Early Post-weaning Period Research Articles

Effects of amphetamine on locomotor activity in adult and juvenile alcohol-preferring and -nonpreferring rats

The objective of this study was to determine whether functional differences exist in amphetamine-induced locomotor activity between alcohol-naive alcohol-preferring (P) and -nonpreferring (NP) rats during postnatal development and during adulthood. Using a between-subjects design, 20- and 28-day-old P and NP rats (male and female counterbalanced, n=11–16/line) were habituated for 30 min in a photocell activity field. Each rat received subcutaneous injections of saline or 0.3, 0.6 or 1.2 mg/kg d-amphetamine (AMPH) and were then tested for an additional 30 min. Because of age and line differences in basal locomotor activity, total activity counts during the 30-min postdrug period were standardized using Z-score transformations. In the 20- and 28-day-old rats, dose-dependent locomotor activity increases after AMPH injections were obtained at both ages, although activity levels were greater in the 20-day-old pups. The 20-day-old female NP rats showed greater AMPH-induced increases in locomotor activity than P rats, whereas at 28 days of age, male NP rats showed greater activity levels than P rats to AMPH. For the adult P and NP rats ( n=8/line/gender), a within-subject design was used. In the adults, the NP line had higher locomotor activity than the P line following AMPH injection, and male rats were activated more by AMPH than female rats. The results suggest that functioning of the DA system in the adult P line is reduced compared to the adult NP line, and this line difference is also observed to some degree at an early postweaning developmental period.

The effect of duration and temperature of simulated transport on the performance of early-weaned piglets

The responses and tolerances of early-weaned piglets to transport are not well documented. It is believed that the additive stress incurred by transporting early-weaned pigs predisposes them to increased disease risk and compromised performance. The objective of this work was to investigate the relationship between two primary transportation stressors, duration and temperature, and their effects on piglet performance. Prior to housing on flatdecks at 30°C, transport was simulated by placing 17-d (± 1 d) old weaned piglets into wooden boxes of dimensions 1.2 × 1.2 × 1 m (space allowance range 0.18 to 0.36 m2 pig–1) with straw bedding. Two trials were conducted. In each trial, 96 piglets were randomly mixed in groups of four and assigned to the following simulated transport durations: no transport (control), 6 h, 12 h (Trial 2 only) and 24 h. Piglets undergoing transport simulation were kept at one of the following temperatures 20, 25 (Trial 1 only), 30 and 35°C. There was a significant interactive effect between transport duration and temperature upon liveweight change in the first 24 h in both trials (P < 0.01). The animals that incurred the greatest liveweight deficit after weaning relative to untransported control groups were predominantly either those that had been transported for 24 h at high transport temperatures (35 and 30°C) or those transported for 6 h at 20°C in Trial 1 and 35°C in Trial 2. The effect of simulated transport was measureable for up to 5 d post-transport. However, by 14 d post-transport there was no detectable influence of transport treatment on feed consumption or weight gain (P > 0.05). The study found piglets are able to recover and perform adequately in the early post-weaning period if extremes of transport duration and temperature are avoided. Key words: Transportation, early weaning, temperature, piglets

Formic acid–sodium benzoate preserved slaughterhouse offal and supplementary folic acid in mink diet

Slaughterhouse offal from beef slaughtering was preserved with 16 g kg −1 formic acid and 3 g kg −1 sodium benzoate and used in mink feed throughout the production year. The ensiled offal replaced 0, 0.5 or 1.0 parts of the frozen stored slaughterhouse offal in the basal diet, the level of which was 300, 200 and 260 g kg −1 during growth-furring, breeding and lactation periods respectively. Because folic acid has a central role in formate metabolism and in all tissues of high rate of cellular division during the breeding and lactation periods, each diet group was further divided into two levels of supplemented dietary folic acid, 1.3 or 10 mg kg −1 DM. Weaning weights of male kits and lactating females, as well as early post-weaning growth and hemoglobin development were negatively affected by the highest silage level, providing 4.5 g formic acid kg −1 feed. The pelt quality of the groups that received silage was comparable to or even better ( p < 0.10) than that of the group of untreated, frozen stored offal. Both male and female reproduction performance was good and unaffected by the dietary manipulations. The effect of increased dietary folic acid could not be seen in any of the reproduction parameters or in pre-weaning kit growth. Extensive formation of free fatty acids in the silage due to high ash content of the offal and probable subsequent fat saponification are suggested to be among the factors that led to the slightly impaired growth performance of the animals, especially before weaning. In conclusion, ensiling of slaughterhouse offal with formic acid and sodium benzoate is an applicable preservation method and inhibits microbiological spoilage for several weeks. The lower performance of animals during lactation and early postweaning periods indicates that the high level of use of silage containing formic acid should be limited to the period from approximately 12 weeks of age to pelting.

