Development of GABAergic neurons from the ventricular zone in the superior colliculus of the mouse

Carbachol increases locus coeruleus activation by targeting noradrenergic neurons, inhibitory interneurons and inhibitory synaptic transmission

The locus coeruleus (LC) consists of noradrenergic (NA) neurons and plays an important role in controlling behaviours. Although much of the knowledge regarding LC functions comes from studying behavioural outcomes upon administration of muscarinic acetylcholine receptor (mAChR) agonists into the nucleus, the exact mechanisms remain unclear. Here, we report that the application of carbachol (CCh), an mAChR agonist, increased the spontaneous action potentials (sAPs) of both LC-NA neurons and local inhibitory interneurons (LC I-INs) in acute brain slices by activating M1/M3 mAChRs (m_1/3 AChRs). Optogenetic activation of LC I-INs evoked inhibitory postsynaptic currents (IPSCs) in LC-NA neurons that were mediated by γ-aminobutyric acid type A (GABA_A ) and glycine receptors, and CCh application decreased the IPSC amplitude through a presynaptic mechanism by activating M4 mAChRs (m₄ AChRs). LC-NA neurons also exhibited spontaneous phasic-like activity (sPLA); CCh application increased the incidence of this activity. This effect of CCh application was not observed with blockade of GABA_A and glycine receptors, suggesting that the sPLA enhancement occurred likely because of the decreased synaptic transmission of LC I-INs onto LC-NA neurons by the m₄ AChR activation and/or increased spiking rate of LC I-INs by the m_1/3 AChR activation, which could lead to fatigue of the synaptic transmission. In conclusion, we report that CCh application, while inhibiting their synaptic transmission, increases sAP rates of LC-NA neurons and LC I-INs. Collectively, these effects provide insight into the cellular mechanisms underlying the behaviour modulations following the administration of muscarinic receptor agonists into the LC reported by the previous studies.

Inhibitory interneurons regulate phasic activity of noradrenergic neurons in the mouse locus coeruleus and functional implications

KEY POINTS

The locus coeruleus (LC) contains noradrenergic (NA) neurons that respond to novel stimuli in the environment with phasic activation to initiate an orienting response; phasic LC activation is also triggered by stimuli, representing the outcome of task-related decision processes, to facilitate ensuing behaviours and help optimize task performance. Here, we report that LC-NA neurons exhibit bursts of action potentials in vitro resembling phasic LC activation in vivo, and the activity is gated by inhibitory interneurons (I-INs) located in the peri-LC. We also observe that inhibition of peri-LC I-INs enhances prepulse inhibition and axons from cortical areas that play important roles in evaluating the cost/reward of a stimulus synapse on both peri-LC I-INs and LC-NA neurons. The results help us understand the cellular mechanisms underlying the generation and regulation of phasic LC activation with a focus on the role of peri-LC I-INs.

ABSTRACT

Noradrenergic (NA) neurons in the locus coeruleus (LC) have global axonal projection to the brain. These neurons discharge action potentials phasically in response to either novel stimuli in the environment to initiate an orienting behaviour or stimuli representing the outcome of task-related decision processes to facilitate ensuing behaviours and help optimize task performance. Nevertheless, the cellular mechanisms underlying the generation and regulation of phasic LC activation remain unknown. We report here that LC-NA neurons recorded in brain slices exhibit bursts of action potentials that resembled the phasic activation-pause profile observed in animals. The activity was referred to as phasic-like activity (PLA) and was suppressed and enhanced by blocking excitatory and inhibitory synaptic transmissions, respectively. These results suggest the existence of a local circuit to drive PLA, and the activity could be regulated by the excitatory-inhibitory balance of the circuit. In support of this notion, we located a population of inhibitory interneurons (I-INs) in the medial part of the peri-LC that exerted feedforward inhibition of LC-NA neurons through GABAergic and glycinergic transmissions. Selective inhibition of peri-LC I-INs with chemogenetic methods could enhance PLA in brain slices and increase prepulse inhibition in animals. Moreover, axons from the orbitofrontal and prelimbic cortices, which play important roles in evaluating the cost/reward of a stimulus, synapse on both peri-LC I-INs and LC-NA neurons. These observations demonstrate functional roles of peri-LC I-INs in integrating inputs of the frontal cortex onto LC-NA neurons and gating the phasic LC output.

