Dopaminergic innervation and binding in the rat cerebellum

Cerebellar Metabolic Connectivity during Treadmill Walking before and after Unilateral Dopamine Depletion in Rats

Compensatory changes in brain connectivity keep motor symptoms mild in prodromal Parkinson's disease. Studying compensation in patients is hampered by the steady progression of the disease and a lack of individual baseline controls. Furthermore, combining fMRI with walking is intricate. We therefore used a seed-based metabolic connectivity analysis based on 2-deoxy-2-[18F]fluoro-D-glucose ([18F]FDG) uptake in a unilateral 6-OHDA rat model. At baseline and in the chronic phase 6-7 months after lesion, rats received an intraperitoneal injection of [18F]FDG and spent 50 min walking on a horizontal treadmill, followed by a brain PET-scan under anesthesia. High activity was found in the cerebellar anterior vermis in both conditions. At baseline, the anterior vermis showed hardly any stable connections to the rest of the brain. The (future) ipsilesional cerebellar hemisphere was not particularly active during walking but was extensively connected to many brain areas. After unilateral dopamine depletion, rats still walked normally without obvious impairments. The ipsilesional cerebellar hemisphere increased its activity, but narrowed its connections down to the vestibulocerebellum, probably aiding lateral stability. The anterior vermis established a network involving the motor cortex, hippocampus and thalamus. Adding those regions to the vermis network of (previously) automatic control of locomotion suggests that after unilateral dopamine depletion considerable conscious and cognitive effort has to be provided to achieve stable walking.

Dopamine receptors of the rodent fastigial nucleus support skilled reaching for goal-directed action

The dopaminergic (DA) system regulates both motor function, and learning and memory. The cerebellum supports motor control and the acquisition of procedural memories, including goal-directed behavior, and is subjected to DA control. Its fastigial nucleus (FN) controls and interprets body motion through space. The expression of dopamine receptors has been reported in the deep cerebellar nuclei of mice. However, the presence of dopamine D1-like (D1R) and D2-like (D2R) receptors in the rat FN has not yet been verified. In this study, we first confirmed that DA receptors are expressed in the FN of adult rats and then targeted these receptors to explore to what extent the FN modulates goal-directed behavior. Immunohistochemical assessment revealed expression of both D1R and D2R receptors in the FN, whereby the medial lateral FN exhibited higher receptor expression compared to the other FN subfields. Bilateral treatment of the FN with a D1R antagonist, prior to a goal-directed pellet-reaching task, significantly impaired task acquisition and decreased task engagement. D2R antagonism only reduced late performance post-acquisition. Once task acquisition had occurred, D1R antagonism had no effect on successful reaching, although it significantly decreased reaching speed, task engagement, and promoted errors. Motor coordination and ambulation were, however, unaffected as neither D1R nor D2R antagonism altered rotarod latencies or distance and velocity in an open field. Taken together, these results not only reveal a novel role for the FN in goal-directed skilled reaching, but also show that D1R expressed in FN regulate this process by modulating motivation for action.

The Discovery of the Monoaminergic Innervation of the Cerebellum: Convergence of Divergent and Point-to-Point Systems

This Cerebellar Classic highlights the landmark discovery of the innervation of the cerebellar cortex and cerebellar nuclei by noradrenergic and serotoninergic axons emanating, respectively, from the locus coeruleus and the raphé nuclei. Since then, modulation of the activity of cerebellar neurons by the monoamine systems has been studied extensively, as well as their reorganization and modifications during development, plasticity, and disease. The discovery of noradrenergic and serotoninergic innervation of the cerebellum has been a crucial step in understanding the neurochemical relationships between brainstem nuclei and the cerebellum, and the attempts to treat cerebellar ataxias pharmacologically. The large neurochemical repertoire of the cerebellum represents one of the complexities and challenges in the modern appraisal of cerebellar disorders.

Gene expression changes in the cerebellum are associated with persistent post-injury pain in adolescent rats exposed to early life stress

Chronic pain develops following injury in approximately 20% of adolescents, at twice the rate in females than males. Adverse childhood experiences also increase the risk for poor health outcomes, such as chronic pain. Emerging literature suggests the cerebellum to be involved in pain processing, however detailed explorations into how the cerebellum contributes to pain are lacking. Therefore, this study aimed to characterise chronic pain outcomes and cerebellar gene expression changes following early life stress and injury in both sexes. The adverse childhood experience of neglect was modelled using a maternal separation (MS) paradigm, which was combined with a subsequent injury (mild traumatic brain injury (mTBI) or plantar incision surgery) in adolescent male and female Sprague-Dawley rats. We measured behavioural nociceptive sensitivity, systemic modulators of pain such as calcitonin gene-related protein (CGRP) and Substance P, as well as gene expression of IL1β, GFAP, GR, MR, GABRA1, CNR1, MAOA, and DAT1 in the cerebellum to examine associations between pain and neuroinflammation, the stress response, inhibitory neurotransmission, and monoaminergic function. We found increases in mechanical nociceptive sensitivity following plantar incision surgery. Sex differences were observed in anxiety-like behaviour and neuroinflammation, whereas systemic pain modulators showed cumulative effects with the addition of stressors. Most interestingly however, the increases in nociceptive sensitivity were associated with the suppressed expression of cerebellar genes that regulate stress, inhibition, cannabinoid function, and dopaminergic function, alongside sex-dependent distinctions for genes involved in inflammation and injury. This study highlights a novel link between nociception and molecular function in the cerebellum. Further investigation into how the cerebellum contributes to pain in males and females will facilitate novel therapeutic insights and opportunities.

Assessing the Location, Relative Expression and Subclass of Dopamine Receptors in the Cerebellum of Hemi-Parkinsonian Rats

Parkinson's Disease (PD) is a neurodegenerative disease with loss of dopaminergic neurons in the nigrostriatal pathway resulting in basal ganglia (BG) dysfunction. This is largely why much of the preclinical and clinical research has focused on pathophysiological changes in these brain areas in PD. The cerebellum is another motor area of the brain. Yet, if and how this brain area responds to PD therapy and contributes to maintaining motor function fidelity in the face of diminished BG function remains largely unanswered. Limited research suggests that dopaminergic signaling exists in the cerebellum with functional dopamine receptors, tyrosine hydroxylase (TH) and dopamine transporters (DATs); however, much of this information is largely derived from healthy animals and humans. Here, we identified the location and relative expression of dopamine 1 receptors (D1R) and dopamine 2 receptors (D2R) in the cerebellum of a hemi-parkinsonian male rat model of PD. D1R expression was higher in PD animals compared to sham animals in both hemispheres in the purkinje cell layer (PCL) and granule cell layer (GCL) of the cerebellar cortex. Interestingly, D2R expression was higher in PD animals than sham animals mostly in the posterior lobe of the PCL, but no discernible pattern of D2R expression was seen in the GCL between PD and sham animals. To our knowledge, we are the first to report these findings, which may lay the foundation for further interrogation of the role of the cerebellum in PD therapy and/or pathophysiology.

