Part of an international team, a researcher from IISc has unravelled the neuro-molecular mechanism of social defence by honeybees when exposed to isoamyl acetate, the main component in the alarm pheromone. The level of serotonin and dopamine in the honeybee brain increases in response to isoamyl acetate, thereby increasing the stinging behaviour in bees. Bees are more likely to sting due to an increase in serotonin in the central brain.
The role of the sting alarm pheromone released by honeybees in the face of a threat in eliciting the defensive behaviour in bees is already known. Now, researchers have unravelled the neuro-molecular mechanism of social defence by honeybees when exposed to the sting alarm pheromone.
The team of researchers led by Prof. Martin Giurfa from the Universite de Toulouse in Toulouse, France has found that smelling isoamyl acetate, the main component in the alarm pheromone, increases the level of serotonin and dopamine in their brain, which, in turn, increases the stinging behaviour in bees and thus repels a threat.
By itself, the alarm pheromone does not behave as a stimulus, but increases the likelihood of bees guarding the hive to repel a threat by stinging.
Bees collected from four hives that were involved in defending the colony were used in the study. When exposed to isoamyl acetate in the lab, guard bees collected from two colonies exhibited greater proclivity to sting than bees from the other two colonies. While the level of dopamine did not show significant difference across brain regions in bees from the four colonies, the level of serotonin was significantly elevated in the central brain (where olfactory processing takes place) in bees that exhibited greater proclivity to sting. This was the first indication of serotonin’s role in the defensive behaviour.
The link between serotonin and responsiveness to threat was verified in individual bees that showed defensive and passive behaviour by measuring the amount of serotonin and dopamine in the brain. Both serotonin and dopamine were significantly elevated in the central brain following exposure to isoamyl acetate. Serotonin level was higher even in the optic lobes and sub-oesophageal zone of bees that showed greater defensive behaviour.
“This established the relationship between higher serotonin level following exposure to isoamyl acetate and stinging behaviour,” says Dr. Souvik Mandal, a PhD scholar from Bengaluru’s Indian Institute of Science (IISc). He was a visiting scientist at the Universite de Toulouse to perform behavioural experiments on the bees and one of the authors of a paper published in the journal Proceedings of the Royal Society B.
“We confirmed that stinging behaviour of bees by topically introducing in the thorax different concentrations of serotonin and dopamine. Compared with controls, the aggressive behaviour was greatly enhanced when highest concentration of serotonin and dopamine was topically applied,” says Dr. Mandal.
The researchers introduced antidotes to serotonin and dopamine to reconfirm their role in the stinging behaviour. “The aggressive behaviour reduced significantly when we introduced the antidotes,” he says. This helped confirm that varying the content of serotonin or dopamine directly affects the stinging behaviour of honeybees. Bees were more likely to sting due to an increase in serotonin in the central brain and to a “lesser extent” by dopamine.
“This is the first time we have begun to understand the neural mechanism by which the alarm pheromone attracts other guard bees to stage a coordinated stinging attack to defend the colony,” Dr. Mandal says.