Distinct origin of GABA-ergic neurons in forebrain of man, nonhuman primates and lower mammals

In this mini-review we present recent data about origin of GABA-ergic (gama-aminobutyric acid) neurons in the mammalian forebrain, including the diencephalon and telencephalon. The interest in GABA-ergic neurons, which in cerebral cortex mostly correspond to local circuit neurons (interneurons), has increased in the past decade. Many studies have shown that in lower mammals all hippocampal and almost all neo-cortical GABA-ergic neurons are born in the specific region named ganglionic eminence, and not locally in proliferative layers all around telencephalic vesicle. The ganglionic eminence, that represents a region with thick proliferative-subventricular layer in the ventral (basal) part of telencephalon, was classically thought to give neurons to basal ganglia and septal nuclei, whereas proliferative layers of dorsal telencephalon give neurons to cerebral cortex including hippocampus. It was thought that neurons migrate from proliferative layer to their target region following a radial orientation. However, data in lower mammals showed that this is the case only for glutamatergic principal cells, i.e. projection neurons. GABA-ergic neurons use long distance tangentional migration, parallel to pial surface to reach, from ganglionic eminence, their targeting layer in the cerebral cortex. Especially intriguing, but frequently neglecting, several studies suggest that mammalian evolution might use different developmental rules to provide GABA-ergic neurons to an expending brain. In this review we focus on specific events underlying GABA-ergic neuron development in human and non-human primates. Disturbances of the GABAergic network are found in many neurological and psychiatric disorders, some of them might result from altered production or migration of these neurons during development. Therefore, it is crucial to understand human-specific mechanisms that regulate the development of GABA-ergic neurons.     

Excitatory projection neuron subtypes control the distribution of local inhibitory interneurons in the cerebral cortex

In the mammalian cerebral cortex, the developmental events governing the integration of excitatory projection neurons and inhibitory interneurons into balanced local circuitry are poorly understood. We report that different subtypes of projection neurons uniquely and differentially determine the laminar distribution of cortical interneurons. We find that in Fezf2?/? cortex, the exclusive absence of subcerebral projection neurons and their replacement by callosal projection neurons cause distinctly abnormal lamination of interneurons and altered GABAergic inhibition. In addition, experimental generation of either corticofugal neurons or callosal neurons below the cortex is sufficient to recruit cortical interneurons to these ectopic locations. Strikingly, the identity of the projection neurons generated, rather than strictly their birthdate, determines the specific types of interneurons recruited. These data demonstrate that in the neocortex individual populations of projection neurons cell-extrinsically control the laminar fate of interneurons and the assembly of local inhibitory circuitry.     

GABAergic interneuron to astrocyte signalling: a neglected form of cell communication in the brain

GABAergic interneurons represent a minority of all cortical neurons and yet they efficiently control neural network activities in all brain areas. In parallel, glial cell astrocytes exert a broad control of brain tissue homeostasis and metabolism, modulate synaptic transmission and contribute to brain information processing in a dynamic interaction with neurons that is finely regulated in time and space. As most studies have focused on glutamatergic neurons and excitatory transmission, our knowledge of functional interactions between GABAergic interneurons and astrocytes is largely defective. Here, we critically discuss the currently available literature that hints at a potential relevance of this specific signalling in brain function. Astrocytes can respond to GABA through different mechanisms that include GABA receptors and transporters. GABA-activated astrocytes can, in turn, modulate local neuronal activity by releasing gliotransmitters including glutamate and ATP. In addition, astrocyte activation by different signals can modulate GABAergic neurotransmission. Full clarification of the reciprocal signalling between different GABAergic interneurons and astrocytes will improve our understanding of brain network complexity and has the potential to unveil novel therapeutic strategies for brain disorders.     

