The venom of the parasitoid wasp Ampulex compressa produces interesting changes in the motivational state of its prey, the cockroach Periplaneta americana. We are studying the effects of the venom on the central nervous system of the cockroach. The wasp stings a cockroach first in the thorax and then in the head, towards the brain. Using radiolabeling of the wasp venom, we have shown that the wasp injects its venom in the cockroach first into the thoracic locomotory ganglion, and then into the brain. The venom has three sequential behavioral effects which are described below.
The thoracic sting induces a transient flaccid paralysis lasting 3-5 min. Crude venom injected in the thorax blocks spontaneous and evoked activity in a postural leg motoneuron for roughly 3 min. Crude venom had no effect on action potential propagation, while blocking both synaptic mediated and agonist-evoked post-synaptic potentials. Hence, the venom produces a post-synaptic block of cholinergic transmission lasting 3-5 min.
After recovering from the transient paralysis, for approximately the next 30 min, the cockroach grooms almost continuously. This excessive grooming is due to venom injection into the brain. We suggest that the venom induces prolonged grooming by stimulating dopamine receptors in the cockroach because the dopamine agonist SKF 82958 induces excessive grooming when injected directly into the brain; the dopamine antagonist flupentixol greatly reduces venom-induced grooming; dopamine is present in the venom.
About 30 min following the sting, the cockroach enters a long-lasting lethargic state during which initiation of escape behavior is suppressed in the thoracic portion of the escape circuitry.
By studying the effect of the venom on a relatively simple model system, like the cockroach escape circuit and behavior, it should be possible to determine the neural mechanisms for change in responsiveness, which is a prime question in the study of motivation.