Genetic mechanisms of drinking and feeding

Sweet taste liking is associated with subjective response to amphetamine in women but not men

RATIONALE AND OBJECTIVE

Preference for sweet taste rewards has been linked to the propensity for drug use in both animals and humans. Here, we tested the association between sweet taste liking and sensitivity to amphetamine reward in healthy adults. We hypothesized that sweet likers would report greater euphoria and stimulation following D-amphetamine (20 mg) compared to sweet dislikers.

METHODS

Men (n = 36) and women (n = 34) completed a sweet taste test in which they rated their liking of various concentrations of sucrose and filtered water (0.05, 0.10, 0.21, 0.42, and 0.83 M). Participants who preferred the highest concentration were classified as "sweet likers." All others were classified as "sweet dislikers." They then completed four sessions in which they received D-amphetamine (20 mg) and placebo in alternating order, providing self-report measures of euphoria and stimulation on the Addiction Research Center Inventory (ARCI) at regular intervals. We conducted linear mixed effects models to examine relationships between sweet liking and drug-induced euphoria and stimulation.

RESULTS

Sweet likers reported significantly greater amphetamine-induced euphoria than did sweet dislikers among women. By contrast, sweet liking was not associated with amphetamine response in men. No associations with stimulation were observed.

CONCLUSION

The association between sweet preference and amphetamine response in women is consistent with animal studies linking sweet taste preference and drug reward and also fits with observations that individuals who use drugs show a preference for sweet tastes. Whether the sex difference is related to circulating hormones, or other variables, remains to be determined.

Drinking of flavored solutions by high preferring (WHP) and low preferring (WLP) alcohol-drinking rats

BACKGROUND

Selective breeding alcohol-preferring (P) and alcohol-nonpreferring (NP) rats showed a strong preference for the sucrose solutions, whereas P rats intake greater amounts than NP rats. The aim of this study was the estimation of selectively bred ethanol-preferring (WHP - Warsaw High Preferring) and ethanol-nonpreferring (WLP - Warsaw Low Preferring) rats for their preference for various tastes.

METHODS

The oral drinking of the following substances was studied at a range of concentrations: sucrose (0.5-64.0 g/100 ml), NaCl (0.025-3.2 g/100 ml), citric acid (0.008-2.048 g/l), and sucrose octaacetate (0.002-0.512 g/l) solutions. Separate groups of 7-8 rats from each line were investigated of each of the four tastes. The investigated solutions were presented continuously keeping water and food always available. Concentrations of the various flavors were doubled every 48 h.

RESULTS

Rats from WHP and WLP lines clearly revealed the preference for the sucrose solution and the highest preference was at the 4.0 and 8.0 g/100ml sucrose concentration. Similar to sucrose, both lines exposed strong preference for the NaCl solution and this preference enhanced together with the increase of the NaCl concentration. Nevertheless their preference for the NaCl solutions decreased when the concentration of NaCl reached 1.600 g/100 ml. Both lines of rats did not differ in citric acid or sucrose octaacetate intake at any of the concentrations studied.

CONCLUSION

Selective breeding of rats (WHP) for high and rats (WLP) for low ethanol drinking is favorably correlated with the drinking of sweet and salty solutions and negatively correlated with the consumption of sour and bitter tastes.

Perception of sweet taste is important for voluntary alcohol consumption in mice

To directly evaluate the association between taste perception and alcohol intake, we used three different mutant mice, each lacking a gene expressed in taste buds and critical to taste transduction: alpha-gustducin (Gnat3), Tas1r3 or Trpm5. Null mutant mice lacking any of these three genes showed lower preference score for alcohol and consumed less alcohol in a two-bottle choice test, as compared with wild-type littermates. These null mice also showed lower preference score for saccharin solutions than did wild-type littermates. In contrast, avoidance of quinine solutions was less in Gnat3 or Trpm5 knockout mice than in wild-type mice, whereas Tas1r3 null mice were not different from wild type in their response to quinine solutions. There were no differences in null vs. wild-type mice in their consumption of sodium chloride solutions. To determine the cause for reduction of ethanol intake, we studied other ethanol-induced behaviors known to be related to alcohol consumption. There were no differences between null and wild-type mice in ethanol-induced loss of righting reflex, severity of acute ethanol withdrawal or conditioned place preference for ethanol. Weaker conditioned taste aversion (CTA) to alcohol in null mice may have been caused by weaker rewarding value of the conditioned stimulus (saccharin). When saccharin was replaced by sodium chloride, no differences in CTA to alcohol between knockout and wild-type mice were seen. Thus, deletion of any one of three different genes involved in detection of sweet taste leads to a substantial reduction of alcohol intake without any changes in pharmacological actions of ethanol.

