BEHAVIORAL BIOLOGY 19, 405--410 (1977), Abstract No. 6243

BRIEF REPORT

Effects of Prefattening on Ovariectomy-lnduced Hyperphagia and Weight Gain in Rats 1

RUTH NOEL z AND ALISON FLEMING 3"4

Department of Psychology, University of California, Berkeley, California 94720

The hypothesis that ovariectomy acts primarily to alter body weight and not food consumption was tested by ovariectomizing animals whose weight had been forcibly elevated prior to surgery. While both prefattened and control groups gained weight after ovariectomy, prefattened animals ate less and gained weight at a significantly slower rate. These data are discussed in terms of hormonal modula- tion of the body weight set-point.

The gonadal hormones contribute to energy balance regulation in female rats (see Wade, 1976). Over the estrous cycle, rats exhibit reliable and consistent fluctuations in food consumption, activity level, and body weight. During proestrus, when levels of estradiol are high, food intake declines, activity levels increase, and animals lose weight (Wade, 1972). During diestrus, when endogenous estradiol is low, food consumption and body weight increase and activity decreases (Wang, 1923; Slonaker, 1924; Brobeck, Wheatland, and Strominger, 1967; Kennedy and Mitra, 1963). These fluctuations in energy balance are mediated by ovarian hormones; ovariectomy increases food intake and body weight and decreases wheel running (Kakolewski, Cox, and Valenstein, 1968), whereas subsequent estradiol replacement reverses this pattern (Zucker, 1969, 1970; Stern and Murphy, 1972; Tartellin and Gorski, 1973; Wade, 1975).

While these relationships suggest a primary effect of estradiol on food intake, the suppressive effects of estradiol on food consumption depend on the animals' body weight (Redick, Nussbaum, and Mook, 1973; Wade, 1972; Zucker, 1970; Landau and Zucker, 1976). This was indicated

x This study was undertaken as part of a senior honor's thesis by Ruth Noel. It was supported by Postdoctoral Fellowship MH-42175 from the National Institute of Mental Health awarded to A. Fleming and by Grant HD-02982 from the National Institute of Health to Irving Zucker. Many thanks to Darlene Frost, Margaret Roisman, and Elise Ravel for their technical assistance and to Maryann Wells for her help with the manuscript.

Present address: Tufts Medical School, Medford, Massachusetts 02155. Present address: Psychology Department, Erindale College, University of Toronto,

Mississuaga, Ontario, Canada, L5L 1C6. To whom requests for reprints should be addressed.

405 Copyright ~) 1977 by Academic Press, Inc. AI1 rights of reproduction in any form reserved. ISSN 0091-6773

406 NOEL AND FLEMING

by the observation that ovariectomized animals show only a transient increase in food intake; once an elevated body weight is attained, food consumption is reduced to control levels and is adequate to maintain the elevated body weight (Mook, Kennedy, Roberts, Nussbaum, and Rodier, 1972; Tartellin and Gorski, 1973). Conversely, estradiol seems to have suppressive effects on animals facing either impending or real obesity (Redick et al . , 1973; Landau and Zucker, 1976); if estradiol benzoate is administered at the time of ovariectomy, it prevents subsequent hyper- phagia and weight gain, and, if it is injected after the ovariectomized animal has become obese, it depresses food intake until the animals return to control body weights (Mook et al . , 1972; Tartellin and Gorski, 1973; Wade, 1975). It has no effect on food consumption in intact animals (Hervey and Hervey, 1965), in adrenalectomized-ovariectomized (adx- ovx) animals who maintain a low body weight (Redick et al . , 1973), or in ovariectomized animals raised in large litters who regulate at relatively low body weights (Zucker, 1970). However, if adx-ovx animals are made mildly obese by means of an attractive palatable diet, estradiol will exert suppressive effects on food intake (Redick et al . , 1973). These studies suggest that the effects of ovariectomy and estrogen on food consumption are dependent on the animal's body weight.

If, in fact, ovariectomy acts primarily to elevate the body weight at which the animal regulates (the body weight "set-point") and only secon- darily affects food consumption, one would predict that animals made obese prior to ovariectomy would not increase their food intake and body weight at the same rate as control ovariectomized animals. Also, exoge- nous estradiol would be expected to exert a suppressive effect on food intake and body weight in intact animals made obese through dietary means. The present paper is concerned with the first of these predictions.

Two- to three-month-old Sprague-Dawley virgin female rats were ob- tained from the Holtzman Company (Madison, Wisconsin); they weighed between 260-300 g at the start of the experiment.

On arrival, animals were housed in 8 x 8 x 12-in. cages in a room maintained on a LD 12:12 cycle (lights on at 0900 hr daily) and at a temperature of 23°C. The cages were equipped with a graduated water tube and a Wahmann feeder (LC-278), and animals were permitted ad lib access to both food and water. A nutritionally balanced powdered diet (S/L white diet, 24% protein, 6.73% fat) obtained from the Simonson Laboratories was used. Measurements of body weight (to the nearest 1.0 g) and food intake (to the nearest 0.1 g) were obtained daily between 0900 and 1100 hr throughout the experiment. Initially, an attempt was made to prefatten animals by providing them with a palatable high-fat diet (a combination of their powdered diet mixed together with vegetable short- ening and peanut butter in varying proportions). Although animals ate the diet, their rate of weight gain was slow, and, so, a force-feeding regimen

PREFATTENING AND OVX-INDUCED WEIGHT GAIN 407

was initiated. The two groups which were prefattened were force-fed a moderate carbohydrate liquid diet developed by Cohn and Joseph (1958).

