. Biology of Reproduction, lecture on Sexual Behavior

Biology of Reproduction

Fall 1998

text: Human Reproductive Biology 2nd Edition - R.E. Jones: Read pages 430-451 for this lecture

X. Development of Sexual Behavior 			back to  IX. Sexual Differentiation

	A. T (from testes) masculinizes the brain (hypothalamus)

		1. converted to E2 intracellularly

			a. aromatase

		2. E2 binding to E-R produces male sexual behavior

			a. female rats have a-feto-protein ® binds to E2
			   and keeps it from entering the brain

		3. Critical Period = Window of Effectiveness

			a. time during development when "T"
			   has an effect masculinizin the brain
			    producing male behavior


(time after fertilization)
Hamster 16 16-21 4.5
Mouse 19-20 20 7.5
Rat 21-22 18-28 5.0
Guinea Pig 68 30-35 5.0
Human 270 T goes up 84-126 21

			b. fetal hormone organize sexual behavior in males, 

				i. postpubertal hormones maintain behavior
				   in males and females 

			c. Evidence for critical period in humans

				i. Guevedoces

					(1) raised as females, plenty of T,
					   male behavior as adults

				ii. Adrenogenital Syndrome 

					(1) high androgens - more male-like behavior

				iii. Progestogens (given to prevent loss of pregnancy)
				     have some androgenic activity,
					 females born to women with this treatment
					 have more male-like behavior

				iv. Estrogen +feedback in homosexuals

					(1) other studies have refuted this finding,
					   and even suggested positive feedback
					    in heterosexual males

XI. Sexual Orientation

	A. Genes associated with homosexuality in males 

		1. X chromosome region Xq28  (tip of  long arm)
		   shared by some homosexual brothers

			a. heritability: 53% of the variance
			   in homosexuality is genetic

				i. X-linked recessive

			b. adopted brothers of homosexuals have a higher
			   than normal incidence of homosexuality

		2. genes code for proteins (mostly enzymes)

			a. genetic background represents a predisposition

				i. triggered or supressed by external factors

	B. Genes and homosexuality in females

		1. 52% heritability
		   associated with same-sex orientation in women

	C. Testicular Feminization Syndrome

		1. no androgen receptors  (Xq11)

			a. normal testis, female external genitalia

		2. XY heterosexual female

	D. neuroanatomical sexual dimorphism:
	   3rd  interstitial nucleus of the mPOA  (=INAH3)

		1. smaller in women

		2. smaller in homosexual men

	E. multimorphisms and sexual orientation: SCN

		1. shape (not size) of SCN differ
		   in heterosexual males and females

			a. AVP secreting cells

		2. SCNAVP 2x larger in homosexual men

		3. SCN = biological clock
		  ® homosexual men arise and retire earlier

			a. SCN also regulates reproductive function + rhythmicity

	F. Differences in hypothalamus suggest hormonal variability

		1. fetal environment can influence androgen and estrogen levels

			a. may influence masculinization during critical period

				i. maternal stress (­ CRH, ACTH and F)
				   decreases androgen and estrogen levels

					(1) advance fetal adrenal conversion of DHEA to F

					(2) ­ homosexuality in males and females

				ii. however, Human male without aromatase
				   was heterosexual (N = 1)

					(1) DA may be able to organize morphological
					   and functional sex differences
					    in the abscence of steroid

				iii. masculinization limits E provoked LH surge

					(1) 50% of homosexual men had LH surge

					(2) 33% of heterosexual men had LH surge

						(a) those with lowest [T] (both groups)

							(i) no masculinization of human brain?

							(ii) E will never provoke LH surge in male rat

	G. Fetal environment can affect non-steroidal events

		1. ­ # of older brothers ­ chance of homosexuality

			a. each older brother increases chance by 33%

				i. multiply by chance of homosexuality (1-2%)
				   to get overall increase

		2. progressive immunization of mothers
		   to minor Y-linked histocompatibility antigens

			a. ­ effects of H-Y antigen antibodies
			   on sexual differentiation of the brain

				i. H-Y antigen was once thought to determine gonadal sex

					(1) before discovery of  TDF

XII. Puberty