Brain stimulation reward performance and sucrose maintained behaviors in alcohol-preferring and -nonpreferring rats

BACKGROUND

The relation between ethanol (EtOH) preference and sensitivity to brain stimulation reward (BSR) was examined under multiples schedules of reinforcement in the current study. For comparison, the study also examined the relation between EtOH preference and motivation for a sweet, palatable sucrose solution under similar schedules of reinforcement.

METHODS

To investigate sensitivity to BSR performance, alcohol-preferring and -nonpreferring rats were tested using the curve-shift (rate-frequency) paradigm under several intensity levels during a 20-min session. Animals were first trained under an optimal current intensity, which produced maximal responding (i.e., 100%) across a series of descending frequencies (i.e., 300-20 Hz). BSR was then evaluated at 100%, 75%, and 50% of the optimal current. The sensitivity of the curve-shift method was further evaluated under the animal's optimal current using the FR1, FR6, and FR12 schedules. To examine responding for the sucrose solution, a separate group of alcohol-preferring and -nonpreferring rats was initially stabilized on an FR1 schedule and then subsequently on FR6 and FR12 schedules.

RESULTS

The results demonstrated that reducing the reinforcing efficacy of BSR via reduction in current intensity/reinforcement schedule produced marked orderly rightward shifts in the rate-frequency curves relating responding to stimulation frequency in both rat lines. However, no differences were found between the lines with either manipulation. Specifically, both lines demonstrated orderly reductions in response rate and increases in BSR threshold parameters (i.e., half maximal frequency/responding, minimum and maximum frequencies). In contrast to BSR, genetic selection for EtOH preference was highly associated with responding for the sweet, palatable sucrose solution. The association was even more salient as the reinforcement schedule increased (i.e., reward cost).

CONCLUSION

The results demonstrate that responding for BSR is not associated with EtOH preference, insofar as alcohol-preferring and -nonpreferring rats respond similarly under an array of reinforcement schedules and current intensities. In contrast, genetic selection for EtOH preference is highly associated with responding for a palatable sucrose reward, and the relation increases as the reward cost for the sucrose increases. These findings suggest that similar/overlapping mechanisms of action regulate the reinforcing properties of EtOH and sucrose but that overlapping yet distinct neuronal mechanism may modulate the reward characteristics of BSR and EtOH preference.

Sweet liking, novelty seeking, and gender predict alcoholic status

BACKGROUND

The relationship between a hedonic response to sweet taste and a propensity to excessive alcohol drinking is supported by both animal and human studies. There is evidence indicating that the tendency to rate more concentrated sweet solutions as the most pleasurable (i.e., sweet liking) is associated with the genetic vulnerability to alcoholism. However, sweet liking by itself is insufficient to predict the alcoholic status of the individual. Our previous study indicated that alcoholic status can be predicted by a combination of hedonic response to sweet taste and personality profile as measured by the Tridimensional Personality Questionnaire (TPQ). This study was designed to further test this hypothesis.

METHODS

Participants were 165 patients admitted to a residential treatment program for the treatment of alcoholism, drug dependence and/or interpersonal problems secondary to substance-abusing family members. In addition to a routine medical examination, on the 24th day after admission, patients completed the TPQ, the standard sweet taste test was conducted, and paternal family history of alcoholism was evaluated.

