Hormones and Behavior in Development

Endocrine glands are glands that secrete hormones directly into the bloodstream. This means that through the secretions of endocrine glands, any tissue in the body that is oxygenated by the blood can also potentially be influenced by the presence of endocrine hormones. 

Regulation of the secretion of endocrine hormones is a very complicated process involving several chemical. The hormone itself is defined by its chemical structure, and a hormone's chemical structure is central to its interaction with two other mediating influences-- two other factors that are essential to the hormone's effect on behavior.  One is the target organ.  This is the tissue in the body that can be influenced by the chemical structure of the hormone.  Remember because it is carried by the blood the hormone gets almost everywhere, but its effects can be quite specific to a particular tissue.  The testes, for example, secrete testosterone because they are sensitive to another hormone whose action is somewhat specific to the testes.  So the testes are a target organ for another hormone that stimulates the secretion of testosterone.  Specific target organs regulate hormone action through receptors.  Receptors are usually protein chemicals that are concentrated in some tissues, but not in others.  They make it possible for the hormone to cross cell membranes and enter the cells in the tissues of the target organ. Receptors are often very, very specific to a particular hormone.  Not always (see the Crews article), but often.  So, a chemical receptor that receives estradiol is different from a receptor that receives testosterone.  A receptor that receives a stress hormone, like cortisol, is different from a receptor that receives testosterone or estradiol.  Tissues of the body in which the concentration of various receptors is high are tissues that can be affected by the presence of the hormone.  The brains of many vertebrates are full of estrogen and testosterone receptors. And these brain receptors enable the hormones to have an impact on behavior.

The human clinical syndrome called the androgen insensitivity syndrome illustrates how critically the action of hormones depends upon the presence of receptors.  In androgen insensitivity syndrome a genetically normal XY male -- a male with otherwise normal sex chromosomes -- completely lacks the receptors that enable tissues to detect the presence of testosterone in the blood.  These men have normal amounts of circulating testosterone, but because of some other abnormality, the receptors necessary to recognize the chemical and usher it into the cell are missing.  Such males' genitals are partially feminized by the absence of the receptor.  If the problem isn’t caught shortly after birth, they will be mistaken as females in childhood, they may be raised as females, and at puberty, when they’re expected to menstruate, nothing happens.  During embryonic and post-natal development, the presence of fetal testosterone usually influences male-like secondary sex characteristics, a male-like brain, etc.  As embryos and in early development, males who have androgen insensitivity do not get the chemical stimulus that would influence male development because without receptors their cells are insensitive to the presence androgen in the blood. Various male-like sexual characteristics fail to develop. Instead some female characteristics develop. This illustrates how central receptors can be to the action of hormones.

But the story is still more complicated, because there are other sets of chemicals involved in the actions of hormones:  These are the enzymes.  It is very rare, especially with the gonadal steroids, that the hormones themselves hang around for very long.  They are produced regularly -- synthesized in the body on a daily and sometimes hourly basis--and they are quickly metabolized or broken down after synthesis.  Enzymes are involved in both the synthesis and the breakdown or metabolism of hormones. 

The chemicals are synthesized from chemical precursors and broken down into metabolic products. With the steroid hormones-- like testosterone--the chemical precursor is another hormone, and the product of metabolism can also be another hormone.  The chemical precursor of testosterone is progesterone.   Males (and females) produce testosterone out of progesterone in the presence of the appropriate enzyme. The chemical conversion from the precursor to testosterone depends upon the presence of progesterone and the presence of the enzyme. If there is something wrong with the enzyme, the male can have too much progesterone and not enough testosterone. Progesterone serves as a hormone in its own right, but it also serves as a chemical precursor to testosterone. 

The metabolic by product of testosterone metabolism can also be another hormone. In the presence of certain enzymes, in both males and females testosterone is broken down into estradiol.  So when males metabolize testosterone it can be converted not only to another hormone, but to another hormone that is often thought to have opposite effect of testosterone itself.  It is now known that the brains of many of male birds and mammals show high concentrations of estradiol receptors that are activated when an enzyme called aromatase converts testosterone to estradiol. The estradiol is then received by brain estradiol receptors to influence male behavior.  There’s evidence, for example, that male courtship behavior in some species of birds and mammals depends on the presence of circulating testosterone, which must first be converted to estradiol in the brain to affect behavior.  So the active chemical in the brain, even in the male, is often not testosterone, but estradiol.  Without the enzyme the conversion will not occur and the behavioral effect will be lacking.

So several chemicals are always involved in hormone action: the hormone itself, tissues in the target organ containing receptors, enzymes necessary for synthesis and metabolism of the hormone, and precursors and metabolites, which themselves often are other hormones (whose presence depends on enzymes).

The process is further complicated by the fact that the coordination of behavior through hormones is achieved by a complicated integration of chemicals with the environment through the brain.  The brain can trigger the production of the hormones themselves in a complicated process that involves brain centers like the hypothalamus, which is a central endocrine control organ in the brain. There are four layers of regulation in this process. Understanding this can show you how important the social environment can be in mediating hormonal influences on behavior during both adulthood and development.

