Chapter 11 Infundibular nucleus (arcuate nucleus), subventricular nucleus and median eminence (Fig. 11A)

2014 Ch 11

20/11/03

11:30 am

Page 249

Handbook of Clinical Neurology, Vol. 79 (3rd Series Vol. 1) The Human Hypothalamus: Basic and Clinical Aspects, Part I D.F. Swaab, author © 2003 Elsevier B.V. All rights reserved

1 2 3 4 5 6 7 8 9 101 1 2 3 4 5 6 7 8 9 201 1 2 3 4 5 6 7 8 9 301 1 2 3 4 5 6 7 8 9 401 1 2 3 4 5 6 7 8 911

CHAPTER 11

Infundibular nucleus (arcuate nucleus), subventricular nucleus and median eminence (Fig. 11A)

situated outside the blood–brain barrier (Youngstrom and Nunez, 1987). The infundibular nucleus in both pre- and postmenopausal women contains some 505,000–520,000 neurons (Abel and Rance, 2000). The infundibular nucleus is chemically characterized by the presence of (pre)pro-opiomelanocortin neurons (Pilcher et al., 1988; Sukhov et al., 1995). Good markers for this nucleus are, e.g. -MSH (Désy and Pelletier, 1978; Pelletier et al., 1987; Fig. 11.2a), a peptide that inhibits feeding; the feeding-stimulating peptide, neuropeptide-Y (NPY; Figs. 11.3, 11.4 and 11.5A); and galanin (Gai et al., 1990) (Fig. 11.2b); while beta-melanotropin (-MSH), corticotropin (ACTH), -endorphin, -endorphin, -lipotropin ( LPH), -MSH and proenkephalin are also found in this nucleus (Bloch et al., 1978; Pelletier et al., 1978; Bugnon et al., 1979; Pelletier and Désy, 1979; Osamura et al., 1982; Sukhov et al., 1995; Abel and Rance, 1999; Bernstein et al., 2002). A high density of delta sleep-inducing peptide containing fibers is present in this nucleus (Najimi et al., 2001b). In addition, a small number of dynorphincontaining neurons were found. In the human fetus, neurons that stain with anti--endorphin or anti-17–39 ACTH were found from the 11th week of fetal life onwards (Bugnon et al., 1979). The sites of fiber termination of the opiomelanocortin neurons agree with the brain sites where pain relief was obtained in human beings by deep brain stimulation (Pilcher et al., 1988; see Chapter 31.2). Moreover, -MSH and its melanocortin (MC)-4 receptors are presumed to be involved in weight homeostasis by inhibiting feeding behavior (Schiöth et al., 1997; Goldstone et al., 2002; Chapter 23). Agouti-related peptide (AGRP; Figs. 11.3 and 11.4) is a high-affinity antagonist of the hypothalamic MC4 receptor, and it causes obesity in mice through antagonism of these receptors. -MSH neurons thus seem

The infundibular nucleus is involved in reproduction, pain, eating behavior and metabolism, thyroid hormone feedback growth, and dopamine regulation. In addition, the infundibular nucleus is continuous with the stalk/median eminence region that contains the portal capillaries of the adenohypophysis (Chapter 17.1). (a) Chemoarchitecture The horseshoe-shaped infundibular (or arcuate) nucleus surrounds the lateral and posterior entrance of the infundibulum (Fig. 11.1; Abel and Rance, 2000) and is 249

2014 Ch 11

250 1 2 3 4 5 6 7 8 9 101 1 2 3 4 5 6 7 8 9 201 1 2 3 4 5 6 7 8 9 301 1 2 3 4 5 6 7 8 9 401 1 2 3 4 5 6 7 8 911

20/11/03

11:30 am

Page 250

D.F. SWAAB

Fig. 11A. Vesalius infundibulum. Vesalius’ representation of the infundibulum of the pituitary (citation Anderson and Haymaker, 1974; Fig. 39, with permission.) Vesalius: “In this small figure we have depicted the pelvis or cup (cyathus) set upright by which the pituita of the brain distills into the gland underlying it, and then we have sketched in the 4 ducts carrying the pituita down from the gland through foramina in the neighborhood of the gland. Therefore A indicates the gland (not shown in this figure) into which the pituita is distilled and B, the pelvis along which it is led. C, D, E, F are the passages provided for the easier exit of the pituita passing down there.” (Andreas Vesalius, De Humani Corporis Fabrica, Basilae 1543; p. 621.)

to exert a tonic inhibition of feeding behavior (Fan et al., 1997; Huszar et al., 1997; Chapter 23). AGRP, but not NPY, colocalizes with NPY in the human infundibular nucleus (Fig. 11.4; Goldstone et al., 2002). Another peptide that is involved in feeding behavior is cocaineand amphetamine-regulated transcript (CART). It inhibits food intake and is found in the infundibular nucleus and other human hypothalamic nuclei (Hurd and Fagergren, 2000; Elias et al., 2001). In addition, hypocretin fibers are found in this brain area (Moore et al., 2001). The infundibular nucleus also contains choline acetyltransferase-containing neurons (Tago et al., 1987) and benzodiazepine-binding sites (Najimi et al., 2001), while the luteinizing hormone-releasing hormone (LHRH = gonadotropin-releasing hormone)-containing cell bodies are mainly found in the ventral portion (Barry, 1977; Najimi et al., 1990; Rance et al., 1994; Dudás et al., 2000). LHRH neurons are mainly localized in the infundibular lip, which is the cell-sparse zone at the very base of the hypothalamus, between the pial surface and the ventral