RESULTS

Sweet liking was strongly associated with a paternal history of alcoholism. The odds of receiving an alcohol dependence diagnosis were shown to increase, on the average, by 11% for every one-point increase in the TPQ novelty-seeking score in sweet-liking but not in sweet-disliking subjects. Gender contributed independently to the probability of alcohol dependence, with males exhibiting higher rates of alcoholism than females.

CONCLUSIONS

These findings support the hypothesis that a hedonic response to sweet taste is associated with a genetic risk for alcoholism. Alcoholic status may be predicted by a combination of sweet liking, the TPQ novelty-seeking score, and gender in a mixed group of alcoholic, polysubstance-dependent, and psychiatric patients.

Association Between Sweet Preference and Paternal History of Alcoholism in Psychiatric and Substance Abuse Patients

BACKGROUND

The relationship between preference for stronger sweet solutions and propensity to excessive alcohol drinking is supported by both animal and human studies. This study was designed to test the hypothesis that sweet preference is associated with the genetic risk of alcoholism as measured by a paternal history of alcoholism.

METHODS

Participants were 180 patients admitted to a residential treatment program for the treatment of alcoholism, drug dependence, or psychiatric conditions. In addition to a routine medical examination, patients completed the standard sweet preference test twice (on the 9th and 24th days after admission), and the family history of alcoholism was evaluated.

RESULTS

Sweet preference was shown to be stable over time. It was strongly associated with a paternal history of alcoholism, with family history-positive patients approximately 5 times more likely to prefer stronger sweet solutions than family history-negative subjects. Such factors as dependence on alcohol, cocaine, opiates, cannabis, other drugs (including prescription drugs), and tobacco smoking, as well as demographics (gender and age), did not significantly interfere with association between sweet preference and paternal history of alcoholism.

CONCLUSIONS

These findings provide some support for the hypothesis that preference for stronger sweet solutions is associated with a genetic predisposition to alcoholism as measured by a paternal history of alcoholism.

Family History of Alcoholism and Response to Sweets

BACKGROUND

The relationship between a hedonic response to sweet tastes and a propensity to excessive alcohol drinking is supported by both animal and human studies. This study was designed to test the hypothesis that the genetic risk for alcoholism as measured by a paternal history of alcoholism in young social drinkers is associated with sweet-liking, defined as rating the strongest offered sucrose solution (i.e., 0.83 M) as the most palatable during the standard sweet test.

METHODS

Participants were 163 subjects (39% male) without a lifetime history of alcohol or drug abuse or dependence. Eighty-one subjects had a paternal history of alcoholism (FH+), and 82 did not (FH-). Each subject rated a series of sucrose solutions for intensity of sweetness and palatability. Subjects were categorized as sweet-likers if they rated the highest sucrose concentration as the most pleasurable.

RESULTS

The estimated odds of being a sweet-liker were 2.5 times higher for FH+ than for FH- subjects. FH+ subjects disliked the tastes of the two weakest offered sucrose concentrations (0.05 and 0.10 M), whereas FH- subjects reported these tastes to be neutral.

CONCLUSIONS

The results of this study support the hypothesis that sweet-liking is associated with a genetic vulnerability to alcoholism.

Differential responding for brain stimulation reward and sucrose in high-alcohol-drinking (HAD) and low-alcohol-drinking (LAD) rats

BACKGROUND

This study examined the associations among selective breeding for alcohol preference, intake of sweet solutions, and responding for brain stimulation reward (BSR), a nonoral reinforcer, in alcohol-preferring high-alcohol-drinking (HAD)-1 and nonpreferring low-alcohol-drinking (LAD)-1 rats.

METHODS

Adult male HAD-1 and LAD-1 rats were trained to lever press for medial forebrain bundle stimulation. Current intensity was varied in separate sessions to generate a rate/intensity function. To further examine BSR responding, the animals responded for stimulation at 100 Hz and at a fixed current intensity on an FR1 schedule. In subsequent sessions, the schedule was increased to FR6 and then to FR12. To examine responding for the sucrose solution, we trained a separate group of HAD-1/LAD-1 rats to bar press for sucrose on an FR1 schedule. Similar to the BSR experiment, in following sessions, the schedule was increased to an FR6 and then to an FR12 schedule.

