Naloxone enhancement of punishment in the rat

Rats were trained to lever press on a variable interval one minute schedule of reinforcement for food pellets. Subsequently in punishment sessions, lever presses also produced electric grid shocks on a fixed ratio 20 schedule. With punishment, lever pressing levels were reduced by about 15%. When naloxone was administered prior to punishment sessions, shocks reduced lever pressing levels by about 50% following the 3 mg/kg dose and by aboaut 80% following the 10 mg/kg dose. This naloxone-induced enhancement of the effect of punishment was probably mediated through an antagonism of naturally-occurring brain peptides with opiate-like activities.     

Environmental enrichment decreases responding for visual novelty

Previous research has demonstrated that rats reared in an enriched condition (EC) with novel objects and social partners self-administer less amphetamine compared to rats raised in an isolated condition (IC). However, it is unclear if the enrichment-induced decrease in stimulant self-administration generalizes to non-drug rewards such as those provided by novel environmental stimuli. In the current study, EC, IC, and social condition (SC) rats were raised from 21 to 51 days of age before being tested in a two-lever operant conditioning chamber in which responding on one lever (active lever) resulted in illumination of a cue light. In Experiment 1, rats were initially assessed for baseline responding (no contingency) and then the contingent light was introduced. EC rats responded less than IC rats for the contingent light stimulus; however, EC rats also displayed a lower rate of baseline responding. In Experiment 2, rats were trained initially to lever press for a sucrose reward to decrease differences in baseline responding. While sucrose pretraining decreased baseline response differences between groups, EC rats still responded less for the contingent light stimulus than IC or SC rats. These results suggest that environmental enrichment decreases the incentive value of visual novelty.     

Behavioral chaining in the goat (Capra hircus)

Five male castrated goats were trained to perform a behavioral chain. In Experiment I the goats were trained to run in an indoor U-shaped runway. The chain consisted of jumping over three hurdles, walking on a raised walkway, running through two 55 gallon barrels and entering an operant chamber and pressing a response panel on a fixed ratio 10 for a food reward. In Experiment II, a larger outdoor runway was used and the components of the chain were reordered. Before the beginning of Experiment I the goats were trained by the method of successive approximations to press a response panel. It took five to seven 1 h sessions to shape this response. It took 16 sessions to train the goats to perform all components of the behaviour chain under stimulus control. Once the goats reached this point, there were no significant variations in performance of the operant task or the time to complete the chain. Although the serial and geometric arrangement of the components of the chain were altered in Experiment II, it took nine sessions to train the goat to perform the altered chain. The results of this study demonstrate that goats can be easily trained to perform a complex behavioral chain. Once the chain has been learned, the goats readily adapt to alterations in the arrangement of the components and the location of the runway.     

Passive immunization against nicotine attenuates nicotine discrimination

Ten rats were trained in a two lever operant chamber to press different levers after a nicotine injection (0.14 mg/kg s.c.) or a saline injection on an FR10 schedule. The rats were then injected i.p. with either 150 mg nicotine-specific IgG or the same amount of control IgG from non-immunized rabbits. On successive days, they were retested with both levers active after a saline injection, a full training dose of nicotine and a half dose of nicotine (0.07 mg/kg s.c.). After saline injection, both groups pressed the saline lever almost exclusively. After each of the nicotine doses, the immunized rats performed a significantly lower percentage of their lever presses on the nicotine lever than did non-immunized rats. The results suggest that passive immunization can interfere with the stimulus properties of nicotine.     

Shape discrimination in sheep and calves

Sheep and calves have been trained, using an operant conditioning method in which they pressed panels with their muzzles in order to obtain food, to discriminate between simultaneously presented shapes. In the discrimination task the animals faced two response panels upon which were projected the shapes to be discriminated. Only presses on the panel associated with the correct shape were reinforced, and after each reinforcement the position of the positive stimulus was randomly varied. The results obtained demonstrated that sheep and calves can discriminate between a variety of similar shapes.     

The Effect of Ratio and Interval Training on Pavlovian-Instrumental Transfer in Mice

Conditional stimuli (CS) that are paired with reward can be used to motivate instrumental responses. This process is called Pavlovian-instrumental transfer (PIT). A recent study in rats suggested that habitual responses are particularly sensitive to the motivational effects of reward cues. The current experiments examined this idea using ratio and interval training in mice. Two groups of animals were trained to lever press for food pellets that were delivered on random ratio or random interval schedules. Devaluation tests revealed that interval training led to habitual responding while ratio training produced goal-directed actions. The presentation of CSs paired with reward led to positive transfer in both groups, however, the size of this effect was much larger in mice that were trained on interval schedules. This result suggests that habitual responses are more sensitive to the motivational influence of reward cues than goal-directed actions. The implications for neurobiological models of motivation and drug seeking behaviors are discussed.     

