Effect of short-term prefeeding and body weight on wheel running and responding reinforced by the opportunity to run in a wheel

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1 Behavioural Processes 67 (2004) 1 10 Effect of short-term prefeeding and body weight on wheel running and responding reinforced by the opportunity to run in a wheel Terry W. Belke a,, W. David Pierce b, K. Jensen c a Department of Psychology, Mount Allison University, Sackville, NB, Canada E4L 1C7 b University of Alberta, Edmonton, Alta., Canada c Max Planck Institute for Evolutionary Anthropology, Leipzig, Germany Received 20 August 2003; received in revised form 23 January 2004; accepted 26 January 2004 Abstract A biobehavioural analysis of activity anorexia suggests that the motivation for physical activity is regulated by food supply and body weight. In the present experiment, food allocation was varied within subjects by prefeeding food-deprived rats 0, 5, 10 and 15 g of food before sessions of lever pressing for wheel-running reinforcement. The experiment assessed the effects of prefeeding on rates of wheel running, lever pressing, and postreinforcement pausing. Results showed that prefeeding animals 5 g of food had no effect. Prefeeding 10 g of food reduced lever pressing for wheel running and rates of wheel running without a significant change in body weight; the effect was, however, transitory. Prefeeding 15 g of food increased the animals body weights, resulting in a sustained decrease of wheel running and lever pressing, and an increase in postreinforcement pausing. Overall the results indicate that the motivation for physical activity is regulated by changes in local food supply, but is sustained only when there is a concomitant change in body weight Elsevier B.V. All rights reserved. Keywords: Prefeeding; Body weight; Wheel-running reinforcement; Lever press; Rat 1. Introduction Activity anorexia occurs when rats are placed on food restriction and provided with the opportunity to run. The initial effect is that food intake is reduced, body weight declines and wheel running increases. As running escalates, food intake drops off and body weight plummets downward, further augmenting wheel running and suppressing food intake. The result of this cycle is emaciation and, if allowed to continue, the eventual death of the animal (Epling and Pierce, 1991). Corresponding author. Tel.: ; fax: address: tbelke@mta.ca (T.W. Belke). A biobehavioural analysis of activity anorexia involves evolution and natural selection (Pierce, 2001). For organisms faced with sporadic reductions in food supply (e.g., unpredictable, non-seasonal famines or droughts), natural selection would have favoured increased physical activity (see Mrosovsky and Barnes, 1974 for cyclic reductions in food supply, hibernation, and anorexia; also see Mrosovsky and Sherry, 1980 for a review of natural anorexia). The movement from one decimated food patch to an alternative location offering higher net returns is one of the central principles of optimal foraging theory (Pyke et al., 1977). That is, animals that travelled or migrated under conditions of food restriction and loss of body weight would have contacted food, survived and reproduced; other animals that continued to forage in a food depleted /$ see front matter 2004 Elsevier B.V. All rights reserved. doi: /j.beproc

2 2 T.W. Belke et al. / Behavioural Processes 67 (2004) 1 10 location or patch had less reproductive success. Furthermore, natural selection would have favoured anorexia during times of travel induced by severe food shortages. Under famine conditions, there would be a net negative energy balance between foraging for small, difficult to obtain food items and travelling to a more abundant food source. Animals that stopped to search for food along the way often would use up their energy stores and die. Those that kept going rather than foraging more frequently would have contacted a stable and abundant food source increasing their reproductive fitness (see Epling and Pierce, 1991; Pierce and Epling, 1996). Both the laboratory model and the biobehavioural analysis of activity anorexia suggest that motivation for physical activity or travel is sensitive to changes in food supply and body weight. Numerous studies (e.g., Belke, 1996; Collier, 1970; Jakubczak, 1967; Moskowitz, 1959; Price, 1976; Treichler and Hall, 1962) have shown that physical activity level is sensitive to fluctuations in food supply sufficient to induce changes in body weight. For example, Moskowitz (1959) showed a linear relationship between physical activity, measured as wheel revolutions per hour, and percentage of body weight from an estimated normal weight. The correlation