Differential rearing alters taste reactivity to ethanol, sucrose, and quinine

Abstract

Rationale: Early-life environment influences reinforcer and drug motivation in adulthood; however, the impact on specific components of motivation, including hedonic value ("liking"), remains unknown.

Objectives: The current study determined whether differential rearing alters liking and aversive responding to ethanol, sucrose, and quinine in an ethanol-naïve rat model.

Methods: Male and female rats were reared for 30 days starting at postnatal day 21 in either an enriched (EC), isolated (IC), or standard condition (SC). Thereafter, all rats had indwelling intraoral fistulae implanted and their taste reactivity to water, ethanol (5, 10, 20, 30, 40% v/v), sucrose (0.1, 0.25, 0.5 M), and quinine (0.1, 0.5 mM) was recorded and analyzed.

Results: EC rats had higher amounts of liking responses to ethanol, sucrose, and quinine and higher amounts of aversive responses to ethanol and quinine compared to IC rats. While EC and IC rats' responses were different from each other, they both tended to be similar to SCs, who fell in between the EC and IC groups.

Conclusions: These results suggest that environmental enrichment may enhance sensitivity to a variety of tastants, thereby enhancing liking, while isolation may dull sensitivity, thereby dulling liking. Altogether, the evidence suggests that isolated rats have a shift in the allostatic set-point which may, in part, drive increased responding for a variety of rewards including ethanol and sucrose. Enriched rats have enhanced liking of both sucrose and ethanol suggesting that enrichment may offer a unique phenotype with divergent preferences for incentive motivation.

Keywords: Differential rearing; Enrichment; Environment; Ethanol; Hedonic value; Motivation; Post-weaning social isolation; Quinine; Sucrose; Taste reactivity.

Fig. 1

A representative example of a substance presentation for an individual animal on an individual trial with members of the other groups yoked to the same presentation order. Please refer to the methods section for more details concerning the counterbalancing.

Fig. 2

The interior of the taste reactivity chamber from the perspective of the video camera.

Fig. 3

Mean ± S.E.M. a Total hedonic and c total aversive taste reactivity responses to ethanol (5–40% v/v) between enriched (EC), isolated (IC), and standard condition (SC) rats. Individual-level data and trend lines from the linear model for rats in each group are shown separate from mean data for ease of data illustration for both b total hedonic and d total aversive taste reactivity responses to ethanol. Hedonic responses (a, b): EC rats had significantly more hedonic responses to alcohol than IC rats (***p < .001). ECs also had a significantly greater rate of decline in hedonic responses compared to SCs as ethanol concentration increased (^bp = .005). Aversive responses (c, d): As alcohol concentration increased, aversive responses increased (^###p < .001). ECs had significantly greater aversive taste responses to ethanol compared to ICs (*p < .05).

Fig. 4

Mean ± S.E.M. a Total hedonic and c total aversive taste reactivity responses to sucrose (0.1–0.5 M) between enriched (EC), isolated (IC), and standard condition (SC) rats. Individual-level data and trend lines from the linear model for rats in each group are shown separate from mean data for ease of data illustration for both b total hedonic and d total aversive taste reactivity responses to sucrose. Hedonic responses (a, b): As sucrose concentration increased, hedonic responses increased (^##p < .01). ECs had significantly more hedonic responses to sucrose than ICs (**p < .01). Aversive responses (c, d): Aversive responding to sucrose was low, no significant differences were observed.

Fig. 5

Mean ± S.E.M. a Total hedonic and c total aversive taste reactivity responses to quinine (0.1–0.5 mM) between enriched (EC), isolated (IC), and standard condition (SC) rats. Individual-level data and trend lines from the linear model for rats in each group are shown separate from mean data for ease of data illustration for both b total hedonic and d total aversive taste reactivity responses to quinine. Hedonic responses (a, b): As quinine concentration increased, hedonic responses decreased (^#p < .05). ECs had significantly more hedonic responses to quinine than ICs (**p < .01). Aversive responses (c, d): As quinine concentration increased, aversive responses increased (^#p < .05). ECs had significantly greater aversive taste responses to quinine compared to ICs (**p < .01).

Fig. 6

Mean ± S.E.M. a Total hedonic and c total aversive taste reactivity responses to water between enriched (EC), isolated (IC), and standard condition (SC) rats during the initial (water 1) and final (water 2) water sessions. Individual-level data for rats in each group and for each session are shown separate from mean data for ease of data illustration for both b total hedonic and d total aversive taste reactivity responses to water. Hedonic responses (a, b): ECs and SCs significantly increased hedonic responding from the initial to the final water session (^†p < .017) and showed greater responding during the final water session compared to ICs (*p < .017). Aversive responses (c, d): Aversive responses increased from the initial to the final water session (^###p < .001).