The ontogeny of feeding in rats: I. Ingestive and behavioral responses to oral infusions.

Reports on 3 experiments with Charles River rat pups. When milk infusions were made through oral cannulas in the front of their mouths, 1–20 day old Ss actively ingested the diet, and their intake was related to the length of deprivation. Ss decreased their ingestive responding after they had consumed large volumes of milk. In addition, 1-, 3-, and 6-day-old Ss, when 24-hr deprived, exhibited an intense behavioral activation in response to milk infusion. The behavioral activation appeared to be stimulated primarily by taste and the opportunity to swallow. Milk infusions did not produce activation in older Ss; their behavior was more exclusively ingestive and food directed. Results demonstrate that (a) from birth, rat pups are capable of an active form of ingestion, independent of normal suckling from the mother; (b) such ingestion is controlled by physiological factors; (c) food has arousing properties in young animals; and (d) as pups grow older, their ingestive responding is refined from a generalized and nondirected activation to specific and directed feeding responses. (57 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)     

Ontogeny of preference for nutritive over nonnutritive suckling in albino rats.

Investigated nutritive vs nonnutritive suckling in 48 10–21 day old Sprague-Dawley rat pups in a spatial discrimination task. Ss preferred nutritive to nonnutritive suckling on their anesthetized mother at l7 and 21 days of age, but no preference was apparent in 10- and 12-day-old Ss. Rearing in isolation from the mother during Days 11–26 did not impair the development of preference for nutritive suckling in 16 17-day-old Ss. Thus, the appetitive component of suckling of infant rats, like the consummatory component, appears to come under the direct control of nutritional factors at the start of the weaning period. (21 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Increased fat intake during lactation modifies hypothalamic-pituitary-adrenal responsiveness in developing rat pups: a possible role for leptin

High fat feeding reportedly enhances hypothalamus-pituitary-adrenal (HPA) responses to stress in adult rats. The present study tested whether elevated fat intake during suckling could have short and/or long lasting consequences on HPA regulation in the offspring. Mothers were fed either a control (C; 5% fat) or high fat (HF; 20% fat) diet during the last week of gestation and throughout lactation. After weaning (day 21), pups from C and HF mothers were fed a chow diet. Offspring from both C- and HF-fed mothers were tested for ACTH and corticosterone responses to stress on postnatal days 10 and 35. We found that HF feeding produced higher lipid levels in the milk of HF compared with C lactating rat dams and that offspring of these mothers had significantly increased retroperitoneal fat pad weight and relative adipose mass on day 21 as well as elevated plasma leptin levels on days 10 and 21 of age. After weaning, pups from the HF mothers had lower plasma leptin levels than those from C mothers. Maternal dietary fat affected HPA responsiveness in the offspring in an age-related manner. Neonatal pups (day 10) from the HF mothers exhibited a reduction in the ACTH and corticosterone responses to ether stress. However, in 35-day-old offspring from HF-fed dams, stress-induced ACTH secretion was increased compared with that in pups from the C-fed mothers. These results demonstrate that maternal diet and increased fat intake through the milk are important regulators of HPA responsiveness in neonates and prepubertal rats. During neonatal life, the blunted stress responsiveness seen with elevated fat intake and the resulting high leptin levels might protect the pups from excessive HPA activation. After removal of the maternal dietary influence and reduced leptin levels, enhanced ACTH stress responses are observed as in adult rats fed a HF diet. Because of the inverse relationship between plasma levels of leptin and HPA responses in pups, the possibility exists that the effects of the HF diet on stress responsiveness are mediated by changes in leptin exposure during development.     

Conditioned aversions and their memories in 5-day-old rats during suckling.

In 4 experiments, 216 5-day-old Sprague-Dawley rat pups equipped with tongue cannulae placed either 2 mm rostral or 4–6 mm caudal to the intermolar eminence received sweet or salty solutions while suckling. This ingestion was followed by intraperitoneal injections of .3 M LiCl or isotonic saline. Ss with anterior cannulae took in considerably less fluid than controls when tested 5 or 16 days later. A series of control groups demonstrated that this acquired aversion was associative in nature. Ss with posterior cannulae did not form the association. It is suggested that the failure of 5-day-old Ss with posterior cannulae to form associations while suckling was not due to the prevention of conditioning by the act of suckling but instead resulted from the failure of the fluid to reach anterior taste receptors when injected into the posterior oropharynx, where the nipple normally empties its contents. Findings are discussed in terms of the transfer of information obtained during suckling to feeding and drinking during and after weaning. (30 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Diet palatability modulates sham feeding in VMH-lesion and normal rats: Implications for finickiness and evaluation of sham-feeding data.

