Characterization of synaptic connections between cortex and deep nuclei of the rat cerebellum in vitro

Alteration of medial temporal lobe metabolism related to Alzheimer's disease and dementia with lewy bodies

BACKGROUND

Association of medial temporal lobe (MTL) metabolism with Alzheimer's disease (AD) and dementia with Lewy bodies (DLB) has not been evaluated considering their mixed disease (MD).

METHODS

131 patients with AD, 133 with DLB, 122 with MD, and 28 normal controls (NCs) underwent neuropsychological tests, assessments for parkinsonism, cognitive fluctuation (CF), and visual hallucinations (VH), and 18F-fluorodeoxyglucose PET to quantify MTL metabolism in the amygdala, hippocampus, and entorhinal cortex. The effects of AD and DLB on MTL metabolism were evaluated using general linear models (GLMs). Associations between MTL metabolism, cognition, and clinical features were evaluated using GLMs or logistic regression models separately performed for the AD spectrum (NC + AD + MD), DLB spectrum (NC + DLB + MD), and disease groups (AD + DLB + MD). Covariates included age, sex, and education.

RESULTS

AD was associated with hippocampal/entorhinal hypometabolism, whereas DLB was associated with relative amygdalar/hippocampal hypermetabolism. Relative MTL hypermetabolism was associated with lower attention/visuospatial/executive scores and severe parkinsonism in both the AD and DLB spectra and disease groups. Left hippocampal/entorhinal hypometabolism was associated with lower verbal memory scores, whereas right hippocampal hypometabolism was associated with lower visual memory scores in both the AD spectrum and disease groups. Relative MTL hypermetabolism was associated with an increased risk of CF and VH in the disease group, and relative amygdalar hypermetabolism was associated with an increased risk of VH in the DLB spectrum.

CONCLUSIONS

Entorhinal-hippocampal hypometabolism and relative amygdala-hippocampal hypermetabolism could be characteristics of AD- and DLB-related neurodegeneration, respectively.

KCa1.1 channels contribute to optogenetically driven post-stimulation silencing in cerebellar molecular layer interneurons

Using cell-attached recordings from molecular layer interneurons (MLI) of the cerebellar cortex of adult mice expressing channel rhodopsin 2, we show that wide-field optical activation induces an increase in firing rate during illumination and a firing pause when the illumination ends (post-stimulation silencing; PSS). Significant spike rate changes with respect to basal firing rate were observed for optical activations lasting 200 ms and 1 s as well as for 1 s long trains of 10 ms pulses at 50 Hz. For all conditions, the net effect of optical activation on the integrated spike rate is significantly reduced because of PSS. Three lines of evidence indicate that this PSS is due to intrinsic factors. Firstly, PSS is induced when the optical stimulation is restricted to a single MLI using a 405-nm laser delivering a diffraction-limited spot at the focal plane. Secondly, PSS is not affected by block of GABA-A or GABA-B receptors, ruling out synaptic interactions amongst MLIs. Thirdly, PSS is mimicked in whole-cell recording experiments by step depolarizations under current clamp. Activation of Ca-dependent K channels during the spike trains appears as a likely candidate to underlie PSS. Using immunocytochemistry, we find that one such channel type, KCa1.1, is present in the somato-dendritic and axonal compartments of MLIs. In cell-attached recordings, charybdotoxin and iberiotoxin significantly reduce the optically induced PSS, while TRAM-34 does not affect it, suggesting that KCa1.1 channels, but not KCa3.1 channels, contribute to PSS.

Altered Synaptic Transmission and Excitability of Cerebellar Nuclear Neurons in a Mouse Model of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is generally regarded as a muscle-wasting disease. However, human patients and animal models of DMD also frequently display non-progressive cognitive deficits and high comorbidity with neurodevelopmental disorders, suggesting impaired central processing. Previous studies have identified the cerebellar circuit, and aberrant inhibitory transmission in Purkinje cells, in particular, as a potential site of dysfunction in the central nervous system (CNS). In this work, we investigate potential dysfunction in the output of the cerebellum, downstream of Purkinje cell (PC) activity. We examined synaptic transmission and firing behavior of excitatory projection neurons of the cerebellar nuclei, the primary output of the cerebellar circuit, in juvenile wild-type and mdx mice, a common mouse model of DMD. Using immunolabeling and electrophysiology, we found a reduced number of PC synaptic contacts, but no change in postsynaptic GABAA receptor expression or clustering in these cells. Furthermore, we found that the replenishment rate of synaptic vesicles in Purkinje terminals is reduced in mdx neurons, suggesting that dysfunction at these synapses may be primarily presynaptic. We also found changes in the excitability of cerebellar nuclear neurons. Specifically, we found greater spontaneous firing but reduced evoked firing from a hyperpolarized baseline in mdx neurons. Analysis of action potential waveforms revealed faster repolarization and greater after-hyperpolarization of evoked action potentials in mdx neurons, suggesting an increased voltage- or calcium- gated potassium current. We did not find evidence of dystrophin protein or messenger RNA (mRNA) expression in wild-type nuclear neurons, suggesting that the changes observed in these cells are likely due to the loss of dystrophin in presynaptic PCs. Together, these data suggest that the loss of dystrophin reduces the dynamic range of synaptic transmission and firing in cerebellar nuclear neurons, potentially disrupting the output of the cerebellar circuit to other brain regions and contributing to cognitive and neurodevelopmental deficits associated with DMD.

Modulatory Effects of Monoamines and Perineuronal Nets on Output of Cerebellar Purkinje Cells

Classically, the cerebellum has been thought to play a significant role in motor coordination. However, a growing body of evidence for novel neural connections between the cerebellum and various brain regions indicates that the cerebellum also contributes to other brain functions implicated in reward, language, and social behavior. Cerebellar Purkinje cells (PCs) make inhibitory GABAergic synapses with their target neurons: other PCs and Lugaro/globular cells via PC axon collaterals, and neurons in the deep cerebellar nuclei (DCN) via PC primary axons. PC-Lugaro/globular cell connections form a cerebellar cortical microcircuit, which is driven by serotonin and noradrenaline. PCs' primary outputs control not only firing but also synaptic plasticity of DCN neurons following the integration of excitatory and inhibitory inputs in the cerebellar cortex. Thus, strong PC-mediated inhibition is involved in cerebellar functions as a key regulator of cerebellar neural networks. In this review, we focus on physiological characteristics of GABAergic transmission from PCs. First, we introduce monoaminergic modulation of GABAergic transmission at synapses of PC-Lugaro/globular cell as well as PC-large glutamatergic DCN neuron, and a Lugaro/globular cell-incorporated microcircuit. Second, we review the physiological roles of perineuronal nets (PNNs), which are organized components of the extracellular matrix and enwrap the cell bodies and proximal processes, in GABA release from PCs to large glutamatergic DCN neurons and in cerebellar motor learning. Recent evidence suggests that alterations in PNN density in the DCN can regulate cerebellar functions.

Functional Abnormalities of Cerebellum and Motor Cortex in Spinal Muscular Atrophy Mice

Spinal muscular atrophy (SMA) is a devastating genetic neuromuscular disease. Diffuse neuropathology has been reported in SMA patients and mouse models, however, functional changes in brain regions have not been studied. In the SMNΔ7 mouse model, we identified three types of differences in neuronal function in the cerebellum and motor cortex from two age groups: P7-9 (P7) and P11-14 (P11). Microelectrode array studies revealed significantly lower spontaneous firing and network activity in the cerebellum of SMA mice in both age groups, but it was more profound in the P11 group. In the motor cortex, however, neural activity was not different in either age group. Whole-cell patch-clamp was used to study the function of output neurons in both brain regions. In cerebellar Purkinje cells (PCs) of SMA mice, the input resistance was larger at P7, while capacitance was smaller at P11. In the motor cortex, no difference was observed in the passive membrane properties of layer V pyramidal neurons (PN5s). The action potential threshold of both types of output neurons was depolarized in the P11 group. We also observed lower spontaneous excitatory and inhibitory synaptic activity in PN5s and PCs respectively from P11 SMA mice. Overall, these differences suggest functional alterations in the neural network in these motor regions that change during development. Our results also suggest that neuronal dysfunction in these brain regions may contribute to the pathology of SMA. Comprehensive treatment strategies may consider motor regions outside of the spinal cord for better outcomes.

