Enhancement of sensory preconditioning by a moderate dose of ethanol in infant and juvenile rats

Integrating and fragmenting memories under stress and alcohol

Stress can powerfully influence the way we form memories, particularly the extent to which they are integrated or situated within an underlying spatiotemporal and broader knowledge architecture. These different representations in turn have significant consequences for the way we use these memories to guide later behavior. Puzzlingly, although stress has historically been argued to promote fragmentation, leading to disjoint memory representations, more recent work suggests that stress can also facilitate memory binding and integration. Understanding the circumstances under which stress fosters integration will be key to resolving this discrepancy and unpacking the mechanisms by which stress can shape later behavior. Here, we examine memory integration at multiple levels: linking together the content of an individual experience, threading associations between related but distinct events, and binding an experience into a pre-existing schema or sense of causal structure. We discuss neural and cognitive mechanisms underlying each form of integration as well as findings regarding how stress, aversive learning, and negative affect can modulate each. In this analysis, we uncover that stress can indeed promote each level of integration. We also show how memory integration may apply to understanding effects of alcohol, highlighting extant clinical and preclinical findings and opportunities for further investigation. Finally, we consider the implications of integration and fragmentation for later memory-guided behavior, and the importance of understanding which type of memory representation is potentiated in order to design appropriate interventions.

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.

Sleep disturbances after acute exposure to alcohol in mothers' milk

The results of previous research in our laboratory revealed that breast-fed infants experience significantly less active sleep after exposure to alcohol in their mothers' milk than do breast-fed infants not exposed to alcohol. The present study tested the hypothesis that infants would compensate for such reductions if their mothers then refrained from drinking alcohol. To this end, 23 breast-fed infants from 3 to 5 months of age and their mothers were tested on 2 days separated by 1 week. A small, computerized movement detector, an actigraph, was placed on the infants' left ankles to monitor sleep and activity patterning after which they were bottle fed mother's milk alone (control condition) on 1 test day and mother's milk containing 32 mg of ethanol per 100 ml--the average concentration detected in human milk after lactating women drank an acute dose (0.3 g/kg) of alcohol--on the other. The infants' behaviors were monitored for the next 24 h; the first 3.5 h of monitoring on each test day took place at the Monell Center. Consistent with previous findings, infants exhibited significantly less active sleep during the 3.5 h immediately after exposure to alcohol in mothers' milk compared with the control condition; the decrease in active sleep was observed in all but 4 of the infants tested. Compensatory increases in active sleep were then observed in the next 20.5 h, when mothers refrained from drinking alcohol. Although the mechanisms underlying the reduction in sleep remain to be elucidated, these findings demonstrate that short-term exposure to small amounts of alcohol in mothers' milk produces distinctive changes in the infants' sleep-wake patterning.

Effects of exposure to alcohol in mother's milk on infant sleep

OBJECTIVE

To test the hypothesis that exposure to alcohol in breast milk affects infants' sleep and activity levels in the short term.

METHODS

Thirteen lactating women and their infants were tested on 2 days, separated by an interval of 1 week. On each testing day, the mother expressed 100 mL of milk, while a small, computerized movement detector called an actigraph was placed on the infant's left leg to monitor sleep and activity patterning. After the actigraph had been in place for approximately 15 minutes, the infants ingested their mother's breast milk flavored with alcohol (32 mg) on one testing day and breast milk alone on the other. The infants' behaviors were monitored for the next 3.5 hours.

RESULTS

The infants spent significantly less time sleeping during the 3.5 hours after consuming the alcohol-flavored milk (78.2 minutes compared with 56.8 minutes after feeding alcohol in breast milk). This reduction was apparently attributable to a shortening in the longest sleeping bout (34.5 compared with 56.7 minutes for sleeping after breast milk alone) and the amount of time spent in active sleep (25.8 minutes compared with 44.2 minutes after breast milk alone); the decrease in active sleep was observed in all but 2 of the 13 infants tested.

CONCLUSIONS

Although the mechanisms underlying the reduction in sleep remain to be elucidated, this study shows that short-term exposure to small amounts of alcohol in breast milk produces distinctive changes in the infant's sleep-wake patterning.

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.

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.

Mechanisms of alcohol abuse and alcoholism in adolescents: a case for developing animal models

This paper reviews the ontogeny of neurotransmitter systems and neuropharmacological challenge within transmitter systems and discusses the actions of alcohol on such systems during the juvenile through adolescent periods. To place the animal research within the context of human development, an attempt is made to first examine some fundamental principles of developmental research as they relate to the adolescent period. Evidence presented from animal studies indicates that unique neurochemical and behavioral changes are occurring during postnatal development, including adolescence, that could mediate the response to alcohol. The limited number of studies on the neurochemical and behavioral response to alcohol during adolescence has employed rats and has been restricted by the relatively brief adolescent period in that species. While one alternative is to use primates, it is suggested that innovative behavioral paradigms be developed for adolescent animals in other species to study behaviors such as alcohol self-administration or alcohol stimulus discrimination. It is also suggested that existing behavioral models that are more easily adapted to younger age ranges (e.g., conditioned place preference, conditioned taste aversion, thermal response to ethanol) be extended to make ontogenetic comparisons through adolescence and adulthood. This may further our understanding of alcohol's immediate consequences during each maturational stage and, more important, the contribution of early alcohol exposure to excessive drinking and abnormal cognitive and social functioning during subsequent stages of development.

Disruptive influence of norepinephrine depletion on sensory preconditioning, but not first-order conditioning, in preweanling rats

Learning and memory processes are known to be influenced by the action of norepinephrine (NE). The present study evaluated the influence of neonatal administration of the NE neurotoxin DSP4 on sensory preconditioning (SPC) in 16- and 28-day-old rats. Rats were subcutaneously administered 50 mg/kg DSP4 or saline within 24 h after birth and tested at 16 or 28 days in two experiments. The results from Experiment 1 indicated that nondepleted 16-day-old rats exhibited strong SPC, whereas this conditioning was disrupted by neonatal administration of DSP4. No SPC was seen in either neonatal treatment group at 28 days of age, confirming previous reports of an ontogenetic decline in SPC. In Experiment 2, no effects of DSP4 were observed on first-order conditioning at either age. The brains of representative subjects were separated into forebrain, cerebellum, and brain stem samples and were analyzed to determine the extent of NE depletion. This analysis showed that DSP4 produced a marked reduction in NE concentration in the forebrain, but not in the cerebellum or brain stem. The results of this study suggest a role for forebrain NE in SPC in preweanling rats. This modulatory noradrenergic influence appears to be exerted on the formation of associations between two relatively neutral stimuli during the preexposure phase, given the absence of an effect of DSP4 on primary conditioning.