Douglas G. Mook

SOME DETERMINANTS OF PREFERENCE AND AVERSION IN THE RAT

Of the events that accompany or follow the act of ingestion and that could therefore serve as “satiety” signals in the regulation of intake, two major classes can be distinguished: 1 ) oropharyngeal events, such as taste, smell, and proprioceptive feedback from the consummatory response, and 2) postingestional events, such as distension of the stomach by the material swallowed, and the hydrational and/or metabolic changes that follow its absorption. Both sets of factors are known to influence feeding and drinking, and attempts to assess the relative contribution of each set of factors have received a great deal of experimental attention. For example, in the rat, Adolph (1947) found that food injected directly into the stomach, bypassing the taste receptors, was about as effective as food eaten normally in suppressing further intake. More recently, Epstein (1960) and Epstein and Teitelbaum (1964) have shown that rats can regulate food and water intake in the total absence of mouth factors by pressing a bar to inject water or liquid diet directly into the stomach. On the other hand, oropharyngeal factors can play a powerful role in determining ingestion if they are permitted to operate. Their influence is most apparent in animals whose regulatory systems have been damaged, as by hypothalamic lesions. Such animals frequently are markedly responsive to small differences in taste and texture which the normal rat will ignore (Teitelbaum & Epstein, 1963). But even the intact rat will, for example, select and ingest large quantities of a saccharin solution, whose major physiological effect is on the taste receptors (Sheffield & Roby, 1950). Many more such investigations could be cited, for the influence on ingestion of these two sets of factors has been a subject of continuing interest, and several of the participants in this monograph have concerned themselves with it. The experiments reported here continue the attempt to identify the contributions of oral and postingestional events, and their interactions, to the regulation of intake. In them, both oral and postingestional mechanisms have been allowed to operate as they normally would do, but they have been dissociated and independently varied. This procedure permits direct assessment of the interaction of oral and postingestional events, as well as of the influence on ingestion of each set of factors in isolation. The method by which this dissociation is produced is shown in FIGURE 1 (Mook, 1963). The Figure shows a rat prepared with a chronic esophageal fistula, so that material swallowed escapes through an opening in the throat, and with an intragastric cannula, which permits both nutritional maintenance and the experimental intubation of material directly into the stomach (for details of preparation and maintenance, see Mook, 1963). The animal’s gastric cannula is attached by flexible tubing to a normally closed solenoid valve, whose input in turn is connected to a fluid reservoir suspended above the test cage. One or more drinking cylinders can be attached to the cage itself, permitting the rat to drink. As the animal laps at the drinking spout, he operates a low-voltage “drink-

Effect of hyperglycemia on pain threshold in alloxan-diabetic rats

&NA; Insulin‐dependent diabetes mellitus (IDDM) is associated with several complications, including painful diabetic neuropathy. Both animal and human investigations suggest an altered pain response in IDDM. Furthermore, it has been suggested that glucose may be an important mediating factor in these painful symptoms. In the present study, pain threshold was assessed via tail flick latency in alloxan‐diabetic and control rats. In addition, tail flick latency was determined under conditions of both hyperglycemia and euglycemia in diabetic rats. Conditions of hyperglycemia resulted in a significant decrease in the tail flick latency of alloxan‐diabetic rats. In contrast, tail flick latency was significantly increased in diabetic rats following normalization of blood glucose levels. It is concluded that elevated blood glucose levels contribute to a decrease in pain threshold in alloxan‐diabetic rats.

Estradiol induced suppression of feeding in the female rat: Dependence on body weight.

Abstract Ovariectomy in the adult female rat leads to a transient increase in food intake and an elevated level of body weight. Estrogen treatment blocks or reverses these changes. Currently, estrogen is seen to suppress feeding directly, but earlier findings suggest that estrogens effect is dependent upon an elevated level of body weight. We demonstrate that 1 μg estradiol, which has suppressant effects on food intake and body weight in ovariectomized (OVX) rats, has no such effects in ovariectomized-adrenalectomized (OVX-ADX) rats, which do not gain excess weight spontaneously. However, estradiol does have a suppressant effect on these measures in OVX-ADX rats, if they are made mildly obese by dietary means. Therefore, estradiol suppresses feeding only in the face of actual or impending obesity. It probably affects the system(s) concerned with the long-term regulation of body weight; but it does not act directly on the mechanisms which mobilize or inhibit feeding.

