Work: a weak reinforcer

Humans have a basic physical and psychological need to move the body: Physical activity as a primary drive

Physical activity, while less necessary for survival in modern times, is still essential for thriving in life, and low levels of movement are related to numerous physical and mental health problems. However, we poorly understand why people move on a day-to-day basis and how to promote greater energy expenditure. Recently, there has been a turn to understand automatic processes with close examination of older theories of behavior. This has co-occurred with new developments in the study of non-exercise activity thermogenesis (NEAT). In this narrative review, it is hypothesized that psycho-physiological drive is important to understand movement in general and NEAT, specifically. Drive, in short, is a motivation state, characterized by arousal and felt tension, energizing the organism to acquire a basic need. Movement is a biological necessity, like food, water, and sleep, but varies across the lifespan and having the greatest impact before adolescence. Movement meets various criteria for a primary drive: (a) deprivation of it produces feelings of tension, such as an urge or craving, known as affectively-charged motivation states, and particularly the feelings of being antsy, restless, hyper or cooped up, (b) provision of the need quickly reduces tension - one can be satiated, and may even over-consume, (c) it can be provoked by qualities of the environment, (d) it is under homeostatic control, (e) there is an appetite (i.e., appetence) for movement but also aversion, and (f) it has a developmental time course. Evidence for drive has mainly come from children and populations with hyperkinetic disorders, such as those with anorexia nervosa, restless legs syndrome, and akathisia. It is also stimulated in conditions of deprivation, such as bed rest, quarantine, long flights, and physical restraint. It seems to be lacking in the hypokinetic disorders, such as depression and Parkinson's. Thus, drive is associated with displeasure and negative reinforcement, subsuming it within the theory of hedonic drive, but it may fit better within new paradigms, such as the WANT model (Wants and Aversions for Neuromuscular Tasks). Recently developed measurement tools, such as the CRAVE scale, may permit the earnest investigation of movement drive, satiation, and motivation states in humans.

Rolling out physical exercise and energy homeostasis: Focus on hypothalamic circuitries

Energy balance is the fine regulation of energy expenditure and energy intake. Negative energy balance causes body weight loss, while positive energy balance promotes weight gain. Modern societies offer a maladapted way of life, where easy access to palatable foods and the lack of opportunities to perform physical activity are considered the roots of the obesity pandemic. Physical exercise increases energy expenditure and, consequently, is supposed to promote weight loss. Paradoxically, physical exercise acutely drives anorexigenic-like effects, but the mechanisms are still poorly understood. Using an evolutionary background, this review aims to highlight the potential involvement of the melanocortin system and other hypothalamic neural circuitries regulating energy balance during and after physical exercise. The physiological significance of these changes will be explored, and possible signalling agents will be addressed. The knowledge discussed here might be important for clarifying obesity aetiology as well as new therapeutic approaches for body weight loss.

Motivation States for Physical Activity and Sedentary Behavior: Desire, Urge, Wanting, and Craving

To better explain daily fluctuations in physical activity and sedentary behavior, investigations of motivation are turning from social cognitive frameworks to those centered on affect, emotion and automaticity, such as the Affect and Health Behavior Framework (AHBF), Integrated Framework and Affective-Reflective Theory (ART). This shift has necessitated: (a) re-examination of older theories and their constructs, such as drives, needs and tensions and (b) an inspection of competing theories from other fields that also attempt to explain dynamic changes in health behaviors. The Dynamical Model of Desire, Elaborated Intrusion Theory and others commonly share with AHBF the idea that human behavior is driven strongly by desires and/or the similar concepts of wants, urges, and cravings. These affectively-charged motivation states (ACMS) change quickly and may better explain physical activity behavior from one moment to the next. Desires for movement predominantly derive from negative but also positive reinforcement. Data from clinical populations with movement dysfunction or psychiatric disorders provides further evidence of these drivers of movement. Those with Restless Legs Syndrome, akathisia, tic disorders and exercise dependence all report strong urges to move and relief when it is accomplished. Motor control research has identified centers of the brain responsible for wants and urges for muscular movement. Models elaborated herein differentiate between wants, desires, urges and cravings. The WANT model (Wants and Aversions for Neuromuscular Tasks) conceptualizes desires for movement and rest as varying by magnitude, approach or avoidance-orientation (wants versus aversions) and as occupying independent dimensions instead of opposite ends of the same axis. For instance, one hypothetically might be in a state of both high desire for movement and rest simultaneously. Variations in motivation states to move and rest may also be associated with various stress states, like freezing or fight and flight. The first validated instrument to measure feelings of desire/want for movement and rest, the CRAVE Scale (Cravings for Rest and Volitional Energy Expenditure) is already shedding light on the nature of these states. With these advances in theory, conceptual modeling and instrumentation, future investigations may explore the effects of desires and urges for movement and sedentary behavior in earnest.

