Social bonds have a profound impact on our everyday life. People differ in their capacity for establishing and maintain strong, intimate relationships. These differences are characterized as attachment styles. Particularly insecure attachment style may lead to developmental and psychiatric disorders, as well as to addictive behavior (Mikulincer & Shaver 2007), which cause large societal expenses due to treatment and sickness leaves. In addition to anti-nociception and reward, the endogenous opioid system has been proposed to regulate social bonding in mammals. That could explain the link between social distress and physical pain (Panksepp & Nelson 1997). However, the role of the endogenous opioid system in human social behavior remains poorly understood (Machin & Dunbar 2011).

The aim of this thesis was to investigate opioidergic basis of affiliative behavior with positron-emission tomography (PET). We approached the phenomenon from two perspectives: by 1) measuring how prosocial behavior affects endogenous opioid peptide release and 2) quantifying whether regional differences in opioid receptor availability explain differences in adult attachment styles. PET was used for quantifying endorphin release after social laughter manipulation (Study I) and levels of brain μ-opioid receptor (MOR) in baseline state (Study II). In the methodological part of the thesis (Study III), PET and MRI data were combined. Voxel-based morphometry (VBM) based indices of gray matter density (GMD) were correlated with tracer binding potentials (BPND)

Social laughter increased endorphin release in brain regions such as thalamus, caudate nucleus and putamen, in subcortical areas and in frontal cortices (Study I). Adult avoidant attachment style correlated negatively with brain MOR availability in thalamus, anterior, middle and posterior cingulate cortices, and medial and lateral prefrontal cortices (Study II). In study III, gray matter density (GMD and radiotracer binding (BPND) correlated positively in multiple brain areas, suggesting a link between brain’s regional macrostructure and molecular functioning.

In sum, these results highlight the crucial role of endogenous opioid system in human prosocial functioning. Furthermore, they show that PET and anatomical MR provide complementary information regarding brains molecular organization, stressing the importance of fusion imaging for understanding brain basis of sociability.