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Ovariectomy-induced changes in food motivation in the rat
HORMONES
AND
BEHAVIOR
9, 120- 129 (1977)
Ovariectomy-Induced Motivation
Changes in Food in the Rat’
STEVEN K. GALES AND ANTHONY SCLAFANI Department
of Psychology,
Brooklyn Brooklyn,
College of the City University New York II210
of New York,
Following ovariectomy, adult female rats did not increase their food-reinforced bar pressing during 30-min tests, but they responded significantly more frequently for food than did control animals during 2- and 24-hr bar-press tests. Meal pattern data obtained during the 24-hr test demonstrate that ovariectomy increases meal size and decreases meal frequency, although the reduction in the number of meals did not fully compensate for the alteration in meal size. These findings suggest that ovariectomy does not increase the motivation to initiate a meal, but does result in the taking of a larger meal. The implication of these findings to body-weight set-point interpretations of ovarian obesity are briefly discussed.
Numerous studies have suggested that ovarian hormones significantly influence the food intake and body weight regulation of adult female rats, and more recent evidence now confirms that estradiol is the principle steroid involved (see Wade, 1976). Following the reduced estrogen levels produced by ovariectomy, rats become hyperphagic and obese (Kakolewski, Cox, and Valenstein, 1968; Leshner and Collier, 1973), and the elevation in food intake and body weight of pregnant, pseudopregnant, and lactating rats has also been attributed to a reduction in estrogen’s feeding inhibitory effects (Ota and Yokoyama, 1967; Wade and Zucker, 1969). Further, in intact cycling rats, it has long been known that food consumption increases during the diestrous stage of the ovarian cycle when estrogen secretion is diminished (Brobeck, Wheatland, and Strominger, 1947). Conversely, anorexia and weight loss have been reported following systemic (Landau and Zucker, 1976; Tarttelin and Gorski, 1973), subcutaneous (Dubuc, 1974; Montemurro, 1971), oral (Bull, Hurley, Kennet, Tamplin, and Williams, 1974), and in* This research was supported by National Institute of Mental Health Grant No. MH21563 and by Research Foundation of the City University of New York Grant Nos. 10103 and 11182. 2 To whom requests for reprints should be addressed at his present address: Obesity Research Center, St. Luke’s Hospital, Amsterdam Avenue at 114th Street, New York, N.Y. 10025. 120 Copyright @ 1977 by Academic Press, Inc. All rights of reproduction in any form reserved.
ISSN 0018-506X
OVARIECTOMY
AND FOOD MOTIVATION
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trahypothalamic estrogen administration (Beatty, O’Briant, and Vilberg, 1974; Jankowiak and Stern, 1974; Wade and Zucker, 1970) in ovariectomized animals, as well as during the estrous stage in intact rats when plasma estrogen levels are elevated (Tarttelin and Gorski, 1971). Despite the considerable evidence in support of estrogenic modulation of feeding in rats and other species (Czaja and Goy, 1975), little attention has been focused on the food-motivational effects produced by changes in ovarian hormone levels. Two studies have reported that cycling rats decrease their food-reinforced responding during estrous (Harris and Heistad, 1970; Jennings, 1973), but there are no published studies that have specifically examined the effects of ovariectomy on food-motivated behavior. Hodos and Valenstein (1960) reported that estrogen replacement does not influence bar pressing for food in ovariectomized rats, but no evidence was presented showing that the ovariectomized rats were hyperphagic at the time of testing, and the results are difficult to interpret since the subjects were concurrently being tested for intracranial selfstimulation. In a recent meal pattern study, Kenny and Mook (1974) reported that ovariectomy increases food-reinforced bar pressing in a free-feeding situation, but they did not examine the effects of deprivation on bar pressing for food. The purpose of the present study, therefore, was to investigate the effects of ovariectomy on food motivation. This was accomplished by comparing the food-reinforced bar-pressing performance of ovariectomized and control rats under a variety of test conditions. EXPERIMENT Method
1
Subjects Fifteen adult female CFE rats (Carworth Co.), about 100 days of age and weighing from 231 to 261 g, served as subjects. During the course of the experiment, one rat became ill and died, and an additional animal was discarded from the study because it failed to gain weight following ovariectomy. The subjects were individually housed in wire-mesh cages in an air-conditioned colony room with an ambient temperature of 23 t 2°C and were maintained on a 12-hr light-dark cycle. Apparatus Three identical Scientific Prototype operant chambers (23 x 22 x 20 cm) located in sound-attenuating cubicles were used. Each chamber contained a single response lever which, when depressed, permitted 5 set of access to a dipper cup containing 0.1 ml of milk diet. This diet, prepared daily, consisted of 300 ml of tap water mixed with an equal volume of
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sweetened condensed milk (Bordens, Inc.) and fortified with 0.6 ml of Polyvisol multivitamin supplement (Mead Johnson Co.). Procedure
Rats were allowed 10 days of free access to Purina chow and water in their home cage while food intake and body weight were measured daily. At the end of this baseline period, the animals were food restricted for several days until their body weights were reduced to 80% of their ad fibirum level. Subjects were then trained to bar press for milk rewards on a VI-I-min schedule, during daily 30-min test sessions. Body weights were maintained at 80% of the baseline level by giving the rats a ration of chow in their home cage 2 hr after each session. Water was available at all times in the home cage. The subjects were divided into two groups equated for food intake, body weight, and response rate, and they received either ovariectomy (OVX; n = 7) or sham operations (CON; n = 6). Ovariectomy was performed under Equi-Thesin anesthetic (2.5 ml/kg body wt) and involved bilateral removal of the ovaries, ovarian fat, and periovarial sac through dorsal incisions. Sham operations were performed in which identical incisions were made, and the ovaries, ovarian fat, and periovarial sac were exteriorized but left intact. Following surgery, the rats were tested in the operant chambers first at 80% and then at 90% of their baseline body weights during two successive 1Zday periods. The subjects were next given unlimited access to chow for 1 week and were tested for 12 additional days in the operant chambers, with food available ad lib in the home cage. Results and Discussion The bar-pressing results of this experiment are summarized in Table 1. Preoperatively, both groups were responding at very similar rates when maintained at 80% of their baseline body weights. Following surgery, the TABLE 1 Effect of Ovariectomy on Bar-Pressing Responses of Rats Maintained at Either 80%, 90%, or nd fib Body Weight Levels” Group
CON ovx
n
6 7
Preoperative 80% body wt
80% body wt
Postoperative 90% body wt
Ad lib body wt
456 443
470 424
305 288
167 128
D Bar-pressing responses measured over 30-min period. Response rates are based on the mean bar presses emitted on a VI-1-min reinforcement schedule during the final 5 days under each body weight condition.
