Membrane excitability and fear conditioning in cerebellar Purkinje cell

The Role of Cerebellar Intrinsic Neuronal Excitability, Synaptic Plasticity, and Perineuronal Nets in Eyeblink Conditioning

Evidence is strong that, in addition to fine motor control, there is an important role for the cerebellum in cognition and emotion. The deep nuclei of the mammalian cerebellum also contain the highest density of perineural nets-mesh-like structures that surround neurons-in the brain, and it appears there may be a connection between these nets and cognitive processes, particularly learning and memory. Here, we review how the cerebellum is involved in eyeblink conditioning-a particularly well-understood form of learning and memory-and focus on the role of perineuronal nets in intrinsic membrane excitability and synaptic plasticity that underlie eyeblink conditioning. We explore the development and role of perineuronal nets and the in vivo and in vitro evidence that manipulations of the perineuronal net in the deep cerebellar nuclei affect eyeblink conditioning. Together, these findings provide evidence of an important role for perineuronal net in learning and memory.

Intrinsic plasticity of Purkinje cell serves homeostatic regulation of fear memory

Two forms of plasticity, synaptic and intrinsic, are neural substrates for learning and memory. Abnormalities in homeostatic plasticity cause severe neuropsychiatric diseases, such as schizophrenia and autism. This suggests that the balance between synaptic transmission and intrinsic excitability is important for physiological function in the brain. Despite the established role of synaptic plasticity between parallel fiber (PF) and Purkinje cell (PC) in fear memory, its relationship with intrinsic plasticity is not well understood. Here, patch clamp recording revealed depression of intrinsic excitability in PC following auditory fear conditioning (AFC). Depressed excitability balanced long-term potentiation of PF-PC synapse to serve homeostatic regulation of PF-evoked PC firing. We then optogenetically manipulated PC excitability during the early consolidation period resulting in bidirectional regulation of fear memory. Fear conditioning-induced synaptic plasticity was also regulated following optogenetic manipulation. These results propose intrinsic plasticity in PC as a novel mechanism of fear memory and elucidate that decreased intrinsic excitability in PC counterbalances PF-PC synaptic potentiation to maintain fear memory in a normal range.

Cerebellar interneurons control fear memory consolidation via learning-induced HCN plasticity

While synaptic plasticity is considered the basis of learning and memory, modifications of the intrinsic excitability of neurons can amplify the output of neuronal circuits and consequently change behavior. However, the mechanisms that underlie learning-induced changes in intrinsic excitability during memory formation are poorly understood. In the cerebellum, we find that silencing molecular layer interneurons completely abolishes fear memory, revealing their critical role in memory consolidation. The fear conditioning paradigm produces a lasting reduction in hyperpolarization-activated cyclic nucleotide-gated (HCN) channels in these interneurons. This change increases intrinsic membrane excitability and enhances the response to synaptic stimuli. HCN loss is driven by a decrease in endocannabinoid levels via altered cGMP signaling. In contrast, an increase in release of cerebellar endocannabinoids during memory consolidation abolishes HCN plasticity. Thus, activity in cerebellar interneurons drives fear memory formation via a learning-specific increase in intrinsic excitability, and this process requires the loss of endocannabinoid-HCN signaling.

Dynamic alteration of intrinsic properties of the cerebellar Purkinje cell during the motor memory consolidation

Intrinsic plasticity of the cerebellar Purkinje cell (PC) plays a critical role in motor memory consolidation. However, detailed changes in their intrinsic properties during memory consolidation are not well understood. Here, we report alterations in various properties involved in intrinsic excitability, such as the action potential (AP) threshold, AP width, afterhyperpolarization (AHP), and sag voltage, which are associated with the long-term depression of intrinsic excitability following the motor memory consolidation process. We analyzed data recorded from PCs before and 1, 4, and 24 h after cerebellum-dependent motor learning and found that these properties underwent dynamic changes during the consolidation process. We further analyzed data from PC-specific STIM1 knockout (STIM1^PKO) mice, which show memory consolidation deficits, and derived intrinsic properties showing distinct change patterns compared with those of wild-type littermates. The levels of memory retention in the STIM1^PKO mice were significantly different compared to wild-type mice between 1 and 4 h after training, and AP width, fast- and medium-AHP, and sag voltage showed different change patterns during this period. Our results provide information regarding alterations in intrinsic properties during a particular period that are critical for memory consolidation.

