Monoamine-producing tumors

Monoamine synthesis

The catecholamines and serotonin share similar pathways of biosynthesis and metabolism, including in some steps the same enzymes. Catecholamines are synthesized from the amino acid, tyrosine, and serotonin from tryptophan ( Fig. 53.1 ). The rate-limiting step in catecholamine biosynthesis involves conversion of tyrosine to 3,4-dihydroxyphenylalanine ( l -dopa ) by the enzyme, tyrosine hydroxylase. A related enzyme, tryptophan hydroxylase, catalyzes conversion of tryptophan to 5-hydroxytryptophan (5-HTP) in the first step of serotonin synthesis.

Conversion of l -dopa to dopamine and of 5-hydroxytryptophan to serotonin is catalyzed by aromatic-L-amino acid decarboxylase, an enzyme with a wide tissue distribution and broad substrate specificity for aromatic amino acids. The dopamine and serotonin formed in the cytoplasm are then transported into vesicular storage granules, where the amines are available for exocytotic release. The dopamine formed in noradrenergic neurons and chromaffin cells is further converted to norepinephrine by dopamine β-hydroxylase, an enzyme with a unique presence in vesicular storage granules. The noradrenergic neurochemical phenotype of central noradrenergic neurons and peripheral sympathetic nerves depends on both translocation of dopamine into storage granules and the presence of dopamine β-hydroxylase.

The additional presence of phenylethanolamine N -methyltransferase (PNMT) in adrenal medullary chromaffin cells leads to further conversion of norepinephrine to epinephrine. Because PNMT is a cytosolic enzyme, this step depends on leakage of norepinephrine from vesicular storage granules into the cell cytoplasm, where the amine can then be available for N -methylation. Epinephrine is then translocated into chromaffin granules, where it is stored awaiting release.

Monoamine storage and release

Storage of catecholamines and serotonin in vesicular granules is facilitated by two vesicular monoamine transporters. Both transporters have a wide specificity for different monoamine substrates. The driving force for vesicular monoamine transport is provided by an adenosine triphosphate (ATP)–dependent vesicular membrane proton pump, which maintains an H ⁺ electrochemical gradient between the cytoplasm and granule matrix. Disruption of this gradient in situations of energy depletion and lowered intracellular pH—such as occurs with ischemia, anoxia, or cyanide poisoning—results in a rapid and massive loss of monoamines from storage vesicles into the neuronal cytoplasm.

Contrary to usual depictions, vesicular stores of catecholamines and serotonin do not exist in a static state until exocytotic release. Rather, vesicular stores of monoamines exist in a highly dynamic equilibrium with the surrounding cytoplasm; passive outward leakage of monoamines into the cytoplasm is counterbalanced by inward active transport under the control of vesicular monoamine transporters ( Fig. 53.2 ). Monoamines share the acid environment of the storage granule matrix with ATP, peptides, and proteins, the best known of which are the chromogranins.

The process of exocytosis at neuroeffector junctions of sympathetic varicosities and other monoamine-producing cells is dictated by the cell surface expression of specialized docking proteins that interact with other proteins on the surface of secretory vesicles. The process is stimulated by an influx of Ca ²⁺ , which in neurons is primarily controlled by nerve impulse–mediated membrane depolarization, and in adrenal medullary cells by acetylcholine release from innervating splanchnic nerves. The wide variety of voltage-, receptor-, G-protein–, and second messenger–operated Ca ²⁺ channels provide numerous points for regulation of Ca ²⁺ -triggered exocytosis. Consequently, a variety of peptides, neurotransmitters, and humoral factors provide additional mechanisms for stimulation of exocytosis or may act to modulate nerve impulse–stimulated release of monoamines. Dopamine, norepinephrine, and serotonin also modulate their own release through occupation of autoreceptors. Regulation of monoamine release and synthesis is closely coordinated, thereby ensuring appropriate replenishment of the amines lost through exocytosis.

Release of catecholamines and serotonin may also occur by calcium-independent nonexocytotic processes involving increased loss of monoamines from storage vesicles into the cytoplasm and reversal of normal inward carrier-mediated transport to outward transport of monoamines into the extracellular environment. Examples of this process include the release of catecholamines induced by sympathomimetic amines, such as tyramine and amphetamine. Excessive release of catecholamines, which accompanies hypoxic ischemia, occurs in part through a similar mechanism, but involving disruption of the pH gradient between the vesicular core and cytoplasm.

