Biopsychology of Sex Behavior

1. Biopsychology of Sex Behavior Lecture 2: The Sexual Brain LeVay, Ch. 5-9

2. Chapter 5 • The hypothalamus is located above pituitary (master gland), below thalamus, behind optic chiasm, in front of mammillary bodies, at base of brain • Although it’s a small structure, it plays an important role in sex, feeding, drinking, cardiovascular performance, control of body temperature, stress, emotional responses, growth, and other functions • Some structures: • Suprachiasmatic nucleus: “biological clock • Supraoptic nucleus: synthesizes oxytocin, which promotes orgasm, parturition, and milk letdown • Median eminence, preoptic area, and other areas secrete releasing hormones or inhibiting hormones, which directly control hormone release from the pituitary

4. Chapter 5 • The hypothalamus (and other brain structures) can be studied using a variety of techniques: • Immunohistochemistry • Hybridization histochemistry plus autoradiography • Golgi technique (for local connections) Stains only a few cells, but fills them completely • Inject single neurons w/ dye after recording from them • Horseradish peroxidase for long connections

5. Chapter 6 • Many of the neuronal circuits that mediate sexual intercourse lie in the spinal cord • Copulation is a series of reflexes and behaviors that must be expressed at the proper time. The brain controls and modulates these reflexes, as well as the subjective experienced that accompany copulation (especially orgasm) • Components of coitus in humans: • (1) erection of penis • (2) engorgement of the walls of the vagina and the labia majora, lubrication of the vagina by glandular secretions and transudation; erection of clitoris • (3) insertion of penis into vagina (intromission) • (4) pelvic thrusting • (5) elevation of the uterus, with a consequent forward & upward rotation of the mouth of cervix • (6) ejaculation of semen into the vagina • (7) orgasm, an intensely pleasurable sense of climax and release

6. Chapter 6 • In brain research, a sexual dimorphism is an anatomical difference between the sexes. This term can also be used to describe other differences (e.g., behavioral differences) • Sexual dimorphism of external genitalia: • Males: two long muscles are attached to penis (ischiocavernosus & bulbocavernosus muscles)—make erect penis moveable, participate in erection, and participate in ejaculation • Females: ischiocavernosus is much smaller, and bulbocavernosus forms circular, constrictive fibers around vagina • Erection of penis & clitoris is hemodynamic, helped by ischiocavernosus & bulbocavernosus muscles • Sexual dimorphism of sexual behavior: • Males: In rats and other mammals, the male approaches female from rear, mounts her, grasps her back, inserts penis, and thrusts w/ pelvic muscles • Females: In response to stimulation from the male, the female rat will typically flex her back in a U-shape (lordosis), exposing her genital area for intromission by the male. Females also express proceptive behaviors.

8. Chapter 6 • Lordosis in rats is a reflex, like the knee-jerk reflex • However, unlike the knee-jerk reflex, the circuitry that controls the lordosis reflex is not contained entirely within the spinal cord • Muscles involved in lordosis are general-purpose muscles; they are also involved in other nonsexual behaviors (e.g., locomotion) • The muscles of the back and the spinal motor neurons that innervate them are similar in the sexes (i.e., they are not sexually dimorphic) • However, in female rats, the lordosis reflex can only be elicited under the appropriate hormonal conditions—this behavior is controlled by the cyclical changes in circulating levels of steroid hormones (estrogens & progestins) • If a rat’s ovaries are removed , sexual behavior, including lordosis reflex, disappears but can be restored by administration of these hormones • The main site of action of these hormones in facilitating the expression of the lordosis reflex is the hypothalamus (particularly the ventromedial nucleus of the hypothalamus, VMN).

9. Chapter 6 • Sexual dimorphism of motor neurons in spinal cord: • Males: certain motor neurons in spinal cord innervate these muscles. Men contain a larger number of these motor neurons than are found in women. • Females: women have fewer of these motor neurons relative to men

10. Chapter 6 • Breedlove & Arnold: early in development, ischiocavernosus & bulbocavernosus muscles and the motor neurons that innervate them are present equally in both sexes • Critical period (a few days before birth till about 5 days after birth): • Females: absence of testosterone (T) in female pups; muscles & motor neurons regress and die • Males: presence of circulating T in male pups; muscles are prevented from dying by T & this action secondarily prevents the motor neurons from dying. Muscles & neurons survive & grow.

