C.  Appetite and Satiety

    The demonstration that discrete regions of the hypothalamus control food intake were based on the early studies of Hetherington and Ranson (279) in 1940 showing that localized lesions of the hypothalamus result in obesity.  These observations were seemingly confirmed in man when Reeves and Plum (280) reported that a discrete lesion (hypothalamic hamartoma) involving the hypothalamic ventromedial nucleus in a 28 year old woman was associated with increased food consumption and profound obesity.  In contrast, bilateral lesions of the lateral hypothalamus in animals result in anorexia and weight loss (279). Thus, the concept of a hypothalamic ventromedial nucleus satiety center and lateral hypothalamic orexigenic center that can be influenced by peripheral signals was developed, and dominated thinking about the hypothalamic control of feeding for several decades.
It was not until 1994, however, when the discovery of leptin revolutionized thinking on the mechanisms governing appetite and satiety (281).  Leptin serves as an important humeral signal that reflects body fat stores, and by acting on discrete regions in the hypothalamus, orchestrates the behavioral, metabolic, and neuroendocrine adaptations to nutrient availability (224-225, 282-284).  Thus, during nutrient abundance, leptin secretion is increased, leading to decreased appetite and increased caloric disposal, whereas nutrient insufficiency leads to decreased leptin secretion, resulting in increased appetite, energy conservation, and a shift to a neuroendocrine profile that facilitates metabolic adaptation.
The site of leptin’s action is the mediobasal hypothalamus, but primarily the hypothalamic arcuate nucleus via specific receptors (Ob-Rb) that influence the activities of two separate groups of neurons with opposing functions through the phosphatidylinositol 3-kinase (PI3K) signaling pathway and by phosphorylating signal transducer and activator of transcription 3 (STAT3) (285,286).  These neurons include -MSH-producing neurons that co-express CART, and AGRP neurons that co-express NPY (225).  These neurons send monosynaptic projections to identical target regions within discrete regions of the hypothalamus where the signals are integrated and then relayed by independent pathways to regions of the brain governing feeding behavior, energy expenditure, and hypophysiotropic function (225,282-284).  When circulating leptin levels are suppressed, such as during fasting, expression of genes encoding proteins that promote weight loss, and energy expenditure, -MSH and CART, are reduced simultaneously with a marked increase in the genes encoding proteins that promote weight gain and reduce energy expenditure, AGRP and NPY.  Cooperation between the opposing components of the regulatory system governing appetite and satiety is underscored by the biological action of AGRP as both a competitive antagonist and inverse agonist at melanocortin receptors (MC3r and MC4r) (224,287).  Thus, during fasting, the rise in AGRP cooperates in down regulation of melanocortin signaling by antagonizing the action of -MSH concurrently with inhibition of the POMC gene.  Reciprocal connections between the arcuate nucleus NPY/AGRP neurons and -MSH/CART neurons also are present (288), suggesting an even greater complexity to this regulatory system.
It is becoming increasingly recognized that the melanocortin signaling system may be the predominant regulatory system governing appetite and satiety.  Whereas animals with targeted deficiency of NPY have an essentially normal phenotype and intact responses to fasting (289), animal models with targeted deletion of the type 4 melanocortin receptor (MC4r) and in humans bearing mutations that interfere with the function of the MC4r, the POMC gene, or the processing enzymes necessary to generate a fully mature -MSH, develop a severe obesity syndrome (290-293).  Loss of tone in the melanocortin signaling system as a result of senescence of the arcuate nucleus POMC neurons may also explain the tendency for weight gain with aging (294,295).  Conversely, recent studies by Wisse et al (296) and Marks et al (297) have demonstrated that cancer cachexia can be prevented in experimental animals by the administration of melanocortin receptor antagonists.  Maintaining adequate tone in the melanocortin signaling system, therefore, would appear to have an especially important role in the maintenance of normal body weight.
The arcuate nucleus is the main sensor of circulating levels of leptin, and by projecting to several different regions in the brain, provide the mechanism whereby leptin is capable of integrating a host of responses involved in energy homeostasis.  The arcuate-PVN projection pathway has an important role in regulation of the thyroid axis (see above).  Thus, hypophysiotropic TRH neurons in the medial parvocellular PVN receive direct projections from NPY/AGRP- and -MSH/CART-producing neurons in the arcuate nucleus, altering the set point at which circulating thyroid hormone inhibits TRH (144217-219).  The end result during fasting is suppression of the HPT axis as a way to reduce energy expenditure.  Other targets include neurons in the anterior and ventral parvocellular subdivisions of the PVN on the basis that these neurons show phosphorylation of CREB following the central administration of -MSH (298).  Both subdivisions receive a high density of axons containing -MSH and AGRP derived from the arcuate nucleus (299,300).  Thus, these regions may be involved in some of the other actions of leptin including the regulation of feeding and energy disposal.   This concept is supported by the observation that focal injections of -MSH or -MSH agonists directly into the PVN reduces feeding and can fully replicate the reduced feeding responses following icv administration (301).  Conversely, -MSH antagonists injected into the PVN have a potent effect to increase feeding (302).  Since anterior parvocellular PVN neurons project to the limbic system (lateral septum and amygdala) (303,304), it is possible that this part of the PVN is involved in the behavioral manifestations of feeding.  The ventral parvocellular subdivision is involved in the regulation of the autonomic nervous system through descending projections to brainstem and spinal cord targets (137,304,305).  This region, therefore, may be involved in the regulation of energy disposal by controlling heat loss from brown adipose tissue through effects on uncoupling protein-1 (UCP-1) (306) and by affecting lipolysis and proteolysis in white fat and muscle, respectively (307).
