REPRODUCTION & SEXUAL BEHAVIOR

TABLE OF CONTENTS

Updated: Mar. 18, 2008

LECTURE INFORMATION

KEY CONCEPTS IN THIS LECTURE

1. Mammalian reproduction is regulated by the pulsatile release of hypothalmic Gonadotropin Releasing Hormone (GnRH). GnRH causes release of two tropic hormones, Follicle Stimulating Hormone (FSH) and Lutenizing Hormone (LH) from the anterior pituitary. These hormones affect gonadal development. Gonadal hormones in turn feedback to alter hypothalamic-hypophyseal function.

Male reproductive regulation is relatively direct. GnRH causes secretion of FSH and LH which causes testicular growth and spermatogenesis. Two gonadal hormones, Testosterone and Inhibin, negatively feed-back on the hypothalamus and anterior pituitary, respectively.

Female reproduction has two feedback loop associated with the mammalian estrous cycle (menstral cycle in humans). FSH stimulates follicle formation and development of ova; the follicle secretes increasing titers of estradiol (E2) stimulate development of the uterine lining as well as positively feeding back to the hypothalamic-hypophyseal axis producing a LH surge. This surge causes ovulation (Day 2 in the estrous cycel of a rodent). The remaining follicle cells transform into a corpus luteum which secretes E2 and progesterone (P). These hormones stimulate development of the uterine lining. Progesterone inhibits FSH secretion (and thus development of ova) as well as uterine contractions. If fertilization fails to occur, the corpus luteum disintegrates after 2 days, P titers decrease, and FSH stimulates development of additional ova.

If fertilization and implantation occur, the placenta develops and secretes E2 (mammary gland development), and P. P inhibits FSH, prolactin (PRL), the neurohormone, Oxytocin), and uterine contraction. A third hormone secreted early in pregnancy is Human Chorionic Gonadatropin (hCG or just CG) which extends the life of the corpus luteum and inhibits ova development. At partition, P drops resulting in disinhibition of PRL (maintains mammary glands and milk production) and Oxytocin (milk ejection via a neural reflex).

2. Since most mammals (e.g., mice and rats) are nocturnal, sexual behavior is regulated largely by tacile, olfactory, and auditiory cues, not visual cues. Even though some mammals breed more or less continuously (mice), sexual behavior is confined to female's estrous phase. In other species, females are in estrous only once during a specific season (e.g, fall in elk). In all cases, however, pheromones play a important role in sexual behavior.

3. Sexual differentiation of the nervous system occurs between males and females, such as Spinal Nucleus Bulbocavernosus (SNB) in the male spinal cord or the enlarged Sexual Dimorphic Nucleus (SDN) in the male hypothalamus. The Medial Preoptic area (MPA), Amygdala (AMG), and Basal Nucleus of the Striatum Terminalis (bNST) are critical for male sexual behavior. These brain areas are T-sensitive. Female sexual behavior (e.g., lordosis behavior) is regulated in part by the Ventromedial hypothalamus (VMH) and the AMG.

4. Parental behavior, such as nesting, is often influenced by PRL. Non-hormonal events (e.g. pheromone communication between a cow and her calf or ultrasound of mouse pups) also play important roles in parental behavior.

