Failure to Find Ethanol-Induced Conditioned Taste Aversion in Honey Bees (Apis mellifera L.)

Abstract

Background: Conditioned taste aversion (CTA) learning is a highly specialized form of conditioning found across taxa that leads to avoidance of an initially neutral stimulus, such as taste or odor, that is associated with, but is not the cause of, a detrimental health condition. This study examines if honey bees (Apis mellifera L.) develop ethanol (EtOH)-induced CTA.

Methods: Restrained bees were first administered a sucrose solution that was cinnamon scented, lavender scented, or unscented, and contained either 0, 2.5, 5, 10, or 20% EtOH. Then, 30 minutes later, we used a proboscis extension response (PER) conditioning procedure where the bees were taught to associate either cinnamon odor, lavender odor, or an air-puff with repeated sucrose feedings. For some bees, the odor of the previously consumed EtOH solution was the same as the odor associated with sucrose in the conditioning procedure. If bees are able to learn EtOH-induced CTA, they should show an immediate low level of response to odors previously associated with EtOH.

Results: We found that bees did not develop CTA despite the substantial inhibitory and aversive effects EtOH has on behavior. Instead, bees receiving a conditioning odor that was previously associated with EtOH showed an immediate high level of response. While this demonstrates bees are capable of one-trial learning common to CTA experiments, this high level of response is the opposite of what would occur if the bees developed a CTA. Responding on subsequent trials also showed a general inhibitory effect of EtOH. Finally, we found that consumption of cinnamon extract reduced the effects of EtOH.

Conclusions: The honey bees' lack of learned avoidance to EtOH mirrors that seen in human alcoholism. These findings demonstrate the usefulness of honey bees as an insect model for EtOH consumption.

Keywords: Alcohol; Cinnamon; Conditioned Taste Aversion; Honey Bee; Taste Aversion Learning.

Figure 1

Diagram of the experimental methods. Subjects first entered a pre-conditioning procedure and consumed a sucrose solution scented with the pre-conditioning stimulus (PS) that contained 0% to 20% ethanol. After a 30-minute delay, subjects entered a 12-trial conditioning procedure. During the conditioning procedure subjects received a conditioned stimulus (CS) odor, followed by a sucrose unconditioned stimulus (US) feeding. Combinations of PS, dose and CS lead to the four groups of the experiment.

Figure 2

Percent of subjects in the same-stimulus group responding to the unconditioned and conditioned stimuli. The odor the bees received as a conditioned stimulus during the conditioning trials was the same odor that they received in the pre-conditioning solution. The key shows the percent ethanol dose the bees received in the 10 µl 2 M sucrose solution during the pre-conditioning procedure.

Figure 3

Percent of subjects in the different-stimulus group responding to the unconditioned and conditioned stimuli. The odor the bees received as a conditioned stimulus during the conditioning trials was a different odor than they received in the pre-conditioning solution. The key shows the percent ethanol dose the bees received in the 10 µl 2 M sucrose solution during the pre-conditioning procedure.

Figure 4

Percent of subjects in the air-control group responding to the unconditioned and conditioned stimuli. For these bees, an unscented air-puff functioned as the conditioned stimulus during the conditioning trials. The key shows the percent ethanol dose the bees received in the 10 µl 2 M sucrose solution during the pre-conditioning procedure.

Figure 5

Regression lines of probability of unconditioned response, for all groups combined, as a function of pre-conditioning stimulus and ethanol dose. The key shows the percent ethanol dose the bees received in the 10 µl 2 M sucrose solution during the pre-conditioning procedure.