Sex and the CNS

W. J. Wilson – November 2004

1. Sexual Anatomy Organized by Early Hormonal Environment

1. Testosterone during an early critical period masculinizes and defeminizes

                                                               i.      Genitalia

                                                              ii.      Internal reproductive organs

                                                            iii.      Brain

                                                            iv.      Adult behavior (?)

2. Critical period at about Weeks 9 – 10 in human gestation
3. Critical period for rats: Day of birth!

2. Activation of Sexual Behavior in Adulthood
   1. If masculinized in critical period, testosterone à male sexual behavior
   2. If feminized in critical period, estradiol and progesterone à female sexual behavior
   3. Above is true for nonhuman mammals.  What about humans?

3. Sexual Differences in the Brain

1. Sexually Dimorphic Nucleus (SDN) – Gorski (1978)

                                                               i.      Closely associated with preoptic area (hypothalamus)

                                                              ii.      Larger n. and more cells in male rat than in female

                                                            iii.      Testosterone in critical period is responsible for the difference in nonhuman animals

4. Where is sexual behavior controlled?
   1. Male sexual behavior:  preoptic area

                                                               i.      Lesion à reduction of male behavior

                                                              ii.      Stimulation à increased male sexual behavior

                                                            iii.      Testosterone receptors are here

2. Female sexual behavior: ventromedial hypothalamus

                                                               i.      Lesion à reduction of female sexual behavior

                                                              ii.      Stimulation à increased female sexual behavior

                                                            iii.      Estrogen receptors are here

5. Sexual orientation
   1. LeVay (1991) INAH-3 (corresponds in humans to SDN in rats) larger and more neurons in heterosexual males than in homosexual males

                                                               i.      Previously known that INAH-3 was larger in males than in females

2. Maybe size of INAH-3 determines sexual orientation

                                                               i.      LeVay: large INAH-3 causes attraction to women; small INAH-3 causes attraction to men

3. BUT: LeVay’s finding was correlational, and therefore cannot address causation.  Maybe sexual behavior or orientation causes changes in size of INAH-3
4. Unlikely, until Breedlove (1997) showed that cells in SNB change size as a result of sexual behavior

                                                               i.      Spinal nucleus of the bulbocavernosus controls penile musculature

                                                              ii.      Larger in males than in females

                                                            iii.      Size dependent on testosterone presence or absence during critical period

                                                            iv.      Male rats with 10-day (?) access to receptive females had smaller neurons in SNB

1.        all males were castrated and received equal amounts of testosterone