Wistar-Kyoto Dams Research Articles

Whole transcriptome analysis of adrenal glands from prenatal glucocorticoid programmed hypertensive rodents

Prenatal glucocorticoid exposure is associated with the development of hypertension in adults. We have previously demonstrated that antenatal dexamethosone (DEX) administration in Wistar-Kyoto dams results in offspring with increased blood pressure coupled with elevated plasma epinephrine levels. In order to elucidate the molecular mechanisms responsible for prenatal DEX-mediated programming of hypertension, a whole-transcriptome analysis was performed on DEX programmed WKY male adrenal glands using the Rat Gene 2.0 microarray. Differential gene expression (DEG) analysis of DEX-exposed offspring compared with saline-treated controls revealed 142 significant DEGs (109 upregulated and 33 downregulated genes). DEG pathway enrichment analysis demonstrated that genes involved in circadian rhythm signaling were most robustly dysregulated. RT-qPCR analysis confirmed the increased expression of circadian genes Bmal1 and Npas2, while Per2, Per3, Cry2 and Bhlhe41 were significantly downregulated. In contrast, gene expression profiling of Spontaneously Hypertensive (SHR) rats, a genetic model of hypertension, demonstrated decreased expression of Bmal1 and Npas2, while Per1, Per2, Per3, Cry1, Cry2, Bhlhe41 and Csnk1D were all upregulated compared to naïve WKY controls. Taken together, this study establishes that glucocorticoid programmed adrenals have impaired circadian signaling and that changes in adrenal circadian rhythm may be an underlying molecular mechanism responsible for the development of hypertension.

Early Postnatal Gut Microbiota Determines SHR Hypertension

Hypertension is regulated by immunological components. We have shown that spontaneously hypertensive rats (SHR) display a genetic abnormality in a large population of proinflammatory CD161+ immune cells. Although early postnatal gut microbiota is important for development of the immune system, this relationship has not been studied in hypertension. Cross‐fostering SHR pups by normotensive Wistar‐Kyoto (WKY) dams lowers the blood pressure in adulthood when cross‐fostering is initiated at a very young age. In this study, we tested whether early alteration in postnatal gut microbiota affect s the immune system and blood pressure of SHR. We first examined the microbial populations in the fecal samples of SHR and normotensive control WKY using 16s rDNA sequencing. We found that the newborn SHR (1‐week old) have abnormal gut microbiota that is qualitatively and quantitatively different from the newborns of normotensive WKY. The representation of the predominant bacterial phylum Proteobacteria was significantly less in 1‐week old SHR pups than in WKY (94% Proteobacteria in WKY vs. 65% in SHR neonates). Even within the phylum Proteobacteria, the colonizing genera in WKY and SHR differed dramatically. Whereas WKY microbiota was predominantly comprised of Escherichia‐Shigella (92.5%), SHR microbiota was represented by Klebsiella (34.7%) and Haemophilus (27.8%). In contrast, the representation of phylum Fermicutes in the neonatal SHR gut was greater than WKY (5% Fermicutes in WKY vs. 31% in SHR). We cross‐fostered newborn SHR pups by lactating WKY dams and measured the resulting changes in the gut microbiota, immune system and blood pressure. Cross‐fostering caused a dramatic shift in 1‐week old cross‐fostered SHR gut microbiota. The two major bacterial genera of phylum Proteobacteria, Klebsiella and Haemophilus, present in self‐fostered SHR pups, were depleted after cross‐fostering, and were replaced by the predominant genera of WKY (Escherichia‐Shigella). A proinflammatory CD161+ immune cell population in the spleen of cross‐fostered SHR was also reduced (30.7% in self‐fostered SHR vs. 12.6% in cross‐fostered SHR at 30 weeks of age) as was the systolic blood pressure in adult cross‐fostered SHR (SHR 204 mmHg vs. cross‐fostered SHR 177 mmHg, p= 0.02, n= 4 each group). Thus, cross‐fostering of SHR pups by WKY dams shifted the composition of their gut microbiota toward WKY within one week. Additionally, reduction in proinflammatory immune cells and lowering of systolic blood pressure were observed in cross‐fostered SHR. In summary, SHR have a congenital defect in their immune system that results in greater proinflammatory response associated with hypertension. Gut microbiota of the SHR is remarkably different from that of normotensive WKY, and likely contributes to SHR blood pressure by modulating early development of the immune system.Support or Funding InformationNational Institutes of Health Program Project Grant to FMA (HL 14388) VA Merit Review Award to MWC (1 I01BX001414), American Heart Association Innovative Research Grant to MVS (16IRG27260323).

