Disruption of diurnal feeding and weight gain cycles in weanling rats by ventromedial and dorsomedial hypothalamic lesions

Inputs and Outputs of the Mammalian Circadian Clock

Circadian rhythms in mammals are coordinated by the central circadian pacemaker, the suprachiasmatic nucleus (SCN). Light and other environmental inputs change the timing of the SCN neural network oscillator, which, in turn, sends output signals that entrain daily behavioral and physiological rhythms. While much is known about the molecular, neuronal, and network properties of the SCN itself, the circuits linking the outside world to the SCN and the SCN to rhythmic outputs are understudied. In this article, we review our current understanding of the synaptic and non-synaptic inputs onto and outputs from the SCN. We propose that a more complete description of SCN connectivity is needed to better explain how rhythms in nearly all behaviors and physiological processes are generated and to determine how, mechanistically, these rhythms are disrupted by disease or lifestyle.

Resilience in the suprachiasmatic nucleus: Implications for aging and Alzheimer's disease

Many believe that the circadian impairments associated with aging and Alzheimer's disease are, simply enough, a byproduct of tissue degeneration within the central pacemaker, the suprachiasmatic nucleus (SCN). However, the findings that have accumulated to date examining the SCNs obtained postmortem from the brains of older individuals, or those diagnosed with Alzheimer's disease upon autopsy, suggest only limited atrophy. We review this literature as well as a complementary one concerning fetal-donor SCN transplant, which established that many circadian timekeeping functions can be maintained with rudimentary (structurally limited) representations of the SCN. Together, these corpora of data suggest that the SCN is a resilient brain region that cannot be directly (or solely) implicated in the behavioral manifestations of circadian disorganization often witnessed during aging as well as early and late progression of Alzheimer's disease. We complete our review by suggesting future directions of research that may bridge this conceptual divide and briefly discuss the implications of it for improving health outcomes in later adulthood.

Cafeteria diet-induced obesity reduces leptin-stimulated NADPH-diaphorase reactivity in the hypothalamic arcuate nucleus of rats

Leptin is an adipokine that plays an important role in the regulation of energy homeostasis. The failure of endogenous and exogenous leptin to mediate its effects (for example, at suppressing appetite and decreasing body weight) has been termed leptin resistance. Hyperleptinemia and leptin resistance can be well demonstrated in animals in which obesity is induced by consumption of a palatable, high-calorie diet (e.g., cafeteria diet-induced obesity). Since leptin receptor signaling is known to be impaired in the hypothalamic arcuate nucleus (ARC) of obese rodents, we investigated the effect of leptin on nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) reactivity in the ARC of male Wistar rats with cafeteria diet-induced obesity. Our results have shown that after intraperitoneal administration of leptin, the number of NADPH-d positive neurons in the ARC was significantly lower in obese rats compared with that observed in normal weight rats. Additionally, we have found that leptin-induced NADPH-d staining in ARC neurons and the adjacent ependyma was decreased in obese rats. The results presented here suggest that the ability of leptin to activate nitric oxide synthase in neurons within the ARC as well as tanycytes and ependymal cells of the third ventricle is reduced in rats made obese by a cafeteria diet. We speculate that impairment in leptin-induced NO production presents a potential mechanism, involved in the pathogenesis of obesity and obesity-related disease states.

Hypothalamic AMPK as a Mediator of Hormonal Regulation of Energy Balance

As a cellular energy sensor and regulator, adenosine monophosphate (AMP)-activated protein kinase (AMPK) plays a pivotal role in the regulation of energy homeostasis in both the central nervous system (CNS) and peripheral organs. Activation of hypothalamic AMPK maintains energy balance by inducing appetite to increase food intake and diminishing adaptive thermogenesis in adipose tissues to reduce energy expenditure in response to food deprivation. Numerous metabolic hormones, such as leptin, adiponectin, ghrelin and insulin, exert their energy regulatory effects through hypothalamic AMPK via integration with the neural circuits. Although activation of AMPK in peripheral tissues is able to promote fatty acid oxidation and insulin sensitivity, its chronic activation in the hypothalamus causes obesity by inducing hyperphagia in both humans and rodents. In this review, we discuss the role of hypothalamic AMPK in mediating hormonal regulation of feeding and adaptive thermogenesis, and summarize the diverse underlying mechanisms by which central AMPK maintains energy homeostasis.

