Differential number of glycine- and GABA-immunopositive neurons and terminals in the deep cerebellar nuclei of normal and Purkinje cell degeneration mutant mice

The AGTPBP1 gene in neurobiology

The function of the Agtpbp1 gene has mainly been delineated by studying Agtpbp1pcd (pcd) mutant mice, characterized by losses in cerebellar Purkinje and granule cells along with degeneration of retinal photoreceptors, mitral cells of the olfactory bulb, thalamic neurons, and alpha-motoneurons. As a result of cerebellar degeneration, cerebellar GABA and glutamate concentrations in Agtpbp1pcd mutants decreased while monoamine concentrations increased. The salient behavioral phenotypes include cerebellar ataxia, a loss in motor coordination, and cognitive deficits. Similar neuropathogical and behavioral profiles have been described in childhood-onset human subjects with biallelic variants of AGTPBP1, including cerebellar ataxia and hypotonia.

Hyper-Formation of GABA and Glycine Co-Releasing Terminals in the Mouse Cerebellar Nuclei after Deprivation of GABAergic Inputs from Purkinje Cells

GABA and glycine are inhibitory neurotransmitters. However, the mechanisms underlying the formation of GABAergic and glycinergic synapses remain unclear. The influence of GABAergic input deprivation on inhibitory terminal formation was investigated using Purkinje cell (PC)-specific vesicular GABA transporter (VGAT) knockout (L7-VGAT) mice, in which GABA release from PCs diminishes in an age-dependent manner. We compared the late development of GABAergic and glycinergic terminals in the cerebellar nucleus (CN) between control and L7-VGAT mice. In the control CN, the density of glutamate decarboxylase (GAD)-positive dots remained unchanged between postnatal 2 months (P2M) and 13 months (P13M), whereas glycine transporter 2 (GlyT2)-positive dots increased in density during this time frame. No difference in the density of GlyT2-positive dots was observed between control and L7-VGAT mice at P2M, but the density was significantly higher in the L7-VGAT fastigial nuclei (FN) than the control FN at P13M. When VGAT was absent from PC terminals, GlyT2-positive dots included GAD and VGAT and formed synapses. These results indicated that GABAergic terminals were formed by P2M, glycinergic terminals were actively formed after P2M, and more glycinergic terminals were formed in the L7-VGAT FN than in the control FN, suggesting that the increased glycinergic terminals may derive from interneurons within the FN and may also release GABA. These results suggest that the deprivation of GABAergic inputs from PCs may accelerate the formation of co-releasing terminals derived from interneurons and that the inhibitory terminal numbers and types may be regulated by the quantity of functional GABAergic inputs.

Optogenetic Approaches to Target Specific Neural Circuits in Post-stroke Recovery

Stroke is a leading cause of death and disability in the USA, yet treatment options are very limited. Functional recovery can occur after stroke and is attributed, in part, to rewiring of neural connections in areas adjacent to or remotely connected to the infarct. A better understanding of neural circuit rewiring is thus an important step toward developing future therapeutic strategies for stroke recovery. Because stroke disrupts functional connections in peri-infarct and remotely connected regions, it is important to investigate brain-wide network dynamics during post-stroke recovery. Optogenetics is a revolutionary neuroscience tool that uses bioengineered light-sensitive proteins to selectively activate or inhibit specific cell types and neural circuits within milliseconds, allowing greater specificity and temporal precision for dissecting neural circuit mechanisms in diseases. In this review, we discuss the current view of post-stroke remapping and recovery, including recent studies that use optogenetics to investigate neural circuit remapping after stroke, as well as optogenetic stimulation to enhance stroke recovery. Multimodal approaches employing optogenetics in conjunction with other readouts (e.g., in vivo neuroimaging techniques, behavior assays, and next-generation sequencing) will advance our understanding of neural circuit reorganization during post-stroke recovery, as well as provide important insights into which brain circuits to target when designing brain stimulation strategies for future clinical studies.

Reorganization of Synaptic Connections and Perineuronal Nets in the Deep Cerebellar Nuclei of Purkinje Cell Degeneration Mutant Mice

The perineuronal net (PN) is a subtype of extracellular matrix appearing as a net-like structure around distinct neurons throughout the whole CNS. PNs surround the soma, proximal dendrites, and the axonal initial segment embedding synaptic terminals on the neuronal surface. Different functions of the PNs are suggested which include support of synaptic stabilization, inhibition of axonal sprouting, and control of neuronal plasticity. A number of studies provide evidence that removing PNs or PN-components results in renewed neurite growth and synaptogenesis. In a mouse model for Purkinje cell degeneration, we examined the effect of deafferentation on synaptic remodeling and modulation of PNs in the deep cerebellar nuclei. We found reduced GABAergic, enhanced glutamatergic innervations at PN-associated neurons, and altered expression of the PN-components brevican and hapln4. These data refer to a direct interaction between ECM and synapses. The altered brevican expression induced by activated astrocytes could be required for an adequate regeneration by promoting neurite growth and synaptogenesis.

Functional classification of neurons in the mouse lateral cerebellar nuclei

The deep cerebellar nuclei (DCN) are at the center of the cerebellum not only anatomically but also functionally. Classical anatomical studies have described different types of DCN neurons according to their expression of various marker proteins, but only recently have we begun to characterize these different cell types according to their electrophysiological properties. These efforts have benefited greatly from the availability of transgenic mouse lines that express green fluorescent protein under the control of the glutamic acid decarboxylase (GAD67) and glycine transporter (GlyT2) promoters, which are markers for GABAergic and glycinergic neurons, respectively. These studies have identified several types of neurons within the lateral cerebellar nuclei, each of which exhibits distinct active membrane properties. In addition to their differential use of neurotransmitters (glutamate, GABA, or glycine), these cell types also receive and provide synaptic information from different sources and to different targets.

