CHAPTER 10: Neural and Neuroendocrine Control of the Internal Milieu

• Hormones are signaling molecules that reach their target cells via the bloodstream.

• The hypothalamus is the key regulator of the endocrine system by releasing hormones directly into the blood and by regulating the anterior pituitary.

• The hypothalamic–neurohypophyseal (posterior pituitary) system secretes two peptide hormones directly into the blood, vasopressin and oxytocin.

• To regulate the anterior pituitary, hypothalamic neurons synthesize peptide-releasing and inhibitory factors.

• The hypothalamic–pituitary–adrenal (HPA) axis comprises corticotropin-releasing factor (CRF), released by the hypothalamus; adrenocorticotropic hormone (ACTH), released by the anterior pituitary; and glucocorticoids, released by the adrenal cortex.

• The hypothalamic–pituitary–thyroid axis consists of hypothalamic thyrotropin-releasing hormone (TRH), the anterior pituitary hormone thyroid-stimulating hormone (TSH), and the thyroid hormones triiodothyronine (T₃) and tetraiodothyronine or thyroxine (T₄).

• The hypothalamic–pituitary–gonadal axis comprises hypothalamic gonadotropin-releasing hormone (GnRH), the anterior pituitary luteinizing hormone (LH) and follicle-stimulating hormone (FSH), and the gonadal steroids.

• Prolactin is synthesized and released by the anterior pituitary and triggers lactation in females; its release is inhibited by dopamine acting at D₂ dopamine receptors.

• Growth hormone, also released by the anterior pituitary, is under the stimulatory control of growth hormone–releasing hormone (GHRH) and inhibitory control of somatostatin.

• The hypothalamus contains a complex network of peptide-releasing neurons that regulate feeding and energy expenditure.

• Leptin, released by adipocytes, regulates the long-term state of the body’s energy reserves largely via its actions on the hypothalamus.

• The arcuate nucleus of the hypothalamus plays a key role in feeding and energy expenditure. It contains one set of neurons that express proopiomelanocortin (POMC) that suppresses feeding and another set that express neuropeptide Y (NPY) that stimulates feeding.

• The POMC product, melanocortin (also called α-melanocyte-stimulating hormone) and NPY, in turn, regulate several other feeding peptides (eg, melanin-concentrating hormone) to produce their ultimate effects on food intake and energy metabolism.

• The hypothalamus responds to inflammatory cytokines by causing fever and sickness behavior.

The brain controls bodily function and behavior through the motor system, the autonomic nervous system (Chapter 9), and hormones released by neuroendocrine neurons. Hormones are signaling molecules that reach their target cells by means of the bloodstream. They comprise three major classes: peptides, neurotransmitter-like small molecules such as epinephrine (adrenalin), and steroid-like molecules. Through its neuroendocrine system, the brain contributes to body homeostasis, regulates growth and maturation, and facilitates the body’s response to environmental stressors, illness, and injury.

Afferent Signals

The hypothalamus, located at the base of the diencephalon, is in a good position to integrate information about the state of the organism from diverse sources. It receives information about the external environment from forebrain sensory systems and about noxious stimuli from nociceptors ascending in the spinal cord and brainstem (Chapter 11). The hypothalamus is activated by subcortical circuits that process emotional responses such as fear, reward, and disgust (eg, from ingesting noxious food), and receives information about the internal milieu from brainstem nuclei, and directly from the blood in those regions where the blood–brain barrier gives way to fenestrated capillaries, for example, the median eminence at the base of the hypothalamus and near the circumventricular organs (Chapter 2). The hypothalamus also has reciprocal relationships with monoamine and cholinergic nuclei in the brainstem and basal forebrain that regulate states of arousal and attention (Chapter 6). Because they are lipophilic, circulating steroid hormones from the adrenal cortex, gonads, and adipocytes can cross directly into the brain and bind receptors (Chapter 4) in the hypothalamus and in other brain areas (eg, hippocampus and amygdala), many of which send projections to the hypothalamus.

The hippocampus, amygdala, septal nuclei, and nucleus accumbens, each of which responds to salient environmental events (Chapters 15 and 16), all project to the hypothalamus. Such connections enable adaptive responses; for example, they allow the hypothalamus to direct the release of stress hormones and activate the sympathetic nervous system in response to a threat. Communication between these regions and the hypothalamus is largely reciprocal, which permits information about the internal milieu to influence cognition, emotion, and reward. Thus, for example, detection of low glucose levels might contribute to a trip to the refrigerator. In addition to the many types of afferent signals directed toward the hypothalamus, there are extensive intrahypothalamic connections, which are involved in coordinating the many hormonal and autonomic outputs of this brain region.

Efferent Signals

The output of the neuroendocrine hypothalamus consists of peptide hormones released into the circulation. Hypothalamic neurons can affect bodily functions directly by releasing vasopressin or oxytocin into the systemic circulation via the posterior pituitary or indirectly via releasing hormones 10–1 into the portal hypophyseal circulation and thence to the anterior pituitary. Neuroendocrine neurons are specialized for the purpose of transducing neural signals into hormonal signals; in response to depolarization, they release peptides into the bloodstream by means of axons that terminate on capillaries rather than at synapses 10–1. These peptides are considered hormones because they travel through the bloodstream to reach their receptors. Many of these same peptides also act as neurotransmitters when they are released from other neurons in the brain and act at synapses.

