Postweanling Rats Research Articles

Differential Recruitment of the Infralimbic Cortex in Recent and Remote Retrieval and Extinction of Aversive Memory in Post-Weanling Rats.

BackgroundWe previously showed that the infralimbic medial prefrontal cortex (IL-mPFC) plays an important role in recent and remote memory retrieval and extinction of conditioned odor aversion (COA) and contextual fear conditioning (CFC) in adult rats. Because the mPFC undergoes maturation during post-weaning, here, we aimed to explore (1) whether post-weanling rats can form recent and remote COA and CFC memory, and (2) the role of the IL-mPFC in mediating these processes.MethodsTo investigate the retrieval process, we transiently inactivated the IL-mPFC with lidocaine prior to the retrieval test at either recent or remote time points. To target the consolidation process, we applied the protein synthesis inhibitor after the retrieval at recent or remote time points.ResultsOur results show that the post-weanling animals were able to develop both recent and remote memory of both COA and CFC. IL-mPFC manipulations had no effect on retrieval or extinction of recent and remote COA memory, suggesting that the IL has no effect in COA at this developmental stage. In contrast, the IL-mPFC played a role in (1) the extinction of recent, but not remote, CFC memory, and (2) the retrieval of remote, but not recent, CFC memory. Moreover, remote, but not recent, CFC retrieval enhanced c-Fos protein expression in the IL-mPFC.ConclusionsAltogether, these results point to a differential role of the IL-mPFC in recent and remote CFC memory retrieval and extinction and further confirm the differences in the role of IL-mPFC in these processes in post-weanling and adult animals.

Effects of saccharin supplementation on body weight, sweet receptor mRNA expression and appetite signals regulation in post-weanling rats

Non-nutritive sweeteners have been considered to promote diet healthfulness by delivering a pleasant sweet taste without calories. We investigated the effects of long term supplementation with drinks containing saccharin on body weight and possible mechanisms of the effects in post-weanling rats. Our results showed that saccharin solution intake increased food intake and energy intake in male rats. In males, saccharin solution intake increased TIR3 mRNA expression in the taste buds and ghrelin receptor mRNA expression both in the taste buds and hypothalamus, whereas no effects were observed in females. These results suggest the effects of saccharin solution exposure on food intake and body weight gain may be different in developmental males and females. In males, peripheral sweet taste receptors and both peripheral and central ghrelin receptors may be involved in the effect of saccharin solution intake to promote food intake and weight gain.

Pyrroloquinoline quinone nutritional status and parameters important to mitochondrial function

Pyrroloquinoline quinone (PQQ) deficiency influences the mitochondrial content of heart and liver (Stites T et al., J Nutr., 2006) and impacts the metabolism of lysine and other amino acids oxidized via mitochondrial and peroxisomal pathways (Bauerly K et al., Biochim Biophys Acta, 2006). Because of mitochondria’s central role in metabolism, we investigated the effect of PQQ deficiency on mitochondriogenesis. Herein are data for serum glucose and fatty acids (FA) in rats fed amino-acid based diets with (2 mg PQQ/Kg diet) or devoid of PQQ for 3–5 weeks in post-weanling rats (fed and/or fasted states). Serum glucose, free fatty acids (FFA), RQ, triglycerides (TG), sphingomyelin (SM), total energy expenditure (EE) per metabolic body size (Wt3/4), relative mitochondrial amount, and expression of serine palmitoyl transferase (SPT) were estimated. Mitochondria content decreased 20% in PQQ deficient rats. Although no differences in serum glucose and FFA levels were observed (PQQ+ vs. PQQ−, fed or fasted), EE was significantly lower in PQQ deficiency. In the fed state, EE was positively correlated with mitochondrial amount and less in PQQ deficiency. Composition of FFA, TG, and SM were altered with PQQ deficiency, suggesting disturbances in long-chain FA metabolic pathways; serum TG were elevated while serum SM tended to be lower with corresponding decreases in liver SPT activity and mRNA levels in deficient rats. These data are consistent with our previous observations regarding PQQ status, mitochondrial function and associated activities. Funded in part by Mitsubishi, Inc., CA Vitamin Settlement Fund, and NDC.

