Neuropeptide Y (NPY) regulates physiological processes via receptor subtypes (Y 1 , Y 2 , Y 4 , Y 5 , and y 6 ). The Y 5 receptor is well known for its role in appetite. Based on expression in the limbic system, we hypothesized that the Y 5 receptor might also modulate stress sensitivity. We identified a novel Y 5 receptorselective antagonist, Lu AA33810 [N-[[trans-4-[(4,5-dihydro ]-hPancreatic Polypeptide. In Sprague-Dawley rats subjected to the social interaction test, Lu AA33810 (3-30 mg/kg p.o.) produced anxiolyticlike effects after acute or chronic treatment. In Flinders sensitive line rats, chronic dosing of Lu AA33810 (10 mg/kg/day i.p.) produced anxiolytic-like effects in the social interaction test, plus antidepressant-like effects in the forced swim test. In Wistar rats exposed to chronic mild stress, chronic dosing of Lu AA33810 (3 and 10 mg/kg/day i.p.) produced antidepressant-like activity, i.e., normalization of stress-induced decrease in sucrose consumption. We propose that Y 5 receptors may function as part of an endogenous stress-sensing system to mediate social anxiety and reward or motivational deficits in selected rodent models.Neuropeptide Y (NPY) is a 36-amino acid transmitter belonging to the pancreatic polypeptide family, along with peptide YY (PYY) and pancreatic polypeptide (PP). NPY has widespread distribution throughout the central nervous system and periphery and has been demonstrated to modulate numerous physiological processes (e.g., appetite, metabolism, mood, and reproduction) via G protein-coupled receptors (primarily G i/o type). Receptor subtypes for NPY and relatedThe authors are affiliated with Lundbeck Research USA as either employees or paid collaborators. Lundbeck has a research program to study the therapeutic potential of the Y 5 receptor as a drug target.