GABA-immunoreactive neurons in the rat cerebellum: a light and electron microscope study

A candidate projective neuron type of the cerebellar cortex: the synarmotic neuron

Previous studies on the granular layer of the cerebellar cortex have revealed a wide distribution of different subpopulations of less-known large neuron types, called "non-traditional large neurons", which are distributed in three different zones of the granular layer. These neuron types are mainly involved in the formation of intrinsiccircuits inside the cerebellar cortex. A subpopulation of these neuron types is represented by the synarmotic neuron, which could play a projective role within the cerebellar circuitry. The synarmotic neuron cell body map within the internal zone of the granular layer or in the subjacent white substance. Furthermore, the axon crosses the granular layer and runs in the subcortical white substance, to reenter in an adjacent granular layer, associating two cortico-cerebellar regions of the same folium or of different folia, or could project to the intrinsic cerebellar nuclei. Therefore, along with the Purkinje neuron, the traditional projective neuron type of the cerebellar cortex, the synarmotic neuron is candidate to represent the second projective neuron type of the cerebellar cortex. Studies of chemical neuroanatomy evidenced a predominant inhibitory GABAergic nature of the synarmotic neuron, suggesting that it may mediate an inhibitory GABAergic output of cerebellar cortex within cortico-cortical interconnections or in projections towards intrinsic cerebellar nuclei. On this basis, the present minireview mainly focuses on the morphofunctional and neurochemical data of the synarmotic neuron, and explores its potential involvement in some forms of cerebellar ataxias.

The Cerebellar Dopaminergic System

In the central nervous system (CNS), dopamine (DA) is involved in motor and cognitive functions. Although the cerebellum is not been considered an elective dopaminergic region, studies attributed to it a critical role in dopamine deficit-related neurological and psychiatric disorders [e.g., Parkinson's disease (PD) and schizophrenia (SCZ)]. Data on the cerebellar dopaminergic neuronal system are still lacking. Nevertheless, biochemical studies detected in the mammalians cerebellum high dopamine levels, while chemical neuroanatomy studies revealed the presence of midbrain dopaminergic afferents to the cerebellum as well as wide distribution of the dopaminergic receptor subtypes (DRD1-DRD5). The present review summarizes the data on the cerebellar dopaminergic system including its involvement in associative and projective circuits. Furthermore, this study also briefly discusses the role of the cerebellar dopaminergic system in some neurologic and psychiatric disorders and suggests its potential involvement as a target in pharmacologic and non-pharmacologic treatments.

Compartmentalized Input-Output Organization of Lugaro Cells in the Cerebellar Cortex

Purkinje cells (PCs) are principal cerebellar neurons, and several classes of interneurons modulate their activity. Lugaro cells (LCs) are one such inhibitory interneuron with distinctive cytology and location, but still most enigmatic among cerebellar neurons. Here we serendipitously produced a novel transgenic mouse line, where a half of Yellow Cameleon (YC)(+) cells in the cerebellar cortex were judged to be LCs, and YC(+) LCs were estimated to constitute one-third of the total LC populations. Neurochemically, two-thirds of YC(+) LCs were dually GABAergic/glycinergic, with the rest being GABAergic. Beneath the PC layer, they extended a sheet of somatodendritic meshwork interconnected with neighboring LCs by adherens junctions, and received various inputs from climbing fibers, mossy fibers, granule cell axons, recurrent PC axons, Golgi cell axons, LC axons, and serotonergic fibers. Intriguingly, somatodendritic elements of individual LCs preferentially extended within a given cerebellar compartment defined by aldolase C expression. In turn, YC(+) LCs projected a dense lattice of ascending and transverse axons to the molecular layer, and innervated molecular layer interneurons (basket and stellate cells) and Golgi cells, but not PCs. Of note, ascending axons profusely innervated individual targets within a cerebellar compartment, while transverse axons ran across several compartments and innervated targets sparsely. This unique circuit configuration highlights that LCs integrate various excitatory, inhibitory, and modulatory inputs coming to the belonging cerebellar compartment and that, as an interneuron-selective interneuron, LCs can effectively disinhibit cerebellar cortical activities in a compartment-dependent manner through inhibition of inhibitory interneurons selectively targeting PCs and granule cells.

Lack of molecular-anatomical evidence for GABAergic influence on axon initial segment of cerebellar Purkinje cells by the pinceau formation

The axon initial segment (AIS) of cerebellar Purkinje cells (PCs) is embraced by ramified axons of GABAergic basket cells (BCs) called the pinceau formation. This unique structure has been assumed to be a device for the modulation of PC outputs through electrical and/or GABAergic inhibition. Electrical inhibition is supported by enriched potassium channels, absence of sodium channels, and developed septate-like junctions between BC axons. The neurochemical basis for GABAergic inhibition, however, has not been well investigated. Here we addressed this issue using C56BL/6 mice. First, we confirmed previous observations that typical synaptic contacts were rare and confined to proximal axonal portions, with the remaining portions being mostly covered by astrocytic processes. Then we examined the expression of molecules involved in GABAergic signaling, including GABA synthetic enzyme glutamic acid decarboxylase (GAD), vesicular GABA transporter vesicular inhibitory amino acid transporter (VIAAT), cytomatrix active zone protein bassoon, GABA receptor GABA(A)Rα1, and cell adhesion molecule neuroligin-2. These molecules were recruited to form a functional assembly at perisomatic BC-PC synapses and along the AIS of hippocampal and neocortical pyramidal cells. GAD and VIAAT immunogold labeling was five times lower in the pinceau formation compared with perisomatic BC terminals and showed no accumulation toward the AIS. Moreover, bassoon, neuroligin-2, and GABA(A)Rα1 formed no detectable clusters along the ankyrin-G-positive AIS proper. These findings indicate that GABAergic signaling machinery is organized loosely and even incompletely in the pinceau formation. Together, BCs do not appear to exert GABAergic synaptic inhibition on the AIS, although the mode of action of the pinceau formation remains to be explored.

Prenatal development of NMDA receptor composition and function in trigeminal neurons

The prenatal development of neural circuits for rhythmical oral-motor behaviors used for feeding is essential for the survival of the newborn mammal. The N-methyl-D-aspartate (NMDA) receptor plays a critical role in brainstem circuits underlying postnatal oral-motor behaviors. To understand a role for the NMDA receptor in the emergence of sucking behavior we conducted physiological and immunohistochemical experiments using fetal rats. Physiology experiments examined the development of the NMDA dose response of the brainstem circuit responsible for generating rhythmical trigeminal activity by recording trigeminal motor outputs using an in vitro preparation. The high dose of NMDA agonist bath application affected the mean cycle duration of rhythmical trigeminal activity (RTA) at both embryonic day (E) 18-19 and E20-21 in comparison with standard concentration of NMDA agonist. NMDA receptor immunohistochemistry studies, using antibodies directed against subunits NR1, NR2A, NR2B, NR3A and NR3B were performed to determine the prenatal regulation of NMDA subunits in trigeminal motoneurons (Mo5), and mesencephalic trigeminal neurons (Me5) between E17 to E20. In Mo5, NR1, NR2A, NR2B and NR3A immunoreactivity was observed throughout the time frame sampled. NR3B immunoreactivity was not observed in Mo5 or Me5. In Mo5, there was a significant decrease in the percentage of NR2B immunoreactive neurons between E17 and E20, and a concurrent increase in the NR2A/NR2B ratio between E17 and E20. In Me5, NR1, NR2A and NR3A immunoreactivity was observed throughout the time frame sampled; a significant decrease in the percentage of NR2A immunoreactive neurons between E17 and E20, and NR3A immunoreactive neurons between E17 and E18 occurred. The timing of subunit changes between E17 and E18 is coincident with the prenatal emergence of rhythmical jaw movements, and in vitro rhythmical trigeminal activity, shown in earlier studies. Our data suggest that NMDA receptor plays an important role in the development and function of prenatal oral-motor circuits.