Cerebellar Circuits for Classical Fear Conditioning

Accumulating evidence indicates that the cerebellum is critically involved in modulating non-motor behaviors, including cognition and emotional processing. Both imaging and lesion studies strongly suggest that the cerebellum is a component of the fear memory network. Given the well-established role of the cerebellum in adaptive prediction of movement and cognition, the cerebellum is likely to be engaged in the prediction of learned threats. The cerebellum is activated by fear learning, and fear learning induces changes at multiple synaptic sites in the cerebellum. Furthermore, recent technological advances have enabled the investigation of causal relationships between intra- and extra-cerebellar circuits and fear-related behaviors such as freezing. Here, we review the literature on the mechanisms underlying the modulation of cerebellar circuits in a mammalian brain by fear conditioning at the cellular and synaptic levels to elucidate the contributions of distinct cerebellar structures to fear learning and memory. This knowledge may facilitate a deeper understanding and development of more effective treatment strategies for fear-related affective disorders including post-traumatic stress or anxiety related disorders.

A novel, ataxic mouse model of ataxia telangiectasia caused by a clinically relevant nonsense mutation

Ataxia Telangiectasia (A-T) and Ataxia with Ocular Apraxia Type 1 (AOA1) are devastating neurological disorders caused by null mutations in the genome stability genes, A-T mutated (ATM) and Aprataxin (APTX), respectively. Our mechanistic understanding and therapeutic repertoire for treating these disorders are severely lacking, in large part due to the failure of prior animal models with similar null mutations to recapitulate the characteristic loss of motor coordination (i.e., ataxia) and associated cerebellar defects. By increasing genotoxic stress through the insertion of null mutations in both the Atm (nonsense) and Aptx (knockout) genes in the same animal, we have generated a novel mouse model that for the first time develops a progressively severe ataxic phenotype associated with atrophy of the cerebellar molecular layer. We find biophysical properties of cerebellar Purkinje neurons (PNs) are significantly perturbed (e.g., reduced membrane capacitance, lower action potential [AP] thresholds, etc.), while properties of synaptic inputs remain largely unchanged. These perturbations significantly alter PN neural activity, including a progressive reduction in spontaneous AP firing frequency that correlates with both cerebellar atrophy and ataxia over the animal’s first year of life. Double mutant mice also exhibit a high predisposition to developing cancer (thymomas) and immune abnormalities (impaired early thymocyte development and T-cell maturation), symptoms characteristic of A-T. Finally, by inserting a clinically relevant nonsense-type null mutation in Atm, we demonstrate that Small Molecule Read-Through (SMRT) compounds can restore ATM production, indicating their potential as a future A-T therapeutic.

A Highly Efficacious PS Gene Editing System Corrects Metabolic and Neurological Complications of Mucopolysaccharidosis Type I

Our previous study delivered zinc finger nucleases to treat mice with mucopolysaccharidosis type I (MPS I), resulting in a phase I/II clinical trial (ClinicalTrials.gov: NCT02702115). However, in the clinical trial, the efficacy needs to be improved due to the low transgene expression level. To this end, we designed a proprietary system (PS) gene editing approach with CRISPR to insert a promoterless α-l-iduronidase (IDUA) cDNA sequence into the albumin locus of hepatocytes. In this study, adeno-associated virus 8 (AAV8) vectors delivering the PS gene editing system were injected into neonatal and adult MPS I mice. IDUA enzyme activity in the brain significantly increased, while storage levels were normalized. Neurobehavioral tests showed that treated mice had better memory and learning ability. Also, histological analysis showed efficacy reflected by the absence of foam cells in the liver and vacuolation in neuronal cells. No vector-associated toxicity or increased tumorigenesis risk was observed. Moreover, no off-target effects were detected through the unbiased genome-wide unbiased identification of double-stranded breaks enabled by sequencing (GUIDE-seq) analysis. In summary, these results showed the safety and efficacy of the PS in treating MPS I and paved the way for clinical studies. Additionally, as a therapeutic platform, the PS has the potential to treat other lysosomal diseases.