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AND
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ovariectomized animals displayed a small decrease in bar pressing, and controls exhibited a slight increase in responding at the 80% weight level, but neither change reached significance. When tested at the 90% weight level, both.groups decreased responding and even further reduced their bar pressing when tested at ad libitum body weight levels. No significant between-group differences in bar pressing emerged under any of these test conditions [F (1, 12) = 1.Ol, P > 0.051 and, as expected, the deprivation effect was significant [F (3, 12) = 22.02, P < 0.011. During the week of free feeding prior to the ad Zibitum bar-press test, the OVX rats ate more chow [22.4 vs 18.5 g; t (11) = 3.68, P < 0.011 and gained more weight [33.6 vs 26.8 g; t (11) = 2.77, P < 0.011 than did CON rats. The OVX rats continued to overeat [22.9 vs 18.4 g; t (11) = 5.60, P < 0.011 and outgain CON animals throughout the 1Zday ad libitum test period [ 16.0 vs 7.5 g; t(l1) = 4.34, P < 0.011. These results indicate that ovariectomized rats, although hyperphagic when food is freely available, do not bar press more than do intact controls for milk rewards, when tested under either deprived or nondeprived conditions. The results also reveal that ovariectomized rats are as responsive as normal animals to changes in the deprivation state. That is, the OVX and CON groups displayed similar reductions in their barpressing performance as body weight levels were increased. EXPERIMENT
2
The finding of the first experiment failed to reveal enhanced food motivation in ovariectomized rats. Since some evidence indicates that ovariectomized rats bar press more than do controls for food pellets during 24-hr tests on a fixed-ratio schedule (Kenney and Mook, 1974), the present experiment investigated the food-motivational effects of ovariectomy using a similar food reward and reinforcement schedule during both short- and long-term test sessions. Method Subjects
Sixteen adult female CFE rats (Cat-worth Co.), about 100 days of age and weighing from 255 to 278 g, served as subjects. Data are presented for 13 animals that completed the study. Two rats were discarded because of postoperative illness, and the data for one additional rat were not included since it failed to display ovarian obesity following surgery. Apparatus
Subjects were tested in six identical Lehigh Valley operant chambers (Model No. 143-22) housed in sound-attenuating cubicles. Each chamber
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SCLAFANI
contained two response levers, and depression of the right bar was programmed to deliver a Standard Formula 45mg Noyes pellet (4.3 kcal/g) to a food cup adjacent to the lever. During the 24-hr test, water was available through a metal drinking tube which protruded 3 cm inside the chamber and was situated 4 cm from the food cup. The operant chambers were maintained on the same 12-hr light-dark cycle as that of the animal colony, and the temperature inside each chamber was approximately 23°C. Procedure Thirty-minute test. Rats were allowed 2 weeks of free access to Purina chow in their home cage, while food intake and body weight were measured daily. Following this preoperative baseline period, the animals were food deprived to 80% of their ad libitum body weights and were trained to bar press for Noyes pellets on an FR-32 schedule during daily 30-min sessions. The subjects were then divided into two groups equated for food intake, body weight, and rate of responding and were ovariectomized (OVX; n = 6) or sham ovariectomized (CON; it = 7) as described in Expt 1. Postoperatively, the animals were tested for 10 days on the FR schedule, with their body weight limited to the 80% level. Body weights were maintained at this level by giving the rats a ration of chow in their home cage 2 hr after each daily test session. Twenty-four-hour test. Following the above test, the animals were given unlimited access to chow in their home cage for 2 weeks. They were then returned to the operant chambers and were tested on the FR-32 schedule during 24-hr sessions for 15 days. An Esterline-Angus event recorder was used to obtain meal pattern data during the last 5 days of this period. Day and night meal frequencies were determined, with a meal being defined as a bout of eating containing pauses of no longer than 10 min. Mean daily meal size was calculated by dividing daily food intake by the total number of meals consumed over the 24-hr period. Two-hour test. The subjects were tested for 5 additional days on the FR schedule using 2-hr sessions. Bar-press responses were recorded during the first 30 min of each session as well as over the total 2-hr period. The rats were returned to their home cages for the remaining 22 hr of each day, where water, but no food, was made available. RESULTS
Thirty-Minute Test The bar-press findings of this and the other two tests are summarized in Fig. 1. Preoperatively, both groups responded at similar rates on the FR-32 schedule when maintained at the 80% body weight level. Following
OVARIECTOMY
I
AND FOOD MOTIVATION
30 minTEST
l-
24 hr.TEST
125
l-
t
L
BAR PRESSTESTS
FIG. 1. The mean FR-32 bar-pressing rates of ovariectomized (darkened bars) and sham-operated controls (blank bars) during the final 5 days of the 30-min, 24-hr, and 2-hr tests. The lower segments of the bars for the 2-hr test represent the response rate for the first 30 min.
