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Neonatal monosodium glutamate-induced lesions of hypothalamus increase intestinal fat absorption in adult mice
EXPERIMENTAL
NEUROLOOY
79, 141-151 (1983)
Neonatal Monosodium Glutamate-Induced Lesions of Hypothalamus Increase Intestinal Fat Absorption in Adult Mice KAZUHIKO TANAKA, MORIMI SHIMADA,AKIKO SASAHARA, NORIAKI OYA, YOSHIHIDEFUJIYAMA,* AND SHIROHOSODA*” Departments of Pediatrics and *Internal Medicine (II), Shiga University of Medical Science, Otsu. Japan Received May I 7. 1982; revision received July 27, 1982 The hypothalamic lesion induced by monosodium dutamate injection during the suckling period affected the absorption of fat from the intestine. Mice in group 1, in which the hypothalamic lesion included not only the arcuate nucleus, but also the basal preoptic area and two-thirds of the ventromedial nucleus, showed more efficient rates of fat absorption with high serum concentrations of immunoreactive insulin than the control. Mice in group 2, in which the hypothalamic lesion was confined mostly to the arcuate nucleus, also showed a significant increase in fat absorption, but elevated immunoreactive insulin was not found in the serum of these mice.
INTRODUCTION Data from a number of laboratories have indicated that excessive amounts of monosodium glutamate (MSG) cause extensive damage to the hypothalamus of mice when this chemical is administered in the suckling period (2,9, 14,21). Some authors also mentioned that mice thus treated subsequently developed obesity and/or endocrinological abnormalities ( 18, 20, 22, 23). A few papers disclosed, furthermore, that mice thus treated were normophagic or slightly hypophagic (8,20,23). There is, however, no study dealing with the question of why these mice become obese in spite of hypophagia. We studied this question, with special reference to the effect of the hypothalamic lesion on fat absorption from the intestine. Abbreviations: IRI-immunomactive insulin, MSG-monosodium glutamate. I Please send correspondence to Kazuhiko Tanaka, M.D., Department of Pediatrics, Shiga University of Medical Science, Tsukinowa-cho, Seta, Otsu, Shii-ken, 520-21 Japan. 141 0014-4886/83/010141-11$03.00/O CopyrightQ 1983by Academic Press, Inc. Au r@hts of repduction
in any form a-earned.
TANAKA
ET AL.
total fecal excretion of ‘3’1-Triolein (%I I 22 20 IB 16 14. 12 10 8.
24
72
hours
FIGURE 1 total fecal excretion of ‘3’1-Triolein (%)
Ilow*
24
48
72
FIGURE 2 FIGS. l-3. Fat absorption tests. FIG. 1. Fecal recovery rate of 13’1after 0.2 ml test fat was administered to lo-week-old mice neonatally treated with monosodium glutamate (MSG). The vertical lines represent fl SD, N = 5. 0 - 0, group 1 (MSG at 1 to 5 days of age); X - - - X, group 2 (MSG at 6 to 10 days of age; 0 - 0, control. FIG. 2. Fecal recovery rate of 13’1 after 0.006 ml/g of test fat was administered to lo-weekold mice neonatally treated with MSG. The vertical lines represent +I SD, N = 10. Symbols as in Fig. 1. FlG. 3. Fecal recovery rate of 13’1at various ages after 0.006 ml/g of test fat was administered to mice treated neonatally with MSG. The vertical lines represent fl SD, N = 10. Symbols asinFig. 1.