Neonatal quinpirole treatment impairs morris water task performance in early postweanling rats: relationship to increases in corticosterone and decreases in neurotrophic factors

Background Past studies from this laboratory have shown that quinpirole administration from postnatal day (P) 1–21 produces persistent supersensitization of the dopamine D 2 receptor that persists throughout the animal's lifetime. Methods In Experiment 1, both male and female rats were treated with quinpirole or saline from P1–21 and tested on the place and match-to-place versions of the Morris water task (MWT) from P22–28. In Experiment 2, both male and female rats were administered either acute or chronic injections of quinpirole (1 mg/kg) or saline beginning on P1 until analysis for corticosterone (CORT) on P7, 14, or 21. Results Neonatal quinpirole treatment produced deficits on both versions of the MWT compared with saline control. One day after behavioral testing, brain tissue was harvested, and the hippocampus was analyzed for nerve growth factor (NGF) and brain-derived nerve growth factor (BDNF); NGF was found to be significantly decreased by neonatal quinpirole treatment. Acute or chronic quinpirole treatment on P14 produced a larger increase in CORT than controls and produced larger increases in CORT than control rats on P21. Conclusions These results demonstrate that neonatal quinpirole treatment produces cognitive deficits that could be related to decreases in hippocampal NGF and increases in CORT, resulting in abnormalities in hippocampal development.

Maturation-dependent acquired coronary structural alterations and atherogenesis in the Dahl sodium-sensitive hypertensive rat.

The Dahl sodium-sensitive hypertensive rat exhibits atherogenic lesions after the initiation of a high-sodium/high-fat diet. This study was designed to gauge the effect of a preadolescent high-fat diet on the postadolescent rate of atherogenesis after supplementation of the diet with sodium. Fifty-three Dahl S male rats were assigned to 2 dietary groups for the postweaning to early adolescence period (3 to 12 weeks): 29 to a standard diet (low-fat/low-sodium) and 24 to a high-fat/low-sodium diet. At age 9 weeks (just after puberty), animals from the high-fat group exhibited a relatively diminished density of coronary elastic fibers. There was no evidence of either lipid or monocytic infiltration of the subendothelial space. At age 12 weeks, most or all of the remaining animals in both groups were switched to a high-sodium/high-fat diet and were sampled through the following 8 weeks for the appearance of arterial lipid. After the switch, the high-fat-conditioned animals developed more extensive atherosclerotic pathological lesions more rapidly than their prepubertal standard-diet counterparts. The importance of the animal's stage of maturation in this effect was underscored by the observation that delaying onset of the high-fat diet to early adolescence resulted in no ultimate difference from the pubertal controls in elastic fiber density. The maturation-dependent high-fat conditioning of these postweanling rats correlated with an accelerated rate of atherogenesis on the initiation of the high-sodium/high-fat diet, possibly as a direct result of an alteration in arterial elasticity.

Ontogeny of spatial navigation in rats: A role for response requirements?

Three experiments investigated the role of response requirements in the Morris water maze for pre- and postweanling rats. Fischer-344N pups were required to locate a hidden platform using extramaze cues in a tank modified for the pups' immature response repertoire. Weanlings (20-22 days) displayed spatial learning in a pool 1/2 the size of the adults' (Experiment 1); by 26-28 days of age, probe performance was comparable to adults' on quadrant preference and platform-crossing measures. Preweanlings (17 days), in a pool 1/3 the original size, significantly reduced escape latencies and displayed quadrant preference and platform-crossing scores indicative of spatial navigation. These results suggest that despite its protracted postnatal development, the preweanling hippocampus allows neural integration of visual-spatial information; however, the capacity to demonstrate such learning is dependent on task parameters and the pup's response repertoire.

Influence of age on duodenal brush border membrane and specific activities of brush border membrane enzymes in Wistar rats.

