ionotropic

Ionotropic describes a class of receptors that function as ligand-gated ion channels. When an appropriate chemical messenger binds to the extracellular site, the receptor undergoes a conformational change that opens an ion channel within the protein, allowing ions to pass directly across the cell membrane. This produces rapid changes in membrane potential and fast synaptic signals, typically on the millisecond timescale.

Most ionotropic receptors are oligomeric protein complexes. Common examples include nicotinic acetylcholine receptors, GABA_A receptors, glycine receptors, and ionotropic glutamate receptors (NMDA, AMPA, and kainate receptors). Some are permeable to cations such as Na+ and K+, others are permeable to anions such as Cl-. The channel’s ion selectivity and subunit composition influence current flow, pharmacology, and kinetics. Subunit assembly can also affect desensitization and sensitivity to modulators.

Activation by neurotransmitters is fast and brief, typically triggering excitatory (depolarizing) or inhibitory (hyperpolarizing) responses depending on ion flux. In the nervous system, ionotropic receptors mediate fast synaptic transmission at many synapses and at the neuromuscular junction.

Ionotropic receptors are distinct from metabotropic (G protein-coupled) receptors, which influence cell signaling via second messengers and a slower, indirect opening or closing of ion channels. Pharmacologically, ligands can act as agonists or competitive antagonists, and many ionotropic receptors are subject to allosteric modulation by anesthetics, neurosteroids, or other agents.