Fetal protein malnutrition impairs acquisition of a DRL task in adult rats

Developing rats were either malnourished or adequately nourished during the prenatal period by feeding their dams diets of 6% (low) or 25% (adequate) casein content 5 weeks prior to mating and throughout pregnancy. All pups received adequate nutrition from the day of birth onwards. Beginning at 160 days of age male rats were tested in a DRL-18 sec operant task. It was found that prenatal malnutrition impaired performance during acquisition, though asymptomatic levels were not significantly different. Subsequent limited hold DRL-18 sec schedules in which late as well as early responses went unrewarded indicated that the timing ability of the prenatally malnourished rats was excellent and similar to that of the controls. These late effects of fetal protein malnutrition are discussed in terms of the difficulty in adapting to the change from CRF to DRL-18 sec (possibilities include a reduced ability to develop a timing strategy or increased sensitivity to the change in reinforcement contingencies), while retaining the ability to time responses accurately once the task was acquired.     

Pontine and thalamic influences on fluid rewards: I. Operant responding for sucrose and corn oil

The reward strength of orosensory sucrose and corn oil was measured using fixed and progressive ratio operant schedules. Because the orosensory effects of the stimuli were of interest, Experiment 1 compared operant responses for sucrose in sham and real feeding rats. The results demonstrated that rats would work for sucrose solutions without the accompanying postingestive effects. Furthermore, the break points for high concentrations of sucrose (1.0 M or 2.0 M) were significantly higher in sham feeding rats than in real feeding controls. Experiment 2 investigated the role of the parabrachial nucleus (PBN) and of the thalamic orosensory area (TOA) in sucrose and corn oil reward. During free access, rats with PBN lesions (PBNx) licked significantly less sucrose solution than their controls, but both groups ingested a similar volume of corn oil emulsion. When an operant was imposed, these same PBNx rats failed to respond for sucrose and continued only modestly for corn oil. In contrast, the TOA lesioned rats (TOAx) showed no impairment in responding for sucrose or corn oil during either the free access or operant sessions. Furthermore, rats with TOA lesions demonstrated significantly higher break points for sucrose than did their controls. Together, the data imply that the PBN but not the TOA is critical for the perception of, or responding to the reward value of sucrose and corn oil.     

Prenatal malnutrition and sleep states in adult rats: effects of restraint stress

Independently, prenatal malnutrition and psychological/physical stress have been shown to affect sleep architecture in adult rats. As malnutrition and stress commonly co-exist in malnourished human populations, the objective of the present study was to ascertain the combined effects of these two insults by examining sleep-wake parameters following a brief restraint stress in prenatally protein malnourished rats. The male offspring of rats provided with a protein deficient diet (6% casein) for 5 weeks prior to mating and throughout pregnancy were implanted with recording electrodes beginning at postnatal day 90. Polygraph recordings were obtained to quantify sleep states during the first 4 h of the dark phase of the cycle on 2 consecutive days. The first followed a 24-h habituation session to the recording chamber (baseline). The second occurred at the same time of day but followed 20 min of restraint stress in a Plexiglas tube. During baseline, prenatally malnourished rats spent more time in rapid eye movement sleep (REMS) in the first 2 h after "lights off" (block 1), and greater amounts of wakefulness (W) with a corresponding reduction in slow wave sleep (SWS) in the second two hours (block 2), as compared with controls. Following stress, the sleep architecture of both groups of rats remained unaltered in block 1 relative to their baseline day. In block 2, both groups exhibited significant reductions in SWS and REMS with significantly greater reductions being expressed in the prenatally malnourished group (most dramatically, REMS was completely eliminated). These findings suggest that sleep disturbances may be more severe in those malnourished human populations subjected to acutely stressful experiences.     