MATERIALS AND METHODS The animals used were a strain of ICR-JCL mice. Monosodium glutamate (Merck) was administered in a 10% aqueous solution. [‘3’I]triolein
HYPOTHALAMIC
LESION
AND
FAT
ABSORPTION
143
total fecal excr6tion of ‘3tl-Triolein (Xl 22 LO 16 16, 14, 12, 101 6 6 4 2I
1: 7
IO ege
13
16
in WQQkS
FIGURE
3
was used as a nonabsorbable marker to estimate fat absorption. Its radiochemical purity determined by thin layer chromatography was 95%. Details of the experimental procedures were described elsewhere (23). Eighty-one newborn male mice were divided into three groups. A group of 26 mice (group 1) had five successive injections of 2 mg MSG/g body weight at 1 to 5 days of age. The 26 mice in group 2 had five successive injections of 2 mg MSG/g body weight at 6 to 10 days of age. The MSG was injected subcutaneously in the back in each animal. The remaining 29 mice, which had no treatment, served as the control. Two different experiments were carried out. Experiment a was designed to study the rate of fat absorption after a certain dose per mouse. At 10 weeks of age, five mice from each group were fasted overnight, then 0.2 ml olive oil containing 2 PCi [‘3’I]triolein was administered by stomach tube under light ether anesthesia. After treatment, these animals were placed in individual metabolic cages designed to collect feces without contamination of urine and to avoid food loss by spillage. They had free access to mouse chow and water. Feces were collected at 24,48, and 72 h after administration of radioactive triolein. In experiment b, 10 mice from each group received intragastrically 0.006 ml olive oil containing 0.06 &i [“‘I]triolein/g body weight at 7, 10, 13, and 16 weeks of age. Feces were collected at 24,48, and 72 h after administration of radioactive triolein. Fat absorption was calculated by the isotope balance method based on the total fecal recovery of the label of the test fat during the collection period.
144
7
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ET AL.
IO
13
16
age in weeks
FIG. 4. Ratios of body weight:body length of mice treated neonatally with MSG. The vertical lines represent f 1 SD; N = 10. 0, group 1 (MSG at 1 to 5 days of age); H, group 2 (MSG at 6 to 10 days of age); n , control.
Body length (from nose to anus) and weight were measured at 7, 10, 13, and 16 weeks of age and the ratio body weight:length was calculated. Food consumption was evaluated at 7, 10, 13, and 16 weeks of age. The remaining mice in each group were used for measurement of immunoreactive insulin (IRI) at 10 weeks of age. Blood samples were drawn g/mouse/day
7 6I
7
IO age
I3 in
I6
weeks
FIG. 5. Measurements of food consumption of mice neonatally treated with MSG. The vertical lines represent f 1 SD, N = 10. Symbols as in Fig. 4.
HYFOTHALAMLC
LESION AND FAT ABSORPTION
145
26 24. 22, 20. 18. 16. 14. 12. 10. 8. 8. 4. 2. .
control
group I
PUP 2 FIG. 6. Measurements of immunoreactive insulin on lo-week-old mice neonatally treated with MSG. The vertical lines represent +1 SD, N = 10. Symbols as in Fig. 4.
after overnight fasting, Serum IRI was measured by the double antibody radioimmunoassay according to the method of Hales and Randle (1 l), using a cross-reaction with human insulin. The amount of IRI was obtained by direct reading on the standard curve for rat insulin. Statistical analysiswas by Student’s t test. RESULTS When 0.2 ml test fats was given intragastrically to five lO-week-old mice, the ratio of the total fecal recoveries of 13’1to the marker in the test dose in the control group (2 1.1 f 1.6%) exceededthat in the two experimental groups (6.5 + 3.9%in group 1and 5.5 + 2.4% in group 2, respectively). The difference in the ratio between the control and two experimental groups was statistically significant (P < 0.001). Although the fecal recovery ratio of 13’1in group 2 was lessthan that in group 1, the difference in the ratio between these two groups was not statistically significant (Fig. 1). As shown in Fig. 2, when 0.006 ml test fats/g body weight were given intragastrically to IO-week-old mice, the total fecal recovery ratio of “‘1 tQ the marker in the initial dosein the control group was 18.1 f 5.3%compared with 14.6 f 4.0% in group 1 and 11.6 f 4.3% in group 2. The fecal recovery ratio of 13’1in the control group was more than that in the experimental
146
TANAKA
ET AL.
FIG.7. Frontal sections through the medial nucleus of the hypothalamus of 115day-old mice (hematoxylin and eosin, X 100). A-control. B-group 1. Note the disappearance of the arcuate nucleus (na) and ventral two-thirds of the ventromedial nucleus (nvm). C-group 2. Note the disappearance of the arcuate nucleus.
groups throughout the entire period tested, and values in group 2 were less than those in group 1 except at 13 weeks of age. Although the difference in the ratio between the control and group 1 at 13 weeks of age (P < 0.05) and between the control and group 2 at 10 and 13 weeks of age (P < 0.05, P < 0.05, respectively) was statistically significant, a significant difference in the ratio was not found between the two experimental groups (Fig. 3). Figure 4 shows the ratio of body weight and length of mice in each group.
HYPOTHALAMIC
LESION AND FAT ABSORPTION
147
FIG. ‘I.-Continued.
