Juan Carlos Molina

Chemosensory Factors Influencing Alcohol Perception, Preferences, and Consumption

This article presents the proceedings of a symposium at the 2002 RSA/ISBRA Meeting in San Francisco, California, co-organized by Julie A. Mennella and Alexander A. Bachmanov of the Monell Chemical Senses Center. The goal of this symposium was to review the role that chemosensory factors (taste, smell, and chemical irritation) play in the perception, preference, and consumption of alcohol. The presented research focused on both humans and laboratory animals and used a variety of approaches including genetic, developmental, pharmacological, behavioral, and psychophysical studies. The presentations were as follows: (1) Introduction and overview of the chemical senses (Julie A. Mennella and Alexander A. Bachmanov); (2) Taste reactivity as a measure of alcohol palatability and its relation to alcohol consumption in rats (Stephen W. Kiefer); (3) Early learning about the sensory properties of alcohol in laboratory animals (Juan Carlos Molina); (4) Early learning about the sensory properties of alcohol in humans (Julie A. Mennella); (5) Genetic dissection of the ethanol-sweet taste relationship in mice (Alexander A. Bachmanov and Michael Tordoff); and (6) Human genetic variation in taste: connections with alcohol sensation and intake (Valerie B. Duffy and Linda M. Bartoshuk). The symposium concluded with a general discussion.

Nursing From an Ethanol-Intoxicated Dam Induces Short- and Long-Term Disruptions in Motor Performance and Enhances Later Self-Administration of the Drug

RATIONALE During interactions with an ethanol-intoxicated dam, preweanling rats encode ethanol-related chemosensory information. These experiences have been observed to enhance subsequent recognition of ethanols chemosensory properties and to modulate learning about ethanol. OBJECTIVE The present study tested the effects of ethanol-related nursing experiences on motor function in later infancy and adolescence and on ethanol intake during adolescence. METHODS Wistar-derived rats were reared by dams intragastrically administered with ethanol (2.5 g/kg) or with water during postnatal days (PDs) 3, 5, 7, 9, 11, and 13. Later in infancy or in adolescence, these rats were tested on a motor coordination task (Accelerod) while either sober or acutely intoxicated with ethanol (1 g/kg). During adolescence, animals had simultaneous access to varying ethanol concentrations (3, 4, 5, or 6% v/v) and water. RESULTS Both infants and adolescents that had been reared by ethanol-intoxicated dams exhibited dramatic behavioral impairments in the Accelerod task when compared with the offspring of water control dams. Ethanol intoxication disrupted motor performance in both age groups, but this effect was independent of prior maternal treatment. When tested for voluntary ethanol intake as adolescents, those with prior nursing experiences with an intoxicated dam ingested more ethanol than adolescents reared by sober dams. CONCLUSIONS Early experiences with alcohol comprising interactions with an alcohol-intoxicated dam result in motor impairment and enhanced ethanol intake later in life.

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.

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 ethanols 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 ethanols palatability and ingestion, but also facilitates the acquisition of response-stimulus associations where the drug acts as an intraoral reinforcer.

Relationship between ethanol-induced activity and anxiolysis in the open field, elevated plus maze, light-dark box, and ethanol intake in adolescent rats.

It is yet unclear if ethanol-induced motor stimulation in the open field (OF) merely reflects psychomotor stimulating effects of the drug or if this stimulation is driven or modulated by ethanols antianxiety properties. In the present study, adolescent rats were administered with different ethanol doses or remained untreated. They were sequentially assessed in the OF, elevated plus maze (EPM), and light-dark box (LDB) and then assessed for ethanol intake. The aims were to assess the relationship between measures of ethanol-induced activity and anxiolysis, analyze ethanol intake as a function of prior ethanol exposure, and associate behavioral responsiveness in these apparatus with ethanol intake during adolescence. The results suggested that the enhanced exploration of the OF observed after 2.5 and 3.25 g/kg ethanol reflected a motor-stimulating effect that appeared to be relatively independent of anxiolysis. The 1.25 g/kg dose induced motor stimulation in the OF and anti-anxiety effects in the EPM, but these effects were relatively independent. The 0.5 g/kg ethanol dose exerted significant anxiolytic effects in the EPM in the absence of stimulating effects in the OF. A multivariate regression analysis indicated that adolescents with a higher frequency of rearing behavior in the OF, higher percentage of open arm entries in the EPM, and lower propensity to enter the central area of the OF exhibited greater ethanol intake. These results indicate that the OF is a valid procedure for the measurement of ethanol-induced stimulation, and provide information toward characterizing subpopulations of adolescents at risk for initiating alcohol drinking.

