Effects of amphetamine, dexfenfluramine, diazepam, and dietary manipulations on responding reinforced by stimuli paired with food in nonhuman primates

Attention Bias Test Measures Negative But Not Positive Affect in Sheep: A Replication Study

Simple Summary

Attention bias tests may provide a practical measure of emotional states in livestock. An attention bias test has been developed as a measure of negative emotional states in sheep. This study aimed to determine whether the test could also be used to assess positive emotional states. Our results indicated that the attention bias test was unable to differentiate control animals from those in drug-induced positive states (p > 0.05). However, our findings further supported the suggestion that the attention bias test may measure negative emotional states in sheep. With further refinement, the attention bias test may be a useful tool to assess and improve livestock welfare.

Abstract

An attention bias test has been developed as a measure of negative affective states in sheep. The test measures an individual’s allocation of attention between a threatening (previous location of a dog) and positive (conspecific photo) stimulus over a 3 min period. This study replicated a previously inconclusive study, to determine whether the test could assess positive affective states under more controlled conditions and with a younger population of animals. Pharmacological treatments were used to induce anxious, calm, happy, and control affective states prior to entering the attention bias test arena (n = 20/treatment). We hypothesized that sheep in positive and negative affective states could be differentiated using key measures of attention during testing, including vigilance (head at or above shoulder height) and duration looking towards the valenced stimuli. Anxious sheep were more vigilant than control animals during attention bias testing as predicted (linear mixed effects model, p = 0.002), but the positive groups did not differ from controls (p > 0.05). There was no effect of treatment on looking behaviors (p > 0.05). We suggest this attention bias test paradigm can assess negative but not positive affect in sheep and that modifications to the ethogram or stimuli are needed to more clearly characterize the direction of attention during testing.

The influence of pharmacologically-induced affective states on attention bias in sheep

When an individual attends to certain types of information more than others, the behavior is termed an attention bias. The occurrence of attention biases in humans and animals can depend on their affective states. Based on evidence from the human literature and prior studies in sheep, we hypothesized that an attention bias test could discriminate between pharmacologically-induced positive and negative affective states in sheep. The test measured allocation of attention between a threat and a positive stimulus using key measures of looking time and vigilance. Eighty 7-year-old Merino ewes were allocated to one of four treatment groups; Anxious (m-chlorophenylpiperazine), Calm (diazepam), Happy (morphine) and Control (saline). Drugs were administered 30 min prior to attention bias testing. The test was conducted in a 4 × 4.2 m arena with high opaque walls. An approximately life-size photograph of a sheep was positioned on one wall of the arena (positive stimulus). A small window with a retractable opaque cover was positioned on the opposite wall, behind which a dog was standing quietly (threat). The dog was visible for 3 s after a single sheep entered the arena, then the window was covered and the dog was removed. Sheep then remained in the arena for 3 min while behaviors were recorded. Key behaviors included time looking toward the dog wall or photo wall, duration of vigilance behavior and latency to become non-vigilant. In contrast with our hypothesis, no significant differences were found between treatment groups for duration of vigilance or looking behaviors, although Anxious sheep tended to be more vigilant than Control animals (P < 0.1) and had a longer latency to become non-vigilant (P < 0.001). A total of 24 of 80 animals were vigilant for the entire test duration. This censoring of data may explain why no differences were detected between groups for vigilance duration. Overall, a lack of difference between groups may suggest the test cannot discriminate positive and negative states in sheep. We suggest that the censoring of vigilance duration data, the use of insufficient drug doses, the potential influence of background noise and the age of the sheep may explain a lack of difference between groups. Due to these potential effects, it remains unclear whether the attention bias test can detect positive states in sheep.

Towards a more practical attention bias test to assess affective state in sheep

Tests for attention bias potentially offer more rapid assessment of affective state in animals than existing cognitive methods. An attention bias test has previously been developed for sheep and validated as a measure of anxious states. The 3 minute test assessed behavioural responses of sheep in an enclosed arena after brief exposure to the threat of a dog. Experiment 1 of the current study aimed to refine the previously developed method, removing the need for a habituation period and shortening the test duration. Sheep were given either an anxiolytic drug, an anxiogenic drug or a control treatment prior to testing to induce contrasting affective states. Differences in behaviour were found between the treatment groups within the first 45s of the test, indicating the original test duration could be shortened from 180 s. During testing, 36 of 40 animals in the control and anxiolytic groups ate the novel feed offered in the test, indicating it is not necessary to habituate animals to a feed container. Experiment 2 aimed to confirm the responses measured in the test were primarily towards the dog rather than other aspects of the test environment. Sheep exposed to an empty window at the beginning of the test behaved differently to those which were exposed to a dog, indicating sheep behaviour in the test is at least partially a response to the dog. A third group of sheep were also tested with the dog immediately after having small data loggers attached to their necks. Behaviour of these sheep did not differ from the sheep tested without loggers, indicating data logger attachment did not impact their behaviour in the test. In both experiments, treatments did not appear to modify activity (zones crossed), which we propose indicates the test was primarily detecting valence of the affective state rather than arousal.

