The Nervous System · Part One · How It Is Built and Fueled
Lesson 08 / 61
GABA and Glutamate: The Excitation to Inhibition Balance and the Cells That Hold It
One supply chain, two transmitters, and the ratio every circuit runs on.
Glutamate is the principal excitatory transmitter of the central nervous system and GABA is its principal inhibitory counterweight. Between them they carry most fast synaptic signaling. What a circuit can do at any moment follows the ratio between the two rather than the level of either, and cortex holds that ratio steady while both sides swing. The Unified Model of Tone reads the defended ratio as tone at the scale of a single neuron.
Ionotropic glutamate receptors
AMPA, NMDA, kainate
GABA receptors
GABA-A chloride channels, metabotropic GABA-B
Glutamate cycling rate
0.74 micromol per minute per gram, about half of TCA flux
Two modes of inhibition
Phasic synaptic events, standing tonic conductance
Glutamate and GABA
Glutamate is the amino acid released at excitatory synapses. GABA is built from it in a single enzymatic step by glutamic acid decarboxylase, which makes the inhibitory transmitter a chemical derivative of the excitatory one. Neither can be synthesized from glucose by the neuron that releases it. The carbon arrives as glutamine from astrocytes, so both sides of the balance draw on one glial supply.
The excitation to inhibition ratio
The running proportion of excitatory to inhibitory drive reaching a neuron, held in place by where the inhibition sits. Forty percent of a CA1 pyramidal cell's inhibitory synapses concentrate perisomatically, on the cell body and axon initial segment, where a single terminal cancels the whole summed total and not one branch of it. Because the quantity is a proportion, a cell can take twice the excitation and twice the inhibition and reply as before. A measured level of either transmitter says little on its own.
01Synapse counts and conductance
Glutamate and GABA set the ratio every circuit runs on
Glutamate and GABA are the two amino acid transmitters that carry the overwhelming majority of fast synaptic signaling in the brain. Glutamate depolarizes the postsynaptic membrane through AMPA, NMDA, and kainate receptors. GABA hyperpolarizes the cell through GABA-A chloride channels and GABA-B receptors. Almost every circuit, from cortex to brainstem, is tuned by the moment to moment proportion between them.
The anatomy of the balance has been counted. One CA1 pyramidal cell in the rat hippocampus carries about 12,000 micrometers of dendrite and receives around 30,000 excitatory and 1,700 inhibitory inputs Megías 2001. Excitation outnumbers inhibition roughly eighteen to one, and the cell still spends most of its time silent. Forty percent of the inhibitory synapses concentrate in the perisomatic region, and only inhibitory terminals reach the cell body and the axon initial segment, where one veto lands on the whole sum.
Clinical neuroscience frames the neuron as a summing junction. Thousands of glutamatergic and GABAergic synapses converge on one input region, and only when depolarization crosses threshold do voltage sensitive calcium channels open and vesicles release. The balance drifts with metabolism, with afferent traffic, and with fatigue, which is why one network can be precise in one state and noisy in another.
The net balance holds steady while excitation and inhibition both move
Membrane conductance in ferret prefrontal cortex changed by 21 nanosiemens on average during the Up states of the slow oscillation. The reversal potential of that network activity held at minus 37 millivolts for hundreds of milliseconds Haider 2006. Intracellular recording through the same states showed excitatory and inhibitory conductance rising and falling in proportion. Both sides moved. The ratio did not.
The Unified Model of Tone reads that defended proportion as the central integrative state, the running sum of every excitatory and inhibitory influence converging on one cell. Neurophysiology already names the quantity, and the neuron and the central integrative state carries it in full. The model identifies it as tone read at the scale of a single neuron, which is why the same proportion reappears at the scale of a network and of a whole organism.
Tonic inhibition sets the floor
Inhibition runs in two modes at once. Low concentrations of ambient GABA persistently activate certain GABA-A subtypes sitting away from synapses, generating a tonic conductance that runs alongside the phasic synaptic response Farrant 2005. That standing conductance sets resting excitability before any single synapse fires. Excitation has to rise above a floor, and GABA decides where the floor sits.
02Findings
What the research shows
Measured values for the two transmitters, their shared supply, and the ratio between them.
