Classically conditioned cockroaches

The name Ivan Pavlov should ring a few bells for anybody who has studied psychology. About 100 years ago, Pavlov, a Russian physiologist, performed a series of classic experiments which showed that dogs could be taught to respond in a particular way to a given stimulus. Pavlov was therefore the first to demonstrate a form of behaviour modification called classical conditioning.

Aversion therapy is a form of behaviour modification used by psychiatrists which based on classical (or Pavlovian) conditioning. For example, in dealing with a patient prone to alcoholism, the therapist might repeatedly pair alcohol with an emetic drug (one that induces vomiting), in the hope that the association between the two would elicit in the patient a feeling of sickness that leads to the avoidance of alcoholic drinks. In the U.S., aversion therapy was used to “treat” homosexuals and sexual deviants until rather recently: the American Psychological Association declared that it was dangerous and ineffective in 1994.

Classical conditioning and aversion therapy are effective in modifying behaviour. However, if the association between the two stimuli is not constantly reinforced, the elicited response becomes weaker and eventually disappears, in a process called extinction. Pavlov’s method has been demonstrated not only in humans and in dogs, but also in an invertebrate – the sea slug Aplysia californica. And now, researchers from Tohoku University in Sendai, Japan show that such conditioned responses can also be elicited in an insect – they have created Pavlov’s cockroach.

Just as Pavlov conditioned his dogs to salivate in response to the sound of a bell, the Japanese team conditioned their cockroaches to salivate when presented with a particular smell. They exposed the cockroaches to vanilla and peppermint odours. During exposure to one of them, the insects were also presented with a sucrose solution. After repeated pairings, they learnt to associate that odour with the sucrose solution, so that, when the odour was placed on its antennae, it would salivate in anticipation of a sugary drink. Exposure to the other odour, however, elicited no such response. The researchers even measured the amounts of saliva secreted by their experimental cockroaches in response to the odour associated with the sucrise solution: 100-200 nanolitres of saliva per second.

In the cockroach, the salivary glands are innervated by three types of neurons, each of which releases a different neurotransmitter – one type, called SN1, synthesizes and releases dopamine; the second, termed SN2, uses GABA; and the third uses serotonin. The authors had previously shown that these salivary neurons fire in response to conditioned stimuli, and that their activity caused the cockroach to salivate. And last year, researchers from the University of Edinburgh showed that cells in the hypothalamus of the rat fire in response to both food and anticipation of it. But little else is known about the neural bases of classical conditioning. Significantly, this study shows that the cellular mechanisms for controlling salivation, and perhaps the other autonomic functions, in the cockroach are somewhat similar to those of vertebrates. So researchers may have a simple animal model for those same mechanisms in the mammalian brain.

Reference:

Watanabe, H. & Mizunaim, M. (2007). Pavlov’s cockroach: Classical conditioning of salivation in an insect. PLoS ONE 2(6): e529. doi:10.1371/journal.pone.0000529. [Full text]