Distribution of type 1 cannabinoid receptor-expressing neurons in the septal-hypothalamic region of the mouse: colocalization with GABAergic and glutamatergic markers

Type 1 cannabinoid receptor (CB1) is the principal mediator of retrograde endocannabinoid signaling in the brain. In this study, we addressed the topographic distribution and amino acid neurotransmitter phenotype of endocannabinoid-sensitive hypothalamic neurons in mice. The in situ hybridization detection of CB1 mRNA revealed high levels of expression in the medial septum (MS) and the diagonal band of Broca (DBB), moderate levels in the preoptic area and the hypothalamic lateroanterior (LA), paraventricular (Pa), ventromedial (VMH), lateral mammillary (LM), and ventral premammillary (PMV) nuclei, and low levels in many other hypothalamic regions including the suprachiasmatic (SCh) and arcuate (Arc) nuclei. This regional distribution pattern was compared with location of γ-aminobutyric acid (GABA)ergic and glutamatergic cell groups, as identified by the expression of glutamic acid decarboxylase 65 (GAD65) and type 2 vesicular glutamate transporter (VGLUT2) mRNAs, respectively. The MS, DBB, and preoptic area showed overlaps between GABAergic and CB1-expressing neurons, whereas hypothalamic sites with moderate CB1 signals, including the LA, Pa, VMH, LM, and PMV, were dominated by glutamatergic neurons. Low CB1 mRNA levels were also present in other glutamatergic and GABAergic regions. Dual-label in situ hybridization experiments confirmed the cellular co-expression of CB1 with both glutamatergic and GABAergic markers. In this report we provide a detailed anatomical map of hypothalamic glutamatergic and GABAergic systems whose neurotransmitter release is controlled by retrograde endocannabinoid signaling from hypothalamic and extrahypothalamic target neurons. This neuroanatomical information contributes to an understanding of the role that the endocannabinoid system plays in the regulation of endocrine and metabolic functions.

The metabolic basis of dual periodicity of feeding in rats

This article examines how the depletion and replenishment of various energy stores give rise to periodic eating and how constant body-energy levels are maintained over time.

Measures of the energy expended throughout the 24-hour feeding pattern in rats indicate that two different energy stores (one of small capacity and one of large) determine two superimposed feeding periodicities: one from meal to meal (prandial), the other from day to night (nycthemeral). The article reviews how experimental overrepletion or overdepletion of gastrointestinal content, blood glucose, or body fats affect food intake. These data suggest that gastrointestinal content determines both meal size and meal-to-meal periodicity. Other evidence indicates that glucose uptake rate in tissues, which is modulated by fat synthesis and fat mobilization, affects the periodic onset of feeding and the difference between nocturnal and diurnal postprandial satiety.

There follows an examination of the neuroendocrine bases for the interacting mechanisms governing energy input and output balance and of the role of the ventromedial hypothalamus in body-fat regulation and the lateral hypothalamus in feeding.

Substance P and neurokinin-1 immunoreactivities in the neural circadian system of the Alaskan northern red-backed vole, Clethrionomys rutilus

The suprachiasmatic nucleus (SCN) of the hypothalamus houses the main mammalian circadian clock. This clock is reset by light-dark cues and stimuli that evoke arousal. Photic information is relayed directly to the SCN via the retinohypothalamic tract (RHT) and indirectly via the geniculohypothalamic tract, which originates from retinally innervated cells of the thalamic intergeniculate leaflet (IGL). In addition, pathways from the dorsal and median raphe (DR and MR) convey arousal state information to the IGL and SCN, respectively. The SCN regulates many physiological events in the body via a network of efferent connections to areas of the brain such as the habenula (Hb) in the epithalamus, subparaventricular zone (SPVZ) of the hypothalamus and locus coeruleus of the brainstem-areas of the brain associated with arousal and behavioral activation. Substance P (SP) and the neurokinin-1 (NK-1) receptor are present in the rat SCN and IGL, and SP acting via the NK-1 receptor alters SCN neuronal activity and resets the circadian clock in this species. However, the distribution and role of SP and NK-1 in the circadian system of other rodent species are largely unknown. Here we use immunohistochemical techniques to map the novel distribution of SP and NK-1 in the hypothalamus, thalamus and brainstem of the Alaskan northern red-backed vole, Clethrionomys rutilus, a species of rodent currently being used in circadian biology research. Interestingly, the pattern of immunoreactivity for SP in the red-backed vole SCN was very different from that seen in many other nocturnal and diurnal rodents.

Determinants affecting physical activity levels in animal models

Weight control is dependent on energy balance. Reduced energy expenditure (EE) associated with decreased physical activity is suggested to be a major underlying cause in the increasing prevalence of weight gain and obesity. Therefore, a better understanding of the biological determinants involved in the regulation of physical activity is essential. To facilitate interpretation in humans, it is helpful to consider the evidence from animal studies. This review focuses on animal studies examining the biological determinants influencing activity and potential implications to human. It appears that physical activity is influenced by a number of parameters. However, regardless of the parameter involved, body weight appears to play an underlying role in the regulation of activity. Furthermore, the regulation of activity associated with body weight appears to occur only after the animal achieves a critical weight. This suggests that activity levels are a consequence rather than a contributor to weight control. However, the existence of an inverse weight-activity relationship remains inconclusive. Confounding the results are the multifactorial nature of physical activity and the lack of appropriate measuring devices. Furthermore, many determinants of body weight are closely interlocked, making it difficult to determine whether a single, combination, or interaction of factors is important for the regulation of activity. For example, diet-induced obesity, aging, lesions to the ventral medial hypothalamus, and genetics all produce hypoactivity. Providing a better understanding of the biological determinants involved in the regulation of activity has important implications for the development of strategies for the prevention of weight gain leading to obesity and subsequent morbidity and mortality in the human population.

