Cerebellar interpositus nucleus exhibits time-dependent errors and predictive responses

Learning is a functional state of the brain that should be understood as a continuous process, rather than being restricted to the very moment of its acquisition, storage, or retrieval. The cerebellum operates by comparing predicted states with actual states, learning from errors, and updating its internal representation to minimize errors. In this regard, we studied cerebellar interpositus nucleus (IPn) functional capabilities by recording its unitary activity in behaving rabbits during an associative learning task: the classical conditioning of eyelid responses. We recorded IPn neurons in rabbits during classical eyeblink conditioning using a delay paradigm. We found that IPn neurons reduce error signals across conditioning sessions, simultaneously increasing and transmitting spikes before the onset of the unconditioned stimulus. Thus, IPn neurons generate predictions that optimize in time and shape the conditioned eyeblink response. Our results are consistent with the idea that the cerebellum works under Bayesian rules updating the weights using the previous history.

Glycogen in Astrocytes and Neurons: Physiological and Pathological Aspects

Brain glycogen is stored mainly in astrocytes, although neurons also have an active glycogen metabolism. Glycogen has gained relevance as a key player in brain function. In this regard, genetically modified animals have allowed researchers to unravel new roles of this polysaccharide in the brain. Remarkably, mice in which glycogen synthase is abolished in the brain, and thus devoid of brain glycogen, are viable, thereby indicating that the polysaccharide in this organ is not a requirement for survival. While there was growing evidence supporting a role of glycogen in learning and memory, these animals have now confirmed that glycogen participates in these two processes.The association of epilepsy with brain glycogen has also attracted attention. Analysis of genetically modified mice indicates that the relation between brain glycogen and epilepsy is complex. While the formation of glycogen aggregates clearly underlies epilepsy, as in Lafora Disease (LD), the absence of glycogen also favors the occurrence of seizures.LD is a rare genetic condition that affects children. It is characterized by epileptic seizures and neurodegeneration, and it develops rapidly until finally causing death. Research into this disease has unveiled new aspects of glycogen metabolism. Animal models of LD accumulate polyglucosan bodies formed by aberrant glycogen aggregates, called Lafora bodies (LBs). The abolition of glycogen synthase (GS) prevents the formation of LBs and the development of LD, thereby indicating that glycogen accumulation underlies this disease and the associated symptoms, and thus establishing a clear relation between the accumulation of glycogen aggregates and the incidence of seizures.Although it was initially accepted that LBs were essentially neuronal, it is now evident that astrocytes also accumulate polyglucosan aggregates in LD. However, the appearance and composition of these deposits differs from that observed in neurons. Of note, the astrocytic aggregates in LD models show remarkable similarities with corpora amylacea (CA), a type of polyglucosan aggregate observed in the brains of aged mice and humans. The abolition of GS in mice also impedes the formation of CA with age and at the same time prevents the formation of a number of protein aggregates associated with aging. Therefore CA may play a role in age-related neurological decline.

Compromised Survival of Cerebellar Molecular Layer Interneurons Lacking GDNF Receptors GFRα1 or RET Impairs Normal Cerebellar Motor Learning

The role of neurotrophic factors as endogenous survival proteins for brain neurons remains contentious. In the cerebellum, the signals controlling survival of molecular layer interneurons (MLIs) are unknown, and direct evidence for the requirement of a full complement of MLIs for normal cerebellar function and motor learning has been lacking. Here, we show that Purkinje cells (PCs), the target of MLIs, express the neurotrophic factor GDNF during MLI development and survival of MLIs depends on GDNF receptors GFRα1 and RET. Conditional mutant mice lacking either receptor lose a quarter of their MLIs, resulting in compromised synaptic inhibition of PCs, increased PC firing frequency, and abnormal acquisition of eyeblink conditioning and vestibulo-ocular reflex performance, but not overall motor activity or coordination. These results identify an endogenous survival mechanism for MLIs and reveal the unexpected vulnerability and selective requirement of MLIs in the control of cerebellar-dependent motor learning.

