Efficacy of Venlafaxine and Deep Brain Stimulation Against the Effects of Hippocampal Lesion with Ibotenic Acid in Animals Exposed to the Chronic Mild Stress Model of Depression.

Introduction: Dysfunction of the pathway between the ventral hippocampus (vHPC) and medial prefrontal cortex (mPFC) may be responsible for the weaker or lack of efficacy of antidepressant drugs in patients suffering from treatment-resistant depression. This study aims to evaluate the behavioural effects of vHPC lesion with ibotenic acid (IBO) in animals subjected to the chronic mild stress (CMS) procedure and treated with either chronic venlafaxine or acute deep brain stimulation (DBS) in the mPFC. In addition, electrophysiological studies are expected to reveal neuromodulatory effects on the function and plasticity of mPFC neurons in response to stress, lesion, and deep brain stimulation (DBS).

Methods: Wistar Han rats were exposed to the chronic mild stress model of depression and IBO lesion in vHPC. The effects of both procedures were evaluated in a series of behavioural tests (sucrose test, elevated plus maze, novel object recognition, and social interaction) and in electrophysiological recordings (field potential recording and LTP induction).

Results: The CMS procedure caused a decrease in sucrose consumption, deficits in cognitive function and social interaction, and increased anxiety. The lesion in vHPC with IBO resulted in similar behavioral changes. Repeated (5 weeks) administration of venlafaxine (10 mg/kg, IP) reversed these deficits in stressed animals but was only partially effective in reversing the effects of IBO lesion in HPC. In contrast, the neuromodulation strategy with DBS of the mPFC produced a robust reversal of all behavioural changes observed in both stressed and lesioned rats. The CMS did not affect the amplitude of Field potentials in mPFC slices, but the induction of Long-Term Potentiation was impaired in these animals. The IBO lesion significantly reduced the amplitude of Field potentials as compared to unstressed rats. Both repeated venlafaxine and acute DBS normalized these effects of the IBO lesion.

Discussion: Observed effects were fully normalized by DBS in mPFC but not by venlafaxine, which only partially reversed the IBO lesion-induced effects. The weaker sensitivity of vHPC-lesioned animals to the therapeutic action of venlafaxine provides further evidence that insufficient transmission from the vHPC to the mPFC could be responsible for antidepressant non-response.

Conclusion: These data support the hypothesis that resistance to antidepressant treatment may result from the inability of antidepressants to fully activate the impaired vHPC-PFC pathway, which could be overcome by the neuromodulatory properties of deep brain stimulation.