Influence of Early Postnatal Gut Microbiota on Immune System in SHR Hypertension

Hypertension exhibits a prominent immunological component. We have recently shown that spontaneously hypertensive rats (SHR) have a genetic abnormality whereby they are born with a large population of proinflammatory CD161+ immune cells. Although early postnatal maternal environment is important for development of hypertension, its impact on the immune system is not known. Cross‐fostering SHR pups by normotensive Wistar‐Kyoto (WKY) dams lowers the blood pressure in adulthood only if the cross‐fostering is initiated at a very young age. In this study, we used SHR as a genetic model of hypertension to test whether early postnatal gut microbiota affect the immune system and blood pressure of SHR. Since cross‐fostering permanently changes the gut microbiota of the fostered offspring, we first examined the microbial populations in the fecal samples of SHR and normotensive control WKY using 16s rDNA sequencing. We found that the newborn SHR (1‐week old) have abnormal gut microbiota that is qualitatively and quantitatively different from the newborns of normotensive WKY. The representation of predominant bacterial phylum Proteobacteria was less in 1‐week old SHR pups (94% Proteobacteria in WKY vs. 65% in SHR neonates). Even within the phylum Proteobacteria, the colonizing genera in WKY and SHR differed dramatically. In contrast, the representation of phylum Fermicutes in the neonatal SHR gut was greater than WKY (5% Fermicutes in WKY vs. 31% in SHR). We also observed a dramatic shift in 1‐week old SHR gut microbiota when we cross‐fostered WKY and SHR pups beginning at 1‐day of age. Two major bacterial genera of phylum Proteobacteria present in self‐fostered SHR pups, but absent in WKY, were depleted after cross‐fostering, and were replaced by other bacteria predominant in WKY gut. Investigation of immune cells in the spleen of cross‐fostered SHR using cell surface markers and flow cytometry showed that there was a clear reduction in CD161+ cells (30.7% in self‐fostered SHR vs. 12.6% in cross‐fostered SHR at 30 weeks of age), a marker associated with inflammation. Thus, cross‐fostering of SHR pups by WKY dams shifted the composition of their gut microbiota toward WKY within one week. Additionally, preliminary results suggest lower systolic blood pressure (tail cuff pressure) in cross‐fostered SHR. In summary, SHR are born with a defect in their immune system that results in greater proinflammatory response associated with hypertension. Moreover, the gut microbiota of the SHR is remarkably different from that of normotensive WKY, and cross‐fostering of SHR by WKY dams favorably changes the gut microbiome and immune regulation.Support or Funding InformationNational Institutes of Health Program Project Grant to FMA (HL 14388) VA Merit Review Award to MWC (1 I01 BX001414), American Heart Association Innovative Research Grant to MVS (16IRG27260323).This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Distinct effects of early-life experience and trait aggression on cardiovascular reactivity and recovery

We previously demonstrated independent effects of early-life experience (ELE) and trait aggression (TA) on resting heart rate (HR) and mean arterial pressure (MAP) in rats. The present study examined the effects of TA and ELE on stress-evoked cardiovascular reactivity and recovery. Pups born to Wistar-Kyoto dams were exposed to daily 180-min periods of maternal separation (MS) during the first two weeks of life, and aggression was assessed in adult offspring using the resident-intruder test. Radiotelemetry was then used to record stress-evoked HR and MAP responses in response to: strobe light, novel environment, intruder rat, or restraint. Maximal HR and MAP responses were quantified as indices of reactivity, and exponential decay curves were fitted to determine decay constants as a measure of recovery. Strobe light was the weakest stressor, evoking the lowest increases in MAP and HR, which were significantly greater in MS-exposed rats irrespective of TA. In contrast, reactivity to and recovery from exposure to a novel environment or an intruder were significantly influenced by TA, but not ELE. TA animals exhibited greater reactivity in both of these paradigms, with either decreased (novel environment) or increased (intruder) recovery. Restraint stress induced the largest changes in HR and MAP with the slowest recovery, and these responses were shaped by a significant ELE x TA interaction. These data indicate that cardiovascular reactivity and recovery are influenced by ELE, TA, or ELE x TA interaction depending on stressor aversiveness as well as its physical and psychological dimensions.

