A central role for norepinephrine in the modulation of cerebellar learning tasks

Central amygdala contributes to stimulus facilitation and pre-stimulus vigilance during cerebellar learning

Our previous studies found that the central amygdala (CeA) modulates cerebellum-dependent eyeblink conditioning (EBC) using muscimol inactivation. We also found that CeA inactivation decreases cerebellar neuronal activity during the conditional stimulus (CS) from the start of training. Based on these findings, we hypothesized that the CeA facilitates CS input to the cerebellum. The current study tested the CS facilitation hypothesis using optogenetic inhibition with archaerhodopsin (Arch) and excitation with channelrhodopsin (ChR2) of the CeA during EBC in male rats. Optogenetic manipulations were administered during the 400 ms tone CS or during a 400 ms pre-CS period. As predicted by the CS facilitation hypothesis CeA inhibition during the CS impaired EBC and CeA excitation during the CS facilitated EBC. Unexpectedly, CeA inhibition just prior to the CS also impaired EBC, while CeA excitation during the pre-CS pathway did not facilitate EBC. The results suggest that the CeA contributes to CS facilitation and vigilance during the pre-CS period. These putative functions of the CeA may be mediated through separate output pathways from the CeA to the cerebellum.

Resting-State Changes in Aging and Parkinson's Disease Are Shaped by Underlying Neurotransmission: A Normative Modeling Study

BACKGROUND

Human healthy and pathological aging is linked to a steady decline in brain resting-state activity and connectivity measures. The neurophysiological mechanisms that underlie these changes remain poorly understood.

METHODS

Making use of recent developments in normative modeling and availability of in vivo maps for various neurochemical systems, we tested in the UK Biobank cohort (n = 25,917) whether and how age- and Parkinson's disease-related resting-state changes in commonly applied local and global activity and connectivity measures colocalize with underlying neurotransmitter systems.

RESULTS

We found that the distributions of several major neurotransmitter systems including serotonergic, dopaminergic, noradrenergic, and glutamatergic neurotransmission correlated with age-related changes across functional activity and connectivity measures. Colocalization patterns in Parkinson's disease deviated from normative aging trajectories for these, as well as for cholinergic and GABAergic (gamma-aminobutyric acidergic) neurotransmission. The deviation from normal colocalization of brain function and GABAA correlated with disease duration.

CONCLUSIONS

These findings provide new insights into molecular mechanisms underlying age- and Parkinson's-related brain functional changes by extending the existing evidence elucidating the vulnerability of specific neurochemical attributes to normal aging and Parkinson's disease. The results particularly indicate that alongside dopamine and serotonin, increased vulnerability of glutamatergic, cholinergic, and GABAergic systems may also contribute to Parkinson's disease-related functional alterations. Combining normative modeling and neurotransmitter mapping may aid future research and drug development through deeper understanding of neurophysiological mechanisms that underlie specific clinical conditions.

Nigella sativa Oil Improves Motor Skill Learning of Albino Mice: In Vivo and In Silico Investigations

Experimental evidences demonstrated that Nigella sativa oil (NSO) can restore neuronal integrities and processes by increasing the neuronal density, decreasing apoptosis, preventing inflammatory processes, and improving the neurogenic cells in the hippocampus. This refurbishment enhances the learning process and memory. The antioxidant defense mechanism of NSO slows down the process of neurodegeneration and motor deficit. The present study aimed to investigate the effects of NSO on motor skill learning using the single pellet reaching task method on Swiss albino mice, followed by in silico studies. Mice (total of 16) were randomly divided into the control group and treatment group (n = 8). The treatment group received 1 ml/kg b.w. NSO orally once daily for 7 days, and a control group received 1 ml/kg normal saline instead of NSO in a similar manner. The average success rate due to ingestion of NSO in the treatment group mice increased significantly (P < 0.05) compared to controlled mice. Molecular docking analysis revealed that thymoquinone, carvacrol, thymohydroquinone, p-cymene, and t-anethole have binding affinities for the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPA-R) that ranges from (-5.1 to -6.2) kcal/mol, which is comparable to the reference ligand glutamic acid binding affinity with AMPA-R (-6.6 kcal/mol). Thymoquinone and carvacrol formed hydrogen bonds with AMPA receptor at TYR61, SER142, and SER143 residues, comparable to the binding affinity of glutamic acid. ADMET analysis reported that all the compounds have higher bioavailability (>90%) and can cross the BBB easily (logBB> 0.3). Based on our experimental data and in silico report, we concluded that the enhanced motor skill learning effects of NSO are due to presence of potent antioxidants-thymoquinone and carvacrol-which might serve as AMPA receptor agonists. These phytoconstituents may play role in synaptic strengthening and promote experience-dependent motor skill learning.

