Context-dependent latent inhibition in preweanling rats.

Preexposure to a conditioned stimulus (CS) usually weakens conditioning, an effect known as latent inhibition. Similar to other learning interference effects, latent inhibition has been characterized as context-dependent, which means that the magnitude of this effect can be attenuated by changing the context between the different phases of the procedure (e.g., preexposure and conditioning). Latent inhibition has been found with a variety of procedures in infant rats, but the few studies that examined the context-dependency of this phenomenon during this ontogenetic period found no context-change effect. The present study explored the context-dependency of latent inhibition during infancy using a conditioned taste aversion preparation and employing contexts enriched with distinctive odors to increase the possible efficacy of the context manipulation. Experiment 1 showed that three preexposures to the CS (saccharin) were sufficient to retard conditioning to the same CS, although this effect was also observed in a control group preexposed to an alternative taste stimulus (saline), in comparison with a non-preexposed control group. In Experiment 2a, the CS-preexposure effect was found to be specific to the preexposed CS when the number of preexposures was increased. This effect was revealed as context-dependent in Experiment 2b, since it was attenuated by changing the context between preexposure and conditioning. The present result is consistent with recent studies showing the context-dependency of extinction in preweanling rats, thus demonstrating these animals' capacity to learn about context early on in their development.

Amniotic fluid can act as an appetitive unconditioned stimulus in preweanling rats.

Studies in humans and animals indicate that exposure to flavors in the amniotic fluid during the later gestational period may induce preferences for those flavors. Considering that during the last prenatal period the amniotic fluid contains substances that activate the opioid system, and that this system plays a critical role in the acquisition of olfactory preferences early in life, it has been hypothesized that the amniotic fluid may acquire appetitive unconditioned properties during this period. This has been tested in an experiment in which preweanling rats were exposed to alcohol odor (CS) paired or unpaired with the intraoral infusion of amniotic fluid (US) collected on gestational day 20. The pairing of these two stimuli induced an enhanced palatability of alcohol's flavor as well an increased intake of the drug. These results support the idea that amniotic fluid acquires appetitive unconditioned properties during the last days of gestation and suggest that associative mechanisms involving the amniotic fluid could be underlying odor and taste preferences acquired through fetal exposure.

Effects of stimulus preexposure on the generalization of conditioned taste aversions in infant rats.

Generalization of a conditioned taste aversion in infant rats and how this is affected by stimulus preexposure was investigated in a series of experiments. In Experiment 1 generalization of a conditioned aversion between two tastes (sweet and salty) was found, and the effect of tastes preexposure was a reduction in generalization (Experiment 2). However, when these tastes were combined with a common taste (acid) that was less (Experiment 3) or more intense (Experiment 3b), the effect of stimulus preexposure was a stronger generalization of the conditioned aversion. In this case, a reduction on generalization was again observed by increasing the number of preexposure trials to the taste compounds (Experiment 4). In all cases the generalization levels were directly related to the effect of stimulus preexposure on the acquisition rate of conditioning. It can be concluded that, with the appropriate parameters, a reduction of generalization of a conditioned taste aversion can be obtained after taste exposure in preweanling rats.

Repeated exposure to moderate doses of alcohol in the rat fetus: evidence of sensitization to toxic and chemosensory aspects of alcohol.

Two experiments tested responsiveness of the rat fetus to alcohol on gestational day (GD) 20 after previous maternal administration of low-to-moderate doses of alcohol (1 or 2 g/kg during GDs 17 to 19). Maternal alcohol intoxication on GD 20 resulted in substantial suppression of spontaneous fetal motor activity. The analysis of mouthing movements showed that this suppression of movement was further enhanced by exposure to alcohol during the previous 3 days of gestation (GDs 17 to 19). A second experiment indicated that this same exposure to alcohol during GDs 17 to 19 induced fetuses on GD 20 to emit more mouthing movements in response to intraoral infusions of alcohol. A moderate amount of prenatal alcohol exposure apparently sensitizes the fetus to the pharmacological and orosensory effects of alcohol.

Alcohol-mediated tactile conditioned aversions in infant rats: devaluation of conditioning through alcohol-sucrose associations.