In addition to the PVN, leptin-responsive neurons in the arcuate nucleus synapse on two separate populations of neurons in the lateral hypothalamus that produce melanin-concentrating hormone (MCH) and orexin (308).  These neurons project to multiple regions of the brain including the cerebral cortex, midbrain and pons (309).  MCH acts as an endogenous stimulator of food intake and its mRNA is increased during fasting (310,311) whereas orexin promotes arousal responses (312) that would have an essential role in permitting food seeking behavior during periods of nutrient deficiency.  Leptin-responsive, CART-producing neurons in the arcuate nucleus also project directly to the intermediolateral cell column of the spinal cord (313), indicating their importance in autonomic control.
Leptin receptors are also expressed by other hypothalamic nuclear groups including the caudal part of the hypothalamic dorsomedial nucleus (DMN) and the dorsomedial division of the ventromedial nucleus (VMN) (314).  The DMN has extensive projections to the PVN, particularly portions involved in autonomic control, as well as direct brainstem projections to the dorsomotor complex of the vagus (315).  In addition, the DMN receives extensive projections from the arcuate nucleus, including projections from NPY/AGRP- and -MSH/CART-leptin-responsive neurons (221,222).  As lesions of the DMN produce hypophagia and reduce linear growth, this nucleus has an important role in the homeostatic control of feeding behavior (316).  The DMN may also mediate the anorexic effect of cholecystokinin-8 (CCK-8) (317), through projections from CCK-8-producing neurons in the superior lateral subdivision of the parabrachial nucleus in the pons (318).  The VMN has been long implicated in the regulation of feeding behavior as lesions of the VMN produce hyperphagia (319).  However, these observations were likely due to transection of surrounding fiber pathways.  Because the dorsomedial portion of the VMN projects to the subparaventricular zone, it has been proposed that leptin-sensitive VMN neurons may have a role in circadian rhythms (225).
The brainstem also contributes to leptin-regulated neuroendocrine responses involved in feeding, particularly the dorsal vagal complex (DVC) (320).  The DVC is an important relay center in the brainstem for visceral sensory information carried by the vagus nerve from the liver GI tract, but also receives descending information from the forebrain including the PVN (305).  Glucagon-like peptide (GLP-1)-producing neurons in the DVC have been shown to be leptin responsive (321), and this peptide has potent anorexic effects (322).  NPY-producing neurons in the hypothalamic arcuate nucleus have been proposed as the primary target for GLP-1 due to the high concentration of GLP-1 receptors in this region and because GLP-1 can inhibit NPY-induced feeding (323,324).  However, the arcuate nucleus contains relatively few GLP-1-containing nerve terminals as compared to the PVN and DMN (325), and discrete injections of GLP-1 into the PVN are capable of suppressing feeding (326).
In addition to leptin, a number of gut peptides contribute to the central regulation of appetite and satiety (Table 10), either by acting directly on the hypothalamic arcuate nucleus transmitted through the NTS (327).  Among these are insulin, cholecystokinin (CCK), peptide YY (3-36), pancreatic polypeptide (PP), GLP-1, oxyntomodulin and ghrelin (328).  Like leptin, levels of insulin vary with adiposity (329) and are suppressed by fasting and increased by eating.  In addition, the intracerebroventricular administration of insulin reduces food intake and body weight (330) and prevents fasting-induced increases of NPY and AGRP mRNA in arcuate nucleus neurons (331).  Both CCK and pYY are secreted by the gut in response to feeding and may have a role in the termination of eating, CCK through effects on receptors in visceral sensory axons that travel in the vagus nerve (332) and pYY through blood-borne inhibitory signals exerted through Y2 receptors on NPY/AGRP neurons in the hypothalamic arcuate nucleus (333).  In addition to the central GLP-1 system described above, GLP-1 is also released into the circulation after a meal from the distal small intestine and colon, decreasing food intake through a vagal mechanism (334).  Oxyntomodulin, which like GLP-1 derives from the same intestinal cells by posttranslational processing of proglucagon, binds to the GLP-1 receptor and has similar actions on reducing food intake that are abolished in the GLP-1 receptor knockout mouse (335).  However, this peptide also appears to increase energy expenditure, raising the possibility of its particular utility in the treatment of obesity (336).  As its name suggests, PP derives from the pancreas and inhibits food intake by binding to Y4 receptors in the DVC and arcuate nucleus and through effects mediated by the vagus nerve (337).  Ghrelin, produced primarily by the stomach, is secreted directly into the bloodstream, but as opposed to all other identified gut-derived peptides, ghrelin stimulates food intake and is highest just prior to a meal and falls after eating (338).  Thus, ghrelin may have a primary role in meal initiation.  Its target is also NPY/AGRP neurons in the hypothalamic arcuate nucleus, increasing the expression of their mRNAs (339).  However, vagotomy abolishes ghrelin-stimulated eating (340), indicating that ghrelin may also signal through the brainstem.  Hypersecretion of ghrelin has been proposed as a mechanism for the morbid obesity associated with the Prader Willi syndrome (341).  Nevertheless, treatment of Prader Willi patients with somatostatin analogues that suppresses circulating levels of ghrelin, does not improve the hyperphagia associated with this disorder (342), indicating that other mechanisms are responsible or that compensatory mechanisms take place.  A second peptide derived from the ghrelin gene has been recently identified, obestatin, but its role in the regulation of appetite requires further characterization.  Curiously, obestatin appears to suppress food intake (343).  Nesfatin-1 has also been found to co-localize with ghrelin in gastric cells (344) and has potent satiety effects (345).  While circulating nesfatin-1 can cross the blood-brain-barrier and inhibit NPY neurons in the arcuate nucleus (346), the peptide is highly expressed in the lateral hypothalamus in MCH neurons (347) and in the PVN and may be under the regulation of -MSH (345).