Effect of Noradrenaline on the Facial Stimulation-Evoked Mossy Fiber-Granule Cell Synaptic Transmission in Mouse Cerebellar Cortex

Noradrenaline is an important neuromodulator in the cerebellum. We previously found that noradrenaline depressed cerebellar Purkinje cell activity and climbing fiber–Purkinje cell synaptic transmission in vivo in mice. In this study, we investigated the effect of noradrenaline on the facial stimulation-evoked cerebellar cortical mossy fiber–granule cell synaptic transmission in urethane-anesthetized mice. In the presence of a γ-aminobutyrate_A (GABA_A) receptor antagonist, air-puff stimulation of the ipsilateral whisker pad evoked mossy fiber–granule cell synaptic transmission in the cerebellar granular layer, which expressed stimulus onset response, N1 and stimulus offset response, N2. Cerebellar surface perfusion of 25 μM noradrenaline induced decreases in the amplitude and area under the curve of N1 and N2, accompanied by an increase in the N2/N1 ratio. In the presence of a GABA_A receptor blocker, noradrenaline induced a concentration-dependent decrease in the amplitude of N1, with a half-maximal inhibitory concentration of 25.45 μM. The noradrenaline-induced depression of the facial stimulation-evoked mossy fiber–granule cell synaptic transmission was reversed by additional application of an alpha-adrenergic receptor antagonist or an alpha-2 adrenergic receptor antagonist, but not by a beta-adrenergic receptor antagonist or an alpha-1 adrenergic receptor antagonist. Moreover, application of an alpha-2 adrenergic receptor agonist, UK14304, significantly decreased the synaptic response and prevented the noradrenaline-induced depression. Our results indicate that noradrenaline depresses facial stimulation-evoked mossy fiber–granule cell synaptic transmission via the alpha-2 adrenergic receptor in vivo in mice, suggesting that noradrenaline regulates sensory information integration and synaptic transmission in the cerebellar cortical granular layer.

Disrupted functional connectivity of the locus coeruleus in healthy adults with parental history of Alzheimer's disease

Prevention and early treatment strategies for Alzheimer's disease (AD) are hampered by the lack of research biomarkers. Neuropathological changes in the Locus Coeruleus (LC) are detected early in AD, and noradrenaline plays a neuroprotective role in LC projecting areas. We assessed functional connectivity (FC) of the brainstem in asymptomatic individuals at familial risk for AD hypothesizing that FC of the LC will be decreased in relation to not-at-risk individuals. Thirty-one offspring of patients with late-onset AD (O-LOAD) (22 females; mean age ± SD = 50.36 ± 8.32) and 28 healthy controls (HC) (20 females; mean age ± SD = 53.90 ± 8.44) underwent a neurocognitive evaluation and a resting-state functional magnetic resonance imaging acquisition. In FC analyses we evaluated whole-brain global connectivity of the brainstem area, and subsequently assessed seed-to-voxel FC patterns from regions showing between-group differences. O-LOAD individuals scored worse in neurocognitive measures of memory and overall functioning (p_FDR<0.05). In imaging analyses, we observed that O-LOAD individuals showed decreased global connectivity in a cluster encompassing the left LC (peak = -4, -34, -32, p_TFCE<0.05). Seed-to-voxel analyses revealed that this finding was largely explained by decreased connectivity between the LC and the cerebellar cortex. Moreover, FC between the LC and the left cerebellum correlated positively with delayed recall scores. FC between the LC and the cerebellar cortex is decreased in the healthy offspring of patients with LOAD, such connectivity measurements being associated with delayed memory scores. The assessment of FC between the LC and the cerebellum may serve as a biomarker of AD vulnerability.

