Effect of vanadium on regional brain glucose utilization in rats

Physiology&Behavior,Vol. 54, pp. 407-409, 1993

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Effect of Vanadium on Regional Brain Glucose Utilization in Rats P. M A R F A I N G - J A L L A T l A N D L. P E N I C A U D

Laboratoire de Physiopathologie de la Nutrition, C N R S URA 307, Universit~ Paris VII, 2 Place Jussieu, 75251 Paris Cedex 05, France Received 14 O c t o b e r 1992 MARFAING-JALLAT, P. AND L. PENICAUD. Effect of vanadium on regional brain glucose utilization in rats. PHYSIOL BEHAV 54(2) 407-409, 1993.--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 carded out to characterize which specific brain areas are implicated in this increased glucose utilization. Atier 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. Vanadate

Vanadyl

Food intake

Fluid intake

Brain areas

THE biological role of vanadium, an essential trace element, remains unclear. However, vanadate has been shown to be a potent insulin mimetic agent in peripheral tissue [for review, see (3)]. It has also been shown that oral administration ofvanadate suppresses food intake in rats (1,2,10,12) concurrently with a stimulation of the rate of hexose uptake by the whole brain (10). Based on these results, Meyerovitch et al. (10) suggest that the inhibitory effect of vanadate on food intake might be linked to its action on glucose uptake in the CNS. The aim of the present work was to extend the investigation of the vanadate effect on glucose utilization in specific brain areas, particularly those known to be involved in the feeding response. In the first experiment, the conditions of oral administration of vanadate used by Meyerovitch et al. (10) were applied. However, it was observed that vanadate, given in the drinking solution, induced a drastic decrease of fluid intake and, consequently, of vanadate consumption. In a second experiment, rats received an IP injection of vanadyl, twice daily. This compound, less toxic than vanadate (11), could be the active form of vanadium.

Glucose utilization

catheters were implanted under ketamine anesthesia (Imalg~ne, M6rieux, Lyon, France, 12.5 mg/100 g b.wt.). Rats were then allowed 3 days to recover from surgery before the beginning of the vanadate treatment. Body weight, food, and fluid intakes were recorded daily. Three groups of rats were used. In a first group, nine rats were given an aqueous solution 0.2 mg/ml of sodium metavanadate (Na3VO4, Merck, Darmstadt, Germany) as drinking solution for 3 days. In a pilot experiment, the addition of NaCl in the drinking solution did not improve fluid consumption in vanadate-treated rats. Thus, we decided to use normal water in further experiments. The second group was constituted by six rats which were given, twice daily, an IP injection of vanadyl sulphate (VOSO4 5H20, 12.5 mg/kg b.wt.) for 3 days, which corresponds to 4.6 mg vanadium/kg b.wt./day in 0.9% (w/v) NaCI solution. A third group of rats (n = 6) received plain water and were paired to treated rats concerning the food and were used as pair-fed controis. In addition of this treatment, two of them were given twice daily an injection of saline. The results of the six rats being similar, they were pooled.

METHOD The study was performed on adult female Wistar rats weighing initially 225 + 4 g. They were individually housed in plastic cages where they had free access to drinking bottle and standard laboratory chow powder (UAR Villemoisson, France). For blood sampling and injection of tritiated 2-deoxyglucose, intrajugular

Determination of Brain Glucose Utilization On day 4, after 3 h of food deprivation, the unrestrained and conscious rats were administered with 2 deoxy-[ 1-3H] glucose (20 #Ci/rat CEA Saclay, France) in 200 #1 of 0.9% NaCI as a

Requests for reprints should be addressed to P. Marfaing-Jallat.

