Metabolic and behavioral responses in pre-weanling rats following alteration of maternal diet

In rat pups, blockade of fatty acid oxidation by administration of 2-mercaptoacetate (MA) produces an increase in independent ingestion by 12 days of age. In contrast, administration of methyl palmoxirate (MP) fails to increase intake or to alter oxidation of fatty acids in young rats due to the high proportion of medium chain triglycerides (MCT) in rat milk. In the present experiments, the composition of rat milk was altered by placing dams on a high fat (HF) diet to examine the development of ingestive responding in rat pups following administration of MP. Following delivery of MP (0.5--10 mg/kg), pups were placed in a cage inside an incubator for 1, 3, or 6 h, and then received an intake test consuming a commercial half-and-half milk diet from the floor of test containers. Blood was collected from additional groups of pups for measurement of beta-hydroxybutyrate (beta HBA) and glucose levels. The results demonstrated that administration of MP produced significant reductions in beta HBA levels after 3 h in 12- and 15-day-old pups, but behavioral responses were noted only in pups aged 15 days. Similar results were obtained following administration of MA to pups reared by dams on HF diets; physiological responses were observed at 12 and 15 days of age, but behavioral responses were not observed after administration of MA until 15 days of age. Taken together, these results suggest that (1) changes in fatty acid oxidation may represent an early metabolic signal that can influence intake in rat pups and (2) alteration of the dam's diet produces physiological and behavioral changes in the pups.     

Energetic limits on reproduction: Maternal food intake

We examined the factors influencing maternal food intake and pup growth in Norway rats. Mother rats allowed pups in naturally large litters to grow at a slower rate than pups in naturally small litters. Pups reared by dams in a warm ambience (26°C) gained weight more slowly than dams at 22°C, and maternal food intake but not weight gain was depressed in the high ambient temperature. Pup growth at 18°C was unimpaired, with those dams eating no more and gaining no less weight than dams at 22°C. Nest material, however, was found to be essential for the successful rearing of young at cooler ambient temperatures. While restriction of food during gestation resulted in a marginally lower weight gain for the pups during the first 2 weeks postpartum, the dams appeared not to mobilize corporal stores or increase their food intake during lactation. Heavy body weight mothers did not eat any more, nor did they gain any less weight nor rear larger pups than light body weight dams. Rat mothers increased their consumption of a diet diluted with non-nutritive fiber to equal the nutritive intake of their controls, with their pups not differing in their growth rate. Pups reared by dams eating a high quality diet grew faster than pups with dams on the control diet. Food intake by mother rats is required during lactation relative to the amount of milk that is delivered to the pups, rather than to an absolute amount of food. Lactating females with a concurrent pregnancy neither increase their food intake nor appear to mobilize their corporal stores to deal with the added energetic drain of pregnancy. Indeed, their young grew somewhat more quickly than pups nursed by dams that were simply lactating. Taken as a whole, these results suggest that Norway rat dams apparently do not monitor and defend a maximal pup growth rate. Rather, rat dams seem to continue to defend their own homeostasis, and by doing so, allow the young to grow and survive under a wide variety of circumstances.     

Maternal effects in infant and adult phenotypes of 5HT1A and 5HT1B receptor knockout mice

The influence of the pre- and postweaning maternal environment on the offspring's phenotype was examined in 5-HT1A and 5-HT1B receptor knockout mice (KO1A and KO1B, respectively). We have previously shown that, when born to and raised by homozygous dams of the same genotype, adult KO1A are more anxious than wild-type (WT) mice, and adult KO1B are hyperactive and slightly less anxious than WT mice. We extend our studies here to the behavioral results of the offspring's own genotype, when the dam's genotype is constant, and the effects of the dam's genotype when the offspring's genotype is constant. In Experiments 1 and 2, KO1A-/- pups produced less ultrasonic vocalizations (USV) than controls in an isolation test on postnatal Day 7 when born to and reared by KO1A dams, either -/- or +/-. Heterozygous F1 pups reared by KO1A-/- dams produced more USV and were less anxious in the plus-maze at 2 to 3 months of age than F1 pups born to and reared by WT dams (Experiment 3). F1 pups reared by KO1B-/- dams produced less USV and were more anxious in the plus-maze than F1 pups reared by WT dams (Experiment 4). The results support a role for maternal effects that may comprise direct effects such as the dam's behavior and nutritional care of the pup, and possibly more complex indirect effects through the establishment of idiosyncratic dam-pup dyadic interactions. We recommend that breeding techniques that rely on same genotype (mutant-mutant or WT-WT) breeding pairs not be used to generate offspring when the focus of research is the study of gene function, but rather when familial effects need to be studied.     

