Abstracts

Levetiracetam (LEV) is an antiepileptic drug indicated for the adjunctive treatment of partial onset seizures in adults and children age 4 and older, which exhibits good tolerance and no known drug interactions. In addition to its antiepileptic effects, LEV has also been shown to possess therapeutic potential for the treatment of neuropathic pain. While the molecular mechanisms of LEV[apos]s action are not fully known, it is known to interact with a specific binding site, synaptic vesicle protein 2A, which is a critical component in the neurotransmitter release process. Intracellular Ca²⁺ plays a major role in a variety of neuronal functions including excitability and neurotransmission. LEV has been shown to reduce Ca²⁺ release through IP[sub]3[/sub] receptors from the intracellular stores of endoplasmic reticulum (ER). In this study, we investigated the effects of LEV on Ca²⁺ release through ryanodine receptor (RyR), which is a Ca²⁺-regulated Ca²⁺ release channel in the ER, in sensory neurons of rat dorsal root ganglion (DRG)., All procedures were approved by the Institutional Animal Care and Use Committee. Freshly dissociated lumbar DRG sensory neurons (L1-L6) from adult Sprague-Dawley rats were used in this study. Changes in intracellular Ca²⁺ were measured using the fluorescent Ca²⁺-sensitive dye, Fura-2. Caffeine (5 mM) was used to stimulate Ca²⁺ release through the RyR. LEV was used at a final concentration of 100 [mu]M., In the presence of 2 mM extracellular Ca²⁺, stimulation of sensory neurons with caffeine elicited two types of responses. Small diameter, capsaicin-sensitive neurons showed a biphasic increase in intracellular Ca²⁺: an initial rapid rise (peak) due to release of Ca²⁺ from the ER, followed by a sustained Ca²⁺ rise due to Ca²⁺ entry through the capacitative Ca²⁺ entry (CCE) pathway. Large diameter neurons showed only a rapid but transient rise in intracellular Ca²⁺ due to ER Ca²⁺ release. Extracellular application of LEV did not elicit any changes in intracellular Ca²⁺ in either type of neurons. However, pretreatment of cells with LEV resulted in inhibition of caffeine-induced Ca²⁺ responses in both types of neurons. In small neurons, LEV reduced the peak Ca²⁺ response by 27% and the sustained Ca²⁺ response by 32%. In large neurons, LEV reduced the caffeine-induced Ca²⁺ response by 35%. These effects are similar to the inhibitory effects of the general anesthetic drug propofol, which also exhibits anti-epileptic activity. The mechanism(s) by which LEV exerts its effects on RyR-mediated Ca²⁺ signaling is not known, however, these effects are also mimicked by the activation of protein kinase C (PKC)., Our results demonstrate that the antiepileptic drug LEV inhibits intracellular Ca²⁺ signaling mediated by RyR and CCE pathways in sensory neurons. The LEV-mediated regulation of Ca²⁺ signaling is similar to that mediated by PKC., (Supported by American Heart Association (National Center).)