Physiology&Behavior,VoL 51, pp. 607-61 I, 1992
0031-9384/92 $5.00 + .00 Copyright© 1992 PergamonPressLtd.
Printed in the USA.
The Effects of Amphetamine on Body Weight and Energy Expenditure J A M E S R. JONES,1 W I L L I A M F. C A U L
Department of Psychology AND J A M E S O. HILL
Department of Pediatrics, Vanderbilt University, Nashville, TN 37240 Received 13 M a y 1991 JONES, J. R., W. F. CAUL AND J. O. HILL. The effects ofarnphetamine on body weightand energy expenditure. PHYSIOL BEHAV 51(3) 607-611, 1992.--Although there is evidence suggesting that, in addition to suppressing food consumption, amphetamine reduces body weight by increasing energy expenditure, there is little consistency among the few studies examining that factor. In this experiment, the effect of amphetamine on daily energy consumption, within-day body weights, and hourly measures of metabolic rate (MR) and respiration quotient (RQ) were assessed. Daytime drug injections decreased total energy consumption, produced biphasic changes over time in MR, and persistently lowered RQ values. In contrast, nighttime injections of drug had little effect on energy consumption and MR but did reduce RQ for the first 4 postinjection hours. These effects show that amphetamine effects interact with the circadian organization of behavior and suggest that rodent studies of anorectic agents have more relevance for humans if drugs are given during the night, when rats are normally awake and eating. From this study, it seems clear that amphetamine reduces body weight by altering metabolic rate and fat metabolism in rats when the drag is given during the day. Amphetamine Energy expenditure Respiration quotient
Metabolic rate
Body weight
Indirect calorimetry
Biphasic response
used as indirect indices of metabolic rate (i.e., energy expenditure), little consistency exists among the few investigations that have used these measures. Whereas Niemegeers and Janssen (9) and Waterman (13) found that both activity levels and oxygen consumption increased as a function of amphetamine administration, Yehuda and Kahn (15) reported finding no change in oxygen consumption and increased carbon dioxide production in restrained rats given 15 mg/kg amphetamine at room temperature. Still another study reported that, even though amphetamine increased activity in C57BL/6J mice, it actually lowered energy expenditure (1). Thus, although the suggestions that amphetamine reduces body weight by increasing energy expenditure and that this effect interacts with timing of drug administration certainly seem reasonable, data specifying the conditions under which this is the case are lacking. The focus of this experiment was two-fold. First, the study was designed to assess the time course of amphetamine's effects on energy expenditure and fat metabolism using indirect calorimetry. Second, by administering drug both during the day and the night, the study determined whether or not differences observed in body weights following daytime versus nighttime drug administration can be accounted
RECENT experiments examining amphetamine's effects on food consumption and body weight using procedures that allowed both increases and decreases to be measured relative to controls showed: (a) that orderly changes in within-day temporal patterns of eating occurred over a 12-day amphetamine treatment period (2,7); (b) that the initial anorexia produced by the drug was followed by hyperphagia as a result of repeated drug administration (2,7); (c) that amphetamine reduced body weight via mechanisms other than its effects of food consumption (2-5, 7), and (d) that this effect interacts with the time of day of drug administration (8). The most obvious explanation of this interaction is that weight loss not attributable to decreased food consumption is a function of amphetamine's effect on energy expenditure and that this factor interacts with the time of day of drug administration. There is ample evidence that amphetamine increases activity levels in animals over a wide range of doses (11,12), which would also increase energy expenditure. One would expect that, because the drug suppresses eating, animals would have to mobilize fat stores to meet their energy needs (10). Although oxygen usage and carbon dioxide release can be
tRequests for reprints should be addressextto James R. Jones, Ph.D., Department of Psychology, A&S Psychology Building, Vanderbilt University, Nashville, TN 37240.
607
608
JONES, CAUL AND HILL
for by the drug's differential effects on metabolism. METHOD
Subjects Eight male Sprague-Dawley rats were purchased from SASCO, Inc. at approximately 60 days of age. The animals were housed in plastic cages with continuous access to laboratory chow and tap water.
Apparatus The indirect calorimeters used (5) were Plexiglas cylinders through which approximately 1 l/min air flow was maintained using a diaphragm vacuum pump (Thomas Industries, Sheboygan, WI) measured by a mass flowmeter (Teledyne HastingRaydist, Hampton, VA). Every 6 rain, the air from each of the four chambers was alternately routed to oxygen (Model No. S-3A) and carbon dioxide (Model No. CD-3A) analyzers (Ametek, Pittsburgh, PA). Output from the analyzers and flow meters was recorded on computer using a Lab Master AD converter interface (Scientific Solutions, Inc., Solon, OH). Metabolic rate and respiration quotient were automatically calculated using the equation of Weir (14). The chambers were equipped with removable food jars so that the animals' food consumption could be assessed. When subcutaneous injections were given, they were of 1-ml/kg volume and were either distilled water or 5-mg/kg d-amphetamine sulfate dissolved in distilled water.