Purkinje cell dopaminergic inputs to astrocytes regulate cerebellar-dependent behavior

Dopamine has a significant role in motor and cognitive function. The dopaminergic pathways originating from the midbrain have received the most attention; however, the relevance of the cerebellar dopaminergic system is largely undiscovered. Here, we show that the major cerebellar astrocyte type Bergmann glial cells express D1 receptors. Dopamine can be synthesized in Purkinje cells by cytochrome P450 and released in an activity-dependent fashion. We demonstrate that activation of D1 receptors induces membrane depolarization and Ca2+ release from the internal store. These astrocytic activities in turn modify Purkinje cell output by altering its excitatory and inhibitory synaptic input. Lastly, we show that conditional knockout of D1 receptors in Bergmann glial cells results in decreased locomotor activity and impaired social activity. These results contribute to the understanding of the molecular, cellular, and circuit mechanisms underlying dopamine function in the cerebellum, revealing a critical role for the cerebellar dopaminergic system in motor and social behavior.

Cerebellar dopamine D2 receptors regulate social behaviors

The cerebellum, a primary brain structure involved in the control of sensorimotor tasks, also contributes to higher cognitive functions including reward, emotion and social interaction. Although the regulation of these behaviors has been largely ascribed to the monoaminergic system in limbic regions, the contribution of cerebellar dopamine signaling in the modulation of these functions remains largely unknown. By combining cell-type-specific transcriptomics, histological analyses, three-dimensional imaging and patch-clamp recordings, we demonstrate that cerebellar dopamine D2 receptors (D2Rs) in mice are preferentially expressed in Purkinje cells (PCs) and regulate synaptic efficacy onto PCs. Moreover, we found that changes in D2R levels in PCs of male mice during adulthood alter sociability and preference for social novelty without affecting motor functions. Altogether, these findings demonstrate novel roles for D2R in PC function and causally link cerebellar D2R levels of expression to social behaviors.

Dopaminergic regulation of vestibulo-cerebellar circuits through unipolar brush cells

While multiple monoamines modulate cerebellar output, the mechanistic details of dopaminergic signaling in the cerebellum remain poorly understood. We show that dopamine type 1 receptors (Drd1) are expressed in unipolar brush cells (UBCs) of the mouse cerebellar vermis. Drd1 activation increases UBC firing rate and post-synaptic NMDAR -mediated currents. Using anatomical tracing and in situ hybridization, we test three hypotheses about the source of cerebellar dopamine. We exclude midbrain dopaminergic nuclei and tyrosine hydroxylase-positive Purkinje (Pkj) cells as potential sources, supporting the possibility of dopaminergic co-release from locus coeruleus (LC) axons. Using an optical dopamine sensor GRABDA2h, electrical stimulation, and optogenetic activation of LC fibers in the acute slice, we find evidence for monoamine release onto Drd1-expressing UBCs. Altogether, we propose that the LC regulates cerebellar cortex activity by co-releasing dopamine onto UBCs to modulate their response to cerebellar inputs. Pkj cells directly inhibit these Drd1-positive UBCs, forming a dopamine-sensitive recurrent vestibulo-cerebellar circuit.

The AGTPBP1 gene in neurobiology

The function of the Agtpbp1 gene has mainly been delineated by studying Agtpbp1pcd (pcd) mutant mice, characterized by losses in cerebellar Purkinje and granule cells along with degeneration of retinal photoreceptors, mitral cells of the olfactory bulb, thalamic neurons, and alpha-motoneurons. As a result of cerebellar degeneration, cerebellar GABA and glutamate concentrations in Agtpbp1pcd mutants decreased while monoamine concentrations increased. The salient behavioral phenotypes include cerebellar ataxia, a loss in motor coordination, and cognitive deficits. Similar neuropathogical and behavioral profiles have been described in childhood-onset human subjects with biallelic variants of AGTPBP1, including cerebellar ataxia and hypotonia.

Cerebellar tDCS as Therapy for Cerebellar Ataxias

In recent years, a growing body of literature has investigated the use of non-invasive brain stimulation (NIBS) techniques to influence cerebellar activity and the effects of cerebellar stimulation on other brain regions through its multiple complex projections. From the early 1990s, with the discovery of the so-called cerebellar inhibition (CBI), several studies have focused their attention on the use of cerebellar NIBS as treatment for different motor disorders. Cerebellar ataxias (CAs) represent the prototypical clinical manifestation of cerebellar alterations, but other movement disorders, such as Parkinson's disease, essential tremor, and dystonia have also been associated with alterations of networks which include the cerebellum, or of the cerebellum itself. Cerebellar transcranial direct current stimulation (ctDCS) could indeed represent an economical, non-invasive therapeutic tool with minimal side effects, thus improving the clinical management of patients and their quality of life. Studies show that ctDCS is effective as a therapeutic option for motor symptoms in patients with CAs, and especially in those with less severe forms, suggesting that ctDCS efficacy could result from augmented neuronal compensation, which itself relies on preserved cerebellar volume. Evidence for the efficacy of ctDCS is less conclusive for the other aforementioned motor disorders, although preliminary results are promising. Future studies should adopt more rigorous methods (e.g., larger sample sizes, double blinding, better characterization of the sample, reliable biomarkers), in order to allow the scientific community to derive higher-quality evidence on the efficacy of ctDCS as a therapeutic option for motor disorders.

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.

Effects of Parkinson Disease on Blur-Driven and Disparity-Driven Vergence Eye Movements

ABSTRACT

Synchronous movements of the 2 eyes in the opposite direction, disconjugate movements such as vergence, facilitate depth perception. The vergence eye movements are affected in Parkinson disease (PD). Visual blur (accommodation) and fusion (retinal disparity) are important triggers for the vergence. The neural circuit responsible for blur-driven and disparity-driven vergence is tightly coupled. We investigated the effect of PD on these 2 vergence paradigms. In the experiment involving 14 patients with PD and 6 healthy controls, substantial differences between blur-driven and disparity-driven vergence were found. The gain (ratio of actual vs desired eye movements) was reduced in patients with PD in case of disparity-driven vergence but not in blur-driven vergence. The latency of disparity-driven vergence onset was significantly longer for patients with PD compared with healthy controls. Four strategies were used to drive disparity-driven vergence: a) pure disconjugate vergence, b) conjugate saccadic movements, c) disconjugate vergence followed by saccadic movements, and d) conjugate saccades followed by disconjugate vergence movements. Blur-driven vergence had only 2 strategies: a) conjugate saccades followed by disconjugate vergence and b) conjugate saccadic movements only. The results are consistent with the prediction that PD primarily affects disparity-driven vergence, but there are some effects on the strategies to execute blur-driven vergence. We speculate that the deep cerebellar nuclei and the supraoculomotor area of the midbrain that carry the disparity-driven and blur-driven vergence are affected in PD. It is possible to modulate their function through projections to the subthalamic nuclei.

