Brief prenatal ethanol exposure alters behavioral sensitivity to the kappa opioid receptor agonist (U62,066E) and antagonist (Nor-BNI) and reduces kappa opioid receptor expression.

BACKGROUND: Approximately 10 to 15% of women consume alcohol (ethanol [EtOH]) during pregnancy in the United States. Even low amounts of EtOH consumption during pregnancy can elicit long-term consequences. Prenatal experience with as few as 3 drinks has been associated with increase problem drinking in adulthood. Such effects are corroborated in rodents; however, the underlying neural adaptations contributing to this effect are not clear. In the current set of experiments, we investigated whether changes in EtOH responding following prenatal EtOH exposure involved kappa opioid receptor activation and expression. METHODS: Sprague-Dawley rats were prenatally exposed to low levels of alcohol (1.0 g/kg) during late gestation (gestational days 17 to 20 [GD17-20]) via intragastric intubation of pregnant dams. Following birth, EtOH intake, kappa- and mu-opioid-induced place conditioning, and kappa opioid receptor expression in mesolimbic brain regions were assessed in infant rats (postnatal days 14 to 15 [PD14-15]) that were offspring of dams given EtOH, vehicle, or untreated, during pregnancy. RESULTS: Animals exposed to prenatal alcohol drank more alcohol later in life and exhibited significant changes in the kappa opioid system. While control subjects found kappa opioid activation aversive, animals exposed to EtOH prenatally exhibited either no aversion or appetitive responding. Further analysis revealed that synaptosomal kappa opioid receptor expression was significantly decreased in brain areas implicated in responding to EtOH. CONCLUSIONS: Overall, these data suggest that prenatal EtOH affects kappa opioid function and expression and that these changes may be involved in increased drinking later in life.

Binge ethanol intoxication heightens subsequent ethanol intake in adolescent, but not adult, rats.

A question still to be answered is whether ethanol initiation has a greater effect on ethanol consumption if it occurs during adolescence than in adulthood. This study assessed the effect of ethanol initiation during adolescence or adulthood on voluntary ethanol consumption when animals were still within the same age range. Adolescent or adult rats were given 5, 2, or 0 ethanol exposures. The animals were tested for ethanol consumption through two-bottle choice tests, before undergoing a 1-week deprivation. A two-bottle assessment was conducted after the deprivation. Adolescents, but not adults, given two ethanol administrations during initiation exhibited significantly higher ethanol intake during the pre-deprivation period. These adolescents also exhibited a threefold increase in ethanol intake after 7 days of drug withdrawal, when compared with controls. These findings suggest that very brief experience with binge ethanol intoxication in adolescence, but not in adulthood, impacts later predisposition to drink.

Maternal isolation during the first two postnatal weeks affects novelty-induced responses and sensitivity to ethanol-induced locomotor activity during infancy.

Animals exposed to chronic maternal separation (MS) exhibit enhanced ethanol self-administration and greater hormonal and behavioral responsiveness to stress in adulthood. Whether the effects of MS are immediately evident in infancy or whether they appear only later on development is still an unanswered question This study tested sensitivity to ethanol's behavioral stimulating effects in infant rats that experienced MS from postnatal Day 1-14. MS infants exhibited significantly greater reactivity to the motor stimulating effects of 1.25 g/kg ethanol than control animals, yet greater motor suppression after 2.5 g/kg ethanol. Baseline level of response to novelty was altered in MS infants, in a nor-binaltorphimine insensitive manner, that is, despite modified activity of the kappa-opioid system. These results indicate that the consequences of chronic maternal isolation emerge early in ontogeny, affecting ethanol sensitivity in infancy.

Ethanol-induced locomotor activity in adolescent rats and the relationship with ethanol-induced conditioned place preference and conditioned taste aversion.

