Differential involvement of endogenous opioids in sucrose consumption and food reinforcement

Abstract

Endogenous opioids within the central nervous system are postulated to mediate hedonic aspects of feeding behavior. To identify the relevant endogenous opioid receptor ligands, mice lacking one or two of the opioid peptide families beta-endorphin, enkephalins or dynorphins were tested for sucrose preference in a two-bottle free-choice drinking paradigm under drug-naïve conditions and following treatment with an opioid antagonist (1 mg/kg naloxone i.p.) or saline. Basal sucrose consumption was unaltered in all of the knockout genotypes compared to their congenic wild-type C57BL/6 littermates during 0.5 and 6 h access to a bottle containing 2, 4, 8, or 16% sucrose and a second bottle containing water. Moreover, all mutant genotypes and wildtype mice exhibited a similar compensatory decrease in overnight food intake following the extra caloric load from 6 h sucrose access. Although these basal responses to sucrose were unaffected by the knockout genotypes, naloxone reduced sucrose consumption by 50% compared to saline treatment during the first 0.5 h in wild-type and beta-endorphin knockout mice, but had no effect in enkephalin knockouts, beta-endorphin and enkephalin double knockouts, or dynorphin knockouts. These data suggest that naloxone reduces sucrose consumption in wild-type mice by blocking endogenous enkephalin and/or dynorphin signaling, but not beta-endorphin. Dynorphin knockouts in the current study had bar-pressing responses for a palatable food reinforcer in an operant procedure under free-feeding conditions similar to wild-type mice while we found in a previous study that beta-endorphin and enkephalin knockout mice had reduced motivation to respond [Hayward MD, Pintar JE, Low MJ. Selective reward deficit in mice lacking beta-endorphin and enkephalin. J Neurosci 2002;22:8251-8258.]. We conclude from these studies directly comparing three strains of opioid peptide knockout mice that enkephalin and dynorphin can modulate sucrose preference but are not necessary to support sucrose consumption. However, dynorphin was not necessary to support wildtype levels of operant responding suggesting that only enkephalin and beta-endorphin modulate conditioned food reinforcement.

Figure 1

Two-bottle free-choice drinking for four sucrose concentrations by opioid peptide knockout mice, normalized for body weight. (A.) The volume of sucrose consumed/kg body weight (means ± SEM) after 0.5 h of access by Enk^+/+End^+/+, Enk^+/+End^−/−, Enk^−/− End^+/+, and Enk^−/− End^−/− mice did not differ among genotypes across four concentrations of sucrose. (B.) The volume of sucrose consumed/kg body weight (means ± SEM) after 6 h of access by Enk^+/+End^+/+, Enk^+/+End^−/−, Enk^−/− End^+/+, and Enk^−/− End^−/− mice did not differ among genotypes across four concentrations of sucrose. (C.) The volume of sucrose consumed/kg body weight (means ± SEM) after 0.5 h of access by Dyn^+/+ and Dyn^−/− mice did not differ between genotypes across four concentrations of sucrose. (D.) The volume of sucrose consumed/kg body weight (means ± SEM) after 6 h of access by Dyn^+/+ and Dyn^−/− mice did not differ between genotypes across four concentrations of sucrose.

Figure 2

The influence of naloxone on two-bottle free-choice drinking for four sucrose concentrations by opioid peptide knockout mice normalized for body weight. Mice were treated with an i.p. injection of saline or 1 mg/kg of naloxone and the volume of sucrose consumed after 0.5 h was measured. All data are expressed as mean volume/kg body weight (± SEM). (A.) Naloxone significantly reduced consumption of 8% and 16% sucrose by Enk^+/+End^+/+ mice. (B.) Naloxone significantly reduced 4%, 8%, and 16% sucrose consumption by Enk^+/+End^−/− mice. (C.) Naloxone did not reduce sucrose consumption at any concentration in Enk^−/− End^+/+mice. (D.) Naloxone did not reduce sucrose consumption at any concentration in Enk^−/− End^−/− mice. (E.) Naloxone significantly reduced 16% sucrose consumption by Dyn^+/+ mice. (F.) Naloxone did not significantly reduce sucrose consumption at any concentration in Dyn^−/− mice. * P<0.05 by Fisher’s PLSD post hoc test on drug treatment effect at one sucrose concentration.

Figure 3

Body weight of the opioid peptide knockout mice in the two-bottle free-choice tests. Mice were weighed at the beginning of the study and mean body weights are given (± SEM). (A.) Female and Male Enk^+/+End^−/− and Enk^−/− End^−/− were heavier than their respective wildtypes. *P<0.05 by Fisher’s PLSD post hoc compared to the Enk^+/+End^+/+. (B.) Female and Male Dyn^+/+ and Dyn^−/− body weights did not differ between genotypes but differed between sex.

Figure 4

Total daily caloric intake during the two-bottle free-choice tests. Total calories from food and sucrose were calculated and expressed as mean kilocalories ± SEM for each 24 h period. All genotypes had similar daily caloric intake during the testing (A.) Enk^+/+End^+/+, Enk^+/+End^−/−, Enk^−/− End^+/+, and Enk^−/− End^−/− mice. (B.) Dyn^+/+ and Dyn^−/− mice.

Figure 5

Operant responding for food reinforcers by opioid knockout mice. (A.) Dyn^+/+ and Dyn^−/− mice were trained to respond under an FR-1 for food pellets for seven days while maintained under restricted access to food in their home cage. Data are expressed as mean number of reinforcers earned (±SEM). Both sexes and genotypes learned to bar press to receive a food pellet at the same rate. (B.) After the completion of training Dyn^+/+ and Dyn^−/− mice were maintained under ad lib access to food and water in their home cage and tested daily for 10 d under a PR3 schedule of reinforcement. Data are expressed as mean breakpoint across all 10 d (±SEM). Breakpoints under this schedule were not significantly different between genotypes or sexes.