To examine age-related changes in the morphology of intestinal brush border membrane (BBM; microvilli) and specific activities of intestinal BBM enzymes including alkaline phosphatase (ALP), gamma-glutamyl transpeptidase (gamma-GT), and disacchridase, four groups of Wistar rats were sacrificed at 2.5 wk, 5 wk, 5 mon and 23 mon. In an electron microscopic examination, morphologically a less dense BBM structure in the duodenum of rats aged 23 mon was observed than that of rats aged 5 mon. Specific activity of ALP in the duodenum from 5-mon-old rats was significantly higher than from rats aged 2.5 wk and 23 mon. The mucosal tissues from 5-wk-old rats had significantly higher specific activity of gamma-GT than did tissues from the other ages. In sucrase and maltase specific activities, 5-mon-old rats had higher activities of these enzymes than other age groups, especially 2.5-wk- and 23-mon-old rats. There was also a significant effect of site on intestinal BBM enzyme activities in post-weanling rats. Regional gradients of ALP and gamma-GT along the entire small intestine (duodenum > jejunum > ileum) were remarkable. Disaccharidase activities peaked in the jejunum and declined toward both the duodenum and ileum. Taken together the result obtained here suggested that 5-mon-old rats had the most elevated intestinal function. This result also strongly indicated that the structure of the intestinal BBM and development of intestinal BBM enzymes in Wistar rate were markedly influenced by age during the postnatal period.

Protein-restricted diet alters concentration of plasma membrane glycoproteins in rat liver.

Malnutrition is known to have adverse effects on the physiology and morphology of the liver. The aim of this investigation was to examine the effect of protein restriction on the content of plasma membrane proteins residing in the sinusoidal and bile canalicular domains of rat liver. Post-weanling rats maintained on low protein isocaloric diets showed marked growth retardation concomitant with reduced liver protein concentration compared to control animals. The content of leucine aminopeptidase, a bile canalicular enzyme, and asialoglycoprotein receptor, a sinusoidal receptor, in livers of protein-restricted rats was 66% and 50%, respectively, of control livers. In contrast, the relative concentrations of dipeptidyl peptidase IV and a cell adhesion molecule (GP 110), both canalicular proteins, were 160% and 121%, respectively, in rat livers upon protein restriction. After a 4-week rehabilitation period, the concentrations of all canalicular membrane proteins were similar to those in control livers, while the sinusoidal receptor was only 68% of control values. Protein restriction was found to adversely affect the concentrations of protein constituents, but not their localization in the hepatocyte plasma membrane. In general, altered concentrations of hepatocyte membrane proteins were reversed on the administration of a normal protein diet.

Exogenous calcium protects postweanling rats from salicylate-induced changes in auditory conditioning

During Days 26–32 (postnatal), 18 rats drank more of a 0.05-M calcium-enriched solution when it was given to them ad lib or 1 h daily, but not when it was given 15 min daily, compared with 18 pups with similar access to plain water only. In a second experiment, acquisition and extinction of auditory conditioned suppression were observed in thirty-six 25-day-old pups, half of them reared with the calcium supplement throughout gestation and preweanling, and the other half reared with tap water. Twenty-two hours before training, the subjects were fluid deprived and exposed to continuous noise of 60 dB (SPL). Two hours prior to each session, the subjects received either injections of sodium salicylate before both sessions (n = 6 for each fluid type), saline before acquisition and salicylate before the extinction session (n= 6), or saline before both sessions (n = 6). Suppression of the approach to drink at the end of an alleyway paired the offset of the 60-dB noise and footshock. Calcium afforded subjects protection from salicylate-induced changes in auditory behavior during both acquisition and extinction.

Effect of early dietary deficiency in polyunsaturated fatty acids on two lectin binding sites in the small intestine of postweanling rats.