Widespread neuronal expression of branched-chain aminotransferase in the CNS: implications for leucine/glutamate metabolism and for signaling by amino acids

Transamination of the branched-chain amino acids produces glutamate and branched-chain alpha-ketoacids. The reaction is catalyzed by branched-chain aminotransferase (BCAT), of which there are cytosolic and mitochondrial isoforms (BCATc and BCATm). BCATc accounts for 70% of brain BCAT activity, and contributes at least 30% of the nitrogen required for glutamate synthesis. In previous work, we showed that BCATc is present in the processes of glutamatergic neurons and in cell bodies of GABAergic neurons in hippocampus and cerebellum. Here we show that this metabolic enzyme is expressed throughout the brain and spinal cord, with distinct differences in regional and intracellular patterns of expression. In the cerebral cortex, BCATc is present in GABAergic interneurons and in pyramidal cell axons and proximal dendrites. Axonal labeling for BCATc continues into the corpus callosum and internal capsule. BCATc is expressed by GABAergic neurons in the basal ganglia and by glutamatergic neurons in the hypothalamus, midbrain, brainstem, and dorsal root ganglia. BCATc is also expressed in hypothalamic peptidergic neurons, brainstem serotoninergic neurons, and spinal cord motor neurons. The results indicate that BCATc accumulates in neuronal cell bodies in some regions, while elsewhere it is exported to axons and nerve terminals. The enzyme is in a position to influence pools of glutamate in a variety of neuronal types. BCATc may also provide neurons with sensitivity to nutrient-derived BCAAs, which may be important in regions that control feeding behavior, such as the arcuate nucleus of the hypothalamus, where neurons express high levels of BCATc.     

Quantitative analysis of glutamatergic and GABAergic neurons expressing 5-HT(2A) receptors in human and monkey prefrontal cortex

5-hydroxytryptamine (5-HT) or serotonin 2A receptors play an important role in modulation of prefrontal cortex (PFC) activity and have been implicated in the physiopathology of psychiatric disorders. There is no quantitative information on the percentage of glutamatergic and GABAergic cells that express 5-HT(2A) receptors in human and monkey PFC. We have used double in situ hybridization to quantify the mRNA co-localization of 5-HT(2A) receptor with the glutamatergic transporter vesicular glutamate transporter 1, and with the GABAergic marker glutamic acid decarboxylase 65/67 and in parvalbumin and calbindin GABAergic cell populations. Our results show that nearly every glutamatergic cell (86-100%) in layers II-V expressed 5-HT(2A) receptor mRNA in both species. This percentage was lower in layer VI (13-31%). In contrast, not all the GABAergic interneurons (13-46%) expressed 5-HT(2A) receptor mRNA. This receptor was expressed in 45-69% of parvalbumin and in 61-87% of calbindin positive cells. These results indicate that, while the majority of glutamatergic neurons can be sensitive to 5-HT action via 5-HT(2A) receptors, this modulation occurs only in a limited population of GABAergic interneurons and provides new neuroanatomical information about the role played by serotonin through 5-HT(2A) receptors in the PFC and on the sites of action for drugs such as antipsychotics and antidepressants used in treatment of psychiatric disorders.     

Cellular and molecular guidance of GABAergic neuronal migration from an extracortical origin to the neocortex.

Formation of the normal mammalian cerebral cortex requires the migration of GABAergic inhibitory interneurons from an extracortical origin, the lateral ganglionic eminence (LGE). Mechanisms guiding the migratory direction of these neurons, or other neurons in the neocortex, are not well understood. We have used an explant assay to study GABAergic neuronal migration and found that the ventricular zone (VZ) of the LGE is repulsive to GABAergic neurons. Furthermore, the secreted protein Slit is a chemorepellent guiding the migratory direction of GABAergic neurons, and blockade of endogenous Slit signaling inhibits the repulsive activity in the VZ. These results have revealed a cellular source of guidance for GABAergic neurons, demonstrated a molecular cue important for cortical development, and suggested a guidance mechanism for the migration of extracortical neurons into the neocortex.     

Developmental downregulation of GABAergic drive parallels formation of functional synapses in cultured mouse neocortical networks

Networks of cortical neurons in vitro spontaneously develop synchronous oscillatory electrical activity at around the second week in culture. However, the underlying mechanisms and in particular the role of GABAergic interneurons in initiation and synchronization of oscillatory activity in developing cortical networks remain elusive. Here, we examined the intrinsic properties and the development of GABAergic and glutamatergic input onto presumed projection neurons (PNs) and large interneurons (L-INs) in cortical cultures of GAD67-GFP mice. Cultures developed spontaneous synchronous activity already at 5-7 days in vitro (DIV), as revealed by imaging transient changes in Fluo-3 fluorescence. Concurrently, spontaneous glutamate-mediated and GABA(A)-mediated postsynaptic currents (sPSCs) occured at 5 DIV. For both types of neurons the frequency of glutamatergic and GABAergic sPSCs increased with DIV, whereas the charge transfer of glutamatergic sPSCs increased and the charge transfer of GABAergic sPSCs decreased with cultivation time. The ratio between GABAergic and the overall charge transfer was significantly reduced with DIV for L-INs and PNs, indicating an overall reduction in GABAergic synaptic drive with maturation of the network. In contrast, analysis of miniature PSCs (mPSCs) revealed no significant changes of charge transfer with DIV for both types of neurons, indicating that the reduction in GABAergic drive was not due to a decreased number of functional synapses. Our data suggest that the global reduction in GABAergic synaptic drive together with more synaptic input to PNs and L-INs during maturation may enhance rhythmogenesis of the network and increase the synchronization at the level of population bursts.     