The liquid diet was administered through an infant feeding tube (Ar- gyle, 8 Fr) twice daily between 1000 and 1200 hr and 1700 and 1900 hr. During the first days of force feeding, the volume injected at any one time varied between 5 and 8 ml. During the last 5 days, 10-15-ml meals were administered. Force feeding lasted 8 days.

When the force-feeding regimen ended, animals were divided into four groups. Seven force-fed animals were bilaterally ovariectomized by a lateral approach (ovx-obese); five were sham-ovx (sham-obese). Two additional groups of animals not undergoing the force-feeding procedure were also ovariectomized (ovx-reg, n = 5) or sham-operated (sham-reg, n = 5). Surgery was performed under ether anesthesia.

Food intake and body weight measurements which were initiated prior to the force-feeding regimen were continued for 30 days after surgery. Group differences on postoperative Days 6, 12, 18, 24, and 30 were evaluated using the Mann-Whitney U test. Changes over successive 6-day intervals (Days 6, 12, 18, 24, and 30) were analyzed by the Wilcoxen matched-pairs signed-ranks test; all differences achieved a P value of ~< 0.05.

Force-fed animals (designated obese) gained an average of 35 g over the 8-day force-feeding period, whereas control animals gained only 9 g. By the end of the force-feeding period, obese animals weighed a mean of 321 g, whereas controls weighed a mean of 292 g. Also, during force-feeding, ad lib food intake decreased from a mean of 13.6 g on the last day ofad lib feeding to a mean of 1.27 g on the last day of force-feeding. During the 8-day period, untreated control animals continued to eat between 10-15 g daily.

Figures 1 and 2 illustrate body weights and food intake for the four groups over the first 12 days after surgery and at 6-day intervals thereafter through Day 30. Body weights increased significantly for each ovx group from postoperative Days 6 to 12; however, the increments for the ovx-reg group were significantly greater than for the ovx-obese group (35.2 vs 15.7 g). By the twelfth day after ovariectomy, the two groups did not differ in body weight (ovx-reg, ~" = 322 g; ovx-obese, X = 320 g), and subsequent rates of body weight gain for the two groups were similar.

Both ovx groups ate significantly more on the twelfth than on the sixth day after ovariectomy, but they did not differ from one another in the extent of the increase (7.82 vs 7.92 g). On both the sixth and twelfth days, ovx-obese animals ate significantly less food than ovx-reg animals. From Day 18 through Days 24 and 30, both groups ate similar quantities of food.

Sham-obese animals weighed consistently more than sham-reg females throughout the 30-day period, but, on individual 6-day comparisons,

408 N O E L AND F L E M I N G

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differences were not significant after the sixth postoperat ive day. Simi- larly, the sham-reg animals consumed consistently more food than sham-obese rats until the thirtieth day after surgery; however , again, differences between the groups were significant only on the sixth day (sham-reg, ~" = 19.3 g; sham-obese, _~ = 14.74 g).

While ovx-reg animals weighed more than the sham-reg animals on the twelfth postoperat ive day, this difference achieved significance only on the eighteenth postoperat ive day; also differences remained significant throughout the twenty-fourth and thirtieth postsurgical days. A similar

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PREFATTENING AND OVX-INDUCED WEIGHT GAIN 409

trend was observed for the food intake scores; ovx-reg animals ate less than sham-reg animals on the sixth postoperative day; however, by post- operative Day 12, this trend had reversed, and, by Day 18, ovx animals were eating significantly more than sham-reg animals; this difference was maintained throughout the subsequent twenty-fourth and thirtieth days.

Ovx-obese and sham-obese groups did not differ significantly in body weight on Days 6, 12, 18, or 24; however, differences were significant by the postoperative Day 30. In contrast, ovx-obese animals ate significantly more food than the sham-obese animals on Days 12, 18, and 24.

While both ovariectomized groups showed weight gain and hyper- phagia following surgery, in both cases, there was a considerable delay before differences between ovx and sham groups achieved significance. This is probably due to the fact that ovx animals were more debilitated by the surgery than were their sham controls; they tended to lose more weight during the immediate postoperative period and, initially, seemed less inclined to eat. However, once recovery had occurred, both 0variec- tomized groups gained weight rapidly.

The fact that the sham-obese animals ate consistently less throughout the 30-day period than the sham-reg group again demonstrates the phe- nomenon of regulation. Sham-obese animals did not show the gradual increment in body weight seen in the sham-reg group; instead they main- tained an elevated body weight which remained relatively constant over the 30-day postoperative period. Over a longer postoperative period, it is likely that body weight and food intake differences between the two sham groups would have disappeared entirely.

These studies indicate that if animals are prefattened prior to ovariec- tomy, their rate of weight gain is reduced in comparison to the rate of the weight gain of animals not prefattened. Differences in weight gain are reflected in the differential food intakes of the obese and nonobese groups. However, once the two groups attain comparable body weights, the level of their food consumption and subsequent rate of weight gain are similar. On the basis of these results, one would predict that, had the obese group been force-fed to body weights normally achieved by ovariectomized animals when they reach asymptotic levels, they would have shown no increase in food intake or body weight if subsequently ovariectomized.

Thus these results support the hypothesis that ovarian hormones primarily affect body weight and not food consumption. They further indicate a gonadal hormonal involvement in the determination of a set- point around which the animal regulates its body weight (see Wade, 1976).

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410 NOEL AND FLEMING

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