Effects of motherless rearing on basal and stress-induced corticosterone secretion in rat pups

Role of tactile stimulation during the preweaning period on the development of the peripheral sensory sural (SU) nerve in adult artificially reared female rat

Maternal deprivation, as a result of the artificial rearing (AR) paradigm, disturbs electrophysiological and histological characteristics of the peripheral sensory sural (SU) nerve of infant and adult male rats. Such changes are prevented by providing tactile or social stimulation during isolation. AR also affects the female rat's brain and behavior; however, it is unknown whether this early adverse experience also alters their SU nerve development or if tactile stimulation might prevent these possible developmental effects. To assess these possibilities, the electrophysiological and histological characteristics of the SU nerve from adult diestrus AR female rats that: (i) received no tactile stimulation (AR group), (ii) received tactile stimulation in the anogenital and body area (AR-Tactile group), or (iii) were mother reared (MR group) were determined. We found that the amplitude, but not the area, of the evoked compound action potential response in SU nerves of AR rats was lower than those of SU nerves of MR female rats. Tactile stimulation prevented these effects. Additionally, we found a reduction in the outer diameter and myelin thickness of axons, as well as a large proportion of axons with low myelin thickness in nerves of AR rats compared to the nerves of the MR and AR-Tactile groups of rats; however, tactile stimulation only partially prevented these effects. Our data indicate that maternal deprivation disturbs the development of sensory SU nerves in female rats, whereas tactile stimulation partially prevents the changes generated by AR. Considering that our previous studies have shown more severe effects of AR on male SU nerve development, we suggest that sex-associated factors may be involved in these processes.

Disentangling developmental effects of play aspects in rat rough-and-tumble play

Animal play encompasses a variety of aspects, with kinematic and social aspects being particularly prevalent in mammalian play behaviour. While the developmental effects of play have been increasingly documented in recent decades, understanding the specific contributions of different play aspects remains crucial to understand the function and evolutionary benefit of animal play. In our study, developing male rats were exposed to rough-and-tumble play selectively reduced in either the kinematic or the social aspect. We then assessed the developmental effects of reduced play on their appraisal of standardized human-rat play ('tickling') by examining their emission of 50 kHz ultrasonic vocalizations (USVs). Using a deep learning framework, we efficiently classified five subtypes of these USVs across six behavioural states. Our results revealed that rats lacking the kinematic aspect in play emitted fewer USVs during tactile contacts by human and generally produced fewer USVs of positive valence compared with control rats. Rats lacking the social aspect did not differ from the control and the kinematically reduced group. These results indicate aspects of play have different developmental effects, underscoring the need for researchers to further disentangle how each aspect affects animals.

Tactile stimulation prevents disruptions in male rat copulatory behavior induced by artificial rearing

Early life social interactions in gregarious mammals provide an important source of stimulation required for the development of species-typical behaviors. In the present study, complete deprivation of maternal and littermate contact through artificial rearing was used to examine the role of early social stimulation on copulatory behavior and the ejaculate in adult rats. We found that artificially reared naïve male rats were sexually motivated; nevertheless, they did not acquire the level of sexual experience that typically occurs during copulatory training. Disrupted expression of sexual experience of artificially reared rats was demonstrated by an inconsistent pattern of ejaculatory behavior across training tests. Artificial tactile stimulation applied during isolation prevented this disruption and rats achieved ejaculation in most copulatory tests. Despite the irregularity of ejaculatory behavior in isolated rats, their sperm count and seminal plug were similar to control maternally reared (sexually experienced) and artificially-reared rats that received tactile stimulation. These results suggest that tactile sensory information provided by the mother and/or littermates to the offspring is crucial for the development of copulatory behavior. The absence of social and/or tactile stimulation during early life compromises the ability of male rats to gain sexual experience in adulthood.

The link between brain acidosis, breathing and seizures: a novel mechanism of action for the ketogenic diet in a model of infantile spasms

