Functional and molecular characterization of a novel afterdepolarizing current in hippocampal pyramidal neurons

In the hippocampus, pyramidal neuron excitability can be regulated in a fast and dynamic manner during the acquisition and integration of information. In particular, firing patterns are shaped by the combination of coincident inputs and intrinsic electrical properties of the cell. Afterpotentials represent one such intrinsic mechanism whereby the simultaneous activation of Ca2+- and voltagegated conductances following firing activity determines the membrane potential of the neuron for up to several seconds and influences further spike output. In the present study, the use of whole-cell patch clamp electrophysiology and pharmacology reveals the occurrence of a novel afterdepolarizing (ADP) mixedcationic current in cultured hippocampal neurons and slices. This conductance, named IADP, has a time course in the order of hundreds of milliseconds and can be visualized upon pharmacological suppression of the afterhyperpolarizing (AHP) currents mediated by SK and KCNQ channels and underlying the medium afterhyperpolarization (mAHP) in these neurons. IADP is activated by intracellular Ca2+ rises as demonstrated using BAPTA in the pipette solution, is carried mainly by Na+ ions and is positively modulated by temperatures in the physiological range. The pharmacological profiling of IADP indicates that a transient receptor potential channel (TRP) mediates a component of this current. In particular, the actions of the specific TRPM2 channel agonist adenosine diphosphate ribose (ADPR), in both cultured neurons and acute slices, point at the involvement of this cationic channel in the generation of a post-spike depolarization in these neurons. This result was corroborated by evidence from a heterologous expression system and confirmed in the culture preparation by genetic manipulation of the functional expression of TRPM2 channels using a dominant negative mutant subunit. Finally, experiments in hippocampal slices using the specific agonist ADPR demonstrate that, under physiological conditions, IADP indeed coexists and interacts with currents underlying the mAHP and that they act in unison to regulate early spike frequency adaptation in CA1 pyramidal cells. Thus, TRPM2 channels mediate a Ca2+-activated cationic current in hippocampal pyramidal cells, which contributes, at least in part, to a medium duration ADP with important implications for the regulation of neuronal firing activity