Two minimally invasive strategies to implant guide cannulas for multiple injections in deep brain areas

Temporal lobe epilepsy (TLE) is characterized by seizures that originate in temporal structures and that are pharmacoresistant in ∼ 40 % of patients. In the context of a preclinical study aimed at developing an innovative therapy to treat TLE, we needed to perform multiple intracranial injections in the rat ventral CA3 (vCA3). To reduce invasiveness and to increase the precision reproducibility when multiple injections are performed over time, we opted for the implantation of guide cannulas. In the conventional approach, the guide cannula is implanted close to the target zone damaging the brain tissue along the route of the cannula insertion. This is a particularly relevant issue in our study because vCA3 is situated deep in the rat brain. The damage caused by the standard procedure would severely compromise the integrity of the hippocampal tissue necessary for the effectiveness of the therapeutic intervention. To overcome this problem, we developed, in TLE adult rats, two novel approaches to implant guide cannulas more superficially: the "above dentate gyrus (DG)" and the "above hippocampus (HPC)" strategies. The target brain area was then reached with the thinner infusion needle, resulting in minimally invasive approaches. We demonstrated by immunofluorescence that both novel surgical approaches enable injections of different agents into the ventral hippocampus with excellent precision and reproducibility. Being this aspect comparable between the two approaches, we concluded that the "above HPC" strategy must be preferred due to its lower invasiveness. Behavioral tests confirmed that memory, locomotion and anxiety level were not affected by the cannula-induced damage.