Thyroid hormone should probably also be considered as a circulating hormone involved in appetite regulation through actions on the hypothalamus.  Thyrotoxicosis is commonly associated with hyperphagia both in experimental animal models and humans (348-352), although the mechanism(s) by which this occurs are uncertain.  Ishii et al (350) have shown an increase in hypothalamic NPY mRNA following the administration of thyrotoxic doses of T3, perhaps secondary to T3-induced increase in neuronal uncoupling protein 2 in NPY neurons (353).  Nevertheless, T3-associated hyperphagia is only partially attenuated by a NPY receptor antagonist, indicating that other mechanisms must also be operable.  The hypothalamus is richly endowed with thyroid hormone receptors (354,355), and nuclear T3 concentrations in hypothalamic extracts are elevated following the systemic administration of thyrotoxic doses of T3 or T4 (356).  Recent studies by Kong et al (352) have demonstrated that the systemic administration of superphysiologic doses of T3 increase immediate early gene expression in the hypothalamic ventromedial nucleus.  In addition, microinjection of T3 into the hypothalamic ventromedial nucleus induces a 4-fold increase in food intake during the first hour following injection.  These data suggest a direct effect of T3 on the hypothalamus to induce feeding.  Evidence that the nuclear T3 concentrations in the hypothalamus are elevated following the systemic administration of thyrotoxic doses of T3 or T4 (356) further substantiates this hypothesis.  The coincidence that rats display a nocturnal pattern of feeding and that hypothalamic deiodinase activity follows a circadian pattern that increases during the night which may have the effect of increasing local tissue levels of T3 (352,357), provides circumstantial evidence for the potential importance of thyroid hormone to regulate feeding by increasing local, neural tissue concentrations of T3.
Nutrient sensing by the brain also appears to contribute to the regulation of appetite and satiety.  Hypoglycemia increases food intake, presumably as a result of glucose deprivation on brainstem catecholamine neurons which project to the hypothalamus (358); the amino acid, L-leucine, but not other branched chain amino acids exerts its anorexic actions by activating the mammalian target of rapamycin, mTORC1 pathway, in arcuate nucleus NPY/AGRP neurons by reducing NPY and AGRP  (359); and fatty acids inhibit food intake by suppressing AMP-activated protein kinase (AMPK) in arcuate nucleus which similarly reduces NPY (360).  Alpha-MSH-producing neurons in the arcuate nucleus are also excited by glucose, mediated closure of ATP-sensitive potassium (KATP) channels (361).  The importance of this mechanism has recently been shown by Parton et al (362), demonstrating that expression of a mutant form of the KATP channel subunit Kir6.2b in -MSH-producing neurons that impairs ATP-mediated closure of KATP channels, results in impaired glucose tolerance.
A summary diagram of the complex mechanisms involved in the regulation of appetite and satiety is shown in Fig. 35.

Fig. 35.  Schematic drawing of major pathways involved in the regulation of appetite and satiety.  Leptin, a white adipose derived cytokine, circulates in the blood stream and binds to receptors on NPY/AGRP- and -MSH/CART-producing neurons in the hypothalamic arcuate nucleus (ARC) to orchestrate a series of responses mediated by downstream centers including the paraventricular nucleus (PVN) to control thyroid hormone secretion, feeding behavior, and energy conservation, and by lateral hypothalamic (LH) orexin- and MCH-producing neurons to control arousal responses and feeding behavior (not shown).  Leptin also binds to receptors in the dorsal vagal complex (DVC) that contribute to modulation of the PVN and/or the ARC.  Gut-derived peptides and nutrients also contribute to the regulation of appetite and satiety through effects on the arcuate nucleus or NTS.  The orexigenic effect of thyroid hormone may be mediated on the ventromedial nucleus (VMN).

D.  Lactation

     Suckling is a well recognized physiological stimulus for increased prolactin secretion from the anterior pituitary gland (363).  This stimulus is relayed to the hypothalamus via the spinal cord and brainstem (364,365), although the pathways have not been precisely elucidated.  Using c-fos as a marker for neuronal activation, several potential relay centers in the brainstem have been identified including the ventrolateral medulla (A1 catecholamine cell group), locus coeruleus, lateral parabrachial nucleus, caudal portion of the paralemniscal nucleus, and lateral and ventrolateral portions of the caudal part of the periaqueductal gray (366).  By inhibiting tuberoinfundibular dopamine neurons in the arcuate nucleus and increasing prolactin releasing factors in tuberoinfundibular neurons in the hypothalamic PVN and posterior pituitary (367), these signals permit high circulating levels of prolactin.  Prolactin is not only important for the synthesis and maintenance of milk secretion, but contributes to the physiologic responses that complement lactation including the development of material behavior and inhibition of reproductive function (363).  These effects are facilitated by upregulation of prolactin receptors in the choroid plexus during lactation (368), allowing increased entry of prolactin from the circulation into the brain, as well as in several hypothalamic nuclear groups including the medial preoptic nucleus, PVN, supraoptic nucleus and arcuate nucleus (369).  In particular, binding of prolactin to its receptor in the medial preoptic nucleus may be important for maternal behavior, as antagonizing prolactin receptors with a specific prolactin receptor antagonist injected directly into the preoptic area delays the onset of material behavior (370).  In addition, inhibition of pulsatile gonadotropin secretion associated with lactation may be at least partly mediated by the effects of prolactin on GnRH neurons in this region (363), although suckling-induced inhibition of kisspeptin gene expression in arcuate nucleus neurons by yet an unknown mechanism may also be contributory (371).
Suckling also results in a marked increase in NPY gene expression in arcuate nucleus neurons (372,373), mediated both by neuronal afferents relayed from the brainstem and by the fall in circulating levels of leptin brought about by negative energy balance resulting from milk production (374).   As described above (see Appetite and Satiety), NPY is a powerful orexigenic substance.  Its increase during suckling, therefore, has been proposed to explain the hyperphagia associated with lactation (374), although other changes are also observed in arcuate nucleus neurons that may contribute to hyperphagic responses including an increase in AGRP gene expression and decrease in POMC and CART mRNAs (375,376), similar to that described above for fasting.  NPY-containing axon terminals of arcuate nucleus origin also richly innervate the medial preoptic region and can be found in close proximity to GnRH neurons (377).  The suckling-induced rise in NPY, therefore, may also contribute to inhibition of reproductive function (376) through direct actions on GnRH neurons by binding to NPY Y5 receptors (378).
The circuitry involved in the coordinated responses to suckling is illustrated in Fig. 36 .