Alterations in GABAergic biomarkers in the autism brain: research findings and clinical implications

Autism is a pervasive developmental disorder characterized by repetitive stereotyped behavior, social-emotional deficits, and delayed or absent language abilities. There are known neuropathologies in the autism brain affecting limbic, cerebellar, and cortical structures but the neurochemical profile of affected individuals, revealed in postmortem tissue studies, is only recently emerging. One major component that appears highly impacted in autism is the GABAergic system. It is now apparent that there are widespread significant effects in many distributed regions in the autism brain revealed by histochemical, autoradiographic, and biochemical studies. The key synthesizing enzymes for GABA, glutamic acid decarboxylase type 65 and 67 (GAD65 and GAD67), are decreased in the cerebellum and closer examination of mRNA levels revealed that it is largely due to decreases in Purkinje cells and a subpopulation of larger dentate neurons as measured by in situ hybridization studies. Other cell types had either normal GAD levels (Golgi cells, smaller dentate interneurons, and stellate cells) or increased levels (basket cells). GABA receptor density, number, and protein expression are all decreased in the cerebellum and in select cortical areas. GABA(A) and GABA(B) subunit protein expression was significantly reduced in cerebellum, BA 9 and BA 40. Benzodiazepine binding sites were significantly reduced in the hippocampus and anterior cingulate cortex (BA 24). Taken together, data from these studies suggest that there is a marked dysregulation of the inhibitory GABA system in the autism brain affecting particular biomarkers localized to specific cell types and lamina likely influencing circuitry and behavior.

GlyT2+ neurons in the lateral cerebellar nucleus

The deep cerebellar nuclei (DCN) are a major hub in the cerebellar circuitry but the functional classification of their neurons is incomplete. We have previously characterized three cell groups in the lateral cerebellar nucleus: large non-GABAergic neurons and two groups of smaller neurons, one of which express green fluorescence protein (GFP) in a GAD67/GFP mouse line and is therefore GABAergic. However, as a substantial number of glycinergic and glycine/GABA co-expressing neurons have been described in the DCN, this classification needed to be refined by considering glycinergic neurons. To this end we took advantage of a glycine transporter isoform 2 (GlyT2)-eGFP mouse line that allows identification of GlyT2-expressing, presumably glycinergic neurons in living cerebellar slices and compared their electrophysiological properties with previously described DCN neuron populations. We found two electrophysiologically and morphologically distinct sets of GlyT2-expressing neurons in the lateral cerebellar nucleus. One of them showed electrophysiological similarity to the previously characterized GABAergic cell group. The second GlyT2+ cell population, however, differed from all other so far described neuron types in DCN in that the cells (1) are intrinsically silent in slices and only fire action potentials upon depolarizing current injection and (2) have a projecting axon that was often seen to leave the DCN and project in the direction of the cerebellar cortex. Presence of this so far undescribed DCN neuron population in the lateral nucleus suggests a direct inhibitory pathway from the DCN to the cerebellar cortex.

Cerebellum-related characteristics of Scn8a-mutant mice

Among ten sodium channel alpha-subunit genes mapped in human and mouse genomes, the SCN8A gene is primarily expressed in neurons and glia. Mice with two types of Scn8a null mutations--Scn8a ( med ) and Scn8a ( medTg )--live for only 21-24 days, but those with incomplete mutations-Scn8a ( medJ ) and Scn8a ( medJo )--and those with knockout of Scn8a only in cerebellar Purkinje cells live to adult age. We review here previous work on cerebellum and related regions of Scn8a mutant mice and include some newer immunohistochemical and microchemical results. The resurgent sodium current that underlies the repeated firing of Purkinje cells is reduced in Scn8a mutant and knockout mice. Purkinje cells of mutant mice have greatly reduced spontaneous activity, as do the analogous cartwheel cells of the dorsal cochlear nucleus. Up-regulation of GABA(A) receptors in regions to which Purkinje cells project may partially compensate for their decreased activity in the mutant mice. The somata of cerebellar Purkinje cells of Scn8a ( medJ ) and Scn8a ( medJo ) mice, as revealed by PEP-19 immunoreaction, are slightly smaller than normal, and their axons, especially in Scn8a ( medJo ) mice, sometimes show enlargements similar to those in other types of mutant mice. Density of GABA-like immunoreactivity is decreased in Purkinje somata and regions of termination in deep cerebellar and vestibular nuclei of Scn8a ( medJ ) mice, but measured GABA concentration is not significantly reduced in microdissected samples of these regions. The concentrations of taurine and glutamine are significantly increased in cerebellar-related regions of Scn8a ( medJ ) mice, possibly suggesting up-regulation of glial amino acid metabolism.

Compartmentation of the cerebellar nuclei of the mouse

The cerebellar nuclei integrate inhibitory input from Purkinje cells with excitatory input from mossy and climbing fiber collaterals and are the sole cerebellar output. Numerous studies have shown that the cerebellar cortex is highly compartmentalized into hundreds of genetically determined, reproducible topographic units--transverse zones and parasagittal stripes--that can be identified through the expression patterns of numerous molecules. The Purkinje cell stripes project to the cerebellar nuclei. However, there is no known commensurate topographic complexity in the cerebellar nuclei. Rather, conventional anatomical descriptions identify four major subdivisions--the medial, anterior and posterior interposed, and lateral nuclei--together with a few intranuclear subdivisions. To begin to address the apparent complexity gap, we have used a panel of antigens and transgenes to reveal a reproducible molecular heterogeneity in the mouse cerebellar nuclei. Based on the differential expression patterns, singly and in combination, a new cerebellar nuclear topographic map has been constructed. This reveals the subdivision of the cerebellar nuclei into at least 12 reproducible expression domains. We hypothesize that such heterogeneity is the counterpart of the zones and stripes of the cerebellar cortex.

