Brain transections selectively alter ingestion and behavioral activation in neonatal rats.

Food-deprived neonatal rats actively ingest milk that is infused into their mouths through intraoral cannulas. This ingestion is accompanied by behavioral activation. The involvement of various brain regions in ingestion and activation was examined in 2-day-old Charles River pups by making transections along the neuraxis from the olfactory bulbs to the anterior pons. Following a 24-hr deprivation period, a series of oral milk infusions was given, and milk intake and activity were measured. Intake was severely reduced only in Ss with diencephalic transections. Cuts in front of or behind the diencephalon resulted in normal or slightly decreased intake. In contrast, activity tended to decline as the level of the transection became more caudal. Thus ingestion and its accompanying behavioral activation could be separated neuroanatomically. These results suggest that 2 brain mechanisms are involved in the ingestive response of the infant rat, one in the diencephalon and another caudal to the mesencephalon. However, behavioral activation appears less discretely organized, involving most of the neuraxis. (28 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The ontogeny of feeding in rats: II. Independent ingestive behavior.

In 4 experiments, Charles River rat pups 3 days of age and older consumed large quantities of milk, sucrose, and wet mash when the Ss were placed in warm containers in which one of these diets had been spread on the floor. The volume of Ss" intake was directly related to the severity of food deprivation. Ingestive behavior occurred in the early part of the 30-min test, but later in the test, Ss stopped feeding and became quiescent. Their ingestive behavior thus resembled that of adults in (a) the motor aspects of feeding responses (licking and lapping), (b) the dependency on deprivation for intake, and (c) the pattern of intake termination. However, in Exp IV, when diet was restricted to a small area of the test container, young Ss consumed little diet. They did not appear able to direct or focus their ingestive responding. It was not until 9–12 days of age that Ss successfully consumed milk from a localized source. (29 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The ontogeny of feeding in rats: V. Influence of texture, home odor, and sibling presence on ingestive behavior.

Determined with Charles River rat pups in 3 experiments that terry-cloth texture, home odor, and the presence of siblings modulated Ss' ingestive behavior. Unlike warmth, which affected ingestion in pups until at least 15 days of age, the relative importance of other cues varied with age. At 3 days, ingestion was dependent on warmth but was not influenced by the other cues. At 6 days, texture and home odor enhanced ingestive behavior (intake, activity, mouthing, and probing), but the presence of siblings had no effect. Home odor or terry-cloth texture did not alter the ingestive behavior of 12-day-olds, but the presence of siblings enhanced milk intake. Thus, during development, the external sensory controls for ingestion became progressively more complex. Warmth served as a primary permissive cue for ingestion, but as Ss grew older, such other cues as odor, texture, or social stimuli also gained significance. (33 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Acute hunger of rat pups elicits increased kyphotic nursing and shorter intervals between nursing bouts: Implications for changes in nursing with time postpartum.

Earlier findings, based on limited behavioral observations, indicate that nursing behavior in rats declines dramatically in duration over time postpartum--despite increasing ingestion of milk by rat pups to meet their growth and metabolic needs--although hungry pups elicit more nursing than do well-nourished pups. The authors compared the nursing pattern in detail for 6 hr on Days 7 and 14 and induced hunger in pups acutely with mammary-duct-ligated dams unable to provide milk. Compared with Day 7, on Day 14, supine nursing and the interval between nursing bouts increased, whereas hovering over pups and kyphotic nursing decreased. When pups were increasingly hungry, these age-related changes were counteracted. Thus, the ingestive motivation of pups largely regulates the nursing pattern over time. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The ontogeny of feeding in rats: III. Thermal determinants of early ingestive responding.

High ambient temperatures (38°C) stimulated high spontaneous levels of activity and high frequencies of behaviors normally associated with ingestion (mouthing and probing), particularly in young, 3- and 6-day-old Charles River CD rat pups (Exp I). The level of spontaneous behavior was highly correlated with body temperature and also depended on deprivation condition. Temperature played an important role in determining responses to food stimuli as well. When Ss were fed by oral infusion (Exp II) or by placing milk on the floor beneath them (Exp III), warm ambient temperatures were required for active ingestion. In Exp IV, body temperature and ambient temperature were manipulated independently to assess their relative importance for Ss' feeding behavior. Ss with a low (29°C) or normal (34°C) core temperature at the start of testing were fed in either a 24°C or a 34°C ambience. Regardless of body temperature, Ss' levels of intake, activity, mouthing, and probing were higher in a warm than in a cool ambience. Therefore, the suppressed ingestive behavior of Ss fed at cool temperatures occurred not simply because Ss became hypothermic and inactive. Perceived warmth appears to be a significant contextual cue that regulates pups' responses to food stimuli. (29 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Investigation of the functions of coprophagy in juvenile rats.