surgery, the OVX rats displayed a small increase while CON animals exhibited a small decrease in responding, but neither change was significant. Twenty-Four-Hour
Test
When food was made available ad libitum both groups rapidly regained the weight lost during the previous deprivation period. By the end of this 2-week period, the OVX rats weighed slightly, but not significantly, more than did the CON animals (294 vs 282 g) and were eating reliably more food than were CON animals [24.9 vs 21.6 g; t (11) = 3.23, P < 0.011. During the 24-hr bar-press test, the OVX rats continued to overeat by responding significantly more frequently [t (11) = 2.72, P < 0.051 on the FR schedule than did the CON rats. Five of the six ovariectomized rats gained weight during this test period, while only two of the seven shamovariectomized animals increased their body-weight (weight change = 8.8 vs -4.1 g; t (I 1) = 3.61, P < 0.01). The meal pattern findings are summarized in Fig. 2. The ovariectomized rats consumed significantly fewer meals [t (11) = 3.18, P < 0.011 than did the controls. This reduced meal frequency was due primarily to a decrease in nightime meal taking [t (11) = 3.61, P < 0.011, and the daytime meal frequency of the OVX rats was not reliably different from that of the CON subjects. While 24 hr meal frequency was depressed, the
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GALE AND SCLAFANI
wx
CON
wx
CON
FIG. 2. Day and night meal frequency (left) and 24-hr meal size (right) of ovariectomized and sham-ovariectomized rats bar pressing for food pellets on the FR-32 reinforcement schedule.
ovariectomized rats consumed significantly larger meals [t (11) = 2.64, P < 0.05) than did the controls. This pattern of decreased meal number and increased meal size was observed in all of the ovariectomized rats. Two-Hour Test When limited to 2 hr/day in the operant chambers for 5 consecutive days, the OVX rats bar pressed more [t (11) = 2.64, P < 0.051 for food than did the CON rats. Furthermore, the mean response rate of the OVX rats exceeded [t (11) = 3.19, P < 0.011 that of the CON animals, even during the initial 30-min segment of this test. Compared with their bar pressing performance during the first postoperative 30-min test, the ovariectomized rats responded more frequently [t (5) = 2.92, P < 0.051, while the controls responded at nearly the same rate, during the 30-min segment of the present 2-hr test. The daily 2-hr bar press tests did not provide sufficient food to maintain body weight, and both groups lost similar amounts of weight (OVX = 11.8 and CON = 13.3 g) over the 5-day period. DISCUSSION
The results of the first postoperative test indicate that ovariectomy does not increase bar pressing for food during 30-min tests in deprived rats, and this confirms the findings obtained in Expt 1 using a different reward and a different reinforcement schedule. However, when tested in 24- and 2-hr sessions, the ovariectomized rats did respond significantly more frequently for food than did the sham-ovariectomized controls. This suggests that ovariectomized rats are more motivated to eat than control animals, but a 30-min test interval may be too short to reveal the differ-
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ence. This is not surprising, since the degree of hyperphagia produced by ovariectomy is relatively mild (approximately l&20% increase in the present study) and may be difficult to observe during a brief test period. At the same time, however, analysis of the initial 30-min segment of the 2-hr test indicates that ovariectomized rats will bar press more than controls during short test sessions under some conditions. The meal pattern data obtained during the 24-hr test indicate that ovariectomy significantly increases meal size, but decreases meal frequency. The decreased meal frequency did not fully compensate for the increased meal size however, and the ovariectomized rats consumed more food and gained more weight than did the sham-ovariectomized controls. These results confirm the findings of Kenney and Mook (1974) who used several different feeding situations to examine the meal patterns of ovariectomized rats. Similar results have also recently been reported by Blaustein and Wade (1976) who further observed that ovariectomized rats continue to eat large meals, even during the static phase of their obesity when food intake returns to preoperative levels. These meal pattern findings in the ovariectomized rat are consistent with the report of Drewett (1974) that intact rats increase meal size and decrease meal frequency during diestrus. The alterations in meal patterns produced by changes in estrogen availability have suggested that this hormone influences food intake primarily by affecting the rate of satiety during a meal (Blaustein and Wade, 1976; Drewett, 1974; Kenney and Mook, 1974). The reduced meal frequency observed following ovariectomy and during the diestrous stage of the ovarian cycle, according to this view, may represent an attempt to compensate for the increased meal size. It is interesting to note that ovariectomy has been reported to increase selectively norepinephrine synthesis in the anterior hypothalamus (Bapna, Neff, and Costa, 1971) and microinjections of norepinephrine into this site increase feeding when infused during, but not between, spontaneously initiated meals (Ritter and Epstein, 1975). Thus, it is possible that the larger meal size of the ovariectomized rat is brought about by an alteration in brain norepinephrine. The pattern of increased meal size and decreased meal frequency displayed by ovariectomized rats during ad libitum feeding may help to explain why they typically do not bar press more frequently for food, relative to controls, during short-term tests. That is, such tests primarily measure the animal’s willingness to initiate a meal, and ovariectomized rats may differ from intact animals not in their motivation to begin a meal, but in their willingness to take a larger meal. Thus, long-term tests which are sensitive to changes in meal size are more likely to reveal increased food motivation in ovariectomized animals than are short-term tests. Finally, the failure of ovariectomy to reliably increase 1~1foodreinforced responding soon after surgery during short-term bar-press tests