Prenatal glucocorticoid exposure programs adrenal PNMT expression and adult hypertension.

Prenatal exposure to glucocorticoids (GCs) programs for hypertension later in life. The aim of the current study was to examine the impact of prenatal GC exposure on the postnatal regulation of the gene encoding for phenylethanolamine N-methyltransferase (PNMT), the enzyme involved in the biosynthesis of the catecholamine, epinephrine. PNMT has been linked to hypertension and is elevated in animal models of hypertension. Male offspring of Wistar-Kyoto dams treated with dexamethasone (DEX) developed elevated systolic, diastolic and mean arterial blood pressure compared to saline-treated controls. Plasma epinephrine levels were also elevated in adult rats exposed to DEX in utero. RT-PCR analysis revealed adrenal PNMT mRNA was higher in DEX exposed adult rats. This was associated with increased mRNA levels of transcriptional regulators of the PNMT gene: Egr-1, AP-2, and GR. Western blot analyses showed increased expression of PNMT protein, along with increased Egr-1 and GR in adult rats exposed to DEX in utero. Furthermore, gel mobility shift assays showed increased binding of Egr-1 and GR to DNA. These results suggest that increased PNMT gene expression via altered transcriptional activity is a possible mechanism by which prenatal exposure to elevated levels of GCs may program for hypertension later in life.

When early life growth restriction in rats is followed by attenuated postnatal growth: effects on cardiac function in adulthood.

Epidemiological and experimental studies demonstrate that intrauterine growth restriction (IUGR) followed by accelerated postnatal growth leads to increased risk of developing cardiac disease in adulthood. The aim of this study was to examine the effect of early life growth restriction on cardiac structure and function in young adult rats. IUGR was induced in Wistar Kyoto dams through administration of a low protein diet (LPD; 8.7% casein) during pregnancy and lactation; controls received a normal protein diet (NPD; 20% casein). Cardiac function and structure were assessed in female NPD (n = 7) and LPD (n = 7) offspring at 18 weeks of age by echocardiography and pressure-volume techniques, and systolic blood pressure by tail-cuff sphygmomanometry. LPD offspring remained significantly smaller throughout life compared to controls. There were no differences in the levels of systolic blood pressure, left ventricular cardiac dimensions, heart rate, ejection fraction and fractional shortening of the cardiac muscle between the investigated groups. Aortic peak systolic velocity was significantly reduced in the LPD group (P = 0.02). Our findings support the idea that the programming of adult cardiovascular disease can be prevented or delayed in IUGR offspring when postnatal growth trajectory resembles that of in utero.

Sex differences in the fetal programming of hypertension: role of adrenal phenylethanolamine N‐methyltransferase gene expression (680.21)

Prenatal exposure to high levels of glucocorticoids (GCs) predisposes to hypertension in later life. GCs regulate catecholamine biosynthesis and are critical for blood pressure homeostasis. This study examined whether prenatal exposure to elevated GCs influences the development of adult hypertension via alterations in phenylethanolamine N‐methyl transferase (PNMT), a gene that is linked to hypertension. Pregnant Wistar‐Kyoto dams were injected with 100 μg/kg/day of the synthetic GC dexamethasone (DEX) in their third trimester. Blood pressure and weights of the offspring were measured from week 3‐18, at which point the animals were sacrificed. Total RNA and protein were extracted from the adrenals, and the expression of PNMT and its regulatory transcription factors analyzed by qPCR and Western blot. Results reveal that in utero exposure to DEX promotes the development of hypertension in the offspring. A more pronounced increase in blood pressure was seen in males than females. The mRNA transcripts for PNMT, and its transcriptional regulators Egr‐1, Sp1, GR and AP‐2, were elevated in the adrenals of these animals compared to saline controls, with differences in both sexes. Preliminary Western data also shows upregulation of PNMT and the transcription factor Egr‐1. These results suggest that prenatal GC exposure increases adrenal PNMT gene expression via altered transcriptional regulatory mechanisms.Grant Funding Source: Supported by CIHR

Evidence of altered biochemical composition in the hearts of adult intrauterine growth-restricted rats

