Cerebellar Golgi cells in the rat: receptive fields and timing of responses to facial stimulation

Electrophysiological characterization of the cerebellum in the arterially perfused hindbrain and upper body of the rat

In the present study, a non-pulsatile arterially perfused hindbrain and upper body rat preparation is described which is an extension of the brainstem preparation reported by Potts et al., (Brain Res Bull 53(1):59-67), 1. The modified in situ preparation allows study of cerebellar function whilst preserving the integrity of many of its interconnections with the brainstem, upper spinal cord and the peripheral nervous system of the head and forelimbs. Evoked mossy fibre, climbing fibre and parallel fibre field potentials and EMG activity elicited in forelimb biceps muscle by interpositus stimulation provided evidence that both cerebellar inputs and outputs remain operational in this preparation. Similarly, the spontaneous and evoked single unit activity of Purkinje cells, putative Golgi cells, molecular interneurones and cerebellar nuclear neurones was similar to activity patterns reported in vivo. The advantages of the preparation include the ability to record, without the complications of anaesthesia, stabile single unit activity for extended periods (3 h or more), from regions of the rat cerebellum that are difficult to access in vivo. The preparation should therefore be a useful adjunct to in vitro and in vivo studies of neural circuits underlying cerebellar contributions to movement control and motor learning.

Alcohol excites cerebellar Golgi cells by inhibiting the Na+/K+ ATPase

Alcohol-induced alterations of cerebellar function cause motor coordination impairments that are responsible for millions of injuries and deaths worldwide. Cognitive deficits associated with alcoholism are also a consequence of cerebellar dysfunction. The mechanisms responsible for these effects of ethanol are poorly understood. Recent studies have identified neurons in the input layer of the cerebellar cortex as important ethanol targets. In this layer, granule cells (GrCs) receive the majority of sensory inputs to the cerebellum through the mossy fibers. Information flow at these neurons is gated by a specialized pacemaker interneuron known as the Golgi cell, which provides divergent GABAergic input to thousands of GrCs. In vivo electrophysiological experiments have previously shown that acute ethanol exposure abolishes GrC responsiveness to sensory inputs carried by mossy fibers. Slice electrophysiological studies suggest that ethanol causes this effect by potentiating GABAergic transmission at Golgi cell-to-GrC synapses through an increase in Golgi cell excitability. Using patch-clamp electrophysiological techniques in cerebellar slices and computer modeling, we show here that ethanol excites Golgi cells by inhibiting the Na(+)/K(+) ATPase. Voltage-clamp recordings of Na(+)/K(+) ATPase currents indicated that ethanol partially inhibits this pump and this effect could be mimicked by low concentrations of ouabain. Partial inhibition of Na(+)/K(+) ATPase function in a computer model of the Golgi cell reproduced these experimental findings. These results establish a novel mechanism of action of ethanol on neuronal excitability, which likely has a role in ethanol-induced cerebellar dysfunction and may also contribute to neuronal functional alterations in other brain regions.

A realistic large-scale model of the cerebellum granular layer predicts circuit spatio-temporal filtering properties

The way the cerebellar granular layer transforms incoming mossy fiber signals into new spike patterns to be related to Purkinje cells is not yet clear. Here, a realistic computational model of the granular layer was developed and used to address four main functional hypotheses: center-surround organization, time-windowing, high-pass filtering in responses to spike bursts and coherent oscillations in response to diffuse random activity. The model network was activated using patterns inspired by those recorded in vivo. Burst stimulation of a small mossy fiber bundle resulted in granule cell bursts delimited in time (time windowing) and space (center-surround) by network inhibition. This burst–burst transmission showed marked frequency-dependence configuring a high-pass filter with cut-off frequency around 100 Hz. The contrast between center and surround properties was regulated by the excitatory–inhibitory balance. The stronger excitation made the center more responsive to 10–50 Hz input frequencies and enhanced the granule cell output (with spikes occurring earlier and with higher frequency and number) compared to the surround. Finally, over a certain level of mossy fiber background activity, the circuit generated coherent oscillations in the theta-frequency band. All these processes were fine-tuned by NMDA and GABA-A receptor activation and neurotransmitter vesicle cycling in the cerebellar glomeruli. This model shows that available knowledge on cellular mechanisms is sufficient to unify the main functional hypotheses on the cerebellum granular layer and suggests that this network can behave as an adaptable spatio-temporal filter coordinated by theta-frequency oscillations.

The energy use associated with neural computation in the cerebellum

The brain's energy supply determines its information processing power, and generates functional imaging signals, which are often assumed to reflect principal neuron spiking. Using measured cellular properties, we analysed how energy expenditure relates to neural computation in the cerebellar cortex. Most energy is used on information processing by non-principal neurons: Purkinje cells use only 18% of the signalling energy. Excitatory neurons use 73% and inhibitory neurons 27% of the energy. Despite markedly different computational architectures, the granular and molecular layers consume approximately the same energy. The blood vessel area supplying glucose and O(2) is spatially matched to energy consumption. The energy cost of storing motor information in the cerebellum was also estimated.

Dynamics of fast and slow inhibition from cerebellar golgi cells allow flexible control of synaptic integration

Throughout the brain, multiple interneuron types influence distinct aspects of synaptic processing. Interneuron diversity can thereby promote differential firing from neurons receiving common excitation. In contrast, Golgi cells are the sole interneurons regulating granule cell spiking evoked by mossy fibers, thereby gating inputs to the cerebellar cortex. Here, we examine how this single interneuron class modifies activity in its targets. We find that GABA(A)-mediated transmission at unitary Golgi cell --> granule cell synapses consists of varying contributions of fast synaptic currents and sustained inhibition. Fast IPSCs depress and slow IPSCs gradually build during high-frequency Golgi cell activity. Consequently, fast and slow inhibition differentially influence granule cell spike timing during persistent mossy fiber input. Furthermore, slow inhibition reduces the gain of the mossy fiber --> granule cell input-output curve, while fast inhibition increases the threshold. Thus, a lack of interneuron diversity need not prevent flexible inhibitory control of synaptic processing.

Characteristics of responses of Golgi cells and mossy fibers to eye saccades and saccadic adaptation recorded from the posterior vermis of the cerebellum

The anatomical organization of the granular layer of the cerebellum suggests an important function for Golgi cells (GC) in the pathway conveying mossy fiber (MF) afferents to Purkinje cells. Based on such anatomic observations, early proposals have attributed a role in "gain control" for GCs, a function disputed by recent investigations, which assert that GCs instead contribute to oscillatory mechanisms. However, conclusive physiological evidence based on studies of cerebellum-dependent behavior supporting/dismissing the gain control proposition has been lacking as of yet. We addressed the possible function of this interneuron by recording the activity of a large number of both MFs and GCs during saccadic eye movements from the same cortical area of the monkey cerebellum, namely the oculomotor vermis (OMV). Our cellular identification conformed to previously established criteria, mainly to juxtacellular labeling studies correlating physiological parameters with cell morphology. Response patterns of both MFs and GCs were highly heterogeneous. MF discharges correlated linearly with eye saccade metrics and timing, showing directional preference and precise direction tuning. In contrast, GC discharges did not correlate strongly with the metrics or direction of movement. Their discharge properties were also unaffected by motor learning during saccadic adaptation. The OMV therefore receives a barrage of information about eye movements from different oculomotor areas over the MF pathway, which is not reflected in GCs. The unspecificity of GCs has important implications for the intricacies of neuronal processing in the granular layer, clearly discrediting their involvement in gain control and instead suggesting a more secluded role for these interneurons.