Progenitors from the postnatal forebrain subventricular zone differentiate into cerebellar-like interneurons and cerebellar-specific astrocytes upon transplantation

Forebrain subventricular zone (SVZ) progenitor cells give rise to glia and olfactory bulb interneurons during early postnatal life in rats. We investigated the potential of SVZ cells to alter their fate by transplanting them into a heterotypic neurogenic and gliogenic environment-the cerebellum. Transplanted cells were examined 1 to 7 weeks and 6 months post transplantation. Forebrain progenitors populated the cerebellum and differentiated into oligodendrocytes, cerebellar-specific Bergmann glia and velate astrocytes, and neurons. The transplanted cells that differentiated into neurons maintained an interneuronal fate: they were GABA-positive, expressed interneuronal markers, such as calretinin, and exhibited membrane properties that are characteristic of interneurons. However, the transplanted interneurons lost the expression of the olfactory bulb transcription factors Tbr2 and Dlx1, and acquired a cerebellar-like morphology. Forebrain SVZ progenitors thus have the potential to adapt to a new environment and integrate into diverse regions, and may be a useful tool in transplantation strategies.

Highly-restricted, cell-specific expression of the simian CMV-IE promoter in transgenic zebrafish with age and after heat shock

Promoters with high levels of ubiquitous expression are of significant utility in the production of transgenic animals and cell lines. One such promoter is derived from the human cytomegalovirus immediate early (CMV-IE) gene. We sought to ascertain if the simian CMV-IE promoter (sCMV), used extensively in non-mammalian vertebrate research, also directs intense, widespread expression when stably introduced into zebrafish. Analysis of sCMV-driven expression revealed a temporal and spatial pattern not predicted by studies using the hCMV promoter in other transgenic animals or by observations of early F0 embryos expressing injected sCMV-reporter plasmids. Unexpectedly, in transgenic fish produced by both integration of linearized plasmid or Tol2-mediated transgenesis, sCMV promoter expression was generally observed in a small population of cells in telencephalon and spinal cord between days 2 and 7, and was thereafter confined to discrete regions of CNS that included the olfactory bulb, retina, cerebellum, spinal cord, and lateral line. In skeletal muscle, intense transgene expression was not observed until well into adulthood (>2-3 months post-fertilization). One final unexpected characteristic of the sCMV promoter in stable transgenic fish was tissue-specific responsiveness of the promoter to heat shock at both embryonic and adult stages. These data suggest that, in the context of stable transgenesis, the simian CMV-IE gene promoter responds differently to intracellular regulatory forces than other characterized CMV promoters.

Protective effect of IL-18 on kainate- and IL-1 beta-induced cerebellar ataxia in mice

The pathogenesis of sporadic cerebellar ataxia remains unknown. In this study, we demonstrate that proinflammatory cytokines, IL-18 and IL-1beta, reciprocally regulate kainate-induced cerebellar ataxia in mice. We show that systemic administration of kainate activated IL-1beta and IL-18 predominantly in the cerebellum of mice, which was accompanied with ataxia. Mice deficient in caspase-1, IL-1R type I, or MyD88 were resistant to kainate-induced ataxia, while IL-18- or IL-18R alpha-deficient mice displayed significant delay of recovery from ataxia. A direct intracerebellar injection of IL-1beta-induced ataxia and intracerebellar coinjection of IL-18 counteracted the effect of IL-1beta. Our data firstly show that IL-18 and IL-1beta display differential direct regulation in kainate-induced ataxia in mice. Our results might contribute toward the development of a new therapeutic strategy for cerebellar ataxia in humans.

Effects of prenatal exposure to the CB-1 receptor agonist WIN 55212-2 or CO on the GABAergic neuronal systems of rat cerebellar cortex

The aim of this study was to assess the effects of prenatal exposures to cannabinoids or carbon monoxide (CO) in an animal experimental model reproducing the environmental conditions in which a fetus develops whose mother, during pregnancy, ingests by smoking low doses of cannabinoids or CO. Particular attention was devoted to analyses of the long-term effects of the exposures at the level of the cerebellar cortex, where already during prenatal development the GABAergic neuronal systems may be modulated by both cannabinoids and CO. Three groups of rats were subjected to the following experimental conditions: exposure to cannabinoids by maternal treatment during pregnancy with the cannabinoid CB-1 receptor agonist WIN 55212-2 (WIN) (0.5 mg/kg/day, s.c.); exposure to CO by maternal exposure during pregnancy to CO (75 parts per million, by inhalation); and exposure to WIN+CO at the above doses and means of administration; a fourth group was used as control. The body weight of dams, length of pregnancy, litter size at birth, body weight and postnatal mortality of pups were monitored in order to evaluate possible effects of the exposures on reproduction and on prenatal and postnatal development. In the different groups, the long-term effects of the exposures were studied in adult rats (120-150 days) by light microscopy analyses of the structure of the cerebellar cortex and of the distribution in the cortex of markers of GABAergic neurons, such as GAD and GABA itself. Results. Exposures to WIN or CO did not affect reproduction or prenatal/postnatal development. Moreover, the exposed rats showed no structural alterations of the cerebellar cortex and displayed qualitative distribution patterns of GAD and GABA immunoreactivities similar to those of the controls. However, quantitative analyses indicated significant changes of both of these immunoreactivities: in comparison with the controls, they were significantly increased in WIN-exposed rats and reduced in CO-exposed rats, but not significantly different in WIN+CO-exposed rats. The changes were detected in the molecular and Purkinje neuron layers, but not in the granular layer. Prenatal exposures of rats to WIN or CO, at doses that do not affect reproduction, general processes of development and histomorphogenesis of the cerebellar cortex, cause significant changes of GAD and GABA immunoreactivities in some GABAergic neuronal systems of the adult rat cerebellar cortex, indicating selective up-regulation of GABA-mediated neurotransmission as a long-term consequence of chronic prenatal exposures to cannabinoids or CO. Because the changes consist of overexpression or, vice versa, underexpression of these immunoreactivities, functional alterations of opposite types in the GABAergic systems of the cerebellum following exposure to WIN or CO can be postulated, in agreement with the results of behavioral and clinical studies. No changes in immunoreactivities were detected after prenatal exposure to WIN and CO in association.

Anatomy of the posterior caudal lobe of the cerebellum and the eminentia granularis posterior in a mormyrid fish

The cerebellum of mormyrid fish is of interest for its large size and unusual histology. The mormyrid cerebellum, as in all ray-finned fishes, has three subdivisions--valvula, corpus, and caudal lobe. The structures of the mormyrid valvula and corpus have been examined previously, but the structure of the mormyrid caudal lobe has not been studied. The mormyrid caudal lobe includes a posterior caudal lobe associated with the electrosense and an anterior caudal lobe associated with lateral line and eighth nerve senses. In this article we describe cellular elements of the posterior caudal lobe and of the eminentia granularis posterior (EGp) in the mormyrid fish Gnathonemus petersii. The EGp gives rise to the parallel fibers of the posterior caudal lobe. We used intracellular injection of biocytin, extracellular injection of biotinylated dextran amine, and immunohistochemistry with antibodies to gamma-aminobutyric acid, inositol triphosphate receptor I, calretinin, and Zebrin II. The histological structure of the posterior caudal lobe is markedly irregular in comparison to that of the corpus and the valvula, and a tight modular organization of cerebellar elements is less apparent here. Most Purkinje cell bodies are in the middle of the molecular region. Their dendrites are only roughly oriented in the sagittal plane, extend both ventrally and dorsally, and branch irregularly. Climbing fibers terminate only on smooth dendrites near the soma. Most Purkinje cell axons terminate locally on eurydendroid cells that project outside the cortex. The results provide an additional variant to the already large set of different cerebellar and cerebellum-like structures.

GABA immunoreactivity in auditory and song control brain areas of zebra finches

Inhibitory transmission is critical to sensory and motor processing and is believed to play a role in experience-dependent plasticity. The main inhibitory neurotransmitter in vertebrates, GABA, has been implicated in both sensory and motor aspects of vocalizations in songbirds. To understand the role of GABAergic mechanisms in vocal communication, GABAergic elements must be characterized fully. Hence, we investigated GABA immunohistochemistry in the zebra finch brain, emphasizing auditory areas and song control nuclei. Several nuclei of the ascending auditory pathway showed a moderate to high density of GABAergic neurons including the cochlear nuclei, nucleus laminaris, superior olivary nucleus, mesencephalic nucleus lateralis pars dorsalis, and nucleus ovoidalis. Telencephalic auditory areas, including field L subfields L1, L2a and L3, as well as the caudomedial nidopallium (NCM) and mesopallium (CMM), contained GABAergic cells at particularly high densities. Considerable GABA labeling was also seen in the shelf area of caudodorsal nidopallium, and the cup area in the arcopallium, as well as in area X, the lateral magnocellular nucleus of the anterior nidopallium, the robust nucleus of the arcopallium and nidopallial nucleus HVC. GABAergic cells were typically small, most likely local inhibitory interneurons, although large GABA-positive cells that were sparsely distributed were also identified. GABA-positive neurites and puncta were identified in most nuclei of the ascending auditory pathway and in song control nuclei. Our data are in accordance with a prominent role of GABAergic mechanisms in regulating the neural circuits involved in song perceptual processing, motor production, and vocal learning in songbirds.