Operant behavior of the rat can be controlled by the configuration of objects in an animated scene displayed on a computer screen

We developed a novel behavioral task in which rats learn to recognize the configuration of objects in an animated scene displayed on a computer screen. The scene consisted of a moving bar and a stationary rectangle. Rats deprived of food were trained to press a lever for reward in a small chamber located in front of the screen. Lever presses were rewarded only when the bar was at the rectangle. Rats anticipated the reward by gradually increasing frequency of lever pressing as the bar approached the rectangle. Control experiments showed that neither the timing nor the discrimination of rewarded and non-rewarded periods as two discrete conditions explain behavior of the rat. Because the changes in the scene were generated by movement of the object, the presented task could be used for studying neural structures involved in spatial behavior of rats using virtual reality technology.     

The role of contextual cues in operant responding in rats

Following lever-press training on a variable-interval 60-second schedule of food presentation, groups of rats either remained in their home cages or were exposed to the operant chamber, from which lever and food had been removed, for five sessions. The lever was replaced in the chamber and rats from Group 1 (exposure to chamber) and Group 3 (home cage) were returned to the variable-interval schedule. Although response rates in test sessions were somewhat lower than at the end of training, there was no statistically significant difference in rates for either group. Rats in Group 2 (exposure to chamber) and Group 4 (home cage) received two test sessions of extinction. During the first session, Group 2 rates of lever pressing were significantly higher than Group 4 rates. These findings do not support the view that associations between contextual cues and the reinforcer serve to energize instrumental behavior (Pearce & Hall, 1979), and provide only minimal support for the view that contextual cues control responses that compete with the operant (Mills, 1980).     

An Operant Conditioning Method for Studying Auditory Behaviors in Marmoset Monkeys

The common marmoset (Callithrix jacchus) is a small New World primate that has increasingly been used as a non-human model in the fields of sensory, motor, and cognitive neuroscience. However, little knowledge exists regarding behavioral methods in this species. Developing an understanding of the neural basis of perception and cognition in an animal model requires measurement of both brain activity and behavior. Here we describe an operant conditioning behavioral training method developed to allow controlled psychoacoustic measurements in marmosets. We demonstrate that marmosets can be trained to consistently perform a Go/No-Go auditory task in which a subject licks at a feeding tube when it detects a sound. Correct responses result in delivery of a food reward. Crucially, this operant conditioning task generates little body movement and is well suited for pairing behavior with single-unit electrophysiology. Successful implementation of an operant conditioning behavior opens the door to a wide range of new studies in the field of auditory neuroscience using the marmoset as a model system.     

A High Throughput In Vivo Assay for Taste Quality and Palatability

Taste quality and palatability are two of the most important properties measured in the evaluation of taste stimuli. Human panels can report both aspects, but are of limited experimental flexibility and throughput capacity. Relatively efficient animal models for taste evaluation have been developed, but each of them is designed to measure either taste quality or palatability as independent experimental endpoints. We present here a new apparatus and method for high throughput quantification of both taste quality and palatability using rats in an operant taste discrimination paradigm. Cohorts of four rats were trained in a modified operant chamber to sample taste stimuli by licking solutions from a 96-well plate that moved in a randomized pattern beneath the chamber floor. As a rat’s tongue entered the well it disrupted a laser beam projecting across the top of the 96-well plate, consequently producing two retractable levers that operated a pellet dispenser. The taste of sucrose was associated with food reinforcement by presses on a sucrose-designated lever, whereas the taste of water and other basic tastes were associated with the alternative lever. Each disruption of the laser was counted as a lick. Using this procedure, rats were trained to discriminate 100 mM sucrose from water, quinine, citric acid, and NaCl with 90-100% accuracy. Palatability was determined by the number of licks per trial and, due to intermediate rates of licking for water, was quantifiable along the entire spectrum of appetitiveness to aversiveness. All 96 samples were evaluated within 90 minute test sessions with no evidence of desensitization or fatigue. The technology is capable of generating multiple concentration–response functions within a single session, is suitable for in vivo primary screening of tastant libraries, and potentially can be used to evaluate stimuli for any taste system.     

Do peptide-induced changes in feeding occur because of changes in motivation to eat?

