Neurodevelopmental effects of octopamine on isolation-induced social behaviors in Pheidole dentata

Ants are eusocial insects that live in constant contact with their nestmates. Social behaviors like trophallaxis, allogrooming, and self-grooming function to transfer cuticular hydrocarbons onto other ants in the colony (Boulay et al. 2000a; Boulay et al. 1999, Lenoir and Hefetz, 2001; Boulay et al., 2000b). Hydrocarbon profile homogenization allows them to discriminate a nestmate from an intruder (Dahbi et al, 1999; Boulay et al. 2000a; Boulay and Lenoir, 2001). Given the importance of continuous contact with nestmates, prolonged periods of social isolation can be highly stressful and deadly (Boulay et al. 1999).
Isolation increases the duration of social behaviors, and Octopamine (OA), a norepinephrine analog in invertebrates, modulates this process (Farooqui, 2012, Boulay et al. 2000a, Boulay et al. 2000b, Wada-Katsumata, 2011). The exact mechanism through which OA mediates isolation-induced social behaviors is unknown. To study the behavioral mechanism of OA on social behavior, we conducted two sets of experiments. We acutely treated adult Pheidole dentata workers with 15nL of 2.4% OA (in N,N-Dimethylformamide, DMF) following 5-day isolation, and recorded their social interactions. To determine the developmental effects of OA, we treated pupa with 15nL of 1.8% OA (in DMF) and allowed them to eclose and mature. The developed adults were then isolated for 5 days and behaviorally tested.
We found an increase in duration of social behaviors, particularly allogrooming, in adults treated with OA. Surprisingly, DMF significantly lowered the duration of social behaviors compared to controls, but OA rescued these behaviors, such that OA treated ants did not significantly differ from controls or vehicle group. Therefore topical acute OA treatment had a behavior enhancing effect in adults (both isolated and non-isolated) and a protective effect during development. Detrimental effects of DMF due to chronic exposure has been well documents in mammals, however, we show that acute DMF treatment during development can be detrimental and OA may be protective against the damage.