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Food intake, body weight and lordosis in male and female Mongolian gerbils: Effects of ovarian steroids
Physiology & Behavior, Vol. 35, pp. 767-774. Copyright©Pergamon Press Ltd., 1985. Printed in the U.S.A.
0031-9384/85 $3.00 + .00
Food Intake, Body Weight and Lordosis in Male and Female Mongolian Gerbils: Effects of Ovarian Steroids L Y N H. R A I B L E A N D B O R I S B. G O R Z A L K A 2
Department o f Psychology, University o f British Columbia, Vancouver, British Columbia, V6T 1 Y7, Canada R e c e i v e d 18 O c t o b e r 1984 RAIBLE, L. H. AND B. B. GORZALKA. Food intake, body weight and lordosis in male and female Mongolian gerbils: Effects of ovarian steroids. PHYSIOL BEHAV 35(5) 767-774, 1985.--The effects of gonadectomy and ovarian hormone treatment on food intake, body weight, and lordosis in male and female Mongolian gerbils were examined. In female gerbils, a significant decrease in food intake and body weight was observed after ovariectomy, with estradiol benzoate (1, 10, or 100/zg/day) increasing food intake in a dose dependent fashion. However, the dose of estrogen (1/zg) that restored food intake and body weight to control levels in ovariectomized animals was lower than that required to elicit maximal sexual receptivity. Progesterone, when given in conjunction with estrogen, significantly facilitated the effect of estrogen on food intake without further altering body weight. In male gerbils, castration produced a significant but transient increase in body weight and a delayed increase in food intake. Unlike female gerbils, male gerbils exhibited no significant alterations in food intake, body weight, or lordosis in response to treatment with ovarian steroids. The present results are compared to those obtained in other species. Estrogen
Progesterone
Ovariectomy
Castration
G O N A D A L steroids have been reported to influence food intake and body weight in a variety of species [20]. Ovariectomy results in a weight gain while estrogen treatment reduces body weight in female rats [18, 19, 21], mice [22], and guinea pigs [16]. In male rats, castration can lead to reductions in body weight and food intake [4,8]. Although treatment with low to moderate doses of testosterone propionate (TP) restores intake and weight to normal levels, high doses o f TP reduce weight [4]. It has been hypothesized that, with high doses of TP, sufficient aromatization of testosterone to estradiol may occur and that it is this estrogen that exerts the weight reducing effects [4,20]. This hypothesis is supported by evidence that progesterone (P), in addition to attenuating the effects o f estradiol benzoate (EB) in ovariectomized female rats [14, 15, 19] also attenuates the effects of high doses of TP in castrated male rats [4]. Although both androgens and the major ovarian hormones can alter food intake and body weight in the rat, evidence that ovarian hormones tend to be more effective suggests that these are the primary hormonal regulators of food intake and body weight in that species [20]. Ovariectomy and EB treatment have also been found to lead to weight gain and loss, respectively, in the female hamster [5,12]. The effects are less pronounced than those observed in the rat which may, according to Morin and Fleming, explain why some researchers (e.g., [9, 17, 24]) have failed to observe significant effects [12]. Unlike male rats,
Food intake
Body weight
Lordosis
male hamsters exhibit increases in food intake and body weight after castration [9, 17, 24]. Furthermore, EB appears to have no effect on body weight or food intake in the castrated male hamster [9,24] whereas TP treatment is inhibitory [24]. These results suggest that androgens, rather than estrogens, are the more important hormonal regulators o f food intake and body weight in the hamster [20]. Maass and Wade examined food intake and body weight changes in response to castration and androgen treatment in male Mongolian gerbils and in response to the administration of ovarian hormones in ovariectomized female gerbils [11]. Castration was found to increase body weight without altering food intake. This effect is somewhat similar to that observed in the male hamster, which also exhibits a weight increase after castration [16,24]. However, unlike the male hamster, the male gerbil exhibits no change in food intake or body weight in response to TP administration [11]. Maass and Wade also observed that ovariectomized, EB-treated gerbils gained significantly more weight and ate significantly more than ovariectomized gerbils given vehicle injections [11]. Even more intriguing was the finding that P, given in conjunction with EB, resulted in food intake levels significantly above those exhibited by animals given EB alone [11]. Whereas other female rodents exhibit a reduction in body weight and food intake in response to EB and an attenuation of this effect when also given P, female gerbils exhibit an increase in weight and intake in response to EB, with P
Whis research was supported by a Natural Sciences and Engineering Research Council of Canada Grant to B. B. Gorzalka. ~Requests for reprints should be addressed to B. B. Gorzalka.
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R A I B L E AND G O R Z A L K A
eliciting a further increase in food intake. The results of Maass and Wade are of considerable interest as they suggest that female gerbils are unlike other female rodents studied to date in their hormonal regulation of food intake and body weight [11]. However, the physiological significance of these results remains to be determined. Neither the effects of ovariectomy nor the relative functions of EB and P in restoring presurgical levels of food intake and body weight have been assessed. Although the results obtained by Maass and Wade [11] are provocative, further study may help clarify the nature of the hormonal regulation of food intake and body weight in the gerbil. The present experiments were designed to examine the effects of gonadectomy, EB dose, and the effects of P, administered alone and in combination with EB, on food intake and body weight in female Mongolian gerbils. Lordosis behavior was also examined to allow a comparison of the relative effects of ovarian hormones on food intake and sexual receptivity. Moreover, a parallel study in male gerbils will permit assessment of sex differences in responsiveness to ovarian hormones. G E N E R A L METHOD Male and female Mongolian gerbils (Meriones unguiculatus) were obtained from Tumblebrook Farms (West Brookfield, MA) at 90 days of age and housed individually in standard laboratory single cages with wire bottoms. A reversed 12:12-hr light-dark cycle was maintained throughout the experiment with food and water available ad lib. To prevent unconsumed food pellets from dropping through the cage bottoms, only pellets weighing more than 4.5 g were used. Food was placed inside the cages, rather than in external food hoppers, to insure that, during consumption, pellet fragments would fall onto the collection paper below each cage. Food intake was determined every 24 hr by subtracting the weight of the food remaining inside and below each cage from the weight of the food supplied 24 hr before. Food was weighed to the nearest 0.01 g on an analytical balance. Immediately after consumption data were collected, animals were weighed to the nearest 0.5 g on a triple beam balance and, where appropriate, received subcutaneous injections. Three days after the animals were introduced to the colony, daily measurements of food intake and body weight commenced and continued throughout the experiment. The first four days of measurements provided baseline data that were used to subdivide the animals into four matched groups. Animals were then either gonadectomized while under ether anesthesia (experimental groups) or sham gnnadectomized (control group). During phases of hormone administration, intact control animals (IC) received injections of the vehicle, 0.01 cc peanut oil. The doses of EB employed were 1/xg (group E 1), 10/zg (group E 10), or 100/zg (group El00) daily. These approximate low, moderate, and high doses of EB with respect to sexual receptivity in the absence of progesterone [13]. A dose of 100/~g P was used to determine the potential effects of progesterone administration on food intake and body weight in EB-primed animals. Tests of lordosis occurred on Days 3-12 of EB treatment. Several hours after the onset of the dark cycle, animals were tested with a sexually vigorous male gerbil in a 30 cm (L) × 30 cm (W) x 45.5 cm (H) Plexiglas chamber. Tests were terminated after 20 rain or ten mounts. For all but a few animals, tests were completed before the 20 min maximum