between body weight and wheel revolutions was 0.99 between 60 and 90% of ad libitum body weight. Collier (1970) reported a 0.79 correlation between log weight loss and log distance run over a range of 5 20% body weight loss. Sclafani and Rendel (1978) found that this linear relationship emerged at different body weight levels in lean, normal, and obese rats and that the rate of increase in physical activity was lowest in obese rats and highest in lean animals. While a relationship between body weight loss and activity level is well established, other research suggests that physical activity level may also be sensitive to changes in food supply that are of a shorter term. For example, Kanarek and Collier (1983) showed that running varied with meal frequency. Rats were provided access to running wheels and fed one meal of 60 min, two 30 min meals or four meals of 15 min. Results showed that the greater the meal frequency the more food the animals consumed; more food consumption raised body weight and decreased wheel running over days. Because body weight changed with meal frequency, the changes in wheel running cannot be attributed uniquely to variation in food supply and its allocation. Boer (1989, also reported in Pierce and Epling, 1996), however, indicated that wheel running is sensitive to fluctuations in 4-day food supply. Based on a procedure to induce voluntary prolonged running (Russell et al., 1987), five 6-week-old rats were given the opportunity to run on a wheel with an attached side cage. Over 10 days the animals were fed a fixed-restricted amount of food (15 or 18 g) until wheel running stabilised around the criterion of 6000 revolutions of a 1.1 m wheel (6600 m in a day). On day 10, some animals were fed 15 g of food and others received 18 g, depending on their level of wheel running (above or below 6000 revolutions). Over the next 4 days, the rats were maintained on 15 or 18 g of food; following this period, the daily amount of food was triturated up or down relative to the initial fixed amount, and each time the new food supply was maintained over 4 days. Each titration was followed by a return to the initial fixed amount of food. For example, one rat received the food values of 15, 14, 15, 16, 15, 13, 15, 17, 15, 12, 15, 18, 15, 11, 15, 19, 15, 10, 15 and 20 g, for consecutive 4-day periods. Results showed that the level of wheel running tracked the changes in food supply increasing when food supply dropped and decreasing when the amount of food jumped. Generally, the larger the change in food supply the greater the adjustment in wheel running (up or down). Boer found that changes in wheel running were sensitive to adjustment in food supply but were not associated with local changes in body weight over the 4-day periods. Body weight and wheel running were, however, inversely correlated over the course of the experiment, confirming the findings of previous studies. In addition to experiments on fluctuations in 4-day food supply and meal frequency, there are studies of the immediate effects of a prior meal (prefeeding) on physical activity. In one study, Taylor (1973) investigated the regulation of running speed in a straight alley by short-term food supply over successive days. Experimental conditions involved no prefeeding, prefeeding a nutritive substance (N) and prefeeding a non-nutritive substance (NN). In his first experiment, the N group showed a lower rate of increase in running speed over successive days compared to the other two groups. Under the prefeeding condition

3 T.W. Belke et al. / Behavioural Processes 67 (2004) (free access to food for 30 min), the N group gained weight and ate considerably more than animals in the NN condition. In a second experiment designed to equate the groups on body weight, the N and NN groups were given 8 g of their respective substances 30 min prior to a session. Under these conditions, body weight remained constant, and the groups did not differ in increases in running speed over successive days. Taylor s results suggest that in the absence of a change in body weight, short-term adjustments in food supply do not affect running. Note, however, that the amount of food and non-nutritive substance was held constant at 8 g rather than varied making it questionable to rule out short-term food supply as a controlling variable of physical activity. The present study addresses the motivation to run and its regulation by short-term food supply and body weight. In the current experiment, rats were fed chow prior to sessions of lever pressing maintained by the opportunity to run in a wheel. For each session, animals pressed a lever on tandem fixed ratio (FR) 1 variable interval (VI) schedules for wheel-running reinforcement. In contrast to previous studies, we show the effects of prefeeding on lever pressing for wheel running a measure of motivation to run independent of the physical activity itself. 