Monoamine uptake and metabolism

The primary mechanism limiting the life span of catecholamines in the extracellular space is uptake by active transport (see Fig. 53.2 ). Uptake is facilitated by transporters that belong to two large families with mainly neuronal or extraneuronal locations. Neuronal uptake of monoamines involves the dopamine transporter at dopaminergic neurons, the norepinephrine transporter at noradrenergic neurons, and the serotonin transporter at serotonergic neurons. These same transporters are also present at some non-neuronal locations, including adrenal chromaffin cells, endothelial cells of the lungs, specialized cells of the gastrointestinal tract, and some blood cells such as platelets. However, most extraneuronal uptake of monoamines is facilitated by a second set of transporters belonging to the organic cation transporter family. These latter transporters are expressed exclusively at extraneuronal locations and act on a broader range of substrates than neuronal monoamine transporters.

The neuronal monoamine transporters provide the principal mechanism for rapid termination of the signal in neuronal transmission, whereas transporters at extraneuronal locations are more important for limiting the spread of the signal and for clearance of catecholamines from the bloodstream. In addition to terminating the actions of released monoamines, the plasma membrane monoamine transporters at neuronal locations function in sequence with vesicular monoamine transporters to recycle catecholamines for re-release (see Fig. 53.2 ). Thus most of the norepinephrine released and recaptured by sympathetic nerves is sequestered back into storage vesicles, thereby reducing the requirements for synthesis of new transmitter.

Plasma membrane monoamine transporters also function as part of metabolizing systems, requiring the additional actions of enzymes for irreversible inactivation of the released amines. For both neuronal and extraneuronal metabolizing systems, inactivation of catecholamines and serotonin occurs in a serial arrangement with uptake followed by metabolism.

Metabolism of catecholamines occurs by a multiplicity of pathways resulting in a wide array of metabolites ( Fig. 53.3 ). Deamination of catecholamines by monoamine oxidase (MAO) yields reactive aldehyde intermediates that are further metabolized to acids by aldehyde dehydrogenase, or to glycols by aldehyde or aldose reductase. The aldehyde intermediate formed from dopamine is a good substrate for aldehyde dehydrogenase, but not aldehyde or aldose reductase. In contrast, the aldehyde intermediate formed from the β-hydroxylated catecholamines—norepinephrine and epinephrine—is a good substrate for aldehyde or aldose reductase, but a poor substrate for aldehyde dehydrogenase. Therefore dopamine is preferentially metabolized to dihydroxyphenylacetic acid (DOPAC), whereas norepinephrine and epinephrine are preferentially deaminated to 3,4-dihydroxyphenylglycol (DHPG).

Catechol- O -methyltransferase (COMT) is responsible for the second major pathway of catecholamine metabolism, catalyzing O -methylation of dopamine to methoxytyramine, norepinephrine to normetanephrine, and epinephrine to metanephrine. COMT is not present in monoamine-producing neurons, which contain exclusively MAO, but is present along with MAO in most extraneuronal tissues. The membrane-bound isoform of COMT, which has high affinity for catecholamines, is especially abundant in adrenal chromaffin cells. As a result of the preceding and other differences in expression of metabolizing enzymes, catecholamines produced at neuronal and adrenal medullary locations follow different pathways of metabolism ( Fig. 53.4 ).

Neuronal deamination pathways are quantitatively far more important than extraneuronal pathways for metabolism of the catecholamines synthesized at neuronal locations, such as the norepinephrine produced in sympathetic nerves. The reasons for this are twofold: first, much more norepinephrine released by sympathetic nerves is removed by neuronal uptake than by extraneuronal uptake; second, under resting conditions, most of the norepinephrine metabolized intraneuronally is derived from transmitter leaking from storage vesicles rather than from transmitter recaptured after exocytotic release. Consequently, most of the norepinephrine produced in the body is metabolized initially to DHPG, mainly from transmitter deaminated intraneuronally after leakage from storage vesicles or after release and reuptake. Most circulating DHPG is derived from sympathetic nerves, with relatively small contributions from the brain (<5%) and the adrenals (<7%).

