Disrupted patterns of consummatory behavior in rats with fornix transections

The feeding and drinking behavior was examined in male rats with fornix transections and sham-operated control rats. Total food and water consumption was recorded but supplemented by a pattern analysis of feeding and drinking behavior. The behavior of the rats was continuously monitored during four hour morning and afternoon sessions under ad lib access and during a two hour session following adaptation to a restricted access feeding schedule. Rats with fornix transections were more active and exhibited increased frequencies of rearing, eating and drinking. The increased meal frequency in rats with fornix transections was accompanied by decreased meal durations and a reduction in the length of intermeal intervals. Total food and water consumption was unaffected by fornix transection as were the duration of sleep bouts and the frequencies of grooming, sleeping and carrying shavings. Fornix transections also reduced food carrying and food hoarding but only under conditions of restricted food access. The results suggest that fornix transection does not alter major homeostatic regulatory mechanisms nor does it alter the components of feeding and drinking behavior. Fornix transection alters, instead, the organization of microregulatory feeding and drinking patterns.     

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.     

Meal patterns of cats encountering variable food procurement cost.

The meal patterns of 2 cats in a laboratory habitat with variable foraging costs were examined in a foraging paradigm in which subjects could initiate meals at any time by completing a predetermined number of bar presses (the procurement price) and then could eat any amount. From meal to meal, the procurement price either was fixed or varied among a geometric series of five prices. As the fixed price or the mean of the variable prices increased, meal frequency decreased and meal size increased; daily intake was unaffected. Within variable-price schedules, meal size was not related to the just-paid procurement price. These results suggest that cats respond to the global rather than to the local cost structure of their habitat. They appear to respond to an average of the prices encountered, initiating meals of a frequency and size appropriate to that average. This was true even when the average price was high, meals were infrequent, and thus price encounters were widely separated in time. Therefore, the time window over which the consequences of behavior can affect behavior is longer than often conceived, at least in economies in which the animal controls its intake and the frequency, size, and distribution of its meals.     

Procurement time as a determinant of meal frequency and meal duration.

Foraging involves the expenditure of both time and effort in the acquisition of food; animals typically modify their meal patterns so as to reduce these expenditures or costs. The contribution of time, as compared with effort, to the overall cost perceived by an animal is not known. We investigated the effect of foraging time as a cost independent of effort by measuring the meal patterns of rats living in a laboratory foraging simulation in which they earned all their daily intake. They pressed a bar once to initiate an interval (procurement interval) leading to the presentation of a large cup of food from which they could eat a meal of any size. As the length of the interval increased from 1 s to 46 hr, meal frequency decreased regularly. Meal size increased in a compensatory fashion, and total daily intake was conserved through an interval of 23 hr. The changes in meal frequency occurred because of changes in the rat's latency to bar press after each meal. The functions relating meal frequency and size to the procurement interval were of the same shape as those seen when cost is the completion of a bar-press requirement, which entails the expenditure of both effort and time. When the bar-press requirement was increased to 10, meal frequency was reduced, but time and effort did not appear to simply add together in the rat's perception of cost. These data reveal that time is preceived to be a cost by rats foraging in this laboratory environment. These results suggest that the time parameters of foraging are different from those of consumption.     

Changes in feeding and foraging patterns as an antipredator defensive strategy: a laboratory simulation using aversive stimulation in a closed economy.

The effects of the risk of electric shock on the meal patterns of rats living in an operant chamber were investigated. Rats could obtain food by working on a response lever that provided reinforcement according to chained fixed-ratio continuous reinforcement schedules that allowed the animals control over meal size. Using a two-compartment operant chamber with a safe nesting area and manipulanda area with a grid floor, shock could be correlated with responding on the schedule. Shocks (less than or equal to 1.25 per hour) were scheduled to occur randomly throughout the day, independent of the rat's behavior. Shock caused a reorganization of meal patterns such that the animals took less frequent but larger meals. This pattern reduced the time the animals spent at risk without compromising caloric balance. Similar changes in feeding pattern were obtained in both hooded and albino rats. Exposure to shock in a separate chamber did not produce these behavioral modifications. The magnitude of shock-induced alterations of meal patterns was greater with chained fixed-ratio 90 continuous reinforcement than with chained fixed-ratio 10 continuous reinforcement. Additionally, the rats seemed to be able to reduce food intake but increase caloric efficiency, such that the reduced food intake did not have deleterious effects on maintenance of body weight. These behavioral modifications reduced the number of shocks received from that which would have been expected if meal pattern changes had not occurred. We suggest that this technique may provide a useful laboratory simulation of the impact that the risk of predation has on foraging behavior.     

