Genetic influences on lordosis, a mammalian
social behavior, are amenable for study because
of the relative simplicity of both stimuli and
response. The neural circuit for lordosis
involves a supraspinal loop, which is controlled
by an estrogen- and progesterone-dependent
signal from the medial hypothalamus and results
in heightened sexual motivation. In turn, this
involves elevated states of arousal, defined by
increased sensory alertness, motor activity and
emotional reactivity. Mice in which the gene
encoding the form of the estrogen receptor (ERa)
has been knocked out show that ERa is crucial
for lordosis behavior. Comparing ERa, ERb and
double knockouts reveals that different patterns
of sexual behaviors in mice require different
patterns of ER activity. Understanding how
hormonal and genetic effects on deep
motivational and arousal processes contribute to
their effects on specific sexual and aggressive
behaviors pose significant challenges for mouse
functional genomics.
A large body of reliable neurobiological
results has been enabled by the analysis of
hormonal and genetic influences on lordosis, a
simple reproductive behavior. Lordosis is the
vertebral dorsiflexion performed by female
quadrupeds in response to adequate stimuli from
a reproductively competent male. Biologically it
is important because it permits fertilization
and, therefore, reproduction. Strategically it
is well chosen for analysis because it depends
on the activity of estrogenic hormones
facilitated by progestins. Therefore it serves
as a virtual expression system for the actions
of these steroid hormones and research in this
field has been enhanced and accelerated by the
tools of steroid chemistry and biochemical
endocrinology.
Furthermore, the behavior involves simple
responses that are triggered by simple stimuli.
All are manageable in the laboratory and,
crucially, all arerelatively easy to study.
Because of these advantages the hormonal, neural
and genetic determinants of lordosis have been
reported in detail. In addition to the spinal
circuitry required, there is an obligatory
supraspinal loop that brings somatosensory
stimuli involved in this reproductive behavior
to the medullary reticular formation and the
midbrain central gray. This circuit, which
governs female reproductive behavior, is
dependent on the interactions of estrogen and
progestins with ER and progesterone receptors
within hypothalamic neurons. Analysis of the
behavioral functions at the molecular level
depends upon the activity of these nuclear
hormone receptors as transcription factors. Thus
estrogenic effects on hypothalamic neurons that
govern female reproductive behavior requires the
synthesis of mRNA and protein and the molecular
actions of estrogens and progestins in the
hypothalamus and basal forebrain guarantee a
biologically adaptive synchrony between
reproductive behavior and the
pituitaryÐovarian mechanisms that cause
ovulation.
« It is
ironic that testosterone "the male sex hormone,"
is more closely associated with the yawning rate
than with the mounting or intromitting rates
» Charles Phoenix
Sexual
steroids
exert several effects on both central
dopaminergic and oxytocinergic systems by acting
either at the genomic or membrane level