E. Thermoregulation

    The hypothalamus is the primary locus for coordinating thermoregulatory information and integrating thermoregulatory responses.  It continually monitors local brain temperature through temperature sensitive neurons and by utilizing thermoreceptors in the skin and spinal cord, and then orchestrates a series of responses to maintain normal, core body temperature by utilizing the autonomic nervous system, altering behavior, and through neuroendocrine responses.  Thyroid hormone is a necessary component for heat regulation since in its absence (myxedema), hypothermia commonly develops.
Although thermosensitive neurons can be found throughout the hypothalamus (379) the most important locus is the preoptic region including neurons in medial and lateral portions of the preoptic nucleus, anterior hypothalamus including the perifornical region, and nearby regions of the septum.  Preoptic cooling increases heat production by inducing shivering, or by nonshivering thermogenesis mediated by sympathetic activation uncoupling proteins-1 (UCP-1) in brown adipose tissue that allows mitochondria to generate heat from ATP, and by increasing intermediary metabolism in muscle and other parenchymal organs.  In addition, cooling induces heat retention responses by cutaneous vasoconstriction and redirecting blood flow from cutaneous to deep vascular beds, results in to behavioral responses (seek warmer environment, put on more cloths, increase food intake), and in some animal species can increase thyroid thermogenesis by activating the hypothalamic-pituitary-thyroid axis (380).  Conversely, preoptic warming reduces heat production and increases heat loss responses through vasoconstriction, sweating, increased respiration (panting), inhibition of UCP-1 in brown adipose tissue, and specific behavioral responses (379).
Two types of thermosensitive neurons can be found in the preoptic region, warm sensitive neurons that increase their firing rate when preoptic temperature rises and cold sensitive neurons that increase their firing when preoptic temperature falls (379).  However, the warm sensitive neurons predominate both in cell number and in importance of the regulatory responses for both heat loss and heat production mechanisms.  Thus, lesions involving the preoptic region per se are commonly characterized by abnormalities in heat dissipation and lead to hyperthermia and elevated temperature in brown adipose tissue (381).  Surprisingly, the neurotransmitter/peptide mediators and pathways mediating thermoregulatory responses are not precisely known.  When injected directly into the preoptic area, however, a number of different substances can induce hypothermic or hyperthermic responses (Table 11).  Since thermoregulation involves the coordination of multiple responses that can differ between animal species (i.e. panting in the dog, increased salivation in the rat which can be applied to the fur to enhance evaporative heat loss, sweating in man), it is logical that several different pathways are employed at the same time.  Evidence suggests that efferent pathways governing shivering involve ipsilateral and crossed fibers (382) that traverse the median forebrain bundle to terminate in the posterior hypothalamus, using GABA as a neurotransmitter (383).  The pathway continues caudally through the midbrain, dorsolateral to the red nucleus, and interacts with reticulospinal neurons.  It then proceeds through the reticulospinal tract, to innervate -motor neurons in the ventral horn of the spinal cord.  Regulation of heat production in brown adipose tissue also proceeds from preoptic neurons through the medial forebrain bundle to hypothalamic nuclear groups involved in autonomic regulation, particularly the PVN, but also the DMN and VMN (383).  The PVN has direct efferent projections to preganglionic neurons in the intermediolateral column of the spinal cord which give rise to the sympathetic innervation of brown adipose tissue (384).  Regulation of cutaneous blood flow also proceeds from thermosensitive neurons in the preoptic region through axons descending in the medial forebrain bundle, but likely relayed to neurons in ventrolateral portions of the midbrain periaqueductal gray (PAG) before proceeding to sympathetic preganglionic neurons in the spinal cord.  PAG neurons show strong c-fos induction following unilateral preoptic region heating (385) and induce cutaneous vasodilatation when stimulated (386).  Preoptic warming also inhibits vasoconstrictor neurons in the medullary raphe (raphe magnus and pallidus) (387) that have projections directly to preganglionic sympathetic neurons in the intermediolateral column of the spinal cord (388).   Pathways mediating behavioral changes associated with thermoregulation are unknown.

