Effect of dorsomedial hypothalamic lesions before and after placement of obesity-producing ventromedial hypothalamic lesions in the weanling male rat

Bilateral electrolytic lesions in the ventromedial hypothalamic nuclei (VMN) were produced in two groups of weanling male rats. Two groups of rats received lesions in the dorsomedial hypothalamic nuclei (DMN). Six days later, one of the VMN groups received DMN lesions (VMN and DMN), while VMN lesions were produced in one of the DMN groups (DMN and VMN). Sham-operated animals served as controls.The animals were maintained for 18 days during which time food intake was measured. On the 19th day they were decapitated and the following parameters determined: carcass fat, plasma insulin, glucose, triglyceride and cholesterol, and body weight and length. DMN lesions failed to attenuate the ‘principal’ parameters of the weanling rat ventromedial syndrome: hyperinsulinemia, hyperlipidemia and increased carcass fat. This was irrespective of the sequence of the two hypothalamic operations. Food intake, body weight and length proved amenable to DMN lesions only when DMN were produced prior to VMN lesions, but it is noteworthy that in these animals plasma insulin and lipid levels remained elevated despite a reduced food intake. Neither of the groups with double lesions had glucose values which were significantly different from those of the controls. The data point to a profound autonomy of the ventromedial hypothalamic nuclei and their related circuits in the development of the weanling rat VMN syndrome.     

Hypothalamus and Brown Fat: White and Brown Adipose Tissue Lipolysis in Weanling Rats with Dorsomedial Hypothalamic Lesions

Weanling male Sprague-Dawley rats received bilateral electrolytic lesions in the dorsomedial hypothalamic nuclei (DMN); sham-operated animals served as controls. The animals were fed lab chow and given tap water ad libitum. Fourteen days after the hypothalamic operation they were weighed and measured to assess ponderal and linear growth gains and Lee Index and were sacrificed on the following day. Body weight, body weight gain over two weeks, nose-tail length and gain in nose-tail length, and food intake were all highly significantly reduced in DMNL rats in comparison with controls. Lee Index and efficiency of food utilization were normal, however. Epididymal fat pads weighed less in both absolute and relative (percentage body weight) terms than in controls. Basal lipolysis was increased and epinephrine-stimulated lipolysis was decreased in DMNL rats, as was the protein content of the epididymal fat pads. Lipid content was normal, however. Interscapular brown adipose tissue (BAT) was significantly lighter in DMNL rats than in controls in absolute terms, but all other parameters measured were normal, as were plasma glucose, glycerol, and free fatty acids. Comparison with results from rats that received ventromedial hypothalamic lesions shortly after weaning indicates a differential effect in most epididymal fat pad parameters but similarities to changes in BAT. These data add to previous demonstrations of normal responses to homeostatic challenges in the growth-retarded, hypophagic-hypodipsic rat with lesions in the dorsomedial hypothalamic nucleus.     

Six-month follow-up in weanling rats with ventromedial and dorsomedial hypothalamic lesions: somatic, endocrine, and metabolic changes.

Thus far both the weanling rat ventromedial and dorsomedial syndrome have been investigated in only two and three-week studies. The present data are derived from a six-month study. Food intake and body weight and length changes were recorded weekly or bi-weekly and endocrine and metabolic parameters were assessed at sacrifice. Rats that had received ventromedial hypothalamic lesions shortly after weaning showed all the changes noted in previous short-term studies, i.e. normal body weight gains and food intake but increased carcass fat content. In addition, they manifested a slight hyperglycemia, normal free fatty acid levels and slowingdown of lipogenesis. Rats that had received dorsomedial lesions shortly after weaning also showed changes previously reported in short-term experiments. These include reduced ponderal and linear growth and food intake, but normal carcass composition and growth hormone levels. In addition, they showed a slight improvement of hypophagia with time, and at sacrifice hypoglycemia and low free fatty acid levels. Incorporation of glucose into fat pad glycogen and CO2 was decreased. In diaphragm, glucose incorporation was increased in both total lipid and fatty acids. The data demonstrate that both hypothalamic syndromes change with time, but not necessarily in a progressive manner. They also show that destruction of two so closely appositioned hypothalamic structures brings about entirely different and, in some parameters, opposite changes.     