Regulation of autism-relevant behaviors by cerebellar-prefrontal cortical circuits

Cerebellar dysfunction has been demonstrated in autism spectrum disorders (ASDs); however, the circuits underlying cerebellar contributions to ASD-relevant behaviors remain unknown. In this study, we demonstrated functional connectivity between the cerebellum and the medial prefrontal cortex (mPFC) in mice; showed that the mPFC mediates cerebellum-regulated social and repetitive/inflexible behaviors; and showed disruptions in connectivity between these regions in multiple mouse models of ASD-linked genes and in individuals with ASD. We delineated a circuit from cerebellar cortical areas Right crus 1 (Rcrus1) and posterior vermis through the cerebellar nuclei and ventromedial thalamus and culminating in the mPFC. Modulation of this circuit induced social deficits and repetitive behaviors, whereas activation of Purkinje cells (PCs) in Rcrus1 and posterior vermis improved social preference impairments and repetitive/inflexible behaviors, respectively, in male PC-Tsc1 mutant mice. These data raise the possibility that these circuits might provide neuromodulatory targets for the treatment of ASD.

Developmental Changes in Serotonergic Modulation of GABAergic Synaptic Transmission and Postsynaptic GABAA Receptor Composition in the Cerebellar Nuclei

Outputs from the cerebellar nuclei (CN) are important for generating and controlling movement. The activity of CN neurons is controlled not only by excitatory inputs from mossy and climbing fibers and by γ-aminobutyric acid (GABA)-based inhibitory transmission from Purkinje cells in the cerebellar cortex but is also modulated by inputs from other brain regions, including serotonergic fibers that originate in the dorsal raphe nuclei. We examined the modulatory effects of serotonin (5-HT) on GABAergic synapses during development, using rat cerebellar slices. As previously reported, 5-HT presynaptically decreased the amplitudes of stimulation-evoked inhibitory postsynaptic currents (IPSCs) in CN neurons, with this effect being stronger in slices from younger animals (postnatal days [P] 11-13) than in slices from older animals (P19-21). GABA release probabilities accordingly exhibited significant decreases from P11-13 to P19-21. Although there was a strong correlation between the GABA release probability and the magnitude of 5-HT-induced inhibition, manipulating the release probability by changing extracellular Ca²⁺ concentrations failed to control the extent of 5-HT-induced inhibition. We also found that the IPSCs exhibited slower kinetics at P11-13 than at P19-21. Pharmacological and molecular biological tests revealed that IPSC kinetics were largely determined by the prevalence of α₁ subunits within GABA_A receptors. In summary, pre- and postsynaptic developmental changes in serotonergic modulation and GABAergic synaptic transmission occur during the second to third postnatal weeks and may significantly contribute to the formation of normal adult cerebellar function.

Synchronicity and Rhythmicity of Purkinje Cell Firing during Generalized Spike-and-Wave Discharges in a Natural Mouse Model of Absence Epilepsy

Absence epilepsy is characterized by the occurrence of generalized spike and wave discharges (GSWDs) in electrocorticographical (ECoG) recordings representing oscillatory activity in thalamocortical networks. The oscillatory nature of GSWDs has been shown to be reflected in the simple spike activity of cerebellar Purkinje cells and in the activity of their target neurons in the cerebellar nuclei, but it is unclear to what extent complex spike activity is implicated in generalized epilepsy. Purkinje cell complex spike firing is elicited by climbing fiber activation and reflects action potential firing in the inferior olive. Here, we investigated to what extent modulation of complex spike firing is reflected in the temporal patterns of seizures. Extracellular single-unit recordings in awake, head-restrained homozygous tottering mice, which suffer from a mutation in the voltage-gated Ca_V2.1 calcium channel, revealed that a substantial proportion of Purkinje cells (26%) showed increased complex spike activity and rhythmicity during GSWDs. Moreover, Purkinje cells, recorded either electrophysiologically or by using Ca²⁺-imaging, showed a significant increase in complex spike synchronicity for both adjacent and remote Purkinje cells during ictal events. These seizure-related changes in firing frequency, rhythmicity and synchronicity were most prominent in the lateral cerebellum, a region known to receive cerebral input via the inferior olive. These data indicate profound and widespread changes in olivary firing that are most likely induced by seizure-related activity changes in the thalamocortical network, thereby highlighting the possibility that olivary neurons can compensate for pathological brain-state changes by dampening oscillations.

The effect of bilirubin on the excitability of mitral cells in the olfactory bulb of the rat

Olfactory dysfunction is a common clinical phenomenon observed in various liver diseases. Previous studies have shown a correlation between smell disorders and bilirubin levels in patients with hepatic diseases. Bilirubin is a well-known neurotoxin; however, its effect on neurons in the main olfactory bulb (MOB), the first relay in the olfactory system, has not been examined. We investigated the effect of bilirubin (>3 μM) on mitral cells (MCs), the principal output neurons of the MOB. Bilirubin increased the frequency of spontaneous firing and the frequency but not the amplitude of spontaneous excitatory postsynaptic currents (sEPSCs). TTX completely blocked sEPSCs in almost all of the cells tested. Bilirubin activity was partially blocked by N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepro pionic acid (AMPA) receptor antagonists. Furthermore, we found that bilirubin increased the frequency of intrinsic firing independent of synaptic transmission in MCs. Our findings suggest that bilirubin enhances glutamatergic transmission and strengthens intrinsic firing independent of synaptic transmission, all of which cause hyperexcitability in MCs. Our findings provide the basis for further investigation into the mechanisms underlying olfactory dysfunction that are often observed in patients with severe liver disease.

Abnormal high-frequency burst firing of cerebellar neurons in rapid-onset dystonia-parkinsonism

Loss-of-function mutations in the α3 isoform of the Na(+)/K(+) ATPase (sodium pump) are responsible for rapid-onset dystonia parkinsonism (DYT12). Recently, a pharmacological model of DYT12 was generated implicating both the cerebellum and basal ganglia in the disorder. Notably, partially blocking sodium pumps in the cerebellum was necessary and sufficient for induction of dystonia. Thus, a key question that remains is how partially blocking sodium pumps in the cerebellum induces dystonia. In vivo recordings from dystonic mice revealed abnormal high-frequency bursting activity in neurons of the deep cerebellar nuclei (DCN), which comprise the bulk of cerebellar output. In the same mice, Purkinje cells, which provide strong inhibitory drive to DCN cells, also fired in a similarly erratic manner. In vitro studies demonstrated that Purkinje cells are highly sensitive to sodium pump dysfunction that alters the intrinsic pacemaking of these neurons, resulting in erratic burst firing similar to that identified in vivo. This abnormal firing abates when sodium pump function is restored and dystonia caused by partial block of sodium pumps can be similarly alleviated. These findings suggest that persistent high-frequency burst firing of cerebellar neurons caused by sodium pump dysfunction underlies dystonia in this model of DYT12.

Muscimol microinjection into cerebellar fastigial nucleus exacerbates stress-induced gastric mucosal damage in rats

AIM

To investigate the effects of microinjection of the GABA(A) receptor agonist muscimol into cerebellar fastigial nucleus (FN) on stress-induced gastric mucosal damage and the underlying mechanism in rats.

METHODS

Stress-induced gastric mucosal damage was induced in adult male SD rats by restraining and immersing them in cold water for 3 h. GABA(A) receptor agonist or antagonist was microinjected into the lateral FN. The decussation of superior cerebellar peduncle (DSCP) was electrically destroyed and the lateral hypothalamic area (LHA) was chemically ablated by microinjection of kainic acid. The pathological changes in the gastric mucosa were evaluated using TUNEL staining, immunohistochemistry staining and Western blotting.

RESULTS

Microinjection of muscimol (1.25, 2.5, and 5.0 μg) into FN significantly exacerbated the stress-induced gastric mucosal damage in a dose-dependent manner, whereas microinjection of GABA(A) receptor antagonist bicuculline attenuated the damage. The intensifying effect of muscimol on gastric mucosal damage was abolished by electrical lesion of DSCP or chemical ablation of LHA performed 3 d before microinjection of muscimol. Microinjection of muscimol markedly increased the discharge frequency of the greater splanchnic nerve, significantly increased the gastric acid volume and acidity, and further reduced the gastric mucosal blood flow. In the gastric mucosa, further reduced proliferation cells, enhanced apoptosis, and decreased anti-oxidant levels were observed following microinjection of muscimol.