Under normal circumstances, there is a diurnal variation of body temperature, highest in late afternoon and early evening and lowest in morning upon arising.  The hypothalamic suprachiasmatic nucleus controls this rhythm, and would appear to do so through direct projections to the dorsal portion of the subparaventricular zone, a region just ventral to the PVN.  Thus, bilateral focal lesions in the dorsal subparaventricular zone disrupts the circadian variation of body temperature, whereas bilateral lesions of the PVN, itself, is without effect (389).  Since the subparaventricular zone has prominent projections to the preoptic area (390), it is presumed that the suprachiasmatic nucleus relays information to thermosensitive neurons in the preoptic region by a multisynaptic pathway involving the subparaventricular zone (389).  However, the precise targets in the preoptic region from subparaventricular zone neurons have not yet been identified.
The set point for temperature regulation is also sensitive to circulating levels of sex steroids, with core body temperature falling just prior to the midcycle surge in women and rising during the luteal phase (391,392).  Estrogen, itself, would appear to be responsible for the fall in temperature in the late follicular phase by increasing the firing rate of preoptic warm sensitive neurons (393), whereas progesterone increases temperature in the luteal phase by decreasing the firing rate of preoptic warm sensitive neurons and, perhaps, increasing the firing rate of cold sensitive neurons (394).  Both steroids readily pass the blood-brain barrier and thereby are presumed to act directly on thermosensitive neurons in the preoptic nucleus (395).  Not unexpectedly, therefore, the lack of estrogen in the postmenopausal period gives rise to altered thermoregulatory responses (hot flashes) that occurs in over 80% of this group, and can be readily reversed by the administration of estrogen.  It has been proposed that in the absence of estrogen, the sensitivity of warm sensitive neurons to even small increases in body temperature is increased (396).  Along these lines, it is of interest that in one study, the frequency of hot flashes was greatest during the afternoon and evening when body temperature normally rises under the influence of the suprachiasmatic nucleus circadian pacemaker (397).  The therapeutic response of some women with postmenopausal hot flashes to clonidine (398), an 2-adrenergic agonist, would indicate that catecholamines also participate in the heat loss responses.
Under certain circumstances, there is an adaptive advantage to elevate body temperature beyond the normal physiologic range that is highly conserved among animal species.  Such is the situation during infection, when fever is a necessary response to facilitate recovery by improving the efficiency of immune cells and impairing replication of microorganisms (399,400).  This homeostatic response is achieved by altering the thermoregulatory set point in medial preoptic neurons, but through a different mechanism than described above.  Under these circumstances, it is proposed that circulating endotoxin and proinflammatory cytokines interact with specific receptors on vascular endothelial cells and/or subendothelial microglia in the OVLT, resulting in activation of cyclooxygenase and production of PGE2 (401,402).  PGE2 released into the surrounding tissue binds to neighboring warm sensitive neurons in the median preoptic nucleus that express EP3 prostaglandin receptors (403), which by reducing GABAergic inhibition of thermogenic neurons in the hypothalamic paraventricular and dorsomedial nuclei and/or brainstem rostral ventromedial medulla, influence sympathetic preganglionic neurons in the spinal cord that contribute to the generation of fever (404,405).  The proposed mechanism is schematized in Fig 37.

Fig. 37. Schematic drawing showing major pathways of the temperature control center.  Heat sensors in the median preoptic hypothalamus (MnPO) project to neurons in the hypothalamic paraventricular nucleus (PVN) and dorsomedial nucleus (DMN) or directly to the raphe pallidus (RPa) in the brainstem to control autonomic responses mediated through preganglionic neurons in the spinal cord.

    Blatteis et al (406) suggest an alternative mechanism for fever induction in which PGE2 release into the preoptic region is mediated by norepinephrine, arising in noradrenergic (A2 cell group) neurons in the ventrolateral medulla.  This is based on the observation that in guinea pigs, the intra-preoptic microdialysis of 2-receptor antagonists potentiate the febrile response to LPS (407).   The mechanism proposed involves activation of hepatic branches of the vagus nerve by mediators (possibly PGE2) produced by liver Kupffer cells following the systemic administration of LPS.  Vagal afferent signals are then carried to the nucleus tractus solitarius in the brainstem, and after projecting to noradrenergic neurons in the ventrolateral medulla (A2), ascend in the ventral noradrenergic pathway and medial forebrain bundle to terminate in the preoptic region.  Ek et al (407) have also demonstrated in the rat that the intravenous administration of interleukin-1 is capable of activating vagal sensory neurons in the nodose ganglion and can be attenuated by inhibitors of prostaglandin synthesis.
In addition to inducing fever, endotoxin simultaneously activates an endogenous, counterregulatory, antipyretic response, to prevent body temperature from rising too severely.  This is largely achieved by stimulating the hypothalamic-pituitary-adrenal axis (see above) that exerts a dampening effect on the cytokine response, but more specifically by the direct antipyretic actions of -MSH within the CNS (408).  The latter situation occurs only in association with cytokine activation, as -MSH has no effect on temperature regulation in the absence of fever (408,409).  Alpha-MSH arises from the neuronal population in the hypothalamic arcuate nucleus (227), and while it is unknown precisely where -MSH exerts its actions, the preoptic region including the VMPO is heavily innervated by axon terminals containing -MSH, suggesting a direct effect on thermosensitive neurons (410).  Alpha-MSH is also contained in axons that heavily innervate autonomic regulatory neurons in the parvocellular PVN and the hypothalamic DMN, providing an alternative route for regulatory control over vasomotor responses and heat generation.
While it is clear that the transient receptor potential ion channels, such as TRP vanilloid 1 (TRPV1) and related receptors, are involved as heat receptors in the periphery, it remains controversial whether these receptors contribute to thermoregulatory responses at the level of the preoptic nucleus (411).  TRPV1 mRNA is present in the hypothalamus, although at low levels, and administration of capsaicin, a ligand for TRPV1, directly into the preoptic hypothalamus induces hypothermic responses, suggesting activation of warm-sensitive neurons (412,413).  Nevertheless, mice deficient in TRPV1 show normal thermoregulation when placed in a warm environment (414), raising doubt about the physiologic importance of TRPV1 in central thermoregulatory responses.  

