Hypothalamic control of liver glycogen metabolism in adult and aged rats

Effect of slaughter age and postmortem aging time on tenderness and water-holding capacity of yak (Bos grunniens) longissimus thoracis muscle

The present study investigated the effect of slaughter age (2.43 ± 0.20, 4.15 ± 0.19, 6.62 ± 0.18, 10.59 ± 0.74 years) and postmortem aging time (1, 24, and 72 h) on the tenderness and water-holding capacity (WHC) of yak longissimus thoracis muscles to determine the most suitable age for slaughter to ensure product consistency. Under conventional postmortem aging conditions (4 °C), muscles of each age group exhibited the effect of cold shortening. Once the cold shortening occurred, the age effect on thickening muscle fiber and developing cross-links of collagen, considered to intensify the meat toughness, became less important. Owing to greater carcass weight and intramuscular fat, muscles of the older carcass (over 6-year-old) were less influenced by the cold shortening effect during the chilling process and showed lessened sarcomere contraction, delayed formation of drip loss channels, and increased level of myofibril fragmentation index (MFI) and myofiber structural disintegration, resulting in greater tenderness and WHC, especially 6-7 years group. Aging of 72 h structurally disintegrated the collagen cross-linking and integrity of muscle fibers and elevated the MFI, improving the meat tenderness. Therefore, the suitable slaughter age for yak is 6-7 years old and after 72 h aging, improved quality of yak meat can be obtained.

Ventromedial Nucleus of the Hypothalamus Neurons Under the Magnifying Glass

The ventromedial nucleus of the hypothalamus (VMH) is a complex brain structure that is integral to many neuroendocrine functions, including glucose regulation, thermogenesis, and appetitive, social, and sexual behaviors. As such, it is of little surprise that the nucleus is under intensive investigation to decipher the mechanisms which underlie these diverse roles. Developments in genetic and investigative tools, for example the targeting of steroidogenic factor-1-expressing neurons, have allowed us to take a closer look at the VMH, its connections, and how it affects competing behaviors. In the current review, we aim to integrate recent findings into the literature and contemplate the conclusions that can be drawn.

Refeeding after caloric restriction reverses altered liver glucose release

CONTEXT

Caloric restriction increases liver glucose release (LGR), but it is not known if this is a permanent condition.

OBJECTIVE

To investigate if refeeding after caloric restriction reverses the high LGR.

MATERIALS AND METHODS

Rats were organised in six-pups litters (GC); 12-pups litters with either 50% caloric restriction from 21 to 80 days of age (GR) or fed at will from 50 to 80 days of age (GRL). Liver perfusion was made at the age of 80 days.

RESULTS

LGR was higher in the GR both during basal and adrenaline-stimulated conditions. Refeeding after caloric restriction decreased it to values close to those of GC rats.

DISCUSSION

The altered LGR of GR rats was reversed by refeeding (group GRL). The influence of hypothalamic neuropetides on these hepatic changes is suggested.

CONCLUSIONS

Enhanced LGR under caloric restriction is not programmed by early feeding; instead, it is determined by the current nutritional conditions.

Hypothalamic responses to long-chain fatty acids are nutritionally regulated

Central administration of the long-chain fatty acid oleic acid inhibits food intake and glucose production in rats. Here we examined whether short term changes in nutrient availability can modulate these metabolic and behavioral effects of oleic acid. Rats were divided in three groups receiving a highly palatable energy-dense diet at increasing daily caloric levels (below, similar, or above the average of rats fed standard chow). Following 3 days on the assigned diet regimen, rats were tested for acute biological responses to the infusion of oleic acid in the third cerebral ventricle. Three days of overfeeding virtually obliterated the metabolic and anorectic effects of the central administration of oleic acid. Furthermore, the infusion of oleic acid in the third cerebral ventricle failed to decrease the expression of neuropeptide Y in the hypothalamus and of glucose-6-phosphatase in the liver following short term overfeeding. The lack of hypothalamic responses to oleic acid following short term overfeeding is likely to contribute to the rapid onset of weight gain and hepatic insulin resistance in this animal model.

