Inactivation of the superior cerebellar peduncle blocks expression but not acquisition of the rabbit's classically conditioned eye-blink response

Inaugural Review Prize 2023: The exercise hyperpnoea dilemma: A 21st-century perspective

During mild or moderate exercise, alveolar ventilation increases in direct proportion to metabolic rate, regulating arterial CO2 pressure near resting levels. Mechanisms giving rise to the hyperpnoea of exercise are unsettled despite over a century of investigation. In the past three decades, neuroscience has advanced tremendously, raising optimism that the 'exercise hyperpnoea dilemma' can finally be solved. In this review, new perspectives are offered in the hope of stimulating original ideas based on modern neuroscience methods and current understanding. We first describe the ventilatory control system and the challenge exercise places upon blood-gas regulation. We highlight relevant system properties, including feedforward, feedback and adaptive (i.e., plasticity) control of breathing. We then elaborate a seldom explored hypothesis that the exercise ventilatory response continuously adapts (learns and relearns) throughout life and ponder if the memory 'engram' encoding the feedforward exercise ventilatory stimulus could reside within the cerebellum. Our hypotheses are based on accumulating evidence supporting the cerebellum's role in motor learning and the numerous direct and indirect projections from deep cerebellar nuclei to brainstem respiratory neurons. We propose that cerebellar learning may be obligatory for the accurate and adjustable exercise hyperpnoea capable of tracking changes in life conditions/experiences, and that learning arises from specific cerebellar microcircuits that can be interrogated using powerful techniques such as optogenetics and chemogenetics. Although this review is speculative, we consider it essential to reframe our perspective if we are to solve the till-now intractable exercise hyperpnoea dilemma.

Cerebellar contribution to the regulation of defensive states

Despite fine tuning voluntary movement as the most prominently studied function of the cerebellum, early human studies suggested cerebellar involvement emotion regulation. Since, the cerebellum has been associated with various mood and anxiety-related conditions. Research in animals provided evidence for cerebellar contributions to fear memory formation and extinction. Fear and anxiety can broadly be referred to as defensive states triggered by threat and characterized by multimodal adaptations such as behavioral and cardiac responses integrated into an intricately orchestrated defense reaction. This is mediated by an evolutionary conserved, highly interconnected network of defense-related structures with functional connections to the cerebellum. Projections from the deep cerebellar nucleus interpositus to the central amygdala interfere with retention of fear memory. Several studies uncovered tight functional connections between cerebellar deep nuclei and pyramis and the midbrain periaqueductal grey. Specifically, the fastigial nucleus sends direct projections to the ventrolateral PAG to mediate fear-evoked innate and learned freezing behavior. The cerebellum also regulates cardiovascular responses such as blood pressure and heart rate-effects dependent on connections with medullary cardiac regulatory structures. Because of the integrated, multimodal nature of defensive states, their adaptive regulation has to be highly dynamic to enable responding to a moving threatening stimulus. In this, predicting threat occurrence are crucial functions of calculating adequate responses. Based on its role in prediction error generation, its connectivity to limbic regions, and previous results on a role in fear learning, this review presents the cerebellum as a regulator of integrated cardio-behavioral defensive states.

The Rules of Cerebellar Learning: Around the Ito Hypothesis

As a tribute to Masao Ito, we propose a model of cerebellar learning that incorporates and extends his original model. We suggest four principles that align well with conclusions from multiple cerebellar learning systems. (1) Climbing fiber inputs to the cerebellum drive early, fast, poorly-retained learning in the parallel fiber to Purkinje cell synapse. (2) Learned Purkinje cell outputs drive late, slow, well-retained learning in non-Purkinje cell inputs to neurons in the cerebellar nucleus, transferring learning from the cortex to the nucleus. (3) Recurrent feedback from Purkinje cells to the inferior olive, through interneurons in the cerebellar nucleus, limits the magnitude of fast, early learning in the cerebellar cortex. (4) Functionally different inputs are subjected to plasticity in the cerebellar cortex versus the cerebellar nucleus. A computational neural circuit model that is based on these principles mimics a large amount of neural and behavioral data obtained from the smooth pursuit eye movements of monkeys.

A FN-MdV pathway and its role in cerebellar multimodular control of sensorimotor behavior

The cerebellum is crucial for various associative sensorimotor behaviors. Delay eyeblink conditioning (DEC) depends on the simplex lobule-interposed nucleus (IN) pathway, yet it is unclear how other cerebellar modules cooperate during this task. Here, we demonstrate the contribution of the vermis-fastigial nucleus (FN) pathway in controlling DEC. We found that task-related modulations in vermal Purkinje cells and FN neurons predict conditioned responses (CRs). Coactivation of the FN and the IN allows for the generation of proper motor commands for CRs, but only FN output fine-tunes unconditioned responses. The vermis-FN pathway launches its signal via the contralateral ventral medullary reticular nucleus, which converges with the command from the simplex-IN pathway onto facial motor neurons. We propose that the IN pathway specifically drives CRs, whereas the FN pathway modulates the amplitudes of eyelid closure during DEC. Thus, associative sensorimotor task optimization requires synergistic modulation of different olivocerebellar modules each provide unique contributions.

Delay eyeblink conditioning depends on the simplex lobule-interposed nucleus pathway in the cerebellum. Here, the authors show that the vermis-fastigial nucleus-medullary reticular nucleus pathway modulates the conditioned and unconditioned eyelid closure during delay eyeblink conditioning.

Multiple components in direction learning in smooth pursuit eye movements of monkeys

We analyzed behavioral features of smooth pursuit eye movements to characterize the course of acquisition and expression of multiple neural components of motor learning. Monkeys tracked a target that began to move in an initial "pursuit" direction and suddenly, but predictably, changed direction after a fixed interval of 250 ms. As the trial is repeated, monkeys learn to make eye movements that predict the change in target direction. Quantitative analysis of the learned response revealed evidence for multiple, dynamic, parallel processes at work during learning. 1) The overall learning followed at least two trial courses: a fast component grew and saturated rapidly over tens of trials, and a slow component grew steadily over up to 1,000 trials. 2) The temporal specificity of the learned response within each trial was crude during the first 100 trials but then improved gradually over the remaining trials. 3) External influences on the gain of pursuit initiation modulate the expression but probably not the acquisition of learning. The gain of pursuit initiation and the expression of the learned response decreased in parallel, both gradually through a 1,000-trial learning block and immediately between learning trials with different gains in the initiation of pursuit. We conclude that at least two distinct neural mechanisms drive the acquisition of pursuit learning over 100 to 1,000 trials (3 to 30 min). Both mechanisms generate underlying memory traces that are modulated in relation to the gain of pursuit initiation before expression in the final motor output. NEW & NOTEWORTHY We show that cerebellum-dependent direction learning in smooth pursuit eye movements grows in at least two components over 1,100 behavioral learning repetitions. One component grows over tens of trials and the other over hundreds. Within trials, learned temporal specificity gradually improves over hundreds of trials. The expression of each learning component on a given trial can be modified by external factors that do not affect the underlying memory trace.