2. Method 2.1. Subjects Eight male Wistar rats selected from an initial group of 21 rats based on running rates served as experimental subjects. Rats were approximately 75 days old, and between 317 and 360 g in body weight, at the beginning of the experiment (i.e., training). The animals were maintained at approximately 80% of their initial free-feeding body weight throughout the experiment Apparatus The apparatus consisted of four running wheels (three Med Associates ENV 041, one Wahmann) located in soundproof shells. Each wheel was 35.5 cm in diameter. A retractable lever (Med Associates ENV-112) was mounted directly at the opening of each wheel (7 cm 9 cm). The lever extended 1.8 cm into the wheel chamber and when extended was located 8.5 cm above the floor of the wheel chamber. A solenoid-operated brake was attached to the base of each wheel. When the solenoid was operated, a rubber tip attached to a metal shaft contacted the outer rim of the wheel and caused the wheel to stop. A microswitch attached to the wheel frame recorded wheel revolutions. A 24 V dc light was mounted on each side of the frame of the wheel, 20 cm above the base, to illuminate the interior of the wheel and the area of the lever. The design of the apparatus is similar to that portrayed in Fig. 1 in Pierce et al. (1986). Control of experimental events and the recording of data were handled by a Digital PDP 8A computer in an adjacent room Procedure Training and baseline Initially 21 rats were given free access to a running wheel for 30 min each day for 15 days and the number of wheel revolutions was recorded. From this group of 21 rats, 14 were selected as subjects based on the number of revolutions (>500 revolutions/30 min). These 14 subjects were shaped to press a lever in a standard operant conditioning chamber. Each lever press produced a 0.1 ml of 10% sucrose solution. When the rats reliably pressed the lever, the schedule of reinforcement was shifted from a continuous reinforcement schedule (CRF) to a VI 10 s schedule. Throughout the period of lever training (i.e., 15 days), subjects continued to run in wheels for 30 min sessions just prior to sessions in the operant chamber. When lever pressing for sucrose solution appeared to be established, the procedure was discontinued. At this time, the retractable lever in the wheel chamber was made operative and a single lever press produced the opportunity to run in the wheel for 60 s. A session consisted of 50 opportunities to run for 60 s. Each session was divided into five components with 10 reinforcements in each component. Between each component there was a 2 min timeout. During this period, the wheel could not turn (brake activated), the lever retracted, and the lights at the side of the wheel were extinguished. The rats were trained to respond on five VI schedules for wheel running through the following steps. A CRF schedule was in effect in each component for

4 4 T.W. Belke et al. / Behavioural Processes 67 (2004) days; next, the schedule in each component was changed to a variable-ratio 5-response schedule (VR 5). After 4 days, the schedule in each component was changed to a VI schedule. With the change from a response to time-based reinforcement, the operation of the schedule was modified so that the reinforcement interval did not start timing until the first response was made (i.e., tandem FR 1 VI schedules). Thus, after the termination of reinforcement, a new interval was selected, but did not begin to time out until the first press of the lever. This modification was necessary due to the markedly longer postreinforcement pauses (PRP) that tend to follow the termination of wheel-running reinforcers. Without this modification, the reinforcement interval would most likely time out during the PRP and convert a short duration interval schedule into a continuous reinforcement schedule. The initial sequence of VI schedules for the five components was VI 15 s, VI 7.5 s, VI 5 s, VI 7.5 s and VI 15 s. Over the following 15 days, VI 30 s and VI 60 s schedules were introduced to arrive at the final sequence of interval schedules in the baseline condition: VI 60 s, VI 15 s, VI 5 s, VI 7.5 s, and VI 30 s. The programmed interreinforcement intervals for the VI schedules approximated an exponential distribution (Fleshler and Hoffman, 1962). Lever presses and time were recorded for each interval. Wheel revolutions were counted for each obtained opportunity to run. Rats were exposed to the series of reinforcement schedules, described