Prenatal protein malnourished rats show changes in sleep/wake behavior as adults

Prenatal protein malnutrition significantly elevates brain levels of serotonin in rats, and these levels remain elevated throughout their lives. This biogenic amine is involved in the regulation of many physiological functions, including the normal sleep/wake cycle. The present study examined the effects of prenatal protein malnutrition on the sleep/wake cycle of freely moving adult rats. Six prenatally protein malnourished (6% casein) and 10 well-nourished (25% casein) male rats (90-120-day-old) were chronically implanted with a standard set of electrodes (to record cortical electroencephalogram, neck muscle electromyogram, electrooculogram, and hippocampal theta wave) to objectively measure states of sleep and wakefulness. Six-hour polygraphic recordings were made between 10.00 and 16.00 h; a time when the rats normally sleep. Prenatally malnourished rats spent 20% more time in slow wave sleep (SWS) compared to the well-nourished rats. The total percentage of time spent in rapid eye movement (REM) sleep was 61% less in prenatally malnourished rats compared to well-nourished control rats. These findings demonstrate the adverse consequences of prenatal protein malnutrition on the quality and quantity of adult sleep in rats. These sleep changes are potentially detrimental to normal social behavior and cognitive functions. Prenatally malnourished rats are an excellent animal model to study the role of endogenous serotonin in the regulation of the normal sleep/wake cycle.     

Reinforcement value of sucrose measured by progressive ratio operant licking in the rat

Progressive ratio (PR) schedules, which require increasing numbers of responses for successive reinforcements, are widely used to measure the reward value of foods, fluids, and drugs in operant lever-pressing tasks. The present study evaluated a PR operant licking task as a measure of sweet taste reward. In Experiment 1, food deprived rats were offered sucrose to drink on PR lick or fixed ratio (FR) lick schedules (30 min/day). In Experiment 2, nondeprived rats were offered sucrose to drink on PR or FR schedules and free access to water and food 23 h/day. In both experiments, the FR rats increased and then decreased their sucrose solution intake as concentration increased from 1% to 32% or 64%. The PR rats, in contrast, showed a near-linear increase in sucrose solution intake, lick rates, and break points (highest ratio completed) as a function of sucrose concentration. The PR rats drank less sucrose than did the FR rats although they emitted more total licks at the highest concentration tested. These results are similar to those reported with PR lever-pressing tasks. Thus, PR operant licking, which requires minimal training and equipment, is a useful alternate measure of fluid reward in rodents.     

Prenatal morphine exposure differentially alters learning and memory in male and female rats

The present study tested the hypothesis that exposure to morphine on prenatal days 11-18 impairs performance on tasks requiring learning and memory in adult male and female rats. In Experiment 1, a symmetrical maze was used to measure learning. In Experiment 2, an eight-arm radial maze was used to assess working spatial memory. The results of Experiment 1 demonstrated that prenatal morphine exposure reduces the time needed to complete the trials, but does not affect the accuracy of performance in male rats. In contrast, prenatal drug treatment had no effects on either the time or the accuracy of performance in female rats. In Experiment 2, both male and female morphine-exposed rats needed more time to complete regular trials (no delay) than controls; however, morphine exposure in male rats did not affect performance on tasks requiring memory, measured with delay trials, but hindered it in ovariohysterectomized (OVX) female rats. In OVX females, replacement injections of both estrogen and progesterone restored the impairment of performance on delay trials produced by prenatal morphine exposure. Thus, the present study demonstrates that prenatal morphine exposure differentially alters performance of adult male and female rats on tasks requiring learning and spatial memory.     

Foster mother care but not prenatal morphine exposure enhances cocaine self-administration in young adult male and female rats

The present study was designed to investigate cocaine self-administration in adult male and female rats exposed prenatally to morphine. Pregnant dams were injected two times a day with either saline, analgesic doses of morphine or no drug at all (controls) on gestation Days 11-18. One day after birth, litters were cross-fostered such that control dams were paired with one another and their litters were crossed; saline- and morphine-treated dams were paired and half of each saline litter was crossed with half of each morphine litter. Thus, each mother (control, saline, and morphine) raised half of her own and half of the adopted litter. At the age of 60 days, males and females were trained first to lever press for sucrose pellets and then for cocaine. Once the lever-pressing behavior was learned and baseline level of this activity was established, animals received a cocaine (.5 mg/kg per infusion) reward for each correct response on the active lever during the next 9-day session. The data demonstrate that adult control, saline- and morphine-exposed male rats self-administer cocaine at a similar rate independent of their prenatal treatment. Adult female rats self-administer cocaine at a higher rate than male rats. Further, saline- and morphine-exposed females in diestrus self-administer more than females in proestrus phase of the estrous cycle, while control females show no such differences. In addition, fostering induces increase in cocaine self-administration in all groups of male rats regardless of prenatal drug exposure. In females, the only fostering-induced increase is in prenatally saline-exposed female rats raised by morphine-treated foster mother. Thus, our results suggest that the prenatal drug exposure does not induce changes in lever-pressing behavior for cocaine reward in adult male and female rats, but it sensitizes the animals to postnatal stimuli such as gonadal hormones and/or rearing conditions that result in increased drug self-administration.     