CONCLUSION

Cerebellar FN participates in the regulation of stress-induced gastric mucosal damage, and cerebello-hypothalamic circuits contribute to the process.

Synchrony and neural coding in cerebellar circuits

The cerebellum regulates complex movements and is also implicated in cognitive tasks, and cerebellar dysfunction is consequently associated not only with movement disorders, but also with conditions like autism and dyslexia. How information is encoded by specific cerebellar firing patterns remains debated, however. A central question is how the cerebellar cortex transmits its integrated output to the cerebellar nuclei via GABAergic synapses from Purkinje neurons. Possible answers come from accumulating evidence that subsets of Purkinje cells synchronize their firing during behaviors that require the cerebellum. Consistent with models predicting that coherent activity of inhibitory networks has the capacity to dictate firing patterns of target neurons, recent experimental work supports the idea that inhibitory synchrony may regulate the response of cerebellar nuclear cells to Purkinje inputs, owing to the interplay between unusually fast inhibitory synaptic responses and high rates of intrinsic activity. Data from multiple laboratories lead to a working hypothesis that synchronous inhibitory input from Purkinje cells can set the timing and rate of action potentials produced by cerebellar nuclear cells, thereby relaying information out of the cerebellum. If so, then changing spatiotemporal patterns of Purkinje activity would allow different subsets of inhibitory neurons to control cerebellar output at different times. Here we explore the evidence for and against the idea that a synchrony code defines, at least in part, the input–output function between the cerebellar cortex and nuclei. We consider the literature on the existence of simple spike synchrony, convergence of Purkinje neurons onto nuclear neurons, and intrinsic properties of nuclear neurons that contribute to responses to inhibition. Finally, we discuss factors that may disrupt or modulate a synchrony code and describe the potential contributions of inhibitory synchrony to other motor circuits.

A computational mechanism for unified gain and timing control in the cerebellum

Precise gain and timing control is the goal of cerebellar motor learning. Because the basic neural circuitry of the cerebellum is homogeneous throughout the cerebellar cortex, a single computational mechanism may be used for simultaneous gain and timing control. Although many computational models of the cerebellum have been proposed for either gain or timing control, few models have aimed to unify them. In this paper, we hypothesize that gain and timing control can be unified by learning of the complete waveform of the desired movement profile instructed by climbing fiber signals. To justify our hypothesis, we adopted a large-scale spiking network model of the cerebellum, which was originally developed for cerebellar timing mechanisms to explain the experimental data of Pavlovian delay eyeblink conditioning, to the gain adaptation of optokinetic response (OKR) eye movements. By conducting large-scale computer simulations, we could reproduce some features of OKR adaptation, such as the learning-related change of simple spike firing of model Purkinje cells and vestibular nuclear neurons, simulated gain increase, and frequency-dependent gain increase. These results suggest that the cerebellum may use a single computational mechanism to control gain and timing simultaneously.

Lateral hypothalamic area mediated the aggravated effect of microinjection of Baclofen into cerebellar fastigial nucleus on stress gastric mucosal damage in rats

Cerebellum, primarily believed as a subcortical somatic motor center, is increasingly considered to be implicated in visceral activities. However, little is known about its regulation on gastrointestinal organs. In this research, we investigated the aggravated effect of microinjection of gamma-aminobutyric acid receptor subtype B (GABA(B)R) agonist, Baclofen into cerebellar fastigial nucleus (FN) on stress gastric mucosal damage (SGMD) and its possible regulatory mechanism. The gastric mucosal damage index was chosen to indicate the severity of gastric mucosal injure. Immunohistochemistry and transferase-mediated dUTP-biotin nick-endlabeling (TUNEL) methods were used to detect the variations of lateral hypothalamic area (LHA) and gastric mucosa. It had been demonstrated that FN participates in regulation of SGMD via its GABA(B)R and GABA neural pathway, which passes through the decussation of superior cerebellar peduncle and projects to the GABA receptors in LHA. Meanwhile, celiac sympathetic nerve involves in this process via mediating neural discharge, which results in the decrease of gastric mucosal blood flow. Additionally, apoptosis, proliferation and oxidation in gastric mucosa, and gastric acid contribute in the mechanism. It could be expected that these results might suggest insights to the cerebellar and hypothalamic function, and the treatment of gastrointestinal diseases.

A decrementing form of plasticity apparent in cerebellar learning

Long-term synaptic plasticity is believed to underlie the capacity for learning and memory. In the cerebellum, for example, long-term plasticity contributes to eyelid conditioning and to learning in eye movement systems. We report evidence for a decrementing form of cerebellar plasticity as revealed by the behavioral properties of eyelid conditioning in the rabbit. We find that conditioned eyelid responses exhibit within-session changes that recover by the next day. These changes, which increase with the interstimulus interval, involve decreases in conditioned response magnitude and likelihood as well as increases in latency to onset. Within-subject comparisons show that these changes differ in magnitude depending on the type of training, arguing against motor fatigue or changes in motor pathways downstream of the cerebellum. These phenomena are also observed when stimulation of mossy fibers substitutes for the conditioned stimulus, suggesting that changes take place within the cerebellum or in downstream efferent pathways. Together, these observations suggest a plasticity mechanism in the cerebellum that is induced during training sessions and fades within 23 h. To formalize this hypothesis more specifically, we show that incorporating a short-lasting potentiation at the granule cell to Purkinje cell synapses in a computer simulation of the cerebellum reproduces these behavioral effects. We propose the working hypothesis that the presynaptic form of long-term potentiation observed at these synapses is reversed by time rather than by a corresponding long-term depression. These results demonstrate the utility of eyelid conditioning as a means to identify and characterize the rules that govern input to output transformations in the cerebellum.

Rebound discharge in deep cerebellar nuclear neurons in vitro

Neurons of the deep cerebellar nuclei (DCN) play a critical role in defining the output of cerebellum in the course of encoding Purkinje cell inhibitory inputs. The earliest work performed with in vitro preparations established that DCN cells have the capacity to translate membrane hyperpolarizations into a rebound increase in firing frequency. The primary means of distinguishing between DCN neurons has been according to cell size and transmitter phenotype, but in some cases, differences in the firing properties of DCN cells maintained in vitro have been reported. In particular, it was shown that large diameter cells in the rat DCN exhibit two phenotypes of rebound discharge in vitro that may eventually help define their functional roles in cerebellar output. A transient burst and weak burst phenotype can be distinguished based on the frequency and pattern of rebound discharge immediately following a hyperpolarizing stimulus. Work to date indicates that the difference in excitability arises from at least the degree of activation of T-type Ca(2+) current during the immediate phase of rebound firing and Ca(2+)-dependent K(+) channels that underlie afterhyperpolarizations. Both phenotypes can be detected following stimulation of Purkinje cell inhibitory inputs under conditions that preserve resting membrane potential and natural ionic gradients. In this paper, we review the evidence supporting the existence of different rebound phenotypes in DCN cells and the ion channel expression patterns that underlie their generation.

Synaptic inhibition, excitation, and plasticity in neurons of the cerebellar nuclei

Neurons of the cerebellar nuclei generate the non-vestibular output of the cerebellum. Like other neurons, they integrate excitatory and inhibitory synaptic inputs and filter them through their intrinsic properties to produce patterns of action potential output. The synaptic and intrinsic features of cerebellar nuclear cells are unusual in several respects, however: these neurons receive an overwhelming amount of basal and driven inhibition from Purkinje neurons, but are also spontaneously active, producing action potentials even without excitation. Moreover, not only is spiking by nuclear cells sensitive to the amount of inhibition, but the strength of inhibition is also sensitive to the amount of spiking, through multiple forms of long-term plasticity. Here, we review the properties of synaptic excitation and inhibition, their short-term plasticity, and their influence on action potential firing of cerebellar nuclear neurons, as well as the interactions among excitation, inhibition, and spiking that produce long-term changes in synaptic strength. The data provide evidence that electrical and synaptic signaling in the cerebellar circuit is both plastic and resilient: the strength of IPSPs and EPSPs readily changes as the activity of cerebellar nuclear cells is modified. Notably, however, many of the identified forms of plasticity have an apparently homeostatic effect, responding to perturbations of input by restoring cerebellar output toward pre-perturbation values. Such forms of self-regulation appear consistent with the role of cerebellar output in coordinating movements. In contrast, other forms of plasticity in nuclear cells, including a long-term potentiation of excitatory postsynaptic currents (EPSCs) and excitation-driven increases in intrinsic excitability, are non-homeostatic, and instead appear suited to bring the circuit to a new set point. Interestingly, the combinations of inhibitory and excitatory stimuli that potentiate EPSCs resemble patterns of activity predicted to occur during eyelid conditioning, suggesting that this form long-term potentiation, perhaps amplified by intrinsic plasticity, may represent a cellular mechanism that is engaged during cerebellar learning.

