Biogenic amines have widespread effects on numerous behaviors, but their
natural functions are often unclear. We investigated the role of octopamine
(OA), the invertebrate analog of noradrenaline, on initiation and maintenance
of aggression in male crickets of different social status. The key-releasing
stimulus for aggression is antennal fencing between males, a behavior
occurring naturally on initial contact. We show that mechanical antennal
stimulation (AS) alone is sufficient to initiate an aggressive response
(mandible threat display). The efficacy of AS as an aggression releasing
stimulus was augmented in winners of a previous fight, but unaffected in
losers. The efficacy of AS was not, however, influenced by OA receptor (OAR)
agonists or antagonists, regardless of social status. Additional experiments
indicate that the efficacy of AS is also not influenced by dopamine (DA) or
serotonin (5HT). In addition to initiating an aggressive response, prior AS
enhanced aggression exhibited in subsequent fights, whereby AS with a male
antenna was now necessary, indicating a role for male contact pheromones. This
priming effect of male-AS on subsequent aggression was dependent on OA since
it was blocked by OAR-antagonists, and enhanced by OAR-agonists. Together our
data reveal that neither OA, DA nor 5HT are required for initiating aggression
in crickets, nor do these amines influence the efficacy of the natural
releasing stimulus to initiate aggression. OA's natural function is restricted
to promoting escalation and maintenance of aggression once initiated, and this
can be invoked by numerous experiences, including prior contact with a male
antenna as shown here