interval. Receptivity scores were calculated by determining the lordosis quotient (LQ): the ratio of female lordotic responses to male mounts with pelvic thrusting, multiplied by 100. Thus, an LQ near 100 indicates a high level of receptivity while an LQ near 0 indicates a low level of receptivity. Since IC animals did not receive EB treatment and would not be receptive daily, they were not tested for lordosis behavior. Near the end of the treatment interval with EB alone and toward the beginning of subsequent treatment with E B + P , several animals in groups El0 and El00 became ill and were sacrificed. Autopsy indicated evidence of internal hemorrhage, presumably related to the chronic EB treatment. Consequently, injections were terminated for all animals in those groups and data from that phase were excluded from analysis. Those groups were excluded from further testing. If animals were obviously ill near the beginning of a treatment phase, their individual scores from the prior phase were also excluded. As a result of these procedures, all El0 males and El0 and El00 females were terminated from the study during the E B + P phase whereas El00 males were terminated during the EB phase. Analyses of variance were performed on the data provided by females and by males in groups IC, El, and El0 and, for females, group El00 during the first two phases of each experiment (gonadectomy and EB treatment). Separate analyses were also performed on data generated by groups IC and E 1 throughout all phases of each experiment. All subsidiary analyses used the Newman-Keul's procedure with a set at 0.05. EXPERIMENT 1 Work in our laboratory on the sexual behavior of female gerbils has indicated that ovariectomized gerbils are less sensitive to EB than ovariectomized rats. In gerbils, administration of 10 ttg EB for eight consecutive days, unaccompanied by progesterone, results in moderate levels of receptivity [13]. In rats, similar or even greater levels of receptivity can be induced by as little as 0.8/zg EB given for eight consecutive days [1, 6, 7]. It is possible that the lesser sensitivity to EB in female gerbils is not restricted to sexual behavior. Therefore, in the present experiment, the differential effects of EB dose on sexual behavior, food intake and body weight are examined. METHOD
Once body weights had stabilized after surgery (Phase 1, 48 days in length), EB treatments (Phase 2, 33 days in length) were initiated and continued until the average body weight of E1 animals was similar to that of the IC animals. During Phase 3 (30 days in length), animals in groups El, El0, and El00 received 100/zg P daily in conjunction with their respective EB doses while IC animals continued to receive injections of the vehicle. When body weight had again stabilized, EB was withdrawn (Phase 4, 22 days in length) and E1 animals continued to receive 100/xg P daily (groups El0 and El00 had been terminated by this phase). Finally, P was withdrawn (Phase 5, 19 days in length) and the effects of no hormone treatment observed. After exclusion of some animals as described under the General Method section, the resultant numbers of subjects for food intake and body weight data were 8 for groups IC and El0, and 7 for groups El and El00. As sexual testing occurred in the early stages of EB treatment, before any adverse effects of EB were
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FIG. 1. Effects of ovariectomy (Ovex) and ovarian steroid administration on food intake and l~ody weight. Prior to surgery (S), baseline (B) data were recorded until food intake and body weights had stabilized. The top panel represents the first phase in the sequence of treatments; the bottom panel represents the last phase. Three day averages for intact control (IC) and ovariectomized animals receiving 1/zg estradiol benzoate (El) during estradiol benzoate (EB) phases are presented for each phase of the experiment. Progesterone (P) was administered at a dose of 100/zg. Three day averages for groups El0 (10 /zg EB) and El00 (100 /zg EB) are presented for the fh'st two phases of the experiment.
observed, the number of animals for groups E l , E l 0 , and El00 used in this analysis was 9. RESULTS AND DISCUSSION
Analyses of variance for food intake and body weight were performed for the IC, E l , E l 0 , and El00 groups for Phases 1 and 2 only. Additional analyses of variance were performed for the IC and E1 groups for phases 1-5.
All Groups, Phases 1-2 Three day averages for body weight and food intake are presented for all groups in Fig. 1. An examination of the data suggests that E l , E l 0 , and El00 animals exhibited a depressed level of body weight and food intake after ovariectomy while EB treatment increased weight and intake. Analysis of the body weight data indicated a significant group x days interaction, F(24,208)=4.01, p<0.0001.
Further analysis of this interaction revealed that IC animals weighed significantly more than E l , E l 0 , and El00 animals from Day 36 of Phase 1 (ovariectomy) through Day 12 of Phase 2 (EB treatment), p<0.05. By Day 30 of Phase 2, E l , E l 0 , and El00 animals had body weights similar to those of IC animals but El00 animals weighed significantly more than E1 and E l 0 animals (/9<0.05). Thus, ovariectomy depressed body weight while EB treatment restored it to, or increased it beyond, control levels, depending upon the dose. The analysis of variance also indicated a significant effect of Phase on body weight, F(1,26)= 170.79, p<0.0001. An analysis of the food intake data revealed a significant group x Phase interaction, F(3,26)=6.11, p=0.003, with further analysis indicating that El00 animals ate more than all other groups during the EB phase (o<0.05) while E1 and E l 0 animals were found to consume about the same as control animals. A significant effect of Phase on food intake was also found, F(1,26)=42.43, p<0.0001.
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FIG. 2. Effects of ovariectomy (Ovex) and ovarian steroid administration on food intake and body weight in female gerbils relative to levels exhibited by intact control animals. Daily measures of food intake and body weight were averaged to create five data points per treatment phase, presented on the abscissa. The ordinate represents the ratio of group El (1/zg estradinl benzoate, EB) means to intact control means, expressed as a percentage. The dose of progesterone (P) used was 100/xg.
IC and E1 Groups, Phases 1-5
The ratio of means for EI:IC, expressed as a percentage, is presented in Fig. 2. In addition to the effects described previously for Phases 1 and 2, it appears that the addition of progesterone (Phase 3) increased food intake without further altering body weight. Withdrawal of EB (Phase 4) decreased body weight without further altering food intake and withdrawal of P (Phase 5) appeared to cause no further weight loss and an eventual return of normal food consumption. Analyses of the weight data of groups IC and E1 for all phases of the experiment indicated a significant group x Phase interaction, F(4,52)=6.17, p =0.0004. Further analysis of this interaction revealed that, in addition to the effects previously described, IC and E1 animals were of similar weight during Phase 3 (EB+P). It was also found that E1 animals weighed less than IC animals during Phases 4 and 5 (P and no hormone treatmen0. The finding that the body weight of E1 animals during Phase 4 did not differ from that of Phase 5 suggests that the weight decrease observed in Phase 4 was due, in part to the withdrawal of EB treatment rather than to an effect of P. Other effects found to be significant for body weight data were Phase, F(4,52)=33.73, p<0.0001, days, F(20,260) =15.91, p<0.0001, and group × days, F(20,260)=4.77, p<0.0001. Analysis of the food intake data of groups IC and E1 for all phases of the experiment indicated a significant group x Phase interaction for food intake, F(4,52)=2.96, p=0.03. Further analysis of this interaction indicated that, in addition to the effects previously described, El animals ate significantly more than IC animals during Phase 3 09<0.05). E1 animals also ate more during E B + P treatment than they had when given EB alone (p<0.05), suggesting that P acted in a synergistic or an additive manner with EB in regulating food intake. However, as indicated by the analysis of the b o d y weight data, this effect of progesterone did not extend to body weight. The analysis of the group x Phase interaction also revealed that E1 animals ate more than IC animals during Phases 4 and 5 (P and no hormone treatment; p<0.05). Thus, the reduced weight of IC animals during these phases was not due to a decrease in food intake. Although the withdrawal of
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FIG. 3. Sexual receptivity in female gerbils administered various doses of estradiol benzoate (EB). Receptivity is expressed as the lordosis to mount ratio, multiplied by 100. Animals received daily injections of 1, 10, or 100/zg EB beginning on Day 1. Testing occurred on Days 3-12 of EB administration.