Cooperation of the vestibular and cerebellar networks in anxiety disorders and depression

The discipline of affective neuroscience is concerned with the neural bases of emotion and mood. The past decades have witnessed an explosion of research in affective neuroscience, increasing our knowledge of the brain areas involved in fear and anxiety. Besides the brain areas that are classically associated with emotional reactivity, accumulating evidence indicates that both the vestibular and cerebellar systems are involved not only in motor coordination but also influence both cognition and emotional regulation in humans and animal models. The cerebellar and the vestibular systems show the reciprocal connection with a myriad of anxiety and fear brain areas. Perception anticipation and action are also major centers of interest in cognitive neurosciences. The cerebellum is crucial for the development of an internal model of action and the vestibular system is relevant for perception, gravity-related balance, navigation and motor decision-making. Furthermore, there are close relationships between these two systems. With regard to the cooperation between the vestibular and cerebellar systems for the elaboration and the coordination of emotional cognitive and visceral responses, we propose that altering the function of one of the systems could provoke internal model disturbances and, as a result, anxiety disorders followed potentially with depressive states.

Cerebellar BDNF Promotes Exploration and Seeking for Novelty

Background

Approach system considered a motivational system that activates reward-seeking behavior is associated with exploration/impulsivity, whereas avoidance system considered an attentional system that promotes inhibition of appetitive responses is associated with active overt withdrawal. Approach and avoidance dispositions are modulated by distinct neurochemical profiles and synaptic patterns. However, the precise working of neurons and trafficking of molecules in the brain activity predisposing to approach and avoidance are yet unclear.

Methods

In 3 phenotypes of inbred mice, avoiding, balancing, and approaching mice, selected by using the Approach/Avoidance Y-maze, we analyzed endogenous brain levels of brain derived neurotrophic factor, one of the main secretory proteins with pleiotropic action. To verify the effects of the acute increase of brain derived neurotrophic factor, balancing and avoiding mice were bilaterally brain derived neurotrophic factor-infused in the cortical cerebellar regions.

Results

Approaching animals showed high levels of explorative behavior and response to novelty and exhibited higher brain derived neurotrophic factor levels in the cerebellar structures in comparison to the other 2 phenotypes of mice. Interestingly, brain derived neurotrophic factor-infused balancing and avoiding mice significantly increased their explorative behavior and response to novelty.

Conclusions

Cerebellar brain derived neurotrophic factor may play a role in explorative and novelty-seeking responses that sustain the approach predisposition.

Granule cells control recovery from classical conditioned fear responses in the zebrafish cerebellum

Although previous studies show that the cerebellum is involved in classical fear conditioning, it is not clear which components in the cerebellum control it or how. We addressed this issue using a delayed fear-conditioning paradigm with late-stage zebrafish larvae, with the light extinguishment as the conditioned stimulus (CS) and an electric shock as the unconditioned stimulus (US). The US induced bradycardia in the restrained larvae. After paired-associate conditioning with the CS and US, a substantial population of the larvae displayed CS-evoked bradycardia responses. To investigate the roles of the zebrafish cerebellum in classical fear conditioning, we expressed botulinum toxin or the Ca2+ indicator GCaMP7a in cerebellar neurons. The botulinum-toxin-dependent inhibition of granule-cell transmissions in the corpus cerebelli (CCe, the medial lobe) did not suppress the CS-evoked bradycardia response, but rather prolonged the response. We identified cerebellar neurons with elevated CS-evoked activity after the conditioning. The CS-evoked activity of these neurons was progressively upregulated during the conditioning and was downregulated with repetition of the unpaired CS. Some of these neurons were activated immediately upon the CS presentation, whereas others were activated after a delay. Our findings indicate that granule cells control the recovery from conditioned fear responses in zebrafish.