Weaning of pigs: duodenal myoelectrical activity during the change from sow's milk to solid feed

Duodenal electromyography (emg) was studied in conscious piglets during the suckling and postweaning period. In suckling piglets, emg showed regular, repeating migrating myoelectric complex (MMC) cycles, each of which was composed of the classical phases I, II and III. During the early postweaning period (1-10 days) feeding with solid feed influenced the duodenal emg. The duration of phase I after weaning was significantly shorter than that before weaning (3.3 vs. 10.6 min, P less than or equal to 0.01), representing a decrease in duration from 14.6 to 4.6% of the whole MMC (P less than or equal to 0.01). On the other hand, phase II increased from 79.9 to 89.6% (P less than or equal to 0.01) after the change in diet. Feeding sow's milk did not disrupt the MMC pattern, however voluntary feed intake of solid feed induced continuous spiking activity of the duodenum for a period of 124 min.

Purkinje Cell Impairment Induced by Early Movement Restriction

In the present work the effects of movement restriction imposed during the early postweaning period on both Purkinje cell dendritic development and exploratory behavior were analyzed. Male and female Sprague-Dawley albino rats were reared either in isolated-restricted or social-standard environments from postnatal day 18 to 30. On the 31st postnatal day, all rats were behaviorally evaluated by the open-field test and then sacrificed under deep ether anesthesia. Vermian cerebellar sections were later stained with the Golgi-Cox-Sholl method and the Purkinje cell dendritic morphology was quantified under light microscopy. The results indicate that early somatomotor restriction severely impairs both exploratory behavior and Purkinje cell dendritic growth.

Effect of timing and duration of dietary conjugated linoleic acid on mammary cancer prevention.

Conjugated linoleic acid (CLA) is a minor fatty acid found predominantly in the form of triglycerides in beef and dairy products. Previous work by Ip and co-workers showed that free fatty acid-CLA at < or = 1% in the diet is protective against mammary carcinogenesis in rats. The present study verified that the anticancer activities of free fatty acid-CLA and triglyceride-CLA are essentially identical. This is an important finding, because it rules out a nonspecific free fatty acid effect. In terms of practical implication, we can continue the in vivo research with the less-expensive free fatty acid-CLA without compromising the physiological relevance of the data. A primary objective of this report was to investigate how the timing and duration of CLA feeding might affect the development of mammary carcinogenesis in the methylnitrosourea (MNU) model. We found that exposure to 1% CLA during the early postweaning and pubertal period only (from 21 to 42 days of age) was sufficient to reduce subsequent tumorigenesis induced by a single dose of MNU given at 56 days of age. This period incidentally corresponds to a time of active morphological development of the mammary gland to the mature state. In contrast to the above observation, a continuous intake of CLA was required for maximal inhibition of tumorigenesis when CLA feeding was started after MNU administration, suggesting that some active metabolite(s) of CLA might be involved in suppressing the process of neoplastic promotion/progression.

Interhemispheric Structural Asymmetry Induced by a Lateralized Reaching Task in the Rat Motor Cortex

The effects of a lateralized reaching task on the morphological structure of the rat motor cortex were studied during the early postweaning period. Our results show that the consistent use of one forelimb accounts for a significant decrease in the numerical density of cells and an increase in cortical thickness of the contralateral 'forelimb' motor cortex. As a consequence of the early motor training the cell distribution, which is lower rostrally than caudally in nontrained hemispheres, is reversed in the trained hemispheres. This may be interpreted as the specific motor training triggering a higher neuronal branching in the corresponding cortical region. The present findings may further the understanding of the mechanisms involved in the generation of morphological brain asymmetries.

