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Hormones & Sexual Behaviour
Author Paul Kenyon


Visit the SALMON Bookshop for recommended books on this topic
Overview:

A quick glance at the title might lead you to believe that this is going to be a very interesting lecture. Well it is interesting, but not in the way you think!

You will see how the role of hormones in behaviour has been elucidated by removing the endocrine glands, observing the animals behaviour and then replacing hormones to reverse the effects of the original intervention. This is a fundamental scientific strategy (remove - observe - replace - observe) and crops up in many behavioural experiments.

A very important question is: Do the effects of these hormone manipulations reflect an effect of hormones on the animal's brain, or are the effects secondary to changes in peripheral tissues? It would be nice to believe that we were observing central effects - after all we are principally interested in the brain. But the possibility that the animal's behaviour is affected by more mundane peripheral effects needs to be experimentally verified. Studies which address this issue are described.

Learning objectives
After reading this material you should be able to:
  • describe the sexual behaviour of female rats
  • operationally distinguish between receptivity, proceptivity and attractiveness
  • describe the sexual behaviour of male rats
  • list 4 measures of male rat sexual performance
  • distinguish between time limited, series limited and satiation tests of sexual behaviour
  • draw a diagram to illustrate the effects of estrogen injection dose on sexual behaviour in female adult rats
  • describe the effect of ovariectomy & estrogen replacement on rat uterine weight
  • describe the effects of castration on sexual behaviour of male rodents
  • describe the effect of castration and testosterone replacement on seminal vesicle weight and papillae of glans penis in male rats
  • explain the importance of studies involving central injection of hormones 


Hormones and human male sexual behavior 

Heim and Hursch (1979), reviewed the effects of castration on men convicted of sex crimes.

  • 50+% stopped exhibiting sexual behavior shortly after castration
  • about 25% showed a gradual decline in sexual behaviour
  • around 10% showed no change in sexual behaviour after castration

Similar effects have been shown in rodents. There are significant individual difference between animals in their sexual behaviour which are not related to circulating level of testosterone and which persist after castration and hormone replacement therapy (see Nelson, 1995)

Davidson et al (1982), studied the effects of androgen injections in men who had very low levels of testosterone circulating in their bodies (called hypogonadal men).
Six patients each received:
  • 100 mg of testosterone enanthate ( a long lasting androgen) dissolved in oil
  • 400 mg of testosterone enanthate dissolved in oil
  • placebo (the oil vehicle used to dissolve the androgen)

This study used a within-subjects design; each patient received each treatment. There was a gap of six weeks between treatments, and the order of treatments varied between patients to control for treatment order (carry over) effects.

The diagram shows that injection of testosterone enanthate increased plasma testosterone levels. Notice that the effect is dose-dependent, and temporary. Peak effects were seen seven days after injection.

The hypogonadal men kept daily diaries of their sexual behaviour and penile erections. The biggest effects of testosterone injections were reported one week after injection. This corresponds to the time of peak effect on plasma testosterone level.

This diagram shows these effects. Notice that the effects are dose-dependent, and that the values do not reach 100% for any of the measures, even at the highest dose of testosterone (400 mg).
Of course one possibility is that not enough testosterone was injected. This is unlikely because studies indicate that plasma testosterone levels as high as 1500 ng/100 ml (the level seen after injection of 400 mg testosterone enanthate) are rare in normal men.

In fact the most surprising thing about male testosterone level is its huge range - shown in red in this diagram . In general there is a tendency for testosterone level to increase during teenage years and the 20s. it usually remains steady in middle life, but after 60 there is a decline to the level seen in young boys. Nevertheless as this diagram shows there are a lot of 90 year old men out there with teenage levels of circulating testosterone! (Vermeulen et al, 1972).

Global Variation in Male Testosterone and Age by Peter T. Ellison et al at Harvard contains a nice diagram showing the decline in male salivary testosterone level as a function of age.

