Individuals with epilepsy experience recurrent and unprovoked seizures characterized by uncontrolled, excessive neurological activity. Seizures can be debilitating and often resistant to available drug therapies. Temporal lobe epilepsy commonly initiates from foci within the hippocampus, making it a potential target for surgical treatments in cases where common anti-epileptic drugs have failed to produce desired results. The field of neuroscience is rich with studies of the hippocampus, commonly performed using rodent animal models. Most commonly, the hippocampal slice preparation has been used to study the rodent hippocampus in vitro. This preparation, although robust, severs longitudinal hippocampal networks such as the recurrent excitation network within CA3. With the unfolded hippocampus preparation described in this manuscript, entire hippocampi from young mice can now be kept alive for an extended period of time. A portion of this study examines the role of orthogonal pathways preserved in the unfolded hippocampus in the propagation of epileptiform waves across the tissue.
The unfolded hippocampus preparation presents a unique opportunity to study the planar network of pyramidal cells extending from CA1 to CA3. To facilitate this, a micro-electrode array of 8 by 8 recording channels was developed with spikes that penetrate into the pyramidal cell layer for recording. This recording array improves on currently available in vitro arrays by being both penetrating and transparent to light. An amplifier was designed and built for the array with 64 individual amplifiers for the electrodes in the array with a bandwidth of 0.5 Hz to 4 kHz.
The data presented in this manuscript suggest that the unfolded hippocampus preparation maintains normal electrophysiology and as such is useful for the study of the same. Evoked responses showed an excellent correlation to responses seen in slices, indicating preservation of neurons. Experimental wave propagation data suggests that a synaptically dependent 4-AP induced epileptiform wave is generated in the CA3 and propagates across the pyramidal cell matrix across CA3 and into CA1. Further, this epileptiform wave propagation can be arrested by a selective local transverse lesion of the CA3. Finally, tests done with the completed array system show it is capable of recording epileptiform events across the CA1–CA3 in a more robust manner and with lower noise than with traditional voltage sensitive dye RH-414. Together, this work furthers the field of hipppocampal study as it relates to the CA3–CA1 and CA3–CA3 networks and enables a more efficient and detailed analysis of activity in these regions through the use of the recording array system described here.