The usual paradigm in which peptides are tested for their effect on food intake involves measuring intake of readily available food. In the lever press apparatus, the subjects must "work" to get food. Such work has traditionally been used as a means of measuring motivation. Mice were trained to press a lever for milk reinforcement. After achieving a stable level of performance, we tested the effects of gastrin-related peptide (GRP), bombesin (BBS) and cholecystokinin octapeptide (CCK-8) on lever pressing. All three peptides suppressed lever pressing for milk reinforcement. Prefeeding mice with milk increased the suppression of lever pressing to a greater extent in peptide-treated mice than in saline-treated mice. As the duration of prefeeding increased, lever pressing decreased. When mice were required to make more lever presses to obtain milk, both saline- and CCK-8-treated mice increased their lever pressing. However, saline-treated mice pressed at a higher rate than CCK-8-treated mice. Unlike the results obtained with saline and CCK-8, administration of a known gustatory adversant, lithium chloride, suppressed lever pressing to the same degree in mice fed or not fed prior to training. The results are consistent with the hypothesis that these peptides act as satiety agents.     

Operant model of frustrated expected reward in mice

One aspect of the addictive process that has not been thoroughly investigated is the consequence of the frustrated state occurring when the drug is not available. The present study aimed to validate a novel operant model of frustrated expected reward in mice. C57BL/6J mice were trained in operant conditioning maintained by chocolate-flavoured pellets or cocaine. After the completion of high rates of responding on a progressive ratio schedule, the reward was unexpectedly withheld. The consequences of this frustrated behaviour on anxiety, aggressiveness, perseveration, extinction and reinstatement were investigated. Mice exposed to the frustrated event perseverated in the operant responses and showed increased aggressiveness in the resident-intruder test. These animals also showed higher rates of cue-induced reinstatement of drug seeking. The present study provides a reliable operant model in mice to evaluate a frustrated state following reward unavailability. This animal model could be useful to study the behavioural and neurochemical consequences related to the emotional states generated during the omission of a highly expected reward.     

A comparison of the effects of discriminative and Pavlovian inhibitors and excitors on instrumental responding

In two experiments, the effects of Pavlovian or discriminative conditioned inhibitors on operant responding were investigated in rats. Experiment 1 found that a Pavlovian conditioned inhibitor for food suppressed food-reinforced lever pressing more than a non-differentially trained control stimulus did. Experiment 2 demonstrated that an operant discriminative inhibitor produced greater suppression of lever pressing than a Pavlovian conditioned inhibitor. Experiment 2 also found that compounding an operant discriminative stimulus (SD) for food-reinforced responding with another SD for food-reinforced responding resulted in more additive summation than when an SD was compounded with a Pavlovian conditioned excitor for food. The results of these experiments support two-factor theories that postulate that incentive and response discriminative processes summate algebraically when the processes are inhibitory or excitatory.     

Progressive-ratio responding for palatable high-fat and high-sugar food in mice

Foods that are rich in fat and sugar significantly contribute to over-eating and escalating rates of obesity. The consumption of palatable foods can produce a rewarding effect that strengthens action-outcome associations and reinforces future behavior directed at obtaining these foods. Increasing evidence that the rewarding effects of energy-dense foods play a profound role in overeating and the development of obesity has heightened interest in studying the genes, molecules and neural circuitry that modulate food reward. The rewarding impact of different stimuli can be studied by measuring the willingness to work to obtain them, such as in operant conditioning tasks. Operant models of food reward measure acquired and voluntary behavioral responses that are directed at obtaining food. A commonly used measure of reward strength is an operant procedure known as the progressive ratio (PR) schedule of reinforcement. In the PR task, the subject is required to make an increasing number of operant responses for each successive reward. The pioneering study of Hodos (1961) demonstrated that the number of responses made to obtain the last reward, termed the breakpoint, serves as an index of reward strength. While operant procedures that measure changes in response rate alone cannot separate changes in reward strength from alterations in performance capacity, the breakpoint derived from the PR schedule is a well-validated measure of the rewarding effects of food. The PR task has been used extensively to assess the rewarding impact of drugs of abuse and food in rats (e.g., 6-8), but to a lesser extent in mice. The increased use of genetically engineered mice and diet-induced obese mouse models has heightened demands for behavioral measures of food reward in mice. In the present article we detail the materials and procedures used to train mice to respond (lever-press) for a high-fat and high-sugar food pellets on a PR schedule of reinforcement. We show that breakpoint response thresholds increase following acute food deprivation and decrease with peripheral administration of the anorectic hormone leptin and thereby validate the use of this food-operant paradigm in mice.     