NR3 protein expression in trigeminal neurons during postnatal development

The N-methyl-d-aspartate (NMDA) receptor plays an important role in the generation of rhythmical oral motor activities. To compliment our previous studies, we examined the developmental regulation of NR3A and NR3B expression in trigeminal motoneurons (Mo5) and mesencephalic trigeminal neurons (Me5). NR3A-immunoreactive neurons were observed at all ages in both nuclei, decreasing in Mo5 and caudal Me5 after P14, and increasing in rostral Me5. NR3B protein expression only emerged in Mo5 after P21-23. Results indicate that NR3A and NR3B expression is differentially regulated between Mo5 and Me5 coincident with the transition from suckling to chewing.

Co-localization of glycine and gaba immunoreactivity in interneurons in Macaca monkey cerebellar cortex

Previous work demonstrates that the cerebellum uses glycine as a fast inhibitory neurotransmitter [Ottersen OP, Davanger S, Storm-Mathisen J (1987) Glycine-like immunoreactivity in the cerebellum of rat and Senegalese baboon, Papio papio: a comparison with the distribution of GABA-like immunoreactivity and with [3H]glycine and [3H]GABA uptake. Exp Brain Res 66(1):211-221; Ottersen OP, Storm-Mathisen J, Somogyi P (1988) Colocalization of glycine-like and GABA-like immunoreactivities in Golgi cell terminals in the rat cerebellum: a postembedding light and electron microscopic study. Brain Res 450(1-2):342-353; Dieudonne S (1995) Glycinergic synaptic currents in Golgi cells of the rat cerebellum. Proc Natl Acad Sci U S A 92:1441-1445; Dumoulin A, Triller A, Dieudonne S (2001) IPSC kinetics at identified GABAergic and mixed GABAergic and glycinergic synapses onto cerebellar Golgi cells. J Neurosci 21(16):6045-6057; Dugue GP, Dumoulin A, Triller A, Dieudonne S (2005) Target-dependent use of coreleased inhibitory transmitters at central synapses. J Neurosci 25(28):6490-6498; Zeilhofer HU, Studler B, Arabadzisz D, Schweizer C, Ahmadi S, Layh B, Bosl MR, Fritschy JM (2005) Glycinergic neurons expressing enhanced green fluorescent protein in bacterial artificial chromosome transgenic mice. J Comp Neurol 482(2):123-141]. In the rat cerebellum glycine is not released by itself but is released together with GABA by Lugaro cells onto Golgi cells [Dumoulin A, Triller A, Dieudonne S (2001) IPSC kinetics at identified GABAergic and mixed GABAergic and glycinergic synapses onto cerebellar Golgi cells. J Neurosci 21(16):6045-6057] and by Golgi cells onto unipolar brush and granule cells [Dugue GP, Dumoulin A, Triller A, Dieudonne S (2005) Target-dependent use of coreleased inhibitory transmitters at central synapses. J Neurosci 25(28):6490-6498]. Here we report, from immunolabeling evidence in Macaca cerebellum, that interneurons in the granular cell layer are glycine+ at a density of 120 cells/linear mm. Their morphology indicates that they include Golgi and Lugaro cell types with the majority containing both glycine and GABA or glutamic acid decarboxylase. These data are consistent with the proposal that, as in the rat cerebellum, these granular cell layer interneurons corelease glycine and GABA in the primate cerebellum. The patterns of labeling for glycine and GABA within Golgi and Lugaro cells also indicate that there are biochemical sub-types which are morphologically similar. Further, we find that glycine, GABA and glutamic acid decarboxylase identified candelabrum cells adjacent to the Purkinje cells which is the first time that this interneuron has been reported in primate cerebellar cortex. We propose that candelabrum cells, like the majority of Golgi and Lugaro cells, release both glycine and GABA.

Heterogeneity of glycinergic and gabaergic interneurons in the granule cell layer of mouse cerebellum

Interneurons of the cerebellum granule cell layer (GCL) form distinct populations. Golgi cells extend dendrites in the molecular layer (ML) and innervate granule cells. In contrast, Lugaro cells have dendrites confined to the GCL but innervate interneurons in the ML, and globular cells have both their dendrites and axons in the ML. The latter cells were described recently and remain poorly characterized. Although several neurochemical markers have been associated selectively with GCL interneurons, it is unclear how they relate to their morphological classification and neurochemical phenotype (glycinergic and/or gamma-aminobutyric acid [GABA]ergic). Here, we performed a detailed characterization of GCL interneurons in mice expressing enhanced green fluorescent protein (GFP) in glycinergic and GABAergic neurons, respectively. By using immunofluorescence for metabotropic glutamate receptor 2 (mGluR2) and neurogranin as markers, we demonstrate the existence of five non-overlapping subsets of Golgi cells: about 65% are glycinergic/GABAergic and co-express both markers. Two small subsets (5-10%) also contain both neurotransmitters but express only mGluR2; they are distinguished by cell body size and location in the GCL. The fourth subset (15%) is GABAergic only and expresses neurogranin. The fifth subset (5%) is glycinergic only and lacks both markers. Thus, the heterogeneity of Golgi cells suggests that they belong to specific functional circuits and are differentially regulated by mGluRs and Ca(2+)-calmodulin-dependent signaling pathways. In contrast to Golgi cells, Lugaro and globular cells are glycinergic/GABAergic and lack mGluR2 and neurogranin. They each represent at least 15% of GCL interneurons and extensively innervate stellate and basket cells, but not Purkinje cells, emphasizing their contribution to inhibitory control of ML interneurons.

Mediodorsal thalamic afferents to layer III of the rat prefrontal cortex: synaptic relationships to subclasses of interneurons

The mediodorsal nucleus of the thalamus (MD) represents the main subcortical structure that projects to the prefrontal cortex (PFC) and it regulates key aspects of the cognitive functions of this region. Within the PFC, GABA local circuit neurons shape the activity patterns and hence the "memory fields" of pyramidal cells. Although the connections between the MD and PFC are well established, the ultrastructural relationships between projecting fibers from the MD and different subclasses of GABA cells in the PFC are not known. In order to address this issue in the rat, we examined MD axons labeled by tract-tracing in combination with immunogold-silver to identify different calcium-binding proteins localized within separate populations of interneurons. Electron micrographic examination of PFC sections from these animals revealed that MD terminals made primarily asymmetric synapses onto dendritic spines and less commonly onto dendritic shafts. Most of the dendrites receiving MD synaptic input were immunoreactive for parvalbumin (ParV), whereas MD synapses onto dendrites labeled for calretinin or calbindin were less frequent. We also observed that some MD terminals were themselves immunoreactive for calcium-binding proteins, again more commonly for ParV. These results suggest that the MD exerts a dual influence on PFC pyramidal cells: direct inputs onto spines and an indirect influence mediated via synapses onto each subclass of interneurons. The apparent preferential input to ParV cells endows MD afferents with a strong indirect inhibitory influence on pyramidal neuron activity by virtue of ParV cell synapses onto soma, proximal dendrites, and axon initial segments.

Potassium channel subunit Kv3.2 and the water channel aquaporin-4 are selectively localized to cerebellar pinceau

The pinceau is a cerebellar structure formed by descending GABA-ergic basket cell axonal terminals converging on the initial axonal segment of Purkinje cell. Although basket cells exert a powerful inhibitory influence on the output of the cerebellar cortex, the function and mode of action of the pinceau are not understood because the majority of basket cell axons fail to make identifiable synaptic contacts with the Purkinje cell axon. Several proteins were previously reported to cluster specifically in this area, including a number of voltage-activated potassium channel subunits. In this study, we used immunohistochemistry, electron microscopy, and electron tomography to examine the ultrastructural localization of a novel voltage-gated potassium channel subunit, Kv3.2, in the pinceau. We found strong, selective localization of Kv3.2 to basket cell axons. Additionally, because potassium buffering is often conducted through water channels, we studied the extent of a brain-specific water channel, aquaporin-4 (AQP4), using confocal and electron microscopy. As expected, we found AQP4 was heavily localized to astrocytic processes of the pinceau. The abundance of potassium channels and AQP4 in this area suggests rapid ionic dynamics in the pinceau, and the unusual, highly specialized morphology of this region implies that the structural features may combine with the molecular composition to regulate the microenvironment of the initial segment of the Purkinje cell axon.