Patch choice as a function of procurement cost and encounter rate.

The effects of patch encounter rate on patch choice and meal patterns were studied in rats foraging in a laboratory environment offering two patch types that were encountered sequentially and randomly. The cost of procuring access to one patch was greater than the other. Patches were either encountered equally often or the high-cost patch was encountered more frequently. As expected, rats exploited the low-cost patch on almost 100% of encounters and exploited the high-cost patch on a percentage of encounters that was inversely proportional to its cost. Meal size was the same at both patches. Surprisingly, when low-cost patches were rare, the rats did not increase their use of high-cost patches. This resulted in spending more time and energy searching for patches and a higher average cost per meal. The rats responded to this increased cost by reducing the frequency and increasing the size of meals at both patches and thereby limited total daily foraging cost and conserved total intake.     

Response variation in instrumental extinction in rats with fornix transections

The present experiment examined spatial and response variation interpretations of the large difference in magnitude between operant and instrumental extinction deficits seen in hipocampally damaged animals. Latency measures and detailed behaviors of rats with fornix transections were compared to those of control rats during acquisition and extinction in an enriched spatial runway. Control rats, in comparison to rats with fornix transections, exhibited larger increases in start box, runway, and goal box latencies, but no spatial gradients were found. Control rats also exhibited less goal persistence and more response variation following the transition to extinction. The results suggest no fundamental difference between operant and instrumental deficits following hippocampal damage, but an interaction between strategies employed and traditional response measures.     

Brain transections selectively alter ingestion and behavioral activation in neonatal rats

Food-deprived neonatal rats actively ingest milk that is infused into their mouths through intraoral cannulas. This ingestion is accompanied by behavioral activation. The involvement of various brain regions in ingestion and activation was examined by making transections along the neuraxis from the olfactory bulbs to the anterior pons in 2-day-old rats. Following a 24-hr deprivation period, a series of oral milk infusions was given, and milk intake and activity were measured. Intake was severely reduced only in animals with diencephalic transections. Cuts in front of or behind the diencephalon resulted in normal or slightly decreased intake. In contrast, activity tended to decline as the level of the transection became more caudal. Thus ingestion and its accompanying behavioral activation could be separated neuroanatomically. These results suggest that two brain mechanisms are involved in the ingestive response of the infant rat, one in the diencephalon and another caudal to the mesencephalon. However, behavioral activation appears less discretely organized, involving most of the neuraxis.     

Role of fatty acid oxidation in control of meal pattern

To characterize the role of fatty acid oxidation in the control of food intake, we investigated the effect of 2-mercaptoacetate, which inhibits fatty acid oxidation, on meal patterns and cumulative food intake in rats. Rats were fed either a medium fat (MF, 18% fat) or a low fat (LF, 3.3% fat) diet. Mercaptoacetate (400 mumole/kg body wt), intraperitoneally injected in the middle of the bright or at the onset of the dark phase of the diurnal lighting cycle, increased cumulative food intake in MF rats by shortening the latency to eat after injection and the duration of the subsequent intermeal interval (IMI) without affecting the size of the first meal. Mercaptoacetate, injected in the middle of the bright phase, reduced the latency to eat but did not affect the duration of the subsequent IMI or cumulative food intake in LF rats. A higher dose of mercaptoacetate (600 mumole/kg body wt), initially increased and later decreased cumulative food intake in MF rats. The initial increase in food intake was due to shorter IMIs; the subsequent decrease in food intake was due to smaller meals after mercaptoacetate injection than after control injection. The results indicate that a drop in fatty acid oxidation caused by mercaptoacetate triggers a meal. This implicates fatty acid oxidation in the maintenance of postprandial satiety.     

Effects of fornix transection on spontaneous and trained non-matching by monkeys

Three monkeys with fornix transection and three normal control monkeys performed a series of tasks which were variations of delayed non-matching. Experiment 1 showed that even at short retention intervals fornix transection impaired the spontaneous tendency to explore novel objects. Experiment 2 provided differential reward for non-matching and showed that the fornix-transected monkeys learned and performed non-matching normally even though the sample-match retention intervals were long throughout the experiment. Experiment 3 showed that non-matching performance was transiently more disrupted in fornix-transected than in normal monkeys when the testing procedure was changed, in a variety of ways, while maintaining the basic non-match rule. Experiment 4 required the monkeys to discriminate objects they had displaced from objects they had seen but not displaced; fornix transection produced in this task a substantial and stable impairment. These four experiments require a revised interpretation of the effects of fornix transection upon recognition memory and exploration. Particularly they contradict the hypothesis, suggested by previous experiments, that fornix transection produces a defect in discrimination of stimulus familiarity in long-term but not in short-term memory. They suggest rather that fornix transection impairs memory of instrumental responses.     