Organization of the Hypothalamic–Pituitary Unit

The hypothalamus and pituitary gland can be seen as an interacting unit 10–2. In humans, the pituitary has two divisions: the neurohypophysis or posterior pituitary, which contains the axon terminals of neurons projecting from the hypothalamus, and the adenohypophysis or anterior pituitary, which is composed primarily of endocrine cells specialized to synthesize and release particular hormones. Rodents, but not humans, also possess a pituitary intermediate lobe.

In the neurohypophyseal system, the peptide hormones vasopressin and oxytocin are synthesized in magnocellular (large) neurons of the supraoptic nucleus (SON) and paraventricular nucleus (PVN) and transported to capillary-associated nerve terminals in the posterior pituitary, where they are released directly into the systemic circulation. In the adenohypophyseal system, peptide-releasing factors 10–1, which are synthesized in hypothalamic neurons, are transported to nerve terminals in the median eminence; when the hypothalamic neurons are stimulated, peptide hormones are released into the hypophyseal–portal circulation, which drains into the blood vessels of the anterior pituitary. Such peptides act to stimulate or inhibit the release of anterior pituitary hormones into the systemic circulation. After reaching their targets, pituitary hormones cause the release of additional hormones, which subsequently act on diverse tissues of the body. Hypothalamic releasing factors that cause pituitary hormone release also have trophic influences on anterior pituitary cells; likewise, hormones of the anterior pituitary gland have trophic influences on their target cells. Thus, hypersecretion of a hormone can lead to hypertrophy or hyperplasia of target endocrine cells, whereas the absence of a hormone can lead to atrophy.

The hypothalamic–neurohypophyseal system secretes two major peptide hormones: vasopressin, also known as antidiuretic hormone (ADH), and oxytocin. Vasopressin and oxytocin are synthesized in distinct magnocellular neurons of the PVN and SON of the hypothalamus, and are carried by axoplasmic flow to nerve endings in the posterior pituitary. Vasopressin also is synthesized in a distinct subset of parvocellular (small) PVN neurons that project to the median eminence. Vasopressin released from these neurons acts together with corticotropin-releasing factor (CRF; also known as corticotropin-releasing hormone [CRH]) to stimulate release of adrenocorticotropic hormone (ACTH; also known as corticotropin). Like most peptide transmitters, vasopressin and oxytocin are derived from large precursor proteins, which are cleaved to form active peptides. Associated cleavage events also produce carrier proteins, or neurophysins, which are coreleased with the peptides. Actions of vasopressin and oxytocin within the brain, where they act as neuropeptide neurotransmitters, are discussed in Chapter 7.

Vasopressin

Release of neurohypophyseal vasopressin occurs in response to central and peripheral blood osmolality and blood pressure. Stimuli such as hemorrhage, pain, stress, and strenuous exercise can lead to vasopressin release. The primary systemic action of vasopressin occurs at the distal nephron within the kidney, where it increases reabsorption of water across the collecting duct epithelium. Many drugs can interfere with vasopressin action. Lithium inhibits vasopressin action by inhibiting the renal V₂ vasopressin receptor–linked adenylyl cyclase. Lithium thereby causes increased urine flow and, in some individuals, nephrogenic diabetes insipidus (the production of more than 3 L of urine per day). Other drugs, including nicotine when taken in high doses, may cause a syndrome of inappropriate vasopressin (ADH) secretion (SIADH), which results in water retention and can lead to dilutional hyponatremia severe enough to produce seizures. Vasopressin and oxytocin have additional actions in the nervous system, including regulation of social behaviors such as affiliation (Chapter 7).

Oxytocin

Oxytocin acts during parturition to cause uterine contractions. Synthetic forms of this hormone (pitocin) are used to induce labor. Oxytocin also mediates milk letdown in lactating mothers. Nipple stimulation causes oxytocin release, which in turn causes the myoepithelial cells of the nipple to contract, resulting in the ejection of milk. The release of oxytocin is subject to classical conditioning; thus, a lactating mother may experience milk letdown in response to the cry of a baby.

The hypothalamic–pituitary–adrenal (HPA) axis 10–3 produces a cascade of hormones that are central to the stress response 10–1. This cascade begins with CRF synthesized and released by parvocellular neurons of the PVN of the hypothalamus. These neurons project to the median eminence, permitting CRF to stimulate the corticotroph cells of the anterior pituitary to synthesize and release ACTH into the systemic circulation. ACTH then acts on the adrenal cortex to release glucocorticoids, cortisol in humans, and corticosterone in rodents. Glucocorticoids function as stress hormones by causing cells to become catabolic, that is, to break down energy stores in order to release glucose. They also increase arousal and inhibit inflammation. (Inflammation has a metabolic cost, and acute inflammation might impede fight-or-flight behaviors. Inflammatory processes, which may include swelling that limits movement, are nevertheless adaptive as a component of the healing processes after an emergency has passed.)