The emotional cerebellum

Great attention has been given so far to cerebellar control of posture and of skilled movements despite the well-demonstrated interconnections between the cerebellum and the autonomic nervous system. Here is a review of the link between these two structures and a report on the recently acquired evidence for its involvement in the world of emotions. In rodents, the reversible inactivation of the vermis during the consolidation or the reconsolidation period hampers the retention of the fear memory trace. In this region, there is a long-term potentiation of both the excitatory synapses between the parallel fibres and the Purkinje cells and of the feed-forward inhibition mediated by molecular layer interneurons. This concomitant potentiation ensures the temporal fidelity of the system. Additional contacts between mossy fibre terminals and Golgi cells provide morphological evidence of the potentiation of another feed-forward inhibition in the granular layer. Imaging experiments show that also in humans the cerebellum is activated during mental recall of emotional personal episodes and during learning of a conditioned or unconditioned association involving emotions. The vermis participates in fear learning and memory mechanisms related to the expression of autonomic and motor responses of emotions. In humans, the cerebellar hemispheres are also involved at a higher emotional level. The importance of these findings is evident when considering the cerebellar malfunctioning in psychiatric diseases like autism and schizophrenia which are characterized behaviourally by emotion processing impairments.

Involving the cerebellum in cocaine-induced memory: pattern of cFos expression in mice trained to acquire conditioned preference for cocaine

Because of its primary role in drug-seeking, consumption and addictive behaviour, there is a growing interest in identifying the neural circuits and molecular mechanisms underlying the formation, maintenance and retrieval of drug-related memories. Human studies, which focused on neuronal systems that store and control drug-conditioned memories, have found cerebellar activations during the retrieval of drug-associated cue memory. However, at the pre-clinical level, almost no attention has been paid to a possible role of the cerebellum in drug-related memories. In the present study, we ought to fill this gap by aiming to investigate the pattern of neuronal activation (as revealed by cFos expression) in different regions of the prefrontal cortex and cerebellum of mice trained to develop conditioned preference for an olfactory stimulus (CS+) paired with cocaine. Our results indicate that CS+ preference was directly associated with cFos expression in cells at the apical region of the granule cell layer of the cerebellar vermis; this relationship being more prominent in some specific lobules. Conversely, cFos+ immunostaining in other cerebellar regions seems to be unrelated to CS+ preference but to other aspects of the conditioning procedure. At the prefrontal cortex, cFos expression seemed to be related to cocaine administration rather than to its ability to establish conditioned preference. The present results suggest that as it has been observed in some clinical studies, the cerebellum might be an important and largely overlooked part of the neural circuits involved in generating, maintaining and/or retrieving drug memories.

Cholesterol and copper affect learning and memory in the rabbit

A rabbit model of Alzheimer's disease based on feeding a cholesterol diet for eight weeks shows sixteen hallmarks of the disease including beta amyloid accumulation and learning and memory changes. Although we have shown that feeding 2% cholesterol and adding copper to the drinking water can retard learning, other studies have shown that feeding dietary cholesterol before learning can improve acquisition and feeding cholesterol after learning can degrade long-term memory. We explore the development of this model, the issues surrounding the role of copper, and the particular contributions of the late D. Larry Sparks.

Fear conditioning-related changes in cerebellar Purkinje cell activities in goldfish

BACKGROUND

Fear conditioning-induced changes in cerebellar Purkinje cell responses to a conditioned stimulus have been reported in rabbits. It has been suggested that synaptic long-term potentiation and the resulting increases in firing rates of Purkinje cells are related to the acquisition of conditioned fear in mammals. However, Purkinje cell activities during acquisition of conditioned fear have not been analysed, and changes in Purkinje cell activities throughout the development of conditioned fear have not yet been investigated. In the present study, we tracked Purkinje cell activities throughout a fear conditioning procedure and aimed to elucidate further how cerebellar circuits function during the acquisition and expression of conditioned fear.

METHODS

Activities of single Purkinje cells in the corpus cerebelli were tracked throughout a classical fear conditioning procedure in goldfish. A delayed conditioning paradigm was used with cardiac deceleration as the conditioned response. Conditioning-related changes of Purkinje cell responses to a conditioned stimulus and unconditioned stimulus were examined.

