Neurotoxicity induced by bupivacaine via T-type calcium channels in SH-SY5Y cells.

There is concern regarding neurotoxicity induced by the use of local anesthetics. A previous study showed that an overload of intracellular calcium is involved in the neurotoxic effect of some anesthetics. T-type calcium channels, which lower the threshold of action potentials, can regulate the influx of calcium ions. We hypothesized that T-type calcium channels are involved in bupivacaine-induced neurotoxicity. In this study, we first investigated the effects of different concentrations of bupivacaine on SH-SY5Y cell viability, and established a cell injury model with 1 mM bupivacaine. The cell viability of SH-SY5Y cells was measured following treatment with 1 mM bupivacaine and/or different dosages (10, 50, or 100 µM) of NNC 55-0396 dihydrochloride, an antagonist of T-type calcium channels for 24 h. In addition, we monitored the release of lactate dehydrogenase, cytosolic Ca(2+) ([Ca2+]i), cell apoptosis and caspase-3 expression. SH-SY5Y cells pretreated with different dosages (10, 50, or 100 µM) of NNC 55-0396 dihydrochloride improved cell viability, reduced lactate dehydrogenase release, inhibited apoptosis, and reduced caspase-3 expression following bupivacaine exposure. However, the protective effect of NNC 55-0396 dihydrochloride plateaued. Overall, our results suggest that T-type calcium channels may be involved in bupivacaine neurotoxicity. However, identification of the specific subtype of T calcium channels involved requires further investigation.

Introduction

Regional anesthetics have been used widely in clinical settings and as postoperative analgesics, because of their reduced systemic effects [1], [2]. However, local anesthetics may cause neurotoxicity, such as transient neurological syndrome (TNS), and cauda equina syndrome, which has raised concerns about their use [3], [4]. One multicenter study found that the incidence of TNS was approximately 8.1%, which resulted in pain or sensory abnormalities in the lower back, buttocks, or lower extremities, with symptoms beginning after spinal anesthesia and lasting for hours to 4 days [5]. Although there is low incidence of anesthetic-induced cauda equina syndrome, it results in severe damage to neurons [6]–[8].

Local anesthetics can cause cell apoptosis, induce the release of reactive oxygen species and lactate dehydrogenase (LDH) [9], [10]. Several studies have shown that lidocaine, bupivacaine, tetracaine, dibucaine, and procaine can induce apoptosis [11]. The underlying mechanisms of local anesthetic neurotoxicity are not clearly understood. Previous studies indicated that intracellular calcium overload is involved in local anesthetic-induced neurotoxicity [12], [13]. Extracellular calcium influx and intracellular calcium store release are the most important factors for local anesthetic-induced calcium overload. Also, an influx of extracellular calcium can induce calcium-dependent release of intracellular calcium stores [14], [15].

The main route of extracellular calcium influx into cells is via voltage-dependent calcium channels (VDCCs) [16]. Currents arising from VDCCs are subdivided into two major classes based on the membrane potential at which they become activated: high-voltage activated (HVA), which are further divided into L-, P-, Q-, N- and R-subtypes, and low-voltage activated (LVA) or transient (T-type) Ca2+ currents, which are further divided into Cav3.1, Cav3.2 and Cav3.3 [17]. The T subtype of VDCCs are known to perform several roles in neurons, such as lowering the threshold for action potentials, promoting burst firing, oscillatory behavior, and enhancing synaptic excitation [17]. With electrophysiological characteristics, such as activation at resting potential, T-type calcium channels act as pacemakers in many pathological and physiological conditions [18], [19]. This pacemaker-like activity of T-type calcium channels allows them to regulate the excitability of neurons. T-type calcium channels can be activated at the resting potential, and then extracellular calcium ions enter into the cells by T-type calcium channels. On the one hand, cell membrane depolarization induced by T-type currents activates the HVA channels and promotes extracellular calcium ion entry into the cell. On the other hand, T-type currents prime calcium-induced calcium release (CICR) [20].