Comprehensive and cell-type-based characterization of the dorsal midbrain during development

The layer structure has been intensively characterized in the developing neocortex and cerebellum based on the various molecular markers. However, as to the developing dorsal midbrain, comprehensive analyses have not been intensely carried out, and thus, the name as well as the definition of each layer is not commonly shared. Here, we redefined the three layers, such as the ventricular zone, intermediate zone and marginal zone, based on various markers for proliferation and differentiation in embryonic dorsal midbrain. Biphasic Ki67 expression defines the classical VZ, in which there is clear separation of the mitotic and interphase zones. Next, we mapped the distribution of immature neurons to the defined layers, based on markers for glutamatergic and GABAergic lineage. Interestingly, Tbr2 and Neurog2 were expressed in the postmitotic neurons. We also report that active (phosphorylated) JNK is a useful marker to demarcate layers during the embryonic stage. Finally, we validated the final arrival layers of the migratory glutamatergic and GABAergic neurons. These results form a foundation for analyses of brain development, especially in the proliferation and migration of excitatory and inhibitory neurons in the dorsal midbrain.

Neural precursor cells form integrated brain-like tissue when implanted into rat cerebrospinal fluid

There is tremendous interest in transplanting neural precursor cells for brain tissue regeneration. However, it remains unclear whether a vascularized and integrated complex neural tissue can be generated within the brain through transplantation of cells. Here, we report that early stage neural precursor cells recapitulate their seminal properties and develop into large brain-like tissue when implanted into the rat brain ventricle. Whereas the implanted cells predominantly differentiated into glutamatergic neurons and astrocytes, the host brain supplied the intact vasculature, oligodendrocytes, GABAergic interneurons, and microglia that seamlessly integrated into the new tissue. Furthermore, local and long-range axonal connections formed mature synapses between the host brain and the graft. Implantation of precursor cells into the CSF-filled cavity also led to a formation of brain-like tissue that integrated into the host cortex. These results may constitute the basis of future brain tissue replacement strategies.

Nikorn Pothayee et al. show that early neural precursor cells (NPCs) derived from the embryonic telencephalon or midbrain can develop into brain-like tissue when implanted into the rat brain ventricle. Telencephalon-derived NPCs also form brain tissue in the host cortex when implanted into a CSF-filled cavity formed by cortical ablation.

Exposure to Sevoflurane Affects the Development of Parvalbumin Interneurons in the Main Olfactory Bulb in Mice

Sevoflurane is widely used in adult and pediatric patients during clinical surgeries. Although studies have shown that exposure to sevoflurane impairs solfactory memory after an operation, the neuropathological changes underlying this effect are not clear. This study detected the effect of sevoflurane exposure on the development of calcium-binding proteins-expressing interneurons in the main olfactory bulb (MOB). We exposed neonatal mice to 2% sevoflurane at two different developmental time points and found that exposing mice to sevoflurane at postnatal day (PD) 7 significantly decreased the expression of GAD67 and parvalbumin (PV) in the olfactory bulb (OB) but did not alter the expression of calretinin (CR) or calbindin D28k (CB). The number and dendritic morphology of PV-expressing interneurons in the MOB were impaired by exposure to sevoflurane at PD7. However, exposure to sevoflurane at PD10 had no effect on calcium-binding protein expression or the number and dendritic morphology of PV-expressing interneurons in the MOB. These results suggest that exposing neonatal mice to sevoflurane during a critical period of olfactory development affects the development of PV-expressing interneurons in the MOB.

Mesencephalic GABA neuronal development: no more on the other side of oblivion

Midbrain GABA neurons, endowed with multiple morphological, physiological and molecular characteristics as well as projection patterns are key players interacting with diverse regions of the brain and capable of modulating several aspects of behavior. The diversity of these GABA neuronal populations based on their location and function in the dorsal, medial or ventral midbrain has challenged efforts to rapidly uncover their developmental regulation. Here we review recent developments that are beginning to illuminate transcriptional control of GABA neurons in the embryonic midbrain (mesencephalon) and discuss its implications for understanding and treatment of neurological and psychiatric illnesses.

