Kalynn M. Schulz

Pubertal hormones modulate the addition of new cells to sexually dimorphic brain regions.

New cells, including neurons, arise in several brain regions during puberty in rats. Sex differences in pubertal addition of cells coincide with adult sexual dimorphisms: for each region, the sex that gains more cells during puberty has a larger volume in adulthood. Removing gonadal hormones before puberty eliminates these sex differences, indicating that gonadal steroids direct the addition of new cells during puberty to maintain and accentuate sexual dimorphisms in the adult brain.

Gonadal hormones masculinize and defeminize reproductive behaviors during puberty in the male Syrian hamster

Three experiments were conducted to test whether testicular hormones secreted during puberty masculinize and defeminize the expression of adult reproductive behavior. Experiment 1 tested the hypothesis that gonadal hormones during puberty masculinize behavioral responses to testosterone (T) in adulthood. Male hamsters were castrated either before puberty (noTduringP) or after puberty (TduringP). All males were implanted with a 2.5-mg T pellet 6 weeks following castration and tested once for masculine reproductive behavior 7 days after the onset of T replacement. TduringP males displayed significantly more mounts, intromissions, and ejaculations than noTduringP males. Experiment 2 tested the hypothesis that gonadal hormones during puberty defeminize behavioral responses to estrogen (EB) and progesterone (P). Eight weeks following castration, noTduringP and TduringP males were primed with EB and P and tested for lordosis behavior with a stud male. Behavioral responses of males were compared to that of ovariectomized (OVX) and hormone primed females. NoTduringP males and OVX females displayed significantly shorter lordosis latencies than TduringP males. Experiment 3 investigated whether prolonged T treatment or sexual experience could reverse the deficits in masculine behavior caused by the absence of T during puberty. Extending the T treatment from 7 to 17 days did not ameliorate the deficits in masculine behavior caused by absence of T during puberty. Similarly, when the level of sexual experience was increased from one to three tests, the deficits in masculine behavior persisted. These studies demonstrate that gonadal hormones during puberty further masculinize and defeminize neural circuits and behavioral responsiveness to steroid hormones in adulthood.

Pubertal hormones, the adolescent brain, and the maturation of social behaviors: Lessons from the Syrian hamster

Conventional wisdom holds that gonadal steroid hormones organize and sexually differentiate neural circuits perinatally, and at puberty they activate these circuits to facilitate expression of social behaviors. Using the Syrian hamster to study the role of pubertal hormones in behavioral maturation, we have found that pubertal hormones also organize the adolescent brain. Initial studies revealed that male reproductive behavior cannot be activated by gonadal steroids prepubertally, indicating that the brain acquires behavioral responsiveness during adolescence. Subsequent experiments demonstrated that the presence of gonadal hormones during adolescence masculinize and defeminize behavioral responses of males to hormones in adulthood. Preliminary data also suggest that ovarian hormones defeminize but do not masculinize behavioral responses of females to hormones in adulthood. Furthermore, pubertal hormones program the adult expression of agonistic behaviors that are both steroid-dependent and steroid-independent in adulthood. Thus, the interaction between pubertal hormones and the adolescent brain is key for the maturation of adult social behaviors, and perturbations in the timing of this interaction have long-lasting consequences on adult behavior.

Adolescents and androgens, receptors and rewards

Adolescence is associated with increases in pleasure-seeking behaviors, which, in turn, are shaped by the pubertal activation of the hypothalamo-pituitary-gonadal axis. In animal models of naturally rewarding behaviors, such as sex, testicular androgens contribute to the development and expression of the behavior in males. To effect behavioral maturation, the brain undergoes significant remodeling during adolescence, and many of the changes are likewise sensitive to androgens, presumably acting through androgen receptors (AR). Given the delicate interaction of gonadal hormones and brain development, it is no surprise that disruption of hormone levels during this sensitive period significantly alters adolescent and adult behaviors. In male hamsters, exposure to testosterone during adolescence is required for normal expression of adult sexual behavior. Males deprived of androgens during puberty display sustained deficits in mating. Conversely, androgens alone are not sufficient to induce mating in prepubertal males, even though brain AR are present before puberty. In this context, wide-spread use of anabolic-androgenic steroids (AAS) during adolescence is a significant concern. AAS abuse has the potential to alter both the timing and the levels of androgens in adolescent males. In hamsters, adolescent AAS exposure increases aggression, and causes lasting changes in neurotransmitter systems. In addition, AAS are themselves reinforcing, as demonstrated by self-administration of testosterone and other AAS. However, recent evidence suggests that the reinforcing effects of androgens may not require classical AR. Therefore, further examination of interactions between androgens and rewarding behaviors in the adolescent brain is required for a better understanding of AAS abuse.