Ca currents activated by spontaneous firing and synaptic disinhibition in neurons of the cerebellar nuclei

In neurons of the cerebellar nuclei, long-term potentiation of EPSCs is induced by high-frequency synaptic excitation by mossy fibers followed by synaptic inhibition by Purkinje cells. Induction requires activation of synaptic receptors as well as voltage-gated Ca channels. To examine how Purkinje-mediated inhibition of nuclear neurons affects Ca levels during plasticity-inducing stimuli, we have combined electrophysiology, Ca imaging, and pharmacology of cerebellar nuclear neurons in mouse cerebellar slices. We find that spontaneous firing generates tonic Ca signals in both somata and dendrites, which drop during 500 ms, 100 Hz trains of Purkinje IPSPs or hyperpolarizing steps. Although the presence of low-voltage-activated (T-type) Ca channels in nuclear neurons has fostered the inference that disinhibition activates these channels, synaptic inhibition with a physiological chloride equilibrium potential (E(Cl)) (-75 mV) fails to hyperpolarize neurons sufficiently for T-type channels to recover substantially. Consequently, after IPSPs, Ca signals return to baseline, although firing is accelerated by approximately 20 Hz for approximately 300 ms. Only after hyperpolarizations beyond E(Cl) does Ca rise gradually beyond baseline, as firing further exceeds spontaneous rates. Cd(2+) (100 microm), which nearly eliminates L-type, N-type, P/Q-type, and R-type Ca currents while sparing approximately one-half the T-type current, prevents Ca changes during and after hyperpolarizations to E(Cl). Thus, high-frequency IPSPs in cerebellar nuclear neurons evoke little postinhibitory current through T-type channels. Instead, inhibition regulates Ca levels simply by preventing action potentials, which usually permit Ca influx through high-voltage-activated channels. The decreases and restoration of Ca levels associated with Purkinje-mediated inhibition are likely to contribute to synaptic plasticity.

Nothing can be coincidence: synaptic inhibition and plasticity in the cerebellar nuclei

Many cerebellar neurons fire spontaneously, generating 10-100 action potentials per second even without synaptic input. This high basal activity correlates with information-coding mechanisms that differ from those of cells that are quiescent until excited synaptically. For example, in the deep cerebellar nuclei, Hebbian patterns of coincident synaptic excitation and postsynaptic firing fail to induce long-term increases in the strength of excitatory inputs. Instead, excitatory synaptic currents are potentiated by combinations of inhibition and excitation that resemble the activity of Purkinje and mossy fiber afferents that is predicted to occur during cerebellar associative learning tasks. Such results indicate that circuits with intrinsically active neurons have rules for information transfer and storage that distinguish them from other brain regions.

Modulatory effects of serotonin on GABAergic synaptic transmission and membrane properties in the deep cerebellar nuclei

Cerebellar outputs from the deep cerebellar nuclei (DCN) are critical for generating and controlling movement. DCN neuronal activity is primarily controlled by GABAergic inhibitory transmission by Purkinje cells in the cerebellar cortex and is also modulated by nerve inputs originating from other brain regions within and outside the cerebellum. In this study, we examined the modulatory effects of 5-HT on GABAergic synapses in the DCN. 5-HT decreased the amplitude of stimulation-evoked inhibitory postsynaptic currents (eIPSCs) in DCN neurons, and this effect was abolished by a 5-HT(1B) antagonist, SB 224289. The decrease in IPSC amplitude was associated with an increased paired-pulse ratio of the IPSC. 5-HT also decreased the frequency of miniature IPSCs without altering the amplitude. These data suggest that 5-HT presynaptically inhibited GABA release. Furthermore, 5-HT elicited a slow inward current in DCN neurons. Pharmacological studies showed that 5-HT activated the 5-HT(5) receptor, which is positively coupled to G protein and elicited the slow inward current through enhancement of hyperpolarization-activated cation channel activation. Finally, we examined the effects of 5-HT on the spike generation that accompanies repetitive stimulation of inhibitory synapses. 5-HT increased the spontaneous firing rate in DCN neurons caused by depolarization. Increase in the 5-HT-induced tonic firing relatively decreased the contrast difference from the rebound depolarization-induced firing. However, the inhibitory transmission-induced silencing of DCN firing remained during the conditioning stimulus. These results suggest that 5-HT plays a regulatory role in spike generation and contributes to the gain control of inhibitory GABAergic synapses in DCN neurons.

Mechanisms of potentiation of mossy fiber EPSCs in the cerebellar nuclei by coincident synaptic excitation and inhibition

Neurons of the cerebellar nuclei receive synaptic excitation from cerebellar mossy fibers. Unlike in many principal neurons, coincident presynaptic activity and postsynaptic depolarization do not generate long-term potentiation at these synapses. Instead, EPSCs are potentiated by high-frequency trains of presynaptic activity applied with postsynaptic hyperpolarization, in patterns resembling mossy-fiber-mediated excitation and Purkinje-cell-mediated inhibition that are predicted to occur during delay eyelid conditioning. Here, we have used electrophysiology and Ca imaging to test how synaptic excitation and inhibition interact to generate long-lasting synaptic plasticity in nuclear cells in cerebellar slices. We find that the extent of plasticity varies with the relative timing of synaptic excitation and hyperpolarization. Potentiation is most effective when synaptic stimuli precede the postinhibitory rebound by approximately 400 ms, whereas with longer intervals, or with a reverse sequence, EPSCs tend to depress. When basal intracellular Ca is raised by spontaneous firing or reduced by voltage clamping at subthreshold potentials, potentiation is induced as long as the synaptic-rebound temporal sequence is maintained, suggesting that plasticity does not require Ca levels to exceed a threshold or attain a specific concentration. Although rebound and spike-dependent Ca influx are global, potentiation is synapse specific, and is disrupted by inhibitors of calcineurin or Ca-calmodulin-dependent protein kinase II, but not PKC. When IPSPs replace the hyperpolarizing step in the induction protocol, potentiation proceeds normally. These results lead us to propose that synaptic and inhibitory/rebound stimuli initiate separate processes, with local NMDA receptor-mediated Ca influx "priming" synapses, and Ca changes from the inhibition and rebound "triggering" potentiation at recently activated synapses.

A spiking network model for passage-of-time representation in the cerebellum

In Pavlovian delay eyeblink conditioning, the cerebellum represents the passage-of-time (POT) between onsets of conditioned and unconditioned stimuli (CS and US, respectively). To study possible computational mechanisms of the POT representation we built a large-scale spiking network model of the cerebellum. Consistent with our previous rate-coding model, we found two conditions necessary for the present model to represent the POT with a dynamic population of active granule cells: (i) long temporal integration of input signals; and (ii) random recurrent connections between granule and Golgi cells. When these conditions were satisfied, a nonrecurrent sequence of active granule cell populations was generated in response to a CS and, conversely, the POT from the CS onset was able to be read out from the sequence. Specifically, simulated N-methyl-D-aspartate (NMDA) channels with a long decay time constant at granule and Golgi cells were responsible for the long temporal integration. Thus, blocking the NMDA channels or ablating Golgi cells impaired the POT representation. Simulated glomerulus structure made POT representation robust against noise in mossy fibre inputs. Long-term potentiation induced at mossy fibre synapses on granule cells also served to enhance the robustness. We reproduced some experimental results of Pavlovian delay eyeblink conditioning using the present model. These results suggest that the recurrent network in the granular layer and NMDA channels in granule and Golgi cells play an essential role in the timing mechanisms in the cerebellum, whereas the glomerulus serves to realize a robust representation of time.

