Viewed from a great height as we scurry about, it can be hard to tell the difference between human and ant societies. We both live in highly organised communities surrounded by architectural structures where each individual has a well-defined role. However, Brian Entler and colleagues, from The University of Scranton, USA, wondered whether we may also share a darker side to our characters. The team explains that some insects are capable of forming addictions if the drug is delivered simultaneously with food, teaching us essential lessons about the neurobiology of addiction. However, humans have the ability to become addicted to drugs that provide pleasure alone, and no other species seem to be able to develop such an addiction without the added temptation of food. Knowing that ants are keen to scurry around searching for titbits even if their escapades are unsuccessful, Entler, John Cannon and Marc Seid wondered whether the highly motivated insects could get hooked on a life of self-administered drug dependency.
Tempting Florida carpenter ants to visit a feeder full of tasty sugar solution, Entler gradually removed the sugar while simultaneously increasing the concentration of highly addictive morphine over a 6-day period in a bid to get them hooked. Initially, the ants lost interest in the feeders as the sugar concentration declined; however, once the insects were completely weaned off the sugar and only the morphine remained, the ants’ interest in the feeder was rekindled. And when Entler offered the ants a choice of snacks – a feeder full of tasty sugar solution versus another containing the drug – their preferences were abundantly clear as they headed for the morphine-laced feeder to feed their habit despite the lack of nutrition. However, when Entler offered ants that had never touched the drug the same option, they clearly sought out the sweet treat. The ants were certainly attracted to morphine, but were they getting the reward-seeking ‘high’ of true junkies or did they just prefer the taste of morphine water?
To answer the question, Entler analysed the ants’ brains to find out whether they were producing any of the tell-tale neurotransmitters that indicate addiction, and found that the morphine-fed ants had higher levels of dopamine, which directs reward-seeking behaviour: the ants were true junkies. ‘This study establishes ants as the first non-mammalian model of self-administration that is truly analogous to mammals’, says the team, who are keen to begin untangling the basic neural circuits that drive drug addiction in ants and probably humans.