Maternally Reared Research Articles

Optimization of Milk Substitutes for the Artificial Rearing of Chinese Tree Shrews (Tupaia belangeri chinensis).

Simple SummaryThe Chinese tree shrew, a squirrel-like mammal, has been widely used as a laboratory animal in biological research. However, the low survival rate of the pups has seriously hindered the establishment of inbred lines of this species and further limited its wider use. We found a milk substitute appropriate for artificial rearing of Chinese tree shrew pups independent of any obvious adverse effects on their survival, health, and reproductive performance compared to those of the maternally reared pups. The successful optimization of a milk substitute for the artificial rearing of Chinese tree shrew pups may increase the availability of this experimental animal.The Chinese tree shrew (Tupaia belangeri chinensis) has the potential to replace the use of non-human primates in biomedical research. To increase the availability of this species, we have undertaken the ambitious task of establishing inbred lines of the Chinese tree shrew; however, we have been hindered by a low survival rate of inbred pups. Here, we report our artificial rearing (AR) of Chinese tree shrew pups using four different milk substitutes: the formula described by Tsang and Collins (milk TC) and three commercially available milk substitutes intended for possums (milk A and milk C) and for guinea pigs (milk B). We compared the effects of these milk substitutes and maternal milk on the daily milk consumption, growth performance, and survival of the pups. We also assessed the life span and reproductive performance of the F1 individuals given the best milk substitute as compared to the maternally reared (MR) pups. Milk B was found to be appropriate for AR. Pups fed with milk B had a high survival rate at the weaning age compared to those fed with the other milk substitutes. The AR pups fed with milk B had a life span similar to that of MR pups. AR females fed with milk B had an earlier age of the first reproduction, a larger number of litters, and a higher rate of survival of the offspring at the weaning age compared with the MR females. The successful optimization of a milk substitute for AR of Chinese tree shrew pups will undoubtedly facilitate the wide usage of this experimental animal.

Early postnatal development of electrophysiological and histological properties of sensory sural nerves in male rats that were maternally deprived and artificially reared: Role of tactile stimulation.

Early adverse experiences disrupt brain development and behavior, but little is known about how such experiences impact on the development of the peripheral nervous system. Recently, we found alterations in the electrophysiological and histological characteristics of the sensory sural (SU) nerve in maternally deprived, artificially reared (AR) adult male rats, as compared with maternally reared (MR) control rats. In the present study, our aim was to characterize the ontogeny of these alterations. Thus, male pups of four postnatal days (PND) were (1) AR group, (2) AR and received daily tactile stimulation to the body and anogenital region (AR-Tactile group); or (3) reared by their mother (MR group). At PND 7, 14, or 21, electrophysiological properties and histological characteristics of the SU nerves were assessed. At PND 7, the electrophysiological properties and most histological parameters of the SU nerve did not differ among MR, AR, and AR-Tactile groups. By contrast, at PND 14 and/or 21, the SU nerve of AR rats showed a lower CAP amplitude and area, and a significant reduction in myelin area and myelin thickness, which were accompanied by a reduction in axon area (day 21 only) compared to the nerves of MR rats. Tactile stimulation (AR-Tactile group) partially prevented most of these alterations. These results suggest that sensory cues from the mother and/or littermates during the first 7-14 PND are relevant for the proper development and function of the adult SU nerve. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 78: 351-362, 2018.

Early postnatal experience and DRD2 genotype affect dopamine receptor expression in the rat ventral striatum

Dopamine systems can be altered by experiences such as early life adversity. The intensity of these effects seems to vary as a function of interactions between genetic and environmental influences. In a series of experiments we have investigated the effects of genetic variants and early life adversity on several biobehavioral outcomes. Here we investigated the presence of single nucleotide polymorphisms (SNPs) in the gene coding for dopamine D2 receptors (DRD2) and the interaction between these variants with early life adversity on the expression of D2 receptors in the striatum. Time-mated pregnant female rats underwent restraint stress (gestational days 10–21) or were left undisturbed. Following parturition rat pups were maternally reared (MR) or artificially reared (AR). Subsequent to adult behavioral testing, rats were genotyped and their brains were processed (autoradiography) for D2 receptor expression. We found three variants in the DRD2 gene and these variants interacted with early adversity to affect D2 receptor expression in the nucleus accumbens. Specifically, artificially reared rats with AG DRD2 variant showed significantly higher D2 expression compared to mother reared rats with the AG DRD2 variant as well as the artificially reared rats with a GG DRD2 variant. These findings show that adult D2 expression is significantly influenced by the interaction of DRD2 SNPs and early developmental factors. These finding may explain why there are significant individual differences in the impact of early life adversity on dopamine-dependent processes and disorder vulnerabilities.

