<Li> GABA in which the receptor is part of a ligand - gated ion channel complex </Li> <Li> GABA metabotropic receptors, which are G protein - coupled receptors that open or close ion channels via intermediaries (G proteins) </Li> <P> Neurons that produce GABA as their output are called GABAergic neurons, and have chiefly inhibitory action at receptors in the adult vertebrate . Medium spiny cells are a typical example of inhibitory central nervous system GABAergic cells . In contrast, GABA exhibits both excitatory and inhibitory actions in insects, mediating muscle activation at synapses between nerves and muscle cells, and also the stimulation of certain glands . In mammals, some GABAergic neurons, such as chandelier cells, are also able to excite their glutamatergic counterparts . </P> <P> GABA receptors are ligand - activated chloride channels: when activated by GABA, they allow the flow of chloride ions across the membrane of the cell . Whether this chloride flow is depolarizing (makes the voltage across the cell's membrane less negative), shunting (has no effect on the cell's membrane potential), or inhibitory / hyperpolarizing (makes the cell's membrane more negative) depends on the direction of the flow of chloride . When net chloride flows out of the cell, GABA is depolarising; when chloride flows into the cell, GABA is inhibitory or hyperpolarizing . When the net flow of chloride is close to zero, the action of GABA is shunting . Shunting inhibition has no direct effect on the membrane potential of the cell; however, it reduces the effect of any coincident synaptic input by reducing the electrical resistance of the cell's membrane . Shunting inhibition can "override" the excitatory effect of depolarising GABA, resulting in overall inhibition even if the membrane potential becomes less negative . It was thought that a developmental switch in the molecular machinery controlling concentration of chloride inside the cell changes the functional role of GABA between neonatal and adult stages . As the brain develops into adulthood, GABA's role changes from excitatory to inhibitory . However, this theory of excitatory GABA in the developing brain has been questioned and subsequent studies in live neonatal rodents have directly shown GABA to be inhibitory in its action (see next section). </P>

Levels of gaba and its effects on brain activity