This study was designed to determine whether dietary lipids influence the development of intestinal cell glycosylation, in relationship to diet-induced changes in phospholipid fatty acid composition. The ability of two different lectins, wheat germ agglutinin (WGA) and Maackia amurensis agglutinin (MAA), to combine specifically with particular carbohydrate residues was used to investigate the surface characteristics of epithelial cells of rats fed different dietary lipids from birth to 6 weeks of age. Diets contained 5% (weight) peanut oil (PO), rich in n-6 fatty acids; salmon oil (SO), rich in n-3 fatty acids; hydrogenated palm oil (HPO), deficient in both n-6 and n-3 fatty acids or a PO and rapeseed oil (RO) mixture (PRO), the control diet. Pieces of jejunal and ileal villi were excised from postweanling rats and prepared for lectin histochemical study. Concurrently, epithelial cells were removed from jejunal and ileal segments for determining their phospholipid fatty acid compositions. Polyunsaturated fatty acid (PUFA) deficiency was evidenced in the HPO group by the appearance of eicosatrienoic acid (20:3n-9) in both jejunal and ileal phospholipids, which paralleled the decrease in arachidonic acid content. Accretion of 18:1n-9 and 20:3n-9 in cell phospholipids of group HPO was not sufficient to match the unsaturation level in rats fed nonhydrogenated vegetable oils (PRO, PO) or fish oil (SO). The lectin histochemical study showed that WGA strongly labelled the brush border membrane microvilli whereas binding of MAA was specific to goblet cells and mucus. Regardless of the type of diet, WGA binding was weaker in the ileum than in the jejunum. In comparison to all other groups, WGA-labelling of villi was less intense in the jejunum and disappeared almost completely in the ileum of HPO-fed rats. Although SO- and PO-fed rats had, respectively, very low and high ratios of n-6 to n-3 in their intestinal phospholipids, binding of WGA in both groups was not markedly different from that in the control (PRO). MAA-labelling was very intense in jejunal and ileal villi of n-3-fed (SO) rats, whereas it was strongly attenuated in the n-3- and n-6 deficient (HPO) group. These results suggest that intestinal glycosyltransferase activities involved in cell differentiation were altered relative to the overall unsaturation index of dietary fatty acids. Alterations of epithelial glycosylation mainly resulted from a drop in total n-6 and n-3 fatty acids, although it may be speculated that there is a specific effect of n-3 fatty acids.

Cardiac levels of fibronectin, laminin, isomyosins, and cytochrome c oxidase of weanling rats are more vulnerable to copper deficiency than those of postweanling rats

The relative quantities of cardiac laminin, fibronectin, cytochrome c oxidase (CCO), and isomyosin types were studied by gel electrophoresis in male rats fed copper-deficient diets beginning either from the time of weaning for 5 weeks or from 5 weeks postweaning for 6 weeks with one group of copper-repleted rats. Increased levels of fibronectin and V 3 isomyosin but decreased levels of CCO subunit IV and laminin were found in weanling copper-depleted rats. In contrast, postweanling copper-depleted rats exhibited only increased levels of fibronectin and decreased levels of cardiac CCO subunit IV. Repletion of copper-deficient rats for 6 weeks was not sufficient to restore CCO subunit IV to the same level as controls. These results confirm that biochemical lesions in the basal laminae are a result of copper restriction. The decreased nuclear encoded subunits of CCO may help explain some of the mitochondrial pathology observed in dietary copper restriction. Increased V 3 isomyosin levels with low ATPase activity may help to conserve to a limited extent the ATP levels in copper-deficient cardiac tissue. These protein changes are consistent with the known morphological alterations of hearts from copper-restricted rats.