Examined the capacity for taste cues to modulate the food intake of 24 ventromedial hypothalamic (VMH) and normal Long-Evans rats. Ss were implanted with a chronically indwelling gastric cannula and sham fed (cannula open) or normally fed (cannula closed) liquid diets varying in sucrose content. VMH Ss were maintained at control body-weight levels. For both groups, the discrepancy in consumption between sham and normal feeding situations depended on the sweetness of the diet. The implications of this finding for studies using sham feeding to assess putative feeding control signals are discussed. VMH lesions exaggerated the sensory control of food intake. Under sham-feeding conditions, increases in the sweetness of the diet led to disproportionately large increments of food intake in VMH Ss relative to controls. Data support the existence of a finickiness component in the VMH syndrome and allude to the nature of the physiological disturbance underlying this behavior change. (36 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Ontogeny of thermal and olfactory determinants of huddling in the rat.

In Exp I, in standardized tests of huddling behavior, 64 5-, 10-, 15-, and 20-day-old Sprague-Dawley rat pups spent substantial and equivalent amounts of time with an immobile rat or a heated, fur-covered tube, which suggests that the conspecific and inanimate stimuli were equally attractive to the pups. In Exp II, with 32 Ss, 2-choice preference tests showed developmental differences in attraction. Younger Ss preferred to huddle with the warmer, inanimate target, whereas older Ss preferred the conspecific. The emergent conspecific preference appears mediated by attraction to species odors. In Exp III (64 Ss), the 5- and 10-day-olds huddled equally with an immobile rat and an immobile gerbil (stimuli with similar thermal and tactile properties), but older Ss preferred the conspecific. In Exp IV, with 16 15-day-old and 16 20-day-old Ss, intranasal zinc sulfate treatment eliminated preference for the conspecific but did not disrupt huddling per se. Results from all experiments show that thermal cues were sufficient to elicit huddling at all ages, but olfactory cues became a more salient influence before weaning. An ontogenetic transition from "physiological" to "filial" huddling is discussed in terms of changes in sensory control of early behavior. (22 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Mechanisms for enhanced feeding in the cold in rats.

The effect of environmental cold upon feeding, food-motivated behavior, and gastric clearance of food was studied in 36 female Sherman rats in 4 experiments. Ss ate liquid diet in either a 5 or 22Deg. C ambient temperature (T-sub(a)) following (a) 24-hr food deprivation at T)a) of either 5 or 22Deg. C or (b) 0- or 48-hr food deprivation at T-sub(a) 22Deg. C. Ss ate more at 5 than at 22Deg. C regardless of the ambient temperature during deprivation. Ss increased feeding in the cold by increasing meal frequency but not meal size. Cold (5Deg. C) also enhanced the urge to eat. Ss barpressed for food more often in the cold on a VI 30-sec schedule when gastric clearance of food was prevented by a pyloric noose and when food cleared from the stomach normally. Quinine adulteration of food suppressed food intake only in the 22Deg. C. Cold (5Deg. C) enhanced gastric clearance of liquid food within 20 min of ingestion. Results support the view that the peripheral sensation of cold is an adequate stimulus for feeding and that cold T-sub(a) can stimulate feeding directly by increasing the urge to eat and indirectly, secondary to increased gastric clearance. (20 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Behaviorally functional opioid systems in infant rats: I. Evidence for olfactory and gustatory classical conditioning.

Conducted 2 series of experiments to characterize the behavioral function of opioid systems in neonatal rats. In the 1st series, the reinforcing properties of exogenous opioids were investigated in 112 5-day-old pups. Ss' ability to associate the novel taste of saccharin, received while suckling, with intraperitoneal morphine injections was assessed. Results show that Ss that received 0.5 ml of saccharin prior to morphine administration ingested considerably more saccharin on Day 10 than did control rats. The 2nd set of experiments was conducted to determine whether 144 rat pups could associate a novel odor with morphine administration. Five days after conditioning, that stimulus was highly preferred by morphine-treated Ss compared with saline control Ss. Thus positive associations were formed with either a novel taste stimulus experienced while suckling or with an odor experienced during social isolation. Conditioning was cue specific and was retained for at least 5 days. The formation of these associations was blocked with opioid antagonists given prior to conditioning. Data suggest behaviorally functional opioid receptors and raise the possibility of a functional role of the endogenous opioids in motivational processes in infant rats. (38 ref) (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)     

Dehydration inhibits suckling behavior in weanling rats.