EB might have been partially responsible for the weight decrease observed in Phase 4, it is also possible that P was exerting a metabolic effect which required elevated food intake to counteract an increase in metabolic rate. The present experiment does not allow for the differentiation of these possibilities. Other effects found to be significant for food intake data were Phase, F(4,52)=10.56, p<0.0001, and days, F(20,260) = 1.83, p =0.02. Overall, these results suggest that ovariectomy decreases body weight and food intake whereas estrogen serves to increase body weight and food intake, effects opposite to those found in other laboratory rodents. Furthermore, rather than attenuating the effects of EB, P appears to act in an additive and/or synergistic manner with EB to increase food intake without altering body weight. Lordosis Behavior
Lordosis quotients are presented in Fig. 3. An examination of the data suggests that only 100/~g EB, administered chronically, was sufficient to elicit high levels of receptivity. Furthermore, there appears to be no difference in receptivity elicited by the 1/~g or 10/~g EB doses. Analysis of the data revealed a significant effect of group, F(2,24)=10.11, p=0.0007, a significant effect of days, F(9,216)=46.83, p<0.0001, and a significant group x days interaction, F(18,216)=4.70, p<0.0001. Further analysis revealed that the 100/~g EB group differed from the 1/~g and 10/~g EB groups (p<0.05). The two lower doses did not differ from each other. Thus, it would appear that the dose of EB that adequately restores food intake and body weight after ovariectomy (1 ~g) does not fully restore sexual receptivity. Furthermore, the female gerbil would appear to be less sensitive to EB than the female rat, which exhibits high levels of receptivity when given EB chronically in doses substantially lower than 10/zg per day [1]. EXPERIMENT 2 Ovarian hormones were found to exert an effect on food intake and body weight in the female gerbil (Experiment 1). It is possible that they may also exert effects on consumma,
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FIG. 4. Effects of castration and ovarian steroid administration on food intake and body weight in male gerbils. Prior to surgery (S), baseline (B) data were recorded until food intake and body weights had stabilized. The top panel represents the first phase in the sequence of treatments; the bottom pane] represents the last phase. Three day averages for intact control ([C) and castrated animals r e c e i v ~ ! /~g estradiol benzoate (El) during estradiol benzoate (EB) phases are presented for each phase of the experiment. Progesterone (P) was administered at a dose of ]00/~g. Three day averages for group El0 (10/~g EB) are presented only for the first two phases of the expefimem.
tory behavior and body weight regulation in the male gerbil. Evidence indicates that, while castration increases body weight in the male gerbil, androgen replacement has little influence on food intake and body weight in male castrates [11]. However, it may be that both female and male gerbils are more sensitive to the major ovarian hormones with respect to food intake and body weight, as appears to be the case in the rat [20]. Thus, the following experiment was carried out to investigate the possible effects of estrogen and progesterone on these responses in the male gerbil. In addition, the effects of castration were examined in an attempt to replicate the finding o f Maass and Wade that castration increases body weight, a result seemingly inconsistent with the failure of androgen treatment to influence food intake and body weight [11]. METHOD In order that any effects observed in the male could be compared to effects seen in the female, initiation o f the five treatment phases followed the time course established for Experiment 1. F o r example, EB was administered for the same number of days after surgery in males as for females and continued the same length o f time before Phase 3 started. However, the detrimental effects of the 100/~g EB dose were observed to manifest themselves more rapidly in the males
than they did in the females and thus, group El00 was terminated before the completion of the EB phase and was eliminated from all experimental analyses. All analyses performed were otherwise identical to those done for Experiment 1. The resulting numbers of animals per group were 10 for groups IC and E l 0 and 7 for group E l . Because neither 1 nor 10/~g EB elicited sexual receptivity in male gerbils, no analysis was performed on these data. RESULTS AND DISCUSSION
IC, El, EIO Groups, Phases 1-2 Three day averages for food intake and body weight data are presented in Fig. 4. A n examination of the data for groups IC, E1 and E l 0 suggests that castration had a transient faeilitatory effect on body weight and that EB administration appeared to increase food intake only slightly. Although castrates and castrates given EB appeared to eat more than intact animals, an analysis of.variance indicated no significant effect of castration on food intake. This m a y have been due, in part, to the within subjects nature of the design, which could have increased the variance when different treatment phases were collapsed to determine an overall castration effect. Indeed, a Kruskal-Wallis test performed on the IC, E1 and E l 0 data for Phases 1 and 2 revealed a significant effect of castration of food intake,
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FIG, 5. Effects of castration and ovarian steroid administration on food intake and body weight in male gerbils relative to levels exhibited by intact control animals. Daily measures of food intake and body weight were averaged to create five data points per treatment phase, presented on the abscissa. The ordinate represents the ratio of group El (1/zg estradiol benzoate, EB) means to intact control means, expressed as a percentage. The dose of progesterone (P) used was 100/zg.
Z=4.68, p<0.05. An analysis of body weight data revealed a significant effect of Phase, F(1,24)= 19.65, p=0.0002, and a significant effect of days, F(8,192)=2.98, p=0.0004. An analysis of variance performed on the food intake data indicated a significant effect of days, F(8,192)=7.79, p<0.0001. The group x days interaction approached significance, F(16,192)= 1.66, p <0.06. IC and El Groups, Phases 1-5 Figure 5 shows the ratio of food intake and body weight means for group E1 relative to group IC, expressed as a percentage. Examination of the data for groups IC and E1 in all phases of the experiment indicate that castration had only a transient effect on body weight and that EB, EB+P, and perhaps P treatments had no effect on body weight. Food intake levels for E 1 animals were elevated throughout the experiment. An analysis of variance performed on the body weight data revealed a significant group x days interaction, F(20,300)=3.37, p<0.0001. Further analysis of this interaction indicated that E 1 animals weighed more than IC animals only through Day 3 of the castration phase (p<0.05). Weights of IC and E1 animals did not differ during the EB and EB+P phases. However, beginning with Day 15 of the P only phase and continuing through Day 6 of the P withdrawal phase, IC animals weighed significantly more than E1 animals (p<0.05). By Day 18 of the P withdrawal phase, this had reversed, with IC animals now weighing less than E1 animals (p<0.05), A significant effect of Phase on body weight was also found, F(4,60)=28.03, p<0.0001. Analysis of food intake data revealed a significant Phase x group interaction, F(4,60)=2.77, p<0.04. Further analysis of this interaction indicated that food intake for E 1 animals was significantly greater than that of controls during the final phase of the experiment (p<0.05). Other effects found to be significant were Phase, F(4,60)=6.05, p =0.0004, and days, F(20,300)=4.69, p<0.0001. Overall, these results suggest that castration may have only temporary effects on body weight. Ovarian hormones appeared to exert little effect on food intake and body weight
GENERAL DISCUSSION The results of the present experiments indicate that female gerbils exhibit a significant depression in food intake and body weight after ovariectomy while male gerbils exhibit a transient increase in body weight after castration. Ovariectomized female gerbils treated chronically with EB exhibited a dose-dependent increase in food intake and body weight, with body weight eventually reaching levels displayed by intact controls. Thus, EB appears to serve a restorative function in the ovariectomized gerbil. Castrated male gerbils exhibited no significant alterations in food intake or body weight in response to EB treatment. In the female gerbil, the administration of P in conjunction with EB produced no significant alterations in body weight although it did produce a significant increase in food intake. In the male gerbil, the administration of P in conjunction with EB appeared to exert no effect on food intake or body weight. Treatment of ovariectomized females with P alone produced mean body weights below those of controls while mean food intake remained above that displayed by controls. The withdrawal of P led to a gradual return of body weight to control levels while food intake remained elevated. In the male gerbil, P administration appeared to produce a decrease in body weight observed after ovariectomy are due to the removal of food intake and a gradual return of body weight to control levels. The EB-induced increase in food intake and body weight observed in females in Experiment 1 is consistent with the results obtained by Maass and Wade [ll]. The present results also indicate that the decreases in food intake and body weight observed after ovariectomy are due to the removal of endogenous estrogens and that EB treatment serves a restorative function. When administered in conjunction with EB, P was found to increase food intake but not body weight in the female. These results are similar to those obtained by Maass and Wade [ll]. This suggests that estrogen and progesterone may interact to alter metabolic activity. Thus, the animal may require greater food intake in order to maintain body weight. The results of Experiment 1 further extend the f'mdings of Maass and Wade [11] by indicating that the administration of EB alone is sufficient to restore both the food intake and body weight of ovariectomized animals to control levels. Indeed, the further increase in food intake observed in animals also receiving progesterone is not necessary for the restoration of body weight to control levels. Analysis of the P only phase suggested that the decrease in female body weight during this phase may be due, in part, to the prior removal of EB. This possibility was strengthened by the finding that the withdrawal of P did not lead to a significant increase in body weight, which one would expect if P administration were responsible for the decrease in body weight. Females did, however, exhibit a return to control levels of food intake after P withdrawal, indicating that progesterone may have been partially responsible for the elevated food intake levels during the P only phase. The fmding that ovariectomy depresses, and EB treatment increases, food intake and body weight in the female gerbil is in direct contrast to results obtained in other rodent species. In other female rodents, ovariectomy has been