Adolescent rats exhibit ethanol-induced locomotor activity (LMA), which is considered an index of ethanol's motivational properties likely to predict ethanol self-administration, but few studies have reported or correlated ethanol-induced LMA with conditioned place preference (CPP) by ethanol at this age. The present study assessed age-related differences in ethanol's motor stimulating effects and analyzed the association between ethanol-induced LMA and conventional measures of ethanol-induced reinforcement. Experiment 1 compared ethanol-induced LMA in adolescent and adult rats. Subsequent experiments analyzed ethanol-induced CPP and conditioned taste aversion (CTA) in adolescent rats evaluated for ethanol-induced LMA. Adolescent rats exhibit a robust LMA after high-dose ethanol. Ethanol-induced LMA was fairly similar across adolescents and adults. As expected, adolescents were sensitive to ethanol's aversive reinforcement, but they also exhibited CPP. These measures of ethanol reinforcement, however, were not related to ethanol-induced LMA. Spontaneous LMA in an open field was, however, negatively associated with ethanol-induced CTA.

Ethanol-mediated appetitive conditioning in infant rats, but not corticosterone release, is dependent on route of ethanol administration.

A recent study found appetitive reinforcement in infant rats given 1.0 but not 2.0 g/kg ethanol and only when ethanol was delivered intragastrically (i.g., but not if intraperitoneally, i.p.; Nizhnikov, Pautassi, Truxell, & Spear [2009] Alcohol 43, 347-358). Corticosterone release could modulate ethanol's motivational effects. The goal of this study was to replicate the differential capability of i.g. vs. i.p. ethanol to induce conditioning and to find hormonal correlates underlying this phenomenon. Experiment 1 confirmed that 1.0 g/kg ethanol induced conditioned preference in infant rats when given i.g. but not i.p. In Experiment 2 corticosterone was assessed at 20, 40, 60, or 120 min after ethanol (0.0, 0.5, 1.0, and 2.0 g/kg, i.g. or i.p.). Route of administration failed to alter corticosterone release. The 2.0 g/kg, but not 0.5 or 1.0 g/kg, ethanol dose evoked heightened corticosterone release. The results confirm the differing motivational effects associated with i.g. and i.p. ethanol. These effects do not seem to be related to differential corticosterone responsiveness.

Ontogenetic differences in sensitivity to LiCl- and amphetamine-induced taste avoidance in preweanling rats.

When amphetamine is associated with a tastant conditioned stimulus, rats learn to avoid the taste even when employing doses that promote conditioned place preference. One hypothesis raised to account for this effect proposes that taste avoidance induced by amphetamine may be motivated by fear. A sensitive period has been identified in the rat (until postnatal day 10) in which infants learn conditioned appetitive effects to stimuli to which aversions are conditioned after this period. Exogenous administration of corticosterone within this period reverses this effect, generating aversive conditioning. In the present study, we tested conditioning of aversions to amphetamine or LiCl, within and after the sensitive period (Experiments 1 and 2). A third experiment evaluated unconditioned rejection of an aversive quinine solution within the sensitive period. Finally, we tested whether corticosterone administration before conditioning modulates amphetamine-induced taste avoidance. After the sensitive period, infant rats rejected the solution paired with amphetamine or LiCl after 2 conditioning trials, but within the sensitive period, aversions were conditioned only by LiCl and after 4 conditioning trials. Amphetamine-induced taste avoidance was not observed even when corticosterone was administered before conditioning. Additionally, during the sensitive period, a low LiCl dose promoted conditioned taste preference. According to Experiment 3, parameters employed in this study were suitable to yield rejection of aversive solutions within the sensitive period. These results suggest that during the sensitive period, there is a notable resistance to the acquisition of taste avoidance induced by amphetamine. The present experimental framework may represent a useful tool for studying mechanisms underlying taste avoidance and aversion effects.

Ontogeny of ethanol intake in alcohol preferring (P) and alcohol nonpreferring (NP) rats.

There is a scarcity of research on ethanol affinity in alcohol-preferring (P) rats before weaning and it is unknown if neonate P rats exhibit ethanol intake preferences comparable to those observed in adult P rats. This study examined ethanol intake in P and alcohol-nonpreferring (NP) rats 3 hr after birth (Experiment 1, surrogate nipple test), at postnatal days (PD) 8, 12, and 18 (Experiment 2, consumption from the floor procedure) and at adolescence (Experiment 3, two-bottle choice test at PD32). The high-preference genotype was readily expressed 3 hr after birth. P neonates drank twice as much ethanol as their NP counterparts. This heightened ethanol preference transiently reversed at P8, reemerged as weaning approached (P18) and was fully expressed during adolescence. These results help to clarify the ontogeny of genetic predisposition for ethanol. Genetic predisposition for higher ethanol intake in P than in NP rats seems to be present immediately following birth.