Aspects of the neuroendocrine cerebellum: expression of secretogranin II, chromogranin A and chromogranin B in mouse cerebellar unipolar brush cells

Morphologically distinct neuron classes can be subdivided in sublineages by differential chemical phenotypes that correlate with functional diversity. Here we show by immunocytochemistry that chromogranin A (CgA) chromogranin B (CgB) and secretogranin II (SgII), the principal granins situated in neuronal secretory granules and large dense-core vesicles, are widely but differentially expressed in cells of the mouse cerebellum and terminals of cerebellar afferents. While CgA and CgB were nearly panneuronal, SgII was more restricted in distribution. The cells most intensely immunoreactive for SgII were a class of small, excitatory interneurons enriched in the granular layer of the vestibulocerebellum, the unipolar brush cells (UBCs), although larger neurons likely to be a subset of the Golgi-Lugaro-globular cell population were also distinctly immunopositive; by contrast, Purkinje cells and granule cells were, at best, faintly stained and, stellate, basket cells were unstained. SgII was also present in subsets of mossy fibers, climbing fibers and varicose fibers. Neurons in the cerebellar nuclei and inferior olive were distinctly positive for the three granins. Double-labeling with subset-specific cell class markers indicated that, while both CgA and CgB were present in most UBCs, SgII immunoreactivity was present in the calretinin (CR)-expressing subset, but lacked in metabotropic glutamate receptor 1alpha (mGluR1alpha)-expressing UBCs. Thus, we have identified an additional cell class marker, SgII, which serves to study subtype properties in the UBC population. The abundance of SgII in only one of the two known subsets of UBCs is remarkable, as its expression in other neurons of the cortex was moderate or altogether lacking. The data suggest that the CR-positive UBCs represent a unique neuroendocrine component of the mammalian cerebellar cortex, presumably endowed with transynaptically regulated autocrine or paracrine action/s. Because of the well-known organization of the cerebellar system, several of its neuron classes may represent valuable cellular models to analyze granin functions in situ, in acute slices and in dissociated cell and organotypic slice cultures.

Decreased GAD65 mRNA levels in select subpopulations of neurons in the cerebellar dentate nuclei in autism: an in situ hybridization study

The laterally positioned dentate nuclei lie in a key position in the cerebellum to receive input from Purkinje cells in the lateral cerebellar hemisphere participating in both motor and cognitive functions. Although neuropathology of the four cerebellar nuclei using Nissl staining has been qualitatively reported in children and adults with autism, surprisingly the dentate nuclei appeared less affected despite reported reductions in Purkinje cells in the posterolateral cerebellar hemisphere. To determine any underlying abnormalities in the critically important GABAergic system, the rate-limiting GABA synthesizing enzyme, glutamic acid decarboxylase (GAD) type 65 was measured via in situ hybridization histochemistry in dentate somata. GAD65 mRNA labeling revealed two distinct subpopulations of neurons in adult control and autism postmortem brains: small-sized cells (about 10-12 microm in diameter, presumed interneurons) and larger-sized neurons (about 18-20 microm in diameter, likely feedback to inferior olivary neurons). A mean 51% reduction in GAD65 mRNA levels was found in the larger labeled cells in the autistic group compared with the control group (P=0.009; independent t-test) but not in the smaller cell subpopulation. This suggests a disturbance in the intrinsic cerebellar circuitry in the autism group potentially interfering with the synchronous firing of inferior olivary neurons, and the timing of Purkinje cell firing and inputs to the dentate nuclei. Disturbances in critical neural substrates within these key circuits could disrupt afferents to motor and/or cognitive cerebral association areas in the autistic brain likely contributing to the marked behavioral consequences characteristic of autism.

GABAergic synaptic communication in the GABAergic and non-GABAergic cells in the deep cerebellar nuclei

The deep cerebellar nuclei (DCN) are the final integrative units of the cerebellar network. The strongest single afferent to the DCN is formed by GABAergic Purkinje neuron axons whose synapses constitute the majority of all synapses in the DCN, with their action strongly regulating the intrinsic activity of their target neurons. Although this is well established, it remains unclear whether all DCN cell groups receive a functionally similar inhibitory input. We previously characterized three types of mouse DCN neurons based on the expression of glutamic acid decarboxylase isoform 67 (GAD67), their active membrane properties and morphological features. Here we describe the GABAergic synapses in these cell groups and show that spontaneous GABAergic synaptic activity can be seen in all three cell types. Since the majority of DCN neurons fire action potentials spontaneously at high frequencies both in vivo and in vitro, we expected that spontaneous GABAergic synaptic activities mediated by intra-DCN synaptic connections could be uncovered by their sensitivity to TTX. However, TTX had little effect on spontaneous synaptic activity. It seems, therefore that functional GABAergic connectivity within the DCN is sparse and/or weak at least under our experimental conditions. Even though present in all cell types, the spontaneous GABAergic events showed significant differences between the cell types. The synaptic currents in GABAergic cells had lower amplitude, lower frequency and slower kinetics than those of non-GABAergic cells. These differences could not be sufficiently explained by considering only cell size differences or a differential GABA(A)-receptor alpha-subunit composition. Rather, the main differentiating factor appears to be the dendritic localization of GABAergic synapses in the GABAergic cells.

Glycinergic synaptic currents in the deep cerebellar nuclei

Despite evidence of local glycinergic circuits in the mature cerebellar nuclei the result of their activation remains unknown. Here, using whole cell recordings in rat cerebellar slices we demonstrated that after postnatal day 17 (>P17) glycinergic IPSCs can be readily evoked in large deep cerebellar nuclear neurons (DCNs), in the same way as in neonatal DCNs (P7-P10). Spontaneous glycinergic IPSCs were very rare but direct presynaptic depolarization by superfusion with elevated potassium concentration or application of 4-aminopyridine consistently evoked strychnine sensitive IPSCs. Glycinergic IPSCs showed fast kinetics in >P17 DCNs while were significantly slower in neonatal DCNs. Immuno-histochemical investigations using a specific marker for glycinergic fibers and terminals showed low density of immuno-fluorescent puncta, putative glycinergic boutons surrounding P18-P23 DCNs, in agreement with the rare spontaneous synaptic activity. But putative glycinergic boutons were present in critical areas for the control of spike generation. In contrast to adult and neonatal DCNs, glycinergic IPSCs could not be induced in juvenile DCNs (P13-P17) despite similar perisomatic immuno-staining pattern and expression of glycinergic receptors to >P17 DCNs. The latter results demonstrate substantial postnatal development of glycinergic cerebellar nuclei circuits. The cerebellum is involved in rapidly controlling ongoing movements. For that function, it is thought important the temporal and spatial precision of its output, which is carried to target structures by DCNs. The present study, by demonstrating fast glycinergic IPSCs in mature DCNs, points to the activation of glycinergic microcircuits as one of the possible mechanism involved in the spatio-temporal control of cerebellar output.