F.  Circadian Rhythmicity

    Circadian rhythms are genetically determined, cyclic modifications of specific physiological functions and behaviors, generated through endogenous mechanisms in nearly all living organisms (415,416).  The basic organization of the circadian timing system includes an endogenous rhythm generator or pacemaker (also called endogenous clock or zeitgeber), a light-dark receptive system to entrain the endogenous clock to the time of day mediated by retinal photoreceptors (mainly cones) and visual pathways (retinohypothalamic pathway), and an efferent neural system coupling the pacemaker activity with effector systems in the brain that give rise to specific physiological functions and behaviors (415,416).  The master clock in mammals is the hypothalamic suprachiasmatic nucleus (SCN), a small, paired nucleus embedded in the dorsal surface of the optic chiasm.  Contained within this nucleus are multiple, small neurons that produce autonomous, self-sustaining oscillations synchronously firing to generate a common rhythmic output, perhaps mediated by the local release of GABA (417,418).  If the SCN is lesioned bilaterally, "free-running circadian rhythmicity” is produced, characterized by disruption of the sleep-wake cycle and loss of predictable daily oscillations in feeding, drinking, melatonin secretion and the secretion of some anterior pituitary hormones (419,420).  Normal rhythmicity can be restored if the SCN is transplanted back into the lesioned animals (421).  Molecular mechanisms for the endogenous pacemaker activity of SCN neurons has been attributed to clock genes that currently include period (per), Clock, Cryptochrome (Cry), and Bmal (415,416).
Two different subdivisions of the SCN have been described, a ventrolateral and dorsomedial subdivision (415).  The ventrolateral subdivision or "core”, receives the major input to the SCN, including a massive projection of pituitary adenyl cyclase-activating peptide (PACAP)- and nitric oxide (NO)-containing axons from the retinohypothalamic pathway, GABA- and NPY-containing axonal projections from the intergeniculate leaflet of the thalamus, and serotonin neurons from the midbrain raphe (415,416,422).  These inputs have an important role in modulating the endogenous rhythms of the individual SCN pacemaker cells during the day / night alternance and as a result of changes in locomotor activity (423). The dorsomedial subdivision or "shell”, primarily serves as the field for afferent information coming from the limbic system (hippocampus, bed nucleus of the stria terminalis, septum) and the hypothalamus, itself (422).  It is likely that through these inputs, cognitive and emotional information may exert phase-shifting effects on SCN pacemaker activity (424).  Both subdivisions are composed of a heterogeneous population of immunocytochemically distinct neurons. The ventrolateral SCN contains neurons that express vasoactive intestinal polypeptide, gastric-releasing peptide and GABA (415).  Dorsomedial neurons express arginine vasopressin, angiotensin II, somatostatin and GABA (415).  However, while ventrolateral subdivision neurons receive light information, most of these neurons do not produce rhythmic patterns (425).  In contrast, the dorsomedial subdivision does contain rhythmic neurons, particularly apparent for arginine vasopressin-producing neurons in which the peptide peaks during the day and is lowest at night (426).  This rhythmic pattern is partly secondary to the presence of binding sites for clock genes in the argenine vasospressin promoter region (427), but also dependent upon synaptic transmission from other SCN neurons (428), perhaps those in the ventrolateral subdivision through intra-SCN connections (429).
The SCN has massive projections to three major regions of the diencephalon.  The most important is the hypothalamic subparaventricular zone (SPVZ).  Projections from the dorsal SPVZ reach the medial preoptic hypothalamus and are involved in the regulation of body temperature set-point and food-dependent energy intake (424).  In contrast, projections from the ventral SPVZ heavily innervate the hypothalamic dorsomedial nucleus and, to lesser extent, the midline thalamus, midbrain reticular formation and basal forebrain.  These outputs entrain photic stimuli with changes in food intake, rest/locomotion behavior, sleep-wake phases and pituitary hormone secretions. The second major projection is a GABAergic fiber tract to the hypothalamic PVN, and is related to the secretion of melatonin from the pineal gland (424,430,431) by way of a multisynaptic pathway involving dorsal parvicellular neurons in the PVN, preganglionic cholinergic neurons in the intermediolateral cell column of the spinal cord, and postganglionic noradrenergic neurons in the superior cervical ganglion (431,432,433).  Melatonin is of importance as a humoral signal that feeds back on the SCN through melatonin receptors expressed in this nucleus, to facilitate sleep onset by communicating information concerning initiation and length of the dark phase  (415,416).  In addition, it regulates immune function, is of particular importance for reproductive activity in animals with seasonal breeding patterns (434,435), and participates in accomodating pituitary function with the shift to torpor in hybernators (436,437).  A third major projection is directed to the medial and lateral tuberal hypothalamus, primarily the ventromedial nucleus, arcuate nucleus and lateral hypothalamic area.  These fibers are belived to influence the regulation of the neuroendocrine tuberoinfundibular and neurohypophysial secretions (422).  Figure 38 schematically depicts an integrated view of the mammalian circadian timing system and the main physiological functions and behaviors it controls.

Fig. 38. Schematic drawing showing an integrated view of the mammalian circadian timing system and the main neuroendocrine responses, physiological functions and behaviors under its control. AII = angiotensin II; APv = anterior periventricular nucleus of the hypothalamus; AVP = arginine vasopressin; BNST = bed nucleus of the stria terminalis; CAL = calretinin; ENK = enkephalin; GABA = aminobutiric acid; GRP = gastrin-releasing peptide; GLU = glutamic acid; 5-HT = 5-hydroxytryptamine or serotonin; HYP = hypothalamus, IGL = intergeniculate leaflet; NO = nitric oxide; NPY = neuropeptide Y; PACAP = pituitary adenylyl cyclase-activating peptide; PTA = pretectal area; PVN = hypothalamic paraventricular nucleus; rht = retinohypothalamic tract; SCN = suprachiasmatic nucleus; SP = substance P; SPVZ = hypothalamic subparaventricular zone; SRIF = somatostatin; VIP = vasoactive intestinal polypeptide.

