Background: Certain anesthetics exhibit neurotoxicity in the brains of immature but not mature animals. γ-Aminobutyric acid (GABA), the primary inhibitory neurotransmitter in the adult brain, is excitatory on immature neurons via its action at the GABAA receptor, depolarizing the membrane potential and inducing a cytosolic Ca2+ increase ([Ca2+]i), because of a reversed transmembrane chloride gradient. Recent experimental data from several rodent studies have demonstrated that exposure to isoflurane during an initial phase causes neuronal excitotoxicity and apoptosis. GABAA receptor-mediated synaptic voltage-dependent calcium channels' (VDCCs) overactivation and Ca2+ influx are involved in these neural changes.
Methods: We monitored [Ca2+]i using Fluo-4 AM fluorescence imaging. Using whole-cell patch clamp techniques, IVDCC (voltage-dependent calcium channel currents) were recorded from primary cultures of rat hippocampal neurons (5-day culture) exposed to isoflurane. To further investigate the neurotoxicity of high cytosolic-free calcium after isoflurane in a dose- and time-dependent manner, the possibility of increased caspase-3 levels was evaluated by Western blot and quantitative real-time polymerase chain reaction. Statistical significance was assessed using the Student t test or 1-way analysis of variance followed by the Tukey post hoc test.
Results: Under control conditions, isoflurane enhanced the GABA-induced [Ca2+]i increase in a dose-dependent manner. Dantrolene and nicardipine markedly inhibited this enhancement mediated by isoflurane. Moreover, in Ca2+-free media, pretreatment with isoflurane did not show any influence on the caffeine-induced increase of [Ca2+]i. Similarly, using whole-cell recording, isoflurane increased the peak amplitude of IVDCC in the cultured neurons from rat hippocampus by depolarization pulses. Isoflurane (0.25, 0.5, 0.75, and 1 minimum alveolar concentration [MAC]) potentiated IVDCC peak current amplitude by 109.11%±9.03%, 120.56%±11.46%, 141.33%±13.87%, and 146.78%±15.87%, respectively. To analyze variation in protein levels, the effect of treatments with isoflurane on caspase-3 activity was dose- and time-dependent, reaching a maximal caspase-3 activity after exposure to 1 MAC for 6 hours (P<0.001). However, in the mRNA levels, hippocampal caspase-3 mRNA levels began to be significantly increased in isoflurane-treated developing rat hippocampal neurons after 6 hours of exposure to 0.25 MAC isoflurane (P<0.001).
Conclusions: Isoflurane-mediated enhancement of GABA-triggered [Ca2+]i release results from membrane depolarization with subsequent activation of VDCCs and further Ca2+-induced Ca2+ release from the ryanodine-sensitizing Ca2+ store. An increase in [Ca2+]i, caused by activation of the GABAA receptor and opening of VDCCs, is necessary for isoflurane-induced calcium overload of immature rat hippocampal neurons, which may be involved in the mechanism of an isoflurane-induced neurotoxic effect in the developing rodent brain.