The Synaptic and Intrinsic Cellular Mechanisms of Persistent Firing in Neurogliaform Cells

While persistent firing in glutamatergic neurons has been well-characterized, the intrinsic and synaptic mechanisms driving this phenomenon in neurogliaform cells (NGFCs), a subtype of GABAergic interneurons, remain unclear. This study investigates the mechanisms underlying persistent firing in hippocampal NGFCs. Whole-cell current-clamp recordings were performed on acute brain slices from C57BL/6J mice to examine the electrophysiological properties of NGFCs in the hippocampal stratum lacunosum-moleculare (SLM). Pharmacological interventions, including T-type calcium channel blocker ML218 and 5-hydroxytryptamine (5-HT) receptor antagonist olanzapine, were used to dissect the mechanisms of persistent firing. Biocytin labeling and confocal microscopy were employed to confirm neuronal morphology and location. The study revealed that persistent firing in NGFCs is induced by a long-lasting delayed afterdepolarization (L-ADP), which depends on T-type calcium channels (intrinsic mechanism) and is modulated by 5-HT receptors (synaptic mechanism). Persistent firing was observed in 62.96% of SLM neurons and was abolished by ML218 or olanzapine. The findings bridge a gap in understanding how inhibitory interneurons contribute to memory processes. The dual-mechanism framework (T-type channels and 5-HT receptors) aligns with prior work on glutamatergic systems but highlights unique features of GABAergic persistent firing. These insights advance the understanding of inhibitory circuit dynamics and their potential role in cognitive functions, paving the way for further research into interneuron-specific memory encoding.