These multiple forms (isoforms or subtypes) of phosphodiesterase were isolated from rat brain using polyacrylamide gel electrophoresis in the early 1970s,[REF][REF] and were soon afterward shown to be selectively inhibited by a variety of drugs in brain and other tissues [REF].
The potential for selective phosphodiesterase inhibitors to be used as therapeutic agents was predicted in the 1970s [REF].