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The signals controlling survival of molecular layer interneurons (MLIs) are unclear

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Whether MLIs are involved in normal cerebellar function was unclear

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Purkinje cells express GDNF, and survival of MLIs depends on GDNF receptors GFRα1 and RET

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Requirement of MLIs for cerebellar-dependent motor learning

Functional basis of associative learning and its relationships with long-term potentiation evoked in the involved neural circuits: Lessons from studies in behaving mammals

While contemporary neuroscience is paying increasing attention to subcellular and molecular events and other intracellular phenomena underlying the acquisition, storage, and retrieval of newly acquired motor and cognitive abilities, parallel attention should be paid to the study of the electrophysiological phenomena taking place at selected cortical and subcortical neuronal and synaptic sites during the precise moment of learning acquisition, extinction, and recall. These in vivo approaches to the study of learning and memory processes will allow the proper integration of the important information collected from in vitro and delayed molecular studies. Here, we summarize studies in behaving mammals carried out in our laboratory during the past ten years on the relationships between experimentally evoked long-term potentiation (LTP) and activity-dependent changes in synaptic strength taking place in hippocampal, prefrontal and related cortical and subcortical circuits during the acquisition of classical eyeblink conditioning or operant learning tasks. These studies suggest that different hippocampal synapses are selectively modified in strength during the acquisition of classical, but not instrumental, learning tasks. In contrast, selected prefrontal and striatum synapses are more directly modified by operant conditioning. These studies also show that besides N-methyl-D-aspartate (NMDA) receptors, many other neurotransmitter, intracellular mediating, and transcription factors participate in these two types of associative learning. Although experimentally evoked LTP seems to prevent the acquisition of classical eyeblink conditioning when induced at selected hippocampal synapses, it proved to be ineffective in preventing the acquisition of operant conditioned tasks when induced at numerous hippocampal, prefrontal, and striatal sites. The differential roles of these cortical structures during these two types of associative learning are discussed, and a diagrammatic representation of their respective functions is presented.

Disease-specific monoclonal antibodies targeting glutamate decarboxylase impair GABAergic neurotransmission and affect motor learning and behavioral functions

Autoantibodies to the smaller isoform of glutamate decarboxylase (GAD) can be found in patients with type 1 diabetes and a number of neurological disorders, including stiff-person syndrome, cerebellar ataxia and limbic encephalitis. The detection of disease-specific autoantibody epitopes led to the hypothesis that distinct GAD autoantibodies may elicit specific neurological phenotypes. We explored the in vitro/in vivo effects of well-characterized monoclonal GAD antibodies. We found that GAD autoantibodies present in patients with stiff person syndrome (n = 7) and cerebellar ataxia (n = 15) recognized an epitope distinct from that recognized by GAD autoantibodies present in patients with type 1 diabetes mellitus (n = 10) or limbic encephalitis (n = 4). We demonstrated that the administration of a monoclonal GAD antibody representing this epitope specificity; (1) disrupted in vitro the association of GAD with γ-Aminobutyric acid containing synaptic vesicles; (2) depressed the inhibitory synaptic transmission in cerebellar slices with a gradual time course and a lasting suppressive effect; (3) significantly decreased conditioned eyelid responses evoked in mice, with no modification of learning curves in the classical eyeblink-conditioning task; (4) markedly impaired the facilitatory effect exerted by the premotor cortex over the motor cortex in a paired-pulse stimulation paradigm; and (5) induced decreased exploratory behavior and impaired locomotor function in rats. These findings support the specific targeting of GAD by its autoantibodies in the pathogenesis of stiff-person syndrome and cerebellar ataxia. Therapies of these disorders based on selective removal of such GAD antibodies could be envisioned.