boundary of the infundibular nucleus (Rance et al., 1994). The LHRH neurons are innervated by TH-containing neurons, probably derived from the supraoptic, paraventricular (PVN) and periventricular regions and by CRH fibers. Catecholamines, substance-P and CRH may influence LHRH function (Dudás and Merchenthaler, 2001, 2002a,b). LHRH neurons are found in the human fetal hypothalamus from 9–13 weeks of pregnancy onwards (Bugnon et al., 1976; Paulin et al., 1977). In addition, Leu-enkephalin fibers have synaptic-like contacts with LHRH neurons in the infundibular nucleus (Dudás and Merchenthaler, 2003). Both -endorphin and LHRH were found to be present in the same infundibular neurons in 17- to 26-week-old human fetuses (Leonardelli and Tramu, 1979). Hypothalamic LHRH concentrations and content were reported to remain stable between 2.5 and 21 hours after death (Parker and Porter, 1982), which would allow the investigation of this system in the human brain. Later studies showed, however, that the postmortem stability of LHRH is present in the preopticseptal regions but not in the mediobasal hypothalamus (Rance et al., 1994; Rance and Uswandi, 1996). Opioid peptides also play a role in reproduction and sexual behavior (Chapter 24). Intracerebroventricular administration of -MSH in the rat induces erection (Mizusawa et al., 2002). -Endorphin is involved in the regulation of LHRH release and thus of the menstrual cycle (Chapter 24.1). Endogenous opioid peptides, in particular -endorphin, inhibit the release of LHRH, while the activity of -endorphin neurons is, in its turn, stimulated by estrogens (Rasmussen, 1992; Abel and Rance, 1999). The endogenous LHRH pulse generator, which gives rise to LHRH pulses with a periodicity of 60–100 min, is located entirely in the mediobasal hypothalamus (Rasmussen, 1992). LHRH is present in at least two isoforms (Chen et al., 1998; Yahalom et al., 1999). The 2 isoforms (GnRHI and II can potentially stimulate gonadotropin release in the monkey to a similar degree (Densmore and Urbanski, 2003). In addition, neurons containing somatostatin, substance-P (Bouras et al., 1986, 1987; Mai et al., 1986; Mengod et al., 1992; Chawla et al., 1997), neurotensin (Saper, 1990), galanin (Gai et al., 1990) and some scattered CRH neurons (Mihály et al., 2002) are found. Adrenomedullin (Satoh et al., 1996) and TRH fibers (Fliers et al., 1994), TRH-binding sites (Najimi et al., 1991), VIP-binding sites (Sarrieau et al., 1994) and oxytocin-binding sites (Loup et al., 1991) are also observed in the infundibular nucleus. Androgen receptors are more prominent in the infundibular nucleus

2014 Ch 11

20/11/03

11:30 am

Page 251

INFUNDIBULAR NUCLEUS, SUBVENTRICULAR NUCLEUS AND MEDIAN EMINENCE

1 2 3 4 5 6 7 8 9 101 1 2 3 4 5 6 7 8 9 201 1 2 3 4 5 6 7 8 9 301 1 2 3 4 5 6 7 8 9 401 1 2 3 4 5 6 7 8 911

251

Fig. 11.1. (A and C) Photomicrographs of Nissl-stained sections from the hypothalamus of a young female. (B and D) Computer-assisted tracings of the sections on the left. The boundary of the infundibular nucleus is outlined. The sections are numbered in successive rostral-to-caudal intervals with the section number at the bottom of each tracing. Abbreviations: fx, fornix; ic, internal capsule; INF, infundibular nucleus; LTN, lateral tuberal nuclei; ot, optic tract; PVN, paraventricular nucleus; TMN, tuberomamillary nucleus; 3V, third ventricle. Bar = 3 mm. (From Abel and Rance, 2000; Fig. 2.)

of men than women (Fig. 6.2). Estrogen receptor (ER) and - cytoplasmic staining intensity were more intense in young women than in men (Kruijver et al., 2002, 2003). There is an abundant presence of thyroid hormone receptors (TR) in the infundibular nucleus. In mammals, 4 isoforms have been reported: TR1, TR1, TR2 and

the nonligand-binding TR2. In the infundibular nucleus all TR isoforms are present. The staining is mainly cytoplasmic, with some nuclear staining. TR2 staining was the most intense. The presence of TR is probably related to the neuroendocrine feedback of thyroid hormone (Fliers et al., 2001; Figs. 8.39 and 8.40).

2014 Ch 11

252 1 2 3 4 5 6 7 8 9 101 1 2 3 4 5 6 7 8 9 201 1 2 3 4 5 6 7 8 9 301 1 2 3 4 5 6 7 8 9 401 1 2 3 4 5 6 7 8 911

20/11/03

11:30 am

Page 252

D.F. SWAAB

Fig. 11.2. -MSH- (a) and galanin (b)-staining neurons in the infundibular (arcuate) nucleus. (M. Fodor, unpublished results.) Bar = 200 m.

Numerous growth hormone-releasing hormone (GHRH = somatocrinin) and NPY fibers and neurons (Fig. 11.5A) are found in the arcuate nucleus (Pelletier et al., 1984; Pelletier et al., 1986; Ciofi et al., 1988, 1990; Abe et al., 1990; Walter et al., 1990; Goldstone et al., 2003; Fig. 11.1), but the density of NPY fibers by far exceeds that of the GHRH fibers. GHRH neurons are activated during prolonged illness (Goldstone et al., 2003). There is also colocalization of dopamine or GABA in subpopulations of NPY neurons. These neurotransmitters/ neuromodulators are thought to play a role in the LHRH

