Brain electrical activity after acute hippocampal stimulation in awake rats.

Objective: We describe the electrocorticographic findings after hippocampal stimulation in normal awake rats.

Methods: Six male Wistar rats were implanted bilaterally with neocortical and hippocampal electrodes. The animals were submitted to hippocampal low- and high-frequency stimulation.

Results: Recruiting responses were seen in the ipsilateral hippocampus after unilateral low-frequency (6 Hz) hippocampal stimulation with low voltage (0.3 V). These recruiting responses could be seen at the contralateral hippocampus with slightly higher voltage (0.5 V) and over the ipsilateral neocortex with stimulation with 1.2 V. Bilateral neocortical recruiting responses were noted at stimuli voltage of 1.5 V. There were no recruiting responses after high-frequency stimulation (130 Hz). A dorsal column (DC) shift, characterized by baseline oscillation without brain activity modification, was noted in all animals after hippocampal stimulation with frequency higher than 60 Hz. The increase of stimulation frequency from 6 to 130 Hz (1.2 V, 300 µ sec) showed progressive reduction in the amplitude and disappearance of the time-locked recruiting responses, especially from around 60 Hz.

Conclusions: Bilateral hippocampal and cortical recruiting responses were easily obtained in all animals after low-frequency hippocampal unilateral stimulation. High-frequency stimulation did not give rise to recruiting response, although a DC shift was noted. The fact that unilateral hippocampal stimulation might lead to bilateral limbic system modulation suggested that unilateral stimulation might be enough in many situations. Our findings suggested that high-frequency stimulation was more likely to be effective than low-frequency stimulation regarding the potential inactivation of the hippocampus. These findings might prove relevant to the determination of the adequate parameters for stimulation using hippocampal deep brain stimulation (DBS) in the future. An increase in our knowledge on the physiologic mechanisms underlying DBS might be translated into more rational clinical approaches.