Methods and compositions to activate a genetically designated target cell (or population of target cells) artificially, in vivo or in vitro, employ triggering of heterologous stimulus-gated ion channels to activate the cells. The stimulus-gated ion channels are suitably TRPV1, TRPM8 or P2X2. A stimulus which leads to opening or “gating” of the ion channel can be a physical stimulus or a chemical stimulus. Physical stimuli can be provided by heat, or mechanical force, while chemical stimuli can suitably be a ligand, such as capsaicin for TRPV1 or ATP for P2X2, or a “caged ligand,” for example a photolabile ligand derivative, in which case a physical signal in the form of light is used to provide the chemical signal. Selective activation of the cell may be used for various applications including neuronal and neuroendocrine mapping and drug screening.