Stimulus-dependent activation of c-Fos in neurons and glia in the rat cerebellum

Cerebellar contributions to a brainwide network for flexible behavior in mice

The cerebellum regulates nonmotor behavior, but the routes of influence are not well characterized. Here we report a necessary role for the posterior cerebellum in guiding a reversal learning task through a network of diencephalic and neocortical structures, and in flexibility of free behavior. After chemogenetic inhibition of lobule VI vermis or hemispheric crus I Purkinje cells, mice could learn a water Y-maze but were impaired in ability to reverse their initial choice. To map targets of perturbation, we imaged c-Fos activation in cleared whole brains using light-sheet microscopy. Reversal learning activated diencephalic and associative neocortical regions. Distinctive subsets of structures were altered by perturbation of lobule VI (including thalamus and habenula) and crus I (including hypothalamus and prelimbic/orbital cortex), and both perturbations influenced anterior cingulate and infralimbic cortex. To identify functional networks, we used correlated variation in c-Fos activation within each group. Lobule VI inactivation weakened within-thalamus correlations, while crus I inactivation divided neocortical activity into sensorimotor and associative subnetworks. In both groups, high-throughput automated analysis of whole-body movement revealed deficiencies in across-day behavioral habituation to an open-field environment. Taken together, these experiments reveal brainwide systems for cerebellar influence that affect multiple flexible responses.

A comparison of how deep brain stimulation in two targets with anti-compulsive efficacy modulates brain activity using fMRI in awake rats

Deep brain stimulation (DBS) is an established neuromodulatory intervention against otherwise treatment-refractory obsessive-compulsive disorder (OCD). Several DBS targets, all of which are part of brain networks connecting basal ganglia and prefrontal cortex, alleviate OCD symptoms. Stimulation of these targets is thought to unfold its therapeutic effect by modulation of network activity through internal capsule (IC) connections. Research into DBS-induced network changes and the nature of IC-related effects of DBS in OCD is needed to further improve DBS. Here, we studied the effects of DBS at the ventral medial striatum (VMS) and IC on blood-oxygen level dependent (BOLD) responses in awake rats using functional magnetic resonance imaging (fMRI). BOLD-signal intensity was measured in five regions of interest (ROIs): medial and orbital prefrontal cortex, nucleus accumbens (NAc), IC area, and mediodorsal thalamus. In previous rodent studies, stimulation at both target locations resulted in a reduction of OCD-like behavior and activation of prefrontal cortical areas. Therefore, we hypothesized that stimulation at both targets would result in partially overlapping BOLD responses. Both differential and overlapping activity between VMS and IC stimulation was found. Stimulating the caudal part of the IC resulted in activation around the electrode, while stimulating the rostral part of the IC resulted in increased cross-correlations between the IC area, orbitofrontal cortex, and NAc. Stimulation of the dorsal part of the VMS resulted in increased activity in the IC area, suggesting this area is activated during both VMS and IC stimulation. This activation is also indicative of VMS-DBS impacting corticofugal fibers running through the medial caudate into the anterior IC, and both VMS and IC DBS might act on these fibers to induce OCD-reducing effects. These results show that rodent fMRI with simultaneous electrode stimulation is a promising approach to study the neural mechanisms of DBS. Comparing the effects of DBS in different target areas has the potential to improve our understanding of the neuromodulatory changes that take place across various networks and connections in the brain. Performing this research in animal disease models will lead to translational insights in the mechanisms underlying DBS, and can aid improvement and optimization of DBS in patient populations.

T-type calcium channels as therapeutic targets in essential tremor and Parkinson's disease

Neuronal action potential firing patterns are key components of healthy brain function. Importantly, restoring dysregulated neuronal firing patterns has the potential to be a promising strategy in the development of novel therapeutics for disorders of the central nervous system. Here, we review the pathophysiology of essential tremor and Parkinson's disease, the two most common movement disorders, with a focus on mechanisms underlying the genesis of abnormal firing patterns in the implicated neural circuits. Aberrant burst firing of neurons in the cerebello-thalamo-cortical and basal ganglia-thalamo-cortical circuits contribute to the clinical symptoms of essential tremor and Parkinson's disease, respectively, and T-type calcium channels play a key role in regulating this activity in both the disorders. Accordingly, modulating T-type calcium channel activity has received attention as a potentially promising therapeutic approach to normalize abnormal burst firing in these diseases. In this review, we explore the evidence supporting the theory that T-type calcium channel blockers can ameliorate the pathophysiologic mechanisms underlying essential tremor and Parkinson's disease, furthering the case for clinical investigation of these compounds. We conclude with key considerations for future investigational efforts, providing a critical framework for the development of much needed agents capable of targeting the dysfunctional circuitry underlying movement disorders such as essential tremor, Parkinson's disease, and beyond.