The role of Tal2 and Tal1 in the differentiation of midbrain GABAergic neuron precursors

Midbrain- and hindbrain-derived GABAergic interneurons are critical for regulation of sleep, respiratory, sensory-motor and motivational processes, and they are implicated in human neurological disorders. However, the precise mechanisms that underlie generation of GABAergic neuron diversity in the midbrain–hindbrain region are poorly understood. Here, we show unique and overlapping requirements for the related bHLH proteins Tal1 and Tal2 in GABAergic neurogenesis in the midbrain. We show that Tal2 and Tal1 are specifically and sequentially activated during midbrain GABAergic neurogenesis. Similar to Gata2, a post-mitotic selector of the midbrain GABAergic neuron identity, Tal2 expression is activated very early during GABAergic neuron differentiation. Although the expression of Tal2 and Gata2 genes are independent of each other, Tal2 is important for normal midbrain GABAergic neurogenesis, possibly as a partner of Gata2. In the absence of Tal2, the majority of midbrain GABAergic neurons switch to a glutamatergic-like phenotype. In contrast, Tal1 expression is activated in a Gata2 and Tal2 dependent fashion in the more mature midbrain GABAergic neuron precursors, but Tal1 alone is not required for GABAergic neuron differentiation from the midbrain neuroepithelium. However, inactivation of both Tal2 and Tal1 in the developing midbrain suggests that the two factors co-operate to guide GABAergic neuron differentiation in a specific ventro-lateral midbrain domain. The observed similarities and differences between Tal1/Tal2 and Gata2 mutants suggest both co-operative and unique roles for these factors in determination of midbrain GABAergic neuron identities.

Effective induction of cells expressing GABAergic neuronal markers from mouse embryonic stem cell

Successful derivations of specific neuronal and glial cells from embryonic stem cells have enormous potential for cell therapies and regenerative medicine. However, the low efficiency, the complexity of induction method, and the need for purification represent obstacles that make their application impractical. In this study, we found that PDGFRα(+) cells derived from mouse embryonic stem cells (mESC) can serve as a useful source from which to induce cells that express γ-aminobutyric-acid (GABA)-releasing (GABAergic) neuronal markers. PDGFRα(+) cells were induced from mESC on collagen IV-coated plates in mesenchymal stem cell (MSC) culture medium with limited exposure to retinoic acid, sorted by fluorescence-activated cell sorter and maintained in MSC culture medium containing Y-27632, a Rho-associated kinase inhibitor. We found that supplementation of vascular endothelial growth factor, fibroblast growth factor-basic, and sodium azide (NaN3) to MSC culture medium effectively differentiated PDGFRα(+) cells into cells that express GABAergic neuronal markers, such as Pax2, Dlx2, GAD67 NCAM, and tubulin-βIII, while markers for oligodendrocyte (Sox2) and astrocyte (Glast) were suppressed. Immunostaining for GABA showed the majority (86 ± 5%) of the induced cells were GABA-positive. We also found that the PDGFRα(+) cells retained such differentiation potential even after more than ten passages and cryopreservation. In summary, this study presents a simple and highly efficient method of inducing cells that express GABAergic neuronal markers from mESC. Together with its ease of maintenance in vitro, PDGFRα(+) cells derived from mESC may serve as a useful source for such purpose.

Molecular regulation of GABAergic neuron differentiation and diversity in the developing midbrain

The midbrain GABAergic neurones control several aspects of behaviour, play important roles in psychiatric disease and are targets of medical treatments as well as drugs of abuse. However, their molecular diversity and regulation of development are only beginning to be understood. In this review, we briefly introduce distinct subpopulations of the midbrain GABAergic neurones and discuss knowledge on their development, including the developmental origins of midbrain GABAergic neurones as well as transcriptional regulatory mechanisms guiding their differentiation and identity. Important GABAergic neuron subpopulations are found within the dopaminergic (DA) nuclei in the ventral midbrain. GABAergic substantia nigra pars reticulata is the main output pathway of the basal ganglia system regulating voluntary movements. Recent studies have also highlighted importance of the GABAergic neurones associated with the ventral tegmental area for the control of DA neuron activity and motivated behaviours. Interestingly, the development of the GABAergic neurones associated with the DA nuclei is very different from the rest of the midbrain. Knowledge on developmental regulation can lead to insights into the molecular, structural and functional diversity of the midbrain GABAergic neurones and their subpopulations, cell groups of great physiological and medical interest.