Effect of chronic maternal diazepam treatment on the development of stress-induced antinociception in young rats

The use and abuse of benzodiazepines is widespread and we have begun to address whether maternal exposure to these drugs influences the development of opioid systems. We have studied the effect of maternal diazepam treatment on stress-induced antinociception in the neonatal offspring. Diazepam (1 or 10 mg/kg) was administered twice daily to mothers from conception. Pre- and postweanling rat pups were assessed for opioid-mediated stress-induced antinociception by 3-min swimming and measuring nociception using the tail immersion test. In preweanling rats there was stress-induced antinociception in both vehicle- and diazepam-treated animals but in diazepam-treated groups (1 and 10 mg/kg) this was insensitive to reversal by the opioid antagonist naloxone, suggesting that nonopioid systems are operating this response. In postweanling rats a similar insensitivity to naloxone was observed in 1 mg/kg diazepam-treated groups; with 10 mg/kg diazepam there was no significant antinociception. The results suggest that maternal diazepam treatment interferes with the development of stress-mediated responses and that part of this toxicity is due to actions on opioid systems in the CNS.

Age-dependent differences in the rate of recovery of striatal dopamine D 1 and D 2 receptors after inactivation with EEDQ

Recovery of striatal dopamine D 1 and D 2 binding sites in 10-, 16-, and 39-day-old rats was measured 1, 2, 4, and 8 days after irreversible antagonism with N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). Ontogenetic and EEDQ-induced changes in D 1 and D 2 binding sites were determined by Scatchard analyses using six concentrations of either [ 3H]SCH 23390 or [ 3H]spiperone. Twenty-four hours after EEDQ (7.5 mg/kg) treatment, a significant depletion of both dopamine D 1 and D 2 receptors was found for all age groups; however, the magnitude of the depletion was greater in 39-day-old rats than in the two preweanling age groups. Both 16- and 39-day-old rats showed significant recovery of dopamine D 1 and D 2 receptors by the eighth day after EEDQ treatment, but the 16-day-old rats showed a faster recovery of dopamine D 1 receptors than did the 39-day-olds. Unexpectedly, 10-day-old rats did not show any evidence of receptor recovery, as the percent control values for these animals did not change across the 8-day recovery period. Pretreatment with the dopamine D 1 receptor antagonist SCH 23390 and the dopamine D 2 receptor antagonist sulpiride was sufficient to protect dopamine D 1 and D 2 receptors from EEDQ-induced inactivation. Protein values and receptor affinity (p K d values) were not affected by EEDQ treatment at any of the ages tested. Therefore, these results indicated that the rate of dopamine receptor repopulation varies across ontogeny, with 10-day-old rats exhibiting slower recovery than older rat pups or postweanling rats.

A developmental study on stress-induced antinociception measured by the tail electric stimulation test

The possible influence of weaning on the development of different neural mechanisms involved in stress-induced antinociception (SIA) was studied. Male Wistar albino rats were used for studies on adult and pre- and postweanling rats of 20 and 25 days of age, respectively. Animals were stressed by warm-water (20C) swimming for 3-min periods. Antinociception wasd assessed by the tail electric stimulation test. The thresholds for the motor response (tail withdrawal) (TW), vocalization during stimulus (V), and vocalization afterdischarge (VAD) were recorded. These responses are considered to be integrated at spinal, medulla oblongata, and diencephalon-rhinencephalon levels, respectively. In 20-day-old neonates, swimming stress only induced significant increases in the VAD thresholds that were not significantly reveresed by naloxone (NAL) (1 mg/kg). Twenty-five-day-old rats showed increased thresholds for the three nociceptive responses after stress, the effects on TW and V being antagonized by NAL. Adult rats subjected to stress showed increased thresholds for the three responses, an effect that was antagonized by NAL in all cases. These results suggest that the weaning period might be critical for the development of the mechanisms mediating SIA. Besides, a different involvement of opioid systems throughout development, particularly in relation to the affective/emotional component of pain, is also suggested.

Salicylate-induced phantom auditory effects on reinforced behavior in pre- and postweanling rats

In two behavioral experiments, normal hearing or salicylate-induced phantom auditory perception were investigated in young pigmented rats. Twenty-four hours prior to training in Experiment 1, 45 preweanling pups (14–21 days of age) were exposed to continuous background noise. They were injected with sodium salicylate or saline either before or after training to suppress approach responses to a lactating dam during noise offset periods. The sequence of injection initiation affected acquisition and extinction rates, and the oldest subjects behaved similarly to adults reported in studies of salicylate-induced auditory effects. Adapting the task in Experiment 2 for 18 postweanling pups 32 days of age produced greater suppression and prolonged extinction when salicylate was administered prior to both acquisition and extinction sessions, whereas subjects injected with saline before training and salicylate before extinction showed rapid recovery from suppression. The experiments support the notion that the auditory system undergoes functionally important change at around 15–16 days of age.