03One glutamine supply chain
Glutamate and GABA are built from one precursor the neuron cannot make
Neurons cannot build transmitter glutamate or GABA from glucose Bak 2006. The carbon arrives as glutamine from astrocytes. The glutamate-GABA-glutamine cycle exists because the cell that releases the transmitter cannot make it from scratch.
The loop runs with two branches. Released glutamate is taken into the astrocyte and converted to glutamine by glutamine synthetase. Glutamine returns to the neuron, where phosphate activated glutaminase regenerates glutamate. In GABAergic neurons that same glutamate is decarboxylated to GABA by glutamic acid decarboxylase. Nitrogen travels the loop alongside the carbon, which is how the cycle also moves ammonia between the two cell types Bak 2006.
In vivo carbon-13 NMR puts the stoichiometry between oxidative glucose metabolism and glutamate neurotransmitter cycling in rat cortex close to 1 to 1 Sibson 1998. Most of what the cortex burns fuel for is glutamatergic signaling and the recycling behind it. Carbon-13 spectroscopy puts glutamate cycling at 0.74 micromol per minute per gram, roughly half of TCA cycle flux, and the rate climbs 60 percent on activation Morris 2003. What the charge itself costs, before any transmitter moves, is counted on the resting and action potential.
Part of the GABA produced in the brain derives from glial glutamine Morris 2003. One carbon skeleton feeds the excitatory transmitter and the inhibitory one, so metabolism and excitability move together. A shortfall in fuel shows up as a shift in the balance long before it shows up as a dead cell.
The dependency also runs the other way. A neuron cut off from glutamine cannot refill its vesicles, and the excitatory and inhibitory sides of the ledger thin together.
Astrocytic uptake ends the signal and sets how far it spreads
Astrocytes take up most released glutamate, and the transporters that feed the cycle also decide how long the transmitter stays in the cleft and how far it reaches past the synapse that released it. Transporter density and the fraction of a synapse a glial process covers both differ by region, so identical release leaves two regions with two different excitation to inhibition ratios. Neuroglia and the neurovascular unit counts the transporter molecules and the coverage. Excitotoxicity takes up what happens when the clearance fails.
One enzyme carries most of the inhibitory supply
Two isoforms of glutamic acid decarboxylase make GABA, and they do not share the load evenly. In newborn mice lacking GAD67, decarboxylase activity fell to 20 percent of normal and cortical GABA content to 7 percent Asada 1997. Deleting one enzyme stripped 93 percent of the cortex's inhibitory transmitter. Deleting GAD65 left brain GABA content unchanged and produced only a slight rise in seizure susceptibility. The inhibitory supply runs on one dominant enzyme with a reserve behind it.
The Unified Model of Tone takes the shared supply chain as the reason this balance behaves as one quantity. Excitation and inhibition are two outputs of a single metabolic loop, so a change in fuel, in glial function, or in glutamine transport moves both sides at once. What is left free to vary is the proportion between them, and the proportion is what the model reads.
04NMDA receptors and plasticity
NMDA receptors gate plasticity while AMPA receptors carry the traffic
AMPA receptors carry the fast, brief excitatory current of ordinary signaling, and NMDA receptors detect coincidence, opening fully only when the membrane is already depolarized. The same molecule therefore means two different things at one synapse, depending on the state the membrane is in when it arrives.
The gate is a magnesium ion sitting in the pore. At the physiological extracellular concentration of about 1 millimolar, magnesium blocks the open NMDA channel in cultured mouse central neurons, and the block deepens steeply with hyperpolarization Nowak 1984. Depolarization expels the ion and the channel conducts. Withdrawing magnesium from the bath removes most of the voltage sensitivity of the NMDA response in cultured spinal cord neurons Mayer 1984. No separate voltage sensor inside the membrane is required.
Blocking the receptor separates learning from signaling. In rat hippocampal slices, APV at doses that abolished responses to N-methyl-aspartate left the Schaffer collateral EPSP untouched and prevented long term potentiation after high frequency stimulation Collingridge 1983. Routine transmission continued unchanged. The synapse could no longer learn.
Long term potentiation was first shown in the dentate area of the anesthetized rabbit. Conditioning trains ran at 10 to 20 per second for 10 to 15 seconds, or at 100 per second for 3 to 4 seconds. Either train potentiated the granule cell population response in 15 of 18 animals Bliss 1973. The potentiation lasted from 30 minutes to 10 hours.