Consensus Paper: Novel Directions and Next Steps of Non-invasive Brain Stimulation of the Cerebellum in Health and Disease

The cerebellum is involved in multiple closed-loops circuitry which connect the cerebellar modules with the motor cortex, prefrontal, temporal, and parietal cortical areas, and contribute to motor control, cognitive processes, emotional processing, and behavior. Among them, the cerebello-thalamo-cortical pathway represents the anatomical substratum of cerebellum-motor cortex inhibition (CBI). However, the cerebellum is also connected with basal ganglia by disynaptic pathways, and cerebellar involvement in disorders commonly associated with basal ganglia dysfunction (e.g., Parkinson's disease and dystonia) has been suggested. Lately, cerebellar activity has been targeted by non-invasive brain stimulation (NIBS) techniques including transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) to indirectly affect and tune dysfunctional circuitry in the brain. Although the results are promising, several questions remain still unsolved. Here, a panel of experts from different specialties (neurophysiology, neurology, neurosurgery, neuropsychology) reviews the current results on cerebellar NIBS with the aim to derive the future steps and directions needed. We discuss the effects of TMS in the field of cerebellar neurophysiology, the potentials of cerebellar tDCS, the role of animal models in cerebellar NIBS applications, and the possible application of cerebellar NIBS in motor learning, stroke recovery, speech and language functions, neuropsychiatric and movement disorders.

Mapping reward mechanisms by intracerebral self-stimulation in the rhesus monkey (Macaca mulatta)

The objective of the study was to identify brain structures that mediate reward as evidenced by positive reinforcing effects of stimuli on behavior. Testing by intracerebral self-stimulation enabled monkeys to inform whether activation of ~2900 sites in 74 structures of 4 sensorimotor pathways and 4 modulatory loop pathways was positive, negative or neutral. Stimulation was rewarding at 30% of sites, negative at 17%, neutral at 52%. Virtually all (99%) structures yielded some positive or negative sites, suggesting a ubiquitous distribution of pathways transmitting valence information. Mapping of sites to structures with dense versus sparse dopaminergic (DA) or noradrenergic (NA) innervation showed that stimulation of DA-pathways was rewarding or neutral. Stimulation of NA-pathways was not rewarding. Stimulation of association areas was generally rewarding; stimulation of purely sensory or motor structures was generally negative. Reward related more to structures' sensorimotor function than to density of DA-innervation. Stimulation of basal ganglia loop pathways was rewarding except in lateral globus pallidus, an inhibitory structure in the negative feedback loop; stimulation of the cerebellar loop was rewarding in anterior vermis and the spinocerebellar pathway; and stimulation of the hippocampal CA1 loop was rewarding. While most positive sites were in the DA reward system, numerous sites in sparsely DA-innervated posterior cingulate and parietal cortices may represent a separate reward system. DA-density represents concentrations of plastic synapses that mediate acquisition of new synaptic connections. DA-sparse areas may represent innate, genetically programmed reward-associated pathways. Implications of findings in regard to response habituation and addiction are discussed.

The Cerebellar Dopaminergic System

In the central nervous system (CNS), dopamine (DA) is involved in motor and cognitive functions. Although the cerebellum is not been considered an elective dopaminergic region, studies attributed to it a critical role in dopamine deficit-related neurological and psychiatric disorders [e.g., Parkinson's disease (PD) and schizophrenia (SCZ)]. Data on the cerebellar dopaminergic neuronal system are still lacking. Nevertheless, biochemical studies detected in the mammalians cerebellum high dopamine levels, while chemical neuroanatomy studies revealed the presence of midbrain dopaminergic afferents to the cerebellum as well as wide distribution of the dopaminergic receptor subtypes (DRD1-DRD5). The present review summarizes the data on the cerebellar dopaminergic system including its involvement in associative and projective circuits. Furthermore, this study also briefly discusses the role of the cerebellar dopaminergic system in some neurologic and psychiatric disorders and suggests its potential involvement as a target in pharmacologic and non-pharmacologic treatments.

Regulation of GluA1 phosphorylation by d-amphetamine and methylphenidate in the cerebellum

Prescription stimulants, such as d-amphetamine or methylphenidate are used to treat suffering from attention-deficit hyperactivity disorder (ADHD). They potently release dopamine (DA) and norepinephrine (NE) and cause phosphorylation of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunit GluA1 in the striatum. Whether other brain regions are also affected remains elusive. Here, we demonstrate that d-amphetamine and methylphenidate increase phosphorylation at Ser845 (pS845-GluA1) in the membrane fraction of mouse cerebellum homogenate. We identify Bergmann glial cells as the source of pS845-GluA1 and demonstrate a requirement for intact NE release. Consequently, d-amphetamine-induced pS845-GluA1 was prevented by β1-adenoreceptor antagonist, whereas the blockade of DA D1 receptor had no effect. Together, these results indicate that NE regulates GluA1 phosphorylation in Bergmann glial cells in response to prescription stimulants.

Catecholaminergic Innervation of the Lateral Nucleus of the Cerebellum Modulates Cognitive Behaviors

The cerebellum processes neural signals related to rewarding and aversive stimuli, suggesting that the cerebellum supports nonmotor functions in cognitive and emotional domains. Catecholamines are a class of neuromodulatory neurotransmitters well known for encoding such salient stimuli. Catecholaminergic modulation of classical cerebellar functions have been demonstrated. However, a role for cerebellar catecholamines in modulating cerebellar nonmotor functions is unknown. Using biochemical methods in male mice, we comprehensively mapped TH+ fibers throughout the entire cerebellum and known precerebellar nuclei. Using electrochemical (fast scan cyclic voltammetry), and viral/genetic methods to selectively delete Th in fibers innervating the lateral cerebellar nucleus (LCN), we interrogated sources and functional roles of catecholamines innervating the LCN, which is known for its role in supporting cognition. The LCN has the most TH+ fibers in cerebellum, as well as the most change in rostrocaudal expression among the cerebellar nuclei. Norepinephrine is the major catecholamine measured in LCN. Distinct catecholaminergic projections to LCN arise only from locus coeruleus, and a subset of Purkinje cells that are positive for staining of TH. LC stimulation was sufficient to produce catecholamine release in LCN. Deletion of Th in fibers innervating LCN (LCN-Th-cKO) resulted in impaired sensorimotor integration, associative fear learning, response inhibition, and working memory in LCN-Th-cKO mice. Strikingly, selective inhibition of excitatory LCN output neurons with inhibitory designer receptor exclusively activated by designer drugs led to facilitation of learning on the same working memory task impaired in LCN-Th-cKO mice. Collectively, these data demonstrate a role for LCN catecholamines in cognitive behaviors.SIGNIFICANCE STATEMENT Here, we report on interrogating sources and functional roles of catecholamines innervating the lateral nucleus of the cerebellum (LCN). We map and quantify expression of TH, the rate-limiting enzyme in catecholamine synthesis, in the entire cerebellar system, including several precerebellar nuclei. We used cyclic voltammetry and pharmacology to demonstrate sufficiency of LC stimulation to produce catecholamine release in LCN. We used advanced viral techniques to map and selectively KO catecholaminergic neurotransmission to the LCN, and characterized significant cognitive deficits related to this manipulation. Finally, we show that inhibition of excitatory LCN neurons with designer receptor exclusively activated by designer drugs, designed to mimic Gi-coupled catecholamine GPCR signaling, results in facilitation of a working memory task impaired in LCN-specific TH KO mice.