Hypothalamic lesions in the weanling rat alter pancreatic response to glucose

Circulating levels of insulin and glucagon were monitored daily in weanling rats bearing bilateral radiofrequency lesions of the hypothalamic region comprising the ventral pole of the dorsomedial nucleus and at least one third of the dorsal pole of the ventromedial nucleus (V-DMH). Plasma insulin levels in the animals with lesions were significantly elevated by the eighth post-lesion day while plasma glucagon levels were significantly reduced by the 13th day. An intravenous glucose bolus administered to conscious unrestrained animals with lesions had no significant effect on circulating insulin levels but resulted in a dramatic increase in circulating glucagon levels. The IV glucose injections had no significant effect on circulating glucagon levels in the sham-lesioned and unoperated controls while the plasma insulin levels in both control groups were significantly elevated. After a glucose challenge in vitro (300 mg%), insulin release by islets from the lesioned animals showed only a slight increase whereas glucagon release was paradoxically increased. These results provide evidence for an abnormal glucose-sensing function of the pancreatic islet after hypothalamic lesions.     

Ventromedial and dorsomedial hypothalamic syndromes in the weanling rat: Is the “center” concept really outmoded?

This report juxtaposes findings from weanling rats with precise lesions in the ventromedial (VMNL rats) to data of weanling rats with lesions in the dorsomedial (DMNL) hypothalamic nuclei. Despite the proximity of the two nuclei their destruction produces opposite effects in most cases but similar responses in other parameters. Absolute and relative food intake are normal in VMNL rats yet they become obese in the face of normal body weight gains. DMNL rats show both reduced absolute food intake and body weight but normal relative food intake and body composition. Both VMNL and DMNL cause reduced linear growth and running wheel activity. DMNL rats defend their lower body weight set point against various challenges and maintain normal body composition. Organ growth in both absolute and relative terms is reduced in VMNL rats. In DMNL rats relative organ growth is normal. Pancreatic growth, protein/pancreas and content and concentrations of several pancreatic enzymes are normal in DMNL but reduced in VMNL rats. Mean 24-hour plasma growth hormone (GH) and corticosterone (B) levels are reduced and insulin levels are greatly elevated in VMNL rats; prolactin (PRL) levels are normal. In DMNL rats, GH, B, insulin and somatomedin activity are normal but PRL is elevated. Circadian rhythms of GH, insulin and triiodothyronine are normal in DMNL rats but B levels are disrupted, as they are in VMNL rats. Glucose incorporation and oxidation in adipose tissue of VMNL rats are enhanced in VMNL rats but normal in DMNL rats. Gluconeogenesis in VMNL rats is enhanced as early as 4 hours post-operatively; in DMNL rats it is normal at this time and several weeks thereafter. Basal lipolysis in epididymal fat pads is elevated in both VMNL and DMNL rats but epinephrine-stimulated lipolysis is elevated in VMNL and decreased in DMNL rats. Both VMNL and DMNL rats show normal basal and epinephrine-stimulated lipolysis in interscapular brown adipose tissue. Several hepatic enzymes are normal in DMNL and depressed in VMNL rats. The above data suggest that the DMN and its circuitry are part of an "organismic" set point system with a "true" body weight and no fat set point, as seems to be the case in the VMNL rat.     

