Effects of diet upon hypothalamic vulnerability to goldthioglucose in male mice

Orexin stimulates breathing via medullary and spinal pathways

A central neuronal network that regulates respiration may include hypothalamic neurons that produce orexin, a peptide that influences sleep and arousal. In these experiments, we investigated 1) projections of orexin-containing neurons to the pre-Bötzinger region of the rostral ventrolateral medulla that regulates rhythmic breathing and to phrenic motoneurons that innervate the diaphragm; 2) the presence of orexin A receptors in the pre-Bötzinger region and in phrenic motoneurons; and 3) physiological effects of orexin administered into the pre-Bötzinger region and phrenic nuclei at the C3–C4 levels. We found orexin-containing fibers within the pre-Bötzinger complex. However, only 0.5% of orexin-containing neurons projected to the pre-Bötzinger region, whereas 2.9% of orexin-containing neurons innervated the phrenic nucleus. Neurons of the pre-Bötzinger region and phrenic nucleus stained for orexin receptors, and activation of orexin receptors by microperfusion of orexin in either site produced a dose-dependent, significant ( P < 0.05) increase in diaphragm electromyographic activity. These data indicate that orexin regulates respiratory activity and may have a role in the pathophysiology of sleep-related respiratory disorders.

The glial drug methionine sulfoximine reduces goldthioglucose lesions in mice

A toxic form of glucose, goldthioglucose (GTG), damages the arcuate nucleus and nucleus of the solitary tract (NTS) after passage through permeable circumventricular capillaries in adjacent structures, but the mechanism of action is unclear. To see if GTG initially acts upon glia to cause lesions, adult male mice were given either saline or a drug--methionine sulfoximine (MS)--that causes glia, but not neurons, to generate intracellular deposits of glycogen from glutamate. One day later, mice were given 0.3 mg/g of GTG and the subsequent severity of GTG-induced arcuate and NTS lesions was examined. The MS-treated group had significantly smaller GTG lesions in both brain sites, suggesting that a MS-induced alteration in glial metabolism produced a diminished uptake of GTG. Uptake of GTG via a glial glucose transport system may lead to lesions by disturbing the heme metabolism of a specialized subset of glia present in these regions. Factors affecting glial glucose transport in these areas may thus modulate both effects of GTG and effects of glucose upon the regulation of feeding.

Gold thioglucose-induced brain lesions in hamsters

Previous studies suggest species differences in the central neural effects of gold thioglucose (GTG). To further assess these differences, we studied the effects of single intraperitoneal injections of GTG on the brains of golden hamsters (Mesocricetus auratus). Within 48 hr of treatment, each of the GTG doses tested (500, 1000 and 1500 mg/kg) had produced lesions in the ventromedial hypothalamus and area postrema. These findings are of interest since they identify the hamster as the first animal known to be susceptible to GTG-induced lesion formation, but resistant to the hyperphagia commonly associated with glucose analog glucoprivation.

Sensitivity to satiating and taste qualities of glucose in obese Zucker rats

A general "glucoreceptor" defect, demonstrable in pancreatic islet and taste cells, may contribute to the metabolic and taste abnormalities of adult onset diabetes and possibly, if present at the level of the hypothalamus, could produce hyperphagia and the obesity seen in diabetics. To determine if a glucoreceptor defect generally accompanies obesity and glucose intolerance, behavioral responsiveness to glucose was examined in nine obese and nine lean female Zucker rats. Daily food and fluid intake were measured during three two-bottle preference tests, in which rats chose between water and one of several glucose solutions (1%, 3%, and 12%). Taste responsiveness to glucose of obese rats appeared normal; however, increased satiating effects of glucose were found in obese rats, possibly due to an enhanced delivery of glucose to neurons that inhibit feeding, caused by glucose intolerance. Also, obese rats had (a) increased brain weights, and (b) increased volumes of ventromedial and paraventricular hypothalamic nuclei. These findings, perhaps explainable by an increased delivery of nutrients to the developing brain, indicate that the hyperphagia of Zucker rats is due neither to an overt hypothalamic lesion nor to insensitivity to glucose.

The effect of digitoxose on feeding behavior

The effect of digitoxose (DIG) on food intake, gold thioglucose (GTG) lesion formation in the ventromedial hypothalamus (VMH), and VMH glucose oxidation in vitro was investigated in mice. DIG significantly decreased the amount of food ingested during the day compared to controls (p less than 0.01). DIG had no effect on nocturnal feeding. GTG lesion formation in the VMH and VMH glucose oxidation were not altered by DIG treatment. These results suggest that DIG alters daytime feeding behavior by affecting extrahypothalamic or peripheral glucoreceptor sites.

Resistance of the gerbil hypothalamus to gold thioglucose lesion formation

Mongolian gerbils do not develop ventromedial hypothalamic (VMH) lesions, as seen in mice, when challenged with gold thioglucose (GTG, 800, 1600 and 3600 mg/kg IP). Insulin administration or bilateral adrenalectomy, procedures which sensitize nonresponsive mice to GTG, have no effect in the gerbil given GTG. These findings suggest that the gerbil VMH differs significantly in structure and/or function compared with the mouse by not possessing glucoreceptive cells sensitive to GTG.

The developmental and neural determinants of the effects of estrogen on feeding behavior in the rat: a theoretical perspective

Conceptually, the neural regulation of feeding behavior is proposed to be a function of the activities of a long-term (day to day) and a short-term (meal to meal) control system. Although these two control systems are presumably involved in a continuous and dynamic interaction, they can be behaviorally and anatomically separated by specific regulatory challenges and brain lesions. Utilizing this general regulatory model for the hypothalamic control of feeding behavior, the effects of estrogen on a variety of behavioral indices of energy regulation are reviewed and discussed. The effects of ovarian hormones on the feeding behavior of both prepubertal and adult female rats when faced with a series of regulatory tests shown to provide specific information about the long- and short-term control of feeding behavior leads to the following conclusions. Modulation of feeding behavior by ovarian hormones is detectable well before the time of puberty in the female rat and is expressed in terms of a 4-day periodicity that is very similar to adult animals. Estrogen appears to modulate feeding behavior primarily by modifying the long-term control of feeding behavior or else the manner in which nutrients are integrated into this long-term system. It is further proposed that estrogen acts upon metabolically specific and unique neural elements in the hypothalamus whose function is to translate error signals derived from the long-term integration of body nutrients into appropriate readjustments in feeding behavior. Although this dual-regulatory model for the hypothalamic control of feeding behavior can account for the majority of the effects of estrogen on feeding behavior, further studies suggest that an "extrahypothalamic" mechanism must mediate certain aspects of the neural control of feeding behavior as well as the behavioral effects of estrogen. Specifically, the present model system can account for the role of carbohydrates in the neural control of feeding behavior, but fats and possibly proteins can modify feeding behavior independent of the hypothalamus. Likewise, estrogen can influence the efficiency with which nutrient loads of fats and proteins can modify subsequent feeding behavior and therefore this proposed "extrahypothalamic" mechanism may mediate those effects of estrogen on feeding behavior that cannot be accounted for by the present dual-regulatory model.