Tonic activation of GABAB receptors reduces release probability at inhibitory connections in the cerebellar glomerulus

In the cerebellum, granule cells are inhibited by Golgi cells through GABAergic synapses generating complex responses involving both phasic neurotransmitter release and the establishment of ambient gamma-aminobutyric acid (GABA) levels. Although at this synapse the mechanisms of postsynaptic integration have been clarified to a considerable extent, the mechanisms of neurotransmitter release remained largely unknown. Here we have investigated the quantal properties of release during repetitive neurotransmission, revealing that tonic GABA(B) receptor activation by ambient GABA regulates release probability. Blocking GABA(B) receptors with CGP55845 enhanced the first inhibitory postsynaptic current (IPSC) and short-term depression in a train while reducing trial-to-trial variability and failures. The changes caused by CGP55845 were similar to those caused by increasing extracellular Ca(2+) concentration, in agreement with a presynaptic GABA(B) receptor modulation of release probability. However, the slow tail following IPSC peak demonstrated a remarkable temporal summation and was not modified by CGP55845 or extracellular Ca(2+) increase. This result shows that tonic activation of presynaptic GABA(B) receptors by ambient GABA selectively regulates the onset of inhibition bearing potential consequences for the dynamic regulation of signal transmission through the mossy fiber-granule cell pathway of the cerebellum.

Patterns and pauses in Purkinje cell simple spike trains: experiments, modeling and theory

We review our recent experimental and modeling results on how cerebellar Purkinje cells encode information in their simple spike trains and present a theory of the function of pauses and regular spiking patterns. The regular spiking patterns were discovered in extracellular recordings of simple spikes in awake and anesthetized rodents, where it was shown that more than half of the spontaneous activity consists of short epochs of regular spiking. These periods of regular spiking are interrupted by pauses, which can be tightly synchronized among nearby Purkinje cells, while the spikes in the regular patterns are not. Interestingly, pauses are affected by long-term depression of the parallel fiber synapses. Both in modeling and slice experiments it was demonstrated that long-term depression causes a decrease in the duration of pauses, leading to an increase of the spike output of the neuron. Based on these results we propose that pauses in the simple spike train form a temporal code which can lead to a rebound burst in the target deep cerebellar nucleus neurons. Conversely, the regular spike patterns may be a rate code, which presets the amplitude of future rebound bursts.

Timing in the cerebellum: oscillations and resonance in the granular layer

The brain generates many rhythmic activities, and the olivo-cerebellar system is not an exception. In recent years, the cerebellum has revealed activities ranging from low frequency to very high-frequency oscillations. These rhythms depend on the brain functional state and are typical of certain circuit sections or specific neurons. Interestingly, the granular layer, which gates sensorimotor and cognitive signals to the cerebellar cortex, can also sustain low frequency (7-25 Hz) and perhaps higher-frequency oscillations. In this review we have considered (i) how these oscillations are generated in the granular layer network depending on intrinsic electroresponsiveness and circuit connections, (ii) how these oscillations are correlated with those in other cerebellar circuit sections, and (iii) how the oscillating cerebellum communicates with extracerebellar structures. It is suggested that the granular layer can generate oscillations that integrate well with those generated in the inferior olive, in deep-cerebellar nuclei and in Purkinje cells. These rhythms, in turn, might play a role in cognition and memory consolidation by interacting with the mechanisms of long-term synaptic plasticity.

Genetic manipulation study of information processing in the cerebellum

The cerebellar circuitry consists of two main excitatory glutamatergic pathways. The inputs of mossy fibers and climbing fibers converge on Purkinje cells and deep cerebellar nuclei. In this circuitry, Golgi interneurons suppress granule cell excitability via the inhibitory GABA transmitter. A novel technique termed reversible neurotransmission blocking (RNB) was genetically established, in which granule cell transmission to Purkinje cells was selectively and reversibly blocked in the mouse cerebellar circuitry. This study revealed that Purkinje cells are essential for expression of conditioned eye-blink motor learning but that this memory is acquired and stored in deep cerebellar nuclei. A different technique termed immunotoxin-mediated cell targeting (IMCT) was developed to selectively ablate Golgi cells from the mouse cerebellar network. The study disclosed that excitatory glutamate receptors and inhibitory GABA receptors cooperatively act at Golgi cell-mossy fiber-granule cell synapses and are indispensable for motor coordination and adaptation. Finally, gene targeting of mGluR2 displayed that the metabotropic glutamate receptor acts collaboratively with the ionotropic AMPA receptors at granule cell-Golgi cell synapses and is crucial for the spatiotemporal regulation in the mouse cerebellar circuitry. The neural information is thus hierarchically regulated and integrated at different levels of the cerebellar network.

Characterization in vivo of bilaterally branching pontocerebellar mossy fibre to Golgi cell inputs in the rat cerebellum

Golgi cells regulate the flow of information from mossy fibres to the cerebellar cortex, through a mix of feedback and feedforward inhibitory actions on granule cells. The aim of the current study was to examine mossy fibre input to Golgi cells, in order to assess their impact on switching Golgi cells into feedforward behaviour. In urethane-anaesthetized rats, extracellular recordings were made from Golgi cells in Crus II (n = 18). Spikes were evoked in all Golgi cells by microstimulation within the contralateral hemispheral cortex, via branches of mossy fibres that terminate in both cerebellar hemispheres. The latencies of these responses were very short, consistent with a monosynaptic mossy fibre contact [average onset latency 2.3 +/- 0.1 ms (SEM)]. The same stimuli had no measurable effect on spike responses of nearby Purkinje cells (n = 12). Systematic mapping in the contralateral cerebellar hemisphere (Crus Ib, IIa, IIb and the paramedian lobule) usually revealed one low-intensity stimulus 'hotspot' (12-35 microA) from which short-latency spikes could be evoked in an individual Golgi cell. Microinjections of red and green retrograde tracers (latex beads, approximately 50-150 nL injection volume) made at the recording site and the stimulation hotspot resulted in double-labelled neurons within the pontine nuclei. Overall, this suggests that subsets of pontine neurons supply mossy fibres that branch to both hemispheres, some of which directly target Golgi cells. Such an arrangement may provide a common feedforward inhibitory link to temporally couple activity on both sides of the cerebellum during behaviour.

Axonal Na+ channels ensure fast spike activation and back-propagation in cerebellar granule cells

In most neurons, Na+ channels in the axon are complemented by others localized in the soma and dendrites to ensure spike back-propagation. However, cerebellar granule cells are neurons with simplified architecture in which the dendrites are short and unbranched and a single thin ascending axon travels toward the molecular layer before bifurcating into parallel fibers. Here we show that in cerebellar granule cells, Na+ channels are enriched in the axon, especially in the hillock, but almost absent from soma and dendrites. The impact of this channel distribution on neuronal electroresponsiveness was investigated by multi-compartmental modeling. Numerical simulations indicated that granule cells have a compact electrotonic structure allowing excitatory postsynaptic potentials to diffuse with little attenuation from dendrites to axon. The spike arose almost simultaneously along the whole axonal ascending branch and invaded the hillock the activation of which promoted spike back-propagation with marginal delay (<200 micros) and attenuation (<20 mV) into the somato-dendritic compartment. These properties allow granule cells to perform sub-millisecond coincidence detection of pre- and postsynaptic activity and to rapidly activate Purkinje cells contacted by the axonal ascending branch.