Mere odor exposure learning in the rat neonate immediately after birth and 1 day later.

Rat pups are more resistant to retroactive associative interference 3 hr after birth than 24 hr later [Cheslock et al. [2004] Developmental Science, 7, 581-598]. The present experiments tested the effect of age, retention interval and dam presence during the retention interval on odor-induced motor activity subsequent to mere odor exposure. Rats were exposed to an hour of odor immediately after birth or approximately 1 day later and tested after a given retention interval (3 or 27 hr [Exp 1]; 0, 30, 75, or 180 min [Exp. 2]). They spent the retention interval either in the presence or absence of a foster dam (Exp. 1 and 3). After the retention interval, pups were tested in a 4-min activity test including a 2-min baseline period and 2 min of odor exposure. Overall activity was scored during tape-playback. Odor-exposed pups were more active than nonexposed pups during reexposure to the odor during testing, but this was true only for P0 pups. In contrast, P1 pups without prior odor exposure were active during testing and behaviorally quieted in the presence of the odor they were previously exposed to. Though 1 day apart, newborn rats just hours old lack many of the experiences that a 1-day-old has had including nursing, huddling, and being groomed. These experiences are associated with, among other stimuli, a barrage of olfactory cues (e.g., colostrum, saliva, dander, feces, and urine). P0 and P1 pups also differ in their proximity from the birthing experience and associated neurochemical changes. The age-related pattern of responding to odors based on previous odor exposure was discussed in relation to these and other possibilities.

A comparison between taste avoidance and conditioned disgust reactions induced by ethanol and lithium chloride in preweanling rats.

Adult rats display taste avoidance and disgust reactions when stimulated with gustatory stimuli previously paired with aversive agents such as lithium chloride (LiCl). By the second postnatal week of life, preweanling rats also display specific behaviors in response to a tastant conditioned stimulus (CS) that predicts LiCl-induced malaise. The present study compared conditioned disgust reactions induced by LiCl or ethanol (EtOH) in preweanling rats. In Experiment 1 we determined doses of ethanol and LiCl that exert similar levels of conditioned taste avoidance. After having equated drug dosage in terms of conditioned taste avoidance, 13-day-old rats were given a single pairing of a novel taste (saccharin) and either LiCl or ethanol (2.5 g/kg; Experiment 2). Saccharin intake and emission of disgust reactions were assessed 24 and 48 hr after training. Pups given paired presentations of saccharin and the aversive agents (ethanol or LiCl) consumed less saccharin during the first testing day than controls. These pups also showed more aversive behavioral reactions to the gustatory CS than controls. Specifically, increased amounts of grooming, general activity, head shaking, and wall climbing as well as reduced mouthing were observed in response to the CS. Conditioned aversive reactions but not taste avoidance were still evident on the second testing day. In conclusion, a taste CS paired with postabsorptive effects of EtOH and LiCl elicited a similar pattern of conditioned rejection reactions in preweanling rats. These results suggest that similar mechanisms may be underlying CTAs induced by LiCl and a relatively high EtOH dose.

Dopamine receptors modulate ethanol's locomotor-activating effects in preweanling rats.