Origin of endocrine-metabolic changes in the weanling rat ventromedial syndrome

Destruction of the ventromedial hypothalamic nuclei (VMN) in the weanling rat without injury to the median eminence results in a series of somatic, endocrine, and metabolic changes that are characterized by normal food and water intake but decreased linear growth, normal body weight but increased carcass fat and reduced carcass protein, lean body mass, and water. The endocrine alterations comprise hyerinsulinemia in the face of normoglycemia, hypertrigly ceridemia and hypercholesterolemia and reduced growth hormone levels. The metabolic changes include greater oxidation of glucose and incorporation into lipid and reduced palmitate oxidation but increased incorporation into lipid. Weanling rats with VMN lesions are normophagic in absolute terms, relative body weight and per metabolic unit, but their nocturnal feeding and weight gain cycles are disrupted and their locomotor activity is reduced. The VMN are involved in the long-term control of feeding – as in the mature rat – as shown by intragastric preloading studies and dietary density manipulations, glucose preference tests and intraperitoneal injections with glucose. Hyperinsulinemia and hypertrigly ceridemia are present four days after the VMN operation in the presence of subnormal food intake and plasma glucose levels. Manipulations of the fat content of the diet revealed that the hyperlipidemia is of both endogenous and exogenous origin and that lipoprotein lipase is increased; a 48-hour fast reduced the hyperlipidemia to control levels, however. This suggests that weanling VMN rat tissue may have an impaired ability to take up circulating lipid. An increased incorporation of glycerol into lipid may be due to induction of glycerokinase by hyperinsulinemia. Adipose tissue of weanling VMN rats showed neither depressed lipolysis nor diminished lipolytic activity per milligram of tissue protein. Glucose oxidation and incorporation into adipose tissue is increased in several tissues in vitro and these is enhanced glucose disappearance from plasma and incorporation into tissue lipids in vivo. These changes develop within a short time after lesion production and persist at least partially up to six months: glucose utilization in liver increases already four hours after the operation whereas it takes 72 hours to commence in adipose tissue. Insulin resistance is not apparent either in vivo or in vitro. The decreased growth hormone levels are not critical to the metabolic changes, nor is the hyperinsulinemia totally necessary. The metabolic changed also appear on several different types of diet and persist with fasting. The latter does not reduce insulin sensitivity of VMN rat tissues, whereas it does so in normal rats. Mature VMN rats develop the same metabolic changes even in the absence of hyperphagia. The metabolic alterations can be blocked by pharmacologic doses of glucocorticoids, but are enhanced by the administration of estrogen. Glucose transport across cell membranes does not appear to be a critical step in the enhanced glucose activity and increased glycolysis. However, pyruvate metabolism does not appear to be enhanced in weanling VMN rat tissue. Enhanced gluconeogenesis is suggested by increased ureagenesis, bicarbonate incorporation into plasma glucose and liver glycogen and greater alanine incorporation. The weanling rat ventromedial syndrome appears to be the result of a hypothalamic-autonomic nervous system disturbance rather than due to a disruption of ?classic”? neuroendocrine mechanisms.     

Neurons in the hypothalamic paraventricular nucleus mediate the serotonergic stimulation of renin secretion

The aim of the present study was to resolve which hypothalamic nucleus is necessary for the serotonergic control of renin secretion. RU 24969 is considered a serotonin (5-HT1A/5-HT1B) agonist, while p-chloroamphetamine is a 5-HT releaser. Both drugs reliably elevate plasma levels of renin when injected peripherally. Previous studies suggest that serotonergic neurons, projecting to the hypothalamus, mediate the effect of p-chloroamphetamine on renin secretion. Discrete cell-selective lesions were made with ibotenic acid in three hypothalamic sites: the paraventricular, the dorsomedial or the ventromedial nuclei. Two weeks after surgery rats were injected with RU 24969 (5 mg/kg, i.p.) or p-chloroamphetamine (8 mg/kg, i.p.). The renin response to both RU 24969 and p-chloroamphetamine was significantly reduced in rats with histologically verified paraventricular lesions compared to vehicle treated controls. In contrast, the renin response to p-chloroamphetamine remained unchanged in rats with either dorsomedial or ventromedial hypothalamic lesions. Thus, these results are consistent with the hypothesis that 5-HT receptors located on cell bodies in the paraventricular nucleus mediate the renin response to a serotonin agonist and releaser. Furthermore, they confirm previous studies that suggest that 5-HT neurons regulate renin secretion through central receptors.     

