Modeling temporal lobe epilepsy with hippocampal sclerosis in rats using the selective neurotoxin stable substance P‐saporin

OBJECTIVE: Temporal lobe epilepsy with hippocampal sclerosis (TLE-HS+) is a common and often refractory form of human epilepsy. In the hippocampus, the neurotoxin stable substance P-saporin (SSP-SAP) targets hilar inhibitory interneurons and has been used to determine whether hilar neuron loss in an otherwise normal brain is sufficient to initiate the epileptogenic process. We determined the time course of SSP-SAP-induced epileptogenesis, as well as the loss of principal cells and astrogliosis, two defining features of TLE-HS+. METHODS: We started with 70-day-old male and female Sprague Dawley rats and injected SSP-SAP into four unilateral sites along the longitudinal axis of the rat dentate gyrus. We determined how long SSP-SAP is detectably present in inhibitory interneurons and then analyzed the electroencephalographic characteristics and behavioral expression of the "reactive" seizures that developed several days after SSP-SAP injection. We then quantified the number of self-generated 7-Hz epileptiform events over 3 months. Finally, we counted the numbers of principal cells and astrocytes up to 2 months postinjection. RESULTS: SSP-SAP was internalized within 2 h after injection and was immunocytochemically undetectable by 5 days. Rats exhibited spontaneous electrographic and behavioral reactive seizures between days 4 and 6 after SSP-SAP injection, with most seizures having a Racine score of either 1 or 2. After the early seizures had abated, the number and duration of self-generated 7-Hz epileptiform events, which were associated with behavioral arrest, progressively increased over the 3-month observation period, indicating epileptogenesis. We also observed the progressive loss of CA1 and CA3 neurons and the progressive increase in astrocytes over time, two defining features of HS, a common pathological observation in TLE patients. SIGNIFICANCE: This study confirms that the SSP-SAP model reproduces the defining features of human TLE and that a primary, selective, and longitudinally extensive γ-aminobutyric acidergic defect is sufficient to trigger epileptogenesis that results in TLE-HS+.