Intracellular labeling of neurons in the medial accessory olive of the cat: III. Ultrastructure of axon hillock and initial segment and their GABAergic innervation

cATR Tracing Approach to Identify Individual Intermediary Neurons Based on Their Input and Output: A Proof-of-Concept Study Connecting Cerebellum and Central Hubs Implicated in Developmental Disorders

Over the past decades, it has become increasingly clear that many neurodevelopmental disorders can be characterized by aberrations in the neuro-anatomical connectome of intermediary hubs. Yet, despite the advent in unidirectional transsynaptic tracing technologies, we are still lacking an efficient approach to identify individual neurons based on both their precise input and output relations, hampering our ability to elucidate the precise connectome in both the healthy and diseased condition. Here, we bridge this gap by combining anterograde transsynaptic- and retrograde (cATR) tracing in Ai14 reporter mice, using adeno-associated virus serotype 1 expressing Cre and cholera toxin subunit B as the anterograde and retrograde tracer, respectively. We have applied this innovative approach to selectively identify individual neurons in the brainstem that do not only receive input from one or more of the cerebellar nuclei (CN), but also project to the primary motor cortex (M1), the amygdala or the ventral tegmental area (VTA). Cells directly connecting CN to M1 were found mainly in the thalamus, while a large diversity of midbrain and brainstem areas connected the CN to the amygdala or VTA. Our data highlight that cATR allows for specific, yet brain-wide, identification of individual neurons that mediate information from a cerebellar nucleus to the cerebral cortex, amygdala or VTA via a disynaptic pathway. Given that the identified neurons in healthy subjects can be readily quantified, our data also form a solid foundation to make numerical comparisons with mouse mutants suffering from aberrations in their connectome due to a neurodevelopmental disorder.

Cytoskeletal organization of axons in vertebrates and invertebrates

The maintenance of axons for the lifetime of an organism requires an axonal cytoskeleton that is robust but also flexible to adapt to mechanical challenges and to support plastic changes of axon morphology. Furthermore, cytoskeletal organization has to adapt to axons of dramatically different dimensions, and to their compartment-specific requirements in the axon initial segment, in the axon shaft, at synapses or in growth cones. To understand how the cytoskeleton caters to these different demands, this review summarizes five decades of electron microscopic studies. It focuses on the organization of microtubules and neurofilaments in axon shafts in both vertebrate and invertebrate neurons, as well as the axon initial segments of vertebrate motor- and interneurons. Findings from these ultrastructural studies are being interpreted here on the basis of our contemporary molecular understanding. They strongly suggest that axon architecture in animals as diverse as arthropods and vertebrates is dependent on loosely cross-linked bundles of microtubules running all along axons, with only minor roles played by neurofilaments.

Theoretical relation between axon initial segment geometry and excitability

In most vertebrate neurons, action potentials are triggered at the distal end of the axon initial segment (AIS). Both position and length of the AIS vary across and within neuron types, with activity, development and pathology. What is the impact of AIS geometry on excitability? Direct empirical assessment has proven difficult because of the many potential confounding factors. Here, we carried a principled theoretical analysis to answer this question. We provide a simple formula relating AIS geometry and sodium conductance density to the somatic voltage threshold. A distal shift of the AIS normally produces a (modest) increase in excitability, but we explain how this pattern can reverse if a hyperpolarizing current is present at the AIS, due to resistive coupling with the soma. This work provides a theoretical tool to assess the significance of structural AIS plasticity for electrical function.

Using subthreshold events to characterize the functional architecture of the electrically coupled inferior olive network

The electrical connectivity in the inferior olive (IO) nucleus plays an important role in generating well-timed spiking activity. Here we combined electrophysiological and computational approaches to assess the functional organization of the IO nucleus in mice. Spontaneous fast and slow subthreshold events were commonly encountered during in vitro recordings. We show that whereas the fast events represent intrinsic regenerative activity, the slow events reflect the electrical connectivity between neurons (‘spikelets’). Recordings from cell pairs revealed the synchronized occurrence of distinct groups of spikelets; their rate and distribution enabled an accurate estimation of the number of connected cells and is suggestive of a clustered organization. This study thus provides a new perspective on the functional and structural organization of the olivary nucleus and a novel experimental and theoretical approach to study electrically coupled networks.