Basal CRF release is controlled partly by the circadian pacemaker, located in the suprachiasmatic nucleus (SCN) of the hypothalamus in mammals (Chapter 13). Maximum secretion of CRF occurs in the morning in humans near the time of waking; maximum secretion in rodents, which are nocturnal, typically occurs in the early evening. Superimposed on normal circadian patterns of circulating CRF levels is CRF release precipitated by stress. (Stressors also stimulate release of vasopressin by PVN neurons. Like CRF, vasopressin is a secretagogue for ACTH.)

PVN neurons receive stimulatory inputs from the amygdala and inhibitory inputs from the hippocampus (see 10–3). They also receive noradrenergic and other monoamine projections from the brainstem that relay information about the salience of stimuli. The effects of CRF and related peptides are mediated by two distinct G protein–coupled receptors, called CRF₁ and CRF₂. CRF binds selectively to CRF₁; the CRF-like peptide, urocortin 1, binds both CRF₁ and CRF₂; and urocortins 2 and 3 bind CRF₂ (Chapter 7). CRF activates release of ACTH via CRF₁. Urocortin 1 appears to mediate stress-induced anorexia. Urocortin 2 relaxes coronary arteries and has been proposed as a treatment for congestive heart failure. CRF₁ antagonists have failed in clinical trials as antidepressant or anxiolytic drugs; the CRF₁ receptors that are most relevant to the control of emotional behavior are likely located in the amygdala and other limbic brain regions rather than in the pituitary (Chapter 15).

Acth and Glucocorticoids

CRF and vasopressin promote the synthesis of proopiomelanocortin (POMC) and release of its peptide derivatives. Glucocorticoids can feed back to inhibit POMC transcription. POMC is a large precursor protein (7–4 in Chapter 7) that gives rise to ACTH as well as β-endorphin and α-melanocyte-stimulating hormone (α-MSH; also called melanocortin). ACTH stimulates the adrenal cortex via G_s-coupled ACTH receptors to synthesize and secrete adrenal glucocorticoids. A cAMP-regulated pathway supports adrenal steroidogenesis by controlling transcription of the major synthetic enzymes. ACTH also stimulates the secretion of other adrenocortical steroids, including aldosterone (a major mineralocorticoid) and adrenal androgens.

Glucocorticoid Synthesis

Dietary cholesterol is the primary starting material for the biosynthesis of glucocorticoids and other adrenal steroids 10–4. Cholesterol enters one of three biosynthetic pathways in the adrenal cortex, which lead to the production of glucocorticoids (cortisol), mineralocorticoids (aldosterone), and adrenal androgens (eg, dehydroepiandrosterone). Cells within different zones of the adrenal cortex express distinct synthetic enzymes that catalyze the production of these specific hormones 10–2.

Glucocorticoid Receptors

The primary mode of action of all steroid hormones is to bind cytoplasmic receptors that, on activation, translocate to the nucleus to regulate gene expression (Chapter 4). Glucocorticoids bind to two different cytosolic receptors: the glucocorticoid receptor (GR) and mineralocorticoid receptor (MR), both of which are members of the large superfamily of steroid hormone (nuclear) receptors (Chapter 4). Despite their names, the MR has a 10-fold higher affinity for glucocorticoids than the GR. Consequently, MR binding sites may be saturated, even with relatively low levels of circulating glucocorticoids, whereas significant GR binding requires high circulating glucocorticoid levels as might occur with a significant stressor or with peak levels of secretion under circadian control. In neurons that express both receptors, such as the hippocampus and amygdala, heterodimers may form with intermediate affinity.

The GR is expressed in essentially every tissue in the body including the brain, but with different levels of expression. Indeed, the lower-affinity GR is anatomically well positioned to exert feedback control when levels of glucocorticoids are very high because it is concentrated in the PVN itself, in the hippocampus, which feeds back negatively on the PVN indirectly by activating GABAergic inputs, and on brainstem monoamine neurons (Chapter 6) that also project to the PVN.

MR expression is more restricted than GR expression; in addition to the brain, it is expressed in the kidney, gut, and heart. Some actions of GR and MR are DNA binding–dependent and others occur independently of DNA binding. After they bind hormone, these receptors are freed from chaperone proteins including heat shock proteins, and are subsequently translocated to the nucleus 10–5. In the nucleus, MR and GR may bind as homodimers or heterodimers to their specific glucocorticoid response elements (GREs) in the regulatory regions of genes; alternatively, they may inhibit or enhance the functions of other transcription factors by means of protein–protein interactions. Thus, GRs may inhibit the actions of AP-1, CREB, NF-κB, and other transcription factors.