Pretrial functional connectivity differentiates behavioral outcomes during trace eyeblink conditioning in the rabbit

Fluctuations in neural activity can produce states that facilitate and accelerate task-related performance. Acquisition of trace eyeblink conditioning (tEBC) in the rabbit is enhanced when trials are contingent on optimal pretrial activity in the hippocampus. Other regions which are essential for whisker-signaled tEBC, such as the cerebellar interpositus nucleus (IPN), somatosensory and prelimbic cortices, may also show optimal connectivity prior to successful performance. Functional magnetic resonance imaging (fMRI) was acquired in nine rabbits during tEBC on the first and tenth days of initial training and once again after a 30-d, training-free hiatus. Data acquired during the intertrial interval was parsed depending on whether or not a conditioned response (CR) occurred on the upcoming trial and seed-based functional connectivity was calculated among the IPN, hippocampus, somatosensory, and prelimbic cortices. Functional connectivity between the left somatosensory cortex and right IPN, regions critical for establishing and producing CRs evoked by right vibrissae vibration and right corneal airpuff, was significantly negative prior to successful, CR trials as compared with unsuccessful, non-CR trials. Differences were not observed for any of the other possible combinations of connectivity. Our results demonstrate that specific pretrial functional connectivity exists within the rabbit brain and differentiates between upcoming behavioral response outcomes. Online analysis of network fluctuations has the potential to be used as the basis for therapeutic interventions to facilitate learning and memory.

Localization and characterization of an essential associative memory trace in the mammalian brain

We argue here that we have succeeded in localizing an essential memory trace for a basic form of associative learning and memory - classical conditioning of discrete responses learned with an aversive stimulus - to the anterior interpositus nucleus of the cerebellum. We first identified the entire essential circuit, using eyelid conditioning as the model system, and used reversible inactivation, during training, of critical structures and activation of pathways to localize definitively the essential memory trace. This discovery and the associated studies have: 1) shown that the essential cerebellar circuit applies equally to all mammals studied, including humans; 2) shown that this cerebellar circuit holds for the learning of any discrete behavioral response elicited by an aversive US, not just eyelid closure; 3) identified the essential circuit and process for reinforcement for this form of learning; 4) shown that this form of learning and its essential cerebellar circuitry is phylogenetically very old; 5) solved the long-standing puzzle of where memory traces are formed in the brain when the CS is electrical stimulation of the cerebral cortex in conditioning; 6) shown that this cerebellar circuitry forms the essential neural substrate for the behavioral phenomenon of "blocking", and hence, 7) provides the first clear neural instantiation of the Rescorla-Wagner learning algorithm; 8) shown that the fundamental neural process underlying this form of learning is a strengthening of preexisting pathways, and 9) shown that the basic mechanism underlying this strengthening is the formation of new excitatory synapses. This article is part of a Special Issue entitled SI: Brain and Memory.

Cerebellar learning mechanisms

The mechanisms underlying cerebellar learning are reviewed with an emphasis on old arguments and new perspectives on eyeblink conditioning. Eyeblink conditioning has been used for decades a model system for elucidating cerebellar learning mechanisms. The standard model of the mechanisms underlying eyeblink conditioning is that there two synaptic plasticity processes within the cerebellum that are necessary for acquisition of the conditioned response: (1) long-term depression (LTD) at parallel fiber-Purkinje cell synapses and (2) long-term potentiation (LTP) at mossy fiber-interpositus nucleus synapses. Additional Purkinje cell plasticity mechanisms may also contribute to eyeblink conditioning including LTP, excitability, and entrainment of deep nucleus activity. Recent analyses of the sensory input pathways necessary for eyeblink conditioning indicate that the cerebellum regulates its inputs to facilitate learning and maintain plasticity. Cerebellar learning during eyeblink conditioning is therefore a dynamic interactive process which maximizes responding to significant stimuli and suppresses responding to irrelevant or redundant stimuli. This article is part of a Special Issue entitled SI: Brain and Memory.

The ontogeny of associative cerebellar learning

Ontogenetic changes in associative cerebellar learning have been examined extensively using eyeblink conditioning in infant humans and rats. The cerebellum is essential for eyeblink conditioning in adult and infant animals. The cerebellum receives input from the conditional stimulus (CS) through the pontine mossy fiber projection and unconditional stimulus (US) input through the inferior olive climbing fiber projection. Coactivation of the CS and US pathways induces synaptic plasticity in the cerebellum, which is necessary for the conditional response. Ontogenetic changes in eyeblink conditioning are driven by developmental changes in the projections of subcortical sensory nuclei to the pontine nuclei and in the inhibitory projection from the cerebellar deep nuclei to the inferior olive. Developmental changes in the CS and US pathways limit the induction of learning-related plasticity in the cerebellum and thereby limit acquisition of eyeblink conditioning.

Cerebellum and conditioned reflexes

The central assumption of existing models of motor learning in the cerebellum is that cerebellar mossy fibres signal information about the context in which a movement is to be performed and climbing fibres signal in relation to a movement error. This leads to changes in the responsiveness of Purkinje cells, which on the next occasion will generate a corrected output in a given context. Support for this view has come mainly from work on adaptation of the vestibulo-ocular reflex. The discovery that classically conditioned eyeblink responses depend critically on the cerebellum offers the possibility to study the learning of a novel behaviour, rather than modification of an existing reflex. After repeated pairing of a neutral stimulus, such as a tone, with a blink-eliciting stimulus, the tone will acquire the ability to elicit a blink on its own. We review evidence from studies employing a wide variety of techniques that the cerebellum is critical in this type of learning as well as evidence that mossy and climbing fibres have roles assigned to them in cerebellar learning models.

Neural circuitry and plasticity mechanisms underlying delay eyeblink conditioning

Pavlovian eyeblink conditioning has been used extensively as a model system for examining the neural mechanisms underlying associative learning. Delay eyeblink conditioning depends on the intermediate cerebellum ipsilateral to the conditioned eye. Evidence favors a two-site plasticity model within the cerebellum with long-term depression of parallel fiber synapses on Purkinje cells and long-term potentiation of mossy fiber synapses on neurons in the anterior interpositus nucleus. Conditioned stimulus and unconditioned stimulus inputs arise from the pontine nuclei and inferior olive, respectively, converging in the cerebellar cortex and deep nuclei. Projections from subcortical sensory nuclei to the pontine nuclei that are necessary for eyeblink conditioning are beginning to be identified, and recent studies indicate that there are dynamic interactions between sensory thalamic nuclei and the cerebellum during eyeblink conditioning. Cerebellar output is projected to the magnocellular red nucleus and then to the motor nuclei that generate the blink response(s). Tremendous progress has been made toward determining the neural mechanisms of delay eyeblink conditioning but there are still significant gaps in our understanding of the necessary neural circuitry and plasticity mechanisms underlying cerebellar learning.

