Physiology and Behavior, Vol. 13, pp. 769-772. Brain Research Publications Inc., 1974. Printed in the U.S.A.

Independence of Food Intake and Obesity Following Ventromedial Hypothalamic Lesions

in the Rat

BERNARD M. RABIN

Department o f Psychology, University o f Maryland Baltimore County Baltimore, Maryland 21228

(Received 22 March 1974)

RABIN, B. M. Independence of food intake and obesity following ventromedial hypothalamic lesions in the rat. PHYSIOL. BEHAV. 13(6) 769-772, 1974. - Lesions of the ventromedial nucleus of the hypothalamus that do not cause overeating cause significantly greater accumulations of body fat in male rats maintained on an ad lib diet than in controls. Lesioned rats that overeat show a still greater percentage of body fat. The results indicate that obesity caused by medial hypothalamic lesions can result from both primary metabolic disturbances and overeating; and that experimental obesity consistently results from ventromedial hypothalamic lesions independently of the development of hyperphagia.

Obesity Food intake HVM lesions Body fat

WHILE lesions of the ventromedial nucleus of the hypo- thalamus (HVM) cause the development of obesity, the basis of the obesity has not been clearly established. Start- ing with Brobeck et al. [4] most investigators have argued that obesity and all other metabolic changes are a direct result of the significantly increased food intake. The opposite position has been taken by Kennedy [12] who proposed that the increased food intake reflects changes in the metabolic systems of the animal which lead to the development of obesity. In the lipostatic theory, Kennedy proposed that the effect of lesions of the HVM is to disrupt energy metabolism in such a way that the animal overeats until a new, but considerably higher, level of balance between food intake and energy expenditure can be achieved.

Attempts to assess the relationship between food intake and obesity have generally involved the technique of limiting the food intake of rats with HVM lesions to deter- mine whether or not the animals will still become obese in the absence of increased food intake. The results of these studies have been consistent in showing that limiting the food intake of animals with HVM lesions does not prevent the development of obesity (e.g. [8,16]). However, this technique involves putting an animal on what has been termed a "stuff and starve" diet [10] which produces metabolic changes in rats independent of any effects of HVM lesions. Recent work has shown that it is possible to destroy the HVM of male rats without producing hyper- phagia [ 13 ], thereby providing a technique for assessing the

effects of such lesions on metabolic processes independent of effects on food intake in rats with unrestricted access to food.

METHOD

Bilateral lesions were made in the HVM of 24 male albino rats (275-310 g) using techniques which previously have been shown to reliably produce hyperphagia (stainless steel anodes) and techniques which do not consistently cause hyperphagia (platinum anodes or radio frequency power) [13]. In 8 control rats an electrode was passed through the HVM, but no current was turned on. Animals were housed in individual cages with water continually available. They were maintained on an ad lib diet of Purina rat chow. Food intake and weight gain were measured daily for each rat over a survival period of 45 days postlesion.

Obesity was determined by the technique developed by Rathbun and Pace [ 14] on the basis of the specific gravity of the animal. At the time the animal was sacrificed, specific gravity was calculated by measuring the rat's dis- placement in water. Since fat has a lower specific gravity than other body constituents, a lower specific gravity relative to the animal's weight indicates a higher percentage of body fat. There is a good correlation (r = 0.97) between percentage body fat measured by this technique and by more conventional techniques, so that while the absolute values may not be exact, the relative differences between the experimental groups should be reliable.

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FIG. 1. Reconstructions of representative lesions. Stippled area indicates the extent of the lesion. H hyperphagic animals: (1) food intake - 46.2 g/day, 43% body fat; (2) 48.0 g/day, 41% body fat; (3) 40.3 g/day, 33% body fat. N-H non- hyperphagic animals: (4) food intake - 28.3 g/day, 25% body fat; (5) 26.0 g/day, 26% body fat: (6) 29.2 g/day, 19% body fat.