Is the inferior olive central to essential tremor? Yes

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

The Pathophysiology and Treatment of Essential Tremor: The Role of Adenosine and Dopamine Receptors in Animal Models

Essential tremor (ET) is one of the most common neurological disorders that often affects people in the prime of their lives, leading to a significant reduction in their quality of life, gradually making them unable to independently perform the simplest activities. Here we show that current ET pharmacotherapy often does not sufficiently alleviate disease symptoms and is completely ineffective in more than 30% of patients. At present, deep brain stimulation of the motor thalamus is the most effective ET treatment. However, like any brain surgery, it can cause many undesirable side effects; thus, it is only performed in patients with an advanced disease who are not responsive to drugs. Therefore, it seems extremely important to look for new strategies for treating ET. The purpose of this review is to summarize the current knowledge on the pathomechanism of ET based on studies in animal models of the disease, as well as to present and discuss the results of research available to date on various substances affecting dopamine (mainly D3) or adenosine A1 receptors, which, due to their ability to modulate harmaline-induced tremor, may provide the basis for the development of new potential therapies for ET in the future.

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

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

Pramipexole Reduces zif-268 mRNA Expression in Brain Structures involved in the Generation of Harmaline-Induced Tremor

Essential tremor is one of the most common neurological disorders, however, it is not sufficiently controlled with currently available pharmacotherapy. Our recent study has shown that pramipexole, a drug efficient in inhibiting parkinsonian tremor, reduced the harmaline-induced tremor in rats, generally accepted to be a model of essential tremor. The aim of the present study was to investigate brain targets for the tremorolytic effect of pramipexole by determination of the early activity-dependent gene zif-268 mRNA expression. Tremor in rats was induced by harmaline administered at a dose of 15 mg/kg ip. Pramipexole was administered at a low dose of 0.1 mg/kg sc. Tremor was measured by Force Plate Actimeters where four force transducers located below the corners of the plate tracked the animal's position on a Cartesian plane. The zif-268 mRNA expression was analyzed by in situ hybridization in brain slices. Harmaline induced tremor and increased zif-268 mRNA levels in the inferior olive, cerebellar cortex, ventroanterior/ventrolateral thalamic nuclei and motor cortex. Pramipexole reversed both the harmaline-induced tremor and the increase in zif-268 mRNA expression in the inferior olive, cerebellar cortex and motor cortex. Moreover, the tremor intensity correlated positively with zif-268 mRNA expression in the above structures. The present results seem to suggest that the tremorolytic effect of pramipexole is related to the modulation of the harmaline-increased neuronal activity in the tremor network which includes the inferior olive, cerebellar cortex and motor cortex. Potential mechanisms underlying the above pramipexole action are discussed.

Purkinje cell misfiring generates high-amplitude action tremors that are corrected by cerebellar deep brain stimulation

Tremor is currently ranked as the most common movement disorder. The brain regions and neural signals that initiate the debilitating shakiness of different body parts remain unclear. Here, we found that genetically silencing cerebellar Purkinje cell output blocked tremor in mice that were given the tremorgenic drug harmaline. We show in awake behaving mice that the onset of tremor is coincident with rhythmic Purkinje cell firing, which alters the activity of their target cerebellar nuclei cells. We mimic the tremorgenic action of the drug with optogenetics and present evidence that highly patterned Purkinje cell activity drives a powerful tremor in otherwise normal mice. Modulating the altered activity with deep brain stimulation directed to the Purkinje cell output in the cerebellar nuclei reduced tremor in freely moving mice. Together, the data implicate Purkinje cell connectivity as a neural substrate for tremor and a gateway for signals that mediate the disease.

Current Opinions and Consensus for Studying Tremor in Animal Models

Tremor is the most common movement disorder; however, we are just beginning to understand the brain circuitry that generates tremor. Various neuroimaging, neuropathological, and physiological studies in human tremor disorders have been performed to further our knowledge of tremor. But, the causal relationship between these observations and tremor is usually difficult to establish and detailed mechanisms are not sufficiently studied. To overcome these obstacles, animal models can provide an important means to look into human tremor disorders. In this manuscript, we will discuss the use of different species of animals (mice, rats, fruit flies, pigs, and monkeys) to model human tremor disorders. Several ways to manipulate the brain circuitry and physiology in these animal models (pharmacology, genetics, and lesioning) will also be discussed. Finally, we will discuss how these animal models can help us to gain knowledge of the pathophysiology of human tremor disorders, which could serve as a platform towards developing novel therapies for tremor.