Although calcium channel blockers (CCB) can cause cancer cell growth, they can inhibit the neuronal apoptosis in several neuron injury models [21]–[23]. For example, the L-type voltage-gated calcium channel blocker, nifedipine, lowered the intracellular Ca2+ concentration of the cerebellar granule cells treated with kainate from 1543 nM to 764 nM and reduced kainate neurotoxicity. Yagami and colleagues found that S-312-d, another L-type voltage sensitive calcium channel blocker, rescued cortical neurons from apoptosis induced by beta amyloid and human group II A secretory phospholipase A2. The neuroprotective effects of CCB were shown by lowering the intracellular Ca2+ concentration. We conjectured that T-type calcium channels, with the pacemaker-like activity, may be involved with the calcium overload of local anesthetic-induced neurotoxicity. In this study, we hypothesize that neurotoxicity induced by bupivacaine involves T-type calcium channels. Therefore, we employed an in vitro model of cytotoxicity using SH-SY5Y cells treated with bupivacaine. In addition, we monitored the effect of NNC 55-0396 dihydrochloride, a highly selective T-type calcium channel blocker that does not significantly alter currents mediated by other subtypes of calcium channels [24], on cell viability, LDH release, cytosolic Ca2+ ([Ca2+]i), apoptosis, and caspase-3 expression, following bupivacaine treatment.

Materials and Methods

Materials

The SH-SY5Y cell line was purchased from Shanghai Institutes for Biological Sciences (Shanghai, China). Goat polyclonal anti-caspase-3 antibody was purchased from Santa Cruz Biotechnology (Santa Cruz, CA, USA), bupivacaine hydrochloride and NNC 55-0396 dihydrochloride were purchased from Sigma (St. Louis, MO, USA) and Boston Biochem (Boston, MA, USA), respectively. Other reagents used in this study were as follows: DMEM/F12 medium and fetal bovine serum (Gibco, Grand Island, NY, USA), 3-(4,5-dimethyl-2- thiazolyl)-2,5-diphenyl-2-tetrazolium bromide (MTT; Beyotime, Nantong, China), Quest Fluo-8 AM ester (AAT Bioquest Inc., Sunnyvale, CA, USA), Hoechst 33258 (Beyotime), annexin V-FITC and propidium iodide (KeyGEN, Nanjing, China), and the LDH cytotoxicity detection kit (Beyotime). All other reagents were from commercial suppliers and of standard biochemical quality.

Cell Culture

SH-SY5Y cells were cultured in DMEM/F12 medium with 15% (v/v) fetal bovine serum, 100 units/mL of penicillin and 100 µg/mL of streptomycin, and maintained in a humidified 5% CO2 incubator at 37°C. The medium was replaced every 2 days.

Viability of the Cell Treated with Different Concentration Bupivacaine

To investigate the effects of different bupivacaine concentrations on SH-SY5Y cell viability, we treated SH-SY5Y cells with 0.1, 0.5, 0.75, 1, 2, 5, or 10 mM bupivacaine for 24 h. The effects of the different bupivacaine concentrations on SH-SY5Y cell viability were evaluated by the MTT assay.

Experimental Classification

Bupivacaine hydrochloride in powder form was dissolved in medium with serum resulting in the final concentration of bupivacaine being 1 mM, based on the results from the cell viability experiment. Cells were pretreated with 10, 50, or 100 µM NNC 55-0396 dihydrochloride (NNC) 30 min prior to treatment with either the culture medium containing 1 mM bupivacaine or an equivalent amount of medium alone for 6, 12, or 24 h. The experimental groups were: (1) NNC 55-0396 dihydrochloride 100 µM (S+NNC 100 group), (2) bupivacaine (S+B group), (3) bupivacaine+NNC 10 μΜ (S+B+NNC 10 group), (4) bupivacaine+NNC 50 μΜ (S+B+NNC 50 group), (5) bupivacaine+NNC 100 µM (S+B+NNC 100 group), and (6) non-pretreated (S group).

MTT Assay

Cell viability was measured using the MTT assay as previously described [25]. The cells were seeded into 96-well plates at a concentration of 5×103 cells/well with 100 µL culture medium per well. The cells were exposed to either 1 mM bupivacaine or an equivalent amount of medium for 6, 12, or 24 h. MTT (20 µL) was added to each well and incubated at 37°C for 4 h. The optical density of the homogenous purple solution was measured using a spectrophotometer (Bio-Tek, Winooski, VT, USA). The control group without bupivacaine treatment was set as 100% cell survival and all other groups were normalized to the corresponding control values.

