The genetic relationships between ethanol preference, acute ethanol sensitivity, and ethanol tolerance in Drosophila melanogaster

Abstract

The relationship between alcohol consumption, sensitivity, and tolerance is an important question that has been addressed in humans and rodent models. Studies have shown that alcohol consumption and risk of abuse may correlate with (1) increased sensitivity to the stimulant effects of alcohol, (2) decreased sensitivity to the depressant effects of alcohol, and (3) increased alcohol tolerance. However, many conflicting results have been observed. To complement these studies, we utilized a different organism and approach to analyze the relationship between ethanol consumption and other ethanol responses. Using a set of 20 Drosophila melanogaster mutants that were isolated for altered ethanol sensitivity, we measured ethanol-induced hyperactivity, ethanol sedation, sedation tolerance, and ethanol consumption preference. Ethanol preference showed a strong positive correlation with ethanol tolerance, consistent with some rodent and human studies, but not with ethanol hyperactivity or sedation. No pairwise correlations were observed between ethanol hyperactivity, sedation, and tolerance. The evolutionary conservation of the relationship between tolerance and ethanol consumption in flies, rodents, and humans indicates that there are fundamental biological mechanisms linking specific ethanol responses.

Figure 1

Ethanol consumption preference of 20 Drosophila ethanol sensitivity mutants. (A) Ethanol preference of the control strain and mutant 6-6 in a 3-day assay. Line 6-6 showed decreased preference compared with the control on day 3 (p < 0.05, t-test, n = 18). (B) Ethanol PI on day 3 for each mutant tested (n = 18 for mutants, n = 36 for control).

Figure 2

Sensitivity to ethanol-induced hyperactivity. (A) Exposure to a moderate concentration of 47% ethanol vapor causes locomotor hyperactivity in flies. Line 8–222 exhibited decreased maximum ethanol hyperactivity and line 9–91 exhibited increased maximum hyperactivity compared with the control (p < 0.001, t-tests, n = 10). (B) Maximum ethanol-induced hyperactivity of each mutant (n = 10 for mutants, n = 20 for control).

Figure 3

Sensitivity to ethanol-induced sedation in naive flies. (A) Flies exhibit sedation during exposure to a high concentration of ethanol vapor (67%). Line 2–67 exhibited decreased sedation sensitivity (p < 0.001) and line 5–10 exhibited increased sedation sensitivity (p < 0.001) compared with the control (t-tests comparing ST50 values, n = 8–9). (B) ST50 of each mutant (n = 8–13 for mutants, n = 16 for control).

Figure 4

Tolerance to ethanol-induced sedation. (A) Flies given two exposures to 73% ethanol vapor become less sensitive to sedation during the second exposure (dotted lines, open symbols) as compared to the first exposure (solid lines, filled symbols), reflecting tolerance. Two tolerance mutants are shown: both 5–21 and 2–10 exhibited sedation sensitivity similar to the control during the first exposure (p > 0.05 for each mutant vs. control ST25, t-tests), but during the second exposure respectively showed increased sensitivity (=decreased tolerance) or decreased sensitivity (=increased tolerance) relative to the control (p < 0.01 for each mutant vs. control ST25, t-tests). (B) Ethanol sedation tolerance for each mutant (n = 5–7 for mutants, n = 18 for control).

Figure 5

Ethanol pharmacokinetics of the mutants. Flies were exposed to 47% ethanol vapor for 15 min and their internal ethanol concentration was measured (n = 3).

Figure 6

Ethanol consumption preference correlates positively with ethanol tolerance. (A) Ethanol preference on day 3 correlated positively with ethanol tolerance (r = 0.664, p < 0.01, Pearson's correlation, n = 18). (B) Average ethanol preference across all three days of the preference assay correlated positively with ethanol tolerance (r = 0.707, p = 0.001, Pearson's correlation, n = 18).