Plasticity of GABAergic terminals in Deiters' nucleus of weaver mutant and normal mice: a quantitative light microscopic study

BDNF Influence on Adult Terminal Axon Sprouting after Partial Deafferentation

BDNF is a neurotrophin family member implicated in many different neuronal functions, from neuronal survival during development to synaptic plasticity associated with processes of learning and memory. Its presence in the oculomotor system has previously been demonstrated, as it regulates afferent composition of extraocular motoneurons and their firing pattern. Moreover, BDNF expression increases after extraocular motoneuron partial deafferentation, in parallel with terminal axon sprouting from the remaining axons. To elucidate whether BDNF could play an active role in this process, we performed partial deafferentation of the medial rectus motoneurons through transection of one of the two main afferents, that is, the ascending tract of Deiters, and injected BDNF into the motoneuron target muscle, the medial rectus. Furthermore, to check whether BDNF could stimulate axon sprouting without lesions, we performed the same experiment without any lesions. Axon terminal sprouting was assessed by calretinin immunostaining, which specifically labels the remaining afferent system on medial rectus motoneurons, the abducens internuclear neurons. The results presented herein show that exogenous BDNF stimulated terminal axon growth, allowing the total recovery of synaptic coverage around the motoneuron somata. Moreover, calretinin staining in the neuropil exceeded that present in the control situation. Thus, BDNF could also stimulate axonal sprouting in the neuropil of intact animals. These results point to an active role of BDNF in plastic adaptations that take place after partial deafferentation.

Is the inferior olive central to essential tremor? Yes

We consider the question whether the inferior olive (IO) is required for essential tremor (ET). Much evidence shows that the olivocerebellar system is the main system capable of generating the widespread synchronous oscillatory Purkinje cell (PC) complex spike (CS) activity across the cerebellar cortex that would be capable of generating the type of bursting cerebellar output from the deep cerebellar nuclei (DCN) that could underlie tremor. Normally, synchronous CS activity primarily reflects the effective electrical coupling of IO neurons by gap junctions, and traditionally, ET research has focused on the hypothesis of increased coupling of IO neurons as the cause of hypersynchronous CS activity underlying tremor. However, recent pathology studies of brains from humans with ET and evidence from mutant mice, particularly the hotfoot17 mouse, that largely replicate the pathology of ET, suggest that the abnormal innervation of multiple Purkinje cells (PCs) by climbing fibers (Cfs) is related to tremor. In addition, ET brains show partial PC loss and axon terminal sprouting by surviving PCs. This may provide another mechanism for tremor. It is proposed that in ET, these three mechanisms may promote tremor. They all involve hypersynchronous DCN activity and an intact IO, but the level at which excessive synchronization occurs may be at the IO level (from abnormal afferent activity to this nucleus), the PC level (via aberrant Cfs), or the DCN level (via terminal PC collateral innervation).

Increased Purkinje Cell Complex Spike and Deep Cerebellar Nucleus Synchrony as a Potential Basis for Syndromic Essential Tremor. A Review and Synthesis of the Literature

We review advances in understanding Purkinje cell (PC) complex spike (CS) physiology that suggest increased CS synchrony underlies syndromic essential tremor (ET). We searched PubMed for papers describing factors that affect CS synchrony or cerebellar circuits potentially related to tremor. Inferior olivary (IO) neurons are electrically coupled, with the degree of coupling controlled by excitatory and GABAergic inputs. Clusters of coupled IO neurons synchronize CSs within parasagittal bands via climbing fibers (Cfs). When motor cortex is stimulated in rats at varying frequencies, whisker movement occurs at ~10 Hz, correlated with synchronous CSs, indicating that the IO/CS oscillatory rhythm gates movement frequency. Intra-IO injection of the GABA_A receptor antagonist picrotoxin increases CS synchrony, increases whisker movement amplitude, and induces tremor. Harmaline and 5-HT_2a receptor activation also increase IO coupling and CS synchrony and induce tremor. The hotfoot17 mouse displays features found in ET brains, including cerebellar GluRδ2 deficiency and abnormal PC Cf innervation, with IO- and PC-dependent cerebellar oscillations and tremor likely due to enhanced CS synchrony. Heightened coupling within the IO oscillator leads, through its dynamic control of CS synchrony, to increased movement amplitude and, when sufficiently intense, action tremor. Increased CS synchrony secondary to aberrant Cf innervation of multiple PCs likely also underlies hotfoot17 tremor. Deep cerebellar nucleus (DCN) hypersynchrony may occur secondary to increased CS synchrony but might also occur from PC axonal terminal sprouting during partial PC loss. Through these combined mechanisms, increased CS/DCN synchrony may plausibly underlie syndromic ET.