The Effect of Artificial Rearing on Gut Microbiota in a Mouse Pup-in-a-Cup Model

In this paper, the mouse pup-in-a-cup model was improved for younger mouse pups, and the effect of artificial rearing on gut microbiota development was evaluated. Intragastric cannulas were placed through the esophagus into 3-day-old C57BL/6J mice (n=48), and the mice were artificially reared (AR) with mouse milk substitute (MMS). Littermate pups (n=20) were maternally reared (MR) as controls. The feces of 3-day-old pups were analyzed by combining the PCR-denaturing gradient gel electrophoresis (DGGE) fingerprinting technique and sequencing of 16s rRNA gene fragments. After 11 days of artificial rearing, 37 of 48 pups were still alive. There were no significant changes in the number of DGGE bands or the Shannon index between the two groups. However, several bands in the AR group were obviously different from those in the MR group in the DGGE profile. These results demonstrate that it is possible to implant intragastric cannulas into 3-day-old C57BL/6J mice pups. However, the variation in the gut microbiota composition is non-negligible, even though the AR pups grow well.

Inadequate early social experience increases the incentive salience of reward-related cues in adulthood

The mechanisms by which childhood abuse and/or neglect become risk factors for the development of drug addiction, problem gambling, and other disorders of behavioral inhibition are unknown. The loss of behavioral inhibition is often triggered by reward-related cues that acquire incentive salience. This study examined whether inadequate early-life social experience in rats affects the incentive salience of reward-related cues. Rats were deprived of early-life social experience with the mother and litter through artificial-rearing (AR). A group of AR rats (AR + STM) received additional tactile stimulation that mimicked maternal licking, a critical component of rat maternal care. Control rats were maternally reared (MR). The incentive salience attributed to a food cue was measured in adult rats using a conditioned approach task, where a conditional stimulus (CS; lever) was paired with food delivery, and in a conditional reinforcement task. The dependent measures were approach towards the CS (sign-tracking) versus approach towards the place of food delivery (goal-tracking) and instrumental responding for the CS. AR rats made significantly more sign-tracking responses than MR rats. AR rats also made more instrumental responses when reinforced with the CS. AR + STM rats’ responses were intermediate to MR and AR rats. Thus, inadequate early-life social experience enhanced the incentive salience of a reward-related cue in adulthood. Replacement of maternal licking partially reversed this effect. These results highlight a potential link between early-life social adversity and susceptibility to disorders of behavioral inhibition.

Early-life maternal separation and social isolation produce an increase in impulsive action but not impulsive choice.

Early life environment, events, and context, such as mother-offspring relationship, can have profound effects on future behavior and physiology. We investigated the effects of long-term maternal and social separation, through artificial rearing, on adult impulsivity. Rats were maternally reared (MR) or artificially reared (AR) and half of the AR rats were provided with replacement somatosensory stimulation intended to simulate maternal licking. There are at least 2 forms of impulsivity and we compared rats on 1 test of impulsive action (differential reinforcement of low rates of responding-DRL-20s) and 2 tests of impulsive choice (delay discounting and fixed consecutive number schedule-FCN). We found that AR rats are more action impulsive; however, this effect can be reduced by maternal licking-like stimulation. In contrast, AR rats did not display an increase in impulsive choice. Overall, these experiments show that early life maternal and social separation have different effects on the 2 forms of impulsivity.

