Differential excitability and voltage-dependent Ca2+ signalling in two types of medial entorhinal cortex layer V neurons

The entorhinal cortex (EC) is a key structure in memory formation, relaying sensory information to the hippocampal formation and processed information to the neocortex. EC neurons in the deep layers modulate the transfer of sensory information by the superficial layers and the dentate gyrus, and form the output to the neocortex. Here we characterize two types of EC layer V neurons by their fluorescence morphology, electrophysiology and intracellular Ca2+ signalling using intracellular recording and Ca2+ imaging. Pyramidal neurons show, in response to depolarizing current pulses, regular firing with strong adaptation and a fast and medium afterhyperpolarization (AHP) which are separated by a depolarizing notch and, with hyperpolarizing current injection, a transient sag. Multipolar cells respond to depolarization with delayed firing with very weak adaptation and have no depolarizing notch between fast and medium AHP and no sag with hyperpolarization. The delayed firing was blocked by 30 muM 4-aminopyridine, indicating mediation by the D-type potassium current. Subthreshold depolarization evoked membrane potential oscillations of 2-5 Hz in both cell types and an increase in [Ca2+](i) of 37 nM in pyramidal and 59 nM in multipolar neurons. Repetitive firing at 10 Hz for 30 s increased [Ca2+](i) in pyramidal and multipolar neurons by 194 and 295 nM, respectively. Differential temporal firing and Ca2+ signalling suggest specific information processing and synaptic memory storage possibilities in these two layer V cell types of the EC.