Epidemiological studies clearly link intrauterine growth restriction with increased risk of cardiac disease in adulthood. The mechanisms leading to this increased risk are poorly understood; remodeling of the myocardium is implicated. The aim was to determine the effect of early life growth restriction on the biochemical composition of the left ventricular myocardium in adult rats. Wistar Kyoto dams were fed either a low protein diet (LPD; 8.7 % casein) or normal protein diet (NPD; 20 % casein) during pregnancy and lactation; from weaning, the offspring were fed normal rat chow. At 18 weeks of age, the biochemical composition of the hearts of NPD control (n = 9) and LPD intrauterine growth-restricted (n = 7) offspring was analyzed using Fourier Transform Infrared (FTIR) micro-spectroscopy. Body weights at postnatal day 4 were significantly lower and remained lower throughout the experimental period in the LPD offspring compared to controls. FTIR analysis of the infrared absorption spectra across the whole "fingerprint" region (1,800-950 cm(-1)) demonstrated wider variation in absorbance intensity in the LPD group compared to controls. In particular, there were marked differences detected in the protein (1,540 cm(-1)), lipid (1,455 and 1,388 cm(-1)), proteoglycan (1,228 cm(-1)) and carbohydrate (1,038 cm(-1)) bands, indicating increased lipid, proteoglycan and carbohydrate content in the growth-restricted myocardium. In conclusion, changes in the biochemical composition of the myocardium provide a likely mechanism for the increased vulnerability to cardiovascular disease in offspring that were growth restricted in early life.

Effect of cross‐fostering on renal anti‐oxidant/oxidant status and development of hypertension in spontaneously hypertensive rats

1. The hypotensive effect of cross-fostering in spontaneously hypertensive rats (SHR) is thought to involve adjustments in renal function. However, its association with renal anti-oxidant/oxidant balance during cross-fostering is not known. 2. The present study examined the effect of cross-fostering and in-fostering of 1-day-old offspring between SHR and Wistar-Kyoto (WKY) dams on renal anti-oxidant/oxidant status and systolic blood pressure (SBP). Renal anti-oxidant/oxidant status and SBP were determined in the offspring from 4-16 weeks of age. 3. Cross-fostered SHR had significantly lower SBP than in-fostered SHR at 6, 8 and 12 weeks, but not at 16 weeks (127 ± 1 vs 144 ± 2, 138 ± 1 vs 160 ± 1, 174 ± 2 vs 184 ± 2 and 199 ± 2 vs 194 ± 3 mmHg at 6, 8, 12 and 16 weeks, respectively). No differences in SBP were evident between cross-fostered and in-fostered WKY rats. There were no significant differences in levels of thiobarbituric acid-reactive substances (TBARS), protein carbonyl and total anti-oxidant status (TAS) or superoxide dismutase, catalase, glutathione peroxidase (GPx), glutathione S-transferase and glutathione reductase activity between cross-fostered and in-fostered SHR or WKY offspring. However, compared with WKY rats, catalase activity was higher at 6 and 16 weeks, TAS was higher at 16 weeks and GPx activity and TBARS were lower at 16 weeks in SHR. 4. It appears that cross-fostering of SHR offspring to WKY dams during the early postnatal period causes a transient delay in the rise in blood pressure in SHR and that this does not involve the renal anti-oxidant/oxidant system.

Elevated vascular resistance and afterload reduce the cardiac output response to dobutamine in early growth-restricted rats in adulthood

Epidemiological studies have linked intra-uterine growth restriction (IUGR) with an increased risk of CVD later in life. The aim of the present study was to examine the effect of maternal protein restriction on cardiac function in adulthood during dobutamine (DOB) stimulation. IUGR was induced in Wistar Kyoto dams through administration of a low-protein diet (LPD; 8.7 % casein) during pregnancy and lactation; the control group received a normal-protein diet (NPD; 20 % casein). At 14 weeks of age, cardiac function was assessed in male and female NPD (eight females and eight males) and LPD offspring (ten females and ten males) by pressure volumetry using an anaesthetised closed-chest approach. We determined mean arterial pressure (MAP), heart rate and left ventricular pressure-volume indices under baseline conditions and DOB stimulation (2 and 4 μg/kg per min). During β-adrenergic activation in LPD offspring, increases in cardiac output (CO, P < 0.018) and stroke volume (SV, P < 0.005) were attenuated in comparison with NPD offspring, while increases in ejection fraction and the maximal rate of ventricular pressure development were not affected. LPD females maintained a smaller end-diastolic volume (P < 0.017). MAP did not differ between the groups and did not change significantly during DOB infusion. Arterial elastance and total peripheral resistance decreased in all rats but remained significantly elevated in LPD offspring (P < 0.015 and < 0.01). Early growth restriction did not affect ventricular contractility but led to an increased afterload and impaired the ability to increase SV and CO during β-adrenergic stimulation.