Environmental Training and Synaptic Functions in Young and Old Brain: A Presynaptic Perspective

BACKGROUND

Aging is an unavoidable, physiological process that reduces the complexity and the plasticity of the synaptic contacts in Central Nervous System (CNS), having profound implications for human well-being. The term "cognitive reserve" refers to central cellular adaptations that augment the resilience of human brain to damage and aging. The term "Cognitive training" indicates the cultural, social and physical stimulations proposed as add-on therapy for the cure of central neurological diseases. "Cognitive training" reinforces the "cognitive reserve" permitting to counteract brain impairments and rejuvenating synaptic complexity. The research has begun investigating the clinical impact of the "cognitive training" in aged people, but additional work is needed to definitively assess its effectiveness. In particular, there is a need to understand, from a preclinical point of view, whether "cognitive training" promotes compensatory effects or, alternatively, if it elicits genuine recovery of neuronal defects. Although the translation from rodent studies to the clinical situation could be difficult, the results from pre-clinical models are of high clinical relevance, since they should allow a better understanding of the effects of environmental interventions in aging-associated chronic derangements in mammals.

CONCLUSION

Data in literature and the recent results obtained in our laboratory concerning the impact of environmental stimulation on the presynaptic release of noradrenaline, glutamate and gamma amino butyric acid (GABA) suggest that these neurotransmitters undergo different adaptations during aging and that they are differently tuned by "cognitive training". The impact of "cognitive training" on neurotransmitter exocytosis might account for the cellular events involved in reinforcement of "cognitive reserve" in young and old animals.

Noradrenaline depresses spontaneous complex spikes activity of cerebellar Purkinje cells via α2-adrenergic receptor in vivo in mice

Locus coeruleus (LC) noradrenergic neurons afferents release noradrenaline (NA) in the cerebellar cortex for modulating cerebellar neuronal circuitry function. Our previous study found that NA inhibited the spontaneous simple spikes activity of cerebellar Purkinje cells (PC) through activation of molecular layer interneurons (MLIs) in vivo in mice. We here examined the effects of NA on spontaneous complex spikes (CSs) activity of cerebellar PC in urethane-anesthetized mice by electrophysiology recording technique and pharmacological methods. Our results showed that cerebellar surface perfusion of NA significantly reduced the number of spikelets and the area under curve (AUC) of the spontaneous CSs. Application of nonselective adrenergic receptor (AR) antagonist, phentolamine, abolished the NA-induced inhibition of CSs. However applying a nonselective β-AR blocker, propranolol, failed to prevent the NA-induced inhibition of CSs activity. The NA-induced inhibition of CSs activity was not blocked by α1-AR antagonist, prazosin, but it was abolished by α2-AR antagonist, yohimibine. Moreover, application of α2-AR agonist, UK14304 induced a depression of CSs activity and mimicked the NA-induced inhibition of CS activity. These results indicate that NA regulates spontaneous CSs activity of cerebellar PCs via activation of α2-AR in vivo in mice. Our present results suggest that noradrenergic neurons of LC may modulate the outputs of cerebellar PCs via inhibition of CSs activity.

The cerebellum in Alzheimer's disease: evaluating its role in cognitive decline

The cerebellum has long been regarded as essential only for the coordination of voluntary motor activity and motor learning. Anatomical, clinical and neuroimaging studies have led to a paradigm shift in the understanding of the cerebellar role in nervous system function, demonstrating that the cerebellum appears integral also to the modulation of cognition and emotion. The search to understand the cerebellar contribution to cognitive processing has increased interest in exploring the role of the cerebellum in neurodegenerative and neuropsychiatric disorders. Principal among these is Alzheimer's disease. Here we review an already sizeable existing literature on the neuropathological, structural and functional neuroimaging studies of the cerebellum in Alzheimer's disease. We consider these observations in the light of the cognitive deficits that characterize Alzheimer's disease and in so doing we introduce a new perspective on its pathophysiology and manifestations. We propose an integrative hypothesis that there is a cerebellar contribution to the cognitive and neuropsychiatric deficits in Alzheimer's disease. We draw on the dysmetria of thought theory to suggest that this cerebellar component manifests as deficits in modulation of the neurobehavioural deficits. We provide suggestions for future studies to investigate this hypothesis and, ultimately, to establish a comprehensive, causal clinicopathological disease model.

