We are working with an investigator on using RNAi to relieve pain and his study prompted some discussion about the length of time before an effect is seen. It is important to note that these types of experiments take time to see an effect. In studying the pain response, the effect may take even longer.
Dorn et al (Nucleic Acids Res. 2004 Mar 16;32) examined the ability of siRNA to relieve chronic neuropathic pain targeting P2X3 in vivo. They showed a significant effect on the ability of the siRNA to relieve the pain; however there was no significant effect until around 6 days post.
Another study by Hemmings-Mieszczak et al (Nucleic Acids Research, 2003, Vol. 31, No. 8 2117-2126) showed that in an in vitro model, oligofectamine-mediated transfection of siRNA resulted in 60±90% downregulation of P2X3, but only after about 3 days.
So a key factor in any siRNA transfection study is to carry the experiment out long enough, which in in vivo work can be longer than for in vitro studies. As Mark points out, "Normally in cell culture we would look at mRNA levels at 24 h and protein levels at 48-72 h post transfection, depending upon the cell division rate and the half life of the protein. Long lived proteins in non-dividing cells could theoretically take multiple administrations of reagent over a week or more to knock down your targeted protein in vivo."