Frank Beach

Frank Beach
In order to understand the role of hormones in our behaviour scientists have conducted fundamental research using laboratory animals. Frank Beach is generally considered to be the founding father of the study of the effects of hormones on behaviour - an area of scientific investigation that is known today as behavioural endocrinology or psychoendocrinology. His first book, Hormones and Behavior (1948), summarizes all that was known at the time and organized the field for all subsequent investigators. Beach devoted his whole life to basic research and justified his endeavours as follows:

"To what degree should my choice of research work be governed by human needs, by social imperatives, and how am I going to justify spending all of my energies on any research that does not bear directly on pressing human problems.... The solution, or rationalization, that I have finally come up with is that it is a perfectly worthwhile way of spending one's life to do your level best to increase human knowledge, and it is not necessary nor is it always even desirable to be constrained by possible applicability of what you find to immediate problems. This may sound very peculiar to some young people, but it is a value judgement which I myself have made and which I can live with."


As you probably realize, Frank Beach is my hero, and I have spent a large part of my research career following up his fascinating work on the role of sensory systems in controlling reproductive behaviours.

Point to ponder
Why are you interested in the study of behaviour?
Do you think it would be possible to pursue a research career in the 1990s along the lines followed by Beach?
What do you think is the general publics' perception of science today, and why?

Sexual behaviour in the female rat
The estrus cycle lasts four or five days in the rat, and is associated with variations in the level of the hormones estrogen and progesterone released by the ovaries. Around mid-cycle there is a 12-15 hour period of estrus or 'heat' during which the female is receptive to the male. Estrus is timed to occur when eggs are released from the ovaries (ovulation) to increase the likelihood that they will be fertilized by sperm from the male rat.

At one time it was thought that females were 'passive recipients' of male ardour. Many early studies measured female sexual behaviour in terms of 'lordosis'. Lordosis refers to a characteristic posture in which the female rodent arches her back and moves her tail to permit penetration by the male. Lordosis is a reflex which usually occurs when a male grasps the female's flanks.

Points to ponder
bite.gif (13871 bytes)Why do you think - at one time - female rats were viewed as passive recipients of male sexual sexual behaviour?
What does this tell us about the effect of the influence of researchers' attitudes and values?
Can you think of other examples of this effect?

Because lordosis is elicited by males this gives the impression that females are passive players in a sexual encounter. However, the female rat often plays a very active role in initiating copulation. If she is in estrus she may:

 

In the first part of this lecture we will examine the role of estrogen and progesterone in the sexual behaviours of female rats. This diagram shows that estrogen and progesterone levels increase before ovulation, and are at a relatively high level during estrus. A number of experiments have examined the roles of estrogen and progesterone on the sexual behaviours of female rats.

These studies employ the hormone-replacement strategy. Female rats have their ovaries removed under general anaesthetic in an operation called ovariectomy.

A female rat who has lost here ovaries in this way is said to be ovariectomised. Male rats do not mate with ovariectomised females, presumably because they do not come into estrus (heat).

This may be a consequence of the loss of estrogen and progesterone which are normally secreted by the ovaries. To investigate this possibility, ovariectomised rats are injected with estrogen or estrogen and progesterone before being given a test of their sexual behaviour. A diagram of the approximate location of the ovaries and uterus has been superimposed on this photograph - Ignore the suckling pups.


 


Effects of estrogen on sexual behaviour in female rats

In this experiment (Davidson et al, 1968) adult female rats with ovaries removed (ovariectomy) were given daily injections of estradiol benzoate.

Five groups each received a different dose over a 12 day period.

On days 8, 10 and 12 each rat was given about 10 test trials with a sexually active male.

The behavioural measure was the lordosis score, that is, the percentage of trials on which the female showed a lordosis response when the male attempted to mount her.

Scores are mean performance over the three test days.