The Emerging Role of Altered Cerebellar Synaptic Processing in Alzheimer's Disease

The role of the cerebellum in Alzheimer's disease (AD) has been neglected for a long time. Recent studies carried out using transgenic mouse models have demonstrated that amyloid-β (Aβ) is deposited in the cerebellum and affects synaptic transmission and plasticity, sometimes before plaque formation. A wide variability of motor phenotype has been observed in the different murine models of AD, without a consistent correlation with the extent of cerebellar histopathological changes or with cognitive deficits. The loss of noradrenergic drive may contribute to the impairment of cerebellar synaptic function and motor learning observed in these mice. Furthermore, cerebellar neurons, particularly granule cells, have been used as in vitro model of Aβ-induced neuronal damage. An unexpected conclusion is that the cerebellum, for a long time thought to be somehow protected from AD pathology, is actually considered as a region vulnerable to Aβ toxic damage, even at the early stage of the disease, with consequences on motor performance.

Differential regulations of vestibulo-ocular reflex and optokinetic response by β- and α2-adrenergic receptors in the cerebellar flocculus

Norepinephrine modulates synaptic plasticity in various brain regions and is implicated in memory formation, consolidation and retrieval. The cerebellum is involved in motor learning, and adaptations of the vestibulo-ocular reflex (VOR) and optokinetic response (OKR) have been studied as models of cerebellum-dependent motor learning. Previous studies showed the involvement of adrenergic systems in the regulation of VOR, OKR and cerebellar synaptic functions. Here, we show differential contributions of β- and α2-adrenergic receptors in the mouse cerebellar flocculus to VOR and OKR control. Effects of application of β- or α2-adrenergic agonist or antagonist into the flocculus suggest that the β-adrenergic receptor activity maintains the VOR gain at high levels and contributes to adaptation of OKR, and that α2-adrenergic receptor counteracts the β-receptor activity in VOR and OKR control. We also examined effects of norepinephrine application, and the results suggest that norepinephrine regulates VOR and OKR through β-adrenergic receptor at low concentrations and through α2-receptor at high concentrations.

Noradrenergic modulation of the parallel fiber-Purkinje cell synapse in mouse cerebellum

The signals arriving to Purkinje cells via parallel fibers are essential for all tasks in which the cerebellum is involved, including motor control, learning new motor skills and calibration of reflexes. Since learning also requires the activation of adrenergic receptors, we investigated the effects of adrenergic receptor agonists on the main plastic site of the cerebellar cortex, the parallel fiber-Purkinje cell synapse. Here we show that noradrenaline serves as an endogenous ligand for both α1-and α2-adrenergic receptors to produce synaptic depression between parallel fibers and Purkinje cells. On the contrary, PF-EPSCs were potentiated by the β-adrenergic receptor agonist isoproterenol. This short-term potentiation was postsynaptically expressed, required protein kinase A, and was mimicked by the β2-adrenoceptor agonist clenbuterol, suggesting that the β2-adrenoceptors mediate the noradrenergic facilitation of synaptic transmission between parallel fibers and Purkinje cells. Moreover, β-adrenoceptor activation lowered the threshold for cerebellar long-term potentiation induced by 1 Hz parallel fiber stimulation. The presence of both α and β adrenergic receptors on Purkinje cells suggests the existence of bidirectional mechanisms of regulation allowing the noradrenergic afferents to refine the signals arriving to Purkinje cells at particular arousal states or during learning.

