According to a study published this fall, ants can get addicted to morphine. The subject sounds absurd, but the study was interesting - or “novel” - for a few different reasons: first of all, behavioral studies related to addiction and reward-seeking behavior are almost always performed in mammalian system. Secondly, similar studies performed in invertebrate systems (such as the previously described bees and cocaine study), are always done in the presence of some sort of non-drug reward. In other words, most studies tracks how an insect responds to a food-based reward (like sucrose) in the presence of drug, rather than how the insect responds to just the drug itself; this seemingly minor detail can potentially obscure the motivations behind drug-seeking behavior. Finally, rather than simply perform behavioral studies on these ants, the researchers also acquired chemical data supporting their hypothesis that ants exposed to morphine are actually undergoing neurochemical changes analogous to changes undergone by humans addicted to morphine.
In brief, Entler et al. performed a series of behavioral experiments to examine how ants previously exposed to morphine respond to rewards of morphine versus sucrose. Most of these experiments involved variations on the following: ants that are systematically exposed to ("trained on") morphine are given access to dishes either containing morphine or sucrose, or a mixture. These morphine-trained ants consistently select the morphine dish - even in the absence of the more conventional ant reward, sucrose. Ants that were never exposed to morphine, however, showed no preference for the drug. Additionally, researchers detected a 2-fold increase in dopamine levels solely in the brains of the morphine-trained ants – providing evidence that morphine exposure is likely affecting chemical processes in the ant brain.
The take home message of this article is that, like vertebrates, invertebrates are inclined to become addicted to, and self-administer morphine. Furthermore, the chemical effects of this behavior are conserved across a broad and diverse range of animal species, suggesting that non-mammalian systems may prove valuable in untangling the neurochemical and behavioral underpinnings of drug use and addiction.
From an ecological perspective this study is also rather thought-provoking. As discussed in "bees on cocaine", plants are thought to produce psychoactive molecules as a deterrent to insect predators. But do ants actually encounter morphine in their normal, non-lab lives? And what happens when an insect becomes addicted to this molecule? Does it provide any benefit for the plant? Does it significantly alter the lifestyle of the insect? While this particular paper does not aim to address these types of questions, I think they're worth considering. The effects of addiction in a purely ecological context might actually hold some interesting insights into the evolution of this seemingly paradoxical behavior.