The receptor can be activated by a range of simpleamino acids including glycine,β-alanine andtaurine, and can be selectively blocked by the high-affinity competitive antagoniststrychnine.[2]Caffeine is a competitive antagonist of GlyR.[3]Cannabinoids enhance the function.[4]
The proteinGephyrin has been shown to be necessary for GlyR clustering at inhibitory synapses.[5][6] GlyR is known to colocalize with theGABAA receptor on somehippocampalneurons.[5] Nevertheless, some exceptions can occur in the central nervous system where the GlyR α1 subunit and gephyrin, its anchoring protein, are not found indorsal root ganglionneurons despite the presence ofGABAA receptors.[7]
Glycine and its receptor were first suggested to play a role in inhibition of cells in 1965.[8] Two years later, experiments showed that glycine had a hyperpolarizing effect on spinal motor neurons[9] due to increased chloride conductance through the receptor.[10] Then, in 1971, glycine was found to be localized in the spinal cord using autoradiography.[11] All of these discoveries resulted in the conclusion that glycine is a primary inhibitory neurotransmitter of the spinal cord that works via its receptor.
(a): shows three agonists and one antagonist of the glycine receptor. (b): the fetal form of the receptor is made up of five α2 subunits, while the adult form is made up of both α1 and β subunits.
Strychnine-sensitive GlyRs are members of a family ofligand-gated ion channels. Receptors of this family are arranged as fivesubunits surrounding a central pore, with each subunit composed of fourα helicaltransmembrane segments.[12] There are presently four known isoforms of theligand-binding α-subunit (α1-4) of GlyR (GLRA1,GLRA2,GLRA3,GLRA4) and a single β-subunit (GLRB). The adult form of the GlyR is the heteromeric α1β receptor, which is believed to have a stoichiometry (proportion) of three α1 subunits and two β subunits[13] or four α1 subunits and one β subunit.[14] The embryo form on the other hand, is made up of five α2 subunits.[15] The α-subunits are also able to form functionalhomopentamers inheterologous expression systems inAfrican clawed frogoocytes or mammaliancell lines, which are useful for studies of channelpharmacokinetics andpharmacodynamics.[14] The β subunit is unable to form functional channels without α subunits but determines the synaptic localization of GlyRs and the pharmacological profile of glycinergic currents.[16]
In mature adults,glycine is an inhibitory neurotransmitter found in the spinal cord and regions of the brain.[15] As it binds to a glycine receptor, a conformational change is induced, and the channel created by the receptor opens.[17] As the channel opens, chloride ions are able to flow into the cell which results inhyperpolarization. In addition to this hyperpolarization, which decreases the likelihood of action potential propagation, glycine is also responsible for decreasing the release of both inhibitory and excitatory neurotransmitters as it binds to its receptor.[18] This is called the "shunting" effect and can be explained byOhm's law. As the receptor is activated, the membrane conductance is increased and the membrane resistance is decreased. According to Ohm's law, as resistance decreases, so does voltage. A decreased postsynaptic voltage results in a decreased release of neurotransmitters.[18]
In developing embryos, glycine has the opposite effect as it does in adults. It is an excitatory neurotransmitter.[18] This is because chloride has a more positive equilibrium potential in early stages of life due to the high expression ofNKCC1. This moves one sodium, one potassium and two chloride ions into the cell, resulting in a higher intracellular chloride concentration. When glycine binds to its receptor, the result is an efflux of chloride, instead of an influx as it happens in mature adults. The efflux of chloride causes the membrane potential to become more positive, or depolarized. As the cells mature, the K+-Cl- cotransporter 2 (KCC2) is expressed, which moves potassium and chloride out of the cell, decreasing the intracellular chloride concentration. This allows the receptor to switch to an inhibitory mechanism as described above for adults.[18]
Disruption of GlyR surface expression or reduced ability of expressed GlyRs to conduct chloride ions results in the rare neurological disorder,hyperekplexia. The disorder is characterized by an exaggerated response to unexpected stimuli which is followed by a temporary but complete muscular rigidity often resulting in an unprotected fall. Chronic injuries as a result of the falls are symptomatic of the disorder.[1] A mutation in GLRA1 is responsible for some cases ofstiff person syndrome.[19]
