Discovered in 1950’s high brain concentration, not used in protein synthesis. Unique neutral charge (isoelectric point 7.3)

Balance of excitation and inhibition Balance of excitation and inhibition. Often GABA and glutamate inputs are paired. Plus modulation by monoamines and 5-HT. GABA influences glutamate transmission two ways—temporally and spatially.

Imbalance between excitation and inhibition/glutamate and GABA lead to MANY disorders of the nervous system. Both are high priority targets for neuropsychiatric therapeutics.

Replace this slide GABA is essentially everywhere and most neurons respond to GABA transmitter. Synthesizing neurons can be identified by GABA synthesis enzyme, glutamic acid decarboxylase (GAD). Local circuit neurons: interneurons in cortex, cb, hp, amy, sc Projection neurons: striatal output neurons, purkinje cells of cb, amy to BNST, nb to cortex

GABA is synthesized from glutamate by glutamic acid decarboxylase.

GAD levels exceed GABA production, meaning that GAD activity can be modulated. GABA a-ketoglutarate transminase (GABA-T) degrades GABA in neurons and glia.

2 genes for GAD (65kD and 67kD) GAD-65 is membrane associated—mitochondria and synaptic vesicle Found high in terminals and associated with the plasma membrane transporter GAD-65-mediated synthesis may be regulated by protein-phosphate interactions upon demand for GABA GAD-67 is cytosolic, produces non-vesicular GABA Remember, glutamine recycling in glia allows conversion of gln to glutamate by PAG (phosphate-activated glutaminase), then glutamate is converted to GABA by GAD enzymes

excluded from dendrites in hippocampal neurons. Endogenous rat GAD67 is targeted to Golgi membranes and presynaptic clusters and excluded from dendrites in hippocampal neurons. Evidence that GAD-67 is localized to vesicles. With GAD-65. Kanaani J et al. J Cell Biol doi:10.1083/jcb.200912101

Importance of GAD isozymes GAD-65 knockout mice Increased seizure susceptibility Impaired experience dependent plasticity GAD-67 knockout mice Normal brain activity Cleft palate Reduced GAD-67 in psychiatric disorders (schizophrenia and bipolar)

Mitochondrial and cytoplasmic processes work together to regulate production of GABA and glutamate in both neurons and glia. GABA may be degraded to glutamate in glia, then reuptaken by GABAergic cells as a GAD substrate Or converted to glutamine and taken up by glutaminergic neurons.

VIAAT: vesicular uptake of GABA Storage in vesicles.

Regulation of GABA transmission GABA-T inhibition in neurons increases GABA current in receptive neurons in cultures. Inhibition of GAD reduces response. Depolarization of cells with Potassium increases GABA current in receptive neurons and GABA-T inhibition enhances response. Vigabatrin: inhibits GABA-Transaminase (prolongs GABA), used as anti-epileptic Semicarbazide: GABA synthesis inhibitor

Inactivation of GABA transmission GABA plasma membrane transporter: GAT

Notice increased decay time for the IPSC C. After high frequency stimulation, subsequent pulses produce current that smaller, takes longer to recover—much slower decay time.

Uptake of GABA by Ce GAT-1 in cell culture Time course (A) and ion dependence (B) of GABA uptake via ceGAT-1 in HRPE cells. A, cells were transfected with either vector alone (pSPORT) (•) or vector carrying ceGAT-1 cDNA (○). Uptake of [3H]GABA (2.5 μm) was determined in these transfected cells for different time periods in uptake medium containing 140 mm NaCl. B, cells were transfected with either vector alone (pSPORT) or vector carrying the ceGAT-1 cDNA insert. Uptake of GABA (2.5 μm)

Pathological dysregulation of GABA Under high activity, ion gradients can be reversed, leading to transporters running backward—require less energy. Though this can happen with monoamine transporters, it may occur more easily with the GAT