Identifying profiles of recovery from reward devaluation in rats

Abstract

In humans and other mammals, the unexpected loss of a resource can lead to emotional conflict. Consummatory successive negative contrast (cSNC) is a laboratory model of reward devaluation meant to capture that conflict. In this paradigm, animals are exposed to a sharp reduction in the sucrose concentration of a solution after several days of access. This downshift in sucrose content leads to behavioral responses such as the suppression of consumption and physiologic responses including elevation of corticosterone levels. However, response heterogeneity in cSNC has yet to be explored and may be relevant for increasing the validity of this model, as humans demonstrate clinically meaningful heterogeneity in response to resource loss. The current analysis applied latent growth mixture modeling to test for and characterize heterogeneity in recovery from cSNC among rats (N=262). Although most animals exhibited recovery of consummatory behavior after a sharp drop in consumption in the first postshift trial (Recovery class; 83%), two additional classes were identified including animals that did not change their consumption levels after downshift (No Contrast class; 6%), and animals that exhibited an initial response similar to that of the Recovery class but did not recover to preshift consumption levels (No Recovery class; 11%). These results indicate heterogeneity in recovery from reward loss among rats, which may increase the translatability of this animal model to understand diverse responses to loss among humans.

Keywords: Animal models; Consummatory successive negative contrast; Individual differences; Latent growth mixture modeling; Reward loss.

Figure 1

Consummatory performance measure in terms of cumulative contact with the sipper tube (goal-tracking time, in seconds) in two groups of rats exposed to a 32-to-4% sucrose downshift (32-4) or to unshifted 4% sucrose (4-4). The cSNC effect is illustrated by the difference in consummatory behavior during Trials 11-15, when animals in both groups receive access to 4% sucrose, but differ in terms of prior reward history.

Figure 2

Goal-tracking time (s) during each postshift trial (4% sucrose) was subtracted from the goal-tracking time (s) of each animal in its last preshift trial (32% sucrose) in downshifted animals from the three classes of recovery identified. The classes (percentage of total sample) were labeled according to their postshift performance: Recovery (83%), No Recovery (11%), and No Contrast (6%). The sample size for each class was 217, 30, and 15 animals, respectively. Change scores are estimated means derived from the latent growth mixture model.

Figure 3

Three-class solution of mean goal-tracking time (s) for 10 preshift trials and 5 postshift trials in downshifted animals (32-4) and unshifted controls (4-4). The classes were labeled according to their postshift performance: Recovery, No Recovery, and No Contrast. The performance of unshifted controls exposed always to 4% sucrose (4-4) was added as reference.