The mean ratios in group 1 and group 2 exceeded the control by 7 weeks of age and the difference in the ratio between the two experimental groups and the control group became statistically significant after 10 weeks of age (P < 0.001). Although the mean ratio in group 1 was slightly smaller than that of group 2 at 7 weeks of age, it exceeded group 2 at 10 weeks and thereafter; the difference became statistically significant (P < 0.05, P < 0.001, respectively). The daily intake of diet was measured for selected mice in each group (Fig. 5). Although mice in the two experimental groups took less food than those in the control group throughout this experiment, the difference be-
148
TANAKA
FIG.
ET AL.
I.-Continued.
tween the two experimental groups and the control group was statistically insignificant except between group 2 and the control group at 13 and 16 weeks of age (P < 0.05, P < 0.05, respectively). The daily dietary intake of mice in group 2 was slightly less than that in group 1 throughout the entire period tested; the difference was statistically insignificant. When serum concentrations of IRI were measured at 10 weeks of age, amounts of this hormone in the mice in group 1 were significantly more than those in group 2 and the control (13.9 f 11.0 rU/ml vs. 6.5 + 1.6 rU/ml and 5.5 + 0.4 &J/ml; P < 0.05, P < 0.05, respectively). The dif-
HYPOTHALAMIC
LESION AND FAT ABSORPTION
149
ference in serum concentration of IRI between group 2 and the control group was, however, statistically insignificant (Fig. 6). When the mice in group 1were killed at 17weeksof age,the ventromedial portion of the preoptic area, the arcuate nucleus, and the ventral two-thirds of the ventromedial nucleus were scanty in cellularity and showed severe atrophy. On the other hand, the hypothalamic lesion of mice in group 2 was mostly confined to the arcuate nucleus (Fig. 7). DISCUSSION In the previous study, we demonstrated that the mice in which the hypothalamic lesion was confined mostly to the arcuate nucleus became obese, but not so noticeably asthe mice in which the hypothalamic lesion extended to the ventral two-thirds of the ventromedial nucleus in addition to the arcuate nucleus (23). From that finding, we could conclude that the development of overt obesity in mice treated with MSG was intimately related to the damage in the ventromedial nucleus in addition to that in the arcuate nucleus. We also confirmed that mice treated with MSG did not consume more food than control mice, and that mice with the lessseverelesion were rather hypophagic throughout the entire period tested, as Olney (20) and Bunyan et al. (8) reported. According to the data obtained by electrolytic lesions and gold thioglucase-inducedlesions in the hypothalamus, hyperphagia and obesity are considered to be associatedwith pathology of the hypothalamus ( 1, 7, 10, 12, 16, 17). Obesemice that have been treated with MSG during their suckling period are, however, normophagic or rather hypophagic. One reason why mice treated with MSG become obese, in spite of hypophagia, may be explained by the results of the fat absorption test in the present study. It has been demonstrated that electrical stimulation of the ventromedial hypothalamus in the rabbit brain results in an increase in lipolysis (13), and that electrolytic destruction of this region in the rat brain is followed by a decreasein mobilization in free fatty acid (6, 19). Those studies suggestthat the ventromedial hypothalamus plays an important role for regulation of lipid metabolism (6, 13, 15, 19,24). The lesion induced by MSG lies in the ventromedial hypothalamus. Our experiment, however, demonstrated that not only the mice in group 1 but also the mice in group 2 showed more efficient rates of fat absorption in the intestine than the control group. This finding suggeststhat the ventromedial hypothalamus, especiallythe arcuate nucleus, may play an important role for regulation of fat absorption in the intestine. Some authors have shown that stimulation of the parasympathetic ner-
150
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ET
AL.