Contrasting effects of ibotenate lesions of the paraventricular nucleus and subparaventricular zone on sleep-wake cycle and temperature regulation

The suprachiasmatic nucleus (SCN), the circadian pacemaker for the brain, provides a massive projection to the subparaventricular zone (SPZ), but the role of the SPZ in circadian processes has received little attention. We examined the effects on circadian rhythms of sleep, body temperature, and activity in rats of restricted ibotenic acid lesions of the ventral or dorsal SPZ that spared the immediately adjacent paraventricular hypothalamic nucleus (PVH) and the SCN. Ventral SPZ lesions caused profound reduction of measures of circadian index of sleep (by 90%) and locomotor activity (75% reduction) but had less effect on body temperature (50% reduction); dorsal SPZ lesions caused greater reduction of circadian index of body temperature (by 70%) but had less effect on circadian index of locomotor activity (45% reduction) or sleep (<5% reduction). The loss of circadian regulation of body temperature or sleep was replaced by a strong ultradian rhythm (period approximately 3 hr). Lesions of the PVH, immediately dorsal to the SPZ, had no significant effect on any circadian rhythms that we measured, nor did the lesions affect the baseline body temperature. However, the fever response after intravenous injection of lipopolysaccharide (5 microg/kg) was markedly decreased in the rats with PVH lesions (66.6%) but not dorsal SPZ lesions. These results indicate that circadian rhythms of sleep and body temperatures are regulated by separate neuronal populations in the SPZ, and different aspects of thermoregulation (circadian rhythm and fever response) are controlled by distinct anatomical substrates.

Organization of neural inputs to the suprachiasmatic nucleus in the rat

The circadian timing of the suprachiasmatic nucleus (SCN) is modulated by its neural inputs. In the present study, we examine the organization of the neural inputs to the rat SCN using both retrograde and anterograde tracing methods. After Fluoro-Gold injections into the SCN, retrogradely labeled neurons are present in a number of brain areas, including the infralimbic cortex, the lateral septum, the medial preoptic area, the subfornical organ, the paraventricular thalamus, the subparaventricular zone, the ventromedial hypothalamic nucleus, the posterior hypothalamic area, the intergeniculate leaflet, the olivary pretectal nucleus, the ventral subiculum, and the median raphe nuclei. In the anterograde tracing experiments, we observe three patterns of afferent termination within the SCN that correspond to the photic/raphe, limbic/hypothalamic, and thalamic inputs. The median raphe projection to the SCN terminates densely within the ventral subdivision and sparsely within the dorsal subdivision. Similarly, areas that receive photic input, such as the retina, the intergeniculate leaflet, and the pretectal area, densely innervate the ventral SCN but provide only minor innervation of the dorsal SCN. A complementary pattern of axonal labeling, with labeled fibers concentrated in the dorsal SCN, is observed after anterograde tracer injections into the hypothalamus and into limbic areas, such as the ventral subiculum and infralimbic cortex. A third, less common pattern of labeling, exemplified by the paraventricular thalamic afferents, consists of diffuse axonal labeling throughout the SCN. Our results show that the SCN afferent connections are topographically organized. These hodological differences may reflect a functional heterogeneity within the SCN.

Faster gastric emptying of a liquid meal in rats after hypothalamic dorsomedial nucleus lesion