Adult-litter relations in rats reared in single and dual-chambered cages

BLU:(LE) rats were reared in cages which allowed movement of adults, but not pups, between 2 chambers. Some were reared in cages containing a virgin female (aunt) in addition to the mother. Although prenatal preferences for location and food and water consumption were for the side on which the litter was eventually delivered and reared, most postnatal food and water consumption occurred on the nonlitter side. Adult-litter contact decreased from 90% of Day 1 to 40% of Day 21, decreasing more rapidly during the dark hours. Presence of aunts increased activity of mothers, but did not affect adult-litter contact. Pups in the dual cages displayed higher rates of solid food consumption at age 17 days and lower brain weights at 21 days than pups in single cages.     

Nest temperature: effects on maternal behavior, pup development, and interactions with handling

These studies compared the patterns of mother-young contact under different thermal environments and assessed the implications of the differing patterns of contact for pup development and the dam's behavior. In the first experiment, the warm surface dramatically reduced the total amount of contact time between dams and litters and the dam's food intake, but the effects on the pups were limited to smaller adrenal glands. A replication of the first experiment in which daily weighing and handling of the pups was eliminated produced no differences in development. In a third experiment, during which dams had access to a warm surface outside of the next area and the daily weighing and handling was resumed, the pups showed smaller adrenal glands, lower body temperatures and less thermoregulatory ability. A comparison of results suggests that handling increased the weight of adrenal glands of pups reared under the unmanipulated condition, but not the warm-rearing condition.     

Sibling inhibition of hoarding in postweaning hamster pups (Mesocricetus auratus)

If hamster pups are placed in individual cages at weaning (21 days of age) they begin to hoard food immediately; within 2-6 days, they hoard 90% of the food they take from their food dish in the course of a 1-hr test, consuming only 10% of it. Pups that remain with their littermates after weaning in large group cages do not hoard food until they are placed in individual cages, when hoarding starts immediately. The inhibitory effect of littermates is just as pronounced in hamsters that have been allowed to hoard food in individual cages for 14 days after weaning and are then regrouped into litters. If litters are housed in divided cages that prevent physical interactions among littermates, but allow the interchange of olfactory, auditory, and some visual cues, hoarding is suppressed to an intermediate level. These results show that the presence of siblings inhibits the expression of hoarding, partly as a result of direct physical interactions and partly through the agency of sensory cues. The onset of hoarding following the dispersal of young from the nest cannot be explained as a motivational consequence of the young no longer having access to the mother's food hoard.     

Early weaning does not accelerate the expression of nursing-related taste aversions

Preweanling rat pups do not display an aversion to a flavor conditioned stimulus (CS) paired with illness if the CS is presented during the act of suckling. In contrast, 20-day-old pups do form such a conditioned taste aversion while suckling (Martin & Alberts, 1979). The dissolution of the nursing-related "blockade" of toxiphobia correlates with the onset of solid food intake. Moreover, prevention of weaning prolongs the blockade; ingestive experience with solid food is necessary for the expression of nursing-related taste aversions in 26-day-old "food naive" pups (Gubernick & Alberts, 1984). The present experiments tested the possibility that premature weaning to solid food might accelerate the onset of nursing-related toxiphobia. Pups were weaned at 13 days of age and ingested only food and water. These prematurely weaned pups received taste aversion conditioning while suckling on Day 16, but showed no aversion to the CS flavor during a later food test. Thus, conditions that lead to early weaning (ingestion of solid food) do not accelerate the onset of taste aversions to mother's milk.     

Maternal malnutrition in the rat: Effects on food intake and body weight

The effects of dietary protein level on food intake and body weight were examined in adult female rats during a 35-day pre-mating period and during gestation and lactation. During the pre-mating period, no differences in daily food intake were observed among rats fed a 6% casein, 8% casein or 25% casein diet. However, during this period, rats fed the 6% casein diet gained significantly less weight than those with ad lib access to the 8% or 25% casein diets or than rats pair-fed the 25% casein diet in amounts equivalent to that consumed by rats in the 6% or 8% casein groups. Additionally, rats fed the 6% casein diet displayed decreased efficiency of energy utilization, calculated as weight gain per 100 kilocalories consumed, relative to rats fed the 8% or 25% casein diets. No differences in food intake were observed among the groups during gestation. However, rats fed the 6% casein diet gained less weight than rats fed the 8% or 25% casein diets. During lactation rats fed either the 6% or 8% casein diet consumed significantly less food than animals given the 25% casein diet ad lib. During the second week of lactation, rats receiving ad lib access to the 25% casein diet gained weight while those receiving the 6% or 8% casein diets continued to lose weight. At parturition, body weights of pups did not differ as a function of dietary condition. However, by day 12 of life, pups whose dams had ad lib access to the 25% casein diet weighed significantly more than pups whose dams consumed the 6% or 8% casein diet or whose dams were pair-fed the 25% casein diet in amounts equivalent to those consumed by rats fed the 6% or 8% casein diet.     

