Chapter 3: Hormonal Influences on Female Sex Behavior

The gonadal steroids play an important role in altering the survival or death of neurons within the brain as well regulating the development of neuronal processes (such as the number and length of dendrites and dendritic spines, or the degree of axonal branching). In this regard, we can say that steroid hormones have organizational effects that produce sex differences in the connections and functions of the brain. Gonadal steroids also have activational effects; these activational effects are transient and are thought to coordinate behavior with either an internal event, such as ovulation, or an external event, such as the presence of a sexual partner.

Chapter 3 discusses the various components of female sex behavior and consider the role of estrogen and progesterone in stimulating the display of this behavior.

Female sex behavior can be divided into 2 components: 1) precopulatory behaviors, also considered courtship behaviors, are important for the subsequent display of copulation, and 2) copulatory behaviors, which in the female involve the display of reflexive postures.

Precopulatory Behaviors:
In the female rat, precopulatory or courtship behaviors include a female's willingness to approach a male and to show solicitatious behaviors, known as proceptive behavior.

There are three components to proceptive behavior: approach (in which the female approaches the male), orientation (female orients to the male in such a way that the male may sniff and groom the female's anogenital region), whenthe male approaches the female, the female will often show "ear wiggling" which is an extremely rapid vibration of the head which makes it appear as though the ears are wiggling; and runaway (in which female shows hopping and darting; hopping is a rapid hop with almost rigid legs, and this is combined with fast "darting" movements away from the male).

This sequence of events: approach, orientation and runaway shown by the female rat, will induce a male rat to chase after the female, and to engage in copulation.

Estrogen is important in female precopulatory behaviors in 3 m ways: 1) estrogen increases a female's willingness to approach a male to induce solicitatious behaviors, 2) estrogen can enhance the "attractivity" of the female: estrogen priming can induce the production of stimuli that make her more attractive to the male (odors or pheromones, vocalizations), and 3) estrogen 'primes' for progesterone.

Progesterone has a biphasic effect: initially. First there is a transient rise in progesterone (following priming by estrogen) which stimulates sexually receptivity. Second, prolonged secretion of progesterone will inhibit sexual receptivity (this inhibitory effect is also associated with the role of progesterone in maintaining pregnancy, this makes sense at some level that if you a pregnant there is no reason to expend energy on mating; especially if the significance of mating is to reproduce).
Copulatory Behaviors
Figure 1.1 In rodents and a variety of other mammals, female sex behavior involves the display of lordosis (lordotic reflex ). Lordosis is characterized by immobility on the part of the female along with an arching of the back and hindleg extension that elevates the rump and head. In some species (like the rat), this may involve deviation of the tail to one side.

Lordosis is typically elicited when a male mounts the female; but in some cases, lordosis can be seen prior to male contact or with manual stimulation of the females flank and hindquarters.

Lordosis can be quantified in terms of several features: 1) strength of the lordotic posture, 2) the frequency of lordosis, 3) the duration of lordosis during a test period with a sexually active male. In rats, it is common to calculate a lordosis quotient (LQ), which is simply the ratio of the number of lordosis postures shown by a female in response to a fixed number of mounts (usually 10) times 100. For example, if a female shows 5 lordotic responses to 10 mounts, then that female rat is said to have a lordosis quotient of 50 (moderately receptive). What this ratio implies is that a male rat can mount a female rat when she is not showing lordosis. This is not true in other species.

Reason for quantifying lordosis is to obtain a measure of the degree to which the female is receptive. Female receptivity can vary for a number of reasons; in the lab, such variations are due to manipulations hormones and/or manipulations of the brain.
In many mammalian species, females show precopulatory and copulatory behaviors at specific times during their reproductive cycle. In rodents, behavioral estrus is the term given to the period of sexual receptivity (and the time at which a female shows precopulatory and copulatory behaviors); sometimes referred to as heat. Behavioral estrus is linked to estrus whichrefers to the day in which ovulation occurs. There are a series of ovarian events that determines when an animal comes into estrous, this series of events is known as the estrus cycle.

As we will discuss, ovarian hormones, estrogen and progesterone, play an important role in coupling behavioral estrus (sex behavior) with ovulation: both responses being necessary for reproduction to occur.

Phases of the ovarian cycle:
In animals that ovulate spontaneously, the duration of an ovarian cycle can vary: 4 days (mice, rats, hamsters), 16 days (guinea pigs), 28 day (humans). There are two main phases that reflect specific effects occurring during the particular phase, as well as a periovulatory period associated with the occurrence of ovulation.

