Abstract Body:

Introduction: Maintaining strong pair-bonded relationships has been shown to offer survivorship advantages^1,2, while their disruption has been associated with significant health risks and mortality³. However, the neurobiological mechanisms underlying these associations, specifically, their impact on oxytocin and kappa opioid receptors (KORs) are heavily understudied. Titi monkeys, a socially monogamous new-world non-human primate species, exhibit signs of adult pair-bonding, including the ability of their partner to buffer their stress response (social buffering)^4,5. The PET radiotracer [¹¹C]GR103545 has been used to obtain an in vivo assessment of the KOR ^6–8. We hypothesized that [¹¹C]GR103545 PET binding will differ in stress versus social buffering conditions in titi monkeys. To enable testing this hypothesis, we developed an image analysis pipeline to assess [¹¹C]GR103545 uptake characteristics regionally in the brain of the titi monkey.

Methods: Adult titi monkeys (N=20; 10 males and 10 females) were subjected to [¹¹C]GR103545 dynamic 110-min brain PET scans (PiPET, Brain Biosciences) at (1) baseline, and after induction of a stressor (blood draw) in the (2) presence (buffering condition), and (3) absence (stress condition) of their pair-bonded partner. The injected dose was 47.7±5.7MBq/kg. PET data had an isotropic voxel size of 0.8mm³ and were motion-corrected. Brain MRI data were obtained (1.5T system, General Electric) at a single timepoint (voxel size 0.3×0.3×1.0mm³) to facilitate anatomical segmentation. The following image processing steps were applied to the MRI scans: mapping to titi monkey reference space via rigid registration; interpolation to isotropic voxel size (0.3mm³); skull-stripping; bias correction (N4ITK); and diffeomorphic warping of a titi monkey brain atlas with 46 regions (created by our group) using the Advanced Normalization Tools Suite. The warped regional labels were then exported to PMOD (v4.3) and region-specific, non-displaceable binding potential (BP_ND) was calculated using Simplified Reference Tissue Model (SRTM), with the cerebellum as the reference region. Primary analysis included 7 regions implicated in social bonds, namely, the nucleus accumbens, caudate, hypothalamus, cingulate cortex, hippocampus and amygdala. A mixed effects model with Geisser-Greenhouse correction was used, with correction for multiple comparisons via the two-stage linear step-up method⁹. Statistical significance was considered for p<0.05.

Results: The segmentation of the MRI took 12 min, while the co-registration of PET and MRI took approximately 7 min per scan. The analysis presented below represents results obtained from 54 out of the 60 scans.

In comparing the baseline versus the stressed condition, the BP_ND was higher in the hypothalamus (p=.02) in the latter. The BP_ND was higher in the caudate (p=.01), hypothalamus (p=.005) and cingulate cortex (p=.001) in the buffered condition versus baseline. No other differences between the groups were noted. Interestingly, however, when the groups were split by sex, the BP_ND was higher only in males in the hippocampus (p=.01) and amygdala (p=.005) in the buffered condition versus baseline. Additionally, for the buffered versus stress condition, BP_ND was higher only in males in the hippocampus (p=.01), caudate (p=.008) and amygdala (p=.002). Finally, the BP_ND was higher in males compared to females in the hypothalamus (p=.05), hippocampus (p=.002) and amygdala (p=.04) for the buffered condition.

Conclusions: Differences in [¹¹C]GR103545 binding were observed across several brain regions under stress and social buffering, highlighting the role of the KOR (and potentially downstream oxytocin pathways) in the presence/absence of a pair-bonded partner. Sex-based analyses were essential to appreciate differences across groups. Higher KOR binding in males in the buffering condition could be indicative of a dependence on the buffer to alleviate stress-related dysphoria. This study contributes to our continued effort to develop imaging tools to understand the complexities of social bonds using this highly relevant monkey model.

Author