Short-term plasticity after partial deafferentation in the oculomotor system

Medial rectus motoneurons are innervated by two main pontine inputs. The specific function of each of these two inputs remains to be fully understood. Indeed, selective partial deafferentation of medial rectus motoneurons, performed by the lesion of either the vestibular or the abducens input, initially induces similar changes in motoneuronal discharge. However, at longer time periods, the responses to both lesions are dissimilar. Alterations on eye movements and motoneuronal discharge induced by vestibular input transection recover completely 2 months post-lesion, whereas changes induced by abducens internuclear lesion are more drastic and permanent. Functional recovery could be due to some kind of plastic process, such as reactive synaptogenesis, developed by the remaining intact input, which would occupy the vacant synaptic spaces left after lesion. Herein, by means of confocal microscopy, immunocytochemistry and retrograde labeling, we attempt to elucidate the possible plastic processes that take place after partial deafferentation of medial rectus motoneuron. 48 h post-injury, both vestibular and abducens internuclear lesions produced a reduced synaptic coverage on these motoneurons. However, 96 h after vestibular lesion, there was a partial recovery in the number of synaptic contacts. This suggests that there was reactive synaptogenesis. This recovery was preceded by an increase in somatic neurotrophin content, suggesting a role of these molecules in presynaptic axonal sprouting. The rise in synaptic coverage might be due to terminal sprouting performed by the remaining main input, i.e., abducens internuclear neurons. The present results may improve the understanding of this apparently redundant input system.

Effects of Selective Deafferentation on the Discharge Characteristics of Medial Rectus Motoneurons

Medial rectus motoneurons receive two main pontine inputs: abducens internuclear neurons, whose axons course through the medial longitudinal fasciculus (MLF), and neurons in the lateral vestibular nucleus, whose axons project through the ascending tract of Deiters (ATD). Abducens internuclear neurons are responsible for conjugate gaze in the horizontal plane, whereas ATD neurons provide medial rectus motoneurons with a vestibular input comprising mainly head velocity. To reveal the relative contribution of each input to the oculomotor physiology, single-unit recordings from medial rectus motoneurons were obtained in the control situation and after selective deafferentation from cats with unilateral transection of either the MLF or the ATD. Both MLF and ATD transection produced similar short-term alterations in medial rectus motoneuron firing pattern, which were more drastic in MLF of animals. However, long-term recordings revealed important differences between the two types of lesion. Thus, while the effects of the MLF section were permanent, 2 months after ATD lesioning all motoneuronal firing parameters were similar to the control. These findings indicated a more relevant role of the MLF pathway in driving motoneuronal firing and evidenced compensatory mechanisms following the ATD lesion. Confocal immunocytochemistry revealed that MLF transection produced also a higher loss of synaptic boutons, mainly at the dendritic level. Moreover, 2 months after ATD transection, we observed an increase in synaptic coverage around motoneuron cell bodies compared with short-term data, which is indicative of a synaptogenic compensatory mechanism of the abducens internuclear pathway that could lead to the observed firing and morphological recovery.SIGNIFICANCE STATEMENT Eye movements rely on multiple neuronal circuits for appropriate performance. The abducens internuclear pathway through the medial longitudinal fascicle (MLF) and the vestibular neurons through the ascending tract of Deiters (ATD) are a dual system that supports the firing of medial rectus motoneurons. We report the effect of sectioning the MLF or the ATD pathway on the firing of medial rectus motoneurons, as well as the plastic mechanisms by which one input compensates for the lack of the other. This work shows that while the effects of MLF transection are permanent, the ATD section produces transitory effects. A mechanism based on axonal sprouting and occupancy of the vacant synaptic space due to deafferentation is the base for the mechanism of compensation on the medial rectus motoneuron.

