The present invention provides modified cyclic nucleotide gated (CNG) channels. In particularly preferred embodiments, the modified CNG channels exhibit increased sensitivity and specificity for cAMP, as compared to wild-type CNG channels. In additional embodiments, regulation by Ca.sup.2+-calmodulin has been removed in the modified CNG channels. Convenient optical methods for detecting changes in cAMP, taking advantage of the Ca.sup.2+ permeability of the channel are also provided by the present invention. In addition, electrophysiological methods are further provided.