Glucose transporter 2 (GLUT 2): expression in specific brain nuclei

In the brain, certain neurons appear to be sensitive to changes in local and/or plasma glucose concentration. The alterations in the electrical activity of these neurons probably depend on the existence of 'glucose sensors', which may be one of the glucose transporters described so far. Because of suitable kinetic properties, we hypothesized that the glucose transporter 2 (GLUT 2) may well constitute one of the cerebral 'glucose sensors'. In this study, it was demonstrated, using the polymerase chain reaction, that GLUT 2 mRNAs are present in a limited number of brain nuclei, including the nucleus tractus solitarius, the motor nucleus of the vagus, the paraventricular hypothalamic nucleus, the lateral hypothalamic area, the arcuate nucleus and the olfactory bulbs. These localizations were confirmed by immunocytochemistry, but the cerebral distribution of GLUT 2-like immunoreactivity was far larger than initially expected. Furthermore, electron microscopic observations showed that, within the regions examined, GLUT 2 was localized to a restricted population of astrocytes. The localization of GLUT 2 in regions previously connected with feeding behavior supports an indirect role for GLUT 2 in 'glucose sensing' in these specific cerebral structures.

Decrease in ethanol consumption by naloxone in naive and dependent rats

Acute effects of the opiate antagonist, naloxone, on alcohol intake have been examined and compared in naive and behaviorally dependent rats. In naive rats the aversion to an 8% alcohol solution exhibited in a 30 min presentation was selectively augmented by an IP administration of naloxone (1 mg/kg) 30 min before a morning drinking session. In other rats, behavioral dependence was established by 15 days of IG administration of intoxicating doses of alcohol. This dependence was exhibited by a sustained preference for ethanol for 6 days. Naloxone (1 mg/kg) abolishes the acquired preference for ethanol tested during an 8 hour day time presentation. These effects of naloxone on alcohol intake in ethanol naive and dependent rats are interpreted in relation to a general non-specific action of naloxone on preferred or aversive flavoured solutions.

The effect of naloxone on intragastric ethanol self-administration

The acute effect of 1.25 and 2.50 mg/kg naloxone was tested in a group of male Wistar rats readily self-administering 10% w/v ethanol intragastrically following 12 days of forced ethanol intoxication. Compared to saline pretreatment, naloxone did not alter 24 hr intakes of food, water or ethanol. However, both does strongly and significantly inhibited lever pressing for ethanol during 2 hr following pretreatment. The results indicate that naloxones's inhibition of ethanol intake does have a transient postabsorptive component, although this component is unlikely to be specific to ethanol.

Induction of high voluntary ethanol intake in dependent rats

The experimental conditions under which oral high intake may be induced in previously intoxicated rats have been investigated. Seventeen rats were administered intragastrically with 10 g/kg/day of ethanol for 15 days. At cessation of treatment, they were presented a single bottle of alcoholic solution (10% v/v) during 24 hr. For the following 6 days, they received either an ethyl alcohol solution or water in alternation for 8 hours each. Ethanol treated rats exhibited a high oral intake of ethanol equivalent to the previously injected doses. Controls displayed a significantly lower intake of ethanol. It is concluded that the suppression of the withdrawal state by an initial priming oral intake of ethanol in physically dependent rats is a condition for the development of a conditioned taste preference for ethanol as a basis for the behavioral dependence.

Failure of naloxone to affect initial and acquired tolerance to ethanol in rats

The effects of naloxone on the initial sensitivity to ethanol and the subsequent changes in tolerance induced by prior ethanol treatment were studied in rats by using the drinking test for assessing behavioral effects. Naloxone (N) was administered concurrently with ethanol (E) in either 5 successive single doses spaced at 2-day intervals (Experiment 1) or during 4 days of chronic i.v. infusion (E: 9 g/kg, N: 9 mg/kg in 6 injections per day, Experiment 2). Both treatments produced an increase in nervous tolerance toward ethanol. However naloxone was found to exert no effect on either initial or acquired tolerance toward ethanol. The results are discussed in terms of possible relations between the CNS mechanisms involved in tolerance to morphine and ethanol.