The effects of dorsomedial hypothalamic (DMH) nucleus lesion on body weight, plasma glucose levels, and the gastric emptying of a liquid meal were investigated in male Wistar rats (170-250 g). DMH lesions were produced stereotaxically by delivering a 2.0-mA current for 20 s through nichrome electrodes (0.3-mm tip exposure). In a second set of experiments, the DMH and the ventromedial hypothalamic (VMH) nucleus were lesioned with a 1.0-mA current for 10 s (0.1-mm tip exposure). The medial hypothalamus (MH) was also lesioned separately using a nichrome electrode (0.3-mm tip exposure) with a 2.0-mA current for 20 s. Gastric emptying was measured following the orogastric infusion of a liquid test meal consisting of physiological saline (0.9% NaCl, w/v) plus phenol red dye (6 mg/dl) as a marker. Plasma glucose levels were determined after an 18-h fast before the lesion and on the 7th and 15th postoperative day. Body weight was determined before lesioning and before sacrificing the rats. The DMH-lesioned rats showed a significantly faster (P < 0.05) gastric emptying (24.7% gastric retention, N = 11) than control (33.0% gastric retention, N = 8) and sham-lesioned (33.5% gastric retention, N = 12) rats, with a transient hypoglycemia on the 7th postoperative day which returned to normal by the 15th postoperative day. In all cases, weight gain was slower among lesioned rats. Additional experiments using a smaller current to induce lesions confirmed that DMH-lesioned rats had a faster gastric emptying (25.1% gastric retention, N = 7) than control (33.4% gastric retention, N = 17) and VMH-lesioned (34.6% gastric retention, N = 7) rats. MH lesions resulted in an even slower gastric emptying (43.7% gastric retention, N = 7) than in the latter two groups. We conclude that although DMH lesions reduce weight gain, they do not produce consistent changes in plasma glucose levels. These lesions also promote faster gastric emptying of an inert liquid meal, thus suggesting a role for the DMH in the regulation of gastric motility.

Attenuation of the expression of circadian rhythms by chronic outputs from the VMH in rats

To alter neural networks in a restricted area of the brain, we previously developed the hydro-polymer gel (PG) implantation technique. In this study, we found that bilateral or unilateral injection of PG (1.6 microliters) into the ventromedial nucleus of the hypothalamus (VMH), contrary to injections into the preoptic area (POA) or posterior hypothalamic area (PHA), severely attenuated circadian changes of locomotor activity in blinded female rats. In addition, PG injection into the VMH also suppressed circadian changes in serum melatonin levels and induced persistent estrus. The effect of PG injection into the VMH on free-running rhythm was blocked by a complete cut around the VMH or a dorsal cut of the VMH but not by anterior or posterior cuts of the VMH. These results suggest that PG injection into the VMH induces some form of neural output via a dorsal route of the VMH that affects the generation of circadian activity rhythm.

Organization of projections from the ventromedial nucleus of the hypothalamus: a Phaseolus vulgaris-leucoagglutinin study in the rat

The organization of projections from the four parts of the ventromedial nucleus (VMH) and a ventrolaterally adjacent region tentatively identified as the tuberal nucleus (TU) have been analyzed with small injections of the anterograde axonal tracer Phaseolus vulgaris-leucoagglutinin (PHA-L). Extrinsic and intranuclear projections of each part of the VMH display clear quantitative differences, whereas the overall patterns of outputs are qualitatively similar. Overall, the VMH establishes massive intrahypothalamic terminal fields in other parts of the medial zone, tending to avoid the periventricular and lateral zones. The ventrolateral VMH is more closely related to other parts of the hypothalamus that also express gonadal steroid hormone receptors, including the medial preoptic, tuberal, and ventral premammillary nuclei, whereas other parts of the VMH are more closely related to the anterior hypothalamic and dorsal premammillary nuclei. All parts of the VMH project to the zona incerta (including the A13 region) and parts of the midline thalamus, including the paraventricular and parataenial nuclei and nucleus reuniens. The densest inputs to the septum are to the bed nuclei of the stria terminalis, where the ventrolateral and central VMH innervate the anteroventral and anterodorsal areas and transverse and interfascicular nuclei, whereas the anterior and dorsomedial VMH innervate the latter two. The central, lateral, and medial amygdalar nuclei receive substantial inputs from various parts of the VMH. Other regions of the telencephalon, including the nucleus accumbens and the piriform-amygdaloid, infralimbic, prelimbic, anterior cingulate, agranular insular, piriform, perirhinal, entorhinal, and postpiriform transition areas, also receive sparse inputs. All parts of the VMH send a massive, topographically organized projection to the periaqueductal gray. Other brainstem terminal fields include the superior colliculus, peripeduncular area, locus coeruleus, Barrington's nucleus, parabrachial nucleus, nucleus of the solitary tract, and the mesencephalic, pontine, gigantocellular, paragigantocellular, and parvicellular reticular nuclei. The projections of the TU are similar to, and a subset of, those from the VMH and are thus not nearly as widespread as those from adjacent parts of the lateral hypothalamic area. Because of these similarities, the TU may eventually come to be viewed most appropriately as the lateral component of the VMH itself. The functional implications of the present findings are discussed in view of evidence that the VMH plays a role in the expression of ingestive, affective, and copulatory behaviors.