as the final sequence in the previous paragraph, within the same session to obtain response and reinforcement rates on each schedule. The purpose was to fit Herrnstein s (1970, 1974) hyperbolic matching law equation to these rates in order to generate estimates of the parameters, k and Re, that are used to assess changes in reinforcement efficacy (Heyman and Monaghan, 1987). The form of the hyperbolic matching law equation is B 1 = kr 1 (1) R 1 + Re Response rates are measured over a series of reinforcement schedules and Eq. (1) is fit to the obtained reinforcement rates (R 1 ) and response rates (B 1 ). In Eq. (1), k is defined as the asymptotic rate of responding while Re is the rate of reinforcement associated with one half the asymptotic level of responding. Heyman and Monaghan (1987) suggested that k and Re can be interpreted as an indices of motor and motivational components of a reinforced response. Wilkinson s (1961) method of estimating the parameters of a hyperbolic function was used to generate k and Re values. Belke (2000), however, showed that the parameter estimates vary with the order of the schedules when wheel running is used as reinforcement and suggested that changes in efficacy of wheel running as a reinforcer within the session affect the estimated parameters. Consequently, k and Re estimates generated using wheel-running as a reinforcer should interpreted with caution. As a result, the effects of prefeeding in the present article were principally based on session level data Prefeeding conditions One hour prior to the start of the session, animals were given 5, 10, or 15 g of Purina TM rat chow. Each prefeeding condition remained in effect for five consecutive days. After each condition, the rats were returned to baseline feeding for 6 days and then exposed to the next prefeeding value. Half the subjects received the prefeeding conditions in an order of 5, 10, and 15 g while the other half received the order 10, 5, and 15 g. The 15 g prefeeding condition was arranged last because, unlike the 5 and 10 g conditions, 15 g of food was more than average daily intake required to maintain 80% free-feeding body weight. For the 5 and 10 g conditions, after each session, the rats were fed the difference between the amount of food consumed at prefeeding and the amount of food necessary to maintain 80% free-feeding body weight. For example, if an animal required 11 g of food to maintain 80% body weight and ate 5 g of food at prefeeding, then after the session, the rat received 6 g of chow. In this way, body weight over a 24 h period was maintained at 80% of free-feeding level in the 5 and 10 g prefeeding conditions. It was not possible to hold body weight at 80% for the 15 g condition because the ration exceeded the requisite amount; consequently, the 15 g condition always occurred last. All animals were weighed prior to prefeeding, before the commencement of a session, and at the termination of each session. This procedure allowed measures of body weight before prefeeding, weight

5 T.W. Belke et al. / Behavioural Processes 67 (2004) gained from prefeeding, and weight lost from wheel running and metabolism during the session. 3. Results Wheel-running rates for and within each prefeed condition are presented in Table 1 and Fig. 1a, respectively. Table 1 shows wheel-running rates averaged over the 5 days of each prefeed condition and, in the case of the 0 g condition, the 5 days before prefeeding manipulations began. These data suggest that prefeeding affected subsequent wheel running when the amount fed was greater than 5 g. A repeated measures analysis of variance (ANOVA) revealed a significant effect of prefeeding on wheel-running rate over sessions, F (3,21) = 44.66, P = Post hoc Dunnett s t-tests showed that wheel-running rates were significantly reduced in the 10 and 15 g conditions compared with the 0 g baseline, t (21) = 4.77, P = 0.001, and t (21) = 10.04, P = 0.001, respectively. Fig. 1a shows the average wheel-running rates across successive days of each prefeeding condition. While wheel-running rates collapsed across days showed that running was decreased by 10 and 15 g, wheel-running rates within each condition showed that the effect on running differed. In the 10 g prefeeding condition, wheel-running rates were reduced on the first day but recovered toward baseline levels over successive days. For the 10 g condition, wheel-running Fig. 1. (a) Mean wheel-running rates (revolutions/min), (b) local lever-pressing rates (presses/min), (c) median postreinforcement pauses (s), and (d) body weight (g) prior to prefeeding across the five successive days of the 0, 5, 10, and 15 g prefeed conditions. Standard errors are provided for means.