Vestibular ganglion neurons survive hair cell defects in jerker, shaker, and Varitint-waddler mutants and downregulate calretinin expression

Bipolar neurons (BNs) in the vestibular ganglion (VG) connect vestibular hair cells with the central nervous system (CNS). Disturbed function and cell loss in central vestibular target areas or in the vestibular periphery involve BNs either retro- or anterogradely. However, the impact of central vestibular disturbances or hair cell defects on the maintenance of BNs is poorly understood. In the present study the volume of the VG, the size and total number of BNs, and the number of BNs expressing the calcium-binding protein calretinin (Calr) were quantified stereologically in the cerebellar mutants purkinje cell degeneration (pcd/pcd), weaver (wv/wv), and Lurcher (Lc/+), and in the vestibular mutants jerker (je/je), shaker-1 (sh/sh), and Varitint-waddler (Va/+). In all the different mutant mice investigated the total number of BNs did not differ from that of wildtypes. In contrast, the number of Calr-positive BNs was significantly reduced in je/je (23%) and sh/sh (33%) mutants. Reduced cell size was apparent in sh/sh mutants and the volume of the VG significantly decreased in je/je mice. Calr was virtually absent from calyx endings in the vestibular periphery of je/je, sh/sh, and Va/+ mutants, whereas in wildtypes and cerebellar mutants many calyces displayed intense Calr labeling. These results imply that the survival of BNs is apparently unaffected by the peripheral and central target defects found in the mutants investigated. Whether the decrease in Calr expression may reflect biochemical adaptations in response to input disturbances or a specific loss of large BNs is discussed.

Morphological and electrophysiological properties of GABAergic and non-GABAergic cells in the deep cerebellar nuclei

The deep cerebellar nuclei (DCN) integrate inputs from the brain stem, the inferior olive, and the spinal cord with Purkinje cell output from cerebellar cortex and provide the major output of the cerebellum. Despite their crucial function in motor control and learning, the various populations of neurons in the DCN are poorly defined and characterized. Importantly, differences in electrophysiological properties between glutamatergic and GABAergic cells of the DCN have been largely elusive. Here, we used glutamate decarboxylase (GAD) 67-green fluorescent protein (GFP) knock-in mice to unambiguously identify GABAergic (GAD-positive) and non-GABAergic (GAD-negative, most likely glutamatergic) neurons of the DCN. Morphological analysis of DCN neurons patch-clamped with biocytin-containing electrodes revealed a significant overlap in the distributions of the soma sizes of GAD-positive and GAD-negative cells. Compared with GAD-negative DCN neurons, GAD-positive DCN neurons fire broader action potentials, display stronger frequency accommodation, and do not reach as high firing frequencies during depolarizing current injections. Furthermore, GAD-positive cells display slower spontaneous firing rates and have a more shallow frequency-to-current relationship than the GAD-negative cells but exhibit a longer-lasting rebound depolarization and associated spiking after a transient hyperpolarization. In contrast to the rather homogeneous population of GAD-positive cells, the GAD-negative cells were found to consist of two distinct populations as defined by cell size and electrophysiological features. We conclude that GABAergic DCN neurons are specialized to convey phasic spike rate information, whereas tonic spike rate is more faithfully relayed by the large non-GABAergic cells.

Morphological and immunohistochemical characterization of interneurons within the rat trigeminal motor nucleus

Three series of experiments were carried out to characterize interneurons located within the trigeminal motor nucleus of young rats aged 5-24 days. Cholera toxin injections were made bilaterally into the masseter and, sometimes, digastric muscles to label motoneurons. In the first set of experiments, thick slices were taken from the pontine brainstem and cholera toxin-positive and cholera toxin-negative neurons located inside the trigeminal motor nucleus were filled with biocytin through whole-cell recording patch electrodes. Positively identified motoneurons (cholera toxin+) of various shapes and sizes always had a thick, unbranched axon that entered the motor root following a tight zigzag course. Many cholera toxin-negative neurons were also classified as motoneurons after biocytin filling based on this particularity of their axon. These are probably either fusimotor motoneurons or motoneurons supplying other jaw muscles. The cholera toxin-negative neurons classified as interneurons differed markedly from motoneurons in that they had thin, usually branched axons that supplied the ipsilateral reticular region surrounding the trigeminal motor nucleus (peritrigeminal area), the main trigeminal sensory nucleus, the trigeminal mesencephalic nucleus, the medial reticular formation of both sides, and the contralateral medial peritrigeminal area. Most often, their dendrites were arranged in bipolar arbors that extended beyond the borders of the trigeminal motor nucleus into the peritrigeminal area. Immunohistochemistry against glutamate, GABA and glycine was used to further document the nature and distribution of putative interneurons. Immunoreactive neurons were uniformly distributed throughout the rostro-caudal extent of the trigeminal motor nucleus. Their concentration seemed greater toward the edges of the nucleus and they were scarce in the digastric motoneuron pool. Glutamate- outnumbered GABA- and glycine-immunoreactive neurons. There was no clear segregation between the three populations. In the final experiment, 1,1'-dioctadecyl-3,3,3',3'-tetra-methylindocarbocyanine perchlorate crystals were inserted into one trigeminal motor nucleus in thick slices and allowed to diffuse for several weeks. This procedure marked commissural fibers and interneurons in the contralateral trigeminal motor nucleus. Together these results conclusively support the existence of interneurons in the trigeminal motor nucleus.