RESULTS

No significant differences were observed between the two rat lines across a range of current intensities. As the reinforcement schedule increased, HAD-1 rats exhibited a dramatic decrease in BSR responding, whereas the LAD-1 rats displayed a more protracted reduction. In contrast to BSR, marked elevations in responding were observed for sucrose as the schedule increased. However, in HAD-1 rats, response rates were similar on the FR6 and FR12 schedules, whereas LAD-1 rats showed a reduction in response rates from the FR6 to FR12 schedule. Furthermore, HAD-1 rats exhibited significantly more responses compared with LAD-1 rats across the three reinforcement schedules. An analysis of the response profile for the three reinforcement schedules suggested that few if any postreinforcement pauses were exhibited when the reinforcer was BSR compared with sucrose in both lines.

CONCLUSION

Medial forebrain bundle BSR is a powerful reinforcer in both HAD-1 and LAD-1 lines. However, BSR responding was not associated with selective breeding for alcohol preference. In contrast, selective breeding for alcohol preference was associated with sucrose consumption, especially as the amount of work increased. The lack of correspondence between BSR and sweet taste rewards in HAD-1 and LAD-1 lines may suggest important differences yet an overlapping brain reward mechanism in the control of motivated behaviors in these selected lines.

Preference for higher sucrose concentrations in cocaine abusing-dependent patients

BACKGROUND

Animal studies suggest that preference for relatively high concentrations of sweet solutions and lack of control over sweet solution consumption is related to a preference for alcohol over water. There also is evidence in humans that alcoholics prefer high concentration sweet solutions. This study was designed to determine whether patients with cocaine use disorder also prefer high concentrations of sweet solutions.

METHODS

Sixteen patients with cocaine abuse/dependency were compared with 16 inpatient controls with an affective disorder as to their preferences for increasing concentrations of sucrose solutions. Patients were administered aqueous sucrose solutions ranging from 0.05 to 0.83 M. They were then asked to rate their degree of preference for, and the degree of sweetness of each solution.

RESULTS

Cocaine abusing/dependent patients significantly more often preferred the highest sucrose concentration (0.83 M).

CONCLUSIONS

The above information suggests that cocaine abusing/dependent patients, like alcoholics, and in contrast to inpatient controls, share a preference for high concentrations of sucrose.

Fluoxetine reduces saccharin-induced elevation of fluid intake in alcohol-preferring Fawn-Hooded rats

Previous work has established that saccharin and alcohol intakes are highly correlated in a variety of rat strains. In addition, it has been shown that alcohol-preferring rats consume saccharin beyond the limit of their normal daily fluid intake (DFI). It has been hypothesized that alcohol-preferring rats have impaired control over consumption of reinforcing substances, which may be related to a deficiency of brain serotonin. In the present study, we examined the effect of the serotonin reuptake inhibitor fluoxetine (2.5, 5.0, 10.0 mg/kg, IP, twice a day) on saccharin intake in alcohol-preferring Fawn-Hooded (FH) rats. It was confirmed that alcohol preferring FH rats almost triple their DFI when saccharin/water choice was introduced. Treatment with fluoxetine resulted in a dose-dependent decrease in saccharin intake to, but not below, the normal level of their DFI. No significant effects of fluoxetine on water intake were observed. Despite a significant (up to 69%) decrease in saccharin intake, only a minimal reduction (< 4%) in saccharin preference occurred. We conclude that fluoxetine reduces the exessive elevation of fluid intake observed at the presence of the palatable saccharin solution in Fawn-Hooded rats. These findings may provide more evidence for the involvement of the serotonergic system in the brain in exessive drinking of rewarding substances.

