Individual variation in the propensity to attribute incentive salience to a food cue: influence of sex

Abstract

There is considerable individual variation in the propensity of animals to attribute incentive salience to discrete reward cues, but to date most of this research has been conducted in male rats. The purpose of this study was to determine whether sex influences the propensity to attribute incentive salience to a food cue, using rats from two different outbred strains (Sprague-Dawley [SD] and Heterogeneous Stock [HS]). The motivational value of a food cue was assessed in two ways: (i) by the ability of the cue to elicit approach toward it and (ii) by its ability to act as a conditioned reinforcer. We found that female SD rats acquired Pavlovian conditioned approach behavior slightly faster than males, but no sex difference was detected in HS rats, and neither strain showed a sex difference in asymptotic performance of approach behavior. Moreover, female approach behavior did not differ across estrous cycle. Compared to males, females made more active responses during the test for conditioned reinforcement, although they made more inactive responses as well. We conclude that although there are small sex differences in performance on these tasks, these are probably not due to a notable sex difference in the propensity to attribute incentive salience to a food cue.

Keywords: Conditioned reinforcement; Goal-tracker; Pavlovian conditioned approach; Sex differences; Sign-tracker.

Figure 1. Pavlovian conditioned approach index scores in male and female SD and HS rats

PCA index scores (group mean ± SEM) across 8 daily sessions of training and mean PCA index (± SEM) score over the last two sessions of training subdivided by sex (female, white; male, black) and strain: (A, B) Sprague Dawley (n = 48/ sex) and (C,D) Heterogeneous Stock (female, n = 24; male, n = 25). * indicates significant difference between sexes during training session (p < 0.05).

Figure 2. Individual variation in Pavlovian conditioned approach behavior

PCA index scores (−1.0 to +1.0) for every individual rat, averaged from the last two sessions of Pavlovian training. Sprague Dawley (SD) (A) females (n = 48) and (B) males (n = 48), and Heterogeneous Stock (HS) (C) females (n = 24) and (D) males (n = 25).

Figure 3. Lever (CS)-directed behavior (sign-tracking) during Pavlovian training in Sprague Dawley rats designated as sign-trackers

Group means ± SEM for Sprague Dawley female sign-trackers (white, PCA index score between 0 and 1.0, n = 44) and male sign-trackers (black, PCA index score between 0 and 1.0, n = 44) for: (A) probability of approaching the lever (CS) during 8 s CS period, (B) number of lever contacts, and (C) latency to first lever contact after CS presentation. For all measures, there was a significant effect of session (p < 0.001). * indicates significant difference between males and females during training session (p < 0.05).

Figure 4. Lever (CS)-directed behavior (sign-tracking) vs. food magazine-directed behavior (goal-tracking) during Pavlovian training in Heterogeneous Stock rats

Group means ± SEM for HS female sign-trackers (white circles, PCA index score between 0 and 1.0, n = 16) and goal-trackers (white squares, PCA index between −1.0 and 0, n = 8), and male sign-trackers (black circles, PCA index score between 0 and 1.0, n = 14) and goal-trackers (black squares, PCA index between −1.0 and 0, n = 11) for: (A) probability of approaching the lever (CS) during 8 s CS period, (B) number of lever contacts, (C) latency to first lever contact after CS presentation, (D) probability of approaching the food magazine during the 8 s CS period, (E) number of head entries into the food magazine during the 8 s CS period, and (F) latency to first head entry into the food magazine entry after CS presentation. There was no main effect of sex or a significant sex x session interaction, for any measure in the acquisition of either ST or GT CRs.

Figure 5. Pavlovian conditioned approach index scores within a subset of Sprague Dawley females throughout estrous cycle

PCA index scores (group mean + SEM) from training sessions 5 through 8 re-ordered based on estrus cycle phase (n = 28).

Figure 6. Test for conditioned reinforcement

Group means + SEM for active and inactive responses by females (white) and males (black) within strains: (A) Sprague Dawley (n = 32/ sex) and (B) Heterogeneous Stock (female, n = 24; male, n = 25 ). * indicates significant difference compared to females within active or inactive responses (p < 0.05).