Mathematical analysis of energy regulatory pattens of normal and diabetic rats.

Analysis of feeding patterns indicated that diabetic (alloxan) hyperphagia is characterized by doubling of meal sizes with no change in feeding frequency. Correlation of meal sizes and intermeal intervals did not provide any systematic relationships for either normal or diabetic rats. When equations of the general form Y = A + Bcos (X) were fit to successive satiety ratios (postmeal interval/meal size), the diabetic rats showed significantly lower A coefficients, reflecting a lower average level of satiety, as well as significantly lower B coefficients, reflecting less systematic variability in the satiating value of food around the average level. It is concluded that the major regulatory deficit in diabetic animals is a chronic reduction in the long-term signal of body nutrient repletion.     

The relationship between feeding rate and patch choice.

Rats in a laboratory foraging simulation searched for sequential opportunities to feed in two patches that differed in the rate at which food pellets were delivered (controlled by fixed-interval schedules) and in the size of the pellets. The profitability of feeding in each patch was calculated in terms of time (grams per minute) and in terms of effort (grams per bar press). These values were the result of the imposed fixed interval, the size of the pellets, and the rate at which the rats pressed the bar in each condition. The rats ate more food and larger meals, but not more frequent meals, at the patch offering the higher rate of food consumption, calculated as grams per minute. The relative intake at any patch was a function of the relative rate of intake during meals at that patch compared to the other patch. Rats respond to explicit manipulations of feeding time in the same manner as they respond to manipulations of feeding effort.     

Learning by honeybees (Apis mellifera) on arrival at and departure from a feeding place

The question of when in the course of a visit to a feeding place foraging honeybees (Apis mellifera) learn about its location was studied in Experiment 1 by moving the animals a short distance after they arrived and began to feed. A preference for the arrival place developed, although less rapidly than in control animals for which the arrival and departure places were the same. In Experiments 2-5, a distinctive object was used to define the location of the feeding place. When the object was removed after arrival or introduced only after arrival, the animals learned less about its color and shape than did control animals for which it was present throughout each visit. The results contradict the claim that honeybees learn about certain characteristics of a feeding place only on arrival and about others only on departure.     

Place and response learning of rats in a Morris water maze: differential effects of fimbria fornix and medial prefrontal cortex lesions

The question examined in this study is concerned with a possible functional dissociation between the hippocampal formation and the prefrontal cortex in spatial navigation. Wistar rats with hippocampal damage (inflicted by a bilateral lesion of the fimbria fornix), rats with damage to the medial prefrontal cortex, and control-operated rats were examined for their performance in either one of two different spatial tasks in a Morris water maze, a place learning task (requiring a locale system), or a response learning task (requiring a taxon system). Performance of the classical place learning (allocentric) task was found to be impaired in rats with lesions of the fimbria fornix, but not in rats with damage of the medial prefrontal cortex, while the opposite effect was found in the response learning (egocentric) task. These findings are indicative of a double functional dissociation of these two brain regions with respect to the two different forms of spatial navigation. When the place learning task was modified by relocating the platform, the impairment in animals with fimbria fornix lesions was even more pronounced than before, while the performance of animals with medial prefrontal cortex lesions was similar to that of their controls. When the task was again modified by changing the hidden platform for a clearly visible one (visual cue task), the animals with fimbria fornix lesions had, at least initially, shorter latencies than their controls. By contrast, in the animals with medial prefrontal cortex damage this change led to a slight increase in escape latency.     

Within-session meal-size effects on induced drinking.

As a control for the effects of session duration and hunger on the relation between food magnitude and induced drinking, four food-deprived rats were exposed to a variable-time 50-s schedule of food delivery in which the size of each food delivery varied randomly within sessions. Food-related behavior and schedule-induced drinking per opportunity were examined as functions of meal size and postfood time. All rats showed an inverted-U-shaped relation between drinking per opportunity and meal size. This relation was caused by variation in the percentage of intervals that contained drinking and by variation in the number of drinking bouts per interval, rather than by bout duration or by the amount of drinking within those intervals that actually contained drinking. Head-in-feeder time increased linearly with meal size. Schedule-induced drinking was entrained by food delivery in 3 of 4 subjects; the entrainment was due to regulation of the starting time of each drinking bout rather than to regulation of bout duration.     