FOOD INTAKE AND LORDOSIS IN GERBILS found to increase food intake and body weight [5, 12, 16, 18, 20, 21, 22]. In addition, progesterone has been found to attenuate rather than facilitate the effects of EB in other female rodents [20] . Knowledge of the mechanisms underlying these species differences may aid in refining present models of the hormonal mediation of food intake and body weight in rodents. The results of Experiment 2 indicated that male gerbils exhibit a transient increase in body weight after castration, a result somewhat similar to that observed in the hamster [9]. Maass and Wade observed a longer lasting increase in body weight in their gerbil castrates [11]. In addition, Maass and Wade observed no significant effect of castration on food intake in the male gerbil [11] while our results indicate that castration may exert a delayed effect on food intake. A delayed effect of castration on behavior is not unusual and has been reported for alterations in food intake and body weight in male rats [4] and for sexual behavior and scent marking in male gerbils [23]. The finding that castrated male gerbils exhibited no significant alterations in food intake or body weight in response to EB treatment is of some interest, particularly in light of Maass and Wade's findings [11]. Maass and Wade found that castrated male gerbils failed to respond to testosterone propionate or dihydrotestosterone propionate treatment [11]. These results plus those of Experiment 2 suggest that neither androgens nor estrogens play a major role in the regulation of food intake in the male gerbil. This is in apparent contrast to results obtained in male rats and hamsters, where the effects of castration on food intake and body weight can be reversed by the administration of testosterone or estradiol [4, 8, 9, 24]. Also apparently in contrast with data for male rats and hamsters is the finding that P, when administered alone, produces a decrease in the body weights of castrated male gerbils. That the observed decrease in body weight was due to P treatment is supported by the finding that the withdrawal of P led to a gradual increase in body weight. It should be noted that eventually these animals surpassed the control group in body weight. Even more unusual was the finding that, in the male gerbil, the withdrawal of P led to a significant increase in food intake. The general elevation of food intake in male castrates throughout the experiment makes this result difficult to interpret. However, there is no indication that P administration was having any more effect on food intake or body weight than EB or EB given in conjunction with P, treatments which themselves appeared to have little effect. Yet, even though the administration of P did not appear to decrease food intake, its withdrawal apparently led to increases in food intake. It is possible that ovarian steroids were exerting a slight inhibitory effect on food intake and, perhaps, body weight that was masked by (or was masking) a delayed effect of castration. If so, it would not be the withdrawal of P p e r se that led to alterations in food intake and body weight. Rather, it would be the complete withdrawal of all ovarian steroids that produced effects. This type of inhibition might partially explain the rather unusual result obtained by Maass and Wade [11]. In their study, the administration of TP to castrated male gerbils produced no observable alteration in food intake or body
773 weight. However, when Maass and Wade ceased TP treatment, male body weight increased [11]. Presumably, the delayed effects of castration in either experiment would have been an increase in food intake and/or body weight if steroid treatment had not been initiated. In regards to sexual receptivity, the present experiments indicated that castrated male gerbils exhibited essentially no female sexual behavior during chronic EB administration whereas female gerbils exhibited high levels of receptivity when given 100/~g EB/day. Doses of 1 and 10/~g EB/day led to moderate receptivity. Thus, although both 1 and 10/zg EB/day were sufficient for the restoration of food intake and body weight to control levels, these doses were not capable of eliciting maximal receptivity. This is in contrast to the female rat, where chronic administration of less than 1 /xg EB/day can elicit high levels of receptivity [1, 6, 7] as well as significantly alter food intake and body weight [10,19]. It would appear that the neural substrates regulating sexual responding in the female gerbil are either less sensitive to EB or more progesterone-dependent than those of the female rat. Furthermore, the marked difference in sensitivity to estrogen seen between the system regulating food intake and body weight and the system regulating sexual responding is a feature not apparent in the female rat. Male gerbils did not exhibit significant female sexual behavior when treated chronically with EB. This is in contrast to the male rat which will show low levels of female sexual behavior when treated chronically with doses such as 20/zg EB/kg/day [2]. The 10 fig EB dose used in the present study represents a dose of about 120/zg/kg/day. Although this dose is substantially larger than that used by Eriksson and S6dersten [2], it failed to elicit any appreciable level of sexual receptivity in castrated male gerbils. Thus, it would appear that both male and female gerbils are less sensitive than male and female rats to the effects of EB on female sexual responding. This again indicates that sexual receptivity in the gerbil may be more like that of the hamster, which also exhibits less sensitivity to EB and is more progesteronedependent than the rat [3]. In summary, ovarian steroids were found to exert a major effect on food intake and body weight in the female gerbil whereas the effects of ovarian steroids on food intake and body weight in the male gerbil remain equivocal. In the female gerbil, EB treatment appears to serve a restorative function in the ovariectomized animal, bringing food intake and body weight back to presurgical levels. Our results also indicate that the system regulating food intake and body weight in the gerbil is more sensitive to estrogen than the system regulating female sexual behavior. Finally, the present results suggest that exogenous quantities of estrogen which induce receptivity in male and female rats are insufficient for the induction of receptivity in male and female gerbils. It seems likely that ovarian steroids play a major role in the hormonal mediation of food intake and body weight in the female gerbil. Although the results of Experiment 2 and those obtained by Maass and Wade [11] suggest that the hormonal regulation of food intake and body weight in the male gerbil differs from that observed in other male rodents, a great deal of study will be required before the mechanisms underlying these differences can be determined.
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RAIBLE AND GORZALKA REFERENCES
1. Davidson, J. M., C. H. Rogers, E. R. Smith and C. J. Bloch. Stimulation of female sexual behavior in adrenalectomized rats with estrogen alone. Endocrinology 82: 193-195, 1968. 2. Eriksson, H. and P. S6dersten. A failure to facilitate lordosis behavior in adrenalectomized and gonadectomized estrogenprimed rats with monoamine-synthesis inhibitors. Horm Behav 4: 89-97, 1973. 3. Feder, H. H., H. Siegel and G. N. Wade. Uptake of [6,7-3H] estradiol-17/3 in ovariectomized rats, guinea pigs, and hamsters: Correlation with species differences in behavioral responsiveness to estradiol. Brain Res 71: 93-103, 1974. 4. Gentry, R. T. and G. N. Wade. Androgenic control of food intake and body weight in male rats. J Comp Physiol Psychol 90: 18-25, 1976. 5. Gerall, A. A. and A. R. Thiel. Effects of perinatal gonadal secretions on parameters of receptivity and weight gain in hamsters. J Comp Physiol Psychol 89: 580--589, 1975. 6. Gorzalka, B. B. and L. H. Raible. Facilitation of lordosis behavior in rats by social isolation: Adrenal mediation. Physiol Behav 27: 603-607, 1981. 7. Gray, D. S. and B. B. Gorzalka. Adrenal steroid interactions in female sexual behavior: A review. Psychoneuroendocrinology 5: 157-175, 1980. 8. Kakolewski, J. W., V. C. Cox and E. S. Valenstein. Sex differences in body weight changes following gonadectomy of rats. Psychol Rep 22: 547-554, 1968. 9. Kowaleski, K. Effect of pre-pubertal gonadectomy and treatment with sex hormones on body growth, weight of organs and skin collagen of hamsters. Acta Endocrinol (Copenh) 61: 48-56, 1969. 10. Landau, I. T. and I. Zucker. Estrogenic regulation of body weight in the female rat. Horm Behav 7: 29-39, 1976. 11. Maass, C. A. and G. N. Wade. Effects of gonadal hormones on eating and body weight in Mongolian gerbils (Meriones unguiculatus). Horm Behav 9: 178-187, 1977. 12. Morin, L. P. and A. S. Fleming. Variation of food intake and body weight with estrous cycle, ovariectomy, and estradioi benzoate treatment in hamsters (Mesocricetus auratus). J Comp Physiol Psychol 92: 1-6, 1978.