Ultrastructure of the serotoninergic system of the motor area of the cerebral cortex

Owing to the microscopical investigation, using selective neurotoxin 5,7-dihydroxytryptamine, it has been possible to reveal the serotoninergic system and targets of its innervation in the rat cerebral cortex motor area. The serotoninergic axonal varicosities and synaptic boutons are present in all layers of the neocortex. Their large amount is revealed in the I and II layers. The terminals form contacts with dendrites of small size, sometimes they terminate on the head of the spines, as well as on bodies of neurons in different layers. According to their position and ultrastructural organization these neurons are, perhaps, pyramidal, that is glutamatergic, and those less in their size--refer to interneurons and can be GABAergic ones. Basing on own data and those of the literature, concerning the existence of nonsynaptic link for transmission of serotoninergic effects, a conclusion is made that a coordinating functioning of the synaptic and non-synaptic intercellular integrative mechanisms ensure a wide range of functions of the serotoninergic system in the cerebral cortex.     

Functional Dissection of Glutamatergic and GABAergic Neurons in the Bed Nucleus of the Stria Terminalis

The bed nucleus of the stria terminalis (BNST)—a key part of the extended amygdala—has been implicated in the regulation of diverse behavioral states, ranging from anxiety and reward processing to feeding behavior. Among the host of distinct types of neurons within the BNST, recent investigations employing cell type- and projection-specific circuit dissection techniques (such as optogenetics, chemogenetics, deep-brain calcium imaging, and the genetic and viral methods for targeting specific types of cells) have highlighted the key roles of glutamatergic and GABAergic neurons and their axonal projections. As anticipated from their primary roles in excitatory and inhibitory neurotransmission, these studies established that the glutamatergic and GABAergic subpopulations of the BNST oppositely regulate diverse behavioral states. At the same time, these studies have also revealed unexpected functional specificity and heterogeneity within each subpopulation. In this Minireview, we introduce the body of studies that investigated the function of glutamatergic and GABAergic BNST neurons and their circuits. We also discuss unresolved questions and future directions for a more complete understanding of the cellular diversity and functional heterogeneity within the BNST.     

Inhibitory or excitatory? Optogenetic interrogation of the functional roles of GABAergic interneurons in epileptogenesis

Alteration in the excitatory/inhibitory neuronal balance is believed to be the underlying mechanism of epileptogenesis. Based on this theory, GABAergic interneurons are regarded as the primary inhibitory neurons, whose failure of action permits hyperactivity in the epileptic circuitry. As a consequence, optogenetic excitation of GABAergic interneurons is widely used for seizure suppression. However, recent evidence argues for the context-dependent, possibly “excitatory” roles that GABAergic cells play in epileptic circuitry. We reviewed current optogenetic approaches that target the “inhibitory” roles of GABAergic interneurons for seizure control. We also reviewed interesting evidence that supports the “excitatory” roles of GABAergic interneurons in epileptogenesis. GABAergic interneurons can provide excitatory effects to the epileptic circuits via several distinct neurological mechanisms. (1) GABAergic interneurons can excite postsynaptic neurons, due to the raised reversal potential of GABA receptors in the postsynaptic cells. (2) Continuous activity in GABAergic interneurons could lead to transient GABA depletion, which prevents their inhibitory effect on pyramidal cells. (3) GABAergic interneurons can synchronize network activity during seizure. (4) Some GABAergic interneurons inhibit other interneurons, causing disinhibition of pyramidal neurons and network hyperexcitability. The dynamic, context-dependent role that GABAergic interneurons play in seizure requires further investigation of their functions at single cell and circuitry level. New optogenetic protocols that target GABAergic inhibition should be explored for seizure suppression.     