DHPG is further O -methylated by COMT in non-neuronal tissues to 3-methoxy-4-hydroxyphenylglycol (MHPG), a metabolite also produced to a limited extent by deamination of normetanephrine and metanephrine (see Fig. 53.4 ). The latter O -methylated metabolites are produced in much smaller amounts compared to DHPG, and only at extraneuronal locations; their single largest source is adrenal chromaffin cells, which account for more than 90% of circulating metanephrine and 24 to 40% of circulating normetanephrine. Within the adrenals, normetanephrine and metanephrine are produced in a similar manner to the production of DHPG within sympathetic nerves, from norepinephrine and epinephrine leaking from storage granules into the chromaffin cell cytoplasm.

The MHPG produced from DHPG and metanephrines may be sulfate-conjugated or metabolized to vanillylmandelic acid (VMA), the latter pathway catalyzed by the sequential actions of hepatic alcohol dehydrogenase and aldehyde dehydrogenase. At least 90% of the VMA formed in the body is produced in the liver, mainly from hepatic uptake and metabolism of circulating DHPG and MHPG.

In contrast to production of VMA, production of homovanillic acid (HVA) from dopamine depends mainly on O -methylation of the deaminated metabolite of dopamine, 3,4-DOPAC, and to a lesser extent on deamination of methoxytyramine, the O -methylated metabolite of dopamine (see Fig. 53.3 ). As a result, HVA is formed in multiple tissues, with about 30% of both circulating and urinary HVA arising from mesenteric organs and about 15% from the brain.

With the exception of VMA, all catecholamines and their metabolites are metabolized to sulfate conjugates by a specific sulfotransferase isoenzyme (SULT1A3). The SULT1A3 isoenzyme is found in high concentrations in gastrointestinal tissues; therefore these tissues represent a major source of sulfate conjugates.

In humans, VMA and the conjugates of MHPG represent the main end products of norepinephrine and epinephrine metabolism ( Table 53.1 ). HVA and conjugates of HVA are the main metabolic end products of dopamine metabolism. These end products and the other conjugates are eliminated mainly by urinary excretion. As a result, their circulatory clearance is slow and plasma concentrations are high relative to those of the precursor amines.

Serotonin is not a substrate for COMT and follows simpler pathways of metabolism than those for catecholamines ( Fig. 53.5 ). Deamination of serotonin by MAO to the aldehyde intermediate is preferentially followed by oxidation to 5-hydroxyindoleacetic acid (5-HIAA) catalyzed by aldehyde dehydrogenase. Reduction to 5-hydroxytryptophol (5-HTOL) normally represents a relatively minor pathway, so that the major urinary excretion product of serotonin metabolism is 5-HIAA. Serotonin can also undergo sulfation or glucuronidation, but these reactions account for a minor portion of the metabolism. In the pineal gland serotonin can be further converted to N -acetyl serotonin, and ultimately to melatonin.

Peptide secretory products

Apart from catecholamines, adrenal medullary chromaffin cells, sympathetic neurons, and their tumor derivatives store and secrete a wide array of neuropeptides and proteins. Peptides include enkephalins, β-endorphin, neuropeptide Y, substance P, vasoactive intestinal peptide (VIP), neurotensin, galanin, atrial natriuretic peptide, pituitary adenylate cyclase–activating peptide, adrenomedullin, and corticotrophin. These peptides are secreted together with the catecholamines and may be involved in local autocrine or paracrine regulation of adrenal medullary and cortical function.

The major soluble proteins within chromaffin vesicles belong to the family of chromogranins and secretogranins, which consist of several secretory acidic glycoproteins. It is the presence of these granins that is responsible for the characteristic electron dense nature of chromaffin storage granules visible in electron micrographs. Chromogranin A represents the index member of the chromogranin/secretogranin family. It is present in almost all aminergic secretory granules, including those of adrenal medullary cells, sympathetic neurons, and enterochromaffin cells of the diffuse neuroendocrine system.

Chromogranin A is a 439-amino-acid protein with a molecular weight of 49 to 80 kDa, the variation depending on glycosylation and phosphorylation status. ^, Granins maintain regulated secretion of signaling molecules according to three processes: (1) facilitation of secretory granule formation; (2) calcium- and pH-mediated sequestration and/or re-solubilization of peptide hormones or biogenic amines; and (3) regulation of neuropeptide and peptide hormone processing through modulation of prohormone convertase activity. Granins contain multiple protease and peptidase cleavage sites, and upon intra- or extracellular cleavage give rise to a series of daughter peptides with distinct functions.

