A device for the continuous recording of solid food ingestion

Central nervous control of voluntary food intake

The central nervous system is the integrator of most of the actions of the animal and as such plays a vital rôle in the control of voluntary food intake. Much of the work to understand how intake is controlled has been carried out with rats but that which has been done with pigs is included. The first experiments used electrolytic lesions in the designation of the ‘hunger centre’ and the ‘satiety centre’. Recent work has identified the paraventricular nucleus as a sensing site for experimental manipulations. Chemical stimulation of the brain has also been carried out to try to gain understanding of the rôle of neurotransmitters. Noradrenaline (NA) stimulates intake when given into many sites. Serotonin (5-HT) inhibits intake and has been claimed to play a rôle in the selection of macronutrients but 5-HT must now be interpreted in the light of the existence of several different subtypes of 5-HT receptors. Dopamine appears to moderate the hedonic response of eating. Numerous peptides are active in the brain where their rôle as neuromodulators may be quite different from their function in the periphery and at least three types of opioid receptors are implicated with kappa antagonists producing the most potent facilitatory effects. Neuropeptide Y and peptide YY produce massive orexigenic effects which readily overcome peripheral satiety factors. The brain cannot control intake in isolation. It receives inputs in the blood stream, such as glucose, as well as via the nervous system, both from the special senses and from visceral organs such as stomach, intestines and liver. Taste and olfaction are important in diet selection and a specific appetite for protein has been demonstrated in the pig.

A revolving food pellet test for measuring sensorimotor performance in rats

A revolving food pellet (RFP) test is presented, measuring the ability of rats to eat food pellets suspended from a horizontal bar in their home cage. This easy to make and economical device evaluates the efficiency of bilateral front paw coordination while standing. During the beginning of a ten day testing period, rats exposed to the RFP chamber had a lower intake of food and decreased body weight than rats housed in a standard home cage. With continued practice, the experimental group increased their food intake and body weight. During this time, the rats learned to control the revolving pellets by stabilizing them with their front paws and chewing on them. This apparatus is suitable for assessing a form of sensorimotor learning, involving the efficiency of front paw reaching, grasping, and holding movements, together with appropriate postural adjustments and biting movements. This test was sensitive to brain lesioning, as rats with bilateral lesions of the cerebellar fastigial nucleus were impaired.

A computerized system for the continuous recording and analysis of feeding, drinking, diuresis, and locomotor activity

A system has been designed to study the intake of liquids and solid foods in rats that allows for the analysis of feeding and drinking episodes over time. In addition, it monitors locomotor activity and diuresis. This system is particularly useful for the study of the differential effects of drugs on various parameters of feeding and drinking. Because it allows for the monitoring of all behaviors for periods of 24 consecutive hours without disturbing the animals, it can be applied to the study of light/dark cycles of ingestive behavior. The apparatus consists of rat cages (equipped with photocells, bottles, electronic balances, and a funnel to collect urine), computer interfaces, an AT-compatible microcomputer for data collection, and a VAX system for analysis.

A contact eatometer for automated continuous recording of feeding behavior in rats

A new contact eatometer designed and built in our laboratory is described. The system makes possible the reliable continuous recording of feeding behavior in rats from the time of weaning, without significantly affecting food intake- and growth-related variables. Improved time discrimination and reliability of records--along with the small financial outlay necessary to manufacture the device--make the system appropriate for chronobiological studies which involve the simultaneous use of a large number of animals.

A system for automated recording and analysis of feeding behavior

We have designed and implemented a system that utilizes a network of top-loading balances digitally interfaced to a Macintosh computer. The system simultaneously collects two forms of data which allow the evaluation of the animal's biting and/or licking behavior in addition to cumulative food intake and meal patterns. The system is capable of resuming data acquisition following a power failure without user intervention. Plexiglas cages utilized with the system features adjustable tunnel feeders and are appropriate for use with small rodents. Given appropriate caging, the system may be utilized to evaluate the feeding behavior of other species.