Ventral lateral geniculate input to the medial pons is necessary for visual eyeblink conditioning in rats

The conditioned stimulus (CS) pathway that is necessary for visual delay eyeblink conditioning was investigated in the current study. Rats were initially given eyeblink conditioning with stimulation of the ventral nucleus of the lateral geniculate (LGNv) as the CS followed by conditioning with light and tone CSs in separate training phases. Muscimol was infused into the medial pontine nuclei (MPN) after each training phase to examine conditioned response (CR) retention to each CS. The spread of muscimol infusions targeting the MPN was examined with fluorescent muscimol. Muscimol infusions into the MPN resulted in a severe impairment in retention of CRs with the LGNv stimulation and light CSs. A less severe impairment was observed with the tone CS. The results suggest that CS information from the LGNv and light CSs is relayed to the cerebellum through the MPN. Retrograde tracing with fluoro-gold (FG) showed that the LGNv and nucleus of the optic tract have ipsilateral projections to the MPN. Unilateral inputs to the MPN from the LGNv and nucleus of the optic tract may be part of the visual CS pathway that is necessary for visual eyeblink conditioning.

The role of the cerebellar interpositus nucleus in short and long term memory for trace eyeblink conditioning

In previous studies the cerebellar interpositus (IP) nucleus, but not the hippocampus, was shown to be necessary both for initial learning and retention and for long-term retention of the standard delay eyeblink conditioned response (CR). However, in the trace eyeblink CR procedure, the hippocampus is also necessary for initial learning and retention, but not for long-term retention. Here the authors evaluate the role of the IP nucleus in both initial learning and retention, and in long-term retention of the trace eyeblink CR, using muscimol infusion to reversibly inactivate the IP nucleus. For the short-term study, there were two subgroups, the first sequentially passed through acquisition, inactivation, and reacquisition phases, whereas the second subgroup went through inactivation, acquisition, and inactivation phases. For the long-term study, the rabbits acquired the CR and then rested for a month. Next, they were distributed into two subgroups: with or without retention training, and finally went through inactivation and reacquisition phases. The results showed that the prelearning IP nucleus inactivation prevented the acquisition of the trace CR, whereas the postlearning inactivation reversibly abolished the expression of both the short- and long-term CR.

Mechanisms of cognitive alterations in hyperammonemia and hepatic encephalopathy: therapeutical implications

Patients with liver diseases (e.g. cirrhosis) may present hepatic encephalopathy (HE), an alteration in cerebral function which is a consequence of previous failure of liver function. Patients with minimal or clinical HE present different levels of cognitive impairment. Hyperammonemia is considered a main contributor to the neurological alterations in HE. Animal models of chronic HE (e.g. rats with portacaval shunts) or of "pure" hyperammonemia also show impaired cognitive function. The studies summarized here show that the impairment of some types of cognitive function in chronic HE is due to the impaired function of the glutamate-nitric oxide-cGMP pathway in brain. Both hyperammonemia and neuroinflammation contribute to the impairment of the pathway and of cognitive function. Treatment of rats with chronic HE or hyperammonemia with inhibitors of phosphodiesterase 5 restores the function of the glutamate-nitric oxide-cGMP pathway and cGMP levels in brain as well as the ability to learn a Y maze conditional discrimination task. The same beneficial effects may be obtained by treating the rats chronically with an anti-inflammatory, ibuprofen. As the function of this pathway is also altered in brain of patients died in HE, this alteration would also contribute to cognitive impairment in patients with HE. Increasing cGMP by using inhibitors of phosphodiesterase 5 (PDE-5) or anti-inflammatories (under safe conditions) would be therefore a new therapeutic approach to improve learning and memory performance in individuals with minimal or clinical HE.

Interaction between Purkinje cells and inhibitory interneurons may create adjustable output waveforms to generate timed cerebellar output

We develop a new model that explains how the cerebellum may generate the timing in classical delay eyeblink conditioning. Recent studies show that both Purkinje cells (PCs) and inhibitory interneurons (INs) have parallel signal processing streams with two time scales: an AMPA receptor-mediated fast process and a metabotropic glutamate receptor (mGluR)-mediated slow process. Moreover, one consistent finding is an increased excitability of PC dendrites (in Larsell's lobule HVI) in animals when they acquire the classical delay eyeblink conditioning naturally, in contrast to in vitro studies, where learning involves long-term depression (LTD). Our model proposes that the delayed response comes from the slow dynamics of mGluR-mediated IP3 activation, and the ensuing calcium concentration change, and not from LTP/LTD. The conditioned stimulus (tone), arriving on the parallel fibers, triggers this slow activation in INs and PC spines. These excitatory (from PC spines) and inhibitory (from INs) signals then interact at the PC dendrites to generate variable waveforms of PC activation. When the unconditioned stimulus (puff), arriving on the climbing fibers, is coupled frequently with this slow activation the waveform is amplified (due to an increased excitability) and leads to a timed pause in the PC population. The disinhibition of deep cerebellar nuclei by this timed pause causes the delayed conditioned response. This suggested PC-IN interaction emphasizes a richer role of the INs in learning and also conforms to the recent evidence that mGluR in the cerebellar cortex may participate in slow motor execution. We show that the suggested mechanism can endow the cerebellar cortex with the versatility to learn almost any temporal pattern, in addition to those that arise in classical conditioning.

Developmental exposure to polychlorinated biphenyls or methylmercury, but not to its combination, impairs the glutamate-nitric oxide-cyclic GMP pathway and learning in 3-month-old rats

Prenatal exposure to polychlorinated biphenyls (PCBs) or methylmercury (MeHg) contaminated food may affect brain development, leading to long-term alterations in cognitive function. Both types of contaminants, PCBs and MeHg, are often found together contaminating food, especially fish in some polluted areas. Exposure to combinations of neurotoxicants may exert different effects on the developing nervous system than exposure to individual contaminants. Developmental exposure (during pregnancy and lactation) to PCB126 or PCB153 impairs learning ability when the rats are 3 months old. Impairment of learning seems to be a consequence of impairment of the function of the glutamate-nitric oxide (NO)-cGMP pathway in brain in vivo. The aims of the present work were 1) to assess whether perinatal exposure to MeHg also affects the function of the glutamate-NO-cGMP pathway in brain in vivo analyzed by in vivo brain microdialysis and/or the ability to learn the Y maze task when the rats are 3 months old, and 2) to assess whether perinatal exposure to combinations of MeHg with PCB153 or PCB126 potentiates, decreases or does not modify the effects of the individual neurotoxicants. Perinatal exposure to PCB126, PCB153 or MeHg impaired the function of the glutamate-NO-cGMP pathway in cerebellum and learning ability. However, co-exposure to PCB126+MeHg or PCB153+MeHg inhibits the impairment of the pathway or learning ability. These results support that the function of this pathway modulates learning of the Y maze task. Moreover, they show that co-exposure to these PCBs and MeHg does not exacerbate, but reduces the effects on the ability to learn this task.

