Divergent pathways mediate 5-HT1A receptor agonist effects on close social interaction, grooming and aggressive behaviour in mice: Exploring the involvement of the oxytocin and vasopressin systems

Background: 5-HT1A receptor (5-HT1AR) abnormalities are implicated in aggression, and there has been considerable interest in developing 5-HT1AR agonists for treating aggression. Endogenous oxytocin (OXT) released upon stimulation of 5-HT1ARs in the hypothalamus mediates at least some of the effects of 5-HT1AR agonists on social behaviour. Aims: Given 5-HT1AR, OXT receptor (OXTR) and vasopressin V1a receptor (V1aR) agonists can all reduce aggression, the current study aimed to determine whether the anti-aggressive effects of 5-HT1AR stimulation can also be explained by downstream actions at OXTRs and/or V1aRs in a mouse model of non-territorial, hyper-aggressive behaviour. Methods: Male Swiss mice (N=80) were socially isolated or group housed for six weeks prior to the start of testing. Testing involved placing two unfamiliar weight- and condition-matched mice together in a neutral context for 10 minutes. Results: Social isolation led to a pronounced increase in aggressive behaviour, which was dose-dependently inhibited by the 5-HT1AR agonist 8-OH-DPAT (0.1, 0.3 and 1 mg/kg intraperitoneally (i.p.)), with accompanying increases in close social contact (huddling) and grooming. The effects of 8-OH-DPAT on aggression, huddling and grooming were blocked by pretreatment with a selective 5-HT1AR antagonist (WAY-100635; 0.1 mg/kg i.p.). The anti-aggressive effects of 8-OH-DPAT were unaffected by an OXTR antagonist (L-368,899; 10 mg/kg i.p.), whereas the effects on huddling and grooming were inhibited. Pretreatment with a V1aR antagonist (SR49059; 20 mg/kg i.p.) had no effect. Conclusions: Our study suggests that stimulation of endogenous oxytocin is involved in the effects of 5-HT1AR activation on close social contact and grooming but not aggression.