Relation between responsiveness to neurotransmitters and complexity of epileptiform activity in rat hippocampal CA1 neurons

Purpose: Our previous works suggested that sensitivity of neurons with chaotic firing patterns to stimuli is significantly greater than that in neurons with periodic firing patterns, which shows that responsiveness of neurons may depend on the complexity of the firing series. This study was performed to determine the relation between responsiveness of the hippocampal CA1 neurons with epileptiform activity (EA) to neurotransmitters and their complexity of firing series. Methods: Firing series of CA1 neurons were recorded extracellularly in rat hippocampal slice. Approximate entropy was used to describe the complexity of the interspike interval (ISI) series. EA was induced by local application of penicillin (1,000 IU/ml). The change of firing rate induced by neurotransmitters (glutamate and γ-aminobutyric acid) was compared with that of the degree of complexity of ISI series in the process of EA. Results: The excitatory responses to glutamate and the inhibitory responses to γ-aminobutyric acid in CA1 neurons appeared to be decreased during the process of penicillin-induced EA. However, during this same process, the approximate entropy of the ISI series also was decreased significantly. Conclusions: The results suggest that the reduced responses to neurotransmitters of the CA1 neurons appear to be closely related to the onset of EA. Furthermore, these neurons show that the changes in responsiveness are closely parallel to the decrease of degree of complexity of firing series during penicillin epileptogenesis.