Puberty: A Finishing School for Male Social Behavior

Abstract: The classical view of steroid‐dependent organization of brain and behavior holds that gonadal steroid hormones, acting during an early critical period of development, cause permanent structural changes in neural circuits that determine behavioral responses to hormones in adulthood. This classical view has been modified to incorporate evidence that organizational effects of steroids can occur outside of the established perinatal critical period and that multiple critical periods may exist during development. Experiments in this laboratory indicate that steroid‐dependent organization of neural circuits underlying male social behaviors occurs during puberty. This work shows that adult‐typical reproductive and flank marking behaviors cannot be activated by gonadal steroids in male Syrian hamsters prior to puberty, suggesting that developmentally timed processes during puberty render the nervous system responsive to activating effects of gonadal steroids in adulthood. Additional experiments demonstrate that the presence or absence of gonadal hormones during puberty is a major factor in the ability of steroids to activate reproductive and flank marking behavior in adult male hamsters and in androgen receptor expression within the neural circuit underlying these behaviors. Thus, gonadal hormones during puberty appear to exert long‐lasting changes in neural circuits that are responsible for the programming of activational responses to steroids later in adulthood. A two‐stage model for maturation of male social behaviors is proposed: a perinatal critical period for sexual differentiation of neural circuits, followed by the pubertal period, during which gonadal steroids further organize the circuits to enhance behavioral responsiveness to hormones in adulthood. Whether puberty is a critical period for the proposed second wave of steroid‐dependent organization of behavioral circuits remains to be determined.

Testosterone Programs Adult Social Behavior before and during, But Not after, Adolescence

Whereas the adolescent brain is a major target for gonadal hormones, our understanding of hormonal influences on adolescent neural and behavioral development remains limited. These experiments investigated how variations in the timing of testosterone (T) exposure, relative to adolescence, alters the strength of steroid-sensitive neural circuits underlying social behavior in male Syrian hamsters. Experiment 1 simulated early, on-time, and late pubertal development by gonadectomizing males on postnatal d 10 and treating with SILASTIC brand T implants for 19 d before, during, or after adolescence. T treatment before or during, but not after, adolescence facilitated mating behavior in adulthood. In addition, preadolescent T treatments most effectively increased mating behavior overall, indicating that the timing of exposure to pubertal hormones contributes to individual differences in adult behavior. Experiment 2 examined the effects of preadolescent T treatment on behavior and brain regional volumes within the mating neural circuit of juvenile males (i.e. still preadolescent). Although preadolescent T treatment did not induce reproductive behavior in juvenile males, it did increase volumes of the bed nucleus of the stria terminalis, sexually dimorphic nucleus, posterodorsal medial amygdala, and posteroventral medial amygdala to adult-typical size. In contrast, juvenile anterodorsal medial amygdala and ventromedial hypothalamus volumes were not changed by preadolescent T treatment yet differed significantly in volume from adult controls, suggesting that further maturation of these brain regions during adolescence is required for the expression of male reproductive behavior. Thus, adolescent maturation of social behavior may involve both steroid-independent and -dependent processes, and adolescence marks the end of a postnatal period of sensitivity to steroid-dependent organization of the brain.

Maternal stress during pregnancy causes sex-specific alterations in offspring memory performance, social interactions, indices of anxiety, and body mass.

Prenatal stress (PS) impairs memory function; however, it is not clear whether PS-induced memory deficits are specific to spatial memory, or whether memory is more generally compromised by PS. Here we sought to distinguish between these possibilities by assessing spatial, recognition and contextual memory functions in PS and nonstressed (NS) rodents. We also measured anxiety-related and social behaviors to determine whether our unpredictable PS paradigm generates a behavioral phenotype comparable to previous studies. Female Sprague-Dawley rats were exposed to daily random stress during the last gestational week and behavior tested in adulthood. In males but not females, PS decreased memory for novel objects and novel spatial locations, and facilitated memory for novel object/context pairings. In the elevated zero maze, PS increased anxiety-related behavior only in females. Social behaviors also varied with sex and PS condition. Females showed more anogenital sniffing regardless of stress condition. In contrast, prenatal stress eliminated a male-biased sex difference in nonspecific bodily sniffing by decreasing sniffing in males, and increasing sniffing in females. Finally, PS males but not females gained significantly more weight across adulthood than did NS controls. In summary, these data indicate that PS differentially impacts males and females resulting in sex-specific adult behavioral and bodily phenotypes.