Behavior and Immune Status of MRL Mice in the Postweaning Period

The notion that the MRl-lpr substrain is a useful model of behavioral and cognitive deficits found in systemic autoimmune diseases is supported by the recent findings of behavioral dysfunction in autoimmune MRL-lpr mice compared to their congenic control MRL +/+ mice. However, it has not been established whether the altered behavioral profile in MRL-lpr mice is the result of the autoimmune process itself or reflects a subtle difference in genetic background or both. To address the question whether MRL-lpr mice are born with behavioral dysfunction the present study compares the behavior of the two MRL substrains in the early postweaning period, when their immune status does not show detectable difference. Results show that the prediseased (4- to 6-week-old) MRL-lpr mice are not distinguishable from the congenic MRL +/+ controls on most behavioral measures except for speed of locomotion and novelty-induced hyperactivity in activity monitors. The immune status of the two substrains is also similar except for a lower hematocrit in the MRL-lpr group. Surprisingly, low amounts of antinuclear and brain-reactive antibodies (possibly transferred from diseased mothers) were detected in the serum of about 50% of the mice in both groups. The lack of major differences in behavior in the premorbid period suggests that appearance of previously reported behavioral dysfunction in the disease state reflects the presence of autoimmunity, time-determined genetic activation, or both.

Reproductive and developmental toxicity studies of tazobactam/piperacillin or tazobactam (3)--Perinatal and postnatal study in rats with intraperitoneal administration

Tazobactam (TAZ) is a newly developed beta-lactamase inhibitor and piperacillin (PIPC) is an antibiotics which is used in clinical field widely. The combination of TAZ and PIPC (TAZ/PIPC), which is combined with TAZ and PIPC at rate of 1:4, has been developed because of PIPC is unstable to various beta-lactamases. Perinatal and postnatal toxicity were studied in rats given daily intraperitoneal doses of TAZ/PIPC (200, 800 or 1600 mg/kg/day) or TAZ (40, 320 or 1280 mg/kg/day). TAZ/PIPC or TAZ were given from day 17 of pregnancy through day 21 of lactation. Total daily doses were administered in two equally divided doses. In this study, evaluation of the late stage of gestation, parturition, lactation and maternal behavior in adult rats and postnatal evaluation of the growth and development, and reproductive performance of the F1 generation occurred. In the TAZ/PIPC, maternal toxicity (decreased food consumption) was observed at 800 and 1600 mg/kg groups during perinatal period. A slight decrease in body weight gain during perinatal period and increased pup mortality and decreased pup weight in lactation period were observed at 1600 mg/kg group. An increase in stillbirths also was observed at 1600 mg/kg group. In the TAZ, maternal toxicity (decreased food consumption) was observed at all dosage groups during perinatal period. A decrease in body weight gain also were observed during perinatal period at 1280 mg/kg group. At maternotoxic doses of 320 and 1280 mg/kg groups, decreased pup weight were observed during lactation period. An increase in stillbirths also was observed at 1280 mg/kg group. Transient, significant decrease in pup body weights at 1280 mg/kg group in early postweaning period. No other effects occurred for the F1 generation rats. In conclusion, perinatal development and postnatal growth and development of offspring were affected only at the intermediate and high doses that caused maternal toxicity in this study. Therefore it is seemed that non-observed effect dose levels (NOELs) of TAZ/PIPC for dams is less than 200 mg/kg/day and that of TAZ is less than 40 mg/mg/day, and NOELs of TAZ/PIPC is 200 mg/kg/day and that of TAZ is 40 mg/kg/day for offspring under the condition of this study.

No persistent effect of preweaning nutrition on postweaning food intake, feeding efficiency, or body energy stores in Long-Evans rats

The purpose of this study was to determine the relative importance of pre- and postweaning nutrition on body mass, body fat, and feeding efficiency in Long-Evans rats up to a period of 18 weeks following weaning. Female rats were bred and pups were redistributed to form large (14–19 pups), normal (11–13 pups) and small (4 pups) litter groups. Weaned rats were housed as pairs (40 pairs) or singletons ( n = 16) and fed either a mixed-fat diet (36.6% fat) or a standard chow diet (13.5% fat). Food intake, body mass, and feeding efficiency were measured at 4, 8, 12, and 18 weeks postweaning. Total body fat and depot fat pad mass were also measured at 18 weeks postweaning. At weaning, pups from small litters were fatter ( p < 0.001), and had a greater mass ( p < 0.03) than pups from large litters. There were no persistent effects of preweaning litter size after covarying for preweaning mass on body mass, and postweaning growth, food intake, feeding efficiency, or body fat accretion. Male rats ingesting the mixed-fat diet had a greater body mass ( p < 0.05), greater body fat accretion ( p < 0.008) and a higher feeding efficiency ( p < 0.001) than their chow-fed counterparts, despite an overall lower energy intake ( p < 0.05). Female rats ingesting the mixed-fat diet had a lower food energy intake ( p < 0.005) and a greater feeding efficiency ( p < 0.001) than chow-fed rats during the early postweaning period, only. Thus, postweaning nutrition may play a more important role in postweaning adult mass and depot fat in freely eating rats than early nutritional experiences.