Plasma corticosterone levels during extinction of a lever-press response in hippocampectomized rats

Male Long-Evans rats with bilateral lesions of the hippocampus or of the cortex overlying the hippocampus and unoperated rats were trained to lever press for water reinforcement on a continuous reinforcement schedule, and then extinguished. The hippocampectomized animals responded at higher rates than controls during early acquisition sessions, the difference decreasing over training and increasing again during extinction. Plasma levels of corticosterone were determined in the resting (non-stressed) state on ad lib and deprivation watering schedules, and following ether anesthesia, exposure to a novel environment, a reinforced operant session and the first extinction operant session. The results indicated that hippocampal lesions produced by aspiration cause no essential deficit in pituitary-adrenal function. However, the hippocampectomized rats failed to exhibit the normal elevation of plasma corticosterone during the first extinction session. This finding contradicts proposals that hippocampectomy increases the frustrative-emotional response to unmet reward expectancies. It is supportive of attentional process theories of hippocampal function.     

Rapid acquisition of operant conditioning in 5-day-old rat pups: a new technique articulating suckling-related motor activity and milk reinforcement

Newborn rats are capable of obtaining milk by attaching to a surrogate nipple. During this procedure pups show a gradual increase in head and forelimb movements oriented towards the artificial device that are similar to those observed during nipple attachment. In the present study the probability of execution of these behaviors was analyzed as a function of their contingency with intraoral milk infusion using brief training procedures (15 min). Five-day-old pups were positioned in a smooth surface having access to a touch-sensitive sensor. Physical contact with the sensor activated an infusion pump which served to deliver intraoral milk reinforcement (Paired group). Yoked controls received the reinforcer when Paired neonates touched the sensor. Paired pups trained under a continuous reinforcement schedule emitted significantly more responses than Yoked controls following two (Experiment 1) or one training session (Experiment 2). These differences were also observed during an extinction session conducted immediately after training. The level of maternal deprivation before training (3 or 6 hr) or the volume of milk delivered (1.0 or 1.5 microl per pulse) did not affect acquisition or extinction performances. In addition, it was observed that the rate of responding of Paired pups during the early phase of the extinction session significantly predicted subsequent levels of acceptance of the reinforcer. These results indicate that the frequency of suckling-related behaviors can be rapidly modified by means of associative operant processes. The operant procedure here described represents an alternative tool for the ontogenetic analysis of self-administration or behavior processes of seeking.     

Social and sexual motivation in the mouse

Does a sexual encounter have reward value for a learned operant response? Ovariectomized female mice with or without estradiol replacement were trained to perform a bar-contact operant response for either male or female targets. Response rates of females with estradiol replacement did not differ from those of females without estradiol replacement or females responding for access to females. Reflexive receptive sexual behavior remained responsive to estradiol replacement. Experiment 2 demonstrated that socially isolated females would respond faster for access to a female target than when group housed. Finally, the oxytocin blocker, atosiban, reduced both operant and reflexive social behavior. These results converge on the conclusion that the operant reward value of social and sexual contact is primarily social.     

Operant and Pavlovian control of visual stimulus orienting and food-related behaviors in rats with lesions of the amygdala central nucleus

The effects of superimposing operant reward and omission contingencies on 2 Pavlovian conditioned responses evoked by a visual conditioned stimulus paired with food were examined in rats with lesions of the amygdala central nucleus (CN). In sham-lesioned rats, the frequency of an orienting response, rearing, was increased by reward contingencies and decreased by omission contingencies, compared with yoked Pavlovian controls. In contrast, in CN-lesioned rats, rearing was not affected by either operant contingency and occurred at lower levels with Pavlovian procedures alone than in sham-lesioned rats. Nevertheless, CN-lesioned and sham-lesioned rats showed similar increases in the frequency of conditioned food-cup behavior with reward contingencies, similar decreases with omission contingencies, and similar levels of that response with Pavlovian procedures.     

Lesions of the pedunculopontine tegmental nucleus increase sucrose consumption but do not affect discrimination or contrast effects.

