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HYPOTHALAMIC NEUROANATOMY AND LIMBIC INPUTS
The hypothalamus is part of the diencephalon. It lies rostral to the midbrain and caudal to the forebrain. The
hypothalamus is bounded dorsally by the thalamus, posteriorly by the mammillary bodies, and anteriorly by the lamina
terminalis and optic chiasm, and the third ventricle splits the hypothalamus bilaterally (Figs. 1-1 and 1-2). The
hypothalamus receives rich input from the autonomic areas and reticular nuclei of the brain stem, particularly the
catecholaminergic cell groups (many of which have neuropeptides as cotransmitters, such as galanin and NPY). The
hypothalamus also receives dense innervation from the limbic areas of the forebrain, including the hippocampus,
amygdala, septum, and orbitofrontal cortex.
Figure 1-1 Saggital section of the human brain, including the pituitary and pineal glands.
(Adapted from Johnson MH, Everitt BJ. Essential Reproduction, ed 5. Blackwell Science, 2000, Fig. 6.1.)
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Figure 1-2 Schematic, three-dimensional view of the human hypothalamus, pituitary, and portal capillary system showing the approximate
locations of the major nuclei. GnRH, gonadotropin-releasing hormone.
(Adapted from Johnson MH, Everitt BJ. Essential Reproduction. Oxford, Blackwell Science, 2000, Fig. 6.2.)
The hypothalamus serves as the primary site for the integration and regulation of many important physiologic
processes. These include homeostatic control of temperature, metabolism, and body weight, aspects of
cardiovascular function, physiologic adaptation to stress, regulation of growth, reproduction (including sexual behavior),
and lactation. Although the regulation of these complex processes depends on the circuitry of the hypothalamus (and
its afferent inputs), the control of these systems cannot be defined on the basis of strict anatomic criteria.
The hypothalamus comprises distinct nuclei (collections of cell bodies), including the supraoptic, paraventricular,
suprachiasmatic, ventro- and dorsomedial, and arcuate nuclei. The suprachiasmatic nucleus (SCN; Fig. 1-3) is the site
of the brain's circadian “clock.” Cells in the SCN receive input from the retinohypothalamic pathway, through which the
brain keeps track of the diurnal rhythm of light and dark and controls rhythmic cycles of activity and hormone secretion
(e.g., sleep–wake and CRH/ACTH/cortisol rhythms). Subgroups of neurons in the SCN that express VIP and arginine
vasopressin project to different parts of the hypothalamus to coordinate diverse physiologic functions, including activity
rhythms and the preovulatory GnRH/LH surge (at least in rodent and ruminant species). Neurons in the SCN express
genes that display endogenous rhythmicity, approximating a 24-hour period—hence, the term circadian, with
components circa (“about”) and dian (“day”). The clock and per genes, among many others expressed by cells in the
SCN, generate pacemaker activity, which may be entrained by external cues (e.g., light or activity) to help the body
keep track of time. Cells in the SCN express estrogen receptor α (ERα) and are believed to be involved in the
neuroendocrine regulation of gonadotropin secretion, at least in rodent and ruminant species, where the evidence is
most compelling.[4-6]
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Figure 1-3 Rostral (A), mid (B), and caudal (C) coronal sections of the human hypothalamus. AHA, anterior hypothalamic area; LAT HYP,
lateral hypothalamus; POA, preoptic area; VMN, ventromedial nucleus.
(Adapted from Johnson MH, Everitt BJ. Essential Reproduction, ed 5. Blackwell Science, 2000, Fig. 6.3.)
The arcuate nucleus (ARC; see Figs. 1-2 to 1-5) is the nodal point for the regulation of many complex physiologic
functions.[7-9] The ARC comprises many phenotypically distinct groups of neurons—including cells that produce
pro-opiomelanocortin (POMC) and its derivatives (e.g., β-endorphin and the melanocortin α-melanocyte–stimulating
hormone, α-MSH), NPY, GHRH, kisspeptin, GALP, and dopamine. Most, if not all, neurons in the ARC express two or
more neuropeptides. For example, NPY-expressing cells also express agouti-related peptide (AgRP). POMC neurons
coexpress cocaine- and amphetamine-regulated transcript (CART). GHRH neurons coexpress galanin, and kisspeptin
neurons coexpress both dynorphin and neurokinin B. Dopamine-containing neurons are concentrated in the
tuberoinfundibular track within the ARC, and these cells play a critical role in the neuroendocrine regulation of prolactin
secretion. Certain neurons whose cell bodies reside in the ARC project to other areas within the hypothalamus,
including the preoptic area and paraventricular nucleus (e.g., NPY neurons project to the paraventricular nucleus).