LDH Assay

LDH activity was determined using an LDH cytotoxicity detection kit after cells were exposed to 1 mM bupivacaine, or an equivalent amount of medium for 6, 12, or 24 h [26]. The incubation solution was collected from the 12-well plates at the end of each experiment, and then centrifuged at 13,000×g for 10 min. The supernatant (100 µL) was transferred to 96-well plates and incubated with the same amount of reaction mixture. LDH activity was determined using a colorimetric assay at an absorbance wavelength of 492 nm and a reference wavelength of 655 nm using a spectrophotometer (Bio-Rad Laboratories, Hercules, CA, USA). Background absorbance from the cell-free buffer solution was subtracted from all absorbance measurements. After removal of the buffer from 12-well plates, 1% (v/v) Triton X-100 solution was applied to the remaining cells. The percentage of LDH released into the incubation buffer was calculated as follows: spontaneously released LDH into the buffer/(spontaneously released LDH into the buffer+intracellular LDH released by Triton X-100).

Measurements of Cytosolic Ca2+

Cytosolic Ca2+ ([Ca2+]i) from each group after treatment, with or without drugs for 24 h, was measured with Quest Fluo-8 AM ester. Briefly, a 5 mM stock solution of Quest Fluo-8 AM ester was prepared in high-quality anhydrous DMSO and a 10 µM working solution was prepared in Hanks and HEPES buffer (HHBS). The Quest Fluo-8 AM ester reagent concentration was 5 uM. The cells were incubated with the Quest Fluo-8 AM ester for 20 min at room temperature. Cells were washed twice in HHBS to remove excess probe. The experiments were analyzed at excitation and emission wavelengths of 490 and 525 nm, respectively. To determine either the free calcium concentration in the solution ([Ca2+]i) or the Kd of a single-wavelength calcium indicator, the following equation was used: [Ca2+]i = Kd[F−Fmin]/Fmax−F]. Where F is the fluorescence of the indicator at experimental calcium levels, Fmin is the fluorescence in the absence of calcium and Fmax is the fluorescence of the calcium-saturated probe. The dissociation constant (Kd) is a measure of the affinity of the probe for calcium, which is provided in the kit manual.

Detection of Apoptosis by Flow Cytometry

After cells were treated as described above for 24 h, the cells were seeded into 24-well plates at a concentration of 5×105 cells/well, with 500 µL culture medium per well. Cells were rinsed with phosphate buffered saline (PBS) and collected. Each pellet was resuspended in 500 µL binding buffer. In addition, 5 µL annexin V-FITC and 5 µL propidium iodide were added to each well. After a 5 min incubation, apoptotic cell death was measured by flow cytometry.

Apoptotic Cell Death Detected with Hoechst 33258

Cells in 24-well plates were rinsed 3 times with PBS and stained with Hoechst 33258. Subsequently, the cells were examined and photographed under a fluorescence microscope (Nikon ECLIPSE TE2000-u, Tokyo, Japan) with a UV excitation wavelength of 300–500 nm. Apoptotic cells were defined on the basis of nuclear morphology changes: chromatin condensation and fragmentation. The number of apoptotic and normal cells was counted manually by researchers blinded to the treatment schedule. For each well, at least 5 different fields were examined and the apoptosis rate was expressed as the percentage of apoptotic cells to the total number of cells counted.