G.  Sleep-Wake Cycle

    Sleep is a natural state of altered consciousness, easily reversible, self-regulating and characterized by a stereotypic posture, decrease in voluntary motor activity and increase in arousal threshold.  In mammals and man, sleep periods are cyclically coupled to periods of wakefulness, giving rise to a circadian sleep-wake cycle.  Electrophysiologically, sleep is characterized by a progressively slower, higher voltage, and more synchronized electrical activity of the cortex (alpha waves – stage 1, spindle and k-complexes – stage 2, delta waves – stages 3 and 4) as opposed to wakefulness where fast, low voltage, and desynchronized electrical activity prevails (beta waves).  Only relatively brief times are spent in sleep-wake transitions.  Episodes of partial arousal without wakefulness occur during sleep, and are characterized by desynchronized electrical cortical activity resembling the EEG pattern of wakefulness and the initial sleep phase (theta waves).  This arousal is coupled to rapid eye movements (REM) and loss of muscle tone (except for respiratory and inner ear muscles).  In contrast, non-REM or NREM sleep is devoid of involuntary eye movements and muscle tone resumes, leading to deep sleep  (438).
The basic neural organization of the sleep-wake cycle relies on two adjacent areas of the neuraxis, the diencephalon-basal forebrain and the mid-rostral brainstem, collectively expressing two, stable, firing states to produce either rest or arousal, with a tendency to avoid intermediate conditions (flip-flop switch).  In this manner, inappropriate behavioral fluctuations that might endanger survival are avoided, favoring discrete and rapid changes between sleep and waking profiles (or between REM and NREM sleep) above a background of slow and continuous variations in circadian and homeostatic inputs (either photic or non-photic autonomic, endocrine-metabolic and immune stimuli) (438,439).
The neural structures participating in this flip-flop switch establish reciprocal, feedback circuitries and can be classified as sleep- and wakefulness-promoting centers, the latter including specific cell groups that trigger and shut off the REM arousal state during sleep.  The primary sleep center is localized in the preoptic hypothalamus, and involves GABA- and galanin-containing neurons in the ventrolateral preoptic nucleus or VLPO (440).  A secondary sleep center is located in the midline thalamus (visceral or limbic thalamus), primarily in the dorsomedial (441) and reticular (442) thalamic nuclei. 
Wakefulness centers are numerous and in large part belong to the ascending reticular activating system of Moruzzi and Magoun (443).  This system can be subdivided in two different components, including monoaminergic and cholinergic cell groups in the pontine and mesencephalic (or limbic midbrain area) reticular formation.  The monoaminergic neurons comprise the noradrenergic locus coeruleus (LC), the serotoninergic median and dorsal raphe (DR) and parabrachial nucleus (PBN), and the dopaminergic ventral periacqueductal grey (vPAG) in the tegmentum of the mesencephalon. The cholinergic neurons are located in the pontine tegmentum, specifically in the laterodorsal (LDT) and peduncolopontine tegmental  (PPT) nuclei, respectively.  In addition to pontine and mesencephalic reticular nuclei, wakefulness centers also involve histaminergic neurons in the posterior hypothalamus (tuberomammillary nucleus or TMN), peptidergic cell groups (orexin/hypocretin and melanin-concentrating hormone or MCH) in lateral hypothalamic area (LHA)/perifornical area (PF), and cholinergic and GABAergic neurons in the basal forebrain (nucleus basalis of Maynert and magnocellular preoptic nucleus in the substantia innominata, medial septal nucleus and nucleus of the diagonal band of Broca)  (439).
Axons from the primary sleep center (VLPO) travel caudally towards the wakefulness centers of the posterior hypothalamus (TMN) and ponto-mesencephalic reticular formation (LC, DR) via the medial forebrain bundle (MFB) (444).  Their cells of origin can be traced primarily to the VLPO core (VLPOc), but also extend to the periphery of this nuclear group (VLPOex) that innervate the LC, DR and mesopontine tegmentum (LDT and PPT).  Synaptic contacts in the LC and DR are primarily established by galanin-containing and to a lesser extent, GABAergic VPLO inputs (445-448).  VPLO synaptic contacts in the LDT and PPT are made with interneurons, likely disinhibiting inhibitory influences originating in TMN, LC and DR.  The signals are then directed to the principal cholinergic cells of the mesopontine tegmentum  (448,449). 
Fibers from the secondary sleep center (thalamic DM and reticular nuclei) course either in the periventricular system or in the MFB (thalamic peduncle of the ansa peduncolaris) to reach the periventricular and lateral preoptic hypothalamus (444) and mesopontine tegmental nuclei (450), respectively. Some of these axons contain glutamate and may enter the corona radiata to innervate the prefrontal cortex (451, 452), that reproject back to the lateral hypothalamus (453).
Axons from the wakefulness cell groups (ponto-mesencephalic, hypothalamic and telencephalic) enter the MFB and travel rostrally through 2 pathways.  The first is through a dorsal route, mainly provided by cholinergic mesopontine tegmental nuclei (LDT and PPT) and, to a lesser extent, by cholinergic and GABAergic basal forebrain neurons.  Axons innervate primarily the intralaminar and reticular thalamic nuclei, that diffusely reproject to the cortex.  The second is through a ventral route arising from aminergic pontine and mesencephalic nuclei (LC, DR-PBN, vPAG), aminergic (histamine) and peptidergic (orexin/hypocretin and MCH) cell groups in the posterior and lateral hypothalamus (TMN and LHA/PF, respectively), as well as by the majority of cholinergic and GABAergic basal forebrain neurons.  These fibers cross the lateral hypothalamus and basal forebrain to reach all cortical areas (438,453).  Finally, peptidergic (orexin/hypocretin and MCH) neurons in the LHA/PF also enter the MFB to heavily innervate aminergic and cholinergic cell groups in the posterior hypothalamus (TMN), pons-mesencephalon (LC, DR) and pontine tegmentum (LDT, PPT), respectively (454). 
Initiation of sleep and maintenance of deep sleep are driven by the VLPO, via inhibition of monoaminergic centers in the posterior hypothalamus (TMN) and ponto-mesencephalic reticular formation (primarily LC and DR).  Rodents with excitotoxic lesions of the VLPO show a reduction of NREM sleep that closely correlates with the loss of Fos-immunoreactive neurons in the VLPOc (448,449).  Indeed, pontine and mesencephalic reticular nuclei (including the vPAG) and the cholinergic basal forebrain cells are responsible for electrical brain desynchronization, arousal and the waking state through their excitatory dorsal- and ventral-projecting axonal pathways to the cortex (454,455).
Orexin/hypocretin cells in the LHA/PF contribute to the waking state by stimulating the posterior hypothalamic (TMN) and ponto-mesencephalic (LC, DR) aminergic waking centers, thus reinforcing cortical arousal.  Excitatory orexin 1 and 2 receptors have been found in all brainstem and basal forebrain wakefulness centers (456,457), injection of orexin/hypocretin into these areas increases neuronal firing (458,459) while its administration into the preoptic hypothalamus presynaptically inhibits VLPO excitability (460), and gene knockout for orexin/hypocretin, mutations in the orexin 2 receptor gene or absence of CSF orexin are associated with narcolepsy in mammals and man (461-463). 
The VLPO also stimulates cholinergic neurons in the LDT and PPT to induce REM bursts, thus favoring arousal without wakefulness.  In particular, outputs from LDT and PPT (and possibly from part of the basal forebrain cells) are excitatory to thalamic neurons projecting to the cortex.  When the VLPO activates the reticular pontotegmental nuclei, the transthalamic sensory transmission may easily diffuse to the cortical mantle, leading to enhancement of cortical arousal during conditions of synchronized brain activity (NREM to REM shift) (438,454).  Conversely, loss of Fos-immunoreactive neurons in the VLPOex correlates with a reduction in REM sleep episodes (448,449). 
In contrast, cholinergic neurons in the basal forebrain (medial septum and diagonal band of Broca) are implicated in the generation of specific electrical activity during REM episodes (theta waves) in response to stimulation by brainstem aminergic inputs (464).  Also MCH neurons in the LHA/PF contribute to the REM sleep by inhibiting the ponto-mesencephalic monoaminergic inputs to the cortex.  This inhibition amplifies arousal without wakefulness (REM phase) mediated by mesopontine tegmental centers (LDT and PPT) (438).  A reciprocal, negative feedback circuitry is established between pontine aminergic cell groups (LC area and tegmental area), leading to episodic switch between NREM and REM phases (465).  Finally, thalamic DM and reticular neurons come into play to coordinate either REM or NREM sleep with other behavioral and endocrine regulations (466), as well as to increase arousal threshold by reinforcing the inhibitory action of the VLPO on rostral mesencephalic waking centers (442).  Consistently, excitotoxic lesions of the thalamic DM induce persistent insomnia in cats (467) and its degeneration in humans gives rise to fatal familial insomnia, a disorder characterized by loss of rhythmicity in sleep-wake cycle, body temperature, blood pressure and anterior pituitary secretions (468). Figure 39 shows a simplified and integrated view of the neural circuitry controlling the mammalian sleep-wake cycle.