Effect of chemical stimulation of the dorsomedial hypothalamic nucleus on blood plasma glucose, triglycerides and free fatty acids in rats

The effects of chemical stimulation of the dorsomedial hypothalamic nucleus (DMH) on blood plasma concentration of glucose, triglycerides, insulin, and free fatty acids (FFA) were investigated in anesthetized adult Wistar rats. Microinjection of 12.5 nmol of norepinephrine into the DMH increased blood plasma concentration of glucose and FFA, decreased triglycerides, and did not change plasma insulin within 5 min; after 20 min, blood glucose and FFA reached control values. Microinjection of epinephrine (12.5 nmol) into the DMH also increased blood plasma glucose concentration and decreased triglycerides after 5 min. These effects are probably mediated by beta-adrenergic mechanisms, because they were prevented by beta-adrenergic antagonist propranolol, but not by alpha-adrenergic antagonist prazosin. Microinjection into the DMH of glutamate, dopamine, or acetylcholine failed to cause any change in those metabolic parameters, corroborating the hypothesis that the DMH is part of a beta-adrenergic pathway involved in short-term modulation of the availability of glucose and FFA.

The effect of a ventral medial hypothalamic lesion on the insulin-induced hypotensive response in normal rats

The cardiovascular responses to insulin-induced hypoglycemia were studied in normal and ventral medial hypothalamic (VMH)-lesioned rats. The goal of this study was to investigate the role of the VMH in mediating the insulin-induced decreases in cardiovascular tone. Male Wistar rats were anesthetized with urethane/chloralose. Following the induction of anesthesia, the trachea, femoral artery, and femoral vein were cannulated. The femoral artery was attached to a pressure transducer for cardiovascular monitoring. The cardiovascular activity was recorded using a Modular Instruments Micro 5000 signal processing system. The mean arterial pressure and pulse pressures and heart rate were evaluated. In control studies, a stable plasma glucose and blood pressure were obtained with urethane/chloralose anesthesia for the duration of the experiments. Insulin (2.0 or 5.0 U/kg) significantly decreased the plasma glucose as well as the blood pressure. In VMH-lesioned rats, the lesions were accomplished by radiofrequency, and the cardiovascular response to insulin-induced hypoglycemia was investigated 1 or 6 weeks later. There was no difference in the cardiovascular response to insulin-induced hypoglycemia between the low or high insulin dose after 1 week in VMH-lesioned animals. The low dose after 6 weeks in VMH-lesioned animals did not produce a change in the mean arterial pressure response compared with controls. The pulse pressure was higher than in the sham-lesioned animals, and the plasma glucose response was greater.(ABSTRACT TRUNCATED AT 250 WORDS)

Ventromedial hypothalamic lesions in rats suppress counterregulatory responses to hypoglycemia

The central nervous system has been implicated in the activation of counterregulatory hormone release during hypoglycemia. However, the precise loci involved are not established. To determine the role of the ventromedial hypoglycemia, we performed hypoglycemic clamp studies in conscious Sprague-Dawley rats with bilateral VMH lesions produced by local ibotenic acid injection 2 wk earlier. Rats with lesions in the lateral hypothalamic area, frontal lobe, sham operated (stereotaxic needle placement into hypothalamus without injection), and naive animals served as control groups. The clamp study had two phases. For the first hour plasma glucose was fixed by a variable glucose infusion at euglycemia (approximately 5.9 mM). Thereafter, for an additional 90 min, glucose was either allowed to fall to (a) mild hypoglycemia (approximately 3.0 mM) or (b) more severe hypoglycemia (approximately 2.5 mM). Glucagon and catecholamine responses of lateral hypothalamic area-, frontal lobe-lesioned, sham operated, and naive animals were virtually identical at each hypoglycemic plateau. In contrast, glucagon, epinephrine, and norepinephrine responses in the VMH-lesioned rats were markedly inhibited; hormones were diminished by 50-60% during mild and by 75-80% during severe hypoglycemia as compared with the other groups. We conclude that the VMH plays a crucial role in triggering the release of glucagon and catecholamines during hypoglycemia.

