Abstracts

Rationale: Transmeningeally delivered muscimol can prevent and terminate focal neocortical seizures in rats and monkeys, suggesting that this compound is a viable candidate for localized drug treatment of intractable focal epilepsy (Ludvig et al. 2009, Epilepsia 50:678-693). This preliminary study aimed to provide insight into the cellular electrophysiological effects of transmeningeally applied muscimol in the neocortex of freely-moving rats. In vivo cellular electrophysiology offers sensitive means to monitor the time-course of transmeningeal muscimol actions, while can reveal unwanted local tissue reactions not apparent in behavioral and EEG studies. Methods: Five rats were implanted with an epidural cup over the right somatosensory/motor cortex, followed by the stereotaxic insertion of a microelectrode array into layers III - V underneath the cup. After a 3-day recovery, artifact-free extracellular and EEG recordings were performed in the behaving animals, with the data collected by the SciWorks system of DataWave Technologies. First, acetylcholine (Ach; 50 - 100 mM; 50 microliter) was delivered via the epidural cup to induce brief local EEG seizures, with or without the preceding delivery of muscimol (0.8 - 2.5 mM; 50 microliter; 1 min before Ach) into the cup. These procedures were performed partly to test proper transmeningeal drug diffusion and partly to test the seizure-preventing efficacy of the used muscimol. After washout and a 2-day interval, 1 mM muscimol (50 microliter) was delivered into the cup and the drug effects were monitored for 24-48 hours. A microperfusion pump was used for all drug administrations. Data were analyzed with the SciWorks software, furnishing material for our proprietary analysis program, as well. Results: Ach alone caused local EEG seizures, which were fully prevented with muscimol administration prior to Ach delivery. Muscimol in the used concentration range silenced the recorded neurons within 2 - 10 min (Fig. 1). In the following 8 hours this effect was maintained, with the recorded cells generating bursts at irregular (0.5 - 10-min) intervals. Neuronal activity returned to pre-muscimol patterns within 24 - 48 hours (Fig. 1). Abnormal EEG activity did not develop during muscimol exposure, but the rat’s movement was suppressed. Conclusions: The present study showed that transmeningeally delivered muscimol, in an antiepileptic concentration range, predominantly inhibits neuronal firing in the neocortex: a sustained effect lasting for 8 - 24 hours. This confirms that in vivo cellular electrophysiological monitoring is a useful tool for determining the time-course of drug effects following transmeningeal/neocortical delivery. It has also become clear that while intracranial muscimol is a powerful seizure-preventing compound, the delivery conditions for this compound should be refined so that it can retain its antiepileptic properties without inducing potentially adverse effects on neuronal firing in the treated neocortex.