Activation of Ryanodine Receptors is Required for PKA-mediated Downregulation of A-type K+ Channels in Rat Hippocampal Neurons

Abstract

A-type K+ channels (IA channels) contribute to learning and memory mechanisms by regulating neuronal excitabilities in the CNS, and their expression level is targeted by Ca2+ influx via synaptic NMDA receptors (NMDARs) during long-term potentiation (LTP). However, it is not clear how local synaptic Ca2+ changes induce IA downregulation throughout the neuron, extending from the active synapse to the soma. In this study, we tested if two major receptors of endoplasmic reticulum (ER), ryanodine (RyRs), and IP3 (IP3R) receptors, are involved in Ca2+-mediated I-A downregulation in cultured hippocampal neurons of rats. The downregulation of I-A channels was induced by doubling the Ca2+ concentration in culture media (3.6mM for 24 hrs) or treating with glycine (200 mu M for 3min) to induce chemical LTP (cLTP), and the changes in I-A peaks were measured electrophysiologically by a whole-cell patch. We confirmed that Ca2+ or glycine treatment significantly reduced I-A peaks and that their effects were abolished by blocking NMDARs or voltage-dependent Ca2+ channels (VDCCs). In this cellular processing, blocking RyRs (by ryanodine, 10 mu M) but not IP(3)Rs (by 2APB, 100 mu M) completely abolished IA downregulation, and the LTP observed in hippocampal slices was more diminished by ryanodine rather than 2APB. Furthermore, blocking RyRs also reduced Ca2+-mediated PKA activation, indicating that sequential signaling cascades, including the ER and PKA, are involved in regulating IA downregulation. These results strongly suggest a possibility that RyR contribution and mediated IA downregulation are required to regulate membrane excitability as well as synaptic plasticity in CA3-CA1 connections of the hippocampus. (C) 2017 Wiley Periodicals, Inc.