Queen pheromone modulates brain dopamine function in worker honey bees

Abstract

Honey bee queens produce a sophisticated array of chemical signals (pheromones) that influence both the behavior and physiology of their nest mates. Most striking are the effects of queen mandibular pheromone (QMP), a chemical blend that induces young workers to feed and groom the queen and primes bees to perform colony-related tasks. But how does this pheromone operate at the cellular level? This study reveals that QMP has profound effects on dopamine pathways in the brain, pathways that play a central role in behavioral regulation and motor control. In young worker bees, dopamine levels, levels of dopamine receptor gene expression, and cellular responses to this amine are all affected by QMP. We identify homovanillyl alcohol as a key contributor to these effects and provide evidence linking QMP-induced changes in the brain to changes at a behavioral level. This study offers exciting insights into the mechanisms through which QMP operates and a deeper understanding of the queen's ability to regulate the behavior of her offspring.

Fig. 1.

Honey bee queen surrounded by a retinue of workers attracted to her by QMP. The schematics show that one component of QMP, HVA, bears a striking structural resemblance to dopamine.

Fig. 2.

Brain dopamine (DA) levels in 2-day-old worker bees. (A) QMP exposure reduces levels of dopamine in the brain. (B) Young hive bees exposed to a mated queen show lower brain dopamine levels than those exposed to a virgin queen. (C) Reduction of brain dopamine levels after exposure to HVA. (D) Brain dopamine levels are not affected by exposure to HOB. (E) Reduction of brain dopamine levels from exposure to HVA and HOB combined. Data are expressed as means ± SEM. Numbers in each bar represent n values. P values show the significance of differences between groups as determined by two-tailed Student's t tests.

Fig. 3.

Age-related changes in dopamine receptor gene mRNA levels quantified by Northern blot analysis. Comparison of transcript levels in the brains of newly emerged adults (NE), 1-day-old workers (1 day), 2-day-old workers (2 day), and foragers (For; generally >21 days old) reveals age-related changes in the expression of Amdop1 (A), Amdop2 (B), and Amdop3 (C). Data are expressed as means ± SEM with a sample size of four independent RNA samples for each group. Overall significance was determined by one-way ANOVA with Tukey's tests used for post hoc comparisons. Letters above the bars indicate differences between groups. Groups that share letters are not significantly different.

Fig. 4.

QMP modulation of dopamine receptor gene expression. (A–C) Northern blot analysis of gene transcript levels in 2-day-old workers. (A) Amdop1 mRNA levels are significantly lower in 2-day-old QMP-treated bees than in age-matched controls. (B and C) No significant differences in Amdop2 (B) or Amdop3 (C) mRNA levels were identified among QMP-treated bees and controls. (D–F) Gene expression levels determined by using quantitative RT-PCR. (D) Amdop1 transcript levels are selectively reduced by exposure to QMP. (E and F) Differences in Amdop2 (E) and Amdop3 (F) mRNA levels between QMP-treated bees and controls are not significant. Data are expressed as mean levels ± SEM with a sample size of three for each group. Statistical comparisons were performed by using two-tailed Student's t tests.

Fig. 5.

Responses of isolated mushroom body calyces to dopamine monitored by using measurements of intracellular cAMP. Calyces from QMP-exposed bees (gray bars) and from control bees (white bars) were exposed to either 10 μM dopamine (A) or 10 μM HVA (B). Note that dopamine-evoked responses are strikingly different in QMP-treated bees versus controls and that the effects of dopamine are mimicked by HVA. Data are expressed as mean levels ± SEM with a sample size of six for each group. P values refer to within-group differences between cAMP levels detected in dopamine-treated or HVA-treated tissues and those detected in tissues that were not exposed to dopamine or HVA.

Fig. 6.

QMP-induced changes in activity. Activity levels in 4-day-old QMP-exposed bees were significantly lower than in age-matched control (untreated) bees. The effects of QMP were partially reversed by treating QMP-exposed bees with l-dopa. Data are expressed as mean activity levels ± SEM with a sample size of 20 for each group. Overall significance was determined by one-way ANOVA followed by Tukey's tests for post hoc comparisons. Letters above the bars indicate differences between groups. Groups that share a letter are not significantly different.