release and thus in the menstrual cycle (Kalra et al., 1997). NPY-positive neurons are mainly found in the medial and lateral parts of the infundibular nucleus. The NPY fibers are thicker and innervation is much more dense than the GHRH innervation. The NPY fibers in the median eminence are mainly restricted to the internal zone and only scarcely innervate the neurovascular zone (Fig. 11.5A), whereas the GHRH fibers do innervate the latter area. The NPY neurons thus do not seem to have, for the larger part, any neurohormonal projection (Bloch et al., 1986; Ciofi et al., 1988; Dudás et al., 2000), in contrast to observations in great apes, such as chimpanzee, gorilla and orang- utan, that do have NPY fibers that seem to terminate in a spray-like fashion near the portal vasculature (Tigges et al., 1997). NPY is the most active peptide in terms of food intake stimulation tested to date (Chapter 23). NPY concentrations are elevated in genetically obese hyperphagic rats as compared to their lean counterparts in the lateral hypothalamus, dorsomedial nucleus and PVN. In the rat, NPY terminals have also been found in these areas. Injection of NPY in the medial or lateral hypothalamus stimulates food intake robustly (Bernardis and Bellinger, 1996). However, the major focus of NPY effects on feeding behavior is located in the perifornical area (Stanley et al., 1993; Chapter 14). For the family of NPY receptors, see Chapter 23. Fasting increases NPY levels in the PVN and arcuate nucleus. These levels are reduced in tumor-bearing anorexic rats (Balasubramaniam, 1997). Another major stimulatory hypothalamic feeding

Fig. 11.3. NPY and AGRP in the human hypothalamus. Immunocytochemical (ICC) staining for (a) NPY and (b) AGRP in the human hypothalamus from a control male (no. 94-118). Note the overlap in the distribution of cell bodies and fibers staining for NPY and AGRP in the infundibular nucleus (INF) and inner layer of the median eminence (ME). The area outlined by arrows indicates the region of the INF/ME. OT = optic tract, 3V = third ventricle. Bar represents 2 mm. Note that there is no NPY or AGRP ICC staining in the outer layer of the median eminence (oME). (From Goldstone et al., 2002; Fig. 1, with permission.)

2014 Ch 11

20/11/03

11:30 am

Page 253

INFUNDIBULAR NUCLEUS, SUBVENTRICULAR NUCLEUS AND MEDIAN EMINENCE

1 2 3 4 5 6 7 8 9 101 1 2 3 4 5 6 7 8 9 201 1 2 3 4 5 6 7 8 9 301 1 2 3 4 5 6 7 8 9 401 1 2 3 4 5 6 7 8 911

Fig. 11.4. AGRP but not -MSH colocalizes with NPY in human hypothalamic neurons. (A,B) Human infundibular nucleus from control subject (no. 93-025m, 68-year-old male), double-stained for NPY mRNA (black silver grains, in situ hybridization (ISH) emulsion autoradiography) with (A) antisense probe or (B) sense probe, and AGRP peptide (brown immunocytochemistry (ICC) staining), with blue thionine counterstaining. (C, D) Human infundibular nucleus from control subject (no. 94-118, 49-year-old male), double-stained for NPY mRNA (black silver grains, ISH emulsion autoradiography) with antisense probe, and (C) AGRP peptide (brown ICC staining), or (D) -MSH peptide (brown ICC staining), with blue thionine counterstaining. (E,F) Human infundibular nucleus from obese PWS subject (no. 95-104, 51-year-old male), doublestained for NPY mRNA (black silver grains, ISH emulsion autoradiography) with antisense probe, and (E) AGRP peptide (brown ICC staining), or (F) -MSH peptide (brown ICC staining), with blue thionine counterstaining. Note that AGRP peptide- containing cells express NPY mRNA using an antisense probe (A), but that there is no specific ISH signal with the NPY sense probe following AGRP ICC (B). Note that while almost all AGRP peptide-containing cells express NPY mRNA, some neurons stain only for NPY mRNA (black arrow), in both control (C) and PWS (E) subjects. Note that by contrast, NPY mRNA is not colocalized in -MSH neurons in either control (D) or PWS (F) subjects, with cells staining for only NPY mRNA (black arrow) or for only -MSH peptide (open arrow). Bar = 20 m. (Goldstone et al., 2002; Fig. 1, with permission.)

neuropeptide localized in the infundibular nucleus is AGRP, an endogenous -MSH antagonist at the MC-4 receptor, which is colocalized with NPY neurons (Goldstone et al., 2002; Fig. 11.4). These neurons are involved in energy balance and are inhibited by leptin.

253

NPY, AGRP and -MSH fibers densely innervate the TRH neurons in the PVN and may alter the set point of these neurons during fasting (Mihaly et al., 2000). The presence of these peptides and of NPY5 receptors in the infundibular nucleus indicates a possible involvement in feeding behavior (Jacques et al., 1998). We observed a decrease in the activity of NPY and AGRP neurons in Prader–Willi syndrome, consistent with an inhibitory action of elevations in peripheral signals such as plasma leptin and insulin. In addition, we found that the activity of NPY/AGRP neurons increased appreciably with longer disease durations of premorbid illness, possibly induced by decreased levels of leptin (Chapter 1, Fig. 1.12; Goldstone et al., 2002; Chapter 23). In case of respiratory failure and severe dyspnea, increased concentrations of NPY have been reported in the infundibular nucleus (Corder et al., 1990). In patients with a variety of illnesses, including nonthyroidal illness, a negative correlation was found between serum leptin concentrations and NPY mRNA in the infundibular nucleus. In addition, a positive correlation was observed between total TRH mRNA in the PVN and NPY immunoreactivity. This suggests a role for decreased NPY input from the infundibular nucleus in resetting the thyroid hormone feedback on hypothalamic TRH cells in the infundibular nucleus (Fliers et al., 2001). It is interesting to note that a mechanism involving NPY in the fetus is now proposed for the initiation of parturition. The decreased levels of fetal glucose, increased levels of cortisol and changes in leptin are presumed to activate NPY neurons in the fetal infundibular nucleus, activating the fetal hypothalamopituitary adrenal (HPA) axis and thus initiating the cascade that will lead to birth (McMillen et al., 1995; Warnes et al., 1998; Chapter 8.5, see also the Hippocrates quote, 8.5a). Dopamine, produced by neurons of the infundibular nucleus (A12), reaches the pituitary via the hypophysial portal blood (Chapter 17.1c) and inhibits the prolactin release of the pituitary lactotrophs. Dopamine neurons are present already early in development, i.e. at a gestational age of 4–6 weeks (Zecevic and Verney, 1995). Catecholamine fluorescence, based upon the presence of dopamine, is present from the 10th fetal week in the infundibular region and from the 13th week in the median eminence (Hyyppö, 1972). Human fetal hypothalami suppress prolactin release from 16 weeks of pregnancy onwards (McNeilly et al., 1977; Ben-Jonathan and Hnasko, 2001). Prolactin is already detectable at 5–6 weeks of gestation. During gestation