EFFECTS OF ZERANOL IMPLANTATION ON GROWTH, FEED CONVERSION, TESTICULAR DEVELOPMENT AND BEHAVIORAL TRAITS OF YOUNG BULLS FED FOR SLAUGHTER

An experiment was conducted to determine whether implantation with zeranol would reduce the aggressive behavior and probability of producing dark-cutting carcasses that characterize slaughter bulls. Thirty-six bull calves were allocated to each of four treatments – nonimplanted bulls, steers castrated at an average age of 101 d, bulls implanted with 36 mg zeranol at an average of 101, 170, 263 and 319 d of age and bulls implanted on the latter three dates only. The results reported include observations of weight gains, linear measurements, scrotal circumferences, serum testosterone concentrations, feed consumption, feed efficiency ratios and behavioral traits. Bulls exceeded steers in weight gain during a feedlot test and in body length after 140 d on feed, but neither implantation treatment significantly affected the growth of bulls. Feed consumption and conversion efficiency were lower in steers than in bulls, but were not significantly affected by implantation. Scrotal circumference measurements and serum testosterone concentrations suggested that implantation reduced testicle development during the preweaning and early postweaning periods, but that this effect diminished or disappeared during the later postweaning period. Implantation depressed aggressive and sexual activity between days 111 and 155 of the feedlot test. When evaluated at 13–14 mo of age, both implanted groups reacted to preslaughter mixing of nonfamiliar animals with equivalent head bunting to, but less mounting activity than, nonimplanted bulls. At 15–16 mo of age, however, the implanted bulls exhibited more head bunting than nonimplanted bulls, and the early implanted group exhibited fully equivalent mounting activity to nonimplanted bulls. The dark-cutting carcass condition was associated with mounting activity during preslaughter lairage. Key words: Slaughter bull, zeranol, growth, linear measurements, testicular development, behavior

Incidental learning is impaired during early-life undernutrition.

Rats were either well-nourished (control) or undernourished (PU) during the suckling and early post-weaning periods (birth to 45 days), after which all animals were fed ad libitum. From 25 to 45 days half of the rats in each group were exposed continuously in their home cases to shape stimuli (triangles and circles). Half of each of these 4 groups were given "reminders" of the stimuli through exposure for 2 h every 10-12 days from 55 to 131 days. Beginning on day 132, ability to discriminate these same visual stimuli was tested using a nonappetitively motivated version of the Lashley jumping stand technique. Discrimination learning per se was unimpaired by previous undernutrition (control and PU rats not viewing the stimuli prior to training performed equally well). However, early-life exposure led to improved learning performance only in control rats; it had no effect on the performance of PU rats (significant exposure X nutrition interaction). These findings indicate that undernutrition interfered with early "incidental learning", that is, learning about features of the environment which are not, at the time, biologically relevant. Finally, retention of the discrimination was assessed 9 weeks after the rats had attained criterion performance. There were no effects of nutrition or prior exposure on long-term memory.

Food intake, growth and body composition in Australian Merino sheep selected for high and low weaning weight. 3. Energy balance

ABSTRACTUsing a simplified energy balance equation, changes in energetic efficiency and energy expenditure were examined as a function of stage of maturity in rams and ewes from flocks of Merino sheep selected for high (weight-plus) and low (weight-minus) weaning weight and from a randomly bred control flock.Specific combustion energy of the body (i.e. MJ/kg of body weight) was an increasing linear function of stage of maturity, which increased at a slightly faster rate in the weight-minus than in the weight-plus animals (15·1 v. 13·1 MJ/kg per unit of maturity), and at a faster rate in ewes than in rams (17·2 v. 11·3 MJ/kg per unit of maturity). At maturity, all strains had a similar mean specific combustion energy (18·3 MJ/kg), whereas the ewes were greater than the rams (20·1 v. 16·6 MJ/kg).Thermochemical efficiency (TCE, defined as the gain in body energy per unit of metabolizable energy intake) was a quadratic function of stage of maturity. In the early post-weaning period, the weight-plus had a greater TCE than the weight-minus animals and the maximum TCE occurred at an earlier stage of maturity in the weight-plus than in the weight-minus animals (0·40 v. 0·45 maturity). The ewes had a higher TCE than the rams at all stages of maturity, although the differences decreased as the animals matured. The maximum TCE occurred at a later stage of maturity in the ewes than in the rams (0·46 v. 0·38 maturity).After scaling for differences in mature size there was little difference between the strains in the rate of energy expenditure, with the exception that at the later stages of maturity the weight-minus animals had a slightly greater rate of energy expenditure. After scaling for differences in mature size, the rams had a greater rate of energy expenditure than the ewes from weaning to maturity.

