Delay Discounting and Polydipsia in Spontaneously Hypertensive and Lewis Rats

Abstract

The 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).