As their name suggests, glucocorticoids increase plasma glucose levels during periods of stress, including starvation; they do so by exerting catabolic effects on target tissues. Glucocorticoids also suppress the immune system by decreasing the synthesis and release of cytokines. In addition, they appear to affect the brain by rapidly increasing alertness and enhancing cognition. Chronic excessive glucocorticoid release or deficiency produces significant illness 10–2. Abnormally elevated CRF secretion and glucocorticoid levels may contribute to symptoms of depression in some patients (Chapter 15).

The synthesis and release of thyroid hormones are controlled by the hypothalamic–pituitary–thyroid (HPT) axis 10–6. Hypothalamic neurons synthesize and release thyrotropin-releasing hormone (TRH; also called thyrotropin-releasing factor [TRF]), a three-amino-acid peptide that is stored in axon terminals in the median eminence of the hypothalamus until it is released into the pituitary portal circulation. Normal TRH release is pulsatile and occurs in a circadian pattern; in humans, the highest levels of this hormone are released during the night. In the anterior pituitary gland, TRH binds to its receptors on thyrotroph cells and stimulates the release of the peptide hormone thyrotropin, also called thyroid-stimulating hormone (TSH). Neurons of the neuroendocrine hypothalamus synthesize and release somatostatin, a peptide that inhibits release of TSH and of growth hormone from the anterior pituitary (see 10–1). Somatostatin also functions as a neuropeptide neurotransmitter in other brain regions including the hippocampus (see 7–4, Chapter 7).

Because biologically active TSH requires glycosylation, it is described as a glycoprotein hormone. It is composed of two subunits; the α subunit is identical to that of two other pituitary hormones, luteinizing hormone (LH) and follicle-stimulating hormone (FSH). The β subunit is unique to each of these dimeric hormones and provides specificity for receptor binding. TRH stimulates the synthesis of both α and β chains of TSH in thyrotroph cells.

Once released in response to TRH, TSH exerts trophic effects on cells of the thyroid gland, via activation of G_s-coupled TSH receptors. This action in turn stimulates the synthesis and release of two thyroid hormones: triiodothyronine (T₃) and tetraiodothyronine, or thyroxine (T₄) 10–6B. T₄ is the predominant form of thyroid hormone secreted by the thyroid gland; it is subsequently converted to T₃ in target cells. All of the steps in the synthesis of these thyroid hormones are stimulated by TSH. Thyroid hormones are released into the general circulation to act on all tissues in the body. They also provide feedback control of transcription of the prepro-TRH gene by hypothalamic neurons and of the two subunits of TSH within pituitary thyrotrophs. Low levels of thyroid hormone stimulate transcription, and high levels lead to inhibition. Of the two thyroid hormones, T₃ exerts the more potent effects; consequently, altered rates of conversion of T₄ to T₃ can regulate the activity of the HPT axis.

The most common forms of hypothyroidism are caused by failure of the thyroid gland and are best detected through the measurement of serum TSH levels. These may become elevated in order to drive a failing thyroid gland even before levels of T₄ and T₃ become abnormal. Hypothyroidism caused by hypothalamic or pituitary failure is characterized by low levels of TSH. Hyperthyroidism is indicated by elevated levels of T₄ and T₃, which must be corrected to measure free thyroid hormone, because a considerable fraction of T₄ and T₃ in plasma is bound to thyroid-binding globulin or thyroglobulin (TBG) and is physiologically inactive.

Thyroid hormones exert genomic effects by regulating the transcription of target genes, and nongenomic effects by regulating intracellular ion concentrations and glucose transport. Genomic effects are mediated by two major thyroid hormone receptors: TR_α and TR_β, each of which has two splice forms. Like GR and MR, these receptors are members of the steroid hormone receptor superfamily. Like other ligand-regulated transcription factors, TR_α and TR_β bind to a specific DNA element, known as the thyroid hormone–response element (TRE), which is found in the promoter region of many genes. The receptors bind as homodimers or, with other nuclear receptor proteins such as retinoic acid receptors (termed retinoid X receptors), as heterodimers to stimulate or repress the transcription of target genes.

The action of thyroid hormones in the body is unusual in that these hormones are constitutively active. During stress or illness, however, thyroid axis function is suppressed, resulting in less secretion of TSH and reduced conversion of T₄ to T₃ in peripheral tissues. During illness, binding of T₄ to TBG also increases, in turn further decreasing levels of free hormone. These are believed to be adaptations aimed at conserving energy. Some similar adaptations may occur during starvation.

Reproductive function is controlled by the hypothalamic–pituitary–gonadal (HPG) axis 10–7 through the release of gonadotropin-releasing hormone (GnRH), previously known as luteinizing hormone–releasing hormone (LHRH). GnRH-expressing neurons are located in the medial basal hypothalamus and arcuate nucleus, and project to the median eminence. GnRH acts on gonadotrophs of the anterior pituitary gland to stimulate the synthesis and release of the gonadotropins, LH and FSH. GnRH release must be pulsatile to activate pituitary gonadotrophs. Accordingly, long-acting GnRH agonists such as leuprolide, which produce continuous stimulation of GnRH receptors, cause their desensitization and thereby inhibit release of LH and FSH. Leuprolide is used clinically to treat testosterone-sensitive prostate cancers and also to delay puberty in children who have precocious onset of puberty. A third gonadotropic hormone, chorionic gonadotropin, is secreted by the syncytiotrophoblast cells of the placenta during pregnancy; its presence in urine forms the basis of most pregnancy tests.