Somatic parameters, organ growth, and plasma substrates in weanling rats with lateral hypothalamic lesions one month postoperatively

Somatic and some metabolic aspects of the syndrome that follows bilateral destruction of the lateral hypothalamic area (LHA) have been studied primarily in mature rats. Fewer data are available for the weanling rat. Weanling Sprague-Dawley rats received small (10 mC) bilateral electrolytic lesions (LHAL). Sham-operated controls were pair-gained to LHAL rats (CON-PG) or fed ad lib (CON-ADLIB). All rats were killed 1 month after LHAL. Both LHAL and CON-PG weighed less, had less carcass fat, and were shorter than CON-ADLIB. Also, LHAL were somewhat, but significantly (SIGN), shorter than CON-PG. Rats with LHAL has less carcass protein than CON-ADLIB in percent but not in absolute terms. Liver, epididymal fat pads, diaphragm, kidneys adrenals, testes, spleen, and heart grew SIGN smaller in LHAL vs. CON-ADLIB, but in no instance was there a SIGN difference between LHAL and CON-PG. In body weight percentage, some of these differences (liver, kidneys, heart) were not SIGN. Both LHAL and CON-PG had larger adrenals than CON-ADLIB and both LHAL and CON-PG had SIGN less protein in their livers, epididymal fat pads, and diaphragm than CON-ADLIB. In organ weight percentage, however, LHAL rats had more protein in their livers and fat pads than CON-ADLIB and LHAL rats had less protein in fat pads than CON-PG in absolute but not in percent organ weight terms. Plasma glucose was similar in all groups, but LHAL had SIGN lower triglycerides and total cholesterol than CON-ADLIB.(ABSTRACT TRUNCATED AT 250 WORDS)

Hormone and somatic changes in rats pair-fed to growth retarded dorsomedial hypothalamic nuclei-lesioned rats

Rats with dorsomedial hypothalamic nuclei lesions (DMNL) are hypophagic and have reduced linear and ponderal growth, but have normal body composition and anabolic hormone concentrations. Previous studies have shown rats pair-fed to levels consumed (70-80% of ad lib) by DMNL rats, using a meal-feeding paradigm, have abnormal body composition and hormone concentrations. Whether the noted changes were due to restriction per se or method of food presentation was uncertain. In the present study, one group of sham-operated rats was pair fed (SHPF) by a computer-operated system that presented 45 mg food pellets in the exact amount and pattern as their DMNL yoked partner; another sham-operated group was ad lib fed (SHAD). At the end of Experiment 1 (11 days) and Experiment 2 (3 weeks) blood was collected for hormone and metabolite analyses; body compositions were also determined. Unlike an earlier report, the DMNL and SHPF groups had normal percentage body fat. Percentage carcass protein was similar in all groups at 11 days, but slightly elevated in DMNL rats at 3 weeks. Also, in contrast to an earlier study, plasma-free fatty acid levels were comparable in DMNL and SHPF rats. Plasma insulin was normal in the DMNL and SHPF rats at 11 days, but was lowered (p < 0.05) in the SHPF group at 3 weeks. Plasma thyroxine was reduced (p < 0.01) in the SHPF group at 11 days but returned to normal by 3 weeks. Thyroxine and triiodothyronine levels were normal in the DMNL groups. Plasma corticosterone levels were similar in all groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Projections of the suprachiasmatic nucleus to stress-related areas in the rat hypothalamus: a light and electron microscopic study

The purpose of the present study was to investigate the sites in the hypothalamus where the suprachiasmatic nucleus (SCN) may influence corticosteroid secretion. In spite of the well established, SCN-mediated, daily rhythms in adrenocorticotrophic hormone (ACTH) and corticosteroid secretion, previous studies determining the projections of the suprachiasmatic nucleus failed to illustrate direct connections with corticotrophin-releasing hormone neurons (CRH). In order to identify where in the central nervous system the SCN may influence corticosteroid secretion, areas were selected that contained SCN efferents contacting neurons involved in the stress response. To achieve this in the present study, SCN efferents were visualized by Pha-L tract-tracing, together with the neurons involved in the stress response by immunocytochemical staining for c-fos protein. The sites where these efferents contacted c-fos-positive neurons were established by light microscopic double staining and electron microscopic immunocytochemical studies. It appeared that apart from the medial parvocellular area of the paraventricular nucleus (PVN) of the hypothalamus, many more regions showed fos-positive neurons. Sites where SCN efferents contacted such neurons are limited only to areas immediately adjacent to these putative CRH neurons but are not concentrated on these neurons themselves. These areas consist of the periventricular and rostral PVN together with the dorsomedial hypothalamus: all three regions are known to project into the PVN. Therefore, it is concluded that the SCN transmits its information related to corticosteroid secretion via interneurons in and around the PVN to the CRH-containing neurons, rather than by a direct interaction with these neurons themselves.