Attention-Deficit/Hyperactivity Disorder

Attention-deficit/hyperactivity disorder (ADHD) is a neurobiological condition of childhood onset with the hallmarks of inattention, impulsivity, and hyperactivity. Inattention includes excessive daydreaming, disorganization, and being easily distracted. Impulsivity manifests as taking an action before fully thinking of the consequences. Hyperactivity includes an excessive rate of speech and motor activity. Complications of ADHD include academic failure, low self-esteem, poor work performance, substance abuse, criminal justice issues, and social problems. ADHD is predominately due to decreased activity in the frontal lobe. Dopamine and norepinephrine are the main neurotransmitters involved in the pathophysiology of ADHD. Pharmacological treatment of ADHD includes psychostimulants, norepinephrine reuptake inhibitors, α2 agonists, bupropion, and monoamine oxidase inhibitors. The most effective medications are the psychostimulants. Nonpharmacological treatment of ADHD includes coaching, providing structure, academic accommodations, and work accommodations.

Erasing synapses in sleep: is it time to be SHY?

Converging lines of evidence strongly support a role for sleep in brain plasticity. An elegant idea that may explain how sleep accomplishes this role is the "synaptic homeostasis hypothesis (SHY)." According to SHY, sleep promotes net synaptic weakening which offsets net synaptic strengthening that occurs during wakefulness. SHY is intuitively appealing because it relates the homeostatic regulation of sleep to an important function (synaptic plasticity). SHY has also received important experimental support from recent studies in Drosophila melanogaster. There remain, however, a number of unanswered questions about SHY. What is the cellular mechanism governing SHY? How does it fit with what we know about plasticity mechanisms in the brain? In this review, I discuss the evidence and theory of SHY in the context of what is known about Hebbian and non-Hebbian synaptic plasticity. I conclude that while SHY remains an elegant idea, the underlying mechanisms are mysterious and its functional significance unknown.

Noradrenergic control of associative synaptic plasticity by selective modulation of instructive signals

Synapses throughout the brain are modified through associative mechanisms in which one input provides an instructive signal for changes in the strength of a second coactivated input. In cerebellar Purkinje cells, climbing fiber synapses provide an instructive signal for plasticity at parallel fiber synapses. Here, we show that noradrenaline activates alpha2-adrenergic receptors to control short-term and long-term associative plasticity of parallel fiber synapses. This regulation of plasticity does not reflect a conventional direct modulation of the postsynaptic Purkinje cell or presynaptic parallel fibers. Instead, noradrenaline reduces associative plasticity by selectively decreasing the probability of release at the climbing fiber synapse, which in turn decreases climbing fiber-evoked dendritic calcium signals. These findings raise the possibility that targeted presynaptic modulation of instructive synapses could provide a general mechanism for dynamic context-dependent modulation of associative plasticity.

Developmental enhancement of alpha2-adrenoceptor-mediated suppression of inhibitory synaptic transmission onto mouse cerebellar Purkinje cells

Noradrenaline (NA) modulates glutamatergic and GABAergic transmission in various areas of the brain. It is reported that some alpha2-adrenoceptor subtypes are expressed in the cerebellar cortex and alpha2-adrenoceptors may play a role in motor coordination. Our previous study demonstrated that the selective alpha2-adrenoceptor agonist clonidine partially depresses spontaneous inhibitory postsynaptic currents (sIPSCs) in mouse cerebellar Purkinje cells (PCs). Here we found that the inhibitory effect of clonidine on sIPSCs was enhanced during postnatal development. The activation of alpha2-adrenoceptors by clonidine did not affect sIPSCs in PCs at postnatal days (P) 8-10, when PCs showed a few sIPSCs and interneurons in the molecular layer (MLIs) did not cause action potential (AP). In the second postnatal week, the frequency of sIPSCs increased temporarily and reached a plateau at P14. By contrast, MLIs began to fire at P11 with the firing rate gradually increasing thereafter and reaching a plateau at P21. In parallel with this rise in the rate of firing, the magnitude of the clonidine-mediated inhibition of sIPSCs increased during postnatal development. Furthermore, the magnitude of the clonidine-mediated firing suppression in MLIs, which seemed to be mediated by a reduction in amplitude of the hyperpolarization-activated nonselective cation current, I(h), was constant across development. Both alpha2A- and alpha2B-, but not alpha2C-, adrenoceptors were strongly expressed in MLIs at P13, and P31. Therefore, the developmental enhancement of the clonidine-mediated inhibition of sIPSCs is attributed to an age-dependent increase in AP-derived sIPSCs, which can be blocked by clonidine. Thus, presynaptic activation of alpha2-adrenoceptors inhibits cerebellar inhibitory synaptic transmission after the second postnatal week, leading to a restriction of NA signaling, which is mainly mediated by alpha1- and beta2-adrenoceptors in the adult cerebellar neuronal circuit.