Timing and plasticity in the cerebellum: focus on the granular layer

Two of the most striking properties of the cerebellum are its control in timing of motor operations and its ability to adapt behavior to new sensorimotor associations. Here, we propose a 'time-window matching' hypothesis for granular layer processing. Our hypothesis states that mossy fiber inputs to the granular layer are transformed into well-timed spike bursts by intrinsic granule cell processing, that feedforward Golgi cell inhibition sets a limit to the duration of such bursts and that these activities are spread over particular fields in the granular layer so as to generate ongoing time-windows for proper control of interacting motor domains. The role of synaptic plasticity would be that of fine-tuning pre-wired circuits favoring activation of specific granule cell groups in relation to particular time windows. This concept has wide implications for processing in the olivo-cerebellar system as a whole.

Synaptic and cellular properties of the feedforward inhibitory circuit within the input layer of the cerebellar cortex

Precise representation of the timing of sensory stimuli is essential for rapid motor coordination, a core function of the cerebellum. Feedforward inhibition has been implicated in precise temporal signaling in several regions of the brain, but little is known about this type of inhibitory circuit within the input layer of the cerebellar cortex. We investigated the synaptic properties of feedforward inhibition at near physiological temperatures (35 degrees C) in rat cerebellar slices. We establish that the previously uncharacterized mossy fiber-Golgi cell-granule cell pathway can act as a functional feedforward inhibitory circuit. The synchronous activation of four mossy fibers, releasing a total of six quanta onto a Golgi cell, can reset spontaneous Golgi cell firing with high temporal precision (200 mus). However, only modest increases in Golgi cell firing rate were observed during trains of high-frequency mossy fiber stimulation. This decoupling of Golgi cell activity from mossy fiber firing rate was attributable to a strong afterhyperpolarization after each action potential, preventing mossy fiber-Golgi cell signaling for approximately 50 ms. Feedforward excitation of Golgi cells induced a temporally precise inhibitory conductance in granule cells that curtailed the excitatory action of the mossy fiber EPSC. The synaptic and cellular properties of this feedforward circuit appear tuned to trigger a fast inhibitory conductance in granule cells at the onset of stimuli that produce intense bursts of activity in multiple mossy fibers, thereby conserving the temporal precision of the initial granule cell response.

Tactile stimulation evokes long-term synaptic plasticity in the granular layer of cerebellum

Several forms of long-term synaptic plasticity [long-term potentiation (LTP) and long-term depression (LTD)] have been reported in the cerebellar circuit in vitro, but their determination in vivo was still lacking in most cases. Here we show that, in the urethane-anesthetized rat, appropriate patterns of facial tactile stimulation as well as intracerebellar electrical stimulation can induce LTP and LTD in local field potentials recorded from the granular layer of Crus-IIa. LTD prevailed in control conditions, whereas LTP prevailed during local application of gabazine. No relevant plasticity was observed when gabazine and APV were coapplied. The pharmacological and kinetic properties of LTP and LTD in vivo were compatible with those reported in the granule cell layer in vitro (Mapelli and D'Angelo, 2007), suggesting that NMDA receptor-dependent plasticity was generated at the mossy fiber-granule cell synapse under the inhibitory control of the Golgi cell circuit. Interestingly, LTP and LTD were able to regulate the response latency to tactile stimulation, as expected from computational modeling of the expression mechanisms (Nieus et al., 2006). This result suggests that LTP and LTD could regulate the spatiotemporal pattern of granular layer responses to mossy fiber inputs.

The critical role of Golgi cells in regulating spatio-temporal integration and plasticity at the cerebellum input stage

The discovery of the Golgi cell is bound to the foundation of the Neuron Doctrine. Recently, the excitable mechanisms of this inhibitory interneuron have been investigated with modern experimental and computational techniques raising renewed interest for the implications it might have for cerebellar circuit functions. Golgi cells are pacemakers with preferential response frequency and phase-reset in the theta-frequency band and can therefore impose specific temporal dynamics to granule cell responses. Moreover, through their connectivity, Golgi cells determine the spatio-temporal organization of cerebellar activity. Finally, Golgi cells, by controlling granule cell depolarization and NMDA channel unblock, regulate the induction of long-term synaptic plasticity at the mossy fiber – granule cell synapse. Thus, the Golgi cells can exert an extensive control on spatio-temporal signal organization and information storage in the granular layer playing a critical role for cerebellar computation.

Functions of interneurons in mouse cerebellum

The output signal of Purkinje cells is conveyed by the modulated discharge of simple spikes (SSs) often ascribed to mossy fiber-granule cell-parallel fiber inputs to Purkinje cell dendrites. Although generally accepted, this view lacks experimental support. We can address this view by controlling afferent signals that reach the cerebellum over climbing and mossy fiber pathways. Vestibular primary afferents constitute the largest mossy fiber projection to the uvula-nodulus. The discharge of vestibular primary afferent mossy fibers increases during ipsilateral roll tilt. The discharge of SSs decreases during ipsilateral roll tilt. Climbing fiber discharge [complex spikes (CSs)] increases during ipsilateral roll tilt. These observations suggest that the modulation of SSs during vestibular stimulation cannot be attributed directly to vestibular mossy fiber afferents. Rather we suggest that interneurons driven by vestibular climbing fibers may determine SS modulation. We recorded from cerebellar interneurons (granule, unipolar brush, Golgi, stellate, basket, and Lugaro cells) and Purkinje cells in the uvula-nodulus of anesthetized mice during vestibular stimulation. We identified all neuronal types by juxtacellular labeling with neurobiotin. Granule, unipolar brush, stellate, and basket cells discharge in phase with ipsilateral roll tilt and in phase with CSs. Golgi cells discharge out of phase with ipsilateral roll tilt and out of phase with CSs. The phases of stellate and basket cell discharge suggests that their activity could account for the antiphasic behavior of CSs and SSs. Because Golgi cells discharge in phase with SSs, Golgi cell activity cannot account for SS modulation. The sagittal array of Golgi cell axon terminals suggests that they contribute to the organization of discrete parasagittal vestibular zones.

Fast-reset of pacemaking and theta-frequency resonance patterns in cerebellar golgi cells: simulations of their impact in vivo

The Golgi cells are inhibitory interneurons of the cerebellar granular layer, which respond to afferent stimulation in vivo with a burst-pause sequence interrupting their irregular background low-frequency firing (Vos et al., 1999a. Eur. J. Neurosci. 11, 2621–2634). However, Golgi cells in vitro are regular pacemakers (Forti et al., 2006. J. Physiol. 574, 711–729), raising the question how their ionic mechanisms could impact on responses during physiological activity. Using patch-clamp recordings in cerebellar slices we show that the pacemaker cycle can be suddenly reset by spikes, making the cell highly sensitive to input variations. Moreover, the neuron resonates around the pacemaker frequency, making it specifically sensitive to patterned stimulation in the theta-frequency band. Computational analysis based on a model developed to reproduce Golgi cell pacemaking (Solinas et al., 2008Front. Neurosci., 2:2) predicted that phase-reset required spike-triggered activation of SK channels and that resonance was sustained by a slow voltage-dependent potassium current and amplified by a persistent sodium current. Adding balanced synaptic noise to mimic the irregular discharge observed in vivo, we found that pacemaking converts into spontaneous irregular discharge, that phase-reset plays an important role in generating the burst-pause pattern evoked by sensory stimulation, and that repetitive stimulation at theta-frequency enhances the time-precision of spike coding in the burst. These results suggest that Golgi cell intrinsic properties exert a profound impact on time-dependent signal processing in the cerebellar granular layer.

