Introduction
People have suspected that substances in the body contribute to behavior for a long time. During the fifth century bce., Hippocrates suggested in his humoral theory, that personality was determined by four body fluids: phlegm, black bile, yellow bile, and blood. The dominance of one of the fluids was associated with a specific behavior pattern, and a proportionate distribution of the fluids resulted in a balanced personality. This theory has contributed terms such as “phlegmatic,” “bilious,” and “good-humored” to describe personality types and states of mind.
Aristotle is reported to have performed castration experiments on both fowl and men to alter behavior. He believed that something produced by the testes caused typically male behavior. Several nineteenth century researchers continued the study of the connection between the testes and male reproductive behavior. In 1849, Arnold Adolphe Berthold implanted testes into the abdomens of castrated cockerels. Successful transplantation restored typical “male” behaviors such as crowing and combativeness.
During the late nineteenth and early twentieth centuries, knowledge of behavior and its causes increased. The science of ethology, which focuses on animal behavior, came into existence. In the early 1900s, John B. Watson founded a branch of psychology that became known as behavior science. This area of psychology concentrated on human behavioral studies. Eventually, ethology and behavior science contributed to biopsychology, also known as psychobiology, or biological psychology, a branch of psychology that analyzes data from neurosciences, genetics, endocrinology, and physiology in the quest for biological explanations of behavior, thoughts, and feelings. Biopsychology embraces several subdivisions. Physiological psychology focuses on nervous system and endocrine system research. Psychopharmacology specializes in the effects of drugs on the nervous system and, ultimately, on behavior. The development of therapeutic drugs is a goal of this discipline. The neuropsychologist studies the effects of brain damage on behavior. Psychophysiology differs from physiological psychology in that the psychophysiologist uses only human subjects while the physiological psychologist experiments on laboratory animals. Early research in physiological psychology focused on the nervous system, but it soon became evident that the endocrine system also influenced behavior and that the two systems were integrated and had coordinated effects on behavior. The classic endocrine system consists of ductless glands that produce chemical substances called hormones. The hormones elicit physiological responses, either locally or at some distant target site. When acting at a distance, the hormones travel to the site by way of the circulatory system.
Hans Selye, a Canadian scientist, proposed a direct connection between the endocrine system and behavior. In 1946, he described physiological events that were triggered by stress. This set of bodily changes became known as the general adaptation syndrome. The syndrome involved the mobilization of the autonomic nervous system, the adrenal glands, and the anterior lobe of the pituitary.
As research continued, data on the role of the endocrine system in determining behavior began to accumulate. Researchers continue to look to the endocrine system to provide clues about the causes of psychiatric diseases and the efficacy of hormone therapy in treating the diseases, as well as in altering behavior patterns.
Invertebrates
Among most invertebrates (animals without backbones), endocrine glands are not in evidence. There is, however, an invertebrate endocrine system. Specialized cells known as neurosecretory cells serve as endocrine tissue. The cells, which resemble neurons (the functional cells of the nervous system) are hormone producers. In invertebrate animals such as the hydra and planaria, the secretions (hormones) of the neurosecretory cells seem to influence growth and may be the underlying cause of the tremendous powers of regeneration possessed by the animals. There are indications that the development of sexuality, the laying of eggs, and the release of sperm may be under hormonal control in these animals. Attempts to establish the link between hormones and invertebrate behavior when the hormones are produced by neurosecretory cells have inherent problems. A historic method of studying hormone influence involves removal of the secreting organ, which causes a hormone deficit. Changes in physiology, behavior, or both are observed. Utilization of this method was complicated by the difficulty in removing all the functioning neurosecretory cells. Modern gene deletion or knockout technologies, which can remove a hormone or its receptor from a single tissue as well the whole animal, allow a more thorough assessment of the effects of hormone deficit.
Hormone effects are observable and measurable in the more developed invertebrates such as the Arthropoda. Studies carried out on insects and crustaceans indicate the presence of both neurosecretory cells and endocrine glands. Among the behaviors and activities controlled by the hormones released from either the cells or the glands are molting, sexual differentiation, sexual behavior, water balance, and diapause. Because arthropods are encased in an outer skeletal structure, it is necessary for the animals to shed their outer structure to grow. During the growth years, the animals go through cycles of shedding the outer skeleton (molting), growing, and reforming an outer coat. There is evidence that insects are under hormonal control when they enter a state of diapause, or arrested behavior in adverse times.
Vertebrates
All vertebrates (animals with backbones) have a well-developed and highly organized endocrine system. The classic endocrine system consists of the pituitary, the pineal, the thyroid, the thymus, the pancreas, a pair of adrenals (each adrenal acts as two glands—the adrenal cortex produces unique hormones and functions independently of the adrenal medulla), a pair of parathyroids, and a pair of ovaries or testes. Most tissues in the body produce hormones that help the central nervous system integrate needs and function of the organism. Adipose tissue hormones signal the level of stored energy to the brain, which determines the satiety or hunger necessary to maintain energy stores. Endocrine tissue in the gastrointestinal tract readies the system for the digestive process. During a pregnancy, the placental tissue assumes an endocrine function. Although the kidneys do not produce a hormone directly, they release an enzyme that converts a blood protein into a hormone that stimulates red blood cell production.
