing of distinctive tastants. It appears that chronic alcohol exposure during early ontogeny provides specific sensory-related alcohol information that later modulates alcohol intake patterns.

Neonatal responsiveness to alcohol odor and infant alcohol intake as a function of alcohol experience during late gestation

It has been previously suggested that maternal alcohol intoxication during the last days of pregnancy promotes fetal experiences that include chemosensory processing of the drug. In this study pregnant Wistar-derived rats were administered saline or one of two alcohol doses (1 or 2 g/kg) during gestational days 17-20. Immediately after birth, pups were tested in regard to motor-eliciting properties of the odor of amniotic fluid or alcohol, or of these stimuli presented as a configuration. Saline controls showed significantly shorter duration of overall motor activity and head movements when stimulated with the biological cue (amniotic fluid) than when exposed to a novel stimulus (ethanol alone or configured with the amniotic fluid). The opposite pattern was found in pups with prenatal experience with the higher alcohol dose. In a second experiment, the impact of similar alcohol treatments on infant consumption of different tastants, including alcohol and a configuration of sucrose and quinine, was tested. This configuration appears to mimic psychophysical properties of ethanol. Consumption of water, sucrose, or quinine was unaffected by the prenatal status of the subjects. Antenatal alcohol experience with the lower alcohol dose (1 g/kg) increased both alcohol and sucrose-quinine consumption. The 2 g/kg alcohol animals also ingested more sucrose-quinine relative to saline controls. As a whole, the results confirm the hypothesis that an intrauterine alcohol sensory memory selectively affects neonatal recognition of the alcohol's olfactory attributes and infant intake of either alcohol or solutions that share certain sensory equivalence with this psychopharmacological agent.

Behavioral detection of low concentrations of ethanol in milk in the preweanling rat

Previous animal models testing infantile reactivity to ethanol (EtOH) in maternal milk used EtOH doses that vastly exceeded levels actually encountered in a mildly or moderately intoxicated dam. The present study assessed whether 12- and 16-day-old rats are capable of detecting EtOH in milk at levels actually recorded in an intoxicated dam. Experiment 1 determined representative levels of EtOH in maternal milk as a function of maternal intragastric administration of EtOH (0.5-3.0 g/kg). Experiment 2A assessed generalization of conditioned taste aversions accrued with a high level of EtOH (6%) in either water or milk vehicles towards lower, more representative EtOH levels obtained from Experiment 1. With body weight gain as the dependent measure, conditioned aversions to milk were evident with the milk vehicle, but there was no detection of EtOH at any level at either age. Detection of the high level of EtOH (6%) in milk, however, was observed by 16 day olds within an habituation paradigm (Exp. 2b) via cardiac and behavioral (locomotion, mouthing) indexes. In Experiment 3 application of Experiment 2's more sensitive, behavioral index to assess generalization of the conditioned taste aversions revealed detection of a lower, more representative concentration of EtOH (175 mg%) in milk in 16-day-old rats. Overall the results show that the unweaned rat is capable of detecting very low concentrations of EtOH in milk and can modify their behavior accordingly. The expression of this capability is not, however, homogeneous across different response indexes. In conjunction with prior research it is clear that the infant rat's perception of EtOH in milk, including the very low levels of EtOH found in maternal milk during mild or moderate intoxication, is a relevant experience for generating new responses towards EtOH.