Five experiments examined possible functions of the ingestion of maternal anal excreta by juvenile rats. Ss were 24 recently parturient Long-Evans rats and their 440 pups. No strong support was found for hypotheses suggesting that (a) maternal excreta serves as a major transition diet from mother's milk to solid food, (b) ingestion of maternal excreta influences pup diet selection at weaning, or (c) ingestion of maternal excreta is a necessary condition for inoculation of pups with enteric bacteria. Some support was found for the hypothesis that maternal excreta can serve as a short-term emergency food supply for rat pups after weaning. It is proposed that pup ingestion of maternal anal excreta may not be a functionally meaningful unit of behavior in preweaning rats. Allocoprophagy may be one facet of a broader pattern of oral exploration in which functional significance resides. (23 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Neural systems for early independent ingestion: Regional metabolic changes during ingestive responding and dehydration.

Regional neural metabolic activity, as assessed by semiquantitative deoxyglucose (DG) autoradiography, was related to dehydration and ingestive behavior in 6-day-old rat pups. During simple ingestive responding, changes in relative DG uptake (representative of changes in neural metabolic activity) occurred primarily in hindbrain sensory and motor nuclei. Producing cellular dehydration resulted in activity changes primarily in the basal forebrain (FB). When Ss were dehydrated and allowed to ingest during the DG-uptake period, activity changes were seen in the hindbrain and FB areas that responded to ingestion or dehydration alone, as well as in regions that were not affected by either manipulation alone. Extracellular dehydration produced fewer and different FB responses in neural metabolic activity. During ingestion, the only effects of extracellular dehydration that overlapped with those of cellular dehydration appeared in circumventricular hypothalamic regions and brain stem motor nuclei. There appeared to be only a limited final common pathway for these 2 types of dehydration-induced drinking. Findings in infant rats depict distributed neural systems subserving ingestion and responding to state change and provide a starting point early in ontogeny for the developmental analysis of neural substrates of ingestive systems. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Ontogeny of suckling in rats: Transitions toward adult ingestion.

Studied the suckling behavior of neonatal Sprague-Dawley rats on their own anesthetized mother from the day of birth until after weaning (35 days). Even newborn Ss were capable of nipple attachment without maternal assistance. Before 11–23 days of age, pups deprived of suckling for 22 hrs and nondeprived pups quickly attached to their mother's nipples, sucked, and remained attached to the nipple though no milk delivery occurred. Then behavior underwent at least 3 changes: (a) After 11–23 days of age, latency to attach became considerably elevated in nondeprived pups; (b) a 2nd change in latency occurred at 23–25 days of age, when nondeprived pups no longer even attached to the nipples of anesthetized mothers; and (c) about 14 days of age, deprived pups began to shift from 1 nipple to the next after initial attachment. These developmental changes were seen in other test situations in which pups were placed directly on a nipple and not required to search and when various periods of deprivation were utilized. These transitions were not critically dependent on the onset of visual function or on the pup's experience with food other than mother's milk. Suckling, therefore, in not an unmodified reflex but is an appetitive behavior that undergoes a series of changes during development. These transitions constitute major developmental events in the ontogeny of rat ingestive behavior. (48 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Prenatal expression of species-typical action patterns in the rat fetus (Rattus norvegicus).

We investigated sensory and behavioral responsiveness of the rat fetus. On Days 19, 20, or 21 of gestation, rat fetuses received intraoral infusions of a biologically important stimulus, milk, or a novel chemical stimulus, lemon. Using a technique to directly observe behavior in utero, we found that rat fetuses discriminate between intraoral infusions of milk and lemon, exhibiting different levels and patterns of overall activity after infusion. Milk was found to evoke a low magnitude, delayed increase in overall fetal activity from Day 19 through Day 21, whereas lemon evoked a high-magnitude, spiked pattern of activity that diminished from Day 19 to Day 21. Late in gestation these two stimuli elicited species-typical action patterns. Milk infusions elicited a stretch response much like the one shown by pups at the nipple; lemon infusions elicited face wiping typical of older pups and adults exposed to aversive gustatory stimuli. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Sodium homeostasis in chronic decerebrate rats.