11. Morris, J. A., Jordan, C. L., & Breedlove, S. M. (2004). Nature Neuroscience7, 1034 - 1039

13. Chapter 6 • Ejaculation: • Seminal fluid is transported to back of urethra (not necessary or sufficient for orgasm) • Ejaculation is accomplished by the contractions of the bulbocavernosus and ischiocavernosus muscles, as well as of the muscular walls of the urethra • Semen is ejected against the mouth of the cervix • Ejaculation is accompanied by the intense emotional sense of climax and release known as orgasm

14. Chapter 6 • A recently discovered feature of orgasm is that is accompanied by a massive release of the hormone oxytocin from the pituitary gland • Oxytocin is synthesized by neurons in two nuclei of the hypothalamus, the supraoptic nucleus (SON) & paraventricular nucleus (PVN). • Oxytocin is transported from the cells bodies of these neurons down their axons to nerve terminals located in the pituitary gland—this hormone is released into the bloodstream • Oxytocin release at ejaculation blocked by systemically administered naloxone (Murphy); male subjects reported feeling less pleasurable orgasms during naloxone treatment. Murphy’s finding suggests that release of oxytocin at orgasm may be under the control of the endorphin system • Sexual climax in women and men is similar, and some women may also ejaculate fluid at orgasm (controversial). For at least a few minutes after orgasm in men, penile erection and ejaculation are impossible—this is the refractory period. In male rats, a male will mount a new female more speedily than he will remount the same female with whom he has just copulated—this is called the Coolidge Effect • Main difference between male and female orgasms is that in women it is not necessarily followed by a refractory period—many females are multiorgasmic

15. Chapter 7 • Courtship: a system of communication that is used by one sex to induce mating in a receiving conspecific (i.e., member of the same species). Information about species, sex, physiological condition, and readiness to mate is exchanged during courtship • Females typically select males who express certain qualities that likely favor the survival of future offspring; male’s role in courtship tends to be more active • However, female often plays an active role too • Receptivity: state of responsiveness to the sexual advances of a male; the female simply permits, or does not permit, copulation in response to the male’s advances (e.g., lordosis behavior) • Proceptivity: degree to which a female seeks out the male and initiates sexual behavior (e.g., hopping, darting, & ear wiggling by female rats)

16. Chapter 7 • Birdsong: Male songbirds sing more frequent and elaborate songs than do females • At least 2 functions: maintain territory & attract females • Nuclei that are responsible for generating these songs are generally larger in males than in females • This sexual dimorphism results from differences in hormone levels during early life. Mark Gurney reported that administration of masculinizing steroids to newly hatched female zebra finches resulted in the development of song nuclei of a size typical for males • Female song birds have vocal systems in the brains, even in species where the females do not sing; females may use their vocal systems to analyze the males’ songs that they hear; If an adult canary is treated w/ masculinizing hormones, she can be induced to sing

17. Chapter 7 • Eye contact: In primates, including humans, eye contact is a mutual behavior that is loaded with significance (powerful cue to sexual arousal) • Primates have developed precise mechanism for determining from visual image of another individual’s eye, whether or not that individual is looking at them • In female marmoset monkeys, courtship behaviors (e.g., staring at male) are dependent on the female’s hormonal state—she is more likely to stare at male around time of ovulation • In males, eye contact is less likely to occur if he is in the refractory period soon after ejaculating, or if his hypothalamus is lesioned

18. Chapter 7 • Sexual Attractiveness: Pomerantz found that if a male rhesus monkey is housed with two estrous females, he will direct all his sexual advances to one of the two females; the preferred female also displays more proceptive behaviors than the other female. There was no absolute difference in attractiveness between females; different males did not consistently prefer the same female

19. Chapter 8 • Maternal behavior: why do mothers devote so much effort to their offspring? • From en evolutionary perspective, mothers devote so much to offspring because they have made a large investment in their offspring by the time they are born—they are compelled to protect this investment • In rats, onset of maternity is accompanied by reorganization of brain by at least 4 hormones: estrogen, progesterone, prolactin, & oxytocin • Estrogen and progesterone (P) rise during pregnancy, then P levels decrease just before parturition; at the same time, prolactin and oxytocin levels rise • Prolactin: hormone secreted by cells in pituitary gland; prepares the mammary glands for secretion of milk, & in conjunction w/ estrogen stimulates maternal behavior • Oxytocin: small peptide hormone that is made by cells in the supraoptic & paraventricular nuclei of hypothalamus. Axons run down into pituitary gland, where oxytocin is released into the bloodstream; oxytocin is also released in the other brain regions. Oxytocin is involved in parturition (hormone stimulates the contraction of the muscles of the uterus); oxytocin causes ejection of milk from mammary glands into the nipples; oxytocin promotes maternal behavior