A sensory-enhanced context allows renewal of an extinguished fear response in the infant rat

Studies of extinction in preweanling rats have failed to find ABA-renewal in a fear conditioning paradigm. This result supports the hypothesis postulating ontogenetic qualitative differences in experimental extinction. A similar result in adult rats led to the conclusion that ABA-renewal requires contexts A and B to differ in several types of features, including odor cues. Recently we reported experimental evidence of the renewal of an extinguished taste aversion response in infant rats employing contexts which differ in their odor content. The present study examines the possibility of renewing an extinguished fear response in infant rats when contexts A and B do not include (Experiment 1) or include (Experiment 2) an explicit odor. Results showed absence of renewal when using standard contexts (without explicit odors, Experiment 1). However, when contexts A and B varied also in their odor content, the ABA-renewal procedure was effective in reinstating the extinguished CR (Experiment 2). Thus, it can be concluded that the sensory content of the context determines the observation of renewal in the infant rat, a result that is coherent with previous observations in the adult rat. As a whole, these results challenge our understanding of extinction as a learning process that is qualitatively different in preweanling rats than in later stages of ontogeny.

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.

Adolescent rats are resistant to the development of ethanol-induced chronic tolerance and ethanol-induced conditioned aversion.

The analysis of chronic tolerance to ethanol in adult and adolescent rats has yielded mixed results. Tolerance to some effects of ethanol has been reported in adolescents, yet other studies found adults to exhibit greater tolerance than adolescents or comparable expression of the phenomena at both ages. Another unanswered question is how chronic ethanol exposure affects subsequent ethanol-mediated motivational learning at these ages. The present study examined the development of chronic tolerance to ethanols hypothermic and motor stimulating effects, and subsequent acquisition of ethanol-mediated odor conditioning, in adolescent and adult male Wistar rats given every-other-day intragastric administrations of ethanol. Adolescent and adult rats exhibited lack of tolerance to the hypothermic effects of ethanol during an induction phase; whereas adults, but not adolescents, exhibited a trend towards a reduction in hypothermia at a challenge phase (Experiment 1). Adolescents, unlike adults, exhibited ethanol-induced motor activation after the first ethanol administration. Adults, but not adolescents, exhibited conditioned odor aversion by ethanol. Subsequent experiments conducted only in adolescents (Experiment 2, Experiment 3 and Experiment 4) manipulated the context, length and predictability of ethanol administration. These manipulations did not promote the expression of ethanol-induced tolerance. This study indicated that, when moderate ethanol doses are given every-other day for a relatively short period, adolescents are less likely than adults to develop chronic tolerance to ethanol-induced hypothermia. This resistance to tolerance development could limit long-term maintenance of ethanol intake. Adolescents, however, exhibited greater sensitivity than adults to the acute motor stimulating effects of ethanol and a blunted response to the aversive effects of ethanol. This pattern of response may put adolescents at risk for early initiation of ethanol intake.

Neonatal experiences with ethanol intoxication modify respiratory and thermoregulatory plasticity and affect subsequent ethanol intake in rats.

Different studies have focused on the deleterious consequences of binge-like or chronic exposure to ethanol during the brain growth spurt period (third human gestational trimester) that in the rat corresponds to postnatal days (PDs) 3-10. The present study analyzed behavioral and physiological disruptions caused by relatively brief binge-like exposures (PDs 3, 5, and 7) with an ethanol dose lower (3.0 g/kg) than those frequently employed to examine teratological effects during this stage in development. At PD 9, pups were exposed to ethanol doses ranging between .0-3.0 g/kg and tested in terms of breathing patterns and thermoregulation. At PDs 11 and 12, ethanol intake was examined. The main findings were as follows: i) pre-exposure to the drug resulted in brief depressions in breathing frequencies and an exacerbated predisposition toward apneic episodes; ii) these effects were not dependent upon thermoregulatory alterations; iii) early ethanol treatment increased initial consumption of the drug which also caused a marked hypothermia that appeared to regulate a subsequent decrement in ethanol consumption; and iv) ethanol exposure retarded overall body growth and even one exposure to the drug (PD 9) was sufficient to reduce brain weights although there were no indications of microcephaly. In conjunction with studies performed during the late gestational period in the rat, the results indicate that relatively brief binge-like episodes during a critical window of brain vulnerability disrupts the respiratory network and exacerbates initial acceptance of the drug. In addition, ethanol treatments were not found to induce tolerance relative to respiratory and thermal disruptions.

Transition from ethanol-induced sensitization to tolerance across early and late infancy in the rat

Drugs of abuse, as cocaine or amphetamine, induce locomotor sensitization during infancy and adulthood of the rat. This effect during the preweanling period is observed only after a short interval of time between training and testing. We recently reported short-term locomotor sensitization induced by ethanol in pups chronically exposed to the drug during the second postnatal week of life. The present series of experiments was designed to explore the persistence of the sensitization effect across the preweanling period. Pups were chronically exposed to ethanol in five consecutive days during the second or the third postnatal weeks, and their locomotor activity was evaluated in an open field 3, 8 or 15days later. Our results showed that, contrarily to what has been observed with other drugs during infancy, sensitization to ethanol persisted at least 8days in rats exposed to the drug during the second postnatal week. Surprisingly, in older pups, the same procedure induced tolerance instead sensitization. This ontogenetic model offers a potentially interesting tool for studying within the same species, how tolerance and sensitization are interrelated, and how these effects affect ethanol-mediated reinforcement and ethanol intake during ontogeny.