Detection of Caspase-3 Protein Expression by Western Blotting

Culture flasks or plates were quickly rinsed with chilled PBS. Cells were collected using a plastic cell scraper, removed, and lysed in lysis buffer A (20.0 mmol/L Tris-HCl, 1.0 mmol/L Na3VO4, 1.5 mmol/L MgCl2, 10.0 mmol/L KCl, 0.1 mmol/L ethylenediaminetetraacetic acid (EDTA), 0.1 mmol/L ethylene glycol tetraacetic acid (EGTA), 0.5 mmol/L phenylmethylsulfonyl fluoride (PMSF), and 0.02% (w/v) protease inhibitor cocktail (pH 7.9)). After addition of 90 µL NP-40 (10% (v/v)), samples were shaken for 30 sec and then centrifuged at 800×g for 15 min at 4°C. The supernatants were centrifuged at 10 000×g for 1 h at 4°C. The samples were then homogenized in lysis buffer B (20.0 mmol/L Tris-HCl, 0.03 mmol/L Na3VO4, 2.0 mmol/L MgCl2, 10.0 mmol/L KCl, 2.0 mmol/L EDTA, 2.0 mmol/L EGTA, 2.0 mmol/L PMSF, 0.1% (v/v) Triton X-100, 5.0 mmol/L NaF, and 0.02% (w/v) protease inhibitor cocktail). The samples were centrifuged at 10,000×g for 1 h at 4°C, and the supernatants were used for western blot analysis. Protein concentration was determined using the Bradford method, and protein samples were stored at −80°C. Protein samples were dissolved in 4× sample buffer (250 mmol/L Tris-HCl, 200 mmol/L sucrose, 300 mmol/L dithiothreitol, 0.01% (w/v) Coomassie brilliant blue-G, and 8% (w/v) SDS, pH 6.8), and were subsequently denatured at 95°C for 5 min. Equivalent amounts of protein were separated on a 7.5% (w/v) sodium dodecyl sulfate polyacrylamide gel (SDS-PAGE) and transferred onto nitrocellulose membranes. The membranes were incubated overnight at 4°C with either goat polyclonal anti-caspase-3 antibody or anti-β-actin (1∶500; Santa Cruz Biotechnology, Santa Cruz CA, USA). The membranes were washed extensively with Tris-buffered saline/Tween-20 and incubated for 2 h in peroxidase-conjugated rabbit anti-goat IgG secondary antibody (1∶500; Santa Cruz Biotechnology) at room temperature. The immune complexes were detected by enhanced chemiluminescence, and membranes were then exposed to X-ray film. Quantification of protein bands was conducted by scanning the films and importing the images into Adobe Photoshop software (Adobe, San Jose, California, USA). Scanning densitometry was used for semi-quantitative analysis of the data. Caspase-3 protein was normalized to β-actin levels.

Statistical Analysis

Results are presented as the mean±SD. Factorial design ANOVA (SPSS 11.0 statistical software, IBM, USA) was used to analyze the data from the MTT assay, LDH assay, apoptosis assay and western blot assay. Multiple comparisons tests were performed by LSD. A probability value of P<0.05 was considered to be statistically significant.

Results

Cell Viability

Viability of SH-SY5Y cells dose-dependently decreased with increasing concentrations of bupivacaine. Treatment with 0.1, 0.5, 0.75, 1, 2, 5, and 10 mM bupivacaine for 24 h resulted in 58±5%, 47±6%, 39±5%, 27±4%, 15±3%, 5±2%, and 2±1% viability, respectively (Fig. 1).

Figure 1

The effect of increasing concentrations of bupivacaine on SH-SY5Y cell viability.

SH-SY5Y cells were exposed to different concentrations of bupivacaine (0.1, 0.5, 0.75, 1, 2, 5, and 10 mM). The viability of the cells declined with increasing bupivacaine concentration.

Compared with the S group, cell viability of SH-SY5Y cells in the S+NNC 100 group was not significantly different at 6, 12, and 24 h. However, 1 mM bupivacaine caused marked cell injury, and cell viability in the S+B group was 47±7%, 36±5% and 26±5% at 6, 12, and 24 h, respectively. Compared with the S+B group, NNC 55-0396 dihydrochloride pretreatment with the three different concentrations protected SH-SY5Y cells against bupivacaine-induced cell injury at 6, 12, and 24 h. Viability of SH-SY5Y cells treated with NNC 55-0396 dihydrochloride improved to 60±8%, 48±6% and 35±4% in the S+B+NNC 10 group, 70±7%, 61±7%, and 45±4% in the S+B+NNC 50 group, and 67±7%, 62±7% and 46±4% in the S+B+NNC 100 group, respectively. Although there was a significant difference between SH-SY5Y cells in the S+B+NNC 10 group and the S+B+NNC 50 and S+B+ NNC 100 groups, there were no significant differences between SH-SY5Y cells in the S+B+NNC 50 and S+B+NNC 100 groups (Fig. 2).

