Opioid-glutamate interactions modulate intrinsically rewarded communication in European starlings

Affiliative, non-sexual vocal communication is essential for social learning and bonding, yet relatively little is known about its neural bases. Past research on songbirds demonstrates that song produced in affiliative, non-sexual flocks correlates positively with a bird's intrinsic reward state, and the link between song and reward is regulated by mu opioid receptors (MORs) in the medial preoptic area (MPOA). RNAseq studies also reveal a strong positive association between affiliative song and glutamatergic gene expression in MPOA, with NMDA receptor 1 (GRIN1) and glutamate receptor scaffolding protein (Shank2) correlating strongly with affiliative singing. In male European starlings we tested the hypothesis that MOR–glutamate interactions in the MPOA regulate affiliative communication. Peripheral administration of the MOR agonist fentanyl increased affiliative singing and calling behavior while reducing stress-related behavior, beak wiping. The effects on vocal but not stress-related behavior were prevented by administration of the NMDA receptor antagonist MK-801. c-Fos mapping revealed that fentanyl increased neural activity in caudal MPOA (cMPOA) and interconnected regions implicated in social behavior, song production, and reward, and that these effects were reversed by MK-801. To identify a local site of MOR-glutamate interaction, we used AAV shRNA to downregulate Shank2 in MPOA. This manipulation prevented fentanyl from stimulating affiliative singing behavior, also without affecting stress reduction, suggesting that MOR–glutamate interactions in the MPOA facilitate social communication but not associated stress reduction. Together these findings provide the first experimental evidence that MOR–glutamate interactions within the MPOA, particularly the cMPOA, contribute to affiliative, rewarding vocal communication. Significance Statement Affiliative, non-sexual vocal communication is fundamental to social cohesion and well-being, yet its neural mechanisms are poorly understood. This study identifies a critical role for interactions between mu opioid receptors and glutamatergic signaling in promoting non-sexual, rewarding vocal communication. In a songbird experimental system, we demonstrate that NMDA receptor activity and the postsynaptic glutamate receptor scaffolding protein Shank2 are required for opioid facilitation of affiliative song. Our findings further reveal the medial preoptic area, particularly its caudal subdivision, to be an important site of this opioid-glutamate interaction. Together, these results extend opioid–glutamate interaction frameworks beyond classic reward circuits and suggest the existence of a conserved neural mechanism supporting affiliative social behavior across vertebrates.