Non-specific enhancement of ethanol-induced taste aversion by naloxone

The conditioned taste aversion paradigm (CTA) was used to examine the effects of naloxone on ethanol-induced aversion towards a saccharine solution (3 conditioning and 11 extinction trials). Six groups of rats received conditioning trials consisting of two IP injections after saccharine presentation of different combinations of either ethanol (E: 1.75 g/kg), LiCl (L: 12 mEq/kg, 0.1 M), naloxone (N: 10 mg/kg) or saline (S); S-S, S-N, E-S, E-N, L-S and L-N. Naloxone by itself produced no aversion to the saccharin flavor. Based on the onset and extinction of aversion, naloxone significantly enhanced ethanol but also LiCl-induced CTA. The comparative data argues in favor of different mechanisms of action (1) between the aversive central effects of ethanol and morphine and (2) between ethanol's acute behavioral effects and negatively reinforcing properties. Enhancement of ethanol and LiCl-induced CTA by naloxone is compatible with hypernociceptive action of the opiate-antagonist and with the pain-modulating role of opiates in the CNS.

Altered conditioned taste aversion and glucose utilization in related brain nuclei of diabetic GK rats

The impact of diabetes and especially hyperglycemia on brain glucose utilization and insulin binding are still not clear. This is probably due to the fact that most studies have been performed in streptozotocin treated rats that are highly hyperglycemia and that could have an effect per se on the brain. The aim of the present work was to measure, in vivo, glucose utilization and insulin binding in different areas of the brain of the spontaneously diabetic GK rats that present a moderate hyperglycemia. Brain insulin receptors number was not changed in the brain of GK rats. By contrast, an increased glucose utilization was present in the external plexiform and the intergranular layers of the olfactory bulbs, as well as in the amygdaloid of the GK rats. These structures are involved in conditioned taste aversion, which was found to be greatly altered in the diabetic rats. These results sustain the hypothesis of impaired neuropsychological functions in diabetic patients particularly in term of learning and memory.

Development of VMH obesity: in vivo insulin secretion and tissue insulin sensitivity

Euglycemic-hyperinsulinemic clamps coupled with an injection of [2-3H]deoxyglucose were performed in rats 1 or 6 wk after lesion of the ventromedial hypothalamus (VMH) and their age-matched controls. In the basal state, glucose utilization was not different in controls and VMH rats in all the tissues studied except in white adipose tissue where it was greatly increased after the lesion. When insulinemia was clamped at 850 microU/ml, glucose utilization was less important in glycolytic and normal in oxidative muscles in animals 1 wk after the lesion (VMH1) compared with controls. In animals 6 wk after the lesion (VMH6), all the muscles utilized less glucose than those of controls. In white adipose tissue, glucose utilization was increased twice more in VMH1 and returned to normal in VMH6. These data demonstrate a progressive development of insulin resistance in muscles. Simultaneously, there is a transient insulin hypersensitivity in white adipose tissue. This, together with a hypersecretion of insulin, could contribute to the development of body fat mass by redirecting glucose towards adipose tissue.

Ethanol preference following hypothalamic stimulation: relation to stimulation parameters and energy balance

Rats were given 5, 10, 20 and 30 min daily sessions of lateral hypothalamic stimulation. Approximately half of the rats showed a large and highly significant increase in their total intake of and preference for 10% v/v ethanol which was continuously available in their home cages. In terms of latency, total consumption and preference for ethanol, 10 min of daily stimulation produced a much greater enhancement than did 30 min. The ethanol drinking rats used more energy per unit of body weight which suggests that the stimulation and/or the ethanol itself may have increased energy expenditure. Simply changing the diet from powdered chow to identical composition pellets produced a large reduction in both total ethanol intake and preference. Reinstating the powdered diet produced a rapid reinstatement of ethanol drinking. These data are discussed in terms of ethanol's role in modulating stimulation induced changes in energy balance.

A bioassay of ethanol-dependence in rats

A bioassay for the quantitative assessment of ethanol dependence in rat is proposed. It is based upon the alleviation by ethanol of the withdrawal syndrome in previously intoxicated animals. The conditioning by ethanol of a taste preference, linearly related to the duration of the previous chronic intoxication, provides a reliable measure of the level of ethanol dependence. In addition, the results display the neurobehavioural mechanism by which self-maintained intoxication is established in physically dependent rats and humans.