Glutamic acid decarboxylase and GABA immunoreactivities in the cerebellar cortex of adult rat after prenatal exposure to a low concentration of carbon monoxide

Glutamic acid decarboxylase and GABA immunoreactivities were qualitatively and quantitatively evaluated in the cerebellar cortex of adult rats prenatally exposed to a low concentration of carbon monoxide (75 parts per million). Carbon monoxide-exposed and control rats were perfused with modified Bouin's fluid and their cerebella were embedded in paraffin. Sections from the vermis of each cerebellum were stained with Toluidine Blue or assayed with anti-glutamic acid decarboxylase 65/67 or with anti-GABA antisera. In the Toluidine Blue-stained sections, no differences were observed in the microscopic structure of the cerebellar cortex between carbon monoxide-exposed rats and controls. The distribution patterns of glutamic acid decarboxylase and GABA immunoreactivities in the cerebellar cortex of the treated animals were qualitatively comparable to those of the controls, and in accordance with previous descriptions of glutamic acid decarboxylase and GABA immunoreactivities in the rat cerebellar cortex. However, quantitative analyses demonstrated a significant reduction of immunoreactivities to both substances in the exposed rats in comparison with the controls. The reduction regarded: in the molecular layer, the number of glutamic acid decarboxylase/GABA-immunoreactive neuronal bodies and of axon terminals and the area they covered; in the Purkinje neuron layer, the number and the area covered by glutamic acid decarboxylase/GABA immunoreactive axon terminals. The differences detected in the prenatally exposed adult rats could be due to carbon monoxide-induced impairment of the differentiation of cerebellar GABA synthesizing neurons. A consequently diminished synthesis of GABA might account for some behavioral disorders detected in adult rats submitted to the same experimental procedure.

A comparison of possible markers for chandelier cartridges in rat medial prefrontal cortex and hippocampus

Chandelier neurons and their characteristic arrays of axonal terminals, known as cartridges, have been implicated in a variety of psychiatric and neurological disorders including schizophrenia and epilepsy. As a result, these neurons have been extensively examined in the brains of several species using a range of markers. However, these markers have not been systematically compared in a single species for their robustness in labelling chandelier cell cartridges. We have therefore examined several markers, reported to label chandelier arrays in primates, for their capacity to mark these structures in rat medial prefrontal cortex and hippocampus. These studies revealed that cartridge-like structures were labelled by parvalbumin and GAT-1 immunohistochemistry in both medial prefrontal cortex and hippocampus of the rat brain. Additionally, GAD65 immunohistochemistry labelled array-like structures preferentially in the dentate gyrus. In contrast, PSA-NCAM, calbindin and GAD67 immunohistochemistry did not reveal any array-like structures in either region of rat brain. These observations indicate that the various immunological markers previously used to visualise chandelier cell cartridges in primates are not equally efficient in labelling these structures in the rat brain, and that GAT-1 immunohistochemistry is the most robust means of visualising chandelier cell cartridges in the regions examined. These are important considerations for quantitative studies in animal models of neurological disorders where chandelier neurons are implicated.

Pathogenesis of peroxisomal deficiency disorders (Zellweger syndrome) may be mediated by misregulation of the GABAergic system via the diazepam binding inhibitor

Background

Zellweger syndrome (ZS) is a fatal inherited disease caused by peroxisome biogenesis deficiency. Patients are characterized by multiple disturbances of lipid metabolism, profound hypotonia and neonatal seizures, and distinct craniofacial malformations. Median live expectancy of ZS patients is less than one year. While the molecular basis of peroxisome biogenesis and metabolism is known in considerable detail, it is unclear how peroxisome deficiency leads to the most severe neurological symptoms. Recent analysis of ZS mouse models has all but invalidated previous hypotheses.

Hypothesis

We suggest that a regulatory rather than a metabolic defect is responsible for the drastic impairment of brain function in ZS patients.

Testing the hypothesis

Using microarray analysis we identify diazepam binding inhibitor/acyl-CoA binding protein (DBI) as a candidate protein that might be involved in the pathogenic mechanism of ZS. DBI has a dual role as a neuropeptide antagonist of GABA(A) receptor signaling in the brain and as a regulator of lipid metabolism. Repression of DBI in ZS patients could result in an overactivation of GABAergic signaling, thus eventually leading to the characteristic hypotonia and seizures. The most important argument for a misregulation of GABA(A) in ZS is, however, provided by the striking similarity between ZS and "benzodiazepine embryofetopathy", a malformation syndrome observed after the abuse of GABA(A) agonists during pregnancy.

Implications of the hypothesis

We present a tentative mechanistic model of the effect of DBI misregulation on neuronal function that could explain some of the aspects of the pathology of Zellweger syndrome.

Developmental regulation of connexins 26, 32, 36, and 43 in trigeminal neurons

The transition from sucking to chewing during postnatal development is accompanied by changes in masticatory muscle activity patterns. We previously demonstrated that changes in numerous parameters of chemical synapses among neurons, and intrinsic membrane properties of neurons, comprising brainstem oral-motor circuits are coincident with changes in masticatory muscle activity patterns. Considering recent findings that implicate a role for gap junctions in early locomotor and respiratory behaviors, our present study focuses on the developmental regulation of connexin proteins in trigeminal neurons as a first step in understanding a role for gap junctions in developing oral-motor circuits used for ingestive behaviors. We conducted immunohistochemistry studies to examine connexin (Cx) 26, 32, 36, and 43 expression in trigeminal motor and mesencephalic trigeminal nuclei during postnatal development at the light and electron microscopic levels. Postnatal days (P) 1, 6, 14, 21, and adult mice were used. Cx32, 36, and 43 expression was developmentally regulated in the trigeminal motor nucleus, while Cx26 expression remained high throughout postnatal development. In the mesencephalic trigeminal nucleus, Cx26, 32, and 43 expression was intense throughout development, with only Cx36 showing a developmental regulation. Ultrastructural examination of neonatal trigeminal motoneurons and mesencephalic trigeminal neurons revealed connexin expression in cell membranes, cytoplasm, and cell nuclei (Cx43, Cx32). Our results show that connexin proteins are differentially regulated between trigeminal motoneurons and mesencephalic trigeminal neurons during development, and suggest a possible role for gap junctions in the development of trigeminal neurons and the function and maturation of oral-motor circuits.

Subcellular localization of GABA receptors in the central nervous system using post-embedding immunohistochemistry

The following detailed protocol can be applied to demonstrate the localization of GABA receptors in CNS neurons at the ultrastructural level. While others have investigated receptors at the electron microscopic level using immunocytochemical techniques, the appearance of the tissue is usually poor and analyses of the distribution of receptors is limited. The methodology described in this paper allows for optimal preservation of the tissue while retaining immunogenicity. It does this, in part, by utilizing a balanced salt solution washout in conjunction with fixation. When the ionic composition of a fixative solution differs from extracellular fluids, like in most fixation protocols for electron microscopy, ultrastructural changes may occur in the tissue. Balanced salt solutions, like the Tyrode solution used here, helps maintain the normal extracellular environment allowing the fixing agent to reach sufficient concentration to bring about permanent and more optimal fixation even when reduced amounts of glutaraldehyde are required to preserve antigenicity. Therefore, unlike many protocols for post-embedding immunoelectronmicroscopy, this method allows for superior preservation of tissue ultrastructure compared to results previously published by others.