All gonadotropins are dimeric glycoprotein hormones that consist of a common α subunit (identical to the α subunit of TSH) and unique β subunits. GnRH stimulates the synthesis of both types of subunit and the glycosylation step required for their biologic activity. The targets of LH and FSH are the gonads. In the ovary, LH induces ovulation of the mature follicle and acts on theca cells to promote the synthesis of androgen precursors necessary for estrogen production 10–8A. Such precursors diffuse into nearby granulosa cells, where FSH stimulates the production of the steroid hormone estrogen. LH also sustains the ruptured follicle that forms the corpus luteum by stimulating progesterone synthesis; FSH stimulates development of the mature follicle. In the testes, LH stimulates testosterone biosynthesis in Leydig cells 10–8B.

Feedback control of gonadotropin secretion is complex, particularly in females, in whom such control varies during the different stages of the menstrual cycle. It comprises a complex integration of influences exerted by the gonadal steroids (estradiol, progesterone, and testosterone) and by the peptide hormones (inhibin and activin), which also are produced by the gonads. Moreover, GnRH release is influenced by several neurotransmitter and neuromodulatory agents, including catecholamines, endogenous opioids, γ-aminobutyric acid (GABA), neuropeptide Y (NPY), and possibly dopamine.

Among the actions that can be traced to gonadal steroids, their participation in gene transcription is best understood. Like glucocorticoids, gonadal steroids can regulate transcription either directly or indirectly. In some cases, they bind to estrogen or androgen response elements in the regulatory regions of specific target genes and thereby influence the expression of those genes. In other cases, gonadal steroids affect transcription by interacting with AP-1, CREB, or other transcription factor families and in turn altering the expression of genes controlled by those transcription factors.

Gonadal steroids regulate neural function and behavior in adults. For example, changing levels of estrogen can have significant effects on mood 10–3. They may also affect neural morphology; in female rats, for example, estradiol increases the density of dendritic spines on CA1 hippocampal pyramidal neurons. The physiologic and behavioral impacts of these changes are not yet clear.

A long-acting form of progesterone, hydroxyprogesterone (Depo-Provera), and the selective estrogen receptor modulator (SERM) tamoxifen are used for reversible chemical castration of male sex offenders. The main use of Depo-Provera is for long-acting birth control in females, and of tamoxifen is for the treatment of estrogen receptor–positive breast cancer.

Diverse androgen derivatives (anabolic steroids) are used to increase muscle mass and athletic performance; such steroids often are used illegally and can produce deleterious effects on the brain. Anabolic steroids appear to be reinforcing, and hence potentially addicting, in some individuals; moreover, they may induce irritability and anger. A fraction of users, including females, have reported hypomanic symptoms during use and depression during withdrawal.

Rapid Actions of Steroids

In addition to activating their nuclear receptors, glucocorticoids and gonadal steroids bind to plasma membrane proteins to produce more rapid effects. There is evidence, for example, that certain nuclear receptors, in addition to being in the cytoplasm and nucleus, are also located on the plasma membrane where they respond to hormone binding by regulating intracellular signaling cascades. In addition, steroids can act directly on several other types of plasma membrane proteins, including ion channels, to produce rapid physiologic effects. Finally, metabolites of the steroid hormone progesterone, referred to as neurosteroids, produce a range of effects on the brain through regulation of GABA_A receptors 10–4.

Prolactin, like growth hormone and placental lactogen, is a member of the somatotropic hormone family. It is synthesized and released by lactotrophs of the anterior pituitary gland. In males, serum prolactin levels normally are low throughout life. In females, such levels typically are only slightly higher but increase markedly during pregnancy. Pregnancy-related increases decline at term unless the mother breast-feeds; suckling causes marked increases in prolactin levels, which serve to initiate lactation. Hypothalamic control of prolactin is primarily inhibitory. It is mediated by dopamine released into the portal circulation at the median eminence by neurons of the tuberoinfundibular dopaminergic system, which originates in the arcuate nucleus (10–9; see also Chapter 6). In the pituitary, dopamine binds to D₂ dopamine receptors to inhibit prolactin synthesis and release. TRH from the hypothalamus and vasoactive intestinal peptide (VIP), which is produced locally in the pituitary, act to increase the release of prolactin.

Although prolactin is released in response to stress, both the mechanism and function of such release in humans remain unclear. Hyperprolactinemia is most commonly caused by tumors, or microadenomas, of pituitary lactotrophs or by D₂ dopamine receptor antagonists, most commonly antipsychotic drugs. It can result in galactorrhea and dysmenorrhea in females. In males, it may produce galactorrhea and impotence. Because prolactin is under inhibitory control of dopamine, hyperprolactinemia can be treated with dopamine D₂ receptor agonists, such as bromocriptine.