A novel α-conotoxin MII-sensitive nicotinic acetylcholine receptor modulates [(3) H]-GABA release in the superficial layers of the mouse superior colliculus

Mouse superficial superior colliculus (SuSC) contains dense GABAergic innervation and diverse nicotinic acetylcholine receptor subtypes. Pharmacological and genetic approaches were used to investigate the subunit compositions of nicotinic acetylcholine receptors (nAChR) expressed on mouse SuSC GABAergic terminals. [(125) I]-Epibatidine competition-binding studies revealed that the α3β2* and α6β2* nicotinic subtype-selective peptide α-conotoxin MII-blocked binding to 40 ± 5% of SuSC nAChRs. Acetylcholine-evoked [(3) H]-GABA release from SuSC crude synaptosomal preparations is calcium dependent, blocked by the voltage-sensitive calcium channel blocker, cadmium, and the nAChR antagonist mecamylamine, but is unaffected by muscarinic, glutamatergic, P2X and 5-HT3 receptor antagonists. Approximately 50% of nAChR-mediated SuSC [(3) H]-GABA release is inhibited by α-conotoxin MII. However, the highly α6β2*-subtype-selective α-conotoxin PIA did not affect [(3) H]-GABA release. Nicotinic subunit-null mutant mouse experiments revealed that ACh-stimulated SuSC [(3) H]-GABA release is entirely β2 subunit-dependent. α4 subunit deletion decreased total function by >90%, and eliminated α-conotoxin MII-resistant release. ACh-stimulated SuSC [(3) H]-GABA release was unaffected by β3, α5 or α6 nicotinic subunit deletions. Together, these data suggest that a significant proportion of mouse SuSC nicotinic agonist-evoked GABA-release is mediated by a novel, α-conotoxin MII-sensitive α3α4β2 nAChR. The remaining α-conotoxin MII-resistant, nAChR agonist-evoked SuSC GABA release appears to be mediated via α4β2* subtype nAChRs.

The transcription factor, Lmx1b, promotes a neuronal glutamate phenotype and suppresses a GABA one in the embryonic trigeminal brainstem complex

Achieving an appropriate balance between inhibitory and excitatory neuronal fate is critical for development of effective synaptic transmission. However, the molecular mechanisms dictating such phenotypic outcomes are not well understood, especially in the whisker-to-barrel cortex neuraxis, an oft-used model system for revealing developmental mechanisms. In trigeminal nucleus principalis (PrV), the brainstem link in the whisker-barrel pathway, the transcription factor Lmx1b marks glutamatergic cells. In PrV of Lmx1b knockout mice (-/-), initial specification of glutamatergic vs. GABAergic cell fate is normal until embryonic day 14.5. Subsequently, until the day of birth, glutamatergic markers (e.g., VGLUT2) stain significantly fewer PrV neurons, whereas, GABAergic markers (Pax2 and Gad1) stain significantly more PrV cells, notably in Lmx1b null PrV cells. These changes also occurred in Lmx1b/Bax double-/- mice, where PrV cells are rescued from Lmx1b-/- induced apoptosis; thus, effects upon excitatory/inhibitory cell ratios do not reflect a cell death confound. Electroporation-induced ectopic expression of Lmx1b in an array of sites decreases numbers of neurons that express GABAergic markers, but increases VGLUT2+ cell numbers or stain intensity. Thus, Lmx1b is not involved in the initial specification of glutamatergic cell fate, but is essential for maintaining a glutamatergic phenotype. Other experiments suggest that Lmx1b acts to suppress Pax2, a promoter of GABAergic cell fate, in a cell-autonomous manner, which may be a mechanism for maintaining a functional balance of glutamatergic and GABAergic cell types in development.

Cytoarchitectural disruption of the superior colliculus and an enlarged acoustic startle response in the Tuba1a mutant mouse

The Jenna mutant mouse harbours an S140G mutation in Tuba1a that impairs tubulin heterodimer formation resulting in defective neuronal migration during development. The consequence of decreased neuronal motility is a fractured pyramidal cell layer in the hippocampus and wave-like perturbations in the cerebral cortex. Here, we extend our characterisation of this mouse investigating the laminar architecture of the superior colliculus (SC). Our results reveal that the structure of the SC in mutant animals is intact; however, it is significantly thinner with an apparent fusion of the intermediate grey and white layers. Birthdate labelling at E12.5 and E13.5 showed that the S140G mutation impairs the radial migration of neurons in the SC. A quantitative assessment of neuronal number in adulthood reveals a massive reduction in postmitotic neurons in mutant animals, which we attribute to increased apoptotic cell death. Consistent with the role of the SC in modulating sensorimotor gating, and the circuitry that modulates this behaviour, we find that Jenna mutants exhibit an exaggerated acoustic startle response. Our results highlight the importance of Tuba1a for correct neuronal migration and implicate postnatal apoptotic cell death in the pathophysiological mechanisms underlying the tubulinopathies.