Two experiments examined physiological and behavioral concomitants of sodium need in supracollicularly transected and pair-fed intact male Sprague-Dawley rats. Chronic decerebrate Ss, like intact Ss, reduced their urine sodium output when placed on a sodium-deficient diet. Similarly, 24 hrs after sodium loading, decerebrate and intact Ss excreted comparable levels of the excess sodium. In the 2 hrs immediately following loading, decerebrate Ss excreted less sodium. In contrast, behavioral aspects of sodium homeostasis were completely absent in chronic decerebrate Ss. In separate experiments, intraoral intake and taste-reactivity responses elicited by intraoral infusions of NaCl were measured during sodium-replete and sodium-deficient conditions. In response to oral infusions of NaCl, intact Ss consumed significantly more and produced greater numbers of ingestive taste-reactivity responses when they were sodium deficient than when they were sodium replete. The same sodium-depletion treatments in chronic decerebrate Ss, however, altered neither the intraoral intake of NaCl nor the frequency of NaCl-elicited ingestive taste-reactivity responses. Results suggest that the behavioral compensatory responses that follow changes in the internal sodium state depend on forebrain mechanisms. (53 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Milk availability modulates weaning in the Norway rat (Rattus norvegicus).

Beginning on Day 14 postnatally, 368 Sprague-Dawley rat pups were exposed to different levels of milk supply by changing litter size of 10 dams from 8 to 4 or 12 (Exp I) or limiting temporally Ss' access to a lactating dam (1 of 36) to 8, 14, or 24 hrs/day (Exp II). Both manipulations accelerated weaning in milk-deprived Ss. By adding solid food and water to their diet, early weaning Ss compensated for the negative energy consequences of milk reduction and achieved premanipulation growth rates. Milk availability thus appears to affect weaning, and it is suggested that the developmental changes in the nutritive energy balance between mother and offspring contribute to the emergence of independent ingestion. (43 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Taste reactivity as a dependent measure of the rapid formation of conditioned taste aversion: A tool for the neural analysis of taste-visceral associations.

Investigated the ability of animals to form taste aversions following neural manipulations. In Exp 1, 10 rats received intraoral infusions of sucrose every 5 min starting immediately after the injection of LiCl. 12 controls were injected with NaCl. Oromotor and somatic taste reactivity behaviors were videotaped and analyzed. Lithium-injected Ss decreased their ingestive taste reactivity over time; aversive behavior increased. Controls maintained high levels of ingestive responding and demonstrated virtually no aversive behavior following sodium injection. Ss were tested several days later for a conditioned taste aversion (CTA). Rats previously injected with lithium demonstrated significantly more aversive behavior than controls. Exp 3 revealed that when similarly treated rats were tested for a CTA while in a lithium-induced state, difference in the ingestive behavior was observed. In Exp 2, naive rats were injected with NaCl or LiCl but did not receive their 1st sucrose infusion for 20 min. Ss also received infusions at 25 and 30 min postinjection. There were no differences in the task reactivity behavior displayed. Rats dramatically changed their oromotor responses to sucrose during the period following LiCl administration, provided the infusions started immediately after injection, a change attributable to associative processes. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Suckling behavior and intake control in the developing rat pup.

Studied the behavioral development of suckling and intake control in 2 experiments with Charles River CD strain rat pups. Ss were observed at the initiation, during the course, and at the termination of suckling from their anesthetized mothers. Diet was delivered intraorally through a fine tongue cannula which enabled control of timing and volume. The control of diet intake and the behavior at termination of suckling showed correlated changes from 5 to 20 days of age. When deprived of suckling (and food and water) for 8 hr, 5- and 10-day-old Ss consumed large volumes of diet (10% of body weight or greater) and terminated suckling only in the presence of extreme gastrointestinal filling. These Ss were immediately lethargic and slept after intake termination. Five-day-old Ss persisted in reattaching to the nipple when manually stimulated; 10-day-old Ss eventually refused to reattach. In contrast, 20-day-old Ss consumed more moderate volumes of diet (5% of body weight). These Ss also remained awake for a period after feeding and engaged in the exploratory and grooming activities characteristic of adult rats at the termination of feeding. These observations demonstrate major changes in suckling behavior during development. They suggest that intake control processes shift from indirect to direct and become more effective and specifically food intake related in older pups. (20 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Milk delivery schedules and stomach preloading alter patterns of suckling behavior by newborn rats on a surrogate nipple.

Newborn rat pups tested before suckling experience attached to and ingested milk from the surrogate nipple. Time attached to the nipple and amount of milk ingested depended on the schedule of milk infusion through the nipple. More frequent milk infusions resulted in more frequent disengagements from the nipple during the test, less time attached to the nipple, and less body weight gain. The initial patterns of attachment behavior- continuous or intermittent-were reproduced later when rats were tested on the surrogate nipple. Preloading of the stomach with milk effectively altered both attachment and ingestion from the nipple, whereas preloading with the same amount of water had no effect on suckling behavior. The data suggest that newborn rats flexibly adjust their attachment behavior to peculiarities of milk delivery through the surrogate nipple and reproduce the initial attachment pattern when reexposed to the surrogate nipple. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Habituation of oral responding in adult rats.