The prolactin receptor belongs to the family of glycophosphatidylinositol (GPI)-linked receptors, which modulate the actions of many cytokines, including ciliary neurotrophic factor, leukemia inhibitory factor, and leptin (Chapter 8). Activation of GPI-linked receptors leads to the activation of the protein tyrosine kinase Janus kinase (JAK), which phosphorylates and activates the transcription factors called signal transducers and activators of transcription (STATs). This JAK–STAT signaling pathway is described in greater detail in Chapters 4 and 8.

The production of growth hormone by the somatotrophs of the anterior pituitary is under the stimulatory control of growth hormone–releasing hormone (GHRH) and the inhibitory control of somatostatin 10–10. GH release must be pulsatile to stimulate skeletal growth.

GHRH is produced by cells of the infundibular and arcuate nuclei of the hypothalamus and is released at the median eminence. It binds to its G protein–linked receptor on the somatotroph membrane and stimulates transcription of growth hormone mRNA and release of mature growth hormone. Somatostatin, produced by neurons of the anterior periventricular region of the hypothalamus, is also released into the portal circulation at the median eminence; it acts via G protein–linked receptors coupled to G_i/o on somatotrophs to inhibit the release of growth hormone. Growth hormone provides negative feedback by stimulating somatostatin release. Growth hormone effects on target tissues are partly mediated by insulin-like growth factor 1 (IGF-1)/somatomedin C, which among other actions suppresses the release of growth hormone from the pituitary.

Growth hormone is necessary for longitudinal growth; accordingly, levels of this hormone are high in children, reach maximal levels during adolescence, and decline during adulthood. In addition to the growth of long bones, the metabolic effects of this hormone, which generally are anabolic in nature, lead to increased muscle mass and decreased body fat. Growth hormone exhibits normal circadian patterns of release, but is also released in response to stressors, such as exertion, or emotional stress.

The growth hormone receptor, like the prolactin receptor, is GPI-linked. It contains a single transmembrane domain, and has a large extracellular N terminus involved in hormone binding and a large intracellular C terminus. Hormone binding causes the receptor to dimerize and results in the activation of the JAK–STAT pathway. Growth hormone has direct effects on some target tissues, and its induction of IGF-1 in peripheral tissues mediates some of its growth-promoting and anabolic actions. Certain forms of dwarfism are caused by loss-of-function mutations of the growth hormone receptor.

The regulation of energy balance involves the exquisite coordination of food intake and energy expenditure. Experiments in the 1940s and 1950s showed that lesions of the lateral hypothalamus reduced food intake; hence, the normal role of this brain area is to stimulate feeding and decrease energy utilization. In contrast, lesions of the medial hypothalamus, especially the ventromedial nucleus (VMH) but also the PVN and dorsomedial nucleus (DMH), increased food intake; hence, the normal role of these regions is to suppress feeding and increase energy utilization. Yet discovery of the complex networks of neuropeptides 10–3 and other neurotransmitters acting within the hypothalamus and other brain regions to regulate food intake and energy expenditure began in earnest in 1994 with the cloning of the leptin (ob, for obesity) gene. Indeed, there is now explosive interest in basic feeding mechanisms given the epidemic proportions of obesity in our society, and the increased toll of the eating disorders, anorexia nervosa and bulimia. Unfortunately, despite dramatic advances in the basic neurobiology of feeding, our understanding of the etiology of these conditions and our ability to intervene clinically remain limited.

Leptin

Parabiosis studies, in which the circulation of two animals is joined, suggested that humoral factors are involved in energy balance. Rats with a medial hypothalamic lesion become obese; when a rat with a VMH lesion had its circulation joined with an unlesioned rat, the normal surgically joined circulatory partner ate less and lost weight. It was hypothesized that fat mass, which represents energy stores, releases a humoral “lipostatic” factor that inhibits feeding.

Obese (ob/ob) and diabetic (db/db) mice are two strains homozygous for different recessive mutations. Both strains are diabetic and typically weigh more than three times those of wild-type mice even when fed an identical diet. When their circulations were joined, it could be concluded that the ob/ob mice lacked a circulating “lipostatic” factor and that the db/db mice were insensitive to it. The missing factor in the ob/ob mouse proved to be leptin, while the db/db mouse lacks the leptin receptor.

Leptin is produced primarily by adipocytes. Plasma levels of leptin correlate with adipose tissue mass; in both humans and rodents, such levels increase with increased adipose tissue and decrease with weight loss 10–11.

Administration of leptin over time results in a dose-dependent decrease in body weight. Leptin levels do not change abruptly with meals; this finding distinguishes alterations in leptin from short-term satiety signals such as levels of glucose, amino acids, or cholecystokinin (CCK) 10–5. Mutations in leptin or in the leptin receptor in humans, which are very rare, produce morbid obesity and failure to undergo puberty.