13. Raible, L. H. Gonadal hormone regulation of behavior in the Mongolian gerbil. M.A. Thesis, University of British Columbia, 1982. 14. Roberts, S., N. J. Kenny and D. G. Mook. Overeating induced by progesterone in the ovariectomized, adrenalectomized rat. Horm Behav 3: 267-276, 1972. 15. Ross, G. E. and I. Zucker. Progesterone and the ovarianadrenal modulation of energy balance in rats. Horm Behav 5." 43--62, 1974. 16. Slob, A. K., R. W. Goy and J. J. van der Werff ten Bosch. Sex differences in growth of guinea pigs and their modification by neonatal gonadectomy and prenatally administered androgen. J Endocrinol 58: 11-19, 1973. 17. Swanson, H. H. Effects ofpre- and post-pubertal gonadectomy on sex differences in growth, adrenal and pituitary weight of hamsters. J Endocrinol 39: 555-564, 1967. 18. Tarttelin, M. F. and R. A. Gorski. The effects of ovarian steroids on food and water intake and body weight in female rats. Acta Endocrinol (Copenh) 72: 551-568, 1973. 19. Wade, G. N. Some effects of ovarian hormones on food intake and body weight in female rats. J Comp Physiol Psychol 88: 183-193, 1975. 20. Wade, G. N. Sex hormones, regulatory behaviors, and body weight. In: Advances in the Study of Behavior, edited by J. Rosenblatt, R. Hinde, E. Shaw and C. G. Beer. New York: Academic Press, 1976, pp. 201-279. 21. Wade, G. N. and I. Zucker. Development of hormonal control over food intake and body weight in female rats. J Comp Physiol Psychol 70: 213-220, 1970. 22. Wright, P. and C. Turner. Sex differences in body weight following gonadectomy and goldthioglucose injections in mice. Physiol Behav 11: 155-159, 1973. 23. Yahr, P., A. Newman and D. R. Stepbens. Sexual behavior and scent marking in male gerbils: Comparison of changes after castration and testosterone replacement. Horm Behav 13: 175-184, 1979. 24. Zucker, I., G. N. Wade and R. Ziegler. Sexual and hormonal influences on eating, taste preferences, and body weight of hamsters. Physiol Behav 8: 101-111, 1972.
0031-9384/85 $3.00 + .00
Food Intake, Body Weight and Lordosis in Male and Female Mongolian Gerbils: Effects of Ovarian Steroids L Y N H. R A I B L E A N D B O R I S B. G O R Z A L K A 2
Department o f Psychology, University o f British Columbia, Vancouver, British Columbia, V6T 1 Y7, Canada R e c e i v e d 18 O c t o b e r 1984 RAIBLE, L. H. AND B. B. GORZALKA. Food intake, body weight and lordosis in male and female Mongolian gerbils: Effects of ovarian steroids. PHYSIOL BEHAV 35(5) 767-774, 1985.--The effects of gonadectomy and ovarian hormone treatment on food intake, body weight, and lordosis in male and female Mongolian gerbils were examined. In female gerbils, a significant decrease in food intake and body weight was observed after ovariectomy, with estradiol benzoate (1, 10, or 100/zg/day) increasing food intake in a dose dependent fashion. However, the dose of estrogen (1/zg) that restored food intake and body weight to control levels in ovariectomized animals was lower than that required to elicit maximal sexual receptivity. Progesterone, when given in conjunction with estrogen, significantly facilitated the effect of estrogen on food intake without further altering body weight. In male gerbils, castration produced a significant but transient increase in body weight and a delayed increase in food intake. Unlike female gerbils, male gerbils exhibited no significant alterations in food intake, body weight, or lordosis in response to treatment with ovarian steroids. The present results are compared to those obtained in other species. Estrogen
Progesterone
Ovariectomy
Castration
G O N A D A L steroids have been reported to influence food intake and body weight in a variety of species [20]. Ovariectomy results in a weight gain while estrogen treatment reduces body weight in female rats [18, 19, 21], mice [22], and guinea pigs [16]. In male rats, castration can lead to reductions in body weight and food intake [4,8]. Although treatment with low to moderate doses of testosterone propionate (TP) restores intake and weight to normal levels, high doses o f TP reduce weight [4]. It has been hypothesized that, with high doses of TP, sufficient aromatization of testosterone to estradiol may occur and that it is this estrogen that exerts the weight reducing effects [4,20]. This hypothesis is supported by evidence that progesterone (P), in addition to attenuating the effects o f estradiol benzoate (EB) in ovariectomized female rats [14, 15, 19] also attenuates the effects of high doses of TP in castrated male rats [4]. Although both androgens and the major ovarian hormones can alter food intake and body weight in the rat, evidence that ovarian hormones tend to be more effective suggests that these are the primary hormonal regulators of food intake and body weight in that species [20]. Ovariectomy and EB treatment have also been found to lead to weight gain and loss, respectively, in the female hamster [5,12]. The effects are less pronounced than those observed in the rat which may, according to Morin and Fleming, explain why some researchers (e.g., [9, 17, 24]) have failed to observe significant effects [12]. Unlike male rats,
Food intake
Body weight
Lordosis
male hamsters exhibit increases in food intake and body weight after castration [9, 17, 24]. Furthermore, EB appears to have no effect on body weight or food intake in the castrated male hamster [9,24] whereas TP treatment is inhibitory [24]. These results suggest that androgens, rather than estrogens, are the more important hormonal regulators o f food intake and body weight in the hamster [20]. Maass and Wade examined food intake and body weight changes in response to castration and androgen treatment in male Mongolian gerbils and in response to the administration of ovarian hormones in ovariectomized female gerbils [11]. Castration was found to increase body weight without altering food intake. This effect is somewhat similar to that observed in the male hamster, which also exhibits a weight increase after castration [16,24]. However, unlike the male hamster, the male gerbil exhibits no change in food intake or body weight in response to TP administration [11]. Maass and Wade also observed that ovariectomized, EB-treated gerbils gained significantly more weight and ate significantly more than ovariectomized gerbils given vehicle injections [11]. Even more intriguing was the finding that P, given in conjunction with EB, resulted in food intake levels significantly above those exhibited by animals given EB alone [11]. Whereas other female rodents exhibit a reduction in body weight and food intake in response to EB and an attenuation of this effect when also given P, female gerbils exhibit an increase in weight and intake in response to EB, with P
Whis research was supported by a Natural Sciences and Engineering Research Council of Canada Grant to B. B. Gorzalka. ~Requests for reprints should be addressed to B. B. Gorzalka.
