Artificially Reared Rats Research Articles

Dietary Protein Restriction in Artificially Reared Neonatal Rats Causes a Reduction of Insulin-Like Growth Factor-I Gene Expression*

To determine in neonates the effects of protein restriction on growth, serum IGF-I, and IGF-I gene expression, we adapted a technique for rearing neonatal rats (days 6-18 of life) artificially by continuous infusion of milk through a gastrostomy. The artificially reared (AR) animals were given isocaloric diets containing 8%, 13.5%, or 18% lactalbumin protein. The AR rats were compared to rats reared by their mothers (MR) for 18 days. The growth of AR rats was related to the amount of dietary protein, with the pups given 18% protein having the best growth (25.55 g gained during the 12 days of the study) and those given 8% protein having the worst (13.42 g). The 13.5% protein-fed animals were intermediate in weight gain (18.39 g). The weight gains of the 18% and 8% protein-fed pups were significantly different from that of the MR animals (18.37 g). An identical pattern of tail length growth was noted among the groups. Mean serum IGF-I concentrations followed the same pattern (MR, 1.66 U/ml on day 18 of life; 18% AR, 2.53; 13.5% AR, 1.52; 8% AR, 1.31). Liver IGF-I mRNA was rank-ordered identically with weight gain and serum IGF-I [MR, 23.10 pg/micrograms poly(A+) RNA; 18% AR, 27.66; 13.5% AR, 21.02; 8% AR, 18.76]. Unexpectedly, the 7.5-kilobase IGF-I mRNA size class showed a 2- to 3-fold higher abundance in all groups of AR rats compared to that in MR controls (P less than 0.01), suggesting that this IGF-I size class is regulated independently of the other species. The reductions in serum IGF-I and IGF-I mRNA during protein restriction of neonatal rats suggest that these responses are mediated by decreased IGF-I gene expression at the level of transcription or RNA stabilization.

Effects of early life undernutrition in artificially-reared rats: 3. Further studies of growth and behaviour

Four groups of rat pups were reared: mother-reared control (well-fed) and undernourished (MRC, MRU) and artificially-reared control and undernourished (ARC, ARU). AR rats were fed expressed rats' milk (days 5–7), mixtures of rats' milk and milk substitute (days 8–16), and milk substitute only (days 17–20) by intermittent infusion via a gastric cannula. Undernutrition, imposed from 5–25 days, was effected initially by underfeeding the mother (MRU) or by infusing restricted quantities of milk (ARU). Weaning was at 21 days and undernutrition from 21–25 days was by restricting the supply of solid food. All rats were fed ad lib from 25 days. From 18 weeks, male rats were subjected to 3 tests of behaviour: spontaneous alternation, conditioned food preference, and visual discrimination (Lashley stand). There were negative effects of both undernutrition and artificial rearing on body growth in weight and length. For two aspects of behaviour, preference for carbohydrate relative to protein (greater in U rats) and latency to jump (longer in U rats) the effects of early undernutrition were the same in both MR and AR animals. Hence, these effects cannot have been mediated through alterations in maternal care.

Hormonal regulation of adaptive intestinal growth in artificially reared rat pups.

Adaptive growth and precocious expression of sucrase activity occur in the small intestine of artificially reared (AR) rat pups fed a hormone-free diet. The physiological mechanisms underlying adaptive intestinal growth were studied. Day 12 rat pups that received jejunal isografts subcutaneously on day 0 were subjected to artificial feeding and were killed on day 16. Crypt cellularity and DNA labeling index in isografts from AR, but not from mother-fed, rats increased significantly to levels found in in situ host jejunum of AR rats, indicating that humoral regulatory mechanisms are responsible for intestinal cell proliferation in AR pups. Radioimmunoassays of serum corticosterone, thyroxine, insulin, and gastrin and of gastric gastrin contents revealed that only serum corticosterone concentrations were significantly elevated, suggesting that corticosterone plays a critical role for intestinal growth. To examine this possibility directly, day 12 rats were adrenalectomized (ADX) and AR by continuous infusion of diets containing 0, 10, or 50 micrograms/ml corticosterone. Serum corticosterone concentrations paralleled the infused doses of corticosterone. Jejunal DNA labeling index increased in all ADX AR rats at day 13 in a dose-dependent manner. Increased jejunal DNA labeling index was maintained on day 14 in intact AR rats and ADX AR rats fed 10 micrograms/ml corticosterone but not in ADX AR rats receiving 0 or 50 micrograms/ml. We conclude that endogenous corticosterone is one of the systemic factors responsible for the adaptive increase in intestinal growth of AR rats.