    Major regulators of the neural machinery for the sleep-wake cycle are the circadian timing system (CTS), feedback regulation exerted by locomotor activity and sleep-wake cycle itself onto the CTS, and homeostatic mechanisms endogenous to the sleep and wakefulness neural circuitries (sleep homeostat) (438).  The CTS is primarily governed by the hypothalamic suprachiasmatic nucleus (SCN) and its widespread CNS projections, including those that trigger melatonin secretion from the pineal gland (see section on circadian rhythmicity).  In rats, sleep-wake rhythmicity is adaptively coordinated with feeding behavior.  In fact, light sensitive, SCN projections to the ventral subparaventricular zone (469) innervate food-entrainable neurons in the hypothalamic dorsomedial nucleus (469,470) that sends stimulatory glutamate-containing inputs to LHA orexin neurons and inhibitory GABAergic inputs to VLPO neurons (471).  In this manner, photic-dependent stimuli can be integrated with nonphotic stimuli to establish a rest/sleep-locomotion/wakefulness cycle that is ideal for nutritional success (472). In turn, locomotor activity may feed back onto the SCN by activating NPY-containing outputs in the intergeniculate leaflet of the thalamus and serotoninergic projections in the median raphe nucleus to entrain the sleep-wake cycle with levels of exploratory behavior (473).  Finally, some cellular and molecular mechanisms such as the intracellular accumulation of adenosine, may inhibit aminergic waking centers and activate the VLPO (474).  This and a number of circulating endocrine, metabolic and immune inputs controlled by the CTS may influence circadian clock genes like Clock, Cry1/Cry2, Bmal1 (and possibly many others), known to act as positive and negative transcriptional regulators of the molecular machinery producing endogenous rhythmicity (438,475).  Knockout mice for these genes in the SNC and related neural pathways, in fact, show either an increase (Clock) or decrease (Cry and Bmal1) in NREM sleep (476-478), supporting an important role for the timing genes in homeostatic and circadian regulation of the sleep-wake cycle.