Enhancement of sensory preconditioning by a moderate dose of ethanol in infant and juvenile rats

Little is known regarding the effects of acute ethanol on learning and memory among infant and juvenile rats. Two experiments were designed to evaluate the influence of various doses of ethanol administration on sensory preconditioning and primary conditioning in preweanling (16-day-old) and postweanling (28-day-old) rats. Moderate doses of ethanol facilitated conditioning at both ages. In the absence of ethanol, sensory preconditioning was not statistically significant among postweanlings in Experiment 1, although the phenomenon was clear and robust among preweanlings. Sensory preconditioning was facilitated by administration of doses of 0.8 and 1.2 g/kg ethanol in preweanlings and a dose of 1.6 g/kg ethanol in postweanlings, whereas sensory preconditioning was impaired in preweanlings by a high dose (2.4 g/kg) of ethanol. This reflected a shift to the right, between the preweaning and the postweaning periods, in the dose-response curve. It was determined in Experiment 2 that the effect of ethanol on sensory preconditioning could not be explained by stimulus generalization or an effect of ethanol on first-order conditioning, confirming the effect of ethanol on learning of the odor-odor association in the preexposure phase. The basis for a dose-dependent biphasic effect of ethanol on sensory preconditioning is discussed.

Tellurium Blocks Cholesterol Synthesis by Inhibiting Squalene Metabolism: Preferential Vulnerability to This Metabolic Block Leads to Peripheral Nervous System Demyelination

Inclusion of 1.1% elemental tellurium in the diet of postweanling rats produces a peripheral neuropathy due to a highly synchronous primary demyelination of sciatic nerve; this demyelination is followed closely by remyelination. Sciatic nerves from animals fed tellurium for various times were removed and incubated ex vivo for 1 h with [14C]acetate, and radioactivity incorporated into individual lipid classes was determined. In nerves from rats exposed to tellurium, there was a profound and selective block in the conversion of radioactive acetate to cholesterol. Another radioactive precursor, [3H]water, gave similar results. We suggest that tellurium feeding inhibits squalene epoxidase activity and that the consequent lack of cholesterol destabilizes myelin, thereby causing destruction of the larger internodes. Ex vivo incubation experiments were also carried out with liver slices. As with nerve, tellurium feeding caused accumulation in squalene of label from radioactive acetate, whereas labeling of cholesterol was greatly inhibited. Unexpectedly, however, incorporation of label from [3H]water into both squalene and cholesterol was increased. Relevant is the demonstration that liver was the primary site of bulk accumulation of squalene, which accounted for 10% of liver dry weight at 5 days. Thus, accumulation of squalene (and other mechanisms, possibly including up-regulation of cholesterol biosynthetic pathways) drives squalene epoxidase activity at normal levels in liver even in the presence of inhibitors of this enzyme. This is reflected by continuing incorporation of [3H]water into cholesterol; incorporation of this precursor takes place at many of the postsqualene biosynthetic steps for sterol formation. [14C]Acetate entering the sterol pathway before squalene in liver is greatly diluted in specific activity when it reaches the large squalene pool, and thus increased squalene epoxidase activity does not transfer significant 14C label to sterols. In contrast to the situation with liver, synthesis of sterols is markedly depressed in sciatic nerve, and squalene does not accumulate to high levels.

Oxytocin selectively increases holding and licking of neonates in preweanling but not postweanling juvenile rats.