Pogo: a novel spontaneous ataxic mutant mouse

The Pogo (pogo/pogo) mouse is a naturally occurring neurological mutant from a Korean wild-type mouse characterized by loss of balance and motor coordination due to dysfunction of the cerebellum. The Pogo mutation is believed to be an allele of P/Q-type calcium channel mutants such as tottering, leaner, and rolling mouse Nagoya. These mutants have been served as mouse models for a group of neurodegenerative diseases. The overall aim of this minireview is to summarize our current understanding of the ataxic Pogo mouse. To address this issue, we first describe the discovery of Pogo mouse and its morphological and behavioral defects. Then, we focus on the abnormal expression of several molecules in the Pogo cerebellum, including tyrosine hydroxylase, glutamate, corticotrophin-releasing factor, and 5-hydroxytryptamine. Much of this review is concerned with the functional implications of these ectopic molecules in the Pogo cerebellum.

Histaminergic and glycinergic modulation of GABA release in the vestibular nuclei of normal and labyrinthectomised rats

Vestibular compensation (the behavioural recovery that follows unilateral vestibular de-afferentation), is facilitated by histamine, and is associated with increased central histamine release and alterations in histamine H(3) receptor expression in the vestibular nuclei. However, little is known of the effects of histamine on neurotransmission in the vestibular nuclei, and the mechanisms by which histamine may influence compensation are unclear. Here we examined the modulatory effects of histaminergic agents on the release of amino acid neurotransmitters in slices of the medial vestibular nucleus (MVN) prepared from normal and labyrinthectomised rats. The release of GABA, but not glutamate, glycine or aspartate, was robustly and reproducibly evoked by a high-K(+) stimulus applied to normal MVN slices. Histamine inhibited the evoked release of GABA, both through a direct action on presynaptic H(3) receptors (presumably located on GABAergic terminals), and through a novel, indirect pathway that involved the increased release of glycine by activation of postsynaptic H(1)/H(2) receptors (presumably on glycinergic neurons). After unilateral labyrinthectomy (UL), the direct H(3) receptor-mediated inhibition of GABA release was profoundly downregulated in both ipsi-lesional and contra-lesional MVNs. This effect appeared within 25 h post-UL and persisted for at least 3 weeks post-UL. In addition, at 25 h post-UL the indirect glycinergic pathway caused a marked suppression of GABA release in the contra-lesional but not ipsi-lesional MVN, which was overcome by strychnine. Stimulation of histamine H(3) receptors at 25 h post-UL restored contra-lesional GABA release to normal, suggesting that acutely after UL H(3) receptors may strongly modulate glycinergic and GABAergic neurotransmission in the MVN. These findings are the first to demonstrate the modulatory actions of the histaminergic system on neurotransmission in the vestibular nuclei, and the changes that occur during vestibular system plasticity. During vestibular compensation, histaminergic modulation of glycine and GABA release may contribute to the rebalancing of neural activity in the vestibular nuclei of the lesioned and intact sides.

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.

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.

