Differential activation of serotonergic neurons during short- and long-term gregarization of desert locusts.

Serotonin is a neurochemical with evolutionarily conserved roles in orchestrating nervous system function and behavioural plasticity. A dramatic example is the rapid transformation of desert locusts from cryptic asocial animals into gregarious crop pests that occurs when drought forces them to accumulate on dwindling resources, triggering a profound alteration of behaviour within just a few hours. The onset of crowding induces a surge in serotonin within their thoracic ganglia that is sufficient and necessary to induce the switch from solitarious to gregarious behaviour. To identify the neurons responsible, we have analysed how acute exposure to three gregarizing stimuli--crowding, touching the hind legs or seeing and smelling other locusts--and prolonged group living affect the expression of serotonin in individual neurons in the thoracic ganglia. Quantitative analysis of cell body immunofluorescence revealed three classes of neurons with distinct expressional responses. All ganglia contained neurons that responded to multiple gregarizing stimuli with increased expression. A second class showed increased expression only in response to intense visual and olfactory stimuli from conspecifics. Prolonged group living affected a third and entirely different set of neurons, revealing a two-tiered role of the serotonergic system as both initiator and substrate of socially induced plasticity. This demonstrates the critical importance of ontogenetic time for understanding the function of serotonin in the reorganization of behaviour.