The Effects of Intravermis Cerebellar Microinjections of Dopaminergic Agents in Motor Learning and Aversive Memory Acquisition in Mice

The cerebellum receives dopaminergic innervation and expresses the five types of described dopaminergic receptors. The cerebellar function involves both motor movement and cognition, but the role of cerebellar dopaminergic system on these processes remain unclear. The present study explores the behavioral responses to intracerebellar microinjection of dopaminergic agents in motor and emotional memory. For this, naïve Swiss mice had their cerebellar vermis implanted with a guide canula, received a intravermis microinjection of Dopamine, D1-like antagonist SCH-23390 or D2-like antagonist Eticlopride, and underwent a behavioral analysis of motor learning (by a Rotarod and balance beam learning protocol) or aversive memory acquisition (by the inhibitory avoidance task). The mixed-effects analysis was used to evaluate groups performance, followed by Tukey’s post hoc when appropriated. In this study, Dopamine, SCH-23390 and Eticlopride at the doses used did not affected motor control and motor learning. In addition, the administration of Dopamine and SCH-233390 had no effects on emotional memory acquisition, but the animals that received the highest dose of Eticlopride had an improvement in aversive memory acquisition, shown by a suppression of its innate preference for the dark compartment of the inhibitory avoidance apparatus following an exposure to a foot shock. We propose that cerebellar dopaminergic D2 receptors seem to participate on the modulation of aversive memory processes, without influencing motor performance at the doses used in this study.

From back to front: A functional model for the cerebellar modulation in the establishment of conditioned preferences for cocaine-related cues

It is now increasingly clear that the cerebellum may modulate brain functions altered in drug addiction. We previously demonstrated that cocaine-induced conditioned preference increased activity at the dorsal posterior cerebellar vermis. Unexpectedly, a neurotoxic lesion at this region increased the probability of cocaine-induced conditioned preference acquisition. The present research aimed at providing an explanatory model for such as facilitative effect of the cerebellar lesion. First, we addressed a tracing study in which we found a direct projection from the lateral (dentate) nucleus to the ventral tegmental area (VTA) that also receives Purkinje axons from lobule VIII in the vermis. This pathway might control the activity and plasticity of the cortico-striatal circuitry. Then we evaluated cFos expression in different regions of the medial prefrontal cortex and striatum after a lesion in lobule VIII before conditioning. Additionally, perineuronal net (PNN) expression was assessed to explore whether the cerebellar lesion might affect synaptic stabilization mechanisms in the medial prefrontal cortex (mPFC). Damage in this region of the vermis induced general disinhibition of the mPFC and striatal subdivisions that receive dopaminergic projections, mainly from the VTA. Moreover, cerebellar impairment induced an upregulation of PNN expression in the mPFC. The major finding of this research was to provide an explanatory model for the function of the posterior cerebellar vermis on drug-related memory. In this model, damage of the posterior vermis would release striatum-cortical networks from the inhibitory tonic control exerted by the cerebellar cortex over VTA, thereby promoting drug effects.

Cannabinoid CB1 receptor mediates METH-induced electrophysiological and morphological alterations in cerebellum Purkinje cells

Our previous studies on cannabinoid type1 receptor (CB1R) activation on Methamphetamine (METH)-induced neurodegeneration and locomotion impairments in male rats suggest an interaction between CB1Rs and METH. However, the role of these receptors in METH-neurotoxicity has not been fully identified. Therefore, the purpose of the present study is to investigate the involvement of CB1Rs in these effects. We conducted an electrophysiological study to evaluate functional interactions between METH and CB1Rs using whole-cell patch current clamp recording. Furthermore, we designed the Nissl staining protocol to assess the effect of METH on the basic cerebellar Purkinje cell structure. Our findings revealed that METH significantly increased the action potential half-width, spontaneous interspike intervals, first spike latency, and decreased the rebound action potential and spontaneous firing frequency. Using CB1R agonist and antagonist, our results showed a significant interaction with some of the electrophysiological alterations induced by METH. Further, Nissl staining revealed that the exposure to the combination of METH and SR141716A resulted in the necrotic cell death. Results of the current study raises the possibility that METH consumption profoundly affect the intrinsic membrane properties of cerebellar Purkinje neurons and cannabinoid system manipulations may counteract some of these effects. In summary, our findings provide further insights into the modulatory role of the endocannabinoid system in METH-induced neurologic changes, which can be used in the development of potential therapeutic interventions for METH dependence.

Purkinje Cell-Specific Knockout of Tyrosine Hydroxylase Impairs Cognitive Behaviors

Tyrosine hydroxylase (Th) expression has previously been reported in Purkinje cells (PCs) of rodents and humans, but its role in the regulation of behavior is not understood. Catecholamines are well known for facilitating cognitive behaviors and are expressed in many regions of the brain. Here, we investigated a possible role in cognitive behaviors of PC catecholamines, by mapping and testing functional roles of Th positive PCs in mice. Comprehensive mapping analyses revealed a distinct population of Th expressing PCs primarily in the posterior and lateral regions of the cerebellum (comprising about 18% of all PCs). To identify the role of PC catecholamines, we selectively knocked out Th in PCs using a conditional knockout approach, by crossing a Purkinje cell-selective Cre recombinase line, Pcp2-Cre, with a floxed tyrosine hydroxylase mouse line (Thlox/lox) to produce Pcp2-Cre;Thlox/lox mice. This manipulation resulted in approximately 50% reduction of Th protein expression in the cerebellar cortex and lateral cerebellar nucleus, but no reduction of Th in the locus coeruleus, which is known to innervate the cerebellum in mice. Pcp2-Cre;Thlox/lox mice showed impairments in behavioral flexibility, response inhibition, social recognition memory, and associative fear learning relative to littermate controls, but no deficits in gross motor, sensory, instrumental learning, or sensorimotor gating functions. Catecholamines derived from specific populations of PCs appear to support cognitive functions, and their spatial distribution in the cerebellum suggests that they may underlie patterns of activation seen in human studies on the cerebellar role in cognitive function.