Targeted overexpression of the neurite growth-associated protein B-50/GAP-43 in cerebellar Purkinje cells induces sprouting after axotomy but not axon regeneration into growth-permissive transplants

B-50/GAP-43 is a nervous tissue-specific protein, the expression of which is associated with axon growth and regeneration. Its overexpression in transgenic mice produces spontaneous axonal sprouting and enhances induced remodeling in several neuron populations (; ). We examined the capacity of this protein to increase the regenerative potential of injured adult central axons, by inducing targeted B-50/GAP-43 overexpression in Purkinje cells, which normally show poor regenerative capabilities. Thus, transgenic mice were produced in which B-50/GAP-43 overexpression was driven by the Purkinje cell-specific L7 promoter. Uninjured transgenic Purkinje cells displayed normal morphology, indicating that transgene expression does not modify the normal phenotype of these neurons. By contrast, after axotomy numerous transgenic Purkinje cells exhibited profuse sprouting along the axon and at its severed end. Nevertheless, despite these growth phenomena, which never occurred in wild-type mice, the severed transgenic axons were not able to regenerate, either spontaneously or into embryonic neural or Schwann cell grafts placed into the lesion site. Finally, although only a moderate Purkinje cell loss occurred in wild-type cerebella after axotomy, a considerable number of injured transgenic neurons degenerated, but they could be partially rescued by the different transplants placed into the lesion site. Thus, B-50/GAP-43 overexpression substantially modifies Purkinje cell response to axotomy, by inducing growth processes and decreasing their resistance to injury. However, the presence of this protein is not sufficient to enable these neurons to accomplish a full program of axon regeneration.

Pattern of selected calcium-binding proteins in the vestibular nuclear complex of two rodent species

The distribution of immunoreactivity to calbindin, calretinin, and parvalbumin in the vestibular nuclear complex and the adjacent nucleus prepositus hypoglossi was studied in rats and gerbils. The distribution of stained fibers was the same for both rodent species. All three calcium-binding proteins were present in vestibular afferents. However, none of the three proteins were present in all afferent fibers. Many fibers were labeled in the vestibular nerve and in fascicles of the descending vestibular nucleus, as well as ascending fibers in the superior vestibular nucleus and fibers directed to the medial vestibular nucleus. Labeled terminals were present in the medial vestibular nucleus, especially along the ventricular border, in the neuropil of the superior vestibular nucleus, and scattered in the descending and ventral portions of the lateral vestibular nucleus. Calbindin- and parvalbumin-positive terminals, but not calretinin-positive terminals, were present in the neuropil of the dorsal lateral vestibular nucleus, especially surrounding the large neuronal somas. Some of these terminals are presumably from cerebellar Purkinje cells, which were also labeled by both antibodies. The pattern of parvalbumin immunoreactivity was slightly different from that of calbindin, indicating that parvalbumin might be contained in additional fibers. Some neurons in the vestibular nuclear complex were labeled with antibodies to calretinin, but few cells were stained with either calbindin or parvalbumin antibodies. The largest group of calretinin-positive cells was a cluster of small- to medium-sized neurons located in a densely stained mesh of dendrites and terminals in the medial vestibular nucleus, adjacent to the ventricular border.

Electrophysiology and GABA-immunocytochemistry in the vestibular nuclei of normal (C57BL/6J) and Leaner mutant mice