Kv3.1b and Kv3.3 channel subunit expression in murine spinal dorsal horn GABAergic interneurones

GABAergic interneurones, including those within spinal dorsal horn, contain one of the two isoforms of the synthesizing enzyme glutamate decarboxylase (GAD), either GAD65 or GAD67. The physiological significance of these two GABAergic phenotypes is unknown but a more detailed anatomical and functional characterization may help resolve this issue. In this study, two transgenic Green Fluorescent Protein (GFP) knock-in murine lines, namely GAD65-GFP and GAD67-GFP (Δneo) mice, were used to profile expression of Shaw-related Kv3.1b and Kv3.3 K(+)-channel subunits in dorsal horn interneurones. Neuronal expression of these subunits confers specific biophysical characteristic referred to as 'fast-spiking'. Immuno-labelling for Kv3.1b or Kv3.3 revealed the presence of both of these subunits across the dorsal horn, most abundantly in laminae I-III. Co-localization studies in transgenic mice indicated that Kv3.1b but not Kv3.3 was associated with GAD65-GFP and GAD67-GFP immunopositive neurones. For comparison the distributions of Kv4.2 and Kv4.3 K(+)-channel subunits which are linked to an excitatory neuronal phenotype were characterized. No co-localization was found between GAD-GFP +ve neurones and Kv4.2 or Kv4.3. In functional studies to evaluate whether either GABAergic population is activated by noxious stimulation, hindpaw intradermal injection of capsaicin followed by c-fos quantification in dorsal horn revealed co-expression c-fos and GAD65-GFP (quantified as 20-30% of GFP +ve population). Co-expression was also detected for GAD67-GFP +ve neurones and capsaicin-induced c-fos but at a much reduced level of 4-5%. These data suggest that whilst both GAD65-GFP and GAD67-GFP +ve neurones express Kv3.1b and therefore may share certain biophysical traits, their responses to peripheral noxious stimulation are distinct.

Comparison of GAD65 and 67 immunoreactivity in the lumbar spinal cord between young adult and aged dogs

We investigated distribution and age-related changes in two isoforms of GABA synthesizing enzymes, glutamic acid decarboxylase (GAD) 65 and 67, in the lumbar levels (L(5)-L(6)) of the dog spinal cord. Male German shepherds were used at 1-2 years (young adult dogs) and 10-12 years (aged dogs) of age. GAD65 immunoreaction was observed in neuropil, not in cell bodies, in all laminae of the adult lumbar spinal cord: Many punctate GAD65-immunoreactive structures were shown in all laminae. The density of GAD65 immunoreactive structures was highest in laminae I-III, and lowest in lamina VII. In the aged dog, the distribution pattern of GAD65 immunoreactivity was similar to that in the adult dog; however the density of GAD65-immunoreactive structures and its protein levels were significantly increased in the aged lumbar spinal cord. GAD67 immunoreaction in the adult dog was also distributed in all laminae of the lumbar spinal cord like GAD65; however, we found that small GAD67-immunoreactive cell bodies were observed in laminae II, III and VIII. In the aged dogs, GAD67 immunoreactivity and its protein levels were also increased compared to those in the adult group. In conclusion, our results indicate that the distribution of GAD65-immunoreactive structures is different from GAD67-immunoreactive structures and that their immunoreactivity in the aged dogs is much higher than the adult dogs.