Weaning in the Norway rat: relation between suckling and milk, and suckling and independent ingestion

We examined in rat pups the relation between the decline of suckling and the emergence of independent ingestion, and the role of milk delivery conditional on nipple attachment in the maintenance of suckling. Fifteen-day-old litters were reared for either 5 or 10 days, and 20-day-old litters for 5 days by either a thelectomized (no nipples), ligated (nipples but no milk), or intact (nipples and milk) dam. Pups' food and water intakes were monitored daily, and their suckling, feeding, and drinking behaviors were videorecorded for 24 hr in the presence of their foster dam (Day 19 or 24) and for 24 hr in the presence of an intact, lactating dam (Day 20 or 25). There were no differences between treatment conditions with respect to either the onset or rate of increase of independent feeding or drinking. Pups reared by a thelectomized dam for 10 days displayed a pronounced, lasting depression of suckling. Twenty-five-day-old pups reared by a ligated dam displayed suckling levels comparable to those of control pups; in the presence of the ligated dam, however, their tendency to attach to a nipple was notably reduced. The implications of the findings for our understanding of the weaning process are discussed.     

The effects of Cannabis sativa L. seed (hemp seed) on reproductive and neurobehavioral end points in rats

This study determined the effects of maternal dietary intake of hemp seed on reproductive and neurobehavioral end points of Wistar rats. Time-mated rats were fed 100% hemp seed (n = 15), 50% hemp seed (n = 15) or basal diet (n = 15) once a day. The amount of food made available was based on control feed consumption records. All dams remained on their respective diets from premating (14 days) throughout gestation and lactation. After weaning, all pups were given their maternal diet until puberty. Mating and delivery weights of dams in all groups did not show significant changes. Number of pregnancies, number and post-natal survival rate of total rat pups, litter size and milk yield were lower in the group that received 100% hemp seed. Offspring that received 50% hemp seed diet expressed reproductive and neurobehavioral end points from a modified Fox battery earlier than rats on 100% hemp seed or basal diet, except acoustic startle results where no differences appeared. In conclusion, this study shows that hemp seed supplementation does not improve the reproductive and neurobehavioral performances of rats. Pregnant women and nursing mothers should be cautious about the using of Cannabis sativa L. byproducts in their diets.     

The effects of early dietary experience on subsequent protein selection in the rat

Two series of experiments were designed to investigate the effects of protein composition of the first solid food consumed by young rat pups on their subsequent protein intake after weaning. In the first series of experiments, pups were allowed access to the maternal diet between 17 and 21 days of age, then weaned to a choice of 10% and 60% casein diets at 21 days. Pups with access to a maternal diet containing 39% casein selected high levels of protein (calculated as % of total calories selected as protein: 50 +/- 1% (access) vs. 29 +/- 3.5% (no access), p less than 0.05), whereas lower levels of protein were selected following access to a maternal diet containing 26% casein (16 +/- 1.1% (access) vs. 23 +/- 1% (no access), p less than 0.05) or 10% casein (27 +/- 3.9% (access) vs. 39 +/- 3.7% (no access), p less than 0.05). In a second series of experiments, the influence of early diet composition on subsequent protein selection was confirmed by prematurely weaning rats at 17 days of age to either 39% or a 10% casein diet. When given a choice of 10% and 60% casein diets at 21 days, pups selected higher levels of protein if weaned to a 39% casein diet as compared to a 10% casein diet (37 +/- 5.0% vs. 19 +/- 3.5% (p less than 0.05) for the first experiment and 43 +/- 3.0% vs. 26 +/- 4.0% (p less than 0.05) in a second experiment). It was concluded that the level of protein selected by young rats is positively correlated with the protein concentration of their first solid food.     

