Question: The Gab operon is responsible for the conversion of γ-aminobutyrate to succinate, what is the  is the chief inhibitory neurotransmitter in the mammalian central nervous system, called?
Context: Glutamate receptors are synaptic receptors located primarily on the membranes of neuronal cells.  Glutamate (the conjugate base of glutamic acid) is abundant in the human body, but particularly in the nervous system and especially prominent in the human brain where it is the body's most prominent neurotransmitter, the brain's main excitatory neurotransmitter, and also the precursor for GABA, the brain's main inhibitory neurotransmitter.  Glutamate receptors are responsible for the glutamate-mediated postsynaptic excitation of neural cells, and are important for neural communication, memory formation, learning, and regulation. Eugene Roberts (January 19, 1920 – November 8, 2016) was an American neuroscientist.  In 1950, he was the first to report on the discovery of gamma-Aminobutyric acid (GABA) in the brain, and his work was key in demonstrating GABA as the main inhibitory neurotransmitter in the mammalian central nervous system. The Central Nervous System controls most of the functions of the body and mind.  It comprises the brain, spinal cord and the nerve fibers that branch off to all parts of the body.  The Central Nervous System viral diseases are caused by viruses that attack the CNS.  Existing and emerging viral CNS infections are major sources of human morbidity and mortality.  Virus infections usually begin in the peripheral tissues, and can invade the mammalian system by spreading into the peripheral nervous system and more rarely the CNS.  CNS is protected by effective immune responses and multi-layer barriers, but some viruses enter with high-efficiency through the bloodstream and some by directly infecting the nerves that innervate the tissues.  Most viruses that enter can be opportunistic and accidental pathogens, but some like herpes viruses and rabies virus have evolved in time to enter the nervous system efficiently, by exploiting the neuronal cell biology.  While acute viral diseases come on quickly, chronic viral conditions have long incubation periods inside the body.  Their symptoms develop slowly and follow a progressive, fatal course. Mitral cells are neurons that are part of the olfactory system.  They are located in the olfactory bulb in the mammalian central nervous system.  They receive information from the axons of olfactory receptor neurons, forming synapses in neuropils called glomeruli.  Axons of the mitral cells transfer information to a number of areas in the brain, including the piriform cortex, entorhinal cortex, and amygdala.  Mitral cells receive excitatory input from olfactory sensory neurons and external tufted cells on their primary dendrites, whereas inhibitory input arises either from granule cells onto their lateral dendrites and soma or from periglomerular cells onto their dendritic tuft.  Mitral cells together with tufted cells form an obligatory relay for all olfactory information entering from the olfactory nerve.  Mitral cell output is not a passive reflection of their input from the olfactory nerve.  In mice, each mitral cell sends a single primary dendrite into a glomerulus receiving input from a population of olfactory sensory neurons expressing identical olfactory receptor proteins, yet the odor responsiveness of the 20-40 mitral cells connected to a single glomerulus (called sister mitral cells) is not identical to the tuning curve of the input cells, and also differs between sister mitral cells.  The exact type of processing that mitral cells perform with their inputs still a matter of controversy.  One prominent hypothesis is the notion that mitral cells transform the strength of olfactory input into a timing code, where odor concentration is encoded in the phase of mitral cell firing relative to the sniff cycle.  A second (not necessarily exclusive) hypothesis is the idea of decorrelation in the olfactory bulb network, where the olfactory bulb network acts as a dynamical system whose action over time increases some (abstract) measure of distance between representations of highly similar odorants.  Support for the second hypothesis comes primarily from research in zebrafish (where mitral and tufted cells cannot be distinguished). "gamma-"Aminobutyric acid (γ-Aminobutyric acid) (GABA ) is the chief inhibitory neurotransmitter in the mammalian central nervous system.  Its principal role is reducing neuronal excitability throughout the nervous system.  In humans, GABA is also directly responsible for the regulation of muscle tone. The peripheral nervous system (PNS) is one of the two components of the nervous system, the other part is the central nervous system (CNS).  The PNS consists of the nerves and ganglia outside of the brain and spinal cord.  The main function of the PNS is to connect the CNS to the limbs and organs, essentially serving as a relay between the brain and spinal cord and the rest of the body.  Unlike the CNS, the PNS is not protected by the vertebral column and skull, or by the blood–brain barrier, which leaves it exposed to toxins and mechanical injuries.  The peripheral nervous system is divided into the somatic nervous system and the autonomic nervous system.  In the somatic nervous system, the cranial nerves are part of the PNS with the exception of the optic nerve (cranial nerve II), along with the retina.  The second cranial nerve is not a true peripheral nerve but a tract of the diencephalon.  Cranial nerve ganglia originated in the CNS.  However, the remaining ten cranial nerve axons extend beyond the brain and are therefore considered part of the PNS.  The autonomic nervous system is an involuntary control of smooth muscle and glands.  The connection between CNS and organs allows the system to be in two different functional states: sympathetic and parasympathetic. The GABA receptors are a class of receptors that respond to the neurotransmitter gamma-aminobutyric acid (GABA), the chief inhibitory compound in the mature vertebrate central nervous system.  There are two classes of GABA receptors: GABA and GABA.  GABA receptors are ligand-gated ion channels (also known as ionotropic receptors); whereas GABA receptors are G protein-coupled receptors, also called metabotropic receptors. The "gab" operon is responsible for the conversion of γ-aminobutyrate (GABA) to succinate.  The "gab" operon comprises three structural genes – "gabD", "gabT" and "gabP" – that encode for a succinate semialdehyde dehydrogenase, GABA transaminase and a GABA permease respectively.  There is a regulatory gene "csiR", downstream of the operon, that codes for a putative transcriptional repressor and is activated when nitrogen is limiting. The nervous system is the part of an animal that coordinates its actions by transmitting signals to and from different parts of its body.  Nervous tissue first arose in wormlike organisms about 550 to 600 million years ago.  In vertebrates it consists of two main parts, the central nervous system (CNS) and the peripheral nervous system (PNS).  The CNS consists of the brain and spinal cord.  The PNS consists mainly of nerves, which are enclosed bundles of the long fibers or axons, that connect the CNS to every other part of the body.  Nerves that transmit signals from the brain are called "motor" or "efferent" nerves, while those nerves that transmit information from the body to the CNS are called "sensory" or "afferent".  Spinal nerves serve both functions and are called "mixed" nerves.  The PNS is divided into three separate subsystems, the somatic, autonomic, and enteric nervous systems.  Somatic nerves mediate voluntary movement.  The autonomic nervous system is further subdivided into the sympathetic and the parasympathetic nervous systems.  The sympathetic nervous system is activated in cases of emergencies to mobilize energy, while the parasympathetic nervous system is activated when organisms are in a relaxed state.  The enteric nervous system functions to control the gastrointestinal system.  Both autonomic and enteric nervous systems function involuntarily.  Nerves that exit from the cranium are called cranial nerves while those exiting from the spinal cord are called spinal nerves. The GABA receptor (GABAR) is an ionotropic receptor and ligand-gated ion channel.  Its endogenous ligand is γ-aminobutyric acid (GABA), the major inhibitory neurotransmitter in the central nervous system.  Upon activation, the GABA receptor selectively conducts Cl through its pore, resulting in hyperpolarization of the neuron.  This causes an inhibitory effect on neurotransmission by diminishing the chance of a successful action potential occurring.  The reversal potential of the GABA-mediated IPSP in normal solution is −70 mV, contrasting the GABA IPSP.
Answer: GABA