Differential NR2A and NR2B expression between trigeminal neurons during early postnatal development

N-methyl-D-aspartate (NMDA) receptors play an important role in the production of rhythmical trigeminal motor activity resembling suckling and chewing. The developmental relationship between the expression of NMDA receptor subunits and the function of neurons comprising brainstem oral-motor circuitry is not clear. We conducted receptor immunohistochemistry studies to demonstrate the expression of NR2A and NR2B subunits in trigeminal motoneurons (Mo5) and mesencephalic trigeminal neurons (Me5) during the first 2 weeks of development. During this time period, rats begin the transition from suckling to chewing, two distinct motor behaviors. In Mo5, NR2A and NR2B immunoreactivity was observed throughout the time frame sampled. A significant increase in the NR2A:NR2B ratio occurred between P3-4 and P11 due to a reduction in the number of NR2B immunoreactive neurons. The temporal and spatial expression of NR2A and NR2B was differentially regulated between caudal and rostral regions of Me5. In contrast to Mo5, the NR2A:NR2B ratio decreased between P0-1 and P11 in caudal Me5 due to a concurrent increase in the number of NR2A and NR2B immunoreactive neurons. In rostral Me5, NR2A and NR2B immunoreactivity emerged at P3 and P11, respectively. Our data provides further insight into the molecular changes of trigeminal neurons during the transition from suckling to chewing behaviors. The differences in the NR2A:NR2B ratio between Mo5 and Me5 suggest functional differences in these neurons during NMDA-mediated neurotransmission.

GABA immunoreactivity in the human cerebellar cortex: a light and electron microscopical study

The distribution of gamma-aminobutyric acid (GABA) in surgical samples of human cerebellar cortex was studied by light and electron microscope immunocytochemistry using a polyclonal antibody generated in rabbit against GABA coupled to bovine serum albumin with glutaraldehyde. Observations by light microscopy revealed immunostained neuronal bodies and processes as well as axon terminals in all layers of the cerebellar cortex. Perikarya of stellate, basket and Golgi neurons showed evident GABA immunoreactivity. In contrast, perikarya of Purkinje neurons appeared to be negative or weakly positive. Immunoreactive tracts of longitudinally- or obliquely-sectioned neuronal processes and punctate elements, corresponding to axon terminals or cross-sectioned neuronal processes, showed a layer-specific pattern of distribution and were seen on the surface of neuronal bodies, in the neuropil and at microvessel walls. Electron microscope observations mainly focussed on the analysis of GABA-labelled axon terminals and of their relationships with neurons and microvessels. GABA-labelled terminals contained gold particles associated with pleomorphic vesicles and mitochondria and established symmetric synapses with neuronal bodies and dendrites in all cortex layers. GABA-labelled terminals associated with capillaries were seen to contact the perivascular glial processes, basal lamina and endothelial cells and to establish synapses with subendothelial unlabelled axons.

IPSC kinetics at identified GABAergic and mixed GABAergic and glycinergic synapses onto cerebellar Golgi cells

In the rat cerebellum, Golgi cells receive serotonin-evoked inputs from Lugaro cells (L-IPSCs), in addition to spontaneous inhibitory inputs (S-IPSCs). In the present study, we analyze the pharmacology of these IPSCs and show that S-IPSCs are purely GABAergic events occurring at basket and stellate cell synapses, whereas L-IPSCs are mediated by GABA and glycine. Corelease of the two transmitters at Lugaro cell synapses is suggested by the fact that both GABA(A) and glycine receptors open during individual L-IPSCs. Double immunocytochemical stainings demonstrate that GABAergic and glycinergic markers are coexpressed in Lugaro cell axonal varicosities, together with the mixed vesicular inhibitory amino acid transporter. Lugaro cell varicosities are found apposed to glycine receptor (GlyR) clusters that are localized on Golgi cell dendrites and participate in postsynaptic complexes containing GABA(A) receptors (GABA(A)Rs) and the anchoring protein gephyrin. GABA(A)R and GlyR/gephyrin appear to form segregated clusters within individual postsynaptic loci. Basket and stellate cell varicosities do not face GlyR clusters. For the first time the characteristics of GABA and glycine cotransmission are compared with those of GABAergic transmission at identified inhibitory synapses converging onto the same postsynaptic neuron. The ratio of the decay times of L-IPSCs and of S-IPSCs is a constant value among Golgi cells. This indicates that, despite a high cell-to-cell variability of the overall IPSC decay kinetics, postsynaptic Golgi cells coregulate the kinetics of their two main inhibitory inputs. The glycinergic component of L-IPSCs is responsible for their slower decay, suggesting that glycinergic transmission plays a role in tuning the IPSC kinetics in neuronal networks.

AMPA receptor subunit expression in trigeminal neurons during postnatal development

Trigeminal motoneurons (Mo5) and mesencephalic trigeminal neurons (Me5) are important constituents of the neural circuitry responsible for jaw movements. Non-N-methyl-D-aspartate (NMDA) glutamate receptors are a critical component of the brainstem circuitry responsible for reflex and centrally activated jaw movements; however, little is known about the expression of these receptors in neonatal oral-motor circuitry. Receptor immunohistochemistry using affinity-purified polyclonal antibodies directed against GluR1, GluR2/3/4c, and GluR4, respectively, and a monoclonal antibody directed against the GluR2 subunit, were used in rats at postnatal day (P)1, P3, P5, P10, P19-21, P32-35, and P60 to describe the expression of the alpha-amino-d-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor in Mo5 and Me5 neurons. In Mo5, immunoreactivity was noted for all antibodies throughout the time frame sampled. Neurons in caudal portions of Me5 displayed immunoreactivity to each antibody except at P60 when GluR2 immunoreactivity was absent. Neurons located in rostral Me5 displayed GluR2/3/4c and GluR4 immunoreactivity throughout the time frame, GluR1 immunoreactivity emerged at P3 and a transient expression of GluR2 expression was observed between P10 and P32-35. The lack of labeling of some neurons in both regions, coupled with differences in temporal expression, suggests that there are differences in the AMPA receptor phenotype within and between Mo5 and Me5 during postnatal development. Differences in AMPA subunit composition suggest a complex role for AMPA-mediated glutamatergic neurotransmission in brainstem circuits controlling jaw movements.

Neurotransmitters, neuropeptides and calcium binding proteins in developing human cerebellum: a review

Many endogenous neurochemicals that are known to have important functions in the mature central nervous system have also been found in the developing human cerebellum. Cholinergic neurons, as revealed by immunoreactivities towards choline acetyltransferase or acetylcholinesterase, appear early at 23 weeks of gestation in the cerebellar cortex and deep nuclei. Immunoreactivities gradually increase until the first postnatal month. Enkephalin is localized in the developing cerebellum, initially in the fibers of the cortex and deep nuclei at 16-20 weeks and then also in the Purkinje cells, granule cells, basket cells and Golgi cells at 23 weeks onward. Another neuropeptide, substance P, is localized mainly in the fibers of the dentate nucleus from 9 to 24 weeks but substance P immunoreactivity declines thereafter. GABA, an inhibitory neurotransmitter of the central nervous system, starts to appear at 16 weeks in the Purkinje cells, stellate cells, basket cells, mossy fibers and neurons of deep nuclei. GABA expression is gradually upregulated toward term forming networks of GABA-positive fibers and neurons. Catecholaminergic fibers and neurons are also detected in the cortex and deep nuclei at as early as 16 weeks. Calcium binding proteins, calbindin D28K and parvalbumin, make their first appearance in the cortex and deep nuclei at 14 weeks and then their expression decreases toward term, while calretinin appears later at 21 weeks but its expression increases with fetal age. The above findings suggest that many neurotransmitters, neuropeptides and calcium binding proteins (1) appear early during development of the cerebellum; (2) have specific temporal and spatial expression patterns; (3) may have functions other than those found in the mature neural systems; and (4) may be able to interact with each other during early development.

Glutamic acid decarboxylase-positive neuronal cell bodies and terminals in the human cerebellar cortex

The distribution of gamma-aminobutyric acid (GABA) in the human cerebellar cortex was studied using immunohistochemistry for glutamic acid decarboxylase (GAD), the enzyme that catalyses GABA synthesis. Observations by light microscopy revealed, in all layers of the cerebellar cortex, strong, punctate positivity for GAD, related to putative GABAergic nerve terminals, as well as a diffuse cytoplasmic immunoreactivity within neuronal cell bodies. GAD-positive nerve terminals were found in close relationship with the walls of the cerebellar cortex microvessels. Observations by electron microscopy revealed positive nerve terminals in contact with the astrocyte perivascular sheath of capillaries. GAD immunoreactivity was also detected within astroglial perivascular endfeet and endothelial cells. The findings provide further insights into the GABAergic synapses of the circuitry of the human cerebellar cortex. The detection of 'vascular' GAD immunoreactivities suggests that GABAergic mechanisms may regulate cerebellar microvessel function.