All vertebrates have a pituitary. The pituitary is a small, round organ found at the base of the brain. This major endocrine gland interacts with the hypothalamus of the nervous system. Together they modulate many behaviors. The hypothalamus monitors physiological status by receiving neural input and hormone signals from many peripheral tissues. In turn, the hypothalamus signals the pituitary by either neural impulse or chemical messengers called neurotransmitters. The pituitary responds by releasing into the peripheral blood circulation, or stopping release, of pituitary hormones that will have an effect on peripheral physiology to alter the physiological event and influence behavior. The endocrine system exerts its effects by binding to cellular receptors that in turn regulate intracellular biochemistry such as metabolism and gene expression. The human endocrine system is typical of vertebrate endocrine systems and their effect on behavior, although more complex, as reflected by a more complex human system. For example, melanocyte-stimulating hormones, which are generated by the anterior lobe of the pituitary, greatly increase skin pigmentation in amphibians. This creates a protective coloration. In humans, certain types of melanocyte-stimulating hormones may darken the skin, especially in certain hormonal conditions as those found in pregnancy. However, the melanocyte-stimulating hormones are not the cause for racial skin tone variation. Other melanocyte-stimulating hormones affect appetite and sexual arousal. There are enough similarities among human and animal endocrine functions and effects, however, to warrant the use of data from both ethology and human behavioral studies in determining the biological bases for behavior.
Influence on Reproductive Behavior
The influence of the endocrine system on behavior has been studied on many levels. Much of the work has been done on animals. There is, however, a growing body of information on hormonal effects on a variety of human behaviors, including reproductive and developmental behavior, reaction to stress, learning, and memory. Studies carried out in reproductive and developmental biology on both animal and human subjects have substantiated the belief that hormones influence mating behavior, developmental events including sexual differentiation, and female and male sexuality.
Castration experiments have linked the testes with a male mating behavior pattern in animals. The sexually active adult male aggressively seeks and attempts to mount the female whether she is receptive or not. The castrated male retains the ability to mount a female but loses the aggressiveness and persistence of the normal male’s pursuit. The castrated animal may assume more submissive female behavior and even engage in homosexual encounters. Normally, the release of reproductive hormones in the male is noncyclic, whereas in the female it is cyclic. Castrated animals begin to exhibit the female, cyclic pattern of hormone release. The hormonal influence is confirmed by administering androgens (male hormones) to the castrated animals. Male mating behavior and the noncyclic release of hormones returns.
The presence of male hormones has an effect on the female cycle and sexual receptivity. Pheromones are substances secreted on the body of one individual that influence the behavior of another. These chemical messengers function during mate attraction, territoriality, and episodes of fear. Their existence and functions are well documented throughout the animal kingdom, especially among insects and mammals. In experiments using rats, it was shown that the pheromones act in conjunction with male hormones in bringing the female rat to a state of receptivity. The urine of noncastrated males contains androgens. When a male is introduced into a cage of sexually inactive females, the male sends off chemical signals by way of pheromones and the androgen-containing urine. The result is the accelerated onset of estrus, or sexual receptivity, on the part of the females. Castrated males produce pheromones but do not have androgens in their urine. When castrated males are introduced into a cage of inactive females, the estrous cycle is not affected.
Female mammals, with the exception of monkeys, apes, and humans, also experience estrus. Under hormonal control, the female is receptive to the male once or twice a year, when her eggs are available for fertilization. This period of receptivity is known as the estrous phase, or heat. Research shows that the particular female hormone that induces estrus is progesterone.
Hormonal Influences
The work done by researchers in developing contraceptives clarified the role of hormones in the functioning of the human female reproductive system. The system operates in a monthly cycle, during which ovarian and uterine changes occur under hormonal control. These hormones do not affect the woman’s receptivity, which is not limited to fertile periods. Progesterone has effects on the nervous system and may be responsible for changes in mood or behavior.
Testosterone derivatives known as anabolic steroids are illegally used by some athletes in an attempt to increase muscularity, strength, and performance. Although both sexes do experience the desired effects, long-term, high-dosage usage has undesirable consequences. This is particularly true in women, who begin to exhibit a deepening of the voice, a male body shape, and increased body and facial hair. Both men and women can become sterile. Psychotic behaviors and states such as depression and anger have been recorded.
Developmental biologists indicate that hormones exert their influence as early as six or seven weeks into embryonic development. At this point, undifferentiated tissue with the potential of developing into either a female or a male reproductive system will develop into a male system in the presence of testosterone and into a female system in its absence. There is some evidence that the embryonic hormones have an effect on the developing brain, producing either a male or female brain. Functionally, this may account for the cyclic activity of female reproductive hormones and the noncyclic activity of male hormones. A few anatomical differences between male and female brains have been observed in both rats and humans. In the hypothalamus of the brain, there are cell bodies called nuclei. In rats and in humans, these nuclei are larger in males than in females.
Learning and memory can be experimentally affected by hormones. Experiments reveal that chemicals that resemble adrenocorticotropic hormone (ACTH) can extend the memory time of rats. Rats, stimulated by electric shock and provided with an avoidance possibility such as moving into another chamber of a cage or climbing a pole in the center of the cage, were administered ACTH-like molecules. The treated rats were able to remember the appropriate reaction to the stimulus for a longer period of time. In other experiments, rats in a maze were administered vasopressin, a posterior pituitary hormone, which increased their frequency in selecting the correct pathway through the maze.
The effect of vasopressin on human memory is not as clearly defined. There have been positive results with schizophrenic patients and patients with alcohol-induced amnesia. In these cases, memory has been enhanced to a limited degree. There is no evidence that learning and memory in humans will be greatly improved by the administration of vasopressin.
Hormones can also effect changes in eating and sleeping behavior. As the prevalence of obesity continues to grow, the physiological psychologist will be challenged to address integrated approaches. Sleeping disorders are also increasing in prevalence and can affect mood, behavior, and quality of life. As more traditional approaches to illness fail to achieve optimal health, understanding neuroendocrinology will become an important foundation to build effective interventions.
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