Development of the calcium plateau following status epilepticus: role of calcium in epileptogenesis

Abstract

Status epilepticus is a clinical emergency defined as continuous seizure activity or rapid, recurrent seizures without regaining consciousness and can lead to the development of acquired epilepsy, characterized by spontaneous, recurrent seizures. Understanding epileptogenesis--the transformation of healthy brain tissue into hyperexcitable neuronal networks--is an important challenge and the elucidation of molecular mechanisms can lend insight into new therapeutic targets to halt this progression. It has been demonstrated that intracellular calcium increases during status epilepticus and that these elevations are maintained past the duration of the injury (Ca(2+) plateau). As an important second messenger, Ca(2+) elevations can lead to changes in gene expression, neurotransmitter release and plasticity. Thus, characterization of the post-injury Ca(2+) plateau may be important in eventually understanding the pathophysiology of epileptogenesis and preventing the progression to chronic epilepsy after brain injury.

Figure 1. Alterations in intracellular Ca²⁺ concentration immediately following 3 h of low Mg²⁺-induced in vitro SE

(A) Representative whole-cell current-clamp electrophysiological recording obtained from a control neuron and a neuron in low Mg²⁺ or in vitro SE. The neuron in low Mg²⁺ shows epileptiform discharges at a frequency greater than 3 Hz. By contrast, the control neuron shows action potentials intermittently, consistent with baseline firing activity. (B) Bar graph comparing fura-2 340/380 ratios in control neurons versus neurons immediately following 3 h of low Mg²⁺. The low Mg²⁺-treated cells exhibit ratio values of 1.4 ± 0.17 as compared with the control neurons with values of 0.20 ± 0.04 (data expressed as mean ± standard error of the mean). (C) Representative pseudocolor ratio images obtained from control (left panel) and low Mg²⁺-treated (right panel) neurons. Neurons from the low Mg²⁺ group show an increased 340/380 ratio immediately following treatment. SEM: Status epilepticus. (A) reproduced with permission from [83].

Figure 2. SE causes increased [Ca²⁺]_i acutely in the in vivo pilocarpine model

(A) Representative EEG recording from control (vehicle) rats and rats in pilocarpine-induced SE. (B) Bar graph demonstrating that [Ca²⁺]_i is higher in neurons from animals immediately following SE when compared with those from animals in the control group: 850 ± 59 nM and 90 ± 22 nM, respectively (data expressed as mean ± standard error of the mean). [Ca²⁺]_i: Intracellular Ca²⁺ concentration. (A) reproduced with permission from [90] and (B) adapted with permission from [32].

Figure 3. The Ca²⁺ plateau following SE

At one day post-SE, ([Ca²⁺]_i) are significantly elevated when compared with controls, 790 ± 43 nM and 98 ± 10 nM, respectively (all data expressed as mean ± standard error of the mean). A total of 2 days post-SE the [Ca²⁺]_i is 684 ± 47 nM, which is significantly higher than the 96 ± 14 nM observed in the control. At 6 days following SE, the [Ca²⁺]_i is still markedly elevated when compared with controls, yet has fallen slightly from earlier timepoints, 475 ± 36 nM and 100 ± 10 nM, respectively. [Ca²⁺]_i falls slightly at 10 days post-SE to 395 ± 56 nM but is still significantly elevated when compared with the controls with [Ca²⁺]_i of 95 ± 14 nM. Finally, at 30 days post-SE the [Ca²⁺]_i is 361 ± 28 nM versus 106 ± 16 nM in controls. The persistent elevation in [Ca²⁺]_i represents the Ca²⁺ plateau. [Ca²⁺]_i: Intracellular Ca²⁺ concentration; SE: Status epilepticus. Adapted with permission from [32].

Figure 4. Acquired epilepsy is associated with alterations in [Ca²⁺]_i and Ca²⁺ homeostatic mechanisms 24 h post-in vitro SE

(A) Electrophysiological traces using current-clamp technique demonstrate baseline activity in control neurons and SREDs in neurons that had been treated in low Mg²⁺ for 3 h and then returned to normal maintenance media. Resting membrane potential = -61.5 mV; duration of shown trace = 30 min. (B) Bar graph comparing 340/380 ratio values in control versus SE groups 24 h after treatment with Mg²⁺-containing buffer solution or low Mg²⁺, respectively. Neurons in the SE group exhibit a 340/380 ratio of 0.5 ± 0.08 (mean ratio ± standard error of the mean). The ratio from control group neurons is significantly lower at 0.20 ± 0.05. (C) Glutamate stimulation causes increased [Ca²⁺]_i. Peak ratios following glutamate are normalized to 1.0. Recovery from this challenge is hindered in epileptic neurons when compared with controls. [Ca²⁺]_i: Intracellular Ca²⁺ concentration; SE: Status epilepticus; SRED: Spontaneous, recurrent epileptiform discharge. (A) reproduced with permission from [83] and (B) reproduced with permission from [37].

Figure 5. Acquired epilepsy is associated with alterations in [Ca²⁺]_i and Ca²⁺ homeostatic mechanisms one year post-in vivo status epilepticus (SE)

(A) Representative EEG recording from control (vehicle) rats and epileptic rats (following pilocarpine-induced SE). (B) Bar graph with neurons from epileptic animals at 1 year post-SE demonstrating [Ca²⁺]_i of 325 ± 35 nM (data represented as mean ± standard error of the mean), which is significantly higher than neurons from control group. (C) Upon glutamate stimulation, the [Ca²⁺]_i increases and then returns to baseline in neurons from the control group, unlike neurons from the epileptic group which also peak but then fail to return to baseline. [Ca²⁺]_i: Intracellular Ca²⁺ concentration. (B) adapted with permission from [32] and (C) reproduced with permission from [35].