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questions recent set-point interpretations of ovarian obesity (Gale and Scalfani, 1977; Dubuc, 1974; Wade, 1976; Redick, Nussbaum, and Mook, 1973). That is, if ovariectomy increases food intake because it elevates the set point for body weight, then, it should also be expected to increase bar pressing for food, particularly on a VI schedule, since the results of Expt 1 demonstrate that such tests are sensitive to changes in body weight level. It is possible, however, that ovariectomy, rather than producing an abrupt shift, may result in a gradual elevation in body-weight set point, and this may not be detected in short-term bar-press tests. The concept of a sliding set point has been used to describe the weight regulation of hibernating animals (Barnes and Mrosovsky, 1974), and future research can determine whether it is also applicable to the weight changes produced by ovariectomy. REFERENCES Bapna, J., Neff, N. H., and Costa, E. (1971). A method for studying norepinephrine and serotonin metabolism in small regions of the rat brain: Effects of ovariectomy on amine metabolism in the anterior and posterior hypothalamus. Endocrinology 89, 1345-1349. Barnes, D. S., and Mrosovsky, N. (1974). Body weight regulation in ground squirrels and hypothalamically lesioned rats: Slow and sudden set point changes. Physiol. Behav. 12, 251-258. Beatty, W. W., O’Briant, D. A., and Vilberg, T. R. (1974). Suppression of feeding by intrahypothalamic implants of estradiol in male and female rats. Bull. Psychon. Sot. 3, 273-274. Blaustein, J. D., and Wade, G. N. (1976). Ovarian influences on the meal patterns of female rats. Physiol. Behav. 17, 201-208. Brobeck, J. R., Wheatland, M., and Stominger, J. L. (1947). Variations in regulation of energy exchange associated with estrus, diestrus, and pseudopregnancy in rats. Endocrinology
40, 65-72.
Bull, L. S., Hurley, W. L., Kennett, W. S., Tamplin, C. B., and Williams, W. F. (1974). Effect of sex hormones on feed intake in rats. J. Nurr. 104, 968-975. Czaja, J. A., and Goy, R. W. (1975). Ovarian hormones and food intake in female guinea pigs and rhesus monkeys. Horm. Behuv. 6, 329-349. Drewett, R. F. (1974). The meal patterns of the oestrous cycle and their motivational significance. Quurt. J. Exp. Psychol. 26, 489-494. Dubuc, P. U. (1974). Effects of estradiol implants on body weight regulation in castrated and intact female rats. Endocrinology 95, 1733-1736. Gale, S. K., and Sclafani, A. (1977). Comparison of the ovarian and hypothalamic obesity syndromes in the female rat: Effects of diet palatability on food intake and body weight regulation. J. Comp. Physiol. Psychol. 91, 381-392. Harris, W. C., and Heistad, G. T. (1970). Food-reinforced responding in rats during estrus. J. Camp. Physiol.
Psycho/.
70, 202-212.
Hodos, W., and Valenstein, E. S. (1960). Motivational variables affecting the rate of behavior maintained by intracranial stimulation. J. Camp. Physiol. Psychol. 53, 502508. Jankowiak, R., and Stem, J. J. (1974). Food intake and body weight modifications following medial hypothalamic hormone implants in female rats. Physiol. Behav. 12, 875-879. Jennings, W. A. (1973). Estrus anorexia: Single-tube intake and bar press rate in the albino rat. Physiol. Psycho/. 1, 369-312.
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Kakolewski, J. W., Cox, V. C., and Valenstein, E. S. (1968). Sex differences in body weight changes following gonadectomy in rats. Psycho/. Rep. 22, 547-554. Kenney, N. J., and Mook, D. G. (1974). Effects of ovariectomy on meal pattern in the albino rat. J. Comp. Physiol. Psychol. 87, 302-309. Landau, T., and Zucker, I. (1976). Estrogenic regulation of body weight in the female rat. Horm. Behav.
7, 29-39.
Leshner, A. I., and Collier, G. C. (1973). The effects of gonadectomy on the sex differences in dietary self-selection patterns and carcass compositions of rats. Physiol. Behav. 11, 671-676. Montemurro, D. G. (1971). Inhibition of hypothalamic obesity in the mouse with diethylstilbestrol. Can. J. Physiol. Pharmacol. 49, 554-558. Ota, K., and Yokoyama, A. (1967). Body weight and food consumption of lactating rats: Effects of ovariectomy and of arrest and resumption of suckling. J. Endocrinol. 38, 251-261. Redick, J. H., Nussbaum, A. I., and Mook, D. G. (1973). Estradiol induced suppression of feeding in the female rat: Dependence on body weight. Physiol. Behav. 10, 543-557. Ritter, R. C., and Epstein, A. N. (1975). Control of meal size by central noradrenergic action. Proc. Nat. Acad. Sci. USA 72, 3740-3743. Tarttelin, M. F., and Gorski, R. A. (1971). Variations in food and water intake in the normal and acyclic female rat. Physiol. Behav. 7, 847-852. Tarttelin, M. F., and Gorski, R. A. (1973). The effects of ovarian steroids on food and water intake and body weight in the female rat. Acta Endocrinol. 72, 551-568. Wade, G. N. (1976). Sex hormones, regulatory behaviors; and body weight. In J. S. Rosenblatt, R. A. Hinde, E. Shaw, and G. C. Beer (Eds)., Advances in the Study of Behavior, Vol 6. Academic Press, New York. Wade, G. N., and Zucker, I. (1969). Hormonal and developmental influences on rat saccharin preference. J. Comp. Physiol. Psychol. 69, 291-300. Wade, G. N., and Zucker, 1. (1970). Modulation of food intake and locomotor activity in female rats by diencephahc hormone implants. J. Camp. Physiol. Psychol. 72,328-338.