Evaluating pharmacological models of high and low anxiety in sheep

New tests of animal affect and welfare require validation in subjects experiencing putatively different states. Pharmacological manipulations of affective state are advantageous because they can be administered in a standardised fashion, and the duration of their action can be established and tailored to suit the length of a particular test. To this end, the current study aimed to evaluate a pharmacological model of high and low anxiety in an important agricultural and laboratory species, the sheep. Thirty-five 8-month-old female sheep received either an intramuscular injection of the putatively anxiogenic drug 1-(m-chlorophenyl)piperazine (mCPP; 1 mg/kg; n = 12), an intravenous injection of the putatively anxiolytic drug diazepam (0.1 mg/kg; n = 12), or acted as a control (saline intramuscular injection n = 11). Thirty minutes after the treatments, sheep were individually exposed to a variety of tests assessing their general movement, performance in a ‘runway task’ (moving down a raceway for a food reward), response to startle, and behaviour in isolation. A test to assess feeding motivation was performed 2 days later following administration of the drugs to the same animals in the same manner. The mCPP sheep had poorer performance in the two runway tasks (6.8 and 7.7 × slower respectively than control group; p < 0.001), a greater startle response (1.4 vs. 0.6; p = 0.02), a higher level of movement during isolation (9.1 steps vs. 5.4; p < 0.001), and a lower feeding motivation (1.8 × slower; p < 0.001) than the control group, all of which act as indicators of anxiety. These results show that mCPP is an effective pharmacological model of high anxiety in sheep. Comparatively, the sheep treated with diazepam did not display any differences compared to the control sheep. Thus we suggest that mCPP is an effective treatment to validate future tests aimed at assessing anxiety in sheep, and that future studies should include other subtle indicators of positive affective states, as well as dosage studies, so conclusions on the efficacy of diazepam as a model of low anxiety can be drawn.

A novel procedure for assessing the effects of drugs on satiation in baboons: effects of memantine and dexfenfluramine

RATIONALE

Procedures for studying the effects of medications on satiation will assist the development of obesity medications.

OBJECTIVES

Develop a procedure for measuring satiation during consumption of bland and highly palatable food and determine the effect of acute intramuscular administration of dexfenfluramine (DFEN), which increases serotonin levels, and memantine (MEM), which blocks N-methyl-D: -aspartate receptors.

MATERIALS AND METHODS

A modified progressive ratio (PR) procedure was used to track changes in reinforcing strength when a food was consumed. The response requirement increased after each reinforcement, and reinforcing strength was estimated using the breakpoint (BP), which was the last completed response cost. There was one preferred food (sweet candy) and one chow pellet PR session per week. During each session, four male and four female adult baboons experienced three 1-h PR trials, separated by 30 min. Chow pellets were available at all other times. We examined the BP for one to 20 candies or chow pellets. Drug effects were examined when baboons had access to one and ten candies or chow pellets.

RESULTS

BPs for candy were greater than for pellets. Varying the pellet/candy pieces per delivery produced an inverted U-shaped function on the first trial, i.e., maximal BP was observed for three items, and the BP for multiple items, but not a single item, decreased across trials, i.e., BP decreased with food intake and satiation. DFEN and MEM decreased responding with the greatest effects at ten deliveries, suggesting that DFEN and MEM enhanced satiation.

CONCLUSION

Drugs that enhance satiation for several types of food may be particularly effective for decreasing food intake.