Release of feeding by the sweet taste in rats: Oropharyngeal satiety

In rats which are hungry but not thirsty, the intake of a saccharin solution occurs in discrete “meals”. We show that the control of meal size in this case is exclusively oropharyngeal. The augmentation of postingestive factors has no effect on meal size or on intake pattern. This is true whether they are augmented during the intake period by concurrent injections, or prior to it by gavage. Therefore, oropharyngeal factors such as taste can initiate, and also terminate, a “meal” under these special conditions.

Overeating induced by progesterone in the ovariectomized, adrenalectomized rat ☆

Abstract Progesterone causes increased food intake in the intact female rat. Ovariectomy abolishes this effect. We find that adrenalectomy restores it: Ovariectomized, adrenalectomized rats respond to progesterone as do intact rats. Therefore, the effect does not require the ovary or its other products.

Release of Feeding by the Sweet Taste in Rats: The Influence of Body Weight*

Rats that are not deprived of water, or that are overhydrated artificially, drink sweet solutions with avidity. We show that intake of sweet solutions is sensitive to nutritional status, suggesting that the rats are treating the solutions as liquid diets and “eating” rather than drinking them. First, when rats are force-fed to mild obesity, intake of dilute and concentrated saccharin and carbohydrate solutions is progressively reduced. Secondly, when weight loss is forced by restricted feeding, intake of saccharin and dilute carbohydrate is elevated, but intake of concentrated carbohydrate solutions is not. Perhaps the postingestive effects of concentrated sugar solutions set a limit on ingestion, so that intake can be driven below that limit as weight is gained, but cannot rise above it as weight is lost.

Taste Modulation of Fluid Intake

In any discussion of “nonregulatory” or “nonhomeostatic” determinants of behavior, the influence of taste on ingestion is certain to receive at least a mention. And at first glance, the phenomena observed when animals are offered various sapid materials are impressive to one accustomed to think of animals as “finely tuned regulatory machines” (Emits and Corbit, 1973).

Some endocrine influences on hypothalamic hyperphagia

Abstract After combined ovariectomy and adrenalectomy, rats with ventromedial hypothalamic lesions did not express hyperphagia if offered standard pelleted diet. A high-fat diet or a liquid diet would induce them to do so, however, Therefore, adrenal (and ovarian) secretions are not necessary for the expression of hyperphagia. When hyperphagia occurred, it showed the characteristic stasis in body weight at an elevated level. This implies that even after ovariectomy, adrenalectomy, and damage to the brain, the capacity for weight regulation survives.

Release of Feeding by the Sweet Taste in Rats: The specificity of Oral Satiety

Saccharin intake in hungry rats occurs in discrete “meals”, indicating that a period of saccharin drinking produces a temporary “oral satiety”. We show that such satiety does not suppress the intake of solid food (powdered rat chow), of liquid food (milk), or even of a nutritive sweet solution (glucose) unless it is very dilute. Therefore, oral satiety does not reflect the reduction of a generalized hunger state or of a specific carbohydrate hunger. Having drunk one concentration of saccharin to satiety, the rat will resume drinking if offered a higher concentration but not if offered a lower one, even if the lower one is more palatable than the original concentration. Therefore, in the saccharin-satiated rat, the resumption of drinking requires an increase in stimulus intensity, even if it means a decrease in palatability of the solution offered. We suggest that it is gustatory adaptation that terminates a saccharin meal in the hungry rat.

Saccharin drinking by hungry rats: Adjustment to constraints

In food-deprived rats, rate of lapping at a saccharin cylinder declines progressively over a session. The decline is unaffected, within limits, by restricted access: Whether the cylinder is available continuously, or only during alternate 30-sec intervals, the rat adjusts its momentary lap rate so that the amount of lapping within each 5-min period remains the same. It is as if the rat specifies over each brief period how much ingestive behavior is to occur, and then adjusts its lap rate so that actual ingestive behavior matches the specification. The specification in turn moves down as the drinking bout progresses.