Research reportEffects of oil intake in the conditioned place preference test in mice
Introduction
Oily and fatty foods are consumed preferably for their good taste and flavor. Recently, excessive energy intake due to over intake of fat is becoming a major health problem in advanced countries. Fat intake has hardly decreased in these countries, although excessive intake of fat inducing obesity is well known as a risk factor relevant to many diseases such as hyper-cholesterolemia, arteriosclerosis, and stroke [2], [21], [23]. This may be evidence of strong preferable stimuli of fat for humans. In rodents, vegetable oil alone was also preferably taken [8], [27], [32]. Moreover, flavor preferences conditioned by fat in rats and human children have been reported, focusing on the contribution of postingestive effects of fat to its rewarding effects [8], [18], [19], [25]. Thus, previous works have revealed that oils and fats are preferred for their postingestive effects. Recently, we found that mice showed strong preferences for vegetable oils in a two-bottle choice test [39]. When the oils were presented to mice for only 10 min, the mice preferred the oils from the first time. Therefore, the oils might release favorable stimuli in the oral cavity. Moreover, possible orosensory detection of fat was reported [11], [12], [26]. In the present study, we investigated the stimuli related to the oil’s rewarding effects.
The conditioned place preference (CPP) test has been used to study the reinforcing (rewarding) effects of addictive drugs [1], [7], [13], [22], [33], [35], [37], [38], [41]. The CPP test measures reinforcing effects based on changes of preference to a certain environment after an association study of preferable stimuli and the environment in rodents. Usually, a shuttle box which consists of light and dark or white and black boxes is used as two different environments. The association study between the environment and stimuli in rodents is performed by conditioning of stimuli. For example, the association study of drug stimuli is carried out by confinement in one of two boxes after drug administration. Preference for the conditioned environment is measured by changes of time spent in each box after conditioning. Reinforcing effects of foods were also measured by the CPP test in which rodents were given the foods in one of the two boxes in conditioning [15], [22], [31], [34]. In the present study, we investigated the reinforcing effects of oil intake in mice in a similar manner. We used an apparatus consisting of light and dark boxes with a connecting zone joining them, and examined the reinforcing effects of diazepam to confirm that our apparatus is able to detect reinforcing effects of an addictive drug. Mice were also examined for locomotion in conditioning to study non-specific effects affecting acquisition of the CPP.
It is well known that the reinforcing effects of many addictive drugs are mediated via dopaminergic systems [1], [7], and that the food reward is reportedly elicited through these systems [15], [34]. Therefore, we evaluated the effects of dopamine (DA) antagonists on acquisition of the CPP.
Section snippets
Animals
Male ddY mice (Japan SLC, Hamamatsu, Japan) were used at approximately 6 weeks of age. The animals were maintained under controlled conditions: a 12/12 h light–dark cycle, with light beginning at 5 a.m., and at a constant temperature of 23±3°C. Food and water were freely available in their home cages. All tests were carried out during the daytime (09:30–17:00).
Apparatus for the CPP test
We developed an automatic measuring system by modification of a system for the light/dark test [17]. The test chamber (Toyo Sangyo,
Results
Time spent in the diazepam-conditioned box was significantly increased by 169.5±69.8 s (n=9, P=0.041). Conditioning by vehicle injection did not induce significant changes in either box (data not shown).
Mice spent more time in the light box on day 10 with corn oil intake in this box in the conditioning sessions, but no change in the control group was observed in either box (Fig. 1). The time spent in the light box was increased by 144.4±41.2 s (n=16, P=0.003) and 20.6±18.5 s (n=11, P=0.292) in
Discussion
To our knowledge, this is the first report which has demonstrated reinforcing effects of oil intake in mice in the CPP test. The CPP test previously revealed reinforcing effects of food [15], [22], [31], [34] as well as several addictive drugs [1], [7], [13], [22], [33], [35], [37], [38], [41]. In the previous studies, food pellets were given to fasted animals, but the oil was taken ad lib in non-deprived mice in the present study. It should be noted that the oil was taken as a positive
Acknowledgements
This research was supported by a grant from the Japan Society for the Promotion of Science (JSPS-RFTF97L00906).
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