LECTURE OUTLINE

I.  INTRODUCTION TO REPRODUCTIVE ENDOCRINOLOGY

  A.  Male Reproductive Endocrinology -- the simpler of the two
       systems (Overview)
     1.  Endocrinology of Testosterone (T)
          a. Testosterone is the principle androgen in males.  
              Secreted by the Leydig cells in response to LH.   
          b. Testosterone has a number of important functions:
                  1) initiates and maintains spermatogenesis
                  2) inhibits LH secretion
                  3) development of male accessory organs and muscle mass
                  4) stimulates and secondary sexual characteristics
                  5) male sexual behavior
                       a)  Castration reduces sexual behavior. T reverses effect.
                       b) T may act through Estrogen/Dihydrotestosterone (E/DHT)
          c. Regulation of testes function.
                  1) GnRH is released in a pulsatile manner 
                  2) These pulses causes release of FSH and LH
          d. Negative feedback controls tropins
              Inhibin inhibits FSH at anterior pituitary
              Testosterone inhibits LH at anterior pituitary & hypothalamus **
  B.  Female Reproductive Endocrinology -- Female system 
       cycles.
      1. Ovary--source of eggs. 
          a. Other reproductive organs and their functions (oviduct, 
              uterus)
     2. Estrous cycle lasts about 4 days in most rodents
          b. Brief description of the estrous cycle
              1) Eggs develop--nurse cells feed the developing egg.
              2) Follicles and eggs mature by Day 2.  
              3) Follicle bursts--ovulation occurs.
              4) Corpus luteum develops and persists for 2 days, 
                  degenerates.
              5) Cycle is repeated if no fertilization occurs. 
                   Fertilization results in Cleavage/Implantation/Placenta Formation
     3.  Hormonal control of the estrous cycle (Overview)
          a. Three sites: 1) hypothalamus, 2) anterior pituitary, 
              and 3) ovaries
          b. Five hormones: FSH, LH, Estradiol (E2), Progesterone (P),
              and GnRH
          c. Mechanism:  GnRH releases FSH causing proliferation 
              of nurse cells.
              1) Follicle releases E2 stimulating uterine lining.
              2) E2 also causes LH surge (via postive feedback) which
                  causes ovulation.
              3) Corpus luteum forms releases E & P--no ova develop 
                  (negative feedback)
              4) E primes P to control sexual behavior at ovulation
                    a) Pheromones are involved **
     4. Placenta as an endocrine gland--Human Chorionic 
         Gonadotropin (hCG), P, E, etc.
     5. Hormonal changes following birth--no P to inhibit hormonal 
         activity.
          a. Prolactin from anterior pituitary--milk production
          b. Oxytocin from posterior pituitary--uterine contraction
              and later milk ejection--these are both neural reflexes **

II.  DESCRIPTION OF SEXUAL BEHAVIOR IN THE RODENT

  A. Male Behavior
     1. Sniffing and rubbing behaviors by the male
     2. Repeated mounting, intromission, and ejaculation
          a. Becomes refractory with time, except to newly 
              introduced female
  B. Female Behavior
     1. Jumping movements and ear movements preceed mating
     2. Lordosis in a receptive female
          a. Stiff curved back, immobile, feet spread, tail to one side
  C. Role of pheromones in sexual behavior
     1. Vomeronasal organs--a specialized olfactory structure
          a. Projections to Amygdala
     2. Examples of pheromone action (see Carlson)
          a. Whitten Effect--male odor causes females to synchronize
              their estrous cycles 
          b. Bruce Effect--spontaneous abortion with new male
          c. Vanderbergh Effect--accelerated female puberty if a 
              male is present

III.  CONTROL OF MALE SEXUAL BEHAVIOR

  A. Spinal refexes are important in some aspects of male behavior
     1. Example: Ejaculation does not require processing in the brain 
     2. Example: "lordosis-like behavior" in neutered male
     3. Example: Spinal nucleus bulbocavernosus (SNB) in ventral
         root is enlarged in males
  B.  Identifying Brain Areas Controlling Behavior
     1. Review of the Hypothalamus
  C. Medial preoptic area (MPA) in the hypothalamus
     1. Critical for male sexual behavior (lesions block behavior)
     2. Sexual Dimorphic Nucleus (SDN) is enlarged in males
          a. Growth of SDN is regulated by fetal T
          b. T during early development affects adult sexual behavior
  D. Medial amygala (AMG) & BNST are sexually dimorphic 
      (larger in males)
     1. Lesions decrease sexual behavior
     2. T implants restore sex behavior; AMG, BNST, MPA are 
         T-sensitive
     3. BNST and sexual orientation in humans **
  E. Summary

IV.  CONTROL OF FEMALE SEXUAL BEHAVIOR

  A. Ventro-Medial Nucleus (VMN) in hypothalamus controls 
       female behavior
    1. Lesions eliminate lordosis
    2. Estradiol receptors are concentrated here
B. Amygala is also important
    1. Efferents of female behavior via the Periaqueductal Gray
C. Summary