Immunohistochemical studies of the structural bases of inhibition in the central cerebellar nuclei in mice

The distribution of glutamate decarboxylase-immunoreactive structures in the central nuclei of the cerebellum, its first afferent component, was studied at the light and electron microscope levels. Axosomatic, axodendritic, and axospinous synapses were detected, in which the presynaptic parts contained glutamate decarboxylase (GDC); this enzyme is involved in GABA synthesis. Additionally, these investigations revealed axoaxonal synapses in which both poles were GDC-reactive. The central nuclei of the cerebellum were found to have an intrinsic GABAergic system.

Confocal microscopic estimation of GABAergic nerve terminals in the central nervous system

We describe a method for estimating the average proportion of GABAergic terminal area relative to total nerve terminal area with confocal microscopy. Nerve terminal regions were identified with dual colour immunofluorescence on Vibratome sections with an antibody to synaptophysin (SYN), and GABAergic processes, including axon terminals, were identified with an antibody to glutamic acid decarboxylase (GAD). Sections were viewed in an Olympus AX70 microscope attached to a Biorad 1024 MRC scanning confocal system. Images were collected with a 100 x objective from the same tissue locations and imported into the NIH-Image program, where black and white binary images were obtained for co-localisation and quantitation. Measurements were made separately of areas of SYN/GAD (GABAergic terminals) and SYN labelling (all terminals). The relative proportion of GABAergic terminal areas in visual cortex (6.1+/-1%; mean +/- SE), CA1 hippocampus (2.6+/-0.5%) and deep cerebellar nuclei (46.6+/-3%) are consistent with what is known of the relative levels of inhibitory input to these structures. The assumptions that SYN labelling is restricted to axon terminals, and that SYN labels all axon terminals was tested by ultrastructural localisation of SYN in the three brain regions examined. Only 7.4+/-0.4% of SYN-labelled profiles could not be positively identified as synaptic vesicle-containing axon terminals, and between 93.4 and 99.2% of vesiculated axon profiles were SYN-positive. These results suggest that SYN is a very reliable marker for axon terminals, and validates the confocal analytical approach. The confocal method allows rapid sampling of many brain regions and would be suitable for examining terminals containing any neurotransmitter that can be detected immunocytochemically.

K(+) channel expression distinguishes subpopulations of parvalbumin- and somatostatin-containing neocortical interneurons

Kv3.1 and Kv3.2 K(+) channel proteins form similar voltage-gated K(+) channels with unusual properties, including fast activation at voltages positive to -10 mV and very fast deactivation rates. These properties are thought to facilitate sustained high-frequency firing. Kv3.1 subunits are specifically found in fast-spiking, parvalbumin (PV)-containing cortical interneurons, and recent studies have provided support for a crucial role in the generation of the fast-spiking phenotype. Kv3.2 mRNAs are also found in a small subset of neocortical neurons, although the distribution of these neurons is different. We raised antibodies directed against Kv3.2 proteins and used dual-labeling methods to identify the neocortical neurons expressing Kv3.2 proteins and to determine their subcellular localization. Kv3.2 proteins are prominently expressed in patches in somatic and proximal dendritic membrane as well as in axons and presynaptic terminals of GABAergic interneurons. Kv3.2 subunits are found in all PV-containing neurons in deep cortical layers where they probably form heteromultimeric channels with Kv3.1 subunits. In contrast, in superficial layer PV-positive neurons Kv3.2 immunoreactivity is low, but Kv3.1 is still prominently expressed. Because Kv3.1 and Kv3.2 channels are differentially modulated by protein kinases, these results raise the possibility that the fast-spiking properties of superficial- and deep-layer PV neurons are differentially regulated by neuromodulators. Interestingly, Kv3. 2 but not Kv3.1 proteins are also prominent in a subset of seemingly non-fast-spiking, somatostatin- and calbindin-containing interneurons, suggesting that the Kv3.1-Kv3.2 current type can have functions other than facilitating high-frequency firing.

GABA immunoreactivity in mouse barrel field after aversive and appetitive classical conditioning training involving facial vibrissae

We have previously reported that a classical conditioning paradigm involving stimulation of a row of facial vibrissae produced an expansion of the cortical representation of the "trained row", labeled with 2-deoxyglucose (2DG), in layer IV of the barrel field. The present study has examined the pattern of GABA immunoreactivity (GABA-IR) in the cortical representation of row B of the facial vibrissae after (i) 3 days of aversive training, and (ii) 2 months of appetitive training, where stimulation of row B of vibrissae on one side of the snout was used as a conditioned stimulus. The most notable observation was a greater density of GABA-IR cells concentrated in the hollows of the "trained row" B barrels compared to the hollows in the barrel field of the opposite hemisphere in the same mouse. After aversive training, we noted a 2-fold increase in the density of GABA-IR neurons in the hollows of row B; after reward training, the increase amounted to 49%. In contrast, GABA-IR was unchanged in the control groups, which received only stimulation of vibrissae without the unconditioned stimulus. The classification of labeled neurons according to size revealed that the increase in density of GABA-IR neurons was confined to the small (12-15 microm) diameter group. We concluded that the GABAergic system undergoes up-regulation, after both associative learning paradigms, and that the population of small, GABAergic neurons plays an active role in use-dependent plasticity.

The neuron-specific K-Cl cotransporter, KCC2. Antibody development and initial characterization of the protein

The neuron-specific K-Cl cotransporter (KCC2) is hypothesized to function as an active Cl- extrusion pathway important in postsynaptic inhibition mediated by ligand-gated anion channels, like gamma-aminobutyric acid type A (GABAA) and glycine receptors. To understand better the functional role of KCC2 in the nervous system, we developed polyclonal antibodies to a KCC2 fusion protein and used these antibodies to characterize and localize KCC2 in the rat cerebellum. The antibodies specifically recognized the KCC2 protein which is an approximately 140-kDa glycoprotein detectable only within the central nervous system. The KCC2 protein displayed a robust and punctate distribution in primary cultured retinal amacrine cells known to form exclusively GABAAergic synapses in culture. In immunolocalization studies, KCC2 was absent from axons and glia but was highly expressed at neuronal somata and dendrites, indicating a specific postsynaptic distribution of the protein. In the granule cell layer, KCC2 exhibited a distinct colocalization with the beta2/beta3-subunits of the GABAA receptor at the plasma membrane of granule cell somata and at cerebellar glomeruli. KCC2 lightly labeled the plasma membrane of Purkinje cell somata. Within the molecular layer, KCC2 exhibited a distinctly punctate distribution along dendrites, indicating it may be highly localized at inhibitory synapses along these processes. The distinct postsynaptic localization of KCC2 and its colocalization with GABAA receptor in the cerebellum are consistent with the putative role of KCC2 in neuronal Cl- extrusion and postsynaptic inhibition.

Developmental expression of gamma-aminobutyric acid transporters (GAT-1 and GAT-3) in the rat cerebellum: evidence for a transient presence of GAT-1 in Purkinje cells

The cerebellar cortex contains several classes of GABAergic neurons. Previous studies have shown that most GABAergic neurons in this region possess the capacity for gamma-aminobutyric acid (GABA) uptake. The present study determined the postnatal expression of two GABA transporters, GAT-1 and GAT-3, in the cerebellar cortex and deep nuclei of the rat by using immunocytochemistry. Immunoreactivity for GAT-1 and GAT-3 appears at postnatal day 7 (P7), emerges centroperipherally across the cerebellum during the following 2 weeks and reaches an adult-like pattern by P30. The mature patterns are fully established by P45, which for GAT-1 is characterized by immunolabeled profiles localized exclusively to neuropil, mostly in the molecular layer and the pinceaux deep to the Purkinje cell bodies, and for GAT-3 as immunoreactivity distributed in the neuropil of mainly the granular layer. Before the adult patterns are completed, GAT-1 immunoreactivity is present in the somata of Purkinje, Golgi, basket and stellate cells between P7 and P21, while GAT-3 immunoreactivity is distinct in astrocytic somata which are organized in regularly spaced clusters. During this period, there is also a banding pattern in the sagittal plane of GAT-1 immunoreactivity in developing Purkinje cells. The postnatal development of GAT-1 and GAT-3 in the rat cerebellar cortex shares a similar spatiotemporal pattern with other GABAergic parameters, including the GABA synthesizing enzyme, GABA content and uptake. Specifically, the transient expression of GAT-1 in the somata and dendrites of cerebellar GABAergic neurons appears to correlate with the supra-adult levels of whole-tissue GABA uptake capability during development. Further, GAT-1 expression in immature Purkinje cells may play a unique role in regulating GABA's function during development, since mature Purkinje cells do not express GAT-1 or take up GABA.