Recurrent axon collaterals underlie facilitating synapses between cerebellar Purkinje cells

Morphological studies have provided ample evidence for synaptic connections between cerebellar Purkinje cells (PCs), but the functional properties of these synapses remain elusive. We report on direct recordings of synaptically connected PCs in mice cerebellar slices. In PCs filled with a fluorescent dye to aid axon visualization and postsynaptic target identification, presynaptic action potentials elicited unitary inhibitory postsynaptic currents in neighboring PCs in 10% of potential connections tested. In 11 pairs, postsynaptic currents had a delay onset of 1.62 +/- 0.16 ms with respect to the presynaptic spike, a 10-90% rise time of 2.20 +/- 0.33 ms, and a monoexponential decay with a time constant of 13.3 +/- 1.7 ms. Average values for peak current and variance-to-mean ratio were 55 +/- 14 and 30 +/- 3 pA, respectively. In contrast to the depressing nature of the synapse between PCs and deep cerebellar nuclei neurons, PC-PC synapses exhibited strong facilitation operating within a time window of a few milliseconds; paired-pulse ratios for 3- and 20-ms intervals were 1.79 +/- 0.18 and 1.01 +/- 0.14, respectively (n = 6). The facilitation is of presynaptic nature because it is accompanied by a decrease in failure rate. Trains of action potentials evoked in presynaptic varicosities volume-averaged calcium transients whose peak increased 1.7-fold as the frequency increased from 50 to 166 Hz. We suggest that PC-PC synapses are tuned for high fidelity of transmission during bursts of PC activity and that their operation in the cerebellar circuit modulates synchronized PC firing.

Dark rearing alters the short-term synaptic plasticity in visual cortex

The strength of all chemical synapses can be rapidly up- or down-regulated by their recent activities to reshape the postsynaptic signal through a mechanism of short-term facilitation or depression. It is also true that sensory experience is essential for maturation of cortical circuits and function. We studied the effect of sensory experience on the short-term plasticity in brain slices of rat visual cortex. Repetitive stimulation was applied in layer II/III and the response of pyramidal neuron in layer V was examined by whole-cell recording. We deprived the visual experience of rats by rearing the animals in dark environment. We found that dark rearing (DR) had no effects on paired-pulse interactions of either excitatory postsynaptic currents (EPSCs) or inhibitory postsynaptic currents (IPSCs). However, DR increased the level of IPSCs steady-state depression at stimulation frequency of 20Hz or more. This result, together with the finding that DR reduced the excitability of neural circuit in visual cortex, suggested a compensatory mechanism, which may be important in enhancing the network excitability at a time when synapses are immature.

Cerebellar LTD and pattern recognition by Purkinje cells

Many theories of cerebellar function assume that long-term depression (LTD) of parallel fiber (PF) synapses enables Purkinje cells to learn to recognize PF activity patterns. We have studied the LTD-based recognition of PF patterns in a biophysically realistic Purkinje-cell model. With simple-spike firing as observed in vivo, the presentation of a pattern resulted in a burst of spikes followed by a pause. Surprisingly, the best criterion to distinguish learned patterns was the duration of this pause. Moreover, our simulations predicted that learned patterns elicited shorter pauses, thus increasing Purkinje-cell output. We tested this prediction in Purkinje-cell recordings both in vitro and in vivo. In vitro, we found a shortening of pauses when decreasing the number of active PFs or after inducing LTD. In vivo, we observed longer pauses in LTD-deficient mice. Our results suggest a novel form of neural coding in the cerebellar cortex.

Structural and functional alterations of cerebellum following fluid percussion injury in rats

Cerebellum was shown to be vulnerable to traumatic brain injury (TBI) in experimental animals. However, the detailed pathological and functional changes within the cerebellum following TBI are not known. Using our established cerebellum fluid percussion injury (FPI) model, we characterized the temporal pattern and the nature of structural damage following FPI, as well as the functional changes of Purkinje cells in response to climbing fiber activation. Our results showed that 60% of Purkinje cells died within the first 24 h following moderate FPI. In contrast, clusters of densely stained shrunken granule cells were stained positive for terminal deoxynucleotidyl transferase-mediated UTP nick end labeling (TUNEL) in 1, 3 or 7 days following FPI animals. We also observed an accompanying structural damage to the cerebellar white matter tract. Disconnected axonal fibers appeared 1 day post-FPI, and loss of white matter fibers were visible 3 and 7 days post-FPI. Massive accumulation of beta-amyloid precursor protein (betaAPP) was found in the white matter tracts and molecular layer in the cerebellum of 1, 3 or 7 days FPI animals. Our functional study showed that the majority of Purkinje cells from 1 day and all cells from 3 to 7 days post-FPI had distorted membrane potential and synaptic responses to climbing fiber activation. These results suggested that there is a co-related structural and functional deterioration with a specific temporal pattern in the cerebellum following FPI. These observations provide a basis for future mechanistic investigations aiming to realize neuroprotection from cerebellar neuronal death and loss of cerebellar functionality.

Role of GABAB receptors in GABA and baclofen-induced inhibition of adult rat cerebellar interpositus nucleus neurons in vitro

Previous studies suggested that the postsynaptic GABA(B) receptors of deep cerebellar nuclear neurons of adult rats were not activated by selective GABA(B) receptor agonist baclofen or endogenous GABA released by cerebellar cortical Purkinje cells, although the receptors have been demonstrated to exist in the deep cerebellar nuclei. In this study, cerebellar slices of adult rats were prepared for testing effects of GABA, baclofen and muscimol (selective GABA(A) receptor agonist) on cerebellar interpositus nucleus (IN) neurons. Perfusing slices with GABA (10-1000 microM), baclofen (1-30 microM) and muscimol (1-100 microM) respectively produced a dose-dependent inhibitory response on the IN neurons (n = 39, 62 and 50), which was not blocked by low-Ca(2+)/high-Mg(2+) medium (n = 5, 6 and 6), supporting a direct postsynaptic action of these GABAergic agonists. Moreover, both selective GABA(B) receptor antagonist CGP35348 and selective GABA(A) receptor antagonist bicuculline were capable of partially blocking the inhibitory response of IN neurons to GABA (n = 14 and 11), suggesting that the GABA-induced inhibition may contain two components, a GABA(B) receptors-mediated component and a GABA(A) receptors-mediated one. Further experiments revealed that not only muscimol (n = 50) but also baclofen (n = 62) suppressed IN cells' activity. The baclofen-induced inhibition was selectively blocked by CGP35348 (n = 12) but not by bicuculline (n = 8), whereas the muscimol-induced inhibition was selectively antagonized by bicuculline (n = 8) instead of CGP35348 (n = 9). These results indicate that GABA(B) receptors in the IN neurons can be activated not only by GABA but also by baclofen, suggesting that besides GABA(A) receptors, GABA(B) receptors may also be involved in mediating the inhibitory effect of GABA on cerebellar IN neurons of adult rats.

Heterogeneous chloride homeostasis and GABA responses in the median preoptic nucleus of the rat

The median preoptic nucleus (MnPO) is an integrative structure of the hypothalamus receiving periphery-derived information pertinent to hydromineral and cardiovascular homeostasis. In this context, excitability of MnPO neurones is controlled by fast GABAergic, glutamatergic and angiotensinergic projection from the subfornical organ (SFO). Taking advantage of a brain slice preparation preserving synaptic connection between the SFO and the MnPO, and appropriate bicarbonate-free artificial cerebrospinal fluid (CSF), we investigated a possible implication of an active outward Cl- transport in regulating efficacy of the GABA(A) receptor-mediated inhibitory response at the SFO-MnPO synapse. When somata of the MnPO neurones was loaded with 18 mm chloride, stimulation of the SFO evoked outward inhibitory postsynaptic currents (IPSCs) in 81% of the MnPO neurones held at -60 mV. Accordingly, E(IPSC) was found 25 mV hyperpolarized from the theoretical value calculated from the Nernst equation, indicating that IPSC polarity and amplitude were driven by an active Cl- extrusion system in these neurones. E(IPSC) estimated with gramicidin-based perforated-patch recordings amounted -89.2 +/- 4.3 mV. Furosemide (100 microm), a pharmacological compound known to block the activity of the neurone-specific K(+)-Cl- cotransporter, KCC2, reversed IPSC polarity and shifted E(IPSC) towards its theoretical value. Presence of the KCC2 protein in the MnPO was further detected with immunohistochemistry, revealing a dense network of KCC2-positive intermingled fibres. In the presence of a GABA(B) receptor antagonist, high-frequency stimulation (5 Hz) of the SFO evoked a train of IPSCs or inhibitory postsynaptic potentials (IPSPs), whose amplitude was maintained throughout the sustained stimulation. Contrastingly, similar 5 Hz stimulation carried out in the presence of furosemide (50 microm) evoked IPSCs/IPSPs, whose amplitude collapsed during the high-frequency stimulation. Similar reduction in inhibitory neurotransmission was also observed in MnPO neurones lacking the functional Cl- extrusion mechanism. We conclude that a majority of MnPO neurones were characterized by a functional Cl- transporter that ensured an efficient activity-dependent Cl- transport rate, allowing sustained synaptic inhibition of these neurones. Pharmacological and anatomical data strongly suggested the involvement of KCC2, as an essential postsynaptic determinant of the inhibitory neurotransmission afferent to the MnPO, a key-structure in the physiology of the hydromineral and cardiovascular homeostasis.