Inactivation of cerebellar output axons impairs acquisition of conditioned eyeblinks

Acquisition of classically conditioned eyeblink responses (CRs) in the rabbit critically depends on intermediate cerebellum-related neural circuits. A highly efficient method for determining possible sites of plasticity within eyeblink circuits is the reversible inactivation of circuit components during learning. Inactivation of either the HVI region of the cerebellar cortex or the cerebellar interposed nuclei (IN) during learning is known to prevent CR acquisition. In contrast, inactivating cerebellar efferent axons in the brachium conjunctivum (BC) with small injections of tetrodotoxin (TTX) has been reported to have no effect on CR acquisition. This suggested that the intermediate cerebellum is essential for learning CRs and that activity mediated by the BC is not required for this process. Since we previously found that BC inactivation blocks CR extinction we re-examined its role in CR acquisition. To ensure complete and long-lasting inactivation of the BC, we injected before each training session doses of TTX that were larger than those in the previous acquisition study. Contrary to the previous negative findings, we found that this temporary block of axons in the brachium conjunctivum prevented normal acquisition of CRs. Injecting TTX directly in the adjacent lateral lemniscus, which could possibly influence CR acquisition, had no effect on learning. In addition, a functional test of TTX diffusion around the BC indicated that the inactivation did not affect other known parts of eyeblink circuits, such as the cerebellar interposed nuclei, the middle cerebellar peduncle or the contralateral red nucleus. We conclude that this form of associative learning in the rabbit eyeblink system requires extra-cerebellar learning and/or cerebellar learning that depends on the operation of cerebellar feedback loops.

Differential effects of cerebellar inactivation on eyeblink conditioned excitation and inhibition

The neural mechanisms underlying excitatory and inhibitory eyeblink conditioning were compared using muscimol inactivation of the cerebellum. In experiment 1, rats were given saline or muscimol infusions into the anterior interpositus nucleus ipsilateral to the conditioned eye before each of four daily excitatory conditioning sessions. Postinfusion testing continued for four more excitatory conditioning sessions. All rats were given a final test session after muscimol infusions. The muscimol infusions inactivated the cerebellar nuclei, lateral anterior lobe, crus I, rostral crus II, and lobule HVI ipsilateral to the conditioned eye. Acquisition of excitatory conditioning was completely prevented by muscimol inactivation. In experiment 2, there were four experimental phases. Phase 1 consisted of excitatory conditioning. In phase 2, rats were given saline or muscimol infusions before conditioned inhibition training. Phase 3 consisted of continued conditioned inhibition training with no drug infusions. In phase 4, all rats received a retardation test in which the inhibitory stimulus was paired with the unconditioned stimulus. Muscimol infusions blocked the expression of conditioned responses during phase 2. However, robust conditioned inhibition was evident in phases 3 and 4. The findings indicate that conditioned excitation and inhibition depend on different mechanisms.

Restoration of learning ability in hyperammonemic rats by increasing extracellular cGMP in brain

Intellectual function is impaired in patients with hyperammonemia and hepatic encephalopathy. Chronic hyperammonemia with or without liver failure impairs the glutamate-nitric oxide-cGMP pathway function in brain in vivo and reduces extracellular cGMP in brain as well as the ability of rats to learn a Y maze conditional discrimination task. We hypothesized that the decrease in extracellular cGMP may be responsible for the impairment in learning ability and intellectual function and that pharmacological modulation of the levels of cGMP may restore learning ability. The aim of this work was to try to reverse the impairment in learning ability of hyperammonemic rats by pharmacologically increasing extracellular cGMP in brain. We assessed whether learning ability may be restored by increasing extracellular cGMP in brain by continuous intracerebral administration of: (1) zaprinast, an inhibitor of the phosphodiesterase that degrades cGMP or (2) cGMP. We carried out tests of conditional discrimination learning in a Y maze with control and hyperammonemic rats treated or not with zaprinast or cGMP. Learning ability was reduced in hyperammonemic rats, which needed more trials than control rats to learn the task. Continuous intracerebral administration of zaprinast or cGMP restored the ability of hyperammonemic rats to learn this task. Pharmacological modulation of extracellular cGMP levels in brain may be a useful therapeutic approach to improve learning and memory performance in individuals in whom cognitive abilities are impaired by different reasons, for example in patients with liver disease who present hyperammonemia and decreased intellectual function.

Inactivation of the brachium conjunctivum prevents extinction of classically conditioned eyeblinks

It is well established that the intermediate cerebellum is involved in the acquisition of classically conditioned eyeblink responses (CRs). Recent studies that inactivated the interposed nuclei (IN) demonstrated that blocking the intermediate cerebellum also interrupts CR extinction. Is this extinction deficit related to interrupting the information flow to efferent targets of the IN? To address this question, we inactivated axons of IN neurons in the brachium conjunctivum (BC). This treatment blocked the output of the intermediate cerebellum without directly affecting neurons in the deep cerebellar nuclei. Rabbits were trained in a delay classical conditioning paradigm, using a tone as the conditioned stimulus (CS) and a corneal air puff as the unconditioned stimulus (US). Then, the BC was microinjected with a sodium channel blocker, tetrodotoxin, during 4 extinction sessions in which rabbits were presented only with the CS. Tests performed after the 4-day injection period revealed that CRs did not extinguish in BC inactivation sessions but extinguished at a normal rate in the absence of the drug. CRs were then re-acquired. These data show that the normal flow of information along axons of cerebellar nuclear cells is required for CR extinction.

In search of memory traces

The key issue in analyzing brain substrates of memory is the nature of memory traces, how memories are formed, stored, and retrieved in the brain. In order to analyze mechanisms of memory formation it is first necessary to find the loci of memory storage, the classic problem of localization. Various approaches to this issue are reviewed. A particular strategy is proposed that involves a number of different techniques (electrophysiological recording, lesions, electrical stimulation, pathway tracing) to identify the essential memory trace circuit for a given form of learning and memory. The methods of reversible inactivation can be used to localize the memory traces within this circuit. Using classical conditioning of eye blink and other discrete responses as a model system, the essential memory trace circuit is identified, the basic memory trace is localized (to the cerebellum), and putative higher-order memory traces are characterized in the hippocampus.