Infantile spasms (IS) syndrome is a catastrophic, epileptic encephalopathy of infancy that is often refractory to current antiepileptic therapies. The ketogenic diet (KD) has emerged as an alternative treatment for patients with medically intractable epilepsy, though the prospective validity and mechanism of action for IS remains largely unexplored. We investigated the KD's efficacy as well as its mechanism of action in a rodent model of intractable IS. The spasms were induced using the triple-hit paradigm and the animals were then artificially reared and put on either the KD (4:1 fats: carbohydrate + protein) or a control milk diet (CM; 1.7:1). ³¹Phosphorus magnetic resonance spectroscopy (³¹P MRS) and head-out plethysmography were examined in conjunction with continuous video-EEG behavioural recordings in lesioned animals and sham-operated controls. The KD resulted in a peripheral ketosis observed both in the blood and urine. The KD led to a robust reduction in the frequency of spasms observed, with approximately a 1.5-fold increase in the rate of survival. Intriguingly, the KD resulted in an intracerebral acidosis as measured with ³¹P MRS. In addition, the respiratory profile of the lesioned rats on the KD was significantly altered with slower, deeper and longer breathing, resulting in decreased levels of expired CO₂. Sodium bicarbonate supplementation, acting as a pH buffer, partially reversed the KD's protective effects on spasm frequency. There were no differences in the mitochondrial respiratory profiles in the liver and brain frontal cortex measured between the groups, supporting the notion that the effects of the KD on breathing are not entirely due to changes in intermediary metabolism. Together, our results indicate that the KD produces its anticonvulsant effects through changes in respiration leading to intracerebral acidosis. These findings provide a novel understanding of the mechanisms underlying the anti-seizure effects of the KD in IS. Further research is required to determine whether the effects of the KD on breathing and intracerebral acid-base balance are seen in other paediatric models of epilepsy.

The effects of multiple early life stressors on adolescent alcohol consumption

Alcohol use disorder is pervasive and effects the health of millions. Identifying factors such as early life stress that contribute to the development of alcohol use disorder is therefore critical, especially those that contribute to adolescent drinking, a strong predictor of AUD development. The majority of prior studies have examined early life effects on adult drinking, but have not studied intake during adolescence, and no prior studies have examined how the effects of multiple stressors may be additive. Therefore, this study determined if experiencing individual or multiple stressors increases adolescent alcohol intake. Male Long Evans rats underwent either early or late maternal separation (postnatal day 2-9 or 13-20), early adolescent social defeat (PND 30-40), both, or neither. All rats were then given two-hour access to alcohol, and voluntary intake assessed daily in late adolescence (PND 41-51). In adulthood, sensitivity to alcohol's sedative effects was assessed using loss and regain of righting reflex tests. Results indicate that experiencing maternal separation (at either time point) or social defeat increased adolescent alcohol consumption, but experiencing the combined stressors did not, and that no stressor significantly affected body weight during adolescence or loss and regain of righting reflex in adulthood. Overall, this pattern of effects suggests that experiencing any individual early life stressor may increase adolescent alcohol intake, in agreement with prior literature, but that the combined effects of multiple early life stressors may be more complicated.

Resilience priming: Translational models for understanding resiliency and adaptation to early life adversity

Despite the increasing attention to early life adversity and its long-term consequences on health, behavior, and the etiology of neurodevelopmental disorders, our understanding of the adaptations and interventions that promote resiliency and rescue against such insults are underexplored. Specifically, investigations of the perinatal period often focus on negative events/outcomes. In contrast, positive experiences (i.e. enrichment/parental care//healthy nutrition) favorably influence development of the nervous and endocrine systems. Moreover, some stressors result in adaptations and demonstrations of later-life resiliency. This review explores the underlying mechanisms of neuroplasticity that follow some of these early life experiences and translates them into ideas for interventions in pediatric settings. The emerging role of the gut microbiome in mediating stress susceptibility is also discussed. Since many negative outcomes of early experiences are known, it is time to identify mechanisms and mediators that promote resiliency against them. These range from enrichment, quality parental care, dietary interventions and those that target the gut microbiota.

Deconstructing the function of maternal stimulation in offspring development: Insights from the artificial rearing model in rats

This article is part of a Special Issue on "Parental Care". Maternal behavior has an important function in stimulating adequate growth and development of the young. Several approaches have been used in primates and rodents to deconstruct and examine the influence of specific components of maternal stimulation on offspring development. These approaches include observational studies of typical mother-infant interactions and studies of the effects of intermittent or complete deprivation of maternal contact. In this review, we focus on one unique approach using rats that enables the complete control of maternal variables by means of rearing rat pups artificially without contact with the mother or litter, while maintaining stable nutrition, temperature and exposure to stressful stimuli. This artificial rearing model permits the removal and controlled replacement of relevant maternal and litter stimuli and has contributed valuable insights regarding the influence of these stimuli on various developmental outcomes. It also enables the analysis of factors implicated in social isolation itself and their long-term influence. We provide an overview of the effects of artificial rearing on behavior, physiology, and neurobiology, including the influence of replacing maternal tactile stimulation and littermate contact on these outcomes. We then discuss the relevance of these effects in terms of the maternal role in regulating different aspects of offspring development and implications for human research. We emphasize that artificial rearing of rats does not lead to a global insult of nervous system development, making this paradigm useful in investigating specific developmental effects associated with maternal stimulation.