Neuroscience: Exploring the Brain, 3e Chapter 17: Sex and the Brain

Introduction Neural machinery underlying reproduction Reproduction and eating Regulated by subcortical structures Conscious control by cerebral cortex Influence of brain on sex hormones Sexual and reproductive behaviors Male and female brains

Sex and Gender Concept of Gender Biological characteristics and qualities Gender-specific behaviors Self-assessment Societal expectations Genetics (XX: female, XY: male) Hormones Gender-identity Perception of gender

Sex and Gender The Genetics of Sex (Cont’d) Discovery of SRY Location of SRY on Y chromosome Encodes testis-determining factor Causes development of testes and testicular hormones Makes fetus develop as male Default pathway, female Exposure to testosterone masculinizes the brain

Sex and Gender Differentiation of fetus and development of gonads

The Hormonal Control of Sex Hormones regulate physiological processes Sex hormones: Steroids Endocrine glands: Release sex hormones Pituitary gland: Regulates endocrine glands Structure affects function of hormone Male hormone: testosterone Female hormone: estradiol

The Hormonal Control of Sex The Principle Male and Female Hormones Men: High concentration of androgens Women: High concentration of estrogens Testosterone (androgen) + aromatase  estradiol (estrogen) Protein hormones- do not pass through cell membranes, bind surface receptors Steroid hormones- pass through cell membrane, bind cytoplasmic receptors Receptor concentrations vary in different brain regions

The Hormonal Control of Sex Concentration of estradiol receptors in sagittal section of rat brain

The Hormonal Control of Sex The Principle Male and Female Hormones (Cont’d) Males: Testes- release androgen Testosterone – increase at puberty leads to development of secondary sex characteristics Females: Ovaries- secrete estradiol (estrogen) and progesterone (progestin) Blood concentrations of sex hormones vary Males- levels fluctuate daily Females- levels fluctuate, 28-day cycle

The Hormonal Control of Sex Pituitary and Hypothalamus Control of Sex Hormones Gonadotropins: LH and FSH Males- LH produces testosterone; FSH aids sperm maturation Females- LH, FSH cause estrogen secretion Menstrual cycle- Follicular phase, Luteal phase Estrous cycle in nonprimates

The Neural Basis of Sexual Behaviors Reproductive Organs and Their Control Cerebral cortex Neural control of sexual response Spinal cord Mediates sexual response of genitals

The Neural Basis of Sexual Behaviors Neural control of human sex organs

The Neural Basis of Sexual Behaviors Mating Strategies: Ecology Polygyny Male mates with many females Polyandry Female mates with many males Monogamy One mate

The Neural Basis of Sexual Behaviors The Neurochemistry of Reproductive Behavior Prairie voles: Solid ‘family values’; monogamous Meadow voles: Asocial and promiscuous Affected by oxytocin and vasopressin

The Neural Basis of Sexual Behaviors Role of oxytocin and vasopressin receptors in reproductive behavior Prairie voles more oxytocin (female) and vasopressin male receptors Meadow voles, fewer receptors Experiments: Vasopressin antagonists prevent pair- bond formation; increasing vasopressin receptors in meadow voles promotes pair-bond formation.

Why and How Male and Female Brains Differ Sexual Dimorphisms of the CNS Onuf’s nucleus Controls motor neuron pool Sexually dimorphic nucleus (SDN)- in rats, much larger in males INAH in humans analogous to rat SDN

Why and How Male and Female Brains Differ Sexual Dimorphisms of Cognition Evolutionary explanations for cognitive dimorphisms Role of distinctive hormonal environment triggers differences in performance Low estrogen levels Better performance by women in spatial reasoning Relation between cognition and hormones

Why and How Male and Female Brains Differ Sex Hormones, The Brain, and Behavior: Steroids Alter membrane excitability, sensitivity to neurotransmitters, neurotransmitter release Modulate functions of various enzymes, channels and transmitter receptors Diffuse across outer cell membrane Bind to specific steroid receptors in cytoplasm and nucleus

Why and How Male and Female Brains Differ Sex Hormones, The Brain, and Behavior (Cont’d) Prostaglandins - downstream from sex hormones Synthesized from arachadonic acid via COX enzyme Involved in tissue damage, pain/fever Fetal and neonatal rats exposed to COX inhibitors Reduced copulatory behavior in male rats Male-like copulatory behavior in female rats

Why and How Male and Female Brains Differ Sex Hormones, the Brain, and Behavior (Cont'd) Organizational effect of hormones Tend to be irreversible Activational effect of hormones Tend to be temporary

Why and How Male and Female Brains Differ Sex Hormones, The Brain, and Behavior (Cont’d) Mismatches Between Genetic Sex and Hormone Action: Treatment with testosterone in mammals, reduced features of female reproductive behavior Androgen-insensitive genetic males: Defective androgen gene, X chromosome Congenital adrenal hyperplasia genetic females: Abnormally large adrenals overproduce androgens

Why and How Male and Female Brains Differ Sex Hormones, The Brain, and Behavior (Cont’d) Direct Genetic Effects on Sexual Differentiation of the Brain Gynandromorphic zebra finch (naturally occurring) Brain: Female left side, male right Differential gene expression

Why and How Male and Female Brains Differ The Activational Effects of Sex Hormones Temporary modifications in brain organization, structural changes in neurites Men: Rise in testosterone, anticipation of sex, fall in testosterone, decreased sexual interest Women: Rise in estrogen, increased sexual interest

Why and How Male and Female Brains Differ The Activational Effects of Sex Hormones Brain Plasticity and Maternal Behavior

Why and How Male and Female Brains Differ The Activational Effects of Sex Hormones (Cont’d) Estrogens, Neurite Growth, and Disease Toran-Allerand – estradiol treatment of hypothalamic tissue promotes neurite outgrowth Songbirds: Testosterone mediates seasonal fluctuation in size of sexually dimorphic vocal-motor regions.

Why and How Male and Female Brains Differ The Activational Effects of Sex Hormones Gould, Woolley, and McEwen – estradiol treatment has dramatic effects on hippocampal neuronal dendrites

Why and How Male and Female Brains Differ The Activational Effects of Sex Hormones Estrogens, Neurite growth, and Disease (Cont’d) Increase in hippocampal spine numbers coincides with female rat’s peak fertility May play a role in changing reproductive needs Estradiol Protective effect on neurons (in culture) Hypoxia, oxidative stress, toxins, etc.

Concluding Remarks Most human behaviors Not distinctly masculine or feminine Reproduction demands Sex-specific behaviors Sex hormones Crucial role in sexual development and behavior

Sex and Gender The Genetics of Sex Genotype- Male: XY, Female: XX X chromosome larger than Y X contains 1500 and Y contains 50 genes X-linked diseases Occur more often in men than women SRY: Gene on Y chromosome, encodes TDF (testis determing factor)

Why and How Male and Female Brains Differ Sexual Orientation INAH-3 Twice as large in men: Sexually dimorphic INAH-3 in gay men: Similar to that in women Significance: Results difficult to interpret

End of Presentation

The Neural Basis of Sexual Behaviors Reproductive Organs and Their Control (Cont’d) Neurotransmitters: Relax smooth muscle Acetylcholine, vasoactive intestinal polypeptide (VIP), and nitric oxide (NO) Male orgasm: Muscular contractions Ejaculation Female orgasm: Muscular contractions