REVIEWS IS THERE MORE TO GABA THAN SYNAPTIC INHIBITION? David F. Owens* and Arnold R. Kriegstein‡ In the mature brain, GABA (γ-aminobutyric acid) functions primarily as an inhibitory neurotransmitter. But it can also act as a trophic factor during nervous system development to influence events such as proliferation, migration, differentiation, synapse maturation and cell death. GABA mediates these processes by the activation of traditional ionotropic and metabotropic receptors, and probably by both synaptic and non-synaptic mechanisms. However, the functional properties of GABA receptor signalling in the immature brain are significantly different from, and in some ways opposite to, those found in the adult brain. The unique features of the early-appearing GABA signalling systems might help to explain how GABA acts as a developmental signal. The construction of the central nervous system (CNS) historical introduction to the establishment of GABA as results from a series of events that begins with neural a neurotransmitter substance, as well as an overview of induction and cellular proliferation, and ends with the components of the GABA signalling system. Then synapse formation and circuit refinement1. Each step in we discuss ontogenetic changes in GABA signalling the developmental sequence results from an interplay and potential developmental functions of GABA, between cell-intrinsic and cell-extrinsic signalling mech- focusing primarily on the mammalian neocortex, anisms2. Over the past half-century, many secreted mol- although other brain regions are also considered. ecules have been identified and shown to significantly influence individual developmental programmes. GABA as a neurotransmitter Neurotransmitters are a class of secreted molecules that The discovery of direct synaptic inhibition, with GABA might be important signals during nervous system devel- being the first clear example of an inhibitory neuro- opment. Although neurotransmitters are generally asso- transmitter substance, markedly altered views of synap- ciated with neuronal communication in the mature tic transmission, which had previously allowed for only *Laboratory of Molecular brain, multiple transmitter systems have been shown to excitatory signalling molecules10. GABA was first identi- Biology, National Institute of influence different aspects of neuronal maturation in fied in the mammalian brain over half a century Neurological Disorders and various experimental systems3–6. ago11,12. During the 1950s and 1960s, strong evidence Stroke, Building 36, Room 3C09, 36 Convent Drive, The amino acid GABA has long been considered accumulated that GABA was acting as an inhibitory Bethesda, Maryland 20892- to be the main inhibitory neurotransmitter in the neurotransmitter in both vertebrate and invertebrate 4092, USA. adult mammalian brain. However, GABA-mediated nervous systems13. For example, extracts from the ‡ Department of Neurology, signalling has also been implicated in the regulation mammalian brain contained a substance, termed fac- Columbia University College of Physicians & Surgeons, of nearly all the key developmental steps, from cell tor I, that blocked impulse generation in crayfish 630 West 168th Street, Box proliferation to circuit refinement. Considering that stretch receptor neurons14. Factor I was subsequently 31, New York, New York nearly all organisms, ranging from bacteria to shown to be GABA15. In crustaceans, GABA was found 10032, USA. humans, can synthesize GABA, it would be surprising to be approximately 100 times more concentrated in Correspondence to A.R.K. if multiple functions for GABA had not evolved7–9. inhibitory axons than in excitatory ones16,17, and in e-mail: [email protected] Here, we address the role of GABA-mediated signalling response to nerve stimulation, inhibitory nerve termi- doi:10.1038/nrn919 during brain development. We first provide a brief nals were found to secrete GABA18. Similar, although NATURE REVIEWS | NEUROSCIENCE VOLUME 3 | SEPTEMBER 2002 | 715 © 2002 Nature Publishing Group REVIEWS a less compelling, findings of GABA being concentrated Presynaptic Glia and released from inhibitory neurons were also made in the vertebrate brain19. The identification of biosyn- Glutamate thetic and metabolic pathways for GABA showed that the production, release, reuptake and metabolism of GAD GABAA Metabolism this substance occurred in the nervous system13.When GAT GABA was applied to nerve and muscle cells of both GABAB GABA vertebrates and invertebrates, it was generally found to GABA-T Ca2+ channel have inhibitory effects15,20–22 and to produce conduc- VGAT tance changes with ion sensitivities similar to those GABA observed after the activation of inhibitory nerves21,23–27. Finally, in the 1970s, GABA was