Primacy of liver glucosensors in the sympathetic response to progressive hypoglycemia

The impact of hepatic glucose concentration on the sympathetic response to progressive hypoglycemia was examined in chronically cannulated conscious male dogs (n = 6). Graded hypoglycemia was induced via peripheral insulin infusion (30 pmol.kg-1.min-1) with either peripheral (PER) or portal (POR) glucose infusion. Over the 260-min experimental period, arterial glycemia was adjusted from 5.2 +/- 0.1 to 2.5 +/- 0.1 mM in decrements of approximately 0.5 mM every 40 min. Arterial glycemias were not significantly different between PER and POR at any measured level. However, hepatic glycemia was significantly elevated at all times during POR (8.4 +/- 0.8 to 3.4 +/- 0.2 mM) when compared to PER (5.2 +/- 0.2 to 2.5 +/- 0.1 mM). Plasma epinephrine values were significantly greater during PER vs. POR at all arterial glycemias below 4.0 mM. At the lowest level of arterial glycemia studied (2.5 +/- 0.2 mM) the epinephrine response above basal was 3-fold greater for PER (8.7 +/- 1.7 nM) when compared to POR (2.6 +/- 0.6 nM) (P < 0.01). Plasma norepinephrine results were similar for the two protocols, with PER demonstrating a 3-fold greater response above basal when compared to POR at 2.5 mM arterial glycemia (P < 0.05). While the sympathetic response was markedly different between protocols when expressed as a function of arterial glycemia, when expressed as a function of hepatic glycemia this discrepancy was largely eliminated. This latter observation supports the liver as the primary locus for glycemic detection relevant to the sympathoadrenal response when hypoglycemia develops slowly--i.e., over a period of 2-3 h. A comparison of the current findings with our previous observations suggests that the hepatic glucosensors may play a greater role in hypoglycemic counterregulation as the rate of fall in glycemia is less.

Increased beta-endorphin and dynorphin concentrations in discrete hypothalamic regions of genetically obese (ob/ob) mice

Disturbances in hypothalamic beta-endorphin and dynorphin levels were investigated in non-fasted genetically obese (ob/ob) and homozygous lean mice at 14-15 weeks of age. Eight brain regions were microdissected from fresh, unfixed brain slices, and opioid peptide concentrations were determined in tissue micropunches by radioimmunoassay. A two-fold and five-fold increase in beta-endorphin levels in ob/ob versus lean mice were found in the ventromedial and dorsomedial hypothalamic nuclei respectively. Dynorphin levels were comparable between ob/ob and lean mice in the anterior, lateral, ventromedial and paraventricular hypothalamic areas, but a 5-fold increase in dynorphin concentrations was detected in the dorsomedial hypothalamic nucleus of the ob/ob mouse. These results demonstrate that increased concentrations of beta-endorphin and dynorphin occur in discrete hypothalamic nuclei, which are known to influence food intake and glucose homeostasis. This could signify an important central defect contributing to hyperphagia and glucoregulatory dysfunction in obese mice.

Search for the hypoglycemia receptor using the local irrigation approach

To elucidate the loci for the putative glucoreceptors responding to hypoglycemia we introduced 'brain' and 'liver' clamps. Systemic hypoglycemia was induced by insulin infusion while the area of interest (ie. forebrain, hindbrain, portal-hepatic region) was maintained euglycemic via local glucose irrigation. Utilizing this approach, there appear to be no glucoreceptors residing exclusively in either the forebrain or hindbrain which are essential for the sympathoadrenal response to hypoglycemia. This is true for both moderate and severe hypoglycemic conditions. The possibility of a redundant glucoreceptor system within the brain, as suggested by a subsequent study, remains to be confirmed. The portal-hepatic glucoreceptors appear essential to engendering the full counterregulatory response. Establishing euglycemia across the portal-hepatic region inhibits the sympathoadrenal response to moderate hypoglycemia by over 40%. Further, despite prevailing hypoglycemia and significant elevations in counter-regulatory hormones, the liver demonstrated net glucose extraction during the liver clamp, suggestive of overriding neural input to the liver. Thus, the hepatic afferents appear to be very important for the counterregulatory response to hypoglycemia.

Liver phosphorylase activation during early phase of feeding behavior: a neurohumoral regulation

The mechanisms of liver glycogen depletion during the early phase of spontaneous feeding were examined in adult rats. A 40% increase in the activity of phosphorylase a, the active form of a key glycogenolytic enzyme, was detected 5 min after the onset of feeding, with a concomitant decrease of glycogen in the liver after 5 and 15 min. Within 60 min after the onset of feeding, the enzyme activity returned to the basal level and the glycogen content was restored. These metabolic responses were impaired by hepatic sympathetic denervation or by bilateral adrenodemedullation. It was concluded that the onset of meals may evoke an arousal of the sympathoadrenal system including the hepatic innervation, and that activation of this system is important in stimulating liver phosphorylase activity and glycogenolysis during the early phase of spontaneous feeding.

