Cells are rendered sensitive to stimulation by introducing into a non-photoreceptor cell nucleic acid sequences encoding at least an opsin gene product, an arrestin gene product, and the alpha subunit of the heterotrimeric G protein of the G.sub.q family. The introduced sequences are expressed by the cell to yield at least the opsin gene product, the arrestin gene product, and the alpha subunit of the heterotrimeric G protein of the G.sub.q family. Retinal or a derivative thereof capable of bonding with the opsin gene product to form a rhodopsin is provided to the cell. The cell is then irradiated with light having a wavelength capable of converting the rhodopsin to metarhodopsin. The conversion of rhodopsin to metarhodopsin triggers a cascade of intracellular responses within the cell resulting in an increased intracellular concentration of IP.sub.3 and calcium ions.