Exercising for food: bringing the laboratory closer to nature

Traditionally, exercise physiology experiments have borne little resemblance to how animals express physical activity in the wild. In this experiment, 15 adult male rats were divided into three equal-sized groups: exercise contingent (CON), non-exercise contingent (NON) and sedentary (SED). The CON group was placed in a cage with a running wheel, where the acquisition of food was contingent upon the distance run. Every three days the distance required to run to maintain food intake at free feeding levels was increased by 90% in comparison to the previous 3 days. The NON group were housed identically to the CON group, but food acquisition was not dependent upon running in the wheel. Finally, the SED group were kept in small cages with no opportunity to perform exercise. A two-way ANOVA with repeated measures was used to determine significant differences in responses between the experimental phases and treatment groups and ANCOVA to analyse growth and tissue mass variables with body length and body mass used separately as covariates. A post hoc Tukey's test was used to indicate significant differences. A Pearson's correlation was used to test the relationship between the distance travelled by the animal and the distance/food ratio. The level of significance was set at p<0.05 for all tests. The CON group showed the hypothesized correlation between distance required to run to obtain food and their mean distance travelled (p<0.001), during 45 days in contingency phase. The CON group showed a decrease in body mass, rather than an increase as shown by NON and SED groups. The CON group had a significantly lower body temperature (p<0.05) and adiposity (p<0.05) when compared to the other two groups for the same body size. The present experimental model based on animals choosing the characteristics of their physical exercise to acquire food (i.e., distance travelled, speed and duration) clearly induced physiological effects (body characteristics and internal temperature), which are useful for investigating relevant topics in exercise physiology such as the link between exercise, food and body weight.

Exploring Body Weight, Voluntary Activity, and Intrinsic Value in Rats

Modeling activity-based anorexia, food-deprived rats consistently show that activity increases as weight decreases. This effect was explored in 8 food-deprived, Sprague-Dawley rats as potentially mediated by intrinsic value of activity. Running-wheel activity rates were recorded for free-fed weight, reduction to 90% of free-fed weight, and to 80% of free-fed weight. As expected, activity increased as weight decreased. Further, significant differences appeared in the trends of individual run rates when compared across all trials. These individual trends were expressed as varied rates of running, with extreme high and low run rates prevalent. The rewarding nature of exercise itself might serve to predict these trends in individual rats and reveal potential indicators for the development of activity-based anorexia.

Reinforcement value and substitutability of sucrose and wheel running: implications for activity anorexia

Choice between sucrose and wheel-running reinforcement was assessed in two experiments. In the first experiment, ten male Wistar rats were exposed to concurrent VI 30 s VI 30 s schedules of wheel-running and sucrose reinforcement. Sucrose concentration varied across concentrations of 2.5, 7.5, and 12.5%. As concentration increased, more behavior was allocated to sucrose and more reinforcements were obtained from that alternative. Allocation of behavior to wheel running decreased, but obtained wheel-running reinforcement did not change. Overall, the results suggested that food-deprived rats were sensitive to qualitative changes in food supply (sucrose concentration) while continuing to defend a level of physical activity (wheel running). In the second study, 15 female Long Evans rats were exposed to concurrent variable ratio schedules of sucrose and wheel-running, wheel-running and wheel-running, and sucrose and sucrose reinforcement. For each pair of reinforcers, substitutability was assessed by the effect of income-compensated price changes on consumption of the two reinforcers. Results showed that, as expected, sucrose substituted for sucrose and wheel running substituted for wheel running. Wheel running, however, did not substitute for sucrose; but sucrose partially substituted for wheel running. We address the implications of the interrelationships of sucrose and wheel running for an understanding of activity anorexia.

