GABA A receptors are found in neurons that respond to the neurotransmitter gamma-aminobutyric acid (GABA). These receptors facilitate the inhibitory action of GABA, making neurons less likely to fire and having a sedative effect on neurological function. Muscle tone and seizure activity are moderated by the effects of GABA. In addition to the neurotransmitter, many drugs that act as anesthetics or muscle relaxants work by binding to GABA A receptors.
An inhibitory neurotransmitter, GABA is found in the brains of many higher animals, with roughly 40% of all mammalian synapses having receptors for it. Released from a neuron into the synapse, the space where two nerve cells communicate, GABA binds to receptors in the neighboring cell. Situated in the cell membrane, GABA A receptors are ionotropic or channel receptors. Once GABA binds to them, they open up a bit, forming a channel for negatively charged ions to pour into the cell, changing its charge and reducing its electrical excitability and ability to communicate with other cells.
There are different subtypes of GABA A receptors, which vary in the composition of their individual protein subunits. Since this receptor is the target of several important sedative drugs, each of which binds to different parts of the receptor, pharmacologists have classified the GABA A subtypes according to the ways that different compounds attach to them. For instance, benzodiazepines, a class of muscle relaxants, bind to special protein locations on the GABA receptor that are different from those activated by the neurotransmitter GABA itself. Ethanol and some anesthetics also have their own binding proteins located on the GABA A receptors.
The GABA A receptors are important parts of neuronal feedback loops, which monitor and balance neural responses throughout the body, and check the effects of excitatory neurotransmitters. GABA feedback controls muscle tone by inhibiting spastic uncontrolled movements. When muscle relaxants bind to GABA A receptors, they stop cramps and spasms by activating this feedback loop. Because of the generally sedative action of GABA, these drugs can also cause drowsiness and reduced fine motor control.
By regulating neural inhibition in the central and peripheral nervous system, GABA controls when and where neurons are active. This means that when neurons are excited across the brain, as in epileptic seizures, activation of GABA receptors will stop the seizure. Barbiturates and some other anti-convulsive drugs operate in this way, as do anti-anxiety drugs that activate GABA A receptors. Some people take GABA dietary supplements to reduce anxiety, though it is unclear how much of the transmitter can cross the blood-brain barrier to take effect in the brain.