Alteration of the Dopamine Receptors' Expression in the Cerebellum of the Lysosomal Acid Phosphatase 2 Mutant (Naked-Ataxia (NAX)) Mouse

A spontaneous mutation in the lysosomal acid phosphatase (Acp2) enzyme (nax: naked-ataxia) in experimental mice results in delayed hair appearance and severe cytoarchitectural impairments of the cerebellum, such as a Purkinje cell (PC) migration defect. In our previous investigation, our team showed that Acp2 expression plans a significant role in cerebellar development. On the other hand, the dopaminergic system is also a player in central nervous system (CNS) development, including cerebellar structure and function. In the current investigation, we have explored how Acp2 can be involved in the regulation of the dopaminergic pathway in the cerebellum via the regulation of dopamine receptor expression and patterning. We provided evidence about the distribution of different dopamine receptors in the developing cerebellum by comparing the expression of dopamine receptors on postnatal days (P) 5 and 17 between nax mice and wild-type (wt) littermates. To this aim, immunohistochemistry and Western blot analysis were conducted using five antibodies against dopamine receptors (DRD1, -2, -3, -4, and -5) accompanied by RNAseq data. Our results revealed that DRD1, -3, and -4 gene expressions significantly increased in nax cerebella but not in wt, while gene expressions of all 5 receptors were evident in PCs of both wt and nax cerebella. DRD3 was strongly expressed in the PCs' somata and cerebellar nuclei neurons at P17 in nax mice, which was comparable to the expression levels in the cerebella of wt littermates. In addition, DRD3 was expressed in scattered cells in a granular layer reminiscent of Golgi cells and was observed in the wt cerebella but not in nax mice. DRD4 was expressed in a subset of PCs and appeared to align with the unique parasagittal stripes pattern. This study contributes to our understanding of alterations in the expression pattern of DRDs in the cerebellum of nax mice in comparison to their wt littermates, and it highlights the role of Acp2 in regulating the dopaminergic system.

Cerebellar D1DR-expressing neurons modulate the frontal cortex during timing tasks

Converging lines of evidence suggest that the cerebellum plays an integral role in cognitive function through its interactions with association cortices like the medial frontal cortex (MFC). It is unknown precisely how the cerebellum influences the frontal cortex and what type of information is reciprocally relayed between these two regions. A subset of neurons in the cerebellar dentate nuclei, or the homologous lateral cerebellar nuclei (LCN) in rodents, express D1 dopamine receptors (D1DRs) and may play a role in cognitive processes. We investigated how pharmacologically blocking LCN D1DRs influences performance in an interval timing task and impacts neuronal activity in the frontal cortex. Interval timing requires executive processes such as working memory, attention, and planning and is known to rely on both the frontal cortex and cerebellum. In our interval timing task, male rats indicated their estimates of the passage of a period of several seconds by making lever presses for a water reward. We have shown that a cue-evoked burst of low-frequency activity in the MFC initiates ramping activity (i.e., monotonic increases or decreases of firing rate over time) in single MFC neurons. These patterns of activity are associated with successful interval timing performance. Here we explored how blocking right LCN D1DRs with the D1DR antagonist SCH23390 influences timing performance and neural activity in the contralateral (left) MFC. Our results indicate that blocking LCN D1DRs impaired some measures of interval timing performance. Additionally, ramping activity of MFC single units was significantly attenuated. These data provide insight into how catecholamines in the LCN may drive MFC neuronal dynamics to influence cognitive function.

The role of the cerebellum in drug-cue associative memory: functional interactions with the medial prefrontal cortex

Drug-induced Pavlovian memories are thought to be crucial for drug addiction because they guide behaviour towards environments with drug availability. Drug-related memory depends on persistent changes in dopamine-glutamate interactions in the medial prefrontal cortex (mPFC), basolateral amygdala, nucleus accumbens core and hippocampus. Recent evidence from our laboratory indicated that the cerebellum is also a relevant node for drug-cue associations. In the present study, we tested the role that specific regions of the cerebellum and mPFC play in the acquisition of cocaine-induced preference conditioning. Quinolinic acid was used to manage a permanent deactivation of lobule VIII in the vermis prior to conditioning. Additionally, lidocaine was infused into the prelimbic and infralimbic (IL) cortices for reversible deactivation before every training session. The present findings show, for the first time, that the cerebellum and mPFC might act together in order to acquire drug-cue Pavlovian associations. Either a dorsal lesion in lobule VIII or an IL deactivation encouraged cocaine-induced preference conditioning. Moreover, simultaneous IL-cerebellar deactivation prevented the effect of either of the separate deactivations. Therefore, similar to the IL cortex, neural activity in the cerebellum may be crucial for ensuring inhibitory control of the expression of cocaine-related memories.

5-Hydroxytryptamine and Dopamine Neurons in the Cerebellum of the New-Hatching Yangtze Alligator Alligator sinensis

Nissl and immunohistochemical staining methods were used to morphologically characterize the cerebellum of the new-hatching Yangtze alligator, and the cerebellar histological structure and the distribution profiles of 5-hydroxytryptamine (5-HT) and dopamine (DA) neurons were investigated for the first time. The results of cerebellar histological structure showed that there was a ventriculus cerebelli in the cerebellum of the new-hatching Yangtze alligator, the surface of the cerebellar cortex was not very smooth, the cerebellar cortex could be divided into four layers, which include external granular layer, molecular layer, Purkinje cell layer and granular layer, Purkinje cell layer could be characterized specially by multilayer, two cerebellar nuclei termed as the nucleus cerebelli lateralis and the nucleus cerebelli medialis were found in the cerebellar medulla. 5-hydroxytryptamine-immunoreactive (5-HT-IR) and dopamine-immunoreactive (DA-IR) neurons and fibers distributed widely in the cerebellum. The structures and profiles of 5-HT-IR and DA-IR neurons and fibers in the cerebellum of the Yangtze alligator were similar to that reported in other reptiles, but also had some specific features. The abundance of 5-HT and DA in cerebellum suggested that these highly conserved neurotransmitters would play important roles in motor control. Anat Rec, 302:861-868, 2019. © 2018 Wiley Periodicals, Inc.

The Sleeping Cerebellum

We sleep almost one-third of our lives and sleep plays an important role in critical brain functions like memory formation and consolidation. The role of sleep in cerebellar processing, however, constitutes an enigma in the field of neuroscience; we know little about cerebellar sleep-physiology, cerebro-cerebellar interactions during sleep, or the contributions of sleep to cerebellum-dependent memory consolidation. Likewise, we do not understand why cerebellar malfunction can lead to changes in the sleep-wake cycle and sleep disorders. In this review, we evaluate how sleep and cerebellar processing may influence one another and highlight which scientific routes and technical approaches could be taken to uncover the mechanisms underlying these interactions.

Cerebellar granule cells encode the expectation of reward

The human brain contains ~60 billion cerebellar granule cells¹, which outnumber all other neurons combined. Classical theories posit that a large, diverse population of granule cells allows for highly detailed representations of sensorimotor context, enabling downstream Purkinje cells to sense fine contextual changes^2–6. Although evidence suggests a role for cerebellum in cognition^7–10, granule cells are known to encode only sensory^11–13 and motor¹⁴ context. Using two-photon calcium imaging in behaving mice, here we show that granule cells convey information about the expectation of reward. Mice initiated voluntary forelimb movements for delayed water reward. Some granule cells responded preferentially to reward or reward omission, whereas others selectively encoded reward anticipation. Reward responses were not restricted to forelimb movement, as a Pavlovian task evoked similar responses. Compared to predictable rewards, unexpected rewards elicited markedly different granule cell activity despite identical stimuli and licking responses. In both tasks, reward signals were widespread throughout multiple cerebellar lobules. Tracking the same granule cells over several days of learning revealed that cells with reward-anticipating responses emerged from those that responded at the start of learning to reward delivery, whereas reward omission responses grew stronger as learning progressed. The discovery of predictive, non-sensorimotor encoding in granule cells is a major departure from current understanding of these neurons and dramatically enriches contextual information available to postsynaptic Purkinje cells, with important implications for cognitive processing in the cerebellum.