Near the end of the second postnatal week motor activity is increased soon after ethanol administration (2.5 g/kg) while sedation-like effects prevail when blood ethanol levels reach peak values. This time course coincides with biphasic reinforcement (appetitive and aversive) effects of ethanol determined at the same age. The present experiments tested the hypothesis that ethanol-induced activity during early development in the rat depends on the dopamine system, which is functional in modulating motor activity early in ontogeny. Experiments 1a and 1b tested ethanol-induced activity (0 or 2.5 g/kg) after a D1-like (SCH23390; 0, .015, .030, or .060 mg/kg) or a D2-like (sulpiride; 0, 5, 10, or 20 mg/kg) receptor antagonist, respectively. Ethanol-induced stimulation was suppressed by SCH23390 or sulpiride. The dopaminergic antagonists had no effect on blood ethanol concentration (Experiments 2a and 2b). In Experiment 3, 2.5 g/kg ethanol increased dopamine concentration in striatal tissue as well as locomotor activity in infant Wistar rats. Adding to our previous results showing a reduction in ethanol induced activity by a GABA B agonist or a nonspecific opioid antagonist, the present experiments implicate both D1-like and D2-like dopamine receptors in ethanol-induced locomotor stimulation during early development. According to these results, the same mechanisms that modulate ethanol-mediated locomotor stimulation in adult rodents seem to regulate this particular ethanol effect in the infant rat.

High ethanol dose during early adolescence induces locomotor activation and increases subsequent ethanol intake during late adolescence.

Adolescent initiation of ethanol consumption is associated with subsequent heightened probability of ethanol use disorders. The present study examined the relationship between motivational sensitivity to ethanol initiation in adolescent rats and later ethanol intake. Experiment 1 determined that ethanol induces locomotor activation shortly after administration but not if tested at a later post-administration interval. In Experiment 2, adolescent rats were assessed for ethanol-induced locomotor activation on postnatal Day 28. These animals were then evaluated for ethanol-mediated conditioned taste aversion and underwent a 16-day-long ethanol intake protocol. Ethanol-mediated aversive effects were unrelated to ethanol locomotor stimulation or subsequent ethanol consumption patterns. Ethanol intake during late adolescence was greatest in animals initiated to ethanol earliest at postnatal Day 28. Females that were more sensitive to ethanol's locomotor-activating effects showed a transient increase in ethanol self-administration. Blood ethanol concentrations during initiation were not related to ethanol-induced locomotor activation. Adolescent rats appeared sensitive to the locomotor-stimulatory effects of ethanol. Even brief ethanol exposure during adolescence may promote later ethanol intake.

Naloxone and baclofen attenuate ethanol's locomotor-activating effects in preweanling Sprague-Dawley rats.

Heterogeneous rat strains appear to be particularly sensitive to the sedative effects of ethanol as adults and insensitive to ethanol's stimulant effects. Recently, the authors found that ethanol induces stimulant effects in preweanling Sprague-Dawley rats. In adult mice, these effects seem to be governed by the mesocorticolimbic dopaminergic pathway, which can be modulated by means of GABA B agonist (baclofen) or opioid antagonist (naloxone) treatments. This study tested whether these pharmacological treatments might reduce the activating effect of ethanol in preweanling Sprague-Dawley rats. Twelve-day-old pups given naloxone (Experiment 1A) or baclofen (Experiment 1B) before ethanol administration were tested in terms of locomotor activity in a novel environment. Naloxone and baclofen significantly reduced the stimulating effect of ethanol but had no effect on locomotor activity patterns in water-treated controls. Blood ethanol levels were not affected by naloxone or baclofen (Experiment 2). During the preweanling period, opioid and GABA B receptors seem to be involved in the stimulating effect of ethanol.

The consequence of fetal ethanol exposure and adolescent odor re-exposure on the response to ethanol odor in adolescent and adult rats.