When AMPA and NMDA activation coincide, calcium floods the spine and triggers the immediate early gene response, cFos and cJun among them, that consolidates the change. Depolarization also opens voltage sensitive calcium channels in the terminal and drives vesicle exocytosis, so transmitter crosses the synapse only when excitation has overcome the resting balance.
Glutamate alone cannot open the gate. Selective degradation of D-serine with D-amino acid oxidase greatly attenuates NMDA receptor mediated transmission in rat hippocampal tissue, and adding D-serine back restores it Mothet 2000. The coagonist comes from the astrocyte, so the cell that clears the transmitter also holds a key to the receptor that reads it.
GABA decides which coincidences reach threshold
Perisomatic terminals place 40 percent of a pyramidal cell's inhibitory input on the cell body and axon initial segment Megías 2001. That is the last place a coincidence has to survive to count for anything. Beneath that sits the tonic conductance from ambient GABA, holding resting excitability down between events Farrant 2005. Inhibition selects which coincidences reach the threshold that rewrites a synapse.
The Unified Model of Tone reads the NMDA receptor as correspondence made molecular. The receptor answers a match between arriving transmitter and existing membrane state. A large volley into a cell held down by tonic GABA accomplishes nothing, and a modest one arriving while the membrane is already depolarized rewrites the synapse. This receptor is deaf to size on its own. What it answers is timing against the state it finds.
05The inhibitory floor
Inhibition sets the floor that excitation has to stay above
Inhibition is the floor of this balance, and excitation past that floor stops being signal. Glutamate that outruns clearance holds NMDA receptors open, calcium enters past what the cell can buffer, and the machinery that builds plasticity begins to take the neuron apart. One receptor writes the memory and kills the cell, and where a given neuron lands on that line depends on the transporter density of the glia beside it. Excitotoxicity follows the cascade from the calcium entry to the death of the cell. This page stops at the boundary, which is the last point at which the ratio still describes a working circuit.
Cortical GABA works inside a narrow band
The inhibitory floor is held inside a narrow band, and the band has been measured in people. Occipital GABA read by magnetic resonance spectroscopy came to 1.03 mmol per kilogram of brain in patients with complex partial seizures against 1.18 in subjects without epilepsy Petroff 1996. Patients who had seized within a day sat at 0.92, and those seizure free for five years or longer sat at 1.28. About a tenth of a millimole per kilogram separates epilepsy from no epilepsy, and roughly a third of a millimole spans the distance between a seizure inside the day and five years without one. Brain activity takes up what that instrument actually samples and how much weight one value carries.
Epilepsy, stroke, migraine, and peripheral and central sensitization all run through this balance tipping toward unchecked excitation. GABA is the brake against the slide, working through phasic synaptic events and through the extrasynaptic receptors that read ambient transmitter.
Drugs that add inhibition or block excitation act on real receptors and change real currents, and for a seizing brain that change is the point. The Unified Model of Tone grades two kinds of input differently. A benzodiazepine adds chloride conductance to every cell carrying the receptor, and it adds the same conductance to the cortex that had drifted toward excitation and to the cortex sitting where it should. Afferent input matched to the state a segment holds arrives through that segment's own pathways, so it reaches the tissue whose proportion has moved and passes over the tissue whose proportion has not. The grading is of the input, not of the patient.
A ratio inside its range is health, because a network holding that range can answer a strong input and come back to where it started. Outside the range it fails in one of two directions. Too little inhibition and the network answers noise, spreads excitation to its neighbors, and seizes. Too much and it stops answering signal that matters, which is sedation rather than calm. Here the drift is legible in ordinary units, in tenths of a millimole per kilogram of cortical GABA.
06Afferent input and the ratio
Input from the body moves the excitation to inhibition ratio
One session of high intensity interval exercise raised sensorimotor cortex GABA concentration in healthy adults by 20 percent, read by magnetic resonance spectroscopy before and after Coxon 2018. The rise correlated positively with the rise in blood lactate. Dorsolateral prefrontal cortex did not change at all.
The model reads regional specificity of that kind as correspondence between an input and the tissue already holding a state that input can move. Exercise drove the sensorimotor cortex, and the inhibitory side of the balance rose there. Prefrontal cortex received no such drive and held its GABA where it was. A systemic exposure would have moved both.