Effect of postweaning social experience on response of Siberian hamsters (Phodopus sungorus sungorus) toward young.

Between 18 and 50 days of age, Siberian hamsters were housed either with same-sex littermates, opposite-sex littermates, opposite-sex littermates and parents, or opposite-sex littermates, parents, and a younger litter. When adults, these animals were presented with two 3-8-day-old pups for 8 hr, and their responses were recorded. Rearing conditions did not affect behavior of females. Males housed with female littermates in the absence of a younger litter showed fewer pup attacks and more nesting with pups than did males housed only with other males. Thus, housing with females in the early postweaning period may influence males' later (adult) responses toward pups. Only among hamsters housed with same-sex littermates did males and females differ in their response to pups, a result emphasizing that sex differences in behavior may depend on early social rearing.

Lipid fluidity and composition of intestinal microvillus membranes isolated from rats of different ages

The lipid composition and fluidity of microvillus (luminal) membranes isolated from the small intestines of Fisher 344 rats aged 6, 17, and 117 weeks were compared. Lipid fluidity, as assessed by the fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene, was significantly greater in rats aged 6 weeks as compared to 17 or 117 weeks. A lipid thermotropic transition was observed at 17.5 ± 1.3°C in the membranes of the youngest group, approx. 5–6°C lower than that of the older animals. The differences in lipid composition which account for the higher fluidity of the youngest preparations include a decreased cholesterol/phospholipid molar ratio in both the proximal and distal halves of the small intestine and, in the proximal half alone, increases in the lipid/protein ratio and double bond index. The foregoing reduction in cholesterol/ phospholipid ratio derives mainly from a higher content of total phospholipid, and the increment in double bond index results from an increase in arachidonic acid residues. The results demonstrate an age-dependent decrease in fluidity of intestinal microvillus membranes in the early post-weaning period in the rat. This pattern was unlike that of the microvillus membrane p-nitrophenylphosphatase, whose specific activity declined progressively in the older age groups.

Effects of feeding pattern and dietary regimen on growth and adipose tissue cellularity in polygenic obese mice.

The effect of varying feed intake and feeding pattern during the early postweaning period on growth, body composition and adipose tissue cellularity was studied in polygenic obese and control normal mice. Male mice were assigned to the following dietary treatments at 4.5 wk of age: stock diet fed ad libitum(AL), four palatable foods cafeteria-fed(CF), stock diet fed every 2 h by automatic feeders adjusted for maximum intake(MI), fed same procedure as MI but restricted to produce 70% of the gain of mice fed ad libitum(RE), and stock diet fed one meal/d the same amount fed RE mice(PM). Mice were killed after 5 wk on treatment. Cafeteria-fed control mice were heavier (P less than .05) than RE control mice, but they were not different (P greater than .05) from AL, MI and PM control mice, while CF obese mice were heavier and RE obese mice were smaller than AL, MI and PM obese mice (P less than .05). Cafeteria-fed mice were fatter than mice from all other treatments in both the obese and control lines. Maximum intake, PM and RE mice were fatter than AL mice but this effect was only significant in the obese line. Alterations in feeding pattern can affect body composition even though body weight may not show a correlated response. Cafeteria-fed obese mice had larger fat pads and more small (less than 40 micron) and large (greater than 110 micron) adipocytes than other obese mice. Results indicate that the difference in the development of obesity on cafeteria diet was due primarily to genetic effects while the increase in percentage fat after restriction on MI, PM and RE treatments was due mainly to the acute change of feeding pattern.

The influence of age and experience on salt preference of the rat.