The four levels are:

The pituitary gland is the so-called master gland of the endocrine system.  The pituitary gland is the second of the four levels.  It is the little gland that hangs down from the base of the brain right under the hypothalamus.  It’s actually two glands.  They’re divided into an anterior lobe and a posterior lobe, and they operate independently of one another. Both lobes secrete their own hormones. The hormones of anterior pituitary are called trophic hormones.  Once secreted by the anterior pituitary, they move through the bloodstream to the so-called target gland and there stimulate  production of another hormone.  Let’s use the ovaries as an example. The anterior pituitary secretes gonadotrophic hormones that travel to the ovaries (there are receptors for these hormones in the target gland) and only as a consequence of reception of the trophic hormone do the ovaries secrete estradiol.  The gonadal trophic hormones produced by the anterior pituitary are the follicle stimulating hormone (usually abbreviated FSH) and the luteinizing hormone (usually abbreviated LH).  They’re the same in males and females.  So when the anterior pituitary secretes LH into the bloodstream, it circulates to the ovaries, and, if the ovaries have receptors for the luteinizing hormone, they then secrete estrogen (by the same mechanism the testes secrete testosterone that is quickly converted to estrogen in the presence of the right enzyme).  So one of the reasons we think of the pituitary as the master gland is because it controls the target glands. The anterior pituitary and the target glands are the second and third levels of chemical control. 

The first level involves the hypothalamus. The hypothalamus is intimately linked to both the anterior and the posterior lobes of the pituitary. In the case of the anterior lobe, the secretion of the trophic hormones is regulated by yet a third hormone called a releasing hormone.  This hormone is produced in the hypothalamus itself. One hypothalamic hormone, the so-called gonadotrophic releasing hormone (GnRH), travels to the anterior pituitary and stimulates the production of the gonadotrophic hormones by the master gland.  So there are two other hormones involved in stimulating, say, the production of testosterone by the testes: 1) releasing hormones secreted by cells in the hypothalamus; and 2) the anterior pituitary hormones like FSH and LH.  Once FSH and LH are secreted into the bloodstream as a result of the action of the releasing hormone, they travel to and are received by any target gland containing receptors. And with receptors for LH in the ovaries and testes, the ovaries will secrete estradiol and the testes will secrete testosterone.

Keep in mind that the brain is full of receptors for testosterone and estradiol. Here's the fourth level. Once the hormone is dumped into the bloodstream, it goes back up to the areas of the brain and can be received by areas with appropriate receptors.  Two things then happen.  It's presence in the brain can further regulate more (or less) hormone production; and it can be integrated with all kinds of environmental stimuli that also influence the action of the chemicals and of behavior the through sensory processes.

Remember the case of the female finch who deposited different amounts of testosterone in her eggs because she was competing a great deal for nest boxes with other females, versus females who did not have to compete for as many nest boxes.  The sensory input from the competition here is received by the brain and influences the production of the GnRH (the gonadotrophic releasing hormone) by the hypothalamus; that in turn influences the production of luteinizing hormone by the anterior pituitary; that in turn influences the production of testosterone in the female.  The testosterone that she produces goes back to her own brain and in interaction with environmental stimulation triggers her own aggressive behavior (and gets deposited in her eggs to affect her developing young).

Hormones can exert powerful effects on animal behavior, but through these mechanisms the hormones themselves are influenced by social concerns. Social influences can alter chemical ones just as much as the other way around--Causation goes both ways. For example, white-throated sparrows form dominance hierarchies in the winter.  If you allow a group of birds to interact socially and form their dominance hierarchy, you can then determine who is number one and who is number 10 in a dominance hierarchy.  Take number 10 and give him an implant of long acting testosterone, and then put him back in a social setting that allows him to interact with other birds.  If you put him back in the same group of nine other males that he was with before, there is no change either in his aggressive behavior or in his position in the dominance hierarchy.  If on the other hand, you put him with a new group of nine other birds that he’s never met before, he rises to number one or two, very high in the dominance hierarchy.  This is an example of social inertia in the influence of hormones on behavior.  The testosterone that reaches his brain could trigger an increase in his aggressive behavior.  But when the low ranking male meets familiar more aggressive individuals, the impact of the hormone is modulated by the stronger force of the prior experience he’s had with the other birds. Somehow the central influence of sensory input prevents the action of the testosterone from influencing aggression. 

Social influences can also act to increase hormones.  Often these influences occur prenatally or perinatally, and they can have a relatively permanent effect on the development of the brain and behavior  Here the impact of the effect is not restricted to the presence of the hormone.  Rather during development hormones can produce a permanent structural change that persists long after the hormone has been taken away. For example, virtually any species that’s been examined shows sex differences in the structure of the brains of males and females (see the whiptail lizard article).  Among the central features dictating the structural differences in the brains and behavior of males and females are environmental differences that have their impact on hormones early in development.

An example in mammals: mother rats engage in differential rearing of their male versus female offspring.  The differential rearing can have an impact on the hormone production of the young organism. Mother rats normally lick the butts of their rat pups when they are nursing (called anogenital licking), and the amount and duration of the licking is greater for male pups than for female pups, their siblings.  The differences in licking occur independently of any nursing differences or of carrying, retrieval differences, or proximity differences. Mothers lick different amounts, because they detect  different odors in the males and the females. If you treat the female rat pups with testosterone, you can increase the maternal licking of females as the moms begin to react to the girls as if they were boys.  The tactile input associated with social licking alters nervous system and behavioral development via hormones.  When scientists take a paint brush and tickle the rear ends  of the girl pups to increase the amount of stimulation to the level that the mothers would normally give the boys, they find that females as adults, behaved more like males as adults.  They engage in patterns of adult sexual behavior that are more male-like.  The reverse is true in males. If by rendering the mom anosmic or by altering the male pups odor so that the mom can’t smell the testosterone, the males receive less licking than normal. Mom isn’t stimulated to lick, and the males develop as less competent adult breeders.  Their copulatory behavior occurs at a slower pace and they’re less able in adulthood to attract females to mate with them.

The differences in licking produce differences in the central nervous system development in males versus females. The licking is a kind of  social cue that has stimulated the hypothalamic/pituitary/gonadal integration differently in males versus females.