Regional specialization of upper‐layer inhibitory interneuron organization in the human prefrontal cortex

Abstract

GABAergic interneurons play a central role in shaping cortical microcircuit function, yet the anatomical organization of major interneuron populations across different subdivisions of the human prefrontal cortex remains poorly characterized. The present study examined the laminar distribution and regional organization of major GABAergic interneuron populations in the dorsal (Brodmann area 9) and ventral (Brodmann area 14r) portions of the human prefrontal cortex. Postmortem human prefrontal cortex tissue from five normotypical adult male brains was analysed using the following molecular markers: calretinin (CR), parvalbumin (PV), calbindin (CB), and somatostatin (SOM). Double‐labelling immunofluorescence was used to assess overlaps between different GABAergic interneuron populations, and RNAscope in situ hybridization was used to confirm their neurotransmitter phenotype. All analysed populations demonstrated a GABAergic molecular phenotype. CR ⁺ neurons constituted the largest interneuron population in both analysed cortical regions and showed marked supragranular enrichment. CB ⁺ neurons were also concentrated in upper cortical layers and frequently co‐expressed SOM. By contrast, PV ⁺ neurons were concentrated in layers III–IV, whereas SOM ⁺ neurons were distributed more uniformly across layers II–VI. The principal regional difference was a significantly higher proportion of CR ⁺ neurons in BA14r compared with BA9 (10.24% vs. 8.21% of all neurons; p  = 0.0248), while exploratory analyses additionally indicated a higher CR/PV ratio and a lower PV/SOM ratio. These differences were driven predominantly by variation in supragranular interneuron composition, while infragranular organization remained comparatively similar between the two cortical regions. These findings support region‐specific specialization of inhibitory interneuron organization in the human prefrontal cortex and suggest that differences in supragranular inhibitory circuitry may contribute to the distinct functional architecture of dorsal and ventral prefrontal cortico‐cortical networks.