6 6 T.W. Belke et al. / Behavioural Processes 67 (2004) 1 10 Table 1 Mean running rates (revolutions/min), lever-pressing rates (lever presses/min), median postreinforcement pause (s) and body weight (g) per session for each rat for the 0, 5, 10 and 15 g prefeed conditions Rat 0g 5g 10g 15g Revolutions/min A (0.5) 42.7 (1.6) 39.8 (1.8) 27.3 (0.8) A (0.9) 40.9 (0.5) 35.5 (0.7) 30.9 (0.7) A (0.6) 34.0 (0.8) 32.5 (1.0) 26.2 (0.3) A (0.5) 42.8 (0.7) 37.7 (1.5) 37.7 (0.9) A (0.9) 35.1 (0.8) 29.8 (0.5) 25.0 (0.6) A (1.0) 35.0 (1.2) 28.4 (2.0) 23.5 (1.4) A (0.9) 36.6 (0.8) 34.1 (1.3) 28.6 (0.8) A (0.8) 38.4 (0.9) 32.3 (0.8) 29.4 (0.7) Mean 38.4 (1.4) 38.2 (1.3) 33.7 (1.4) 28.6 (1.6) Lever presses/min A (1.0) 31.9 (2.1) 28.8 (2.2) 23.1 (0.5) A (1.8) 38.9 (1.1) 39.7 (2.0) 29.4 (1.6) A (1.3) 24.0 (1.5) 31.4 (2.3) 24.7 (0.8) A (0.5) 30.2 (0.6) 28.1 (0.5) 25.7 (0.7) A (0.6) 35.8 (0.7) 29.1 (0.5) 26.6 (1.3) A (1.2) 38.3 (2.3) 27.0 (2.0) 19.2 (1.1) A (0.6) 37.1 (0.8) 36.7 (0.9) 27.3 (1.1) A (2.5) 53.1 (0.7) 39.4 (2.4) 35.1 (2.6) Mean 35.2 (2.0) 36.2 (3.0) 32.5 (1.9) 26.4 (1.6) Median postreinforcement pause (s) A (28.8) 30.9 (30.4) 33.8 (40.9) 37.1 (41.4) A (29.2) 22.7 (24.1) 23.6 (26.4) 40.3 (65.1) A (11.9) 12.9 (11.6) 14.8 (11.9) 15.8 (14.5) A (18.8) 14.9 (28.3) 18.6 (37.3) 15.8 (42.3) A (15.9) 23.0 (25.5) 30.1 (31.2) 35.2 (47.2) A (24.8) 24.4 (32.8) 29.9 (40.1) 29.0 (49.7) A (24.9) 24.0 (43.2) 32.3 (44.4) 39.2 (77.6) A (54.8) 44.4 (42.9) 45.1 (59.5) 76.7 (66.4) Mean 24.5 (3.9) 24.6 (3.5) 28.5 (3.4) 36.1 (6.8) Body weight (g) A (1.8) (0.9) (1.0) (2.7) A (0.9) (0.2) (0.6) (3.2) A (0.8) (0.4) (2.0) (2.1) A (0.6) (1.2) (0.4) (1.3) A (0.6) (1.5) (0.4) (2.6) A (1.1) (0.7) (0.5) (4.0) A (0.7) (0.7) (0.4) (2.9) A (1.0) (0.7) (0.5) (4.2) Mean (3.9) (3.5) (3.7) (3.7) Standard errors are provided for each mean and interquartile ranges are provided for each median. rates differed across days, F (4,28) = 5.58, P = 0.002, and a linear contrast confirmed that wheel running rates increased across days, F (1,28) = 11.09, P = For the 15 g condition, wheel-running rates were reduced and did not change across days, F (4,28) = 0.69, ns. For the baseline condition, across days, wheel-running rates did not differ, F (4,28) = 0.33, ns. For the 5 g prefeeding condition, running rates also did not differ across days, F (4,28) = 1.81, ns, and did not differ from baseline rates. Lever-pressing rates for and within each prefeed condition are presented in Table 1 and Fig. 1b, respectively. Like wheel-running rates, lever-pressing rates were also affected by prefeeding, F (3,21) = 10.92, P = 0.001; however, relative to 0 g, rates were only lower in the 15 g condition, t (21) = 4.67, P = Lever-pressing rates within prefeeding conditions showed the same pattern as wheel-running rates. Lever-pressing rates did not differ across days in the 0g, F (4,28) = 0.60, ns, or 5 g conditions, F (4,28) = 0.88, ns. For the 10 g condition, lever-pressing rates were reduced on the first day and recovered toward baseline over subsequent days. For the 10 g condition, the overall F-value was not significant, F (4,28) = 2.14, P = 0.10; however, a linear contrast confirmed that, like wheel-running rates, lever-pressing rates increased across successive days, F (1,28) = 6.58, P = For the 15 g condition, rates were reduced, but did not significantly differ across days, F (4,28) = 2.54, ns. Median PRPs for and within each prefeed condition are presented in Table 1 and Fig. 1c, respectively. Median rather than mean PRP durations were used because of the sensitivity of means to extreme values. Since parametric analyses of medians are performed with medians of time data in reaction time experiments, a similar approach was taken with PRP medians. Prefeeding had a significant effect on postreinforcement pause duration, F (3,21) = 7.73, P = 0.001, with longer PRPs in the 15 g condition than in the 0 g baseline, t (21) = 4.205, P = Nonparametric analyses yielded similar results, Friedman ANOVA, χ 2 = (3, n = 8), P = 0.001; Wilcoxon matched pairs, 0 g versus 10 g, Z = 1.96, P = 0.05; 0 g versus 15 g, Z = 2.52, P = Within prefeed conditions, median PRP durations were more varied and less systematic than were running and lever-pressing rates. In general, no systematic trends appear evident for any condition. For the 10 g condition, a paired t-test showed that median PRP durations on the last day did not differ significantly from PRPs on the first day (t (7) = 1.57, ns). For the 15 g condition, although

