Masahiro Imaizumi

Preference for corn oil in olfactory-blocked mice in the conditioned place preference test and the two-bottle choice test.

We studied the effects of olfactory stimuli on preference for corn oil in mice. In the conditioned place preference test, voluntary intake of 100% corn oil by both olfactory normal and ZnSO4-induced olfactory-blocked (anosmic) mice resulted in their place preference for the corn oil-related box. In the olfactory normal mice, place preference was also observed by voluntary intake of linoleic acid as well as of corn oil. In the two-bottle choice test, normal mice showed significant preference for test fluids that contained corn oil at all concentrations (1-10%) tested relative to vehicle alone. However, the lower concentrations (1 and 3%) of corn oil were not preferred in the anosmic mice. These results suggested that stimuli other than olfaction contributed to the rewarding effects of corn oil, but at lower concentrations olfactory stimuli might act as a signal for the oil.

Preference for vegetable oils in the two-bottle choice test in mice.

We evaluated in mice their preference for different vegetable oils using the two-bottle choice test. Mice showed preference for all vegetable oils (at 1% concentration in emulsion) tested, including corn oil, canola oil, and mixed vegetable oil, compared to a control fluid without oil. Mice also significantly preferred highly palatable sucrose solutions (5-20%) in this test. One percent corn oil was selected to the same extent as were these sucrose solutions. Mixtures of 1% corn oil and several concentrations of sucrose were preferentially selected as compared to corn oil or sucrose alone. The concentration-response curves of oil-sucrose mixtures vs sucrose (0.5-20% concentrations) were similar to those of sucrose vs control solutions. These results suggest that 1% corn oil and sucrose at the concentrations tested did not affect each others preference. Chronic intake of 20% sucrose solution, but not 100% corn oil, changed the preference for lower concentrations of each, and both induced remarkable increases in body weight of mice. In conclusion, we showed that mice preferred vegetable oils, and this preference compared favorably with highly palatable sucrose solutions and was not changed by chronic intake of corn oil.

Effects of oil intake in the conditioned place preference test in mice

We investigated the effects of corn oil intake in the conditioned place preference (CPP) test in mice. Voluntary intake of corn oil in the light box in the CPP test showed place preference, but its peroral administration 60 min before conditioning did not show either place preference or aversion. Acquisition of the place preference by corn oil intake was blocked by i.p. injection of SCH 23390 (0.03 mg/kg) and haloperidol (0.1 mg/kg), but not by that of (+/-)-sulpiride (100 mg/kg), (-)-sulpiride (100 mg/kg), and L-741, 626 (1 mg/kg) 15 min before conditioning. These results suggest that stimulation of corn oil in the oral cavity, but not its postingestive effects, have positive reinforcing effects and the stimulation of corn oil is at least partly mediated via dopaminergic systems through the D(1) receptors. Moreover, the present results suggest that the CPP test is useful for the study of preferable stimulation and rewarding effects of food intake.

Effects of oral stimulation with fats on the cephalic phase of pancreatic enzyme secretion in esophagostomized rats

Recent studies suggest that there is a chemical perception of dietary fat in the oral cavity. A much greater response in the cephalic phase of pancreatic secretion is obtained by palatable taste stimuli. To examine the perception of dietary fat in the oral cavity, we determined whether or not oral stimulation with fat alters the cephalic phase of pancreatic enzyme secretion. A cannula was inserted into the common bile duct of rats to collect bile-pancreatic juice, and the animals were esophagostomized to prevent stimulation of the gastrointestinal tract with the test substances. The cephalic phase of pancreatic enzyme secretion was enhanced by oral stimulation with the long chain fatty acids, oleic acid, linoleic acid and linolenic acid, but was unaffected by the middle chain fatty acid, caprylic acid, and long chain fatty acid derivatives, methyl oleate and methyl linoleate. This selectivity is consistent with our previous findings that, in a short term two-bottle preference test, rats prefer long chain fatty acids to triglycerides, middle chain fatty acids or long chain fatty acid derivatives. These results suggest that long chain fatty acids trigger chemical reception in the oral cavity, and the length of the fatty acid chain and the presence or absence of carboxylate groups are related to recognition of fat.

Integration of orosensory and postingestive stimuli for the control of excessive fat intake in mice.

