A study of small and intermediate conductance calcium-activated potassium channels in sensory neurones
The role of small and intermediate conductance calcium-activated potassium channels (SK and IK channels) in dorsal root ganglion (DRG) neurones was examined. Sixteen antibodies raised against human or rat SK/IK channel peptide epitopes were tested for their ability to stain cells expressing channel protein. Of sixteen antibodies, 12 (6 to SKI, 1 to SK2, 2 to SK3 and 3 to IK) were deemed suitable for immunohistochemistry in human or rat tissue. Real-time quantitative PCR (qPCR) of rat DRG cDNA was performed to examine SK/IK expression levels. DRG neurones produce mRNA for all SK/IK channels and these mRNA levels were found to increase during development. Antibody staining experiments using DRG neurones cultured from different aged animals produced a positive stain with the anti-SK3 antibody only. The number of cells that stained positively and the intensity of staining for SK3 increased with age. To investigate possible functional roles for SK/IK channels sensory neurones, action potential afterhyperpolarisations (AHPs) were recorded from cultured DRG and nodose cells. The majority of these AHPs proved to be insensitive to the SK channel blocker UCL 1848. Attempts to block medium duration AHPs in DRG cells using IK and calcium channel blockers, also failed in most cases, suggesting that some other potassium conductances) are responsible. The possibility that SK3 is functional at the terminals of primary afferents was examined next. Spinal cord slices stained with SK/EK channel antibodies revealed positive SK3 staining in the outer laminae of the dorsal horn, where small and large diameter DRG fibres are expected to terminate. In vivo experiments (done by Dr Rie Suzuki, Department of Pharmacology, UCL) using UCL 1848 and l-ethyl-2- benzimidazolinone (1-EBIO an SK channel opener) showed that SK channels are likely to be active at these terminals where they have a functional role in mediating innocuous mechanical and nociceptive responses.