Suppression of ethanol intake in alcohol-preferring rats by prior voluntary saccharin consumption

In a situation offering a free choice between 0.1% saccharin solution and tap water, Fawn Hooded (FH) rats consumed 363.0 +/- 33.5 ml/kg/day of saccharin solution. Subsequently those animals drank 3.0 +/- 0.4 g/kg of ethanol in a free choice between water and 10% ethanol solution. Control FH rats that did not have access to saccharin consumed 5.0 +/- 0.5 between groups was significant: p = 0.006). When control rats were exposed to the choice between 10% ethanol solution and 0.1% saccharin solution for 4 days they consumed 383.7 +/- 27.5 ml/kg/day of saccharin solution and their ethanol intake dropped to 1.2 +/- 0.3 g/kg/day. When these rats were returned back to alcohol/water choice and exposure to saccharin was discontinued, their alcohol intake was still reduced (3.7 +/- 0.3 g/kg/day for at least 10 consecutive days). Exposure of alcohol-experienced alcohol-preferring P rats with high (6.8 +/- 0.5 g/kg/day) and stable alcohol intake to saccharin/water choice for 4 days also resulted in a significant attenuation of their ethanol intake for at least 6 days following saccharin cessation. Thus, voluntary consumption of saccharin can suppress subsequent alcohol intake in both alcohol-naive and alcohol-experienced rats.

Saccharin-induced increase in daily fluid intake as a predictor of voluntary alcohol intake in alcohol-preferring rats

This study examined the relationship between saccharin intake and ethanol consumption in alcohol preferring (P) rats and Fawn Hooded (FH) rats before and after exposure to forced ethanol (10%, v/v) solution. Both groups exhibited large increases (> 2X) in daily fluid intake (DFI) when saccharin (0.1%, w/v) was present and exhibited moderate levels of ethanol intake. Only the P rats significantly increased their ethanol consumption after exposure to ethanol as the sole drinking fluid. Correlational analyses revealed that the absolute intakes of saccharin and ethanol were not significantly correlated in either group, but the increase in DFI in the presence of saccharin was highly correlated with ethanol intake after forced ethanol exposure (r > +0.8; p < 0.05). Similarly, when correlations were conducted for these variables over both the P and FH groups, the correlation between increase in DFI in the presence of saccharin and alcohol intake was significantly higher than that between saccharin and alcohol intakes. Reexamination of previous data from 6 different rat strains also revealed a significant correlation between increase in DFI in the presence of saccharin and ethanol intake. These findings suggest that the dramatic increase in of DFI in the presence of saccharin may be an animal analog of the clinical phenomenon known as a loss of control.

Consumption of sweet, salty, sour, and bitter solutions by selectively bred alcohol-preferring and alcohol-nonpreferring lines of rats

To determine whether selective breeding for high or low oral ethanol consumption is associated with different preferences for nonpharmacological solutions with various flavors, the oral intake of a range of concentrations of sucrose (0.5-64.0 g/100 ml), NaCl (0.025-3.2 g/100 ml), citric acid (0.008-2.048 g/liter), and sucrose octaacetate (0.002-0.512 g/liter) was studied in alcohol-preferring (P) and alcohol-nonpreferring (NP) rats. Separate groups of 7-8 rats from each line were tested for consumption of each of the four flavors. The flavored solutions were presented continuously with water and food always available, and the concentrations were doubled every 48 hr. Although rats from both lines showed a strong preference for the sucrose solutions, P rats consumed greater amounts than NP rats [F(7,98) = 5.57, p < 0.001]. Rats of the P line drank less of the NaCl solutions than NP rats [F(7,98) = 3.88, p < 0.001], but the effect was not as robust as the line differences seen with sucrose. The P and NP rats did not differ in citric acid or sucrose octaacetate intake at any of the concentrations tested. Selective breeding for high oral ethanol preference appears to be positively associated with consumption of sweet solutions and negatively associated with intake of salty solutions.