Procedure Because the variables were manipulated within subjects, the experiment was conducted in two phases in order to counterbalance daytime and nighttime injections. Hence, four of the rats were given daytime injections during the first phase and nighttime injections during the second. The order was reversed for the other four animals. Each phase consisted of three 24-h baseline exposures to the metabolic chambers without injections, a distilled water injection day, followed by an amphetamine injection day. During the two phases, the rats in the daytime condition were removed from the colony room at approximately 0800 h, weighed and then placed into the chamber at 0900 h. The injections were given immediately before placement into the chambers. The rats were removed at 2000 h, reweighed, and returned to the chambers at 2100 h where they remained until 0800 h the next day, at which time they were returned to the colony room. For the rats in the nighttime condition, the procedure was the same except that the rats were first placed into the cages at 2100 h, reweighed at 0800 h the next day, and returned to the colony room at 2000 h. Thus each day was divided into two periods. In the daytime condition, the first period was daylight and the second period was night, whereas for the nighttime condition, the first period was night and the second period was daylight. Each day in the chamber was separated by one day during which the rats were left undisturbed in their home cages. Additionally, between the two phases, the rats were left in their home cages for 7 days. Data were recorded for the last baseline day during which the animals received no injection (NI), the distilled water injection day (DW) and the amphetamine injection day (AM). Body weights were recorded at the beginning of the filst and second periods and at the conclusion of the day. Energy consumption was also measured for each of the two periods. Energy consumption was determined by calculating the amount of energy contained in the powdered food gone from the food jar minus the
amount of energy lost through spillage and feces. The amount of energy lost was determined by bomb calorimetry' of the homogenized and then dried spillage/feces. Because the chambers were only cleaned once, at the end of each day, the energy lost was prorated for the two periods on the basis of the proportion of the total food consumed during each period. The oxygen and carbon dioxide measurements were used to calculate the metabolic rate (MR) and the respiration quotient (RQ). MR reflects the amount of energy expended by the animal, whereas RQ provides an indication of the source of the energy. Low RQ values are associated with fat metabolism, whereas high values represent carbohydrate metabolism. Because the chambers take approximately 20 min to equilibrate after the rats are placed inside, the first two measures of MR and RQ were not included in the analysis. Additionally, so that the data could be analyzed in terms of 2 periods, with 10 h in each period and 10 measurements in each hour, the last few measurements were not included. RESULTS
Body Weight The mean body weights for the animals for both the daytime injection condition and the nighttime condition are presented in Fig. 1. Inspection of these data shows that different patterns of weight change emerged as a function of time of injection, F(2,14)=39.10, p
0031-9384/92 $5.00 + .00 Copyright© 1992 PergamonPressLtd.
Printed in the USA.
The Effects of Amphetamine on Body Weight and Energy Expenditure J A M E S R. JONES,1 W I L L I A M F. C A U L
Department of Psychology AND J A M E S O. HILL
Department of Pediatrics, Vanderbilt University, Nashville, TN 37240 Received 13 M a y 1991 JONES, J. R., W. F. CAUL AND J. O. HILL. The effects ofarnphetamine on body weightand energy expenditure. PHYSIOL BEHAV 51(3) 607-611, 1992.--Although there is evidence suggesting that, in addition to suppressing food consumption, amphetamine reduces body weight by increasing energy expenditure, there is little consistency among the few studies examining that factor. In this experiment, the effect of amphetamine on daily energy consumption, within-day body weights, and hourly measures of metabolic rate (MR) and respiration quotient (RQ) were assessed. Daytime drug injections decreased total energy consumption, produced biphasic changes over time in MR, and persistently lowered RQ values. In contrast, nighttime injections of drug had little effect on energy consumption and MR but did reduce RQ for the first 4 postinjection hours. These effects show that amphetamine effects interact with the circadian organization of behavior and suggest that rodent studies of anorectic agents have more relevance for humans if drugs are given during the night, when rats are normally awake and eating. From this study, it seems clear that amphetamine reduces body weight by altering metabolic rate and fat metabolism in rats when the drag is given during the day. Amphetamine Energy expenditure Respiration quotient
Metabolic rate
Body weight
Indirect calorimetry
Biphasic response
used as indirect indices of metabolic rate (i.e., energy expenditure), little consistency exists among the few investigations that have used these measures. Whereas Niemegeers and Janssen (9) and Waterman (13) found that both activity levels and oxygen consumption increased as a function of amphetamine administration, Yehuda and Kahn (15) reported finding no change in oxygen consumption and increased carbon dioxide production in restrained rats given 15 mg/kg amphetamine at room temperature. Still another study reported that, even though amphetamine increased activity in C57BL/6J mice, it actually lowered energy expenditure (1). Thus, although the suggestions that amphetamine reduces body weight by increasing energy expenditure and that this effect interacts with timing of drug administration certainly seem reasonable, data specifying the conditions under which this is the case are lacking. The focus of this experiment was two-fold. First, the study was designed to assess the time course of amphetamine's effects on energy expenditure and fat metabolism using indirect calorimetry. Second, by administering drug both during the day and the night, the study determined whether or not differences observed in body weights following daytime versus nighttime drug administration can be accounted
RECENT experiments examining amphetamine's effects on food consumption and body weight using procedures that allowed both increases and decreases to be measured relative to controls showed: (a) that orderly changes in within-day temporal patterns of eating occurred over a 12-day amphetamine treatment period (2,7); (b) that the initial anorexia produced by the drug was followed by hyperphagia as a result of repeated drug administration (2,7); (c) that amphetamine reduced body weight via mechanisms other than its effects of food consumption (2-5, 7), and (d) that this effect interacts with the time of day of drug administration (8). The most obvious explanation of this interaction is that weight loss not attributable to decreased food consumption is a function of amphetamine's effect on energy expenditure and that this factor interacts with the time of day of drug administration. There is ample evidence that amphetamine increases activity levels in animals over a wide range of doses (11,12), which would also increase energy expenditure. One would expect that, because the drug suppresses eating, animals would have to mobilize fat stores to meet their energy needs (10). Although oxygen usage and carbon dioxide release can be
tRequests for reprints should be addressextto James R. Jones, Ph.D., Department of Psychology, A&S Psychology Building, Vanderbilt University, Nashville, TN 37240.
