Shock-induced Analgesia in the cockroach (Periplaneta americana)

Comparative psychoneuroimmunology: evidence from the insects

Interactions between immune systems, nervous systems, and behavior are well established in vertebrates. A comparative examination of these interactions in other animals will help us understand their evolution and present adaptive functions. Insects show immune-behavioral interactions similar to those seen in vertebrates, suggesting that many of them may have a highly conserved function. Activation of an immune response in insects results in illness-induced anorexia, behavioral fever, changes in reproductive behavior, and decreased learning ability in a broad range of species. Flight-or-fight behaviors result in a decline in disease resistance. In insects, illness-induced anorexia may enhance immunity. Stress-induced immunosuppression is probably due to physiological conflicts between the immune response and those of other physiological processes. Because insects occupy a wide range of ecological niches, they will be useful in examining how some immune-behavioral interactions are sculpted by an animal's behavioral ecology.

A ganglionic model of "learned helplessness"

The phenomenon known as "learned helplessness" (LH) is seen broadly across the animal kingdom. Some of the basic characteristics of this behavior are: failure to escape shock when it is possible to do so following non-escapable shock; reversion to non-escape behavior even after successful escape; if the animal is given escape/avoidance training prior to being given inescapable shocks, the latter will not interfere with its ability to later show normal escape/avoidance behavior (generally described as an immunization effect); following inescapable shock training the animals often become "passive and still" when confronted with an inescapable shock. These behaviors are seen in intact mammals, lower vertebrates, and invertebrates. In fact, the basic characteristics are even seen in a spinal rat and, with the exception of one characteristic not yet examined, in an isolated thoracic ganglion of an insect. The brain is evidently not essential either in mammals or in invertebrates for demonstrating this behavior. Not only can an insect ganglion show the behavioral characteristics of LH, but the neural information underlying the phenomenon of LH can be shown to transfer from one ganglion innervating one pair of legs to another ganglion innervating a different pair of legs. Thus, how CNS information underlying LH is coded and transferred from one site to another within the CNS can be examined in such a system. The LH model has provided valuable insights into the physiology of depression. This model suggests that human depression is caused by one's lack of control over traumatic events. It is supported by a number of parallels between depression and LH behavior. Tricyclic antidepressants, MAO inhibitors, and ECT, which are effective in treating depression, also can prevent and reverse LH in mammals. It would be important to find out if they are also effective in invertebrate models. The fact that the characteristics of the behavior called LH are seen in invertebrates such as slugs, cockroaches, and locusts provokes other intriguing questions about the presence of cognition at these phylogenetic levels, as well as what animal or preparation constitutes an appropriate model for human depression.