Chronic infusion of norepinephrine into the ventromedial hypothalamus induces obesity in rats

To examine whether chronic increase in the local concentration of certain neurotransmitters in the hypothalamus might lead to some metabolic disorders in peripheral organs, several neurotransmitters and neuropeptides were infused continuously at a rate of 0.48 microliter/h for up to 20 weeks into the unilateral ventromedial (VMH), lateral or paraventricular hypothalamic nucleus of rats. A massive obesity with marked increase in body weight was observed after about 10 weeks when norepinephrine (NE) was infused chronically into the VMH, but not into other hypothalamic loci. Plasma insulin level went up markedly at the early stage of NE infusion and remained high during the infusion period, but plasma glucose did not change appreciably. Similar, but less pronounced effects were observed with epinephrine infusion into the VMH. However, other neurotransmitters and neuropeptides tested had no such effects. The NE-induced obese rats showed hyperphagia and an arrhythmic feeding pattern with increased daytime feeding. Chemical composition and the rate of fatty acid synthesis in the interscapular brown adipose tissue (BAT) were also altered after NE infusion into the VMH: although the triglyceride content was increased, the rate of fatty acid synthesis was decreased, indicating that triglyceride turnover in BAT was greatly reduced. The latter observation suggests that energy dissipation in BAT is impaired and this functional abnormality in BAT may also contribute to the development or maintenance of a new type of hypothalamic obesity induced by NE infusion.

Vasopressin excites ventromedial hypothalamic glucose-responsive neurons in vitro

Effects of vasopressin (AVP), oxytocin (OXY), norepinephrine (NE), and glucose on the single-unit activity of hypothalamic ventromedial nucleus (VMN) in tissue slices were studied. While AVP was exclusively excitatory on 58% of the neurons, OXY could be excitatory or inhibitory and affected only 42% of the neurons. There was no correlation between the responses to these two peptides. Each of these two peptides could desensitize neuronal response to itself, but did not cross-desensitize responses to each other. These results indicate that AVP and OXY do not act on the same population of VMN neurons through the same cellular mechanism. Furthermore, only the responses to AVP were correlated to responses to glucose and NE, two agents relevant to central regulation of feeding. This correlation with responses to feeding-relevant agents and the exclusively excitatory action on the VMN, which is involved in the regulation of feeding, suggest that AVP can play a role in the regulation of feeding, particularly the feeding induced by the injection of NE into the paraventricular nucleus, that is known to alter AVP release.

Central glucagon-induced hyperglycemia is mediated by combined activation of the adrenal medulla and sympathetic nerve endings

Intracerebroventricular (ICV) microinjection of glucagon (0.0025-2.5 micrograms) produced significant dose-dependent hyperglycemia in mice. This hyperglycemic effect was prevented by pretreatment with the sympathetic ganglionic blocker chlorisondamine chloride or bilateral adrenalectomy plus chemical sympathectomy with 6-hydroxydopamine. Similar pretreatments had no effect on the plasma glucose responses to systemic glucagon administration. Pretreatment with somatostatin, which blocks pancreatic glucagon secretion had no effect on the hyperglycemic response to central glucagon administration. The results suggest that the increase in plasma glucose following central glucagon administration is mediated by combined action of adrenal and sympathetic amines to stimulate hepatic glucose production, or additionally to inhibit insulin release from the pancreas. The possible involvement of glucagon in the central nervous system in systemic glucoregulation is discussed.

Actions of feeding-relevant agents on hypothalamic glucose-responsive neurons in vitro

Effects of three feeding-relevant agents on the single-unit activity of the hypothalamic ventromedial nucleus (VMN) were investigated with tissue slice preparations. Changes in ambient glucose concentrations altered the activity of 39% of the 129 VMN neurons recorded. The predominant effect of glucose was facilitation. Norepinephrine (NE) affected all of the 29 glucose-responsive neurons tested, but only some (33/47) of the neurons unresponsive to glucose. Unlike that of glucose, the effects of NE on neuronal activity were distributed across excitatory, inhibitory, or biphasic responses. In vivo estrogen treatment did not affect neuronal responses to glucose, it modulated the type of neuronal responses to NE from glucose-responsive, but not glucose-unresponsive neurons. These results indicate that glucose, NE, and estrogen act on a common, glucose-responsive, population of VMN neurons but, as judged from their effects on neuronal activity, through different cellular mechanisms. Accordingly, glucose-responsive neurons may serve to integrate information mediated by different feeding-relevant agents.