Work with Figure 3.1 from text. We will start by discussing in detail the ovarian cycle of the female rat. I will note differences that exist in other species.

Follicular Phase: Neurons in hypothalamus secrete GnRH (gonadotropin releasing hormone), which stimulates cells in the anterior pituitary to secrete (luteinizing hormone (LH) and follicle-stimulating hormone (FSH); LS and FSH act at the ovary to stimulate development of the egg or oocyte and maturation of the follicle. Follicular cells surround the oocyte and nurture its development; follicular cells also produce estrogen. [FSH-stimulates primary follicles to enlarge and differentiate forming two distinct layers of cells: inner granulosa cell layer (secrete fluid into the antrum of the follicle), and an outer thecal cell layer; LH and FSH work together to produce estrogen: LH stimulates thecal cells to produce androgens, and androgens are converted to estrogens in granulosa cells via aromatase; FSH stimulates process by increasing levels of aromatase). Follicular phase lasts 3 days in rodents to 10-14 days in humans.

Periovulatory Period: As the egg matures, the rate of estrogen secretion increases. This increase in estrogen results in a surge of GnRH released from the hypothalamus, which in turn induces a surge of LH released from the pituitary [what about FSH?, granulosa cels also secrete inhibin which selectively inhibits release of FSH from the pituitary]. The LH surge causes the follicle to rupture, resulting in ovulation. In the female rat, LH also induces a preovulatory surge of progesterone release from the ovaries. The preovulatory surge of progesterone triggers the onset of sexually receptive behaviors in some species (rats and hamsters). The periovulatory period lasts about 12 hours in most rodents; in humans it occurs over a period of 1 to 2 days.

Luteal Phase: Once the follicle has ruptured and expelled its egg into the Fallopian tube, the follicle is transformed into a new endocrine organ called the corpus luteum. The corpus luteum secretes progesterone which is important for implantation of the egg into the uterine wall and the maintenance of pregnancy.

If progesterone levels decrease or if they are blocked by an antagonist, then either the fertilized egg will not be able to implant into the uterus or if it has implanted, it will be spontaneously aborted. The abortion pill, RU-486, produces its effect by blocking the action of progesterone.

Formation of the corpus luteum differs between species. We will come back to this point shortly.

Species differences associated with the periovulatory period:
1. In rats, the LH surge is associated with increased levels of estrogen stimulating a GnRH surge. In other species, such as primates, the LH surge is believed reflect an increased sensitivity of pituitary gland to GnRH (rather than a surge in GnRH from the hypothalamus). [The follicular phase rise in estradiol stimulates synthesis of GnRH receptors and enhances pituitary responsiveness to GnRH; although this is also true of both the deterministic or permissive model (rat or primate models).]

2. In rats and hamsters, a rise in progesterone occurs following LH secretion (to time copulation) and following formation of the corupus uteum following vaginal-cervical stimulation. However, in females of other species, a rise in progesterone occurs only after ovulation with the presence of a functional corpus luteum. In other species, a rise in estrogen is important for timing sexual behavior and ovulation.

The corpus luteum can either be spontaneously functional or not. What do I mean by spontaneously functional?

In some species, the corpus luteum is spontaneously function; that means that following ovulation, the corpus luteum will form and will secrete progesterone (and estrogen) for several days following ovulation. This phenomenon is true for primates/humans; and the corpus luteum is functional for a period of about 10 days. If an egg is fertilized following ovulation, the luteal phase hormones assist with implantation of the fertilized egg within the uterus and the maintenance of pregnancy.
In other species, the corpus luteum is not spontaneously functional; in this case it will regress unless vaginal-cervical stimulation has occurred during copulation. If vaginal-cervical stimulation occurs, then the corpus luteum will be retain and will secrete progesterone. This scenario is true for species like rats and mice.

What do I mean when I say vaginal-cervical stimulation? I am talking about intromissions, which is the insertion of the penis into the vagina. How do we know that vaginal-cervical stimulation is the critical input? First it has been shown that the number of females that become progestational (that is, there corpus luteum secretes progesterone) is dependent upon the number of intromissions. Second, we can stimulate the process of intromission by using a small glass rod. So if we use a glass rod to stimulate the vaginal cervical region of sexually receptive females (primed with estrogen and progesterone), we can stimulate the production of progesterone in these females and maintain formation of the corpus luteum over several days. Normally, a female that mates with a male will receive intromissions followed by ejaculations that will lead to pregnancy; secretion of progesterone by the now functional corpus luteum will enable fertilized eggs to implant into the uterine horn and develop. Stimulation of the vaginal cervical region of females with a rod will produce a state called pseudopregnancy, in which progesterone levels are elevated for a period of 3 days or so before they decline and ovarian cyclicity again occurs.