Enteroendocrine cells also produce a wide repertoire of peptide hormones, which are derived from different peptide precursors. One family of peptides includes cholecystokinin, gastrointestinal peptide, glucagon-like peptide, pancreatic peptide YY, secretin, and neurotensin. Ghrelin and motilin are peptides produced in other cell types mainly involved in hunger and increased gastrointestinal motility. Substance P and somatostatin are found in enteroendocrine cells throughout the gastrointestinal tract. Many of these peptide hormones appear to work in concert with monoamines, such as serotonin and histamine, also produced and, in some instances, co-secreted with the peptides by enteroendocrine cells.

The sympathetic nervous system

Sympathetic nerve transmission operates below the level of consciousness in controlling the physiologic functions of many organs and tissues of the body ( Fig. 53.6 ). The sympathetic nervous system plays a particularly important role in regulating cardiovascular function in response to postural, exertional, thermal, and mental stress. With sympathetic activation, heart rate is increased, peripheral arterioles are constricted, and blood pressure is elevated. Sympathetic signals work in balance with the parasympathetic branch of the autonomic nervous system to maintain a stable internal environment.

Sympathetic outflow is regulated by brain stem centers according to afferent (incoming) sensory signals from peripheral pressure, stretch, chemical, pain, and temperature receptors. Afferent signals are integrated with other neural pathways from higher centers to establish efferent (outgoing) components of reflex arcs in which responses are transmitted by the neurotransmitter acetylcholine released by preganglionic sympathetic neurons that exit the spinal cord and converge on sympathetic ganglia chains along the spinal column or in visceral ganglia (see Fig. 53.6 ). Terminal branches of postganglionic fibers that project into target organs have varicosities that form a rich ground plexus for communication with a large number of effector cells.

Most sympathetic postganglionic nerves liberate norepinephrine as their neurotransmitter. In limited end-organ locations, such as sweat glands, sympathetic nerve endings release acetylcholine. Cholinergic preganglionic sympathetic fibers also innervate the adrenal medulla, stimulating release of epinephrine from chromaffin cells.

The relative contributions of individual organs to sympathetic outflow have been inferred from measurements of norepinephrine released into blood draining those organs. ^{, ,} Based on such measurements the GI tract and other mesenteric organs produce the most norepinephrine (≈37%), but almost all of this is removed from the portal circulation by the liver so little reaches the systemic circulation. The kidneys (≈25%) followed by skeletal muscle (≈11%) provide the next largest contributions to norepinephrine release to the bloodstream, while the adrenals, heart, liver, lungs, and skin each contribute less than 10%.

Changes in sympathoneural release of norepinephrine in response to stressors such as exercise, upright posture, meals, and mental stress occur in distinct regional patterns to allow for appropriate organ-specific responses. ^, Although global sympathetic activation clearly operates in extreme conditions of stress, the differential control of sympathetic output enables an appropriate patterning of physiologic responses according to different behavioral and environmental stressors. Increased sympathoneuronal release of norepinephrine occurs with aging and is also found in disorders such as cardiac failure and hypertension.

Physiologic responses to changes in local release of norepinephrine at neuroeffector junctions or of epinephrine released from the adrenals into the bloodstream depend on the types and locations of adrenergic receptors within tissues ( Table 53.2 ). In peripheral target organs, α ₁ - and β ₁ -adrenergic receptors are strategically located in the immediate vicinity of nerve terminals for rapid responses via signals from the brain. Postsynaptic β ₂ -adrenergic receptors in the heart also allow rapid sympathetic neuroactivation. Extrajunctional α ₂ and β ₂ receptors that are remote from sympathetic nerve varicosities in vascular smooth muscle or platelets may be preferentially influenced by circulating catecholamines such as epinephrine produced by the adrenal gland.

The adrenal medulla

Although often considered a part of the sympathetic nervous system, the adrenal medulla produces and secretes a different catecholamine, epinephrine, with different functions from the norepinephrine secreted by sympathetic nerves. The adrenal medulla and sympathetic nerves are also regulated separately, often in divergent directions in response to different forms of stress.

The human adrenals overlie the superior poles of the kidneys. Each gland consists of an outer lipid-rich cortex and a thin inner central medulla containing chromaffin cells. The adrenal medulla is up to 2 mm thick and accounts for about one tenth of the entire weight of the gland. Blood is supplied to the adrenal medulla by direct arterial supply and through vessels draining from the cortex to the medulla. The latter supplies adrenocortical steroids for regulation of adrenal medullary function. Neural input to the adrenal medulla includes direct innervation by cholinergic fibers that pass through the sympathetic paravertebral chain from preganglionic sympathetic cell bodies of the spinal cord (see Fig. 53.6 ).

