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?

<P0-S0> The "gab" operon is responsible for the conversion of γ-aminobutyrate (GABA) to succinate. <P0-S1>  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. <P0-S2>  There is a regulatory gene "csiR", downstream of the operon, that codes for a putative transcriptional repressor and is activated when nitrogen is limiting. <SEP> <P1-S0> Glutamate receptors are synaptic receptors located primarily on the membranes of neuronal cells. <P1-S1>  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. <P1-S2>  Glutamate receptors are responsible for the glutamate-mediated postsynaptic excitation of neural cells, and are important for neural communication, memory formation, learning, and regulation. <SEP> <P2-S0> Mitral cells are neurons that are part of the olfactory system. <P2-S1>  They are located in the olfactory bulb in the mammalian central nervous system. <P2-S2>  They receive information from the axons of olfactory receptor neurons, forming synapses in neuropils called glomeruli. <P2-S3>  Axons of the mitral cells transfer information to a number of areas in the brain, including the piriform cortex, entorhinal cortex, and amygdala. <P2-S4>  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. <P2-S5>  Mitral cells together with tufted cells form an obligatory relay for all olfactory information entering from the olfactory nerve. <P2-S6>  Mitral cell output is not a passive reflection of their input from the olfactory nerve. <P2-S7>  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. <P2-S8>  The exact type of processing that mitral cells perform with their inputs still a matter of controversy. <P2-S9>  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. <P2-S10>  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. <P2-S11>  Support for the second hypothesis comes primarily from research in zebrafish (where mitral and tufted cells cannot be distinguished). <SEP> <P3-S0> "gamma-"Aminobutyric acid (γ-Aminobutyric acid) (GABA ) is the chief inhibitory neurotransmitter in the mammalian central nervous system. <P3-S1>  Its principal role is reducing neuronal excitability throughout the nervous system. <P3-S2>  In humans, GABA is also directly responsible for the regulation of muscle tone.