The leptin receptor is most densely concentrated in the medial hypothalamus, the region known from lesion studies to be involved in suppression of feeding. Moreover, these nuclei are near the median eminence, a region of the brain with fenestrated capillaries that allows the entry of circulating leptin and other circulating peptides. The receptor is also highly expressed in the arcuate nucleus and more sparsely in other brain regions, including ventral tegmental area (VTA) dopamine neurons, which are important mediators of reward (Chapter 16). As will be seen later, activation of leptin receptors stimulates anorexigenic factors (antifeeding) and suppresses orexigenic factors (profeeding) to produce its powerful effects on feeding and energy utilization. The leptin receptor is a member of the GPI-linked receptor family, and leptin binding causes the activation of the JAK–STAT pathway.

Leptin influences other neuroendocrine functions. For example, ob mice exhibit many abnormalities characteristic of the starvation state, despite the animals’ obesity and hyperphagia. Such mice exhibit decreased body temperature, decreased energy expenditure, infertility, and decreased immune function. Because leptin replacement corrects all of these abnormalities in the ob mouse, it is believed that leptin may play a role in multiple neuroendocrine cascades.

The discovery of leptin raised a great deal of excitement regarding its usefulness in the treatment of obesity. It turned out, however, that the vast majority of obese individuals already have appropriately high levels of leptin, which has turned the field’s attention to other feeding factors, many of which are leptin targets.

Ghrelin

The peptide hormone ghrelin is produced by the cells that line the stomach. Ghrelin secretion increases hours after the end of a meal and stimulates appetite for the next meal. The ghrelin receptor, termed the growth hormone secretagogue receptor (GHSR), is G protein–linked. In addition to stimulating feeding, ghrelin helps maintain glucose during caloric restriction. Ghrelin stimulates feeding by activating NPY and agouti-related peptide (AgRP) neurons in the arcuate nucleus of the hypothalamus (see below) and possibly by acting directly on VTA dopamine neurons in the brain’s reward circuit to increase the motivational value of food and related cues. Ghrelin also increases growth hormone release, that is, it is a growth hormone secretagogue. Ghrelin’s activity depends on its octanoylation by ghrelin O-acyltransferase (GOAT). Among many possible mechanisms, bariatric surgery, in which portions of the stomach are excised or constricted in the treatment of severe obesity, may reduce appetite in part by reducing ghrelin secretion. There is interest in the possible use of ghrelin receptor antagonists or GOAT antagonists, still in early stages of investigation, in the treatment of obesity and of ghrelin itself in the treatment of cachexia.

POMC, NPY, and the Arcuate Nucleus

The arcuate nucleus of the hypothalamus plays a central role in energy balance. It contains two groups of GABAergic neurons that produce opposite effects on feeding and energy balance 10–11B. One group expresses POMC and cocaine- and amphetamine-regulated transcript (CART) and exerts anorexigenic effects and increases energy utilization; these cells are potently stimulated by leptin. The other group expresses NPY and AgRP and exerts orexigenic effects and reduces energy expenditure; these cells are inhibited by leptin and are stimulated by ghrelin.

As described above in the discussion of the HPA axis, POMC is a large precursor that is cleaved to produce ACTH, α-, β-, and γ-MSH, and β-endorphin (see 7–4 in Chapter 7). α- and β-MSH reduce feeding and increase energy expenditure acting primarily via the melanocortin receptor 4 (MC₄), which is G_s-linked), in the arcuate nucleus itself and also in the lateral hypothalamus, PVN, and DMH, those regions of the hypothalamus known to suppress feeding. MC₄ receptors are also highly expressed in the nucleus accumbens, a component of the brain’s reward circuitry (Chapter 16), which may provide a mechanism by which α-MSH suppresses motivation for feeding in addition to suppressing food consumption and increasing energy utilization per se. Knockout of the MC₄ receptor in animal models produces excessive feeding and obesity, and, likewise, very rare loss-of-function mutations in the MC₄ gene cause morbid obesity in humans.

Recent evidence indicates that POMC neurons express the serotonin 5HT_2C receptor, which, when activated, stimulates melanocortin release. This might explain the mechanism by which serotonin-promoting drugs (eg, fenfluramine or fenfluramine–phentermine combinations; Chapter 6) cause weight loss, while many second-generation antipsychotic drugs with 5HT_2C antagonism properties cause severe weight gain. Lorcaserin, a selective 5HT_2C agonist, has recently been approved by the US Food and Drug Administration for the treatment of obesity. Meanwhile, phentermine, in combination with the anticonvulsant drug topiramate (Chapter 19), is also approved for the treatment of obesity, but the mechanism involved remains unknown.

In contrast to melanocortins, NPY is a potent activator of feeding and suppressor of energy expenditure acting at its Y₁ and Y₅ receptors, which are linked to G_i/o. The NPY neurons of the arcuate nucleus also express AgRP that acts as an inverse agonist at the MC₃ and MC₄ receptors. AgRP not only blocks α- and β-MSH binding but also serves as an inverse agonist: it independently inhibits MC₃ and MC₄ function and thereby stimulates feeding and inhibits energy expenditure. In addition to being inhibited by leptin, NPY/AgRP cells are stimulated by ghrelin.