20. Chapter 8 • Onset of maternity is accompanied by reorganization of the brain. Barbara Modney & Glenn Hatton examined supraoptic nucleus • They found that in virgin female rats neurons in the supraoptic nucleus are separated from each other by thin insulating layers produced by glial cells • A few hours before birth of pups, these glial cell layers are withdrawn, and oxytocin-containing neurons come into direct contact with each other • This allows cells to become much more excitable, and new synapses are formed in the supraoptic and paraventricular nuclei • Oxytocin is released in bursts, due to electrical coupling in these nuclei • These structural changes are not permanent; the new synapses eventually disappear and glial cell layers reform between cells • Androgens during development may have a negative effect on organization of circuits for maternal behavior

21. Chapter 8 • Experience can also stimulate maternal (parental) behaviors • If a virgin rat is given access to pups for a period of days, she will eventually exhibit maternal behaviors toward the pups; the same occurs in juvenile rats of either sex. This effect is not dependent on hormones • In primates, learning and social interactions contribute to the later expression of maternal behaviors. Young females will often show maternal behavior to infants that are not their own—this may serve as a chance to rehearse these behaviors • In some circumstances, male primates will show protective behaviors toward infants

22. Chapter 9 • LeVay: There are separate centers within the hypothalamus for the generation of male-typical and female-typical sexual behavior and feelings • Male-typical behaviors are those that are more common in males; whereas female-typical behaviors are those that are more common in females • LeVay: when I speak of a “center” for sex I mean that it is a crucial node in a complex circuit that involves many other brain regions; certain hypothalamic nuclei are critical for the expression of sexual behaviors • Sexual feelings are not generated in the hypothalamus; however, they hypothalamus may play a role in triggering these feelings

23. Chapter 9 • Medial Preoptic Area (MPOA) • Located in the anterior area of hypothalamus; it plays a major role in male-typical behavior • It is a region containing several small nuclei as well as axonal tracts • Lesions of the MPOA have a negative impact on copulatory behaviors in males of most species (i.e., these behaviors are reduced or eliminated) • Lesions of the MPOA do not completely eliminate sex drive; it has been shown that male monkeys with MPOA lesions still masturbate • In ferrets and rat, lesions of the MPOA increased the occurrence of female-typical behaviors, especially when combined with estrogen treatment • Electrical stimulation of the MPOA facilitates copulation • The MPOA does not merely generate a motor response. It generates an internal state in which appropriate signals from the female can readily elicit sexual behaviors—e.g., if the female is not in heat, stimulation of the MPOA has little effect; however, if a male is sitting idly with an estrous female, stimulation of the MPOA of the male elicits mounting and other sexual behaviors • Electrical recording from MPOA: highest activity while pressing button to bring female near; less activity during copulation; none after ejaculation during refractory period • Dorsomedial nucleus, an area that is adjacent to the MPOA, is involved in ejaculation

24. Chapter 9 • Roger Gorski: MPOA is sexually dimorphic in rodents and humans—contains at least one nucleus that is larger in males than females • INAH=interstitial nucleus of anterior hypothalamus • INAH 1=not dimorphic & corresponds to nucleus intermedius • INAH 2= may be dimorphic; but limited to certain age groups • INAH 3=sexually dimorphic; larger in males than females; about a threefold difference in volume between the sexes • Position and appearance of INAH 3 corresponds with sexually dimorphic nucleus (SDN) of the preoptic area (POA) in the rat, which is also sexually dimorphic • Potential reasons why males show more male-typical behaviors than females: androgen levels higher in males; a nucleus in the MPOA is larger in males than in females; differences between males & females in connectivity in MPOA; & different neurotransmitters may be involved in regulating male sexual behavior • Note: anatomical differences and differences in hormone levels may not completely account for sex differences in male-typical behaviors. For example, destruction of the SDN-POA of male rats does not abolish male sexual behavior. In addition, treating females w/ testosterone doe not cause them to display male-typical behaviors; in human females, testosterone actually increases sex drive.

25. Third interstitial nucleus of the anterior hypothalamus (INAH3)

26. Chapter 9 • Ventromedial Nucleus of Hypothalamus • VMN, a relatively large nucleus, is the principal hypothalamic region for female-typical sexual behavior • It is involved in regulating other behaviors (e.g., feeding behavior); ventrolateral portion of the VMN appears to be specifically involved in sex • Damage to the VMN inhibits expression of female-typical sexual behaviors; stimulation of this nucleus elicits sexual behaviors • Expression of female-typical sexual behaviors is dependent on hormonal state; estrogen in the VMN causes neurons to synthesize progesterone receptors; in addition, estrogen causes changes in synaptic connectivity in the VMN • Progesterone then acts in the VMN (by binding to progesterone receptors) and this facilitates the expression of female-typical sexual behaviors (particularly lordosis) • Progesterone does not facilitate lordosis reflex in males, whether they are treated with estrogen or not