Neurotransmitter Receptors in the Amygdala: R&D Systems
A few years ago, a quote by the Dalai Lama went viral. Abnormal functioning of the glutamate/GABA-glutamine cycle can trigger mental can help many people gain back control of their lives, relationships, and careers. In red, connections indicate glutamatergic excitation and blue connections We will quote experimental evidence indicating that GABA is . SNAT1, both absolutely and in relation to the GABA-synthesizing enzyme GAD The gold standard for studies of glutamate-glutamine(GABA) cycling and its The Glutamine–Glutamate (GABA) Shuttle and Its Relation to Glucose Metabolism.
Complementary neuroanatomical work has highlighted the structural basis for this function: All these properties favour synchronous rhythmic inhibition of large populations of principal cells [ 13697174 — 76 ].
It should be noted that the connections between excitatory projection cells and inhibitory interneurons provide an automatic homeostatic mechanisms at the network level. Feed forward or feedback inhibition is driven by excitatory inputs or outputs, respectively, from remote or local excitatory neurons.
- Function of the GABA Neurotransmitter and Everything Else About It
- Neurotransmitter Receptors in the Amygdala
This mechanism does automatically recruit inhibitory neurons in an activity-dependent manner and, hence, balance local activity Figure 1. Key Molecules for GABAergic Signalling The molecular organization of synapses is highly complex, and a complete review would be beyond the scope of this paper.
We will restrict our remarks to some families of molecules that are crucial for understanding homeostatic regulation of GABA concentration Figure 2. Schematic drawing of transmitter release, transport, and synthesis at a GABAergic synaptic terminal. Bottom structure indicates postsynaptic membrane of a target cell POSTfor example, a pyramidal neuron.
Transporters are marked by flanking arrows, and synthesizing or degrading enzymes are marked by a centred arrow. Transporters are colour matched to substrates: GABA is shown as blue particles, glutamate in red, and glutamine in green.
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For other abbreviations, see the main text. Like many other neurotransmitters, GABA acts on ionotropic as well as metabotropic ion channels. GABAA receptors are pentameric ion channels composed out of a large variety of 19 homologous subunits [ 327778 ]. The latter properties are of special interest with respect to GABA concentration. GABAARs with low agonist affinity appear to be clustered at postsynaptic sites, whereas receptors with high affinity are mostly found extrasynaptically [ 4748 ].
The underlying sorting mechanisms are partially known and involve specific subsynaptic sorting signals within the gamma subunit and interactions with postsynaptic scaffolding proteins like gephyrin and collybistin [ 80 — 83 ]. This distinction reflects the different concentrations of GABA at both sites: GABAergic auto- or heteroreceptors have been described at the axon terminals of various neurons, including spinal cord afferents [ 87 ], hippocampal mossy fibres [ 88 ], Schaffer collaterals [ 89 ], cerebellar interneurons [ 90 ], and pituitary terminals [ 91 ].
The effects of such receptors are diverse. Depending on the GABA-induced change in membrane potential and local membrane resistance, presynaptic GABAA receptors may increase or decrease transmitter release [ 92 ]. At presynaptic terminals, activation of Rs reduces GABA release, forming the typical negative feedback loop of autoreceptor-mediated synaptic gain control. Furthermore, receptors can also mediate tonic inhibition, exerting negative control on overall network activity [ 97 ].
We will briefly address each class of molecules involved in these processes. However, upon strong depolarization or altered ion homeostasis, GABA transporters can also reverse direction. This mechanism leads to nonvesicular release of GABA which may be of special importance in pathophysiological situations [ 60, ]. Terminology of GABA transporters is not fully compatible between rats and mice [ ]. As a global rule, GAT-1 is the prevailing neuronal isoform in the rodent brain, and GAT-3 is strongly expressed in glial cells [ — ].
Expression of different GAT isoforms is, however, overlapping, so that selective modulation of one isoform will always affect more than one cell type.
It might therefore turn out impossible to achieve a strictly selective block of glial or neuronal GABA uptake with conventional pharmacological tools. An alternative pathway for enriching GABA in presynaptic terminals is transmitter synthesis from glutamate. Moreover, neurons can synthesize glutamate from glutamine which can also be taken up by specialized transporters see below .
The smaller isoform GAD65 is directly associated to presynaptic vesicles, indicating that glutamate, once present in the presynaptic cytosol, can be rapidly used for vesicular enrichment of GABA.
More recently, glutamine has gained interest as an alternative source of GABA. The amino acid glutamine has long been known as the immediate precursor for glutamate.
Recordings of epileptiform activity in rodent brain slices in vitro have revealed functional evidence for boosting of inhibition by glutamine via this mechanism [ — ].