RESULTS

The majority of Purkinje cells sampled responded to the conditioned stimulus by either increasing or decreasing their firing rates before training. Although there were various types of conditioning-related changes in Purkinje cells, more than half of the cells showed suppressed activities in response to the conditioned stimulus after acquisition of conditioned fear. Purkinje cells that showed unconditioned stimulus-coupled complex-spike firings also exhibited conditioning-related suppression of simple-spike responses to the conditioned stimulus. A small number of Purkinje cells showed increased excitatory responses in the acquisition sessions. We found that the magnitudes of changes in the firing frequencies of some Purkinje cells in response to the conditioned stimulus correlated with the magnitudes of the conditioned responses on a trial-to-trial basis.

CONCLUSIONS

These results demonstrate that Purkinje cells in the corpus cerebelli of goldfish show fear conditioning-related changes in response to a stimulus that had been emotionally neutral prior to conditioning. Unconditioned stimulus-induced climbing fibre inputs to the Purkinje cells may be involved in mediating these plastic changes.

Lobule-specific membrane excitability of cerebellar Purkinje cells

  Cerebellar Purkinje cells (PCs) are the sole output of the cerebellar cortex and function as key to a variety of learning-related behaviours by integrating multimodal afferent inputs. Intrinsic membrane excitability of neurons determines the input-output relationship, and therefore governs the functions of neural circuits. Cerebellar vermis consists of ten lobules (lobules I-X), and each lobule receives different sensory information. However, lobule-specific differences of electrophysiological properties of PC are incompletely understood. To address this question, we performed a systematic comparison of membrane properties of PCs from different lobules (lobules III-V vs. X). Two types of firing patterns (tonic firing and complex bursting) were identified in response to depolarizing current injections in lobule III-V PCs, whereas four distinct firing patterns (tonic firing, complex bursting, initial bursting and gap firing) were observed in lobule X. A-type K(+) current and early inactivation of fast Na(+) conductance with activation of 4-aminopyridine-sensitive conductances were shown to be responsible for the formation of gap firing and initial bursting patterns, respectively, which were observed only in lobule X. In response to current injection, PCs in lobule X spiked with wider dynamic range. These differences in firing pattern and membrane properties probably contribute to signal processing of afferent inputs in lobule-specific fashion, and particularly diversity of discharge patterns in lobule X, as a part of the vestibulocerebellum, might be involved in strict coordination of a precise temporal response to a wide range of head movements.

Involvement of Cerebellum in Emotional Behavior

In the last decade a growing body of data revealed that the cerebellum is involved in the regulation of the affective reactions as well as in forming the association between sensory stimuli and their emotional values. In humans, cerebellar areas around the vermis are activated during mental recall of emotional personal episodes and during learning of a CS-US association. Lesions of the cerebellar vermis may affect retention of a fear memory without altering baseline motor/autonomic responses to the frightening stimuli in both human and animal models. Reversible inactivation of the vermis during the consolidation period impairs retention of fear memory in rodents. Recent findings demonstrate that long-term potentiation (LTP) of synapses in the cerebellar cortex occurs in relation to associative fear learning similar to previously reported data in the hippocampus and amygdala. Plastic changes affect both excitatory and inhibitory synapses. This concomitant potentiation allows the cerebellar cortical network to detect coincident inputs, presumably conveying sensorial stimuli, with better efficacy by keeping the time resolution of the system unchanged. Collectively, these data suggest that the vermis participates in forming new CS-US association and translate an emotional state elaborated elsewhere into autonomic and motor responses.