Distinct requirements for Ascl1 in subpopulations of midbrain GABAergic neurons

Midbrain GABAergic neurons regulate multiple aspects of behavior and play important roles in psychiatric and neurological disease. These neurons constitute several anatomical and functional subpopulations, but their molecular heterogeneity and developmental regulatory mechanisms are poorly understood. Here we have studied the involvement of the proneural gene Ascl1 in the development of the midbrain GABAergic neurons. Analysis of Ascl1 mutant mice demonstrated highly region-specific requirements for Ascl1 for development of different GABAergic neuron subpopulations. Ascl1 is dispensable for the development of the ventral-most midbrain GABAergic neurons associated with dopaminergic nuclei substantia nigra pars reticulata (SNpr) and ventral tegmental area (VTA) GABAergic neurons. In the ventrolateral midbrain, loss of Ascl1 results in markedly delayed neurogenesis in the midbrain domains m3-m5. Within this region, Ascl1 has a unique role in m4, where it also regulates glutamatergic neurogenesis. Our results suggest that the m3-m5 midbrain neuroepithelium gives rise to the GABAergic neuron groups located in the midbrain reticular formation and ventrolateral periaqueductal gray. In contrast to m3-m5, Ascl1 is absolutely required in the dorsal midbrain domains m1-m2, for generation of the GABAergic neurons populating the superior and inferior colliculi as well as dorsal periaqueductal gray. These studies demonstrate different molecular regulatory mechanisms for the distinct midbrain GABAergic neuron subpopulations. Also, our results have implications on understanding the origins of the various midbrain GABAergic neuron groups in the embryonic neuroepithelium.

Gata2 is a tissue-specific post-mitotic selector gene for midbrain GABAergic neurons

Midbrain GABAergic neurons control several aspects of behavior, but regulation of their development and diversity is poorly understood. Here, we further refine the midbrain regions active in GABAergic neurogenesis and show their correlation with the expression of the transcription factor Gata2. Using tissue-specific inactivation and ectopic expression, we show that Gata2 regulates GABAergic neuron development in the mouse midbrain, but not in rhombomere 1, where it is needed in the serotonergic lineage. Without Gata2, all the precursors in the embryonic midbrain fail to activate GABAergic neuron-specific gene expression and instead switch to a glutamatergic phenotype. Surprisingly, this fate switch is also observed throughout the neonatal midbrain, except for the GABAergic neurons located in the ventral dopaminergic nuclei, suggesting a distinct developmental pathway for these neurons. These studies identify Gata2 as an essential post-mitotic selector gene of the GABAergic neurotransmitter identity and demonstrate developmental heterogeneity of GABAergic neurons in the midbrain.

Transient increases of glutamic acid decarboxylase 67 immunoreactivity and its protein levels in the somatosensory cortex after transient cerebral ischemia in gerbils

In this study, we investigated changes in glutamic acid decarboxylase 67 (GAD67) immunoreactivity and its protein levels in the gerbil somatosensory cortex after ischemia/reperfusion. GAD67 immunoreactivity was significantly increased in layers III and V of the somatosensory cortex 12 hr after ischemia/reperfusion. Thereafter, GAD67 immunoreactivity was decreased with time after ischemia/reperfusion. GAD67 immunoreactivity in the somatosensory cortex 4 days after ischemia/reperfusion was similar to that in the sham-operated group. In addition, GAD67 protein levels were also significantly increased 12 hr after transient forebrain ischemia. These results suggest that the transient increase of GAD67 immunoreactivity in layers III and V may be associated with responses to transient ischemia-induced neuronal damage.

Characterization of GABAergic neurons in rapid-eye-movement sleep controlling regions of the brainstem reticular formation in GAD67-green fluorescent protein knock-in mice

Recent experiments suggest that brainstem GABAergic neurons may control rapid-eye-movement (REM) sleep. However, understanding their pharmacology/physiology has been hindered by difficulty in identification. Here we report that mice expressing green fluorescent protein (GFP) under the control of the GAD67 promoter (GAD67-GFP knock-in mice) exhibit numerous GFP-positive neurons in the central gray and reticular formation, allowing on-line identification in vitro. Small (10-15 microm) or medium-sized (15-25 microm) GFP-positive perikarya surrounded larger serotonergic, noradrenergic, cholinergic and reticular neurons, and > 96% of neurons were double-labeled for GFP and GABA, confirming that GFP-positive neurons are GABAergic. Whole-cell recordings in brainstem regions important for promoting REM sleep [subcoeruleus (SubC) or pontine nucleus oralis (PnO) regions] revealed that GFP-positive neurons were spontaneously active at 3-12 Hz, fired tonically, and possessed a medium-sized depolarizing sag during hyperpolarizing steps. Many neurons also exhibited a small, low-threshold calcium spike. GFP-positive neurons were tested with pharmacological agents known to promote (carbachol) or inhibit (orexin A) REM sleep. SubC GFP-positive neurons were excited by the cholinergic agonist carbachol, whereas those in the PnO were either inhibited or excited. GFP-positive neurons in both areas were excited by orexins/hypocretins. These data are congruent with the hypothesis that carbachol-inhibited GABAergic PnO neurons project to, and inhibit, REM-on SubC reticular neurons during waking, whereas carbachol-excited SubC and PnO GABAergic neurons are involved in silencing locus coeruleus and dorsal raphe aminergic neurons during REM sleep. Orexinergic suppression of REM during waking is probably mediated in part via excitation of acetylcholine-inhibited GABAergic neurons.