Carbohydrate metabolism and food intake in food-restricted rats. Relationship between the metabolic events during the meal and the degree of food intake

To study some metabolic features during feeding in food-restricted rats two groups of animals were maintained on a 2 hr feeding/22 hr fast schedule. Group D (n = 38) received a meal every day from 8:00 to 10:00 a.m. Group N (n = 34) was given the meal from 8:00 to 10:00 p.m. The average total amount of food ingested by rats of group N in the two hour period was 6.3 +/- 0.4 whereas Group D ingested 4.8 +/- 0.3 g/100 g b.w. The metabolic pattern also was different in one group as to the other. The basal liver glycogen content when feeding started was considerably lower in the nocturnal group (0.14 +/- 0.02 mg/100 mg of liver tissue) than in the diurnal group (0.44 +/- 0.10 mg/100 mg). Afterwards glycogen increased in both groups but more steeply and intensely in group N. Glycemia increased in group D and was almost invariant in group N. Insulinemia went up in both groups but in group D its peak was higher and occurred 60 minutes after the onset of feeding whereas the peak in group N was much lower and occurred at 90 minutes. There was a clear dissociation between the time courses of insulinemia and glycemia in both groups, especially in group N, which suggests a central control of insulin secretion during feeding that partially unlocks it from blood glucose concentration. The hepatic glycogen content was partially linked to the amount of food ingested but again there was a dissociation between these two variables, inasmuch as a higher glycogen replenishment in the nocturnal group corresponded to a larger food intake.

Circulating free fatty acids, insulin, and glucose during chemical stimulation of hypothalamus in rats

The aim of this study was to investigate plasma free fatty acids (FFA), insulin, and blood glucose during chemical stimulation of the lateral and ventromedial hypothalamic areas (LHA and VMH) in rats. Therefore male Wistar rats were implanted with bilateral cannulas in the LHA or the VMH and into the left and right jugular veins. Freely moving rats were then infused into the LHA and VMH with norepinephrine (NE), epinephrine (E), or acetylcholine or intravenously with NE or E. Before, during, and after the infusions, simultaneous blood samples were taken without disturbing the animals. Infusion of NE into the LHA resulted in a decrease of plasma FFA and a simultaneous increase of insulin. NE infusion in the VMH elicited an increase of plasma FFA, plasma insulin, and blood glucose. E infusion into the LHA did not lead to a change of plasma FFA, whereas insulin and glucose showed an increase. E infusion into the VMH evoked increases of plasma FFA and insulin. Peripheral administration of NE led to a sharp increase of FFA, whereas plasma insulin and blood glucose did not change. E in the periphery elicited an augmentation of plasma FFA and blood glucose and a suppression of insulin during infusion. After termination of E infusion, plasma FFA and glucose levels decreased, whereas plasma insulin showed a sharp increase.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of age on glycogen synthase and phosphorylase activities in rat liver

The activities of glycogen synthase and phosphorylase were determined in homogenates and subcellular fractions of liver from young adult (6 months' old), adult (12 months' old) and aged (24 months' old) rats. The specific activities (enzyme activity expressed as units per mg protein) of the active form of glycogen synthase (synthase a) as well as of total synthase (synthase a + b) were significantly lower in liver homogenates of aged compared to young adult or adult rats. The age-associated decrease in the specific activity of synthase a was most marked in the 10 000 g and 100 000 g particulate fractions of liver. The specific activities of the active form of phosphorylase (phosphorylase a) as well as of total phosphorylase (phosphorylase a + b) were also significantly lower in liver homogenates of aged compared to young adult or adult animals. The age-related decrease in the specific activity of phosphorylase a was most pronounced in the 10 000 g particulate fraction of liver. Analysis of the specific activity ratios of glycogen synthase a/glycogen synthase a + b indicated a disproportionately greater age-related decrement in the active form of this enzyme. No such age-related difference was evident in the specific activity ratios of phosphorylase a/phosphorylase a + b. The age-associated decrease in the activities of glycogen synthase (a or a + b) and phosphorylase (a or a + b) was also discernible when the enzyme activities were expressed as units per g liver; this decrement in enzyme activity was reflected in all subcellular fractions examined. The levels of plasma insulin, plasma glucose and liver glycogen were not significantly different in rats of the three age groups used in these studies. The reduced activities of liver glycogen synthase and phosphorylase in aged rats is indicative of a likely diminution in the turnover of glycogen in liver during aging. Such an age-associated deficit in liver glycogen metabolism may contribute, at least in part, to the generally observed glucose intolerance upon aging.