A characteristic feature of adrenal medullary chromaffin cells is the presence of numerous catecholamine storage granules ranging in size from 100 to 300 nm in diameter. These granules turn brown when exposed to potassium dichromate solutions, ammoniacal silver nitrate, or osmium tetroxide as a result of the oxidation and polymerization of epinephrine and norepinephrine. This process is known as the chromaffin reaction, hence the terms chromaffin cells and chromaffin granules.

The human adrenal medulla produces mainly epinephrine; as a hormone, epinephrine is secreted directly into the bloodstream to act on cells distant from sites of release. Both the proximity of sites of norepinephrine and epinephrine release to adrenoceptors and differences in potencies of action on α- and β-adrenergic receptors contribute to differences in adrenoceptor-mediated responses to the two catecholamines. Consequently, epinephrine exerts its effects on different populations of adrenoceptors than norepinephrine. As a circulating hormone, epinephrine acts potently on β ₂ -adrenergic receptors of the skeletal muscle vasculature, causing vasodilatation. In contrast, norepinephrine released locally within the vasculature causes α ₁ -adrenoceptor–mediated vasoconstriction. Increases in circulating epinephrine during stress may contribute to skeletal muscle vasodilatory responses, but do not substantially contribute to other cardiovascular changes unless the stress is extreme (e.g., hemorrhage). Thus despite the potent hemodynamic actions of epinephrine, under usual day-to-day conditions the adrenal medulla appears to play a minimal role in cardiovascular regulation compared with sympathetic nerves.

Epinephrine released from the adrenals is more important as a metabolic than as a hemodynamic regulatory hormone. In particular, epinephrine stimulates lipolysis, ketogenesis, thermogenesis, and glycolysis. Epinephrine raises plasma glucose concentrations by stimulating glycogenolysis and gluconeogenesis. Epinephrine also has potent effects on pulmonary function, causing β ₂ -adrenoceptor–mediated dilatation of airways. Circulating norepinephrine, in minor part derived from the adrenal medulla and functioning as a hormone, may have additional metabolic actions, but appears to have little importance for cardiovascular regulation compared to the higher concentrations of the amine at sympathoneuroeffector sites.

Despite the apparent importance of the adrenal medulla in homeostasis, particularly in the regulation of metabolism, the medulla in contrast to the adrenal cortex is not vital for survival. Studies in adrenalectomized subjects clearly show that both hemodynamic and glucose counter-regulatory responses to insulin hypoglycemia, exercise, and other manipulations remain intact, despite the absence of epinephrine responses. ^, This contrasts with the severe disturbances of blood pressure regulation that accompany loss of sympathetic nerves.

In contrast to the sympathetic nervous system, the adrenal medulla makes a relatively minor contribution to the overall production and turnover of catecholamines ( Table 53. 3 ). However, because PNMT is expressed mainly in adrenal chromaffin cells, more than 90% of circulating epinephrine is derived from the adrenal medulla. This contrasts with circulating norepinephrine, more than 90% of which is derived from sympathetic nerves.

Enteroendocrine and pulmonary neuroendocrine systems

The main function of the gastrointestinal system is digestion, absorption, and metabolism of nutrients. This requires coordination of a host of biological activities throughout the gastrointestinal tract, as well as synchronization between gut, liver, pancreas, and other organ systems. Cardiac output, vascular tone, and overall gastrointestinal blood flow have to be regulated to ensure optimal nutrient flow. Excess nutrients need to be stored. Metabolic heat needs to be shed. Nutrient demands from all the body’s organ systems need to be met.

Precise regulation of the gastrointestinal system requires both rapid and sustained responses on an autocrine, paracrine, and endocrine/systemic level. This is achieved through an interlocking system of autonomic nervous regulation and a vast network of neuroendocrine cells, which collectively produce a large array of bio-active peptides and a smaller number of biogenic amines, the latter chiefly serotonin, histamine, and compounds related to these two biogenic amines ( Table 53.4 ). Secretory products might be released into neighboring tissues, the gut lumen, the blood stream, or a combination of the above. In terms of sheer number of cells, the gastroenteropancreatic neuroendocrine network represents the largest endocrine organ in the body.

