Anxiolytic-like Effects of the Positive GABAB Receptor Modulator GS39783 Correlate with Mice's Individual Basal Anxiety and Stress Reactivity

Abstract

Positive gamma-aminobutyric acid type B (GABA_B) receptor modulators such as GS39783 have showed anxiolytic-like effects in several studies while such effects were absent in other studies. These conflicting findings led us hypothesize that the anxiolytic-like effects of such compounds depend on the individual basal anxiety and/or the anxiogenic properties of the used tests. The present study addresses this hypothesis by testing GS39783 effects on mice's anxiety-like behavior in a light-dark box. We found that GS39783 had no effects on a whole-group level. However, after grouping the mice for their basal anxiety, GS39783 reduced anxiety-like behavior in the subgroup with highest basal anxiety. Moreover, GS39783 effects correlated with individual basal anxiety. Next, the anxiogenic properties of the light-dark box test were increased by prior stress exposure. Again, GS39783 was not effective on a whole-group level. However, GS39783 had an anxiolytic-like effect in the most stress-responsive subgroup. Moreover, GS39783 effects correlated with individual stress responsiveness. Finally, we show that GS39783 brain levels were within a behaviorally relevant range. Overall, our study demonstrates that GS39783 effects depend on individual basal anxiety and stress responsiveness. This suggests that anxiety tests should generally be designed to capture individual basal anxiety and/or stress responsiveness as well as individual compound effects.

Keywords: GS39783; anxiety; gamma-aminobutyric acid type B receptor; light–dark box; stress.

Figure 5

Timelines of the different experiments. The squares with the mouse symbolize the light–dark box test (10 min test duration), the syringes symbolize the intraperitoneal injections, the red drops symbolize the blood collections by tail cutting (in experiment 4: trunk blood), and the flashes symbolize the stress by exposure to electric stimuli. Abbreviations: D, day; LC-MS, liquid chromatography-mass spectrometry.

Figure 1

Diagrams depicting anxiety-like behavior in the light–dark box during repeated tests. There were no effects of test day, neither when all mice were analyzed together (A), nor when mice were grouped in anxious and non-anxious animals based on their behavior in the first test (B). ** p < 0.01, * p < 0.05, comparisons as indicated. Numbers in the bars represent group sizes for males (top) and females (bottom). Y-axis scale and units in panel (B) are the same as in panel (A).

Figure 2

Diagrams showing the effects of 10 and 30 mg/kg GS39783 on the percent time spent in the bright area of the light–dark box. There were no effects of GS39783 if all animals were analyzed together (A). For further analysis, mice were then grouped based on their basal anxiety, i.e., their behavior after vehicle (B). If mice were grouped for basal anxiety, 30 mg/kg GS39783 had anxiolytic-like effects in the mice of the most anxious group but not in the average and non-anxious groups (C). Furthermore, the individual effects of 30 mg/kg GS39783 were correlated with the behavior after vehicle (D). * p < 0.05, comparisons as indicated. Sexes were pooled since there were no effects of sex (numbers in the bars represent group sizes for males (top) and females (bottom)). Y-axis scale and units in panels (B,C) are the same as in panel (A).

Figure 3

Diagrams showing the effects of 10 and 30 mg/kg GS39783 on the percent time spent in the bright area of the light–dark box and on corticosterone plasma levels 10 days after stress by exposure to electric stimuli. There were no main effects of GS39783 treatment (A). However, the change of light–dark box behavior after stress was very variable (B). Therefore, mice were grouped according to their stress response (C). In the high stress-responsive group, GS39783 treatment significantly increased percent time spent in the bright area of the light–dark box. GS39783 treatment had no effects in the groups with average or low stress responsiveness. Of note, individual stress responsiveness was correlated with the effects of 30 mg/kg GS39783 (D). Corticosterone plasma levels were increased after the stress as well as after the light–dark box tests but were not affected by stress responsiveness and treatment (E). ## p < 0.01, * p < 0.05, comparisons as indicated. Sexes were pooled since there were no effects of sex (numbers in the bars represent group sizes for males (top) and females (bottom)).

Figure 4

Extracted ion chromatograms of mass-to-charge ratio (m/z) 338.1640 corresponding to [M+H]⁺ ion of GS39783 in positive mode electrospray (ESI) from (A) mouse brain extract and (B) plasma sample. Peak areas were used for quantitative purposes. Note the values measured in the peak apex (m/z 338.1647 and m/z 338.1644).