Cerebellum and emotional behavior

Fear conditioning involves learning that a previously neutral stimulus (CS) predicts an aversive unconditioned stimulus (US). Lesions of the cerebellar vermis may affect fear memory without altering baseline motor/autonomic responses to the frightening stimuli. Reversible inactivation of the vermis during the consolidation period impairs retention of fear memory. In patients with medial cerebellar lesions conditioned bradycardia is impaired. In humans, cerebellar areas around the vermis are activated during mental recall of emotional personal episodes, if a loved partner receives a pain stimulus, and during learning of a CS-US association. Moreover, patients with cerebellar stroke may fail to show overt emotional changes. In such patients, however, the activity of several areas, including ventromedial prefrontal cortex, anterior cingulate, pulvinar and insular cortex, is significantly increased relative to healthy subjects when exposed to frightening stimuli. Therefore, other structures may serve to maintain fear response after cerebellar damage. These data indicate that the vermis is involved in the formation of fear memory traces. We suggest that the vermis is not only involved in regulating the autonomic/motor responses, but that it also participates in forming new CS-US associations thus eliciting appropriate responses to new stimuli or situations. In other words, the cerebellum may translate an emotional state elaborated elsewhere into autonomic and motor responses.

Cerebellar Transcranial Direct Current Stimulation Impairs the Practice-dependent Proficiency Increase in Working Memory

How the cerebellum is involved in the practice and proficiency of non-motor functions is still unclear. We tested whether transcranial direct current stimulation (tDCS) over the cerebellum (cerebellar tDCS) induces after-effects on the practice-dependent increase in the proficiency of a working memory (WM) task (Sternberg test) in 13 healthy subjects. We also assessed the effects of cerebellar tDCS on visual evoked potentials (VEPs) in four subjects and compared the effects of cerebellar tDCS on the Sternberg test with those elicited by tDCS delivered over the prefrontal cortex in five subjects. Our experiments showed that anodal or cathodal tDCS over the cerebellum impaired the practice-dependent improvement in the reaction times in a WM task. Because tDCS delivered over the prefrontal cortex induced an immediate change in the WM task but left the practice-dependent proficiency unchanged, the effects of cerebellar tDCS are structure-specific. Cerebellar tDCS left VEPs unaffected, its effect on the Sternberg task therefore seems unlikely to arise from visual system involvement. In conclusion, tDCS over the cerebellum specifically impairs the practice-dependent proficiency increase in verbal WM.

Learning-related long-term potentiation of inhibitory synapses in the cerebellar cortex

Despite the widespread distribution of inhibitory synapses throughout the central nervous system, plasticity of inhibitory synapses related to associative learning has never been reported. In the cerebellum, the neural correlate of fear memory is provided by a long-term potentiation (LTP) of the excitatory synapse between the parallel fibers (PFs) and the Purkinje cell (PC). In this article, we provide evidence that inhibitory synapses in the cerebellar cortex also are affected by fear conditioning. Whole-cell patch-clamp recordings of spontaneous and miniature GABAergic events onto the PC show that the frequency but not the amplitude of these events is significantly greater up to 24 h after the conditioning. Adequate levels of excitation and inhibition are required to maintain the temporal fidelity of a neuronal network. Such fidelity can be evaluated by determining the time window for multiple input coincidence detection. We found that, after fear learning, PCs are able to integrate excitatory inputs with greater probability within short delays, but the width of the whole window is unchanged. Therefore, excitatory LTP provides a more effective detection, and inhibitory potentiation serves to maintain the time resolution of the system.

The effects of fear conditioning on cerebellar LTP and LTD

Long-term potentiation (LTP) and depression (LTD) at parallel fibre-Purkinje cell synapses have been described in vitro in the cerebellar cortex, but the physiological roles of these two forms of plasticity have not been well defined. Here we show that, in cerebellar slices taken from rats that had undergone fear conditioning, there was a significant occlusion of electrically induced LTP at parallel fibre-Purkinje cell synapses. This effect was long-lasting and related to associative processes, as LTP was not occluded in unpaired animals. Notably, in conditioned animals the LTP-inducing protocol produced LTD in some cells instead of LTP. Conversely, synaptic depression induced by conjunctive stimulation of parallel fibers and climbing fibres was impaired in tissue taken immediately following aversive stimulation in both paired and unpaired subjects. This effect was not, however, long-lasting as the incidence and extent of LTD returned to normal levels 24 h after behavioural testing. These findings suggest that LTP takes part in the mechanisms underlying aversive associative memories in the cerebellum.