BACKGROUND: An epidemiologic predictive relationship exists between fetal ethanol exposure and the likelihood for adolescent use. Further, an inverse relationship exists between the age of first experience and the probability of adult abuse. Whether and how the combined effects of prenatal and adolescent ethanol experiences contribute to this progressive pattern remains unknown. Fetal ethanol exposure directly changes the odor attributes of ethanol important for both ethanol odor preference behavior and ethanol flavor perception. These effects persist only to adolescence. Here we tested whether adolescent ethanol odor re-exposure: (Experiment 1) augments the fetal effect on the adolescent behavioral response to ethanol odor; and/or (Experiment 2) perpetuates previously observed adolescent behavioral and neurophysiological responses into adulthood. METHODS: Pregnant rats received either an ethanol or control liquid diet. Progeny (observers) experienced ethanol odor in adolescence via social interaction with a peer (demonstrators) that received an intragastric infusion of either 1.5 g/kg ethanol or water. Social interactions were scored for the frequency that observers followed their demonstrator. Whole-body plethysmography evaluated the unconditioned behavioral response of observers to ethanol odor in adolescence (P37) or adulthood (P90). The olfactory epithelium of adults was also examined for its neural response to five odorants, including ethanol. RESULTS: Experiment 1: Relative to fetal or adolescent exposure alone, adolescent re-exposure enhanced the behavioral response to ethanol odor in P37 animals. Compared to animals with no ethanol experience, rats receiving a single experience (fetal or adolescent) show an enhanced, yet equivalent, ethanol odor response. Fetal ethanol experience also increased olfactory-guided following of an intoxicated peer. Experiment 2: Combined exposure yielded persistence of the behavioral effects only in adult females. We found no evidence for persistence of neurophysiological effects in either sex. CONCLUSION: Fetal ethanol exposure influences adolescent re-exposure, in part, by promoting interactions with intoxicated peers. Re-exposure subsequently enhances ethanol odor responsivity during a key developmental transition point for emergent abuse patterns. While persistence of behavioral effects occurred in females, the level of re-exposure necessary to uniformly yield persistence in both sexes remains unknown. Nonetheless, these results highlight an important relationship between fetal and adolescent experiences that appears essential to the progressive pattern of developing ethanol abuse.

Fetal exposure to moderate ethanol doses: heightened operant responsiveness elicited by ethanol-related reinforcers.

BACKGROUND: Prenatal exposure to moderate ethanol doses during late gestation modifies postnatal ethanol palatability and ingestion. The use of Pavlovian associative procedures has indicated that these prenatal experiences broaden the range of ethanol doses capable of supporting appetitive conditioning. Recently, a novel operant technique aimed at analyzing neonatal predisposition to gain access to ethanol has been developed. Experiment 1 tested the operant conditioning technique for developing rats described by Arias and colleagues (2007) and Bordner and colleagues (2008). In Experiment 2, we analyzed changes in the disposition to gain access to ethanol as a result of moderate prenatal exposure to the drug. METHODS: In Experiment 1, newborn pups were intraorally cannulated and placed in a supine position that allowed access to a touch-sensitive sensor. Paired pups received an intraoral administration of a given reinforcer (milk or quinine) contingent upon physical contact with the sensor. Yoked controls received similar reinforcers only when Paired pups activated the circuit. In Experiment 2, natural reinforcers (water or milk) as well as ethanol (3% or 6% v/v) or an ethanol-related reinforcer (sucrose compounded with quinine) were tested. In this experiment, pups had been exposed to water or ethanol (1 or 2 g/kg) during gestational days 17 to 20. RESULTS: Experiment 1 confirmed previous results showing that 1-day-old pups rapidly learn an operant task to gain access to milk, but not to gain access to a bitter tastant. Experiment 2 showed that water and milk were highly reinforcing across prenatal treatments. Furthermore, general activity during training was not affected by prenatal exposure to ethanol. Most importantly, prenatal ethanol exposure facilitated conditioning when the reinforcer was 3% v/v ethanol or a psychophysical equivalent of ethanol's gustatory properties (sucrose-quinine). CONCLUSIONS: The present results suggest that late prenatal experience with ethanol changes the predisposition of the newborn to gain access to ethanol-related stimuli. In conjunction with prior literature, this study emphasizes the fact that intrauterine experience with ethanol not only augments ethanol's palatability and ingestion, but also facilitates the acquisition of response-stimulus associations where the drug acts as an intraoral reinforcer.

Olfactory learning in the rat immediately after birth: Unique salience of first odors.

