Bidirectional enantioselective effects of the GABAB receptor agonist baclofen in two mouse models of excessive ethanol consumption

Abstract

The GABAB receptor agonist baclofen has been studied extensively in preclinical models of alcohol-use disorders, yet results on its efficacy have been uncertain. Racemic baclofen, which is used clinically, can be broken down into separate enantiomers of the drug. Baclofen has been shown to produce enantioselective effects in behavioral assays, including those modeling reflexive and sexual behavior. The current studies sought to characterize the enantioselective effects of baclofen in two separate models of ethanol consumption. The first was a Drinking-in-the-Dark procedure that provides "binge-like" ethanol access to mice by restricting access to a 2-h period, 3 h into the dark cycle. The second was a two-bottle choice procedure that utilized selectively bred High Alcohol Preferring 1 (HAP1) mice to model chronic ethanol access. HAP1 mice are selectively bred to consume pharmacologically relevant amounts of ethanol in a 24-h two-bottle choice paradigm. The results showed that baclofen yields enantioselective effects on ethanol intake in both models, and that these effects are bidirectional. Total ethanol intake was decreased by R(+)-baclofen, while total intake was increased by S(-)-baclofen in the binge-like and chronic drinking models. Whereas overall binge-like saccharin intake was significantly reduced by R(+)-baclofen, chronic intake was not significantly altered. S(-)-baclofen did not significantly alter saccharin intake. Neither enantiomer significantly affected locomotion during binge-like reinforcer consumption. Collectively, these results demonstrate that baclofen produces enantioselective effects on ethanol consumption. More importantly, the modulation of consumption is bidirectional. The opposing enantioselective effects may explain some of the variance seen in published baclofen literature.

Keywords: Baclofen; Drinking-in-the-dark; Ethanol consumption; Rodent model; Selected line.

Figure 1. R(+)- baclofen reduces binge-like reinforcer intake

A) Acquisition of ethanol and saccharin intake. B) The R(+)- baclofen enantiomer reduced total binge-like ethanol intake in B6 mice at the 10 mg/kg dose. C) The effect on intake was specific to the first hour of consumption. D) The 10 mg/kg R(+)- dose also reduced BECs. E) Total binge-like saccharin intake was also reduced at the 10 mg/kg R(+)- baclofen dose. F) This result was significant at both hours. Means ± SEM are depicted. Asterisk (*) indicates different than 0 mg/kg at p < .05. Double asterisk (**) indicates different than 0 mg/kg at p < .01. Group n's = 9-10.

Figure 2. S(-)- baclofen increases binge-like ethanol intake

A) Acquisition of ethanol and saccharin intake. B) The S(-)- baclofen enantiomer increased total binge-like ethanol intake in B6 mice at the 10 mg/kg dose. C) The effect on intake was specific to the second hour of consumption, with the 3 mg/kg and 10 mg/kg dose increasing intake. D) Dose effects on intakes were not reflected in BECs. E) S(-)- baclofen did not alter total saccharin intake. F) S(-)-baclofen did not alter hourly saccharin intake. Mean ± SEM is depicted. Asterisk (*) indicates different than 0 mg/kg at p < .05. Group n's for panels A-C = 12-13. Group n's for panel D and E = 10.

Figure 3. The separate baclofen enantiomers bidirectionally modulated free-choice ethanol intake in HAP1 mice

A) Acquisition of 24-hour access daily ethanol intake. B) Total 3 hour intake on the drug day indicates that R(+)- baclofen reduced ethanol intake whereas S(-)-baclofen increased drug intake compared to a saline injection. C) One-way ANOVAs at each of the three testing hours revealed dose effects at each hour of consumption. D) BEC following ethanol intake was reduced in the R(+)- baclofen group and trended towards an increase in the S(-)- baclofen group compared to control. Mean ± SEM is depicted. Asterisk (*) indicates different than 0 mg/kg at p < .05. Double asterisk (**) indicates different than 0 mg/kg p < .01. Pound sign (#) indicates a trend of p < .06 compared to 0 mg/kg. Group n's = 9-10.

Figure 4. The separate baclofen enantiomers did not significantly modulate free-choice saccharin intake in HAP1 mice

A) Acquisition of 24-hour access daily saccharin intake. B) Total 3 hour intake following the drug injection. Neither drug group was significantly different from the control group. C) Hourly intake for each of the three testing hours following the drug injection showed no significant effects. Mean ± SEM is depicted. Group n's = 9-10.

Figure 5. The separate baclofen enantiomers did not alter home-cage locomotion in B6 mice

A) Home-cage locomotor scores of ethanol drinking animals on Day 4 following a saline injection. B) Home-cage locomotor scores of ethanol drinking animals on Day 5 following drug injection. C) Home-cage locomotor scores of saccharin drinking animals on Day 4 following saline injection. D) Home-cage locomotor scores of saccharin drinking animals on Day 5 following drug injections. For both reinforcers, there was a main effect of day and time. Locomotion was lower in both groups on Day 5 (panels B and D). Mean ± SEM is depicted. Group n's = 7-8.