Determined the relation between dehydration and suckling behavior in 567 Sprague-Dawley weanling rats 15, 20, and 23 days of age. After 15 days of age, intra- and extracellular dehydration sharply reduced both the number of Ss that suckled and the amount of milk consumed. Rehydration returned both behaviors to control levels. Thus, during the weaning period, the internal determinants of suckling are not homologous with those of drinking, but are more homologous with those governing feeding. (29 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Opiates and medial hypothalamic knife cuts cause hyperphagia through different mechanisms.

In Exp I, male Sprague-Dawley rats were given bilateral parasagittal medial hypothalamic knife cuts (KCs) or a sham procedure and fed a high-fat diet. KC and sham-operated Ss were approximately equally sensitive to the suppressive effects of naloxone (0.1–20 mg/kg, subcutaneously [sc]) on food intake. Ketocyclazocine (0.1–20 mg/kg, sc) generally increased daytime food intake in sham-operated Ss; in contrast, the normal hyperphagia of KC Ss was in most cases either unchanged or decreased by ketocyclazocine. In Exp II, neither diet composition nor hypothalamic KCs significantly affected the feeding responses to naloxone or the stimulatory effects of butorphanol tartrate (0.1–20 mg/kg, sc). It was hypothesized that the differential effects of ketocyclazocine in KC and sham-operated Ss were a consequence of the sedative effects of the drug combined with the elevated baseline of the KC Ss. This hypothesis was supported by Exp III, which showed that ketocyclazocine also reduced nocturnal intake in unoperated Ss and that butorphanol increased intake. That feeding responses to naloxone and butorphanol were essentially unchanged by hypothalamic KCs suggests that the opioid feeding system is independent of the longitudinal feeding inhibitory pathway believed to be involved in KC-induced hyperphagia. (36 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Inhibition of suckling in weaning-age rats: A possible serotonergic mechanism.

Four experiments studied the nature, development, and specificity of serotonergic involvement in the control of suckling behavior in Sprague-Dawley rat pups from 10 to 35 days of age. During development, suckling normally declines after 10 days and is abandoned after 30 days. It was found that (a) methysergide, a serotonin (5-hydroxytryptamine, 5-HT) receptor blocker, reinstated suckling behavior in pups 15 days of age and older; (b) quipazine, a 5-HT receptor agonist, inhibited suckling of pups 10 days of age and older; (c) methysergide pretreatment blocked the quipazine inhibition of suckling; and (d) metergoline, another 5-HT blocker, also stimulated suckling, and fenfluramine, a 5-HT releaser, blocked suckling in deprived pups. Data support the hypothesis that a serotonergic inhibitory mechanism modulates the suckling of weaning-age rats. These pharmacological manipulations of 5-HT appear to alter specific components of suckling behavior involved in its initiation and maintenance but do not appear to alter a general hunger system. (47 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effect of gastric reduction surgery on consummatory behavior in the rat.

When the gastric capacity of 10 male Sprague-Dawley rats were reduced by partitioning a portion of the cardiac region of the stomach, they ate significantly less than did 10 sham-operated Ss during restricted (2-hr) access to either a liquid or a solid diet. Experimental Ss not only ate less during 2-hr food access, they appeared to satiate more quickly than sham-operated Ss. In contrast, when food was continuously available, the food intake of experimental Ss did not differ from that of controls. In addition, the water intake of experimental Ss was not significantly different from that of controls under either ad-lib or restricted access. Thus, reducing stomach capacity by partitioning a portion of the cardiac region appears to produce a relatively specific disruption of feeding behavior under conditions of restricted access. Implications for the study of gastric feeding mechanisms and for the surgical treatment of morbid obesity in humans are discussed. (9 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Suckling behavior in neonatal rats: Psychopharmacological investigations.