AND
BEHAVIOR
9, 120- 129 (1977)
Ovariectomy-Induced Motivation
Changes in Food in the Rat’
STEVEN K. GALES AND ANTHONY SCLAFANI Department
of Psychology,
Brooklyn Brooklyn,
College of the City University New York II210
of New York,
Following ovariectomy, adult female rats did not increase their food-reinforced bar pressing during 30-min tests, but they responded significantly more frequently for food than did control animals during 2- and 24-hr bar-press tests. Meal pattern data obtained during the 24-hr test demonstrate that ovariectomy increases meal size and decreases meal frequency, although the reduction in the number of meals did not fully compensate for the alteration in meal size. These findings suggest that ovariectomy does not increase the motivation to initiate a meal, but does result in the taking of a larger meal. The implication of these findings to body-weight set-point interpretations of ovarian obesity are briefly discussed.
Numerous studies have suggested that ovarian hormones significantly influence the food intake and body weight regulation of adult female rats, and more recent evidence now confirms that estradiol is the principle steroid involved (see Wade, 1976). Following the reduced estrogen levels produced by ovariectomy, rats become hyperphagic and obese (Kakolewski, Cox, and Valenstein, 1968; Leshner and Collier, 1973), and the elevation in food intake and body weight of pregnant, pseudopregnant, and lactating rats has also been attributed to a reduction in estrogen’s feeding inhibitory effects (Ota and Yokoyama, 1967; Wade and Zucker, 1969). Further, in intact cycling rats, it has long been known that food consumption increases during the diestrous stage of the ovarian cycle when estrogen secretion is diminished (Brobeck, Wheatland, and Strominger, 1947). Conversely, anorexia and weight loss have been reported following systemic (Landau and Zucker, 1976; Tarttelin and Gorski, 1973), subcutaneous (Dubuc, 1974; Montemurro, 1971), oral (Bull, Hurley, Kennet, Tamplin, and Williams, 1974), and in* This research was supported by National Institute of Mental Health Grant No. MH21563 and by Research Foundation of the City University of New York Grant Nos. 10103 and 11182. 2 To whom requests for reprints should be addressed at his present address: Obesity Research Center, St. Luke’s Hospital, Amsterdam Avenue at 114th Street, New York, N.Y. 10025. 120 Copyright @ 1977 by Academic Press, Inc. All rights of reproduction in any form reserved.
ISSN 0018-506X
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AND FOOD MOTIVATION
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trahypothalamic estrogen administration (Beatty, O’Briant, and Vilberg, 1974; Jankowiak and Stern, 1974; Wade and Zucker, 1970) in ovariectomized animals, as well as during the estrous stage in intact rats when plasma estrogen levels are elevated (Tarttelin and Gorski, 1971). Despite the considerable evidence in support of estrogenic modulation of feeding in rats and other species (Czaja and Goy, 1975), little attention has been focused on the food-motivational effects produced by changes in ovarian hormone levels. Two studies have reported that cycling rats decrease their food-reinforced responding during estrous (Harris and Heistad, 1970; Jennings, 1973), but there are no published studies that have specifically examined the effects of ovariectomy on food-motivated behavior. Hodos and Valenstein (1960) reported that estrogen replacement does not influence bar pressing for food in ovariectomized rats, but no evidence was presented showing that the ovariectomized rats were hyperphagic at the time of testing, and the results are difficult to interpret since the subjects were concurrently being tested for intracranial selfstimulation. In a recent meal pattern study, Kenny and Mook (1974) reported that ovariectomy increases food-reinforced bar pressing in a free-feeding situation, but they did not examine the effects of deprivation on bar pressing for food. The purpose of the present study, therefore, was to investigate the effects of ovariectomy on food motivation. This was accomplished by comparing the food-reinforced bar-pressing performance of ovariectomized and control rats under a variety of test conditions. EXPERIMENT Method
1
Subjects Fifteen adult female CFE rats (Carworth Co.), about 100 days of age and weighing from 231 to 261 g, served as subjects. During the course of the experiment, one rat became ill and died, and an additional animal was discarded from the study because it failed to gain weight following ovariectomy. The subjects were individually housed in wire-mesh cages in an air-conditioned colony room with an ambient temperature of 23 t 2°C and were maintained on a 12-hr light-dark cycle. Apparatus Three identical Scientific Prototype operant chambers (23 x 22 x 20 cm) located in sound-attenuating cubicles were used. Each chamber contained a single response lever which, when depressed, permitted 5 set of access to a dipper cup containing 0.1 ml of milk diet. This diet, prepared daily, consisted of 300 ml of tap water mixed with an equal volume of
122
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sweetened condensed milk (Bordens, Inc.) and fortified with 0.6 ml of Polyvisol multivitamin supplement (Mead Johnson Co.). Procedure
Rats were allowed 10 days of free access to Purina chow and water in their home cage while food intake and body weight were measured daily. At the end of this baseline period, the animals were food restricted for several days until their body weights were reduced to 80% of their ad fibirum level. Subjects were then trained to bar press for milk rewards on a VI-I-min schedule, during daily 30-min test sessions. Body weights were maintained at 80% of the baseline level by giving the rats a ration of chow in their home cage 2 hr after each session. Water was available at all times in the home cage. The subjects were divided into two groups equated for food intake, body weight, and response rate, and they received either ovariectomy (OVX; n = 7) or sham operations (CON; n = 6). Ovariectomy was performed under Equi-Thesin anesthetic (2.5 ml/kg body wt) and involved bilateral removal of the ovaries, ovarian fat, and periovarial sac through dorsal incisions. Sham operations were performed in which identical incisions were made, and the ovaries, ovarian fat, and periovarial sac were exteriorized but left intact. Following surgery, the rats were tested in the operant chambers first at 80% and then at 90% of their baseline body weights during two successive 1Zday periods. The subjects were next given unlimited access to chow for 1 week and were tested for 12 additional days in the operant chambers, with food available ad lib in the home cage. Results and Discussion The bar-pressing results of this experiment are summarized in Table 1. Preoperatively, both groups were responding at very similar rates when maintained at 80% of their baseline body weights. Following surgery, the TABLE 1 Effect of Ovariectomy on Bar-Pressing Responses of Rats Maintained at Either 80%, 90%, or nd fib Body Weight Levels” Group
CON ovx
n
6 7
Preoperative 80% body wt
80% body wt
Postoperative 90% body wt
Ad lib body wt
456 443
470 424
305 288
167 128
D Bar-pressing responses measured over 30-min period. Response rates are based on the mean bar presses emitted on a VI-1-min reinforcement schedule during the final 5 days under each body weight condition.