The effects of repeated oral stimulation on ingestive responding were investigated in adult rats. A series of brief intraoral infusions of flavored diet was delivered to female rats once every minute through an oral cannula. When the flavor of the infused diet remained constant, significant decreases in mouthing behavior were observed by the end of testing, whereas switching the flavor of the diet during testing resulted in enhanced responding and infusions delivered through gastric cannulas produced minimal effects. Patterns of oral responding were also similar in food-restricted rats. These patterns of responding suggest that adult rats habituate to oral stimulation. Finally, oral habituation led to decreased ingestion, whereas gastric infusions had minimal effects. Thus, oral habituation may represent a mechanism influencing intake in rats at all ages. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Taste aversions to mother's milk: The age-related role of nursing in acquisition and expression of a learned association.

Seven experiments examined the development of taste aversion learning to novel cues contained in mother's milk in 176 laboratory and 377 Sprague-Dawley pups. Ss receiving distinctive milk by experimenter-delivered oral infusions followed by toxicosis formed an aversion to the dam's diet. Robust aversions were learned as early as Day 10 and were retained for at least 11 days. When the same distinctive milk was obtained directly from a foster mother through nursing, only weanling-age Ss formed an aversion. X-ray analysis of nipple location in the mouths of suckling Ss suggested that they receive milk at a similar tongue locus between the ages of 10 and 21 days; flavored milk was then delivered at specific time intervals in controlled quantities through tongue cannulas implanted at loci corresponding to the nipple position. Cannulated preweanling Ss that were attached to a nipple during mild delivery failed to associate the taste cue with illness, whereas both preweanlings off the nipple and weanlings on the nipple acquired aversions to the taste cue in the milk. Results suggest that pups of all ages are incapable of expressing a taste aversion in a nursing situation and that preweanling pups in particular are also deficient in acquiring aversions within a suckling context. The inability of preweanling pups to acquire taste aversions in a nursing situation appears to result from a failure to associate taste cues with illness rather than a failure to detect taste cues obtained from a nipple. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Mother's milk: A medium for transmission of cues reflecting the flavor of mother's diet.

Reported that 24 nursling rat pups poisoned following ingestion of 1/2-cc of milk from a female rat eating a diet different from that of their own mother showed an aversion to that female's diet during weaning. A 2nd experiment showed that pups fed 31/2-cc meals of milk from a female rat and which were not poisoned showed a slightly increased preference for that female's diet at weaning. Results are interpreted as offering additional evidence that (a) a mother's milk contains gustatory cues reflecting the flavor of her diet and (b) these cues are sufficient to influence dietary preference at weaning. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Social facilitation and inhibition to hunger-induced killing by rats.

Gave 162 male Long-Evans rats different types of experiences to alter their mouse- and rat-pup-killing behavior. First, hungry Ss given considerable experience in mouse killing did kill rat pups, whereas inexperienced Ss were unlikely to kill pups. Second, nonkilling experiences subsequently inhibited hunger-induced mouse killing. Nonkilling experience was given by presenting rat pups and mice to food-satiated Ss and by presenting rat pups to hungry Ss. Inhibition was greater when pups were presented to satiated Ss rather than to hungry, and when pups, rather than mice, were used as inhibitory stimuli. (20 ref.) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Predatory attack by rats: Its relationship to feeding and type of prey.

Investigated mouse killing behavior in 5 experiments with male Long-Evans rats (N = 233). Hunger potentiated mouse killing by naive Ss, but not by Ss familiarized with mice before and during food deprivation. Once Ss had been made hungry, mouse killing was unaffected by increasing or decreasing severity of food deprivation or by time of testing with respect to a regular feeding hour. Ss fed either dead mice, powdered chow, or hard pellets while on cyclic food deprivation were about equally likely to kill, showing that hunger does not indirectly potentiate killing by increasing practice of responses like pouncing and biting. Whether hungry or not, killers were likely to eat their prey, whereas nonkillers were unlikely to eat the same prey. Ss killed 12-14 day-old rat pups as often as they killed mice, but killed weanling rat pups less often. Findings question several common notions regarding predatory aggression. (26 ref.) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Development of drinking behavior in preweanling rats.

Studied the development of drinking behavior in 3–20 day old Sprague-Dawley rats by tracing their ability to perform progressively more complex components of the drinking act. Even 3-day-old Ss responded to cellular dehydration by actively swallowing intraoral infusions of milk and water or by licking these fluids when they were spread in a thin film across the floor of the test container. Ss were not able to approach and maintain contact with a distant fluid source until 10–25 days of age. By 20 days of age, dehydration reduced the consumption of milk and other types of food (dehydration anorexia). Results show that young Ss were able to respond to dehydration in a manner that resembled the adult response. (37 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)