Localization in the ventromedial hypothalamic nuclei of an area affecting plasma triglyceride and cholesterol levels

Summary In three experiments exploratory lesions were placed in anterior, medial, dorsal and posterior hypothalamic areas of weanling female rats to determine the localization and the extent of the area affecting plasma triglyceride and cholesterol levels. In another experiment, lesions of various sizes were placed in the ventromedial hypothalamic area to define the size of the lesions necessary to elevate plasma triglyceride and cholesterol levels. Three or more weeks after the hypothalamic operation, trunk blood was obtained after decapitation. Only lesions destroying primarily the ventromedial hypothalamic nuclei resulted in significant elevations of both lipid fractions. Small lesions, confined to the medial area of the VMN in antero-posterior respect did not produce the above alterations, but with increasing lesion size significant positive correlations between lesion size and plasma lipid levels were evident. The data suggest that the ventromedial hypothalamic nuclei are a hypothalamic site which influences lipid metabolism.

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Zusammenfassung In drei Experimenten wurden elektrolytische Läsionen im vorderen bzw. medialen, dorsalen und hinteren Gebiet des Hypothalamus der weiblichen abgesetzten Ratte plaziert, um den Locus zu bestimmen, dessen Zerstörung Hypertriglyceridämie und Hypercholesterinämie verursacht. In einem weiteren Experiment wurden Läsionen verschiedener Größe im ventromedialen Hypothalamus plaziert, um die Ausdehnung der verantwortlichen Nervenmasse zu definieren. Nur Läsionen, welche hauptsächlich den ventromedialen hypothalamischen Kern zerstören, verursachen bedeutende Erhöhungen von Triglyceriden und Cholesterinspiegeln. Kleine Läsionen, welche auf das mittlere Gebiet des ventromedialen Kernes beschränkt sind, verursachen die erwähnten Erscheinungen nicht. Mit größer werdendem Läsionsherd werden die Änderungen der Plasmalipide aber größer. Die Ergebnisse weisen darauf hin, daß die ventromedialen hypothalamischen Kerne und ihr unmittelbares Grenzgebiet den Fettstoffwechsel beeinflussen.

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With 4 Figures     

The role of vagally-mediated hyperinsulinemia in hypothalamic obesity

Evidence that the obesity syndrome which follows ventromedial hypothalamic (VMH) lesions is at least partially the result of a primary metabolic dysfunction is reviewed, as are proposals that the altered metabolism is due to enhanced vagally-mediated insulin release. This hypothesis was based largely on experiments demonstrating the complete reversal of hypothalamic obesity by subdiaphragmatic vagotomy, but subsequent studies have revealed that hypothalamic obesity is not always prevented by prior vagal transections. Interpretation of these discrepant results has been made difficult because of the frequent use of gastric secretion, behavioral, or other indirect tests for completeness of vagotomy. A review of more recent studies which have employed either direct assessment of vagotomy effects on insulin levels, pharmacological blockade of vagal efferent activity, or selective vagotomies indicates that vagally-mediated hyperinsulinemia can account for no more than 40% of the weight gain observed in animals with VMH lesions fed ad libitum, and may not be involved in the obesity that results from some parasagittal VMH knife cuts. It is concluded that vagally-mediated hyperinsulinemia does make a substantial, although not exclusive, contribution to the increased carcass lipid content observed in VMH animals that are food-restricted or pair-fed with control animals.     