Collateral projections from vestibular nuclear and inferior olivary neurons to lobules I/II and IX/X of the rat cerebellar vermis: a double retrograde labeling study

Axon collateral projections to various lobules of the cerebellar cortex are thought to contribute to the coordination of neuronal activities among different parts of the cerebellum. Even though lobules I/II and IX/X of the cerebellar vermis are located at the opposite poles in the anterior-posterior axis, they have been shown to receive dense vestibular mossy fiber projections. For climbing fibers, there is also a mirror-image-like organisation in their axonal collaterals between the anterior and posterior cerebellar cortex. However, the detailed organisation of mossy and climbing fiber collateral afferents to lobules I/II and IX/X is still unclear. Here, we carried out a double-labeling study with two retrograde tracers (FluoroGold and MicroRuby) in lobules I/II and IX/X. We examined labeled cells in the vestibular nuclei and inferior olive. We found a low percentage of double-labeled neurons in the vestibular nuclei (2.1 ± 0.9% of tracer-labeled neurons in this brain region), and a higher percentage of double-labeled neurons in the inferior olive (6.5 ± 1.9%), especially in its four small nuclei (18.5 ± 8.0%; including the β nucleus, dorsal cap of Kooy, ventrolateral outgrowth, and dorsomedial cell column), which are relevant for vestibular function. These results provide strong anatomical evidence for coordinated information processing in lobules I/II and IX/X for vestibular control.

Properties of the nucleo-olivary pathway: an in vivo whole-cell patch clamp study

The inferior olivary nucleus (IO) forms the gateway to the cerebellar cortex and receives feedback information from the cerebellar nuclei (CN), thereby occupying a central position in the olivo-cerebellar loop. Here, we investigated the feedback input from the CN to the IO in vivo in mice using the whole-cell patch-clamp technique. This approach allows us to study how the CN-feedback input is integrated with the activity of olivary neurons, while the olivo-cerebellar system and its connections are intact. Our results show how IO neurons respond to CN stimulation sequentially with: i) a short depolarization (EPSP), ii) a hyperpolarization (IPSP) and iii) a rebound depolarization. The latter two phenomena can also be evoked without the EPSPs. The IPSP is sensitive to a GABA_A receptor blocker. The IPSP suppresses suprathreshold and subthreshold activity and is generated mainly by activation of the GABA_A receptors. The rebound depolarization re-initiates and temporarily phase locks the subthreshold oscillations. Lack of electrotonical coupling does not affect the IPSP of individual olivary neurons, nor the sensitivity of its GABA_A receptors to blockers. The GABAergic feedback input from the CN does not only temporarily block the transmission of signals through the IO, it also isolates neurons from the network by shunting the junction current and re-initiates the temporal pattern after a fixed time point. These data suggest that the IO not only functions as a cerebellar controlled gating device, but also operates as a pattern generator for controlling motor timing and/or learning.

First node of Ranvier facilitates high-frequency burst encoding

In central neurons the first node of Ranvier is located at the first axonal branchpoint, ∼ 100 μm from the axon initial segment where synaptic inputs are integrated and converted into action potentials (APs). Whether the first node contributes to this signal transformation is not well understood. Here it was found that in neocortical layer 5 axons, the first branchpoint is required for intrinsic high-frequency (≥ 100 Hz) AP bursts. Furthermore, block of nodal Na(+) channels or axotomy of the first node in intrinsically bursting neurons depolarized the somatic AP voltage threshold (∼ 5 mV) and eliminated APs selectively within a high-frequency cluster in response to steady currents or simulated synaptic inputs. These results indicate that nodal persistent Na(+) current exerts an anterograde influence on AP initiation in the axon initial segment, revealing a computational role of the first node of Ranvier beyond conduction of the propagating AP.

Encoding of oscillations by axonal bursts in inferior olive neurons

Inferior olive neurons regulate plasticity and timing in the cerebellar cortex via the climbing fiber pathway, but direct characterization of the output of this nucleus has remained elusive. We show that single somatic action potentials in olivary neurons are translated into a burst of axonal spikes. The number of spikes in the burst depends on the phase of subthreshold oscillations and, therefore, encodes the state of the olivary network. These bursts can be successfully transmitted to the cerebellar cortex in vivo, having a significant impact on Purkinje cells. They enhance dendritic spikes, modulate the complex spike pattern, and promote short-term and long-term plasticity at parallel fiber synapses in a manner dependent on the number of spikes in the burst. Our results challenge the view that the climbing fiber conveys an all-or-none signal to the cerebellar cortex and help to link learning and timing theories of olivocerebellar function.