Unlike neuroendocrine cells that are found in the pituitary or associated with the autonomic nervous system, gastroenteropancreatic neuroendocrine cells do not form distinct organs. They can nevertheless be stratified into two main cell populations: (1) those that are dispersed throughout most intestinal structures that have arisen from foregut (including the bronchopulmonary tree and thymus), midgut, and hindgut; and (2) those that reside exclusively within pancreatic islets or (occasionally) the duodenum. There also exist small populations of neuroendocrine cells in other organs (heart, ovaries, testicles, prostate, etc.).

Neuroendocrine cells of the pancreatic-gastrointestinal and respiratory tracts comprise a diffuse system of both endocrine and paracrine cell types from which different types of neuroendocrine tumors arise. The gastrointestinal tract alone contains as many as 12 different enteroendocrine cell types characterized by production of a variety of monoamine and peptide hormones (see Table 53.4 ). ^{, ,} These various secretory products regulate digestive function, food intake, and energy homeostasis. ^, Various enteroendocrine cell types localize to distinct regions. For example, histamine-producing cells represent the major subtype in the acid-producing gastric corpus, along with ghrelin, somatostatin, and serotonin-secreting cells.

More than 95% of the body’s serotonin is produced within the gastrointestinal tract, with significant amounts synthesized and stored in enteroendocrine cells in the gut mucosa. Serotonin is released from these cells in response to mechanical or chemical stimuli, such as the passage of food; this in turn stimulates both intrinsic (via 5-HT _1P and 5-HT ₄ receptors) and extrinsic (via 5-HT ₃ ) vagal sensory nerve fibers. Intrinsic sensory neurons activated by serotonin stimulate the peristaltic reflex and secretion, whereas extrinsic sensory neurons initiate bowel sensations such as nausea, vomiting, abdominal pain, and bloating. The paracrine actions of serotonin are terminated by uptake into epithelial cells by the same serotonin transporter present in serotonergic neurons. Serotonin more widely modulates numerous physiologic systems in the human body from blood clotting, liver regeneration, and bone formation to glucose homeostasis. ^,

Peripheral dopamine systems

It is often presumed that most dopamine in the body is produced within the brain. From measurements of dopamine metabolites in jugular venous blood it is now clear that the contribution of the brain represents less than 20% of the overall production of dopamine within the body ( Fig. 53.7 ). ^, Furthermore, almost all of the dopamine formed in sympathetic nerves and the adrenals is converted to norepinephrine and epinephrine. Thus most of the dopamine and dopamine metabolites in the circulation and excreted into urine are derived from peripheral sources other than sympathetic nerves or the adrenal medulla.

Evidence that dopamine acts as a neurotransmitter outside of the CNS is weak. Instead, it appears that dopamine in the periphery has autocrine or paracrine functions. Findings of increased plasma concentrations of l -dopa, dopamine, and dopamine metabolites after consumption of foods also indicate that dietary constituents may additionally contribute to plasma and urinary levels of dopamine metabolites (see Fig. 53.7 ). Such food sources do not, however, account for the substantial amounts of dopamine produced in peripheral tissues outside of the digestive tract. Diet also does not account for findings in fasting individuals in which large arterial to portal venous increases in plasma concentrations of dopamine and its metabolites indicate substantial production of dopamine within mesenteric organs. These findings are consistent with morphologic studies demonstrating the presence of cells in the gastrointestinal tract that contain dopamine and express components of dopamine signaling pathways, including catecholamine biosynthetic enzymes and specific dopamine receptors and transporters.

Dopamine and dopamine receptor agonists stimulate bicarbonate secretion and protect against ulcer formation, whereas dopamine antagonists augment secretion of gastric acid and promote ulcer development. Dopamine also influences gastrointestinal motility, sodium transport, and gastric and intestinal submucosal blood flow. ^, In the stomach dopamine acts on D1A receptors to stimulate ghrelin secretion. In the pancreas, dopamine modulates exocrine secretion of digestive enzymes and bicarbonate and release of insulin from islet cells. ^, Thus dopamine acts in mesenteric organs as an enteric neuromodulator or paracrine-autocrine substance.