How do intromissions lead to the secretion of progesterone from the ovary? Intromissions by a male rat initiates a neuroendocrine reflex in the female rat; the result of this reflex is to induce repeated surges of PRL which in turn induce an elevated secretion of protesterone from the ovary. Draw out relationship: sensory input associated with intromissions > spinal cord > brain > hypothalamus > neurosecretory cells secrete prolactin-releasing factor into median eminence > PRF reaches cells in the anterior pituitary causing the release of prolactin > prolactin stimulates formation of corpus luteum > secretion of progesterone.

In addition to a follicular phase, periovulatory period, and a luteal phase, primates show a menstrual phase in which menstruation occurs after the fall in progesterone and estrogen secretion, and is associated with regression of the corpus luteum. The long ovarian cycle of primates results in a buildup of the uterine wall (endometrium), and sloughing of the endometrium results in uterine bleeding. In other species (rat), the estrous cycle is shorter, the endometrial wall is thinner and does not bleed, and there is not event analogous to menstruation.

In many species, ovarian hormones coordinate the display of female sex behavior with the occurrence of ovulation. The 4 day estrous cycle provides a nice example of this pattern of coordination.
* metestrus: day following ovulation; low levels of hormones and no sexual activity
* disestrus: day associated with follicular activity and estrogensecretion
* proestrus: rising estrogen leads to surge in GnRH from hypothalamus and a surge in LH from pituitary that will lead to ovulation on the mornin of estrus; onset of behavioral estrus then occurs during the time of ovulation (behavioral estrus referring to the display of precopulatory and copulatory behaviors).
* estrus: day of the cycle when ovulation occurs; behavioral estrus continues into the morning of estrus

There are variations in mechanisms that mediate ovulation.
* spontaneous ovulation: a series of hormonal events lead to the occurrence of ovulation (humans and rats)
* induced ovulation: sexual receptivity occurs in spontaneous cycles, but ovulation does not occur without copulation (cats, rabbits, ferrets)
* induced estrus and ovulation: both behavioral estrus and ovulation are induced by specific stimuli (prairie voles & musk shrew; unique in requirement for androgens)

Induced Ovulation
* includes species such as rabbits, cats and ferrets
* during reproductive season, the ovary produces waves of follicles that are activated to produce mature eggs
* as the ovarian follicles mature, the follicular cells secrete estrogen, and estrogen induces, within a day or more, behavioral estrus leading to mating; in induced ovulators, progesterone is not needed for stimulation of sex behaviors
* vaginal-cervical stimulation associated with copulation (insertion of the penis into the vagina; intromission) activates a neuroendocrine reflex that results in hypothalamic release of GnRH, stimulating the pituitary to release LH that induces the rupture of the follicle and ovulation
* progesterone is not essential for female receptivity in these species, but it may play a role in terminating estrus and it would be involved in maintenance of pregnancy

Induced Estrus and Ovulation
* includes species such as prairie voles
* animals remain reproductively inactive until stimuli from a male are present; stimulus that is critical is the presence of a pheromone in the urine of the male
* exposure to the pheromone leads to an increase in estrogen that primes the female to show sexual receptivity in about 1 day; ovulation is then induced about 10-12 hours following coitus; I assume that vaginal-cervical stimulation associated with copulation activates a neuroendocrine reflex that results in hypothalamic release of GnRH stimulating the pituitary to release LH which induces the rupture of the follicle and ovulation
* progesterone secretion follows ovulation by a day or more; progesterone functions to inhibit receptivity in prairie voles (similar to the process occurring in induced ovulators) and maintenance of pregnancy
* the musk shrew is another species that relies on social stimuli for them induction of lordosis and ovulation
* female musk shrews will begin to show sexual behavior within 1 hour of male exposure
* it is believed that androgens secreted from the ovary are critical for female sex behavior in this species (instead of estrogen)
* androgens secreted by the ovary are then aromatized to estrogens within the brain where they mediate their effects 

How do these changes in hormones associated with estrous cycle affect the


First, it is important to realize that steroid receptors are localized within specific areas of the brain. This localization is important for understanding that specific parts of the brain mediate female sex behavior.