Loss of the cisternal organelle in the axon initial segment of cortical neurons in synaptopodin-deficient mice

The axon initial segment of cortical neurons contains the so-called cisternal organelle, an enigmatic formation of stacked endoplasmic reticulum and interdigitating plates of electron-dense material. This organelle shows many structural similarities to the spine apparatus, a cellular organelle found in a subpopulation of dendritic spines. Whereas roles in calcium signaling and protein trafficking have been proposed for the spine apparatus, little is yet known about the physiological function of its putative axonal counterpart. Considering the structural similarity of these two organelles, we hypothesized that synaptopodin, a protein essential for the formation of the dendritic spine apparatus, could also be a component of the cisternal organelle. By using immunofluorescence microscopy, we found that synaptopodin is indeed located within the axon initial segments of principal neurons in the mouse neocortex and hippocampus. Pre-embedding immunogold labeling demonstrated a close association of synaptopodin immunoreactivity with the dense plates of cisternal organelles. In synaptopodin-deficient mice, ultrastructural analysis of identified axon initial segments of CA1 pyramidal cells revealed a lack of cisternal organelles similar to the reported lack of spine apparatuses in these mutants. However, in vitro patch clamp recording of mutant neurons showed that the lack of cisternal organelles did not lead to any changes in basic electrophysiological parameters of action potentials. Taken together, our data demonstrate that synaptopodin is an essential component of the cisternal organelle of axons and of the dendritic spine apparatus, two organelles that are structurally and molecularly related.

GABAergic and glutamatergic modulation of spontaneous and motor-cortex-evoked complex spike activity

Olivocerebellar activity is organized such that synchronous complex spikes occur primarily among Purkinje cells located within the same parasagittally oriented strip of cortex. Previous findings have shown that this synchrony distribution is modulated by the release of GABA and glutamate within the inferior olive, which probably act by controlling the efficacy of the electrotonic coupling between olivary neurons. The relative strengths of these two neurotransmitters in modulating the patterns of synchrony were compared by obtaining multiple electrode recordings of spontaneous crus 2a complex spike activity during intraolivary injection of solutions containing a GABA(A) (picrotoxin) and/or AMPA [1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide disodium (NBQX)] receptor antagonist. Injection of either antagonist led to increased synchrony between cells located within the same parasagittally oriented approximately 250-microm-wide cortical strip. Picrotoxin also increased complex spike synchrony among cells located in different cortical strips, leading to a less prominent banding pattern, whereas injections of NBQX tended to decrease complex spike synchrony among such cells, enhancing the banding pattern. The relative strength of these two classes of olivary afferents was assessed by first injecting one of the antagonists alone and then in combination with the other. The enhanced banding pattern of complex spike synchrony following injection of NBQX alone remained during the subsequent combined injection of both antagonists. Furthermore, the widespread synchronization of complex spike activity following injection of picrotoxin alone was partially or completely reversed by combined injection of picrotoxin and NBQX. Changes in the climbing fiber reflex induced by the intraolivary injections paralleled the changes observed for spontaneous complex spike activity, indicating that the effects of picrotoxin and NBQX on the synchrony distribution reflect changes in the pattern of effective coupling of inferior olivary neurons and demonstrating that synchronous complex spike activity does not require simultaneous excitatory input to olivary cells. Finally the pattern of synchrony during motor cortical stimulation was examined. It was found that the patterns of synchrony for motor-cortex-evoked complex spike activity were similar to those of spontaneous activity, indicating an important role for electrotonic coupling in determining the response of the olivocerebellar system to afferent input. Moreover, intraolivary injections of picrotoxin increased the spatial distribution of the evoked response. In sum, the results provide evidence for the hypothesis that electrotonic coupling of inferior olivary neurons via gap junctions is the mechanism underlying complex spike synchrony and that this coupling plays an important role in determining the responses of the olivocerebellar system to synaptic input.