OBJECTIVES Long-term preferences for, and reinforcement effects of, undigested fat substitutes were investigated by means of a two-bottle choice test and the conditioned place preference (CPP) test. METHODS We tested intact corn oil and sorbitol fatty acid esters, which have been developed as non-digestible fat substitutes with low energy (1.5 kcal/g). RESULTS Palatability of the sorbitol fatty acid esters was similar to corn oil over 30 min in the short-term two-bottle choice test in mice. However, mice did not continue to eat the fat substitute in the long-term two-bottle choice test, which included postprandial feedback effects. Moreover, sorbitol fatty acid ester did not act as a reinforcer in the CPP test. Mice with 0.1 mL of corn oil placed into their stomachs just before conditioning showed reinforcing effects on taking sorbitol fatty acid ester in the CPP test. However, intragastric administration of corn oil alone without stimulation of the oral cavity did not show any such reinforcing effects against corn oil. CONCLUSIONS These results suggest that the postingestible effects of corn oil are involved in long-term preference and reinforcing effects.

Opioidergic contribution to conditioned place preference induced by corn oil in mice.

We previously reported that voluntary intake of corn oil in the light box showed place preference in the conditioned place preference (CPP) test in mice. In the present study, we investigated the contribution of opioidergic systems to the corn oil-induced CPP in mice. Acquisition of the place preference by corn oil intake was blocked by i.p. injections of an opioid mu antagonist, naloxone (0.1 and 0.3 mg/kg), and delta antagonists, 7-benzylidenenaltrexone (0.5 mg/kg) and naltriben (0.5 mg/kg) 15 min before conditioning. The opioid kappa agonist U-50488H (1 and 3 mg/kg i.p.) also blocked corn oil-induced CPP. Naloxone (1 mg/kg, i.p.) and naltriben (0.5 mg/kg, i.p.) did not affect corn oil intake in the home cage. However, 7-benzylidenenaltrexone (0.5 mg/kg, i.p.) and U-50488H (1 mg/kg i.p.) decreased and increased the corn oil intake, respectively. These results suggested that the rewarding effects of corn oil in the CPP test are at least partially mediated via opioidergic systems through mu and delta receptors. Further, we showed that an opioid kappa agonist reduced the rewarding effects of corn oil in the CPP test in mice, although it increased corn oil intake.

Long-Term Optional Ingestion of Corn Oil Induces Excessive Caloric Intake and Obesity in Mice

Corn oil is well tolerated by mice but tolerance may decrease with excessive ingestion. In the present study, we compared the effects of optional ingestion of excessive corn oil with ingestion of water (control) or a 20% sucrose solution in mice. During the entire study, mice consistently ingested 100% corn oil and incrementally ingested 20% sucrose. Food intake in the corn-oil group was approximately constant but that in the sucrose group was slightly decreased. Body-weight gains in the corn-oil group were higher than those in the control and sucrose groups. At the end of the study, hepatic hypertrophy and fatty liver were present, especially in the corn-oil group, and the visceral fat of mice fed corn oil increased significantly compared with the other two groups. These results suggest that mice, when given a choice, will continue to overeat corn oil over the long term, inducing excessive caloric intake and obesity.

Grooming behavior in mice induced by stimuli of corn oil in oral cavity.

Mice show a strong preference for corn oil, which was thought to be elicited by stimulation in the oral cavity. Grooming behavior is known to be induced by dopamine D(1) stimulation in rodents. Therefore, we evaluated stimulation by corn oil in the oral cavity and the contribution of D(1) receptors to corn-oil-induced grooming in mice. Intraoral injection (0.1 ml) of corn oil induced grooming behavior similarly to SKF 38393 (10 mg/kg i.p.), a D(1) agonist, and both were antagonized by pretreatment with SCH 23390, a D(1) antagonist. However, a higher dose was needed for antagonism of the corn-oil-induced grooming compared with that induced by SKF 38393. Long-chain fatty acids, their methyl esters and alcohol, their triglycerides, mineral oil and silicone oil but not glycerin, a short-chain triglyceride, xanthan gum solution, or sucrose solution also induced grooming in mice. Xanthan gum solution, which was suggested to mask oil-like texture, attenuated the silicone-oil- but not corn-oil-induced grooming when injected intraorally as a mixture with an equal volume of the oil (50% suspension). The silicone-oil-induced grooming was reduced by SCH 23390 similarly to that induced by corn oil. These results suggested that stimulation by the oil-like texture in the oral cavity in mice induced grooming behavior and that it might be mediated at least partially via D(1) receptors. Moreover, stimuli other than texture might also contribute to the corn-oil-induced grooming.