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The physiology of wandering behaviour in Manduca sexta. IV. Hormonal induction of wandering behaviour from the isolated nervous system.

Prior to exposure to ecdysteroids, the isolated central nervous system (CNS) of the fifth instar Manduca sexta larvae exhibited infrequent motor bursts over a 24-h period of extracellular recording from segmental motor nerves. In contrast, the CNS isolated from wandering larvae was characterized by persistent, frequent spontaneous motor bursts throughout the 24-h incubation. The motor bursts generated by the isolated CNS of wandering larvae were similar to those observed in deafferented segments of partially dissected wandering larvae during locomotion. In both cases bursts in the deafferented ganglia were synchronous and had a lower frequency than in intact animals. Removal of the brain from a CNS isolated prior to ecdysteroid exposure resulted in the appearance of spontaneous bursts, which were abolished by removing the suboesophageal ganglion (SEG). By contrast, when the brain was removed from the isolated CNS of wandering larvae, spontaneous bursts ceased. These results parallel the behavioural effects of the same lesions in intact larvae of the respective stages. The CNS isolated from larvae prior to ecdysteroid exposure initiated sustained frequent bursting after an average latency of 15 h following incubation in haemolymph taken from larvae during the interval of ecdysteroid secretion. Incubations of the CNS with 1 microgram ml-1 20-hydroxyecdysone (20-HE) resulted in the onset of the same pattern of sustained motor activity. In a CNS isolated prior to ecdysone release, it was necessary and sufficient to expose the brain to 20-HE in order to induce the state of persistent motor bursts characteristic of wandering. We conclude that the spontaneous persistent motor bursts observed in the isolated CNS of wandering larvae are directly related to the sustained locomotion seen during the wandering behaviour. 20-HE acts directly on the CNS, specifically the brain, to induce this state of neural activity.