Regular patterns in cerebellar Purkinje cell simple spike trains

BACKGROUND

Cerebellar Purkinje cells (PC) in vivo are commonly reported to generate irregular spike trains, documented by high coefficients of variation of interspike-intervals (ISI). In strong contrast, they fire very regularly in the in vitro slice preparation. We studied the nature of this difference in firing properties by focusing on short-term variability and its dependence on behavioral state.

METHODOLOGY/PRINCIPAL FINDINGS

Using an analysis based on CV(2) values, we could isolate precise regular spiking patterns, lasting up to hundreds of milliseconds, in PC simple spike trains recorded in both anesthetized and awake rodents. Regular spike patterns, defined by low variability of successive ISIs, comprised over half of the spikes, showed a wide range of mean ISIs, and were affected by behavioral state and tactile stimulation. Interestingly, regular patterns often coincided in nearby Purkinje cells without precise synchronization of individual spikes. Regular patterns exclusively appeared during the up state of the PC membrane potential, while single ISIs occurred both during up and down states. Possible functional consequences of regular spike patterns were investigated by modeling the synaptic conductance in neurons of the deep cerebellar nuclei (DCN). Simulations showed that these regular patterns caused epochs of relatively constant synaptic conductance in DCN neurons.

CONCLUSIONS/SIGNIFICANCE

Our findings indicate that the apparent irregularity in cerebellar PC simple spike trains in vivo is most likely caused by mixing of different regular spike patterns, separated by single long intervals, over time. We propose that PCs may signal information, at least in part, in regular spike patterns to downstream DCN neurons.

Stochastic description of complex and simple spike firing in cerebellar Purkinje cells

Cerebellar Purkinje cells generate two distinct types of spikes, complex and simple spikes, both of which have conventionally been considered to be highly irregular, suggestive of certain types of stochastic processes as underlying mechanisms. Interestingly, however, the interspike interval structures of complex spikes have not been carefully studied so far. We showed in a previous study that simple spike trains are actually composed of regular patterns and single interspike intervals, a mixture that could not be explained by a simple rate-modulated Poisson process. In the present study, we systematically investigated the interspike interval structures of separated complex and simple spike trains recorded in anaesthetized rats, and derived an appropriate stochastic model. We found that: (i) complex spike trains do not exhibit any serial correlations, so they can effectively be generated by a renewal process, (ii) the distribution of intervals between complex spikes exhibits two narrow bands, possibly caused by two oscillatory bands (0.5-1 and 4-8 Hz) in the input to Purkinje cells and (iii) the regularity of regular patterns and single interspike intervals in simple spike trains can be represented by gamma processes of orders, which themselves are drawn from gamma distributions, suggesting that multiple sources modulate the regularity of simple spike trains.

The spatial organization of long-term synaptic plasticity at the input stage of cerebellum

The spatial organization of long-term synaptic plasticity [long-term potentiation (LTP) and long-term depression (LTD)] is supposed to play a critical role for distributed signal processing in neuronal networks, but its nature remains undetermined in most central circuits. By using multielectrode array recordings, we have reconstructed activation maps of the granular layer in cerebellar slices. LTP and LTD induced by theta-burst stimulation appeared in patches organized in such a way that, on average, LTP was surrounded by LTD. The sign of long-term synaptic plasticity in a given granular layer region was directly correlated with excitation and inversely correlated with inhibition: the most active areas tended to generate LTP, whereas the least active areas tended to generate LTD. Plasticity was almost entirely prevented by application of the NMDA receptor blocker, APV. This suggests that synaptic inhibition, through a control of membrane depolarization, effectively regulates NMDA channel unblock, postsynaptic calcium entry, and the induction of bidirectional synaptic plasticity at the mossy fiber-granule cell relay (Gall et al., 2005). By this mechanism, LTP and LTD could regulate the geometry and contrast of network computations, preprocessing the mossy fiber input to be conveyed to Purkinje cells and molecular layer interneurons.

Dynamic Synchronization of Purkinje Cell Simple Spikes

Purkinje cells (PCs) integrate all computations performed in the cerebellar cortex to inhibit neurons in the deep cerebellar nuclei (DCN). Simple spikes recorded in vivo from pairs of PCs separated by <100 μm are known to be synchronized with a sharp peak riding on a broad peak, but the significance of this finding is unclear. We show that the sharp peak consists exclusively of simple spikes associated with pauses in firing. The broader, less precise peak was caused by firing-rate co-modulation of faster firing spikes. About 13% of all pauses were synchronized, and these pauses had a median duration of 20 ms. As in vitro studies have reported that synchronous pauses can reliably trigger spikes in DCN neurons, we suggest that the subgroup of spikes causing the sharp peak is important for precise temporal coding in the cerebellum.

Cerebellar Golgi cells in the rat receive multimodal convergent peripheral inputs via the lateral funiculus of the spinal cord

We recently showed that the activity of cerebellar Golgi cells can be powerfully modulated by stimulation of peripheral afferents, in a pattern different to local Purkinje cells. Here we have examined the pathways underlying these responses. Graded electrical stimulation of muscle and cutaneous nerves revealed that long-lasting depressions and short-lasting excitations of Golgi cells were evoked by stimulation of cutaneous nerves at stimulus intensities that activated large mechanoreceptive afferents, and grew as additional afferents were recruited. In contrast, none of the neurones responded to stimulation of muscle nerves at intensities that activated group I afferents, although about half responded with long-lasting depressions, but not excitations, to stimuli that recruited group II and III afferents. Selective lesions of the spinal dorsal columns did not affect either of these types of response. After lesions of one lateral funiculus in the lumbar cord the responses evoked by stimulation of the hindlimb contralateral to the lesion were reduced or abolished, leaving responses evoked by ipsilateral hindlimb afferents unaltered. Since both ipsi- and contralateral afferents generate responses in Golgi cells, the convergence from the two sides must occur supraspinally. It is difficult to reconcile these properties with any of the direct spinocerebellar pathways or spinoreticulocerebellar pathways that have been described. Instead, it is likely that the responses are evoked via the multimodal 'wide dynamic range' neurones of the anterolateral system. Golgi cell activity may thus be powerfully enhanced or depressed during arousal via the anterolateral system.

A Novel High Channel-Count System for Acute Multisite Neuronal Recordings

Multisite recording represents a suitable condition to study microphysiology and network interactions in the central nervous system and, therefore, to understand brain functions. Several different materials and array configurations have been proposed for the development of new probes utilized to record brain activity from experimental animal models. We describe new multisite silicon probes that broaden the currently available application base for neuroscientists. The array arrangement of the probes recording sites was extended to increase their spatial resolution. Probes were integrated with a newly developed electronic hardware and novel software for advanced real-time processing and analysis. The new system, based on 32- and 64-electrode silicon probes, proved very valuable to record field potentials and single unit activity from the olfactory-limbic cortex of the in vitro isolated guinea-pig brain preparation and to acutely record unit activity at multiple sites from the cerebellar cortex in vivo. The potential advantages of the new system in comparison to the currently available technology are discussed.