407

408

MARFAING-JAH.AT AND PENICAUI)

TABLE 1 WEIGHT CHANGES, FLUID AND FOOD INTAKE, PLASMA GLUCOSE, AND INSULIN IJ~Vt!l.5 IN VANADATE- OR VANADYL-TREATED AND CONTROL RATS Average Weight Decrease (g)

Control pair-fed rats (n=6) Vanadate rats (n - 9) Vanadyl rats (n - 6)

Fluid Intake (ml/day)

I:ood Intake (g/day)

Plasma Glucose (g/l)

Plasma Insulin (uU/ml)

11.3±5.2

21+3

4.4+1.2

0.84+0.06

17+4

21.8 ± 3.3

10 _+ 2*

4.7 +_ 1.5

0.82 _+ 0.06

17 _+ 1

20.7 _+ 2.8

21 ± I

5.4 _+ 0.6

0.91 _+ 0.03

14 ± 0.5

Values are means +_ SEM. * p < 0.05 in comparison to controls.

bolus, through the catheter. Timed blood samples (1, 3, 5, 10, 20, 40, 60 min) were drawn after the 3H-2DG pulse and assayed for plasma glucose and 3H-2DG concentrations. After the last blood sample, rats were killed by cervical dislocation. The brain was quickly removed and structures sampled on an ice-cooled glass plate following the method of Iversen and Glowinski (6). Seven regions were dissected: olfactory bulbs, frontal cortex, hypothalamus, cerebellum hippocampus, midbrain, and hindbrain. The 2 deoxy-[ I-aH] glucose-6-phosphate content was determined as described previously (4). Plasma glucose concentration was determined by a glucose oxidase method (Boerhinger-Mannheim, Germany) and plasma 2 deoxy-[ 1-3H]glucose and 2 deoxy-[ 1-3H] glucose-6-phosphate concentrations by liquid scintillation spectrophotometry. Plasma insulin was determined by a radioimmunoassay using rat insulin as standard (SB INSI 1, ORIS, Saclay, France). Results are presented as mean values _+ SEM. Mann-Whitney U-test was used for statistical analysis. RESULTS

from 27 _+ 1 to 10 _+ 2 ml/day with a large interindividual variation: three rats consumed less than 5 ml of fluid, four rats drank about 10 ml, and two rats more than 17 ml. This was accompanied by a decrease of body weight of 21.8 + 3.3 g over the 4 days of treatment. Despite a similar reduction of food intake, fluid intake and body weight were m u c h less decreased in pair-fed controls than in vanadate-treated rats (Table 1). The two groups of rats had similar plasma glucose and insulin levels (Table 1). Table 2 shows the rate of 2DG uptake in several brain areas in vanadate-treated rats compared to controls. Glucose uptake was slightly higher in most brain areas studied in the vanadate rats. However, no significant difference was obtained in any areas between the two groups. The results found on fluid intake were, in our opinion, not satisfactory. Indeed, this reduced fluid consumption could be the causal factor for the lower food intake. Consequently the vanadate consumption was lower than wished. We, thus, undertook a second experiment using vanadyl, a less aversive compound.

Vanadate- Treated Rats

Vanadyl- Treated Rats

The effect of oral vanadate consumption (0.2 mg/ml) on food and fluid intake is shown in Table 1. Daily food intake was strongly reduced in comparison to the pretreatment value: 4.7 -+ 1.5 g/day vs. 20 + 0.5 g/day. Fluid intake was also reduced

The IP injection of vanadyl sulphate reduced the daily food intake in comparison to the pretreatment value (5.4 +_ 0.6 g vs. 18.5 --- 0.5 g). Fluid intake was maintained near the normal level (21 + 1 ml vs. 27 + 0.8 ml in the previous control period) and identical to that of pair fed rats. The decrease in body weight was similar to that obtained with sodium vanadate treatment (Table 1). Mean blood glucose and insulin concentrations did not differ from the values of the other two groups, i.e., pair-fed controls and vanadate-treated rats (Table 1). However, in the vanadyl-treated group, cerebral glucose utilization was higher in most of the structures studied compared to the pair-fed controls. A significant increase was observed in the olfactory bulbs (U = 5.5, p < 0.05) in the hypothalamus (U = 2, p < 0.01), and in the hindbrain (U = 4, p < 0.05).

