Puberty and Adolescence in a Lifespan Context

The final chapter zooms out to explore some bigger picture questions about puberty and adolescence that remain unanswered; the authors provide their perspectives on these larger issues. For instance, how do we define the end of adolescence, and how do we know when adulthood has been achieved? Are cultural expectations partially responsible for the protracted nature of human adolescent brain development? How does the timing of puberty (early vs. late bloomers) influence psychosocial development and risk of psychopathology in females and males? What are the potential consequences of medically suspending puberty onset in gender dysphoric youth? Is adolescence an experience-expectant (a specific type of experience is absolutely required for normal development) or experience-dependent (specific experiences influence developmental trajectory) developmental period? Finally, the limitations of animal models for understanding human adolescent development are discussed and experimental approaches for future research are recommended.

Introduction

Chapter 1 provides the context and conceptual framework for the authors’ approach to thinking about the science of puberty and adolescence. The overarching principle is that the transition from childhood to adulthood that occurs during puberty and adolescence involves complex and iterative interactions between the developing brain, hormones, and experience. This chapter first introduces the concepts of puberty and adolescence and discusses how they are separate, yet intricately linked, developmental processes. Examples of how puberty, defined as reproductive maturation, can be dissociated from adolescence, defined as maturation of the cognitive, emotional, and social behaviors associated with adulthood, are discussed. Other examples highlight the recurring interactions between the brain, pubertal hormones, and experience that ultimately result in an adult individual. The chapter then traces the evolution and growth of research on puberty and adolescence during the last half of the 20th century, which started with puberty being studied mainly by endocrinologists and adolescence being studied mainly by psychologists, and progressed to both puberty and adolescence becoming a focus for basic research conducted by psychobiologists and developmental neurobiologists. The advent of magnetic resonance imaging made possible imaging of the human brain in healthy adolescents; this methodological advance led to new knowledge of the scope and timing of adolescent brain development and how it is shaped by pubertal hormones.

Drugs and the Adolescent Brain

Chapter 8 highlights some of the ways in which drugs can influence the adolescent brain and its development. Drug use typically begins during adolescence, in part because of increased risk-taking and reward sensitivity that characterizes adolescence. Adolescent drug use is concerning because the developing brain is often more vulnerable to many of the adverse consequences of drug use, and it is a predictor of drug abuse in adulthood. The chapter discusses how the commonly used drugs alcohol, nicotine, and marijuana acutely affect the brain and behavior differently during adolescence and in adulthood, as well as the longer-term effects on brain structure, cognitive function, and motivation that adolescent drug use may have. Research on the effects of anabolic androgenic steroids on aggressive behavior and of prescription drugs used to treat depression and ADHD during adolescence is also discussed.

Stress and the Adolescent Brain

Chapter 7 considers stress as a modulator of adolescent development. It starts with an overview of the key hormones in the hypothalamic–pituitary–adrenal (HPA) axis and describes responses of the HPA axis and the sympathetic nervous system to stress. The HPA stress response is somewhat different in adolescents compared with adults; adolescents often show heightened stress reactivity and a protracted recovery period after psychological stressors compared to adults. The chapter then reviews research on chronic stress-induced anatomical and functional changes in the hippocampus, prefrontal cortex, and amygdala, three brain regions involved in regulation of the HPA axis and modulation of stress responses. Stress-induced changes in these brain regions include dendritic complexity of pyramidal cells, attenuated long-term potentiation, attention deficits, and changes in fear and depressive-like behaviors; these changes may be long-lasting. The perfect storm alludes to the alignment of three features of adolescence that together may render the adolescent brain especially vulnerable to effects of chronic stress: (a) The quality and quantity of stressors is different during adolescence than in adulthood; (b) stress reactivity is higher during adolescence; and (c) the hippocampus, prefrontal cortex, and amygdala are sensitive to stress hormones and are still developing during adolescence. However, the developing adolescent brain may be more resilient to insult, more responsive to interventions, and more buffered by social support systems.