Suppression of Harmaline Tremor by Activation of an Extrasynaptic GABAA Receptor: Implications for Essential Tremor

Background

Metabolic imaging has revealed excessive cerebellar activity in essential tremor patients. Golgi cells control cerebellar activity by releasing gamma-aminobutyric acid (GABA) onto synaptic and extrasynaptic receptors on cerebellar granule cells. We postulated that the extrasynaptic GABA_A receptor-specific agonist THIP (gaboxadol; 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol) would suppress tremor in the harmaline model of essential tremor and, since cerebellar extrasynaptic receptors contain α6 and δ subunits, would fail to do so in mice lacking either subunit.

Methods

Digitally measured motion power, expressed as 10-16 Hz power (the tremor bandwidth) divided by background 8-32 Hz motion power, was accessed during pre-harmaline baseline, pre-THIP harmaline exposure, and after THIP administration (0, 2, or 3 mg/kg). These low doses were chosen as they did not impair performance on the straight wire test, a sensitive test for psychomotor impairment. Littermate δ wild-type and knockout (Gabrd^+/+, Gabrd^-/-) and littermate α6 wild-type and knockout (Gabra6^+/+, Gabra6^-/- ) mice were tested.

Results

Gabrd^+/+ mice displayed tremor reduction at 3 mg/kg THIP but not 2 mg/kg, and Gabra6^+/+ mice showed tremor reduction at 2 and 3 mg/kg. Their respective subunit knockout littermates displayed no tremor reduction compared with vehicle controls at either dose.

Discussion

The loss of anti-tremor efficacy with deletion of either δ or α6 GABA_A receptor subunits indicates that extrasynaptic receptors containing both subunits, most likely located on cerebellar granule cells where they are highly expressed, mediate tremor suppression by THIP. A medication designed to activate only these receptors may display a favorable profile for treating essential tremor.

Physical exercise promotes proliferation and differentiation of endogenous neural stem cells via ERK in rats with cerebral infarction

Physical exercise is beneficial for the functional recovery of neurons after stroke. It has been suggested that exercise regulates proliferation and differentiation of endogenous neural stem cells (NSCs); however, the underlying molecular mechanisms are still largely unknown. In the present study, the aim was to investigate whether physical exercise activates the extracellular signal-regulated kinase (ERK) signaling pathway to promote proliferation and differentiation of NSCs in rats with cerebral infarction, thereby improving neurological function. Following middle cerebral artery occlusion, rats underwent physical exercise and neurological behavior was analyzed at various time points. Immunofluorescence staining was performed to detect proliferation and differentiation of NSCs, and western blotting was used to analyze cyclin-dependent kinase 4 (CDK4), Cyclin D1, retinoblastoma protein (p-Rb), P-16, phosphorylated (p)-ERK1/2 and c-Fos expression. The results indicated that physical exercise promoted proliferation and differentiation of NSCs, and led to improved neural function. In addition, the expression levels of CDK4, Cyclin D1, p-Rb, p-ERK1/2 and c-Fos were upregulated, whereas the expression of P-16 was downregulated following exercise. U0126, an inhibitor of ERK signaling, reversed the beneficial effects of exercise. Therefore, it may be hypothesized that physical exercise enhances proliferation and differentiation of endogenous NSCs in the hippocampus of rats with cerebral infarction via the ERK signaling pathway.

Stimulation-dependent remodeling of the corticospinal tract requires reactivation of growth-promoting developmental signaling pathways

The corticospinal tract (CST) can become damaged after spinal cord injury or stroke, resulting in weakness or paralysis. Repair of the damaged CST is limited because mature CST axons fail to regenerate, which is partly because the intrinsic axon growth capacity is downregulated in maturity. Whereas CST axons sprout after injury, this is insufficient to recover lost functions. Chronic motor cortex (MCX) electrical stimulation is a neuromodulatory strategy to promote CST axon sprouting, leading to functional recovery after CST lesion. Here we examine the molecular mechanisms of stimulation-dependent CST axonal sprouting and synapse formation. MCX stimulation rapidly upregulates mTOR and Jak/Stat signaling in the corticospinal system. Chronic stimulation, which leads to CST sprouting and increased CST presynaptic sites, further enhances mTOR and Jak/Stat activity. Importantly, chronic stimulation shifts the equilibrium of the mTOR repressor PTEN to the inactive phosphorylated form suggesting a molecular transition to an axon growth state. We blocked each signaling pathway selectively to determine potential differential contributions to axonal outgrowth and synapse formation. mTOR blockade prevented stimulation-dependent axon sprouting. Surprisingly, Jak/Stat blockade did not abrogate sprouting, but instead prevented the increase in CST presynaptic sites produced by chronic MCX stimulation. Chronic stimulation increased the number of spinal neurons expressing the neural activity marker cFos. Jak/Stat blockade prevented the increase in cFos-expressing neurons after chronic stimulation, confirming an important role for Jak/Stat signaling in activity-dependent CST synapse formation. MCX stimulation is a neuromodulatory repair strategy that reactivates distinct developmentally-regulated signaling pathways for axonal outgrowth and synapse formation.