Restriction of early exploratory forays effects specific aspects of spatial processing in weanling hamsters

Normally reared hamsters, but not hamsters reared on a liquid diet, demonstrated spatial memory for the location of odor cues in an allocentric task (Experiment 1). In Experiment 2, an egocentric task, liquid-reared hamsters detected a change in the spatial location of odor cues. In Experiment 3 liquidreared hamsters detected a change in the spatial location of two visual cues under allocentric task conditions. Female hamsters on a liquid diet retrieved their pups more often than dams on solid food, resulting in reduced exploratory opportunities for their pups during the period when olfaction mediates behavior. Hamsters in Experiment 4 experienced a direct restriction of early forays. The restricted-rearing group failed to detect a change in the spatial location of odor cues in an allocentric task. These findings suggest that restriction of early exploratory experience during a narrow period of development results in specific spatial processing deficits.     

Some effects of a split litter cross foster design applied to 15 day old rat pups

Rat litters were divided and cross fostered on a postnatal Day 15 so that each mother had 4 of her own pups and 4 pups of a second litter, until weaning at 21 days. Daily weight and body temperature measurements made between Day 15 and 21 showed no differences between foster pups and pups who stayed with their biological mothers. However, by the age of 30 days, foster pups weighed less than pups reared by their own (biological) mothers; and foster pups at that age did not survive food deprivation as well as the pups reared by their biological mothers. In their Day 30 weights and the capacity to survive food deprivation, foster pups resembled pups that had been permanently separated from their mothers on postnatal Day 15. We conclude that the split cross foster design may introduce variability rather than reduce it. Nonetheless, this design may be useful in the experimental investigation of maternal behavior and mother-pup interactions in the rat.     

Maternal diet affects feeding behaviour of self-selecting weanling rats

Weanling rats have been shown to regulate protein intake when provided the opportunity. The objective of these studies was to determine the influence of protein concentration in the maternal diet on protein selection of the offspring. Dams were fed single diets containing 10, 20, 30, or 40% casein throughout gestation and lactation. Pups from all groups were then allowed to select from diets containing 10 and 60% casein for two weeks. Protein selection in the weanling rat was found to correlate with protein concentration in the maternal diet expressed either as g/100g BW (r=0.48, p<0.05) or protein concentration (r=0.65, p<0.01). In order to differentiate between a gestational and lactational effect a cross-over study was performed. Pups from dams fed 10% casein during gestation and/or lactation selected 17% protein concentration, lower than the 25.1% selected by pups from dams receiving 40% casein through gestation and lactation. Therefore, feeding behaviour, as indicated by protein selection, is influenced by maternal diet during both gestation and lactation. The use of the self-selection paradigm has been proposed as a behavioural model for the investigation of the functional significance of maternal nutrition.     

The ontogeny of independent ingestion in mice: Or,why won't infant mice feed?

Unlike infant rats, which show deprivation-related ingestion in several different test situations, infant mice appeared to be relatively unwilling to feed independently of suckling until 12 days of age. We tested mouse pups that were deprived (of food, water, suckling, and maternal care) for 1, 7, or 24 hr in ingestive tests in which a milk diet was spread on the floor of their test container (Experiment 1). Pups at 3, 6, and 9 days of age consumed small amounts of the diet and showed little increase in intake when deprivation was increased. In contrast (and like rat pups of all ages), mouse pups 12 and 15 days of age actively ingested the diet and increased their intake with increased deprivation. Six-day-old mouse pups were similarly unwilling to ingest a 5% sucrose solution, though 12-day-old pups showed deprivation-related intake (Experiment 2). Cellular dehydration (produced by hypertonic saline injection), a potent stimulus for ingestion in infant rats, did not stimulate ingestion in mice younger than 12 days of age (Experiment 3). Finally, when ingestion was tested with diet infusions made through oral cannulas, mouse pups at 6 and 9 days of age showed only a slight increase in intake with increased deprivation. However, by 12 days of age, pups' ingestion increased markedly with deprivation (Experiment 4). Thus, mouse pups seem to be very different from rat pups with respect to the early existence of ingestive systems. The neural substrates for the ingestive responses that subserve independent ingestion are only minimally present in infant mice or are somehow inhibited.     

Influence of the odorization of the rearing environment on the development of odor-guided behavior in rat pups

The study describes a phenomenon in which attraction of rat pups to artificial odors disappeared after the first week of life. In experiment 1, pups were continually exposed to either: (1) the normal odors present in the litter or (2) the same odors enriched with one of four artificial odors present in the dam's food. Pups were tested daily with the odors of normal or adulterated soiled shavings from their nests. The results show that attraction to the normal test shavings lasted throughout the testing period (PN 1-7). However, pups raised on odorized shavings exhibited an attraction to the artificial odor until day 6 only, not on day 7. In experiment 2, pups were tested with dam's artificially adulterated food. The results show that the artificial odor, and not the food odor, was responsible for the lack of attraction on day 7. Experiment 3 was carried out to determine whether the date on which attraction to artificial odors changed might be specific to postnatal day 7 or whether the duration of odor exposure in test conditions was the important factor. The results suggest that the age of pup is the more important variable.     