Pedunculopontine tegmental nucleus (PPTg) lesions block place preferences to drugs or food only when animals are nondeprived. PPTg lesions also disrupt operant responding, but lesioned rats cannot discriminate active from inactive levers. It is not clear, therefore, whether PPTg lesions block reward or disrupt the ability to differentiate changes in reward magnitude. These hypotheses were tested by measuring sucrose consumption, choice, and contrast effects after PPTg lesions. Both sham and lesioned rats consumed greater amounts of a sucrose solution as the concentration and level of deprivation were increased. Given a choice between 2 solutions, all rats consumed more of the higher concentration. Both groups exhibited contrast effects when the concentration was shifted from 32% to 4% within a session. Somewhat surprisingly, lesions increased sucrose intake when rats were food-restricted. These results suggest that PPTg lesions do not disrupt primary motivation or the ability to evaluate and respond to changes in reward strength.     

Learning and memory in the FMR1 knockout mouse.

The fragile X [fra(X)] syndrome is manifested phenotypically as a developmental disability comprised mainly of moderate-to-severe mental retardation (MR). Deficits are especially evident in auditory and visual short-term memory. Recently, an FMR1 knockout mouse developed by the Dutch-Belgian Fragile X Consortium demonstrated significantly lower visual-spatial abilities than littermate controls. We wondered if these results were associated with learning per se or to performance deficits only. Thus, we examined learning and memory in male FMR1 knockout mice crossbred from Fvb and E129 strains, and in male Fvb control mice, using operant conditioning techniques. In Experiment 1, we demonstrated that two aged male FMR1 knockouts could acquire the necessary bar-press response to discriminate visual (L+) and auditory (N+) stimuli. In Experiment 2, we showed that three naive male knockouts and two naive male controls, all 12 weeks old, also learned to discriminate L+ and N+. A third component, a complex discrimination task, during which light and noise were presented concurrently without reinforcement (LN-) was added to each session. All knockouts acquired both L+ and N+ discriminative responses in fewer sessions and with higher discrimination ratios than either control. Moreover, all knockouts exhibited the typical response pattern associated with complex discrimination (LN-) tasks. However, neither control made the complex discrimination. Our findings were unexpected and raise issues concerning FMR1 mouse strains and their cognitive-behavioral testing.     

Operant responding in rats with dietary obesity: A systematic replication

Two groups of female rats were first trained to bar press for food reward available on fixed ratio (FR) schedules ranging from 1–64. Then one group was fed a variety of palatable supermarket foods for 45 days while the controls were fed laboratory chow. Six of the 8 rats fed supermarket foods became moderately obese, gaining 3 times as much weight as controls. In tests conducted after the differential feeding treatment these moderately obese rats earned fewer reinforcements than controls on each of the FR schedules (1–64). Thus pretraining, which attenuates the deficit in operant performance for food displayed by dynamic stage rats with VMH lesions did not eliminate the deficit in food-motivated performance by rats that became obese by eating palatable foods.     

Effects of ventromedial hypothalamic lesions on restricted feeding behavior in rats

Previous research has shown that VMH lesioned rats overeat when given free access to food yet undereat when daily access is restricted. In the present study VMH lesioned female rats in Experiment 1 and male rats in Experiment 2 consumed more food than controls during both free and restricted access feeding schedules. The factor that most likely accounts for the contrasting outcomes is the extent of hyperphagia displayed by the lesioned rats prior to restricted access feeding. The present data are consistent with the interpretation that VMH lesions increase hunger motivation.     

Schedule dependent and schedule induced behavior at reduced and recovered body weight.

Rats maintained at 80 percent ad lib feeding body weight became hyperdipsic during 60 min FI-1 min food reinforcement sessions in standard rat operant test chambers. Animals were then allowed to return gradually to their ad lib feeding weight and were tested again in the same experimental chambers for 18 daily and 9 weekly sessions. Data were obtained for test sessions of 10, 30 and 60 min. When body weight recovered, schedule dependent lever pressing and eating as well as schedule induced licking and drinking occurred at reduced frequencies. The behavior endured throughout the test session and over periods as long as 80 days. These persistent and robust stereotyped behaviors are evoked by specific environmental stimuli during a resultant increased responsiveness.     

