Sign stimulus activates a peptidergic neural system controlling reproductive behavior in Aplysia.

1. In the marine mollusk Aplysia, egg laying is a complex behavior that lasts for up to several hours. We used behavioral and electrophysiological methods to determine how egg laying occurs in groups of animals and how it is related to other aspects of reproductive behavior. 2. Prolonged contact with an existing egg mass by the lips and tentacles of an animal is a sign stimulus for release of egg-laying behavior and two other fixed action patterns in the same individual, mating as a female during egg laying and mating as a male after egg laying. 3. Prolonged contact with the egg mass initiated repetitive spike activity in bag cell neurons, which are part of a peptidergic neural system that modulates neuronal activity in the CNS for up to several hours. The sign stimulus thus activates the neuromodulatory system, which may serve as an innate releasing mechanism, and an associated internal drive, for control of the behavioral sequence.