Intracerebroventricular injection of insulin or glucose alters insulin-like growth factor II (IGF-II) concentrations in specific hypothalamic nuclei

Peripherally administered insulin has been shown to alter content and gene expression of hypothalamic insulin-like growth factor II (IGF-II) in a region specific manner (Lauterio, TJ. et al., Endocrinology, 126 (1990) 392-398. The objective of this experiment was to determine whether central administration of insulin can modulate hypothalamic IGF-II peptide content. Male Sprague-Dawley rats were implanted with lateral ventricular cannulae and allowed to recover from surgery one week prior to injection. At that point animals were remotely injected with one of the following: (1) synthetic cerebral spinal fluid vehicle (sCSF); (2) 2 mIU porcine insulin (I); (3) glucose (100 mg%) only. Animals were decapitated 30 min after injections and brains were quickly removed, frozen and dissected into specific hypothalamic regions for IGF-II analysis by RIA. Insulin increased IGF-II content in the ventromedial hypothalamic region by 80% (P < 0.001) and paraventricular nucleus by 30% (P < 0.01) compared to sCSF or glucose treatment. Arcuate nucleus and neurointermediary lobe pituitary IGF-II content was decreased with insulin treatment compared to controls (P < 0.01). Insulin had no effect on IGF-II concentrations in the dorsomedial or lateral hypothalamic regions or in the supraoptic and suprachiasmatic nuclei. Peripheral concentrations of glucose, insulin and IGF-II were unaffected by any treatment. Results show that insulin which reaches the brain can alter IGF-II levels in specific regions of the hypothalamus and suggests a possible role for IGF-II in insulin mediated changes in metabolism or hypothalamic hormone secretion.     

Central and peripheral immunoreactivity of melanin-concentrating hormone in hypothalamic obese and lactating rats

Melanin-concentrating hormone (MCH) is believed to be an important orexigenic peptide mainly localized in the lateral hypothalamic area. Its involvement in the hyperphagia induced by hypothalamic lesions and lactation remains unclear. In this study, we investigated MCH immunoreactivity in the hypothalamus using immunohistochemistry and MCH concentration in the peripheral circulation using an enzyme immunoassay in rats with a lesion in the ventromedial hypothalamus or the paraventricular nucleus, and in lactating rats. Bilateral lesions of the ventromedial or paraventricular nuclei were performed using an electrolytic method. Quantification of immunoreactivity by image analysis revealed that the number and mean staining intensity of MCH-immunoreactive neurones in the lateral hypothalamic area and the zona incerta were significantly decreased by both types of lesions compared to sham controls, whereas circulating MCH concentration was not significantly different on day 7 postlesion. By contrast, in lactating rats on days 11-12 postpartum, the expression of MCH in the lateral hypothalamic area and the zona incerta was significantly increased compared to nonlactating controls. Circulating MCH concentration was not changed in lactating rats. These results suggest that hyperphagia induced by lactation, but not hypothalamic lesion, might be induced by excessive expression of MCH in the lateral hypothalamic area and the zona incerta.     

Comparison of the metabolic and behavioral disturbances following paraventricular- and ventromedial-hypothalamic lesions

Lesions of the ventromedial hypothalamus (VMH) result in an obesity syndrome with several metabolic and behavioral manifestations. It has also been reported that damage to the paraventricular hypothalamus (PVH) leads to changes characteristic of obesity. However, little is known about the consequence of PVH lesions, especially in contrast to the extensive documentation of VMH lesion-induced effects. To assess the basic features of the two hypothalamic obesity syndromes, rats underwent VMH, PVH, or sham lesions and, for 15 weeks, were maintained ad lib on a series of test diets. Both lesions groups were hyperphagic and showed similar weight gains. Although both lesion groups became obese (measured by % carcass fat), VMH rats were fatter than PVH animals. Similarly, only VMH rats were hyperinsulinemic. Further tests were conducted in PVH and VMH rats restricted to control body weights. VMH, but not PVH, rats developed a persisting elevation in basal gastric acid secretion. As well, only VMH, and not PVH, animals developed an obesity when restricted to normal weights. These data indicate similarities in PVH and VMH rats maintained ad lib but experiments on restricted animals reveal fundamental differences in the two obesities and point to different etiologies.     