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

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.

Responding for sucrose and wheel-running reinforcement: effect of body weight manipulation

As body weight increases, the excitatory strength of a stimulus signaling an opportunity to run should weaken to a greater degree than that of a stimulus signaling an opportunity to eat. To test this hypothesis, six male albino Wistar rats were placed in running wheels and exposed to a fixed interval 30-s schedule that produced either a drop of 15% sucrose solution or the opportunity to run for 15s as reinforcing consequences for lever pressing. Each reinforcer type was signaled by a different stimulus. The effect of varying body weight on responding maintained by these two reinforcers was investigated by systematically increasing and decreasing post-session food amounts. The initial body weight was 335 g. Body weights were increased to approximately 445 g and subsequently returned to 335 g. As body weight increased, overall and local lever-pressing rates decreased while post-reinforcement pauses lengthened. Analysis of post-reinforcement pauses and local lever-pressing rates in terms of transitions between successive reinforcers revealed that local response rates in the presence of stimuli signaling upcoming wheel and sucrose reinforcers were similarly affected. However, pausing in the presence of the stimulus signaling a wheel-running reinforcer lengthened to a greater extent than did pausing in the presence of the stimulus signaling sucrose. This result suggests that as body weight approaches ad-lib levels, the likelihood of initiation of responding to obtain an opportunity to run approaches zero and the animal "rejects" the opportunity to run in a manner similar to the rejection of less preferred food items in studies of food selectivity.

Relationship between wheel running, feeding, drinking, and body weight in male rats

The amount of wheel running varies widely between rats. Wheel introduction and running also have profound effects on the animal's energy balance. We explored the effects of ad lib wheel access and running levels on feeding, drinking, and body weight in 30 young adult male rats with wheel access and in 30 rats without wheel access. The initial running period (Days 1-8) and a time of stable running [Days 29-32 (DEnd)] were analyzed using both between- and within-group approaches. Initially, wheel access suppressed feeding (by about 25% over the 8 days) but not drinking. There were no significant correlations between the amount of wheel running and the other behaviors. The degree of feeding suppression was also not correlated to the amount of running. When running had stabilized (animal ran from 841 to 13,124 wheel turns per day), food intake was increased by about 0.75 g per 1000 wheel turns. Running at this time correlated positively with feeding and drinking and negatively with body weight and weight gain. In animals without wheel access, feeding and drinking were positively correlated with weight and weight gain, but in animals with wheel access, these correlations were close to zero. Only early running predicted later levels of running but accounted for only 23% of the variance in running. Wheel access has profound but very different immediate and long-term effects on the rats' energy balance.

Responding for sucrose and wheel-running reinforcement: effects of sucrose concentration and wheel-running reinforcer duration

Six male albino rats were placed in running wheels and exposed to a fixed-interval 30-s schedule of lever pressing that produced either a drop of sucrose solution or the opportunity to run for a fixed duration as reinforcers. Each reinforcer type was signaled by a different stimulus. In Experiment 1, the duration of running was held constant at 15 s while the concentration of sucrose solution was varied across values of 0, 2.5. 5, 10, and 15%. As concentration decreased, postreinforcement pause duration increased and local rates decreased in the presence of the stimulus signaling sucrose. Consequently, the difference between responding in the presence of stimuli signaling wheel-running and sucrose reinforcers diminished, and at 2.5%, response functions for the two reinforcers were similar. In Experiment 2, the concentration of sucrose solution was held constant at 15% while the duration of the opportunity to run was first varied across values of 15, 45, and 90 s then subsequently across values of 5, 10, and 15 s. As run duration increased, postreinforcement pause duration in the presence of the wheel-running stimulus increased and local rates increased then decreased. In summary, inhibitory aftereffects of previous reinforcers occurred when both sucrose concentration and run duration varied; changes in responding were attributable to changes in the excitatory value of the stimuli signaling the two reinforcers.