Adverse effects of melatonin on rat pups of Wistar–Kyoto dams receiving melatonin supplementation during pregnancy

This report documents an incidental finding during a study investigating the effects of melatonin supplementation on the development of blood pressure in SHR. Administration of 10 mg/kg/day of melatonin in drinking water during pregnancy to Wistar-Kyoto (WKY) dams caused a loss of more than 50% of the pups by the age of three weeks and 95% by the age of 6 weeks. There was no maternal morbidity or mortality in the two strains or death of any of the SHR pups. No obvious physical defects were present but mean body weight was lower in the surviving WKY rats when compared to that of melatonin supplemented SHR or non-supplemented WKY pups. The reason for the high mortality in WKY pups is uncertain and appears to be strain if not batch specific. There is a need for caution in its use, particularly during pregnancy, and clearly necessitates more detailed studies.

Prenatal thyroxine treatment disparately affects peripheral and amygdala thyroid hormone levels

A prenatal hypothyroid state is associated with behavioral abnormalities in adulthood. Wistar Kyoto (WKY) rats exhibit hypothyroidism and increased depressive and anxiety-like behaviors. Thus, the WKY could illuminate the mechanisms by which the reversal of developmental hypothyroidism in humans and animals results in adult behavioral improvement. We examined the outcome of maternal thyroxine (T4) treatment on thyroid hormone-regulated functions and adult behavior of the WKY offspring. Pregnant WKY dams completed gestation with and without T4 administration and their adult male offspring were tested. Measures included depressive and anxiety-like behaviors, and thyroid hormone (TH) concentrations in both plasma and specific brain regions. In addition, the expression of two proteins affecting thyroid hormone trafficking and metabolism, monocarboxylate transporter 8 (MCT-8) and iodothyronine deiodinase type III (Dio3), and of several behavior-altering molecules, glucocorticoid receptor (GR), prepro-thyrotropin releasing hormone (prepro-TRH) and corticotrophin releasing hormone (CRH), were determined in the hippocampus and amygdala of the offspring. Prenatal T4 treatment of WKYs did not affect adult depressive behavior but increased anxiety-like behavior and decreased plasma levels of THs. In the hippocampus of males treated with T4 in utero, Dio3 and MCT-8 protein levels were increased, while in the amygdala, there were increases of free T4, MCT-8, GR, prepro-TRH protein and CRH mRNA levels. These results show that T4 administration in utero programs adult peripheral and amygdalar thyroid hormone levels divergently, and that the resulting upregulation of anxiety-related genes in the amygdala could be responsible for the exacerbated anxiety-like behavior seen in WKYs after prenatal T4 treatment.

Divergent maternal behavioral patterns in two genetic animal models of depression

Maternal behavior was examined in Flinders Sensitive-Line (FSL) and Wistar–Kyoto (WKY) rats, two different genetic animal models of depression. Behavioral patterns were assessed by undisturbed observations in the nest [Post-Partum Days (PPD) 4 and 9] and post-disturbance observations (“retrieval tests”) on PPD 10. Litters were randomly allocated to a mild chronic-stress condition (limiting available bedding between PPD 2 and 9) or a standard rearing condition. The findings indicated that FSL dams did not differ from control dams in the undisturbed observations. However, in the post-disturbance observations FSL dams exhibited less pup-directed behaviors, a shorter latency to first pup carrying/retrieval and more self-directed behaviors than controls (the latter effect only in dams' interaction with whole litter). In contrast, WKY dams performed more pup-directed activities (e.g., nursing and licking) and less self-directed activities in both the undisturbed and post-disturbance observations (in both dams' interaction with single-pup and with the whole-litter) compared to controls. Accordingly, WKY dams exhibited a shorter latency for first pup-licking bout (in both post-disturbance observations). The early life mild chronic-stress used in the study (‘limited-bedding’) had a minor effect on the dams' behavior. Overall, the study investigated for the first time the maternal behavior of WKY dams and suggests that these dams show an almost opposite behavioral pattern to that of FSL dams. The results are discussed with regard to earlier findings in the FSL strain and behavioral patterns documented in depressed human mothers.