2014 Ch 11

254 1 2 3 4 5 6 7 8 9 101 1 2 3 4 5 6 7 8 9 201 1 2 3 4 5 6 7 8 9 301 1 2 3 4 5 6 7 8 9 401 1 2 3 4 5 6 7 8 911

20/11/03

11:30 am

Page 254

D.F. SWAAB

Fig. 11.5. (A) NPY-immunoreactive neurons and fibers at the level of the infundibular nucleus. Note the high density of fibers in the infundibular nucleus and the fiber- and cell-free zone in the ventral portion of the median eminence. Bar = 400 m. (B) Distribution pattern of somatostatin fibers in the median eminence. The fiber density is much lower in the infundibular nucleus and much denser in the stalk/median eminence region. Bar = 400 m. (NHB 91.201, 25-yearold; preparation F.P.M. Kruijver.)

the levels rise progressively, reaching a peak before birth. An additional rise occurs immediately after birth and persists for several days; it declines during the first postnatal month. During childhood there are no sex differences in serum prolactin levels, but concentrations are twice as high in adult women than in men, due to estrogens. Suckling is the most potent stimulus for prolactin release. Stress conditions also stimulate prolactin release. During sleep, primarily during non-REM periods, the amplitude of the prolactin pulses increases. Drugs that interfere with the dopaminergic transmission affect prolactin homeostasis. Neuroleptics, antihypertensive drugs and

antidepressants stimulate prolactin release, while antiParkinson drugs such as L-DOPA suppress prolactin release. Hyperprolactinemia may be caused by pituitary tumors or processes that interfere with the dopaminergic release into the portal system, such as tumors (Chapter 19), vascular disorders (Chapter 17.2) or pituitary stalk traumata (Chapter 25.4) (Ben-Jonathan and Hnasko, 2001). In addition, prolactin is released after orgasm in men (Krüger et al., 2003). In the stalk/median eminence region, the capillaries of the portal system are present (see Chapter 17.1a). Here, fibers containing, e.g. CRH, LHRH, opiomelanocortins, somatostatin (Fig. 11.5B), GHRH, galanin, TRH, LHRH, substance-P, terminate around the portal capillaries, the detailed distribution and microscopic anatomy of which has not yet been studied. In addition, a rich plexus of delta sleep-inducing peptide was found in the median eminence (Vallet et al., 1990; Najimi et al., 2001b; Dudas and Merchenthaler, 2002a; Goldstone et al., 2003). The central portion of the median eminence is often infiltrated by cell nests of the pars tuberalis of the pituitary (Fig. 11.6). These pars tuberalis cells are already present in the median eminence of a 12-week-old fetus (Thliveris and Curie, 1980). The vasopressin and oxytocin fibers from the SON and PVN also contain the low-affinity neurotrophin receptor P75 in the median eminence, which they pass on their way to the neurohypophysis (Moga and Duong, 1997). The NPY fibers that are derived from neurons in the arcuate nucleus do not innervate the most ventral part of the median eminence, indicating that most NPY fibers do not terminate on portal capillaries but project to other hypothalamic areas (Ciofi et al., 1988; Goldstone et al., 2000; Fig. 11.5a). This in contrast to, e.g. the somatostatin fibers that do not colocalize neurophysin and form numerous endings that stain as caps around the vascular loops of the portal system and as dots around the superficial fibers of the mantel plexus, already in infants (Bugnon et al., 1977). In the median eminence a dense plexus of GHRH fibers is found around the portal capillaries. Growth hormone and insulin-like growth factor I levels are decreased in prolonged critical illness, probably due to diminished GHRH release (Van den Berghe et al., 1997a). GHRH is capable of promoting sleep, although less so in elderly people than in younger subjects (Guldner et al., 1997). Galanin potentiates the GHRH-induced growth hormone release (Todd et al., 2000). In the subpial region, corpora amylacia are frequently present (Cavanagh, 1999; Chapter 2.6)

2014 Ch 11

20/11/03

11:30 am

Page 255

INFUNDIBULAR NUCLEUS, SUBVENTRICULAR NUCLEUS AND MEDIAN EMINENCE

1 2 3 4 5 6 7 8 9 101 1 2 3 4 5 6 7 8 9 201 1 2 3 4 5 6 7 8 9 301 1 2 3 4 5 6 7 8 9 401 1 2 3 4 5 6 7 8 911

255

Fig. 11.6. Some cell nests of the pars tuberalis of the adenohypophysis are integrated in the median eminence (hematoxylin & eosin). Bar = 400 m (NHB 96.235).

