22.4: G-Protein Gated Ion Channels
GPCRs are primarily responsible for our sense of smell, taste, and vision. The binding of a sensory stimulus activates GPCR to stimulate effector proteins, many of which are ion channels in the sensory organs. GPCRs modulate the opening and closing of the target ion channels either directly by binding them, or by releasing second messengers that activate these channels. As ions move across the membrane, the membrane potential is altered, which induces an appropriate response.
Sensory organs, including the nose, are lined by specialized olfactory neurons that help distinguish distinct smells. These neurons contain GPCRs that bind odorants and stimulate olfactory G proteins or Golf. Golfα undergoes a GTP-GDP exchange and activates adenylyl cyclase to produce a second messenger, cAMP. The increased cAMP levels lead to an influx of Na+ ions through cAMP gated cation channels, rapidly increasing the membrane potential, thereby inducing membrane depolarization. This leads to the transmission of nerve impulses to the brain and gives us the perception of different aromas.
For taste perception, the tongue relies on taste receptor cells that transmit information and help distinguish different tastes such as salty, sweet, bitter, or sour. Humans consist of 30 bitter-taste receptors named as T2Rs and one high-affinity sweet-taste receptor, the T1R2-T1R3 heterodimer.
Similarly, visual transduction is mediated by well-known photosensitive GPCRs called Rhodopsins. The rod photoreceptors provide black and white vision in dim light. These rod cells comprise an outer segment with stacks of discs; each disc is closed, membrane-bound, and packed with rhodopsins. Furthermore, a plasma membrane encloses the outer segment that contains cyclic GMP-gated cation channels. In dark conditions, high cyclic GMP levels keep these cation channels open, leading to membrane depolarization by the influx of Na+ and Ca2+ ions. Upon light absorption, rhodopsin undergoes a conformational change and binds G protein transducin (Gt), to activate it. Each GTP-Gtα subunit stimulates cyclic GMP phosphodiesterase (PDE) enzyme. PDE decreases intracellular cyclic GMP levels by hydrolyzing cyclic GMP to 5’-GMP and leads to the closure of cation channels. This restricts the entry of Na+ and Ca2+ ions in the cytosol, thereby increasing the negative charge on the inner side of the membrane while inducing membrane hyperpolarization. Thus, the extracellular light signal is transmitted as an electrical signal by hyperpolarizing the rod cell membrane.