The neuroscience of mammalian associative learning

Mammalian associative learning is organized into separate anatomically defined functional systems. We illustrate the organization of two of these systems, Pavlovian fear conditioning and Pavlovian eyeblink conditioning, by describing studies using mutant mice, brain stimulation and recording, brain lesions and direct pharmacological manipulations of specific brain regions. The amygdala serves as the neuroanatomical hub of the former, whereas the cerebellum is the hub of the latter. Pathways that carry information about signals for biologically important events arrive at these hubs by circuitry that depends on stimulus modality and complexity. Within the amygdala and cerebellum, neural plasticity occurs because of convergence of these stimuli and the biologically important information they predict. This neural plasticity is the physical basis of associative memory formation, and although the intracellular mechanisms of plasticity within these structures share some similarities, they differ significantly. The last Annual Review of Psychology article to specifically tackle the question of mammalian associative learning ( Lavond et al. 1993 ) persuasively argued that identifiable "essential" circuits encode memories formed during associative learning. The next dozen years saw breathtaking progress not only in detailing those essential circuits but also in identifying the essential processes occurring at the synapses (e.g., Bi & Poo 2001, Martinez & Derrick 1996 ) and within the neurons (e.g., Malinow & Malenka 2002, Murthy & De Camilli 2003 ) that make up those circuits. In this chapter, we describe the orientation that the neuroscience of learning has taken and review some of the progress made within that orientation.

Endogenous eye blinks in preadolescents: relationship to information processing and performance

Endogenous blinks--those occurring without apparent provocation--are regulated in adults with respect to the presentation, cognitive loading, and response demands of stimuli. This investigation determined the extent to which similar regulatory and response-related relationships were evident in preadolescents during a visual continuous performance task (CPT). As in adults, increased blink incidence on task, longer blink deferral following stimuli with greater cognitive loading, and blink-facilitated motor responses to imperative stimuli were observed. Reaction times significantly decreased when the button press (BP) occurred near (+/- 200 ms) blink onset and increased across the task period on blink-free but not blink-associated trials. More blinks occurred before motor responses in females, and a reaction time (RT) advantage for males on blink-free trials was maintained across blink-associated conditions. From these results, an interpretation is developed arguing that endogenous blinks are a meaningful and integral component of sensory-motor processing, indexing times of facilitated attentional and motor response capability.

Task-dependent role of the cerebellum in motor learning

This chapter reviews several findings from our laboratory supporting the hypothesis that the cerebellum's role in motor learning is task-dependent. Namely, its contribution is dependent on the specific task being learned. Several studies are reviewed to demonstrate that the effect of temporary or permanent cerebellar lesions on a specific process such as storage varies depending on the behavior. Furthermore, this task-dependency is reflected also in the modulation of Purkinje cells and nuclear neurons recorded during the learning process. The behavioral correlates of this modulation are very paradigm specific. These observations support the above hypothesis and emphasize the importance of paradigm selection in designing experiments focused on elucidating the cerebellum's role in learning a specific motor behavior.

Inhibiting the expression of a classically conditioned behavior prevents its extinction

The underlying neuronal substrates and behavioral properties that might mediate extinction of the classically conditioned eye-blink response (CR) were examined. Four groups of rabbits were trained to perform the CR. Two of the groups then received either three or six sessions of tone-alone extinction training while the motor nuclei that mediate expression of the CR (facial nucleus and accessory abducens) were reversibly inactivated with microinjections of the GABA agonist muscimol. After these inactivation extinction sessions, rabbits received four more extinction sessions without inactivation. Two groups of controls received either three or six extinction sessions while saline vehicle was infused into the motor nuclei, followed by four sessions with no infusions. Saline infusions had no effect on extinction, and controls extinguished the CR normally over the first three to four sessions. In contrast, muscimol inactivation of the motor nuclei completely prevented any performance of CRs during the three or six inactivation extinction sessions. At the start of the four extinction sessions without inactivation, rabbits performed CRs at the same rate and amplitude as controls on their first extinction sessions. The muscimol rabbits then extinguished the CR normally over the four sessions without inactivation. In short, inactivation of the motor nuclei completely prevented any extinction of the eye-blink CR with no effect on subsequent extinction without inactivation. These results are discussed in terms of possible neuroanatomical loci that might mediate the extinction process as well as how effects of manipulating CR performance during extinction may affect the extinction process.

Inhibiting the expression of a classically conditioned behavior prevents its extinction

The underlying neuronal substrates and behavioral properties that might mediate extinction of the classically conditioned eye-blink response (CR) were examined. Four groups of rabbits were trained to perform the CR. Two of the groups then received either three or six sessions of tone-alone extinction training while the motor nuclei that mediate expression of the CR (facial nucleus and accessory abducens) were reversibly inactivated with microinjections of the GABA agonist muscimol. After these inactivation extinction sessions, rabbits received four more extinction sessions without inactivation. Two groups of controls received either three or six extinction sessions while saline vehicle was infused into the motor nuclei, followed by four sessions with no infusions. Saline infusions had no effect on extinction, and controls extinguished the CR normally over the first three to four sessions. In contrast, muscimol inactivation of the motor nuclei completely prevented any performance of CRs during the three or six inactivation extinction sessions. At the start of the four extinction sessions without inactivation, rabbits performed CRs at the same rate and amplitude as controls on their first extinction sessions. The muscimol rabbits then extinguished the CR normally over the four sessions without inactivation. In short, inactivation of the motor nuclei completely prevented any extinction of the eye-blink CR with no effect on subsequent extinction without inactivation. These results are discussed in terms of possible neuroanatomical loci that might mediate the extinction process as well as how effects of manipulating CR performance during extinction may affect the extinction process.

Positron emission tomography scanning in essential blepharospasm

PURPOSE

To localize in the brain using positron emission tomography neuroimaging with (18)fluorodeoxyglucose [PET ((18)FDG)] differences in glucose metabolism between patients with essential blepharospasm (EB) and controls.

DESIGN

Prospective case-control study.

METHODS

Positron emission tomography neuroimaging with (18)fluorodeoxyglucose was performed in 11 patients with EB and 11 controls matched for age and gender. Global analysis of images was used to localize differences in glucose metabolism between groups.

RESULTS

Multiple cortical and subcortical abnormalities were observed in EB patients in comparison with controls. Cortical areas with the largest and most significant clusters of increased glucose uptake were the inferior frontal gyri, right posterior cingulate gyrus, left middle occipital gyrus, fusiform gyrus of the right temporal lobe, and left anterior cingulate gyrus. Cortical areas with the largest and most significant clusters of decreased glucose uptake were the inferior frontal gyri, ventral to the area of increased glucose metabolism. Subcortical abnormalities, consisting of increased glucose uptake, involved the right caudate and consisting of decreased glucose uptake, involved the left inferior cerebellar hemisphere and thalamus.

CONCLUSIONS

Global analysis of positron emission tomography neuroimaging with (18)fluorodeoxyglucose neuroimaging in EB patients in comparison with controls demonstrates a pattern of abnormalities involving several cortical and subcortical areas that control blinking, including the inferior frontal lobe, caudate, thalamus, and cerebellum.