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AND
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ovariectomized animals displayed a small decrease in bar pressing, and controls exhibited a slight increase in responding at the 80% weight level, but neither change reached significance. When tested at the 90% weight level, both.groups decreased responding and even further reduced their bar pressing when tested at ad libitum body weight levels. No significant between-group differences in bar pressing emerged under any of these test conditions [F (1, 12) = 1.Ol, P > 0.051 and, as expected, the deprivation effect was significant [F (3, 12) = 22.02, P < 0.011. During the week of free feeding prior to the ad Zibitum bar-press test, the OVX rats ate more chow [22.4 vs 18.5 g; t (11) = 3.68, P < 0.011 and gained more weight [33.6 vs 26.8 g; t (11) = 2.77, P < 0.011 than did CON rats. The OVX rats continued to overeat [22.9 vs 18.4 g; t (11) = 5.60, P < 0.011 and outgain CON animals throughout the 1Zday ad libitum test period [ 16.0 vs 7.5 g; t(l1) = 4.34, P < 0.011. These results indicate that ovariectomized rats, although hyperphagic when food is freely available, do not bar press more than do intact controls for milk rewards, when tested under either deprived or nondeprived conditions. The results also reveal that ovariectomized rats are as responsive as normal animals to changes in the deprivation state. That is, the OVX and CON groups displayed similar reductions in their barpressing performance as body weight levels were increased. EXPERIMENT
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The finding of the first experiment failed to reveal enhanced food motivation in ovariectomized rats. Since some evidence indicates that ovariectomized rats bar press more than do controls for food pellets during 24-hr tests on a fixed-ratio schedule (Kenney and Mook, 1974), the present experiment investigated the food-motivational effects of ovariectomy using a similar food reward and reinforcement schedule during both short- and long-term test sessions. Method Subjects
Sixteen adult female CFE rats (Cat-worth Co.), about 100 days of age and weighing from 255 to 278 g, served as subjects. Data are presented for 13 animals that completed the study. Two rats were discarded because of postoperative illness, and the data for one additional rat were not included since it failed to display ovarian obesity following surgery. Apparatus
Subjects were tested in six identical Lehigh Valley operant chambers (Model No. 143-22) housed in sound-attenuating cubicles. Each chamber
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contained two response levers, and depression of the right bar was programmed to deliver a Standard Formula 45mg Noyes pellet (4.3 kcal/g) to a food cup adjacent to the lever. During the 24-hr test, water was available through a metal drinking tube which protruded 3 cm inside the chamber and was situated 4 cm from the food cup. The operant chambers were maintained on the same 12-hr light-dark cycle as that of the animal colony, and the temperature inside each chamber was approximately 23°C. Procedure Thirty-minute test. Rats were allowed 2 weeks of free access to Purina chow in their home cage, while food intake and body weight were measured daily. Following this preoperative baseline period, the animals were food deprived to 80% of their ad libitum body weights and were trained to bar press for Noyes pellets on an FR-32 schedule during daily 30-min sessions. The subjects were then divided into two groups equated for food intake, body weight, and rate of responding and were ovariectomized (OVX; n = 6) or sham ovariectomized (CON; it = 7) as described in Expt 1. Postoperatively, the animals were tested for 10 days on the FR schedule, with their body weight limited to the 80% level. Body weights were maintained at this level by giving the rats a ration of chow in their home cage 2 hr after each daily test session. Twenty-four-hour test. Following the above test, the animals were given unlimited access to chow in their home cage for 2 weeks. They were then returned to the operant chambers and were tested on the FR-32 schedule during 24-hr sessions for 15 days. An Esterline-Angus event recorder was used to obtain meal pattern data during the last 5 days of this period. Day and night meal frequencies were determined, with a meal being defined as a bout of eating containing pauses of no longer than 10 min. Mean daily meal size was calculated by dividing daily food intake by the total number of meals consumed over the 24-hr period. Two-hour test. The subjects were tested for 5 additional days on the FR schedule using 2-hr sessions. Bar-press responses were recorded during the first 30 min of each session as well as over the total 2-hr period. The rats were returned to their home cages for the remaining 22 hr of each day, where water, but no food, was made available. RESULTS
Thirty-Minute Test The bar-press findings of this and the other two tests are summarized in Fig. 1. Preoperatively, both groups responded at similar rates on the FR-32 schedule when maintained at the 80% body weight level. Following
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FIG. 1. The mean FR-32 bar-pressing rates of ovariectomized (darkened bars) and sham-operated controls (blank bars) during the final 5 days of the 30-min, 24-hr, and 2-hr tests. The lower segments of the bars for the 2-hr test represent the response rate for the first 30 min.