Executive Function

Chapter 5 focuses on adolescent maturation of cognitive abilities and executive function—the capacity to control and coordinate thoughts and behavior. Executive function emerges from interactions among three major brain regions: the prefrontal cortex (behavioral modulation), amygdala (emotional valence), and ventral striatum (motivation and reward). The triadic model provides a conceptual framework for understanding the neural basis for higher risk-taking by adolescents. This model proposes that adolescent maturation of prefrontal cortex, striatum, and amygdala occurs along different time frames, with the striatum and amygdala maturing sooner than the prefrontal cortex. Thus, early in adolescence, decisions and behaviors are more heavily influenced by rewards and emotions in the face of relative lack of prefrontal control. As the prefrontal cortex matures during late adolescence, decisions and behaviors become more guided by executive function. This chapter also discusses research on the importance of social context and peer pressure in decision-making by adolescents. Finally, the chapter discusses how research showing that prefrontal maturation is protracted (extending into the third decade of life) has influenced court decisions and shaped policy in the U.S. juvenile justice system.

Puberty, Hormones, and the Social Brain

This chapter begins with some history of the field of behavioral neuroendocrinology and traces the origins of the classic organizational-activational hypothesis to explain sexual differentiation of the brain and behavior and hormonal influences on sex-typical social behaviors. The classic hypothesis posits that testicular hormones masculinize and defeminize neural circuits during a perinatal sensitive period, programming sex-typical activational responses to gonadal hormones in adulthood. Research since the mid- to late 1980s shows that a second wave of hormone-dependent organization of the brain and behavior occurs during puberty and adolescence and that ovarian hormones are actively involved in feminization of the brain during the adolescent period of organization. Next, a conceptual framework is presented for studying adolescent development of social cognition (the mental processes by which an individual encodes, interprets, and responds to sensory information from an animal of the same species) in the context of social reorientation, when during adolescence the source of social reward shifts from family to peers. The chapter reviews the literature on what social behaviors and aspects of social cognition are organized by pubertal hormones in males, as well as the nonsocial behaviors that are organized by pubertal hormones in males and females.

Remodeling the Adolescent Brain

Chapter 3 covers the basic neural mechanisms by which the brain undergoes an extreme makeover during adolescence. It starts with the proposition that the nervous system has only so many tools in the toolbox to accomplish this makeover and these tools can be categorized as either progressive or regressive. Progressive tools include neurogenesis, migration, axon outgrowth, and synapse formation. Regressive tools include programmed cell death and experience-dependent synapse elimination. Two analogies are used to help readers understand this process: house remodeling and gardening. These analogies are woven into the concepts of progressive and regressive developmental events, and they can be imagined as mechanisms that result in either gain or loss of function (e.g., a house addition might equal new neurons or new projections) or maximize efficiency and success (pruning of seedlings might equal programmed cell death). Research on increased myelination during adolescence is also discussed.

Puberty

Chapter 2 provides an overview of the neural and endocrine mechanisms that govern the timing and onset of puberty (reproductive maturation). The cells and hormones that comprise the hypothalamic–pituitary–gonadal (HPG) axis are introduced, followed by an explanation of how both negative and positive neuroendocrine feedback loops regulate circulating levels of gonadal steroid hormones in males and females. The rest of the chapter is devoted to mechanisms that govern the timing of puberty and activation of the HPG axis at the onset of puberty. The role of the metabolic hormone leptin as a permissive signal for the timing of puberty, the role of neural excitation and disinhibition in the awakening of the gonadotropin-releasing hormone (GnRH) neurons at the onset of puberty, and the role of the neuropeptide kisspeptin as a proximal driver of HPG axis activation are highlighted. Finally, recent research on hierarchical gene networks that are ultimately responsible for the developmental unfolding of activation of GnRH neurons at puberty onset is reviewed.

Remodeling the Adolescent Brain

Chapter 4 focuses on research based primarily on imaging studies of the human adolescent brain. It highlights the extent of changes that occur, as well as the protracted nature of these changes, which take place over the second and third decades of life. Major themes of this chapter include (a) spatial and temporal differences in the adolescent development of particular brain regions; (b) puberty (i.e., gonadal hormones influence some, but not all, aspects of adolescent brain development); and (c) the timing and rate of developmental changes are critically important. Research on adolescent changes in white matter and connectivity is reviewed. The chapter also addresses aspects of adolescent development that are typical versus atypical, as well as the limitations of imaging approaches to understanding adolescent brain development.