Based on this evolving model of feeding regulation, one would predict that melanocortin agonists or NPY antagonists would be useful in the treatment of obesity. Numerous agents are in development but have not yet been tested extensively in the clinic.

These complex peptidergic networks provide many potential molecular targets for the treatment of obesity, but to date potential treatments have been difficult to develop likely because of compensation within the hypothalamus and brainstem and rapid development of homeostatic adaptations.

Lateral Hypothalamus

Both the POMC and the NPY neurons of the arcuate nucleus project to the lateral hypothalamus. As mentioned earlier, this brain area is the site of potent orexigenic factors. One of the most important is melanin-concentrating hormone (MCH), which stimulates appetite and suppresses energy utilization. MCH receptors, which are G_i/o linked, are enriched in the hypothalamus as well as in the nucleus accumbens. MCH antagonists, beyond serving as potential antiobesity treatments, are being evaluated for their possible antidepressant use, based on a growing number of studies in animal models.

More recently, the lateral hypothalamus was also found to play a central role in arousal. Neurons in this region contain cell bodies that produce the orexin (also called hypocretin) peptides (see also Chapter 6, 6–25). These neurons project widely throughout the brain and are involved in sleep, arousal, feeding, reward, aspects of emotion, and learning. In fact, orexin is thought to promote feeding primarily by promoting arousal. Mutations in orexin receptors are responsible for narcolepsy in a canine model, knockout of the orexin gene produces narcolepsy in mice, and humans with narcolepsy have low or absent levels of orexin peptides in cerebrospinal fluid (Chapter 13).

Lateral hypothalamus neurons have reciprocal connections with neurons that produce monoamine neurotransmitters (Chapter 6). It should not be surprising that arousal and feeding are jointly regulated since feeding is absolutely central to survival, and the wake period for free-living organisms must be associated with the ability to find and consume food.

Dopaminergic Reward Circuits

Reward circuits, which are described in detail in Chapter 16, play a key role in feeding behavior. New, palatable foods cause dopamine release from VTA neurons of the midbrain that project to the nucleus accumbens, prefrontal cortex, and other limbic structures that regulate emotion. Dopamine acts in the nucleus accumbens to attach motivational significance to stimuli associated with rewards (such as food). It acts in the orbital prefrontal cortex to set a value on rewards, and dopamine released from the substantia nigra acts in the dorsal striatum to consolidate efficient motor programs to obtain rewards. It is very interesting then, as mentioned above, that orexin, leptin, and ghrelin receptors are expressed in the VTA, and MC₄ and MCH receptors are enriched in the nucleus accumbens. There is increasing evidence that some of the actions of these feeding peptides are mediated at the level of the VTA–NAc circuit: recent studies, for example, have shown that injection of leptin into the VTA suppresses feeding behavior, while RNA interference (RNAi; Chapter 4)–mediated knockdown of leptin receptors in the VTA increases food intake, sensitivity to highly palatable foods, and locomotor activity.

Inflammation plays a critical role in fighting infection, in wound healing, and in responses to many forms of tissue damage (Chapter 12). At the same time, infections and tissue damage represent substantial challenges to homeostasis. Immune mediators such as cytokines help defend the organism against infection, for example, but can have their own toxic effects if released in excess or as a result of autoimmunity.

Multiple microbial antigens and toxins activate cells of the immune system to secrete cytokines, including interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α), and interleukin-6 (IL-6) (Chapters 8 and 12). Such inflammatory mediators not only act locally at the site of an infection but also act systemically. As has been described in this chapter, the hypothalamus plays the central role in regulating homeostasis; moreover, some regions of the hypothalamus lie outside the blood–brain barrier and are thus able to detect circulating cytokines. It also has been hypothesized that cytokines interact with capillary endothelial cells on the blood side of the blood–brain barrier to produce lipid messengers, such as prostaglandins, that diffuse directly into the brain. As such, the hypothalamus is central in mediating systemic effects of cytokines, including the generation of fever and of “sickness behaviors” such as decreased appetite, reduced activity, fatigue, and altered hormonal output (eg, increased CRF secretion and decreased GnRH and TRH release).

Fever results from the action of prostaglandin PGE₂ acting on EP₃ prostaglandin receptors. Prostaglandins are synthesized by endothelial cells within small venules near the surface of the brain. Anatomically selective genetic deletion of EP₃ receptors within the median preoptic nucleus (MnPO) of the hypothalamus demonstrates that this site is required for the generation of fever. GABAergic MnPO neurons project to and tonically inhibit neurons within the PVN and DMH and to a region of the medulla that regulates sympathetic nervous system activity. Activation of EP₃ receptors disinhibits these neurons, and thereby facilitates thermogenic responses such as constriction of blood vessels in the skin.

Pharmacologic blockade of the enzymes that generate PGE₂ is antipyretic. Such drugs include aspirin and other salicylates, acetaminophen, and nonsteroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen and naproxen. These drugs block production of PGE₂ by inhibiting type 2 cyclooxygenase (COX2), which converts arachidonic acid to prostaglandins. COX2 inhibitors are discussed in greater detail in Chapter 11.