Cerebellar transcranial direct current stimulation in neurological disease

Several studies have highlighted the therapeutic potential of transcranial direct current stimulation (tDCS) in patients with neurological diseases, including dementia, epilepsy, post-stroke dysfunctions, movement disorders, and other pathological conditions.

Because of this technique’s ability to modify cerebellar excitability without significant side effects, cerebellar tDCS is a new, interesting, and powerful tool to induce plastic modifications in the cerebellum.

In this report, we review a number of interesting studies on the application of cerebellar tDCS for various neurological conditions (ataxia, Parkinson’s disease, dystonia, essential tremor) and the possible mechanism by which the stimulation acts on the cerebellum.

Study findings indicate that cerebellar tDCS is a promising therapeutic tool in treating several neurological disorders; however, this method’s efficacy appears to be limited, given the current data.

Automatic and controlled processing in the corticocerebellar system

During learning, performance changes often involve a transition from controlled processing in which performance is flexible and responsive to ongoing error feedback, but effortful and slow, to a state in which processing becomes swift and automatic. In this state, performance is unencumbered by the requirement to process feedback, but its insensitivity to feedback reduces its flexibility. Many properties of automatic processing are similar to those that one would expect of forward models, and many have suggested that these may be instantiated in cerebellar circuitry. Since hierarchically organized frontal lobe areas can both send and receive commands, I discuss the possibility that they can act both as controllers and controlled objects and that their behaviors can be independently modeled by forward models in cerebellar circuits. Since areas of the prefrontal cortex contribute to this hierarchically organized system and send outputs to the cerebellar cortex, I suggest that the cerebellum is likely to contribute to the automation of cognitive skills, and to the formation of habitual behavior which is resistant to error feedback. An important prerequisite to these ideas is that cerebellar circuitry should have access to higher order error feedback that signals the success or failure of cognitive processing. I have discussed the pathways through which such feedback could arrive via the inferior olive and the dopamine system. Cerebellar outputs inhibit both the inferior olive and the dopamine system. It is possible that learned representations in the cerebellum use this as a mechanism to suppress the processing of feedback in other parts of the nervous system. Thus, cerebellar processes that control automatic performance may be completed without triggering the engagement of controlled processes by prefrontal mechanisms.

Cerebellum in levodopa-induced dyskinesias: the unusual suspect in the motor network

The exact mechanisms that generate levodopa-induced dyskinesias (LID) during chronic levodopa therapy for Parkinson’s disease (PD) are not yet fully established. The most widely accepted theories incriminate the non-physiological synthesis, release and reuptake of dopamine generated by exogenously administered levodopa in the striatum, and the aberrant plasticity in the cortico-striatal loops. However, normal motor performance requires the correct recruitment of motor maps. This depends on a high level of synergy within the primary motor cortex (M1) as well as between M1 and other cortical and subcortical areas, for which dopamine is necessary. The plastic mechanisms within M1, which are crucial for the maintenance of this synergy, are disrupted both during “OFF” and dyskinetic states in PD. When tested without levodopa, dyskinetic patients show loss of treatment benefits on long-term potentiation and long-term depression-like plasticity of the intracortical circuits. When tested with the regular pulsatile levodopa doses, they show further impairment of the M1 plasticity, such as inability to depotentiate an already facilitated synapse and paradoxical facilitation in response to afferent input aimed at synaptic inhibition. Dyskinetic patients have also severe impairment of the associative, sensorimotor plasticity of M1 attributed to deficient cerebellar modulation of sensory afferents to M1. Here, we review the anatomical and functional studies, including the recently described bidirectional connections between the cerebellum and the basal ganglia that support a key role of the cerebellum in the generation of LID. This model stipulates that aberrant neuronal synchrony in PD with LID may propagate from the subthalamic nucleus to the cerebellum and “lock” the cerebellar cortex in a hyperactive state. This could affect critical cerebellar functions such as the dynamic and discrete modulation of M1 plasticity and the matching of motor commands with sensory information from the environment during motor performance. We propose that in dyskinesias, M1 neurons have lost the ability to depotentiate an activated synapse when exposed to acute pulsatile, non-physiological, dopaminergic surges and become abnormally receptive to unfiltered, aberrant, and non-salient afferent inputs from the environment. The motor program selection in response to such non-salient and behaviorally irrelevant afferent inputs would be abnormal and involuntary. The motor responses are worsened by the lack of normal subcortico–cortical inputs from cerebellum and basal ganglia, because of the aberrant plasticity at their own synapses. Artificial cerebellar stimulation might help re-establish the cerebellar and basal ganglia control over the non-salient inputs to the motor areas during synaptic dopaminergic surges.

Role of cerebellar dopamine D(3) receptors in modulating exploratory locomotion and cataleptogenicity in rats

Dopamine D(3) receptors are highly expressed in the cerebellum; however, their pathophysiological functions are not fully understood. Here, we conducted microinjection studies to clarify the role of cerebellar D(3) receptors in modulating locomotion and cataleptogenicity in rats. Microinjection of the preferential D(3) agonist 7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT) into lobe 9 of the cerebellum significantly reduced spontaneous locomotor activity with a U-shaped dose-response curve. The intracerebellar microinjection of 7-OH-DPAT did not elicit catalepsy by itself, but markedly potentiated catalepsy induction with a low dose (0.3mg/kg) of haloperidol. The catalepsy enhancement by 7-OH-DPAT occurred in a dose-dependent manner and was not associated with the locomotor inhibition. U-99194A (a selective D(3) antagonist) or AD-6048 (a preferential D(3) vs. D(2) antagonist) antagonized both the catalepsy enhancement and the locomotor inhibition with 7-OH-DPAT. In addition, U-99194A and AD-6048 per se significantly alleviated catalepsy induced by a high dose (0.5mg/kg) of haloperidol. Furthermore, microinjection of 7-OH-DPAT into the nucleus accumbens or the dorsolateral striatum neither affected spontaneous locomotor activity nor haloperidol (0.3mg/kg)-induced catalepsy. The present results illustrate for the first time the role of cerebellar D(3) receptors in modulating cataleptogenicity of antipsychotic agents, implying that blockade of cerebellar D(3) receptors contributes to the reduction of extrapyramidal side effects.