GAD isoforms exhibit distinct spatiotemporal expression patterns in the developing mouse lens: correlation with Dlx2 and Dlx5

Gamma-aminobutyric acid (GABA), the major inhibitory neurotransmitter of the adult nervous system and its biosynthetic enzyme glutamic acid decarboxylase (GAD) are abundantly expressed in the embryonic nervous system and are involved in the modulation of cell proliferation, migration, and differentiation. Here we describe for the first time the expression of GABA and embryonic and adult GAD isoforms in the developing mouse lens. We show that the GAD isoforms are sequentially induced with specific spatiotemporal profiles: GAD65 and embryonic GAD isoforms prevail in primary fibers, while GAD67 is the predominant GAD expressed in the postnatal secondary fibers. This pattern correlates well with the expression of Dlx2 and Dlx5, known as upstream regulators of GAD. GABA and GAD are most abundant at the tips of elongating fibers and are absent from organelle-free cells, suggesting their involvement is primarily in shaping of the cytoskeleton during fiber elongation stages.

Comparison of glutamic acid decarboxylase 67 immunoreactive neurons in the hippocampal CA1 region at various age stages in dogs

The hippocampus is a main brain region concerning learning and memory processes. It is imperative to determine the extent of alterations in number and function of inhibitory GABAergic interneurons in the hippocampus as a function of age. We examined changes in GABAergic neurons in the hippocampal CA1 region at various ages of dogs using glutamic acid decarboxylase 67 (GAD67), which is a rate-limiting enzyme for GABA synthesis. We found only one band in the brain homogenates in dogs as well as mice and rats. GAD67 immunoreactive neurons in 1-year-old dogs were mainly detected in the stratum oriens. In the 6-year-old group, GAD67 immunoreactive neurons were evenly distributed in the CA1 region, and numbers of the neurons were highest among all experimental groups. Thereafter, GAD67 immunoreactive neurons were significantly decreased region with age: GAD67 immunoreactive neurons were scarcely found in the CA1 region in 10-year-old dogs. The reduction of GAD67 immunoreactive neurons in the hippocampal CA1 region may be closely related to highly susceptibility to memory loss in old aged dogs.

Development and migration of GABAergic neurons in the mouse myelencephalon

GABAergic neurons are the major inhibitory interneurons that are widely distributed in the central nervous system. It is well established that they originate from a focal region in the embryonic forebrain during development, and then migrate to other regions such as the neocortex. However, the migration of GABAergic neurons remains obscure in other axial levels of the brain. We examined the early development of myelencephalic GABAergic neurons using glutamate decarboxylase 67 / green fluorescent protein (GAD67-GFP) knocking mice. Observation of fixed tissues in coronal sections and flat whole-mount preparations indicated that, while GFP-positive cells are restricted to the subpial region in the ventral aspect of the myelencephalon at an early stage, they spread dorsally and eventually occupy the entire region of the myelencephalon as development proceeds. We developed a flat-mount in vitro preparation in which these patterns of development could be recapitulated. Transplantation of dorsal myelencephalic tissue of a wildtype embryo to a corresponding region of GAD67-GFP mouse embryos clearly demonstrated invasion of dorsally oriented GABAergic neurons from host to donor tissue. These results indicate that ventral-to-dorsal tangential migration of GABAergic neurons takes place in the myelencephalon. Our results extend the observations in the forebrain that inhibitory and excitatory neurons in a specific brain compartment take distinct migratory paths.