Effects of progesterone on sexual behaviour in female rats

Davidson's experiment tells us that estrogen replacement restores one aspect of the sexual behaviour of ovariectomised rats. But his experiment involved chronic treatment with estrogen over several days. Normally estrogen levels increase a matter of hours before estrus. It could be said that chronic experiments do not tell us much about the role of hormones under natural conditions. Therefore subsequent studies explored the acute effect of estrogen, and the interaction between estrogen and progesterone.

A study by de Jonge et al shows that a single injection of estrogen can elicit lordosis behaviour and that the effect of estrogen is roughly dose dependant - the greater the dose of estrogen injected the higher the females lordosis score.

Perhaps the more interesting finding is that progesterone facilitates the effect of estrogen. If ovariectomised rats are injected with progesterone, they do not show lordosis. But this diagram shows that if progesterone is given in conjunction with estrogen, the lordosis score was increased compared to the score under estrogen alone.


Measuring sexual motivation in the female rat

In the experiments considered so far, only one aspect of female sexual behaviour was explored - the lordosis response. But we now know that even female rats can be strongly motivated to engage in sexual behaviour, and can play an active role in initiating sexual behaviour from males. The article by Doty (1974) was seminal in bringing about this change in attitude towards sexuality in female rats.

In fact hormones have at least three effects on female sexual behaviour. Hormones modify:

This diagram shows a piece of apparatus that can be used to measure female sexual motivation. A male and a female rat are enclosed in individual boxes with gauze windows placed within a larger test apparatus.

The time spent by a test rat in front of each box is recorded through pressure-sensitive pads located outside each small box.

If a female test rat spends more time in front of the box containing a male rat than close to a female, this suggests that the test rat is sexually motivated and exhibiting proceptive behaviour.

If a male test rat spends more time in front of a box containing a female in estrus, than in front of an ovariectomised rat, this suggests that he finds the estrus female more attractive.


Effects of hormones on female proceptive behaviour

This picture shows the results of an experiment which examined the effects of various hormones on the proceptive behaviour of female rats.

Injection of estrogen, testosterone or a combined injection of estrogen and progesterone, increased the amount of time ovariectomised rats spent in the vicinity of a male rat. This suggests that these hormones control proceptive behaviour in the female rat.

In this study de Jonge et al measured the amount of time ovariectomised rats spent in front of cages containing male and female rats during a 15 minute test.

I have expressed their results as the extra amount of time spent in front of the preferred cage. For example, after an injection of testosterone the ovariectomized rats spent on average 100 seconds longer in front of the cage holding the male rat compared to the amount of time spent in front of the cage containing the female.

There are two interesting results in this experiment:

One explanation for this effect of testosterone involves conversion of testosterone into estrogen by a process called aromatization. This is unlikely because, in a further experiment de Jong et al, showed that a synthetic testosterone - that cannot be converted to estrogen also increased proceptive behaviour in ovariectomised rats.

Testosterone may also be involved in human sexual motivation (Carlson, 2001).

Point to ponder
How might you measure the effect(s) of hormones on human sexual behaviour?

Effect of estrogen on growth of rat uterus
How might estrogen affect sexual behaviour? It's tempting to conclude from the results above showing the effect of estrogen on sexual receptivity that estrogen directly stimulates brain cells that control behaviour.

Alternatively the hormone may be having an effect on sensory stimulation from peripheral tissue that changes behaviour without the hormone affecting the brain directly. For example the diagram shows the effect of ovariectomy on rat uterus. Compared to a normal uterus, the uterus in an ovariectomised rat is small and shrivelled. Uterine weight recovers under estrogen replacement therapy.

Can the decline in sexual activity in ovariectomised animals be simply explained as due to pain when the animal is mounted by the male? Another possibility is that ovariectomised animals do not emit scent signals (pheromones) that make them attractive to male animals.

These simply explanations need to be ruled out before we can conclude that the effects of hormones seen in the behavioural experiment involve direct effects of hormones on the brain.