Beyond the basal ganglia: cFOS expression in the cerebellum in response to acute and chronic dopaminergic alterations

The suggestion of an anatomical and functional relationship between the basal ganglia and cerebellum is recent. Traditionally, these structures were considered as neuronal circuits working separately to organize and control goal-directed movements and cognitive functions. However, several studies in rodents and primates have described an anatomical interaction between cortico-basal and cortico-cerebellar networks. Most importantly, functional changes have been observed in one of these circuits when altering the other one. In this context, we aimed to accomplish an extensive description of cerebellar activation patterns using cFOS expression (cFOS-IR) after acute and chronic manipulation of dopaminergic activity. In the acute study, substantia nigra pars compacta (SNc) activity was stimulated or suppressed by intra cerebral administration of picrotoxin or lidocaine, respectively. In addition, we analyzed cerebellar activity after the induction of a parkinsonism model, the tremulous jaw movements. In this model, tremulous jaw movements were induced in male rats by IP chronic administration of the dopamine antagonist haloperidol (1.5mg/kg). Acute stimulation of SNc by picrotoxin increased cFOS-IR in the vermis and cerebellar hemispheres. However, lidocaine did not produce an effect. After 14days of haloperidol treatment, the vermis and cerebellar hemispheres showed an opposite regulation of cFOS expression. Chronic dopaminergic antagonism lessened cFOS expression in the vermis but up-regulated such expression in the cerebellar hemisphere. Overall, the present data indicate a very close functional relationship between the basal ganglia and the cerebellum and they may allow a better understanding of disorders in which there are dopamine alterations.

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.

The cerebellum in Parkinson's disease

Parkinson's disease is a chronic progressive neurodegenerative disorder characterized by resting tremor, slowness of movements, rigidity, gait disturbance and postural instability. Most investigations on Parkinson's disease focused on the basal ganglia, whereas the cerebellum has often been overlooked. However, increasing evidence suggests that the cerebellum may have certain roles in the pathophysiology of Parkinson's disease. Anatomical studies identified reciprocal connections between the basal ganglia and cerebellum. There are Parkinson's disease-related pathological changes in the cerebellum. Functional or morphological modulations in the cerebellum were detected related to akinesia/rigidity, tremor, gait disturbance, dyskinesia and some non-motor symptoms. It is likely that the major roles of the cerebellum in Parkinson's disease include pathological and compensatory effects. Pathological changes in the cerebellum might be induced by dopaminergic degeneration, abnormal drives from the basal ganglia and dopaminergic treatment, and may account for some clinical symptoms in Parkinson's disease. The compensatory effect may help maintain better motor and non-motor functions. The cerebellum is also a potential target for some parkinsonian symptoms. Our knowledge about the roles of the cerebellum in Parkinson's disease remains limited, and further attention to the cerebellum is warranted.

6-OHDA injections into A8-A9 dopaminergic neurons modelling early stages of Parkinson's disease increase the harmaline-induced tremor in rats

The aim of the present study was to examine the influence of a unilateral 6-hydroxydopamine (6-OHDA)-induced partial lesion of both the substantia nigra pars compacta (SNc, A9) and retrorubral field (RRF, A8) on the tremor evoked by harmaline. 6-OHDA (8μg/2μl) was injected unilaterally into the region of the posterior part of the SNc and RRF. Harmaline was administered in a dose of 7.5mg/kg ip on the eighth day after the operation and tremor of forelimbs, head and trunk was measured. We found that the lesion increased intensity of the tremor induced by harmaline but did not influence its character. Stereological examination of the lesion extent revealed losses of dopaminergic (tyrosine hydroxylase-immunoreactive) neurons in the anterior (30%) and posterior (72%) SNc, as well as in RRF (72% on the average). Levels of dopamine and all its metabolites, as well as noradrenaline concentrations, were ipsilaterally moderately decreased in the caudate-putamen in the lesioned animals, however, dopamine and DOPAC in the anterior cerebellum were increased. In the caudate-putamen, the ipsi/contra ratio of dopamine level correlated negatively, while that of dopamine turnover positively with the tremor intensity. However, in the anterior cerebellum an inverse relationship was found. Moreover, this symptom correlated positively with the serotonin level and negatively with the 5-HIAA/serotonin ratio on the contralateral side of the posterior cerebellum. The present results seem to indicate that the modulation of dopaminergic and serotonergic transmissions by the lesion modelling early stages of Parkinson's disease may influence tremor triggered in the cerebellum.

Lesion of the cerebellar noradrenergic innervation enhances the harmaline-induced tremor in rats

Abnormal synchronous activation of the glutamatergic olivo-cerebellar pathway has been suggested to be crucial for the harmaline-induced tremor. The cerebellum receives two catecholaminergic pathways: the dopaminergic pathway arising from the ventral tegmental area/substantia nigra pars compacta, and the noradrenergic one from the locus coeruleus. The aim of the present study was to examine a contribution of the cerebellar catecholaminergic innervations to the harmaline-induced tremor in rats. Rats were injected bilaterally into the cerebellar vermis with 6-hydroxydopamine (6-OHDA; 8 μg/0.5 μl) either alone or this treatment was preceded (30 min earlier) by desipramine (15 mg/kg ip). Harmaline was administered to animals in doses of 7.5 or 15 mg/kg ip. Tremor of forelimbs was measured as a number of episodes during a 90-min observation. Rats were killed by decapitation 30 or 120 min after harmaline treatment. The levels of dopamine, noradrenaline, serotonin, and their metabolites were measured by HPLC in the cerebellum, substantia nigra, caudate–putamen, and frontal cortex. 6-OHDA injected alone enhanced the harmaline-induced tremor. Furthermore, it decreased the noradrenaline level by ca. 40–80% in the cerebellum and increased the levels of serotonin and 5-HIAA in the caudate–putamen and frontal cortex in untreated and/or harmaline-treated animals. When 6-OHDA treatment was preceded by desipramine, it decreased dopaminergic transmission in some regions of the cerebellum while inducing its compensatory activation in others. The latter lesion did not markedly influence the tremor induced by harmaline. The present study indicates that noradrenergic innervation of the cerebellum interacts with cerebral serotonergic systems and plays an inhibitory role in the harmaline-induced tremor.

Past, present and future therapeutics for cerebellar ataxias

Cerebellar ataxias are a group of disabling neurological disorders. Patients exhibit a cerebellar syndrome and can also present with extra-cerebellar deficits, namely pigmentary retinopathy, extrapyramidal movement disorders, pyramidal signs, cortical symptoms (seizures, cognitive impairment/behavioural symptoms), and peripheral neuropathy. Recently, deficits in cognitive operations have been unraveled. Cerebellar ataxias are heterogeneous both at the phenotypic and genotypic point of view. Therapeutical trials performed during these last 4 decades have failed in most cases, in particular because drugs were not targeting a deleterious pathway, but were given to counteract putative defects in neurotransmission. The identification of the causative mutations of many hereditary ataxias, the development of relevant animal models and the recent identifications of the molecular mechanisms underlying ataxias are impacting on the development of new drugs. We provide an overview of the pharmacological treatments currently used in the clinical practice and we discuss the drugs under development.