Pre-embedding staining for GAD67 versus postembedding staining for GABA as markers for central GABAergic terminals

Pre-embedding immunocytochemistry for the active form of glutamate decarboxylase (GAD67) and postembedding staining for gamma-aminobutyric acid (GABA) were compared as markers for central GABAergic terminals in the phrenic motor nucleus, in which phrenic motor neurons had been retrogradely labeled with cholera toxin B-horseradish peroxidase. Nerve terminals with or without GAD67 immunoreactivity were identified in one ultrathin section. GABA was localized with immunogold in an adjacent section after etching and bleaching. GABA labeling density was assessed over 519 GAD67-positive and GAD67-negative nerve terminals in the phrenic motor nucleus. Frequency histograms showed that statistically higher densities of gold particles occurred over most GAD67-positive terminals. However, some GAD67-negative terminals also showed high densities of gold particles, and some GAD67-positive terminals showed low densities. Preabsorption of the anti-GABA antibody with a GABA-protein conjugate, but not with other amino acid-protein conjugates, significantly reduced gold labeling over both GAD67-positive and GAD67-negative terminals. These results show that the presence of GAD67 immunoreactivity correlates strongly with high densities of immunogold labeling for GABA in nerve terminals in the phrenic motor nucleus. Preabsorption controls indicate that authentic GABA was localized in the postembedding labeling procedure. Only a small proportion of intensely GABA-immunoreactive terminals lack GAD67, suggesting that both GAD67 and GABA are reliable markers of GABAergic nerve terminals.

The effects of the tremorgenic mycotoxin penitrem A on the rat cerebellum

Within 10 minutes of intraperitoneal injection of penitrem A (3 mg/kg), rats develop severe generalized tremors and ataxia that persist for up to 48 hours. These are accompanied by a three- to fourfold increase in cerebellar cortical blood flow. Mitochondrial swelling occurs in cerebellar stellate and basket cells within 30 minutes of dosing and persists for more than 12 hours without leading to cell death. From 2 hours, Purkinje cell dendrites show early cytoplasmic condensation accompanied by fine vacuolation of smooth endoplasmic reticulum and enlargement of perikaryal mitochondria. From 6 hours, many Purkinje cells develop intense cytoplasmic condensation with eosinophilia that resembles "ischemic cell change," and from 12 hours, many other Purkinje cells show marked watery swelling. Astrocytes begin to swell from 0.5 hours after injection and show hypertrophy of organelles from 6 hours. Also from 6 hours onward, discrete foci of necrosis appear in the granule cell layer, while permeability of overlying meningeal vessels to horseradish peroxidase becomes evident at 8 hours. All changes are more severe in vermis and paravermis. Despite widespread loss of Purkinje cells, the animals' behavior becomes almost normal within a week. While tremor occurs with doses of 1.5 and 0.5 mg/kg, cellular damage is minimal. The tremor mechanism differs from that of harmaline since destruction of inferior olivary nuclei abolishes neither the tremor response to penitrem A nor the cellular damage. No morphological changes are found in other brain regions. The affinities of penitrem A for high-conductance calcium-dependent potassium channels and for gamma-aminobutyric acid receptors with the probability of resultant excitotoxity are considered to be important underlying factors for these changes.

GABA-ergic transmission in deep cerebellar nuclei

gamma-Aminobutyric acid (GABA) is the inhibitory transmitter released at Purkinje cell axon terminals in deep cerebellar nuclei (DCN). Neurons in DCN also receive excitatory glutamatergic inputs from the inferior olive. The output of DCN neurons, which depends on the balance between excitation and inhibition on these cells, is involved in cerebellar control of motor coordination. Plasticity of synaptic transmission observed in other areas of the mammalian central nervous system (CNS) has received wide attention. If GABA-ergic and/or glutamatergic synapses in DCN also undergo plasticity, it would have major implications for cerebellar function. In this review, literature evidence for GABA-ergic synaptic transmission in DCN as well as its plasticity are discussed. Studies indicate that fast inhibitory postsynaptic potentials (IPSPs) and currents (IPSCs) in neurons of DCN are mediated by GABAA receptors. While GABAB receptors are present in DCN, they do not appear to be activated by Purkinje cell axons. The IPSPs undergo paired-pulse, as well as frequency-dependent, depressions. In addition, tetanic stimulation of inputs can induce a long-term depression (LTD) of the IPSPs and IPSCs. Excitatory synapses do not appear to undergo long-term potentiation or LTD. The LTD of the IPSP is not input-specific, as it can be induced heterosynaptically and is associated with a reduced response of DCN neurons to a GABAA receptor agonist. Postsynaptic Ca2+ and protein phosphatases appear to contribute to the LTD. The N-methyl-D-aspartate receptor-gated, as well as the voltage-gated Ca2+ channels are proposed to be sources of the Ca2+. It is suggested that LTD of GABA-ergic transmission, by regulating DCN output, can modulate cerebellar function.

Expression of a glutamate transporter subtype, EAAT4, in the developing human cerebellum

A glutamate transporter subtype, EAAT4, is closely related to removal of glutamate from the synaptic cleft. Immunohistochemistry for EAAT4 demonstrated the specific distribution and localization of its expression in the developing human cerebellum. Purkinje cells showed faint EAAT4 immunostaining at 17 gestational weeks (GW), which became increasingly intense from 23 GW to the infantile period. In the late fetal to early infantile periods, Purkinje cells showed marked immunoreactivity. After the late infantile period, EAAT4 immunoreactivity was the same in extent as in the adult pattern. Its intracellular localization also changed with development. EAAT4 immunoreactivity was demonstrated in the short processes of Purkinje cells in the early embryonic period, in the cell bodies and dendrites in the late fetal to early infantile periods, and then in the spines after the late infantile period. In the adult cerebellum, immunoreactivity was detected strongly in the spines of Purkinje cells and weakly in the cell bodies. No immunoreactivity was found in the axons or axon terminals of the cells. Thus, the glutamate transporter exhibits developmental changes in its distribution in the cerebellum and its localization in Purkinje cells. EAAT4 immunoreactivity may be related to the dendritic arborization of cells in the molecular layer.

Immunohistochemical studies of GABA and parvalbumin in the developing human cerebellum

The localization of GABA and parvalbumin was studied in the developing cerebellum of human fetuses from 16 to 28 weeks of gestation. The avidin-biotin complex immunohistochemical method combined with silver staining were used to reveal the presence of GABA- and parvalbumin-positive neurons and nerve fibres. As early as the 16th week of gestation, GABA immunopositivity was observed in the cerebellar cortex and the deep nuclei. GABA-positive neurons included Purkinje cells, stellate and basket cells of the cerebellar cortex and neurons in the deep nuclei. The gradient of immunoreactivity increased with the maturing cells, being weak at 16 weeks and becoming markedly pronounced at 28 weeks of gestation. GABA-immunopositive mossy fibres were observed in the granular cell layer at 16 weeks, and by 28 weeks, a robust fibre network was present in the cortex and deep nuclei. Immunohistochemical localization for parvalbumin indicates that weak immunoreactivity was observed in Purkinje cells, stellate and basket cells at 16 weeks of gestation, increasing in intensity with advancing age, notably in the Purkinje cells which had acquired an elaborate arbor of neurites at 28 weeks of gestation. In the deep nuclei, parvalbumin-positive cells and nerve fibres were observed throughout the 16 to 28 week period. These results indicate that GABA- and parvalbumin-positive neurons and fibres appeared as early as 16 weeks of gestation, expressing a high degree of immunoreactivity by the 28 week of fetal age.