(b) Ependyma and internal glia layer The ependymal lining of the infundibular nucleus in adulthood is characterized by parts that are multilayered and folded. This particularity has its basis in early development. In the human fetus, the exhaustion of the matrix layer around the third ventricle begins in the 14-week-old embryo in the anterior and medial hypothalamus. A subependymal zone is present in these areas in 17- to 23-week-old embryos. The one-cell layer ependyme of the third ventricle first appears in the anterior area of the ventromedial nucleus in the 24-week-old embryo, after which the one-cell layer ependyma appears in the regions of the anterior hypothalamus and the remainder of the ventromedial nucleus from 25 to 28 weeks of gestation. In the immediate surroundings of the infundibular nucleus, the exhaustion of the matrix is a continuous process up to the 23rd week. The boundary of the third ventricle at the level of the infundibular nucleus remains to be multilayered, and in all stages of development cells are found that reach into the lumen. Only after birth is the multilayer boundary reduced and at the end of the first year of postnatal life does the one layer ependyma develop at a few sites (Staudt and Stüber, 1977). However, parts of the ependyma of the infundibular nucleus keep their multilayered folded pattern in adulthood (Fig. 11.7). It is this part of the third ventricular wall that is suggested to be a subventricular zone, perhaps capable of adult neurogenesis in the human brain, e.g. by the presence of nestin (Bernier et al., 2000; Gu et al., 2002). However, not all nestin-positive cells are neuroprogenitor cells, and not

Fig. 11.7. Parts of the ependyma of the infundibular nucleus keep a characteristic multilayered folded pattern in adulthood. (Bar represents 100 m.)

all neuroprogenitor cells are nestin-positive (Gu et al., 2002). Beneath the ependymal epithelium an internal glial layer is present (Polak and Azcoaga, 1969). Astrocytes and tarycytes contain ER--positive staining (Donahue et al., 2000). (c) Catecholamines and melanin The infundibular nucleus (or A12 in the nomenclature of Björklund and colleagues) contains catecholaminergic neurons as early as the gestational ages of 4 to 6 weeks (Zecevic and Verney, 1995). These neurons correspond with the tuberoinfundibular dopaminergic

2014 Ch 11

20/11/03

11:30 am

Page 256

256 1 2 3 4 5 6 7 8 9 101 1 2 3 4 5 6 7 8 9 201 1 2 3 4 5 6 7 8 9 301 1 2 3 4 5 6 7 8 9 401 1 2 3 4 5 6 7 8 911

D.F. SWAAB

neurons in the rat, which give rise, in laboratory animals, to catecholaminergic terminals in the median eminence and neurohypophysis. The infundibular nucleus and the internal and external layers of the median eminence are already richly innervated by catecholaminergic fibers in the 3- to 4-month-old human fetus (Hyyppö, 1972; Nobin and Björklund, 1973). In the fourth decade of life, the human infundibular nucleus becomes pigmented by melanin, and the proportion of tyrosine-hydroxylase (TH)positive neurons in the infundibular nucleus that also contain melanin increases with age. Melanin is considered to be a by-product of the synthesis of L-DOPA and thus a postmortem marker for catecholaminergic neurons. However, it should be noted that in the SON and PVN neurons TH is present, but that melanin is not (see Chapter 8.2). In fact, not all catecholaminergic neurons in the infundibular nucleus contain melanin either: only 50–60% of the TH-positive neurons were found to contain melanin as well (Saper and Petito, 1982; Spencer et al., 1985). The infundibular nucleus neurons also contain AADC (Kitahama et al., 1998a), indicating dopamine production, and sepiapterin reductase, an enzyme that catalyzes the final step of the synthesis of tetrahydrobiopterin, the cofactor for phenylalanine hydroxylase, TH, tryptophan hydroxylase and nitric oxide synthase (Ikemoto et al., 2002). In Parkinson’s disease the number of melanincontaining neurons in the arcuate nucleus is not affected (Matzuk and Saper, 1985). However, in spite of this observation, endocrine studies indicate an impairment of the tuberoinfundibular dopaminergic system in Parkinson’s disease (Cusimano et al., 1991). (d) Leptin, the voice of the adipous tissue Leptin is a protein secreted by adipose tissue (see also Chapter 23). It is the product of the ob gene. A significant circadian variation is found with a peak at night and a trough around noon (Zhao et al., 2002). Leptin causes a reduction in body weight, body fat, food intake, serum glucose, and serum insulin, and an increase in metabolic and physical activity when administered into obese (ob/ob) mice with mutations in the leptin gene (Considine et al., 1996; Maffei et al., 1996). In vitro and in vivo experiments in animals have shown that leptin may stimulate luteinizing hormone (LH), follicle-stimulating hormone (FSH), CRH and LHRH, depending on the dosage (Costa et al., 1997b; Yu et al., 1997). Leptin seems to act in a permissive fashion as a metabolic gate to allow

pubertal maturation to proceed – if and when metabolic resources are deemed adequate (Cheung et al., 1997). Body fat is linked with the initiation of increased activity of the hypothalamic-pituitary-gonadal axis at puberty, probably by the effect of leptin on LHRH neurons. Indeed, a homozygous mutation of the human leptin receptor gene that results in a truncated receptor, an early-onset morbid obesity, and a lack of pubertal development was observed. In addition, the secretion of growth hormone and TSH was reduced (Clément et al., 1998). Since leptin inhibits NPY-containing neurons in the arcuate nucleus in ob/ob mice (Schwartz et al., 1996a), the leptin receptor was presumed to be located in this nucleus, also in humans (Couce et al., 1997). However, the long form of the leptin receptor, which has a long intracellular domain essential for intracellular signal transduction, is widely expressed in the human brain. The messenger for this receptor was found not only in the hypothalamus (infundibular, supraoptic, paraventricular, suprachiasmatic and mamillary nucleus), but also in Purkinje cells, the dentate nucleus of the cerebellum, the inferior olive and cranial nerves nuclei in the medulla, the amygdala, and in neurons from both neocortex and entorhinal cortex. The hybridization signal in the ependyma was lower than in neurons. No specific hybridization signal was detected in glial cells (Counce et al., 1997; Burguera et al., 2000). The original hypothesis that this receptor would only be present in the hypothalamus thus needs to be reconsidered. Obese human individuals have fourfold higher leptin levels than lean individuals. However, no difference was found between lean and obese individuals as far as the amount of hypothalamic leptin receptor mRNA was concerned. Moreover, mutations in leptin or in the leptin receptor in the human hypothalamus do not constitute a common cause of obesity in human subjects (Considine et al., 1996; Maffei et al., 1996; Gotoda et al., 1997; Matsuoka et al., 1997). These observations indicate that a lesion of the leptin system is generally not the cause of obesity in human beings. However, one family of Pakistani descent has been described with severe early onset obesity that is attributable to mutation in the gene encoding for leptin, and as such an equivalent of the ob/ob mouse with whom they also share hyperphagia, obesity and hyperinsulinaemia. The pedigree of this family was highly consanguineous. Their serum levels were low in spite of their markedly elevated body mass. A homozygous frameshift mutation in codon 133 at the gene for leptin was found in the two affected prepubertal subjects. The