Acute effect of the anti-addiction drug bupropion on extracellular dopamine concentrations in the human striatum: an [11C]raclopride PET study

Bupropion is an effective medication in treating addiction and is widely used as an aid to smoking cessation. Bupropion inhibits striatal dopamine reuptake via dopamine transporter blockade, but it is unknown whether this leads to increased extracellular dopamine levels at clinical doses in man. The effects of bupropion on extracellular dopamine levels in the striatum were investigated using [(11)C]raclopride positron emission tomography (PET) imaging in rats administered saline, 11 or 25 mg/kg bupropion i.p. and in healthy human volunteers administered either placebo or 150 mg bupropion (Zyban Sustained-Release). A cognitive task was used to stimulate dopamine release in the human study. In rats, bupropion significantly decreased [(11)C]raclopride specific binding in the striatum, consistent with increases in extracellular dopamine concentrations. In man, no significant decreases in striatal [(11)C]raclopride specific binding were observed. Levels of dopamine transporter occupancy in the rat at 11 and 25 mg/kg bupropion i.p. were higher than predicted to occur in man at the dose used. Thus, these data indicate that, at the low levels of dopamine transporter occupancy achieved in man at clinical doses, bupropion does not increase extracellular dopamine levels. These findings have important implications for understanding the mechanism of action underlying bupropions' therapeutic efficacy and for the development of novel treatments for addiction and depression.

Pharmacological characterization and anatomical distribution of the dopamine transporter in the mouse cerebellum

We studied the binding parameters, the pharmacological profile and the anatomical distribution of the dopamine transporter in the mouse cerebellum by using the specific dopamine uptake antagonist [(3)H]GBR12935 and an antidopamine transporter monoclonal antibody. Competition experiments in cerebellar and striatal membrane preparations showed that [(3)H]GBR12935 binds to a specific binding site, sensitive to dopamine and low concentrations of mazindol. The affinity of dopamine for the cerebellar binding site was one order of magnitude lower than the affinity for the striatal binding site. Saturation experiments in cerebellar membrane preparations and thin frozen sections showed that the affinity of [(3)H]GBR12935 for this binding site is similar to its affinity for the striatal dopamine transporter. Saturable binding was lobule specific and in general was higher in the molecular layer compared to the granule cell layer. The immunohistochemical signal was mostly concentrated in the Purkinje cell layer and the cerebellar nuclei. The results suggest that the cerebellar dopamine transporter is similar but not identical to the striatal dopamine transporter and that it is present in the mouse cerebellum in a lobule and lamina specific pattern.

Measurement of methylphenidate-induced change in extrastriatal dopamine concentration using [11C]FLB 457 PET

[(11)C]FLB 457 is a very high-affinity radiotracer that allows the measurement of dopamine D(2/3) receptor availability in regions of the brain where densities are very low, such as the cerebral cortex. It is not known if [(11)C]FLB 457 binding is sensitive to the concentration of endogenous dopamine in humans in a manner analogous to [(11)C]raclopride and [(123)I]IBZM in the striatum. To test this possibility, extrastriatal [(11)C]FLB 457 binding was measured at baseline and after the oral administration of 40 to 60 mg of the psychostimulant methylphenidate (MP) in 12 healthy volunteers using positron emission tomography (PET) in a balanced-order, double-blind design. The dynamic PET data were quantified using a two-tissue compartment model with a metabolite-corrected arterial plasma input function. Two volunteers were excluded because of excessive head movement. In the remainder, MP caused significant reductions in the volume of distribution (VD) in temporal and frontal cortical regions and thalamus, suggesting that [(11)C]FLB 457 binding is sensitive to endogenous dopamine concentration. Moreover, the change in [(11)C]FLB 457 binding after MP correlated with the dose of MP (in mg/kg body weight) in all regions assessed. We conclude that MP in doses within the therapeutic range for the treatment of attention deficit hyperactivity disorder causes increases in dopamine concentrations in extrastriatal regions and that [(11)C]FLB 457 PET may be a useful tool for the assessment of change in dopamine concentration in these areas in humans.

Effects of Methamphetamine on the Cerebellar Cortex: A Preliminary Study

Methamphetamine (METH) targets monoamine nerve terminals and produces motor effects, which are related to changes in catecholamine activity within the basal ganglia. Cerebellum plays an important role in motor control, nonetheless only a few studies investigated the effects of METH in this area. In this article, we report preliminary results on protein expression in the cerebellum following METH administration. In particular, we focused on the rate-limiting catecholamine-synthesizing enzyme tyrosine hydroxylase (TH). By using immunoblotting, we found that METH administration produces a dose-dependent increase of TH within the cerebellar cortex of mice, which is opposite to the decrease of TH within the striatum. Further investigations are needed in order to determine the time course, the cerebellar regions, the cellular (and subcellular) compartments, and the functional role related to these effects.

Dopamine transporters in the cerebellum of mutant mice

In the central nervous system, dopamine is known to play a critical role in motor and cognitive functions. Although the cerebellum plays a role in the control of movement and posture and in cognitive functions, it has not been considered to be a dopaminergic region and the dopamine present was thought to represent a precursor of noradrenaline. However, recent evidence suggests that in the cerebellum there is a small dopaminergic element, whose properties are similar to the well characterized system of striatum. In order to better understand the functional role of this system and to delineate its specific interactions within the cerebellum, the distribution and properties of dopamine transporter (DAT) in the cerebellum of reeler and Purkinje cell degeneration (Nna1pcd) mutant mice, which are characterized by severe loss of different cell populations and abnormalities in synapse formation, have been studied. Kinetic studies revealed that [3H]dopamine is transported into cerebellar synaptosomes prepared from normal mice with affinities similar to that into striatal synaptosomes but with much lower maximal velocities. In reeler cerebellar synaptosomes the number of transport sites is significantly reduced. In Nna1pcd cerebellar synaptosomes the kinetic properties of transport of [3H]dopamine are similar to the normal. However, in vitro quantitative DAT autoradiography revealed a significantly increased binding in cerebellar nuclei, a decreased binding in molecular layer and an unaltered binding in the granule cell layer. These observations confirm a dopaminergic innervation of the cerebellum and contribute to our understanding of the intracerebellar distribution of the dopaminergic system.

Increase of GAP-43 in the rat cerebellum following unilateral striatal 6-OHDA lesion

In order to further characterize synaptic alterations following a severe lesion of the nigrostriatal system, the expression of synaptic marker proteins, synaptophysin and growth-associated protein-43 (GAP-43), was examined in various brain regions of 6-hydroxydopamine (6-OHDA)-treated rats, an animal model of Parkinson's disease. Unilateral nigrostriatal lesioning induced an increase in synaptophysin protein levels by 68% and 106% in the sensorimotor cortex and striatum, respectively, while changes in the level of GAP-43 were not observed. In contrast, 6-OHDA induced a 73% increase in the level of GAP-43 protein in the cerebellum. This increase was also confirmed with immunohistochemistry. The level of synaptophysin in the cerebellum remained unchanged in response to the lesion. These results suggest that a neurotoxic lesion of the nigrostriatal pathway differentially affects the expression of the two synaptic proteins and that plasticity-related changes in this model are not solely restricted to the nigrostriatal system. In addition, these results provide further evidence of the involvement of the cerebellum in the late response to a 6-OHDA lesion.