Effects of central injection of hormones on sexual behaviour:
hormone receptor sitesThere are specific receptor sites in the brain that act as target cells for hormones released by the testes and ovaries. These are shown as red blobs in the diagram.

Receptor sites are particularly concentrated at the base of the brain in an area running from the preoptic area back to the hypothalamus ( but note that several other areas including the midbrain and hippocampus contain hormone receptor sites.)

Injecting estrogen into the hypothalamic area of ovariectomised female rats restores sexual receptivity. This procedure involves lowering a hollow tube ( called a cannula) containing hormone into the brain.

Implanting these hormones into other parts of the brain does not restore sexual behaviour which shows that the effects are site specific. Furthermore the brain implants are not associated with recovery of peripheral tissue that is hormone sensitive, which indicates that the hormone has not leaked into the animals circulation, and supports the conclusion that the restoration of behaviour is the result of the hormone affecting brain tissue that mediates sexual behaviour in normal animals.

How hormones affect the brain to control sexual behaviour is the subject of a separate lecture on your course.


Sexual behaviour in the male rat
Sexual behaviour in male rats consists of three behaviours:

It can be very difficult - even for a trained observer - to distinguish between these behaviours. There are subtle differences in the style of dismounts that help distinguish between mounts and intromissions. After an ejaculation, the male tends to pause longer before dismounting. Every ejaculation is preceded by a number of mounts and intromissions. Typically rats have multiple ejaculations before they reach exhaustion (satiation). After each ejaculation there is a refractory period lasting several minutes. Sexual encounters are divided into a number of 'series' depending upon the number of ejaculations. A series starts with a mount or intromission, and ends with an ejaculation.

Numerous measures of the relationships between mounts, intromissions and ejaculations are recorded to measure sexual motivation and performance. For example:

Tests of sexual behaviour are either:


A model of male sexual motivation

Under normal conditions male rats typically emit an intromission every 20 to 30 seconds, and ejaculate after 10-12 intromissions.

The Swedish psychologist Knut Larsson discovered that if the male was removed from the test area after each intromission, and then replaced 2-3 minutes later, the males ejaculated after fewer - four or five - intromissions.

However, if the males were removed for seven or more minutes, they needed a large number of intromissions to achieve ejaculation, and some failed to ejaculate at all.

Larsson suggested an interesting theory to account for this finding whichexplains the role of intromissions in rat sexual behaviour.

According to Larsson's "accumulating excitation" model the excitation required for a rat to ejaculate increases for several minutes after each intromission, and then declines when there is a longer interval after intromission.

This model can explain why fewer intromissions are required to reach an ejaculation if the rat is prevented from intromitting for 2-3 minutes after his previous intromission. Each successive intromission builds on the accumulated excitation from preceding intromissions.

However if the enforced interval is too long (e.g. 7 minutes) between successive intromissions, excitation may never build up to the level required to trigger ejaculation.

The next diagrams show how intromissions increase excitation leading up to ejaculation.


Effects of castration on sexual behaviour of male rodents
In the experiment shown here, carried out on male guinea pigs (Grunt & Young, 1952), sexual behaviour decreased shortly after castration (gonadectomy) and was re-established by the administration of testosterone to castrated animals.

The hands on the figure point to weeks when these treatment changes occurred.


The role of estrogens in male sexual behaviour

Estrogen and androgen are names given to two important families of hormones. To complicate things further many brain cells contain an enzyme that converts androgens such as testosterone into estradiol (an estrogen). this process is called aromatization. Although it is convenient to refer to androgens and estrogens as 'sex hormones' it is important to point out that males and females produce both types of hormone. Furthermore, sex hormones can be produced in the adrenal glands as well as the gonads (testes and ovaries).

We have already seen that testosterone plays an important role in female sexual motivation. It appears that estrogen is important for maintaining mating in males.

This table shows that only androgens that can be converted to estrogens restore sexual behaviour in castrated male rodents. Furthermore estradiol (an estrogen) also activates mating in castrates.