Abnormalities in the Structure and Function of Cerebellar Neurons and Neuroglia in the Lc/+ Chimeric Mouse Model of Variable Developmental Purkinje Cell Loss

Autism spectrum disorders (ASDs) are a group of neurodevelopmental disorders characterized by impaired and disordered language, decreased social interactions, stereotyped and repetitive behaviors, and impaired fine and gross motor skills. It has been well established that cerebellar abnormalities are one of the most common structural changes seen in the brains of people diagnosed with autism. Common cerebellar pathology observed in autistic individuals includes variable loss of cerebellar Purkinje cells (PCs) and increased numbers of reactive neuroglia in the cerebellum and cortical brain regions. The Lc/+ mutant mouse loses 100 % of cerebellar PCs during the first few weeks of life and provided a valuable model to study the effects of developmental PC loss on underlying structural and functional changes in cerebellar neural circuits. Lurcher (Lc) chimeric mice were also generated to explore the link between variable cerebellar pathology and subsequent changes in the structure and function of cerebellar neurons and neuroglia. Chimeras with the most severe cerebellar pathology (as quantified by cerebellar PC counts) had the largest changes in cFos expression (an indirect reporter of neural activity) in cerebellar granule cells (GCs) and cerebellar nucleus (CN) neurons. In addition, Lc chimeras with the fewest PCs also had numerous reactive microglia and Bergmann glia located in the cerebellar cortex. Structural and functional abnormalities observed in the cerebella of Lc chimeras appeared to be along a continuum, with the degree of pathology related to the number of PCs in individual chimeras.

Tremorolytic effect of 5'-chloro-5'-deoxy-(±)-ENBA, a potent and selective adenosine A1 receptor agonist, evaluated in the harmaline-induced model in rats

AIM

The aim of this study was to examine the role of adenosine A₁ receptors in the harmaline-induced tremor in rats using 5'-chloro-5'-deoxy-(±)-ENBA (5'Cl5'd-(±)-ENBA), a brain-penetrant, potent, and selective adenosine A₁ receptor agonist.

METHODS

Harmaline was injected at a dose of 15 mg/kg ip and tremor was measured automatically in force-plate actimeters by an increased averaged power in the frequency band of 9-15 Hz (AP2) and by tremor index (a difference in power between AP2 and averaged power in the frequency band of 0-8 Hz). The zif-268 mRNA expression was additionally analyzed by in situ hybridization in several brain structures.

RESULTS

5'Cl5'd-(±)-ENBA (0.05-0.5 mg/kg ip) dose dependently reduced the harmaline-induced tremor and this effect was reversed by 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), a selective antagonist of adenosine A₁ receptors (1 mg/kg ip). Harmaline increased the zif-268 mRNA expression in the inferior olive, cerebellar cortex, ventroanterior/ventrolateral thalamic nuclei, and motor cortex. 5'Cl5'd-(±)-ENBA reversed these increases in all the above structures. DPCPX reduced the effect of 5'Cl5'd-(±)-ENBA on zif-268 mRNA in the motor cortex.

CONCLUSION

This study suggests that adenosine A₁ receptors may be a potential target for the treatment of essential tremor.

MSG-Evoked c-Fos Activity in the Nucleus of the Solitary Tract Is Dependent upon Fluid Delivery and Stimulation Parameters

The marker of neuronal activation, c-Fos, can be used to visualize spatial patterns of neural activity in response to taste stimulation. Because animals will not voluntarily consume aversive tastes, these stimuli are infused directly into the oral cavity via intraoral cannulae, whereas appetitive stimuli are given in drinking bottles. Differences in these 2 methods make comparison of taste-evoked brain activity between results that utilize these methods problematic. Surprisingly, the intraoral cannulae experimental conditions that produce a similar pattern of c-Fos activity in response to taste stimulation remain unexplored. Stimulation pattern (e.g., constant/intermittent) and hydration state (e.g., water-restricted/hydrated) are the 2 primary differences between delivering tastes via bottles versus intraoral cannulae. Thus, we quantified monosodium glutamate (MSG)-evoked brain activity, as measured by c-Fos, in the nucleus of the solitary tract (nTS; primary taste nucleus) across several conditions. The number and pattern of c-Fos neurons in the nTS of animals that were water-restricted and received a constant infusion of MSG via intraoral cannula most closely mimicked animals that consumed MSG from a bottle. Therefore, in order to compare c-Fos activity between cannulae-stimulated and bottle-stimulated animals, cannulated animals should be water restricted prior to stimulation, and receive taste stimuli at a constant flow.

