Molecular and structural substrates of the neurotoxic effects of lidocaine injection in the brainstem rostral ventro-medial medulla -

Abstract

Background and Aims: The local anesthetic lidocaine was shown to decrease neuropathic manifestations in rats when injected in the rostral ventromedial medulla (RVM). This was explained by a selective block of descending facilitatory mechanisms that are released during chronic pain. However, recent observations from our laboratory led to challenge this hypothesis and to postulate that observed effects are due to selective block by lidocaine of the inhibitory neurons in the RVM that exert tonic suppression of the descending inhibitory control of pain. The present study investigates the possible toxic effects of lidocaine (2percent) on the RVM neurons.Methods: Several groups of rats (n=4-8) received intra-RVM injections of saline (Sham), kainic acid (1.5µg), or lidocaine (2percent)], and were euthanized at various time intervals following the injections (Days 1, 3 and 7). Tissue samples, isolated from the RVM areas of injected and control rats were, subsequently used to perform one of the following procedures. Experiment 1: Mechanical cell dissociation of the sampled RVM tissue followed by propidium iodide (PI) staining to measure DNA degradation and rhodamine 123 (Rh123) staining to measure changes in mitochondrial membrane potential (MMP). Experiment 2: Immunofluorescence labeling of neurons, gabaergic and glycinergic neurons followed by Hoechst staining to detect late apoptosis using microscopic analysis. Experiment 3: Whole cell extracts from RVM tissues subjected to western blotting for detection of caspases 9,3, 7 and their cleaved products.Results: Lidocaine (2percent) injection in the RVM region induced (1) transient change in MMP followed by recovery; (2) Slight DNA degradation; (3) increase in initiator caspase 9 with a slight but significant cleavage; (4) upregulation of effector caspases 3 and 7 without any cleavage. Kainic acid injection induced comparable but more pronounced effects than those of lidocaine. These include: (1) pronounced DNA degradation at day 1 followed by a decrease in the number of apoptotic