Different forms of decision-making involve changes in the synaptic strength of the thalamic, hippocampal, and amygdalar afferents to the medial prefrontal cortex

Decision-making and other cognitive processes are assumed to take place in the prefrontal cortex. In particular, the medial prefrontal cortex (mPFC) is identified in rodents by its dense connectivity with the mediodorsal (MD) thalamus, and because of its inputs from other sites, such as hippocampus and amygdala (Amyg). The aim of this study was to find a putative relationship between the behavior of mice during the performance of decision-making tasks that involve penalties as a consequence of induced actions, and the strength of field postsynaptic potentials (fPSPs) evoked in the prefrontal cortex from its thalamic, hippocampal, and amygdalar afferents. Mice were chronically implanted with stimulating electrodes in the MD thalamus, the hippocampal CA1 area, or the basolateral amygdala (BLA), and with recording electrodes in the prelimbic/infralimbic area of the prefrontal cortex. Additional stimulating electrodes aimed at evoking negative reinforcements were implanted on the trigeminal nerve. FPSPs evoked at the mPFC from the three selected projecting areas during the food/shock decision-making task decreased in amplitude with shock intensity and animals' avoidance of the reward. FPSPs collected during the operant task also decreased in amplitude (but that evoked by amygdalar stimulation) when lever presses were associated with a trigeminal shock. Results showed a general decrease in the strength of these potentials when animals inhibited their natural or learned appetitive behaviors, suggesting an inhibition of the prefrontal cortex in these conflicting situations.

Altitude acclimatization improves submaximal cognitive performance in mice and involves an imbalance of the cholinergic system

The aim of this work was to reveal a hypothetical improvement of cognitive abilities in animals acclimatized to altitude and performing under ground level conditions, when looking at submaximal performance, once seen that it was not possible when looking at maximal scores. We modified contrasted cognitive tasks (object recognition, operant conditioning, eight-arm radial maze, and classical conditioning of the eyeblink reflex), increasing their complexity in an attempt to find performance differences in acclimatized animals vs. untrained controls. In addition, we studied, through immunohistochemical quantification, the expression of choline acetyltransferase and acetyl cholinesterase, enzymes involved in the synthesis and degradation of acetylcholine, in the septal area, piriform and visual cortexes, and the hippocampal CA1 area of animals submitted to acute hypobaric hypoxia, or acclimatized to this simulated altitude, to find a relationship between the cholinergic system and a cognitive improvement due to altitude acclimatization. Results showed subtle improvements of the cognitive capabilities of acclimatized animals in all of the tasks when performed under ground-level conditions (although not before 24 h), in the three tasks used to test explicit memory (object recognition, operant conditioning in the Skinner box, and eight-arm radial maze) and (from the first conditioning session) in the classical conditioning task used to evaluate implicit memory. An imbalance of choline acetyltransferase/acetyl cholinesterase expression was found in acclimatized animals, mainly 24 h after the acclimatization period. In conclusion, altitude acclimatization improves cognitive capabilities, in a process parallel to an imbalance of the cholinergic system.

Physical exercise protects against Alzheimer's disease in 3xTg-AD mice

Physical exercise is considered to exert a positive neurophysiological effect that helps to maintain normal brain activity in the elderly. Expectations that it could help to fight Alzheimer's disease (AD) were recently raised. This study analyzed the effects of different patterns of physical exercise on the 3xTg-AD mouse. Male and female 3xTg-AD mice at an early pathological stage (4-month-old) have had free access to a running wheel for 1 month, whereas mice at a moderate pathological stage(7-month-old) have had access either during 1 or 6 months. The non-transgenic mouse strain was used as a control. Parallel animal groups were housed in conventional conditions. Cognitive loss and behavioral and psychological symptoms of dementia (BPSD)-like behaviors were present in the 3xTg-AD mice along with alteration in synaptic function and ong-term potentiation impairment in vivo. Brain tissue showed AD-pathology and oxidative-related changes. Disturbances were more severe at the older age tested. Oxidative stress was higher in males but other changes were similar or higher in females. Exercise treatment ameliorated cognitive deterioration and BPSD-like behaviors such as anxiety and the startle response. Synaptic changes were partially protected by exercise. Oxidative stress was reduced. The best neuroprotection was generally obtained after 6 months of exercise in 7-month-old 3xTg-AD mice. Improved sensorimotor function and brain tissue antioxidant defence were induced in both 3xTg-AD and NonTg mice. Therefore, the benefits of aerobic physical exercise on synapse, redox homeostasis, and general brain function demonstrated in the 3xTg-AD mouse further support the value of this healthy life-style against neurodegeneration.