7 T.W. Belke et al. / Behavioural Processes 67 (2004) PRPs do not appear to change systematically across successive days, median PRP duration on the last day was shorter than on the first (t (7) = 3.49, P = 0.01) (Wilcoxon matched pairs, Z = 2.24, P = 0.025). Body weight, measured before prefeeding, for and within each prefeed condition are presented in Table 1 and Fig. 1d, respectively. Analysis of body weight by prefeeding condition revealed a significant effect, F (3,21) = 94.70, P = 0.001, with weight significantly higher than baseline in the 15 g, t (21) = 14.28, P = 0.001, but not the 5 or 10 g conditions, t (21) = 0.76 and 0.85, both ns. Fig. 1d shows that within each condition, body weight was maintained at a constant level over days in the 0, 5, and 10 g prefeeding conditions. In contrast, and as expected, body weight increased across successive days in the 15 g condition. A repeated measures ANOVA with prefeeding condition (0, 5, 10) and days (1 5) the within-subject variables revealed no significant main effect of prefeeding condition, F (2,14) = 0.26, ns, or days, F (4,28) = 0.49, ns and no interaction, F (8,56) = 0.85, ns. However, inclusion of the 15 g prefeeding condition into the analysis yielded significant main effects of prefeeding condition, F (3,21) = 94.54, P = 0.001, and days, F (4,28) = 10.83, P = as well as a significant interaction, F (12,84) = 11.75, P = In addition, a linear contrast confirmed that body weight increased across days in the 15 g condition, F (1,28) = 33.63, P = Fig. 2 presents the hyperbolic curves for the relationship between response and reinforcement rates for each prefeed condition. Table 2 shows the associated estimates of k, Re, and variance accounted for each rat. Fig. 2. Hyperbolic curves relating response and reinforcement rates for the 0, 5, 10, and 15 g prefeed conditions. For this analysis, data for the 0 g condition were averaged from the five sessions that preceded each prefeeding condition. Repeated measures ANOVAs revealed no significant effect of prefeeding on asymptotic levels of responding (k), F (3,21) = 0.86, ns; however, prefeeding did affect Re, F (3,21) = 3.88, P = Relative to baseline, Re was significantly higher in the 15 g condition, Dunnett t (21) = 2.61, P = Increases in the value of Re are generally interpreted as a decrease in the reinforcing efficacy of the programmed reinforcer, the opportunity to run in a wheel. Table 3 shows, for the 5, 10, and 15 g conditions, that the body weights of the animals prior to Table 2 Estimates of k, Re, and variance accounted for each the 0, 5, 10, and 15 g prefeed conditions Rat k Re %VAC 0g 5g 10g 15g 0g 5g 10g 15g 0g 5g 10g 15g A A A A A A A A Mean

8 8 T.W. Belke et al. / Behavioural Processes 67 (2004) 1 10 Table 3 Body weights (g) prior to prefeeding (before), increases in measured weight (g) following the prefeeding period (in), and decrease in weight (g) following the wheel-running session (out) for the 5, 10, and 15 g conditions Rat Prefeed condition 0g 5g 10g 15g Out Before In Out Before In Out Before In Out A3 4.1 (0.6) (0.4) 7.4 (0.5) (1.0) 8.2 (0.7) (0.8) 7.6 (0.4) A9 5.0 (0.4) (0.8) 7.0 (0.8) (1.0) 7.3 (0.7) (1.1) 7.4 (0.7) A (0.5) (1.0) 6.8 (0.4) (1.4) 5.2 (0.9) (1.2) 7.4 (1.3) A (0.8) (0.4) 6.2 (0.8) (1.4) 9.2 (1.0) (2.4) 9.8 (0.9) A (0.8) (0.7) 8.6 (0.2) (0.9) 7.8 (0.6) (1.1) 10.2 (0.9) A (0.6) (0.9) 5.8 (0.8) (1.7) 5.4 (1.8) (0.9) 7.2 (0.6) A (0.7) (1.1) 7.6 (0.6) (0.4) 6.6 (0.6) (1.3) 8.2 (0.7) A (0.9) (0.5) 12.6 (0.4) (1.0) 8.6 (0.9) (1.4) 12.6 (1.2) Mean 4.8 (0.5) (0.8) 7.8 (0.8) (0.8) 7.3 (0.5) (1.5) 8.8 (0.7) For the 0 g condition, only the decrease in weight following the wheel-running session is given. Standard errors are reported in parentheses. prefeeding increase following consumption of the food they were prefed, and decrease following the experimental session (after a session of wheel-running reinforcement). On average, the body weights of the animals increased by 10.00, 18.98, and g in the 5, 10, and 15 g prefeeding conditions, respectively. A repeated measures ANOVA confirmed that body weights increased with amount of prefeeding, F (2,14) = , P = Following sessions of wheel running, weight losses were 4.84, 7.75, 7.30, and 8.80 g on average for the 0, 5, 10, and 15 g prefeeding conditions. Data for the 0 g condition were obtained from the difference between weight going into and coming out of the running wheel taken on the 5 days following the 15 g prefeeding condition. A repeated measures ANOVA revealed a significant effect of prefeeding on body weight loss, F (3,21) = 15.46, P = Post hoc comparisons showed that more weight was lost when the animals were prefed than when they were not, t (21) = 4.83, 4.00, and 6.57 all significant at P = Discussion In the present study, prefeeding regulated wheel running and responding for the opportunity to run; however, the effect depended on (1) the amount of prefeeding and (2) an associated change in body weight. Prefeeding the animals 5 g ( 40%) of their daily food allotment prior to a session without a change in body weight did not reduce wheel running or responding for the opportunity to run. Prefeeding 10 g ( 80%) of their daily food allotment under the same conditions, decreased wheel running and lever pressing for the opportunity to run; however, the decrease was transitory and diminished over successive days. Prefeeding the animals 15 g ( 125%) of their daily food allotment prior to a session produced both immediate and sustained decreases in wheel-running and lever-pressing rates and an increase in postreinforcement pauses. Response rate functions showed no effect of prefeeding on response rate asymptotes (k) and an increase in Re in the 15 g condition as the function rose less rapidly toward the asymptote. The increase in Re suggests that the reinforcing efficacy of wheel running was reduced by the 15 g prefeeding load and associated increase in body weight. One potential confound is that feeding the animals before a session added bulk to their digestive systems and that the decline in wheel running could be a function of the added bulk. Going into the wheels, the body weights of the animals increased an average of 10.00, 18.98, and g in the 5, 10, and 15 g conditions, respectively. These weight gains reflect the amount of food plus water consumed during the prefeeding period. Hamilton (1969) showed that while consumption of non-nutritive bulk decreased running in rats, it did not affect lever pressing for heat reinforcement. In the present experiment, the reduc-

9 T.W. Belke et al. / Behavioural Processes 67 (2004) tion in wheel running could reflect digestive overload, but it is unlikely that changes in lever pressing and postreinforcement pausing are due to added bulk. The results of the present study also appear consistent with those obtained by Taylor (1973) for rats running in a runway. In the absence of a change in body weight prefeeding reduced running speed, though not significantly. In the presence of a change in body weight, prefeeding significantly reduced running speed. The results for the 10 g condition are consistent with the conclusions from Boer (1989) that wheel running is sensitive to food supply without local changes in body weight. That is, in the present study, prefeeding the animals 10 g of food reduced wheel running and this occurred without an increase in weight. However, the decrease in wheel running was transitory with 10 g prefeeding, but