2014 Ch 11

20/11/03

11:30 am

Page 257

INFUNDIBULAR NUCLEUS, SUBVENTRICULAR NUCLEUS AND MEDIAN EMINENCE

1 2 3 4 5 6 7 8 9 101 1 2 3 4 5 6 7 8 9 201 1 2 3 4 5 6 7 8 9 301 1 2 3 4 5 6 7 8 9 401 1 2 3 4 5 6 7 8 911

heterozygous family members were not obese (Montague et al., 1997). In addition, a homozygous leptin missense mutation was observed in a Turkish consanguineous pedigree, which caused not only morbid obesity, but also hypogonadism. The individuals never reached puberty (Strobel et al., 1998). Leptin levels in CSF are strongly correlated to those in plasma in a nonlinear manner. The presence of a saturable mechanism mediating transport of leptin to CSF with reduced efficiency for higher plasma leptin levels has been proposed (Schwartz et al., 1996b). A defect in leptin transport over the blood–brain barrier has been hypothesized as a possible mechanism for the leptin resistence in obese human beings, but this has yet to be demonstrated. Apart from its role as a central satiety agent, leptin is active in the periphery. It modifies insulin sensitivity, tissue metabolism, stress responses and reproductive function, often mediated via hypothalamopituitary axes (Harris, 2000). (e) Origin and localization of LHRH neurons LHRH (also known as GnRH) is a decapeptide that is essential for mammalian reproduction. LHRH is released as a neurohormone from the median eminence into the hypophysial portal system. LHRH cell bodies are concentrated in the preoptic area and basal hypothalamus but are also evident in the septal region, anterior olfactory area and medial amygdaloid nuclei. LHRH-containing fibers are observed in the infundibular region and preoptic area, septum, stria terminalis, ventral pallidum dorsomedial thalamus, olfactory stria and anterior olfactory area (Stopa et al., 1991). Gonadotropic hormone-releasing hormone-associated peptide (GAP)-containing neuron, a 56-amino acid residence from the precursor of LHRH, is found in the human fetal hypothalamus from the 9th week of fetal life. In the adult hypothalamus, GAP neurons coexpressing LHRH are found in the infundibular nucleus, medial preoptic area and paraventricular nucleus (Abe et al., 1990). Observations in Kallmann’s syndrome (i.e. inherited hypogonadotropic hypogonadism) have shown that LHRH neurons in this syndrome fail to migrate from the olfactory placode into the developing brain (Schwanzel-Fukuda et al., 1989; Chapters 24.2, 24.3). The LHRH neurons originate in the epithelium of the fetal medial olfactory pit and normally migrate from the nose into the hypothalamus along terminal nerve fibers rich in neural cell adhesion molecule (N-CAM). At this time these cells

257

frequently have a neuroblastic appearance (Bloch et al., 1992). In the human embryo, LHRH immunoreactivity was detected in the epithelium of the medial olfactory pit and in cells associated with the terminal-vomeronasal nerves at 42 (but not 28–32) days of gestation (SchwanzelFukuda et al., 1996). The first LHRH producing neurons in the human fetal hypothalamus were seen at 9 weeks of gestation (Paulin et al., 1977). From experiments in rodents it was concluded that migration from the olfactory placode would only concern the LHRH neurons in the preoptic and septal regions, whereas the LHRH neurons in the ventral hypothalamus did not seem to come from the nasal placode (Northcutt and Muske, 1994). However, a study of embryonal and fetal rhesus monkey brains showed that two LHRH cell types migrated out of the olfactory placode, one several days earlier than the other. The “late” LHRH neurons reached into the preoptic area and basal hypothalamus in a distribution resembling the one described for mouse and rat. In addition, “early” migrating LHRH neurons were found that went to the septum, preoptic region, stria terminalis, medial amygdala, claustrum, internal capsule and globus pallidus. These LHRH neurons may modulate nonreproductive functions (Quanbeck et al., 1997). In postmenopausal women, the LHRH neurons contain more LHRH mRNA only in the heavily labeled oval to fusiform neurons of the infundibular nucleus, while no difference with premenopausal women was found for gene expression in the sparsely labeled round to oval neurons in the dorsal preoptic-septal region. There was also a significant postmortem degradation of LHRH-mRNA in the neurons of the mediobasal hypothalamus, but not in those of the dorsal preoptic-septal regions. These differential responses of the two types of LHRH neurons provide additional evidence that this concerns two distinct functional subgroups. There is evidence from animal experiments that LHRH may function as a neurotransmitter or neuromodulator in the central regulation of sexual behavior (Rance et al., 1994; Rance and Uswandi, 1996). (f) Subventricular nucleus and postmenopause Now Abraham and Sarah were old, and well stricken in age: It had ceased to be with Sarah after the manner of women. Genesis 18:11

In 1966, Sheehan and Kovacs described neuronal hypertrophy in a subdivision of the infundibular nucleus in postmenopausal women and younger women suffering