Effects of the cannabinoid antagonist SR141716 (rimonabant) and d-amphetamine on palatable food and food pellet intake in non-human primates

The purpose of this study was to determine if a cannabinoid CB(1) receptor antagonist would selectively decrease consumption of highly palatable food in non-human primates. The CB(1) receptor antagonist SR141716 (rimonabant; 0.12-1.0 mg/kg, i.m.) and the stimulant anorectic drug d-amphetamine (0.12-1.0 mg/kg, i.m.) were administered to non-food deprived baboons for the purpose of measuring the effect of each drug on consumption of the normal diet, and a large single meal of a high-carbohydrate candy. Four male and four female baboons had access to food 24 h each day, but they had to complete a two phase operant procedure in order to eat. Responding on one lever during a 30-min appetitive phase was required before animals could start a consumption phase, where responding on another lever led to food delivery, i.e., a meal. Three days a week baboons received a jelly sugar-coated candy (Skittles) during the first meal and then pellets were available in subsequent meals. All baboons ate as many individual candies in one meal as they did pellets throughout the entire day. Acute d-amphetamine and, to a lesser extent, SR141716 decreased both candy intake in a single meal and pellet intake in a single meal and over 24 h. d-Amphetamine, but not SR141716, increased latency to the candy meal and the first pellet meal indicating that the two drugs differentially altered feeding topography. Although males ate more food pellets than females, few other sex differences were observed. Thus, although effective in decreasing food intake, there was no evidence of a specific effect of CB(1) receptor antagonism on consumption of a large meal or a palatable food.

"Tasting and wasting" behavior in non-human primates: aberrant behavior or normal behavior in "times of plenty"

The purpose of this study was to develop a foraging model that engenders large meals. Eight free-feeding baboons were first given periodic access to a chocolate sugar-coated candy (M & Ms) and then a jelly sugar-coated candy (Skittles). Baboons had access to food 24 h each day, but they had to complete a two-phase operant procedure in order to eat. Responding on one lever during a 30-min appetitive phase was required before animals could start a consumption phase, where responding on another lever led to food delivery, i.e., a meal. 3 days a week for 8 or 9 weeks baboons received candy during the first meal and then food pellets were available: a 2 month interval when only pellets were available separated periods of candy access. All baboons ate as much candy in the single candy meal as they did pellets throughout the remainder of the day. Beginning week 5 of M & M access, five baboons began to waste a large number of M & Ms by spitting them out. Baboons wasted few Skittles or pellets. Pellet intake was less, but total caloric intake was greater on days that animals had access to either candy. Pellet, but not candy eating varied between males and females: males began eating pellets sooner in the day, ate more pellet meals and more pellets. Periodic access to a preferred candy food engendered large amounts of candy consumption in all baboons, and periodic access to M & Ms engendered food tasting and wasting behavior in 5 of 8 baboons.

Baclofen decreases feeding in non-human primates

This study examined how the GABA(b) agonist baclofen (0.5-5.6 mg/kg, p.o.), reported to increase food intake in rodents, affected the appetitive and consummatory aspects of feeding of non-human primates. Baboons had access to food 24 h each day, but they had to complete a two-phase operant procedure in order to eat. Responding on one lever during a 30-min appetitive phase was required before animals could start a consumption phase, where responding on another lever led to food delivery, i.e., a meal. Responding during the appetitive phase resulted in presentations of food-related stimuli only. Baclofen increased the latency to the first meal and decreased both appetitive and consummatory behavior. At the largest dose, baclofen induced emesis, indicating that the effects were due to malaise rather than a specific motivational action. In contrast, the positive control diazepam (GABA(a) agonist, 1.0-2.0 mg/kg, i.m.) decreased the latency to the first meal and increased both appetitive (P < 0.07) and consummatory behavior. Although the baclofen-induced decrease in appetitive behavior replicates data obtained in rodents, the baclofen-induced decreases in consummatory behavior do not. The findings suggest that the effects of large doses of baclofen in non-human primates may, in part, be due to non-specific behavioral disruptions.

Effects of sibutramine on the appetitive and consummatory aspects of feeding in non-human primates

This study examined how sibutramine (0.06-4.0 mg/kg, i.m.), a clinically effective weight-loss medication which increases extracellular serotonin and norepinephrine levels, affected the appetitive and consummatory aspects of feeding of non-human primates. The effects were compared to the effects of the positive control dexfenfluramine (2.0-6.0 mg/kg, p.o.), which primarily increases extracellular serotonin levels. Baboons had access to food 24 h each day, but they had to complete a two-phase operant procedure in order to eat. Responding on one lever during a 30-min appetitive phase was required before animals could start a consumption phase, where responding on another lever led to food delivery, i.e., a meal. Responding during the appetitive phase resulted in presentations of food-related stimuli only. Sibutramine increased the latency to the first meal of the session in females, and decreased consummatory behavior without affecting other appetitive behavior in males and females. In contrast, dexfenfluramine, increased the latency to the first meal of the session, and decreased both appetitive and consummatory behavior in males and females. The behavioral mechanism by which sibutramine decreases food intake is distinct from other anorectic drugs, including dexfenfluramine, that have been tested in this paradigm.

