GABA (gamma-aminobutyric acid) is the primary inhibitory neurotransmitter in the brain. Unlike excitatory neurons, which "pass along" a message to the next neuron by causing an Excitatory Post-Synaptic Potential (EPSP), inhibitory neurons make the next neuron less likely to fire by causing an Inhibitory Post-Synaptic Potential (IPSP). So when the arrival of an action potential at the axon terminal of a gabaminergic neuron causes the release of GABA into the synaptic cleft to interact with its post-synaptic receptors, these receptors open channels for chloride ions. The rush of these negatively-charged chloride ions into the receiving neuron makes it more negative inside, and thus less likely to fire.
GABA is recognized by both ligand-gated and G-protein-linked receptors. The ligand-gated GABA receptor is called GABAa and represents a typical ligand-gated receptor molecule. High concentrations of GABA and GABAa receptors are found in the limbic system, an area of the brain where personal feelings and emotional memories are generated and stored. When an individual experiences a strong emotion, or is faced with a threatening challenge, it is the limbic system that interprets the ambiguous physiological responses of the autonomic nervous system that accompany these situations (i.e. change in heart rate, blood pressure, breathing, perspiration, etc.) to determine exactly which emotion is being experienced. Is it fear? Excitement? Anxiety? Anger? By analyzing the particular situation and comparing it to any memories associated with that or similar experiences, the limbic system helps us to identify and name the emotions we feel.
GABA neurons are present throughout the nervous system, from the eye (see http://retina.anatomy.upenn.edu/~lance/Neurochem/gaba.html), to the limbic system. The amygdala (or amygdaloid complex), located in the medial temporal lobe, is one of the most important elements of the limbic system, playing a crucial role in emotion and memory. More specifically, it is thought that the amygdala plays a large role in associating certain stimuli with rewards and in thus moderating some of our behavioral responses. Several studies have illustrated the crucial role that the amygdala plays in modulating the connection between our emotions and our behavior, most specifically in the areas of fear and aggression. In one experiment, rats were conditioned to experience a fear response to a neutral stimulus. This response, characterized by increased blood pressure and periods of "freezing" (immobility), was abolished after lesioning of the amygdala. In a second lesioning experiment, usually aggressive rhesus monkeys became placid and fearless around their caretakers after the removal of their amygdalas. In humans, electrical stimulation of the amygdala has been shown to lead to uncontrolled rage.
The amygdala connects various sensory processing regions with the hypothalamus and brain stem. Its inputs include projections from the olfactory cortex, somatic sensory pathways, and the auditory complex. This diverse sensory information is integrated and processed in the amygdala, where evidence of long term potentiation (LTP), and thus associative learning, has been shown. The processed signals are then sent through projections to the hypothalamus and brain stem. In this way, the amygdala has the ability to control both autonomic and somatic systems.