Cellular and molecular control of dendritic growth and development of cerebellar Purkinje cells

Purkinje cells are the principal neurons of the cerebellar cortex and are characterized by a large and highly branched dendritic tree. For this reason, they have for a long time been an attractive model system to study the regulation of dendritic growth and differentiation. In this article, I will first review studies on different aspects of Purkinje cell dendritic development and then go on to present studies which have aimed at experimentally altering Purkinje cell dendritic development. Some of the cellular and molecular mechanisms which have been shown by these studies to be important determinants of Purkinje cell dendritic development will be discussed, in particular the role of the parallel fiber input, of hormones, and of neuronal growth factors. The organotypic slice culture method will be introduced as an important experimental tool to study Purkinje cell dendritic development under controlled conditions. Using cerebellar slice cultures, protein kinase C (PKC) has been identified as a major determinant of Purkinje cell dendritic development and the contribution of specific isoforms of PKC will be discussed. Finally, it will be shown that Purkinje cell dendritic development in slice cultures does not depend on the activation of glutamate receptors and appears to be independent of the presence of the neurotrophin BDNF. These studies indicate that the initial outgrowth of the Purkinje cell dendritic tree can occur in the absence of signals derived from afferent fibers, but is under control of PKC signaling.

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

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

Maintenance of High-Frequency Transmission at Purkinje to Cerebellar Nuclear Synapses by Spillover from Boutons with Multiple Release Sites

Cerebellar Purkinje neurons maintain high firing rates but their synaptic terminals depress only moderately, raising the question of how vesicle depletion is minimized. To identify mechanisms that limit synaptic depression, we evoked 100 Hz trains of GABAergic inhibitory postsynaptic currents (IPSCs) in cerebellar nuclear neurons by stimulating Purkinje axons in mouse brain slices. The paired-pulse ratio (IPSC(2)/IPSC(1)) of the total IPSC was approximately 1 and the steady-state ratio (IPSC(20)/IPSC(1)) was approximately 0.5, suggesting a high response probability of postsynaptic receptors, without an unusually high release probability. Three-dimensional electron microscopic reconstructions of Purkinje boutons revealed multiple active zones without intervening transporters, suggestive of "spillover"-mediated transmission. Simulations of boutons with 10-16 release sites, in which transmitter from any site can reach all receptors opposite the bouton, replicated multiple-pulse depression during normal, high, and low presynaptic Ca influx. These results suggest that release from multiple-site boutons limits depletion-based depression, permitting prolonged, high-frequency inhibition at corticonuclear synapses.

Purkinje cell synapses target physiologically unique brainstem neurons

The cerebellum controls motor learning via Purkinje cell synapses onto discrete populations of neurons in the deep cerebellar nuclei and brainstem vestibular nuclei. In the circuitry that subserves the vestibulo-ocular reflex, the postsynaptic targets of Purkinje cells, termed flocculus target neurons (FTNs), are thought to be a critical site of learning. Little is known, however, about the intrinsic cellular properties of FTNs, which are sparsely distributed in the medial vestibular nucleus. To identify these neurons, we used the L7 promoter to express a tau-green fluorescent protein fusion protein selectively in Purkinje cells. Fluorescent Purkinje cell axons and terminal boutons surrounded the somata and proximal dendrites of a small subset of neurons, presumed FTNs, in the medial vestibular nucleus. Targeted intracellular recordings revealed that FTNs fired spontaneously at high rates in brain slices (mean, 47 spikes/sec) and exhibited dramatic postinhibitory rebound firing after the offset of membrane hyperpolarization. These intrinsic firing properties were exceptional among brainstem vestibular nucleus neurons but strikingly similar to neurons in the deep cerebellar nuclei, indicating a common role for intrinsic firing mechanisms in cerebellar control of diverse behaviors.

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

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

Depression of inhibitory synaptic transmission between Purkinje cells and neurons of the cerebellar nuclei

Neurons of the cerebellar nuclei have basal firing rates of 10-20 Hz, despite the convergence of many GABAergic Purkinje terminals onto cerebellar nuclear somata and the high spontaneous firing rate of Purkinje neurons. This persistence of firing during a constant barrage of inhibition raises the question of what patterns of Purkinje cell input inhibit nuclear cells most effectively. To explore the hypothesis that synaptic depression moderates inhibition at this synapse, we made whole-cell recordings from cerebellar nuclear neurons in mouse brain slices. IPSCs and IPSPs were elicited by electrically stimulating the corticonuclear tract at 10, 50, and 100 Hz. IPSCs evoked at the mean spontaneous firing rate of Purkinje cells (50 Hz) depressed by approximately 60%. The onset of depression had a fast, frequency-dependent component, from which recovery was rapid (approximately 100 msec), and a slower, frequency-independent component, from which recovery was slow (approximately 10 sec). As stimulation rate increased, steady-state depression increased, but each IPSC decayed less completely between stimuli, producing a "tonic" IPSC. Changes in stimulation rate produced rapid changes in the level of depression. Under current clamp, cerebellar nuclear neurons fired spontaneously. During 50 Hz trains of IPSPs, firing was initially interrupted, but resumed coincident with the onset of depression. Low-frequency trains entrained postsynaptic firing, and high-frequency trains greatly slowed firing, primarily because of the tonic IPSC. Thus, the properties of depression at this synapse appear to limit the sensitivity of nuclear cells to basal inhibition, while allowing the cells to respond to increases and decreases in Purkinje cell activity.

Cerebellar cortical inhibition and classical eyeblink conditioning

The cerebellum is considered a brain structure in which memories for learned motor responses (e.g., conditioned eyeblink responses) are stored. Within the cerebellum, however, the relative importance of the cortex and the deep nuclei in motor learning/memory is not entirely clear. In this study, we show that the cerebellar cortex exerts both basal and stimulus-activated inhibition to the deep nuclei. Sequential application of a gamma-aminobutyric acid type A receptor (GABA(A)R) agonist and a noncompetitive GABA(A)R antagonist allows selective blockade of stimulus-activated inhibition. By using the same sequential agonist and antagonist methods in behaving animals, we demonstrate that the conditioned response (CR) expression and timing are completely dissociable and involve different inhibitory inputs; although the basal inhibition modulates CR expression, the conditioned stimulus-activated inhibition is required for the proper timing of the CR. In addition, complete blockade of cerebellar deep nuclear GABA(A)Rs prevents CR acquisition. Together, these results suggest that different aspects of the memories for eyeblink CRs are encoded in the cerebellar cortex and the cerebellar deep nuclei.

Ionic currents and spontaneous firing in neurons isolated from the cerebellar nuclei

Neurons of the cerebellar nuclei fire spontaneous action potentials both in vitro, with synaptic transmission blocked, and in vivo, in resting animals, despite ongoing inhibition from spontaneously active Purkinje neurons. We have studied the intrinsic currents of cerebellar nuclear neurons isolated from the mouse, with an interest in understanding how these currents generate spontaneous activity in the absence of synaptic input as well as how they allow firing to continue during basal levels of inhibition. Current-clamped isolated neurons fired regularly ( approximately 20 Hz), with shallow interspike hyperpolarizations (approximately -60 mV), much like neurons in more intact preparations. The spontaneous firing frequency lay in the middle of the dynamic range of the neurons and could be modulated up or down with small current injections. During step or action potential waveform voltage-clamp commands, the primary current active at interspike potentials was a tetrodotoxin-insensitive (TTX), cesium-insensitive, voltage-independent, cationic flux carried mainly by sodium ions. Although small, this cation current could depolarize neurons above threshold voltages. Voltage- and current-clamp recordings suggested a high level of inactivation of the TTX-sensitive transient sodium currents that supported action potentials. Blocking calcium currents terminated firing by preventing repolarization to normal interspike potentials, suggesting a significant role for K(Ca) currents. Potassium currents that flowed during action potential waveform voltage commands had high activation thresholds and were sensitive to 1 mm TEA. We propose that, after the decay of high-threshold potassium currents, the tonic cation current contributes strongly to the depolarization of neurons above threshold, thus maintaining the cycle of firing.