Effects of early-life undernutrition in artificially reared rats: subsequent body and organ growth.

1. Four groups of rat pups were reared: mother-reared (MR) control (well-fed) and undernourished (MRC and MRU respectively) and artificially reared (AR) control and undernourished (ARC and ARU respectively). Pups for artificial rearing were fitted with a gastric cannula on postnatal day 5 and were fed, by intermittent gastric infusion, expressed rats' milk (days 5-7), mixtures of rats' milk and milk-substitute (days 8-16), and milk-substitute only (days 17-20). Solid food was available to MR pups throughout and to AR pups from day 14. Undernutrition, imposed from postnatal days 5 to 25, was effected initially by underfeeding the mother (MRU) or by infusing restricted quantities of milk (ARU). Weaning was at 21 d and undernutrition from day 21 to day 25 was by restricting the supply of solid food. All rats were fed ad lib. from 25 d. 2. The developmental milestone, eye-opening, was delayed by undernutrition but unaffected by artificial rearing. 3. Growth curves in body-weight during the refeeding phase were influenced most by previous undernutrition and to a lesser extent (also negatively) by artificial rearing. 4. Fourteen measures of body and organ growth were taken at autopsy at 39 weeks. Twelve measures were affected by nutrition and only four by rearing (weight of whole body, epididymal fat pads, renal fat pads and adrenals). 5. AR rats had lighter epididymal and renal fat pads than MR rats perhaps due to the low fat content of the expressed milk they received early in artificial rearing.

Effects of early life undernutrition in artificially-reared rats 2. Subsequent behaviour

Four groups of rat pups were reared: mother-reared control (well-fed) and undernourished (MRC, MRU) and artificially-reared control and undernourished (ARC, ARU). AR rats were fed expressed rats' milk (days 5–7), mixtures of rats' milk and milk substitute (days 8–16), and milk substitute only (days 17–20) by intermittent infusion via a gastric cannula. Undernutrition, imposed from 5–25 days, was effected initially by underfeeding the mother (MRU) or by infusing restricted quantities of milk (ARU). Weaning was at 21 days and undernutrition from 21–25 days was by restricting the supply of solid food. All rats were fed ad lib from 25 days. From 18 weeks, male rats were subjected to 3 behavioural tests: open field, social behaviour (encounters between rats of like treatment group) and bar-pressing for food (CRF, FR5, VI 1′, VI 2′ and Extinction). Previously undernourished rats responded at higher rates on FR5, received fewer rewards on VI 2′ and displayed more aggression during social interaction. These findings may reflect direct effects of nutrition on brain growth. In the open field and social situations ARU were generally less active than ARC animals, whereas MRU and MRC rats differed little. These results are discussed in terms of MRU pups being maternally “buffered” against the worst effects of undernutrition and ARC rats displaying characteristics of “early stimulated” animals.

Endogenous Corticosterone rather than Dietary Sucrose as a Modulator for Intestinal Sucrase Activity in Artificially Reared Rat Pups

The effects of sucrose and corticosterone on the expression of intestinal sucrase activity in preweanling rat pups were studied using an artificial rearing (AR) technique. When AR rat pups were isocalorically fed diets containing lactose or sucrose, or a carbohydrate-free diet from d 12–16, jejunal and ileal sucrase and maltase activities were induced to similar levels in all AR rats, whereas ileal lactase activity was precociously decreased. In separate experiments, enzyme activities were measured in ileal isografts subcutaneously implanted in littermates at birth. In AR rats fed the lactose diet, these isografts showed sucrase and maltase activities comparable with those of host ileum and also to isografts from AR rats fed the sucrose diet. In contrast, lactase activity was significantly higher in isografts than host ileum in all AR rats. Serum corticosterone levels were significantly elevated in AR rats for 24 h after intragastric cannular implantation. Precocious expression of ileal sucrase activity occurred in corticosterone treated, but not in untreated, adrenalectomized AR rats. In conclusion, dietary sucrose has no specific role in enhancing intestinal sucrase activity, and endogenous corticosterone is responsible for the induction of sucrase activity in AR rats.