Ontogenetic differences in the expression of olfactory-conditioned aversions resulting from a state of acute alcohol intoxication in the rat

Three experiments examined the effects of acute alcohol intoxication on olfactory conditioning in infant (16 days old) and adult rats (60-65 days old). In both age groups, alcohol administration (2 g/kg, IG) prior to conditioning procedures failed to affect subsequent expression of methyl-conditioned aversions in a two-way odor locational test. Nevertheless, whenever this same alcohol dose was administered prior to testing procedures, the expression of the olfactory conditioning was substantially impaired in infants but unaffected in adults. The factorial design allowed rejection of the possibility that ontogenetic differences in alcohol-induced state-dependent retention might explain this developmental difference (Experiment 1). A subsequent experiment (Exp. 2a) revealed major age related differences in terms of peak blood alcohol levels (BALs) that resulted from the 2 g/kg alcohol dose previously employed. As revealed by gas chromatography, infants exhibited significantly higher BALs than adults at time of conditioning and/or testing. Therefore in Experiment 2b, the alcohol dose administered in adults was increased until BALs were comparable to those attained in infants given 2 g/kg. This was accomplished with a 2.5 g/kg alcohol dose, employed in Experiment 2c to evaluate responsiveness of adults in conditioning circumstances identical to those of Experiment 1. Yet even with this dose the adults' expression of the conditioned olfactory aversion was unaffected by the alcohol at the time of testing. Experiment 3 analyzed the effects of alcohol intoxication on infantile and adult motor responses elicited by methyl and a novel odor (lemon), which might conceivably have affected performance in the two-way odor locational tests used to assess olfactory conditioning. The results failed to support the possibility that the previously observed ontogenetic differences in the expression of olfactory-conditioned aversions were due to alcohol's effect on sensorimotor responsiveness. In accordance with prior studies, the present results seem to indicate age-related sensitivity to alcohol's effects on cognitive processes.

Prenatal and postnatal ethanol exposure influences preweanling rats' behavioral and autonomic responding to ethanol odor

The specific question was how prenatal and/or postnatal experience with ethanol influences cardiac and behavioral responses to the odor of ethanol on postnatal day (PD) 16. In each of two experiments, pregnant rats were given ethanol or water on gestational days 17-20. Offspring were exposed on PD12 to one of three conditions: intragastric administration of 6% ethanol, indirect exposure to ethanol from littermates, or no treatment. Results of Experiment 1 indicated that, regardless of prenatal ethanol exposure, 16-day-olds exposed on PD12 either directly or indirectly to ethanol expressed a greater increase in HR in response to ethanol odor than pups not postnatally exposed to ethanol. In Experiment 2, in which a lower ethanol dose was used postnatally, an interaction between pre- and postnatal ethanol exposure was observed; that is, pups exposed pre- and postnatally to ethanol showed the greatest increases in HR and the smallest increases in motor activity in response to ethanol odor. In both experiments motor activity was dissociated from increases in HR. The results are discussed in terms of what is learned, prenatally and postnatally, in association with the chemosensory properties of ethanol.

Context and tone conditioning are selectively impaired by ethanol in the preweanling rat: effects of dose and time of administration

Depending on dose and task requirements, ethanol can have either a facilitative or an impairing influence on learning. Some explanations for this dichotomy have considered ethanol's suppression of behavioral variability and processing of incidental stimuli (e.g., context). The present study examined the effect of ethanol on context and conditioned stimulus (CS) learning in the preweanling rat. To assess state-dependent effects, a drug dissociation design was used. Learning to both context and CS were analyzed within each dose of ethanol (0, 1.2, 1.6, or 2.0 g/kg) and a trend analysis was conducted to determine dose-response relationships as a function of train-test state. The 1.2 g/kg dose of ethanol did not affect conditioning to either the context or the CS. A 1.6 g/kg dose tended to disrupt context, but not CS, conditioning. The influence of 2.0 g/kg ethanol depended on train-test conditions. Ethanol administration prior to training resulted in the stronger impairment of CS learning while context conditioning was most disrupted if ethanol was given only prior to testing. The results suggest that ethanol selectively attenuates processing of stimuli, possibly dependent on relative saliency at the time of testing.