Determinants affecting physical activity levels in animal models

Weight control is dependent on energy balance. Reduced energy expenditure (EE) associated with decreased physical activity is suggested to be a major underlying cause in the increasing prevalence of weight gain and obesity. Therefore, a better understanding of the biological determinants involved in the regulation of physical activity is essential. To facilitate interpretation in humans, it is helpful to consider the evidence from animal studies. This review focuses on animal studies examining the biological determinants influencing activity and potential implications to human. It appears that physical activity is influenced by a number of parameters. However, regardless of the parameter involved, body weight appears to play an underlying role in the regulation of activity. Furthermore, the regulation of activity associated with body weight appears to occur only after the animal achieves a critical weight. This suggests that activity levels are a consequence rather than a contributor to weight control. However, the existence of an inverse weight-activity relationship remains inconclusive. Confounding the results are the multifactorial nature of physical activity and the lack of appropriate measuring devices. Furthermore, many determinants of body weight are closely interlocked, making it difficult to determine whether a single, combination, or interaction of factors is important for the regulation of activity. For example, diet-induced obesity, aging, lesions to the ventral medial hypothalamus, and genetics all produce hypoactivity. Providing a better understanding of the biological determinants involved in the regulation of activity has important implications for the development of strategies for the prevention of weight gain leading to obesity and subsequent morbidity and mortality in the human population.

The currency of procurement cost

As the number of instrtumental responses required to procure access to food is increased, animals decrease the frequency of initiating meals and increase meal size, conserving total intake while limiting the increase in the overall cost of feeding. In two studies, one using wheel turns and one using bar presses as the instrumental response, we asked whether freely feeding laboratory rats measure cost according to the energy or the time they expend. In each study we varied both the price (i.e., number of wheel turns or bar presses) and the force required to make a response (i.e., torque on the wheel or weight of the bar). Price affected both procurement time (from the first to the last procurement response) and procurement work, whereas torque and bar weight affected work without altering time in most cases. Meal patterns were altered by all manipulations of price, but changes in torque and bar weight had little effect on meal patterns, except in the conditions in which they altered procurement time. These results suggest that time is a critical currency of procurement cost in rats.

The effects of changes in housing on feeding and wheel running

The experiments explored the effects on feeding when rats were moved between individual and paired housing. In Experiment 1, rats moved to paired housing showed a 3-day suppression in feeding (initially 23%) compared to chronically individual- or pair-housed rats. In Experiment 2, half of the rats from the two control groups of Experiment 1 were moved between individual and paired housing on alternate days. Only the rats moved to paired housing showed a feeding suppression (initially 40%), but the nature of the suppression differed from Experiment 1: it appeared that only one rat of each pair showed a feeding suppression. Experiment 3 examined simultaneous introduction of running wheels and moves to paired housing. The feeding suppression induced by the move to paired housing was more immediate and shorter lived than the wheel-induced suppression. Unlike wheel access, paired housing produced only a temporary suppression of body weight. These experiments suggest that the relatively simple manipulation of moving rats from individual to paired housing results in a temporary stress-induced decrease in feeding.

Effects of short- and long-term wheel deprivation on running

The effects of wheel deprivation on running were explored. Eight male rats, well habituated to wheels, were each deprived of wheels for periods of 0, 1, 3, and 10 h during the night (Experiment 1) and 0, 1, 3, and 10 days (Experiment 2). In Experiment 1, as wheel deprivation lengthened, wheel running in the first 24 h of access increased. After 10 days of wheel deprivation subsequent daily running decreased (by 70%), and feeding was suppressed for several days. This temporary decline may be due to detraining and the rats physical inability to run more. Experiment 3 with 12 rats found that the running increase after 3-h wheel deprivation was proportional to the amount of running normally occurring during the deprivation period. Over the short-term, running appears to be regulated like other appetitive behaviors.

