A conserved population of genetically defined striatal neurons gates opioid reward.

A longstanding paradox in striatal circuit architecture is that opioid reward depends on μ-opioid receptors (µORs) in nucleus accumbens medium spiny neurons (MSNs), yet µOR function is not explained by the canonical D1/direct and D2/indirect pathway organization. Here, we identify a rare MSN population marked by Chst9 that exhibits exceptionally high expression of the µOR and is conserved across species. Notably, Chst9-MSNs comprise a specialized indirect pathway striatal neuron subtype that is molecularly and spatially distinct from canonical striatal populations. Opioids robustly silence Chst9-MSNs, and selective deletion of Oprm1 from this population abolishes fentanyl-conditioned place preference. These findings establish Chst9-MSNs as a critical substrate for opioid reward and define a new cellular framework for therapies targeting opioid use disorder.