Aversions to alcohol's orosensory cues in infant rats: generalization to compounds of alcohol with sucrose or sodium chloride

Prior research has demonstrated that rat pups perceive alcohol's orosensory consequences during an acute state of intoxication with the drug and are able to associate these orosensory stimuli with aversive reinforcement. The present two experiments tested whether the resulting conditioned aversion to ethanol orosensory consequences generalized to two basic tastants (sucrose or sodium chloride) and if ethanol's orosensory consequences were detected when this agent was configured with these tastants. Conditioned aversions to alcohol were expressed only in rejection of an intraoral infusion of an ethanol solution alone or ethanol in compound with sucrose (experiment 1). A sucrose aversion was recorded in pups that had been subjected to infusions of a sucrose-ethanol compound paired with aversive reinforcement. An aversion to sodium chloride was not induced, however, by analogous procedures (experiment 2). The results indicate that, as in adults, ethanol aversions do not generalize directly to sucrose alone or sodium chloride alone. The infant is, however, capable of detecting the drug in compound with sucrose, and an acquired aversion to ethanol can be transferred to sucrose through ingestion of a sucrose-alcohol compound.

Alcohol in the amniotic fluid prior to cesarean delivery: effects of subsequent exposure to the drug's odor upon alcohol responsiveness

Rat fetuses during the last day of gestation have the capacity to process ethanol and non-ethanol-related chemosensory cues present in the amniotic fluid. Recent studies suggest that the consequences related to cesarean delivery act as an unconditioned stimulus that is associated with these cues. In the first experiment, ethanol neonatal responsiveness assessed through a motor activity test was analyzed in pups that received ethanol or saline in utero proximal to cesarean delivery. Different factors and the interaction among them, were analyzed in this experiment: (i) ethanol concentration administered into the amniotic sac (0, 6, or 18% v/v), (ii) delay between administration and cesarean section (3, 10, or 30 min), and (iii) postnatal exposure to ethanol odor prior to test (0, 7.5, or 15 min). Only animals exposed to ethanol 10 min prior to delivery differed from vehicle-exposed subjects. Subsequent postnatal exposure to ethanol odor attenuated the magnitude of prenatally established effects. In the second experiment it was observed that prenatal ethanol exposure was sufficient to increase ethanol intake during Postnatal Day 11. Again, this effect was strongly attenuated when pups were exposed to the odor of the drug prior to assessment procedures. These results suggest that (i) associations between chemosensory cues in the amniotic fluid and consequences related with perinatal manipulations are likely to occur and (ii) postnatal reexposure to similar cues exerts an effect comparable to an extinction phenomenon.

Enhanced ethanol intake in preweanling rats following exposure to ethanol in a nursing context

Several studies have confirmed that diet selection patterns of adult rats are at least partially established as a result of early experiences with food-related stimuli present in the milk of a lactating female (e.g., Capretta & Rawls, 1974; Galef & Clark, 1972; Galef & Henderson 1972). The present experiments were designed to investigate whether preweanling rats would similarly modify their acceptance of an ethanol solution following exposure to this cue in a nursing context. In Experiment 1, 8-, 12-, and 16-day-old rats were given ethanol, delivered intraorally in compound with milk, while given the opportunity to suckle an anesthetized dam. Subsequent testing revealed that 12- and 16-day-old subjects evidenced enhanced intake of the ethanol relative to controls, while 8-day-olds did not. Finally, the oldest (16 days of age) subjects also expressed a conditioned aversion to the milk when tested 24 hr after conditioning and ethanol-ingestion testing. Experiment 2 demonstrated that the critical factor for ethanol conditioning was the opportunity to suckle, rather than the simultaneous presence of milk. Finally, the aversion to milk observed in Experiment 1 was shown to have resulted from long-delay learning, due to the ingestion of a sufficient dose of ethanol during testing to serve as an aversive unconditioned stimulus (Exp. 3).