Learning capabilities and CA1-prefrontal synaptic plasticity in a mice model of accelerated senescence

SAMP8 mice represent a suitable model of accelerated senescence as compared with SAMR1 animals presenting normal aging. Five-month-old SAMP8 mice presented reflex eyelid responses like those of SAMR1 controls, but were incapable of acquiring classically-conditioned eye blink responses in a trace (230 milliseconds [ms] of interstimulus interval) paradigm. Although SAMP8 mice presented a normal paired-pulse facilitation of the hippocampal CA1-medial prefrontal synapse, an input/output curve study revealed smaller field excitatory postsynaptic potentials (fEPSPs) in response to strong stimulations of the CA1-prefrontal pathway. Moreover, SAMP8 mice did not show any activity-dependent potentiation of the CA1-prefrontal synapse across the successive conditioning sessions shown by SAMR1 animals. In addition, SAMP8 mice presented a functional deficit during an object recognition test, continuing to explore the familiar object when controls moved to the novel one. Alert behaving SAMP8 mice presented a significant deficit in long-term potentiation (LTP) at the CA1-medial prefrontal synapse. According to the present results, SAMP8 mice present noticeable functional deficits in hippocampal and prefrontal cortical circuits directly related with the acquisition and storage of new motor and cognitive abilities.

Aged wild-type and APP, PS1, and APP + PS1 mice present similar deficits in associative learning and synaptic plasticity independent of amyloid load

Wild-type and single-transgenic (APP, PS1) and double-transgenic (APP+PS1) mice were studied at three different (3-, 12-, and 18-month-old) age periods. Transgenic mice had reflex eyelid responses like those of controls, but only 3-month-old mice were able to fully acquire conditioned eyeblinks, using a trace paradigm, whilst 12-month-old wild-type and transgenic mice presented intermediate values, and 18-month-old wild-type and transgenic mice were unable to acquire this type of associative learning. 18-month-old wild-type and transgenic mice presented a normal synaptic activation of CA1 pyramidal cells by the stimulation of Schaffer collaterals, but they did not show any activity-dependent potentiation of the CA3-CA1 synapse across conditioning sessions, as was shown by 3-month-old wild-type mice. Moreover, 18-month-old wild-type and transgenic mice presented a noticeable deficit in long-term potentiation evoked in vivo at the hippocampal CA3-CA1 synapse. The 18-month-old wild-type and transgenic mice also presented a significant deficit in prepulse inhibition as compared with 3-month-old controls. Except for results collected by prepulse inhibition, the above-mentioned deficits were not related with the presence of amyloid beta deposits. Thus, learning and memory deficits observed in aged wild-type and transgenic mice are not directly related to the genetic manipulations or to the presence of amyloid plaques.

Age and sex-related serum changes in nitric oxide: Correlations with serological markers

Serum nitric oxide levels, systematically determined in 200 men and women from 18 to 65 year-old, undergo age and sex changes that strongly correlate with serological markers such as those related with cardiovascular functions and lipid profile.

Classical eyeblink conditioning during acute hypobaric hypoxia is improved in acclimatized mice and involves Fos expression in selected brain areas