2014 Ch 11

258 1 2 3 4 5 6 7 8 9 101 1 2 3 4 5 6 7 8 9 201 1 2 3 4 5 6 7 8 9 301 1 2 3 4 5 6 7 8 9 401 1 2 3 4 5 6 7 8 911

20/11/03

11:30 am

Page 258

D.F. SWAAB

from postpartum hypopituitarism due to pituitary necrosis. This subdivision was named the subventricular nucleus, after its location beneath the floor of the third ventricle, extending from just behind the infundibulum to a short distance in front of the posteromedial tuberal nucleus (see Chapter 13). In women up to the age of 45 and in men of any age, the subventricular nucleus is morphologically only a “potential nucleus”. It consists of small nerve cells with little cytoplasm and no obvious Nissl substance. There are a few scattered, medium-sized nerve cells and only very occasional large ones. However, in certain physiological and pathological states, about half of the small nerve cells hypertrophy into medium-sized neurons or even attain the size of the large tuberomamillary neurons. Their nuclei and cytoplasm increase in size, and clear peripheral Nissl substance is seen (Fig. 11.8). As a result the nucleus becomes recognizable on low-power examination (Rance, 1992). Conventional stainings show that clear hypertrophy of the nucleus occurs in one-third of menopausal women. One-third has medium hypertrophy and one-third has no hypertrophy at all. The subventricular nucleus may hypertrophy with age or starvation, following hypophysectomy, and during the last trimester of pregnancy, but gradually reverts to normal after delivery (Sheehan and Kovacs, 1982). In postmenopausal women the neuronal hypertrophy is considered to be due to the diminished inhibitory action of estrogens. Neuronal hypertrophy has been described in the entire infundibular nucleus of postmenopausal women. Interestingly, the total number of neurons (unilaterally around the 500,000) in the infundibular nucleus of pre- and postmenopausal women did not differ, whereas the mean neuronal volume increased by 40%. The loss of menstrual cyclicity can, consequently, not be explained by degeneration of neurons in the infundibular nucleus as they, in fact, show an adaptive activation due to the loss of the estrogen feedback in menopause (Abel and Rance, 2000). Neuronal hypertrophy has also been described in chronically ill hypogonadal men, in patients with anorexia nervosa (Chapter 23.2) and other causes of gonadal atrophy (Ule and Walter, 1983; Rance, 1992; Rance et al., 1993). Although testosterone levels decline with age, there is great individual variability. Testosterone decline is not a state that is strictly analogous with the strong and sudden decrease in estrogen levels in female menopause (Sternback, 1998). Increase in nucleolar size and multiplication of nucleoli confirm the activation of neurons in post-

Fig. 11.8. Representative photomicrographs of cresyl violet-stained sections of the infundibular nucleus of pre-menopausal (A) and postmenopausal (B) women. The hypertrophied neurons in B are distinguished not only by increased soma size, but also by larger nuclei, nucleoli and increased Nissl substance. Bar, 20 m for both photomicrographs. (From Rance 1992, Fig. 1, with permission.)

menopausal women in this area (Fig. 11.8; Ule et al., 1983; Rance, 1992). In addition, spheroids are frequently present in the nucleus of infundibular nucleus neurons of postmenopausal women. They consist of cytoplasmic protrusions into the nucleus of the activated neurons. These “nucleare spheroids” have already been described by Ortner and Schiebler in 1951 in a patient with parahypopituitarism. The mean cross-sectional area of infundibular neurons in postmenopausal women was 30% greater than in premenopausal women (Rance et al., 1990) which is also a sign of activation. The mean cross-sectional area of the neurons that contain increased amounts of neurokinin-B (NKB), substance-P and estrogen receptor transcripts is even twice as large in postmenopausal women as in premenopausal women. These neurons do not contain LHRH mRNA (Rance et al., 1990; Rance and Scott Young, 1991; Rance, 1992). Immunocytochemically, estrogen receptor- appeared to be located in interneurons rather than in the LHRH neurons (Donohue et al., 2000). However, the LHRH neurons themselves are also activated in postmenopausal women (Rance and Uswandi, 1996; see earlier). Moreover, in one case, that of a young ovariectomized woman, hypertrophy of neurons containing estrogen receptor transcripts was observed that was indistinguishable from the hypertrophy exhibited by postmenopausal women (Rance et al., 1990). At first glance it seems surprising that the LHRH content was reported to be decreased in the hypothalamus of postmenopausal women (Parker and Porter, 1984),

2014 Ch 11

20/11/03

11:30 am

Page 259

INFUNDIBULAR NUCLEUS, SUBVENTRICULAR NUCLEUS AND MEDIAN EMINENCE

1 2 3 4 5 6 7 8 9 101 1 2 3 4 5 6 7 8 9 201 1 2 3 4 5 6 7 8 9 301 1 2 3 4 5 6 7 8 9 401 1 2 3 4 5 6 7 8 911

259

Fig. 11.9. Computer-assisted maps showing the distribution of POMC neurons in representative sections of premenopausal (left) and postmenopausal (right) women. The open arrows mark the approximate position of the tuberoinfundibular sulcus. The insert into the figure on the left is a projection drawing of a human brainstem and cerebellum that has been sectioned in the mid-sagittal plane. The boxed area indicates the location of the hypothalamic sections. Note the decrease in labeled neurons in the postmenopausal infundibular nucleus. ac, anterior commissure; fx, fornix; INF, infundibular nucleus; MB, mamillary body; mt, mamilleothalamic tract; POA, preoptic area; oc, optic chiasm; RCA, retrochiasmatic area; VMN, ventromedial nucleus. (From Abel and Rance, 1999; Fig. 1, with permission.)