607
608
JONES, CAUL AND HILL
for by the drug's differential effects on metabolism. METHOD
Subjects Eight male Sprague-Dawley rats were purchased from SASCO, Inc. at approximately 60 days of age. The animals were housed in plastic cages with continuous access to laboratory chow and tap water.
Apparatus The indirect calorimeters used (5) were Plexiglas cylinders through which approximately 1 l/min air flow was maintained using a diaphragm vacuum pump (Thomas Industries, Sheboygan, WI) measured by a mass flowmeter (Teledyne HastingRaydist, Hampton, VA). Every 6 rain, the air from each of the four chambers was alternately routed to oxygen (Model No. S-3A) and carbon dioxide (Model No. CD-3A) analyzers (Ametek, Pittsburgh, PA). Output from the analyzers and flow meters was recorded on computer using a Lab Master AD converter interface (Scientific Solutions, Inc., Solon, OH). Metabolic rate and respiration quotient were automatically calculated using the equation of Weir (14). The chambers were equipped with removable food jars so that the animals' food consumption could be assessed. When subcutaneous injections were given, they were of 1-ml/kg volume and were either distilled water or 5-mg/kg d-amphetamine sulfate dissolved in distilled water.
Procedure Because the variables were manipulated within subjects, the experiment was conducted in two phases in order to counterbalance daytime and nighttime injections. Hence, four of the rats were given daytime injections during the first phase and nighttime injections during the second. The order was reversed for the other four animals. Each phase consisted of three 24-h baseline exposures to the metabolic chambers without injections, a distilled water injection day, followed by an amphetamine injection day. During the two phases, the rats in the daytime condition were removed from the colony room at approximately 0800 h, weighed and then placed into the chamber at 0900 h. The injections were given immediately before placement into the chambers. The rats were removed at 2000 h, reweighed, and returned to the chambers at 2100 h where they remained until 0800 h the next day, at which time they were returned to the colony room. For the rats in the nighttime condition, the procedure was the same except that the rats were first placed into the cages at 2100 h, reweighed at 0800 h the next day, and returned to the colony room at 2000 h. Thus each day was divided into two periods. In the daytime condition, the first period was daylight and the second period was night, whereas for the nighttime condition, the first period was night and the second period was daylight. Each day in the chamber was separated by one day during which the rats were left undisturbed in their home cages. Additionally, between the two phases, the rats were left in their home cages for 7 days. Data were recorded for the last baseline day during which the animals received no injection (NI), the distilled water injection day (DW) and the amphetamine injection day (AM). Body weights were recorded at the beginning of the filst and second periods and at the conclusion of the day. Energy consumption was also measured for each of the two periods. Energy consumption was determined by calculating the amount of energy contained in the powdered food gone from the food jar minus the
amount of energy lost through spillage and feces. The amount of energy lost was determined by bomb calorimetry' of the homogenized and then dried spillage/feces. Because the chambers were only cleaned once, at the end of each day, the energy lost was prorated for the two periods on the basis of the proportion of the total food consumed during each period. The oxygen and carbon dioxide measurements were used to calculate the metabolic rate (MR) and the respiration quotient (RQ). MR reflects the amount of energy expended by the animal, whereas RQ provides an indication of the source of the energy. Low RQ values are associated with fat metabolism, whereas high values represent carbohydrate metabolism. Because the chambers take approximately 20 min to equilibrate after the rats are placed inside, the first two measures of MR and RQ were not included in the analysis. Additionally, so that the data could be analyzed in terms of 2 periods, with 10 h in each period and 10 measurements in each hour, the last few measurements were not included. RESULTS
Body Weight The mean body weights for the animals for both the daytime injection condition and the nighttime condition are presented in Fig. 1. Inspection of these data shows that different patterns of weight change emerged as a function of time of injection, F(2,14)=39.10, p
