Chronic deep brain stimulation of the rat ventral medial prefrontal cortex disrupts hippocampal–prefrontal coherence

• DBS to infralimbic cortex reduced coherence with hippocampus in 2–4 Hz but not theta frequencies.
• DBS effects were observed after 10, but not 1, days of treatment.
• DBS effects persisted across behaviors and running speeds.
• Reduced 2–4 Hz coherence was correlated with relative 2–4 Hz activity in the VH.
• Coherence was not affected by the serotonin reuptake inhibitor fluoxetine.

Deep brain stimulation (DBS) of the subgenual cingulate gyrus (SCG) has been used to treat patients with treatment-resistant depression. As in humans, DBS applied to the ventromedial prefrontal cortex of rats induces antidepressant-like responses. Physiological interactions between structures that play a role in depression and antidepressant treatment are still unknown. The present study examined the effect of DBS on inter-region communication by measuring the coherence of local field potentials in the rat infralimbic cortex (IL; homologue of the SCG) and one of its major afferents, the ventral hippocampus (VH). Rats received daily IL DBS treatment (100 μA, 90 μs, 130 Hz; 8 h/day). Recordings were conducted in unrestrained, behaving animals on the day before treatment, after 1 and 10 days of treatment, and 10 days stimulation offset. VH-IL coherence in the 2–4 Hz range was reduced in DBS-treated animals compared with shams after 10 days, but not after only 1 day of treatment. No effect of DBS was observed in the 6–10 Hz (theta) range, where coherence was generally high and could be further evoked with a loud auditory stimulus. Finally, coherence was not affected by fluoxetine (10 mg/kg), suggesting that the effects of DBS were not likely mediated by increased serotonin levels. While these data support the hypothesis that DBS disrupts communication between regions important for expectation-based control of emotion, they also suggest that lasting physiological effects require many days of treatment and, furthermore, may be specific to lower-frequency patterns, the nature and scope of which await further investigation.