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Figure 1-4 Schematic illustration of the neurosecretory systems in the human that regulate reproduction. A, The locations of oxytocin cell
bodies, which reside in the hypothalamus, and their fibers, which project to the neurohypophysis. B, The primary locations of gonadotropinreleasing hormone (GnRH) cell bodies in the human hypothalamus and their axons, which terminate near portal capillaries in the median
eminence.
(Adapted from Johnson MH, Everitt BJ. Essential Reproduction, ed 5. Blackwell Science, 2000, Fig. 6.4.)
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Figure 1-5 Diagram of some of the neurotransmitter systems that are believed to play a role in regulating gonadotropin-releasing hormone
(GnRH) secretion. GABA, γ-aminobutyric acid.
(Adapted from Johnson MH, Everitt BJ. Essential Reproduction, ed 5. Blackwell Science, 2000, Fig. 6.18.)
Together, the lateral hypothalamus, dorsomedial nucleus (DMN), ventromedial nucleus (VMN), and parvocellular region
of the paraventricular nucleus (PVN) exert regulatory control over feeding, body weight, and activity rhythms.[10] In
experimental animals (e.g., rats and cats) lesions of the VMN stimulate appetite and cause obesity, whereas
stimulation of the VMN reduces feeding and body weight subsequently declines as a result. The VMN may also play a
role in sexual behavior, particularly in females. The lateral hypothalamus comprises other unique cell groups, including
neurons that produce orexins (also known as hypocretins), which have profound effects on sleep–wake cycles,
feeding, and reward-seeking behavior, and can influence GnRH secretion. Neurons in the parvocellular region of the
PVN produce TRH and CRH, which regulate the hypothalamic–pituitary–thyroid and hypothalamic–pituitary–adrenal
axes, respectively, but both of these neuropeptides also play a critical role in the control of feeding and metabolism.
CRH has been implicated in the stress-induced inhibition of GnRH secretion—perhaps through its interaction with
β-endorphin–producing neurons in the ARC.
Just rostral to the formal boundaries of the hypothalamus lies the medial preoptic area, which contains many GnRH
neurons that project (along with GnRH neurons in the ARC) to the median eminence. (In the primate, GnRH neurons
are widely dispersed in the anterior hypothalamus, medial preoptic area, and ARC, whereas in rodent species, GnRH
cell bodies are restricted to the rostral hypothalamus and medial preoptic area [Figs. 1-4B and 1-5]).
The hypothalamus receives input from many regions of the brain. Ascending noradrenergic projections arise from the
medulla and pons and innervate many nuclear groups within the hypothalamus, including the medial preoptic area and
ARC (see Fig. 1-5). Serotonin projections originate in the midbrain raphe and provide dense innervation of the
hypothalamus, particularly the mammillary complex, periventricular nucleus, ARC, and SCN. The hypothalamus also
receives descending input from several sources. These include projections from the basal forebrain, olfactory tubercle,
piriform cortex, amygdala, and hippocampus.
Hypophysiotropic neurons whose cell bodies reside within the hypothalamus send their projections to the median
eminence, where their secretory products enter the portal vasculature and thus regulate anterior pituitary function. In
the context of reproduction, GnRH neurons, with cell bodies in the preoptic area and ARC, send projections into the
external zone of the median eminence. From nerve terminals in the median eminence, GnRH is secreted into the
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fenestrated capillaries and transported to the anterior pituitary (see Figs. 1-4B and 1-5). Kisspeptin neurons (whose
cell bodies are in the ARC and rostral hypothalamus) interact with GnRH neurons by projecting directly to GnRH cell
bodies, but may also send axoaxonal projections into the zona internal of the median eminence and thereby influence
GnRH secretion by several different mechanisms (see Fig. 1-5). Magnocellular neurons in the PVN and supraoptic
nucleus (SON) send long axons into the neurohypophysis, where they release vasopressin and oxytocin into the
vasculature (see Fig. 1-4A).
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