Repetitive exposures to a surrogate nipple providing nutritive and non-nutritive fluids: effects on suckling behavior of the newborn rat

Exposure to maternal odor enhances intake of a taste that mimicks the sensory attributes of ethanol

Early exposure to ethanol increases subsequent acceptance of this drug. Little attention, however, has been devoted to the interaction of the taste of the drug with other, familiar or non-familiar, odors contingent with ethanol access, particularly early in ontogeny. This study assessed the influence of exposure to maternal odor on intake and grasp responses to an artificial nipple providing a solution (a sucrose-quinine mix) that emulates the taste of alcohol, in 4-day-old rat pups. The results showed that the mother's odor enhanced intake from and seeking responses to an artificial nipple that provided the solution that mimicked the taste of alcohol (Experiment 1). This pattern of results was not evoked by the odor of an unrelated dam (Experiment 2), nor was it observed when the nipple delivered water. The main new finding of the present study is that 4-day-old rats tested in the presence of the mother (and hence exposed to her odor cues) exhibited enhanced seeking and intake of a solution that mimics the chemosensory properties of ethanol.

Pharmacological activation of kappa opioid receptors: aversive effects in adolescent and adult male rats

RATIONALE

The dynorphin (DYN)/kappa opioid receptor (KOR) system is involved in the dysphoric properties of drugs of abuse. Given that adolescents show reduced sensitivity to aversive effects of many drugs, alterations in the DYN/KOR system may contribute to the prevalence of drug use during adolescence.

OBJECTIVES

The present study was designed to assess dysphoric properties of a selective kappa agonist, U62,066, in adolescent and adult rats using both conditioned taste aversion (CTA) and conditioned place aversion (CPA) paradigms.

METHODS

For CTA, water-restricted rats were administered U62,066 following 30 min access to a saccharin solution, with subsequent saccharin consumption used to index aversion. For CPA, animals were allowed access to both compartments of a two-compartment chamber for a 15-min pre- and post-conditioning test. For conditioning, subjects were administered U62,066 prior to confinement to one side of the chamber and saline prior to confinement to the other side for a total of four pairings.

RESULTS

Overall, adolescents displayed reduced sensitivity to the kappa agonist relative to adults. Adults demonstrated taste aversions to the 0.2 and 0.3 mg/kg doses of U62,066, whereas adolescents did not display aversions to any tested doses. Adults demonstrated a place aversion to the 0.1 and 0.2 mg/kg dose of U62,066 when paired with the preferred side of the conditioning chamber. Adolescents did not display aversions to any of the doses tested.

CONCLUSIONS

Reduced sensitivity to DYN/KOR system activation during adolescence may be a contributing factor to the age-typical insensitivity to aversive properties of drugs commonly abused by adolescents.

An appraisal of the strengths and weaknesses of newborn and juvenile rat models for researching gastrointestinal development

Research on the impact of bioactive compounds on the development and functional maturation of the gastrointestinal (GI) tract using newborn and juvenile rats has greatly contributed to the knowledge of GI physiology and to the improved clinical management of both premature and full-term newborns. Of the animal models available, two types have been described for use with young rats – maintenance models and substitution models. Maintenance models are those in which the young are reared with the dam and therefore benefit from continuation of natural nutrition and maternal care. Substitution models are those in which the young are reared in the absence of the dam using artificially formulated milk delivered by various means into specific GI sites. In this review, we describe these models and their operation, and discuss the strengths and weaknesses of each. Attention is also given to questions of scientific validity and some animal welfare issues raised by the use of these models.

Rapid acquisition of operant conditioning in 5-day-old rat pups: a new technique articulating suckling-related motor activity and milk reinforcement

Newborn rats are capable of obtaining milk by attaching to a surrogate nipple. During this procedure pups show a gradual increase in head and forelimb movements oriented towards the artificial device that are similar to those observed during nipple attachment. In the present study the probability of execution of these behaviors was analyzed as a function of their contingency with intraoral milk infusion using brief training procedures (15 min). Five-day-old pups were positioned in a smooth surface having access to a touch-sensitive sensor. Physical contact with the sensor activated an infusion pump which served to deliver intraoral milk reinforcement (Paired group). Yoked controls received the reinforcer when Paired neonates touched the sensor. Paired pups trained under a continuous reinforcement schedule emitted significantly more responses than Yoked controls following two (Experiment 1) or one training session (Experiment 2). These differences were also observed during an extinction session conducted immediately after training. The level of maternal deprivation before training (3 or 6 hr) or the volume of milk delivered (1.0 or 1.5 microl per pulse) did not affect acquisition or extinction performances. In addition, it was observed that the rate of responding of Paired pups during the early phase of the extinction session significantly predicted subsequent levels of acceptance of the reinforcer. These results indicate that the frequency of suckling-related behaviors can be rapidly modified by means of associative operant processes. The operant procedure here described represents an alternative tool for the ontogenetic analysis of self-administration or behavior processes of seeking.

Ontogeny of ingestive behavior

Review of the ontogeny of the controls of independent ingestion reveals that some of the direct and indirect controls of meal size identified in adult rats function in the first three postnatal weeks. The controls appear sequentially and some of them change their potency after they emerge. Indirect controls exerted by metabolism and adiposity do not emerge until the fourth postnatal week or later in the postweaning period. Recent experiments in rats with monogenic obesities involving the leptin and cholecystokinin receptors have demonstrated the usefulness of independent ingestion in the detection of the earliest expression of hyperphagia. Although much remains to be learned about the normal controls of independent ingestion, it is clear that it provides relevant information about the development of normal and abnormal controls of meal size in rodents that is useful for translational research into the controls of meal size in normal and obese children.

Taste differentiation in the context of suckling and independent, adultlike ingestive behavior

The present study compared intake of sweet (saccharin), bitter (quinine), and neutral (water) tastants available either in the context of suckling behavior through a surrogate nipple or independent adultlike feeding through an intraoral cheek cannula in 3-hr-old newborn rats lacking any suckling experience and 24-hr-old rats with regular experience with the dam's nipple. The new technique of online monitoring of fluid flow was applied for assessment of the temporal patterns of ingestion. Newborn and 1-day-old rats tested in the context of suckling behavior showed extremely low intake of quinine, moderate intake of water, and high intake of saccharin. In the same way, they demonstrated low intake of quinine and high intake of saccharin in the context of independent feeding, but intake of water was also high and comparable to that of saccharin. Suckling rats attained higher efficacy of fluid extraction from nipple than pups drinking from cannula. The differential responsiveness to quinine, saccharin, and water in suckling rats was also manifested through attachment behavior, with pups spending less time on the nipple providing quinine and more time on the nipple with saccharin than on the nipple providing water. These results suggest that neonates show taste differentiation as early as 3 hr after birth, and that this taste differentiation is more pronounced in the context of suckling behavior than in the context of adultlike, independent ingestion.