GABAB receptor

GABAB receptors (GABABR) are metabotropic transmembrane receptors for gamma-aminobutyric acid (GABA) that are linked via G-proteins to potassium channels. These receptors are found in the central and peripheral autonomic nervous system.

Functions
They can stimulate the opening of K+ channels which brings the neuron closer to the equilibrium potential of K+, hyperpolarising the neuron. This prevents sodium channels from opening, action potentials from firing, and VDCCs from opening, and so stops neurotransmitter release. Thus GABAB receptors are considered inhibitory receptors.

GABAB receptors can also reduce the activity of adenylyl cyclase and  decrease the cell’s conductance to Ca2+.

GABAB receptors are involved in behavioral actions of ethanol, gamma-hydroxy butyric acid (GHB), and possibly in pain. Recent research suggests that these receptors may play an important developmental role.

Structure
GABABRs are similar in structure to and in the same receptor family with metabotropic glutamate receptors. There are two subtypes of the receptor, GABAB1 and GABAB2, and these appear to assemble as heterodimers in neuronal membranes by linking up by their intracellular C termini.

It is speculated that binding of GABA causes the subunits to swing shut around the agonist like a venus fly trap.

Agonists and antagonists
Baclofen is a GABA analogue which acts as a selective agonist of GABAB receptors, and is used as a muscle relaxant. However, it can aggravate absence seizures, and so is not used in epilepsy.

Saclofen, phaclofen and SCH-50911 are selective GABAB antagonists.