Hormone Aromatized to estrogen? Maintains sexual behaviour
in castrated males?
testosterone
androstenedione
dihydrotestosterone
estradiol

Dihydrotestosterone is thought to play an important role in maintaining the sensitivity of the penis (see Nelson, 1995)

Point to ponder
If you were called as an expert witness in the trial of a pedophiliac, what treatment would you recommend?

Effect of testosterone on seminal vesicle weight
It is tempting to think that the restoration of sexual behaviour in castrated male animals given testosterone replacement therapy is the result of the hormone reactivating 'centers' in the brain that control sexual motivation.

This is an attractive hypothesis because it implies that these experiments will tell us something important about the relationship between brain and behaviour.

However a more parsimonious explanation might be that castration somehow affects tissue outside the brain that is involved in sexual behaviour. For example, this diagram shows the effect of castration and testosterone replacement on seminal vesicle weight.


Effect of hormones on papillae of glans penis in male rats
If you are still not convinced by this argument that recovery of sexual behaviour with testosterone replacement is the result of recovery of peripheral function, consider this diagram which shows the effect of castration and testosterone replacement therapy upon the number of genital papillae and the number of animals continuing to copulate four weeks after castration.

The glans penis (tip of the penis) in the rat is covered in sensitive papillae. Maybe it's the recovery of sensation in this area that is responsible for the recovery of behaviour under testosterone therapy. All scores are expressed as percentages of the averages for normal rats (Data from Beach & Levinson, 1950).

This experiment suggests that the effects of replacement testosterone on sexual behaviour could be mediated by changes in the sensitivity of the rats' penis, rather than an effect of the hormone on brain tissue.

Although dihydrotestosterone is thought to play an important role in maintaining the sensitivity of the penis (see Nelson, 1995), it is now clear that some of the effects of testosterone on behaviour involve the brain. This is a complex area and we only have time in this lecture to give a flavour of the research which led to this conclusion.


Effects of central injection of hormones on male sexual behaviour:
hormone receptor sitesThere are specific receptor sites in the brain that act as target cells for hormones released by the testes and ovaries. These are shown as red blobs in the diagram.

Receptor sites are particularly concentrated at the base of the brain in an area running from the preoptic area back to the hypothalamus ( but note that several other areas including the midbrain and hippocampus contain hormone receptor sites.)

Injecting testosterone into the hypothalamic area of castrated male rats restores their sexual receptivity. This procedure involves lowering a hollow tube ( called a cannula) containing hormone into the brain.

does not restore sexual behaviour.

Cholesterol is used as a control substance in these experiments because it has a similar chemical structure to testosterone. Cholesterol is converted into testosterone by enzymes in the gonads and adrenal glands. These enzymes are not present in brain tissue. Therefore cholesterol is a very good control, or placebo, in this type of experiment.

These experimental results indicate that the effects of testosterone are site specific. Furthermore the brain implants are not associated with recovery of peripheral tissue that is hormone sensitive (e.g. seminal vesicle weight). This indicates that the hormone has not leaked into the animals circulation, and supports the conclusion that the restoration of behaviour is the result of the hormone affecting brain tissue that mediates sexual behaviour in normal animals.

Hypothalamic implants of estrogen restore sexual behaviour in male castrates. This suggests that the effect of brain testosterone implantation involves aromatization.

How hormones affect the brain to control sexual behaviour is the subject of a separate lecture on your course.


