Daily alcohol use by males has been shown to increase sexual arousal and decrease sexual inhibition.
In Fruit Flies.
The current (Jan 2) issue of PLoS ONE includes a paper by Lee et al exploring thye physiological side of changes in sexual behavior under the influence of alcohol, with an eye towards understanding this process in humans, using an animal model.
Alcohol has a strong causal relationship with sexual arousal and disinhibited sexual behavior in humans; however, the physiological support for this notion is largely lacking and thus a suitable animal model to address this issue is instrumental. We investigated the effect of ethanol on sexual behavior in Drosophila. Wild-type males typically court females but not males; however, upon daily administration of ethanol, they exhibited active intermale courtship, which represents a novel type of behavioral disinhibition.
First, the researchers acclimated the flies to a “chamber” which was then moved on top of a Petri dish wiht a cotton pad soaked with ethanol. As the flies absorbed the vapor, they became hyperactive, lost motor control, and then passed out flat on their backs. With exposure to 95% ethanol, all the male flies passed out within 24 minutes, and exposure to 70% ethanol took somewhat longer. With repeated exposure, the flies became somewhat habituated to the effects and took longer to pass out, but eventually reached an equilibrium. The evidence shows that eventually the flies became somewhat addicted to the ethanol.
If the flies were of the fruitless mutant variety, males normally court other males, and nothing seemed to change under the effects of alcohol. Males of the white mutant variety showed some increased courtship behavior oriented towards other males. Flies with inhibited dopamine neurotransmission during exposure to alcohol dramatically decreased their interest in other males.
This led the researchers to suspect that these three factors (two genes and one neurotransmitter) played a role in the process of increased inhibition of sexual orientation towards males, in male fruit flies.
Not surprisingly, male’s behavior in relation to females also changed. Exposure to alcohol increased sexual arousal in relation to females, but reduced sexual performance, giving new meaning to the old saying “Hey, baby, let’s get drunk and screw.” (I quickly add: this linguistic link was not explicitly made by the researchers in this paper, but it was clearly implied.)
Here is a graph showing increased intermale courtship by drunken fruit flies:
Caption: (A) The percentage of males engaged in intermale courtship progressively increased upon additional ethanol treatments in 95% Flypub. Least squares regression showed the significant effect of exposure (r2 = 0.68, p<0.0001, n = 10). (B) CS males subjected to daily ethanol exposure in 70% Flypub exhibited the exposure-dependent increase in intermale courtship but at the significantly reduced levels compared to those challenged with 95% ethanol. Two-way ANOVA revealed the significant effects of ethanol concentration, exposure, and interaction (concentration effect, F1,24 = 54.02, p<0.0001; exposure effect, F3,24 = 138.2, p<0.0001; interaction, F3,24 = 6.7, p = 0.0019; n = 4). Post hoc two-tailed Student t-test showed the significant difference of the courtship scores on the 2nd (p<0.005) or 3rd exposure (p<0.0005) (marked by double asterisks). (C) The chronic-ethanol-treated males displayed active courtship toward the decapitated previously-ethanol-naïve males under the influence of ethanol (chronic to naïve), whereas the ethanol-naïve males, on the 1st ethanol exposure, displayed negligible courtship toward the decapitated chronic-ethanol-exposed males (naïve to chronic). Two-tailed Student t-test showed a significant difference (p<0.0001, n = 6, marked by double asterisks). (D) Two or 4 wk-old CS males exhibited the increased levels of intermale courtship compared to 4 d-old males when tested in 95% Flypub. Two-way ANOVA revealed the significant effects of age and exposure, and a marginal interaction (age effect, F2,36 = 16.6, p<0.0001; exposure effect, F3,36 = 61.7, p<0.0001; interaction, F6,36 = 2.58, p = 0.035; n = 4). Tukey-Kramer tests showed that the intermale courtship activities of 2 and 4 wk-old males were significantly different from that of 4 d-old males on the 2nd exposure (marked by double asterisks).
Here is a graph showing increased sexual arousal with decreased sexual performance by drunk male fruit flies:
Caption: (A) Courtship. The wings of CS males or females were clipped to distinguish the sex. The ethanol-naïve or ethanol-treated males were housed with an equal number of virgin females and subjected to ethanol exposure in 95% Flypub. Two-factor ANOVA revealed the significant effects of partner, exposure, and interaction in both sets of experiments (intact male with wing-clipped female: partner effect, F1,27 = 75.8, p<0.0001; exposure effect, F1,27 = 93.9, p<0.0001; interaction, F1,27 = 53.2, p<0.0001; n = 7. Wing-clipped male with intact female: partner effect, F1,27 = 120.8, p<0.0001; exposure effect, F1,27 = 239.5, p<0.0001; interaction, F1,27 = 43.2, p<0.0001; n = 7). Double asterisks, significant difference by planned Student t-tests. (B) Copulation. CS males under the influence of ethanol on the 1st and 6th exposures displayed significantly reduced copulation with virgin females (ANOVA, F2,20 = 47.7, p<0.0001; double asterisk, significant difference by Tukey-Kramer; n = 7).
One of the researchers, Kyung-An Han, comments:
…our study demonstrates that sexual behavior is not determined only during an organism’s development, but it also can be influenced by a post-developmental environmental factor; in this case, recurring exposure to ethanol … These findings represent the first demonstration of enduring behavioral changes induced by recurring ethanol exposure in a fly model.
As a result of our research with the fruit fly, we are now just beginning to discover the molecular and cellular mechanisms underlying neural changes in the brain that result from the chronic use of alcohol and that result in alcohol addiction and other behavior changes in our fly model.
Lee HG, Kim YC, Dunning JS, Han KA (2008) Recurring Ethanol Exposure Induces Disinhibited Courtship in Drosophila. PLoS ONE 3(1): e1391 doi:10.1371/journal.pone.0001391. PLoS is an Open Access journal, so you can view the article here even if you are not special.
There is a press release on this work here.