Rationale: Temporal lobe epilepsy (TLE) is common, devastating, and often medically refractory. Despite years of work, disease modifying therapies still do not exist. Efforts to develop novel therapies have been hindered by the lack of understanding of the cellular basis for TLE. Similar challenges have plagued the field of learning and memory. However, within the past decade, multiple investigations have identified the cellular basis of memory to be the memory engram – a subpopulation of neurons that are connected in a multi-focal network and prove critical for memory development and expression. Building on this work, we ask if a similar cellular network may underlie TLE, namely the epileptic engram.
Methods: To identify the cells responsible for epilepsy development, we utilized the intra-amygdala kainic acid status epilepticus (SE) mouse model in which a brief episode of SE (45 minutes) results in spontaneous recurrent seizure (SRS) development days later. We hypothesized that the hippocampal cells active after SE but before development of SRS might constitute the epileptic engram. To identify such cells, we performed immunohistochemistry for the immediate early gene c-Fos (a known marker of cellular activity) at various time points after SE (1 to 96 hours). We further focused on c-Fos positive cells at 24 hours and performed immunohistochemistry for NeuN, parvalbumin (PV), somatostatin (SST), and glial fibrillary acidic protein (GFAP) to characterize these cells as neuronal, interneurons, and astrocytes.
Results: Immediately following SE, we found a pattern of c-Fos expression in hippocampus bilaterally that starts quite robustly within the dentate gyrus, hilus, and various pyramidal layers and remains stable for approximately six hours. By twelve hours, the majority of c-Fos expression has subsided except for subpopulations of CA3 and CA1 mainly ipsilateral to KA-infuse amygdala. The observed CA3 expression persists until about 24-48 hours post-SE and CA1 expression remains until about 72 hours post-SE. The c-Fos positive cells present at 24 hours comprised a mix of neuronal (NeuN positive) and non-neuronal (GFAP positive) cells. Of the neurons, a subset were positive for either PV or SST. Notably, only a portion of all PV or SST positive cells in the hippocampus were also positive for c-Fos.
Conclusions: In contrast to the fleeting increases of c-Fos expression induced by a brief seizure, status epilepticus in the current model causes persistent c-Fos expression that lasts for 48-72 hours. Interestingly, the sites of
persistent cFos expression coincide with sites from which spontaneous recurrent seizures are thought to arise. These c-Fos positive cells comprise a mix of neurons (both excitatory and inhibitory) as well as astrocytes. Future work will be dedicated towards determining whether these cells are directly contributing to epileptogenesis and could prove to be a novel cellular therapeutic target.
Funding: NIH TL1 Scholar Grant