Figure 2

SH-SY5Y cell viability following treatment with 1 mM bupivacaine (%, mean±S.D, n = 6).

<0.05 vs. S group; <0.05 vs. S+NNC 100 group; <0.05 vs. S+B group; <0.05 vs. S+B+NNC 10 group; <0.05 vs. time point of 6 hours; <0.05 vs. time point of 12 h.

LDH Activity

SH-SY5Y cells in the S group showed marked LDH release, with extracellular LDH being 8.8±1.9%, 9.2±1.6%, and 10.1±1.2% of total LDH at 6, 12, and 24 h, respectively. In addition, there was no significant change in LDH release in SH-SY5Y cells pretreated with 100 μΜ NNC 55-0396 dihydrochloride for 6, 12, and 24 h. However, incubation with 1 mM bupivacaine increased extracellular LDH release to 20.7±2.1%, 27.1±2.8% and 31.3±2.9% at 6, 12, and 24 h, respectively. Interestingly, SH-SY5Y cells pretreated with NNC 55-0396 dihydrochloride resulted in a reduction in LDH release following bupivacaine treatment. Extracellular LDH at 6, 12, and 24 h was 17.3±1.6%, 22.2±2.7% and 25.3±1.6%, respectively, in the S+B+NNC 10 group; was 14.3±1.8%, 16.7±1.6% and 20.1±1.7% in the S+B+NNC 50 group; and was 13.5±1.9%, 16.5±2.1% and 20.8±1.9% in the S+B+NNC 10 group. Although there was a significant difference between SH-SY5Y cells in the S+B+NNC 10 group and the S+B+NNC 50 and S+B+NNC 100 groups, there were no significant differences between SH-SY5Y cells in the S+B+NNC 50 and S+B+NNC 100 groups (Fig. 3).

Figure 3

Bupivacaine treatment leads to the release of LDH.

SH-SY5Y cells were either pretreated with the indicated concentrations of NNC 55-0396 dihydrochloride or left untreated prior to 1 mM bupivaine treatment for 24 h. LDH release was determined by the level of LDH activity present in the culture media. (%, mean±S.D, n = 6). <0.05 vs. S group; <0.05 vs. S+NNC100 group; <0.05 vs. S+B group; <0.05 vs. S+B+NNC 10 group; <0.05 vs. time point of 6 h; <0.05 vs. time point of 12 h.

Changes in Cytosolic Ca2+

[Ca2+]i in SH-SY5Y cells in the S group and S+NNC 100 group was 358±25 nM and 372±32 nM, respectively. However, [Ca2+]i in the S+B group increased dramatically after treatment with 1 mM bupivacaine for 24 h to 715±35 nM. SH-SY5Y cells pretreated with NNC 55-0396 dihydrochloride resulted in a reduction of [Ca2+]i following bupivacaine exposure. [Ca2+]i of the cells in the S+B+NNC 10, S+B+NNC 50 and S+B+NNC 100 groups was 657±29 nM, 619±37 nM and 585±39 nM, respectively (See Fig. 4).

Figure 4

Bupivacaine treatment leads to an increase in cytosolic Ca2+ ([Ca2+]i).

SH-SY5Y cells were either pretreated with the indicated concentrations of NNC 55-0396 dihydrochloride or left untreated prior to 1 mM bupivaine treatment for 24 h. [Ca2+]i levels were measured by Quest Fluo-8 AM ester (mean±SD, n = 6)). A: Representative image of Quest Fluo-8 AM ester flow cytometry analysis. B: [Ca2+]i levels in the different treatment groups. <0.05 vs. S group; <0.05 vs. S+NNC 100 group; <0.05 vs. S+B group; <0.05 vs. S+B+NNC 10 group.

