Cholinergic modulation of hippocampal CA1 pyramidal neuron intrinsic excitability

Abstract

Cholinergic stimulation in the hippocampus increases neuronal excitability that contributes to rhythmic activity and seizures. In this dissertation, I used the whole-cell patch clamp techniques in hippocampal CA 1 pyramidal neurons to study the current underlying the cholinergic-dependent plateau potential (PP). When combined with muscarinic stimulation, depolarizing command potentials to evoke Ca²⁺ influx elicited a prolonged inward tail current (Itail). Itail had an extrapolated reversal potential of -20 m V and was abolished by chelating [Ca²⁺]i with BAPTA or removal of [Na+]0 Soluble guanylate cyclase (sGC) inhibitors depressed Itail whereas blocking cGMP specific phosphodiesterase enhanced Itail. Cyclic-nucleotide gated (CNG) channel blockers reversibly depressed Itail, but PKG inhibition had no effect. Thus, I propose that Ca²⁺ influx combined with muscarinic stimulation activates sGC and increases cGMP levels, leading to CNG channel activation and generation of the PP. R-type Ca²⁺ spikes are important in generating dendritic Ca²⁺ transients and burst firing. I show that R-type Ca²⁺ spikes are enhanced by activation of Ml/M3 muscarinic receptors. This enhancement requires increased [Ca²⁺]i and the activation of a Ca²⁺/calmodulin-dependent protein kinase II pathway. Enhanced R-type Ca²⁺ spikes could repetitively fire at theta frequencies (~5-10 Hz) and could contribute to spike generated DAPs and PPs. Blocking R-type Ca²⁺ channels depressed carbachol-induced field potential theta oscillations, implying that enhanced R-type Ca²⁺ spikes play a role in theta current generation. The sensitivity of the PP to the antiepiletic topiramate was investigated. In current clamp, PPs were depressed by therapeutic concentrations of topiramate. Conversely, ItailS evoked in voltage clamp were not depressed but significantly longer voltage steps were required to elicit Itail indicating that the Ca²⁺entry trigger for evoking PPs was depressed. Topiramate had no effect on Ca²⁺ spikes in control conditions, however Ca²⁺ spikes after muscarinic stimulation were reduced. Carbachol enhances R-type Ca²⁺ spikes; therefore, the effect of topiramate was tested on R-type Ca²⁺ channels. Carbachol-enhanced R-type Ca²⁺ spikes were depressed by topiramate. Topiramate also depressed currents mediated by Cav2.3 Ca²⁺ channels expressed in tsA-201 cells. Thus, I have found that topiramate reduces seizure activity in hippocampal neurons through a novel inhibitory action of R-type CA²⁺channels.