An infant rat's chance of survival is increased when it remains close to the nest. Early olfactory learning supports such adaptive behavior. Previous experiments indicated that non-associative odor exposure immediately after birth promoted later attachment to a similarly scented artificial nipple. The goal of the current experiments was to extend these findings on olfactory learning in the hours after birth by: exposing pups to more than one odor exposure (Experiment 1), dissecting the role of timing versus order of odor exposure (Experiment 2), testing the odor specificity of these effects (Experiments 3 and 4), and evaluating associative odor conditioning soon after birth (Experiment 5). Without explicit prior odor experience, pups only hours old do not respond much to a novel odor. Prior non-associative odor experience increases later motor activity to that same odor and to novel odors. Furthermore, these findings may be specific to certain amodal dimensions of the (in our case) lemon odor exposure. Single odor non-associative and associative conditioning was equally effective immediately after birth and during the third postnatal hour. Nevertheless, pups given two mere odor exposures responded to the first one more than the second at test, regardless of whether the exposures began immediately or 2 hr after birth. Possible mechanisms for these findings concerning early olfactory learning are discussed.

Ethanol, saccharin, and quinine: early ontogeny of taste responsiveness and intake.

BACKGROUND: Rat pups demonstrate high levels of immediate acceptance of ethanol during the first 2 weeks of postnatal life. Given that the taste of ethanol is most likely perceived by infant rats as a combination of sweet and bitter, high intake of ethanol early in ontogeny may be associated with age-related enhanced responsiveness to the sweet component of ethanol taste, as well as with ontogenetic decreases in sensitivity to its bitter component. Therefore, the present study compared responsiveness to ethanol and solutions with bitter (quinine) and sweet (saccharin) taste in terms of intake and palatability across the first 2 weeks of postnatal life. METHODS: Characteristic patterns of responsiveness to 10% (v/v) ethanol, 0.1% saccharin, 0.2% quinine, and water in terms of taste reactivity and fluid intake were assessed in rat pups tested on postnatal day (P) 4, 9, or 12 using a new technique of on-line monitoring of fluid flow through a two-channel intraoral cannula. Taste reactivity included analysis of ingestive and aversive responses following six intraoral infusions of the test fluids. This taste reactivity probe was followed by the intake test, in which animals were allowed to voluntarily ingest fluids from an intraoral cannula. RESULTS: Pups of all ages showed more appetitive responses to saccharin and ethanol than to water or quinine. No age-related differences were apparent in taste responsiveness to saccharin and ethanol. However, the age-related pattern of ethanol intake drastically differed from that of saccharin. Intake of saccharin increased from P4 to P9 and decreased substantially by P12, whereas intake of ethanol gradually increased from P4 to P12. Intake of ethanol was significantly lower than intake of saccharin on P9, whereas P12 pups took in more ethanol than saccharin. CONCLUSION: The findings of the present study indicate ontogenetic dissociations between taste reactivity to ethanol and saccharin and intake of these solutions, and suggest that high acceptance of ethanol early in ontogeny may not be associated with its orosensory properties but rather with the pharmacological effects of ethanol.

Analysis of ethanol reinforcement in 1-day-old rats: assessment through a brief and novel operant procedure.

BACKGROUND: An accumulating body of experimental evidence supports the notion that, early in development, heterogeneous rats exhibit heightened affinity for ethanol ingestion and are sensitive to the drug's postabsorptive reinforcing effects. The brevity of this ontogenetic period and the limited behavioral repertoire of the newborn represent obstacles in the examination of these phenomena. In the present study, we developed a novel operant technique aimed at examining the neonatal predisposition to gain access to intraoral infusions of different ethanol solutions and other potential reinforcers. METHODS: In all experiments, 1-day-old rats were placed in a supine position that allowed access to a touch-sensitive sensor. In Paired pups, reinforcers were delivered through an intraoral cannula in a fixed-ratio-1 schedule, based on their physical contact with the sensor. Yoked controls were included to account for overall magnitude of behavioral responding and were given infusions in accord with behavior of the corresponding Paired pup. The reinforcement effect of milk, water and different ethanol solutions (0.75 to 10% v/v) was assessed using a single 15-minute conditioning session. Additional pharmacokinetic studies were conducted to determine blood ethanol concentrations attained during the course of the training session. RESULTS: Within-subjects analysis revealed that Paired pups rapidly learned to increase the probability of behavioral execution to gain access to a biological reinforcer such as milk (Experiment 1). Follow-up experiments indicated that relatively low ethanol concentrations are equally likely to support operant performance (Experiments 2a and 3a). It was also observed that Paired pups exhibited surprisingly high levels of responding during an extinction session, particularly when initially trained with milk or 3% v/v ethanol as reinforcers (Experiment 4). The pharmacokinetic studies indicated that, within a short period of time, ethanol was detectable in blood. Levels attained during conditioning varied as a function of the ethanol concentration utilized as a reinforcer (Experiments 2b and 3b). CONCLUSIONS: The present technique appears to represent a valuable tool for examining ethanol self-administration and seeking behavior of the drug during early ontogeny. The results indicate that newborn rats, subjected to the explicit contingency comprising suckling-related behaviors and intraoral ethanol delivery (Paired pups), rapidly learn to gain access to the drug. These results are not explainable through psychomotor effects of ethanol as Yoked pups consumed similar amounts of ethanol and yet exhibited relatively low and consistent levels of responding. The overall pattern of results extends and validates previous observations of substantial affinity for ethanol during early stages of development, a phenomenon rarely encountered in genetically heterogeneous adult rats.