Linking GABA and glutamate levels to cognitive skill acquisition during development
Using high-resolution recordings of miniature IPSCs in conjunction with pharmacological manipulation of glutamine levels and glutamine transport, these studies showed that glutamine can serve as a source for GABA, especially under conditions of increased synaptic activity.
More recent evidence from rat hippocampal slices showed that the contribution of glutamine to vesicular GABA content is more pronounced in immature tissue, and that glutamine forms a constitutive source of vesicular GABA in immature hippocampal synapses on CA1 pyramidal cells. At later stages, the functional importance seems to be restricted to periods of enhanced synaptic activity [ ].
Additionally, chloride gradients between vesicle lumen and presynaptic cytosol may contribute to the vesicular loading of GABA . On the other side, recent evidence suggests that GABAergic synaptic vesicles are leaky, implying generation of a dynamic equilibrium between accumulation and loss of GABA, given that there is enough time to reach such a steady state .
In summary, there are several different molecular pathways and compartments for enrichment, synthesis, and degradation of GABA Figure 2. The resulting concentration of GABA in synaptic vesicles and in the extracellular space depends on the equilibrium between these mechanisms. It should be clearly stated that the absolute concentrations of GABA in the presynaptic cytosol, in vesicles, and in the extrasynaptic space are not known.
The affinity constants of extrasynaptic GABA receptors may serve as a rough estimate of background concentrations 0. Direct measurements from rat cerebrospinal fluid yielded similar or slightly higher values which may be lower in humans [ ]. The highly dynamic time course of transmitter concentration in the synaptic cleft, on the other hand, has been estimated based on experimental and theoretical work in different types of neurons.
Peak concentrations may be as high as 0. The cytosolic GABA concentration is most difficult to estimate or measure, especially since most of the neuronal GABA pool is used for energy metabolism rather than for synaptic inhibition. It should be explicitly stated that none of the above-given numbers has been directly measured.
Indeed, our knowledge on local GABA concentrations in different compartments is far from sufficient. This is even more concerning when we take into account the enormous heterogeneity of neurons [ 206365 ], the different microarchitecture of different local circuits, and activity-dependent changes in GABA release and ionic homeostasis. A major challenge is the lack of quantitative data about key molecules and structures: How many GABA-uptake molecules are present at a given inhibitory synapse?
What is their distribution with respect to the site of release? What is the precise extracellular volume at the synaptic cleft? How much GABA does go into glia cells and neurons, respectively? An important example for progress in this quantitative molecular approach to subcellular structure and function is the recent work on the vesicular proteasome by Takamori and colleagues [ ]. Regulation of GABA in Physiology and Pathophysiology Different lines of evidence support the view that the cellular and molecular mechanisms mentioned above make important contributions to homeostatic synaptic plasticity.
Pre-learning contents are indicated by open symbols and post-learning contents with filled-in symbols. From Hertz et al. Oxidative metabolism in astrocytes is a sine qua non-for operation of the glutamine—glutamate GABA cycle.
Pioneering studies early in this century Gruetter et al. These studies have been consistently and repeatedly confirmed in both human and rodent brain, and many of the rates are tabulated by Hertz b.
Since the volume occupied by astrocytes is similar to, or smaller, than the relative contribution of these cells to energy metabolism, their rate of oxidative metabolism per cell volume must be as high, if not higher, than that of neurons Hertz, b. This conclusion is consistent with an at least similarly high expression of most enzymes involved in oxidative metabolism of glucose in astrocytic as in neuronal cell fractions freshly obtained from the mouse brain Lovatt et al.
This is consistent with a recent in vivo study by Pardo et al. On the basis of their own and previous immunocytochemical observations in brain tissue by themselves and others Ramos et al. B Proposed expansion by Hertz a of the model shown in A. The expanded model shows astrocytic production of glutamine pathway 1its transfer to glutamatergic neurons without indication of any extracellular space, because there is no other function for extracellular glutamine than astrocyte-to-neuron transfer and extracellular release as the transmitter glutamate pathway 2and subsequent reuptake of glutamate and oxidative metabolism in astrocytes pathway 3with connections between pathways 1 and 3 shown as pathway 4.
Biosynthesis of glutamine is shown in brown and metabolic degradation of glutamate in blue. Redox shuttling and astrocytic release of glutamine and uptake of glutamate are shown in black, and neuronal uptake of glutamine, hydrolysis to glutamate, and its release is shown in red.
Reactions involving or resulting from transamination between aspartate and oxaloacetate OAA are shown in green. Small blue oval is pyruvate carrier into mitochondria and small purple oval malate carrier out from mitochondria. The latter suggestion required exit to the cytosol of mitochondrially located aspartate via the aralar-dependent AGC1 in the MAS. The suggestion of malate—aspartate participation in Figure 3 B was felt to be justified by the finding by Lovatt et al.