Classification and function of GABAergic interneurons of the mammalian cerebral cortex

GABAergic interneurons make up about 20% of neurons in the cortex and are a heterogeneous group of cells. In recent years it has become clear that different populations of interneurons not only provide the balance of excitation and inhibition in neural networks but are also critically important for generation of rhythmic activity, successful processing of sensory information, implementation of synaptic plasticity and a number of other functions. We examine current approaches to classification of interneurons and review the properties and the functional role of basket cells, chandelier cells, neurogliaform interneurons, Martinotti cells, and some other classes of interneurons based on morphological, immunohistochemical, electrophysiological and optogenetic studies. Besides, we consider the opportunities of the selective impact on target population of interneurons and review the data on the role of different types of interneurons in the pathogenesis of epilepsy and schizophrenia.     

Neuron‐type specific cannabinoid‐mediated G protein signalling in mouse hippocampus

Type 1 cannabinoid receptor (CB1) is expressed in different neuronal populations in the mammalian brain. In particular, CB1 on GABAergic or glutamatergic neurons exerts different functions and display different pharmacological properties in vivo. This suggests the existence of neuron‐type specific signalling pathways activated by different subpopulations of CB1. In this study, we analysed CB1 expression, binding and signalling in the hippocampus of conditional mutant mice, bearing CB1 deletion in GABAergic (GABA‐CB1‐KO mice) or cortical glutamatergic neurons (Glu‐CB1‐KO mice). Compared to their wild‐type littermates, Glu‐CB1‐KO displayed a small decrease of CB1 mRNA amount, immunoreactivity and [³H]CP55,940 binding. Conversely, GABA‐CB1‐KO mice showed a drastic reduction of these parameters, confirming that CB1 is present at much higher density on hippocampal GABAergic interneurons than glutamatergic neurons. Surprisingly, however, saturation analysis of HU210‐stimulated [³⁵S]GTPγS binding demonstrated that ‘glutamatergic’ CB1 is more efficiently coupled to G protein signalling than ‘GABAergic’ CB1. Thus, the minority of CB1 on glutamatergic neurons is paradoxically several fold more strongly coupled to G protein signalling than ‘GABAergic’ CB1. This selective signalling mechanism raises the possibility of designing novel cannabinoid ligands that differentially activate only a subset of physiological effects of CB1 stimulation, thereby optimizing therapeutic action.     

How do glutamatergic and GABAergic cells contribute to synchronization in the medial septum?

The medial septum-diagonal band (MSDB) complex is considered as a pacemaker for the hippocampal theta rhythm. Identification of the different cell types, their electro-physiological properties and their possible function in the generation of a synchronized activity in the MSDB is a hot topic. A recent electro-physiological study showed the presence of two antiphasically firing populations of parvalbumin containing GABAergic neurons in the MSDB. Other papers described a network of cluster-firing glutamatergic neurons, which is able to generate synchronized activity in the MSDB. We propose two different computer models for the generation of synchronized population theta oscillation in the MSDB and compare their properties. In the first model GABAergic neurons are intrinsically theta periodic cluster-firing cells; while in the second model GABAergic cells are fast-firing cells and receive periodic input from local glutamatergic neurons simulated as cluster-firing cells. Using computer simulations we show that the GABAergic neurons in both models are capable of generating antiphasic theta periodic population oscillation relying on local, septal mechanisms. In the first model antiphasic theta synchrony could emerge if GABAergic neurons form two populations preferentially innervate each other. In the second model in-phase synchronization of glutamatergic neurons does not require specific network structure, and the network of these cells are able to act as a theta pacemaker for the local fast-firing GABAergic circuit. Our simulations also suggest that neurons being non-cluster-firing in vitro might exhibit clustering properties when connected into a network in vivo. Action Editor: David Golomb     

Retinoic acid functions as a key GABAergic differentiation signal in the basal ganglia

Although retinoic acid (RA) has been implicated as an extrinsic signal regulating forebrain neurogenesis, the processes regulated by RA signaling remain unclear. Here, analysis of retinaldehyde dehydrogenase mutant mouse embryos lacking RA synthesis demonstrates that RA generated by Raldh3 in the subventricular zone of the basal ganglia is required for GABAergic differentiation, whereas RA generated by Raldh2 in the meninges is unnecessary for development of the adjacent cortex. Neurospheres generated from the lateral ganglionic eminence (LGE), where Raldh3 is highly expressed, produce endogenous RA, which is required for differentiation to GABAergic neurons. In Raldh3?/? embryos, LGE progenitors fail to differentiate into either GABAergic striatal projection neurons or GABAergic interneurons migrating to the olfactory bulb and cortex. We describe conditions for RA treatment of human embryonic stem cells that result in efficient differentiation to a heterogeneous population of GABAergic interneurons without the appearance of GABAergic striatal projection neurons, thus providing an in vitro method for generation of GABAergic interneurons for further study. Our observation that endogenous RA is required for generation of LGE-derived GABAergic neurons in the basal ganglia establishes a key role for RA signaling in development of the forebrain.     

