Ended last lecture discussing the effects of estrogens which increase sexual desire and progesterones that decrease sexual desire. It was thought that female sexual behavior was independent of hormones as women (plus a few other species) engage in sexual activity throughout the cycle.
Evidence that hormones do play a role in Female Sexual Behavior
An increase in sexual activity around the time of ovulation and
another smaller increase
around menses may be attritubuted to hormones. The peak in behavior may
coincide with
increased estrogen levels.
Right before ovulation, the follicles are secreting high amounts of
estradiol. High amounts of
androgens are secreted and then converted to estradiol. It could be that
testosterone is the best
aphrodisiac and may be much stronger in women than men.
The post oluvatory decrease in sexual behavior may be an indication
that estrogen increases
sexual desire and progesterone decreases it.
Any of the 3 mechanisms may be involved in sexual desire in the female.
Emotionality may play a role in sexual desire as well. Some women
claim that they can tell
when they're ovulating. During the luteal phase is an increase in body
temperature (only about 1
degree higher).
Stages of sexual response
Human females go through the same stages of sexual response as do males. Female response is more variable.A study done by Masters and Johnson shows 3 patterns.
The stages of sexual response include a general excitement phase, a plateau phase followed by what can be a series of orgasms (without refractory period) followed by a resolution phase.
We know little about hormonal changes that accompany sexual response in females. Brain waves (EEG) are the same in females and in men. You can also stimulate the septum in females as in males to produce orgasm. Oxytocin is released after orgasm which may also be responsible for feelings of satiety.
Sexual Behavior of the Rat
It was generally believed that hormones didn't have much influence on
sexuality in humans or
animals. It was believed that sexual behaviors in animals was limited to
being receptive to the
male. There's a lot of bias in this area and most researchers were men
not women.
Most mammals (animals in general) have only a small period of time in
which they engage in
sexual behavior. It's often mediated by seasons.
Breeding periods may be evolutionarily designed for the animals. If
gestation occurs in winter,
offspring are born in spring when temperatures and climate are most
advantageous to the
offspring.
The period of time associated with development of the follicle and ovulation is referred to as the estrous cycle. The estrous cycle is analogous to the menstrual cycle of the human female. However, most animals (non-primates) will only mate during specific parts of the estrous cycle.
Estrous Cycle
Estrous refers to the entire ovarian cycle in the female. Books can
be very confusing on the
use of estrous and estrus. When they say estrus they probably mean when
an animal is in heat or
proestrus.
Rats engage in multiple cycles throughout a year (once every 4 or 5 days).
| Part of Cycle | Time | Characteristics |
|---|---|---|
| Estrous | ~36 hours | Large numbers of epithelial cells. |
| Diestrus | ~24 hours | Reduced numbers of epithelial cells and presence of increasing numbers of leukocytes. |
| Diestrus II | ~24 hours | Less and Less epithelial cells and more leukocytes. |
| Proestrus | ~12 hours | Nucleated epithelial cells. |
Diestrus I and II and the beginnings of Proestrus are roughly analogous
to follicular phase in humans. At end of proestrus and beginning of estrus ovulation occurs. Unlike humans, there is no luteal phase in non-breeding animals. It is impossible for male to initiate copulation before proestrus because female will fight off the male and there is a membrane
in females (analogous to the human hymen) that obstructs the vaginal opening. It is in proestrus when this membrane dissolves and later in cycle regrows.
Behavioral estrus occurs during vaginal proestrus. During this time females actively seek a male with which to mate. Characteristic behavior of the proceptive female rats include engaging in flirting behaviors: wiggling their ears, hopping around ridgedly or play "tag" with the male.
These sorts of behaviors were somehow missed in most literature on sexual behavior in animals prior to the 1970s. With rats and primates, most sexual behavior is initiated by the female. Male doesn't engage in copulatory behavior unless female initiates behavior.
Schema developed to categorize types of female behaviors.
Proposed by Beach in 1976. Classifying behavior may help understand
the hormones
underlying them.
Attractivity: the stimulus value of a female for a given male.
Attractivity is measured by a PROX score based on the amount of time the male spends in proximity of the female.
Some factors may mediate attraction.
Proceptivity: degree to which females initiate copulation.
Measured in terms of what behaviors the female engages in and what
she'll go through to get a mate. Proceptivity can also be determined by male attractiveness.
Receptivity: state of responsiveness to the sexual initiation
of another.
Measured through lordosis behavior. Only occurs at end of proestrus or
beginning of estrus or
vaginal proestrus.
The Role of Hormones in each of the Stages
Estrogen is highest around ovulation when males find females most attractive.
Injections of progesterone tend to decrease attractiveness.
Attractivity is mediated by chemosensory cues generally present in urine. Also known as pheromones. If you block the ability to smell, females tend not to engage in these behaviors quite as frequently.