Regulation of breathing and autonomic outflows by chemoreceptors

Lung ventilation fluctuates widely with behavior but arterial PCO2 remains stable. Under normal conditions, the chemoreflexes contribute to PaCO2 stability by producing small corrective cardiorespiratory adjustments mediated by lower brainstem circuits. Carotid body (CB) information reaches the respiratory pattern generator (RPG) via nucleus solitarius (NTS) glutamatergic neurons which also target rostral ventrolateral medulla (RVLM) presympathetic neurons thereby raising sympathetic nerve activity (SNA). Chemoreceptors also regulate presympathetic neurons and cardiovagal preganglionic neurons indirectly via inputs from the RPG. Secondary effects of chemoreceptors on the autonomic outflows result from changes in lung stretch afferent and baroreceptor activity. Central respiratory chemosensitivity is caused by direct effects of acid on neurons and indirect effects of CO2 via astrocytes. Central respiratory chemoreceptors are not definitively identified but the retrotrapezoid nucleus (RTN) is a particularly strong candidate. The absence of RTN likely causes severe central apneas in congenital central hypoventilation syndrome. Like other stressors, intense chemosensory stimuli produce arousal and activate circuits that are wake- or attention-promoting. Such pathways (e.g., locus coeruleus, raphe, and orexin system) modulate the chemoreflexes in a state-dependent manner and their activation by strong chemosensory stimuli intensifies these reflexes. In essential hypertension, obstructive sleep apnea and congestive heart failure, chronically elevated CB afferent activity contributes to raising SNA but breathing is unchanged or becomes periodic (severe CHF). Extreme CNS hypoxia produces a stereotyped cardiorespiratory response (gasping, increased SNA). The effects of these various pathologies on brainstem cardiorespiratory networks are discussed, special consideration being given to the interactions between central and peripheral chemoreflexes.

Molecular neuroimaging of post-injury plasticity

Nerve injury induces long-term changes in neuronal activity in the primary somatosensory cortex (S1), which has often been implicated as the origin of sensory dysfunction. However, the cellular mechanisms underlying this phenomenon remain unclear. C-fos is an immediate early gene, which has been shown to play an instrumental role in plasticity. By developing a new platform to image real-time changes in gene expression in vivo, we investigated whether injury modulates the levels of c-fos in layer V of S1, since previous studies have suggested that these neurons are particularly susceptible to injury. The yellow fluorescent protein, ZsYellow1, under the regulation of the c-fos promoter, was expressed throughout the rat brain. A fiber-based confocal microscope that enabled deep brain imaging was utilized, and local field potentials were collected simultaneously. In the weeks following limb denervation in adult rats (n=10), sensory stimulation of the intact limb induced significant increases in c-fos gene expression in cells located in S1, both contralateral (affected, 27.6±3 cells) and ipsilateral (8.6±3 cells) to the injury, compared to controls (n=10, 13.4±3 and 1.0±1, respectively, p value<0.05). Thus, we demonstrated that injury activates cellular mechanisms that are involved in reshaping neuronal connections, and this may translate to neurorehabilitative potential.

A new era for functional labeling of neurons: activity-dependent promoters have come of age

Genetic labeling of neurons with a specific response feature is an emerging technology for precise dissection of brain circuits that are functionally heterogeneous at the single-cell level. While immediate early gene mapping has been widely used for decades to identify brain regions which are activated by external stimuli, recent characterization of the promoter and enhancer elements responsible for neuronal activity-dependent transcription have opened new avenues for live imaging of active neurons. Indeed, these advancements provided the basis for a growing repertoire of novel experiments to address the role of active neuronal networks in cognitive behaviors. In this review, we summarize the current literature on the usage and development of activity-dependent promoters and discuss the future directions of this expanding new field.

Lu AF21934, a positive allosteric modulator of mGlu4 receptors, reduces the harmaline-induced hyperactivity but not tremor in rats

Harmaline induces tremor in animals resembling essential tremor which has been suggested to result from activation of the glutamatergic olivo-cerebellar projection. The aim of the present study was to examine the effects of systemic administration of Lu AF21934, a brain-penetrating positive allosteric modulator of the metabotropic glutamate receptor 4 (mGlu4), on the harmaline-induced tremor and other forms of motor activity in rats using fully automated Force Plate Actimeters. The influence of harmaline on the mGlu4 mRNA expression in the cerebellum and inferior olive was analysed by in situ hybridization. Harmaline at a dose of 15 mg/kg (ip) triggered tremor which was manifested by an increase in the power within 9-15 Hz band and in the tremor index (a difference in power between bands 9-15 Hz and 0-8 Hz). Harmaline induced a biphasic effect on mobility, initially inhibiting the exploratory locomotor activity of rats (0-30 min after administration), followed by an increase in their basic activity. Lu AF21934 (0.5-5 mg/kg sc) did not influence tremor but at doses of 0.5 and 2.5 mg/kg reversed harmaline-induced hyperactivity. MGlu4 mRNA expression was high in the cerebellar cortex and low in the inferior olive. Repeated harmaline (15 mg/kg ip once a day for 5 days] decreased mGlu4 mRNA in the cerebellum and inferior olive. The present study indicates that the mGlu4 stimulation counteracts hyperactivity induced by harmaline which suggests the involvement of cerebellar glutamatergic transmission in this process. In contrast, neuronal mechanisms involved in tremor seem to be insensitive to the stimulation of mGlu4.