Voluntary wheel running: a review and novel interpretation

Voluntary wheel running by animals is an activity that has been observed and recorded in great detail for almost a century. This review shows that it is performed, often with startling intensity and coordination, by a wide variety of wild, laboratory and domestic species with diverse evolutionary histories. However, despite the plethora of published studies on wheel running, there is considerable disagreement between many findings, thus leading to a lack of consensus on explanations of the causality and function. In the initial part of this review, I discuss the internal and external factors that may be involved in the causality of this behaviour, with an emphasis on disparities in both the factual and theoretical development of the subject. I then address the various proposed functions of wheel running, again highlighting evidence to the contrary. This leads to the conclusion that any single theory on the basis of wheel running is likely to be simplistic with little generality. I then present a novel, behaviour-based interpretation in which it is argued that wheel running has no directly analogous naturally occurring behaviour, it is (sometimes) performed for its own sake per se rather than as a redirected or substitute activity, and studies on motivation show that wheel running is self-reinforcing and perceived by animals as 'important'. This review proposes that wheel running may be an artefact of captive environments or of the running-wheel itself, possibly resulting from feedback dysfunction. I also discuss the ubiquity and intensity of its performance, along with its great plasticity and maladaptiveness, all indicating that if it is an artefact, it is nevertheless one of great interest to behavioural science. Copyright 1998 The Association for the Study of Animal Behaviour.

Alternate-day wheel access: effects on feeding, body weight, and running

For rats access to a running wheel results in a pronounced but temporary suppression of feeding. The reasons for the feeding suppression and its temporary nature are unclear. The effects of alternate-day wheel access were explored by comparing feeding and running in 25 male Sprague-Dawley rats given either no wheel access, continuous wheel access, or alternate-day wheel access. With alternate-day wheel access food intake was suppressed on wheel days and elevated on non-wheel days for the full 32 days of the experiment. Body weight decreased on wheel days and showed a large increase on non-wheel days. Acquisition of running over days was similar in both wheel groups and plateaued at the same level, but running was elevated, compared to continuous-access rats, for the first few hours when alternate-day rats were returned to the wheel. These results suggest that wheel-induced feeding suppression is not due to the novelty of the wheel and that this suppression can be extended by providing periods with no wheel access. The temporary nature of feeding suppression in chronic access conditions may be due to secondary longer term motivational changes.

Developmental consequences of diet and activity

The effect of a protein-deficient and a protein-surfeit diet and continuous access to an activity wheel on food intake, growth, and body temperatures of behaviorally thermoregulating White Leghorn chicks was assessed in two experiments. In Experiment 1, both imbalanced-protein diets depressed intake and growth and differentially affected activity relative to a control diet, but activity did not ameliorate the deleterious effect of a high-protein diet on growth. Diet groups with continuous access to a running wheel did not differ on any measure from corresponding inactive dietary control groups. In Experiment 2, these results were replicated in a lower ambient temperature, and an effect of diet on body temperature emerged. Diets that affected spontaneous activity or body temperature also affected death feigning, a predation defense behavior. The data from behaviorally thermoregulating chicks are consistent with previous findings that activity does not depress growth rate in animals who cannot convert a portion of their intake into adipose tissue.

Hippocampal serotonin mediates hypoactivity in dietarily obese hamsters: a possible manifestation of aging?