Neural Substrates of Eyeblink Conditioning: Acquisition and Retention

Classical conditioning of the eyeblink reflex to a neutral stimulus that predicts an aversive stimulus is a basic form of associative learning. Acquisition and retention of this learned response require the cerebellum and associated sensory and motor pathways and engage several other brain regions including the hippocampus, neocortex, neostriatum, septum, and amygdala. The cerebellum and its associated circuitry form the essential neural system for delay eyeblink conditioning. Trace eyeblink conditioning, a learning paradigm in which the conditioned and unconditioned stimuli are noncontiguous, requires both the cerebellum and the hippocampus and exhibits striking parallels to declarative memory formation in humans. Identification of the neural structures critical to the development and maintenance of the conditioned eyeblink response is an essential precursor to the investigation of the mechanisms responsible for the formation of these associative memories. In this review, we describe the evidence used to identify the neural substrates of classical eyeblink conditioning and potential mechanisms of memory formation in critical regions of the hippocampus and cerebellum. Addressing a central goal of behavioral neuroscience, exploitation of this simple yet robust model of learning and memory has yielded one of the most comprehensive descriptions to date of the physical basis of a learned behavior in mammals.

Cerebellar function in consolidation of a motor memory

Several forms of motor learning, including classical conditioning of the eyeblink and nictitating membrane response (NMR), are dependent upon the cerebellum, but it is not known how motor memories are stored within the cerebellar circuitry. Localized infusions of the GABA(A) agonist muscimol were used to target putative consolidation processes by producing reversible inactivations after NMR conditioning sessions. Posttraining inactivations of eyeblink control regions in cerebellar cortical lobule HVI completely prevented conditioning from developing over four sessions. In contrast, similar inactivations of eyeblink control regions in the cerebellar nuclei allowed conditioning to develop normally. These findings provide evidence that there are critical posttraining memory consolidation processes for eyeblink conditioning mediated by the cerebellar cortex.

Parallels between cerebellum- and amygdala-dependent conditioning

Recent evidence from cerebellum-dependent motor learning and amygdala-dependent fear conditioning indicates that, despite being mediated by different brain systems, these forms of learning might use a similar sequence of events to form new memories. In each case, learning seems to induce changes in two different groups of neurons. Changes in the first class of cells are induced very rapidly during the initial stages of learning, whereas changes in the second class of cells develop more slowly and are resistant to extinction. So, anatomically distinct cell populations might contribute differentially to the initial encoding and the long-term storage of memory in these two systems.

Blockade of GABAA receptors in the interpositus nucleus modulates expression of conditioned excitation but not conditioned inhibition of the eyeblink response

The cerebellum and related brainstem structures are essential for excitatory eyeblink conditioning. Recent evidence indicates that the cerebellar interpositus and lateral pontine nuclei may also play critical roles in conditioned inhibition (CI) of the eyeblink response. The current study examined the role of GABAergic inhibition of the interpositus nucleus in retention of CI. Male Long-Evans rats were implanted with a cannula positioned just above or in the anterior interpositus nucleus before training. The rats were trained with two different tones and a light as conditioned stimuli, and a periorbital shock as the unconditioned stimulus. CI training consisted of four phases: 1) excitatory conditioning (8 kHz tone paired with shock); 2) feature-negative discrimination (2 kHz tone paired with shock or 2 kHz tone concurrent with light); 3) summation test (8 kHz tone or 8 kHz tone concurrent with light); and 4) retardation test (light paired with shock). After reaching a criterion level of performance on the feature-negative discrimination (40% discrimination), 0.5 microl picrotoxin (a GABAA receptor antagonist) was infused at one of four concentrations, each concentration infused during separate test sessions. Picrotoxin transiently impaired conditioned responses during trials with the excitatory stimulus (tone) in a dose-dependent manner, but did not significantly impact responding to the inhibitory compound stimulus (tone-light). The results suggest that expression of conditioned inhibition of the eyeblink conditioned response does not require GABAergic inhibition of neurons in the anterior interpositus nucleus.

Comparison of single unit responses to tone, light, and compound conditioned stimuli during rabbit classical eyeblink conditioning

Unit recordings and lesion studies have implicated the cerebellum as an essential site for the acquisition and maintenance of the conditioned eyeblink response. The current study looked at the neural characteristics of conditioned stimulus (CS) processing in the interpositus nucleus of the cerebellum after training New Zealand white rabbits (Oryctolagus cuniculus) in one of two conditioning paradigms: (a) compound conditioning (CMP), a compound CS consisting of light and tone paired with an air puff unconditioned stimulus (US); or (b) stimulus compounding (ALT), alternating blocks of tone CS and light CS trials paired with the air puff US. Single unit responses were recorded during five sessions after the animals had reached an asymptotic level of responding. Animals were tested for behavioral and neural responses to CS alone trials that included tone alone, light alone, and compound tone-light trials. For the CMP group, the compound CS elicited 80 to 90% conditioned eyeblink responses (CRs), whereas the individual tone and light CSs elicited only 40 to 50% CRs. For the ALT group, all three CSs (tone, light, and compound) elicited very high levels of responding of at least 80% CRs. For the CMP group, there were roughly equal numbers of cells responding to all of the CSs. This includes cells that responded exclusively to one, and only one, of the three stimuli and also those cells that responded to combinations of two or more. Cells from the ALT group were far more likely to respond exclusively to only one of the CSs. Both the behavioral and physiological results suggest that the compound tone-light stimulus was processed as a distinct stimulus, separate from the component tone and light. These results are discussed in the context of multisensory processing.

Cerebellar substrates for error correction in motor conditioning

The authors evaluate a mapping of Rescorla and Wagner's (1972) behavioral model of classical conditioning onto the cerebellar substrates for motor reflex learning and illustrate how the limitations of the Rescorla-Wagner model are just as useful as its successes for guiding the development of new psychobiological theories of learning. They postulate that the inhibitory pathway that returns conditioned response information from the cerebellar interpositus nucleus back to the inferior olive is the neural basis for the error correction learning proposed by Rescorla and Wagner (Gluck, Myers, & Thompson, 1994; Thompson, 1986). The authors' cerebellar model expects that behavioral processes described by the Rescorla-Wagner model will be localized within the cerebellum and related brain stem structures, whereas behavioral processes beyond the scope of the Rescorla-Wagner model will depend on extracerebellar structures such as the hippocampus and related cortical regions. Simulations presented here support both implications. Several novel implications of the authors' cerebellar error-correcting model are described including a recent empirical study by Kim, Krupa, and Thompson (1998), who verified that suppressing the putative error correction pathway should interfere with the Kamin (1969) blocking effect, a behavioral manifestation of error correction learning. The authors also discuss the model's implications for understanding the limits of cerebellar contributions to associative learning and how this informs our understanding of hippocampal function in conditioning. This leads to a more integrative view of the neural substrates of conditioning in which the authors' real-time circuit-level model of the cerebellum can be viewed as a generalization of the long-term memory module of Gluck and Myers' (1993) trial-level theory of cerebellar-hippocampal interaction in motor conditioning.