surgery, the OVX rats displayed a small increase while CON animals exhibited a small decrease in responding, but neither change was significant. Twenty-Four-Hour
Test
When food was made available ad libitum both groups rapidly regained the weight lost during the previous deprivation period. By the end of this 2-week period, the OVX rats weighed slightly, but not significantly, more than did the CON animals (294 vs 282 g) and were eating reliably more food than were CON animals [24.9 vs 21.6 g; t (11) = 3.23, P < 0.011. During the 24-hr bar-press test, the OVX rats continued to overeat by responding significantly more frequently [t (11) = 2.72, P < 0.051 on the FR schedule than did the CON rats. Five of the six ovariectomized rats gained weight during this test period, while only two of the seven shamovariectomized animals increased their body-weight (weight change = 8.8 vs -4.1 g; t (I 1) = 3.61, P < 0.01). The meal pattern findings are summarized in Fig. 2. The ovariectomized rats consumed significantly fewer meals [t (11) = 3.18, P < 0.011 than did the controls. This reduced meal frequency was due primarily to a decrease in nightime meal taking [t (11) = 3.61, P < 0.011, and the daytime meal frequency of the OVX rats was not reliably different from that of the CON subjects. While 24 hr meal frequency was depressed, the
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wx
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FIG. 2. Day and night meal frequency (left) and 24-hr meal size (right) of ovariectomized and sham-ovariectomized rats bar pressing for food pellets on the FR-32 reinforcement schedule.
ovariectomized rats consumed significantly larger meals [t (11) = 2.64, P < 0.05) than did the controls. This pattern of decreased meal number and increased meal size was observed in all of the ovariectomized rats. Two-Hour Test When limited to 2 hr/day in the operant chambers for 5 consecutive days, the OVX rats bar pressed more [t (11) = 2.64, P < 0.051 for food than did the CON rats. Furthermore, the mean response rate of the OVX rats exceeded [t (11) = 3.19, P < 0.011 that of the CON animals, even during the initial 30-min segment of this test. Compared with their bar pressing performance during the first postoperative 30-min test, the ovariectomized rats responded more frequently [t (5) = 2.92, P < 0.051, while the controls responded at nearly the same rate, during the 30-min segment of the present 2-hr test. The daily 2-hr bar press tests did not provide sufficient food to maintain body weight, and both groups lost similar amounts of weight (OVX = 11.8 and CON = 13.3 g) over the 5-day period. DISCUSSION
The results of the first postoperative test indicate that ovariectomy does not increase bar pressing for food during 30-min tests in deprived rats, and this confirms the findings obtained in Expt 1 using a different reward and a different reinforcement schedule. However, when tested in 24- and 2-hr sessions, the ovariectomized rats did respond significantly more frequently for food than did the sham-ovariectomized controls. This suggests that ovariectomized rats are more motivated to eat than control animals, but a 30-min test interval may be too short to reveal the differ-
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ence. This is not surprising, since the degree of hyperphagia produced by ovariectomy is relatively mild (approximately l&20% increase in the present study) and may be difficult to observe during a brief test period. At the same time, however, analysis of the initial 30-min segment of the 2-hr test indicates that ovariectomized rats will bar press more than controls during short test sessions under some conditions. The meal pattern data obtained during the 24-hr test indicate that ovariectomy significantly increases meal size, but decreases meal frequency. The decreased meal frequency did not fully compensate for the increased meal size however, and the ovariectomized rats consumed more food and gained more weight than did the sham-ovariectomized controls. These results confirm the findings of Kenney and Mook (1974) who used several different feeding situations to examine the meal patterns of ovariectomized rats. Similar results have also recently been reported by Blaustein and Wade (1976) who further observed that ovariectomized rats continue to eat large meals, even during the static phase of their obesity when food intake returns to preoperative levels. These meal pattern findings in the ovariectomized rat are consistent with the report of Drewett (1974) that intact rats increase meal size and decrease meal frequency during diestrus. The alterations in meal patterns produced by changes in estrogen availability have suggested that this hormone influences food intake primarily by affecting the rate of satiety during a meal (Blaustein and Wade, 1976; Drewett, 1974; Kenney and Mook, 1974). The reduced meal frequency observed following ovariectomy and during the diestrous stage of the ovarian cycle, according to this view, may represent an attempt to compensate for the increased meal size. It is interesting to note that ovariectomy has been reported to increase selectively norepinephrine synthesis in the anterior hypothalamus (Bapna, Neff, and Costa, 1971) and microinjections of norepinephrine into this site increase feeding when infused during, but not between, spontaneously initiated meals (Ritter and Epstein, 1975). Thus, it is possible that the larger meal size of the ovariectomized rat is brought about by an alteration in brain norepinephrine. The pattern of increased meal size and decreased meal frequency displayed by ovariectomized rats during ad libitum feeding may help to explain why they typically do not bar press more frequently for food, relative to controls, during short-term tests. That is, such tests primarily measure the animal’s willingness to initiate a meal, and ovariectomized rats may differ from intact animals not in their motivation to begin a meal, but in their willingness to take a larger meal. Thus, long-term tests which are sensitive to changes in meal size are more likely to reveal increased food motivation in ovariectomized animals than are short-term tests. Finally, the failure of ovariectomy to reliably increase 1~1foodreinforced responding soon after surgery during short-term bar-press tests