Three experiments were conducted to assess the plasticity of ethanol-mediated conditioned aversions to a tactile stimulus in infant rats. Ten- and 11-day-old rats first acquired an aversion to a texture, by virtue of its pairing with alcohol-induced intoxication. This first conditioning phase was followed by an associative devaluation procedure, a second phase in which sucrose was intraorally infused during alcohol-induced intoxication. Pups were then tested for their texture preference. Results indicated that infant rats readily express conditioned aversion to a tactile cue as a result of tactile-alcohol pairings and that this associative learning was not state dependent. When alcohol-texture conditioning was followed by sucrose-alcohol pairings, the magnitude of the texture aversion was dramatically reduced (Experiments 1 and 2). In Experiment 3 citric acid rather than sucrose was paired with alcohol intoxication following texture-alcohol pairings. The results indicated that this procedure strengthened texture conditioned aversions in terms of increased resistance to extinction. Taken as a whole these studies indicate that infants rapidly acquire alcohol-mediated texture aversions and that this memory is malleable and can be reduced or potentiated through manipulation of the representation of alcohol's unconditioned properties.

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.

Perinatal responsiveness to alcohol's chemosensory cues as a function of prenatal alcohol administration during gestational days 17-20 in the rat.

Rat fetuses proximal to birth process alcohol-derived cues when the drug is directly delivered into the amniotic fluid. Prior evidence indicates that chemosensory sensation is detected during gestational Day 17 (GD17). In the present study Wistar-derived pregnant females received 0, 1, or 2 g/kg/day of alcohol (intragastric intubation) during GDs 17-20. Prenatal treatment failed to affect different maternal-fetal and perinatal physical parameters, e.g., placenta weight, umbilical cord length, offspring's body weights, weights and/or size of the olfactory bulbs, cerebral hemispheres, and cerebellum. Alcohol chemosensory responsiveness assessed in a perinatal motor activity test, indicated that pups pretreated with the 1 and 2 g/kg alcohol dose exhibit significant decrements in their activity rate when alcohol odor is presented in the test chamber. Alcohol concentrations in maternal and fetal blood and in the amniotic fluid were also recorded through head-space chromatography 1 h after females received the last intubation procedure (GD20) with the 1 or 2 g/kg alcohol doses. Dose-dependent alcohol concentrations across the different sites of assessment were recorded. As indicated by previous studies, even the alcohol level in the amniotic fluid attained with the 1 g/kg alcohol dose is above threshold values in terms of allowing fetal chemosensory stimulation with alcohol-derived cues. The results suggest that maternal ethanol intoxication during the last days of pregnancy leads to fetal exposure to alcohol's sensory attributes and that this experience subsequently modifies responsiveness to these cues.

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.

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.

Association between chemosensory stimuli and cesarean delivery in rat fetuses: neonatal presentation of similar stimuli increases motor activity.

Recent studies conducted in this laboratory indicated that prenatal chemosensory stimulation followed by cesarean delivery strongly affected postnatal responsiveness to odors derived from the administered substances. The present experiments were performed in order to examine if an associative process was responsible for such effects. In Experiment 1 rat fetuses during Gestational Day 21 were exposed to a tenuous alcohol solution or to a lemon-containing solution either 40 or 10 min prior to cesarean delivery. All subjects were subsequently tested in terms of changes in neonatal motor activity when confronted with the odor of alcohol or lemon. Rats experiencing prenatal cues 10 min prior to delivery exhibited higher and differential responsiveness to the smell of these cues when compared to those experiencing similar solutions 40 min prior to delivery. In Experiment 2 each fetus sequentially experienced both cues. Subsequent tests confirmed that the delay between prenatal sensory experience and birth induction was critical in terms of significantly affecting olfactory-mediated motor responses. The results suggest that consequences related with cesarean delivery act as an unconditioned stimulus capable of being associated with orosensory cues present in the amniotic fluid.

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.

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

A recent study suggested that infant rats process alcohol odor and/or taste during acute ethanol intoxication probably due to ethanol elimination via respiration and salivation. The present set of experiments was meant to analyze the possibility that this orosensory processing may act as a conditioned stimulus when an appetitive reinforcer is paired with the state of intoxication. In the first experiment it was observed that intragastric administration of a mildly intoxicating ethanol dose (1.5 g/kg), paired during postabsorptive time intervals with oral infusion of sucrose, was sufficient to promote a significant preference to ethanol. In Experiment 2 different doses of ethanol were either paired or explicitly unpaired with sucrose administration. The result reported in Experiment 1 was replicated and it was observed that a higher dose (3.0 g/kg) unpaired with the reinforcer resulted in alcohol aversions in terms of alcohol consumption patterns. However, when the reinforcer was paired with this dose, the aversion was inhibited. Finally, in the third experiment results indicated that preexposure to alcohol odor eliminates sucrose-conditioned alcohol preferences. These results indicate that, in physiologically immature rats, alcohol preference can be regulated by prior associative experiences involving the state of intoxication and consequences internal and/or inherent to this state.