From precursors to myelinating oligodendrocytes: contribution of intrinsic and extrinsic factors to white matter plasticity in the adult brain

Oligodendrocyte precursor cells (OPC) are glial cells that metamorphose into myelinating oligodendrocytes during embryogenesis and early stages of post-natal life. OPCs continue to divide throughout adulthood and some eventually differentiate into oligodendrocytes in response to demyelinating lesions. There is growing evidence that OPCs are also involved in activity-driven de novo myelination of previously unmyelinated axons and myelin remodeling in adulthood. In this review, we summarize the interwoven factors and cascades that promote the activation, recruitment and differentiation of OPCs into myelinating oligodendrocytes in the adult brain based mostly on results found in the study of demyelinating diseases. The goal of the review was to draw a complete picture of the transformation of OPCs into mature oligodendrocytes to facilitate the study of this transformation in both the normal and diseased adult brain.

A partial lesion of the substantia nigra pars compacta and retrorubral field decreases the harmaline-induced glutamate release in the rat cerebellum

The aim of the present study was to examine the influence of a partial lesion of both the substantia nigra pars compacta (SNC) and retrorubral field (RRF) on the glutamatergic transmission in the cerebellum and tremor induced by harmaline in rats. 6-Hydroxydopamine (6-OHDA, 8 μg/2 μl) was injected unilaterally into the region of the posterior part of the SNC and RRF. Harmaline was administered in a dose of 30 mg/kg ip on the 8th day after the operation and the extracellular level of glutamate was measured by microdialysis in vivo in the cerebellar vermis. Harmaline induced glutamate release in the cerebellum. The lesion which encompassed 23-37% neurons in the anterior SNC, 52-54% in the posterior SNC and 47-55% in the RRF did not influence the basal extracellular glutamate level but decreased the harmaline-induced release of this neurotransmitter. Tremor evoked by harmaline was also visibly inhibited by the above lesion. The results of the present study seem to indicate that midbrain dopaminergic neurons influence glutamatergic transmission in the cerebellum which may be important for generation of the tremor induced by harmaline.

High and abnormal forms of aggression in rats with extremes in trait anxiety--involvement of the dopamine system in the nucleus accumbens

A better neurobiological understanding of high and abnormal aggression based on adequate animal models is essential for novel therapy and prevention. Selective breeding of rats for extremes in anxiety-related behavior resulted in two behavioral phenotypes with high and abnormal forms of aggression. Rats bred for low anxiety-related behavior (LAB) consistently show highest levels of aggression and little social investigation in the resident-intruder (RI) test, compared with non-selected low-aggressive (NAB) rats. High anxiety-related (HAB) rats also show higher levels of aggression than NAB rats, but to a lesser extent than LAB rats. Accordingly, extremes in inborn anxiety in both directions are linked to an increased aggression level. Further, both LAB and HAB, but not NAB males, display abnormal aggression (attacks towards vulnerable body parts, females or narcotized males), which is particularly prominent in LABs. Also, only in LAB rats, the nucleus accumbens (NAc) was found to be strongly activated in response to the RI test as reflected by increased c-fos and zif268 mRNA expression, and higher local dopamine release compared with NAB males, without differences in local dopamine receptor binding. Consequently, local pharmacological manipulation by infusion of an anesthetic (lidocaine, 20 μg/μl) or a dopamine D2 (haloperidol, 10 ng/μl), but not D1 (SCH-23390 10 ng/μl), receptor antagonist significantly reduced high aggression in LAB rats. Thus, LAB rats are an adequate model to study high and abnormal aggression. In LAB males, this is likely to be linked to hyper-activation of the reward system, as found in psychopathic patients. Specifically, activation of the accumbal dopamine system is likely to underlie the high aggression observed in LAB rats.