P2Y1 receptor switches to neurons from glia in juvenile versus neonatal rat cerebellar cortex

Background

In the CNS, several P2 receptors for extracellular nucleotides are identified on neurons and glial cells to participate to neuron-neuron, glia-glia and glia-neuron communication.

Results

In this work, we describe the cellular and subcellular presence of metabotropic P2Y₁ receptor in rat cerebellum at two distinct developmental ages, by means of immunofluorescence-confocal and electron microscopy as well as western blotting and direct membrane separation techniques. At postnatal day 21, we find that P2Y₁ receptor in addition to Purkinje neurons, is abundant on neuronal specializations identified as noradrenergic by anatomical, morphological and biochemical features. P2Y₁ receptor immunoreactivity colocalizes with dopamine β-hydroxylase, tyrosine hydroxylase, neurofilament light chain, synaptophysin and flotillin, but not with glial fibrillary acidic protein for astrocytes. P2Y₁ receptor is found enriched in membrane microdomains such as lipid rafts, in cerebellar synaptic vesicles, and is moreover visualized on synaptic varicosities by electron microscopy analysis. When examined at postnatal day 7, P2Y₁ receptor immunoreactivity is instead predominantly expressed only on Bergmann and astroglial cells, as shown by colocalization with glial fibrillary acidic protein rather then neuronal markers. At this age, we moreover identify that P2Y₁ receptor-positive Bergmann fibers wrap up doublecortin-positive granule cells stretching along them, while migrating through the cerebellar layers.

Conclusion

Membrane components including purinergic receptors are already known to mediate cellular contact and aggregation in platelets. Our results suggesting a potential role for P2Y₁ protein in cell junction/communication and development, are totally innovative for the CNS.

α-Adrenoceptive Dual Modulation of Inhibitory GABAergic Inputs to Purkinje Cells in the Mouse Cerebellum

Noradrenaline (NA) modulates synaptic transmission in various sites of the CNS. In the cerebellar cortex, several studies have revealed that NA enhances inhibitory synaptic transmission by β-adrenoceptor–and cyclic AMP–dependent pathways. However, the effects of α-adrenoceptor activation on cerebellar inhibitory neurotransmission have not yet been fully elucidated. Therefore we investigated the effects of the α1- or α2-adrenoceptor agonist on inhibitory postsynaptic currents (IPSCs) recorded from mouse Purkinje cells (PCs). We found that the nonselective α-adrenoceptor agonist 6-fluoro-norepinephrine increased both the frequency and amplitude of spontaneous IPSCs (sIPSCs). This enhancement was mostly mimicked by the selective α1-adrenoceptor agonist phenylephrine (PE). PE also enhanced the amplitude of evoked IPSCs (eIPSCs) and increased the frequency but not the amplitude of miniature IPSCs (mIPSCs). Moreover, PE decreased the paired-pulse ratio of eIPSCs and did not change γ-aminobutyric acid (GABA) receptor sensitivity in PCs. Conversely, the selective α2-adrenoceptor agonist clonidine significantly reduced both the frequency and the amplitude of sIPSCs. Neither eIPSCs nor mIPSCs were affected by clonidine. Furthermore, presynaptic cell-attached recordings showed that spontaneous activity of GABAergic interneurons was enhanced by PE but reduced by clonidine. These results suggest that NA enhances inhibitory neurotransmitter release by α1-adrenoceptors, which are expressed in presynaptic terminals and somatodendritic domains, whereas NA suppresses the excitability of interneurons by α2-adrenoceptors, which are expressed in presynaptic somatodendritic domains. Thus cerebellar α-adrenoceptors play roles in a presynaptic dual modulation of GABAergic inputs from interneurons to PCs, thereby providing a likely mechanism for the fine-tuning of information flow in the cerebellar cortex.