Effect of Complete Maternal and Littermate Deprivation on Morphine-Induced Fos-Immunoreactivity in the Adult Male Rat Brain

Previous research has demonstrated that rats reared in isolation from their dam and littermates show altered behavioral responsiveness to both natural and drug-mediated rewards. This study examined the effects of complete maternal deprivation through the use of artificial rearing on neural activation after acute morphine exposure in adulthood. Male rats were either artificially reared (AR) or maternally reared (MR) from postnatal day 5 to 21. In adulthood (4 mo old), rats received a single injection of morphine sulfate (10 mg/kg) or equivolume saline 2 h before perfusion and brain extraction. Neural activation was quantified using Fos immunohistochemistry. Analyses of several brain regions revealed a consistent pattern of differences between AR and MR rats. Specifically, relative to MR rats, AR rats showed significantly greater morphine-induced Fos-immunoreactivity in brain regions associated with the mesocorticolimbic "reward" pathway. These results support the hypothesis that functional activity in reward neurocircuitry can be altered by early life experience.

Motherless rats show deficits in maternal behavior towards fostered pups

Complete maternal deprivation in rats, through artificial rearing (AR), produces deficits in subsequent maternal behavior of the offspring. These deficits are partially reversed when isolated pups are provided with additional tactile stimulation designed to simulate maternal licking (e.g., Gonzalez et al. [2001] Developmental Psychobiology, 38, 11-32). These findings highlight the importance of the early maternal environment in subsequent development. However, given the possibility that prenatal environments may differ between AR and maternally reared (MR) offspring, the deficits in the behavior of AR mothers may be driven by the characteristics of their pups derived from the effects of an altered prenatal environment. Hence differences in the neonatal pups of AR mothers may produce the alterations in the AR maternal behavior. To rule out this possibility, we employed a fostering paradigm where AR and MR mothers received cross-fostered mother-reared pups. AR mothers showed the same level of deficits in maternal behavior towards MR foster pups as they do with their own pups and these deficits were partially reversed with additional tactile stimulation. Hence, maternal behavior deficits reported in mothers who had been reared in isolation are due primarily to the direct effects of the earlier experience on mechanisms regulating their maternal behavior and not to the effects on their offspring.

Interactions between the effects of early isolation rearing and complex housing on adult locomotor activity and sensitivity to amphetamine in rats involve noradrenergic neurotransmission

Increased sensitivity to the locomotor-activating effects of amphetamine in rats with a history of early-life social isolation is commonly attributed to alteration of the dopamine system. The locomotor response to amphetamine may also be due to effects on the noradrenergic system and particularly α-adrenergic receptors. The present study examined whether noradrenergic neurotransmission mediates the increased sensitivity to the locomotor effects of amphetamine resulting from early social isolation and whether this effect can be reversed by later-life social housing experience. Rats reared in complete social isolation (artificially reared, AR) exhibited higher levels of locomotor activity than maternally reared (MR) rats in response to amphetamine (0.25 mg/kg). Increased sensitivity to the locomotor effects of amphetamine in AR rats was reduced by the α-adrenergic receptor antagonist prazosin (0.5 mg/kg). Prazosin alone reduced activity in AR rats to the level of MR rats. Group housing in cages that were more complex than standard laboratory cages reduced activity in both AR and MR rats. Group housing did not decrease the sensitivity of AR rats to the locomotor effects of either amphetamine or prazosin. Differences in activity between rats in standard and complex housing conditions were not altered by drug treatments. These findings indicate that pre-weaning social experience alters the responsiveness of the noradrenergic system to drug challenges, whereas post-weaning housing experience may not, even though ongoing activity is affected. Increased activity and sensitivity to amphetamine resulting from social isolation in early life may be mediated by changes in noradrenergic α-receptor mediated neurotransmission.