localized to mam- malian nerve terminals28, and antibodies raised against GABA GABA-biosynthetic enzymes were shown to be A localized preferentially to known inhibitory neurons13. GABAB GAT K+ channel Postsynaptic The GABA neurotransmitter system FIGURE 1a provides a summary of the GABA neurotrans- b I mitter system. In the mammalian brain, GABA is syn- II thesized primarily from glutamate in a reaction that is catalysed by two glutamic acid decarboxylase (GAD) enzymes, GAD65 and GAD67 (REF.29). GABA is loaded III into synaptic vesicles by a vesicular neurotransmitter transporter (VGAT)30 and is liberated from nerve termi- nals by calcium-dependent exocytosis. However, non- IV vesicular forms of GABA secretion (for example, by reverse transporter action) have also been described and V might be particularly important during development31,32. The effects of GABA can be mediated by the activation of either IONOTROPIC or METABOTROPIC receptors, which can VI 100 µm be localized either pre- or postsynaptically. GABA sig- nals are terminated by reuptake of the neurotransmitter c into nerve terminals and/or into surrounding glial GABA GABA GABA A Cl– B C Cl– cells by a class of plasma-membrane GABA trans- porters (GATs)33; thereafter, GABA is metabolized by a transamination reaction that is catalysed by GABA transaminase (GABA-T)13. G G GABA neurons. There is a wide variety of GABA- producing neurons in the brain. In the neocortex, most Subunits: Subunits: Subunits: α1–6, β1–3, γ1–3, δ, R1a, R1b and R2 ρ1–3 GABA-containing neurons are local interneurons with ε, θ and π few, if any, dendritic spines, and are therefore classified as sparsely spiny, aspiny or smooth cells34–36. These cells are Figure 1 | Components of the GABA signalling pathway. a | Schematic diagram of the further classified into one of several basic groups, such as γ synthesis and transport of GABA ( -aminobutyric acid) at synapses. GABA is synthesized in basket cells, chandelier cells, double bouquet cells, local inhibitory neurons from glutamate by the enzyme glutamic acid decarboxylase (GAD), and is transported into vesicles by a vesicular neurotransmitter transporter (VGAT). GABA can be plexus neurons or neurogliaform cells (FIG. 1b). These released either vesicularly or non-vesicularly (by reverse transport). GABA receptors are located at subtypes differ in their morphology, neurochemical com- 35,37 pre- and postsynaptic sites. GABAB receptors are metabotropic receptors that cause presynaptic position, somatic location and terminal arborization . inhibition by suppressing calcium influx and reducing transmitter release, and achieve postsynaptic GABA-containing cells are distributed throughout the inhibition by activating potassium currents that hyperpolarize the cell. Reuptake of GABA by cortical lamina37–39. In addition, GABA interneurons can surrounding neurons and glia occurs through the activity of GABA transporters (GAT). be classified by intrinsic membrane properties and Subsequently, GABA is metabolized by a transamination reaction that is catalysed by GABA 40,41 transaminase (GABA-T). The metabolism of GABA occurs in both neurons (not shown) and glia. synaptic connectivity . Interestingly, experimental b | Cortical GABA interneurons are phenotypically diverse, as illustrated here by classification on data have indicated that most, if not all, neocortical the basis of morphological features. This figure illustrates (left to right) an interneuron with axonal GABA neurons are generated and migrate not from cor- arcades, a double bouquet cell, three types of basket cell, two chandelier cells, a cell spanning all tical, but rather from subcortical locations42.However, cortical layers and a neurogliaform cell. c | GABA receptors differ in subunit composition and this property might differ across species, as recent work assembly. GABA and GABA receptors are closely related pentameric receptors that carry A C in humans suggests that approximately 65% of neocorti- chloride; however, whereas GABAA receptors are composed of combinations of several subunit types, GABA receptors are composed of only single or multiple ρ-subunits. GABA receptors are cal GABA neurons arise from the neocortical prolifera- C B 43 metabotropic receptors that exist as R1a, R1b and R2 isoforms, and are associated with tive zone . Finally, in addition to local circuit neurons, direct GABA afferents project to the cortex from the G proteins. Native GABAB receptors are dimers composed of one R1 subunit and the R2 subunit. Modified, with permission, from REF.179 © 1987 The Institute of Mind and Behavior. basal forebrain and the ZONA INCERTA44–46. 716 | SEPTEMBER 2002 | VOLUME 3 www.nature.com/reviews/neuro © 2002 Nature Publishing Group REVIEWS GABA synapses. Initial electron-microscopic