Megane/Heslike is required for normal GABAergic differentiation in the mouse superior colliculus

The mouse Mgn protein (Helt) is structurally related to the neurogenic Drosophila hairy and Enhancer of split [h/E(spl)]proteins, but its unique structural properties distinguish it from other members of the family. Mgn expression shows a spatiotemporal correlation with GABAergic markers in several brain regions. We report here that homozygous Mgn-null mice die between the second and the fifth postnatal week of age, and show a complete depletion of Gad65 and Gad67 expression in the superior colliculus and a reduction in the inferior colliculus. Other brain regions, as well as other neural systems, are not affected. The progenitor GABAergic cells appear to be generated in right numbers but fail to become GABAergic neurons. The phenotype of the mice is consistent with reduced GABAergic activity. Thus, our in vivo study provides evidence that Mgn is the key regulator of GABAergic neurons, controlling their specification in the dorsal midbrain. Another conclusion from our results is that the function of Mgn shows a previously unrecognized role for h/E(spl)-related transcription factors in the dorsal midbrain GABAergic cell differentiation. Vertebrate h/E(spl)-related genes can no longer be regarded solely as a factors that confer generic neurogenic properties, but as key components for the subtype-neuronal identity in the mammalian CNS.

Electrophysiological and morphological characteristics of GABAergic respiratory neurons in the mouse pre-Bötzinger complex

The characteristics of GABAergic neurons involved in respiratory control have not been fully understood because identification of GABAergic neurons has so far been difficult in living tissues. In the present in vitro study, we succeeded in analysing the electrophysiological as well as morphological characteristics of GABAergic neurons in the pre-Bötzinger complex. We used 67-kDa isoform of glutamic acid decarboxylase-green fluorescence protein (GAD67-GFP) (Delta neo) knock-in (GAD67(GFP/+)) mice, which enabled us to identify GABAergic neurons in living tissues. We prepared medullary transverse slices that contained the pre-Bötzinger complex from these neonatal mice. The fluorescence intensity of the pre-Bötzinger complex region was relatively high among areas of the ventral medulla. Activities of GFP-positive neurons in the pre-Bötzinger complex were recorded in a perforated whole-cell patch-clamp mode. Six of 32 GFP-positive neurons were respiratory and the remaining 26 neurons were non-respiratory; the respiratory neurons were exclusively inspiratory, receiving excitatory post-synaptic potentials during the inspiratory phase. In addition, six inspiratory and one expiratory neuron of 30 GFP-negative neurons were recorded in the pre-Bötzinger complex. GFP-positive inspiratory neurons showed high membrane resistance and mild adaptation of spike frequency in response to depolarizing current pulses. GFP-positive inspiratory neurons had bipolar, triangular or crescent-shaped somata and GFP-negative inspiratory neurons had multipolar-shaped somata. The somata of GFP-positive inspiratory neurons were smaller than those of GFP-negative inspiratory neurons. We suggest that GABAergic inhibition not by expiratory neurons but by inspiratory neurons that have particular electrophysiological and morphological properties is involved in the respiratory neuronal network of the pre-Bötzinger complex.

Multidirectional and multizonal tangential migration of GABAergic interneurons in the developing cerebral cortex

Most GABAergic interneurons originate from the basal forebrain and migrate tangentially into the cortex. The migratory pathways and mode of interneuron migration within the developing cerebral cortex, however, previously was largely unknown. Time-lapse imaging and in vivo labelling with glutamate decarboxylase (GAD)67-green fluorescence protein (GFP) knock-in embryonic mice with expression of GFP in gamma-aminobutyric acid (GABA)ergic neurons indicated that multidirectional tangential (MDT) migration of interneurons takes place in both the marginal zone (MZ) and the ventricular zone (VZ) of the cortex. Quantitative analysis of migrating interneurons showed that rostrocaudally migrating neurons outnumber those migrating mediolaterally in both of these zones. In vivo labelling with a lipophilic dye showed that the MDT migration in the MZ occurs throughout the cortex over distances of up to 3 mm during a period of a few days. These results indicate that MZ cortical interneurons undergo a second phase of tangential migration in all directions and over long distances, after reaching the cortex by dorsomedial tangential migration. The MDT migration in the MZ may disperse and intermix interneurons within the cortex, resulting in a balanced distribution of interneuron subtypes.