Spatial propagation of interictal discharges along the cortex

Interictal discharges (IIDs) accompany epileptic seizures and highlight the mechanisms of pathological activity. The propagation of IIDs along the neural tissue is not well understood. To simulate IID propagation, this study proposes a new mathematical model that uses the conductance-based refractory density approach for glutamatergic and GABAergic neuronal populations. The mathematical model is found to be consistent with experimental double-patch registrations in the 4-aminopyridine in vitro model of epilepsy. In slices, the spontaneous activity of interneurons leads to their synchronization by means of the depolarizing GABAmediated response, thus initiating IIDs. Modeling reveals a clustering of interneuronal synchronization followed by IIDs with activity fronts that propagate along the cortex. The GABA-mediated depolarization either remains to be subthreshold for the principal neurons and thus results in pure GABAergic IIDs (IID1s) or leads to glutamatergic excitation, thus resulting in another type of IIDs (IID2s). In both the model and experiment, IIDs propagate as waves, with constant activity profiles and velocity. The speed of IIDs is of the order of tens of mm/s and is larger for IID2s than for IID1s (40 and 20?mm/s, respectively). The simulations, consistent with experimental observations, show that the wavelike propagation of IIDs initiated by interneurons is determined by local synaptic connectivity under the conditions of depolarizing GABA.     

Cortical interneuron specification and diversification in the era of big data

Inhibition in the mammalian cerebral cortex is mediated by a small population of highly diverse GABAergic interneurons. These largely local neurons are interspersed among excitatory projection neurons and exert pivotal regulation on the formation and function of cortical circuits. We are beginning to understand the extent of GABAergic neuron diversity and how this is generated and shaped during brain development in mice and humans. In this review, we summarise recent findings and discuss how new technologies are being used to further advance our knowledge. Understanding how inhibitory neurons are generated in the embryo is an essential pre-requisite of stem cell therapy, an evolving area of research, aimed at correcting human disorders that result in inhibitory dysfunction.     

Serotonin 1A receptors in human and monkey prefrontal cortex are mainly expressed in pyramidal neurons and in a GABAergic interneuron subpopulation: implications for schizophrenia and its treatment

Serotonin 1A (5-HT(1A)) receptors are found in high densities in prefrontal cortex. However, their distribution within cortical cell populations is unknown in both humans and primates. We used double in situ hybridization histochemistry to quantify the percentage of glutamatergic and GABAergic neurons expressing 5-HT(1A) receptors in human and monkey prefrontal cortex. Moreover, in the case of the monkey, we also quantified the parvalbumin and calbindin GABAergic subpopulations expressing this receptor. 5-HT(1A) receptor mRNAs were expressed in about 80% of glutamatergic neurons in external layers II and upper III, and in around 50% in layer VI; they were also present in approximately 20% of GABAergic neurons in both species. Although they were found in up to 43% of the calbindin cell subpopulation they were rarely present in parvalbumin cells in monkey prefrontal cortex. The knowledge of the phenotype of the prefrontal cortex (PFC) cells expressing 5-HT(1A) will help understanding serotonin actions in PFC.     

Afferent modulation of dopamine neuron firing patterns

In recent studies examining the modulation of dopamine (DA) cell firing patterns, particular emphasis has been placed on excitatory afferents from the prefrontal cortex and the subthalamic nucleus. A number of inconsistencies in recently published reports, however, do not support the contention that tonic activation of NMDA receptors is the sole determinate of DA neuronal firing patterns. The results of work on the basic mechanism of DA firing and the action of apamin suggest that excitatory projections to DA neurons from cholinergic and glutamatergic neurons in the tegmental pedunculopontine nucleus, and/or inhibitory GABAergic projections, are also involved in modulating DA neuron firing behavior.