To determine whether endogenous opiates mediate hyperactivity in food restricted hamsters and serotonergic fibers innervating the hippocampus mediate hypoactivity in obese hamsters, food restriction and high-fat diet supplementation were used to produce significant body fat changes (8 vs. 21%). The levels and pattern of spontaneous running were examined after IP saline or naloxone HCl (20 mg/kg) and following the infusion of vehicle and 5,7-dihydroxytryptamine creatine sulfate (4 micrograms/2 microliters) into rostromedial septum of mature female hamsters. Septum-medial preoptic area (POA), hippocampus, hypothalamus, and cortex were dissected from the three groups as well as from two additional groups of hamsters receiving vehicle or neurotoxin. Concentrations of serotonin, norepinephrine, and dopamine were measured in these tissues by HPLC method. Fat-fed hamsters were hypoactive relative to food-restricted hamsters. Naloxone had no significant effect on running behavior. Serotonin neurotoxin increased the running activity of fat-fed hamsters to the level displayed by control hamsters by increasing the number of runs, the total activity level, the speed of running and by decreasing the duration of pauses. Neurotoxin led to selective deletion of serotonin in the hippocampus (77%) and parietal cortex (50%). Serotonergic fibers innervating the hippocampus thus appear to mediate the hypoactivity that is induced by dietary obesity in mature hamsters. Since serotonin mediates some other manifestations of aging, and slow weight increases characterize mid-portion of hamster life span, we hypothesize that serotonergic mediation of hypoactivity is another manifestation of aging.

Time budgets in growing chicks

The manner in which rapidly growing chicks distribute their time among diurnal activities was measured in two studies in which the amount of available light was varied to match seasonal extremes. The effectiveness of the chick's time budgets was assessed in terms of the impact on growth. In Experiment 1, 24-hr patterns of feeding, drinking, nesting, and wheel-running were recorded during successive photoperiods with 12, 6, 18, and 12 hr of light when access to a social partner was concurrently available. In Experiment 2, access to a social partner was an exclusive activity. In both studies, chicks' 24-hr behavioral patterns in response to temporal constraints on their diurnal activities were surprisingly plastic, permitting them to defend a normal and high rate of growth. This was accomplished by changes in feeding rate and by nocturnal feeding rather than by systematic elimination of other diurnal activities. When sociality and feeding were competing activities, feeding time decreased and feeding rate increased such that contact with a conspecific as well as rapid growth were defended. The absolute amount of time spent in measured activities was invariant within subjects irrespective of age, photoperiod, or the number and type of activities recorded, providing evidence of time budgets in immature organisms. In both studies, buffer time (the amount of time spent in no activity) emerged as a significant aspect of the daily time budget that is defended through a variety of environmental challenges.

Ambient temperature effects on energetic relations in growing chicks

The effects of different ambient temperature conditions on the diet selection, intake, growth, body temperature, and activity of immature domestic chicks were assessed in two experiments. In Experiment 1, the ambient temperature either remained warm during both the light and dark phases of the photoperiod, as is characteristic in laboratory settings, or was warm during the light phase and cold during the dark phase. The latter condition reflects the daily temperature pattern in natural settings. Chicks exposed to low nocturnal ambient temperatures had lower body temperatures in both phases of the photoperiod, were less active, ate more, selected a higher percentage of carbohydrate in their diets, and grew faster but were less feed-efficient than warm-reared controls. In Experiment 2, the ambient temperature was either cool in both phases of the photoperiod or cool in the light phase and warm in the dark phase. Chicks reared continuously in the cold had lower body temperatures, selected a high-carbohydrate diet, and grew faster, but both rearing groups were relatively inactive. These results show that an animal's body temperature, diet composition, food intake, feed efficiency, and activity reflect its 24-hr energy requirements and are a part of a general strategy of maximizing energy income and minimizing energy expenditure in response to energetic challenges to growth.

Rostral hypothalamic microinfusions of 5,7 dihydroxytryptamine produce anatomically and neurochemically selective depletions of hippocampal serotonin and increase the influence of estrogen and food deprivation on locomotor activity

Ovariectomized Long-Evans rats received bilateral rostral hypothalamic infusions of 5,7-dihydroxytryptamine (5,7-DHT). Neurochemical determination of catecholamines (CA) and indoleamines in the hippocampus, hypothalamus and mesencephalon revealed that 5,7-DHT infusions had no effect on CA content in these areas nor in mesencephalic serotonin or 5-hydroxyindoleacetic acid (5-HIAA). However, the neurotoxin produced significant decreases in hippocampal serotonin and 5-HIAA. Serotonin-depleted animals exhibited an increase in both spontaneous and estradiol-induced wheel running. In addition it was found that serotonin-depleted animals exhibit an enhanced activity response to starvation. Because estrogen is thought to decrease serotonergic transmission, the enhanced activity response to estrogen may be secondary to an estrogen-related exaggeration of the 5,7-DHT-induced serotonin depletion. The increased activity effect of starvation may indicate that serotonin-depleted animals do not effectively mobilize energy stored as lipid.