Spatial distribution and subunit composition of GABA(A) receptors in the inferior olivary nucleus

GABAergic inhibitory feedback from the cerebellum onto the inferior olivary (IO) nucleus plays an important role in olivo-cerebellar function. In this study we characterized the physiology, subunit composition, and spatial distribution of gamma-aminobutyric acid-A (GABA(A)) receptors in the IO nucleus. Using brain stem slices, we identified two types of IO neuron response to local pressure application of GABA, depending on the site of application: a slow desensitizing response at the soma and a fast desensitizing response at the dendrites. The dendritic response had a more negative reversal potential than did the somatic response, which confirmed their spatial origin. Both responses showed voltage dependence characterized by an abrupt decrease in conductance at negative potentials. Interestingly, this change in conductance occurred in the range of potentials wherein subthreshold membrane potential oscillations usually occur in IO neurons. Immunostaining IO sections with antibodies for GABA(A) receptor subunits alpha 1, alpha 2, alpha 3, alpha 5, beta 2/3, and gamma 2 and against the postsynaptic anchoring protein gephyrin complemented the electrophysiological observation by showing a differential distribution of GABA(A) receptor subtypes in IO neurons. A receptor complex containing alpha 2 beta 2/3 gamma 2 subunits is clustered with gephyrin at presumptive synaptic sites, predominantly on distal dendrites. In addition, diffuse alpha 3, beta 2/3, and gamma 2 subunit staining on somata and in the neuropil presumably represents extrasynaptic receptors. Combining electrophysiology with immunocytochemistry, we concluded that alpha 2 beta 2/3 gamma 2 synaptic receptors generated the fast desensitizing (dendritic) response at synaptic sites whereas the slow desensitizing (somatic) response was generated by extrasynaptic alpha 3 beta 2/3 gamma 2 receptors.

Microcircuitry and function of the inferior olive

The inferior olive, which provides the climbing fibers to Purkinje cells in the cerebellar cortex, has been implicated in various functions, such as learning and timing of movements, and comparing intended with achieved movements. For example, climbing-fiber activity could transmit error signals during eye-blink conditioning or adaptation of the vestibulo-ocular reflex, or it could carry motor command signals beating on the rhythm of the oscillating and synchronous firing of ensembles of olivary neurons, or both. In this review, we approach the controversial issue of olivocerebellar function from the perspective of the unique organization of the microcircuitry of the olivary neuropil. The characteristic glomeruli are formed by a core of long dendritic or axonal spines, each of which is innervated by both an inhibitory terminal derived from the hindbrain and an excitatory terminal derived from either an ascending or descending input. The dendritic spines, which originate from dendrites with varicosities carrying dendritic lamellar bodies, are coupled by gap junctions. By drawing a comparison with a computational model by Segev and Rall,which might be applicable to the typical olivary spine with its unique morphological features and combined excitatory and inhibitory input, we propose that the microcircuitry of the inferior olive is capable of functioning both in motor learning and motor timing, but does not directly compare intended with achieved movements.

Electron microscopic serial analysis of GABA presynaptic terminals on the axon hillock and initial segment of labeled abducens motoneurons in the rat

The aim of the present study was to provide a quantitative analysis of the synapses made onto the axon hillock and initial segment of rat abducens motoneurons retrogradely or intracellularly stained with HRP. GABA-immunoreactive terminals contacting these axons were visualized using a postembedding procedure. The presynaptic terminals contained either spherical or pleomorphic vesicles. gamma-Aminobutyric acid (GABA)-immunoreactive axon terminals, which belonged to this last category, were distributed both onto axon hillocks and the proximal part of initial segments. The percentage of axonal membrane covered by synapses ranged from 44.1 to 68.2%. A quantitative analysis performed on a series of ultrathin sectioned terminals contacting the axon of an intracellularly labeled motoneuron revealed a significant correlation between the length of membrane apposition of the terminals and their perimeter or surface area, and also between the area of membrane apposition and terminal volume. GABA-immunoreactive terminals had a mean perimeter and volume that were larger than those of unlabeled axon terminals. The number of active zones was correlated with the area of apposition. Some hypotheses concerning the functional role of the GABAergic innervation of this particular part of the neuron are discussed.