Short-term facilitation and depression in the cerebellum: some observations on wild-type and mutant rodents deficient in the extracellular matrix molecule tenascin C

Short-term plasticity was studied on synapses to Purkinje cells (PC): paired-pulse facilitation in parallel fibers (PF) and paired-pulse depression in climbing fibers (CF). Both phenomena relate to synaptic strength. These forms of short-term plasticity were tested on cerebellar slices in rat by early postnatal synchronous stimulation of olivary neurons (i.e., CFs) with harmaline and by inhibition of a metabotropic glutamate receptor (mGluR) as well as in mice that were deficient in the extracellular matrix glycoprotein tenascin-C. Harmaline stimulation delayed the developmental competition between CF inputs and maintained multiple innervation. Paired-pulse depression of the CF-PC synapse after harmaline treatment was more expressed. However, paired-pulse facilitation in PF-PC synapses remained unchanged. Electrophysiological responses of postsynaptic mGluR1 in CF-PC synapses could be obtained only with AMPA receptors blocked and glutamate uptake impaired. The mGluR1-specific antagonist CPCCOEt suppressed the CF-mGluR EPSC in some PCs and potentiated it in other PCs. CF paired-pulse depression was not changed with CPCCOEt, thus excluding a presynaptic effect. The postsynaptic effect was underlined by CPCCOEt-induced rise in amplitude of EPSC and by a prolongation of its decay time. Tenascins are extracellular matrix glycoproteins that may restrict the regenerative capacity of the nervous tissue. Testing short-term presynaptic plasticity in tenascin-C-deficient mice showed that CF paired-pulse depression was less expressed while PF paired-pulse facilitation was augmented except in a group of cells where there was even depression. The results underline differences in forms of short-term plasticity with regard to susceptibility to diverse modulatory factors.

MAPK Activation in Cerebellar Basket Cell Terminals after Harmaline Treatment

The mitogen-activated protein kinases (MAPKs) are a family of signal transduction mediators that regulate a number of cellular activities, including cell growth and proliferation, differentiation and survival, via phosphorylation (activation) of protein kinases. MAPKs are also recruited during synaptic plasticity and remodeling. In the present study we used Western blotting and immunohistochemistry to examine the effects of harmaline administration on the phosphorylation state of three MAPKs: the extracellular signal-regulated kinase (ERK1/2), c-Jun-N-terminal kinase/stress-activated protein kinase (JNK/SAPK), and p38 MAPK. Harmaline is a tremorigenic drug known to induce enhanced and rhythmic firing of the inferior olive. In rats, synchronous activity of the inferior olive cells induced by harmaline administered for four days from postnatal day 9 to 12 resulted in prolonged maintenance of polyinnervation of Purkinje cells by climbing fibers (axons of olivary cells). Immunohistochemistry showed small but sustained cytoplasmic positivity to phospho-ERK in Purkinje cells and a strong signal for phospho-ERK in the "pinceaux," terminals of the interneuronal basket cells onto Purkinje cells. A similar pattern was observed for JNK/SAPK, while no changes in p38 were noticed. Thus, it was revealed that the activation of two members of the MAPK family in these inhibitory presynaptic terminals is also one consequence of synchronous olivary input to Purkinje cells known to affect developmental plasticity.

Fos expression in the vestibular brainstem: what one marker can tell us about the network

Fos inducible transcription factor expression in rodent brains (rats and gerbils) during manipulations of vestibular input is reviewed. Stimuli included centripetal hypergravity, unilateral labyrinth lesion or semicircular canal plugging, rotational axis cross-coupling (Coriolis forces), high and low rotational vestibulo-ocular reflex gain adaptation, translabyrinth galvanic stimulation, pharmacological manipulation, and combinations thereof. Each type of stimulation elicited unique but partially redundant response patterns in the vestibulo-olivo-cerebellar (VOC) network that reflect the origin and interaction of the labyrinth inputs. On the basis of these patterns, a trained observer can predict what the animal experienced during testing; the patterns of VOC Fos expression reveal a trace of recent genomic activity. Based on principal component analysis, VOC network modules associated with lesion recovery, spatial representation and the calibration of gravity, and optokinetic influences are proposed. Probable and possible gene targets of the Fos protein are also reviewed.

Activity-induced tissue oxygenation changes in rat cerebellar cortex: interplay of postsynaptic activation and blood flow