Moreover, it was calculated based on data by Berkich et al. Equally high levels of mRNA aralar expression is astrocytes were later confirmed, and its protein expression Figure 4 shown in freshly separated astrocytes and neurons from isolated cell fractions Li et al. The separation procedure used selects astrocytes indiscriminately, but among neurons it mainly isolates glutamatergic projection neurons.
These experiments also demonstrated remarkably large differences in aralar expression in young and mature animals. This finding was replicated in cultured astrocytes, whereas homogeneous neuronal cultures are too short-lived to provide meaningful results.
Protein expression of aralar in neuronal and astrocytic cell fractions are similar and develop at identical rates. Neuronal and astrocytic cell fractions were gently isolated from two mouse strains, one expressing a neuronal marker with a specific fluorescence and the second expressing an astrocytic fluorescent signal Lovatt et al.
From Li et al. The model suggested in Figure 3 B is consistent with the important 13C labeling data in the study by Pardo et al. Formation of glutamate from glucose requires glycogenolysis, both in the intact chicken brain Gibbs et al. Absence of glycogen phosphorylase in oligodendrocytes Richter et al. The rate of glycogenolysis in brain Table 1 is not high enough that pyruvate derived from glycogen could be used by the astrocytes as the sole source of pyruvate for carboxylation.
This exceeds the rate of glycogenolysis by at least 10 times. Rather, as in the case of other astrocytic processes requiring activation of specific signaling pathways Xu et al. Pyruvate carboxylation at least in other cell types Garrison and Borland, is also stimulated by noradrenaline, as is astrocytic glycogenolysis Magistretti, ; Subbarao and Hertz, This does not mean that a very brief increase in glutamate content, as shown in Figure 2 might not, at least partly, be derived from glycogen, which showed a simultaneous precipitous and large fall Hertz et al.
Formation of glutamine from glutamate in the astrocytic cytosol is in agreement with the astrocyte-specific expression of glutamine synthetase Norenberg and Martinez-Hernandez,with probable lack of expression in oligodendrocytes confirmed by Derouiche In cultured astrocytes reduced function of the glutamine synthetase after administration of its inhibitor, methionine sulfoximine MSOcauses an increase in glutamate and aspartate formation, the latter probably reflecting increased glutamate oxidation, when glutamine synthesis is inhibited Zwingmann et al.
Increased content of aspartate in brain slices during MSO inhibition has also been shown by Nicklas Glutamine can travel between gap-coupled astrocytes, and the distance it reaches increases during brain activation Cruz et al.
Different transporters have been proposed to direct its transport from astrocytes to neurons, but it now appears well established that glutamine release occurs via the amino acid transporter SN1.
This transporter is densely expressed in astrocytic processes abutting glutamatergic and GABAergic neurons Boulland et al. This topic is discussed in detail in the paper by Chaudhry et al.
Although not shown in Figure 3 B for the sake of simplicitythe subsequent de-amidation of glutamine to glutamate appears to be somewhat complex, probably reflecting the subcellular localization of the phosphate-activated glutaminase PAG. In cultured glutamatergic neurons inhibitor studies have suggested the pathway indicated in Figure 5 Palaiologos et al. This Figure shows conversion of glutamine to glutamate by PAG, followed by a process similar to that occurring in the MAS, with the only exception that the glutamate molecule involved does not originate in the cytosol, but from PAG-activated de-amidation of glutamine in the intermembranaceus space of the mitochondrion.
Evidence that a similar process occurs in freshly isolated mitochondria Bak et al. Metabolic pathway for conversion of glutamine to glutamate in cultured cerebellar granule neurons.
Glutamine enters the intermembranaceus space from the cytosol red arrow at bottom of Figure. Testing stopped as soon as the participant failed on two subsequent sequences of the same length. Test administration took no more than 8 minutes for each participant. All participants were then invited to participate in a separate behavioral testing session.
The IFG was placed based on coronal and axial images to cover the inferior frontal tissue without capturing temporal cortices. None of the voxels exceeded the located gyrus toward the anterior or posterior end. For the IPS, the hand knob of the motor cortex was identified on axial slices [Yousry, et al. We centered the voxel on the sulcus posterior and inferior to this landmark for the motor cortex.
The IOG was localized by angling the voxel along the outer cortical edge away from the calcarine fissure and as ventral as the size of the voxel allowed, avoiding the sinuses and ventricles.
Due to the shapes of some brains, the shim volume centered on the voxel penetrated the ventricles in a few cases to a similar extent in adults and children.GABA Neurotransmitters and Glutamate
Due to individual variations in anatomy, slight differences in the signal contribution from neighboring areas could not entirely be avoided.