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R A I B L E AND G O R Z A L K A
eliciting a further increase in food intake. The results of Maass and Wade are of considerable interest as they suggest that female gerbils are unlike other female rodents studied to date in their hormonal regulation of food intake and body weight [11]. However, the physiological significance of these results remains to be determined. Neither the effects of ovariectomy nor the relative functions of EB and P in restoring presurgical levels of food intake and body weight have been assessed. Although the results obtained by Maass and Wade [11] are provocative, further study may help clarify the nature of the hormonal regulation of food intake and body weight in the gerbil. The present experiments were designed to examine the effects of gonadectomy, EB dose, and the effects of P, administered alone and in combination with EB, on food intake and body weight in female Mongolian gerbils. Lordosis behavior was also examined to allow a comparison of the relative effects of ovarian hormones on food intake and sexual receptivity. Moreover, a parallel study in male gerbils will permit assessment of sex differences in responsiveness to ovarian hormones. G E N E R A L METHOD Male and female Mongolian gerbils (Meriones unguiculatus) were obtained from Tumblebrook Farms (West Brookfield, MA) at 90 days of age and housed individually in standard laboratory single cages with wire bottoms. A reversed 12:12-hr light-dark cycle was maintained throughout the experiment with food and water available ad lib. To prevent unconsumed food pellets from dropping through the cage bottoms, only pellets weighing more than 4.5 g were used. Food was placed inside the cages, rather than in external food hoppers, to insure that, during consumption, pellet fragments would fall onto the collection paper below each cage. Food intake was determined every 24 hr by subtracting the weight of the food remaining inside and below each cage from the weight of the food supplied 24 hr before. Food was weighed to the nearest 0.01 g on an analytical balance. Immediately after consumption data were collected, animals were weighed to the nearest 0.5 g on a triple beam balance and, where appropriate, received subcutaneous injections. Three days after the animals were introduced to the colony, daily measurements of food intake and body weight commenced and continued throughout the experiment. The first four days of measurements provided baseline data that were used to subdivide the animals into four matched groups. Animals were then either gonadectomized while under ether anesthesia (experimental groups) or sham gnnadectomized (control group). During phases of hormone administration, intact control animals (IC) received injections of the vehicle, 0.01 cc peanut oil. The doses of EB employed were 1/xg (group E 1), 10/zg (group E 10), or 100/zg (group El00) daily. These approximate low, moderate, and high doses of EB with respect to sexual receptivity in the absence of progesterone [13]. A dose of 100/~g P was used to determine the potential effects of progesterone administration on food intake and body weight in EB-primed animals. Tests of lordosis occurred on Days 3-12 of EB treatment. Several hours after the onset of the dark cycle, animals were tested with a sexually vigorous male gerbil in a 30 cm (L) × 30 cm (W) x 45.5 cm (H) Plexiglas chamber. Tests were terminated after 20 rain or ten mounts. For all but a few animals, tests were completed before the 20 min maximum
interval. Receptivity scores were calculated by determining the lordosis quotient (LQ): the ratio of female lordotic responses to male mounts with pelvic thrusting, multiplied by 100. Thus, an LQ near 100 indicates a high level of receptivity while an LQ near 0 indicates a low level of receptivity. Since IC animals did not receive EB treatment and would not be receptive daily, they were not tested for lordosis behavior. Near the end of the treatment interval with EB alone and toward the beginning of subsequent treatment with E B + P , several animals in groups El0 and El00 became ill and were sacrificed. Autopsy indicated evidence of internal hemorrhage, presumably related to the chronic EB treatment. Consequently, injections were terminated for all animals in those groups and data from that phase were excluded from analysis. Those groups were excluded from further testing. If animals were obviously ill near the beginning of a treatment phase, their individual scores from the prior phase were also excluded. As a result of these procedures, all El0 males and El0 and El00 females were terminated from the study during the E B + P phase whereas El00 males were terminated during the EB phase. Analyses of variance were performed on the data provided by females and by males in groups IC, El, and El0 and, for females, group El00 during the first two phases of each experiment (gonadectomy and EB treatment). Separate analyses were also performed on data generated by groups IC and E 1 throughout all phases of each experiment. All subsidiary analyses used the Newman-Keul's procedure with a set at 0.05. EXPERIMENT 1 Work in our laboratory on the sexual behavior of female gerbils has indicated that ovariectomized gerbils are less sensitive to EB than ovariectomized rats. In gerbils, administration of 10 ttg EB for eight consecutive days, unaccompanied by progesterone, results in moderate levels of receptivity [13]. In rats, similar or even greater levels of receptivity can be induced by as little as 0.8/zg EB given for eight consecutive days [1, 6, 7]. It is possible that the lesser sensitivity to EB in female gerbils is not restricted to sexual behavior. Therefore, in the present experiment, the differential effects of EB dose on sexual behavior, food intake and body weight are examined. METHOD
Once body weights had stabilized after surgery (Phase 1, 48 days in length), EB treatments (Phase 2, 33 days in length) were initiated and continued until the average body weight of E1 animals was similar to that of the IC animals. During Phase 3 (30 days in length), animals in groups El, El0, and El00 received 100/zg P daily in conjunction with their respective EB doses while IC animals continued to receive injections of the vehicle. When body weight had again stabilized, EB was withdrawn (Phase 4, 22 days in length) and E1 animals continued to receive 100/xg P daily (groups El0 and El00 had been terminated by this phase). Finally, P was withdrawn (Phase 5, 19 days in length) and the effects of no hormone treatment observed. After exclusion of some animals as described under the General Method section, the resultant numbers of subjects for food intake and body weight data were 8 for groups IC and El0, and 7 for groups El and El00. As sexual testing occurred in the early stages of EB treatment, before any adverse effects of EB were
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FIG. 1. Effects of ovariectomy (Ovex) and ovarian steroid administration on food intake and l~ody weight. Prior to surgery (S), baseline (B) data were recorded until food intake and body weights had stabilized. The top panel represents the first phase in the sequence of treatments; the bottom panel represents the last phase. Three day averages for intact control (IC) and ovariectomized animals receiving 1/zg estradiol benzoate (El) during estradiol benzoate (EB) phases are presented for each phase of the experiment. Progesterone (P) was administered at a dose of 100/zg. Three day averages for groups El0 (10 /zg EB) and El00 (100 /zg EB) are presented for the fh'st two phases of the experiment.
observed, the number of animals for groups E l , E l 0 , and El00 used in this analysis was 9. RESULTS AND DISCUSSION
Analyses of variance for food intake and body weight were performed for the IC, E l , E l 0 , and El00 groups for Phases 1 and 2 only. Additional analyses of variance were performed for the IC and E1 groups for phases 1-5.
All Groups, Phases 1-2 Three day averages for body weight and food intake are presented for all groups in Fig. 1. An examination of the data suggests that E l , E l 0 , and El00 animals exhibited a depressed level of body weight and food intake after ovariectomy while EB treatment increased weight and intake. Analysis of the body weight data indicated a significant group x days interaction, F(24,208)=4.01, p<0.0001.
Further analysis of this interaction revealed that IC animals weighed significantly more than E l , E l 0 , and El00 animals from Day 36 of Phase 1 (ovariectomy) through Day 12 of Phase 2 (EB treatment), p<0.05. By Day 30 of Phase 2, E l , E l 0 , and El00 animals had body weights similar to those of IC animals but El00 animals weighed significantly more than E1 and E l 0 animals (/9<0.05). Thus, ovariectomy depressed body weight while EB treatment restored it to, or increased it beyond, control levels, depending upon the dose. The analysis of variance also indicated a significant effect of Phase on body weight, F(1,26)= 170.79, p<0.0001. An analysis of the food intake data revealed a significant group x Phase interaction, F(3,26)=6.11, p=0.003, with further analysis indicating that El00 animals ate more than all other groups during the EB phase (o<0.05) while E1 and E l 0 animals were found to consume about the same as control animals. A significant effect of Phase on food intake was also found, F(1,26)=42.43, p<0.0001.
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FIG. 2. Effects of ovariectomy (Ovex) and ovarian steroid administration on food intake and body weight in female gerbils relative to levels exhibited by intact control animals. Daily measures of food intake and body weight were averaged to create five data points per treatment phase, presented on the abscissa. The ordinate represents the ratio of group El (1/zg estradinl benzoate, EB) means to intact control means, expressed as a percentage. The dose of progesterone (P) used was 100/xg.