Functional neuroimaging relies on the robust coupling between neuronal activity, metabolism and cerebral blood flow (CBF), but the physiological basis of the neuroimaging signals is still poorly understood. We examined the mechanisms of activity-dependent changes in tissue oxygenation in relation to variations in CBF responses and postsynaptic activity in rat cerebellar cortex. To increase synaptic activity we stimulated the monosynaptic, glutamatergic climbing fibres that excite Purkinje cells via AMPA receptors. We used local field potentials to indicate synaptic activity, and recorded tissue oxygen partial pressure (P(tiss,O2)) by polarographic microelectrodes, and CBF using laser-Doppler flowmetry. The disappearance rate of oxygen in the tissue increased linearly with synaptic activity. This indicated that, without a threshold, oxygen consumption increased as a linear function of synaptic activity. The reduction in P(tiss,O2) preceded the rise in CBF. The time integral (area) of the negative P(tiss,O2) response increased non-linearly showing saturation at high levels of synaptic activity, concomitant with a steep rise in CBF. This was accompanied by a positive change in P(tiss,O2). Neuronal nitric oxide synthase inhibition enhanced the initial negative P(tiss,O2) response ('dip'), while attenuating the evoked CBF increase and positive P(tiss,O2) response equally. This indicates that increases in CBF counteract activity-induced reductions in P(tiss,O2), and suggests the presence of a tissue oxygen reserve. The changes in P(tiss,O2) and CBF were strongly attenuated by AMPA receptor blockade. Our findings suggest an inverse relationship between negative P(tiss,O2) and CBF responses, and provide direct in vivo evidence for a tight coupling between activity in postsynaptic AMPA receptors and cerebellar oxygen consumption.

The ventral uvula of the mouse cerebellum: a neural target of ethanol and vestibular stimuli

The present study demonstrates that mice exposed to vertical translation stimulation exhibit a distinct parasagittal pattern of Fos-immunoreactive (Fos-IR) granule cells in the ventral uvula of the cerebellum. This pattern is identical to one produced by acute ethanol treatment. In contrast, yaw stimulation produces an entirely different pattern in this same region of the cerebellum. Similar results are obtained in the light or in total darkness. These results suggest an anatomical and functional organization within the granule cells of the ventral uvula that may be a common neural substrate for some effects of ethanol and particular vestibular stimuli.

A change in the pattern of activity affects the developmental regression of the Purkinje cell polyinnervation by climbing fibers in the rat cerebellum

Pattern of activity during development is important for the refinement of the final architecture of the brain. In the cerebellar cortex, the regression from multiple to single climbing fiber innervation of the Purkinje cell occurs during development between postnatal days (P) 5 and 15. However, the regression is hampered by altering in various ways the morpho-functional integrity of the parallel fiber input. In rats we disrupted the normal activity pattern of the climbing fiber, the terminal arbor of the inferior olive neurons, by administering harmaline for 4 days from P9 to P12. At all studied ages (P15-87) after harmaline treatment multiple (double only) climbing fiber EPSC-steps persist in 28% of cells as compared with none in the control. The ratio between the amplitudes of the larger and the smaller climbing fiber-evoked EPSC increases in parallel with the decline of the polyinnervation factor, indicating a gradual enlargement of the synaptic contribution of the winning climbing fiber synapse at the expense of the losing one. Harmaline treatment had no later effects on the climbing fiber EPSC kinetics and I/V relation in Purkinje cells (P15-36). However, there was a rise in the paired-pulse depression indicating a potentiation of the presynaptic mechanisms. In the same period, after harmaline treatment, parallel fiber-Purkinje cell electrophysiology was unaffected. The distribution of parallel fiber synaptic boutons was also not changed. Thus, a change in the pattern of activity during a narrow developmental period may affect climbing fiber-Purkinje cell synapse competition resulting in occurrence of multiple innervation at least up to 3 months of age. Our results extend the current view on the role of the pattern of activity in the refinement of neuronal connections during development. They suggest that many similar results obtained by different gene or receptor manipulations might be simply the consequence of disrupting the pattern of activity.

Frequency-dependent expression of corticotropin releasing factor in the rat's cerebellum

Corticotropin releasing factor (CRF), localized in extrinsic afferents in the mammalian cerebellum, is defined as a neuromodulator within cerebellar circuits, and appears to be an essential element in the generation of long term depression, a proposed mechanism for motor learning. These physiological studies are based on exogenous application of CRF and do not address potential mechanisms that may influence endogenous release of the peptide. In the present study, immunohistochemistry was used to analyze changes in the lobular distribution of CRF-like immunoreactivity (LIR). In addition radioimmunoassay (RIA) was used to quantify changes in levels of the peptide in the cerebellum following stimulation of the inferior cerebellar peduncle (ICP) at 10 or 40 Hz or the inferior olivary nucleus (ION) at 1, 5, 10, or 20 Hz. Results indicate that there is a greater distribution of CRF-like-immunolabeled climbing fibers, mossy fibers, and astrocytes in all lobules of the cerebellum that is directly related to stimulation frequency. Maximal effects were elicited with 40 Hz ICP and 5-10 Hz ION stimulation. Quantitatively, the RIA studies indicate that there is a significant increase in CRF levels in the vermis, hemispheres and flocculus that correlates closely with stimulation frequency. In conclusion, stimulation of cerebellar afferents induces a significant change in the distribution and levels of CRF-LIR in climbing fibers, mossy fibers and glial cells. This suggests that the modulatory effects ascribed to CRF may influence a greater number of target neurons when levels of activity in afferent systems is increased.