BACKGROUND: Dysregulation of the vasopressin (AVP) system has been implicated in 
the pathogenesis of autistic spectrum disorder (ASD). Apelin is a recently 
discovered neuropeptide that could counteract AVP actions and whose receptors 
are colocalized with vasopressin in hypothalamic magnocellular neurons. Aims of 
the present study were to investigate circulating levels of apelin in patients 
with ASD and to assess their correlation with plasma AVP concentrations.
METHODS: Plasma levels of apelin and AVP were measured in a total of 18 patients 
with ASD and 21 age- and gender-matched healthy comparison subjects. The 
Childhood Autism Rating Scale (CARS) was used to assess the severity of autistic 
symptoms.
RESULTS: Significantly reduced levels of apelin (p < 0.001) and elevated 
concentrations of AVP (p = 0.02) were found in ASD patients as compared to 
controls. Additionally, a significant inverse correlation between apelin and AVP 
levels was found within the ASD group (r = -0.61; p = 0.007), but not in healthy 
participants (r = -0.26; p = 0.25). Multivariate linear regression analysis 
showed that only AVP concentrations independently predicted apelin values in ASD 
individuals (beta = -0.42, t = 2.63, p = 0.014). No correlation was seen between 
apelin levels and CARS scores (r = -0.10; p = 0.68).
CONCLUSIONS: Our findings of a significantly reduced peripheral level of apelin 
coupled with elevated AVP point to a subtle but definite vasopressinergic 
dysfunction in autism that could play a role in the etiopathophysiology of this 
disorder in humans.