Glucose utilization and insulin binding in discrete brain areas of obese rats

The present study was carried out to determine whether genetically obese Zucker rats present changes in brain glucose utilization and/or insulin binding when compared to their lean counterparts. Glucose utilization in the whole brain, determined by measurement of 2-deoxy(1-3H)glucose-6-phosphate, was significantly lower in obese than in lean Zucker rats. In order to precise the structure involved, we then used quantitative autoradiography methods after either (1-14C) 2-deoxyglucose injection or 125I-insulin incubation. In obese rats, local cerebral glucose utilization (LCGU) was significantly decreased in the external plexiform layer (-37%, p < 0.05), in the lateral hypothalamus (-23%, p < 0.05), and in the basolateral amygdaloid nucleus (-30%, p < 0.05). In contrast, no difference in specific insulin binding was found between the two genotypes in any of the areas studied. These results are consistent with some data showing a decrease of LCGU in hyperinsulinemic rats. All together, these data show perturbations of glucose utilization, particularly in structures linked to the regulation of body weight and food intake in obese Zucker rats.

Relationship between initial sensitivity to ethanol and the high alcohol intake in dependent rats

The high spontaneous intake of ethanol, which can be induced in rats after a period of forced administration, may be used to study the altered state created in the C.N.S. by the chronic exposure to ethanol. The relationship between the initial acute sensitivity to ethanol and this induced high oral intake has been examined in rats. Initial sensitivity was determined in two groups of rats either by a test of motor impairment or by alcohol induced hypothermia. After 15 days of daily IG administration of 10 g/kg, rats were submitted to the ethanol presentations which display the high voluntary intake. Two groups of controls were initially tested for their motor impairment or hypothermia respectively under ethanol and then treated for 15 days with saline injections. The results indicate a highly significant negative correlation between initial sensitivity and the level of dependence induced by a chronic treatment and manifested by a voluntary high intake. In control groups, the low intake of ethanol observed in the final test was not correlated to the initial sensitivity to ethanol as tested by hypothermia but weakly correlated to sensitivity measured by motor impairment. The results are discussed in terms of mechanisms which determine the voluntary intake of ethanol in ethanol naive and dependent rats.

Periodicity of chronic ethanol administration as a variable in the induction of dependence in rats

The effect of periodicity of chronic ethanol intragastric administrations on the induction of physical and behavioral dependence on ethanol has been assessed in two groups of rats. In the two groups, rats initially selected for an identical sensitivity to ethanol, were administered daily 10 g per kg b.w. during 15 days. In one group, this daily dose was administered in six intragastric pulses of 1.7 g/kg each, 3 hours apart. In the other group, the same 10 g/kg daily dose was administered in three pulses of 3.33 g/kg, 6 hours apart. In control rats determinations of blood ethanol level in these two treatment conditions showed that the daily area of blood alcohol was 2.5 times higher in the first condition than in the second one. After the 15 days of chronic forced administration, experimental rats were subjected during 6 days to an alternate eight hours single bottle presentation of 10% ethanol solution and water. In this final test of the induced behavioral dependence, rats chronically treated by small frequent doses, displayed the highest intake of ethanol solution versus water. It is concluded that periodicity of a chronic administration or intake of ethanol is an important parameter in the induction of dependence, and the frequent administration of a small daily dose distributed in higher and less frequent unitary administrations.

Effect of vanadium on regional brain glucose utilization in rats

Recent study has indicated that administration of vanadate, an insulomimetic agent, stimulates glucose uptake in the brain, concurrently with a suppression of food intake. The present work was carried out to characterize which specific brain areas are implicated in this increased glucose utilization. After 3 days of vanadate or vanadyl treatment, anaesthetized fasted rats were injected with tritiated 2-deoxyglucose and different brain areas were punched. Vanadate had no effect on glucose utilization but vanadyl significantly increased glucose uptake in the olfactory bulbs, the hypothalamus, and the hindbrain compared to the pair fed controls.

Further study of induced behavioral dependence on ethanol in rats

This study extends previous work from our laboratory on the induction of a high consumption of alcohol in rats, following a chronic treatment by high doses of ethanol. Thirty-three rats exhibited voluntary consumption of ethanol averaging 11 g/kg/24 hr after the chronic treatment. Twenty-two controls submitted to the same procedure of testing drunk 6 g/kg/24 hr. The results are interpreted as a basis for discussing the possible mechanisms involved in alcohol addiction.