Operant responding controlled by milk or milk contaminated with alcohol as positive reinforcers in infant rats

Infant rats during the first, second, or third week of life were tested in operant conditioning with uncontaminated milk or milk supplemented with 6.0% v/v absolute ethanol (EtOH) as the reinforcer. Relative to yoked controls, pups of each age group reinforced on a response-contingent basis exhibited a significantly higher rate of responding with either reinforcer. In terms of amount of reinforcement, milk induced a higher rate of lever pressing than did the EtOH-contaminated compound. Age-related differences in the onset of differential responding for plain milk and EtOH-contaminated milk suggested developmental changes in the effects of alcohol. In a second experiment, forced drinking of milk and EtOH-contaminated milk was compared in similar age groups. Patterns of intake resembled the patterns of operant responding controlled by the same substance in the first experiment. These experiments indicate that the presence of alcohol in milk partially inhibits the reinforcing capacity of uncontaminated milk. Nevertheless, the former compound is still effective as a positive reinforcer during the first weeks of life.

Bradycardiac responses elicited by alcohol odor in rat neonates: influence of in utero experience with ethanol

Two experiments were performed in order to assess neonatal responsiveness to alcohol odor as a function of prior acute in utero experience with this drug. In experiment 1 the amniotic fluid surrounding rat fetuses of 21 days of gestation was either contaminated with alcohol or lemon solutions or with physiological saline. Neonatal presentation of alcohol odor induced stable and relatively low cardiac decelerations in saline and lemon exposed rats. Alcohol prenatal administration resulted in stronger bradycardiac patterns that lasted throughout most of the testing procedure. In experiment 2 these results were replicated and it was observed that nociceptive stimulation paired with alcohol prenatal administration significantly inhibited neonatal responsiveness to ethanol odor. The results indicate that learning relative to alcohol derived cues is likely to occur in utero. Furthermore, expression of acquisition is detected almost immediately after birth when employing an autonomic index frequently related with orienting responses to relevant stimuli.

Acute prenatal experience with alcohol in the amniotic fluid: interactions with aversive and appetitive alcohol orosensory learning in the rat pup

Prenatal alcohol acute contamination of the amniotic fluid and different postnatal manipulations with this drug alter subsequent responsiveness to EtOH's chemosensory cues. In this study, the interaction between prenatal and postnatal alcohol-related experiences was examined. Alcohol administered in the amniotic fluid during gestational Day 21 potentiated subsequent alcohol-odor conditioned preferences resulting from postnatal pairings between the odor and sucrose intraoral infusions. No interaction was attained when examining the impact of the in utero experience with postnatal aversive conditioning defined by alcohol odor-citric acid pairings (Exps. 1a & 1b). In Exp. 2, infantile alcohol aversions derived from a state of acute ethanol intoxication were inhibited by prior alcohol experience in utero. Examination of alcohol levels in fetal trunk blood and the amniotic fluid suggests that the antenatal experience is related to the chemosensory perception of the drug rather than its intoxicating properties (Exp. 3). These results strongly suggest that the alcohol-related memory generated proximal to birth can modulate subsequent learning with the drug.

Alcohol preference and hepatic alcohol dehydrogenase activity in adult Long-Evans rats is affected by intrauterine sibling contiguity

Alcohol preference and hepatic alcohol dehydrogenase activity in adult rats are known to be sexually dimorphic. Intrauterine sibling contiguity (the intrauterine position of a fetus relative to adjacent siblings of the same or opposite sex) alters selected reproductive, behavioral and enzymatic sexual dimorphisms via intersibling sex hormone transfer. We postulated that sibling contiguity would affect alcohol preference and hepatic alcohol metabolism in adult rats. The results of our study demonstrate that adult mMm male Long-Evans rats (genetic male rat developing in utero between two male siblings) had significantly lower ethanol preference, attained higher blood alcohol levels after standard ethanol "challenge" doses and had significantly lower hepatic alcohol dehydrogenase activity than either male siblings developing in utero between two females (fMf) or genetic females developing between two males or between two females (mFm or fFf). Hepatic cytosolic aldehyde dehydrogenase activity was higher in adult female than male rats regardless of nearest neighbor siblings. It is suggested that the differences in ethanol preference and hepatic alcohol dehydrogenase activity between the adult mMm and fMf male rats is due to differences in prenatal hormonal environment which can modulate sexual dimorphisms in alcohol intake and metabolism in the adult.