Hypothalamically-induced insulin release and its potentiation during oral and intravenous glucose loads

Male Wistar rats were provided with bilateral cannulas in the lateral hypothalamic area (LHA) and cannulas in the left and right jugular vein. Freely moving rats provided in this way with cannulas were infused with transmitters in the LHA and with various substances in the blood circulation during simultaneous sampling of blood without disturbing the animals. Infusion of norepinephrine (NE) in the LHA resulted in increased insulin levels while plasma glucagon and blood glucose were nearly not affected. This LHA mediated insulin release was suppressed by atropine injection in the blood circulation suggesting a vagal contribution to the observed phenomenon. Administration of either an oral or i.v. glucose load during noradrenergic stimulation of the LHA elicited an exaggerated insulin response when compared to their controls. This LHA potentiated insulin response during an oral and i.v. glucose load could be suppressed by atropinization of the rats. It is concluded that meal-related stimuli are relayed to the NE-stimulated area of the LHA and that these stimuli modulate the output from this area of the LHA that is concerned with the release of insulin.

Role of autonomic nervous system in the time-dependent hyperglycemia induced by intracranial injection of D-mannitol and D-glucose

Intracranial injection of D-mannitol (MA) or D-glucose (GL) caused photoperiodic hyperglycemia in rats; MA injection elicited hyperglycemia only in the light period, while GL injection induced it only in the dark period. To elucidate the mechanisms of these hyperglycemias, we examined the effect of the autonomic nervous system on them in rats under the L:D (12:12) condition. Propranolol, a beta-adrenergic blocker, and hexamethonium, a ganglion blocker, effectively inhibited hyperglycemia induced in either the light (MA) or dark (GL) period. In contrast, phenoxybenzamine, an alpha-adrenergic blocker, and atropine, a cholinergic blocker, inhibited hyperglycemia induced in the light period (MA), but not that induced in the dark period (GL). These findings suggest that alpha and beta-adrenergic and cholinergic mechanisms are involved in the hyperglycemia induced by MA in the light period, while only the beta-adrenergic mechanism is involved in the hyperglycemia induced by GL in the dark period.

Minireview. The hypothalamus and blood glucose regulation

Complex neural circuits exist in the hypothalamus for the control of metabolic hormones, enzyme activities, and substrate flux that appear to be involved with the normal adaptation to feeding and subsequent blood sugar regulation. Abnormalities within this system have been found in experimental obesity and in human diabetes, and such maladaptive changes are thought to contribute to the pathophysiology of both disorders. The present report is an attempt to present a coherent picture of the manifold factors which may be involved in the homeostatic regulation of blood glucose levels by the hypothalamus. A wide variety of evidence is touched upon here to show that this part of the ancient vertebrate forebrain mediates a neural glucoregulatory mechanism involving the endocrine pancreas, the liver, and the adrenal medulla.

Vagotomy or atropine blocks hypoglycemic effect of insulin injected into ventromedial hypothalamic nucleus

Stereotaxic microinjections of insulin (100 microU) into the ventromedial hypothalamic nucleus (VMN) resulted in rapid decrease, whereas injection of control saline into the same region caused a slight increase of hepatic venous plasma glucose concentration in rats. The hypoglycemic effect of insulin injected into the VMN was eliminated by pretreatment of the animals with atropine but not with propranolol or with phentolamine. Subdiaphragmatic vagotomy also prevented the decrease of hepatic venous plasma glucose concentration seen after microinjection of insulin into the VMN. These results support the hypothesis that the VMN is an insulin-sensitive glucoregulator center or that it is part of one and that the glucoregulatory impulse that originates in the VMN reaches the effector organ, the liver, through the cholinergic fibers of the vagus nerves.