Apoptotic Cell Death Measured by Flow Cytometry

The rate of apoptosis in SH-SY5Y cells from the S and S+NNC 100 group was 12.5±2.7% and 12.9±2.3% respectively. After treatment with 1 mM bupivacaine for 24 h, the rate of apoptosis in the S+B group dramatically increased to 41.6±2.3%. NNC 55-0396 dihydrochloride pretreatment reduced the amount of apoptotic cell death following bupivacaine exposure, and the rates of apoptosis in the S+B+NNC 10, S+B+NNC 50 and S+B+NNC 100 groups were 36.2±3.9%, 28.7±3.2% and 25.1±2.8%, respectively. Although there was a significant difference between SH-SY5Y cells in the S+B+NNC 10 group and the S+B+NNC 50 and S+B+NNC 100 groups, there were no significant differences between SH-SY5Y cells in the S+B+NNC 50 and S+NB+NC 100 groups (Fig. 5).

Figure 5

NNC 55-0396 dihydrochloride protects SH-SY5Y cells from bupivacaine-induced apoptosis.

Cells were either treated with the indicated concentrations of NNC 55-0396 dihydrochloride or left untreated prior to 1 mM bupivaine treatment for 24 h. Apoptosis was measured by Annexin-V staining with flow cytometry (%, mean±SD, n = 6). A: Representative image from the flow cytometric analysis. B: Rates of apoptosis in the different treatment groups. <0.05 vs. S group; <0.05 vs. S+NNC 100 group; <0.05 vs. S+B group; <0.05 vs. S+B+NNC 10 group.

Detection of Apoptosis Using Hoechst 33258

Nuclear alterations of apoptotic cells were observed using Hoechst 33258 nuclear staining. As seen in Figure 6, apoptotic cells were observed to have condensed or segmented nuclei accompanied by bright blue fluorescence. Data analysis revealed similar results to that of flow cytometry (Table 1).

Figure 6

NNC 55-0396 dihydrochloride protects SH-SY5Y cells from bupivacaine-induced nuclear alterations during apoptosis.

Cells were either treated with the indicated concentrations of NNC 55-0396 dihydrochloride or left untreated prior to 1 mM bupivaine treatment for 24 h. Nuclear morphology was evaluated by Hoechst 33258 staining (×200). Apoptotic cells were observed to have condensed or segmented nuclei accompanied by bright blue fluorescence.

Table 1

Apoptosis measured by Hoechst 33258 staining (%, mean±S.D, n = 6).

Group	Apoptosis
S	7.5±1.9
S+NNC 100	7.5±2.3
S+B	49.2±3.0ab
S+B+NNC 10	36.3±2.2abc
S++B+NNC 50	25.5±2.7abcd
S+B+NNC 100	24.5±2.9abcd

P<0.05 vs. S group;

P<0.05 vs. S+NNC 100 group;

P<0.05 vs. S+B group;

P<0.05 vs. S+B+NNC 10 group.

Detection of Caspase-3 Expression by Western Blotting

The expression of cleaved caspase-3 (active form) and procaspase-3 (inactive form) were measured. The expression of procaspase-3 in SH-SY5Y cells in the S group and S+NNC 100 group was markedly higher than in the other groups. After treatment with 1 mM bupivacaine for 24 h, the expression of procaspase-3 in SH-SY5Y cells decreased and the expression of caspase-3 dramatically increased. However, NNC 55-0396 dihydrochloride pretreatment prevented the bupivacaine-induced reduction in procaspase-3. Therefore, NNC 55-0396 dihydrochloride pretreatment inhibited caspase-3 cleavage. Although the effects of NNC 55-0396 dihydrochloride were significantly different between SH-SY5Y cells in the S+B+NNC 10 group and the S+B+NNC 50 and S+B+NNC 100 groups, there were no significant differences between SH-SY5Y cells in the S+B+NNC 50 and S+B+NNC 100 groups (Fig. 7).

Figure 7

Inhibition of T-type calcium channels prevents bupivacaine-induced cleavage of caspase-3.

SH-SY5Y cells were either pretreated with the indicated concentrations of NNC 55-0396 dihydrochloride or left untreated prior to 1 mM bupivaine exposure for 24 h. Procaspase-3 (inactive form) and cleaved caspase-3 (active form) expression was measured by western blot analysis (mean+S.D, n = 6). Lane 1 = S group; Lane 2 = S+NNC 100 group; Lane 3 = S+B group; Lane 4 = S+B+ NNC 10 group; Lane 5 = S+B+NNC 50 group; Lane 6 = S+B+NNC 100 group. <0.05 vs. S group; <0.05 vs. S+NNC 100 group; <0.05 vs. S+B group; <0.05 vs. S+B+NNC 10 group.