References

Supplementary material
HEFCE, the funding body for universities and colleges for the UK, has purchased a 3 year license to IDEAL, the Academic Press online journal library. If you are a member of a UK academic institution (i.e. HEFCE funded) you now have full access rights to this online library which enables you to read the full text of articles in Academic Press journals. Note IDEAL uses your computer's IP internet address to allow access. Consequently you may not gain automatic access if you are a HE student using a PC at home. 
The following articles cover topics raised in the lecture in greater depth:
  • Pfaus, FRANK BEACH AWARD. Homologies of animal and human sexual behavior. Hormones and Behavior, 30,3,187-200,1996. Follow this link to IDEAL
  • V. D. Ramirez,J. Zheng ,Membrane Sex-Steroid Receptors in the Brain, Frontiers in Neuroendocrinology, 17, 4, 402-439,1996. Follow this link to IDEAL

Other online resources

  • Global Variation in Male Testosterone and Age by Peter T. Ellison et al at Harvard contains a nice diagram showing the decline in male salivary testosterone level as a function of age.
  • NEERAJA SANKARAN, The Science Of Sex: What Is It And Who's Doing It? The Scientist, Volume 8, #6, page 3, 21 March 1994
  • Archives of Sexual Behavior, Volume 27 Number 1 Feb 1998. 'Pheromonal Influences on Sociosexual Behavior in Men' Winnifred B. Cutler Erika Friedmann Norma L. McCoy
    The Feb 1998 issue of Archives of Sexual Behavior is available free of charge. According to the authors "Significantly more pheromone than placebo users increased above baseline in sexual intercourse and sleeping with a romantic partner. There was a tendency for more pheromone than placebo users to increase above baseline in petting/affection/kissing, and informal dates, but not in self-stimulation to ejaculation or in formal dates . These initial data need replication but suggest that human male pheromones affected the sexual attractiveness of men to women. "
VIAGRA (sildenafil citrate) 
The Viagra story is interesting - in the context of this lecture - because it emphasizes the importance of peripheral factors in the control of sexual behaviour. 
A pill hailed as a "magic bullet" for the way it combats male impotence has created the greatest demand for a drug in the history of the American pharmaceutical industry. Originally developed as a heart medication, it is the first oral pill for impotence and it works for at least 70 per cent of sufferers. There are side effects. For example, about 16 per cent of men in the tests reported headaches. Another potential problem is that the drugs may mask early warning signs of heart disease.
The Virginia Urology Center explains Viagra's biochemical mechanism as follows: "To achieve an erection the brain produces a chemical (cyclic GMP) that relaxes specific muscles and allows the blood to flow into the penis. After orgasm, another chemical, (Phosphodiesterase) is produced to remove cyclic GMP, so blood can flow out of the penis. Viagra™ is a Phosphodiesterase Type (5 PDE-5) Inhibitor that, increases the time cyclic GMP is available to achieve or maintain an erection for sexual activities." 
The underlying mechanisms of male and female impotence may be similar. Female genitals fill with blood during sexual stimulation just as male genitals do, resulting in engorgement of the clitoris and lubrication of the vagina. Therefore female trials of Viagra are under way in Europe and the U.S

Figure 2. Percentage of Patients Reporting an Improvement in Erections. 
The following description of clinical studies of Viagra appeared on the Pfizer Viagra site 
" The frequency of patients reporting improvement of erections in response to a global question in four of the randomized, double-blind, parallel, placebo controlled fixed dose studies (1797 patients) of 12 to 24 weeks duration is shown in Figure 2. These patients had erectile dysfunction at baseline that was characterized by median categorical scores of 2 (a few times) on principal IIEF questions. Erectile dysfunction was attributed to organic (58%; generally not characterized, but including diabetes and excluding spinal cord injury), psychogenic (17%), or mixed (24%) etiologies. Sixty-three percent, 74%, and 82% of the patients on 25 mg, 50 mg and 100 mg of VIAGRA, respectively, reported an improvement in their erections, compared to 24% on placebo."
Further details can be obtained from the Pfizer Viagra site 
The Virginia Urology Center Impotence/Sexual Dysfunction site has detailed answers to the following questions:

     

  • How does an erection occur? 
  • What is impotence? 
  • What can cause impotence? 
  • How is impotence evaluated? 
Copyright Dr. C.A.P. Kenyon 1994-2006