Estrogen Receptor Null Mice - Endocrine Reviews

June, 1999 ESTROGEN RECEPTOR NULL MICE 395
direct loss of or simply enhanced by the concurrent decrease
in PRL signaling.
Interestingly, the extremely low levels of PRL mRNA in
the anterior pituitary of the �ERKO female are even significantly
less than that observed 14 days after ovariectomy in
the wild-type (282). Therefore, the loss of ER� during development
and differentiation of the lactotrophs in the anterior
pituitary has resulted in a phenotype that is more
severe than that induced by postpubertal ovariectomy, possibly
due to a decrease in lactotroph cell number. It has been
proposed that the lactotroph and somatotroph cell types of
the adult anterior pituitary may be derived from a common
cell that expresses both the genes for GH and PRL during
development (reviewed in Ref. 410). The factors that may be
involved in the terminal differentiation of this stem cell into
a distinct cell type secreting only one of the respective hormones
remain elusive. Because the appearance of the ER and
the ontogeny of PRL expression appear to coincide in the
developing pituitary, estrogen action has been proposed as
a possible factor (410–412). However, a defect in the cell
lineage of the lactotrophs that may be expected due to a loss
of ER� action was not apparent in the �ERKO, as immunostaining
for both PRL and GH localized expression of the
genes to distinct cell types (282).
Furthermore, estrogen has also been shown to stimulate
proliferation of the lactotrophs and PRL-secreting cell lines
(reviewed in Ref. 339). Therefore, since the marked difference
in PRL mRNA levels observed between the �ERKO female
and the ovariectomized wild-type is not apparently due to a
defect in the differentiation of the lactotrophs, it may possibly
be due to a decreased number of lactotrophs in the anterior
pituitary of the �ERKO. Scully et al. (282) provided evidence
against this hypothesis, by once again employing immunohistochemical
staining to illustrate only a modest decrease in
lactotroph cells in the anterior pituitary of the �ERKO mice.
Therefore, ER� action does not appear to be required for
either differentiation or proliferation of the lactotrophs in the
mouse anterior pituitary. However, a recent report by Chun
et al. (413) has illustrated a distinct contrast in the level of
occupied ER required to elicit proliferation and that required
for PRL synthesis in PR1 cells, a PRL-secreting cell line.
Whereas approximately 50% of the cellular pool of ER was
required to be complexed with estradiol for half-maximal
stimulation of the PRL gene, only 0.1% was required to
induce cellular proliferation (413). These results suggest that
the mechanisms required for estrogen-induced lactotroph
proliferation are hypersensitive in this cell line compared
with the mechanisms involved in regulation of the PRL gene
(413). Therefore, it is possible that the small amount of the
active ER� splicing variant known to be present in the
�ERKO (see Section II.C.) has allowed for sufficient estrogen
signaling and lactotroph proliferation during develpment,
resulting in the apparent lack of a somewhat expected phenotype
of decreased lactotroph cell number in the pituitary
of �ERKO mice.
B. Behavior
There are obvious effects of the gonadal steroids on sexual
behavior in vertebrates; however, a more defined knowledge
of these actions has become evident from a series of classical
experimental schemes. These laboratory studies often relied
on perinatal castration and/or developmental exposure to
exogenous steroids followed by studies of the activational
abilities of the different steroids during adulthood. The majority
of such investigations have been carried out in the rat,
but similar results have been described in other species (345,
414). Breifly, studies on sexual behavior in the rat have
shown that 1) castration on the day of birth results in a
feminized adult male that exhibits a female pattern of behavioral
responses when treated with estradiol and progesterone,
and 2) neonatal testosterone or estradiol treatment of
a female results in a masculinized adult that exhibits a malelike
pattern of behaviors and is refractory to estradiol and
progesterone (131, 337). The culmination of the data collected
from such experimental schemes has led to the conclusion
that testosterone secreted from the perinatal testes during a
critical developmental window results in permanent changes
in the hypothalamic nuclei of the brain that mediate male
sexual behavior. However, the data indicating that developmental
exposure to estradiol results in an adult phenotype
that is similar to that elicited by testosterone suggest that
many of the masculinizing effects of perinatal testosterone
may be via local aromatization of the hormone to estradiol
and subsequent activation of the ER signaling pathway (reviewed
in Refs. 406 and 407). In addition, estradiol is also
necessary for normal development of the female brain, although
in lower amounts (131). Therefore, sex steroid-mediated
sexual differentiation of the various regions of the
brain that are critical to behavior relies not only on the nature
of the steroid ligand, but also on the dose and timing of
exposure (348).
Before the availability of the �ERKO mouse, McCarthy et
al. (415) employed an elaborate technique of infusing anti-
ER� oligodeoxynucleotides into the neonatal rat hypothalamus
to elucidate a direct role for ER� in the sexual differentiation
of the female brain. This experimental scheme was
based on the hypothesis that the presence of specific ER�
antisense oligodeoxynucleotides in the hypothalamus would
interfere with proper expression of the ER� gene during a
critical period of differentiation (415). The experimental
groups included neonatal rats treated with testosterone plus
or minus the infusion of the ER� antisense oligodeoxynucleotides.
As adults, those females infused with the ER� antisense
oligodeoxynucleotides exhibited more female sexual
behavior compared with those treated with androgen alone.
The investigators thereby concluded that the reduced ER�
expression protected the infused female rats from the masculinizing
effects of testosterone exposure (415), providing
strong evidence that local aromatization and subsequent estradiol
activation of the ER� pathway plays a primary role
in the masculinization of the rat brain. However, the experimental
scheme of McCarthy et al. does not allow for a direct
comparison with the �ERKO female, due to the caveats discussed
(see Section II.C.1). It is important to recognize that the
�ERKO are deficient in ER� throughout development,
whereas McCarthy’s scheme produced a lack of ER� action
that was only transient and most likely not as complete.
The use of technologies to target individual genes has
created numerous models available for studies in the behav-