Neurological disorders of genetic origin that lead to distinct disarrangements of the cerebellar wiring and cause a specific motor behaviour are likely to differentially influence the response properties and activity of postsynaptic cerebellar target neurons in the deep cerebellar (DCN) and vestibular nuclei (VN). Comparative electrophysiological and morphological analyses of these neurons in different mutants may increase our understanding of the physiological consequences of cell damage to the cerebellum and help to elucidate the relationships between histopathology and severeness of motor impairment. The Leaner mutation removes GABAergic inhibitory input to the VN predominantly originating from Purkinje cells (PC) located in the anterior lobe of the cerebellum and causes extremely severe motor disability when compared to other cerebellar mutants. In the present study the electrophysiological properties of naturally stimulated (sinusoidal head and body rotation) neurons in the VN of Leaner mutants and their corresponding wild-types (C57BL/6J) were investigated. Neuronal activity of VN single units in Leaner mutants is significantly increased over that of wild-types (frequency range 0.2-0.6 Hz) and more pronounced for type I neurons than for type II. Phase relationships and spontaneous activity are similar at these frequencies in both groups. To elucidate the degree of GABAergic input loss of VN-neurons, quantitative morphometric and numerical analyses of GABA-immunopositive synaptic boutons in the lateral VN of Leaner mutants were performed in addition and revealed significantly smaller terminals and a massive decrease (80%) in Leaner mutant terminal numbers compared to controls. In the context of the findings recently obtained in Weaver and Purkinje cell degeneration (PCD) mutants, the results in Leaner suggest that the loss of inhibition due to the PC degeneration in the anterior vermis leads to a differential enhancement of type I and type II target neuron activity in the VN. The overall activity in Leaner is intermediate between PCD (no increase in activity) and Weaver (strong increase of type I but no increase of type II). GABA-immunocytochemistry supports the idea that in Leaner the lost GABAergic PC-innervation of the lateral VN has not been replaced by surviving PCs, which is in contrast to Weaver where sprouting of GABAergic terminals in this nucleus was observed. Substitution of lost cerebellar inhibition by non-cerebellar sources, as suggested in the case of PCD-mutants, is uncertain in Leaner due to the increased activity of type I target neurons. These conditions may contribute, among others, to the severe motor disturbances in Leaner.

Lesion-specific pattern of immunocytochemical distribution of growth-associated protein B-50 (GAP-43) in the cerebellum of Weaver and PCD-mutant mice: lack of B-50 involvement in neuroplasticity of Purkinje cell terminals?

The growth-associated protein B-50 (GAP-43) is thought to play a major role in the development and regeneration of neurons. The participation of B-50 in neuronal plasticity is well documented, especially for monoaminergic systems. However, such an important role for B-50 in GABAergic systems has not been substantiated to date. This study was performed to obtain detailed information about the identity of B-50 immunopositive axons and terminals in the cerebellum and to test the involvement of this protein during plastic changes as observed in the projections of GABAergic Purkinje cells to the lateral vestibular nucleus (LVN). For this purpose mutant mice with specific cerebellar cell loss were used. Weaver mutants (B6CBA wv/wv), PCD-mutants (B6C3Fe pcd/pcd), and their corresponding wild-type mice were investigated with immunocytochemical and immunoblot procedures at the age of 8-23 days and 5-6 months using polyclonal and monoclonal antibodies to B-50. Substantial differences in B-50 distribution were detected between normals and mutants and between young and adult animals. These results demonstrate that the labeling of B-50 is mainly related to the out-growth of parallel fibers and to a minor degree on the ingrowth of non-GABAergic cerebellar afferents. There was no immunocytochemical indication that B-50 is related to Purkinje cells or accompanies the plasticity of the GABAergic innervation of the LVN.

Calbindin D-28k in the lateral vestibular nucleus of mutant mice as a tool to reveal Purkinje cell plasticity

Antibodies against the calcium-binding protein Calbindin D-28k (CaBP) are specific markers of cerebellar Purkinje cells (PC). To identify the origin of CaBP-immunopositive (CaBP+) terminals and fibres in the dorsal part of the lateral vestibular nucleus (dLVN), brains of Purkinje cell degeneration mutants (PCD) were immunoreacted for CaBP using the avidin-biotin method (ABC). In PCD an almost complete loss of CaBP+ fibres and terminals in the dLVN compared to the wildtype and the Weaver mutant was present. Morphometric analysis of CaBP+ synaptic terminals in the dLVN of adult Weaver mutants showed that the maximum and mean terminal size exceeded those in wildtypes by almost twice, which is a far larger difference than in GABA-immunoreacted material. The results show that CaBP-immunoreactivity and terminal size expansion in Weaver are both mainly attributable to PCs. Moreover, it can be concluded that the colocalization of CaBP and GABA in fibres and terminals of the dLVN in normal animals is almost entirely restricted to the PC-innervation of this nucleus. Therefore CaBP-immunocytochemistry is an excellent tool to selectively investigate the direct PC-projections in the dLVN, as it sets off the GABAergic PC-innervation from the total GABAergic innervation of this area.