Background and Objectives Sodium bisulfite (NaHSO₃) was clinically used as a preservative agent for local anesthetics but was later suspected to be neurotoxic. However, recent studies reported that NaHSO₃ reduces the neurotoxicity of local anesthetics. The purpose of this study was to examine the effects of NaHSO₃ with and without procaine on axonal transport in cultured mouse dorsal root ganglion (DRG) neurons.

Methods Experiment 1 served to determine the dose-dependent effects of NaHSO₃ on axonal transport (DRG neurons were treated with 0.01, 0.1, 1, 10, or 20 mM of NaHSO₃), whereas experiment 2 investigated the effect of 0.1 mM NaHSO₃ on the action of local anesthetics on axonal transport (DRG neurons were treated with 1 mM procaine alone, or with 0.1 mM NaHSO₃ plus 1 mM procaine). As an additional experiment, DRG neurons were also treated with 1 mM chloroprocaine alone, or with 0.1 mM NaHSO₃ plus 1 mM chloroprocaine. In these experiments, we analyzed the percent change in the number of anterogradely and retrogradely transported organelles and recorded changes in neurite morphology using video-enhanced microscopy.

Results In experiment 1, NaHSO₃ at more than 1 mM caused cell membrane damage and complete inhibition of axonal transport, whereas 0.1 mM NaHSO₃ maintained axonal transport at 40% to 60% of control with intact cell membrane. In experiment 2, 1 mM procaine alone maintained axonal transport at 90% to 100%. However, application of 1 mM procaine-0.1 mM NaHSO₃ solution resulted in deformation of neurites and with complete cessation of axonal transport. Likewise, although 1 mM chloroprocaine maintain axonal transport at 80% to 100%, 1 mM chloroprocaine-0.1 mM NaHSO₃ arrested axonal transport.

Conclusions NaHSO₃ resulted in a dose-dependent damage to the cell membrane and axonal transport, especially when used in combination with procaine or chloroprocaine.