Ethanol-mediated taste aversions and state-dependency in preweanling (16-day-old) rats

Requirements for conditioning of an ethanol-mediated taste aversion in 16-day-old rat pups were examined. Experiment 1 demonstrated that preweanling rats are capable of acquiring, in two trials, an aversion to a 15% sucrose solution when followed by intragastric intubation of a 1.2 g/kg dose of 17% v/v ethanol, but not when followed by a 0.4 g/kg dose. Comparison was with control animals given sucrose followed by an equivalent volume isocaloric Half and Half. When the 0.4 g/kg dose of ethanol preceded sucrose presentation by 30 min (Experiment 2), the aversion was learned, suggesting that the effective delay between the sucrose and the critical consequences of the ethanol had been too long with the former procedure. Expression of the sucrose aversion required, however, the reinstatement of the context of intoxication--state-dependent retention. Finally, the results of Experiment 3B indicated that, in addition to the association between the sucrose and the aversive consequences of alcohol intoxication, the orosensory cues resulting from alcohol's direct elimination, via such processes as respiration and salivation, became associated with the appetitive properties of the sucrose. This was evidenced by a conditioned increase in preference for ethanol odor. Possible age-related differences in the ability to associate stimuli with alcohol's unconditioned consequences, and in state dependency are discussed.

Acute ethanol contamination of the amniotic fluid during gestational day 21: postnatal changes in alcohol responsiveness in rats

Two experiments investigated the effects of an acute alcohol prenatal experience during gestational Day 21 in the rat. At postnatal Days 8 and 9, this experience was sufficient to significantly increase ethanol odor preference as well as alcohol intake. Fetuses treated with a nonethanol stimulus (lemon) also exhibited changes suggesting increased lemon olfactory acceptance patterns (Exp. 1). Furthermore, when the olfactory component of the solutions experienced in utero were later paired with a novel tactile cue, responsiveness to such cue was strongly affected. Pups prenatally exposed to alcohol exhibited significantly lower tactile preference scores when texture was postnatally paired with ethanol odor when compared to specific controls. This effect was also observed in lemon-treated subjects after pairing defined by lemon-texture trials (Exp. 2). The results reported in alcohol-treated subjects appear not to be related with postabsorptive effects of the drug. It is suggested that sensory prenatal experience with alcohol is responsible for the reported changes in postnatal alcohol responsiveness patterns.

Acute alcohol intoxication paired with aversive reinforcement: ethanol odor as a conditioned reinforcer in rat pups

Previous research has suggested that infant rats process ethanol sensory properties during acute alcohol intoxication. The present study was designed in order to examine if alcohol odor could act as an aversive conditioned stimulus after the organism experiences the state of intoxication paired with nociceptive stimulation (footshock). In a first experiment 11-day-old pups received intragastric alcohol administration (1.5 g/kg). At different postabsorptive intervals footshock was presented (0-30, 30-60, 60-90, or 90-120 min). An explicitly unpaired control group which experienced footshock prior to the state of intoxication was also employed. All animals were subsequently tested in terms of alcohol intake and ethanol locational odor preferences. Both assessments indicated that pups which were exposed to the unconditioned peripheral stimulus 30-60 min after receiving ethanol expressed strong alcohol aversions. In a second experiment pups were exposed to footshock during this postabsorptive interval. Twenty four hours later, pups experienced ambient ethanol odor paired with soft or rough texture surfaces. Differential texture aversions were registered in experimental animals when compared with controls which suffered the state of intoxication explicitly unpaired with footshock, or unpaired presentations of ethanol odor and the tactile stimuli under consideration. These results appear to support the hypothesis concerning orosensory processing during an acute state of intoxication. Additionally it seems that the hedonic value of sensory attributes of this drug varies as a function of associative processes occurring during such a state.