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Corn oil is reported to elicit a conditioned place preference (CPP) in a CPP test in mice. To further investigate a reinforcing effect of corn oil, we studied whether the corn oil acts as a reinforcer under a progressive ratio (PR) schedule in the operant task. BALB/c mice were trained to lever press for sucrose and corn oil. After reaching a stable break-point for 100% corn oil, the PR test was conducted for various concentrations of corn oil (0%-100%). The reinforcing effect of corn oil was increased in a concentration-dependent manner under the PR schedule. A mineral oil and 0.3% xanthan gum as vehicles did not show any reinforcing effect in the PR test, suggesting that oily and viscous texture was not related to the reinforcing property of corn oil. The break-point for corn oil was attenuated by pretreatment with (-)-sulpiride, a D(2) antagonist, in the PR test. On the other hand, SCH23390, a D(1) antagonist, did not influence the break-point. Furthermore, the pretreatment with (-)-sulpiride or SCH23390 did not influence the intake of corn oil in a one-bottle test for 30 min, suggesting that the dopaminergic system is involved in the reinforcing effect but not the consumption of corn oil in mice. In conclusion, operant response to corn oil is concentration-dependently enhanced under the PR schedule. This reinforcing effect of corn oil is at least partly mediated through the dopaminergic systems via D(2) receptors.

Free fatty acids (FFAs) were reported to be recognized in the oral cavity and possibly involved in fatty foods recognition. To understand the importance of oil recognition in the oral cavity, we investigated the effect of various concentrations of a fatty acid or corn oil on fluid intake as well as mice's preferences in a two-bottle choice test and a licking test. Linoleic acid (LA), which is a main component of corn oil, was used as a representative FFA. In the two-bottle choice test between a pair of different concentrations of corn oil, the mice consistently adopted the higher concentration of corn oil. In the licking test for corn oil, the licking rates for the serial concentration of corn oils (0, 1, 5, 10 and 100%) were increased in a concentration-dependent manner. On the other hand, in the two-bottle test for a pair of different concentrations of LA (0, 0.125, 0.25 and 1%), 0.25% and 1% LA were preferred to mineral oil, but 0.25% and 1% LA were preferred equally in mice. In the licking test for LA, the mice showed the largest number of initial lickings for the 1% LA, while the licking rates for the high concentration of LA decreased. These results suggest that mice could discriminate the concentration of corn oil and LA in the oral cavity. We also suggest that pure corn oil is a highly preferable solution, while an optimal concentration of LA according to the preferences of mice is a low-range concentration (0.25-1%).

We investigated the involvement of opioid receptors such as the mu and delta receptors in the predominant elevation of corn oil appetite just after 5-day repeated treatment of corn oil ingestion. Rats were given 5% corn oil emulsified with 0.3% xanthan gum for 20 min at the same hour for 5 consecutive days. A strong appetite for fat was formed after the 5 days presentation, and it was inhibited by naloxonazine, a selective antagonist of the mu-1 receptor, at doses of 3 mg/kg, but not by antagonists of the opioid delta receptor. In days 6, after the formation of a strong appetite for corn oil, an additional injection of naloxonazine suppressed fat intake 0-30, 30-60, 60-90 and 90-150 min after the presentation of the corn oil, but antagonists of the opioid delta receptor did not. These data suggested that the opioid mu receptor is involved in the sharp elevation of corn oil appetite during repeated presentation of corn oil to rats.

We investigated the gene expression dynamics of several hypothalamic neuropeptides associated with appetite regulation when rats are anticipating being fed a corn-oil emulsion. For 5 days at the same hour each day, rats were fed 5% corn oil emulsified with 0.3% xanthan gum or the vehicle for 20 min. On Day 6, the 5% corn oil emulsion or the vehicle (Vehicle) was presented to the rats, some of which (Oil-intake) were allowed to eat it and some of which (Oil-anticipation) were kept from eating it. Despite waiting a corn-oil, the mRNA levels of proopiomelanocortin (POMC), a beta-endorphin precursor, and orexin showed increases, and high levels of mRNAs of POMC and orexin were maintained for 30 min after the corn-oil was placed before the rats, and only gradually decreased through 150 min. However, the mRNA levels of POMC and orexin in the hypothalamus were decreased within 30 min after starting to ingest the corn-oil emulsion. These results suggest that POMC and orexin mRNA expression was induced by the anticipation in rats after learning the palatability of 5% corn oil emulsion, and the induced mRNA expression based on the anticipation was maintained for at least for 30 min as the rats eagerly waited for ingestion.