Activation of GIRK channels by muscarinic receptors and group II metabotropic glutamate receptors suppresses Golgi cell activity in the cochlear nucleus of mice

Granule cells and parallel fiber circuits in the dorsal cochlear nucleus (DCN) play a role in integration of multimodal sensory with auditory inputs. The activity of granule cells is regulated through inhibitory connections made by Golgi cells. Golgi cells in turn probably receive parallel fiber inputs and regulate activity of the DCN. We have investigated the electrophysiological properties of Golgi cells using the whole cell patch-clamp method in slices made from transgenic mice that express green fluorescent protein driven by the promotor of metabotropic glutamate receptor subtype 2. Stimulation of auditory nerve fibers (ANFs) and of parallel fibers evoked glutamatergic excitatory postsynaptic currents (EPSC) through AMPA receptors. The strengths and latencies of these inputs differed, however. ANF stimulation evoked EPSCs after 4.7 +/- 0.4 ms, whereas parallel fiber stimulation evoked EPSCs after 1.4 +/- 0.2 ms that were on average 2.5 times as large. The multiple peaks and prolonged activity suggest the presence of polysynaptic connections between ANFs and Golgi cells. Agonists for group II metabotropic glutamate receptors (mGluRs) and for muscarinic receptors induced membrane hyperpolarization and suppressed the firing of Golgi cells by activating G-protein-coupled inward rectifier K(+) (GIRK) channels. These results strongly suggest that Golgi cells were regulated through the combined activities of glutamatergic and cholinergic synapses, which presumably regulated the temporal firing patterns of granule cells and through them the activity of principal cells of the DCN.

Inhibition of constitutive inward rectifier currents in cerebellar granule cells by pharmacological and synaptic activation of GABABreceptors

gamma-Aminobutyric acid (GABA)(B) receptors are known to enhance activation of Kir3 channels generating G-protein-dependent inward rectifier K(+)-currents (GIRK). In some neurons, GABA(B) receptors either cause a tonic GIRK activation or generate a late K(+)-dependent inhibitory postsynaptic current component. However, other neurons express Kir2 channels, which generate a constitutive inward rectifier K(+)-current (CIRK) without requiring G-protein activation. The functional coupling of CIRK with GABA(B) receptors remained unexplored so far. About 50% of rat cerebellar granule cells in the internal granular layer of P19-26 rats showed a sizeable CIRK current. Here, we have investigated CIRK current regulation by GABA(B) receptors in cerebellar granule cells, which undergo GABAergic inhibition through Golgi cells. By using patch-clamp recording techniques and single-cell reverse transcriptase-polymerase chain reaction in acute cerebellar slices, we show that granule cells co-express Kir2 channels and GABA(B) receptors. CIRK current biophysical properties were compatible with Kir2 but not Kir3 channels, and could be inhibited by the GABA(B) receptor agonist baclofen. The action of baclofen was prevented by the GABA(B) receptor blocker CGP35348, involved a pertussis toxin-insensitive G-protein-mediated pathway, and required protein phosphatases inhibited by okadaic acid. GABA(B) receptor-dependent CIRK current inhibition could also be induced by repetitive GABAergic transmission at frequencies higher than the basal autorhythmic discharge of Golgi cells. These results suggest therefore that GABA(B) receptors can exert an inhibitory control over CIRK currents mediated by Kir2 channels. CIRK inhibition was associated with an increased input resistance around rest and caused a approximately 5 mV membrane depolarization. The pro-excitatory action of these effects at an inhibitory synapse may have an homeostatic role re-establishing granule cell readiness under conditions of strong inhibition.

Different responses of rat cerebellar Purkinje cells and Golgi cells evoked by widespread convergent sensory inputs

While the synaptic properties of Golgi cell-mediated inhibition of granule cells are well studied, less is known of the afferent inputs to Golgi cells so their role in information processing remains unclear. We investigated the responses of cerebellar cortical Golgi cells and Purkinje cells in Crus I and II of the posterior lobe cerebellar hemisphere to activation of peripheral afferents in vivo, using anaesthetized rats. Recordings were made from 70 Golgi cells and 76 Purkinje cells. Purkinje cells were identified by the presence of climbing fibre responses. Golgi cells were identified by both spontaneous firing pattern and response properties, and identification was confirmed using juxtacellular labelling of single neurones (n = 16). Purkinje cells in Crus II showed continuous firing at relatively high rates (25-60 Hz) and stimulation of peripheral afferents rarely evoked substantial responses. The most common response was a modest, long-latency, long-lasting increase in simple spike output. By comparison, the most common response evoked in Golgi cells by the same stimuli was a long-latency, long-lasting depression of firing, found in approximately 70% of the Golgi cells tested. The onsets of Golgi cell depressions had shorter latencies than the Purkinje cell excitations. Brief, short-latency excitations and reductions in firing were also evoked in some Golgi cells, and rarely in Purkinje cells, but in most cases long-lasting depressions were the only significant change in spike firing. Golgi cell responses could be evoked using air puff or tactile stimuli and under four different anaesthetic regimens. Long-lasting responses in both neurone types could be evoked from wide receptive fields, in many cases including distal afferents from all four limbs, as well as from trigeminal afferents. These Golgi cell responses are not consistent with the conventional feedback inhibition or 'gain control' models of Golgi cell function. They suggest instead that cerebellar cortical activity can be powerfully modulated by the general level of peripheral afferent activation from much of the body. On this basis, Golgi cells may act as a context-specific gate on transmission through the mossy fibre-granule cell pathway.

Ionic mechanisms of autorhythmic firing in rat cerebellar Golgi cells

Although Golgi cells (GoCs), the main type of inhibitory interneuron in the cerebellar granular layer (GL), are thought to play a central role in cerebellar network function, their excitable properties have remained unexplored. GoCs fire rhythmically in vivo and in slices, but it was unclear whether this activity originated from pacemaker ionic mechanisms. We explored this issue in acute cerebellar slices from 3-week-old rats by combining loose cell-attached (LCA) and whole-cell (WC) recordings. GoCs displayed spontaneous firing at 1-10 Hz (room temperature) and 2-20 Hz (35-37 degrees C), which persisted in the presence of blockers of fast synaptic receptors and mGluR and GABAB receptors, thus behaving, in our conditions, as pacemaker neurons. ZD 7288 (20 microM), a potent hyperpolarization-activated current (Ih) blocker, slowed down pacemaker frequency. The role of subthreshold Na+ currents (INa,sub) could not be tested directly, but we observed a robust TTX-sensitive, non-inactivating Na+ current in the subthreshold voltage range. When studying repolarizing currents, we found that retigabine (5 microM), an activator of KCNQ K+ channels generating neuronal M-type K+ (IM) currents, reduced GoC excitability in the threshold region. The KCNQ channel antagonist XE991 (5 microM) did not modify firing, suggesting that GoC IM has low XE991 sensitivity. Spike repolarization was followed by an after-hyperpolarization (AHP) supported by apamin-sensitive Ca2+-dependent K+ currents (I(apa)). Block of I(apa) decreased pacemaker precision without altering average frequency. We propose that feed-forward depolarization is sustained by Ih and INa,sub, and that delayed repolarizing feedback involves an IM-like current whose properties remain to be characterized. The multiple ionic mechanisms shown here to contribute to GoC pacemaking should provide the substrate for fine regulation of firing frequency and precision, thus influencing the cyclic inhibition exerted by GoCs onto the cerebellar GL.