Testicular hormone exposure during adolescence organizes flank-marking behavior and vasopressin receptor binding in the lateral septum.

Adolescence is a period during which many social behaviors emerge. One such behavior, flank marking, is a testosterone-modulated scent marking behavior that communicates dominance status between adult male Syrian hamsters. Testosterone modulates flank-marking behavior by altering neural transmission of vasopressin within a forebrain circuit. This study tested whether testicular hormones secreted during adolescence play purely a transient activational role in the display of flank-marking behavior, or whether adolescent steroid hormone secretions also cause long-term organizational changes in vasopressin binding within brain regions underlying flank-marking behavior. We tested this hypothesis by manipulating whether testicular secretions were present during adolescent development and then tested for flank-marking behavior and vasopressin receptor binding within the flank-marking neural circuit in young adulthood. Specifically, males were gonadectomized immediately before or after adolescence, replaced with testosterone 6 weeks following gonadectomy in young adulthood, and behavior tested 1 week later. Adult testosterone treatment activated flank-marking behavior only in males that were exposed to testicular hormones during adolescence. In addition, males exposed to testicular hormones during adolescence exhibited significantly less vasopressin receptor binding within the lateral septum than males deprived of adolescent hormones, suggesting that hormone-dependent remodeling of synapses normally occurs in the lateral septum during adolescence. These data highlight the importance of gonadal steroid hormone exposure during adolescence for the organization of neural circuits and social behavior.

Pubertal testosterone organizes regional volume and neuronal number within the medial amygdala of adult male Syrian hamsters.

The medial amygdala plays a key role in regulating adult social behavior and undergoes structural changes during puberty that may be driven by gonadal hormone secretion during this developmental period. The current study sought to investigate potential organizational effects of testosterone during puberty, activational effects of testosterone in adulthood, and any interactions on regional volume and neuronal number of the medial amygdala. Male Syrian hamsters either did or did not experience endogenous testosterone during pubertal brain development, and then received either testosterone-filled or blank capsules during adulthood 2 weeks before tissue collection. The results show that pubertal testosterone has long-term organizational effects on volume of specific subregions of the medial amygdala such that the presence of pubertal testosterone resulted in 1) decreased volume of the anterior ventral amygdala and, to a lesser extent, the anterior dorsal medial amygdala; and 2) increased volume of the posterior dorsal medial amygdala. Both effects were independent of the presence of testosterone during adulthood. Pubertal testosterone also decreased neuronal number in the anterior dorsal medial amygdala, suggesting a possible mechanism by which pubertal testosterone decreases volume in this subregion. In addition, there was a significant interaction between pubertal and adult testosterone, such that testosterone in adulthood increased the number of neurons in the posterior ventral medial amygdala only in males that did not experience endogenous pubertal testosterone. In conclusion, pubertal testosterone organizes the medial amygdala in a subregion-specific manner, which may contribute to the maturation of adult-typical social behavior.

Medial preoptic area dopaminergic responses to female pheromones develop during puberty in the male Syrian hamster.

Chemosensory cues from receptive females do not elicit similar reactions before and after puberty in male hamsters. While pheromones facilitate a complex display of reproductive behavior in adults, prepubertal males do not engage in these same behaviors. Dopamine (DA) released from the medial preoptic area (MPOA) in response to a receptive female or her odors is an important component of the neural events underlying adult male rat sexual behavior. The current experiment investigated whether increased dopaminergic activity occurs in the adult male hamster MPOA in response to female pheromones, and if so, whether this response is absent in prepubertal males, which do not mate. Sexually nai;ve prepubertal and adult male hamsters were exposed to cotton swabs with or without pheromone from an estrous female for 0, 5, 15, or 25 min, after which brains were collected and frozen on dry ice. The MPOA was micropunched from frozen coronal sections (500 microm), and concentrations of DA and its primary metabolite DOPAC were determined by high-performance liquid chromatography-electrochemical detection. DOPAC was used as an index of dopaminergic activity. DOPAC levels significantly increased in adults after 15 min exposure to pheromone. In contrast, MPOA DOPAC concentrations did not increase in prepubertal males exposed to pheromone. These data demonstrate that the neural processing of sexually relevant chemosensory stimuli matures during puberty. The absence of a DA response to female pheromones prior to puberty may contribute to the inability of prepubertal males to display reproductive behavior.