Postsynaptic currents in deep cerebellar nuclei

Postsynaptic currents were studied by whole cell recordings in visually identified large neurons of the deep cerebellar nuclei (DCN) in slices of 4- to 11-day-old mice. Spontaneous postsynaptic currents were abolished by the GABA(A) receptor antagonist bicuculline and had a single-exponential decay with a mean time constant of 13.6 +/- 3.2 (SD) ms. Excitatory postsynaptic currents (EPSCs) were evoked in 48/56 neurons recorded. The addition of AMPA and N-methyl-D-aspartate (NMDA) receptor antagonists together completely abolished all synaptic responses. In 1 mM [Mg(2+)](o) and at a holding potential of -60 mV, the peak amplitude of the NMDA component of the EPSC (NMDA-EPSC) was 83.2 +/- 21.2% of the AMPA component (AMPA-EPSC). This indicates that in DCN neurons, at a physiological [Mg(2+)](o) and at the resting membrane potential, NMDA receptors contribute to the synaptic signal. AMPA-EPSCs had a linear current-voltage relationship with a reversal potential of +2.3 +/- 0.4 mV and a single-exponential decay with a voltage-dependent time constant that at -60 mV was 7.1 +/- 3.3 ms. In 10 microM glycine and 1 mM [Mg(2+)](o), the I-V relationship of NMDA-EPSCs had a reversal potential of -0.5 +/- 3.3 mV and a maximal inward current at -33.4 +/- 5.8 mV. The apparent dissociation constant (K(D)) of Mg(2+) for the NMDA receptor-channel at -60 mV, measured by varying [Mg(2+)](o), was 135.5 +/- 55.3 microM, and when measured by fitting the I-V curves with a theoretical function, it was 169.9 +/- 119.5 microM. Thus in the DCN, NMDA receptors have a sensitivity to Mg(2+) that corresponds to subunits that are weakly blocked by this ion (epsilon 3 and epsilon 4) of which the DCN express epsilon 4. NMDA-EPSCs had a double-exponential decay with voltage-dependent time constants that at -60 mV were 20.2 +/- 8.9 and 136.4 +/- 62.8 ms. At positive voltages, the time constants were slower and their contributions were about equal, while in the negative slope conductance region of the I-V curve, the faster time constant became predominant, conferring faster kinetics to the EPSC. The weak sensitivity to Mg(2+) of NMDA receptors, together with a relatively fast kinetics, provide DCN neurons with strong excitatory inputs in which fast dynamic signals are relatively well preserved.

Timing mechanisms in the cerebellum: testing predictions of a large-scale computer simulation

We used large-scale computer simulations of eyelid conditioning to investigate how the cerebellum generates and makes use of temporal information. In the simulations the adaptive timing displayed by conditioned responses is mediated by two factors: (1) different sets of granule cells are active at different times during the conditioned stimulus (CS), and (2) responding is not only amplified at reinforced times but also suppressed at unreinforced times during the CS. These factors predict an unusual pattern of responding after partial removal of the cerebellar cortex that was confirmed using small, electrolytic lesions of cerebellar cortex. These results are consistent with timing mechanisms in the cerebellum that are similar to Pavlov's "inhibition of delay" hypothesis.

In vitro biocytin injection into perinatal mouse brain: a method for tract tracing in developing tissue

Injection of biocytin provides an effective method for labeling axonal projections. Several difficulties arise when this technique is employed in fetal or early postnatal animals in vivo, including limited access to injection sites and extended post-injection survival periods. To circumvent these problems, we adapted the technique of extracellular biocytin injection for use in explanted brain hemispheres of developing mice. Briefly, entire brain hemispheres from perinatal mice (E16-P9) were removed and placed in oxygenated aCSF in a brain slice recording chamber. Following visually guided injection of biocytin (2%) into the prelimbic cortex, the brains were then incubated in oxygenated artificial cerebrospinal fluid (aCSF) for varying periods of time and then immersion-fixed in 4% paraformaldehyde and 0.5% glutaraldehyde. The next day, the brains were sectioned and processed for biocytin histochemistry using the avidin-biotin-complex method. We examined the method of injection, electrode type, time of injection, and post-injection incubation period. We found that in E16-P9 animals iontophoresis of biocytin using 8- to 12-megaohm patch clamp electrodes for a duration of 10 min provides optimal axonal labeling. Post-injection incubation times of four or more hours are sufficient for labeling fine caliber collaterals as well as axon bundles that reach distances over 3 mm. In vitro injection of biocytin into explanted brain hemispheres provides a quick and easy method for tract tracing in developing brains.

Interrelated modification of excitatory and inhibitory synapses in three-layer olivary-cerebellar neural network

The model of three-layer olivary-cerebellar neural network with modifiable excitatory and inhibitory connections between diverse elements is suggested. The same Hebbian modification rules are proposed for Purkinje cells, granule (input) cells, and deep cerebellar nuclei (output) cells. The inverse calcium-dependent modification rules for these cells and hippocampal/neocortical neurones or Golgi cells are conceivably the result of the involvement of cGMP and cAMP in postsynaptic processes. The sign of simultaneous modification of excitatory and inhibitory inputs to a cell is opposite and determined by the variations in pre- and/or postsynaptic cell activity. Modification of excitatory transmission between parallel fibers and Purkinje cells, mossy fibers and granule cells, and mossy fibers and deep cerebellar nuclei cells essentially depends on inhibition effected by stellate/basket cells, Golgi cells and Purkinje cells, respectively. The character of interrelated modifications of diverse synapses in all three layers of the network is influenced by olivary cell activity. In the absence (presence) of a signal from inferior olive, the long-term potentiation (depression) in the efficacy of a synapse between input mossy fiber and output cell can be induced. The results of the suggested model are in accordance with known experimental data.

Regulation of the rebound depolarization and spontaneous firing patterns of deep nuclear neurons in slices of rat cerebellum

Current-clamp recordings were made from the deep cerebellar nuclei (DCN) of 12- to 15-day-old rats to understand the factors that mediate intrinsic spontaneous firing patterns. All of the cells recorded were spontaneously active with spiking patterns ranging continuously from regular spiking to spontaneous bursting with the former predominating. A robust rebound depolarization (RD) leading to a Na(+) spike burst was elicited after the offset of hyperpolarizing current injection. The voltage and time dependence of the RD was consistent with mediation by low-threshold voltage-gated Ca(2+) channels. In addition, induction of a RD also may be affected by activation of a hyperpolarization-activated cation current, I(h). A RD could be evoked efficiently after brief high-frequency bursts of inhibitory postsynaptic potentials (IPSPs) induced by stimulation of Purkinje cell axons. IPSP-driven RDs were typically much larger and longer than those elicited by direct hyperpolarizing pulses of approximately matched amplitude and duration. Intracellular perfusion of the Ca(2+) buffer bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) dramatically enhanced the RD and its associated spiking, sometimes leading to a plateau potential that lasted several hundred milliseconds. The effects of BAPTA could be mimicked partly by application of apamin, a blocker of small conductance Ca(2+)-gated K(+) channels, but not by paxilline, which blocks large conductance Ca(2+)-gated K(+) channels. Application of both BAPTA and apamin, but not paxilline, caused cells that were regularly spiking to burst spontaneously. Taken together, our data suggest that there is a strong relationship between the ability of DCN cells to elicit a RD and their tendency burst spontaneously. The RD can be triggered by the opening of T-type Ca(2+) channels with an additional contribution of hyperpolarization-activated current I(h). RD duration is regulated by small-conductance Ca(2+)-gated K(+) channels. The RD also is modulated tonically by inhibitory inputs. All of these factors are in turn subject to alteration by extrinsic modulatory neurotransmitters and are, at least in part, responsible for determining the firing modes of DCN neurons.