Af te r de te rmina t ion of displacement , the rats were perfused with isotonic saline fol lowed by 10% Formal in saline. Serial sections through the hypotha lamus were cut at 40 g m and stained with cresyl violet. Samples of the lesions are presented in Fig. 1.

RESULTS AND DISCUSSION

While all types of HVM lesions can cause hyperphagia, the f requency with which hyperphagia fol lows anodal stainless steel lesions in male rats is greater than the f requency with which it fol lows anodal plat inum or radio- f requency lesions; so that it is possible to obtain two groups of animals, bo th with similar hypotha lamic lesions, but only one of which is hyperphagic. Animals were then assigned to a hyperphagic condi t ion and a non-hyperphagic condi t ion based upon the level of food intake, independent of the techniques used to produce the lesion.

The non-hyperphagic rats had an average daily food intake (measured be tween Days 1 0 - 3 6 , during which t ime food intake levels are most stable) of 29.2 g which was not significantly different f rom the food intake of control rats (Mann-Whitney U = 20, p>0 .05 , two-tailed). The hyper-

phagic rats had an average daily food intake of 44.2 g which was significantly greater than the food intake of the non- hyperphagic rats (U = 0, p ' (0 .001) . Similarly, the weight gain of the non-hyperphagic rats was not significantly different than that of the controls (U = 35, p>0.05) . This data is summarized in Table 1.

Despite the fact that the food intake of control and non-hyperphagic rats was not significantly different , the non-hyperphagic rats had a significantly greater percentage of body fat than did the controls (U = 4, p<0 .001) . As might be expected , the hyperphagic rats with their greater food intake had a significantly greater percentage of body fat than did the non-hyperphagic rats (U = 13.5, p<0 .01) .

These results indicate that the obesity observed follow- ing lesions of the HVM is not comple te ly dependen t upon increased food intake. This conclusion is supported by the correlat ions presented in Table 2. These correlat ions show that for both the cont ro l and non-hyperphagic rats, per- centage body fat is independent of food intake. It is only for the hyperphagic rats that the correlat ion be tween food intake and body fat reaches significance. Thus, while increased food intake can contr ibute to the deve lopment of

TABLE 1

SUMMARY OF LESION CONDITION EFFECTS

Food Total Weight Condition n Intake (g)* Gain (g)

% Body Fat

Control 8 27.5 t 0.60] 115.0 + 4.25

Non-Hyperphagic 10 29.2 + 0.66 118.8 -+ 9.11

Hyperphagic 14 44.2 + 1.71 208.5 ± 13.25

*Average daily food intake between postoperative days 10-36 tMean +- S.E.

14 + 1.7[)

24 ± 1.20

35 ~ 1.92

HYPOTHALAMIC OBESITY IN THE RAT 771

TABLE 2

RANK ORDER CORRELATION COEFFICIENTS (RHO) FOR THE COMPARISON OF FOOD INTAKE AND PERCENT BODY

FAT

Con~fion Rho

Control 0.01

Non-Hyperphagic 0.11

Hyperphagic 0.57 *

*p<0.05

obesity, it does not appear to be the primary determinant of obesity following HVM lesions in the rat.

The finding of increased levels of body fat in non- hyperphagic rats with HVM lesions is concordant with the results of other studies of the effects of HVM lesions in which the development of hyperphagia has been prevented by the use of limited food [8, 9, 16]. However, the procedure of limiting access to food can produce metabolic changes in animals independent of possible changes result- ing from HVM lesions [ 10]. The observation of obesity in the present experiment in adult rats given unlimited access to food indicates that the development of obesity following HVM lesions is independent of both the means used to prevent overeating and the amount of food intake.

A second procedure which has been used to study the metabolic functions of the HVM has been to produce HVM lesions in weanling rats [1,3]. Weanling rats with HVM lesions become obese despite normal levels of food intake. The present results are consistent with these findings and further show that the development of obesity in the absence of hyperphagia is not restricted to weanling rats, but can also be observed in adult rats as well. These findings suggest that experimental obesity always follows HVM lesions even though hyperphagia may not.