GABA, in some cases together with glycine, is used as the inhibitory transmitter by pump cells in the Hering-Breuer reflex pathway of the rat

The Hering-Breuer reflex is one of the fundamental respiratory reflexes and is mediated by second-order relay neurons of the slowly adapting lung stretch receptors. These neurons, which are called pump cells, are located in the nucleus tractus solitarii and include a population of inhibitory neurons. We aimed to determine which transmitter, GABA or glycine, the inhibitory pump cells use. In addition, we examined whether or not second-order relay neurons of the rapidly-adapting lung stretch receptors (RAR-cells), whose excitatory or inhibitory nature is not known, use these inhibitory neurotransmitters. In Nembutal-anesthetized, neuromuscularly blocked and artificially ventilated rats, we labeled pump cells (n=33) and RAR-cells (n=26) with Neurobiotin and processed the tissues for detection of mRNA encoding either glutamic acid decarboxylase isoform 67 (GAD67) or glycine transporter 2 (GLYT2) using in situ hybridization. The pump cells were located in the interstitial nucleus and its vicinity and the RAR-cells in the commissural subnucleus. The majority (64%) of the pump cells examined for GAD67 mRNA and many (26%) of the pump cells examined for GLYT2 mRNA expressed respective mRNAs. Of the eight pump cells in which both mRNAs were double-detected, three expressed both mRNAs and one expressed GAD67 mRNA but not GLYT2 mRNA, the other four expressing neither mRNAs. On the other hand, RAR-cells expressed neither GAD67 mRNA nor GLYT2 mRNA. The results suggest that the inhibitory pump cells are basically GABAergic and some of them may corelease GABA and glycine, and that RAR-cells are neither GABAergic nor glycinergic. These findings expand our understanding of the networks of lung receptor-mediated reflexes including the Hering-Breuer reflex.

Persistent changes in the intrinsic excitability of rat deep cerebellar nuclear neurones induced by EPSP or IPSP bursts

The deep cerebellar nuclei (DCN) are the major output of the cerebellum, and have been proposed as a site of memory storage for certain forms of motor learning. Microelectrode and whole-cell patch recordings were performed on DCN neurones in acute slices of juvenile rat cerebellum. DCN neurones display tonic and bursting basal firing patterns. In tonically firing neurones, a stimulus consisting of EPSP bursts produced a brief increase in dendritic Ca(2+) concentration and a persistent increase in the number of spikes elicited by a depolarizing test pulse, along with a decrease in spike threshold. In intrinsically bursting DCN neurones, EPSP bursts induced an increase in the number of depolarization-evoked spikes in some neurones, but in others produced a change to a more tonic firing pattern. Application of IPSP bursts evoked a large number of rebound spikes and an associated dendritic Ca(2+) transient, which also produced a persistent increase in the number of spikes elicited by a test pulse. Intracellular perfusion of the Ca(2+) chelator BAPTA prevented the increase in intrinsic excitability. Thus, rapid changes in intrinsic excitability in the DCN may be driven by bursts of both EPSPs and IPSPs, and may result in persistent changes to both firing frequency and pattern.

Overall distribution of GLYT2 mRNA-containing versus GAD67 mRNA-containing neurons and colocalization of both mRNAs in midbrain, pons, and cerebellum in rats

We aimed to clarify the overall distribution of glycinergic neurons in the midbrain, pons, and cerebellum in rats, using in situ hybridization for mRNA encoding glycine transporter 2 (GLYT2), which reliably detects glycinergic cell bodies. We combined this method with in situ hybridization for mRNA encoding glutamic acid decarboxylase isoform 67 (GAD67), and have presented for the first time global and detailed views of the distribution of glycinergic neurons in relation to GABAergic neurons. In addition to this single-detection study, we performed double-detection of GLYT2 mRNA and GAD67 mRNA to determine the distribution of neurons co-expressing these mRNAs. We have shown that many areas of the brainstem and cerebellum, not only areas where previous immunohistochemical studies have specified, involve double-labeled neurons with GLYT2 and GAD67 mRNAs. In particular, when lightly labeled GLYT2 mRNA-positive neurons were distributed within the area of GAD67 mRNA-positive neurons, almost all such GLYT2 mRNA-positive neurons were GAD67 mRNA-positive. Areas or neuron groups expressing exclusively GLYT2 mRNA or GAD67 mRNA were rather limited, such as the superior colliculus, nucleus of the trapezoid body, and Purkinje cells. The present study suggests that the corelease of glycine and GABA from single neurons is more widespread than has been reported.

Glycine receptors and glycinergic synaptic transmission in the deep cerebellar nuclei of the rat: a patch-clamp study

To clarify possible glycinergic transmission in the cerebellum, principal neurons in deep cerebellar nuclei (DCN) of sliced cerebella (200 microm in thickness) from rats (aged 2-14 days) were studied using whole cell patch-clamp techniques. When glycine (100 microM) was applied to the DCN neurons from a "Y tube," large outward currents were induced (average peak amplitude of about 600 pA at -40 mV). The currents were blocked by strychnine (1 microM) and showed a reversal potential of -62 mV, which was approximately the estimated Cl- equilibrium potential. The dose-response relation of the currents showed an apparent dissociation constant of 170 microM for glycine and Hill coefficient of 1.6. In the presence of 6-cyano-7-nitroquinoziline-2, 3-dione (CNQX), d-(-)-2-amino-5-phosphonovaleric acid (APV) and bicuculline, which antagonize amino-3-hydroxy-5-methyl-isoxazol-propionate (APMA), N-methyl-d-aspartate (NMDA), and GABAA receptors, respectively, postsynaptic currents sensitive to strychnine (1 microM) were induced in DCN neurons by external perfusion of 20 mM K+ saline. Electrical stimulation of surrounding tissues in DCN evoked definite inhibitory postsynaptic currents (IPSCs) in these neurons. The IPSCs had a reversal potential of -62 mV and showed sensitivities to strychnine and tetrodotoxin. Thus this study has revealed that strychnine-sensitive glycine receptors are expressed in neurons of the DCN of rats and that glycinergic transmission mediated by these receptors is functional in these neurons from stages immediately after birth. The glycinergic innervations are presumably supplied by small interneurons located in the DCN.

