1. What idea are the authors testing?
It has been previously shown that D2R (dopamine receptors) in the striatum have been decreased levels of expression in obese rats and humans. The researchers have hypothesized that obese subjects tend to compensate for the hyposensitivity in the reward circuit by increasing the intake of high-energy foods, which raises dopamine levels in the brain, and contributes largely to obesity. Thus, the researchers will test for the effects of extended consumption of high-energy foods and the addictive behavior of this food consumption in rats.
2. What are their one or two principal experimental methods?
The authors tested the rats by a brain-stimulation reward (BSR) procedure where they were trained for 10-14 days to run on a cycle that shocks the brain of the rat which rewards them for the behavior. The BSR sets the reward threshold, which is the level of stimulation at which the rats will continue to run, and this level remains stable at baseline conditions. Then, the rats (which measure 300-350g) were divided into 3 groups, one with only lab chow access, one with lab chow access and limited (1 hr) access to palatable energy-dense foods, and the third with lab chow access and extended (18-23 hr) access. The test ran for 40 days measuring their weight and BSR reward threshold. Afterwards, they were killed and brains were sliced and immunoblotted to test for D2R density.
In continuation with the experiment, the researchers injected rats with lentiviral vector to deliver a short hairpin interfering RNA in the striatum of each brain hemisphere to knock down D2R. Daily BSR threshold assessment continued for 33 days after virus injections to ensure maximal striatal D2R knockdown. Another group of rats were injected with empty lentivirus vector as a control group. These two groups were divided as previously with chow only, restricted, and extended access, and were tested for BSR and reward threshold....