The degree to which adult taste preferences can be influenced by early taste exposure is a matter of some debate. The present studies characterize the developmental course of saline preference during maturation in the rat and examine whether adult saline preference is influenced by postweaning exposure to saline solutions. Saline intake from weaning to adulthood was examined in Wistar rats allowed continuous access to demineralized water and either .0, .9, 1.8, or 2.7% sodium chloride (NaCl) w/v solutions. When rats were immature they expressed greater preference for saline solutions than they did as adults. It is hypothesized that the overconsumption of saline by young animals is related to lower sensitivity of the taste system to NaCl in the early postweaning period. Animals that had had continuous access to saline from weaning were compared to those that had not received saline exposure early in life, in order to determine whether extensive experience with salt solutions leads to alterations in adult salt preference. These studies failed to reveal any evidence that rearing condition affects adult salt preference; thus adult salt preference may be maturationally determined and relatively independent of postweaning environmental influences.

Large-scale management systems and parasite populations: Coccidia in rabbits

Broiler rabbit production has become an important branch of animal protein and fur production, not only in the traditionally rabbit breeding and consuming countries, but recently in many other countries of the world. The profitability of the rabbit industry is dependent primarily on the good feed conversion. Aspects of this include factors such as morbidity and mortality of infections, adequate knowledge and fulfilment of both accommodation and nutritional requirements, appropriate breeding systems, etc. There is much controversy about the role coccidia in the losses of intensified rabbit breeding enterprises. There is no doubt about the significance of Eimeria stiedai which may cause condemnation of large amounts of large amounts of liver as a result of infection in rabbit colonies of small holders. However, this parasite is rare in large scale rabbit farms. In the latter, intestinal coccidiosis is most frequently — although not invariably — incriminated either as a primary or more often as a predisposing factor of intestinal enteropathies causing severe mortality, mainly in the early post-weaning period. The ability to isolate and maintain species of Eimeria in specifc pathogen-free (SPF) rabbits has made it possible to characterize them, such as the most pathogenic E. intestinalis and E. flavescens: the pathogenic E. magna, E. irresidua and E. piriformis; and the least pathogenic E. perforans, E. neoleporis (syn o E. coecicola and E. media species. Prophylactic measures are dealth with briefly, and laboratory and field trials with the currently most promising rabbit anti-coccidial drugs (Lerbek, robenidine and salinomyciin) are dealt with in more detail.

Comparative Postnatal Growth of Four Ground Squirrel Species

We employed four external measurements plus live weight to study postnatal growth of two montane (Spermophilus armatus, S. columhianus) and two lowland species of ground squirrels (S. elegans, S. richardsonii) from birth through approximately 3 months of age. Mean instantaneous growth rates (IGRs) were negatively correlated with litter size in the preweaning period and positively correlated in the early postweaning period; variances of IGRs were positively correlated with litter size. Montane species tended to be larger than lowland species in linear dimensions, but not in weight. Species IGRs overlapped but those of S. armatus peaked earlier than the other species; IGRs also fluctuated, indicating that growth varied with development. Longitudinal (analysis of data over time) principal components (PC) analyses of character means confirmed the similarity between the four species and revealed allometric growth patterns; Factor 1 represented size, Factor 2 contrasted extremities (hindfoot and tail lengths) and body core (body length and weight), and Factor 3 contrasted support (body and hindfoot lengths and weight) and non-support (tail) structures. Cross-sectional (analysis of data at each time period), stepwise discriminant function (DF) analyses revealed that tail length and weight best discriminated between species. Classifications from DF analyses showed S. elegans and S. richardsonii often were mistaken for one another, whereas S. armatus and S. columbianus were less so. Euclidean distances between species centroids for scores on the first three canonical axes of DF analyses revealed changes of one species with respect to another through time; phenograms based on these distances represented simultaneous growth pattern relationships between the four species.

Temporally spaced responding for food in adult rats after undernutrition early in life

Rats were undernourished during the suckling and early post-weaning periods and were then fed a good quality diet ad libitum from 43 to 300 days when behavioural testing was begun. Male previously undernourished and control rats were tested on three operant conditioning schedules in which low rats of bar-pressing were rewarded with food (differential reinforcement of low rate or DRL schedules).These were DRL 5 sec, DRL 10 sec and DRL 30 sec. On each schedule the response rates of both groups of animals declined over several sessions and their efficiency at obtaining the food rewards increased. However, there were no significant differences between the two groups on either of these measures.This finding indicates that, despite being hyper-responsive in many situations, including some operant conditioning schedules, previously undernourished rats are as capable as controls of witholding responsing when necessary.