Developmental flavor experience affects utilization of odor, not taste in toxiphobic conditioning

Feeding experiences were varied in developing rats and the effects upon flavor neophobia and lithium chloride-induced flavor aversions were observed. In Experiment 1, nursing experience of neonate rats was reduced by artificial feeding via intragastric cannula; the rats then were tested with apple juice paired with lithium chloride injection at weaning or maturity. Conditioned aversions were not affected, but neophobia to novel apple juice was attenuated in artificially-reared rats tested at maturity. In Experiment 2, rats received enriched feeding experience after weaning, which consisted of (a) obtaining many complex flavors, a few of which were paired with poisoning, effortlessly in the home cage, or (b) foraging for various foods on an elevated maze. No dramatic effects on neophobia or conditioned taste aversion for saccharin water were apparent. In Experiment 3, rats were given experience after weaning with vanilla-scented water either paired or unpaired with quinine water, and then tested with the odor of almond or that odor compounded with saccharin water for neophobia and lithium-induced aversions. Flavor-experienced rats exhibited more pronounced odor conditioning and more resistance to extinction of the odor aversion after both simple and compound conditioning. In contrast, saccharin taste aversions were relatively unchanged. Apparently, enriched feeding and drinking experience facilitates the utilization of odor more than taste cues.

Small Intestine of Artificially Reared Rat Pups: Weight Gain and Changes in Alkaline Phosphatase, Lactase and Sucrase Activities during Development

Comparative studies on small intestinal development in artificially reared (AR) and in mother-fed (MF) rats were carried out. When pups at 12 days of age were reared by continuous intragastric infusing of a cow milk formula (CMF), they gained weight equal to MF rats until 20 days; their small intestine weight was approximately 40 and 23% greater than that of MF rats at 16 days and 22 days of age, respectively. Autoradiographic studies indicated that the intestinal DNA synthetic index was 74–85% higher in AR than in MF rats at 16 days; similarly intestinal crypt depth and duodenal villus length were significantly higher in AR rats. The developmental pattern of duodenal alkaline phosphatase activity was similar in both AR and MF rats. In contrast, AR rats showed precocious decrease of lactase activity in the ileum at 16 days and in the jejunum at 18 days; but toal lactase activity in the intestine was not different from MF rats at 16 days. Jejunal sucrase activity in AR rats was precociously induced and elevated. It was concluded that the intestine of AR rats undergoes structural and enzymic modifications. The physiological significance of these changes remains to be elucidated.

Small Intestine of Artificially Reared Rat Pups: Effect of Caloric Intake and Dietary Composition on Growth and Disaccharidase Activities

When rats were fed a cow milk formula (CMF) from 12 to 16 days of age at a caloric intake sufficient to support a body weight gain equal to mother-fed (MF) rats, their small intestine outweighed that of MF rats by 40%. If the caloric intake was reduced by approximately 40%, artificially reared (AR) rats exhibited intestinal weights similar to MF rats, but their body weight was lower than MF rats. Sucrase specific activity was precociously increased in all AR rats; this increase was higher in rats with low caloric intake than rats with high caloric intake. Conversely, lactase specific activity was significantly decreased in the ileum of all AR rats. As compared to MF rats, the total lactase activity was significantly lower only in rats with low caloric intake. When AR rats were fed a modified Ross Carbohydrate-Free formula (RCF) containing the same caloric density of protein, fat, and carbohydrate as rat milk, the intestinal growth and disaccharidase activities were the same as those of AR rats fed CMF. It is concluded that the weight gain of small intestine depends on the caloric intake of AR rats and both CMF and RCF are nutritionally equal in supporting intestinal growth. Changes in sucrase and lactase activities in AR rats appear to have no relation to compositional differences in CMF and RCF.