Purkinje cells in the lateral cerebellum of the cat encode visual events and target motion during visually guided reaching

In this study the receipt of visual information by the lateral cerebellum and its contribution to a motor output was studied using single unit recording of cerebellar cortical neurones in cats trained to perform visually guided reaching. The activity of Purkinje cells and other cortical neurones in the lateral cerebellum was investigated in relation to various aspects of the task, such as visual events, parameters of target movement, and limb and eye movements. Two-thirds (66%) of Purkinje cells tested could signal simple visual events, such as a flash of light. Neurones were also capable of detecting other less potent, but behaviourally important visual events, such as a 'GO' signal (LED brightening). Half of the cells tested were responsive to the on-going motion of the visual target, displaying tonically altered discharge rates for as long as it was moving, and a 'preferred' target velocity. A small proportion of cells showed short latency visual modulation that persisted during the forelimb reach. Anatomical tracing studies confirmed that the recordings were obtained from the D1 zone of crus I. In summary, cells in this region of lateral cerebellar cortex perform simple visual functions, such as event detection, but also more complex visual functions, such as encoding parameters of target motion, and their visual responsiveness is appropriate for a role in accurate visually guided reaching to a moving target.

Temporal characteristics of tactile stimuli influence the response profile of cerebellar Golgi cells

An increasing number of studies have investigated the effect of stimulation parameters on neuronal response properties. Here, we describe the effect of temporal characteristics of tactile stimuli, more specifically the stimulation frequency and duration, on the response profile of simultaneously recorded cerebellar and cerebral cortical units in ketamine-xylazine anaesthetized rats. Long stimulus durations (>50 ms) elicited ON and OFF excitatory components in response to the stimulus onset and offset respectively, in both the cortex and the cerebellum. Golgi cells responded on average 7.5 ms later to the stimulus withdrawal than to the stimulus onset. Furthermore, the corticopontine OFF responses in the cerebellum and OFF responses in the cortex showed congruent latency decreases and amplitude increases for longer stimulus durations (50-200 ms). Decreasing the stimulus frequency similarly affected the latency and amplitude of the responses for inter-stimuli intervals shorter than 200 ms. In view of these results, we speculate that the stimulus offset is regarded as a novel input, because both paradigms resulted in similar response amplitude and latency modifications. Finally, the results suggest that a 100-200 ms time window can be of particular importance for cerebellar processing of information in the somatosensory system.

Synaptic pathways in neural microcircuits

The functions performed by different neural microcircuits depend on the anatomical and physiological properties of the various synaptic pathways connecting neurons. Neural microcircuits across various species and brain regions are similar in terms of their repertoire of neurotransmitters, their synaptic kinetics, their short-term and long-term plasticity, and the target-specificity of their synaptic connections. However, microcircuits can be fundamentally different in terms of the precise recurrent design used to achieve a specific functionality. In this review, which is part of the TINS Microcircuits Special Feature, we compare the connectivity designs in spinal, hippocampal, neocortical and cerebellar microcircuits, and discuss the different computational challenges that each microcircuit faces.

Correlations Between Purkinje Cell Single-Unit Activity and Simultaneously Recorded Field Potentials in the Immediately Underlying Granule Cell Layer

Evidence from both anatomical and physiological studies suggests that the ascending segment of the granule cell axon provides a large, driving input to overlying Purkinje cells. In the current experiments, we used dual recording electrodes to simultaneously record spike activity of Purkinje cells and multiunit field potential activity in the directly underlying granule cell layer. These dual recordings were performed both during periods of spontaneous (“background”) firing and also after peripheral tactile stimulation. The results demonstrate that in the large majority of cases, there is a strong positive correlation between spontaneous Purkinje cell simple spikes and spontaneous activity in the immediately underlying granule cell layer. The strength of this correlation was dependent on both the firing rate of the Purkinje cell as well as on the rate of granule cell layer multiunit activity. In addition, for any given pair of recordings, the correlation seen during spontaneous activity accurately predicted the magnitude and time course of responses evoked by peripheral tactile stimulation. These results provide additional evidence that the synapses associated with the ascending segment of the granule cell axon have a substantial influence on Purkinje cell output. This relationship is considered in the context of our ongoing reevaluation of the physiological relationship between cerebellar granule and Purkinje cells.

Computation of inverse functions in a model of cerebellar and reflex pathways allows to control a mobile mechanical segment

The command and control of limb movements by the cerebellar and reflex pathways are modeled by means of a circuit whose structure is deduced from functional constraints. One constraint is that fast limb movements must be accurate although they cannot be continuously controlled in closed loop by use of sensory signals. Thus, the pathways which process the motor orders must contain approximate inverse functions of the bio-mechanical functions of the limb and of the muscles. This can be achieved by means of parallel feedback loops, whose pattern turns out to be comparable to the anatomy of the cerebellar pathways. They contain neural networks able to anticipate the motor consequences of the motor orders, modeled by artificial neural networks whose connectivity is similar to that of the cerebellar cortex. These networks learn the direct biomechanical functions of the limbs and muscles by means of a supervised learning process. Teaching signals calculated from motor errors are sent to the learning sites, as, in the cerebellum, complex spikes issued from the inferior olive are conveyed to the Purkinje cells by climbing fibers. Learning rules are deduced by a differential calculation, as classical gradient rules, and they account for the long term depression which takes place in the dendritic arborizations of the Purkinje cells. Another constraint is that reflexes must not impede voluntary movements while remaining at any instant ready to oppose perturbations. Therefore, efferent copies of the motor orders are sent to the interneurones of the reflexes, where they cancel the sensory-motor consequences of the voluntary movements. After learning, the model is able to drive accurately, both in velocity and position, angular movements of a rod actuated by two pneumatic McKibben muscles. Reflexes comparable to the myotatic and tendinous reflexes, and stabilizing reactions comparable to the cerebellar sensory-motor reactions, reduce efficiently the effects of perturbing torques. These results allow to link the behavioral concepts of the equilibrium-point "lambda model" [J Motor Behav 18 (1986) 17] with anatomical and physiological features: gains of reflexes and sensori-motor reactions set the slope of the "invariant characteristic," and efferent copies set the "threshold of the stretch reflex." Thus, mathematical and physical laws account for the raison d'etre of the inhibitory nature of Purkinje cells and for the conspicuous anatomical pattern of the cerebellar pathways. These properties of these pathways allow to perform approximate inverse calculations after learning of direct functions, and insure also the coordination of voluntary and reflex motor orders.