V.  PARENTAL BEHAVIOR

A. What is parental behavior?
     1. Review hormonal events at and following birth
          a. Ovary removal decreases parental behavior
          b. Other hormone effects
          c. MPA affects parental behavior
     2. Non-hormonal control of parental behavior
         a. Role of pheromones in parent: Offspring identification 
             (antelope, cow)
         b. Role of ultrasound in retrieval behavior (mice)

VII. DIVERSITY IN SEXUAL BEHAVIOR

A. Monogamy in the prairie vole
   1. Background and pair formation
   2. Oxytocin (OT) & vasopressin (VP) injections mimic effects  (Summary)
       a. OT and VP injections
   3. Role of the limbic system and the nucleus accumbens 
B. Delayed implantation in skunks, badgers, etc.
C. Alternative mating strategies in the garter snake
   1. Mating balls are established when the gravid female releases an
       estrogen-dependent pheromone
   2. "She" males compete with "he" males for the gravid female
D. Many of the principles observed in mammals apply to other vertebrates
     1.  Summary of similarities
     2. Example:  Control of Reproduction in Anolis
          a. Describe the seasonal cycle 
          b. Hormonal regulation of the cycle

STUDY QUESTIONS

Make a table listing the 1) hormone source, 2) type of hormone (peptide/amine or steroid) 3) target tissue(s) and 4) hormone action(s) for the following:
- Testosterone (T)
- Inhibin
- Estradiol (E2)
- Dihydrotestosterone (DHT)
- Progesterone (P)
- Lutenizing hormones (LH)
- Follicle Stimulating Hormone (FSH)
- Prolactin (PRL)
- Oxytocin
- Gonadotropin Releasing Hormone (GnRH)
- Human Chorionic Gonadotropin (HCG) or Chorionic Gonadotropin (CG)
Testosterone (T) can be converted within the cell to either an Estrogen or Dihydrotestosterone (DHT). As a result it is difficult to determine which hormone acts at the target cell. Can you think of approaches or methods that you might use to distinguish the action of T from that of Estrogen or DHT?
Describe the process of implantation and placenta formation. What hormones influence these processes? What are the functions of the placenta? What hormones are secreted by the placenta?
Describe the hormones that play a role in lactation. What factors influence milk secretion and milk ejection?
Summarize the actions of estrogen and progesterone. How are these hormones regulated?
Describe the events of the follicular and luteal phases of the ovarian cycle. Relate each phase of the cycle to the hormonal events occurring at that time. Correlate each phase of the cycle to the events occurring in the ovary and the uterus.
Describe the mechanism regulating testosterone titer. List the specific actions of FSH and LH on the testes.
Why is the formation of the corpus luteum important for successful reproduction?
Inhibin is a peptide hormone secreted by the testis that inhibits the anterior pituitary and specifically FSH release. How would you experimentally establish this role of inhibin in controlling reproduction? Include controls and defend your experimental design.
List the experimental evidence which indicates that the following brain structures are involved in sexual behavior. Identify which are involved in male sexual behavior. --in female sexual behavior.
What is the importance of parental behavior? List some parental behaviors of rodents. What hormones and brain areas regulated parental behavior?
How are estrogen and progesterone important for the maintenance of pregnancy? Why does ovulation and the estrous cycle cease during pregnancy?
What is an "environmental estrogen" and why are they of concern?
In the absence of T at birth, a default condition of "femaleness" results. Given that observation, how would you expect an adult male, castrated at birth, to behave following brain injections of either E2/P or T? Explain. Which brain areas might be involved in eliciting sexual behavior?
Thought question. If you administer E2 at birth to a castrated male, it will develop a normal male behavioral repetoire as an adult. How would you explain that obseration?
You observe that male hamsters exhibit a lot of marking behavior once they passed through puberty, and you wonder if this behavior is regulated by male steroid hormone(s). 1) What specific questions would you ask to get a better idea if your hypothesis is correct, and 2) how would you experimentally establish that this is the case.
List some general ways in which the hormonal control of reproduction and sexual behavior are similar for many vertebrate classes?

ADDITIONAL INFORMATION ON THE INTERNET

Hormones and Sexual Behavior with emphasis on the laboratory rat

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