In the kidneys, dopamine is an established autocrine and/or paracrine effector substance contributing to the regulation of sodium excretion. Unlike other catecholamine systems, production of dopamine in the kidneys is largely independent of local synthesis of l -dopa by tyrosine hydroxylase (see Fig. 53.7 ). Instead, production of dopamine in the kidneys depends mainly on proximal tubular cell uptake of l -dopa from the circulation. The l -dopa is then converted to dopamine by L-aromatic amino acid decarboxylase, the activity of which is up-regulated by a high-salt diet and down-regulated by a low-salt diet. The presence of a renal dopamine paracrine-autocrine system explains the considerable amounts of free dopamine excreted in the urine. Most derives from renal uptake and decarboxylation of circulating l -dopa and reflects the plasma levels of this amino acid and the function of the renal dopamine paracrine/autocrine system. Most urinary free methoxytyramine is similarly derived from renal uptake and metabolism of circulating l -dopa.

Historical introduction

After the first description of neuroblastoma by Virchow in 1864, Marchand made the connection in 1891 between tumors arising from the sympathetic nervous system and those from the adrenal medulla. The clinical picture of tumors in newborns that could spontaneously regress was first sketched by Pepper in 1901. These neuroblastomas are now classified as stage 4S tumors. Within the following decade the more ominous counterpart (now classified as stage 4) became apparent when Hutchinson described ecchymosis and proptosis, typical symptoms of orbital and skeletal metastases from an adrenal primary. In 1910 Wright coined the name “neuroblastoma” originating from embryonal neuroblasts of the sympathetic nervous system, and within 20 years the concept of maturation into ganglioneuroma was introduced by Cushing and Wolbach. In 1957, Mason identified production of catecholamines in a neuroblastoma and, in 1959, VMA was described as a useful tumor marker. ^, Evans, D’Angio, and Randolph proposed the first tumor staging system in 1971. In 1983, Schwab and Bishop identified amplification of MYCN (or N-MYC ), one of first identified human oncogenes. Amplification correlated with high-risk disease; therefore MYCN amplification was incorporated into the international neuroblastoma tumor staging system in 1987.

Incidence and clinical presentation

Neuroblastomas occur almost exclusively in children, accounting for approximately 7 to 10% of childhood cancer and represent the most common malignancy diagnosed in the first year of life. The incidence of neuroblastoma is approximately 10 cases per million children, resulting worldwide in about 10,000 new cases per year. Although familial cases have been reported, the vast majority of neuroblastomas develop sporadically. At presentation, the location of the primary tumor and the extent of the disease determine the signs and symptoms. Patients generally appear well for loco-regional or stage 4S (low risk) and sick with stage 4 (high risk) disease. It is not uncommon for loco-regional tumors to be diagnosed incidentally following an ultrasound performed for unrelated reasons. In high-risk disease, pain from abdominal distension or bone metastases is common, often associated with a limp, weight loss, and anemia. Although stage 4S neuroblastomas in infants (<12 months) often spontaneously regress, they occasionally present with massive hepatomegaly, causing life-threatening respiratory distress or urinary or bowel obstructions.

Primary neuroblastoma may arise anywhere along sympathetic paravertebral chain. Typical locations include the adrenal medulla, the organ of Zuckerkandl, and the ganglion stellatum at the 7th cervical vertebral transverse process. While adrenal primaries are the most common for stage 4 and 4S disease, thoracic and cervical primaries are more common among loco-regional stages.

Typical sites of metastasis for stage 4 neuroblastoma include bone marrow, bone, lymph nodes, liver, rarely in lungs or skin, and even less commonly in the central nervous system. This contrasts with stage 4S disease where liver and skin are more common sites of distant disease, with bone marrow involvement usually scattered and never involving bone itself. Lung metastasis, rare at diagnosis, is more often seen at relapse.

In contrast to pheochromocytoma, severe hypertension is uncommon at diagnosis of neuroblastoma; this presumably results from the limited catecholamine storage capacity of neuroblastoma tumor cells as reflected by electron microscopic findings of sparse populations of chromaffin granules.

Certain syndromes at presentation are associated with a low-risk biology. The opsoclonus myoclonus syndrome (Kinsbourne syndrome) is characterized by rapid, irregular eye movements (“dancing eyes,” may continue during sleep) and/or by myoclonus and ataxia (“dancing feet”). Most of these patients survive their neuroblastoma only to be left with developmental delays, language deficits, and behavioral abnormalities. ^, Horner syndrome (ptosis, miosis, enophthalmos) and heterochromia (difference in color between the two irises) at diagnosis is typically associated with tumors arising from the cervical sympathetic ganglia, a location usually associated with a better than usual prognosis.