Adolescent but not adult rats exhibit ethanol-mediated appetitive second-order conditioning.

BACKGROUND: Adolescent rats are less sensitive to the sedative effects of ethanol than older animals. They also seem to perceive the reinforcing properties of ethanol. However, unlike neonates or infants, ethanol-mediated appetitive behavior is yet to be clearly shown in adolescents. Appetitive ethanol reinforcement was assessed in adolescent (postnatal day 33, P33) and adult rats (P71) through second-order conditioning (SOC). METHODS: On P32 or P70, animals were intragastrically administered ethanol (0.5 or 2.0 g/kg) paired with intra-oral pulses of sucrose (CS(1), first-order conditioning phase). CS(1) delivery took place either 5-20 (early pairing) or 30-45 minutes (late pairing) following ethanol administration. The time interval between CS(1) exposure and ethanol administration was 240 minutes in unpaired controls. On P33 or P71, animals were presented the CS(1) (second-order conditioning phase) in a distinctive chamber (CS(2), second-order conditioning). Then they were tested for CS(2) preference. RESULTS: Early and late paired adolescents, but not adults, had greater preference for the CS(2) than controls, a result indicative of ontogenetic variation in ethanol-mediated reinforcement. During the CS(1)-CS(2) associative phase, paired adolescents given 2.0 g/kg ethanol wall-climbed more than controls. Blood and brain ethanol levels associated with the 0.5 and 2.0 g/kg doses at the onset of each conditioning phase did not differ substantially across age, with mean blood ethanol concentration of 38 and 112 mg%. CONCLUSIONS: These data indicate age-related differences between adolescent and adult rats in terms of sensitivity to ethanol's motivational effects. Adolescents exhibited high sensitivity for ethanol's appetitive effects. These animals also showed ethanol-mediated behavioral activation during the SOC phase. The SOC preparation provides a valuable conditioning model for assessing ethanol's motivational effects across ontogeny.

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.

Motivational effects of intraorally-infused ethanol in rat pups in an operant self-administration task.

Motivational effects of self-administered ethanol have rarely been studied in preweanling rats due primarily to the lack of age-appropriate operant tasks. The present experiments assessed the hedonic effects of intraoral ethanol in infant rats self-administered by activating a touch sensor. On postnatal day (PD) 13 pups were pre-exposed to the drug's pharmacological and/or sensory effects. Operant sessions were conducted during PDs 14-16 (Experiments 1 and 2). Paired animals were placed in chambers equipped with a touch-sensitive disk and received an intraoral infusion of ethanol (3 or 5% v/v, 5 microl) after each sensor contact. Yoked controls were equated for number and distribution of ethanol infusions but had no control over the contingency between operant behavior and intraoral infusion. In Experiment 2, training trials were preceded by a non-reinforced phase. Paired pups performed fewer operant responses than controls and decreased their operant responses across sessions. These results suggest that intraoral self-administered ethanol has an aversive hedonic value in two-week old rats. Operant behavior seems to have been associated with aversive orosensory effects derived from intraoral ethanol infusion.