Polarity of long-term synaptic gain change is related to postsynaptic spike firing at a cerebellar inhibitory synapse

Long-term potentiation and depression (LTP and LTD) in excitatory synapses can coexist, the former being triggered by stimuli that produce strong postsynaptic excitation and the latter by stimuli that produce weaker postsynaptic excitation. It has not been determined whether these properties also apply to LTP and LTD in the inhibitory synapses between Purkinje neurons and the neurons of the deep cerebellar nuclei (DCN), a site that has been implicated in certain types of motor learning. DCN cells exhibit a prominent rebound depolarization (RD) and associated spike burst upon release from hyperpolarization. In these cells, LTP can be elicited by short, high-frequency trains of inhibitory postsynaptic potentials (IPSPs), which reliably evoke an RD. LTD is induced if the same protocol is applied with conditions where the amount of postsynaptic excitation is reduced. The polarity of the change in synaptic strength is correlated with the amount of RD-evoked spike firing during the induction protocol. Thus, some important computational principles that govern the induction of use-dependent change in excitatory synaptic efficacy also apply to inhibitory synapses.

Cross-correlation analysis of neuron connections in the cerebellum of conscious rabbits

Cross-interval and cross-correlation analysis of neuron activity was used to study cell interactions in the cerebellar vermis of conscious rabbits. The cross-correlation method, based on post-synaptic potentials (PSP) from one member of a pair of neurons, supplements the cross-interval analysis used for evaluating the interactions of neurons in terms of action potentials. A total of 16 neuron pairs were analyzed, in which the activity of one neuron was recorded intracellularly and that of the other extra- or intracellularly. Interactions were observed in 10 pairs. Most pairs showed inhibitory interactions. Pairs with inhibitory interactions included those in which the activity of one or both neurons contained complex spikes. Also of interest was the analysis of connections in which influences were excitatory in one direction and inhibitory in the other. In these cases, neuron discharges were invariably simple spikes. Variability and plasticity in the output signals of Purkinje cells were greater than the variability and plasticity of the common inputs to Purkinje cells.

Development of a topographically organized auditory network in slice culture is calcium dependent

Inhibitory and excitatory connections of remarkably precise topographic order are characteristic features of the mammalian auditory system, particularly within the superior olivary complex (SOC). Little is known about the requirements for the correct development of these specific connections. Previous in vivo experiments have demonstrated a high expression of calcium-binding proteins in this system during development, pointing to the need for precise calcium regulation. Here, we have employed an organotypic slice culture from the above neuronal network and analyzed the requirements for the maintenance and development of this system in vitro. When slices from neonatal rats were incubated in standard culture medium for up to 7 days, we found no organotypic features. Only if 25 mM KCl was added to the culture medium, the cytoarchitecture of the nuclei, the neuronal morphology, and the specificity and topography of internuclear connections were indistinguishable from that in vivo. The addition of calcium channel blockers (MgCl2 and nifedipine) to the high-KCl medium reduced organotypicity drastically, indicating that a depolarization-induced increase of intracellular calcium is indispensable. Furthermore, the temporal course of the expression of the calcium-binding protein parvalbumin in culture under high KCl mimics that in vivo, demonstrating developmental processes during incubation. The need for calcium influx into neurons of this auditory network in vitro (which is not seen in other slice culture systems) strengthens the hypothesis that an optimal calcium concentration is exceptionally important in auditory neurons. The effect of KCl in the slice cultures may substitute for input activity regulating intracellular calcium in auditory neurons in vivo.

GABA-ergic transmission in deep cerebellar nuclei

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

GABA(B) receptor activation of Purkinje cells in cerebellar slices

The metabotropic GABA(B) receptors are densely represented in the molecular layer of the cerebellar cortex which contains the dendritic tree of the Purkinje cells (PCs). We report here the results obtained by applying Baclofen, the specific GABA(B) agonist, to PCs recorded intrasomatically in cerebellar slices. Diluted in the perfusion solution or applied by pressure to the molecular layer near to the recorded cell, Baclofen dose-dependently inhibited the PCs as seen by the suppression of Na and Ca dependent action potentials accompanied by a variable membrane hyperpolarization. The weak hyperpolarization was interpreted as due to the dendritic localization of the receptors. These results concerned postsynaptic receptor sites since they persisted after bath applied TTX blocking presynaptic activity. They also persisted in the presence of bicuculline, the GABA(A) antagonist, but they were reduced by bath application of 2-OH saclofen and CGP55845A, both being GABA(B) receptor antagonists. Current clamp experiments revealed a conductance increase with an equilibrium potential consistent with a K+ channel opening. The conclusions were reached that GABA inhibition of the PCs is mediated by GABA(B) receptors in the dendrites and GABA(A) receptors in the soma and dendrites. Therefore, the GABA released by stellate cells modulate PC activity through two inhibitory mechanisms.

Fast (mainly 30-100 Hz) oscillations in the cat cerebellothalamic pathway and their synchronization with cortical potentials

1. Intracellular recordings from 216 thalamocortical (TC) neurones in the ventrolateral (VL) nucleus of intact-cortex and decorticated cats under ketamine-xylazine anaesthesia revealed spontaneously occurring fast oscillations (mainly 30-100 Hz) in 86% of investigated cells. The fast depolarizing events consisted of excitatory postsynaptic potentials (EPSPs), giving rise to fast prepotentials (FPPs) in 22% of neurones, which eventually lead to full-blown action potentials. The frequency of fast events changed by factors of 2-5 in periods as short as 0.3-1.0 s. 2. The spontaneous oscillations were similar to responses evoked in VL relay neurones by stimuli to the afferent cerebellofugal axons in brachium conjunctivum (BC) and were strikingly reduced or abolished after electrolytic lesion of BC axons. 3. The amplitude and duration of fast depolarizing events were significantly reduced during the descending phase of the inhibitory postsynaptic potentials (IPSPs) in TC cells, related to spontaneous spindles or evoked by local thalamic stimulation. 4. Averaged field potentials recorded from motor cortex and triggered by EPSPs and/or action potentials of intracellularly recorded VL cells demonstrated that both spontaneous and BC-evoked fast depolarizations in VL relay neurones were coherent with fast rhythms in cortical area 4. 5. These results show that, in addition to the thalamic and cortical generation sites of the fast (so-called gamma) oscillations, prethalamic relay stations, such as deep cerebellar nuclei, are major contributors to the induction of fast rhythms which depend on the depolarization of thalamic and cortical neurones and which represent a hallmark of brain activation patterns.

Purkinje cell survival and axonal regeneration are age dependent: an in vitro study

Purkinje cells are among the most resistant neurons to axotomy and the most refractory to axonal regeneration. By using organotypic cultures, we have studied age- and environment-related factors implicated in Purkinje cell survival and axonal regeneration. Most Purkinje cells taken from 1- to 5-d-old rats, the period in which these neurons are engaged in intense synaptogenesis and dendritic remodeling, die 1 week after plating, whereas if cultured before or after this period, Purkinje cells survive, even in the absence of deep nuclear neurons, their postsynaptic targets. Cerebellar slices taken from 10-d-old rats and kept in vitro for 1 week acquire a cellular composition resembling mature cerebellum. Their Purkinje cells are resistant to axotomy, but even when confronted with permissive environments (sciatic nerves or fetal cerebellar slices), their axons do not regenerate. In contrast, fetal rat and mouse Purkinje cells are able to regenerate their axons on mature cerebellar slices. This regeneration is massive, and the regrowing axons invade all cerebellar regions of the apposed mature slices, including white matter. These results show that Purkinje cell survival and axonal regeneration are age-related and independent from environmental constraints. Moreover, our observations suggest strongly that the onset of synaptogenesis of Purkinje cell axons could provide a signal to turn off their growth program and that, thereafter, permissive microenvironment alone is unable to reestablish such a program.

Beta-adrenergic modulation of GABAergic inhibition in the deep cerebellar nuclei of F344 rats

The presence of norepinephrine (NE) and NE activated cells, in the deep cerebellar nuclei (DCN) of male F344 rats, was investigated using immunohistochemistry and electrophysiology, during iontophoresis of the beta-adrenergic agonist isoproterenol (ISO). During extracellular electrophysiology, GABA was iontophoretically applied to the cell and ISO was then co-applied in an attempt to modulate the GABAergic inhibition of cell firing in the DCN. Immunohistochemistry was used to detect tyrosine hydroxylase (TH) positive fibers in the DCN. Isoproterenol modulated GABAergic inhibition in 51% of the DCN cells recorded from. In addition, TH-positive fibers that appeared to make contact with DCN cells were found. Therefore, this study demonstrated that functional NE receptors exist in the DCN and NE appears to be present in fibers therein.