Increased Prolactin Receptor Immunoreactivity in the Hypothalamus of Lactating Rats

Prolactin (PRL) exerts numerous effects in the brain including induction of maternal behaviour, increased food intake, and inhibition of GnRH secretion. Knowledge about the distribution of PRL receptors (PRL-R) in the brain will be critical for investigating mechanisms of PRL-brain interactions during lactation. The present study aimed to investigate the distribution of PRL-R in specific hypothalamic nuclei of lactating rats by immunohistochemistry and to compare this distribution with that in dioestrous rats. Rats were perfused with 2% paraformaldehyde and brains were cut into coronal sections (18 microm) for immunostaining. Immunoreactivity was detected by the avidin biotin complex method using mouse monoclonal antibody U5. In dioestrous rats, PRL-R immunoreactivity was observed in the choroid plexus, three hypothalamic nuclei: medial preoptic, periventricular and arcuate, and in the median eminence. The number of labelled profiles per section in the medial preoptic and arcuate nuclei increased significantly (P<0.05) in lactating rats (days 7-10 to post partum) when compared with dioestrous rats. Furthermore, in lactating rats, PRL-R immunoreactive neurons were identified in the cerebral cortex, substantia nigra and numerous additional hypothalamic nuclei including the ventromedial preoptic, ventrolateral preoptic, lateroanterior hypothalamic, ventrolateral hypothalamic, paraventricular hypothalamic, supraoptic, suprachiasmatic, and ventromedial hypothalamic nuclei. These observations assist our understanding of the multiple sites of PRL effects on brain function during lactation.     

Pair-feeding of sham-operated controls to rats with dorsomedial hypothalamic lesions: new evidence for an 'organismic' set point

Weanling male Sprague-Dawley rats received electrolytic lesions in the dorsomedial hypothalamic nuclei (DMNL rats) and sham operations, respectively, and were maintained on lab chow ad libitum for 23 days (Expt. 1) and 55 days (Expt. 2). Following this period, one group of sham-operated controls was pair-fed to the DMNL rats, which were hypophagic compared to a second sham-operated group that was fed ad libitum. This period lasted 27 days (Expt. 1) and 24 days (Expt. 2). Although pair-fed controls were heavier than DMNL rats, they showed normal food utilization (EFU) in Expt. 1 and reduced EFU in Expt. 2. Pair-fed controls also had decreased carcass fat, hypoglycemia, hypoinsulinemia, hypotriglyceridemia and enhanced in vitro epididymal fat pad glucose oxidation when compared to DMNL rats. These data suggest that pair-fed controls cannot cope as efficiently with the reduced influx of substrate as do DMNL rats. Evidently, the reduced amount of food that DMNL rats spontaneously eat is adequate for their reduced body size, whereas this is not the case in the pair-fed controls. This is taken as evidence for a harmonious readjustment by DMN lesions of both body size and food intake and the underlying homeostatic processes and supports our concept of an 'organismic' set point that has been released by the DMN lesions.     

Increases in melanin-concentrating hormone and MCH receptor levels in the hypothalamus of dietary-obese rats