Examined the effects of various neurotransmitter antagonists on suckling behavior of 370 3- and 4-day-old Sprague-Dawley rat pups. Peripheral administration of 3 serotonergic antagonists (methysergide, methiothepin, and metergoline) and a cholinergic antagonist (scopolamine) were observed to markedly reduce suckling behavior of neonatal Ss. These effects appear to be centrally mediated, since intracisternal administration of small doses of all these drugs was observed to suppress suckling. The reduction in suckling induced by these antagonists did not appear to be a result of a debilitating effect of the drugs or to be due to any alteration in body temperature. In contrast, the opiate antagonist naloxone, the dopaminergic antagonist haloperidol, the alpha-noradrenergic antagonist phentolamine, and the beta-noradrenergic antagonist propranolol did not consistently produce any alteration in suckling behavior. Results suggest that the serotonergic and cholinergic systems may be functioning much earlier in ontogeny than previously suggested and may be involved in modulating suckling behavior in the early neonatal period. (26 ref) (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)     

Effects of ethanol intake on retinol concentration in the milk of lactating rats

The effect of the consumption of ethanol (5%) on retinol concentration in milk was studied in the rat on day 12 after delivery, together with the evolution of dam body weight and pup growth rate. Female Wistar rats receiving alcohol (5%) in drinking water during lactation (N = 7) were compared to normal controls fed ad libitum (N = 6). The mean maternal alcohol intake was 3.96 +/- 0.23 g/kg body weight per day. To determine retinol levels in milk we used the Bessey and Lowry method, modified by Araújo and Flores ((1978) Clinical Chemistry, 24:386-392). The pups were separated from dams for a 2-4-h period, after which the dams were injected intraperitoneally with anesthetic and oxytocin. The concentration of retinol in milk was 162.88 +/- 10.60 micrograms/dl in the control group and 60.02 +/- 8.22 micrograms/dl in the ethanol group (P < 0.05). The ethanol group consumed less food than the controls and lost a significant amount of weight during lactation. On days 8, 10 and 12, the body weight of the pups from rats given ethanol (13.46 +/- 0.43, 16.12 +/- 0.48 and 18.60 +/- 0.91 g, respectively) were significantly lower (P < 0.05) than the weight of pups from controls (15.2 +/- 0.44, 18.36 +/- 0.54, 20.77 +/- 0.81 g). These data show that ethanol intake during the suckling period, even at low concentrations, decreases the amount of retinol in milk and, therefore, the amount available to the pups.     

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)     

Evidence for a transitional period for development of persistence in infant rats.

Conducted 4 experiments with a total of 188 rat pups to test the hypothesis that rat pups can learn persistence at age 11 days but cannot express this learning in their behavior until they are a few days older. Exp I used a space-trial procedure to investigate the relative reinforcing effects of milk suckling, dry suckling, and contact without suckling at ages 11 and 14 days. Exp II used the Exp I procedure, manipulating schedule of reward at 2 ages and under 2 of the reward conditions. Exp III manipulated the dry suckling reward schedule in 11- and 14-day-old pups. Exp IV manipulated partial and continuous reinforcement on Day 11 but tested for persistence on Day 14. Results suggest the existence of a traditional period in neonatal rats for the learning of persistence. Partial reinforcement training with suckling on an anesthetized dam as reward induced greater persistence in extinction of the approach response than did continuous reinforcement in rat pups 14 days old but not in 11-day-old pups. Other aspects of this period of development are discussed. (26 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Postsuckling behavioral arousal in weanling rats (Rattus norvegicus).

Twenty-day-old litters and their dams were observed in seminatural habitats consisting of a nest compartment and adjacent open field that contained powdered rat chow. It was found that pups displayed marked bursts of activity after suckling. Independent feeding reliably followed nursing bout termination (Experiment 1). Nipple withdrawal, with or without milk transfer, induced behavioral arousal whereas withdrawal of thermotactile and conspecific odor cues did not (Experiments 2-3). Increased thermogenesis was observed following milk transfer (Experiment 4). Finally, preweanling pups (10- to 12-day-olds) also displayed postsuckling arousal within the confines of the nest; full locomotor expression of this arousal was not evident until weaning age (Experiment 5). It was concluded that postsuckling arousal in weanlings functions to stimulate activities performed away from the nest and suckling, propelling pups into the field where feeding begins. (PsycINFO Database Record (c) 2010 APA, all rights reserved)