These behaviors have typically been studied in lab. It has recently been found that behaviors elicited in lab are different than in the wild. Rats live in colonies.
Almost all females in colony wil
come into behavioral estrus at the same time. Females tend to have thier
offspring all at the same
time. Might promote the survivability of the offspring.
Females also tend to modulate the receptive phase of the cycle
differently in the wild than the lab.
During receptive phase, rats engage in lordosis. In a large colony,
female will iniate copulation,
male will mount and intromit and then the female runs away and then she
will allow the male to
mount her in 3 or 4 minutes. The timing ensures the vaginal code in
which stimulation of the
vagina causes the corpus luteum to be maintained and progesterone
secreted which will encourage
pregnancy.
Social or Environmental Effects
Whitten effect: Whenever a female rat was placed in a cage
with a male the females
typically would not engage in sexual activity with the males until they
had been in cage with him
for 3 or 4 days. Males may actually secrete phermones that triggers the
estrus phase in females
and it takes about 3 days to occur. Also insures that all females tend
to come into estrus at the
same time as found in the wild. Male synchronizes activity. Females, if
there's no male around,
tend to suspend the estrus cycle. If there is no male present, females
cease the ovarian cycle.
Lee-Boot: Similar to Whitten effect except that if only females are in a small group (4 to a cage) they tend to exhibit a prolonged diestrus phase.
Both seem to be mediated by chemosensory cues. If you remove the
olfactory bulbs, there is
no effect.
Bruce effect:When a female is pregnant and the male that sired
her offspring is
removed and strange male comes on the scene, the fetuses are reabsorbed
or aborted and she goes
into estrus. One possible reason for her behavior is that it's not
good for the female to invest
all her energy in having the offspring when the new male will probably
kill them. The same effect
is found in primates. She will spontaneously abort or if she has a
newborn will engage in
infanticide. She doesn't waste her time when male will kill offspring
anyway.
Vandenbergh effect: Mediated by chemosensory cues. Females
will sexually mature
earlier if they are exposed to adult males. The reaction is androgen
based, males release
pheromones that cause an abrupt release of LH. They cut off the amount
of days to reach sexual
matruity by roughly a third.
If you place female pups before they reach their first estrus phase in
an environment with only
females, their puberty is delayed. You can mimc the effects by exposing
the pups to male urine.
Conversely, some chemosensory cues from female urine casues the
supporesion of LH and
FSH thereby slowing maturation.
Neural Mediation of Mating Behavior in the Female
Based on studies of lordosis behavior. Basically, we know that
unlike males who still perform
penile erection and orgasm if you sever the spinal cord, females do not
engage in lordosis
behavior if you cut the spinal cord. There has to be some influence from
higher centers of the
brain for females to engage in lordosis.
Looked at concentrations of estrogens and progesterones in the brain. Found there are cells throughout brain that concentrate estrogens and progesterones.The preoptic area, ventromedial hypothalamus, parts of the pituitary and midbrain have the highest amount of estradiol contcentrating cells. If you lesion the ventromedial nucleus, your reduce the amount of lordossis behavior.
If you lesion the midbrain central gray, you decrease
lordosis behavior.
Sensory receptors (located on the skin of the anogenital region) which send information from the male mounting the female are much larger when estrogen levels are highest in the ovarian cycle. Estrogen promotes the growth in number and size of sensory receptors during proestrus and information related to male mounting the female is relayed to the medullary retiuclar formation.
This area governs muscles that control the
reflexive behavior during lordosis.
Information is sent from medullary retiruclar formation to the midbrain
central gray which receive
input from regions of the hypothalamus. If you input estradiol into the
ventromedial nucleus or
the midbrain central gray, females will engage in lordosis behavior.
Frequency is somewhat less
than rats found in the wild. If you give an injection of estrogen
followed 6-8 hours later by an
injection of progesterone, lordosis behavior is maximized. This can be
greater than lordosis
behavior found in proestrus.
The highest frequency of sexual behavior occurs with estrogen
followed by progesterone.
Perhaps estrogen primes the system to facilitate the later progesterone
injection. Estrogen tends to
cause a change in the electrical activity of the midbrain central gray.
This tends to produce
growth-like appendages on neurons of the VMH. These may be analogous to
dendrites or axons
in these animals.
One way to change activity of the midbrain central gray is to administer
an injection of steroids
which increase the number of receptors it's associated with. Estrogen
injections tend to promote
the increased production of progesterone receptors in the
midbrain central gray.
Estrogen must be present in higher brain structures to cause lordosis behavior. Looks much like organizational effects prenatally. You can change the structure and electrical activity in these areas. Suggests the primary way in which hormones interact with all behaviors is an organizational, activational effect.