Abstract: Dietary oils such as corn oil, olive oil, and canola oil, which primarily contain triacylglycerol and small quantities of fatty acids, are highly palatable to animals. In a previous study, we examined the short-term (60 s) licking behavior of mice and observed that they exhibited a high licking response to a low concentration of fatty acid (linoleic acid), which is comparable to that observed for pure corn oil. This finding suggests that fatty acids contribute to the palatability of dietary oils. In order to supplement our knowledge of the fundamental features of fatty acid palatability in the oral cavity, we assessed the licking behavior of BALB/c mice to investigate the palatability of various types of long-chain fatty acids. The mice showed high licking responses to 1% unsaturated 16- and 18-carbon fatty acids (palmitoleic acid, 16:1; oleic acid, 18:1; linoleic acid, 18:2; and linolenic acid, 18:3), low licking responses to 16- and 20-carbon fatty acids (palmitic acid, 16:0 and arachidonic acid, 20:4), and no significant response to saturated fatty acids (stearic acid, 18:0 and arachidic acid, 20:0) or fatty acid derivatives (methyl linoleate and linole alcohol). Additionally, there were differences in the palatability of 18-carbon unsaturated fatty acids at very low concentrations. At fatty acid concentrations of 0.04% and 0.0625%, the mice showed significant preference for linoleic acid and linolenic acid, but not oleic acid, when compared with mineral oil. These results suggest that mice show high licking responses to 16- and 18-carbon unsaturated long-chain fatty acids at low concentrations. Further, we suggest that sensitivity to fatty acids is affected by the saturated state of the fatty acid, carbon chain length, and terminal carboxyl group. [Copyright &y& Elsevier]

We investigated the effect of β-oxidation inhibition on the fat ingestive behavior of BALB/c mice. Intrapentoneal administration to mice of mercaptoacetate, an inhibitor of fatty acid oxidation, significantly suppressed intake of corn oil but not intake of sucrose solution or laboratory chow. To further examine the effect of mercaptoacetate on the acceptability of corn oil in the oral cavity, we examined short-term licking behavior. Mercaptoacetate significantly and specifically decreased the number of licks of corn oil within a 60-s period but did not affect those of a sucrose solution, a monosodium glutamate solution, or mineral oil. In contrast, the administration of 2-deoxyglucose, an inhibitor of glucose metabolism, did not affect the intake or short-term licking counts of any of the tasted solutions. These findings suggest that fat metabolism is involved in the mechanism underlying the oral acceptance of fat as an energy source. [ABSTRACT FROM AUTHOR]

Abstract: Aims: To determine whether beta-endorphin contributes to the ingestion of and preference for dietary oil, we examined the relationship between the dynamics of beta-endorphin, before and after the ingestion of corn oil, and the intake volume of corn oil. Main methods: Rats were offered 5% corn oil for 20 min for 5 consecutive days so they could acquire a preference for corn oil. On day 6, seven groups of rats were presented with the oil for defined time periods, and we measured the beta-endorphin levels in the serum and cerebrospinal fluid (CSF) before and after the presentation of corn oil as well as the consumed volume of corn oil at defined time points. Key findings: Beta-endorphin levels in serum and CSF were significantly increased 15 min after the ingestion of corn oil, followed by a rapid decrease and maintenance at the basal level throughout the rest of the experimental period. The intake of corn oil was the lowest in the time period of 15–30 min, when the beta-endorphin level reached a peak value. The intake volume of corn oil might be inversely correlated with beta-endorphin levels in serum and CSF. The pretreatment of naloxone, an antagonist of the opioid receptor, decreased the initial licking rate for corn oil and increased the latency for corn oil in the licking test. Significance: The beta-endorphin was rapidly released after oil ingestion, which contributed to the hedonic preference and ingestive behavior for fat. [Copyright &y& Elsevier]