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questions recent set-point interpretations of ovarian obesity (Gale and Scalfani, 1977; Dubuc, 1974; Wade, 1976; Redick, Nussbaum, and Mook, 1973). That is, if ovariectomy increases food intake because it elevates the set point for body weight, then, it should also be expected to increase bar pressing for food, particularly on a VI schedule, since the results of Expt 1 demonstrate that such tests are sensitive to changes in body weight level. It is possible, however, that ovariectomy, rather than producing an abrupt shift, may result in a gradual elevation in body-weight set point, and this may not be detected in short-term bar-press tests. The concept of a sliding set point has been used to describe the weight regulation of hibernating animals (Barnes and Mrosovsky, 1974), and future research can determine whether it is also applicable to the weight changes produced by ovariectomy. REFERENCES Bapna, J., Neff, N. H., and Costa, E. (1971). A method for studying norepinephrine and serotonin metabolism in small regions of the rat brain: Effects of ovariectomy on amine metabolism in the anterior and posterior hypothalamus. Endocrinology 89, 1345-1349. Barnes, D. S., and Mrosovsky, N. (1974). Body weight regulation in ground squirrels and hypothalamically lesioned rats: Slow and sudden set point changes. Physiol. Behav. 12, 251-258. Beatty, W. W., O’Briant, D. A., and Vilberg, T. R. (1974). Suppression of feeding by intrahypothalamic implants of estradiol in male and female rats. Bull. Psychon. Sot. 3, 273-274. Blaustein, J. D., and Wade, G. N. (1976). Ovarian influences on the meal patterns of female rats. Physiol. Behav. 17, 201-208. Brobeck, J. R., Wheatland, M., and Stominger, J. L. (1947). Variations in regulation of energy exchange associated with estrus, diestrus, and pseudopregnancy in rats. Endocrinology
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Bull, L. S., Hurley, W. L., Kennett, W. S., Tamplin, C. B., and Williams, W. F. (1974). Effect of sex hormones on feed intake in rats. J. Nurr. 104, 968-975. Czaja, J. A., and Goy, R. W. (1975). Ovarian hormones and food intake in female guinea pigs and rhesus monkeys. Horm. Behuv. 6, 329-349. Drewett, R. F. (1974). The meal patterns of the oestrous cycle and their motivational significance. Quurt. J. Exp. Psychol. 26, 489-494. Dubuc, P. U. (1974). Effects of estradiol implants on body weight regulation in castrated and intact female rats. Endocrinology 95, 1733-1736. Gale, S. K., and Sclafani, A. (1977). Comparison of the ovarian and hypothalamic obesity syndromes in the female rat: Effects of diet palatability on food intake and body weight regulation. J. Comp. Physiol. Psychol. 91, 381-392. Harris, W. C., and Heistad, G. T. (1970). Food-reinforced responding in rats during estrus. J. Camp. Physiol.
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Hodos, W., and Valenstein, E. S. (1960). Motivational variables affecting the rate of behavior maintained by intracranial stimulation. J. Camp. Physiol. Psychol. 53, 502508. Jankowiak, R., and Stem, J. J. (1974). Food intake and body weight modifications following medial hypothalamic hormone implants in female rats. Physiol. Behav. 12, 875-879. Jennings, W. A. (1973). Estrus anorexia: Single-tube intake and bar press rate in the albino rat. Physiol. Psycho/. 1, 369-312.
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Kakolewski, J. W., Cox, V. C., and Valenstein, E. S. (1968). Sex differences in body weight changes following gonadectomy in rats. Psycho/. Rep. 22, 547-554. Kenney, N. J., and Mook, D. G. (1974). Effects of ovariectomy on meal pattern in the albino rat. J. Comp. Physiol. Psychol. 87, 302-309. Landau, T., and Zucker, I. (1976). Estrogenic regulation of body weight in the female rat. Horm. Behav.
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Leshner, A. I., and Collier, G. C. (1973). The effects of gonadectomy on the sex differences in dietary self-selection patterns and carcass compositions of rats. Physiol. Behav. 11, 671-676. Montemurro, D. G. (1971). Inhibition of hypothalamic obesity in the mouse with diethylstilbestrol. Can. J. Physiol. Pharmacol. 49, 554-558. Ota, K., and Yokoyama, A. (1967). Body weight and food consumption of lactating rats: Effects of ovariectomy and of arrest and resumption of suckling. J. Endocrinol. 38, 251-261. Redick, J. H., Nussbaum, A. I., and Mook, D. G. (1973). Estradiol induced suppression of feeding in the female rat: Dependence on body weight. Physiol. Behav. 10, 543-557. Ritter, R. C., and Epstein, A. N. (1975). Control of meal size by central noradrenergic action. Proc. Nat. Acad. Sci. USA 72, 3740-3743. Tarttelin, M. F., and Gorski, R. A. (1971). Variations in food and water intake in the normal and acyclic female rat. Physiol. Behav. 7, 847-852. Tarttelin, M. F., and Gorski, R. A. (1973). The effects of ovarian steroids on food and water intake and body weight in the female rat. Acta Endocrinol. 72, 551-568. Wade, G. N. (1976). Sex hormones, regulatory behaviors; and body weight. In J. S. Rosenblatt, R. A. Hinde, E. Shaw, and G. C. Beer (Eds)., Advances in the Study of Behavior, Vol 6. Academic Press, New York. Wade, G. N., and Zucker, I. (1969). Hormonal and developmental influences on rat saccharin preference. J. Comp. Physiol. Psychol. 69, 291-300. Wade, G. N., and Zucker, 1. (1970). Modulation of food intake and locomotor activity in female rats by diencephahc hormone implants. J. Camp. Physiol. Psychol. 72,328-338.