Clonidine hyperphagia: neuroanatomic substrates and specific function

Recent studies have indicated that the alpha 2-noradrenergic agonist clonidine (CLON), when peripherally and centrally administered, potentiates feeding in satiated rats in a manner similar to that observed following injection of norepinephrine (NE) into the hypothalamic paraventricular nucleus (PVN). The present experiments examined the effects of CLON on meal patterns and macronutrient selection and compared these findings to earlier NE-stimulated feeding studies. Administration of CLON (25 nmoles), directly into the PVN (n = 5), similar to PVN injected NE, produced an increase in meal size (190%) and feeding duration (164%), with no change in meal frequency. Additional tests were conducted in rats with PVN electrolytic or 6-hydroxydopamine lesions. In Sham rats (n = 16) peripheral CLON (0.05 mg/kg), like NE, produced an increase in food intake and particularly potentiated carbohydrate ingestion. Discrete electrolytic lesions of the PVN (n = 5) abolished this CLON-induced feeding and carbohydrate preference, suggesting that the PVN may be a primary site for CLON-stimulated hyperphagia. Neurotoxin lesions of the PVN (n = 17), which reduced PVN NE levels by 75%, failed to alter peripheral CLON-induced feeding. This and other evidence indicates that this agonist may be acting via postsynaptic alpha 2 receptors in the PVN to potentiate carbohydrate intake, rather than via presynaptic release of NE from nerve endings in the PVN.

Hypothalamic serotonin in the control of meal patterns and macronutrient selection

Serotonin (5-HT) is believed to have an inhibitory influence over feeding behavior. The present experiments were designed to investigate the effects of hypothalamic 5-HT on spontaneously motivated feeding and appetite regulation. Freely-feeding rats were injected with 5-HT or norfenfluramine (NORFENF) directly into the paraventricular hypothalamus (PVN), and precise changes in feeding behavior were monitored by a computer. Following PVN 5-HT or NORFENF injection, animals exhibited a marked suppression in food intake, associated with a decrease in meal size, duration and eating rate, and no change in the frequency of meals consumed. This suggests that brain 5-HT may influence primarily the induction of satiety rather than the suppression of hunger. The effect of drugs presumed to affect brain 5-HT transmission on diet selection was also investigated in groups of rats injected centrally with 5-HT or NORFENF or peripherally with either fenfluramine, quipazine or cyproheptadine. In a series of 2-diet tests, rats centrally injected with 5-HT or NORFENF exhibited a selective suppression of the carbohydrate-rich diets. In animals provided with three pure macronutrient diets, protein, carbohydrate, and fat, systemic administration of serotonergic agents had its greatest impact on fat and carbohydrate ingestion, as compared to protein consumption. These findings support a role for hypothalamic 5-HT in modulating meal patterns and appetite for particular macronutrients.

Amphetamine: effects on meal patterns and macronutrient selection

Catecholaminergic systems, specifically in the region of the lateral perifornical hypothalamus (PFH), have been linked to the inhibition of feeding behavior. The present studies examined the effects of d-amphetamine (AMPH), which is believed to act through the release of endogenous catecholamines (CAs), on spontaneous feeding and appetite regulation in rats. Injection of AMPH directly into the PFH caused a marked suppression of food intake; changes in computer-monitored meal patterns were characterized by an increase in the latency to meal onset and a consequent reduction in meal size and duration. This suggests that hypothalamic AMPH administration may influence primarily the initiation, rather than the termination, of feeding. In other experiments, chronic infusion of AMPH directly into the PFH was shown to suppress 24 hr food intake and body weight gain, indicating the effectiveness of lateral hypothalamic CA stimulation in overriding normal long-term patterns of feeding. The effect of hypothalamic CA stimulation on macronutrient selection was also investigated in groups of rats injected either centrally or peripherally with AMPH, or centrally with the CA agonists, dopamine and epinephrine. Each of these manipulations caused a strong inhibition of protein intake with no effect on carbohydrate, and only a mild suppression of fat ingestion after peripheral AMPH. These selective effects of AMPH on feeding patterns and diet choice, provide support for a role of CA innervation to the lateral hypothalamus in the modulation of natural feeding behavior and macronutrient selection.

