Jan W. Kakolewski

Sex Differences in Taste Preference for Glucose and Saccharin Solutions

Taste preferences of mature male and female rats for caloric and noncaloric sweet solutions have been found to differ. Although females do not drink more water than males, they consume significantly greater quantities of a slightly sweet 3 percent glucose and a very sweet 0.25 percent saccharin solution. When given a choice, males switch their initial preference for a saccharin solution to a preference for a glucose solution after several days, while females maintain a preference for the saccharin solution. Females also prefer significantly higher concentrations of saccharin than males do.

Modification of Motivated Behavior Elicited by Electrical Stimulation of the Hypothalamus

Previous reports demonstrated that hypothalamic stimulation may elicit either eating, drinking, or gnawing and emphasized both the specificity of the neural circuits mediating these behaviors and the similarity to behavior during natural-drive states such as hunger and thirst. We find that, after a period of very consistent elicitation of one of these behaviors, the animal may exhibit an equally consistent alternate behavior. A learning component is implicated in the association of hypothalamic stimulation with a particular behavior pattern.

Polydipsia Elicited by the Synergistic Action of a Saccharin and Glucose Solution

Rats consume significantly more of a solution combining saccharin and glucose than equivalent solutions of these substances presented in separate bottles. Fluid consumption may exceed body weight. Experiments helping to delineate the basis of this synergistic action are presented.

SEX DIFFERENCES IN HYPERPHAGIA AND BODY WEIGHT FOLLOWING HYPOTHALAMIC DAMAGE

Informal observations from several laboratories as well as our own experience have suggested that the degree and frequency of hyperphagia and “obesity” following destruction of the ventromedial area of the hypothalamus (HVM) are greater in female than in male rats. In many laboratories studying hypothalamic hyperphagia and obesity, female animals are used exclusively as experimental subjects. It is difficult, however, to determine whether the choice of female subjects is based upon laboratory “folklore” or controlled observations. One of the earliest studies in this area (Hetherington & Ranson, 1940) used rats of both sexes, but other than comments about different effects on growth, which we will refer to later, no effort was made to report the effects of the lesions separately for males and females. An. examination of the data, however, does reveal evidence indicating that the females exhibited a higher percent increase in body weight than the males over their respective controls. In the studies of Brobeck and colleagues (1943) most of the data presented are from female animals. These authors did refer to a “record weight” (982 g) of a male rat that was lesioned, but this animal was placed on a special pancreatic-protein diet and there was no similarly treated female animal with which to make a cornparison. Studies such as those of Fusco and coworkers (1966), McCann (1953), and Trifar6 and Mikuli2 (1966), which employed males exclusively, have reported no change in rate of weight gain as a consequence of HVM lesions. Bernardis and Skelton (1967) reported a significant increase in body weight and food intake in male rats with HVM lesions, but there was no simultaneously tested comparison group of female animals. It would be a monumental task to review all the research in this area. We have attempted to examine all the published reports of hypothalamic damage in which either food consumption or material related to obesity is presented. Although it is certain that not every study has been found, the majority have been surveyed. As a result we have concluded that while there is no doubt that hyperphagia and obesity may be produced by HVM lesions in males, the likelihood of success and the magnitude of the effect are smaller with males. It may be worth noting in passing that little attention has been paid to the fact that some controversies have arisen over the interpretation of the role of the HVM area that have been confounded with a sex variable. For example, Reynolds (1963 & 1965) proposed that HVM destruction resulted in an irritative lesion. Data supporting this hypothesis were obtained from male rats that did not exhibit hyperphagia or obesity (measured by body weight increase) following nonirritative radio-frequency destruction of the HVM. Hoebel (1965) used female rats in a “refutation” that demonstrated that radio-frequency lesions could produce hyperphagia and obesity. For the most part, the comparisons we have made from the literature on the relative effectiveness of HVM lesions in producing hyperphagia and obesity in males and females involve different laboratories, investigators and animal

CHAPTER 9 – The Hypothalamus and Motivated Behavior1

Eating, drinking and gnawing motivation interchangeability under hypothalamic stimulation, noting role of neural substrate activation

FURTHER STUDIES OF SEX DIFFERENCES IN TASTE PREFERENCES WITH SWEET SOLUTIONS

Two additional studies are described which add to the original report that female rats consume greater quantities of sweet solutions of glucose and saccharin during successive 24-hr. periods than do males. The results justify the following conclusions: (1) the original results obtained with Holtzman rats can be extended to the Charles River strain and (2) females consume greater quantities of a saccharin solution during relatively brief 1-hr. taste preference tests.

Emergence of systemic cues evoking food-associated drinking☆

Abstract Rats maintained on a feeding schedule of eight 1.5-g portions of Purina Lab Chow per day consistently initiate drinking following food ingestion. In order to establish whether systemic cues accompany the onset of drinking, the rats were sacrificed and serum osmolality was determined. The control group was sacrificed at the time of day they normally received the first portion of food. The experimental group was sacrificed at the moment of initiation of drinking following consumption of the first portion of food. The difference in serum osmolality between the groups indicates that a systemic cue had emerged at the time of initiation of drinking.

The motivation underlying eating elicited by lateral hypothalamic stimulation

Abstract Animals that eat a specific food in response to lateral hypothalamic stimulation do not readily switch to another familiar food. Furthermore, such animals exhibit a hesitancy to switch to the same food changed in form as when pellets are ground to a powdered meal. In both instances stimulation began to elicit drinking rather than eating of a different or altered food. We have concluded that in this significant respect, eating elicited by electrical stimulation and hunger are very different.

Emotionally Induced Increases in Effective Osmotic Pressure and Subsequent Thirst

Following a brief period of handling or enclosed rotation, rats increased the frequency of drinking relative to eating. Handling also delayed or eliminated eating behavior in hypoosmotic rats. Osmometric analysis revealed a rapid increase in serum osmolality during stress which may account for the emergence of thirst and disruption of eating.

The relationship between gnawing and food consumption with ventromedial hypothalamic lesions

Abstract Ventromedial hypothalamic lesions in rats increased food intake and decreased wood gnawing. Both feeding and gnawing can be elicited by electrical stimulation of the lateral hypothalamus. The opposite effect of ventromedial hypothalamic lesions on feeding and gnawing is discussed in terms of the relationship between the ventromedial hypothalamus and the lateral hypothalamus.