Biological variation

Neuroblastomas can be broadly classified into three types: type 1 are those that differentiate into ganglioneuroblastomas and ganglioneuromas; type 2 are those regressing spontaneously with time; and type 3 are those progressing in size and spreading to distant sites. The differentiating type (5%) is identified only retrospectively after maturation to ganglioneuroma. The regressing type is typified by stage 4S neuroblastoma, ^, which while waxing and waning in the early months of life eventually completely disappears. This biology is repeated among loco-regional tumors (90 to 95% curable) in spite of residual microscopic disease. Perhaps the most striking indication for the prevalence of regressing biology in neuroblastoma is reflected by the two- to threefold lower incidence of neuroblastoma reported in unscreened newborns versus populations screened for neuroblastoma using urinary VMA or HVA. ^, The difference indicates natural regression of most cases of undetected neuroblastoma in early childhood.

Among detected cases of neuroblastoma, more than 50% belong to the progressive type, typically acquiring chemoresistance and accounting for the majority of deaths from this disease. Despite the large-scale unraveling of neuroblastoma genomics and biology, a robust signature to reliably predict long-term outcome among these high-risk patients has yet to emerge.

Differential diagnosis

The histologic diagnosis of neuroblastoma is unequivocal for the majority of patients where the presence of neuropil is highly distinctive. Neuroblastomas are more primitive tumors than pheochromocytomas and do not express a chromaffin phenotype. For those with a completely undifferentiated pattern, other small blue round cell tumors need to be excluded, including Ewing family of tumors, rhabdomyosarcoma, desmoplastic small round cell tumor, leukemia, and malignant non-Hodgkin lymphoma. Several clinical features can help in the differential diagnosis: (1) these other tumors do not produce elevations of catecholamine metabolites and are not meta-iodobenzylguanidine (MIBG) avid; (2) distinctive tumor locations such as soft tissue mass outside the sympathetic chain or lymph nodes for rhabdomyosarcoma; (3) high white cell count for leukemia; and (4) morphology of bone marrow infiltration in Burkitt lymphoma.

Immunohistochemical findings (synaptophysin, chromogranin, or neuron-specific enolase [NSE]) can also assist with the correct diagnosis. Tumor-specific genetic translocations can also be used to definitively exclude neuroblastoma.

The distinction of necrotic/hemorrhagic neuroblastoma from a posthemorrhage adrenal gland during the newborn period can be difficult. Urinary excretion of VMA and HVA may also be too low to confirm neuroblastoma, particularly if the primary tumor is small. Close follow-up in such cases with ultrasound is warranted. Finally, although adrenal pheochromocytoma and extra-adrenal paragangliomas may produce catecholamines, they do not share the molecular or clinical characteristics typical of neuroblastoma.

Background consideration for biochemical testing

Unlike pheochromocytoma and paraganglioma, where biopsies are contraindicated and initial diagnosis relies crucially on biochemical testing, such testing is of secondary and limited importance for neuroblastoma. Based on an international consensus, a diagnosis of neuroblastoma can be made by either of two ways: (1) histologic confirmation based on histopathology, supplemented by immunohistochemistry, with or without elevations of catecholamine metabolites; or (2) bone marrow aspirate or trephine biopsy containing unequivocal tumor cells (e.g., syncytia) with elevations of catecholamine metabolites. The clinical diagnosis of “suspected neuroblastoma” made in emergency situations based on measurements of catecholamine metabolites or radiographic and functional imaging features (e.g., MIBG) is conditional since ganglioneuroma or pheochromocytoma need to be ruled out.

Biochemical testing for neuroblastoma today continues to rely on measurements of urinary HVA and VMA, tests developed in the mid-20th century. Lack of any significant advance in biochemical testing for neuroblastoma in more than 60 years likely reflects several factors: (1) emphasis of diagnosis on biopsy evidence from masses that are usually discovered on clinical presentation; (2) difficulties in developing new tests in pediatrics where the regulatory hurdles are considerably more complex than for adults; and (3) lack of efficacy of screening programs utilizing biochemical tests for improving outcomes.

As a consequence of the above factors there remains little incentive to develop new and improved blood or urine tests for neuroblastoma. Even if new tests are developed, any motivation to bring such tests into the clinical mainstream must contend with limited enthusiasm from the clinical community responsible for managing and treating children with neuroblastoma. Although invasive, diagnosis based on biopsy is invariably unequivocal whereas that based on urinary HVA and VMA has questionable accuracy and does not provide critical information for disease stratification and subsequent patient management.