Acquisition of eyeblink conditioning is critically dependent on normal function in cerebellar cortical lobule HVI

Classical conditioning of the nictitating membrane response (NMR)/eyeblink response of rabbits is a simple form of cerebellar-dependent, associative motor learning. Reversible inactivations of the cerebellar nuclei and inferior olive have implicated the olivo-cortico-nuclear loop in the acquisition of nictitating membrane conditioning, but the role of the cerebellar cortex in acquisition has not been tested directly. Here we have used local infusions of the water-soluble, disodium salt of 6-cyano-7-nitroquinoxaline-2,3-dione reversibly to block cerebellar cortical AMPA/kainate receptors in lobule HVI during acquisition training. After the drug effects dissipated, there was no evidence that acquisition had taken place; the subjects behaved as if naive. Further training without inactivation then allowed normal acquisition, and further inactivations during performance of conditioned responses abolished these established responses. There was a strong correlation between the inactivation effects on acquisition and subsequent inactivation effects on performance, indicating that the same eyeblink-control cortical microzones are engaged in learning and expressing this behavior. The cortical component of the olivo-cortico-nuclear loop is essential for acquisition of classically conditioned nictitating membrane response learning, and eyeblink control areas in HVI are critical. Our findings are consistent with models of cerebellar learning that assign essential plasticity to the cortex or to a distribution between levels in olivo-cortico-nuclear modules.

Acquisition of eyeblink conditioning is critically dependent on normal function in cerebellar cortical lobule HVI

Classical conditioning of the nictitating membrane response (NMR)/eyeblink response of rabbits is a simple form of cerebellar-dependent, associative motor learning. Reversible inactivations of the cerebellar nuclei and inferior olive have implicated the olivo-cortico-nuclear loop in the acquisition of nictitating membrane conditioning, but the role of the cerebellar cortex in acquisition has not been tested directly. Here we have used local infusions of the water-soluble, disodium salt of 6-cyano-7-nitroquinoxaline-2,3-dione reversibly to block cerebellar cortical AMPA/kainate receptors in lobule HVI during acquisition training. After the drug effects dissipated, there was no evidence that acquisition had taken place; the subjects behaved as if naive. Further training without inactivation then allowed normal acquisition, and further inactivations during performance of conditioned responses abolished these established responses. There was a strong correlation between the inactivation effects on acquisition and subsequent inactivation effects on performance, indicating that the same eyeblink-control cortical microzones are engaged in learning and expressing this behavior. The cortical component of the olivo-cortico-nuclear loop is essential for acquisition of classically conditioned nictitating membrane response learning, and eyeblink control areas in HVI are critical. Our findings are consistent with models of cerebellar learning that assign essential plasticity to the cortex or to a distribution between levels in olivo-cortico-nuclear modules.

Ontogenetic changes in the neural mechanisms of eyeblink conditioning

The rodent eyeblink conditioning paradigm is an ideal model system for examining the relationship between neural maturation and the ontogeny of associative learning. Elucidation of the neural mechanisms underlying the ontogeny of learning is tractable using eyeblink conditioning because the necessary neural circuitry (cerebellum and interconnected brainstem nuclei) underlying the acquisition and retention of the conditioned response (CR) has been identified in adult organisms. Moreover, the cerebellum exhibits substantial postnatal anatomical and physiological maturation in rats. The eyeblink CR emerges developmentally between postnatal day (PND) 17 and 24 in rats. A series of experiments found that the ontogenetic emergence of eyeblink conditioning is related to the development of associative learning and not related to changes in performance. More recent studies have examined the relationship between the development of eyeblink conditioning and the physiological maturation of the cerebellum, a brain structure that is necessary for eyeblink conditioning in adult organisms. Disrupting cerebellar development with lesions or antimitotic treatments impairs the ontogeny of eyeblink conditioning. Studies of the development of physiological processes within the cerebellum have revealed striking ontogenetic changes in stimulus-elicited and learning-related neuronal activity. Neurons in the interpositus nucleus and Purkinje cells in the cortex exhibit developmental increases in neuronal discharges following the unconditioned stimulus (US) and in neuronal discharges that model the amplitude and time-course of the eyeblink CR. The developmental changes in CR-related neuronal activity in the cerebellum suggest that the ontogeny of eyeblink conditioning depends on the development of mechanisms that establish cerebellar plasticity. Learning and the induction of neural plasticity depend on the magnitude of the US input to the cerebellum. The role of developmental changes in the efficacy of the US pathway has been investigated by monitoring neuronal activity in the inferior olive and with stimulation techniques. The results of these experiments indicate that the development of the conditioned eyeblink response may depend on dynamic interactions between multiple developmental processes within the eyeblink neural circuitry.

Parallel neural systems for classical conditioning: support from computational modeling

Classical conditioning has been explained by two main types of theories that postulate different learning mechanisms. Rescorla and Wagner (1972) put forth a theory in which conditioning is based on the ability of the US to drive learning through error correction. Alternatively, Mackintosh (1973) put forth a theory in which the ability of the CS to be associated with the unconditioned stimulus is modulated. We have proposed a reconciliation of these two mechanisms as working in parallel within different neural systems: a cerebellar system for US modulation and a hippocampal system for CS modulation. We developed a computational model of cerebellar function in eyeblink conditioning based on the error correction mechanism of the Rescorla-Wagner rule in which learning-related activity from the cerebellum inhibits the inferior olive, which is the US input pathway to the cerebellum (Gluck et al., 1994). We developed a computational model of the hippocampal region that forms altered representations of conditioned stimuli based on their behavioral outcomes (Gluck & Myers, 1993; Myers et al., 1995). Overall, computational modeling and empirical findings support the idea that, at least in the case of eyeblink conditioning, there may be two different neural systems: the cerebellum which mediates US-based error correction and hippocampus which alters representations of CSs.

