Activation of the Central Medial Thalamic Afferent to the Orbitofrontal Cortex Contributes to Successful Reversal Learning

Abstract

Stress‐related psychiatric disorders, such as major depressive disorder and anxiety disorders, have cognitive symptoms that are not well‐treated by classical antidepressants and cause major detriments to patients’ quality of life. Reversal learning, a form of cognitive flexibility necessary to adapt to a changing environment, is disrupted in stress‐related psychiatric disorders. The orbitofrontal cortex (OFC) mediates reversal learning, and hyperactivity in the OFC is associated with psychiatric disorders in humans. Using a reward‐based discrimination digging task to assess reversal learning in rodents, we have previously reported that chronic stress impairs reversal learning and that inducing long‐term depression in the mediodorsal thalamus to OFC pathway reverses these deficits. However, the circuit‐level mechanisms underlying reversal learning are not well established. Preliminary data using Fos immunohistochemistry, showed a significant decrease in Fos in the lateral OFC following reversal learning, and a significant increase in Fos induction in the central medial thalamus (CM). Converging evidence from other laboratories indicates the CM inhibits the prefrontal cortex. Therefore, we next tested the hypothesis that input to the OFC from the CM, inhibiting the OFC, contributes to successful reversal learning. We used an adeno‐associated virus to deliver an inhibitory (Gi) DREADD or GFP control construct into the CM under the control of the CaMKII promoter, and implanted guide cannulae into the lateral OFC for administration of the DREADD agonist directly at the DREADD‐expressing terminals from the CM. Animals received microinjections of DREADD agonist clozapine‐N‐oxide (300 µM, i.c.) directly preceding the reversal learning task. Activating the Gi DREADD significantly impaired reversal learning, increasing the number of trials it took animals to make six consecutive correct responses. These results suggest that excitatory input from the CM to the OFC is critical for successful reversal learning, indicating a novel role for the CM in cognitive flexibility. Future experiments will investigate the role of thalamic inputs to the OFC in the effects of stress on reversal learning.