Parental enrichment and offspring development: Modifications to brain, behavior and the epigenome

Environmental enrichment in female rodents: considerations in the effects on behavior and biochemical markers

Environmental enrichment (EE) exposes laboratory animals to novelty and complexity through alterations in the physical and social environment, which lead to enhanced sensory, cognitive and physical stimulation. Housing rodents in an EE is a highly recommended practice by governing bodies regulating animal welfare due to a growing body of evidence suggesting its benefits on rodents' wellbeing and the more naturalistic environment that such housing conditions provide. However, most paradigms and hypotheses rely on information currently available from studies performed on male subjects and the information regarding the effects of EE on female rodents' behavior and physiology is limited. Given the variety of EE paradigms described, it is increasingly difficult to ascertain the benefits or possible consequences of enriched housing strategies in females, let alone aid at establishing standardized environments in rodents. This review evaluates the female rodent literature that has examined the outcome of EE on behavior and neurochemistry and aims at identifying key elements to be addressed by future studies. Specifically, results from cognitive behavioral tests as well as commonly used tests of emotionality will be discussed, while also evaluating their relation to changes in neurochemistry and hormones brought on by various EE paradigms. Lastly, the impact of maternal enrichment on both offspring and maternal behavior and physiology will be reviewed.

Bi-parental care contributes to sexually dimorphic neural cell genesis in the adult mammalian brain

Early life events can modulate brain development to produce persistent physiological and behavioural phenotypes that are transmissible across generations. However, whether neural precursor cells are altered by early life events, to produce persistent and transmissible behavioural changes, is unknown. Here, we show that bi-parental care, in early life, increases neural cell genesis in the adult rodent brain in a sexually dimorphic manner. Bi-parentally raised male mice display enhanced adult dentate gyrus neurogenesis, which improves hippocampal neurogenesis-dependent learning and memory. Female mice display enhanced adult white matter oligodendrocyte production, which increases proficiency in bilateral motor coordination and preference for social investigation. Surprisingly, single parent-raised male and female offspring, whose fathers and mothers received bi-parental care, respectively, display a similar enhancement in adult neural cell genesis and phenotypic behaviour. Therefore, neural plasticity and behavioural effects due to bi-parental care persist throughout life and are transmitted to the next generation.

Paternal stress prior to conception alters DNA methylation and behaviour of developing rat offspring

Although there has been an abundance of research focused on offspring outcomes associated with maternal experiences, there has been limited examination of the relationship between paternal experiences and offspring brain development. As spermatogenesis is a continuous process, experiences that have the ability to alter epigenetic regulation in fathers may actually change developmental trajectories of offspring. The purpose of this study was to examine the effects of paternal stress prior to conception on behaviour and the epigenome of both male and female developing rat offspring. Male Long-Evans rats were stressed for 27 consecutive days and then mated with control female rats. Early behaviour was tested in offspring using the negative geotaxis task and the open field. At P21 offspring were sacrificed and global DNA methylation levels in the hippocampus and frontal cortex were analysed. Paternal stress prior to conception altered behaviour of all offspring on the negative geotaxis task, delaying acquisition of the task. In addition, male offspring demonstrated a reduction in stress reactivity in the open field paradigm spending more time than expected in the centre of the open field. Paternal stress also altered DNA methylation patterns in offspring at P21, global methylation was reduced in the frontal cortex of female offspring, but increased in the hippocampus of both male and female offspring. The results from this study clearly demonstrate that paternal stress during spermatogenesis can influence offspring behaviour and DNA methylation patterns, and these affects occur in a sex-dependent manner. Development takes place in the centre of a complex interaction between maternal, paternal, and environmental influences, which combine to produce the various phenotypes and individual differences that we perceive.

Intergenerational health responses to adverse and enriched environments

Health consequences of relative or absolute poverty constitute a definitive area of study in social medicine. As demonstrated in the extreme example of the Dutch Hunger Winter from 1944 to 1945, prenatal hunger can lead to adult schizophrenia and depression. A Norwegian study showed how childhood poverty resulted in a heightened risk of myocardial infarction in adulthood. In England, a study of extended impaired prenatal nutrition indicated three different types of increased cardiovascular risk at older ages. Current animal and human studies link both adverse and enriched environmental exposures to intergenerational transmission. We do not fully understand the molecular mechanisms for it; however, studies that follow up epigenetic marks within a generation combined with exploration of gametic epigenetic inheritance may help explain the prevalence of certain conditions such as cardiovascular disease, schizophrenia, and alcoholism, which have complex etiologies. Insights from these studies will be of great public health importance.

Brain plasticity in the developing brain

The developing normal brain shows a remarkable capacity for plastic change in response to a wide range of experiences including sensory and motor experience, psychoactive drugs, parent-child relationships, peer relationships, stress, gonadal hormones, intestinal flora, diet, and injury. The effects of injury vary with the precise age-at-injury, with the general result being that injury during cell migration and neuronal maturation has a poor functional outcome, whereas similar injury during synaptogenesis has a far better outcome. A variety of factors influence functional outcome including the nature of the behavior in question and the age at behavioral assessment as well as pre- and postinjury experiences. Here, we review the phases of brain development, how factors influence brain, and behavioral development in both the normal and perturbed brain, and propose mechanisms that may underlie these effects.

Humans, fruit flies, and automatons

My response is divided into four sections: (1) is devoted to a potpourri of commentaries that are essentially in agreement with the substance of my target article (with one exception); in (2) I address, in response to one of the commentaries, several issues relating to the use of candidate gene association studies in behavior genetics (in particular those proposing a specific G×E interaction); in (3) I provide a detailed response to several defenses of the twin study methodology; and in (4) I conclude with several reflections on that methodology and the conception of human nature it has fostered.

Experience and the developing prefrontal cortex

The prefrontal cortex (PFC) receives input from all other cortical regions and functions to plan and direct motor, cognitive, affective, and social behavior across time. It has a prolonged development, which allows the acquisition of complex cognitive abilities through experience but makes it susceptible to factors that can lead to abnormal functioning, which is often manifested in neuropsychiatric disorders. When the PFC is exposed to different environmental events during development, such as sensory stimuli, stress, drugs, hormones, and social experiences (including both parental and peer interactions), the developing PFC may develop in different ways. The goal of the current review is to illustrate how the circuitry of the developing PFC can be sculpted by a wide range of pre- and postnatal factors. We begin with an overview of prefrontal functioning and development, and we conclude with a consideration of how early experiences influence prefrontal development and behavior.

Dynamic DNA methylation: a prime candidate for genomic metaplasticity and behavioral adaptation

DNA methylation was once considered to be a static epigenetic modification whose primary function was restricted to directing the development of cellular phenotype. However, it is now evident that the methylome is dynamically regulated across the lifespan: during development as a putative mechanism by which early experience leaves a lasting signature on the genome and during adulthood as a function of behavioral adaptation. Here, we propose that experience-dependent variations in DNA methylation, particularly within the context of learning and memory, represent a form of genomic metaplasticity that serves to prime the transcriptional response to later learning-related stimuli and neuronal reactivation.