Melanin-concentrating hormone (MCH) is a hypothalamic neuropeptide that stimulates feeding and increases body weight in rodents. We studied the role of the system in energy homeostasis and its regulation by the satiety signals, leptin and insulin. We used real-time PCR to measure the hypothalamic expression of MCH and its receptor (MCHR1) in two contrasting models of altered nutritional status, namely, obesity induced by 8 weeks' voluntary overeating and food restriction for 10 days. Diet-fed rats were stratified according to final total fat-pad mass into a 'high fat gain' group (HG) and 'low fat gain' group (LG). MCH mRNA levels were increased by 31% (p>0.05) and 49% (p<0.05) in the LG and HG, respectively, compared with controls. MCHR1 mRNA levels rose by 118% in the LG (p<0.01) and 85% in the HG (p<0.01). There were significant positive correlations (p<0.05) between plasma leptin concentration and both MCH and MCHR1 mRNA levels, and between plasma insulin and MCHR1 expression. A positive correlation was also observed between MCH and MCHR1 mRNA levels (p<0.05). Food-restricted rats showed no significant alterations in the levels of either MCH mRNA or MCHR1 mRNA. In a second experiment, we measured MCH peptide levels in five discrete hypothalamic areas of dietary-obese rats. MCH concentrations were significantly increased in the arcuate nuclei of the HG (p<0.05) and the paraventricular nuclei of both the LG (p<0.05) and HG (p<0.05), compared with their lean counterparts. These results suggest that the MCH system becomes more active in dietary obesity and could be involved in enhancing appetite for palatable food. The possibility that MCH and MCHR1 expression are positively regulated by leptin and insulin, which normally inhibit feeding, is a putative explanation for how appetite for palatable food is able to override mechanisms that prevent the development of obesity.     

Effects of insulin on local cerebral glucose utilization in the rat

The effects of hyperinsulinemia on local cerebral glucose utilization were studied by the quantitative autoradiographic 2-[14C]deoxyglucose method in normal conscious rats under steady-state normoglycemic conditions. Hyperinsulinemia and a steady state of normoglycemia were achieved and maintained during the experimental period by a continuous intravenous (i.v.) infusion of insulin given simultaneously with a programmed i.v. infusion of D-glucose. Hyperinsulinemia under normoglycemic conditions did not change the average rate of glucose utilization in the brain as a whole, but significant increases in local glucose utilization were found selectively in the ventromedial, dorsomedial, and anterior hypothalamic nuclei. The results suggest that a known anatomical pathway linking the dorsomedial and anterior nuclei with the ventromedial nucleus of the hypothalamus may be physiologically activated in response to hyperinsulinemia.     

Effects of hypothalamic lesion on pancreatic autonomic nerve activity in the rat

The effects of hypothalamic lesions and intravenous glucose infusion on the efferent activity of vagal and splanchnic nerves to the pancreas were studied in anesthetized rats. Lesions of the ventromedial hypothalamic (VMH), the dorsomedial hypothalamic (DMH) and the paraventricular (PVN) nuclei increased vagal and reduced splanchnic nerve activity. Lesion of the lateral hypothalamic area (LHA) decreased pancreatic vagal nerve activity, and produced either increased or decreased activity of pancreatic splanchnic nerve. Intravenous glucose infusion increased activity of the vagal nerve and reduced that of the splanchnic nerve. These glucose responses were influenced by hypothalamic lesions only slightly or not at all. The findings suggest that hypothalamic modulation of pancreatic hormone secretion involves both the parasympathetic and sympathetic nervous systems, and provide evidence that not only the VMH and the LHA but also the DMH and the PVN are involved in this mechanism.     

Reduction of insulin binding in the arcuate nucleus of the rat hypothalamus after 6-hydroxydopamine treatment

Insulin receptors are present in the hypothalamus, but the cell types bearing them are unknown. In order to test the hypothesis that some insulin receptors in the hypothalamus are associated with catecholamine terminals, rats were injected with 50 μg or 75 μg doses (intracerebroventricular) of 6-hydroxydopamine (6-OHDA). Control rats received vehicle only. The animals were sacrificed 7 days after injection, and catecholamine and indolamine levels in the hypothalamus were measured by high performance liquid chromatography with electrochemical detection. Localization of specific binding sites for [¹²⁵I]-insulin in the arcuate (ARC), dorsomedial (DMN) and ventromedial (VMN) nuclei were determined by quantitative film autoradiography. Treatment with 6-OHDA resulted in a 70% reduction in hypothalamic norepinephrine content as compared to vehicle-treated controls (P < 0.01). A slight depletion of epinephrine, dopamine and indolamines was also detected. Computerized image analysis of the autoradiograms was used to determine radioactivity bound (DPM/mm²) in each nucleus. Highest binding was in the ARC and DMN, with much lower binding in the VMN. Insulin binding in the ARC of the 6-OHDA-treated group was decreased by 25% compared to controls (P < 0.01). No significant change in insulin binding was observed in the DMN or VMN. The 6-OHDA treatment had no significant effect on weight gain or on plasma insulin levels. The reduction of insulin binding in the ARC after 6-OHDA treatment supports the hypothesis that some insulin binding sites are located on catecholamine terminals in the arcuate nucleus.     

