Deciphering the peculiarities of cell types in the septum

Similar to other brain regions, the neurons in the lateral septum (LS) are of heterogeneous populations. However, their resting membrane potential (RMP) on average is not too far apart. Cells were characterized based on biological markers by using brain slices, as under these in vitro conditions, neurons retain their morphologies. Since the LS neurons are not spontaneously excitable at RMP, the action potentials (APs) were evoked via injections of currents of moderate magnitude during the patch-clamp recordings. In coronal brain slices of rats, a smaller portion of neurons generated a train of APs of complex nature. In order to define the types of neurons with similar phenotypes, we subsequently used the four lines of td-Tomato transgenic mice. The brains of these mice express the promoter fluorophore td-Tomato and enhanced green fluorescent protein (eGFP). Therefore, recordings were conducted in a targeted manner in neurons expressing glutamic acid decarboxylase (GAD), parvalbumin (PV), somatostatin (SOM), or vasoactive intestinal polypeptide (VIP). Similar spike phenotypes that we refer to as type III, in order to distinguish from AP in principal cells - type I and those in interneurons - type II, also exist in mice, substantiating a similitude among rodents. The type III AP is selectively triggered by Ca²⁺ in GAD and SOM-positive neurons. Conclusions are supported by established pharmacologic tools, nimodipine, TTX, and ZD7288, a selective HCN channel antagonist.Collectively, these observations revitalize our knowledge from pioneering studies with regard to the brain of mammals in general and septal structures in particular.