W. G. Hall

Weaning and growth of artificially reared rats

The importance of suckling experience for later feeding in the rat was tested by means of an isolate rearing technique that eliminated oral feeding. Pups reared in the nearly complete absence of suckling and feeding ate and drank at weaning and then grew normally. Furthermore, the characteristics of apparently normal ingestion and growth make the artificially reared rat a useful preparation for other developmental investigations.

Suckling Isn't Feeding, or Is It? A Search for Developmental Continuities

Publisher Summary This chapter describes the suckling behavior of infant rats and some of the recently developed techniques used to study it. It provides a basis for discussion of the relationship between suckling and later ingestive behavior. It is assumed that at least two possible states of affairs could exist: (1) either the suckling system and later ingestion are continuous and based on the same underlying neural structures and organization, with suckling merely undergoing changes that turn it into the adult ingestive system by the time of weaning; or (2) the adult ingestive system is built on a separate ingestive structure and has little in common with that of suckling. The chapter discusses the ways in which suckling behavior differs from later ingestive behavior and points out suggestive differences in the physiological controls and neural substrates of suckling and later ingestion. It shows that another ingestive system does exist in rat pups concurrently with suckling, and while it is not used by the infant during normal development, this system may represent the true forerunner of later ingestive behavior.

Ontogeny of feeding motor patterns in infant rats: an electromyographic analysis of suckling and chewing.

During mammalian ontogeny, there is a transition from suckling to the chewing of food. The question was asked: Is suckling a neuromuscular precursor to chewing, or are suckling and chewing independent systems? Electromyograms (EMGs) were recorded in rat pups of ages 6, 9, 12, 15, 18, and 21 days from the superficial masseter, anterior digastric, sternohyoideus, and genioglossus muscles during suckling and chewing. The EMG patterns of the 3 components of suckling behavior (nipple attachment, rhythmic sucking and the stretch response) are distinctive from one another and reflect the musculoskeletal biomechanics of suckling. Chewing EMGs are present by 12 days of age and attain the adult pattern by 18-21 days of age. During nipple attachment, pups exhibit a motor pattern that is similar to that of adult chewing, but other aspects of suckling differ from chewing in some EMG features. Comparison of EMGs between behaviors and between ages allowed interpretation of the degree of contunity of muscular activity across the suckling-to-chewing transition.

Developmental changes in suckling of rat pups

ONTOGENETIC analysis of mammalian suckling provides a unique opportunity to observe the emergence of appetitive behaviour. Transitions in the nature of suckling may reveal progressive stages in the development of adult ingestive systems. Unfortunately, little information is available regarding the nature of suckling behaviour as it develops, and is eventually replaced by feeding and drinking, because it has proved difficult to separate the behaviour of the altricial infant from the contribution of the mother to the nursing dyad. We report here the developmental characteristics of, and changes in, the behaviour of suckling rats as they locate, apprehend and suckle the nipples of anaesthetised dams. This situation permits the detailed evaluation of rat pup behaviour in isolation from the active maternal role. Even at birth, pups are capable of quickly apprehending and suckling the nipples of an anaesthetised dam. Suckling then undergoes two major transitions. The first, after day 10 post partum, suggests the emergence of a physiological control of suckling behaviour. The second, beginning after day 14, indicates a changing responsiveness to some characteristic of the mothers nipples or milk ejection.

Taste and smell sensations enhance the satiating effect of both a high-carbohydrate and a high-fat meal in humans

The effects of meal sensory properties (tasty vs. bland) and nutrient composition [high-CHO (carbohydrate) vs. high-FAT] on hunger ratings, blood glucose and free fatty acids (FFA), taste perception, and subsequent food intake, were studied in human subjects. Aspartame and vanilla were used to augment meal palatability, yielding four isocaloric liquid meals: bland-FAT, tasty-FAT, bland-CHO, tasty-CHO. Normal-weight, nondieting young adults consumed each of the meals for breakfast on separate days. The main finding was that tasty versions of high-FAT and high-CHO meals were more satiating than nutritionally identical bland meals, as indicated by a greater decrease in hunger ratings following the tasty meals. Changes in blood glucose and FFA were related to meal nutrient composition, but not to meal sensory properties. High-CHO meals tended to be more satiating than high-FAT meals. Consumption of each of the meals produced a similar decrease in pleasantness ratings of food-related tastes. Intake of carbohydrates was significantly higher at a self-selected lunch 5.25 h following a tasty breakfast. These findings indicate that hunger is decreased to a greater extent by meals flavored with aspartame and vanilla relative to nutritionally identical, unflavored meals. The satiety-enhancing effect of oral stimulation was found for both high-FAT and high-CHO meals.

