Abstracts

Rationale: High-frequency oscillations (HFOs, ripples: 80-200 Hz, fast ripples: 250-500 Hz) are thought to be important markers of epileptogenesis and ictogenesis. They are recorded in epileptic patients, in animal models of temporal lobe epilepsy and in in vitro preparations. Here, we addressed the role of neurosteroids in the modulation of HFOs recorded in vitro in the piriform cortex, a highly excitable structure that is implicated in seizure generation and maintenance. Methods: Epileptiform activity was induced in horizontal rat brain slices by bath application of 4-aminopyridine (4AP, 50µM). Pharmacological manipulation of GABAergic signaling was performed under 4AP conditions with the application of THDOC at 100nM, 1µM and 5µM concentrations. Field potential recordings were obtained from the anterior and posterior piriform cortex (n= 6 for each experimental condition). Traces were analyzed offline with pClamp. HFOs were detected with custom built functions in MATLAB. Results: THDOC significantly reduced the duration of ictal-like events in both the anterior and posterior piriform cortex. Under 4AP alone, ictal events were 89.46 ± 9.03s (±SEM) in duration, but with the application of 100nM, 1µM and 5µM THDOC, the duration decreased by 26%, 15% and 27%, respectively. The analysis of HFOs showed that the anterior piriform cortex contained a greater number of HFOs compared to the posterior piriform cortex. On average, 14 ripples and 45 fast ripples were detected per ictal-like event in the anterior region compared to 7 ripples and 14 fast ripples detected in the posterior piriform cortex. THDOC reduced the number of both ripples and fast ripples in the anterior piriform cortex by 70% with no significant difference between concentrations but failed in inducing significant changes in HFOs in the posterior piriform cortex. Conclusions: Our results demonstrate that enhancing extrasynaptic GABAA receptor activity with THDOC (which presumably leads to a greater tonic inhibitory current) decreases the duration of ictal events. These changes appear to influence the occurrence of HFOs in the anterior piriform cortex, possibly due to the differential expression of GABAA receptors and their respective subunits between the two regions. We propose that GABAA receptor mediated signaling can modulate 4AP-induced ictogenesis and HFOs by affecting network excitability.