Oscillatory and epileptiform activity in human and rodent cortical regions in vitro

Abstract

Epilepsy is a chronic neurological disorder in which patients have spontaneous recurrent seizures. Approximately 50 million people worldwide live with epilepsy and of those ~30% fail to adequately respond to anti-epileptic drugs (AEDs), indicating a need for further research. In this study oscillatory and epileptiform activity was explored in the rodent piriform cortex (PC) in vitro, an underexplored brain region implicated in the development of epilepsy. PC gamma oscillations have been studied in both anaesthetised and awake rodents in vivo; however, to date they have not been reported in vitro. Extracellular field potential recordings were made in rodent PC brain slices prepared from 70-100g male Wistar rats in vitro. Application of kainic acid and carbachol reliably induced persistent gamma oscillations (30 – 40 Hz) in layer II of the PC. These oscillations were found to be pharmacologically similar to gamma oscillations previously found in other rodent brain regions in vitro, as they were dependent on GABAA receptors, AMPA receptors and gap junctions. Persistent oscillations were also induced and characterised for the first time in human neuronal tissue in vitro. Human brain slices were prepared from excised tissue from various brain regions (primarily temporal) from paediatric patients undergoing surgery to alleviate the symptoms of drug resistant epilepsy. As in the rodent PC, oscillations were induced by application of kainic acid and carbachol, however, these oscillations were found to be within the beta frequency range (12 – 30 Hz). Despite this difference in frequency band, these beta oscillations were pharmacologically similar to gamma oscillations found in the rodent PC. Seizure-like events (SLEs) were induced in brain slices prepared from 70-100g male Wistar rats via application of zero Mg2+ artificial cerebral spinal fluid (0[Mg]2+ aCSF). The properties of these SLEs were found to be similar between brain regions when recordings were performed in layer II of the anterior and posterior PC and lateral entorhinal cortex (LEC) and the stratum pyramidale of CA1. In the majority of recordings SLEs occurred in the PC before the LEC or CA1 and SLEs were displayed in the PC in a higher proportion of slices than the LEC. The sensitivity of these PC slices to 0[Mg]2+ aCSF was assessed at several stages (24 hours and 1 week (early latent), 4 weeks (mid latent) and 3 months+ (chronic period)) following the reduced intensity status epilepticus (SE) protocol for epilepsy induction compared to age-matched controls (AMCs). A decrease in excitability of the slices was observed in slices prepared from AMC animals with age, as the inter-event interval and latency to first SLE was observed to be longer in slices prepared from aged compared to young AMC animals. Slices prepared from SE animals maintained their youthful hyperexcitability with no difference in IEI or latency to first SLE observed in the early latent period compared to the chronic period. The pharmacoresistance (or sensitivity) of these SLEs to single and double AED challenge was evaluated. Differences in efficacy of the AEDs were found between SE and AMC in the mid-latent period; increased efficacy of Na+ channel modulating AEDs were found in slices prepared from SE compared to AMC animals. The proportion of slices that displayed pharmacoresistance of these SLEs to AEDs was found to be higher in slices prepared from young animals (early latent period and AMCs), and was similar to that found clinically in human patients. The pharmacoresistance of the SLEs to AEDs was lower in slices prepared from older animals (mid latent, chronic and AMCs) compared to young animals (early latent and AMCs). This age-dependent reduction in resistance likely reflects normal alterations in neuronal networks with ageing. SLEs induced in young control PC slices could be exploited as a new in vitro model of drug resistant epilepsy. Overall, oscillatory and epileptiform activity in the PC and human cortex in vitro could be further explored as tools to evaluate the efficacy and mechanism of action of newly developed AEDs, as well as to explore the networks involved in drug resistant epilepsy.