GREATER CORTICAL THICKNESS IN PATIENTS WITH FIRST-EPISODE SCHIZOPHRENIA WHO ARE RESPONSIVE TO ATYPICAL ANTIPSYCHOTICS

Dopamine systems can be altered by experiences such as early life adversity. The intensity of these effects seems to vary as a function of interactions between genetic and environmental influences. In a series of experiments we have investigated the effects of genetic variants and early life adversity on several biobehavioral outcomes. Here we investigated the presence of single nucleotide polymorphisms (SNPs) in the gene coding for dopamine D2 receptors (DRD2) and the interaction between these variants with early life adversity on the expression of D2 receptors in the striatum. Time-mated pregnant female rats underwent restraint stress (gestational days 10–21) or were left undisturbed. Following parturition rat pups were maternally reared (MR) or artificially reared (AR). Subsequent to adult behavioral testing, rats were genotyped and their brains were processed (autoradiography) for D2 receptor expression. We found three variants in the DRD2 gene and these variants interacted with early adversity to affect D2 receptor expression in the nucleus accumbens. Specifically, artificially reared rats with AG DRD2 variant showed significantly higher D2 expression compared to mother reared rats with the AG DRD2 variant as well as the artificially reared rats with a GG DRD2 variant. These findings show that adult D2 expression is significantly influenced by the interaction of DRD2 SNPs and early developmental factors. These finding may explain why there are significant individual differences in the impact of early life adversity on dopamine-dependent processes and disorder vulnerabilities.

The effects of early isolation on sexual behavior and c‐fos expression in naïve male long‐evans rats

Previous findings have demonstrated that the maternal environment is important for the development of male sexual behavior. The present study examined the effects of complete early life isolation and replacement 'stroking' stimulation on male sexual behavior and neural activation as seen by Fos immunoreactivity (Fos-IR). Animals were either artificially reared (AR) with minimal (AR-MIN) or maximal (AR-MAX) body simulation, or maternally reared (MR). In adulthood, animals were either given an exposure to an estrous female (EXP) or left undisturbed (NoEXP). No significant effects of early development were found in sexual behavior; however differences in activation in response to this exposure were observed. AR-MIN animals showed lower Fos-IR in the medial preoptic area and the ventromedial hypothalamus compared to MR animals. AR-MAX animals were not significantly different from either condition. These findings demonstrate that although there are no differences in the quality of the first copulatory exposure between AR and MR animals, the brain's response to this exposure differs in sites within the brain that subserve sexual behavior.

Artificial rearing alters the response of rats to natural and drug-mediated rewards

Artificial rearing (AR) of infant rats permits precise control over key features of the early environment without maternal influence. The present study examined the behavioral response of AR rats towards natural and drug-mediated rewards, as well as their exploratory and affective behaviors. Adolescent AR rats showed increased preference for sucrose consumption relative to chow and demonstrated greater activity in the open field and in the elevated plus-maze compared to maternally reared (MR) rats. With respect to measures of emotionality, AR rats showed enhanced avoidance of the open arms of the plus-maze, indicating increased anxiety, but they did not differ from MR rats in exploring the center of the open field. Adult AR rats displayed a stronger conditioned response to morphine in a place preference test. These findings support the potential of the AR model to contribute to understanding the role of early experience in the development of behavioral motivation.

Thymic stress in artificially reared and maternally reared rat pups

It is well documented that the hypothalamic-pituitary-adrenal (HPA) axis influences immunological responses to stress. Maternal factors have been shown to be necessary for appropriate modulation of the HPA axis in the developing rat. The purpose of this study was to determine whether artificially reared (AR) infant rat pups (a procedure whereby infant rats are gastrostomized and reared independently of maternal factors) have an altered function of the HPA axis in terms of thymocyte apoptosis (programmed cell death) and other indices of thymic stress. AR and maternally reared (MR) Long–Evans rat pups were randomized to control, fasted, stressed, and fasted+stressed treatment groups, as well as an unhandled, MR naive group that served as a baseline control. AR rat pups were significantly heavier than MR ( p<0.001). AR rat pups had significantly lighter thymuses than did the MR pups ( p<0.001) and fasted pups had significantly lighter thymuses than unfasted pups, regardless of whether they were in the AR or MR condition ( p<0.005). AR pups had significantly lower thymic cell numbers and a greater percent of necrotic cells than did MR pups. There were no significant effects of rearing condition on the percent of apoptotic thymocytes. The thymocyte alterations observed in this study between the two rearing conditions suggest that AR reduces thymic weight and cell numbers, which may have consequences for the development of adult cellular immunity.