TABLE 2 GLUCOSE UTILIZATION IN DIFFERENT AREAS OF CONTROL PAIR-FED VANADATE- AND VANADYLTREATED RATS

Olfactory bulbs Frontal cortex Hippocampus Hypothalamus Midbrain (thalamus) Hindbrain Cerebellum

Control (n = 6)

Vanadate (n = 9)

35 +-_4 39 + 4 40 _+ 6 27 + 1 30 +_ 3 30 ± 1 28 ± 2

37 _+ 4 46 _+ 4 34 ± 2 33 ± 4 37 + 2 33 ± 2 31 _+ 1

Results are given as rig. mg min ~; mean +_SEM. • p < 0.05. I P < 0.01 in comparison to controls. ~ •

Vanadyl (n = 6)

47 48 57 49 47 44 36

___ 7* ± 6 ± 13 ± 101" ± 12 ± 4* ± 5

DISCUSSION

The present study was undertaken to determine if concommitantly with its antifeeding effect, vanadium induces changes in glucose utilization of specific brain nuclei. Indeed, we found, in agreement with previous data [review in (3)] that whatever the form of vanadium used (vanadate or vanadyi), food intake and body weight were largely decreased. Furthermore, it should be underlined that when compared to pair-fed nontreated rats, the decrease of body weight of rats submitted to vanadium was

VANADIUM AND BRAIN GLUCOSE UTILIZATION

more marked. This suggests that vanadium not only decreases energy intake but also increases energy expenditure. Another explanation could be that vanadium induces diarrhea and, thus, loss of water (3). But this hypothesis is unlikely, particularly in vanadyl-treated rats that present no evident signs of gastric malaises. It has been shown that brain glucose use, after 2 days of starvation, is slightly lower than in fed rats (7). To eliminate this interaction between food restriction and brain glucose metabolism, a pair-fed group was used as control. In rats treated with 0.2 mg/ml NaVO3, glucose utilization in all brain areas studied was not different from that observed in pair-fed rats. These results are in contrast to those described by Meyerovitch et al., who reported a twofold increased glucose utilization in the whole brain of rats treated with 0.8 mg/ml of vanadium (10). This discrepancy could be due to the different doses of vanadium used. Furthermore, even with a four times lower dose, fluid intake was strongly reduced, rendering the interpretation of the results difi~cult. Indeed, one cannot exclude the possibility that a reduction of water consumption could induce, by itself, alterations in brain glucose utilization. This side effect on fluid intake was probably due to a taste aversion mechanism. Both vanadate toxicity and its effect on fluid intake cast doubt on the results ob-

409 tained with this compound. This had led us to use a vanadyl compound which is less toxic (3). In these conditions, a significant increase of glucose uptake was found in three brain areas, namely the hindbrain (47%), the olfactory bulbs (34%), and the hypothalamus (80%). These results reinforce the idea that the anorexigenic effect of vanadium could be due to an increased glucose uptake by some structures of the central nervous system. Two of the structures cited above, the olfactory bulbs and especially the hypothalamus, have been shown to be largely involved in food intake regulation. The mechanism underlying the effect of vanadium is still unclear. Vanadium, as well as insulin, is able to increase glucose transport in peripheral tissues (1,13). In the CNS, a stimulation of brain glucose uptake is observed only with vanadyl administration. Indeed, hyperinsulinemia naturally occurring in obese Zucker rats or induced in normoglycemic rats is accompanied by no change or a decreased brain glucose utilization (8,9). Thus, as suggested by Meyerovitch, vanadium exerts its effect in the brain via different mechanisms than insulin. In conclusion, the present results showed that vanadyl, together with its antifeeding effect, induced an increase glucose utilization by the brain and, specifically, in some structures related to food intake regulation.

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