Learning- and expectation-related changes in the human brain during motor learning

We have studied a simple form of motor learning in the human brain so as to isolate activity related to motor learning and the prediction of sensory events. Whole-brain, event-related functional magnetic resonance imaging (fMRI) was used to record activity during classical discriminative delay eyeblink conditioning. Auditory conditioned stimulus (CS+) trials were presented either with a corneal airpuff unconditioned stimulus (US, paired), or without a US (unpaired). Auditory CS- trials were never reinforced with a US. Trials were presented pseudorandomly, 66 times each. The subjects gradually produced conditioned responses to CS+ trials, while increasingly differentiating between CS+ and CS- trials. The increasing difference between hemodynamic responses for unpaired CS+ and for CS- trials evolved slowly during conditioning in the ipsilateral cerebellar cortex (Crus I/Lobule HVI), contralateral motor cortex and hippocampus. To localize changes that were related to sensory prediction, we compared trials on which the expected airpuff US failed to occur (Unpaired CS+) with trials on which it occurred as expected (Paired CS+). Error-related signals in the contralateral cerebellum and somatosensory cortex were seen to increase during learning as the sensory prediction became stronger. The changes seen in the ipsilateral cerebellar cortex may be due either to the violations of sensory predictions, or to learning-related increases in the excitability of cerebellar neurons to presentations of the CS+.

Learning about pain: the neural substrate of the prediction error for aversive events

Associative learning is thought to depend on detecting mismatches between actual and expected experiences. With functional magnetic resonance imaging (FMRI), we studied brain activity during different types of mismatch in a paradigm where contrasting-colored lights signaled the delivery of painful heat, nonpainful warmth, or no stimulation. When painful heat stimulation was unexpected, there was increased FMRI signal intensity in areas of the hippocampus, superior frontal gyrus, cerebellum, and superior parietal gyrus that was not found with mismatch between expectation and delivery of nonpainful warmth stimulation. When painful heat stimulation was unexpectedly omitted, the FMRI signal intensity decreased in the left superior parietal gyrus and increased in the other regions. These contrasting activation patterns correspond to two different mismatch concepts in theories of associative learning (Rescorla-Wagner, temporal difference vs. Pearce-Hall, Mackintosh). Searching for interventions to specifically modulate activation of these brain regions therefore offers an approach to identifying new treatments for chronic pain, which often has a substantial associative learning component.

Intracerebellar conditioning--Brogden and Gantt revisited

In 1942, Brogden and Gantt reported that electrical stimulation of cerebellar white matter elicited specific behavioral responses (limb flexion, eyeblink, etc.) and that these movements so elicited could easily be conditioned to a neural tone CS, using standard Pavlovian procedures. This early evidence for the key role of the cerebellum in learning of discrete movements has in recent years been replicated and much extended. It is now clear that the cerebellum is the essential structure for associative learning of discrete movements elicited by peripheral aversive or intracerebellar stimuli and that the memory traces so formed are stored in the cerebellum.

Learning- and cerebellum-dependent neuronal activity in the lateral pontine nucleus

The effects of inactivation of cerebellar deep nuclei and the lateral pontine nucleus on classical eyeblink conditioning with tone or lateral reticular nucleus (LRN) stimulation as conditioned stimuli (CSs) were examined. Inactivation of cerebellar deep nuclei abolished eyeblink conditioned responses (CRs) when the CS was either a tone or LRN stimulation. Inactivation of the lateral pontine nucleus prevented only the acquisition and retention of tone-evoked eyeblink CRs. Multiple-unit recording demonstrated that when LRN stimulation was used as the CS, inactivation of the interpositus nucleus abolished learning-related neuronal activity in the lateral pontine nucleus, whereas inactivation of pontine nucleus had little effect on similar activity in the interpositus nucleus. Thus, the learning-induced neuronal activity in the lateral pontine nucleus was most likely driven by the cerebellar interpositus nucleus.

Microinfusion of protein kinase inhibitor H7 into the cerebellum impairs the acquisition but not the retention of classical eyeblink conditioning in rabbits

Rabbits were infused with H7, a general protein kinase inhibitor, into the region of the cerebellar interpositus nucleus during classical eyeblink conditioning. Acquisition of the conditioned eyeblink response was delayed by the H7 infusion, but the protein kinase inhibitor had no effect on performance of the learned response when infused after asymptotic learning had been reached. These data indicate that protein kinases in the cerebellum are involved in plasticity processes that underlie the learning of this simple conditioned behavior.

Short-lasting conditioned stimulus applied to the middle cerebellar peduncle elicits delayed conditioned eye blink responses in the decerebrate ferret

In delay eye blink conditioning, the conditioned stimulus (CS) ends at the time of the unconditioned stimulus (US). If the CS duration is decreased, there will be a 'trace' period with no ongoing CS before the onset of the US. During this period some neural activity has to continue after the CS offset to: (i) permit association between the CS and the US; and (ii) elicit a conditioned response appearing after the CS offset. In this study we test the role of the cerebellum in maintaining CS activity required for eliciting a conditioned response after the CS offset. Decerebrate ferrets were trained in a delay conditioning paradigm with an electrical stimulation of the forelimb as CS and of the periorbital area as US. The conditioned responses in the upper eyelid were monitored with electromyographical techniques. In well-trained animals, test CSs of short duration down to 0.2 ms were applied to the forelimb or the middle cerebellar peduncle, while the interstimulus interval between CS onset and US onset was kept constant at 300 ms. Test CSs of short duration applied to the forelimb elicited conditioned responses. More importantly, also a short-lasting CS to the middle cerebellar peduncle could elicit conditioned responses. The results indicate that precerebellar CS pathways are not required for maintaining the neural activity that elicits conditioned responses after the CS offset. It is suggested that neurons maintaining such activity are located in the cerebellum, either the cortex alone or the cortex and the deep nuclei.

Learning induces a CDC2-related protein kinase, KKIAMRE

To elucidate molecular mechanisms in learning and memory, we analyzed expression of mRNAs in brains of rabbits undergoing eyeblink conditioning. Infusion of the transcription inhibitor actinomycin D into the cerebellar interpositus nucleus reversibly blocked learning but not performance of the conditioned response. Differential display PCR analysis of cerebellar interpositus RNAs from trained and pseudotrained rabbits identified a 207 bp band that was induced with learning. The fragment was used to isolate a cDNA from a lambdagt11 rabbit brain library containing a 1698 bp open reading frame. The deduced amino acid sequence contains the KKIAMRE motif, which is conserved among cell division cycle 2 (cdc2)-related kinases. These results suggest that there is a new category of cdc2-related kinases in the brain whose function may be important in learning and memory.

Learning induces a CDC2-related protein kinase, KKIAMRE

To elucidate molecular mechanisms in learning and memory, we analyzed expression of mRNAs in brains of rabbits undergoing eyeblink conditioning. Infusion of the transcription inhibitor actinomycin D into the cerebellar interpositus nucleus reversibly blocked learning but not performance of the conditioned response. Differential display PCR analysis of cerebellar interpositus RNAs from trained and pseudotrained rabbits identified a 207 bp band that was induced with learning. The fragment was used to isolate a cDNA from a lambdagt11 rabbit brain library containing a 1698 bp open reading frame. The deduced amino acid sequence contains the KKIAMRE motif, which is conserved among cell division cycle 2 (cdc2)-related kinases. These results suggest that there is a new category of cdc2-related kinases in the brain whose function may be important in learning and memory.