Brown (BAT) and white (WAT) adipose tissue in high-fat junk food (HFJF) and chow-fed rats with dorsomedial hypothalamic lesions (DMNL rats)

Male weanling rats received dorsomedial hypothalamic nucleus lesions (DMNL) or sham operations and were fed for 173 postoperative days a high-fat diet and given a 32% sucrose solution as drinking fluid. This was supplemented with chocolate chip cookies, potato chips and marshmallows. Other DMNL and sham-operated controls were fed lab chow instead of the above high-fat junk food diet (HFJF) and given tap water instead of 32% sucrose solution. All animals were killed on postoperative day 174. Caloric intake per 100 g body weight was similar in all groups; however, the HFJF fed control and DMNL rats had significantly elevated carcass fat. Since HFJF-DMNL rats were not nearly as obese as the HFJF control animals, it appears that the DMNL offered some protection against the HFJF-diet-produced obesity. When their smaller body size is considered. DMN lesions had no effect on brown adipose tissue (BAT) mass in chow-fed or HFJF fed rats, whereas BAT size was significantly enlarged in HFJF-fed control animals. This suggests but does not prove that HFJF-fed controls, but not DMNL rats, may be using dietary-induced thermogenesis (DIT) to attenuate their obesity. We hypothesize that the HFJF-fed DMNL may not be enhancing DIT as reflected in normal BAT size, because they had not attained a degree of fatness to activate this system, or the DMN lesions impaired its activation. Both HFJF-fed groups showed reduced linear growth compared to their counterparts. The reason for stunting is uncertain, but may be related to their low plasma insulin concentrations.

Somatic, endocrine and metabolic changes in controls pair-fed for six weeks to rats with dorsomedial hypothalamic nucleus lesions (DMNL rats)

One group of weanling male Sprague-Dawley rats received lesions in the dorsomedial hypothalamic nuclei (DMNL rats), whereas two additional groups of rats were sham-operated (CON). One of these CON groups was allowed to feed ad lib (CON-ADLIB) while the other CON group was pair-fed for 6 weeks to the DMNL rats (CON-PF). Despite eating the same amount of food as DMNL rats. CON-PF animals had consistently lower body weights and also utilized food energy more poorly than DMNL rats. The CON-PF group also had smaller kidneys and less percent liver protein but more epididymal fat pad percent protein than DMNL rats. Whereas plasma glucose concentrations were comparable among the three groups, insulin levels were significantly higher, and free fatty acid levels lower in CON-PF than in DMNL rats. The CON-PF group incorporated less glucose-U-C14 carbon into liver glycogen but more of the tracer into liver lipid than the DMNL group. Glucose carbon was also incorporated more avidly into epididymal fat pad lipid by CON-PF than by DMNL rats. The data not only confirm previous findings in DMNL rats but in addition show that the neurologically intact rats fed the same amount of food that is eaten spontaneously by DMNL rats show somatic and metabolic alterations that suggest that they cannot cope with this low amount of substrate. The normalcy of the DMNL rats, compared to ad lib-fed sham-operated controls, in all metabolic parameters suggests that the low food intake is indeed "normal" for this preparation and may be the reflection of an "organismic" set point.

Metabolic-endocrine correlates of the lateral hypothalamic syndrome: the first 48 hours

Mature (224 g) male Sprague-Dawley rats received bilateral electrolytic (1 mA for 8 sec) lesions in the lateral hypothalamic area (LHAL) or sham operations (CON). One group of CON was allowed to eat ad lib (CON-ADLIB), a second CON group was pair-fed to the LHAL rats (CON-PF). Tap water was available ad lib. Two days after the operation/sham operation all rats were killed by decapitation. Body weight, body weight change, food intake, carcass fat, liver weight, epididymal fat pad weight, in vitro incorporation of U-C14-glucose into liver total lipid, glycogen and CO2 (oxidation) (DPM, DPM/mg protein) as well as oxidation in fat pad tissue, plasma glucose and insulin were significantly reduced in LHAL and CON-PF rats compared with CON-ADLIB. Glucose carbon incorporation into epididymal fat pad lipid and glycogen were normal in LHAL and CON-PF. Liver protein and plasma free fatty acids (FFA) were both higher in LHAL and CON-PF than in CON-ADLIB groups. Thus, many of the somatic and metabolic changes that appear in the first few days after lesion production are simply due to hypophagia. However, CON-PF rats also exhibited some significant differences from the LHAL group, i.e., their plasma glucose and incorporation of glucose carbon into liver glycogen (DPM) were significantly lower than in LHAL rats; alternatively, plasma FFA levels were higher in CON-PF than in LHAL rats. Also, liver weight/100 g body weight was lower and fat pad weight/100 g body weight was higher in CON-PF than in LHAL rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Normal catch-up growth in rats severely food-restricted prior to lesions of the dorsomedial hypothalamic nucleus: the first 48 hours