Morphological classification of the rat lateral cerebellar nuclear neurons by principal component analysis

The deep cerebellar nuclei (DCN) constitute the major structures by which the cerebellum forwards its output to the rest of the brain. Although the connectivity of the DCN has been well studied, little is known about the interface-the neurons' soma and dendrites-between the DCN's inputs and outputs. We therefore decided to analyze the neurons' somatic and dendritic morphology by applying a multivariate approach (principal component analysis; PCA), in order to define morphological groups possibly related to distinct positions in the nuclear microcircuitry. The PCA was based on intracellularly stained neurons from the rat's lateral DCN and on 19 parameters that described the neurons' morphology. The PCA yielded two principal components that accounted for 46% of the variance. The first component, correlated with soma size, separated the majority of neurons (type I) from a population of small neurons (type II). The second component showed negative correlation with larger cells with more numerous primary dendrites and a more multipolar appearance (type Ia) and positive correlation with smaller neurons with asymmetric dendritic fields and tufted dendrites (type Ib). The preponderance of small somata in our type Ib neurons suggests that these neurons probably correspond to the inferior olive projection neurons. In summary, our results are in agreement with previous classifications, which distinguished projection neurons (type I) from local neurons (type II); furthermore, our results point to a hitherto undescribed dendritic morphological difference in the projection neurons. The latter may be important for understanding the phylogenetic changes seen in the mammalian lateral cerebellar nucleus.

Quantitative organization of neurotransmitters in the deep cerebellar nuclei of the Lurcher mutant

The Lurcher mutant mouse is characterized by a primary selective loss of Purkinje cells, leading to the near total apoptotic death of these neurons. In contrast to the subsequent massive secondary degeneration of the granule cells and the inferior olivary neurons, only mild degeneration occurs in the deep cerebellar nuclei (DCN). However, it is not known to what extent the different populations of DCN neurons-glutamatergic principal projection neurons, gamma-aminobutyric acid (GABA)-ergic inferior olivary projection neurons, and glycinergic neurons-are affected in their neurotransmitter composition. To answer this question we studied the neurotransmitter contents (glutamate, GABA, and glycine) of DCN neurons and the size of synaptic boutons immunohistochemically on serial semithin sections in both Lurcher and wild-type mice. Applying the physical dissector counting method, our results confirmed the mild degeneration (a reduction by 20%) of large glutamatergic neurons and a more pronounced degeneration of GABAergic (by 42%) and glycinergic neurons (by 45%). On the other hand, an analysis of neurons colabeled for both GABA and glycine, revealed that this specific colabeling increased in the Lurcher mutant (by 40%). In addition, both the GABA-immunolabeled (IL) (by 56%) and the glycine-IL (by 45%) synaptic boutons showed an increase in diameter in the mutant. The density of these boutons showed a decrease of 30% each. In summary, the increase in the number of neurons colabeled for GABA and glycine, together with the increase in the size of the inhibitory synaptic boutons, could help in providing the minimum inhibition needed to maintain a residual "cerebellar" functionality in the Lurcher DCN.

Distribution of postsynaptic GABA(A) receptor aggregates in the deep cerebellar nuclei of normal and mutant mice

In the central nervous system, the aggregation of receptors is crucial for synapse formation and function. To study the role of presynaptic terminals in the maintenance of postsynaptic specializations, we analyzed the synaptic contacts between Purkinje cells and neurons of the deep cerebellar nuclei in two in vivo models: the Lurcher and Purkinje cell-deficient (PCD) mutant mice. These mutants lose their Purkinje cells at different postnatal stages. By using confocal scanner microscopy and immunohistochemistry, we studied the distribution of the alpha subunit of the gamma-aminobutyric acid (GABA)(A) receptor (GABA(A)Ralpha1) and gephyrin, one of its anchoring proteins, in relation to the distribution of presynaptic markers, glutamic acid decarboxylase (GAD), or synaptophysin. In Lurcher the distribution of GABA(A) receptor aggregates on the membrane of postsynaptic neurons was not affected by the important loss of GAD-positive terminals, whereas in PCD, the number of large GABA(A) receptor aggregates increased. In both mutants the number of aggregates of gephyrin decreased. Most of these remaining aggregates were clustered to form groups, some of which were in front of GAD-positive terminals. This study shows, for the first time, the localization of GABA(A)R alpha 1 in Lurcher and PCD mutant mice. It clearly establishes that GABA(A)R alpha 1 and gephyrin are differentially affected by deafferentation. Because the receptor aggregates are maintained while the gephyrin aggregates are lost, as a result some receptor aggregates are not associated with any gephyrin. These two postsynaptic components appeared to be regulated by different mechanisms.

Cerebellar input to magnocellular neurons in the red nucleus of the mouse: synaptic analysis in horizontal brain slices incorporating cerebello-rubral pathways

We studied the synaptic input from the nucleus interpositus of the cerebellum to the magnocellular division of the red nucleus (RNm) in the mouse using combined electrophysiological and neuroanatomical methods. Whole-cell patch-clamp recordings were made from brain slices (125-150 microm) cut in a horizontal plane oriented to pass through both red nucleus and nucleus interpositus. Large cells that were visually selected and patched were injected with Lucifer Yellow and identified as RNm neurons. Using anterograde tracing from nucleus interpositus in vitro, we examined the course of interposito-rubral axons which are dispersed in the superior cerebellar peduncle. In vitro monosynaptic responses in RNm were elicited by an electrode array placed contralaterally in this pathway but near the midline. Mixed excitatory post-synaptic potentials (EPSPs)/inhibitory post-synaptic potentials (IPSPs) were observed in 48 RNm neurons. Excitatory components of the evoked potentials were studied after blocking inhibitory components with picrotoxin (100 microM) and strychnine (5 microM). All RNm neurons examined continued to show monosynaptic EPSPs after non-N-methyl-D-aspartate (NMDA) glutamate receptor components were blocked with 10 microM 6,7-dinitroquinoxaline-2,3-dione or 5 microM 2,3-dihydro-6-nitro-7-sulfamoyl-benzo(f)-quinoxaline (NBQX; n=12). The residual potentials were identified as NMDA receptor components since they (i) were blocked by the addition of the NMDA receptor antagonist, D,L-2-amino-5-phosphonovaleric acid (APV), (ii) were voltage-dependent, and (iii) were enhanced by Mg(2+) removal. Inhibitory components of the evoked potentials were studied after blocking excitatory components with NBQX and APV. Under these conditions, all RNm neurons studied continued to show IPSPs. Blockade of GABA(A) receptors reduced but did not eliminate the IPSPs. These were eliminated when GABA(A) receptor blockade was combined with strychnine to eliminate glycine components of the IPSPs. Thus, IPSPs evoked by midline stimulation of the superior cerebellar peduncle, while blocking alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and NMDA receptors, raise the possibility of direct inhibitory inputs to RNm from the cerebellum. In summary we propose that the special properties of the NMDA receptor components are considered important for the generation of RNm motor commands: their slow time course will contribute a steady driving force for sustained discharge and their voltage dependency will facilitate abrupt transitions from a resting state of quiescence to an active state of intense motor command generation.