Effects of inhibitors of carbohydrate absorption or lipid metabolism on meal patterns of Zucker rats

Peripherally active anorectic agents represent a new approach to the pharmacological management of obesity. Two inhibitors of carbohydrate absorption: an alpha-glucosidase inhibitor, acarbose (Bay g 5421) and a alpha-amylase inhibitor, Ro 12-2272, were compared with two novel inhibitors of lipid metabolism: an inhibitor of human pancreatic lipase (Ro 20-0083) and of hepatic fatty acid synthesis (Ro 22-0654). All drugs were presented as diet admixtures over 3 or 4 consecutive days. Total food and water intakes, the temporal pattern of feeding, and the average meal frequency and meal size were measured using computerized data collection procedures. Inhibitors of carbohydrate absorption failed to suppress food intake in either obese or lean Zucker rats and had no effect on the parameters of feeding. In contrast, inhibitors of lipid metabolism reduced food intake by 56-77% by reducing both meal frequency and meal size. Direct inhibition of lipid metabolism may be a viable mechanism for anti-obesity agents.

Meal-taking behavior is related to predisposition to dietary obesity in the rat

The hypothesis that early nutritional experience can determine endogenous patterns of meal-taking behavior and thereby affect predisposition to dietary obesity was tested by raising male Sprague-Dawley rats in litters of 4, 8, and 20, and examining their meal patterns and responsiveness to a high-fat, high-sugar (HFS) diet in adulthood. At 9 months of age, half the rats from each litter size group were given the HFS diet for 16 weeks, while the other half were continued on laboratory chow. As expected, HFS-fed rats gained more weight and developed larger fat depots and more and larger fat cells than did chow-fed controls. Analysis of meal-taking behavior just prior to the introduction of HFS-feeding allowed some of the rats to be classified as "gorgers" or "nibblers" according to their average daily meal size. While on lab chow, gorgers and nibblers showed no differences in body weight gain, but upon being switched to the HFS diet, gorgers gained significantly more weight than did nibblers, and showed a greater degree of fat depot enlargement. These findings suggest that patterns of meal-taking behavior can predict the magnitude of and may contribute to the development of dietary obesity.

Dietary self-selection and meal patterns of obese and lean Zucker rats

Following two weeks of baseline measures on laboratory chow, young obese and lean male Zucker rats were given access to separate macronutrients for 18 weeks. First, the rats were given access to soybean meal, dextrinized starch, and lard for eight weeks. Daily caloric intake of obese rats was greater on laboratory chow than that of lean rats and remained so following the switch to the self-selection diet. While obese and lean rats consumed similar proportions of each macronutrient over the eight-week period, there were substantial changes in the pattern of intake across weeks. Obese rats decreased daily caloric intake primarily by decreasing lard consumption. In contrast, lean rats maintained daily caloric intake while increasing the proportion of calories from fat. The addition of a 25% sucrose solution (weeks 11-18) produced an elevated daily caloric intake for both obese and lean rats, achieved through an increased carbohydrate intake with a simultaneous decrease in fat intake. A diurnal pattern of intake was maintained for each macronutrient. Compared to controls, rats on the self-selection diet did not show differential growth. Measurement of daily meal patterns for soybean meal, sucrose, and lard during the last four days of the experiment showed that obese rats ate significantly larger meals comprised of all three food components than did lean rats. For both obese and lean rats, one-component meals were primarily sucrose with greater intake during the light period. Lean rats showed a greater tendency to indulge in these between-meal "snacks" than did obese rats. These data suggest that palatability and nutrient source as well as length of exposure are critical determiners of nutrient selection and total daily caloric intake.