Milking Procedure Alters the Electrolyte Composition of Spontaneously Hypertensive and Normotensive Rat Milk

A number of groups, including our own, have shown that the severity of hypertension expressed by the adult spontaneously hypertensive rat (SHR), which is primarily considered to be a genetic model of hypertension, can be reduced as a result of exposure to the behavioural and nutritional environment provided by a normotensive foster mother. It has been suggested that the hypertensive influence of the SHR dam may involve increased sodium delivery to the pups and there have been some reports of elevated sodium concentrations in the milk of SHR dams. However, these studies used either a long (≥6 h) dam–pup separation period before collecting milk or repeated milking of the same dams, both of which have been shown to alter the trace element content of rat milk. Therefore, we have compared the electrolyte content of milk collected by these methods with milk derived from SHR and normotensive Wistar–Kyoto (WKY) dams separated from their litters for 2 h prior to a single-milking session. Long separation and repeated milking resulted in variable effects on the electrolyte content of both SHR and WKY dams’ milk, compared with milk collected after 2 h from dams which had not previously been milked. The most notable effects were the abolition of significant strain differences, observed following 2-h separation, for milk sodium (WKY 22.1 ± 1.4 vs. SHR 27.5 ± 2.1 mmol/liter, p < 0.05) and calcium (WKY 92.3 ± 4.3 vs. SHR 69.4 ± 2.9 mmol/liter, p < 0.05) when dams were separated for 6 h or were serially milked. These data suggest that the electrolyte content of SHR and WKY dams’ milk can be altered by the collection procedure and it is recommended that dams be milked on only one occasion following a short separation period from their litter.

Fetal growth retardation and increased placental weight in the spontaneously hypertensive rat

Epidemiological studies have linked low birth weight and increased placental weight with increased risk of hypertension in adult life. It has been proposed that the cardiovascular changes which lead to hypertension are initiated in utero by processes associated with intrauterine growth retardation. The alternative possibility, that hypertension may result from genetic influences which also determine fetal and placental size, has had less support because birth weight is not determined genetically in humans. However, in the spontaneously hypertensive rat (SHR) essential hypertension is known to be transmitted genetically. Fetal and placental weights were, therefore, measured at Day 20 gestation in SHRs and compared with those in the normotensive Wistar Kyoto (WKY) control strain. Fetal weight (1.93 +/- 0.04 g) was significantly (P < 0.001) reduced in SHRs compared with WKY fetuses (2.23 +/- 0.01 g) but placental weight was heavier (P < 0.001) in SHRs (0.347 +/- 0.005 g) than in WKY rats (0.300 +/- 0.006 g) although litter size was not different. As expected, maternal blood pressure recorded under 1% halothane anaesthesia was higher (126 +/- 2.7 mm Hg) in SHR than WKY rats (100 +/- 2.1 mm Hg; 1 mm Hg = 133 Pa). In addition the concentration of maternal blood glucose in SHR was significantly (P < 0.001) higher (4.8 +/- 0.32 mM v. 3.7 +/- 0.11 mM) and the concentration of plasma insulin was significantly (P < 0.05) lower in SHRs (18.8 +/- 3.0 ng mL-1) than in WKY dams (29.4 +/- 3.1 ng mL-1). Thus, the data support human population studies which show an association between adult hypertension and a reduced fetal:placental weight ratio at birth. However, because hypertension in the SHR is genetically determined, these data suggest that fetal growth retardation and increased placental weight may also be determined genetically.

Maternal behavior of spontaneously hypertensive and Wistar—Kyoto normotensive rats: Effects of reciprocal cross-fostering of litters

Maternal behavior of spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) normotensive dams rearing either their natural pups or pups of the opposite strain was assessed when pups were 4-6 days of age. Consistent with previous reports, SHR dams caring for their natural pups were observed nursing and licking their pups more often than WKY dams and were away from their pups less often. The behavior of dams rearing cross-fostered pups demonstrated that this strain difference in patterns of maternal behavior is dependent on pup type. Mothers of both strains shifted their behavior in the direction of the opposite strain's maternal behavior pattern when caring for cross-fostered pups. This shift was most pronounced in SHR dams caring for WKY pups. These data suggest that alterations in patterns of maternal behavior produced by cross-fostering may play an important role in cross-fostering effects on the physiological development of SHRs previously reported.