VOW system or destruction of the sympathetic nervous system is followed by an elevation in insulin (4, 5, 25). Ban has shown that the ventromedial hypothalamus lies in the medial sympathetic zone (3). The serum concentration of IRI in mice in group 1 was significantly higher than that in group 2 and the control in the present experiment, and it did not differ significantly between group 2 and the control group. These findings are consistent with those reports. On the basis of our findings, it may be suggested that the arcuate nucleus plays an important role in regulating fat absorption in the intestine and that there is no correlation between serum IRI and fat absorption in the intestine. REFERENCES 1. ANAND, B. K., AND J. R. BROBECK. 1951. Hypothalamic control of food intake in rats and cats. Yale J. Biol. Med. 24: 123-140. 2. AREES, E., AND J. MAYER. 1970. Monosodium glutamate-induced brain lesions: electron microscopic examination. Science 170: 549-550. 3. BAN, T. 1966. The septo-preoptico-hypothalamic system and its autonomic function. Prog. Brain Rex 21A: l-43. 4. BERNARDIS, L. L., AND J. K. GOLDMAN. 1976. Origin of endocrine-metabolic changes in the weanling rat ventromedial syndrome. J. Neurosci. Res. 2: 9 l-l 16. 5. BRAY, G. A., AND T. F. GALLAGHER, JR. 1975. Manifestations of hypothalamic obesity in man. A comprehensive investigation of eight patients and a review of the literature. Medicine 54: 301-330. 6. BRAY, G. A., AND Y. NISHIZAWA. 1978. Ventromedial hypothalamus modulates fat mobilisation during fasting. Nature (London) 274: 900-902. 7. BROBECK, J. R., J. TEPPERMAN, AND C. N. H. LONG. 1943. Experimental hypothalamic hyperphagia in the albino rat. Yale J. Physiol. 15: 831-853. 8. BUNYAN, J., E. A. MURRELL, AND P. P. SHAH. 1976. The induction of obesity in rodents by means of monosodium glutamate. Br. J. Nutr. 35: 25-39. 9. BURDE, R. M., B. SCHAINKER, AND J. KAYES. 1971. Monosodium glutamate: acute effect of oral and subcutaneous administration on the arcuate nucleus of hypothalamus in mice and rats. Nature (London) 233: 58-60. 10. DEBONS, A. F., I. KRIMSKY, H. J. LIKUSKI, A. FROM, AND R. J. CLOUTIER. 1968. Gold thioglucose damage to the satiety center. Am. J. Physiol. 214: 652-658. 11. HALES, C. N., AND P. J. RANDLE. 1963. Immunoassay of insulin with insulin-antibody precipitate. Biochem. J. 88: 137-146. 12. HOEBEL, B. G., AND P. TIETELBAUM. 1966. Weight regulation in normal and hypothalamic hyperphagic rats. J. Comp. Physiol. Psychol. 61: 189-193. 13. KUMON, A., A. TAKAHASHI, T. HARA, AND T. SHIMAZU. 1976. Mechanism of lipolysis induced by electrical stimulation of the hypothalamus in the rabbit. J. Lipid Rex 17: 551-558. 14. LEMKEY-JOHNSTON, N., AND W. A. REYNOLDS. 1974. Nature and extent of brain lesions in mice related to ingestion of monosodium glutamate. J. Neuropathol. Exp. Neural. 33: 74-97.
15. LUYCKX, A. S., A. DRESSE, A. CESSION-FOSSION,AND P. J. LEFEBVRE. 1975. Catecholamines and exercise-induced glucagon and fatty acid mobilization in the rat. Am. J. Physiol.
229: 376-383.
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16. MAYER,J., ANDN. B. MARSHALL. 1956.Specificityof goldthioglucose for ventromedisl hypothalamic lesions and hyperphagia. Nature (London) 178: 1399-1400. 17. MOGENSON, G. J. 1974.Changingviewsof the roleof the hypothalamus in the control of ingestivebehaviors. Pages 268-293 in Recent Studies of Hypothalamic Function Int. sy?np. Calgfuy, 1973. Karger, Bsse.1. 18. NAGASAWA, H., R. YANAI,ANDS.KIKUYAMA.1974.Irreversibleinhibitionof pituitary prolactinandgrowthhormonesecretion andof mammaryglanddevelopment in mice by monosodium glutamateadministration neonatally.Acta Endocrinol. 75: 249-259. 19. NISHIZAWA, Y., ANDG. A. BRAY.1978.Ventromedialhypothalamiclesionsand the mobilizationof fatty acids.J. Clin. Invest. 61: 714-721. 20. OLNEY,J. W. 1969.Brainlesions,obesity,andotherdisturbances in micetreatedwith monosodium glutamate. Science 164: 7 19-721. 21. OLNEY, J. W., AM) L. G. SHARPE. 1969.Brainlesions in aninfantrhesus monkeytreated with monosodium glutamate.Science166:386-388. 22. REDDING, T. W., A. V. !%CHALLY, A. ARIMURA,ANDI. WAKABAYASHI. 1971.Effectof monosodium glutamate on someendocrine functions.Neuroendocrinology8: 245-255. 23. TANAKA,K., M. SHIMADA,K. NAKAO,ANDT. KUSUNOKI. 1978.Hypothalamiclesion inducedby injectionof monosodium glutamatein sucklingperiodand subsequent development of obesity.Exp. Neural. 62: 191-199. 24. TEIXEIRA,V. L., J. ANTUNES-RODRIGUES, ANDR. H. MICUORINI.1973.Evidencefor centersin the centralnervoussystemthat selectivelyregulatefat mobilizationin the rat. J. Lipid. Res. 14: 672-677. 25. WOODS, S.C.,AND D. PORTE, JR. 1974.Neuralcontrolof theendocrine pancreas. Physiol. Rev. 54: 596-619.
