Psychology Graduate Theses
Is the Lewis (LEW) Rat an Appropriate Control for the Spontaneously Hypertensive Rat (SHR)?The Spontaneously Hypertensive rat (SHR), the most widely accepted rodent-model of Attention Deficit/Hyperactivity disorder (ADHD), is compared with its normotensive control the Wistar Kyoto (WKY) rat looking for between strains differences in impulsive choice. But the WKY is not a proper control for the SHR when the procedure requires locomotion to choose. The SHR has deficiencies in dopamine activity in nucleus accumbens causing lower tolerance to delayed outcomes than the WKY. Locomotion and anomalies in dopamine in the Lewis (LEW) rat are like those in the SHR, suggesting that the LEW is a good control for the SHR. This possibility was analyzed with SHRs and LEWs responding to concurrent-chains procedures. Choice was measured in the initial link where two random interval schedules arranged entries to two terminal links, one delivering 1-food pellet immediately and the other delaying 4-food pellets 0.1, 5, 10, 20, 40, or 80 s. Impulsive choice increased with training, but the SHRs showed faster changes in preference, making more impulsive choices than the LEWs. The hyperbolic-decay model and the generalized matching law fitted the data well. Positive correlations between discounting rate and sensitivity of choice to the immediacy of reinforcement suggests compatibility between the models of choice.
Delay Discounting and Polydipsia in Spontaneously Hypertensive and Lewis RatsThe choices made by Spontaneously Hypertensive Rats (SHRs) were compared with those made by the Lewis rats (LEWs) responding to a concurrent-chains procedure varying the delay to the larger later reinforcement (LLR). Impulsive choice was measured in conditions where a bottle of water was or was not available in the choice situation. Both strains produced discounting functions with proportions of choice decreasing with increasing delay to the LLR. At the beginning of training the LEWs made more impulsive choices than the SHR, but late in training both strains produced similar discounting rates suggesting same levels of impulsivity. Sensitivity of choice to the magnitude of the LLR also increased with extended training in the choice situation. Adding the bottle of water to the choice situation did not affect the impulsive choices made by the SHRs and the LEWs, but both strains developed polydipsia indicating that it was induced by food, with the SHRs drinking substantially more water than the LEWs. Licking mostly occurred in blackouts and before starting the choice cycles, showing a tendency to decrease in the initial and terminal links of the concurrent-chains procedure. Licking persisted when the water was removed from the choice situation, but the spout of the bottle was available for the rats to lick, indicating that water was not necessary to maintain licking. Overall, these findings support the laws of allocation, induction, and covariance (Baum, 2018a, 2018b).