Following a brief period of ad lib (AL) feeding, 45-day-old male Sprague-Dawley rats were either fed AL or food-restricted (REST) for 21 days to 50% of the intake of the AL rats. At this time, some AL and some REST rats received electrolytic lesions in the dorsomedial hypothalamic nuclei (DMNL), whereas other AL and REST rats were sham-operated (CON). Following this, all rats were refed (REF) AL and killed two days later. At this time, DMNL-REST + REF and DMNL-AL weighed as much as CON-REST + REF and CON-AL, whereas the body weight of the DMNL-AL group began to separate from the CON-AL group; carcass lipid and protein were normal among the groups. DMNL-AL laid down more % lipid and % protein/g food eaten than CON-AL; this was not the case in the REST + REF groups. DMNL-AL were hypophagic vs. CON-AL, but DMNL-REST + REF ate as much as CON-REST + REF. Compared to DMNL-AL, DMNL-REST + REF increased their food intake more than four-fold and also utilized food energy more efficiently than DMNL-AL rats. Epididymal fat pads and kidneys were smaller in REST + REF vs. AL groups, irrespective of brain manipulation. Plasma glucose and growth hormone were normal among the groups, but plasma insulin concentrations were higher in REST + REF DMNL and CON groups vs. DMNL-AL and CON-AL, respectively. Glucose incorporation into epididymal fat pad lipid and CO2 and liver lipid was elevated in REST-REF groups vs. respective AL groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Recovery of rats with dorsomedial hypothalamic nucleus lesions (DMNL rats) from body weight restriction: effect of duration of postoperative prerestriction period

The present study was performed to see whether somatic and underlying metabolic-adaptive responses of DMNL rats and sham-operated controls (CON) to body weight restriction and subsequent refeeding could be influenced by the duration of ad lib feeding between lesion production and start of restriction. In contrast to previous studies (42 and 55 days, respectively) this time was reduced to 25 days. Restriction was similar, i.e., 27 days. DMNL rats show the same adaptive capacity in most parameters as do restricted CON. However, this response was at a lower level of absolute body weight, appropriate, so it appears, for their DMNL-induced lower body weight. In some parameters different responses were noted, however, suggesting that the time of ad lib feeding following the DMNL does indeed affect adaptive responses. Notably, this is the case in both DMNL and CON commensurately. Linear growth was reduced by restriction in the present but not in the two previous studies. Food intake showed a pronounced "overshoot" on refeeding but did not previously. Efficiency of food utilization was normal in the present study but depressed previously. A rise in plasma free fatty acids was not evident but was so in previous experiments. We concluded that, although DMNL rats respond to food restriction and recovery like similarly-treated CON, the duration of the ad lib feeding before restriction and/or the absolute age of the animals at that time, do indeed affect some parameters. This may be related to the fact that different aspects of the DMNL syndrome declare themselves in a sequential rather than a simultaneous manner.

Diurnal changes in sympathetic activity. Relation to food intake and to insulin injected into the ventromedial or suprachiasmatic nucleus

The present study was designed to test whether there are diurnal changes in the firing rate of sympathetic nerves to brown adipose tissue and whether these diurnal rhythms influenced the response to insulin injected into the suprachiasmatic nucleus or ventromedial hypothalamic nucleus (VMH). Food intake was highest at the beginning of the dark period (1800-2200 hours) and lowest during the daylight hours (0600-1000 and 1200-1600 hours). The basal sympathetic firing rate was highest at noon (1000-1200 hours) when food intake was lowest. At midnight, when food intake was highest, sympathetic firing rate was lowest. Injection of insulin (77, 144, and 288 pmol) into the VMH produced a dose-dependent depression of sympathetic firing rate at each of the four measurement periods (0400-0600 hours, 1000-1200 hours, 1600-1800 hours, and 2200-2400 hours), but the magnitude of the effect was greater at noon than at night. In contrast, insulin injections into the suprachiasmatic nucleus decreased the sympathetic firing rate at noon but produced a significant increase in the sympathetic firing rate at night. These data show that a diurnal rhythm exists for the sympathetic firing rate. The decrease in firing rate in response to insulin when injected into the VMH is in the same direction but varies in magnitude throughout the day, whereas the responsiveness of the suprachiasmatic nucleus to injections of insulin shows a reversal of response in relation to day/night cycles. The highly significant inverse relationship between basal sympathetic firing rate and food intake suggests that sympathetic activity may be part of an important control system for energy balance.

Circadian regulation of feeding in rats: suprachiasmatic versus ventromedial hypothalamic nuclei

The role of the suprachiasmatic nuclei as a major component of a specific circadian system controlling feeding periodicity is reviewed. Evidence is presented supporting the assumption that the ventromedial hypothalamus and the suprachiasmatic nucleus may act as a constant (tonic) regulator and a circadian modulator respectively of feeding in rats. It is concluded that a specific circadian system differing from the metabolic control mechanism superimposes the circadian periodicity of feeding. A model is put forward for the possible functional relationships between circadian and metabolic (homeostatic) control mechanisms of feeding in rats.