but these peptide measurements do, of course, not distinguish between decreased peptide synthesis or increased release or degradation. The observation that LHRH mRNA expression increases in the infundibular nucleus in postmenopausal women confirms the activation of these peptidergic cells secondary to ovarian failure (Rance and Uswandi, 1996). In older men only a moderate hypertrophy of the infundibular neurons was observed, probably because the drop in estrogen levels in postmenopausal women is much stronger than the decrease in testosterone levels in older men (Rance et al., 1993). The NKBcontaining neurons are proposed to participate in the hypothalamic circuitry, which regulates estrogen-negative feedback on gonadotropin release in human by acting as an interneuron on the cells containing LHRH. Climacteric hot flushes, the episodes of sudden vasodilatation in the facial and upper abdominal skin, are accompanied by increased levels of LHRH and LH (Casper et al., 1979; Ravnikar et al., 1984; Rebar and Spitzer, 1987). Men with testicular insufficiency and women after hypophysectomy also experience hot flushes (Meldrum et al., 1981; Rebar and Spitzer, 1987). The

NKB neurons are presumed to be involved in the initiation of menopausal flushes (Rance, 1992). However, recent animal experiments in the rat have shown that LHRH itself can elicit thermoregulatory skin vasomotion when injected into the septal area (Hosono et al., 1997) and that premenopausal women who received a long-acting LHRH agonist because of endometriosis experienced hot flushes (De-Fazio et al., 1983). The thermoregulatory vasodilatative effect on septal LHRH-receptor seems to point to the preoptic-septal LHRH neurons as better candidates to be related to the etiology of climacteric hot flushes. However, these neurons did not show increased LHRH mRNA in postmenopausal women (Rance et al., 1994; Rance and Uswandi, 1996). It would, therefore, be of interest to know whether arcuate nucleus LHRH neurons might project to the septum or preoptic area. Propiomelanocortin (POMC) peptides, particularly -endorphin, are responsible for an inhibitory “opioid” tone on the secretion of LHRH. Due to age and to the decrease in estrogens, POMC-mRNA decreases in postmenopausal women. This may contribute to both the activation of LHRH neurons and to the menopausal

2014 Ch 11

20/11/03

11:30 am

Page 260

260 1 2 3 4 5 6 7 8 9 101 1 2 3 4 5 6 7 8 9 201 1 2 3 4 5 6 7 8 9 301 1 2 3 4 5 6 7 8 9 401 1 2 3 4 5 6 7 8 911

D.F. SWAAB

Fig. 11.10. Representative photomicrographs of neurons labeled with the POMC probe and counterstained with toluidine blue in the retrochiasmatic area (A) and infundibular nucleus (B). Note the large size, elongated shape and parallel orientation of retrochiasmatic neurons relative to those in the infundibular nucleus. Scale = 20 m. (From Abel and Rance, 1999; Fig. 2, with permission.)

flushes that may thus represent a limited endogenous opioid-withdrawal syndrome (Abel and Rance, 1999; Figs. 11.9 and 11.10). Although, on the basis of the presence of LHRH and estrogen receptor-containing neurons and their changes in postmenopausal women, one would presume a crucial role for the subventricular nucleus in the biological rhythm of the menstrual cycle, such a role has so far not been established. (g) Neurodegeneration and psychiatric disorders In Down’s syndrome a strong decrease in neuronal density and gliosis was observed in the infundibular nucleus (Wisniewski and Bobinski, 1991). The authors relate the reduction in cell number in the infundibular nucleus and VMH to the decreased growth hormone levels in this syndrome (see Chapter 26.5), a possibility that has to be confirmed by determining the total number of GHRH neurons in the infundibular nucleus of Down’s syndrome patients. In autistic patients, swollen axonal terminals (spheroids) were found (Weidenheim et al., 2001). Neurofibrillary Alzheimer pathology was found in the infundibular nucleus and the adjacent median eminence in up to 90% of the males over the age of 60 and in only 8–10% of the females. This pathology occurred in controls, in the absence of neocortical Alzheimer changes. A dense network of large dystrophic neurites with neurofibrillary tangles interspersed among them was identified in the medial basal hypothalamus and the infundibular

Fig. 11.11. Mediobasal hypothalamus, including the infundibular (or arcuate) nucleus, of a 66-year-old male with advanced cytoskeletal pathology stained by Alz-50. Such pathology is rarely present in postmenopausal women, as was first reported by Schultz et al. (1996). In postmenopausal women this area is strongly activated as a result of ovarian failure (Rance, 1992). These observations suggest that the hyperactivity in the mediobasal hypothalamus in postmenopausal women may reduce the risk of developing Alzheimer changes in that area. The bar indicates 0.5 mm.

nucleus. The dystrophic neurites contact small vessels in the mediobasal hypothalamus and form a perivascular plexus of bouton-like structures (Schultz et al., 1996, 1999; Fig. 11.11). We have proposed that the sex difference in the occurrence of Alzheimer changes in the infundibular nucleus may be explained by the activation of NKB and LHRH neurons in this nucleus of postmenopausal women. Hyperactivity of neurons might

2014 Ch 11

20/11/03

11:30 am

Page 261

INFUNDIBULAR NUCLEUS, SUBVENTRICULAR NUCLEUS AND MEDIAN EMINENCE

1 2 3 4 5 6 7 8 9 101 1 2 3 4 5 6 7 8 9 201 1 2 3 4 5 6 7 8 9 301 1 2 3 4 5 6 7 8 9 401 1 2 3 4 5 6 7 8 911

protect them against the neurodegenerative changes observed in Alzheimer’s disease, a principle that was paraphrased as “use it or lose it” (Swaab, 1991; Swaab et al., 1998).

261

The number of -endorphin containing neurons in the infundibular nucleus and their fibers innervating the paraventricular nucleus is reduced in schizophrenia and depression (Bernstein et al., 2002).