Energy balance in an inbred strain of rats: comparison with the Wistar strain

Food intake and body weight gain were examined in two groups of male rats (7 weeks): an inbred strain, Dark Agouti (DA, n = 12) and a noninbred strain, Wistar (n = 13). The animals were allowed to select their diet from separate sources of the three macronutrients protein, fat, and carbohydrate. After 10 days of adaptation to the diets, body weights and food intakes were measured for 3 weeks. During this period, meal patterns were recorded for at least 5 days in each rat. Then, rats were switched to a chow diet (UAR, A.O4) for 10 days. The total caloric intake of DA rats was 60% that of Wistar rats, while their body weight gain was 25% that of Wistar rats (1.3 g/day in DA vs. 5.3 g in Wistar). However, when energy intake was related to total body weight, there was no difference in energy ingestion. It was observed that DA rats ingested mainly proteins (45%) and fats (41%), while Wistar rats ingested an identical proportion of proteins and carbohydrates (40%). The percent of total white adipose tissue to total body weight was identical in both strains (6% on average). Brown adipose tissue thermogenic activity of DA rats was threefold higher than in Wistar rats. This could be one of the elements responsible for the lower body weight gain of this group of rats. Self-selected food intake of the inbred DA strain of rats, in contrast to what was expected, was greatly variable.

What can genetic models tell us about behavioral plasticity?

Biological diversity and learning have played an essential interactive role in the evolution of species, as intra-specific individual differences have exerted a buffering effect towards environmental changes, and learning ability per se has allowed their maintenance. By exploiting biological diversity individuals with defective learning and memory have been produced that allow the study of the neural substrates of encoding mechanisms, as has been done in studies from Drosophila to rodents. Various aspects of this neurogenetic approach are reviewed and pitfalls are indicated. It is clear that genetic models need to be implemented by an integrated multidisciplinary top-down approach based on behavioral, electrophysiological, histochemical, immunocytochemical and neurochemical techniques. Examples are presented from some animal models that illustrate how a systems level analysis of the neural substrates of information processing can be carried out using such an integrated scheme.

Taste preferences in rat lines selected for low and high alcohol consumption

Alcohol-avoiding (ANA), alcohol-preferring (AA), and control Wistar rats were tested sequentially for their initial preferences for single concentration solutions of quinine, saccharin, salt, and citric acid, and then for an ascending series of saccharin concentrations. A similar study was subsequently conducted with the alcohol-nonpreferring (NP) and alcohol-preferring (P) rat lines. Both lines developed for low alcohol consumption drank much less saccharin than their respective lines developed for high alcohol intake when tested with the single concentration and with the ascending series. The ANAs also generally drank less of the bitter, salty, and sour solutions than the AAs or Wistars but little difference was found between the NPs and Ps with the other tastes. The curve relating saccharin consumption to concentration reached a maximum at about the same concentrations for AAs, Wistars, NPs, and Ps but for the ANAs, was shifted to the left. The results support a close relationship between the genetic factors influencing alcohol and saccharin intake in both line pairs. This relationship is probably not caused by saccharin tasting like alcohol to a rat, because other results indicate that the NPs do not have more negative reactions initially to the taste of alcohol, but it might be related to similar mechanisms mediating the reinforcement from sweet tastes and from systemic alcohol.

Strain differences in dietary hyperphagia: interactions with age and experience

The development of susceptibility to dietary hyperphagia was examined in two strains of rats. Juvenile Lewis and CD rats fed a wet diet initially eat less energy than do rats fed the same diet in dry form. As the rats approach adulthood, rats fed the wet diet consume more energy than rats fed the dry diet. Lewis rats began displaying hyperphagia at an earlier age than did CD rats. However, Lewis rats, unlike CD rats, fed dry diet during the juvenile stage and subsequently switched to wet diets, displayed only transient hyperphagia. Variability between animals within a group was substantially smaller in Lewis than in CD rats, indicating that genetic factors may be responsible for the differences between the two strains. Thus, susceptibility to dietary hyperphagia is influenced by interactions between strain of rat, age of testing, and type of diet fed in the juvenile stage.

Why do sugars taste good?