This work attempts to evaluate the cognitive aspects of the acclimatization ability of mice submitted to simulated altitude. Critical altitudes were detected by evaluating open field activity, combined or not with object recognition tasks, at different acute simulated altitudes. Results showed impaired cognitive abilities at approximately 3,733 m and above. To evaluate acclimatization capabilities, mice submitted to hypobaric hypoxia at approximately 5,000 m for 1 wk were tested for learning and memory performances with classical eyeblink conditioning at the same altitude or at land altitude. Results showed total acclimatization in mice conditioned at approximately 5,000 m but no improved performance in those conditioned at land altitudes compared with controls. Selected brain sites of conditioned animals were analyzed by immunohistochemistry to detect expression of the protein product of the protooncogene c-fos (Fos) in relation to both motor learning processes and hypobaric conditions. In the nucleus of the solitary tract, a higher expression of Fos was found in the acute hypobaric conditioned animals than in control conditioned and nonconditioned animals. Similar patterns between groups were found in the other brain areas, mainly in the piriform cortex and area 1 of the cingulate cortex and in the hippocampus. Differences between hemispheres were detected only in acute hypobaric animals. The present results show that acclimatization to high altitude prevents the impairment of classical eyeblink conditioning evoked by hypobaric hypoxic conditions but does not improve this task when acquired under land conditions, although it could diminish the activation requirements for its performance.

Evidence of a decrease in nitric oxide-storage molecules following acute hypoxia and/or hypobaria, by means of chemiluminescence analysis

Nitrate, nitrite, and other nitroso compounds (NOxs) had been proposed as possible nitric oxide (NO) storage molecules. The present work examines, by means of chemiluminescence analysis, changes in NOx serum levels in rats 1 h before and 24, 48, and 72 h after exposure to acute hypobaric hypoxia (HH; barometric pressure [P(B)] 225 mmHg, oxygen partial pressure [PO2] 48 mmHg), normobaric hypoxia (NH; P(B) 716 mmHg [Jaén city], PO2 48 mmHg), hypobaric normoxia (HN; P(B) 225 mmHg, PO2 150 mmHg), and normobaric normoxia (NN; P(B) 716 mmHg, PO2 150 mmHg) the latter as a control group. Results show a decrease in NOx levels, which reached significance 24 h after exposure in HH animals, 4 h after exposure in the HN and NH groups, and persisted after 48 h of exposure in the HN group. NOx determinations were also performed in brain (cerebral cortex, hippocampus, decorticated brain [basal ganglia-brainstem] and cerebellum), liver, kidney, lung, and heart homogenates, 72 h after the experiment, to detect persistent effects when serum NOx levels had returned to basal values. Only in cerebellum (HN group) and hippocampus (HN and NH groups) were NOx levels significantly lower than in controls. We conclude that not only acute hypobaric hypoxia but also either hypobaria or hypoxia alone induce changes in NOx serum levels. Moreover, all three episodes involve a decrease in NOxs, greater and longer-lasting in hypoxia alone than in hypobaria and hypoxia together. The exhaustion of these NO-storage molecules could be critical when, as during a hypoxic episode, the L-arginine/NOS pathway is impaired.

Age-related changes of the nitric oxide system in the rat brain

This work examines the age-related changes of the NO pathway in the central nervous system (CNS), analyzing nitric oxide synthase (NOS) isoform expression, the level of nitrotyrosine-modified proteins, and the NOS activity in the cerebral cortex, decorticated brain (basal ganglia, thalamus, hypothalamus, tegtum and tegmentum) and cerebellum of young, adult and aged rats. Our data demonstrate that the different NOS isoforms are not uniformly expressed across the CNS. In this sense, the nNOS and eNOS isoenzymes are expressed mainly in the cerebellum and decorticated brain, respectively, while the iNOS isoenzyme shows the highest level in cerebellum. Concerning age, in the cerebral cortex nNOS significantly increased its expression only in adult animals; meanwhile, in the cerebellum the eNOS expression decreased whereas iNOS increased in adult and aged rats. No age-related changes in any isoform were found in decorticated brain. NOS activity, determined by nitrate plus nitrite quantification, registered the highest levels in the cerebellum, where the significant increase detected with aging was probably related to iNOS activity. The number of nitrotyrosine-modified immunoreactive bands differed among regions; thus, the highest number was detected in the decorticated brain while the cerebellum showed the least number of bands. Finally, bulk protein nitration increased in cerebral cortex only in adult animal. No changes were found in the decorticated brain, and the decrease detected in the cerebellum of aged animals was not significant. According to these results, the NO pathway is differently modified with age in the three CNS regions analyzed.