Body weight, food intake and energy regulation in exercising and melatonin-treated Siberian hamsters

The coupling among energy intake, storage, and expenditure was examined in male Siberian hamsters (Phodopus sungorus sungorus) given access to running wheels for 14 weeks. Half of the hamsters were injected with melatonin in a schedule that mimics the effects of short photoperiods by decreasing body weight, carcass lipid content, and testis weight. The exercise-induced body weight increases which are seen in Syrian hamsters (Mesocricetus auratus) were not found in Siberian hamsters. Instead, the Siberian hamsters maintained their body weights, compensating for the increased energy demands of exercise by increasing their food intakes. Exercise did not affect carcass composition. Melatonin treatment decreased food intake and carcass lipid stores but did not affect voluntary exercise. The previously reported decrease in testis weight was seen in all melatonin-treated hamsters, but the stimulation of brown adipose tissue growth was not. Thus, exercising Siberian hamsters, unlike exercising Syrian hamsters, appear to exhibit a tight coupling among energy intake, storage, and expenditure.

Ingestive behaviour of Syrian hamsters: advantages of the comparative approach

A brief review of laboratory feeding studies in Syrian hamsters (Mesocricetus auratus) is presented. A number of profound behavioural differences between rats and hamsters are noted. One such difference concerns the diurnal organization of energy input and expenditure. It is suggested that several of these differences might be exploited at the level of neurochemical correlates of feeding, and some preliminary studies are summarized. Finally, some unexpected abnormalities in salt and water intake in regard to the actions of angiotensin II are identified as further areas for comparative investigation.

Opioid modulation of appetite

The discovery of opiate receptors and endogenous opioid peptides within the central nervous system has resulted in a number of speculations concerning the physiological significance of these peptides. In the present article, we review the evidence suggesting a primary role for some of the opioid peptides as regulators of ingestive behavior. In particular, we elaborate a hypothesis in which we suggest that in some species opioid peptides may play a role as a tonic inducer of ingestive behaviors, held in check by a variety of neuropeptides and monoamines. This review explores in detail the role of the opioid peptides as major mediators of the reward system and as a link between reward and feeding behaviors. Finally, a teleological role for opioid peptides in species preservation, which may explain the discrepancies in the role of the opioid peptides in feeding behavior in different species is proposed. It is suggested that the feeding profile of the animal provides important clues as to whether or not the animal has an opiate-sensitive feeding system. We stress that interactions with ingested nutrients and the milieu interieur provide an important means by which animals modulate the opiate-entrained feeding drives.

Interrelationships among activity, food intake and weight gain in genetically obese and lean Zucker rats

In Experiment 1, food deprivation resulting in a 30% reduction in body weight produced significant increases in wheel running in both obese and lean female Zucker rats. In Experiment 2, a new technique, food contingent activity (FR, VI), dramatically increased wheel running in both obese and lean female Zucker rats. This increase in activity was achieved primarily during the dark period. Regardless of changes in activity levels, food intake and body weight gain remained similar to controls. When food was again available ad lib, activity levels rapidly decreased for obese but not lean rats. These results indicate that behavioral interventions alone are not sufficient to correct the obesity of the genetically obese rat.

activity and catch-up growth in hypothyroid rats

Hypothyroidism was induced in 38-day old male rats by feeding the animals a chow diet supplemented with propylthiouracil (PTU, 0.1% by weight) for 43 days. Wheel activity of PTU animals was not significantly different from that of euthyroid, ad lib feeding controls it was significantly lower when compared to pairfed controls. Body weight was significantly lower than that of euthyroid ad lib controls. After 75 days of PTU discontinuation catch-up growth of PTU animals was not complete: body weight and tibia length were significantly lower in the PTU condition in comparison to euthyroid, ad lib feeding condition. However, no difference existed between the catch-up growth of PTU and pairfed animals. It was suggested that growth arrest observed in early hypothyroidism may be partly due to factors nonspecific to thyroxine absence, such as hypophagia.