The ventromedial hypothalamic nucleus is not essential for the prefeeding corticosterone peak in rats under restricted daily feeding

The role of the ventromedial hypothalamic nucleus (VMH) in the generation of the prefeeding corticosterone peak was examined in rats under restricted daily feeding, under which daily meal supply was restricted to a fixed time in the early light period. Rats were lesioned in the VMH bilaterally and subjected to restricted daily feeding during two different post-operative phases and with two different durations of food presentation. A restricted feeding with free access to meal for 4 hr was imposed on the VMH-lesioned rats from 2 to 4 weeks after the lesion, when the daily food intake increased significantly (dynamic phase). The restricted feeding induced the prefeeding hormone peak in sham operated rats, but failed to develop it in the VMH-lesioned rats. On the other hand, the hormone peak appeared in the VMH-lesioned rats subjected to the feeding schedule from 8 to 10 weeks after the lesion, when the daily food intake was not different from the control (static phase). Moreover, the VMH-lesioned rats showed the hormone peak even in the dynamic phase when the access to meal was shortened to 1 hr. These results indicate that the VMH is not essential for the generation of the prefeeding corticosterone peak under restricted daily feeding, and suggest that a special metabolic state observed during the dynamic phase of VMH lesion prevents the development of the feeding-associated oscillation or its expression upon plasma corticosterone level.

Body weight set point studies in weanling rats with dorsomedial hypothalamic lesions (DMNL rats)

In order to further characterize a previously postulated "organismic" set point, weanling DMNL and control (CON) rats were maintained on lab chow ad lib (AL) for 55 post-operative days. Subsequently, some DMNL and CON rats were food-restricted (REST) to 80% of the food intake of their AL-fed counterparts for 24 days. At this point, representative rats from each group were killed by decapitation and the remaining animals were re-fed AL and killed 7 and 22 days thereafter. At the end of REST, both DMNL and CON showed significant weight loss, which was greater in CON than in DMNL rats. After 7 days of refeeding, DMNL rats normalized their body weights but re-fed CON still weighed less than AL-fed CON 22 days after refeeding. Food intake in formerly REST groups overshot on refeeding for 7 days, but this was significant only in DMNL rats. Notably, during this time formerly REST-DMNL ate as much as AL-fed CON. Efficiency of food utilization was normal in DMNL during AL feeding and became reduced on REST as it did in REST-CON. Notably, on refeeding formerly REST-DMNL rats overshot that of AL-fed DMNL rats by the same magnitude as previously REST-CON overshot the values of AL-fed CON. After 22 days of refeeding, this overshoot was still evident in DMNL but not in CON. At the end of the REST period, plasma insulin and glucose were similar in AL-fed DMNL and AL-fed CON. They were significantly and comparably reduced in both REST-DMNL and REST-CON compared to the AL-fed DMNL and AL-fed CON. On refeeding these changes normalized within seven days. At the end of REST, plasma free fatty acid concentrations were higher in REST-DMNL and REST-CON than in AL-fed DMNL and AL-fed CON. After seven days of refeeding they normalized only in formerly REST-CON. Plasma glycerol and total protein were normal throughout all groups, as was carcass protein. Carcass fat was equivalently reduced in both DMNL and CON at the end of REST and normalized 7 days after refeeding. AL-DMNL had the same carcass fat as AL-CON and REST-DMNL had the same carcass fat as REST-CON. In conjunction with previously reported normal anabolic hormone levels the data suggest that DMNL rats are not growth-retarded but are merely scaled down in size without compromise of their homeostatic competence. We take this as strong evidence for the existence of an "organismic" set point.

Similarities in feeding behavior of chronic methamphetamine treated and withdrawn rats to VMH lesioned rats

During chronic methamphetamine (M) administration and after drug withdrawal, total daily food intake, water consumption and body weight increase was measured. Additionally, food intake in the early light phase provided an index of normal circadian rhythmicity. A parallel development of tolerance to the anorexic effect of M and increased feeding in the early light phase occurred. A further abnormal behavior of both chronic M-treated and withdrawn rats was the lack of preference reversal when given free access to concentrated vs. diluted glucose solutions. Both disturbed circadian feeding rhythmicity and inhibition of the glucose preference reversal are found in rats with ventromedial hypothalamic (VMH) lesions. Reduced food intake and diminished body weight increase is a characteristic of lateral hypothalamic lesions. By analogy, chronic M-treatment could be considered to induce a functional state similar to lesions of the medial and lateral hypothalamus, each with a different time course.