A new animal model of binge eating: key synergistic role of past caloric restriction and stress

Dieting and stress are important in the etiology and maintenance of eating disorders, and dieting strongly predicts stress-induced overeating in humans. We hypothesized that caloric restriction and stress interact in a unique manner to promote binge eating. To test this hypothesis, a group of young female rats were cycled through a restriction period (4 days of 66% of control food intake) followed by 6 days of free feeding prior to being stressed by acute foot shock. After three of these cycles, the food intake of rats exposed only to restriction (R), or only to stress (S), did not differ from controls. However, R+S rats that were restricted and refed, despite normal body weight and food intake after free feeding, engaged in a powerful bout of hyperphagia when stressed (Experiment 1). The R + S effect was replicated in an older group of rats (Experiment 2). The hyperphagia was characteristically binge-like, it constituted a 40% selective increase in highly palatable (HP) food (P < .001) over a discrete period of time (within 24 h post-stress), and reflected feeding for reward (higher HP:chow ratio) over metabolic need as occurred after restriction (higher chow:HP ratio). Subsequent experiments revealed that binge eating did not occur if only chow was available (Experiment 3) or if restriction-refeeding (R-R) did not proximally precede stress (Experiment 4). Experiment 5 revealed that a history of R-R cycles followed by only one stress episode was sufficient to increase intake to 53% above controls as early as 2 h after stress (P < .001). This animal model of binge eating should facilitate investigations into the neurochemical changes induced by dieting and environmental stress to produce disordered eating and provide a preclinical tool to test preventive strategies and treatments more relevant to bulimia nervosa, multiple cases of binge eating disorder (BED) and binge-purge type anorexia nervosa.     

Influence of type of reinforcement on operant responding by rats with ventromedial lesions

Bar-pressing on FR schedules for sucrose rewards was studied in rats with ventromedial hypothalamic (VMH) lesions. When both VMH and control rats were maintained on ad lib feeding, their bar-pressing performance for sucrose did not differ, but if the VMH group was maintained at control body weight levels, they responded more frequently for sucrose than controls. In a subsequent experiment performance for Noyes pellets and 32% sucrose was directly compared in VMH and control animals maintained at 85% of their respective postoperative body weight levels. Under these conditions controls responded more frequently than lesioned rats for either type of reinforcement, but the magnitude of the difference was greater with the Noyes pellet reward.     

Sex differences in the effects of high-fat feeding on behavior and carcass composition

Male and female Sprague-Dawley rats were fed a high-fat diet (40% by weight) for 11 weeks beginning at 70–80 days of age. At the end of the 11th week, high-fat fed rats of both sexes were significantly heavier than chow-fed controls. All rats were then food deprived and were trained to bar-press in an operant chamber for Noyes pellets. Testing on fixed ratio (FR) schedules started when their body weights reached 85% of pre-deprivation levels and they pressed bar steadily. At the end of operant testing, all rats were refed their previous diet until body weights returned to pre-deprivation levels. The animals were then sacrificed. Fat pads from retroperitoneal, inguinal and gonadal regions were dissected out and cellularity determined. Carcass composition was analyzed by chemical methods. On the operant apparatus, the high-fat fed female rats (F-HF) behaved more like VMH lesioned obese rats, i.e. decreased bar pressing responses when compared with controls. No difference in operant responding was found between males fed high-fat diet and chow. Fat cell number and size were increased in retroperitoneal and inguinal fat pad for rats fed high-fat diet. In gonadal pads, only cell size was increased. Females on the high-fat diet had higher percentages of body fat than males on the high-fat diet. The behavioral difference in F-HF rats could be attributed to their higher adiposity. The results support previously reported findings on the behavior and adipose tissue cellularity of dietary obese rats.     

Early protein malnutrition in the rat: Behavioral changes during rehabilitation

Behavioral development was studied in 2 groups of rats during the postweaning period. Pups were selected from litters nursed by dams fed either a standard protein (25% casein by weight) or a low protein diet (12% casein) during lactation. Two pups from each litter were housed together and fed the control diet throughout rehabilitation Behavioral observations were made by repeated time-lapse photography at 5-day intervals. Differences in h ome cage behavior were observed at the onset of rehabilitation in the postnatally malnourished rats. Increased feeding behavior was observed during the 1st week of rehabilitation. Locomotor behavior was depressed during the rehabilitation period in the experimental animals. Climbing activity, however, was significantly greater. Self-grooming activity was elevated throughout rehabilitation in post-natally malnourished animals. During the course of rehabilitation, the behavioral differences between the 2 groups gradually disappeared.