902. Modulation of Voltage-Gated Potassium Channels in Dorsal Root Ganglion Neurons by an Inducible Adenoviral Vector In Vivo

Abstract

Injury to the nerve or dorsal root ganglion (DRG) reduces voltage-gated potassium (Kv) channel expression and increases neuronal excitability in DRG neurons. Kv channels are therefore potential targets of gene therapy for chronic pain. An adenoviral vector, containing a reporter gene encoding the enhanced green fluorescent protein (GFP) and a Kv gene, was delivered to the fourth lumbar (L4) DRG of adult rat by microinjection via a small catheter inserted under the epineurium of the spinal nerve 3 to 5 mm from the DRG. The in vivo expression of the transferred gene was controlled by an ecdysone analog via an ecdysone-inducible promoter in the viral vector. Three types of vectors were used. A control vector (AdEGI) contained only the GFP gene. A vector intended to reduce votage gated potassium channels (AdKv1.xDN) had a dominant-negative Kv1.3 gene that disrupted Kv1 channels. A vector to increase inwardly rectifying K current (AdKir2.1) had the human Kir2.1 gene. Sharp-electrode intracellular recordings were obtained in vivo from the ipsilateral L4 DRG neurons. As early as one day, and lasting longer than 2 weeks, injection of the control vector AdEGI produced a slight but significant decrease in force of a Von-Frey filament required to elicit withdrawal of the ipsilateral hindpaw. There was no decrease in withdrawal latency to heating the same paw. Compared with the effects of AdEGI, induction of the AdKv1.xDN produced (1) a further decrease of mechanical withdrawal threshold, and (2) increased neuronal excitability as indicated by a decreased rheobase, higher input resistance, and a wider action potential with a smaller afterhyperpolarization in GFP-labeled medium and large cell bodies of DRG neurons. Whereas induction of the AdKir2.1 produced no significant change in paw withdrawal, it did decrease neuronal excitability in GFP-labeled DRG neurons as indicated by a hyperpolarized resting potential, a higher rheobase and a lower input resistance. By applying adenoviral vectors carrying different types of potassium channel genes, we could either increase or decrease the excitability of DRG neurons, and modulate the pain-related behaviors of the animal. In order for gene therapy to treat neuropathic pain we will need to develop viral vectors that reach a greater number of hyperexcitable neurons.