Weaning in rats: II. Pup behavior patterns

In this study, litters of rat pups and their mothers were continuously video-monitored from Day 14 to Day 35 postpartum in order to describe the behavioral changes that occur during that period. Prior to the onset of solid food intake (Day 18), pups spent most of their active time suckling. During Days 18 to 26, feeding, drinking, grooming, and play-fighting rapidly became frequent daily activities, while suckling gradually began to decline. By Day 28, pups' food and water intake relative to body weight and time allocated to the new behaviors reached asymptote. Pups nevertheless continued to suckle until Day 34. These results suggest that (1) the transition from milk to solid food is embedded in a multitude of behavioral changes; (2) these behavioral changes have distinct temporal characteristics; and (3) the developmental period between Days 14 and 34 can be divided into three phases.     

Diet choice exaggerates food hoarding, intake and pup survival across reproduction

Siberian hamsters increase food intake and hoarding during pregnancy and lactation, perhaps to compensate for large decreases in body fat (approximately 50%). We tested the effects of diet choice on these responses in pregnant, lactating and virgin hamsters housed in a simulated burrow system. Hamsters were offered pellet chow (PC) or a choice of sunflower seeds (SS), rabbit chow (RC) and PC. Pregnant or lactating PC-fed hamsters had increased food intake and hoard size compared with virgins, effects exaggerated by diet self-selection. The pregnancy-induced increases and lactation-induced decreases in body mass were enhanced and diminished by diet self-selection, respectively. Pregnant self-selecting hamsters ate relatively more carbohydrate and less fat and hoarded less carbohydrate and more fat than their virgin counterparts (protein not affected). Lactating and virgin self-selecting hamsters both ate and hoarded relatively more carbohydrate than protein or fat compared with PC-fed hamsters but were not different from each other. Litter and pup sizes were similar at birth, but pups from self-selecting mothers had decreased lipid content (50%) compared with pups from PC-fed mothers, whereas at weaning they were heavier but not fatter. Only lactating PC-fed mothers cannibalized their pups (approximately 60% eaten, 8/10 litters). The pregnancy-induced increased eating and hoarding of carbohydrate may have helped meet immediate energy needs sparing dwindling lipid reserves, whereas the decreased fetal lipid investment may have helped conserve energy in anticipation of the increased demands of lactation. The diet-induced exaggerated caloric intake and food hoard size of lactating hamsters may have promoted pup growth and survival.     

Food selection during recovery from protein restriction in lactation.

Lactating rats were fed a 12% or 25% casein diet. After the pups were weaned on Day 21, dams were given ad lib access to three food cups containing protein, carbohydrate, and fat. Both groups were hyperphagic for three to four weeks. Previously, protein-restricted rats consumed more protein than controls during Weeks 2 and 3, but protein intake returned to control levels as body weight recovered.     

Maternal stress and high-fat diet effect on maternal behavior, milk composition, and pup ingestive behavior

Chronic variable prenatal stress or maternal high-fat diet results in offspring that are significantly heavier by the end of the first postnatal week with increased adiposity by weaning. It is unclear, however, what role maternal care and diet play in the ontogenesis of this phenotype and what contributions come from differences already established in the rat pups. In the present studies, we examined maternal behavior and milk composition as well as offspring ingestive behavior. Our aim was to better understand the development of the obese phenotype in offspring from dams subjected to prenatal stress and/or fed a high-fat (HF) diet during gestation and lactation. We found that dams maintained on a HF diet through gestation and lactation spent significantly more time nursing their pups during the first postnatal week. In addition, offspring of prenatal stress dams consumed more milk at postnatal day (PND) 3 and offspring of HF dams consume more milk on PND 7 in an independent ingestion test. Milk from HF dams showed a significant increase in fat content from PND 10-21. Together these results suggest that gestational dietary or stress manipulations can alter the rat offspring's developmental environment, evidence of which is apparent by PND 3. Alterations in maternal care, milk composition, and pup consumption during the early postnatal period may contribute to long-term changes in body weight and adiposity induced by maternal prenatal stress or high-fat diet.