NEUROLOOY
79, 141-151 (1983)
Neonatal Monosodium Glutamate-Induced Lesions of Hypothalamus Increase Intestinal Fat Absorption in Adult Mice KAZUHIKO TANAKA, MORIMI SHIMADA,AKIKO SASAHARA, NORIAKI OYA, YOSHIHIDEFUJIYAMA,* AND SHIROHOSODA*” Departments of Pediatrics and *Internal Medicine (II), Shiga University of Medical Science, Otsu. Japan Received May I 7. 1982; revision received July 27, 1982 The hypothalamic lesion induced by monosodium dutamate injection during the suckling period affected the absorption of fat from the intestine. Mice in group 1, in which the hypothalamic lesion included not only the arcuate nucleus, but also the basal preoptic area and two-thirds of the ventromedial nucleus, showed more efficient rates of fat absorption with high serum concentrations of immunoreactive insulin than the control. Mice in group 2, in which the hypothalamic lesion was confined mostly to the arcuate nucleus, also showed a significant increase in fat absorption, but elevated immunoreactive insulin was not found in the serum of these mice.
INTRODUCTION Data from a number of laboratories have indicated that excessive amounts of monosodium glutamate (MSG) cause extensive damage to the hypothalamus of mice when this chemical is administered in the suckling period (2,9, 14,21). Some authors also mentioned that mice thus treated subsequently developed obesity and/or endocrinological abnormalities ( 18, 20, 22, 23). A few papers disclosed, furthermore, that mice thus treated were normophagic or slightly hypophagic (8,20,23). There is, however, no study dealing with the question of why these mice become obese in spite of hypophagia. We studied this question, with special reference to the effect of the hypothalamic lesion on fat absorption from the intestine. Abbreviations: IRI-immunomactive insulin, MSG-monosodium glutamate. I Please send correspondence to Kazuhiko Tanaka, M.D., Department of Pediatrics, Shiga University of Medical Science, Tsukinowa-cho, Seta, Otsu, Shii-ken, 520-21 Japan. 141 0014-4886/83/010141-11$03.00/O CopyrightQ 1983by Academic Press, Inc. Au r@hts of repduction
in any form a-earned.
TANAKA
ET AL.
total fecal excretion of ‘3’1-Triolein (%I I 22 20 IB 16 14. 12 10 8.
24
72
hours
FIGURE 1 total fecal excretion of ‘3’1-Triolein (%)
Ilow*
24
48
72
FIGURE 2 FIGS. l-3. Fat absorption tests. FIG. 1. Fecal recovery rate of 13’1after 0.2 ml test fat was administered to lo-week-old mice neonatally treated with monosodium glutamate (MSG). The vertical lines represent fl SD, N = 5. 0 - 0, group 1 (MSG at 1 to 5 days of age); X - - - X, group 2 (MSG at 6 to 10 days of age; 0 - 0, control. FIG. 2. Fecal recovery rate of 13’1 after 0.006 ml/g of test fat was administered to lo-weekold mice neonatally treated with MSG. The vertical lines represent +I SD, N = 10. Symbols as in Fig. 1. FlG. 3. Fecal recovery rate of 13’1at various ages after 0.006 ml/g of test fat was administered to mice treated neonatally with MSG. The vertical lines represent fl SD, N = 10. Symbols asinFig. 1.