Local Field Potential Oscillations in Primate Cerebellar Cortex: Synchronization With Cerebral Cortex During Active and Passive Expectancy

Many brain regions, such as the cerebellum, primary somatosensory cortex (SI), and primary motor cortex (MI), interact to produce coordinated actions. Synchronization of local field potentials (LFPs) in sensorimotor cerebral areas has been related to motor performance, often through 10- to 25-Hz oscillatory LFPs. The macaque cerebellar paramedian lobule (PM) also shows 10- to 25-Hz LFP oscillations, which are modulated in a stimulus–response lever press task to get reward (active condition), but also, albeit differently, in a similarly timed stimulus–reward relation (passive condition). This study focuses on simultaneous LFP activity in primate SI or MI and the PM cerebellum during the active (left- or right-hand lever presses) and passive conditions. Results show a similar modulation pattern of 10- to 25-Hz oscillations in the cerebellum, MI, and SI during the active condition (left or right hand), decreasing after stimulus onset, returning, and again decreasing after movement onset. In the passive condition, when the monkey did not move but got reward, all 3 areas show an oscillatory profile where oscillations increase after stimulus onset and last until reward, denoting a role for these oscillations in passive expectancy. However, synchronization between cerebellar LFPs and SI LFPs is higher during the active condition than during the passive condition, and highest for the interested hand. This greater PM–SI synchronization, when the monkey had to press the lever, could represent a form of cerebro-cerebellar communication, perhaps to serve somatosensory processing to accomplish the task; PM–MI synchronization was less selective for the hand used and might carry a more general type of information.

Between in and out: linking morphology and physiology of cerebellar cortical interneurons

We used the juxtacellular recording and labeling technique of Pinault (1996) in the uvula/nodulus of the ketamine anesthetized rat in an attempt to link different patterns of spontaneous activity with different types of morphologically identified cerebellar cortical interneurons. Cells displaying a somewhat irregular, syncopated cadence of spontaneous activity averaging 4-10 Hz could, upon successful entrainment and visualization, be morphologically identified as Golgi cells. Spontaneously firing cells with a highly or fairly regular firing rate of 10-35 Hz turned out to be unipolar brush cells. We also found indications that other types of cerebellar cortical neurons might also be distinguished on the basis of the characteristics of their spontaneous firing. Comparison of the interspike interval histograms of spontaneous activity obtained in the anaesthetized rat with those obtained in the awake rabbit points to a way whereby the behaviorally related modulation of specific types of interneurons can be studied. In particular, the spontaneous activity signatures of Golgi cells and unipolar brush cells anatomically identified in the uvula/nodulus of the anaesthetized rat are remarkably similar to the spontaneous activity patterns of some units we have recorded in the flocculus of the awake rabbit. The spontaneous activity patterns of at least some types of cerebellar interneurons clearly have the potential to serve as identifying signatures in behaving animals.

Inactivation of calcium-binding protein genes induces 160 Hz oscillations in the cerebellar cortex of alert mice

Oscillations in neuronal populations may either be imposed by intrinsically oscillating pacemakers neurons or emerge from specific attributes of a distributed network of connected neurons. Calretinin and calbindin are two calcium-binding proteins involved in the shaping of intraneuronal Ca2+ fluxes. However, although their physiological function has been studied extensively at the level of a single neuron, little is known about their role at the network level. Here we found that null mutations of genes encoding calretinin or calbindin induce 160 Hz local field potential oscillations in the cerebellar cortex of alert mice. These oscillations reached maximum amplitude just beneath the Purkinje cell bodies and are reinforced in the cerebellum of mice deficient in both calretinin and calbindin. Purkinje cells fired simple spikes phase locked to the oscillations and synchronized along the parallel fiber axis. The oscillations reversibly disappeared when gap junctions or either GABA(A) or NMDA receptors were blocked. Cutaneous stimulation of the whisker region transiently suppressed the oscillations. However, the intrinsic somatic excitability of Purkinje cells recorded in slice preparation was not significantly altered in mutant mice. Functionally, these results suggest that 160 Hz oscillation emerges from a network mechanism combining synchronization of Purkinje cell assemblies through parallel fiber excitation and the network of coupled interneurons of the molecular layer. These findings demonstrate that subtle genetically induced modifications of Ca2+ homeostasis in specific neuron types can alter the observed dynamics of the global network.

mGluR2 postsynaptically senses granule cell inputs at Golgi cell synapses

In the cerebellar circuit, Golgi cells are thought to contribute to information processing and integration via feedback mechanisms. In these mechanisms, dynamic modulation of Golgi cell excitability is necessary because GABA from Golgi cells causes tonic inhibition on granule cells. We studied the role and synaptic mechanisms of postsynaptic metabotropic glutamate receptor subtype 2 (mGluR2) at granule cell-Golgi cell synapses, using whole-cell recording of green fluorescent protein-positive Golgi cells of wild-type and mGluR2-deficient mice. Postsynaptic mGluR2 was activated by glutamate from granule cells and hyperpolarized Golgi cells via G protein-coupled inwardly rectifying K+ channels (GIRKs). This hyperpolarization conferred long-lasting silencing of Golgi cells, the duration and extents of which were dependent on stimulus strengths. Postsynaptic mGluR2 thus senses inputs from granule cells and is most likely important for spatiotemporal modulation of mossy fiber-granule cell transmission before distributing inputs to Purkinje cells.

Temporal organization of activity in the cerebellar cortex: a manifesto for synchrony

The issues of temporal coding and the temporal organization of activity have aroused a great deal of interest in sensory systems, cortex, thalamus, and hippocampus. Strangely, despite the important timing roles attributed to the cerebellum, little consideration has been given to the organization of activity within the cerebellar circuitry. In fact, there is evidence of a remarkable temporal patterning of activity in even the earliest cerebellar recordings. The evidence for the existence of high-frequency oscillations in the cerebellar cortex is reviewed and possible mechanisms are discussed; one involves the synchrony of parallel fiber inputs to Purkinje cells. It is shown how synchronous and oscillatory activity can enable extremely precise timing and also how they can maximize the information storage capacity of the cerebellar cortex.

Peripheral stimuli excite coronal beams of Golgi cells in rat cerebellar cortex

Cerebellar granule cells constitute the largest neurone population of the brain. Their axons run as parallel fibres along the coronal axis, and the one-dimensional spread of excitation that is expected to result from this arrangement is a key assumption of theories of cerebellar function. In many studies using various techniques, however, it was not possible to evoke such a beam-like propagation of excitation with natural stimuli. We recorded, in Crus I and II of anaesthetised rats, pairs of Golgi cells aligned along the parallel fibre axis and synchronising spontaneously. Each pair was subjected to two stimulation protocols: punctate and semi-continuous. Local punctate facial stimulation evoked distinct fast and late responses of variable strength and latency (fast: 4.0-10.2 ms; late: 13.6-22.7 ms). Semi-continuous stimulation with a brush increased the firing rate, and modified the precision and phase of synchronisation. Differences between a pair in response strength and phase to brush stimulation correlated strongly with the difference in latency to punctate stimulation. These observations were reproduced in a model of the granular layer. The stimulus activated a central patch of mossy fibres, and Golgi cells received short- and long-range excitation from mossy and parallel fibres, respectively. The strength and latency of the punctate response of a model Golgi cell were found to vary with its position, reflecting a systematic change in the contribution of mossy and parallel fibres to its excitation with distance from the activated patch. During brush stimulation, model Golgi cells inside the patch fired more precisely synchronised, whereas the other Golgi cells responded with a lag proportional to their distance from the patch, thereby reproducing the experimentally observed changes in synchronisation. Taken together with the previously reported large receptive fields of Golgi cells and with their spontaneous synchronisation, the variable, position-dependent latency of evoked Golgi cell responses indicates a beam-like spread of excitation along the parallel fibres in rat cerebellar cortex.

Cerebellar cortex: computation by extrasynaptic inhibition?