Hypothalamic regulation of lipid metabolism in the rat: effect of hypothalamic stimulation on lipolysis

In unanesthetized rats, electrical stimulation of the ventromedial hypothalamic nucleus (VMH) induced a marked increase in plasma concentration of glycerol, but did not increase the plasma free fatty acid (FFA) concentration, probably owing to a great elevation of plasma lactate which might inhibit the release of FFA from adipose tissue. In anesthetized rats, on stimulation of the VMH there was no remarkable increase in the plasma lactate, and the plasma glycerol and FFA concentrations were both elevated markedly. Electrical stimulation of the lateral hypothalamic nucleus (LH), on the other hand, had no significant effects on plasma glycerol and FFA levels. Bilateral adrenodemedullation did not prevent the lipolytic response to VMH stimulation, although it reduced slightly the increment of plasma glycerol and FFA. However, the lipolytic response was completely blocked by previous treatment of the animals with hexamethonium or propranolol, but not with phentolamine. These results suggest that sympathetic innervation of the adipose tissue is the dominant factor involved in VMH-induced lipolysis in the rat, while the role of the adrenal medulla is subdominant; the effect of VMH stimulation is mainly transmitted through the sympathetic nervous system to beta-adrenergic receptor of the adipose tissue.

Central nervous system regulation of liver and adipose tissue metabolism

Hypothalamic and autonomic nervous regulation of carbohydrate and amino acid metabolism in the liver and of lipid metabolism in adipose tissues is described. The direct neural mechanism underlying this regulation has been evaluated. Electrical stimulation of the ventromedial hypothalamic nucleus (VMH)-splanchnic nerve system causes glycogenolysis in the liver by rapid activation of glycogen phosphorylase, whereas electrical stimulation of the lateral hypothalamic nucleus (LH)-vagus nerve system promotes glycogenesis in the liver by activation of glycogen synthetase, through direct neural and neural-hormonal mechanisms. Studies on chemical coding of the hypothalamic neurones have revealed that norepinephrine-sensitive neurones in the VMH and acetylcholine-sensitive neurones in the LH are specifically involved in the regulation of liver phosphorylase and glycogen synthetase, respectively. Acetylcholine-sensitive neurones of the LH were also found to be concerned in regulation of hepatic tyrosine aminotransferase activity, through intermediation of the cholinergic system in the LH-vagal pathway. Finally, it has been shown that the VMH acts as a regulatory centre for lipolysis in adipose tissues by modulating activation of the sympathetic nervous system. In addition, stimulation of the VMH enhanced lipogenesis in brown adipose tissue preferentially, probably through a mechanism mediated by sympathetic innervation of this tissue. The latter finding suggests that both the breakdown and resynthesis of triglycerides in brown adipose tissue, but not in white adipose tissue, are accelerated by stimulation of the VMH.

Ultrastructure of 6-aminonicotinamide (6-AN)-induced lesions in the central nervous system of rats. III. Alterations of the spinal gray matter lesion with aging

Following a single i.p. injection of 6-AN (10 mg/kg), the anterior horn cells of 20- and 25-month-old rats increased more in size and recovered slower from chromatolytic changes than those of 3-month-old rats. Neurofilamentous hyperplasia of the perikarya was more prominent in aged rats; proliferated neurofilaments were arranged in thick parallel bundles. In the acute stage, reactive and degenerative changes of glial and mesenchymal elements were more conspicuous in 3-month-old rats; however, they disappeared by day 14 with prominent proliferation of hypertrophic astrocytes. The older rats showed less intensity and slower progression of these changes; sponginess and swelling of the astrocytic cytoplasm were still observed at day 14. Our results suggest that these age-dependent changes in the response to neurotoxins are not only induced on the neuron without mitotic phenomena after birth, but also on neuroglial cells. Furthermore, an alteration or reduction in the support of the neuron augments its intensified and delayed susceptibility to neurotoxins.

Circadian rhythm of liver glycogen metabolism in rats: effects of hypothalamic lesions

The effects of electrolytic lesions of the suprachiasmatic nucleus (SCN) and ventromedial nucleus (VMH) of the hypothalamus on the circadian rhythm of liver glycogen metabolism were studied in rats fed during the dark period (2000-0800 h). Bilateral lesions of the SCN did not affect appreciably the circadian rhythmicity in three parameters of liver glycogen metabolism, i.e., glycogen content and glycogen synthetase I and phosphorylase a activities. In contrast, bilateral lesions of the VMH resulted in almost complete loss of circadian rhythmicity in phosphorylase a activity by lowering the high activity normally observed in late evening. VMH lesions also reduced the amplitudes of the rhythms of synthetase I activity and glycogen content, but did not abolish their rhythmicities. These results suggest that the integrity of the VMH, rather than the SCN, is important for regulation or expression of the food-triggered circadian rhythm of liver phosphorylase a activity. The implication of the lateral hypothalamic nucleus in regulation of the circadian rhythm of liver synthetase I is also discussed.