The role of canonical transient receptor potential channels in seizure and excitotoxicity

Abstract

Canonical transient receptor potential (TRPC) channels are a family of polymodal cation channels with some degree of Ca2+ permeability. Although initially thought to be channels mediating store-operated Ca2+ influx, TRPC channels can be activated by stimulation of Gq-coupled G-protein coupled receptors, or by an increase in intracellular free Ca2+ concentration. Thus, activation of TRPC channels could be a common downstream event of many signaling pathways that contribute to seizure and excitotoxicity, such as N-methyl-D-aspartate (NMDA) receptor-mediated Ca2+ influx, or metabotropic glutamate receptor activation. Recent studies with genetic ablation of various TRPC family members have demonstrated that TRPC channels, in particular heteromeric TRPC1/4 channels and homomeric TRPC5 channels, play a critical role in both pilocarpine-induced acute seizures and neuronal cell death. However, exact underlying mechanisms remain to be fully elucidated, and selective TRPC modulators and antibodies with better specificity are urgently needed for future research.

Figure 1

Quantitative comparison of the 1S,3R-ACPD-induced plateau potential in lateral septal neurons in WT and TRPC KO mice. (A) Representative traces showing the normal decay of the membrane potential after spikes generated by a brief depolarizing current pulse under control conditions and the plateau potential after spikes generated by the same depolarizing current pulse following bath perfusion of 30 µM 1S,3R-ACPD. The area-under-the curve (area; shaded) was measured from the end of the depolarizing current pulse to a time point after the end of the plateau potential (typically 1 or 2 s). The mean of total area (shaded with both light and dark gray) in the presence of 1S,3R-ACPD for each lateral septal neuron was plotted in B; (B) The total area in the presence of 1S,3R-ACPD in WT, TRPC1KO, TRPC1/4DKO, TRPC3KO, TRPC5KO, TRPC6KO, TRPC7KO (n = 34, 36, 9, 10, 6, 7, 9 respectively). Note that the TRPC1KO and TRPC1/4DKO groups were significantly different from the WT group (***: p < 0.001, Kruskal-Wallis test and Dunn's Multiple Comparison Test). Traces in A adapted from Figure 3B of [33]. The WT, TRPC1KO and TRPC1/4DKO data in B are replotted from the raw data presented in Figure 3C of [33]. The TRPC3KO, TRPC5KO and TRPC6KO data were reported in [33]. TRPC7KO data is unpublished data that was obtained in a similar manner.

Figure 2

The signaling cascade leading to epileptiform burst firing in lateral septal neuron.

Figure 3

Pilocarpine-induced seizures were significantly reduced in TRPC5KO mice. The time course of pilocarpine-induced seizures in WT, TRPC1KO, TRPC1/4DKO, and TRPC5KO mice after a single injection of pilocarpine (280 mg/kg, i.p.). Pooled data (mean ± SEM) was plotted (n = 18, 14, 6, 19 for WT, TRPC1KO, TRPC1/4DKO and TRPC5KO mice). See Phelan et al. 2012 for description of seizure scale. Note significantly reduced seizure scores in TRPC5KO mice at the late phase after pilocarpine injection (***: p < 0.001, Two-way ANOVA; *: p < 0.05; **: p < 0.01, Bonferroni post hoc tests against WT). The TRPC1KO and TRPC5KO data are adapted from Figure 2 of [70], while the TRPC1/4KO data is from Figure 4A of [33].

Figure 4

A working hypothesis for TRPC5 in seizure and excitotoxicity. We propose that TRPC5 is critical for mGluR1/mGluR5-mediated enhancement of NMDA receptor-dependent long-term potentiation at SC synapses in CA1 and RC synapses in CA3. The enhanced LTP at SC synapses contributes to hyperexcitability and excitotoxicity in CA1 while enhanced LTP at RC synapses in CA3 contributes to the initiation of seizure and excitotoxicity in CA3. EC: entorhinal cortex; DG: dentate gyrus; MF: mossy fiber; RC: recurrent collaterals; SC: Schaffer collaterals; Sub: subiculum.

Figure 5

Neuronal cell death after pilocarpine-induced seizures in the hippocampus of WT, TRPC1KO, TRPC1/4DKO and TRPC5KO mice. Neuronal cell death induced by pilocarpine-induced seizures was assessed in selected groups of mice with comparable average seizure scores above three for WT (3.79 ± 0.14, n = 6), TRPC1KO (3.89 ± 0.13, n = 6), TRPC1/4DKO (3.89 ± 0.20, n = 5), and TRPC5KO mice (3.45 ± 0.10, n = 5). Serial coronal sections (50 µm) from mice with similar pilocarpine-induced seizures were stained with Nissl and surviving neurons (with stained cytoplasm and round nuclei) were counted using Stereologer with a 100X oil-immersion objective. The percentage of surviving neurons was calculated by dividing the cell count in each pilocarpine-treated mouse with the averaged cell count in WT control mice (mice without seizures). There was no significant difference in the number of neurons between WT and untreated KO mice. Pooled data (mean ± SEM) were plotted. Note the significant increase in surviving neurons in the CA1 region of TRPC1/4DKO and TRPC5KO (**: p < 0.01, ANOVA), and the significant increase in surviving neurons in the CA3 region of TRPC1/4DKO (*: p < 0.05, ANOVA). Adapted from data presented in Figure 6 of [33] and Figure 4 of [70] relative to untreated WT mice.