In the cerebellar cortex, inhibitory inputs to granule cells exhibit prominent tonic and spillover components resulting from the activation of extrasynaptic receptors. A recent study shows how extrasynaptic inhibition affects information flow through cerebellar cortex.

Morphological and neurochemical differentiation of large granular layer interneurons in the adult rat cerebellum

The granular layer of the cerebellar cortex consists of densely packed neuronal cells, classified into granule cells and large interneurons. In this study, we provide a comparative survey of large granular layer interneurons in the adult rat cerebellum based on both morphological and neurochemical criteria. To this end, double immunofluorescence histochemistry was performed by combining antibodies against the cytoplasmic antigen Rat-303, calretinin, the metabotropic glutamate receptor mGluR2 and somatostatin. Based on Rat-303/calretinin double immunohistochemistry, three distinct populations of large granular layer interneurons could be discerned: cells immunopositive for Rat-303, calretinin or both. Rat-303 or calretinin single-labeled cells represented Golgi cells and unipolar brush cells, respectively. Rat-303/calretinin double-labeled cells located just underneath the Purkinje cell layer represented Lugaro cells. Morphometrical analysis distinguished two populations of Rat-303-positive Golgi cells according to their location: vermis versus hemisphere. Immunostaining for the metabotropic glutamate receptor mGluR2 combined with Rat-303 or calretinin revealed that the majority of Golgi cells (about 90%) appeared to be mGluR2 positive. Lugaro cells were mGluR2 negative. In addition, a limited population of large polymorphous interneurons in the depth of the granular layer with morphological features resembling Golgi cells also displayed Rat-303/calretinin immunoreactivity and were mGluR2 negative. Double immunohistochemistry for Rat-303 and somatostatin revealed three populations of labeled cells in the depth of the granular layer. Besides double-labeled Golgi cells, Rat-303 or somatostatin single-labeled cells were present. Based on mGluR2/somatostatin and calretinin/somatostatin double immunostainings, Rat-303 single-labeled cells were found to correspond to Rat-303/calretinin-positive, mGluR2-negative Golgi-like cells, while the identity of somatostatin single-labeled cells remained unclear. The data presented in this article elaborate previous reports on the morphological and neurochemical differentiation of large interneurons in the rat cerebellar granular layer. In addition, they indicate that the current classification of these cells into Golgi cells, Lugaro cells and unipolar brush cells does not describe the observed neurochemical heterogeneity.

GABA spillover from single inhibitory axons suppresses low-frequency excitatory transmission at the cerebellar glomerulus

GABA type B receptors (GABA(B)-Rs) are present on excitatory terminals throughout the CNS, but surprisingly little is known about their role in modulating neurotransmission under physiological conditions. We have investigated activation of GABA(B)-Rs on excitatory terminals within the cerebellar glomerulus, a structure where glutamatergic excitatory and GABAergic inhibitory terminals are in close apposition and make axodendritic synapses onto granule cells. Application of the GABA(B)-R agonist baclofen depressed evoked mossy fiber EPSCs by 54% at 1 Hz. The amplitude of miniature EPSCs recorded in tetrodotoxin was unchanged in the presence of baclofen, but the frequency was significantly reduced, indicating a purely presynaptic action of baclofen under our recording conditions. At physiological temperature (37 degrees C) presynaptic GABA(B)-Rs were not tonically activated by spontaneous GABA release from Golgi cells, which fire at approximately 8 Hz in slices at this temperature. However, tonic activation could be induced by blocking GABA uptake or by lowering temperature. GABA(B)-Rs were activated at physiological temperature when Golgi cell firing was increased above the basal level by stimulating a single inhibitory Golgi cell input at 50 Hz, suppressing the mossy fiber-evoked EPSC by 24% at 1 Hz. Furthermore, glutamate release was selectively inhibited at low-frequency mossy fiber inputs (<10 Hz) during Golgi cell stimulation. Our findings suggest that GABA spillover in the glomerulus modulates sensory input to the cerebellar cortex.

Long-term potentiation of intrinsic excitability at the mossy fiber-granule cell synapse of rat cerebellum

Synaptic activity can induce persistent modifications in the way a neuron reacts to subsequent inputs by changing either synaptic efficacy or intrinsic excitability. After high-frequency synaptic stimulation, long-term potentiation (LTP) of synaptic efficacy is commonly observed at hippocampal synapses (Bliss and Collingridge, 1993), and potentiation of intrinsic excitability has recently been reported in cerebellar deep nuclear neurons (Aizenmann and Linden, 2000). However, the potential coexistence of these two aspects of plasticity remained unclear. In this paper we have investigated the effect of high-frequency stimulation on synaptic transmission and intrinsic excitability at the mossy fiber-granule cell relay of the cerebellum. High-frequency stimulation, in addition to increasing synaptic conductance (D'Angelo et al., 1999), increased granule cell input resistance and decreased spike threshold. These changes depended on postsynaptic depolarization and NMDA receptor activation and were prevented by inhibitory synaptic activity. Potentiation of intrinsic excitability was induced by relatively weaker inputs than potentiation of synaptic efficacy, whereas with stronger inputs the two aspect of potentiation combined to enhance EPSPs and spike generation. Potentiation of intrinsic excitability may extend the computational capability of the cerebellar mossy fiber-granule cell relay.

Identification of subunits contributing to synaptic and extrasynaptic NMDA receptors in Golgi cells of the rat cerebellum

1. To investigate the properties of N-methyl-D-aspartate receptors (NMDARs) in cerebellar Golgi cells, patch-clamp recordings were made in cerebellar slices from postnatal day 14 (P14) rats. To verify cell identity, cells were filled with Neurobiotin and examined using confocal microscopy. 2. The NR2B subunit-selective NMDAR antagonist ifenprodil (10 microM) reduced whole-cell NMDA-evoked currents by approximately 80 %. The NMDA-evoked currents were unaffected by the Zn2+ chelator N,N,N',N'-tetrakis-(2-pyridylmethyl)-ethylenediamine (TPEN; 1 microM) suggesting the absence of NMDARs containing NR2A subunits. 3. Outside-out patches from Golgi cells exhibited a population of 'high-conductance' 50 pS NMDAR openings. These were inhibited by ifenprodil, with an IC50 of 19 nM. 4. Patches from these cells also contained 'low-conductance' NMDAR channels, with features characteristic of NR2D subunit-containing receptors. These exhibited a main conductance of 39 pS, with a sub-conductance level of 19 pS, with clear asymmetry of transitions between the two levels. As expected of NR2D-containing receptors, these events were not affected by ifenprodil. 5. The NMDAR-mediated component of EPSCs, evoked by parallel fibre stimulation or occurring spontaneously, was not affected by 1 microM TPEN. However, it was reduced (by approximately 60 %) in the presence of 10 microM ifenprodil, to leave a residual NMDAR-mediated current that exhibited fast decay kinetics. This is, therefore, unlikely to have arisen from receptors composed of NR1/NR2D subunits. 6. We conclude that in cerebellar Golgi cells, the high- and low-conductance NMDAR channels arise from NR2B- and NR2D-containing receptors, respectively. We found no evidence for NR2A-containing receptors in these cells. While NR2B-containing receptors are present in both the synaptic and extrasynaptic membrane, our results indicate that NR1/NR2D receptors do not contribute to the EPSC and appear to be restricted to the extrasynaptic membrane.