MATERIALS AND METHODS The animals used were a strain of ICR-JCL mice. Monosodium glutamate (Merck) was administered in a 10% aqueous solution. [‘3’I]triolein
HYPOTHALAMIC
LESION
AND
FAT
ABSORPTION
143
total fecal excr6tion of ‘3tl-Triolein (Xl 22 LO 16 16, 14, 12, 101 6 6 4 2I
1: 7
IO ege
13
16
in WQQkS
FIGURE
3
was used as a nonabsorbable marker to estimate fat absorption. Its radiochemical purity determined by thin layer chromatography was 95%. Details of the experimental procedures were described elsewhere (23). Eighty-one newborn male mice were divided into three groups. A group of 26 mice (group 1) had five successive injections of 2 mg MSG/g body weight at 1 to 5 days of age. The 26 mice in group 2 had five successive injections of 2 mg MSG/g body weight at 6 to 10 days of age. The MSG was injected subcutaneously in the back in each animal. The remaining 29 mice, which had no treatment, served as the control. Two different experiments were carried out. Experiment a was designed to study the rate of fat absorption after a certain dose per mouse. At 10 weeks of age, five mice from each group were fasted overnight, then 0.2 ml olive oil containing 2 PCi [‘3’I]triolein was administered by stomach tube under light ether anesthesia. After treatment, these animals were placed in individual metabolic cages designed to collect feces without contamination of urine and to avoid food loss by spillage. They had free access to mouse chow and water. Feces were collected at 24,48, and 72 h after administration of radioactive triolein. In experiment b, 10 mice from each group received intragastrically 0.006 ml olive oil containing 0.06 &i [“‘I]triolein/g body weight at 7, 10, 13, and 16 weeks of age. Feces were collected at 24,48, and 72 h after administration of radioactive triolein. Fat absorption was calculated by the isotope balance method based on the total fecal recovery of the label of the test fat during the collection period.
144
7
TANAKA
ET AL.
IO
13
16
age in weeks
FIG. 4. Ratios of body weight:body length of mice treated neonatally with MSG. The vertical lines represent f 1 SD; N = 10. 0, group 1 (MSG at 1 to 5 days of age); H, group 2 (MSG at 6 to 10 days of age); n , control.
Body length (from nose to anus) and weight were measured at 7, 10, 13, and 16 weeks of age and the ratio body weight:length was calculated. Food consumption was evaluated at 7, 10, 13, and 16 weeks of age. The remaining mice in each group were used for measurement of immunoreactive insulin (IRI) at 10 weeks of age. Blood samples were drawn g/mouse/day
7 6I
7
IO age
I3 in
I6
weeks
FIG. 5. Measurements of food consumption of mice neonatally treated with MSG. The vertical lines represent f 1 SD, N = 10. Symbols as in Fig. 4.
HYFOTHALAMLC
LESION AND FAT ABSORPTION
145
26 24. 22, 20. 18. 16. 14. 12. 10. 8. 8. 4. 2. .
control
group I
PUP 2 FIG. 6. Measurements of immunoreactive insulin on lo-week-old mice neonatally treated with MSG. The vertical lines represent +1 SD, N = 10. Symbols as in Fig. 4.
after overnight fasting, Serum IRI was measured by the double antibody radioimmunoassay according to the method of Hales and Randle (1 l), using a cross-reaction with human insulin. The amount of IRI was obtained by direct reading on the standard curve for rat insulin. Statistical analysiswas by Student’s t test. RESULTS When 0.2 ml test fats was given intragastrically to five lO-week-old mice, the ratio of the total fecal recoveries of 13’1to the marker in the test dose in the control group (2 1.1 f 1.6%) exceededthat in the two experimental groups (6.5 + 3.9%in group 1and 5.5 + 2.4% in group 2, respectively). The difference in the ratio between the control and two experimental groups was statistically significant (P < 0.001). Although the fecal recovery ratio of 13’1in group 2 was lessthan that in group 1, the difference in the ratio between these two groups was not statistically significant (Fig. 1). As shown in Fig. 2, when 0.006 ml test fats/g body weight were given intragastrically to IO-week-old mice, the total fecal recovery ratio of “‘1 tQ the marker in the initial dosein the control group was 18.1 f 5.3%compared with 14.6 f 4.0% in group 1 and 11.6 f 4.3% in group 2. The fecal recovery ratio of 13’1in the control group was more than that in the experimental
146
TANAKA
ET AL.
