Sexual Dimorphism in Environmental Exposures and Behavior Bernard Weiss, Ph.D. Department of Environmental Medicine and Environmental Health Sciences Center University of Rochester School of Medicine and Dentistry Rochester, New York

Gonadal Development and Testosterone  The Y chromosome determines testicular development  The SRY gene on Y chromosome encodes testis-determinating factor (TDF)  The fetal testes secrete testosterone, which promotes growth of internal and external sex organs  Ovaries do not make steroids until after birth

Sexual Differentiation of the Brain The undifferentiated brain is basically female, the default structure Fetal testosterone transforms it into the male structure via estradiol The transformation is governed by aromatase (T  E) Alpha-fetoprotein prevents masculinization of females Undifferentiated brain Masculine brain Female brain Fetal testosterone Estradiol aromatase

Sex Differences in Brain Structure  Male brains are more asymmetrical (lateralized)  Several structures show differences in size  The sexually dimorphic nucleus of the preoptic area is larger in males  The anteroventral periventricular nucleus is larger in females  Male brains, corrected for height, are 2-3% larger in males  Female brains have a higher percentage of gray matter, male brains a higher percentage of white matter.

Sex Differences in Gray Matter, White Matter, and Cerebrospinal Fluid (Gur et al, 1999)

Male-Female Performance Differences (Kimura, 1992) Males betterFemales better

Hormonal Determinants of Neurobehavioral Development in Animals  Sexual differentiation of the brain  Sexual behavior  Courtship, mating, motivation  Maternal behavior  Aggressive and attack behaviors  Sensory-motor function  Stress responses  Cognitive function  Learning and performance  Play behavior

EXAMPLES OF ENDOCRINE-DISRUPTING CHEMICALS Phthalates (plasticizers): testicular poisons Alkylphenols (detergents): estrogenic properties Bisphenol A (coatings): estrogenic properties Organochlorine pesticides: estrogenic properties PCBs: thyroid hormone dysfunction PCDDs: anti-androgenic properties Vinclozolin (fungicide): anti-androgen Phytoestrogens (plants): estrogenic properties

Characteristics of Endocrine Disrupters Structurally diverse group Resist degradation Tend to accumulate in fatty tissues Affect developing organisms at low doses Behavior is a sensitive endpoint

Dioxins and Related Compounds

2,3,7,8 TCDD: the Prototypical Disrupter Binds to the intracellular Ah receptor Impairs copulatory behavior in males Impairs genital development in females Modifies operant behavior Alters sex differences in other behaviors

Wheel running activity in the female rat Rodents run spontaneously in wheels Gross output correlated to stages of the estrous cycle Rising levels of circulating estrogen associated with increased running

Reinforcing Value of Access to Running Wheels (Motivation to Run) Method: single maternal dose of 0, 20, 60, or 180 ng/kg TCDD on Gestational Day 18 mean human body burden of dioxins is 13.5 ng/kg female offspring began training at 5 months examined with a series of fixed-ratio schedules of access to running wheels

Earned Running Periods in Female Rats Exposed to Dioxin on GD18 (Markowski et al, 2001)

Benchmark Dose Plot for Fixed-Ratio Access to Running Wheel (Markowski et al, 2001)

Chamber for Schedule-Controlled Operant Behavior

Schedules of Reinforcement, Las Vegas Style

Sex Differences in Fixed-Ratio Performance in Rats Exposed Prenatally to Dioxin (Hojo et al, 2002)

Male-Female Contrasts in Fixed Ratio Response Rate in Rats Exposed to TCDD on GD8

Quadratic Fit and Benchmark Dose for Male-Female Differences on Fixed Ratio Response Rates in Rats Exposed to TCDD on GD8

Endocrine Modulation of Cerebral Lateralization Male Rats –Right cerebral cortex thicker than left (48/49 areas) –Cell counts are higher on right –More estrogen receptors in left neonate cortex –Testes removal on PND1 reverses laterality Female Rats –Left cerebral cortex thicker than right (45/63 areas) –Cell counts higher on left –More estrogen receptors in right neonate cortex –After ovariectomy at birth, left thicker than right in only 2/7 areas

Scheme for Measurement of Cortical Thickness

Measurement of Cortical Thickness for Lateralization Index

Cortical Lateralization (Right-Left) in Female Offspring Exposed to 180 ng/kg TCDD on GD8

Radial Maze Diagram

Radial Maze Errors in Dioxin-Exposed Male and Female Offspring (Seo et al, 2000) Males Females

Sample items on Pre-School Activity Inventory (Vreugdenhil et al, 2002) Toy choices –Guns –Dolls Activities –Playing house –Sports and ball games Behavioral characteristics –Likes pretty thing –Enjoys rough and tumble play

Regression Coefficients for Play Behavior versus PCBs in Cord Blood (Vreugdenhill et al, 2002) BoysGirls Composite Masculine Feminine

Implications of Dioxin’s Neurobehavioral Effects Sexually dimorphic behaviors are sensitive to doses in the range of current human levels Exposures in utero may produce global, permanent alterations in behavior or transient, domain-specific changes Behavioral changes reflect altered patterns of brain development

Is Prenatal Dioxin a Risk Factor for Depression? Impaired serotonin function is the clearest neurochemical risk –SSRIs are the major drug treatments for depression Serotonin is a prime modulator of brain development In adult rodents dioxin disrupts serotonin function –Brain turnover of serotonin is increased –Tryptophan, the amino acid precursor, is increased in brain, plasma Trytophan crosses the placenta, alters fetal brain serotonin Fetal dioxin exposure in mice reduces serotonin neuron numbers

A Rat Model of Depression (Males Only)