Second messenger system
. The “second messenger” system by which a transmitter substance from an initial neuron can activate a second neuron by first causing a transformational change in the receptor that releases the activated alpha (α) subunit of the G protein into the second neuron’s cytoplasm. Four subsequent possible effects of the G protein are shown, including 1, opening an ion channel in the membrane of the second neuron; 2, activating an enzyme system in the neuron’s membrane; 3, activating an intracellular enzyme system; and/or 4, causing gene transcription in the second neuron. Return of the G protein to the inactive state occurs when guanosine triphosphate (GTP) bound to the α subunit is hydrolyzed to guanosine diphosphate (GDP) and the β and γ subunits are reattached to the α subunit.