Neuronal signaling mainly involves the regulation of the mechanics of the CNS (central nervous system). Neurons have evolved unique capabilities for intracellular signaling (communication within the cell) and intercellular signaling (communication between cells). G protein-coupled receptors (GPCRs, such as 5-HT receptor, histamine receptor, opioid receptor) are the largest class of sensory proteins, and thus are important therapeutic targets in neuronal signaling. GPCRs are activated by diverse stimuli, including light, enzymatic processing of their N-termini, and binding of proteins, peptides, or small molecules such as neurotransmitters, and regulate neuronal excitability by indirectly modulating the function of voltage-gated channels, such as voltage-gated calcium channel and transient receptor potential (TRP) ion channels.