Discussion

Generally, nerve damage resulting from local anesthetic exposure is related to the dose, concentration, and the time of exposure to the local anesthetic [27]. The precise mechanism of local anesthetic-induced nerve damage remains unclear. An intracellular overload of calcium may be a contributing factor to local anesthetic-induced nerve injury [12], [13]. Calcium is an important mineral essential for cellular function. Calcium can serve as a chemical signal in cells, and its levels are carefully regulated. One intriguing role of calcium is its ability to trigger apoptosis, a controlled form of cell death. Extracellular calcium ions can enter cells through voltage-dependent calcium channels or ligand-gated calcium channels, and activate calcium-dependent enzymes. Over-activation of these enzymes can cause nerve damage. At the same time, calcium ions entering cells can produce calcium-induced calcium release (CICR), causing an overload of intracellular calcium, and subsequently apoptosis and nerve damage [14].

In the present study, we detected the intracellular Ca2+ concentration with Fluo-8, with absorption and emission peaks at 490 nm and 514 nm, respectively. They can be excited with an argon ion laser at 488 nm, and their emitted fluorescence increases with increasing concentrations of Ca2+.Compared with Fluo-3 or Fluo-4, Fluo-8 is an excellent probe to use with high sensitivity. In this study, we found intracellular Ca2+ concentrations of SH-SY5Y cells treated with 1 mM bupivacaine for 24 h increased sharply and NNC, inhibited the rise of the intracellular Ca2+ concentration and prevented the apoptosis induced by bupivacaine.

The Cav3 family T-type calcium channels generate low-voltage-activated Ca2+ currents, and play an important role in many physiological and pathological processes, such as the regulation of cellular excitability, neurotransmitter secretion and release, motor coordination and function, learning and memory, epilepsy, and neuropathic pain [18], [28]. In our previous study, we monitored the protein and mRNA expression of T-type calcium channels in SH-SY5Y cells [29]. In this study, we found that 1 mM bupivacaine induced apoptosis in SH-SY5Y cells, activated caspase-3, and increased LDH release. However, NNC 55-0396 dihydrochloride, an antagonist of T-type calcium channels, reduced bupivacaine-induced cell injury. Therefore, T-type calcium channels may be involved in the neuronal injury observed following local anesthetic administration.

We found that NNC 55-0396 dihydrochloride protection against bupivacaine-induced apoptosis was dose-dependent. Although 10 μΜ NNC 55-0396 dihydrochloride significantly protected SH-SY5Y cells from 1 mM bupivacaine-induced cell death, the effects of 50 μΜ NNC 55-0396 dihydrochloride were notably enhanced. However, there was no significant difference between 50 μΜ and 100 μΜ NNC 55-0396 dihydrochloride pretreatment, demonstrating that the protection of NNC 55-0396 dihydrochloride exhibited a ceiling effect.

One limitation of this study was that NNC 55-0396 dihydrochloride is not subtype specific, and may have acted on Cav3.1, Cav3.2 and Cav3.3, which are all expressed in SH-SY5Y cells [26]. To investigate the subtypes of T-type calcium channels involved in bupivacaine toxicity, we would like to have employed Cav3.1, Cav3.2 or Cav3.3 specific antagonists. However, to our knowledge, there are currently no drugs available that specifically block Cav3.1, Cav3.2 or Cav3.3. Therefore, genetic engineering to silence subtype gene expression may be necessary to understand the role of specific subtypes in local anesthetic toxicity.

In summary, we found that treatment of SH-SY5Y cells with 1 mM bupivacaine for 24 h resulted in apoptosis, activation of caspase-3 and release of LDH. Interestingly, inhibition of T-type calcium channels with NNC 55-0396 dihydrochloride reduced bupivacaine-induced cell death, suggesting a novel role for these calcium channels in local anesthetic toxicity.