Elevated content and secretion of gut GLI in VMH-lesioned rats.

Gut glucagon-like immunoreactivity (GLI), a supposed intestinal growth factor in the plasma and gastrointestinal tract, and weight of the gastrointestinal tract, an intestinal growth indicator, were examined in ventromedial hypothalamic (VMH)-lesioned rats 1 week after VMH lesions. Postprandial plasma gut GLI in VMH-lesioned rats was significantly higher than that in control rats. The content of gut GLI in all gastrointestinal sections except the duodenum in VMH-lesioned rats was significantly greater than that in control rats. Gel chromatography in the lower portion of the small intestine in which GLI content was the highest of all sections revealed the same pattern in both VMH-lesioned and control rats with two peaks of similar molecular size. Weight of all gastrointestinal sections except the cecum in VMH-lesioned rats significantly increased. These results demonstrated that both gut GLI secretion and production were enhanced in VMH-lesioned rats. The elevated gut GLI release may accelerate growth of the gastrointestinal tract in these animals.     

Zucker obese rats are insensitive to the CRH-increasing effect of oleoyl-estrone

Adult female Zucker lean and obese rats were treated for 14 days with 3.5 nm/kg oleoyl-estrone (OE) in liposomes (Merlin-2) through continuous i.v. injection with osmotic minipumps. Rat wt. and food intake were measured daily. On days 0, 3, 6, 10, and 14, groups of rats were killed and their hypothalamic nuclei [lateral preoptic (LPO), median preoptic (MPO), paraventricular (PVN), ventromedial (VMH), and arcuate (ARC)] were dissected, homogenized, and used for the measurement of corticosterone-releasing hormone (CRH) by radioimmunoassay. The OE treatment decreased food intake by 67.4% in lean and 62.6% in obese rats (means for 14 days). Body wt. decreased steadily in lean and obese rats, the gap between controls and treated rats becoming 11.5% of initial body wt. in the lean and 12.4% in the obese. The levels of CRH in the ARC nucleus were at least 10-fold higher than in the other nuclei. No changes in CRH were observed in any of the nuclei of obese rats, with levels up to day 6 similar to those of lean rats. In the lean rats, the LPO and ARC nuclei showed peaks on day 10, while the MPO showed no changes and the PVN and VMH nuclei showed a progressive increase, to a maximum at the end of the study (day 14). This contrasted with the peak of plasma adrenocorticotropic hormone (ACTH) and corticosterone (day 6 in lean and day 14 in obese rats). There was a definite lack of correlation between the plasma levels of these two hormones and the levels of CRH in the hypothalamic nuclei, and between the latter and the decreases in appetite in the rats. The loss of appetite induced by OE is not necessarily mediated by CRH, because the obese rats show an intense decrease in voluntary food intake but their hypothalamic nuclei CRH levels do not change at all. Hypothalamic nuclei CRH does not, necessarily, mediate the rise in glucocorticoids induced by OE treatment, because this is observed in lean and obese rats, lean rats increases being mismatched with those of hypothalamic CRH. The OE induced changes in hypothalamic CRH require a fully functional leptinergic pathway, because it is not observed in Zucker fa/fa rats lacking a working leptin receptor. This–indirectly–shows that leptin is needed for its synthesis or modulation.