The obesity following HVM lesions has been related to several different changes in the metabolic processes of the lesioned animals. Significant increases in plasma insulin levels have been reported following HVM lesions in wean- ling rats [3] as well as in adult hypophysectomized rats in which hyperphagia was prevented by limiting food intake [9]. These findings, combined with the observation of morphological changes in the pancreas associated with

HVM obesity, suggest that hyperinsulinernia is a primary effect of the lesion leading to the development of obesity. HVM obesity in weanling rats is also associated with changes in glucose and fatty acid metabolism. These changes include hypertriglyceridemia [15], a faster incor- poration of labelled glucose into lipid [5] and decreased free fatty acid release, suggesting a greater utilization of glucose following HVM lesions [ 11 ]. Haessler and Crawford [6] have further reported a specific change in fatty acid composition associated with HVM lesions in adult rats given either unlimited or restricted access to food. Since there was no change for control rats given limited food, these findings suggest that the changes in glucose utilization and fatty acid metabolism must also be considered as primary effects of the lesions independently of possible lesion effects on food intake.

There is some question about the extent to which decreases in spontaneous activity may contribute to the development of obesity in the absence of hyperphagia. Attempts to assess this have, for the most part, used activity wheels to measure activity (e.g. [2,7]). This tech- nique is not completely adequate since rats are generally kept confined to relatively small cages with highly limited opportunities for activity. When activity is measured using standard rat cages placed on an electronic activity unit (Lafayette Instrument Co.), preliminary data indicate that the 24 hr level of activity of HVM obese rats over a 5 week postlesion period is reduced by only 2% from the activity levels of control animals (B. M. Rabin, in preparation). Under these conditions, there is not a significant correlation between obesity (percent body fat) and activity levels of either control (n = 5, rho = 0.40, p>0.05) or HVM-lesioned (n = 8, rho = 0.70, p>0.05) rats. Given the rather small drop in activity levels and the lack of correlation between activity and obesity, it is improbable that the differences in obesity between control and experimental animals could be due to differences in activity levels.

The results support the lipostatic theory only to the extent that they indicate that the HVM is involved in the regulation of the metabolism of ingested foodstuffs. The data do not support the idea that the overeating following HVM lesions is a secondary effect of the metabolic altera- tions. However, the increased levels of body fat found in the non-hyperphagic rats with HVM lesions indicates that the obesity must be considered a primary effect of the lesions and not merely ~ secondary effect of increased food intake. The data also provide additional support indicating that experimental obesity consistently results from HVM lesions even though hyperphagia may not follow such lesions.

REFERENCES

1. Bernardis, L. L. Development of hyperphagia in female rats with ventromedial hypothalamic lesions placed at four differ- ent ages. Experientia 22: 593, 1966.

2. Bernardis, L. L. Hypoactivity as a possible contributing cause of obesity in the weanling rat ventromedial syndrome. Can. J. Physiol. Pharmac. 50: 370-372, 1972.

3. Bernardis, L. L. and L. A. Frohman. Effects of hypothalamic lesions at different loci on development of hyperinsulinemia and obesity in the weanling rat. Z comp. Neurol. 141: 107-116, 1971.

4. Brobeck, J. R., J. Tepperman and C. N. H. Long. Experimental hypothalamic hyperphagia in the albino rat. Yale J. Biol. Med. 15: 831-853, 1943.

5. Frohman, L. A., J. K. Goldman and L. L. Bernardis. Metabo- lism of intravenously injected ~*C-glucose in weanling rats with hypothalamic obesity. Metabolism 21: 799-805, 1972.

6. Haessler, H. A. and J. D. Crawford. Fatty acid composition and metabolic activity of depot fat in experimental obesity. Am. Z Physiol. 213: 255-261, 1967.

772 RABIN

7. Han, P. W. Energy metabolism of tube-fed hypophysectomized rats bearing hypothalamic lesions. Am. J. Physiol. 215: 1343-1350, 1968.

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