GABA gates sensory traffic at the first synapse
GABA gates afferent input before it reaches a second order neuron. GABAergic interneurons make axo-axonic synapses on the terminal arborizations of primary afferent fibers in the cat spinal cord, producing primary afferent depolarization and reducing transmitter release Rudomin 1999. The effect can be confined to single intraspinal collaterals of one muscle spindle afferent. Proprioceptive traffic is held down at its point of entry by the same transmitter that sets cortical excitability, and the cord can do it to one branch at a time.
Segmental input reaches central inhibition measurably. In 10 subjects with subclinical neck pain, spinal manipulation combined with a motor sequence learning task produced a 19 percent decrease in mean reaction time Daligadu 2013. Cerebellar inhibition, read by paired pulse transcranial magnetic stimulation, fell at the same time, and that shift carried the group toward the pattern of the healthy controls. Manipulation was delivered with the learning task rather than tested alone.
We are the practice organized around the nervous system, and the input we deliver is mechanical, graded, and segmental. Spindle and joint afferents from a loaded segment enter the dorsal horn through the same GABAergic gate Rudomin described, and they arrive at cortex where GABA concentration is a number an instrument can read. The Unified Model of Tone states the claim in its own name. What care changes at a segment is the proportion between excitatory and inhibitory drive in the circuits that segment feeds, and glutamate and GABA are the chemistry that proportion is made of.
The commitment is testable in this chemistry. Cortical GABA concentration is readable by spectroscopy, cerebellar inhibition by paired pulse stimulation, and presynaptic inhibition of a spindle afferent one collateral at a time. A segmental input that moves all three together, cortical GABA concentration, cerebellar inhibition and presynaptic inhibition of a spindle afferent, is what establishes them as three readings of one balance. Tone carries the library's general form of that commitment. Here it is narrow: one proportion, reachable from the spine, from the muscles, and from the drugs that bind the receptor.
The brain builds both transmitters from one supply, then spends its day deciding the ratio between them.
07Tone
How this system expresses tone
Three of the nine foundations are read directly in the concentrations and conductances of these two molecules.
Set point
Cortex defends a proportion while the totals swing. Membrane conductance in ferret prefrontal cortex moved 21 nanosiemens during Up states while network reversal potential held at minus 37 millivolts.
Coupling
Excitation and inhibition draw on one carbon skeleton. Glutamate cycling runs at 0.74 micromol per minute per gram, about half of TCA cycle flux, and it feeds both transmitters.
Input quality
Somatic drive moves the inhibitory side directly. High intensity exercise raised sensorimotor cortex GABA by 20 percent in healthy adults and left prefrontal cortex unchanged.
The other six foundations are legible in the same two molecules. Gain: inhibitory terminals on the cell body and axon initial segment decide how much of the arriving excitation survives to the output. Oscillation: excitation and inhibition arrive as paired bursts across the cortical slow rhythm, so the balance is read across a cycle and not at one instant. Prediction: the NMDA receptor counts coincidence, which strengthens the synapse for input that has already proved reliable. Load: glutamatergic signaling and its recycling run close to 1 to 1 with oxidative glucose metabolism in rat cortex. Constraint: neurons cannot build either transmitter from glucose, so what the astrocyte supplies limits what the synapse can send. Time course: channels open in milliseconds, tonic conductance holds across seconds, and a few seconds of patterned input changes synaptic strength for up to 10 hours.
08Across the library
How this page relates to the rest of the library
Where the excitation to inhibition balance is picked up across the library.
The running balance taken as a property of the whole cell, where summation down the dendritic tree turns thousands of excitatory and inhibitory votes into one answer at the trigger zone.
Glutamate and GABA placed beside the monoamines, acetylcholine, and the peptides, with the receptor families that decide what each molecule means at the cell receiving it.
The astrocyte counted as a working cell: transporter molecules per cubic micrometer of tissue, the fraction of each synapse a glial process wraps, and the potassium handling behind both.
What follows when clearance fails, from the calcium route that decides how fast a neuron dies to the stroke chemistry in which the cascade runs at scale.
The spindle and joint afferents that GABAergic axo-axonic terminals hold down one collateral at a time, followed back to the receptors in muscle that generate the traffic.
The same balance read as instrument output: what a spectroscopy value for cortical GABA or Glx samples, why one voxel averages across cell types and compartments, and which interventions move the number.