Figure 3.3 illustrates a sagittal view of a female rat brain showing the relative distribution of estradiol-concentrating cells. Some of the areas that have the highest concentration of of estrogen receptors include the preoptic area, hypothalamus (ventromedial nucleus of the hypothalamus, midbrain and pituitary; the amygdala also shows a high levels of ERs but are not illustrated in this schematic.

We will focus our attention on the ventromedial nucleus of the hypothalamus, a brain region shown to be critical for the facilitatory effects of gonadal steroids on lordosis.
Cells within the lateral part of the VMH contain ERs. ERs are regulated during the estrous cycle. ER mRNA levels are elevated in the VMH, with peak levels observed during proestrus; time when estrogen levels are elevated, progesterone levels are also elevated, and increased estrogen will lead to a surge in GnRH, and LH followed by ovulation during estrus. ER mRNA levels decrease significantly during estrus.

So ER mRNA levels vary with the estrous cycle; this is also true for ER protein. And it is known that the increase in estrogen that occurs during the follicular phase is important for this increase; estrogen stimulates the production of its own receptors.

Interestingly, estrogen also stimulates the production of progesterone receptors within the lateral part of the VMH. Estrogen induced production of progesterone receptors is then associated with the ability of preovulatory surge of progesterone to stimulate lordosis.

We know that the VMH is critical for the facilitatory effects of gonadal steroids on female sex behavior:
* if we lesion the VMH, we see reduction in lordosis
* if we ovariectomized female rats, and then give them estrogen and progesterone implants into the VMH we can reinstate female sex behavior; so hormones don't have to reach other parts of the brain to produce stimulatory effect.
Hormones act at the VMH to stimulate neuronal activity over hours (in ovariectomized females, estrogen injections are given over a period of 1-2 days, followed by a single injection of progesterone which stimulates behavior about 4 hours later).

VMH neurons project to the the midbrain periaqueductal gray. Hormones excite VMH neurons > excites neurons within the midbrain > excites lower motor neurons within the brain stem and spinal cord to stimulate lordosis.

I want to return to the idea that progesterone has a biphasic effect of sexual behavior. We said that in the female rat, that an initial rise in progesterone is important for stimulating sexual receptivity, and that prolonged levels of progesterone can lead to inhibition of receptivity. We think that the mechanism for inhibition of receptivity is associated with the ability of progesterone over a period of time to cause a decrease in the number of estrogen and progesterone receptors. So following a period of mating, females become refractory to the stimulatory effects of estrogen and progesterone due to the inhibitory effect of prolonged exposure of progesterone on estrogen and progesterone receptors.

Review the module explanation of lordosis. Note: although VMH is believed critical for the hormonal display of lordosis, hormones may have additional effects on other brain regions.

The forebrain has been shown to provide largely inhibitory input into the lordosis circuit (as well as the circuit mediating GnRH secretion and ovulation). Such inhibitory effects may be associated with stress which can inhibit reproductive activity. Also, there are a number of pheromones secreted by males or females that can inhibit reproduction in a variety of ways; pheromones would be detected by sensory neurons within the olfactory bulbs which would subsequently carry that information centrally (via pathways in the forebrain) to mediate their inhibitory effects.

There are a number of neurohormones and neurotransmitters that play an important role in regulating female sex behavior. We do not have time to look at this regulation in any detail.

GnRH Neurons

GnRH neurons are scattered throughout the medial preoptic area. GnRH neurons project to the median eminence to stimulate release of LH and FSH (and important for LH surge in species like the rat). GnRH neurons are also known to project to the midbrain central gray, and to facilitate lordosis at those sites. It is possible that GnRH release in this area helps to couple ovulation with behavioral receptivity. This would suggest redundancy in CNS function. Clearly, redundancy may be important to processes like reproduction that is essential for survival of the species.

Noradrenergic Neurons

Norepinephrine neurons located within the brainstem secrete norepinephrine within multiple brain sites including medial preoptic area and the VMH. NEhas been implicated in the release of GnRH (ovulation and possibly behavior), as well as in stimulating lordosis at the level of the VMH.

Dopaminergic Neurons

Dopamine is a catecholamine that may be important for promoting proceptive motor behaviors (ear wiggling, hopping and darting) as well as general movements. Two groups of cells within the midbrain, the ventral tegmental areas and the substantia nigra, produce dopamine and project to the basal ganglia; the basal ganglia are important for the execution of stereotyped movements (or movements that are done in the same way). Gonadal hormones can stimulate dopamine activity that may be important for some motor responses during female sex behavior (not for lordosis, per se).