IC and E1 Groups, Phases 1-5
The ratio of means for EI:IC, expressed as a percentage, is presented in Fig. 2. In addition to the effects described previously for Phases 1 and 2, it appears that the addition of progesterone (Phase 3) increased food intake without further altering body weight. Withdrawal of EB (Phase 4) decreased body weight without further altering food intake and withdrawal of P (Phase 5) appeared to cause no further weight loss and an eventual return of normal food consumption. Analyses of the weight data of groups IC and E1 for all phases of the experiment indicated a significant group x Phase interaction, F(4,52)=6.17, p =0.0004. Further analysis of this interaction revealed that, in addition to the effects previously described, IC and E1 animals were of similar weight during Phase 3 (EB+P). It was also found that E1 animals weighed less than IC animals during Phases 4 and 5 (P and no hormone treatmen0. The finding that the body weight of E1 animals during Phase 4 did not differ from that of Phase 5 suggests that the weight decrease observed in Phase 4 was due, in part to the withdrawal of EB treatment rather than to an effect of P. Other effects found to be significant for body weight data were Phase, F(4,52)=33.73, p<0.0001, days, F(20,260) =15.91, p<0.0001, and group × days, F(20,260)=4.77, p<0.0001. Analysis of the food intake data of groups IC and E1 for all phases of the experiment indicated a significant group x Phase interaction for food intake, F(4,52)=2.96, p=0.03. Further analysis of this interaction indicated that, in addition to the effects previously described, El animals ate significantly more than IC animals during Phase 3 09<0.05). E1 animals also ate more during E B + P treatment than they had when given EB alone (p<0.05), suggesting that P acted in a synergistic or an additive manner with EB in regulating food intake. However, as indicated by the analysis of the b o d y weight data, this effect of progesterone did not extend to body weight. The analysis of the group x Phase interaction also revealed that E1 animals ate more than IC animals during Phases 4 and 5 (P and no hormone treatment; p<0.05). Thus, the reduced weight of IC animals during these phases was not due to a decrease in food intake. Although the withdrawal of
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FIG. 3. Sexual receptivity in female gerbils administered various doses of estradiol benzoate (EB). Receptivity is expressed as the lordosis to mount ratio, multiplied by 100. Animals received daily injections of 1, 10, or 100/zg EB beginning on Day 1. Testing occurred on Days 3-12 of EB administration.
EB might have been partially responsible for the weight decrease observed in Phase 4, it is also possible that P was exerting a metabolic effect which required elevated food intake to counteract an increase in metabolic rate. The present experiment does not allow for the differentiation of these possibilities. Other effects found to be significant for food intake data were Phase, F(4,52)=10.56, p<0.0001, and days, F(20,260) = 1.83, p =0.02. Overall, these results suggest that ovariectomy decreases body weight and food intake whereas estrogen serves to increase body weight and food intake, effects opposite to those found in other laboratory rodents. Furthermore, rather than attenuating the effects of EB, P appears to act in an additive and/or synergistic manner with EB to increase food intake without altering body weight. Lordosis Behavior
Lordosis quotients are presented in Fig. 3. An examination of the data suggests that only 100/~g EB, administered chronically, was sufficient to elicit high levels of receptivity. Furthermore, there appears to be no difference in receptivity elicited by the 1/~g or 10/~g EB doses. Analysis of the data revealed a significant effect of group, F(2,24)=10.11, p=0.0007, a significant effect of days, F(9,216)=46.83, p<0.0001, and a significant group x days interaction, F(18,216)=4.70, p<0.0001. Further analysis revealed that the 100/~g EB group differed from the 1/~g and 10/~g EB groups (p<0.05). The two lower doses did not differ from each other. Thus, it would appear that the dose of EB that adequately restores food intake and body weight after ovariectomy (1 ~g) does not fully restore sexual receptivity. Furthermore, the female gerbil would appear to be less sensitive to EB than the female rat, which exhibits high levels of receptivity when given EB chronically in doses substantially lower than 10/zg per day [1]. EXPERIMENT 2 Ovarian hormones were found to exert an effect on food intake and body weight in the female gerbil (Experiment 1). It is possible that they may also exert effects on consumma,
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FIG. 4. Effects of castration and ovarian steroid administration on food intake and body weight in male gerbils. Prior to surgery (S), baseline (B) data were recorded until food intake and body weights had stabilized. The top panel represents the first phase in the sequence of treatments; the bottom pane] represents the last phase. Three day averages for intact control ([C) and castrated animals r e c e i v ~ ! /~g estradiol benzoate (El) during estradiol benzoate (EB) phases are presented for each phase of the experiment. Progesterone (P) was administered at a dose of ]00/~g. Three day averages for group El0 (10/~g EB) are presented only for the first two phases of the expefimem.
tory behavior and body weight regulation in the male gerbil. Evidence indicates that, while castration increases body weight in the male gerbil, androgen replacement has little influence on food intake and body weight in male castrates [11]. However, it may be that both female and male gerbils are more sensitive to the major ovarian hormones with respect to food intake and body weight, as appears to be the case in the rat [20]. Thus, the following experiment was carried out to investigate the possible effects of estrogen and progesterone on these responses in the male gerbil. In addition, the effects of castration were examined in an attempt to replicate the finding o f Maass and Wade that castration increases body weight, a result seemingly inconsistent with the failure of androgen treatment to influence food intake and body weight [11]. METHOD In order that any effects observed in the male could be compared to effects seen in the female, initiation o f the five treatment phases followed the time course established for Experiment 1. F o r example, EB was administered for the same number of days after surgery in males as for females and continued the same length o f time before Phase 3 started. However, the detrimental effects of the 100/~g EB dose were observed to manifest themselves more rapidly in the males
than they did in the females and thus, group El00 was terminated before the completion of the EB phase and was eliminated from all experimental analyses. All analyses performed were otherwise identical to those done for Experiment 1. The resulting numbers of animals per group were 10 for groups IC and E l 0 and 7 for group E l . Because neither 1 nor 10/~g EB elicited sexual receptivity in male gerbils, no analysis was performed on these data. RESULTS AND DISCUSSION
IC, El, EIO Groups, Phases 1-2 Three day averages for food intake and body weight data are presented in Fig. 4. A n examination of the data for groups IC, E1 and E l 0 suggests that castration had a transient faeilitatory effect on body weight and that EB administration appeared to increase food intake only slightly. Although castrates and castrates given EB appeared to eat more than intact animals, an analysis of.variance indicated no significant effect of castration on food intake. This m a y have been due, in part, to the within subjects nature of the design, which could have increased the variance when different treatment phases were collapsed to determine an overall castration effect. Indeed, a Kruskal-Wallis test performed on the IC, E1 and E l 0 data for Phases 1 and 2 revealed a significant effect of castration of food intake,
772
RAIBLE AND GORZALKA Cast ration
No Hormone
EB+ P
.
130
while withdrawal of P may have served to increase food intake above postcastration levels.
"
.
120 . 110 -
100
1
'°t
1111, 'ill
I"
l = Weight Q= Inlake
80
FIG, 5. Effects of castration and ovarian steroid administration on food intake and body weight in male gerbils relative to levels exhibited by intact control animals. Daily measures of food intake and body weight were averaged to create five data points per treatment phase, presented on the abscissa. The ordinate represents the ratio of group El (1/zg estradiol benzoate, EB) means to intact control means, expressed as a percentage. The dose of progesterone (P) used was 100/zg.
