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Lesions also have more profound effects on food
intake, particularly in females. Neurons of the VMH
are selectively activated by the ovarian hormone,
estrogen, and in female rats the cells respond by pro-
ducing progesterone receptors. This does not occur in
the male VMH. In primates, the hormonal signal may
be somewhat different because loss of the adrenal
glands, a source of androgen hormones, leads to
reduction in copulatory behavior. Although there is
little difference in the absolute size of the VMH, there
is some reason to believe that it becomes sexually
dimorphic during development (Sakuma, 1984). Estra-
diol and testosterone have a dramatic affect on both
neurite outgrowth and dendritic branching in organ-
otypic cultures of the mouse hypothalamus (Toran-
Allerand, 1980; Toran-Allerand et al., 1983).
Sexual behavior in mice that have a disrupted estro-
gen receptor gene (ER-a and ER-b) is significantly atten-
uated. For example, females do not lordose. ER-a
knockout males do initiate copulatory behavior, mount-
ing females at a normal rate, but they rarely achieve an
intromission or an ejaculation. Furthermore, the males
are less aggressive than wild-type males, generally
failing to attack an “'intruder” mouse when it is placed
in the male's home cage (Ogawa et al., 1997). However,
genetic male mice that lack both estrogen receptor genes
display no sexual behavior, including mounting and
ultrasonic vocalizations (Ogawa et al., 2000).
(P4 Testost)
(P1 Castr)
(P21 Castr)
FIGURE 10.18 Sexual dimorphism in the mammalian brain. A.
A hypothalamic structure called the sexually dimorphic nucleus of
the preoptic area (SPN-POA, arrow) is almost six times larger in
male than in female rats. B. In genetic females, the size of SPN-POA
can be increased by treating with testosterone at postnatal day 4. In
genetic males, the SPN-POA can be decreased in size by castrating
at postnatal day 1. (Adapted from Gorski et al., 1978, 1980)
Since the control of gender is cell autonomous in
insects, sexual behavior has been explored from a
genetic perspective. Male fruit flies recognize females
based on an olfactory cue, called a contact pheromone ,
and males will perform stereotyped courtship behav-
ior when they receive this signal. The male will orient
toward a female, tap her abdomen, flutter his wing in
song, and place his proboscis (the mouthparts) on the
female's genitals. If the female is receptive, the male
will then mount her and copulate. How does the
central nervous system create this complex set of
behaviors? One approach to the problem is to create
unusual flies, called mosaics, that have some cells that
are genetically female (XX) and some cells that are
genetically male (XO). By studying many of these
animals, each one with a unique mosaic, it is possible
to determine which brain cells must be male in order
for male or female behaviors to occur (Hall, 1977).
In more recent studies, a genetic trick has been used
to construct a line of animals in which a single part of
The hormonal environment of developing males yields
a larger nucleus, and the dimorphism can be greatly
reduced by castrating genetic males within a few days
of birth (Figure 10.18B). Furthermore, this nucleus can
be enlarged in genetic females when they are treated
with testosterone as neonates (Gorski et al., 1978).
Intracranial implants of estradiol turn out to be as
effective as testosterone in masculinizing the SDN-
POA, and such estradiol-treated females fail to lordose
or ovulate. Presumably, testosterone is converted to
estradiol in the male SDN-POA, whereas the circulat-
ing estradiol in females is bound by a-fetoprotein
(Naftolin et al., 1975).
A second region of the hypothalamus, the ventro-
medial region (VMH), also participates in sexual
behavior. Damage to this region disrupts female cop-
ulatory behaviors, such as lordosis in rats, and stimu-
lation of the region seems to facilitate such behaviors.
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