Effects of dietary and pharmacological manipulations on appetitive and consummatory aspects of feeding in non-human primates

This study examined how pharmacological and behavioral manipulations affect appetitive and consummatory aspects of feeding of baboons. Baboons have access to food 24 h each day, but they must complete a two-phase operant procedure in order to eat. Responding on one lever during a 30-min appetitive phase was required before animals could start a consumption phase, i.e. a meal, where responding on another lever led to food delivery. Responding during the appetitive phase resulted in presentations of food-related stimuli only. Decreasing session length, increased appetitive behavior and increased meal size. Limiting the number of meals to a single 90 min meal each day but increasing the number of food pellets the animals received increased the size of meal, but did not increase appetitive behavior. These findings suggest that time since the previous meal has a greater effect on appetitive behavior than the size of the previous meal. Amphetamine (AMPH), which increases dopamine, decreased food intake at doses that did not affect appetitive behavior, indicating that appetitive and consummatory aspects of eating can be pharmacologically differentiated. Increasing how frequently animals could earn food-related stimuli in the appetitive phase and food in the consummatory phase increased both appetitive and consumatory behavior. Under these conditions, AMPH nearly doubled appetitive behavior at doses that decreased food intake by nearly 50 percent. When animals had one meal, of self-determined duration, meal size increased without affecting appetitive behavior, further demonstrating that appetitive behavior can be independent of the size of the previous meal and not predictive of the size of the subsequent meal. Under these conditions, AMPH decreased food intake at doses that did not affect appetitive behavior. In contrast, dexfenfluramine (DFEN), which increases serotonin, decreased both appetitive and consumatory behavior. Thus, it is possible to independently manipulate the appetitive and consummatory aspects of eating using both pharmacological and behavioral interventions indicating that it may be possible to develop medications that selectively affect appetitive or consummatory aspects of eating.

Palatability-dependent appetite and benzodiazepines: new directions from the pharmacology of GABA(A) receptor subtypes

This paper updates an early review on benzodiazepine-enhanced food intake, published in the first issue of Appetite, and describes the considerable advances since then in the pharmacology of benzodiazepines, their sites and mechanisms of action, and in understanding the psychological processes leading to the increase in food consumption. A great diversity of benzodiazepine receptor ligands have been developed, many of which affect food intake. Agonists can be divided into full agonists (which produce the full spectrum of benzodiazepine effects) and partial agonists (which are more selective in their effects). In addition, inverse agonists have been identified, with high affinity for benzodiazepine receptors but having negative efficacy: these drugs exhibit anorectic properties. Benzodiazepine receptors are part of GABA(A) receptor complexes, and ligands thereby modulate inhibitory neurotransmission in the brain. Molecular approaches have identified a palette of receptor subunits from which GABA(A) receptors are assembled. In all likelihood, benzodiazepine-induced hyperphagia is mediated by the alpha2/alpha3 subtype not the alpha1 subtype. Novel alpha2/alpha3 selective compounds will test this hypothesis. A probable site of action in the caudal brainstem for benzodiazepines is the parabrachial nucleus. Behavioural evidence strongly indicates that a primary action of benzodiazepines is to enhance the positive hedonic evaluation (palatability) of tastes and foodstuffs. This generates the increased food intake and instrumental responding for food rewards. Therapeutic applications may derive from the actions of benzodiazepine agonists and inverse agonists on food procurement and ingestion.

Effects of amphetamine, dexfenfluramine, and diazepam on responding during extinction in nonhuman primates

The effects of pharmacological manipulations on responding under extinction conditions were determined in baboons using a schedule of reinforcement that modeled food acquisition and food consumption. Responding during the initial acquisition component was reinforced by stimuli paired with food, while responding during the latter consumption component was reinforced with food. Certain sessions began with a 7-h extinction phase, where responding in both components produced only the paired stimuli. Dexfenfluramine (DFEN) decreased responding during extinction. Diazepam (DZP) increased responding during extinction. Low doses of amphetamine (AMPH) increased responding during extinction. Thus, DZP and AMPH increased and DFEN decreased the conditioned reinforcing effects of stimuli paired with food.