Preweanling rats exhibit components of maternal behavior (MB) after brief periods of contact with neonates; the latency of onset of MB rises considerably after weaning. Oxytocin (OXT) stimulates MB in adult rats. The effects of intracisternal (IC) administration of OXT (2 micrograms) on pup-directed and other behaviors in preweanling and postweanling juvenile rats were tested. Compared with saline and no treatment, OXT significantly increased active holding of pups in preweanling but not postweanling juvenile rats. No other components of adultlike MB were stimulated by OXT. OXT also decreased inactive touching of pups and robustly increased selfgrooming in juveniles at all ages tested. It is concluded that OXT facilitation of active pup-holding and licking in preweanling rats may be an extension of OXT-induced self-grooming to pups and may also be related to OXT activation of MB in adult rats.

Protein and carbohydrate selection respond to changes in dietary saturated fatty acids but not to changes in essential fatty acids

We previously reported differences in protein and carbohydrates selection patterns in post-weanling rats fed beef tallow or soybean oil-based diets. Two experiments were designed to determine the characteristic of the dietary fat which mediates the selection behavior. For each experiment, dietary fat was 20% (w/w) of diets and fatty acid profiles were obtained by blending fat sources. Rats were randomly assigned to diets (24% protein, 40% carbohydrate) which varied only in fatty acid composition. After 2 weeks, rats selected from 2 diets with the fat composition previously fed, but varying in their protein and carbohydrate composition (55% protein, 4% carbohydrate and 5% protein, 61% carbohydrate). Experiment 1 was designed to test the effect of relative (ω6:ω3 ratios of 1 and 20) and absolute (15% or 4% ω6, 0.7% or 0.2% ω3) differences in essential fatty acids on macronutrient selection patterns. Differences in dietary essential fatty acids had no effect on energy intake or the proportion of energy consumed as protein and carbohydrate. Experiment 2 examined the effect of differences in the level of saturated fat (3–10% diet (w/w)) on protein and carbohydrate selection. Animals selecting from diets with higher levels of saturated fat consumed more energy as protein and less as carbohydrate than rats selecting from diets with lower levels of saturated fat ( p < 0.0001). Regression analysis was used to examine the relationship between percent protein or carbohydrate energy and classes of dietary fat. The strongest relationship existed between percent dietary saturated fat and percent protein or carbohydrate energy ( p < 0.0001). Polyunsaturated:saturated fat ratio was also weakly associated with percent protein and carbohydrate energy ( p < 0.05). Polyunsaturated, monounsaturated, ω6 and ω3 fatty acids were not significantly related to percent protein or carbohydrate energy. These results indicated that protein and carbohydrate selection patterns are altered in response to qualitatively different dietary fatty acids, and that the amount of saturated fat in the diet is the important characteristic of dietary fat mediating the behavioral alteration.

Facilitation of acquisition and retention in preweanling but not postweanling rats by the presence of familiar home-nest material

A series of four experiments examined the effect of the presence of stimuli from the home nest on the acquisition and retention of aversively motivated behaviors in preweanling and adult rats. In Experiment 1, training in the presence of home-nest shavings facilitated acquisition of a T-maze discrimination to escape footshock for 16-day-old rats but not for adults. Experiment 2 demonstrated that the extent to which preweanlings were familiar with the home shavings determines the degree to which these stimuli facilitate spatial discrimination learning. When clean shavings were made more familiar than soiled home-nest stimuli (by changing the shavings every day) clean shavings enhanced discrimination performance, whereas no enhancement of learning by home shavings was observed. Experiment 3 extended the generality of the enhancement effect to a conditioned location aversion and examined the extent to which this facilitative effect was due to the tendency for home-nest shavings to elicit approach responses. Expression of the conditioned aversion was enhanced in subjects conditioned in the presence of home shavings, regardless of whether the home shavings were presented with the CS+, the CS-, or both. Experiment 4 determined that the enhanced expression of learning in the context of home-nest stimuli observed for preweanlings did not occur among subjects trained shortly after weaning. Collectively, these data suggest that whereas the enhancement of learning and retention by familiar home-nest stimuli enjoys generality across a number of conditioning situations, the effect may be limited to a relatively brief period during ontogeny.