FIG.7. Frontal sections through the medial nucleus of the hypothalamus of 115day-old mice (hematoxylin and eosin, X 100). A-control. B-group 1. Note the disappearance of the arcuate nucleus (na) and ventral two-thirds of the ventromedial nucleus (nvm). C-group 2. Note the disappearance of the arcuate nucleus.
groups throughout the entire period tested, and values in group 2 were less than those in group 1 except at 13 weeks of age. Although the difference in the ratio between the control and group 1 at 13 weeks of age (P < 0.05) and between the control and group 2 at 10 and 13 weeks of age (P < 0.05, P < 0.05, respectively) was statistically significant, a significant difference in the ratio was not found between the two experimental groups (Fig. 3). Figure 4 shows the ratio of body weight and length of mice in each group.
HYPOTHALAMIC
LESION AND FAT ABSORPTION
147
FIG. ‘I.-Continued.
The mean ratios in group 1 and group 2 exceeded the control by 7 weeks of age and the difference in the ratio between the two experimental groups and the control group became statistically significant after 10 weeks of age (P < 0.001). Although the mean ratio in group 1 was slightly smaller than that of group 2 at 7 weeks of age, it exceeded group 2 at 10 weeks and thereafter; the difference became statistically significant (P < 0.05, P < 0.001, respectively). The daily intake of diet was measured for selected mice in each group (Fig. 5). Although mice in the two experimental groups took less food than those in the control group throughout this experiment, the difference be-
148
TANAKA
FIG.
ET AL.
I.-Continued.
tween the two experimental groups and the control group was statistically insignificant except between group 2 and the control group at 13 and 16 weeks of age (P < 0.05, P < 0.05, respectively). The daily dietary intake of mice in group 2 was slightly less than that in group 1 throughout the entire period tested; the difference was statistically insignificant. When serum concentrations of IRI were measured at 10 weeks of age, amounts of this hormone in the mice in group 1 were significantly more than those in group 2 and the control (13.9 f 11.0 rU/ml vs. 6.5 + 1.6 rU/ml and 5.5 + 0.4 &J/ml; P < 0.05, P < 0.05, respectively). The dif-
HYPOTHALAMIC
LESION AND FAT ABSORPTION
149
ference in serum concentration of IRI between group 2 and the control group was, however, statistically insignificant (Fig. 6). When the mice in group 1were killed at 17weeksof age,the ventromedial portion of the preoptic area, the arcuate nucleus, and the ventral two-thirds of the ventromedial nucleus were scanty in cellularity and showed severe atrophy. On the other hand, the hypothalamic lesion of mice in group 2 was mostly confined to the arcuate nucleus (Fig. 7). DISCUSSION In the previous study, we demonstrated that the mice in which the hypothalamic lesion was confined mostly to the arcuate nucleus became obese, but not so noticeably asthe mice in which the hypothalamic lesion extended to the ventral two-thirds of the ventromedial nucleus in addition to the arcuate nucleus (23). From that finding, we could conclude that the development of overt obesity in mice treated with MSG was intimately related to the damage in the ventromedial nucleus in addition to that in the arcuate nucleus. We also confirmed that mice treated with MSG did not consume more food than control mice, and that mice with the lessseverelesion were rather hypophagic throughout the entire period tested, as Olney (20) and Bunyan et al. (8) reported. According to the data obtained by electrolytic lesions and gold thioglucase-inducedlesions in the hypothalamus, hyperphagia and obesity are considered to be associatedwith pathology of the hypothalamus ( 1, 7, 10, 12, 16, 17). Obesemice that have been treated with MSG during their suckling period are, however, normophagic or rather hypophagic. One reason why mice treated with MSG become obese, in spite of hypophagia, may be explained by the results of the fat absorption test in the present study. It has been demonstrated that electrical stimulation of the ventromedial hypothalamus in the rabbit brain results in an increase in lipolysis (13), and that electrolytic destruction of this region in the rat brain is followed by a decreasein mobilization in free fatty acid (6, 19). Those studies suggestthat the ventromedial hypothalamus plays an important role for regulation of lipid metabolism (6, 13, 15, 19,24). The lesion induced by MSG lies in the ventromedial hypothalamus. Our experiment, however, demonstrated that not only the mice in group 1 but also the mice in group 2 showed more efficient rates of fat absorption in the intestine than the control group. This finding suggeststhat the ventromedial hypothalamus, especiallythe arcuate nucleus, may play an important role for regulation of fat absorption in the intestine. Some authors have shown that stimulation of the parasympathetic ner-
150
TANAKA
ET
AL.
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