The preference humans and animals show for sweet solutions has been the subject of hundreds of publications. Nevertheless, the evolutionary origin of sweet preference remains enigmatic because of the relatively low nutritional value of sugars and the absence of specific tastes for other, more essential, nutrients. Moderate concentrations of sugars are found in most plant foods because sugars play an important role in plant physiology. Widespread occurrence of sugars in plants is paralleled by widespread preference for sugar solutions in mammals. These observations suggest that preference for sugars evolved because they are common in plants and easy to detect rather than because of any special nutritional merits they offer. Perception of sweetness cannot be used to accurately meter the metabolizable energy or nutritive value of a food.

Initial acceptance of ethanol: gustatory factors and patterns of alcohol drinking

Individual differences related to taste, determined by prior two-bottle tests of quinine and saccharin selection against water, were found to be related to the initial selection of 15% ethanol solution during the first week of access by 60 randomly bred male rats. The 36 rats that drank the least alcohol during the first week (mean +/- SE: 0.49 +/- 0.06 g/kg/day), however, greatly increased their intake during the second and third weeks, to the level of the 24 initially high alcohol drinkers (4.07 +/- 0.39 g/kg/day during 1st week), and the influence of gustatory factors was no longer apparent. Subsequently, the initially low rats drank less alcohol when saccharin was the alternative fluid. The results can be interpreted as showing that initially low rats, that only drank rather large amounts of alcohol after prolonged exposure, resemble Cloninger's Type 1 alcoholics not only in this temporal pattern but also in being high in novelty seeking, and low in harm avoidance and reward dependence, and that the initially high rats that spontaneously drank rather large amounts even in the first week show the opposite characteristics and resemble Type 2 alcoholics. Although these rats are not themselves models for alcoholism, the results nevertheless suggest it might be possible to develop two separate animal models for the two types of alcoholism.

Dietary hyperphagia and obesity: what causes them?

Diets that cause animals to overeat and become obese have been used in many investigations of obesity. Most of this research, however, has concentrated on the consequences rather than the causes of overeating. Furthermore, in most studies, several nutritional variables were manipulated simultaneously, making cause and effect relationship impossible to disentangle. Consequently, progress has been slow. Diets could alter energy intake by virtue of their effects on oral-sensory, gastrointestinal or postabsorptive effects. Palatability is the most popular oralsensory hypothesis but the empirical basis for this hypothesis is particularly weak. A substantial body of evidence is consistent with the possibility that the osmotic effects of diets in the gastrointestinal tract and metabolic postabsorptive factors may play a major role in dietary hyperphagia and obesity. Suggestions for future research directions are offered.

Multiple opiate receptors and pharmacological response to morphine in rats maintained on diets differing in protein concentration

1. The influence of diets differing in protein concentration on the characteristics of mu, delta, and kappa-opiate receptors and on the analgesic and hyperthermic responses to morphine was examined in rats. Three groups of male Sprague-Dawley rats were maintained for four weeks on isocaloric diet containing either 4, 20 or 50% protein. 2. The animals maintained on 4% protein diet weighed 92 +/- 2% of the initial weight at the end of the fourth week, whereas animals maintained on 20% and 50% protein diet weighed 222 +/- 2% and 221 +/- 2%, respectively. The average food intake per 100 g body weight on day 1 of the study in 4, 20 and 50% protein diet group was 5.0 +/- 2.2 g, 10.4 +/- 1.4 g, and 10.0 +/- 1.2 g, respectively. This difference in food intake was not observed during rest of the period of the study. 3. Water intake was higher for the animals maintained on diet containing 50% protein as compared to the other two groups. 4. The analgesic and the hyperthermic response to morphine varied in direct relation to the concentration of protein in the diet. The concentration of morphine in the brain and plasma of animals maintained on the three diets following challenge dose of morphine did not differ. Similarly the Bmax and Kd values for the binding of [3H]naltrexone, [3H]D-Ser2-Thr6-leucine enkephalin and [3H]ethylketocyclazocine to brain membranes prepared from rats kept on 4, 20 and 50% protein concentration did not differ. 5. It is concluded that the diet differing in protein concentration can alter the responses to morphine, and that such altered effects can not be accounted for by the changes in the distribution of morphine in brain and plasma or to the changes in the characteristics of the mu, delta and kappa opiate receptors in the brain.