09Frequently asked
Questions about this topic
What do glutamate and GABA do in the brain?
Glutamate is the principal excitatory transmitter of the central nervous system. It opens AMPA, NMDA, and kainate receptors and drives the target neuron toward firing. GABA is the principal inhibitory transmitter, acting at GABA-A chloride channels and at metabotropic GABA-B receptors to hold the cell back from threshold. Together they carry most fast synaptic signaling in the brain. The two are chemically continuous, since GABA is made from glutamate in one step by glutamic acid decarboxylase, so a single supply feeds both sides of the balance.
What is the excitation to inhibition balance?
The excitation to inhibition balance is the running proportion between excitatory and inhibitory drive reaching a neuron or a network. In ferret prefrontal cortex, membrane conductance changed by about 21 nanosiemens during the Up states of the slow oscillation, with excitation and inhibition rising in proportion. The reversal potential of network activity held at minus 37 millivolts throughout. A single CA1 pyramidal cell receives around 30,000 excitatory and 1,700 inhibitory inputs, and 40 percent of the inhibitory ones sit perisomatically where they cancel the summed total.
Where does the brain get its glutamate and GABA?
Neurons cannot synthesize transmitter glutamate or GABA from glucose on their own. Both are rebuilt through the glutamate-GABA-glutamine cycle. Astrocytes take up released glutamate, convert it to glutamine with glutamine synthetase, and return the glutamine to neurons, where glutaminase regenerates glutamate. GABAergic neurons then decarboxylate that glutamate with glutamic acid decarboxylase, and part of brain GABA derives from glial glutamine directly. Glutamate cycling runs at 0.74 micromol per minute per gram, roughly half of TCA cycle flux, and climbs 60 percent on activation.
What is tonic inhibition?
Tonic inhibition is a standing chloride conductance produced by low concentrations of ambient GABA acting at GABA-A receptor subtypes that sit outside the synapse. It runs continuously, alongside the brief phasic events that arrive when an inhibitory terminal releases a vesicle. The tonic component sets resting excitability before any single synapse fires, which makes it the floor that arriving excitation has to clear. A drug acting at extrasynaptic GABA-A receptors moves that floor for every input at once, without altering synaptic transmission itself.
Why is the NMDA receptor called a coincidence detector?
A magnesium ion blocks the NMDA channel at rest. At the physiological extracellular concentration of about 1 millimolar, that block deepens with hyperpolarization and is relieved by depolarization, so the receptor conducts only when transmitter arrives at an already depolarized membrane. No separate voltage sensor inside the membrane is involved. The receptor also requires a coagonist, and degrading D-serine with D-amino acid oxidase silences NMDA transmission even when glutamate is present. Blocking the receptor with APV leaves ordinary transmission intact and abolishes long term potentiation.
What happens when inhibition fails to hold the floor?
Excitation that runs past the inhibitory floor stops behaving as signal. Glutamate accumulating in the cleft holds NMDA receptors open, calcium enters past what the cell can buffer, and the machinery of plasticity begins to take the neuron apart. The inhibitory margin is narrow to begin with. Occipital GABA sits near 1.18 mmol per kilogram in people without epilepsy and at 0.92 in patients who seized within the day. A third of a millimole per kilogram spans good seizure control and poor.
Can input from the body change GABA levels in the brain?
Input from the body changes GABA levels in the brain, and the change is regional. Magnetic resonance spectroscopy before and after high intensity interval exercise found sensorimotor cortex GABA up by 20 percent in healthy adults, correlated with the rise in blood lactate, while dorsolateral prefrontal cortex did not change. In the cat spinal cord, GABAergic interneurons form axo-axonic synapses on primary afferent terminals and reduce transmitter release, and the effect can be confined to single collaterals of one muscle spindle afferent.
How does the Unified Model of Tone read the excitation to inhibition balance?
The model reads the defended proportion between excitatory and inhibitory drive as tone in its chemical form. Both transmitters are rebuilt from glial glutamine, so a change in fuel or in glial function moves both sides at once and leaves the proportion as the quantity that varies. Cortex protects that proportion while the totals swing, and an input that shifts it changes what every synapse downstream can accomplish. The whole-cell version of the same quantity is the central integrative state.
10The sources
References
Sources: primary literature, linked inline.