Selective Inhibitory Circuit Dysfunction after Chronic Frontal Lobe Contusion

Traumatic brain injury (TBI) is a leading cause of neurologic disability; the most common deficits affect prefrontal cortex-dependent functions such as attention, working memory, social behavior, and mental flexibility. Despite this prevalence, little is known about the pathophysiology that develops in frontal cortical microcircuits after TBI. We investigated if alterations in subtype-specific inhibitory circuits are associated with cognitive inflexibility in a mouse model of frontal lobe contusion that recapitulates aberrant mental flexibility as measured by deficits in rule reversal learning. Using patch clamp recordings and optogenetic stimulation, we identified selective vulnerability in the non-fast spiking, somatostatin-expressing (SOM+) subtype of inhibitory neurons in layer V of the orbitofrontal cortex (OFC) two months after injury. These neurons exhibited reduced intrinsic excitability and a decrease in their synaptic output onto pyramidal neurons. By contrast, fast spiking, parvalbumin-expressing (PV+) interneurons did not show changes in intrinsic excitability or synaptic output. Impairments in SOM+ inhibitory circuit function were also associated with network hyperexcitability. These findings provide evidence for selective disruptions within specific inhibitory microcircuits that may guide the development of novel therapeutics for TBI.