Z=4.68, p<0.05. An analysis of body weight data revealed a significant effect of Phase, F(1,24)= 19.65, p=0.0002, and a significant effect of days, F(8,192)=2.98, p=0.0004. An analysis of variance performed on the food intake data indicated a significant effect of days, F(8,192)=7.79, p<0.0001. The group x days interaction approached significance, F(16,192)= 1.66, p <0.06. IC and El Groups, Phases 1-5 Figure 5 shows the ratio of food intake and body weight means for group E1 relative to group IC, expressed as a percentage. Examination of the data for groups IC and E1 in all phases of the experiment indicate that castration had only a transient effect on body weight and that EB, EB+P, and perhaps P treatments had no effect on body weight. Food intake levels for E 1 animals were elevated throughout the experiment. An analysis of variance performed on the body weight data revealed a significant group x days interaction, F(20,300)=3.37, p<0.0001. Further analysis of this interaction indicated that E 1 animals weighed more than IC animals only through Day 3 of the castration phase (p<0.05). Weights of IC and E1 animals did not differ during the EB and EB+P phases. However, beginning with Day 15 of the P only phase and continuing through Day 6 of the P withdrawal phase, IC animals weighed significantly more than E1 animals (p<0.05). By Day 18 of the P withdrawal phase, this had reversed, with IC animals now weighing less than E1 animals (p<0.05), A significant effect of Phase on body weight was also found, F(4,60)=28.03, p<0.0001. Analysis of food intake data revealed a significant Phase x group interaction, F(4,60)=2.77, p<0.04. Further analysis of this interaction indicated that food intake for E 1 animals was significantly greater than that of controls during the final phase of the experiment (p<0.05). Other effects found to be significant were Phase, F(4,60)=6.05, p =0.0004, and days, F(20,300)=4.69, p<0.0001. Overall, these results suggest that castration may have only temporary effects on body weight. Ovarian hormones appeared to exert little effect on food intake and body weight
GENERAL DISCUSSION The results of the present experiments indicate that female gerbils exhibit a significant depression in food intake and body weight after ovariectomy while male gerbils exhibit a transient increase in body weight after castration. Ovariectomized female gerbils treated chronically with EB exhibited a dose-dependent increase in food intake and body weight, with body weight eventually reaching levels displayed by intact controls. Thus, EB appears to serve a restorative function in the ovariectomized gerbil. Castrated male gerbils exhibited no significant alterations in food intake or body weight in response to EB treatment. In the female gerbil, the administration of P in conjunction with EB produced no significant alterations in body weight although it did produce a significant increase in food intake. In the male gerbil, the administration of P in conjunction with EB appeared to exert no effect on food intake or body weight. Treatment of ovariectomized females with P alone produced mean body weights below those of controls while mean food intake remained above that displayed by controls. The withdrawal of P led to a gradual return of body weight to control levels while food intake remained elevated. In the male gerbil, P administration appeared to produce a decrease in body weight observed after ovariectomy are due to the removal of food intake and a gradual return of body weight to control levels. The EB-induced increase in food intake and body weight observed in females in Experiment 1 is consistent with the results obtained by Maass and Wade [ll]. The present results also indicate that the decreases in food intake and body weight observed after ovariectomy are due to the removal of endogenous estrogens and that EB treatment serves a restorative function. When administered in conjunction with EB, P was found to increase food intake but not body weight in the female. These results are similar to those obtained by Maass and Wade [ll]. This suggests that estrogen and progesterone may interact to alter metabolic activity. Thus, the animal may require greater food intake in order to maintain body weight. The results of Experiment 1 further extend the f'mdings of Maass and Wade [11] by indicating that the administration of EB alone is sufficient to restore both the food intake and body weight of ovariectomized animals to control levels. Indeed, the further increase in food intake observed in animals also receiving progesterone is not necessary for the restoration of body weight to control levels. Analysis of the P only phase suggested that the decrease in female body weight during this phase may be due, in part, to the prior removal of EB. This possibility was strengthened by the finding that the withdrawal of P did not lead to a significant increase in body weight, which one would expect if P administration were responsible for the decrease in body weight. Females did, however, exhibit a return to control levels of food intake after P withdrawal, indicating that progesterone may have been partially responsible for the elevated food intake levels during the P only phase. The fmding that ovariectomy depresses, and EB treatment increases, food intake and body weight in the female gerbil is in direct contrast to results obtained in other rodent species. In other female rodents, ovariectomy has been
FOOD INTAKE AND LORDOSIS IN GERBILS found to increase food intake and body weight [5, 12, 16, 18, 20, 21, 22]. In addition, progesterone has been found to attenuate rather than facilitate the effects of EB in other female rodents [20] . Knowledge of the mechanisms underlying these species differences may aid in refining present models of the hormonal mediation of food intake and body weight in rodents. The results of Experiment 2 indicated that male gerbils exhibit a transient increase in body weight after castration, a result somewhat similar to that observed in the hamster [9]. Maass and Wade observed a longer lasting increase in body weight in their gerbil castrates [11]. In addition, Maass and Wade observed no significant effect of castration on food intake in the male gerbil [11] while our results indicate that castration may exert a delayed effect on food intake. A delayed effect of castration on behavior is not unusual and has been reported for alterations in food intake and body weight in male rats [4] and for sexual behavior and scent marking in male gerbils [23]. The finding that castrated male gerbils exhibited no significant alterations in food intake or body weight in response to EB treatment is of some interest, particularly in light of Maass and Wade's findings [11]. Maass and Wade found that castrated male gerbils failed to respond to testosterone propionate or dihydrotestosterone propionate treatment [11]. These results plus those of Experiment 2 suggest that neither androgens nor estrogens play a major role in the regulation of food intake in the male gerbil. This is in apparent contrast to results obtained in male rats and hamsters, where the effects of castration on food intake and body weight can be reversed by the administration of testosterone or estradiol [4, 8, 9, 24]. Also apparently in contrast with data for male rats and hamsters is the finding that P, when administered alone, produces a decrease in the body weights of castrated male gerbils. That the observed decrease in body weight was due to P treatment is supported by the finding that the withdrawal of P led to a gradual increase in body weight. It should be noted that eventually these animals surpassed the control group in body weight. Even more unusual was the finding that, in the male gerbil, the withdrawal of P led to a significant increase in food intake. The general elevation of food intake in male castrates throughout the experiment makes this result difficult to interpret. However, there is no indication that P administration was having any more effect on food intake or body weight than EB or EB given in conjunction with P, treatments which themselves appeared to have little effect. Yet, even though the administration of P did not appear to decrease food intake, its withdrawal apparently led to increases in food intake. It is possible that ovarian steroids were exerting a slight inhibitory effect on food intake and, perhaps, body weight that was masked by (or was masking) a delayed effect of castration. If so, it would not be the withdrawal of P p e r se that led to alterations in food intake and body weight. Rather, it would be the complete withdrawal of all ovarian steroids that produced effects. This type of inhibition might partially explain the rather unusual result obtained by Maass and Wade [11]. In their study, the administration of TP to castrated male gerbils produced no observable alteration in food intake or body
773 weight. However, when Maass and Wade ceased TP treatment, male body weight increased [11]. Presumably, the delayed effects of castration in either experiment would have been an increase in food intake and/or body weight if steroid treatment had not been initiated. In regards to sexual receptivity, the present experiments indicated that castrated male gerbils exhibited essentially no female sexual behavior during chronic EB administration whereas female gerbils exhibited high levels of receptivity when given 100/~g EB/day. Doses of 1 and 10/~g EB/day led to moderate receptivity. Thus, although both 1 and 10/zg EB/day were sufficient for the restoration of food intake and body weight to control levels, these doses were not capable of eliciting maximal receptivity. This is in contrast to the female rat, where chronic administration of less than 1 /xg EB/day can elicit high levels of receptivity [1, 6, 7] as well as significantly alter food intake and body weight [10,19]. It would appear that the neural substrates regulating sexual responding in the female gerbil are either less sensitive to EB or more progesterone-dependent than those of the female rat. Furthermore, the marked difference in sensitivity to estrogen seen between the system regulating food intake and body weight and the system regulating sexual responding is a feature not apparent in the female rat. Male gerbils did not exhibit significant female sexual behavior when treated chronically with EB. This is in contrast to the male rat which will show low levels of female sexual behavior when treated chronically with doses such as 20/zg EB/kg/day [2]. The 10 fig EB dose used in the present study represents a dose of about 120/zg/kg/day. Although this dose is substantially larger than that used by Eriksson and S6dersten [2], it failed to elicit any appreciable level of sexual receptivity in castrated male gerbils. Thus, it would appear that both male and female gerbils are less sensitive than male and female rats to the effects of EB on female sexual responding. This again indicates that sexual receptivity in the gerbil may be more like that of the hamster, which also exhibits less sensitivity to EB and is more progesteronedependent than the rat [3]. In summary, ovarian steroids were found to exert a major effect on food intake and body weight in the female gerbil whereas the effects of ovarian steroids on food intake and body weight in the male gerbil remain equivocal. In the female gerbil, EB treatment appears to serve a restorative function in the ovariectomized animal, bringing food intake and body weight back to presurgical levels. Our results also indicate that the system regulating food intake and body weight in the gerbil is more sensitive to estrogen than the system regulating female sexual behavior. Finally, the present results suggest that exogenous quantities of estrogen which induce receptivity in male and female rats are insufficient for the induction of receptivity in male and female gerbils. It seems likely that ovarian steroids play a major role in the hormonal mediation of food intake and body weight in the female gerbil. Although the results of Experiment 2 and those obtained by Maass and Wade [11] suggest that the hormonal regulation of food intake and body weight in the male gerbil differs from that observed in other male rodents, a great deal of study will be required before the mechanisms underlying these differences can be determined.
774
RAIBLE AND GORZALKA REFERENCES
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