The timing of meals

In most individuals, food intake occurs as discrete bouts or meals, and little attention has been paid to the factors that normally determine when meals will occur when food is freely available. On the basis of experiments using rats, the authors suggest that when there are no constraints on obtaining food and few competing activities, 3 levels of interacting controls normally dictate when meals will start. The first is the genetically determined circadian activity pattern on which nocturnal animals tend to initiate most meals in the dark. The second is the regularly occurring changing of the light cycle: These changes provide temporal anchors. The third relates to the size of the preceding meal, such that larger meals cause a longer delay until the onset of the next meal. Superimposed on these 3 are factors related to learning, convenience, and opportunity.     

Performance on a fixed-ratio schedule with correlated amount of reward

Four rats were trained to bar press on FR 9 TO 30 sec. They were reinforced with a large or small amount of water according to whether their final IRT was long or short respectively. Four control rats always received the small amount of reinforcement. The control animals produced the high rates of responding typical of fixed-ratio performance. The experimental animals, with one exception, developed superstitious behavior and maintained slow responding throughout the ratio. However, some features of the results pointed to a persistent influence of the factors which favor short IRTs.     

The psychobiology of meals

Meals are considered as bouts of behavior that, although necessary for supplying nutrients to the body, result in undesirable perturbations of homeostatically controlled parameters. If the environment dictates that an animal mainly eat very large meals, these meal-associated perturbations become potentially dangerous. When the opportunity to eat a very large meal is regular and predictable, animals adopt strategies that maximize the efficiency of the process while minimizing the threatening homeostatic disturbances. Hence, prior to the onset of meals, animals elevate their body temperatures, presumably to facilitate critical processes involved in ingestion and/or digestion. Temperature continues to rise during the meal, and as it approaches potentially dangerous levels, the meal is terminated and temperature falls to “safer” levels. Animals also undergo a slow decline of blood glucose prior to the initiation of meals, thus minimizing the postprandial elevation of blood glucose caused by the absorption of ingested carbohydrates. Analogously, prior to meals, animals undergo a decrease of metabolic rate, thus precluding the necessity for postprandial increases of metabolic rate to reach even higher absolute levels. These premeal changes of regulated parameters have been interpreted by others as indicating depletion of one or more energy supplies so that the animal is compelled to eat. Contrary to this, we interpret the changes as ones that enable the animal to prepare adequately to consume a large meal when the environment is predictable.     

Response of the rat brain beta-endorphin system to novelty: importance of the fornix connection

In control rats, a step-down inhibitory avoidance training trial using a 0.8 mA footshock, or simple exposure to the training apparatus without footshock, was followed by a decrease of beta-endorphin-like immunoreactivity measured in the hypothalamus and ventral thalamus. The effect of inhibitory avoidance training was also measured in rats submitted to a brain sham operation, to bilateral transection of the dorsal fornix, to anterior or to posterior hypothalamic deafferentation, to adrenal medullectomy, to an adrenal sham operation, to 16 daily ip injections of 0.2 mg/kg dexamethasone, or to 16 daily ip injections of 1 ml/kg saline. The diencephalic beta-endorphin-like immunoreactivity response to training was abolished by fornix transection and was unaffected by all other treatments. This suggests that the response is not mediated by anterior or posterior neural afferents to the hypothalamus, or by a hypersecretion of epinephrine by the adrenal medullae, or of ACTH by the pituitary gland. The response, instead, appears to require the integrity of the pathway that sends projections from the septo-hippocampal system to the hypothalamus. Previous evidence had suggested that the diencephalic beta-endorphin-like immunoreactivity response to training is a result of novelty, and the septo-hippocampal system has been postulated to play a role in the registration of novelty.     

Transection of direct anterior thalamic afferents from the hippocampus: effects on activity and active avoidance in rats

This investigation demonstrates the importance of the direct hippocampo-anterior thalamic component of the postcommissural fornix in the control of general locomotion and active avoidance. Transection of anterior thalamic afferents from the hippocampal formation (subicular cortex), at the point where they exit from the fornix posterior to the septum, is sufficient to enhance bidirectional active avoidance acquisition and increase general activity. This transection may also interrupt fibers to or from other thalamic nuclei and the anterior septum. However, destruction of connections of the anterior septum with the hippocampus, habenula, and thalamus by transection in the coronal plane anterior to the descending fornix columns, without damage to the subiculothalamic fibers, increases general activity levels without damage to the subiculothalamic fibers, increases general activity levels without affecting active avoidance behavior. The activity increase in this case resembles that seen after septal lesions rather than that seen after hippocampal lesions. Thus, destruction of a single fornix component contributing afferents to the anterior thalamic nuclei reproduces at least part of the hippocampal syndrome. This suggests that these fibers contribute significantly to the control of these behaviors and may mediate active avoidance changes resulting from hippocampal and fornix damage.