was sustained in the 15 g condition where prefeeding generated weight gain. One problem is that wheel-running rates did not systematically decrease as body weight increased from 280 to 300 g on average. This suggests that there could be a lag between changes in body weight and changes in wheel running; such a lag would have implications for Boer s (1989) claim that 4-day changes in food supply regulate wheel running without changes in local body weight. Although we did not find co-variation of wheel-running rates with body weight over the 5 days of 15 g prefeeding, wheel running did decline for these same rats when body weight was raised from 80 to 100% of ad libitum weight (Belke, 1996). These data also address the effect of postsession feeding on wheel running and responding for the opportunity to run. In the present experiment, feeding occurred shortly after the completion of a session; one possibility is that within session lever pressing for wheel running and bouts of running were a chain maintained by postsession feeding as the terminal reinforcement. The results from the present study are inconsistent with this account. In the 10 g condition, running and lever-pressing for the opportunity to run were maintained despite the fact that animals received almost all of their daily food allotment before the session (only 1 2 g postsession). Furthermore, we would have expected the animals to show declines in running and lever pressing over successive days in the 10 g condition as the animal repeatedly experienced the greatly reduced postsession feeding. Instead, running and lever pressing increased. Finally, in the 15 g condition, where the animals received no postsession feeding, lever-pressing and wheel-running rates were only reduced by an average of 25 and 25.5%, respectively. In general, the effect of prefeeding on motivation for physical activity is consistent with the biobehavioural analysis of activity anorexia proposed by Epling and Pierce (1991; also Pierce and Epling, 1996). A change in food supply sufficient to stop food-related searching or dispersal would have involved gains in food intake sufficient to produce stable gains in body weight. If the amount of food produced only transitory weight gains (i.e., through gut-fullness), then the motivation to travel would not decline. Both of these effects were demonstrated in the present experiment for the 10 and 15 g prefeeding conditions. Accordingly, animals would vary their range of foraging based on food density (e.g., quantity and frequency of meals), and abandon the patch only when reduced food supply began to impact body weight. Overall, motivation for physical activity would be most sensitive to changes in food allocation that were sufficient to produce gains (or losses) in animals body weights. Taken together, the results suggest that motivation for physical activity is mediated through changes in food intake and body weight. That is, gains in food consumption decrease the motivation for wheel running; reduced motivation for physical activity is, however, more sustained if food intake is sufficient to increase body weight. In other words, enduring changes in the motivation for physical activity occur when food intake is greater than day-to-day fluctuations in food supply related to foraging. References Belke, T.W., The effect of a change in body weight on running and responding reinforced by the opportunity to run. Psychol. Rec. 46, Belke, T.W., Studies of wheel-running reinforcement: parameters of Herrnstein s (1970) response-strength equation vary with schedule order. J. Exp. Anal. Behav. 73, Boer, D.P., Determinants of excessive activity in activity anorexia. Unpublished Doctoral Dissertation. University of Alberta, Edmonton. Collier, G.H., Work: a weak reinforcer. Trans. NY Acad. Sci. 32, Epling, W.F., Pierce, W.D., Solving the Anorexia Puzzle: A Scientific Approach. Hogrefe and Huber, Toronto, Ont.

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