Horizontal vestibuloocular reflex (VOR) head velocity estimation in Purkinje cell degeneration (pcd/pcd) mutant mice

The horizontal vestibuloocular reflex (VOR) of Purkinje cell degeneration (pcd/pcd) mutant mice, which lack a functional cerebellar cortex, was compared in darkness to that of wild-type animals during constant velocity yaw rotations about an earth-horizontal axis and during sinusoidal yaw rotations about an earth-vertical axis. Both wild-type and pcd/pcd mice showed a compensatory average VOR eye velocity, or bias, during constant velocity horizontal axis rotations, evidence of central neural processing of otolith afferent signals to create a signal proportional to head angular velocity. Eye velocity bias was greater in pcd/pcd mice than in wild-type mice at a low rotational velocity (32 degrees/s), but less at higher velocities (128 and 200 degrees/s). Lesion of the medial nodulus severely attenuated eye velocity bias in two wild-type mice, without attenuating VOR during sinusoidal vertical axis yaw rotations at 0.2 Hz. These results show that while head velocity estimation in mice, as in primates, depends on the cerebellum, pcd/pcd mutant mice develop velocity estimation without a functional cerebellar cortex. We conclude that neural circuits that exclude cerebellar cortex are capable of the signal processing necessary for head angular velocity estimation, but that these circuits are insufficient for normal estimation at high velocities.

Histochemical and immunocytochemical investigations of the marginal nuclei in the spinal cord of pigeons (Columba livia)

In birds there are segmentally organized marginal nuclei at the lateral or ventrolateral border of the spinal cord. In most regions of the spinal cord these nuclei are within the outline of the cord. However, in the lumbosacral region they form accessory lobes protruding into the vertebral canal. Histochemical and immunocytochemical investigations were performed to study the neurochemical features of the marginal nuclei of the pigeon. Despite histological differences (only accessory lobe neurons are embedded in glia-derived glycogen cells), there was no difference in the chemical neuroanatomy of the two types of marginal nuclei. These nuclei contained cholinergic neurons and there was also evidence for a cholinergic innervation. NADPH-diaphorase activity, which is considered to indicate nitric oxide synthesis, was faint in marginal neurons. No serotonin immunoreactivity was found. However, all neurons showed immunoreactivity to glutamate and glycine, and some were immunoreactive to gamma-aminobutyric acid (GABA). A GABAergic innervation of non-GABAergic neurons could also be demonstrated. The lack of difference in the chemical neuroanatomical features between cervical marginal nuclei and lumbosacral accessory lobes suggests a similar origin of all marginal neurons. A comparison with the chemical neuroanatomy of marginal neurons in other vertebrates shows both similarities and differences.

Mutant mice as a model for cerebellar ataxia

Not later than two synapses after their arrival in the cerebellar cortex all excitatory afferent signals are subsequently transformed into inhibitory ones. Guaranteed by the exceedingly ordered and stereotyped synaptic arrangement of its cellular elements, the cerebellar cortex transmits this inhibitory result of cerebellar integration exclusively via Purkinje cells (PCs) in a precise temporal succession directly onto the target neurons of the deep cerebellar and vestibular nuclei. Thus the cerebellar cortex seems to produce a temporal pattern of inhibitory influence on these target neurons that modifies their excitatory action in such a way that an activation of muscle fibers occurs which progressively integrates the intended motion into the actual condition of the motoric inventory. In consequence, disturbances that affect this cerebellar inhibition will cause uncoordinated, decomposed and ataxic movements, commonly referred to as cerebellar ataxia. Electrophysiological investigations using different cerebellar mouse mutants have shown that alterations in the cerebellar inhibitory input in the target nuclei lead to diverse neuronal responses and to different consequences for the behavioural phenotype. A dependence between the reconstitution of inhibition and the behavioural outcome seems to exist. Obviously two different basic mechanisms are responsible for these observations: (1) ineffective inhibition on target neurons by surviving PCs; and (2) enhancement of intranuclear inhibition in the deep cerebellar and vestibular nuclei. Which of the two strategies evolves is dependent upon the composition of the residual cell types in the cerebellum and on the degree of PC input loss in a given area of the target nuclei. Motor behaviour seems to deteriorate under the first of these mechanisms whereas it may benefit from the second. This is substantiated by stereotaxic removal of the remaining PC input, which eliminates the influence of the first mechanism and is able to induce the second strategy. As a consequence, motor performance improves considerably. In this review, results leading to the above conclusions are presented and links forged to human cerebellar diseases.

Purkinje cell degeneration and control mice: responses of single units in the dorsal cochlear nucleus and the acoustic startle response

The cartwheel cell is the most numerous inhibitory interneuron of the dorsal cochlear nucleus (DCN). It is expected to be an important determinant of DCN function. To assess the contribution of the cartwheel cell, we examined the discharge characteristics of DCN neurons and behavioral measures in the Purkinje cell degeneration (pcd) mice, which lack cartwheel cells, and compared them to those of the control mice. Distortion product otoacoustic emissions and auditory brainstem-evoked response thresholds were similar between the two groups. Extracellularly recorded DCN single units in ketamine/xylazine-anesthetized mice were classified according to post-stimulus time histogram (PSTH) and excitatory-inhibitory response area (EI-area) schemes. PSTHs recorded in mouse DCN included chopper, pauser/buildup, onset, inhibited and nondescript types. EI-areas recorded included Types I, II, III, I/III, IV and V. There were no significant differences in the proportions of various unit types between the pcd and control mice. The pcd units had slightly lower thresholds to characteristic frequency tones; however, they had spontaneous rates, thresholds to noise, and maximum driven rates to noise that were similar to those of the control units. Pcd mice had smaller startle amplitudes, but startle latency, prepulse inhibition/augmentation and facilitation by a background tone were comparable between the two groups. From these results, we conclude that DCN function in response to relatively simple acoustic stimuli is minimally affected by the absence of the cartwheel cells. Future studies employing more complex and/or multimodal stimuli should help assess the role of the cartwheel cells.

