Effects of cocaine on honey bee dance behaviour

Abstract

The role of cocaine as an addictive drug of abuse in human society is hard to reconcile with its ecological role as a natural insecticide and plant-protective compound, preventing herbivory of coca plants (Erythroxylum spp.). This paradox is often explained by proposing a fundamental difference in mammalian and invertebrate responses to cocaine, but here we show effects of cocaine on honey bees (Apis mellifera L.) that parallel human responses. Forager honey bees perform symbolic dances to advertise the location and value of floral resources to their nest mates. Treatment with a low dose of cocaine increased the likelihood and rate of bees dancing after foraging but did not otherwise increase locomotor activity. This is consistent with cocaine causing forager bees to overestimate the value of the floral resources they collected. Further, cessation of chronic cocaine treatment caused a withdrawal-like response. These similarities likely occur because in both insects and mammals the biogenic amine neuromodulator systems disrupted by cocaine perform similar roles as modulators of reward and motor systems. Given these analogous responses to cocaine in insects and mammals, we propose an alternative solution to the paradox of cocaine reinforcement. Ecologically, cocaine is an effective plant defence compound via disruption of herbivore motor control but, because the neurochemical systems targeted by cocaine also modulate reward processing, the reinforcing properties of cocaine occur as a ;side effect'.

Fig. 1.

Effect of cocaine (COC) on the likelihood and rate of round dancing by bees returning from a 1.5 mol l^–1 sucrose feeder. Bees were treated with one of three doses of cocaine or octopamine (OA) dissolved in DMF at the feeder. DMF- and sham-treated bees served as controls. N>34 bees per group. Differences between treatment groups were tested with Kruskal-Wallis tests, and comparisons between specific groups were performed using Dunn's post hoc tests. Groups that did not differ (Dunn's post hoc test, P>0.05) are marked by the same letter above the bars. (A) Likelihood of each bee dancing, calculated as the proportion of visits to the feeder that resulted in a dance during the 50-min observation period. Bars represent means and standard error calculated from arcsin-transformed values and consequently are not always symmetrical around the mean. (B) Number of dance circuits per minute. Bars represent means and standard error. Since our statistical comparisons used nonparametric methods, the standard error provides an indication of the distribution of the data but is less indicative of calculated significant differences between groups.

Fig. 2.

Effect of mianserin (MI) on the stimulation of dancing by cocaine (COC). Dance likelihood (A) and dance rate (B) of bees treated with MI and COC for a 1.5 mol l^–1 sucrose feeder were compared with dances of bees treated with COC alone and DMF- and sham-treated controls (N>39 bees per group). Bar plots, analyses and statistical notation as in Fig. 1.

Fig. 3.

Effect of cocaine on locomotion. The movement index measured the amount that DMF- (white bars) and cocaine-treated bees (black bars) moved in a Petri dish during 5 min observation periods 30 and 60 min post-treatment. Bars show mean and standard error. N=20 for each group. Data were analysed with two-way ANOVA. There was no significant effect of drug (F=0.09, P=0.7600) or time (F=0.04, P=0.8418) and no significant interaction between these factors (F=0.83, P=0.3694).

Fig. 4.

Effect of cocaine (COC) on sucrose responsiveness. 20 μl drops of seven different sucrose solutions ranging in concentration from 0.1% sucrose to 60% sucrose were presented to the antennae of harnessed bees in the laboratory. The first sugar solution (1–7) eliciting proboscis extension was the sucrose sensitivity index; therefore a lower value indicates higher sucrose sensitivity. Bars show means and 95% confidence intervals. Differences between treatment groups were tested with Kruskal-Wallis tests (median sucrose sensitivity index: 3 μg cocaine, 3; DMF, 4; untreated, 5. Kruskal-Wallis statistic 24.17, d.f.=2, P<0.001) and comparisons between specific groups were performed using Dunn's post hoc tests. Bars that do not differ significantly (Dunns post hoc test, P>0.05) are marked by the same letter above the bar. Sample size shown in bars.

Fig. 5.

Effect of cocaine (COC) withdrawal on learning. Relative proportion of bees responding correctly in a proboscis extension response (PER) learning assay 20 h after three trials of training. Prior to training, bees were orally treated for 6 days with 10.54 mmol l^–1 octopamine (OA) or 0.66 mmol l^–1 cocaine (COC) in ad libitum 1.5 mol l^–1 sucrose solution, or plain 1.5 mol l^–1 sucrose. (A) Chronic treatment: bees continued to receive treatment during the 20 h interval between training and testing. (B) Withdrawal: treatment withheld during the 20 h interval between training and testing. In each panel the proportion of bees responding correctly is normalized with respect to the untreated control group. Sample size is shown in bars. Groups that did not differ at the 5% confidence interval share the same letter above the bar (pair-wise Fisher's exact tests with Bonferroni correction).