A combined retrograde tracer and GABA-immunocytochemical study of the projection from nucleus interpositus posterior to the posterior lobe C2 zone of the cat cerebellum

The extent to which the cells of origin of the cerebellar nucleocortical pathway are immunopositive for gamma-aminobutyric acid (GABA) was investigated in four cats using retrograde labelling of nucleocortical neurons in combination with immunocytochemistry. Neurons were retrogradely labelled by injection of fluorescent (coumarin)-tagged latex microspheres into the c2 zone in the rostral part of the paramedian lobule. The zone was identified electrophysiologically by the characteristics of the climbing fibre responses evoked on the cerebellar surface by percutaneous stimulation applied to the left and right forepaws in pentobarbitone-anaesthetized animals. Sections of the cerebellum containing the retrogradely labelled neurons were processed for GABA immunocytochemistry using a fluorescent (rhodamine)-tagged immunoglobulin. When viewed with epifluorescence microscopy and appropriate filter blocks the retrogradely labelled nucleocortical neurons could be visualized in the same sections as the GABA-immunopositive neurons. Almost all of a total of 254 labelled nucleocortical neurons were located in nucleus interpositus posterior, where a total of 711 GABAergic neurons were also found. None of these cells contained coumarin-tagged beads and displayed immunoreactivity for GABA (i.e. none was double-labelled). When compared by area of their cell body, the nucleocortical and GABA-immunopositive neurons appeared to form two partially overlapping populations. The mean cell area of the nucleocortical neurons was 620 +/- 233 microns2 (SD), whereas the GABA-immunopositive neurons were much smaller, with a mean cell area of 220 +/- 115 microns2. The results suggest that GABA does not play a major role in the nucleocortical pathway to the c2 zone of the rostral paramedian lobule of the cat cerebellum.

Immunohistochemical evidence for GABA and glycine-containing trigeminal premotoneurons in the guinea pig

Electrophysiological studies have suggested that inhibition of trigeminal motoneurons during mastication and the jaw-opening reflex are mediated by last-order interneurons (premotoneurons) utilizing GABA and glycine [Chandler et al. (1985), Brain Res., 325:181-186; Enomoto et al. (1987), Neurosci. Res., 4:396-412; Goldberg and Nakamura (1968), Experientia, 24:371-373; Kidokoro et al. (1968), J. Neurophysiol., 31:695-708; Nakamura et al. (1978), Exp. Neurol., 61:1-14]. In the present study we performed a series of double-labeling experiments in guinea pigs to determine the location of neurons which contain GABA (gamma aminobutyric acid) or glycine that project to the trigeminal motor nucleus (Mo5). This was accomplished by performing immunohistochemical staining in combination with a retrograde tract tracing technique using colloidal gold bound to inactivated WGA-HRP (wheat germ agglutin-horseradish peroxidase) (gWGA-HRP) as our retrograde tracer. Neurons which had a positive immunoreactivity to GABA or GAD (glutamic acid decarboxylase) and contained the retrograde marker were located in regions adjacent to the Mo5 such as the intertrigeminal, supratrigeminal, peritrigeminal and rostral portions of the parvocellular reticular formation alpha. Neurons which had a positive immunoreactivity to glycine and contained the retrograde marker were identified in the parvocellular reticular formation, the spinal trigeminal nucleus oralis, supratrigeminal and intertrigeminal regions. These data provide anatomical evidence for GABAergic and glycinergic projections to Mo5.

Immunohistochemical localization of glutamate and glutaminase in guinea pig trigeminal premotoneurons

Previous electrophysiological experiments in guinea pigs from our laboratory [11,36,37] have suggested that synaptic transmission between last-order interneurons (premotoneurons) and trigeminal motoneurons during reflex activation or cortically induced rhythmical jaw movements is mediated by excitatory amino acids (EAAs). In the present study, we performed a series of double-labeling experiments in guinea pigs to determine the location of neurons which contain glutamate or glutaminase and project to the trigeminal motor nucleus (Mo5). This was accomplished by combining immunohistochemical staining and standard retrograde tract-tracing techniques. Injections of a retrograde tracer, colloidal-gold bound to inactivated WGA-HRP (gWGA-HRP), into the trigeminal motor nucleus labeled a column of neurons originating adjacent to Mo5, including the supratrigeminal nucleus, intertrigeminal nucleus and the mesencephalic nucleus of V. The column extended caudally into the parvocellular reticular formation and adjacent trigeminal sensory nucleus oralis and oralis gamma subdivision. In all of these regions, immunoreactivity to glutamate or glutaminase was observed co-localized with gWGA-HRP.

Unusual neurofilament composition in cerebellar unipolar brush neurons

During antibody screening on sections of rat cerebellum, we noticed a group of small neurons which exhibited unusual staining properties. They were robustly immunopositive for the high molecular weight neurofilament protein, moderately immunostained with antibodies to the low molecular weight neurofilament protein and alpha-internexin, but only faintly immunoreactive (in PAP sections) or essentially immunonegative (in immunofluorescent sections) with all members of a panel of antibodies directed against the middle molecular weight neurofilament protein. Since neurons generally react equally well with phosphate-independent, (antibodies to) low, middle and high molecular weight neurofilament protein, we conclude that middle molecular weight neurofilament protein is present in these cells in an unusually low relative amount. These cells are found in the granular layer and appear concentrated in the flocculus, ventral paraflocculus, and vermis, particularly in the ventral uvula and nodulus (lobules IXd and X). Previous studies performed by Hockfield defined a population of neurons of similar appearance and distribution using the monoclonal antibody Rat-302, which recognized an uncharacterized 160 kDa protein. We show here that the cells described by Hockfield are identical to those we have found and furthermore that the Rat-302 antibody specifically recognizes the dephosphorylated form of the lysine-serine-proline repeated sequences of high molecular weight neurofilament protein. These cells were studied by pre-embedding immunoelectron microscopy. The nucleus is deeply indented and shows little condensed chromatin. The cytoplasm contains scattered microtubules and a larger number of neurofilaments than expected in a small cell. There are numerous large dense core vesicles, an unusual organelle consisting of ringlet subunits, and relatively little granular endoplasmic reticulum. A thin axon and a single stout dendritic trunk emanate from the perikaryon. Although the cell body and the dendritic shaft may form either complex contacts with mossy fibres (resembling those previously termed en marron synapses) or simple symmetric synapses with small boutons containing pleomorphic vesicles, most of the synaptic relations are established on the shafts of brush-like branchlets that form at the tip of the dendrite and enter one or two glomeruli. Each branchlet forms an extraordinarily extensive asymmetric synapse with the mossy fibre rosette and the subsynaptic region shows a microfibrillar web connected to the postsynaptic density. In addition to other organelles, the branchlets contain numerous mitochondria and large dense core vesicles. Short, non-synaptic appendages with few cytoplasmic organelles emanated from the cell body, dendritic shaft and branchlets.(ABSTRACT TRUNCATED AT 400 WORDS)

Interaction between excitatory and inhibitory amino acids in the ventromedial nucleus of the hypothalamus in inducing hyper-running

We have previously shown that blockade of neurotransmission mediated by gamma-aminobutyric acid (GABA) in the ventromedial nucleus of the hypothalamus (VMH) by the microinjection of a GABAA receptor antagonist, bicuculline methiodide (BM), exclusively induced running activity in the rat. The purpose of the present study was to examine the role of receptors for excitatory amino acids (EAAs) in the VMH in inducing hyper-running and to clarify the interaction between GABA and EAAs in the VMH in controlling running activity. Although the injection of neither N-methyl-D-aspartate (NMDA) nor quisqualate into the VMH induced hyper-running, kainate (KA) produced running activity in a dose-dependent manner with similar features to that induced by BM. This effect of KA was blocked by a non-NMDA receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX). GABA injected simultaneously with KA into the VMH failed to affect hyper-running induced by KA. On the other hand, DNQX significantly suppressed the BM-induced running activity. These results suggest that endogenous EAAs acting on the KA-type receptor in the VMH facilitates running activity and that the release of such EAAs from the nerve terminal is presynaptically inhibited by GABA.

Enkephalinergic innervation of GABAergic neurons in the dorsal raphe nucleus of the rat

The preembedding double immunoreaction method was used to study interrelations of enkephalinergic and GABAergic neuronal elements in the dorsal raphe nucleus of the Wistar albino rat. The enkephalin-like neuronal elements were immunoreacted by the peroxidase-antiperoxidase method and silver-gold intensified, which showed strongly and was specific. The GABA-like immunoreactive neurons were immunoreacted by the peroxidase-antiperoxidase method only. GABA-like neural somata were postsynaptic to both the enkephalin-like immunoreactive and the non-immunoreactive axon terminals. The enkephalin-like immunoreactive axon terminals were also found to synapse GABA-like immunoreactive dendrites. The GABA-like immunoreactive neuronal elements were also found to receive synapses from other non-immunoreactive as well as GABA-like immunoreactive axon terminals. Almost all of the synapses appeared to be asymmetrical. Possible functional activity of interactions among the enkephalinergic, GABAergic, and serotonergic neuronal elements in the dorsal raphe nucleus are discussed.