Hemisynaptic distribution patterns of presenilins and beta-APP isoforms in the rodent cerebellum and hippocampus

Healthy brain neurons co-express Alzheimer's disease (AD) related proteins presenilins (PS) and beta-amyloid precursor protein (beta-APP). Deposition of beta-amyloid and PS in the senile plaques of AD brain and their ability to interact in vitro suggest that AD pathology could arise from a defect in the physiological interactions between beta-APP and PS within and/or between neurons. The present study compares the immunocytochemical distribution of PS (1 and 2) and beta-APP major isoforms (695 and 751/770) in the synapses of the cerebellum and hippocampus of the adult rat and mouse. In the cerebellar cortex of both species, the four molecules are immunodetected in the presynaptic or the postsynaptic compartments of synapses, suggesting that they are involved in interneuronal relationships. In contrast, PS and beta-APP are postsynaptic in almost all the immunoreactive synapses of the hippocampus. The different distribution patterns of these proteins in cerebellar and hippocampal synapses may reflect specific physiological differences, responsible for differential vulnerability of neurons to AD synaptic pathology. Defective interactions between beta-APP and PS at the synapses could impede the synaptic functions of beta-APP, inducing the selective loss of synapses that accounts for cognitive impairment in AD.

Co-localization of glutamate, choline acetyltransferase and glycine in the mammalian vestibular ganglion and periphery

Glutamate (Glu) is considered to be the main transmitter at the central synapses of primary vestibular afferents (PVA) and glycine (Gly) is assumed to play a modulatory role. In the vestibular periphery a transmitter role for acetylcholine (ACh) has been attributed chiefly to vestibular efferents (VE), however only a subset of VE neurons displays immunoreactivity (ir) for choline acetyltransferase (ChAT) and acetylcholine esterase (AChE). Controversial results exist on the presence of these two enzymes in PVA. In this study the presence of Glu, ChAT, Gly and their co-localization in the vestibular ganglia (VG) and end organs of mouse, rat, guinea pig and squirrel monkey were investigated. In the VG all bipolar neurons display strong Glu-ir and the majority of cells show a graded ChAT-ir and Gly-ir in all species examined. ChAT and Gly are present in highly overlapping neuronal populations and with a similar gradation. In the end organs ChAT and Gly are again co-localized in the same sets of fibers and endings. In conclusion, in the vestibular ganglion and end organs ChAT appears also to be present in primary afferents rather than being restricted to efferent processes. ChAT in primary afferents might indicate a modulatory or co-transmitter function of acetylcholine.

Vermectomy enhances parvalbumin expression and improves motor performance in weaver mutant mice: an animal model for cerebellar ataxia

In the Weaver mutant mouse (wv/wv), an animal model for hereditary cerebellar ataxia, electrophysiological experiments have revealed a disorganized output of cerebellar Purkinje cells (the latter using GABA as an inhibitory transmitter) which, by a cascade of mechanisms, was thought to be the cause of the poor motor abilities. In Purkinje cell degeneration mice (pcd/pcd) lacking nearly all Purkinje cells and displaying milder motor deficiencies than wv, in comparison to wild-type mice, a strong increase in parvalbumin- and (co-localized with parvalbumin) glycine-immunopositive somata in the deep cerebellar and vestibular nuclei has recently been found. It was therefore intriguing to investigate whether motor performance in weaver mutants could be ameliorated by applying cerebellar lesions to eliminate the faulty output and to look for a change in transmitter weighting, indicated by a strong increase in parvalbumin-positive somata in areas (the respective target areas) which were formerly devoid of it. Ten Weaver mutants were subjected to cerebellar lesions. After removal of the vermis a total abolition of tremor, a definite improvement in the balance of affected body parts, an increase in locomotor activity when tested in an open-field matrix, and a strong increase in parvalbumin expression in Weaver mutant deep cerebellar and vestibular nuclei in comparison to wild-types have indeed been found. Increase in motor activity (or explorative behaviour) has been placed in relation to learning mechanisms. The increase in parvalbumin expression and the observed improvement in motor abilities and mechanisms probably related to learning underline the hypothesis that any change in the physiological equilibrium of the brain function by removal of input or output related to an assembly of nerve cells leads to a cascade of changes at the transmitter and neuronal level in near or distant connected brain structures.

Dependence of parvalbumin expression on Purkinje cell input in the deep cerebellar nuclei

A complete loss of Purkinje cell (PC) input leads to an increase in expression of the calcium-binding protein parvalbumin (Parv) in neurons of the deep cerebellar nuclei (DCN) of PC degeneration (pcd) mutants. To verify this apparent dependence of Parv expression on PC input in the DCN, the patterns of expression in five other cerebellar mutants (weaver, staggerer, leaner, nervous, and lurcher) with differing grades and chronologies of PC loss were compared. Degree and time course of PC loss and the subsequent denervation of DCN neurons were monitored by using Calbindin D-28k (Calb) immunocytochemistry. Similar to pcd mice, somatal Parv in lurcher mutants increased massively throughout the cerebellar nuclei. In nervous and leaner mutants, somatal Parv was restricted to almost completely denervated nuclear areas, whereas areas with appreciable remnants of PC input were spared. The first appearance of Parv+ somata was closely correlated with the time course of PC degeneration--postnatal day 19 in lurcher mutants and postnatal day 23 in nervous mutants. In staggerer mice, neurons immunopositive for Parv as well as for Calb were present in outer DCN areas, likely representing ectopic PCs rather than DCN neurons. No Parv+ DCN somata were found in weaver mutants at any time. In conclusion, increased expression of somatal Parv in DCN neurons is not restricted to the specific histopathology in pcd mutants but is a common mechanism that is dependent on the topography and severeness of PC-input loss. The functional significance of the Parv increase and its possible contribution to the degree of motor disability among the different mutants are discussed.