Naloxone suppresses feeding and drinking but not wheel running in rats

The effects of naloxone hydrochloride on food and water intake and number of wheel revolutions were measured in male rats. The administration of 10 mg/kg naloxone but not 1 mg/kg suppressed the 3-hr food and water intake in nondeprived rats. Naloxone injections (1 mg/kg or 10 mg/kg) were ineffective in altering the number of wheel revolutions in nondeprived or food deprived rats. These results support the interpretation that the suppressive effects of naloxone previously reported with deprived rats are evident in nondeprived rats and are specific to feeding and drinking.

Food and water deprivation: changes in rat feeding, drinking, activity and body weight

Groups of male rats totally food deprived for periods up to 96 hr, water ad lib do not totally regain body weight lost through deprivation. Degree of body weight defence was estimated by fitting statistically a curve to the predeprivation baseline data and extrapolating the curve over the deprivation and postdeprivation period. From group data, body weight is not fully defended. However, marked individual differences in degree of body weight defence were noted. During food deprivation the daily drinking cycle persists for approximately 48 hr, then a reduction in nocturnal drinking appears. General activity increases but only in the light period. After food restitution, the main hyperphagia is confined to the first post-deprivation day. Additional, subtle long term effects of overeating in the light period are also found. In contrast to food deprived groups, water deprived groups accurately defend body weight. During water deprivation daily food intakes decrease immediately whereas nocturnal activity decreases after approximately 72 hr of deprivation has elapsed. After the termination of water deprivation the immediate response is gross hyperdipsia, but again subtle long term effects are apparent. Total daily food intake increases for many days but the increase is not confined to either light or dark periods. Water intake is similarly affected. These results are evaluated in relation to the notion of a body weight set-point regulating feeding behavior. Some limitations on the value of the findings in terms of species differences and lack of ecological validity are also discussed.

Patterns of eating as a function of the cost of the meal

Rats were required to complete fixed ratio schedules (FR 20-FR 2560) of wheel turns to obtain access to food. By decreasing meal frequency and increasing meal size directly as functions of the fixed ratio requirement, animals controlled total daily food intake and body weight relatively constant until the highest ratio requirement was introduced. These functional changes in feeding patterns provide experimental support for theoretical models of optimal feeding strategies. At the highest ratio requirement, as animals lost weight, they increased running and therefore opportunities to feed, however, food intake continued to decrease with increasing exposure to this schedule. As rats on this schedule initiated feeding each time food became available, but did not eat large enough to this schedule. As rats on this schedule initiated feeding each time food became available, but did not eat large enough meals to maintain body weight, it is suggested that activity may interest with satiety mechanisms to produce termination of meals.

Increased serum growth hormone and somatic growth in exercising adult hamsters

In freely feeding adult hamsters, voluntary exercise induces accelerated somatic growth and increased food consumption that last through several days of retirement. We examined the effects of exercise on serum concentrations of growth hormone (GH) and insulin during ad libitum or restricted intake of food. Serum insulin and GH concentrations were measured by radioimmunoassays in exercising, retired, or sedentary hamsters during ad libitum or restricted intake of food. Linear growth was delayed in food-restricted, exercising hamsters until they were retired and unlimited food consumption was allowed. Serum GH concentrations were increased during exercise and after retirement; serum insulin concentrations were increased only after retirement during both dietary regimes. In food-restricted hamsters, endocrine changes were noted after 4 h of feeding but not after a 14-h fast. We conclude that 1) in adult hamsters voluntary exercise leads to increased secretion of GH even when ingested nutrients are insufficient to support increased growth, and 2) increased secretion of insulin is not related directly to exercise.