Retention of memory traces in rats subjected to 10-minute clinical death procedure

Retention of memory traces was tested on the 12th day after 10-min clinical death (CD) produced by the total cessation of blood circulation in Albino rats previously trained for operant food conditioning in a multiple alternative maze. It was found that the structure of memory trace (sequence of operant actions in the maze), motivation activity and habit organization after the resuscitation were completely preserved in all rats independently of the number of the memory traces formed. Short-term negative changes were revealed only in the character of habit realization: the efficiency and reproduction parameters such as "readiness", mobilization activity, and stability of habit realization had lower values during 2-3 sessions. The rate of their recovery after the CD depended on rat's initial motivation state: the higher was the initial motivation, the faster recovered all the reproduction parameters. It is suggested that the observed negative changes in habit realization were nonspecific and were provoked by the CD-induced hyperexcitability of rats.     

Comparison of anorectic drugs in rats trained to discriminate between satiation and deprivation

Eight male rats were trained to discriminate between the internal states produced by food deprivation of 3 hours (satiation) and that produced by food deprivation of 27 hours duration (deprivation). One lever, in a two-lever operant chamber, had to be pressed to receive reinforcement in the satiation state, whereas pressing the other lever was required when the rat was in the deprivation state. Once the rats were trained, increasing the number of hours of food deprivation, from 1 to 48 hours, resulted in more deprivation-appropriate lever responses in the two-lever operant task. Administration of doses of fenfluramine (0.5-1.5 mg.kg), its active metabolite norfenfluramine (0.25-1.0 mg/kg) or d-amphetamine (0.5-1.5 mg/kg) produced a dose-responsive decrease in deprivation-appropriate responses when each drug/dose was injected (i.p.) 15 min prior to deprivation (27 hours) testing. Norfenfluramine was 1.5 times more potent than fenfluramine which was 1.5 times more potent than amphetamine.     

Ghrelin enhances cue-induced bar pressing for high fat food

Ghrelin is an orexigenic hormone produced by the stomach that acts on growth hormone secretagogue receptors (GHSRs) both peripherally and centrally. The presence of GHSRs in the ventral tegmental area (VTA) suggests that ghrelin signaling at this level may increase the incentive value of palatable foods as well as other natural and artificial rewards. The present investigation sought to determine if ghrelin plays a role in relapse to such foods following a period of abstinence. To achieve this, thirty-six male Long Evans rats were trained to press a lever to obtain a high fat chocolate food reward on a fixed ratio schedule of 1. Following an extinction period during which lever presses were not reinforced, rats were implanted with a cannula connected to a minipump that continuously delivered ghrelin, a GHSR antagonist ([d-Lys-3]-GHRP-6), or saline in the VTA for 14 days. One week later, food reward-associated cues, food reward priming, and an overnight fast were used to induce reinstatement of the lever pressing response. Our results indicate that intra-VTA ghrelin enhances cue-induced reinstatement of responses for palatable food pellets. To the extent that the reinstatement paradigm is considered a valid model of relapse in humans, this suggests that ghrelin signaling facilitates relapse to preferred foods in response to food cues through GHSR signaling in the VTA.     

One-trial instrumental conditioning in rats

A new method for a single-trial operant conditioning task with food reward in rats is described. The non-contingent reward of the first random lever press resulted in a higher response rate in both 24 and 48-hour retention tests as compared to contingently rewarded individuals. The latency to press the lever was shorter in non-contingently rewarded subjects than in contingently rewarded rats. The response rate in the first 5-min interval of the 24-hour retention test was higher in non-contingently rewarded subjects. The neuroleptic haloperidol (500 micrograms/kg., i.p.) given immediately following the "event to be remembered" significantly depresses the response rate compared to that for saline-treated controls. The advantages of the new method are discussed with respect to its applicability for memory research in animals and especially in neurophysiology and psychopharmacology.     

Continuous infusion of cholecystokinin and meal pattern in the rat

Purified natural cholecystokinin (CCK-33) was infused continuously for two days at a rate of 5.9 μg/hr in two rats trained to bar-press for food (Noyes pellet 45 mg) on a fixed ratio of five bar presses to obtain one pellet. The animals also received control surgery and were tested in the operant chamber for two days, one prior to and the other following the CCK-33 treatment. CCK-33 suppressed the number of meals, the total amount of food eaten, and the total duration of time spent eating. However, the size of each meal and the rate of intake were not affected. The CCK effect did not interact with the light-dark phases of diurnal cycle. It appears that a major effect of continuous systemic elevation of CCK-33 is to reduce food intake by prolonging the satiety period rather than by decreasing the individual meal size.