Development of nutritional control of food intake in suckling rat pups

The initiation of suckling (nipple approach and attachment) was not affected by gastric preloads in 10- and 20-day-old rat pups tested with their anesthetized mother. However, food intake was affected. In 10-day-old pups nutritive and nonnutritive loads depressed diet intake during suckling by the same amount and apparently as a result of severe gastric distension. In contrast, the diet intake of 20-day-old pups was differentially depressed by nutrivive preloads, suggesting an emerging postgastric or postabsorptive control of ingestive behavior.

Control of ingestion by oral habituation in rat pups.

The role of habituation of mouthing activity in the control of ingestion was investigated in 6-, 12-, and 18-day-old rat pups. In pups at all ages, oral habituation to a flavored diet inhibited ingestion of a continuous oral infusion of that same diet. Twelve-day-old pups that had orally habituated to a diet continued to consume less of a continuous oral infusion of that diet both 30 min and 3 hr later, and the duration of suppressed ingestion was shown to be dependent on the rate of stimulus presentation during habituation experience. These data suggest that oral habituation may be a diet-specific influence on both intra- and intermeal patterning.

Developmental change in the access to olfactory memories

Memory for a learned odor preference can be functionally confined to one side of the brain in 6-day-old rat pups by preferentially stimulating a single naris and corresponding olfactory bulb during training. We report here that this form of unilateral learning is present only during the first postnatal week; older pups show bilateral recall of unilateral olfactory experience. The maturation of bilateral learning probably depends on the postnatal growth and development of olfactory commissural fibers, because infantlike unilateral learning and memory is reinstated when these commissural fibers are sectioned before training in older pups. Section of commissural fibers after training also resulted in unilateral preferences. This latter finding indicates that the learned odor preference of older pups tested with the untrained naris open depends on access to unilaterally stored memories on the contralateral side, access provided by the newly developed commissural projections.

Evidence that oral and nutrient reinforcers differentially condition appetitive and consummatory responses to flavors

Rats tend to increase their intake of a flavor that has previously been paired with either sweet taste or with caloric repletion. However, it is unclear whether such a change in intake is caused by changes in appetitive behaviors such as orienting and approach, or changes in consummatory behaviors and oral responsiveness. Also, it is unclear whether oral reinforcers (sweetness) and postingestive reinforcers (nutrients) lead to the same kinds of behavioral change. In the current experiments, weanling rats with oral and gastric cannulas repeatedly experienced a flavor paired with either sweetness, high caloric density, or neither. Rats were then tested for differences in appetitive olfactory orienting and consummatory oral responsiveness elicited by the flavor. Results suggest that oral reinforcement (sweetness) produces conditioning of appetitive responding to the flavor, while postingestive reinforcement produces conditioning of consummatory responding. A second experiment indicates that these behavioral changes are specific increases in responsiveness conditioned by flavor + unconditioned stimulus (US) pairing, and are unlikely to be nonspecific effects of daily unconditioned stimulus exposure.

A nutritive control of independent ingestion in rat pups emerges by nine days of age.

The emergence of controls of independent ingestion in rat pups was studied using nutritive (0.6 M glucose in saline or water) and vehicle gastric preloads. Two hours after preloading, ingestive responses were assessed in a 30-minute test of feeding from the floor. In 6-day-olds, all preloads had similar effects on consumption of a milk diet. In 9-day-olds, however, glucose-water preloads inhibited intake compared to water preloads. This inhibition was secondary to an effect on gastric emptying. Glucose preloads also inhibited intake in 12- and 15-day-olds. Behavioral observations indicated that feeding patterns were altered in response to glucose preloads in 9-day-olds. These results provide evidence for the emergence of a nutritive, postgastric control of independent ingestion between 6 and 9 days of age.