In the marine mollusk, Aplysia californica, there is a sequence of behaviors that occurs during mating and egg laying and lasts for up to several hours. Pheromones appear to play a critical role in initiating and coordinating the behaviors between individuals. Within each individual, the behavioral sequence is initiated by a chemical stimulus -- probably a contact pheromone -- located in an egg mass laid by others. In the central nervous system, this stimulus triggers release of neuropeptides from a defined group of neuroendocrine cells and other neurons coupled to them. The actions of the neuropeptides within central nervous system and on peripheral organs serve to orchestrate the behavioral sequence, which in these hermaphrodites consists of mating as a female, laying eggs, and then mating as a male.
The release of neuropeptides corresponds to what ethologists long ago speculated to be an 'innate releasing mechanism.' The innate releasing mechanism is triggered by a sign stimulus -- in this case a contact pheromone in the egg mass. Many of the features reproductive behavior of Aplysiaare also features of instinctive, reproductive behavior in other mollusks and other animals, including humans. In many of these cases it seems likely that there is an innate releasing mechanism that also involves the release of neuropeptides from defined neural circuits. The neuropeptides act on target neurons within the central nervous system to modulate electrical signaling for minutes or hours, and thus regulate aspects of the behavior.